Cement Industry Process Technology - Holderbank Course (2 of 3)
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II
B05 - PT II
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems
C01 - Kiln Systems
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview
Kiln Systems - Overview Urs Gasser PT 99/14501/E 1. PROCESS REQUIREMENTS FOR KILN SYSTEMS 2. PROCESS TYPES 2.1 General 3. WET PROCESS 3.1 General 3.2 Long Wet Process Kilns 3.3 Wet Process Kilns with Slurry Preheaters 4. SEMI WET PROCESS 4.1 General 4.2 Semi Wet Process Long Kilns 4.3 Semi Wet Grate Preheater Kilns 4.4 Semi-Wet Suspension Pre-heater Kiln 5. SEMI DRY PROCESS 5.1 Semi-Dry Process Long Kilns 5.2 Semi-Dry Process Grate Pre-heater Kilns 6. DRY PROCESS 6.1 Long Dry Kilns 6.2 Raw Meal Suspension Preheater Kilns 6.2.1
General
6.2.2
One and two Stage Cyclone Pre-heater Kilns
6.2.3
Four Stage Cyclone Pre-heater Kilns
6.2.4
Precalciner Kilns
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"Holderbank" - Cement Course 2000 SUMMARY Today’s kiln systems for burning cement clinker of major importance use a rotary kiln. Exceptions are vertical shaft kilns still used in certain geographical areas. With very rare exceptions, new plants use the dry process. However, there are still important markets where older wet process plants are predominant (USA, Russia). A first classification of the process can be made based on the water content of the kiln feed:
< 1% water
dry-process
10 ...
12% water
semi-dry-process
17 ...
21% water
semi-wet-process
25 ...
40% water
wet-process
♦ Dry-Process •
Precalciner kiln with 4 to 6 cyclone stages (contemporary technology): ∗ Separate tertiary air duct ∗ 50 - 60% fuel to the precalciner ∗ Large capacities possible > 10000 t/d ∗ Up to 4000 t/d in 1 string ∗ Heat consumption < 3000 kJ/kg possible (6 stages) ∗ Sensitive to circulation phenomena (-> kiln gas bypass!)
•
4-stage cyclone pre-heater kiln (standard technology 1970 to 1980): ∗ Cyclone stages (co-current flow) for raw meal preheating ∗ Large application world wide ∗ Capacities of up to 4500 t/d technically possible ∗ Heat consumption: 3150 to 3350 kJ/kg cli ∗ Sensitive to circulation phenomena (-> kiln gas bypass!)
•
2-stage cyclone pre-heater kiln: ∗ Less sensitive to circulation phenomena than 4-stage pre-heater ∗ Higher heat consumption than pre-heater with more stages
•
Shaft pre-heater kiln: ∗ Counter current heat exchange between hot gas and raw meal ∗ Practical efficiency inferior to cyclone pre-heater
•
Long-dry-kiln: ∗ Rather simple equipment ∗ High dust emission from kiln tube ∗ Without heat exchange internals: high heat consumption of up to 5100 kJ/kg cli ∗ With chains and/or crosses: 4200 kJ/kg cli achievable
♦ Semi-dry and semi wet process •
Grate pre-heater kiln (LEPOL, ACL): ∗ Raw meal must be suitable to be nodulised with water (semi-dry)
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"Holderbank" - Cement Course 2000 ∗ 3450 kJ/kg cli (no waste heat available for primary raw material drying) •
Long rotary kiln and suspension preheater: ∗ Filter cakes fed or slurry injection into vertical dryer; rather rare cases
♦ Wet-process •
Long wet kiln: ∗ Fed with raw meal slurry of approx. 32 - 42% water content ∗ Internal heat transfer improved by chains ∗ High heat consumption of 5300 to 6300 kJ/kg cli due to evaporation of water ∗ Heat consumption reduced by slurry thinners for a slurry with 25 - 30% H2O ∗ Slurry preheaters can reduce kiln size and improve heat exchange
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 1. PROCESS REQUIREMENTS FOR KILN SYSTEMS
1.
PROCESS REQUIREMENTS FOR KILN SYSTEMS
The kiln system has to be designed to cope with the requirements of the chemical process during which the kiln feed material is converted into cement clinker. This process as a whole is endothermic and takes place at maximum material temperatures of 1450°C. Receiving its thermal energy from hot gases of up to 2000°C generated by combusting fuels, it is also referred to as pyroprocess. Type of reaction and temperature development are compiled in “sequence of reactions occurring in a rotary kiln” (table 1) and graphically as the “quasi-qualitative variation of minerals with temperature” (figure 1). The chemical process taking place in the kiln system where raw meal (input) is converted to cement clinker (output) can be subdivided into the following five steps: 1.
Drying
2.
Preheating
3.
Calcining
4.
Sintering
5.
Cooling
Process and equipment has been developed and improved with the aim at performing these steps forever improved economy, which means •
High availability
•
Low heat consumption
•
Low power consumption
•
Higher unit capacity
•
Stable kiln operation
•
Good, uniform clinker quality
Table 1
Sequence of Reactions occurring in a Rotary Kiln
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"Holderbank" - Cement Course 2000 Temperature range (°C)
Type of reaction
Heating Up 20 - 100
Evaporation of free H2O
100 - 300
Loss of physically absorbed water
400 - 900
Removal of structural H2O (H2O and OH groups) from clay minerals
> 500
Structural changes in silicate minerals
600 - 900
Dissociation of carbonates CO2 driven out)
> 800
Formation of belite, intermediate products, aluminate and ferrite
> 1250
Formation of liquid phase (aluminate and ferrite melt)
approx. 1450
Completion of reaction and re-crystallisation of alite and belite
Cooling 1300 - 1240
Crystallisation of liquid phase into mainly aluminate and ferrite
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 2. PROCESS TYPES
2.
PROCESS TYPES
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 2. PROCESS TYPES / 2.1 General
2.1
General
The criterion normally used to distinguish the process types is the moisture of the kiln feed material. Four basically different process types for clinker burning can be defined:
Process Type
Feed Material
Cons.
Dry process
Raw meal
Dry
Semi dry process
Nodules
Moist
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Feed Moisture
Feed System
< 1% H2O
Mechanic, pneumatic
≈ 10 ... 12% H2O
Mechanic, pneumatic
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≈ 10 ... 12% H2O
"Holderbank" - Cement Course 2000 Semi wet process
Filter cake, nodules Moist
≈ 17 ... 21% H2O
Mechanic, pneumatic
Wet process
Slurry
≈ 25 ... 40% H2O
Hydraulic
Liquid
Table 1 gives a general survey of the various rotary kiln systems in operation for industrial clinker production. Shaft kilns, which are still used in China or experimental systems such as sintering grates or fluidised beds, are not considered in the scheme. We can distinguish two main groups of kiln systems: a)
Long kilns with or without internal heat exchanging installation
b)
Short or medium kilns with external preheaters (e.g. suspension preheaters, grates or external slurry preheaters)
The heat consumption of burning depends strongly on the water content of the kiln feed This can be illustrated by the typical specific heat consumption: The fuel consumption of wet kilns is nearly twice as high as for modern dry process suspension pre-heater kilns. The comparison of the heat economy within each process group (dry or wet) shows clearly: The more intensive the heat-exchange for drying and preheating, the lower the heat consumption. Other than based on the feed moisture, kiln systems can be grouped in different ways: Process Type
wet semi wet semi dry dry
>25% H2O in feed 17 - 21% H2O in feed 10 - 12% H2O in feed < 1% H2O in feed
Slurry nodules from slurry nodules from meal raw meal
Production Mode
batch+cont. continuous
< 200 t/d 300 t/d – 10’000 t/d
shaft kilns rotary kilns
2900 kJ/kg cli ( 700 kcal/kg cli)
state of the art system
> 6000 kJ/kg cli (> 1400 kcal/kg cli)
long wet or dry kilns, not optimum operation
20 to 65 kWh/t cli
kiln feed to clinker cooler
Heat Consumption
Power Consumption
OVERVIEW OF KILN AND PROCESS TYPES
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"Holderbank" - Cement Course 2000
When the concept for a new plant is developed, not only the present situation but also the possible future developments of all relevant factors must be taken into account. The following main parameters must be considered when selecting the kiln system: •
Raw material: ∗ moisture content ∗ grindability ∗ homogeneity of deposit ∗ number of components for raw mix ∗ chemical composition (sulphur, chlorides, alkalis, organic compounds etc) ∗ filtration properties of slurry (for semi-wet process only)
•
Plant installation and operating costs
•
Requirements for clinker quality (e.g. low alkali clinker)
•
Aspects of environmental protection (emission of dust, SOx, NOx, etc)
•
Technical standard of the country
Long wet (and dry) rotary kilns are the oldest and most simple type of installation to produce cement clinker. The pyroprocess takes place in a long rotating tube, which has usually internal equipment to improve heat transfer, and, in wet kilns, to reduce dust loss. Unit capacities of up to 2000 t/d are typically achieved, higher outputs are possible, however, they require kilns of gigantic dimensions. Today, economy requires plants for 3000 to 10’000 t/d. Therefore new plants are almost always based on the dry process with preheater, pre-calciner and reciprocating grate cooler. The semi wet process for a new plant could be preferred in special cases, e.g. where raw material with a high natural moisture must be used (e.g. quarry below water level). The three following graphs illustrate the development of the significance of the various processes within the Holderbank group, which can be considered representative of the global situation.
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"Holderbank" - Cement Course 2000
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 3. WET PROCESS
3.
WET PROCESS
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 3. WET PROCESS / 3.1 General
3.1
General
The wet process was the most important process for clinker burning in the past and almost all plants were wet. Heterogeneous quarries and corrective addition were no problem; stirring of the liquid slurry in the slurry tanks provides very good batch-wise blending. Grinding was done in slurry mills, which consume 30%, less energy than dry ball mills, but at higher lining wear rates. © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 The disadvantage of the wet process is the high heat consumption. Compared to e.g. a suspension preheater kiln, the difference is more than 2000 kJ/kg clinker or 60 to 70%! Today, with efficient dry homogenising technology available, the wet process is no longer applied for new plants. Investments as well as operating costs of a wet system are higher than for dry systems of the same output. Technical development allows using more efficient kiln systems even where wet plants would have been built in earlier times. Another reason for preferring the wet process in the past was the production of low alkali cement (alkali content < 0,6%) and the fact that difficult circulation problems are easier to control in wet kilns. Today secondary firing or efficient bypass installations with precalciner are possibilities to keep these problems under control also in modern kiln systems. Because of the lower specific gas volume and the shorter rotary part, rotary kiln dimensions as well as gas handling, dedusting and fuel preparation can be designed accordingly smaller. Although new wet kilns are no longer considered for new plants, they still play an important role in the US as well as in many countries of Eastern Europe and Central Asia. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 3. WET PROCESS / 3.2 Long Wet Process Kilns
3.2
Long Wet Process Kilns
Long wet kilns have been the most commonly used burning reactors for a very long time, but because of the high water content of the feed, their heat consumption is up to twice as high as for modern dry systems. The milled and homogenised raw material is a slurry with a water content of typically 32 to 42% and is pumped to the kiln inlet. In the first zone heat transfer for the evaporation of water is always increased by means of chain systems (extended surface, higher relative velocities, increase of turbulence). The chain systems should also reduce the dust losses and clean the kiln shell. These internal heat exchanger installations require very special know-how, based to a large degree on experience (see separate paper ‘chain systems’). In order to decrease fuel consumption the water content should be kept as low as possible. The limit is normally the pumpability of the slurry. It is basically possible to further reduce the slurry moisture by using slurry thinners. This technology has been successfully applied and will provide an economical advantage if adequate quantities are available at low cost, e.g. as industrial by-product. Example: Beauport (Canada): 28% feed moisture
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"Holderbank" - Cement Course 2000 Wet kilns are relatively insensitive to circulation problems because the critical temperature ranges are in the rotary part of the kiln (see also ‘circulation phenomena’). Low alkali clinker can be produced from high alkali raw material simply by selectively wasting of dust: The highest enriched kiln dust (e.g. from the last precipitator compartment) is removed from the process (i.e. dumped onto a dust pile) as necessary. The rest of the dust can be reintroduced to the kiln by dust scoops or insufflation into the burning zone. Today, discarding dust creates increasing problems because of restrictive permitting of dust piles. Note: Kiln dust cannot just be blended to the slurry because it would react and thicken the slurry. Typical technical data for long wet kilns with chains:
Heat consumption q
5’000 ... 6’300 kJ/kg cli (1’200 ... 1’500 kcal/kg cli)
Kiln exit gas temperature
150° ... 250°C
System pressure drop
0,5 ... 1,0 kPa
Dust emission in % of clinker production
5 ... 100%
Probably the largest wet process kiln in the world is installed at Holnam’s Clarksville plant (Michigan USA). This kiln has a diameter of 7,6 m and a length of 232 m with a daily capacity of about 3’600 t. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 3. WET PROCESS / 3.3 Wet Process Kilns with Slurry Preheaters
3.3
Wet Process Kilns with Slurry Preheaters
External Slurry Preheaters In order to improve the heat exchange between gas and slurry and to reduce the kiln size, external slurry preheaters have been developed by MIAG (Kalzinator) and Krupp (Konzentrator). Both of them are revolving drums with special internal packing. These drums have about the same diameter as the kiln, its length being slightly smaller than the diameter. The capacity of these machines is limited to 800 -1000 t/d and frequently operating problems arise. Very often, external preheaters were large sources of false air. Internal Slurry Preheaters F.L. Smidth designed a slurry pre-heater system integrated into the kiln compartment, which should avoid the disadvantage of external slurry preheaters. In practice, this construction turned out to be very sensitive to clogging. A better system developed by Fives Cail Babcock is installed in the three kilns at Obourg. Lifting buckets and chain curtains produce a slurry curtain that keeps back a high amount of dust and improves heat exchange.
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Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS
4.
SEMI WET PROCESS
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS / 4.1 General
4.1
General
A process is considered semi-wet if the kiln feed is produced from wet slurry. A mechanical water extraction process reduces the water content of the kiln feed to 17 to 21%. A number of filter presses operating batch-wise are commonly used, but also continuous filter band presses or similar equipment would be possible. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS / 4.2 Semi Wet Process Long Kilns
4.2
Semi Wet Process Long Kilns
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"Holderbank" - Cement Course 2000 Principally, long kilns with heat exchanger crosses can be fed with slurry, filter cakes or dry meal. Feeding filter cakes is a straightforward and simple solution and is used by Italcementi in some cases. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS / 4.3 Semi Wet Grate Preheater Kilns
4.3
Semi Wet Grate Preheater Kilns
Most of the semi-wet systems use a grate preheater kiln fed with filter cakes. A grate preheater system includes a short rotary kiln (similar to a four stage preheater kiln) where only calcining and sintering take place. For drying, preheating and partial calcining, a travelling grate is installed in front of the kiln, where heat of the kiln exhaust gases is used. For the semi-wet grate kiln, the slurry must be prepared in a special way so it can be fed to a travelling grate: The pumpable slurry as starting material is fed to filter presses where the moisture content is reduced to approx. 20% applying a filtration pressure of 15 to 20 bar. In a special type of extruder (Siebkneter), the filter cakes are converted into cylindrical nodules (diameter 15 ... 20 mm, length 30 ... 50 mm) and then fed to the preheater-grate. The economy of this way of preparation depends strongly on the filtration properties of the slurry. Operating and performance data are similar to the semi-dry grate preheater systems described under 5.2. Characteristic data of a semi-wet grate pre-heater system: Feed Nodules made from Moisture Content of the Feed
Slurry Filter Cake 10 ... 12%
Heat consumption q
3770 kJ/kg cli (≈ 900 kcal/kg cli)
Exit gas temperature after grate
100° ... 120°C
System pressure drop
2,6 kPa
Example of a semi-wet LEPOL kiln: AB’s kiln 10 at the Lägerdorf plant (Germany) Maximum kiln capacity:
3’600 t/d
Kiln dimensions:
φ 6.0/5.6 m x 90 m
Grate dimensions:
5.6 x 61.7 m
Secondary firing with Fullers earth (special) (Shut down; replaced by semi wet precalciner kiln in 1996)
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Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS / 4.4 Semi-Wet Suspension Pre-heater Kiln
4.4
Semi-Wet Suspension Pre-heater Kiln
The suspension preheater kiln is normally fed with dry meal (details see separate paper). However, there are some rare cases where suspension preheater kilns are fed with nodules prepared from slurry. These nodules should not be too strong because they must be cracked by thermal shock or abrasion before being fed to the kiln system via top stage of the pre-heater. A two-stage pre-heater kiln operated with semi-wet nodules was e.g. the Liesberg plant. There, the nodules were cracked in a vertical dryer before being fed to the preheater. The first modern kiln system using this principle has been built in the late 1980’s by FLS in Aalborg Cement’s RORDAL plant. It is a three stage two string kiln system with precalciner for a capacity of 4000 t/d. The high operating cost of the filter presses has been avoided by directly injecting the slurry into a drier-crusher followed by a vertical drier. The semi-wet process was selected because the raw material (chalk) is mined under water and has very high natural moisture. From the “Holderbank” group: Example of a semi-wet pre-heater/pre-calciner kiln: AB’s kiln 11 at the Lägerdorf plant (Germany) Maximum kiln capacity:
4’500 t/d at 3900 kJ/kg
Kiln dimensions:
φ 4.8 x 65 m; 2 supports, gearless friction drive
Preheater:
3 stages, 2 strings
Utilisation of various alternative fuels in both firings Supplied by Polysius; start-up: 1996 Filter cakes produced in already existing filter-presses
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Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 5. SEMI DRY PROCESS
5.
SEMI DRY PROCESS
The semi-dry process is characterised by the fact that kiln feed nodules are made from dry raw meal. Water is added in order to produce nodules with 10 - 12% moisture. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 5. SEMI DRY PROCESS / 5.1 Semi-Dry Process Long Kilns
5.1
Semi-Dry Process Long Kilns
There are long kilns with heat exchanger crosses fed with nodules. This system was applied by Italcementi and looks very similar to an installation for semi-wet feed material. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 5. SEMI DRY PROCESS / 5.2 Semi-Dry Process Grate Pre-heater Kilns
5.2
Semi-Dry Process Grate Pre-heater Kilns
The grate preheater kiln is by far the most popular semi-dry system. The principle of the grate preheater system for the semi-dry process is identical to the one used for the semi-wet process. What is different is the feed preparation: The dry raw material is mixed with water (10 ... 12%) and nodulised in a drum or preferably on a rotating plate (pan noduliser). This system can be used only for raw materials containing plastic components enabling the formation of nodules that are resistant against thermal shock and abrasion. © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 The main factor influencing plasticity is the mineralogical composition, especially the presence of montmorillonite. On the grate, heat exchange from the gas to the nodules forming a fixed bed layer of approx. 20 cm thickness is excellent. In some grate preheaters, precalcination is done successfully, often using even waste fuels (such as Fullers earth, acid sludge, waste lubricating oils etc.) utilising secondary firing. The only successfully working travelling grate pre-heater was available from Polysius and became known under the name LEPOL system (American licensee: Allis-Chalmers, ACL system).
This principle sketch shows a LEPOL kiln fed with nodules made out of dry raw meal. LEPOL kilns built after 1945 are equipped with two-pass grates; i.e. the exhaust gas is led twice through the nodule bed from top to bottom: The hot kiln gas passes first through a bed of dry and preheated nodules and subsequently, after an intermediary dedusting once again trough a layer of moist incoming nodules. It is believed that the nodules survive throughout the process resulting in a clinker with very uniform size. Furthermore, dust loads in the kiln atmosphere and dust emission out of the system are low. The nodules on the grate let only pass the fine dust while the coarse particles are retained. In cases of increased trace compound concentrations (especially alkali) in the raw material, the fine dust separated in the electrostatic precipitator is largely enriched with them. Only a small amount of dust has to be discarded to reduce the balance of these compounds in the kiln system. This effect makes the LEPOL kiln quite suitable to produce a low alkali clinker with rather low heat consumption. For this reason, it has been chosen in many cases, particularly in the USA. The following limits and disadvantages have to be considered: •
Only raw materials with good plastic properties can be used (semi-wet: filter cake nodules -> good filtration properties are required)
•
The grate chain is subject to wear.
•
Uneven temperature distribution across the grate can cause difficulties.
•
Additional theoretical heat consumption due to the water content of the feed (partially compensated by a low exit gas temperature).
•
Exhaust gases cannot be used in drying and grinding systems.
Characteristic data of a semi-dry grate pre-heater systems: Feed nodules made from © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Feed nodules made from Course 2000 dry raw meal Moisture content of the feed nodules
10 ... 12%
Specific heat consumption q
3450 kJ/kg cli (= 820 kcal/kg cli)
Exit gas temperature after grate
100 ... 120°C
System pressure drop
2.6 kPa
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS
6.
DRY PROCESS
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.1 Long Dry Kilns
6.1
Long Dry Kilns
Without internal heat exchange equipment The simplest kind of dry process installation is the long dry kiln without any internal heat exchange equipment (empty tube). With a heat consumption of 5100 kJ/kg cli (1200 kcal/kg cli) or about 90% of the wet process it must be considered very uneconomical. Advantages might be its simplicity and insensitivity to heavy circulation problems. This kiln type is suitable to be used in combination with waste heat recovery steam boilers for power © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 generation. In that case, the waste heat contained in the hot kiln exhaust gases is further used to produce valuable energy. Characteristic kiln data: Heat consumption q
4500 ... 6000 kJ/kg cli
Kiln gas exit temperature
450° ... 500°C
System pressure drop
0,5 ... 1,0 kPa
(1075 ... 1430 kcal/kg cli)
With internal heat exchange equipment Long dry kilns with internal heat exchange equipment (chains or crosses from steel or ceramic material) represent a more economical solution. Heat consumption of 4200 kJ/kg or even less can be achieved. Other typical operating figures are contained in annex 10. Characteristic kiln data: Heat consumption q
3800 ... 4500 kJ/kg cli
Kiln gas exit temperature
400° ... 450°C
System pressure drop
1,0 ... 1,5 kPa
(910 ... 1075 kcal/kg cli)
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns
6.2
Raw Meal Suspension Preheater Kilns
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns / 6.2.1 General
6.2.1
General
During the last thirty years, the suspension preheater kiln became the dominant clinker manufacturing system. This system is fed by dry raw meal that is preferably prepared in a grinding and drying plant, using the kiln waste gases as a drying medium. This ground and dried raw meal is homogenised and then fed to the preheater where it is suspended in the kiln gas flow, where an extremely effective heat transfer takes place. More information is contained in the special section “Suspension Preheaters”. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns / 6.2.2 One and two Stage Cyclone Pre-heater Kilns
6.2.2
One and two Stage Cyclone Pre-heater Kilns
Characteristic kiln data: one stage:
Heat consumption q
3750 ... 4000 kJ/kg cli
Kiln gas exit temperature
400° ... 500°C
System pressure drop
1,5 ... 2,5 kPa
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(900 ... 950 kcal/kg cli)
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"Holderbank" - Cement Course 2000 two stages:
Heat consumption q
3500 ... 3750 kJ/kg cli
Kiln gas exit temperature
400° ... 450°C
System pressure drop
1,5 ... 2,5 kPa
(850 ... 900 kcal/kg cli)
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns / 6.2.3 Four Stage Cyclone Pre-heater Kilns
6.2.3
Four Stage Cyclone Pre-heater Kilns
Until the mid 1980s, this arrangement belong to the systems with the lowest fuel consumption. It was offered in several configurations with capacities up to 4500 t/d, most of them being combinations of single or twin cyclone stages. The kiln exit gas includes still enough heat to dry raw material up to moisture content of 8% if the mill is running during all the kiln operation time. From this point of view, the remaining relatively high exit gas temperature cannot be considered fully as a loss, because it can substitute an auxiliary firing for raw material drying. The preheater system is installed in a steel or concrete tower with a height of about 60 to 120 m (6 stages) above the kiln inlet, depending on capacity and concept. The four to six stages preheater is most susceptible to circulation problems at presence of excessive concentration of circulation compounds causing clogging problems in the pre-heater system. The sketch shows a conventional four stage cyclone preheater system. In the 1970’s, production lines with more than approx. 2000 t/d had to be built with two parallel preheater strings. Today, one-string installations are possible for up to 4000 t/d. Characteristic operating figures of 4-stage pre-heater kilns: Heat consumption q small units
3350 ... 3550 kJ/kg cli (= 800 ... 850 kcal/kg cli)
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"Holderbank" - Cement Course 2000 (= 800 ... 850 kcal/kg cli) large units
3150 ... 3350 kJ/kg cli (= 750 ... 800 kcal/kg cli)
kiln exit gas temperature
320° ... 350°C
kiln exit gas volume
approx. 1,5 Nm3/kg cli
System pressure drop
4 ... 6 kPa
Dust loss relative to clinker
8 ... 15%
Transition chamber kiln gas temperature
approx. 1100°C
Material temperature
approx. 800°C
Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns / 6.2.4 Precalciner Kilns
6.2.4
Precalciner Kilns
For larger production capacities, a larger portion of the pyroprocess had to be relocated out of the rotary kiln in order to maintain reasonable kiln diameters without excessive thermal load of the burning © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 zone. The process of dissociation of CO2 (calcination) is suitable to take place in a static reactor outside of the rotary kiln. Of the total heat consumption, 60 to 65% are required to achieve about 90% of calcination. 100% calcination must be avoided because clogging problems will seriously disturb kiln operation (beginning of clinker formation). The development of this reactor started with a secondary firing in the kiln riser duct sufficient for 35 to 40% calcination of the meal, combustion air still pulled through the kiln tube (=air through). It was therefore referred to as precalciner (PC) type AT. Only when hot cooler air (= tertiary air) for the PC fuel (= secondary fuel) was taken to the calciner in a separate duct, the so called tertiary air duct, the full benefit of this technology could be used. Today, only this type called PC-AS (=air separate) is considered a real precalciner. The elements of a precalciner kiln system are explained in the sketch. The strongest boost of calciner development was in the seventies in Japan, initiated by the demand for very large units exceeding the potential of conventional kilns with suspension preheaters. Only precalciner technology makes today’s largest units of 10’000 t/d possible. Two process alternatives of precalciner are used: •
in-line calciner (calciner installed in kiln gas flow)
•
separate-line calciner (calciner not passed by kiln gases)
More details on calciner technology are contained in a separate section. The operating data are very close to the ones of the corresponding preheater kiln system. In-line calciners have a tendency to higher gas exit temperature and system pressure drop; however, modern units are equipped with 5 or 6 preheater stages to compensate for this. Characteristic operating data of 4 to 6 stage precalciner kilns: Heat consumption q small units, 4 stage SP
3350 ... 3550 kJ/kg cli (= 800 ... 850 kcal/kg cli)
large units, 5 stage SP
2900 ... 3200 kJ/kg cli (= 700 ... 800 kcal/kg cli)
SP exit gas temp. 6 to 4 st. SP
290° ... 370°C
SP exit gas volume
approx. 1.3 to 1.5 Nm3/kg cli
System pressure drop
4 ... 6 kPa
Dust loss relative to clinker
8 ... 15%
Transition chamber: kiln gas temperature
approx. 1100°C
Material temperature
approx. 800°C
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"Holderbank" - Cement Course 2000 More data of precalciner kiln systems are shown in the section “Precalciners”.
HEAT BALANCE WET / SEMI-DRY / 4-ST. PREHEATER / 5-ST. PREHEATER-PRECALCINER
WET PROCESS kJ/kg cli
Input Fuel kiln combustion
%
SEMI-DRY LEPOL kJ/kg cli
%
4-STAGE SP kJ/kg cli
%
6-STAGE SP-PC kJ/kg cli
%
5560
96.7%
3343
97.6%
3150
97.7%
1180
39.2%
25
0.4%
15
0.4%
13
0.4%
5
0.2%
0
0.0%
0
0.0%
0
0.0%
1775
58.9%
sensible heat
0
0.0%
0
0.0%
0
0.0%
8
0.3%
Kiln feed sensible heat
25
0.4%
30
0.9%
54
1.7%
45
1.5%
73
1.3%
17
0.5%
0
0.0%
0
0.0%
sensible heat Fuel PC combustion
sensible heat of water
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"Holderbank" water- Cement Course 2000 Insufflated air (PA, cooler)
Total inputs
Output
67
1.2%
20
0.6%
6
0.2%
0
0.0%
5750
100%
3425
100%
3223
100%
3013
100%
kJ/kg cli
%
kJ/kg cli
%
kJ/kg cli
%
kJ/kg cli
%
Heat of formation
1750
30.4%
1750
51.1%
1750
54.3%
1750
58.1%
Water evaporation
2370
41.2%
506
14.8%
13
0.4%
8
0.3%
Exhaust gas sens. heat
754
13.1%
314
9.2%
636
19.7%
553
18.4%
Exhaust gas dust sens. heat
25
0.4%
21
0.6%
18
0.6%
29
1.0%
Clinker
59
1.0%
50
1.5%
63
2.0%
83
2.8%
100
1.7%
276
8.1%
423
13.1%
288
9.6%
0
0.0%
160
4.7%
77
2.4%
60
2.0%
- Precalciner (or bottom stage)
0
0.0%
0
0.0%
20
0.6%
20
0.7%
- Kiln (+tertiary air duct)
530
9.2%
200
5.8%
200
6.2%
200
6.6%
10
0.2%
92
2.7%
10
0.3%
10
0.3%
Water cooling
0
0.0%
42
1.2%
0
0.0%
0
0.0%
Other outputs
0
0.0%
0
0.0%
0
0.0%
0
0.0%
152
2.6%
14
0.4%
13
0.4%
12
0.4%
5750
100%
3425
100%
3223
107%
3013
100%
Cooler waste air Radiation and convection : - Preheater
- Cooler
Rest
Total outputs
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"Holderbank" - Cement Course 2000
HISTORICAL DEVELOPMENT
Annex 1
The word cement is more than 2000 years old, but impure lime has been used much longer as a building material. It is historically established, that the Phoenicians used a pozzolanic lime about 700 B.C. and also the Romans produced some sort of cement or hard burned lime. From the medieval ages, it is known that in Holland a type of hydraulic cement was formed out of lime and tuff in dome shaped kilns. Our cement, as we know it today, is now more than 200 years old, “invented” by the Englishman John Smeaton in 1756. It was burned in bottle kilns. The better known inventor of Portland cement was Joseph Aspdin, who patented his burning process in 1824. He also used dome kilns of approx. 36 ft height and 17 ft diameter with a production of 90 bbl (= 15 t) per charge, each of which took several days to produce. Fuel consumption was 50% of clinker weight in coal which corresponds to 15’500 kJ/kg cli (= 3’700 kcal/kg cli). In 1880 an important step forward was made with the development of the continuously working shaft kiln, which had a much better heat economy. An example of such a kiln was the “Dietzsche Etagenofen” which is shown in Annex 1. From 1877 experiments have been conducted with rotary kilns. In 1897 Hurry and Seaman developed the first successfully operating unit of this type in America. These first rotary kilns were wet process kilns with a daily capacity of 50 to 100 tons. Their heat consumption was again very high (about 30% of clinker in coal = 9’500 kJ/kg cli) and they had an incredible dust emission (usually more than one third of the whole production). In order to decrease heat consumption, chain systems were installed in wet kilns to improve heat transfer during drying. Behind long dry kilns, waste heat steam boilers were arranged for the same purpose. It took almost another 30 years, before a further substantial reduction of heat consumption could be achieved by reducing the water content of the feed and by a better heat exchange in the preheating a calcining zone. In 1930 an officer of the army of the tsar, Dr. Lellep, took an important step in this direction. He developed the travelling grate pre-heater, which is fed with moist nodules. This invention was taken over by Polysius and got the name LEPOL kiln. Some years later, there was a Czech patent of a cyclone raw meal pre-heater, and in 1953 Kloeckner-Humboldt-Deutz AG in Germany installed the first suspension pre-heater system for raw meal. This type of kiln now became dominant because of its heat economy and nowadays other systems are only chosen in special cases. In former years, the main reason for the selection of the wet process was, that effective homogenisation of © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 ground raw material was not possible except in the form of slurry. In developing special techniques for dry material homogenisation such as mix beds, mixing chamber silos etc., this factor could be eliminated. Utilising a rather old idea, since about 1966 especially Japanese cement machine manufacturers have designed several successfully working precalcining kiln systems. Calcination is already done in a stationary calciner system, where secondary firing is installed. By this means, it is possible to design kiln systems with a comparatively small rotary part diameter but a very large capacity up to more than 10’000 t/d. Kiln systems built after 1990 include 6-stage preheaters with up to 4000 t/d per string, pure air calciners, designed for a variety of fuels and emission control. Using modern low primary air burners, low pressure drop cyclone designs and high recuperation efficiency coolers allow further reduction of heat and power consumption.
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns
Rotary Kilns U. Gasser PT 98/14362/E 1. General 2. Kiln Dimensioning 3. Mechanical Aspects of Rotary Kilns 3.1 Riding Ring Fixation, Kiln Shell Ovality 3.2 Kiln Seals 3.2.1
Kiln Inlet Seal
3.2.2
Kiln Outlet Seal
3.3 Kiln Drive
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"Holderbank" - Cement Course 2000 SUMMARY After over 100 years, the rotary kiln is used in all cement plants for clinker production. The following properties made it superior to other principles: ♦ suitable to cope with high temperatures ♦ easy to be lined with refractory bricks due to its shape ♦ material transport behaviour ♦ tight to ambient ♦ mechanically relatively simple ♦ large units possible The rotary kiln must be designed for process, combustion and mechanical requirements.
♦ Characteristic figures: ♦ Length L [m] , diameter D [m] and their ratio L/D [-]
♦
♦
♦ Slope [°], speed range [min-1] and drive [kWh]
♦
♦ Dimensioning criteria:
♦ Volume load
♦ [t/(d m3)]
♦
♦ Burning zone load
♦ [t/(d m2)]
♦
♦ Thermal burning zone load
♦ [MW/m2]
♦ ♦ Important mechanical features are: ♦ riding ring fixation ♦ roller station / alignment ♦ seals at inlet and outlet ♦ drive ♦ ♦ With modern precalciner technology, outputs exceeding 10’000 t/d per kiln are possible with diameters still below the 6.5 m of the largest wet kilns. ♦ There is a trend towards short L/D kilns with only two piers mainly because of lower investment. Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 1. GENERAL
1.
GENERAL
Today, all clinker producing installations of industrial size use a rotary kiln. The rotary kiln is still the only feasible way to manage this high temperature process with process material of varying behaviour. One exception is the vertical shaft kiln still used in some parts of the world, e.g. China, however, for © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 small unit capacities only. The other exceptions are few pilot installations based on sintering in a fluidized bed reactor. Like many other great ideas, the rotary kiln was invented towards the end of the 19th century and has found application in many different industries. In 1987, Hurry and Seaman in the USA developed the first successfully working rotary kiln to produce cement clinker. The first rotary cement kilns were using the wet process with one very long kiln tube, making it the dominating single piece of equipment of a plant. With technological progress, the kiln sections used for for drying, heating-up and calcining have gradually been replaced by other types of equipment, the rotary kiln remains to be the most suitable type of machine for the clinkerization process. The rotary kiln has to satisfy three types of requirements: Combustion:
as a combustion chamber for burning zone fuel
Process:
as a reactor for the clinker burning process
(→ retention time)
as a material conveyor
(→ slope, speed)
Mechanical:
stability of shape, carrying load, thermal flexibility, tightness
Remarks: ♦ Even though the rotary kiln is a relatively simple piece of equipment, nobody has developed a complete theoretical/mathematical model of its behaviour and process which would allow correct process simulation and equipment design. ♦ The rotary kiln is still the “heart” of the entire production line. Its OEE (overall equipment efficiency) depending mainly on hourly output and availability, is decisive for the success of a plant. ♦ The rotary kiln is designed to operate 24 hours a day, and the rest of the equipment upstream and downstream has to follow. ♦ Being a major cause for production cost (mechanical maintenance, refractories), a well managed kiln is vital for a successful plant. Figure 1:
Old and new kiln
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 2. KILN DIMENSIONING
2.
KILN DIMENSIONING
The kiln dimensions are defined with diameter D (for kilns with different diameter: burning zone D) and length L: L [m] and D [m]
resp.
L/D [m]
♦ For cement kilns, the actual L/D ratio range is: from 40 (for long wet kilns) to 11 (for modern short kilns with precalciner) ♦ The diameter D is the inner diameter Di of the kiln (steel-) shell. ♦ Process technological dimensioning of a kiln is based on empirical figures and experience from existing installations One limiting factor for the diameter is the mechanical stability of the ‘arch’ of the brick lining. Maximum diameters which can be safely realised with standard size bricks are about 6,5 m. The largest kiln in the “Holderbank” group is 232 m (wet process, 3750 t/d). The following process technological dimensioning criteria are mostly used:
Clinker Production Net Kiln Volume
[t/(d m3)]
Specific Zone Load
Clinker Production Net Burning Zone Cross Section
[t/(d m2)]
Thermal Burning Zone Load
Burning Zone Heat Input Net Bruning Zone Cross Section
[MW/m2)]
Specific Volume Load
Specific volume load and thermal burning zone (BZ) load have no physical significance. They are merely defined to make existing installations comparable. The specific load is indirectly a gas velocity, because generating a certain amount of thermal energy by fuel combustion results in a proportional gas flow which can be calculated. The thermal BZ load per cross section is considered the limiting factor for a modern kiln system. For a certain length/diameter ratio, which is typical for each kiln type, the thermal BZ load it is proportional to the heat load on the inside of the lining surface which is one of the main influencing factor on brick life. The limit usually respected is: © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000
Max. Thermal BZ Load = 6 MW/m2 (=5.16 x 106 kcal/m2 h)
Other absolute limiting values of all the three factors are not known. Each supplier seems to have his own rules of kiln dimensioning. Since no theoretical formulas have been derived to calculate the kiln size on an analytical basis, it is possible, that the present limits of the dimensioning criteria may be surpassed even for the conventional processes. Figure 2:
Long and short L/D kilns
Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 3. MECHANICAL ASPECTS OF ROTARY KILNS
3.
MECHANICAL ASPECTS OF ROTARY KILNS
The following aspects of kiln mechanical design are relevant for the process: ♦ Riding ring fixation, kiln shell ovality ♦ Kiln seals ♦ kiln drive ♦ refractory lining (separate paper) ♦ nose ring (covered in “refractory lining”) Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 3. MECHANICAL ASPECTS OF ROTARY KILNS / 3.1 Riding Ring Fixation, Kiln Shell Ovality
3.1
Riding Ring Fixation, Kiln Shell Ovality
A rotary kiln should be designed as cheaply as possible, yet it must still be rigid to guarantee minimum wear of the lining. This requirement can be met, if the deformation of the kiln shell is reduced to a tolerable limit. The parameter expressing shell deformation at a certain point is the kiln shell ovality
ω
:
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"Holderbank" - Cement Course 2000 Definition of
ω
ω=2 (a - b)
with 2a and 2b as the main axis of an ellipse
:
Investigations have shown, that generally a maximum relative ovality 0,3% is allowed This ovality may be subdivided into two amounts:
ω
of
a) Ovality of the riding ring 3 cm due to external forces allowed value:
ω 10000 t/d ∗ Up to 4000 t/d in 1 string ∗ Heat consumption < 3000 kJ/kg possible (6 stages) ∗ Sensitive to circulation phenomena (-> kiln gas bypass!) ∗ ∗ ∗ ∗ ∗
4-stage cyclone pre-heater kiln (standard technology 1970 to 1980): Cyclone for raw meal preheating Large application world wide Capacities of up to 4500 t/d technically possible Heat consumption: 3150 to 3350 kJ/kg cli Sensitive to circulation phenomena (-> kiln gas bypass!)
∗ ∗
2-stage cyclone pre-heater kiln: Less sensitive to circulating elements than 4-stage pre-heater Higher heat consumption than pre-heater with more stages
♦ Most recent innovations: •
Horizontal cyclone for “low profile” preheaters (Polysius)
•
Dip tube add-on RTS for 30% lower cyclone pressure drop
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 1. GENERAL
1.
GENERAL
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 1. GENERAL / 1.1 History
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"Holderbank" - Cement Course 2000 1.1
History
With dry process, the heat exchange for heating up and calcination takes place between hot kiln gas and dry powder. Since the high dust losses from long dry kilns made it almost impossible to achieve acceptable heat consumption: other heat exchange principles had to be applied. Since the temperature range to be covered is below 1000°C, where the meal behaves normally like dry powder, stationary reactors where the meal is in suspension with the hot gas can be used. The first patent for a suspension preheater using four co-current cyclone stages was applied for in 1932 and issued in 1934 by the patent office in Prague to a Danish engineer employed by FLS. Even though the concept was entirely described in the patent, it took another 20 years for industrial application in 1951 by the company Humboldt, now KHD. Other developments using shaft stages have been abandoned and today, a suspension preheater is actually a cyclone preheater. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 1. GENERAL / 1.2 Trend
1.2
Trend
All new kiln systems and the majority of the ones with start-up date after 1970 are equipped with cyclone pre-heaters. Gradually, older plants with wet kilns or long dry kilns are shut down for good due to their age as well as their high specific production cost The portion of world’s cement produced with kilns using suspension pre-heaters is still growing, as can be seen by the development of the “Holderbank” plants. It looks as if it will exceed 95% one day because no feasible alternative solution changing this development is in sight. In combination with pre-calciners, units of 10’000 t/d have been built using up to four strings, five stages. Typically, 3500 t/d can be handled in one string, in a recent project even 4000 t/d have been proposed.
Figure 1:
Figure 2:
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"Holderbank" - Cement Course 2000
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 2. HEAT EXCHANGE IN A SUSPENSION PREHEATER
2.
HEAT EXCHANGE IN A SUSPENSION PREHEATER
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 2. HEAT EXCHANGE IN A SUSPENSION PREHEATER / 2.1 Counter-Current Heat Exchange (Shaft Stage)
2.1
Counter-Current Heat Exchange (Shaft Stage)
The most efficient type of heat exchange is the counter-current principle. The flows of the heat releasing media and the heat absorbing media are in opposite directions. This provides optimum the temperature difference (=temperature gradient, in theory allowing almost complete heat exchange. In case of a suspension preheater, where powder is suspended in a gas, the heat exchange takes place in a “reactor” vessel where the hot gas enters from below and leaves at the top. The meal to be preheated is fed at the top. The meal retention time depends on distribution across the gas flow and the retention time, which is determined by the gas velocity. In industrial installations, the heat exchange proved to be far below expected, because even distribution of the meal was not achieved, particularly not with large units. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 2. HEAT EXCHANGE IN A SUSPENSION PREHEATER / 2.2 Co-Current Heat Exchange (Cyclone Stage)
2.2
Co-Current Heat Exchange (Cyclone Stage)
Co-current heat exchange takes place if both heat exchanging media flow in the same direction. Because of the rapidly decreasing temperature difference, the meal can never reach gas inlet temperature. Good and reproducible results in industrial installations with this type lead to the predominance of this principle in the cement industry. The heat exchanger is a gas duct with velocities from 10 to 20 m/s, equipped with good meal dispersion devices. The purpose of the cyclone is primarily to separate meal from gas, and not to exchange heat! Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 2. HEAT EXCHANGE IN A SUSPENSION PREHEATER / 2.3 Thermodynamic Limits
2.3
Thermodynamic Limits
Regardless of the type of heat exchange, there is always a thermodynamic imbalance between hot © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 gases from kiln and calciner and cold raw meal. The heat contained available in the hot gas leaving the rotary kiln exceeds the heat required for heating the meal to the temperature levels required for calcination. Another limit must be observed: Because the temperature gradient between gas and meal (T gas > T meal) must always be maintained, a higher calcination degree than 30% cannot be achieved without additional heat input. The following heat balance estimate shall illustrate this: Heat contained in the gas:
2300 kJ/kg cli
(1100°C; 1.3 Nm3/kg cli) Heat to preheat meal to 850°C:
1300 kJ/kg cli
(1.6 kg meal /kg cli) Heat required for 30% calcination:
650 kJ/kg cli
Rest (ideal heat exchange):
350 kJ/kg
(corresponding to 200 °C)
This shows that even if the heat of the gas above 850°C is used for partial calcination (about 30%), there is still excessive heat in the gas which would correspond to 200°C gas temperature. It is obvious that even with a very large number of stages (with accordingly small temperature gradients), there will always be excess heat! This waste heat is lost only for the kiln system, but not for the plant, since it can be used for raw material drying in the mill.
Figure 3:
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES
3.
PREHEATER TYPES
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.1 Preheaters with Shaft Stages
3.1
Preheaters with Shaft Stages
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"Holderbank" - Cement Course 2000 The rather disappointing performance of the shaft stage made it virtually disappear from the market. Many hybrid preheaters were equipped with one or two cyclone stages replacing the shaft stage. Shaft stages at the kiln inlet have the advantage to be less sensitive to build-ups. This could be an advantage in cases where elevated sulfur input in the kiln system must be expected. Several Suppliers built preheaters using shaft stages. Two groups can be distinguished: Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.1 Preheaters with Shaft Stages / 3.1.1 Pure shaft preheaters:
3.1.1
Pure shaft preheaters:
Polysius:
ZAB Dessau:
•
GEPOL
•
Self-supporting structure (no tower required)
•
Vertical tube with restrictions
•
For small capacities (up to ca. 1000 t/d)
•
Some applications in Eastern Europe
•
Similar to GEPOL, but not self-supporting
•
The Deuna plant had originally 4 ZAB shaft preheaters
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.1 Preheaters with Shaft Stages / 3.1.2 Hybrid preheaters:
3.1.2
Hybrid preheaters:
Several suppliers used a combination of shaft and cyclone stages: Polysius:
Bühler-Miag:
Prerov:
•
DOPOL preheater (first generation)
•
The central swirl-pot (second lowest stage) was shaft stage
•
Replaced by DOPOL 90 from 1990
•
Gradually developed into a cyclone preheater
•
Up to ca. 3000 t/d
•
Lowest stage was shaft stage
•
Later often replaced by cyclone stage
•
One large shaft stage with dedusting cyclone
•
Shaft stage selfsupporting
•
Additional cyclone stage possible
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"Holderbank" - Cement Course 2000 • Additional cyclone stage possible
MBM:
•
Separate meal duct to kiln
•
As sensitive to circulation phenomena as a cyclone type
•
Bottom shaft stage with 4 cyclone stages
•
Only hybrid design still on the market
Figure 4:
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.2 Preheaters with Cyclone Stages
3.2
Preheaters with Cyclone Stages
A quasi counter-current heat exchange can be achieved by serial installation of several co-current stages. The result is the multi-stage cyclone preheater as it is generally applied in modern cement plants. In the early years, one and two stage systems have been installed with long kilns, often to avoid problems caused by circulating phenomena. A large number of plants are equipped with four stages; the majority of them were built before 1990. Today, five stage preheaters represent the economical optimum. High raw material moisture leads occasionally to fewer stages, in combination with low temperature dedusting systems, or in areas with high fuel cost, six stages can be more economical. Number of stages depends thus on: ♦ Raw material moisture (i.e. drying heat requirement) ♦ Cost of thermal energy ♦ Cost of electrical energy ♦ Gas handling system (temperature limit, dew point) ♦ Soil conditions (foundations, earthquake zone -> height of structure) If raw material moisture shows significant seasonal variations, it can be economical to equip preheaters with “variable stages”. This is achieved by feeding all or part of the meal to the second highest stage or by skipping a stage. © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000
Note:
Numbering of stages is always from top to bottom:
top stage
=
stage 1.
Exception:
bottom stage
=
stage 1
Polysius:
Figure 5:
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.3 Economical Number of Stages for Cyclone Preheaters
3.3
Economical Number of Stages for Cyclone Preheaters
For many years, the pressure drop across one preheater stage was up to 1,5 kPa (15 mbar). The reason for the 4-stage pre-heater being so widely used is, that it represented an optimum between investment cost (structure height, foundation), pressure drop and heat consumption. . The performance of comparable systems built in about the same period are within a relatively narrow range. About two third of the pressure drop of a stage occurs in the cyclone and depends on its shape/design and the size, the latter being the determining cost factor. New cyclone designs are now on the market with only 0,5 to 1,0 kPa (5-10 mbar) pressure drop per stage. Considering increasing energy cost, it is justified to install 5 or 6 pre-heater stages for new or modified kiln systems. The following table indicated the estimated effect of a 5th and a 6th cyclone stage: 4 to 5st
5 to 6st
Heat consumption
kJ/kg cli
- 80
- 50
Exhaust gas temperature
°C
- 40 to -50
- 20 to -30
Exhaust gas quantity
Nm3/kg cli
- 0,03
- 0,015
Drying capacity in RM
% H2O
from 8 - 6,5
From 6,5 5,5
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.4 Minimum Gas Velocity © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 3.4
Minimum Gas Velocity
Dimensioning of a cyclone preheater is a careful consideration of the importance of separation efficiency, pressure drop, part load operation capability, size of the preheater and cost of the project. It must be mentioned that there is a lowest gas velocity in a cyclone preheater. If operation results in lower figures, the meal will not be lifted by the gas anymore, resulting in poor heat exchange and consequently high heat consumption, but also excessive temperatures . Large dimensions give lower velocities with low pressure drop, but also limit the lowest possible economical production. Figure 6:
Polysius
Figure 7:
FLS
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES
4.
DESIGN FEATURES OF PREHEATER-CYCLONES
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.1 General
4.1
General
Modern preheaters are designed for low pressure drop using the new cyclone design which must still provide good separation efficiency, particularly in the top and the bottom stage. Cyclone inlet velocities © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 are designed in the range of 10 to 15 m/s. It has been found that the total pressure drop of one cyclone stage is caused by about 1/3 by the gas duct (i.e. lifting of the meal) and 2/3 by the cyclone. Since not much can be done regarding lifting of the meal, efforts have been made to improve the cyclone design in order to reduce total pressure drop: the low pressure (drop) cyclone was designed. Cyclone design means to optimize between high separation efficiency, low pressure drop and low cost (i.e. small size). Other than having the correct design parameters, all stages should be equipped with ♦ Dip Tubes (also called ‘immersion tubes’, ‘thimbles’ or ‘vortex finders’) ♦ Meal flaps ♦ Splash boxes (or splash plates). Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.2 Dust Cycles
4.2
Dust Cycles
The entire kiln system is subject to dust cycles. Precondition is gas flow in opposite direction of pulverized process materials. This causes wear, unnecessary material transport and heat losses due to heat exchange in the wrong direction. In the preheater, internal dust cycles due to poor separation efficiency of the cyclones result in less than optimum preheating of meal. Unfortunately, it is almost impossible to measure dust loss from lower cyclones in normal operation. The only indicator is the temperature profiles of gas and meal, but even the meal temperature is not always easy tp measure. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features
4.3
Features
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features / 4.3.1 Splash Box
4.3.1
Splash Box
Early cyclone preheater designs had no splash boxes. Instead, the meal was fed into the gas at a higher point against the gas flow, creating some turbulence with a certain distribution effect. Modern cyclone preheaters must be equipped with correctly designed splash boxes for optimum meal distribution across the gas duct cross section. The principle is based on impact on a plate. In some installations, the bottom plate of the splash box can be adjusted. Note: No splash box must be installed at the kiln inlet! The hot meal from the bottom cyclone must enter the rotary kiln as smoothly as possible. Meal is easily picked up by the kiln gas and will create a dusty transition chamber. Figure 8:
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Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features / 4.3.2 Dip Tube (Immersion Tube, Vortex Finder, Thimble)
4.3.2
Dip Tube (Immersion Tube, Vortex Finder, Thimble)
This integral element of the cyclone has a decisive influence on separation and pressure drop. It makes the gas to follow a 180 to 360° rotation thus creating the desired centrifugal force for the separation effect. In the colder upper stages (stage 1 to 3) it can be designed as simple extension of the outlet gas duct, made from steel plate. These upper stage dip tubes create usually no problems except when the preheater gets overheated, e.g. during start-up. Then, the dip tube can collapse, causing excessive pressure drop. In the hotter lower stages, mild steel ducts from one piece are not suitable. Several segmented designs made from heat resistant steel or ceramic material (Hasle) are available on the market. It is standard today that all stages are equipped with dip tubes. Note: It appears that some designs of segmented dip tubes have a tendency to unhook enabling individual elements to drop and to block the cyclone outlet! For older plants, installing a segmented dip tube in the lower stages is a optimization possibility which is often applied. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features / 4.3.3 Meal Flap
4.3.3
Meal Flap
In order to understand the purpose of the meal flap, the following two aspects must be mentioned: ♦ There is a pressure difference across a cyclone stage, i.e. between two subsequent cyclone gas outlets (maintained by the ID fan). ♦ Without meal, there are two ways the gas can flow from one stage to the next: through gas duct and through meal duct If there was an ideal kiln system, i.e. a system with 100% constant meal flow and never changing operation parameters, the meal duct diameter could be designed for just the meal. The meal would then fill the entire cross section, leaving no opening for the gas. In reality, there are fluctuations of meal and dropping build-ups, requiring oversized meal ducts. It is the purpose of the meal flap to close the free cross section not used by the meal, to avoid gas © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 bypass. There are designs that open only when a certain weight pushes them open, thus creating meal fluctuations. Other designs (see figure) are adjustable so that they move only in case of meal peaks or lumps. Not operational meal flaps cause heat loss and allow build-up formation in meal ducts (circulating elements)! Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features / 4.3.4 Cyclone Shapes
4.3.4
Cyclone Shapes
The separation efficiency of a cyclone gets better with longer dip tube and increasing distance between swirl (cylinder) and dust collecting cone, i.e. with high and slim shapes. The top stage of preheaters is designed for high separation efficiency. In order to save height, most suppliers install twin cyclones with the drawback that meal and gas have to be split. There are a few plants from FLS with only one top cyclone, avoiding this drawback. Figure 9:
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 5. PREHEATER OPERATION
5.
PREHEATER OPERATION
The performance of a preheater is assessed based on the criteria: ♦ Temperature profile (first indicator: exit gas temperature) ♦ Pressure profile ♦ Oxygen profile Table
Typical Gas Temperature Profiles 4 stages
5 stages
6 stages
SP
PC
SP
PC
SP
PC
Stage 1
°C
350
360
300
310
270
280
Stage 2
°C
540
570
490
500
440
460
Stage 3
°C
710
740
630
650
580
600
Stage 4
°C
840
870
750
770
690
710
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"Holderbank" Stage 4 °C - Cement 840 Course 8702000
750
770
690
710
Stage 5
°C
-
-
840
870
770
800
Stage 6
°C
-
-
-
-
840
870
Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 5. PREHEATER OPERATION / 5.1 Operating Problems of Suspension Preheaters
5.1
Operating Problems of Suspension Preheaters
Some reasons for poor preheater performance frequently experienced: ♦ Worn out or non existing immersion tubes (often in bottom stage) ♦ Open inspection doors, leaky gaskets or holes in the pre-heater (cold false air leaks in, can be detected by hissing sound) ♦ Blocked or non existing meal flaps ♦ No splash boxes (specially older preheaters), combined with not optimum position of meal feed point (e.g. old DOPOL) ♦ Excessive dust circulation due to poor separation efficiency of cyclones Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 5. PREHEATER OPERATION / 5.1 Operating Problems of Suspension Preheaters / 5.1.1 Circulation Phenomena.
5.1.1
Circulation Phenomena.
Cyclone preheaters are sensitive to circulation phenomena. Cyclone blockages cause kiln stops resulting in production loss and dangerous cleaning actions. Possible causes are: ♦ Excessive input via feed or fuel (Cl, S, 1 Na, K) ♦ Chemical unbalance (sulphur, alkali ratio) ♦ Unfavourable kiln/burner operation ♦ Unfavourable design geometry of bottom stage and kiln gas riser duct area Countermeasures known today allow to solve the problems are: ♦ Change feed composition or fuel quality ♦ Improve burning conditions ♦ Install automatic cleaning (air cannon, big blasters) at critical locations ♦ Change temperature profile by installing a small secondary burner ♦ Install a kiln gas bypass* system *A bypass system is not desirable since it is expensive and causes loss of heat and material. It is therefore the last solution left and should be only considered if all other measures are not sufficient. The paper ‘circulating phenomena’ contains more details on this rather complex subject. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 6. NEW DEVELOPMENTS
6.
NEW DEVELOPMENTS
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 6. NEW DEVELOPMENTS / 6.1 Horizontal Cyclone
6.1
Horizontal Cyclone
Polysius have developed a “horizontal cyclone”. (not to be mixed up with earlier designs of Kawasaki!) This cyclone is a modified version of the conventional cyclone with the major difference that the gas outlet is also at the bottom, encircling the meal outlet. The heat exchanger duct is still from bottom to top, but the stages can be arranged next to each other instead on top of each other. This allows a significantly lower height of the preheater structure. It is expected that savings in civil cost can be achieved. Additional benefit is possible in cases where the maximum height is restricted (earthquake zones, scenery protection). Only top cyclones on conventional preheaters are in industrial operation, however. Any other experience is yet to be made. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 6. NEW DEVELOPMENTS / 6.2 TRS
6.2
TRS
The Austrian company Zyklontechnik have introduced a dip tube add-on device which will reduce pressure drop across the cyclone (not the entire stage!) by 30% at otherwise unchanged performance. The principle is to avoid the flow around the edge of the dip tube. Instead, the horizontal swirl from the gas inlet is maintained and can continue into the dip tube through an accurately shaped slot in the TRS. Prerequisite is aerodynamically correct cyclone design and very accurate manufacturing of the TRS, which cannot be made locally. The device can be mounted to the bottom of a shortened dip tube. If the inspection opening is large enough, the whole unit can be installed in one piece, otherwise it comes in pieces. Several TRS are in operation (not in preheaters, however) with performance equal to or exceeding the predicted figures. Figure 10:
Figure 11:
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"Holderbank" - Cement Course 2000
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems
Precalcining Systems U. Gasser VA 93/4055/E 1. INTRODUCTION 2. THEORETICAL ASPECTS OF PRECALCINING 2.1 2.1 Calcining of Raw Meal 2.2 Combustion in Precalciner 2.3 Specific Heat Consumption 2.4 True and Apparent Calcination Degree 3. BASIC ARRANGEMENTS OF PRECALCINING SYSTEMS 3.1 AS and AT Systems 3.2 In-Line, Off-Line and Separate Line Calciners 4. FEATURES OF PRECALCINERS 4.1 Main Benefits of Precalciner Technology 4.2 Limitations and Restrictions 4.3 Tertiary Air Damper and Kiln Riser Orifice 4.4 Circulation Problems and Bypass with PC Kilns 5. PRESENT STATE OF PRECALCINER DEVELOPMENT 5.1 Calciners from FCB 5.2 Calciners from FLS - FULLER 5.3 PYROCLON Calciners (KHD) 5.4 PREPOL® Calciners (Polysius) 5.5 Prerov-Calciner 5.6 Conclusion 6. SYNOPSIS OF PRECALCINERS 7. TEST QUESTIONS
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"Holderbank" - Cement Course 2000 SUMMARY When burning cement clinker in a suspension preheater kiln, about 2/3 of the total heat consumed or about 2000 kJ/kg are required for the dissociation of CaCO3 also known as calcination. The idea of precalcination is, to let this process take place before the meal enters the rotary kiln by introducing that part of the fuel, i.e. up to 65%, into a stationary reactor. Because the combustion air (tertiary air) is drawn through a separate duct parallel to the kiln directly from the cooler, the rotary kiln operates at significantly lower specific thermal load and gas flow. The main advantages of precalcination are: ♦ More stable kiln operation due to better kiln control via two separate fuel feed/control points ♦ More stable kiln operation due to controlled meal conditions at kiln inlet ♦ Reduced thermal load of burning zone ♦ Higher kiln availability ♦ Longer life of burning zone refractories ♦ Larger capacity with given kiln dimensions, resp. smaller kiln for given capacity ♦ Possibility of increasing capacity of existing kilns ♦ More favorable conditions regarding circulating element problems ♦ Allows shorter kilns (L/D 30% via precalciner ♦ < 10% via secondary DeNOx burner Experience on an industrial scale only will prove the capability of this system. One of the difficulties is how to control the kiln atmosphere without the gas analysis sampled near the kiln inlet. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 5. PRESENT STATE OF PRECALCINER DEVELOPMENT / 5.5 Prerov-Calciner
5.5
Prerov-Calciner
The Czek company Prerov have developed a new precalciner (Fig. 18). It consists of a precombustion chamber (KKS) and a reaction chamber (KKN) with a vortex chamber and is comparable to Polysius’ PREPOL-AS CC. During 1992, the first installation will be commissioned in Southern Italy. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 5. PRESENT STATE OF PRECALCINER DEVELOPMENT / 5.6 Conclusion
5.6
Conclusion
The development of tube type calciners and vessel type calciners has moved them closer to each other. The tube type calciners have received a swirl pot or a pre-combustion chamber for improved mixing and fuel burning and the vessel type calciners have become longer. The calciner without separate air duct also known as „air through“ actually operating only with 10 - 20% of the total fuel never fulfilled the expectations and has virtually disappeared, together with the planetary cooler. © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 Low NOx calciners have been developed based on the principle of locally reducing atmosphere by means of fuel excess zones. It can be expected that NOx from precalciner combustion can be reduced to around 700 - 800 ppm. Calciners can be designed to reduce NOx generated in the burning zone, or to keep NOx generated in the calciner low, or both. Since further NOx reduction to lower levels require methods such as NH3 injection, temperature control is very important. A modern calciner can be described as follows:
Type:
in-line with pre-combustion chamber
Fuel ratio:
50 - 60% (include. low NOx fuel in case of staged combustion
Fuel dosing:
low fluctuation
Fuel types:
various, including alternative fuels
Combustion environment:
pure air or air/kiln gas mix
Calciner size criteria:
fuel reactivity gas retention time (up to 4 - 5 sec.)
Feature:
enhanced turbulence
Tertiary air:
staged for reducing zone
Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 6. SYNOPSIS OF PRECALCINERS
6.
SYNOPSIS OF PRECALCINERS
The different PC systems as well as their developers and suppliers are summarized in Table 3. During the 1970ies the cement manufacturers greatly contributed to the development of the Japanese PC systems: Until 1985, ot 304 kilns with PC, 83 were located in Japan, totaling 35% of the capacity. This shows the explosive expansion of PC systems in Japan back than. Today, all new kilns have precalciner with tertiary air duct. Table 3
Synopsis of PC Systems
Trade Name
Signification
PASEC SLC
Separate Line Calciner
SLC-S
Separate Line Calciner Special
ILC
In-Line Calciner
ILC-D
In-Line Calciner Downdraft
ILC-E*
In-Line Calciner, Excess Air
Prepol AS
Air Separate
Prepol AS-CC
Controlled Combustion
Prepol AS-MSC
Multi Stage Combustion
Prepol AT*
Air Through
Pyroclon R
Regular = Air Separate
Pyroclon RP
Regular Parallel
© Holderbank Management & Consulting, 2000 Query:
Developer & Licenser
Plant Supplier & Licensee
Voert Alpine / SKET
ACT
F.L. Smidth
F.L. Smidth
Krupp-Polysius
Krupp-Polysius
KHD Humboldt Wedag
KHD Humboldt Wedag
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"Holderbank" Course Pyroclon RP - Cement Regular Parallel2000 Pyroclon R Low NOx Pyroclon R Low Nox with Pyrotop Pyroclon S*
Special = Air Through
EVS-PC (only fuel - oil)
Echangeur à voie sèche avec précalcination
Fives-Cail Babcock
Fives-Cail Babcock
KKS-KKN
n.a.
Prero
Prerov
SF
Suspension Flash Calciner
Ishikawajima-Harima Heavy Ind. Chichibu Cement
Ishikawajima-Harima Heavy Ind. Fuller Company / Fives-Cail Babcock
NSF
New SF
RSP
Reinforced Suspension Preheater
Onoda Cement
Onoda Engineering & Consulting Kawasaki Heavy Industries Allis-Chalmers CLE-Technip
KSV
Kawasaki Spouted Bed and Vortex Chamber
Kawasaki Heavy Industries
Kawasaki Heavy Industries
NKSV
New KSV
MFC
Mitsubishi Fluidized Calciner
Mitsubishi Mining & Cement
Mitsubishi Heavy Industries
GG
Reduction Gas Generator
Mitsubishi Heavy Industries
DDF
Dual Combustion and Denitration Nihon Cement Furnance
Kobe Steel
CSF (CFF)
Chichibu Suspension Flash Calciner
Chichibu Cement
Chichibu Cement (own plants)
SCS
Sumitomo Cross Suspension Preheater and Spouted Furnace Process
Sumitomo Cement
Kawasaki Heavy Industries Ishikawajima-Harima Heavy Industries
*Air through: secondary firing systems
Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 7. TEST QUESTIONS
7.
TEST QUESTIONS
1) Which is the chemical reaction with the highest heat consumption within the clinker burning process? How much does it consume in absolute terms (kJ/kg clinker) and in percent of the total heat consumption of a modern kiln system? 2) Which are the three basic precalciner arrangements and what are their differences? 3) At what temperature does the calcination take place and how much CO2 is totally dissociated from the CaCO3? 4) Which are the benefits of precalciner technology? 5) Which is the most important design criteria for precalciner dimensioning? 6) Explain the term „apparent calcination degree“. How can it be determined and what is its significance? 7) How do the effects of a bypass compare in case of a straight preheater kiln and a precalciner kiln? Fig. 1 Sketch of Dotternhausen Kiln, the first Precalciner (KHD, 1966)
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Table 4:
Temperatures and Process Steps for Clinker Burning
Temperature [°C]
Process Step, Type of Reaction
Heat
20 - 100
Evaporation of free H2O
Endo
100 - 300
Loss of physically absorbed H2O
Endo
400 - 900
Removal of structural water
Endo
Structural changes in silicate minerals
Exo
Dissociation of CO2 from CaCO3
Endo
> 800
Formation of intermediate products Belite, Aluminate and Ferrite
Exo
> 1250
Formation of liquid phase (aluminate and ferrite melt)
Endo
Formation of alite
Exo
Crystallization of liquid phase into mainly aluminate and ferrite
Exo
> 500 600 - 900
1300 - 1240
For numerical calculations, an approximate quantity of CO2 from the raw material (dissociated from the calcites) can be used, regardless of the exact chemical composition. CO2 from raw mat = 0.28 Nm3/kg cli
Table 5:
Energy Balance of Process Steps for Clinker Burning
Endothermic Processes:
kJ/kg cli
kcal/kg cli
Dehydration of clays
165
40
Decarbonisation of calcite
1990
475
Heat of melting
105
25
Heating of raw materials (0 to 1450°C)
2050
490
Total endothermic
4310
1030
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"Holderbank" - Cement Course 2000 Total endothermic
4310
1030
Exothermic Processes:
kJ/kg cli
kcal/kg cli
Recrystallization of dehydrated clay
40
10
Heat of formation of clinker minerals
420
100
Crystallization of melt
105
25
Cooling of clinker
1400
335
Cooling of CO2 (ex calcite)
500
120
Cooling and condensation of H2O
85
20
Total exothermic
2550
610
Net Theor. Heat of Clinker Formation:
kJ/kg cli
kcal/kg cli
Endothermic - exothermic
1760
420
Heat consumption of Kiln System:
kJ/kg cli
kcal/kg cli
Average 4-stage SP system
3300
790
Modern 6-stage SP system
3000
720
Rel. Heat Requirement of Calcination: Average 4-stage SP system
60%
Modern 6-stage SP system
66%
Fig. 12 FCB Low-NOx Precalciner
Fig. 13 FLS
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Fig. 14 FLS Adjustable Kiln Orifice
Fig. 15 Pyroclon
Fig. 16 Pyrotop
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Fig. 17 Polysius
Fig. 18 Prerov
Fig. 19 EVS-PC
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Features of EVS-PC PC system Supplier:
Fives-Cail Babcock
Fig. 20
SF / NSF
Fig. 21
RSP
Features of RSP PC system Suppliers: Onoda Engineering & Consulting Kawasaki Heavy Industries Allis Chalmers Creusot - Loire Entreprises © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 Fig. 22
KSV / NKSV
Features of KSV/NKSV PC system Supplier:
Kawasaki Heavy Industries
Fig. 23
MFC
Fig. 24
GG
Features of GG PC system Supplier:
Mitsubishi Heavy Industries System abandoned
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"Holderbank" - Cement Course 2000 Fig. 25
DD
Fig. 26
CSF
Features of CSF PC system Supplier: Fig. 27
Chichibu Cement in own plants Voest Alpine PASEC System
Fig. 28 FLS
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers
Clinker Coolers U. Gasser / D. Brassel PT 97/14232/E (Revision 1, February 1999) 1. INTRODUCTION 2. GENERAL CONSIDERATIONS 2.1 Heat Flow in a Kiln System 2.2 Definitions 2.3 Calculations 3. GRATE COOLERS 3.1 The Reciprocating Grate Cooler 3.1.1
Principle
3.1.2
History
3.1.3
Conventional Grate Coolers (1980’s)
3.1.4
Typical Grate Cooler Problems
3.1.5
Modern Grate Coolers (1990’s)
3.1.6
Design Highlights of Modern Grate Coolers
3.1.7
Clinker Crushers
3.1.8
Cooler control
3.1.9
Cooler Dedusting
3.1.10 Developments 3.2 The Cross Bar Cooler 3.2.1
Principle
3.2.2
Main features
3.2.3
Strengths and Weaknesses
3.3 The Travelling Grate Cooler 3.3.1
Principle
3.3.2
Strengths and Weaknesses
4. ROTATING COOLERS 4.1 The Rotary Cooler or Tube Cooler 4.1.1
Principle
4.1.2
Design Features
4.1.3
Cooling performance
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"Holderbank" - Cement Course 2000 4.1.4
Strengths / Weaknesses
4.2 The Planetary Cooler 4.2.1
Principle
4.2.2
Historical
4.2.3
Design features
4.2.4
Internal heat transfer equipment (see Fig. 26)
5. VERTICAL COOLERS 5.1 The Gravity Cooler (G - Cooler) 5.2 The Shaft Cooler
SUMMARY Clinker coolers have two tasks to fulfil: ♦ Recuperate as much heat as possible from the hot clinker by heating up the air used for combustion ♦ Cool the clinker from 1400°C to temperatures adequate for the subsequent process equipment, normally to 100 - 200°C. There are mainly two different types of clinker coolers in operation with the following features: Grate coolers ♦ Crossflow heat exchange through horizontal clinker bed with cold air from below. ♦ Cooling airflow exceeding combustion air requirement allows low clinker temperatures, but necessitates excess (waste) air dedusting. ♦ Modern cooler technology with sophisticated plates and forced aeration systems allow combustion air temperatures exceeding 1000°C. ♦ Trend to wider and fewer grates, less cooling air and fixed inlets ♦ Largest units: 10’000 t/d ♦ Travelling grate (Recupol): last unit built around 1980 Rotating coolers ♦ Rotary tube coolers with separate drive or planetary cooler attached to kiln shell ♦ Quasi counter-current flow heat exchange ♦ Cooling air determined by combustion air, no waste air ♦ Heat exchange (recuperation) determined by condition of internal heat transfer equipment ♦ Limited unit size, up to 3000 t/d ♦ Planetary cooler not suitable for precalciner technology ♦ Practically no new installation built anymore Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 1. INTRODUCTION
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"Holderbank" - Cement Course 2000 1.
INTRODUCTION
The clinker cooler is a vital part of the kiln system and has a decisive influence on the performance of the plant. Three key indicators characterize a good cooler: ♦ Maximum heat recuperation ♦ Minimum cooling air flow ♦ Unrestricted availability There have been periodic changes in trends during the past decades. Grate coolers were first introduced by Fuller Company (USA) around 1930. While its design was continuously being optimized, the grate cooler became the predominant type in the 1950's. In the late 1960's, the planetary cooler gained popularity which reached its peak in the 1970's, mainly due to its simplicity. Larger unit capacities with precalciner technology made the grate cooler the preferred solution again. A wave of grate cooler reengineering starting in the mid 1980's has generated a much improved grate cooler technology as well as a new situation on the suppliers' side. New problems were experienced and have been or are being solved. Since cement plants have life cycles of 40 years and more, numerous units of each cooler type, planetary, rotary or grate cooler of old or new designs, will remain in operation for many more years. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 2. GENERAL CONSIDERATIONS
2.
GENERAL CONSIDERATIONS
The clinker cooler has the following tasks to fulfil: ♦ Process internal heat recuperation by heat transfer from clinker to combustion air ♦ Reduce clinker temperature to facilitate clinker handling and storage ♦ Provide maximum cooling velocity to avoid unfavorable clinker phases and crystal size Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 2. GENERAL CONSIDERATIONS / 2.1 Heat Flow in a Kiln System
2.1
Heat Flow in a Kiln System
The importance of the cooler as a heat recuperator can be well demonstrated with a heat flow (Sanki) diagram. Figure 1
Clinker cooler and kiln system
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Figure 2
Energy turnover (Grate cooler)
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 2. GENERAL CONSIDERATIONS / 2.2 Definitions
2.2
Definitions
♦ As for other components of the kiln system, specific figures for clinker coolers refer to 1 kg of clinker. This eliminates the influence of plant size and allows direct comparison of clinker coolers of different types and sizes. ♦ Cooling air is the air which passes the clinker thus being heated up while cooling the clinker. It corresponds approximately to the combustion air requirement, only grate coolers allow additional air for better cooling. ♦ Primary air is the air which is required for proper functioning of the burner. Ambient air insufflated by a separate small fan plus the air from a pneumatic transport system, amounting from < 10% up to > 30% of the air required to combust that fuel. Some precalciner burners are equipped with primary air fans (for cooling) as well. ♦ Secondary air is the hot air entering the rotary kiln via clinker cooler. Its flow is determined by the combustion of the burning zone fuel. While cooling the clinker, it reaches temperatures of 600 to over 1000°C, depending on type and condition of the cooler. ♦ Tertiary air is that part of the combustion air which is required for combusting the precalciner fuel. It is extracted from kiln hood or cooler roof, and then taken along a duct (=tertiary air duct) parallel © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 to the kiln to the precalciner. It reaches temperatures near or equal to the level of the secondary air. ♦ Middle air (grate cooler only) is extracted from the cooler roof if drying of process materials requires a temperature level which is higher than the waste air. If the quantity is small, up to 450°C can be expected at normal cooler operation. ♦ Waste air (grate cooler only) is also called cooler exit air or cooler excess air. The total cooling airflow from the fans is normally higher than the flow required for combustion (=tertiary + secondary air). The extra air, which has normally a temperature of 200 to 300°C, must be vented to ambient via a dedusting system. ♦ False air is cold air entering the system via kiln outlet seal, burner opening, casing or clinker discharge. It either dilutes secondary air thus reducing recuperated heat or adds load to the waste air system of grate coolers. ♦ Specific air volumes are airflows per kg of clinker (m3/kg cli, Nm3/kg cli). Independent of the kiln size, airflows of cooler systems can be directly compared. ♦ Specific loads express the relation of clinker production to a characteristic dimension of the cooler (t/d m, t/d m2, t/d m3). Exact definitions vary with cooler type. ♦ Radiation losses from the cooler casing/shell are particularly important for planetary coolers, where they actively support the cooling of the clinker. ♦ Efficiency expresses the quality of heat transfer from clinker to the air which is used for combustion in the burning zone and precalciner firing. Remark: Since the heat recuperated is proportional to hot air used for combustion and temperature, an efficiency figure is only meaningful if it is related to a heat consumption figure (resp. a combustion airflow). Figure 3
Clinker coolers - Definitions
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 2. GENERAL CONSIDERATIONS / 2.3 Calculations
2.3
Calculations
The calculations below are examples of heat balance investigations:
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"Holderbank" - Cement Course 2000 •
Heat in hot clinker Qcli :
Qcli = mcli* cpcli* (tcli - t ref)
Example with mcli =1 kg/h: tcli = 1400°C: Qcli = 1 kg/h * 1.090 kJ/kg°C * (1400°C-20°C) = 1504 kJ/h
•
Heat in hot air Qair :
Qair = Vair* cpair* (tair - t ref)
Example with V air = 1Nm3/h: tair = 1066°C: Qair = 1 Nm3/h * 1.421 kJ/Nm3°C * (1066°C-20°C) = 1486 kJ/h
•
Radiation loss Qrad :
Q rad =CR * ε * A {(t/100)4 4
(t0/100) }
Grate cooler Qrad = 20 kJ/kg cli (from experience)
Cooler efficiency ηcooler
ηcooler =
Q combustion air ∑ Qloss = 1− Q clinker from kiln Q clinker from kiln
The secondary (+ tertiary) air requirements are dictated by the amount of fuel fed to the burners. Per this definition, the efficiency of a cooler is getting better with increasing kiln heat consumption. It is thus obvious that a cooler efficiency figure is only meaningful if the corresponding heat consumption (or airflow) is indicated. Example:
production
5000 t/d
heat consumption
3000 kJ/kg cli
secondary and tertiary air temperatures
1066°C
Primary air main burner
10%
PC fuel ratio
60%
False air and excess air neglected (not realistic!) Q comb air: V Comb air
= 3000 MJ/kg cli * 0.26 Nm3/MJ * 5000/24*103 kg/h * (1-0.4*0.1) = 156'000 Nm3/h
t comb
air
Q comb air
= 1066°C → q combustion air = 1.421 kJ/Nm3° * (1066-20)° = 1486 kJ/Nm3 = V comb air * q comb air = 1486*156'000 kJ/h = 231'816 GJ/h
Q clinker: m clinker
= 5000 t/d /24 h/d *103 kg/t = 208'333 kg/h
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3 m "Holderbank" Cement Course 2000 =-5000 t/d /24 h/d *10 kg/t = 208'333 kg/h clinker
t clinker from
= 1400°C → q clinker from kiln = 1.09 kJ/kg° * (1400-20)° = 1504 kJ/kg
kiln
Q
clinker
Efficiency η
Figure 4
= 208'333 * 1504 kJ/kg = 313'333 GJ/h = 231'816 / 313'333 * 100% = 74.0%
Clinker cooler typical data (4-stage SP Kiln, 2’000 t/d)
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS
3.
GRATE COOLERS
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler
3.1
The Reciprocating Grate Cooler
The reciprocating grate cooler is the most widely applied type and is exclusively used for new plants. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.1 Principle
3.1.1
Principle
♦ The following major system components can be distinguished: •
Casing with kiln hood and connections for air at different temperature levels
•
Reciprocating grate with drive system
•
Aeration system with fans, undergrate compartments and direct air ducts
•
Riddling (= fall through) extraction system with hoppers, gates and transport
•
Clinker crusher
♦ Material transport The clinker is pushed by the vertical part of the front edge of the preceding plate. The entire grate consists of a combination of fixed and moving rows which results in a quasi-continuous motion of the clinker bed. © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 ♦ Heat exchange Heat exchange from clinker to air is according to the cross current principle. The cooling air penetrates the clinker bed which is laying on the grate from underneath and leaves it at the surface. While passing through the hot clinker, the air is accumulating heat which is transferred from the clinker. ♦ Cooling air Normally, ambient air is blown to underneath of the grate plates loaded with clinker by a number of cooling air fans. Delivery pressure must be sufficient to penetrate the clinker bed and to compensate for the expansion (increase of actual volume) of the air from heating it up Under ideal conditions, the required cooling air depends directly from the desired clinker temperature. One part of the cooling air is used for combustion in the kiln, the rest is cleaned and vented to ambient, unless it is further used, e.g. for drying. ♦ Cooling curve A simplified mathematical model for clinker cooling in a conventional, optimized grate cooler gives the relation between cooling air quantity and clinker temperature as follows:
T cli− Tamb = exp[− (Vair / 0.77)] Tcli in − Tamb with
= clinker temperature at cooler inlet
°C
T amb
= ambient temperature
°C
V air
= cooling air quantity
Nm3/kg cli
T cli in
The above approximation (curve Fig. 17: Tcli = 1400°C) has been found to give satisfactory results for conventional grate coolers from various suppliers. Figure 5
Reciprocating Grate Cooler: Design Features
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.2 History
3.1.2
History
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"Holderbank" - Cement Course 2000 It was the Fuller Company (USA) who introduced the first reciprocating grate cooler in the late 1930's with a grate slope of 15°. Fluidized material running down the grate leads to 10° grate inclination. The 10° cooler was predominantly used until the mid 1950's. Problems were encountered with those 10° coolers when the clinker was fine and started to fluidize. As an attempt to solve this problem, wedge grate plates were used. Another drawback of those 10° coolers was the building height required for larger units. In the mid 1950's, the first horizontal grate coolers were introduced. They were initially just 10° grates installed horizontally with accordingly reduced conveying capacity. Some of these coolers were severely damaged by overheating, due to fluidization and accumulation of hot fine clinker at the feed end. This drawback of the horizontal cooler lead to the development of the so-called combi cooler. Is has one (or formerly two) inclined grates with normally 3° slope, followed by one or two horizontal grates. Not all suppliers followed the same philosophies, so all three concepts (all horizontal, combi and all inclined) can be found all over the world. The planetary cooler boom period in the 1970's came to an end, when large production capacities were in demand. Precalciner technology required grate coolers which eventually needed to be reengineered again. Problems related to the clinker distribution, growing awareness of heat and power consumption as well as the demand for higher availability forced the suppliers to introduce new solutions. Initiated by the new company IKN, the grate cooler technology underwent significant changes since the mid 1980's. Modern grate plates, forced (direct) aeration and better gap design were introduced by all cooler makers helping to reduce cooling airflow and cooler size. The new approach lead to better recuperation in most cases. However, serious wear problems with the new systems forced most of the companies to modify their solutions once again. Today, in the mid 1990's, we are still gaining experience with latest designs. The ultimate solution would be the waste air free grate cooler with unlimited flexibility and availability. However, right now the cement industry would be happy with smooth operation, high recuperation, low cooling air and no cooler related kiln stops. Figure 6
Various configurations of reciprocating grate coolers
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.3 Conventional Grate Coolers (1980’s)
3.1.3
Conventional Grate Coolers (1980’s)
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.3 Conventional Grate Coolers (1980’s) / 3.1.3.1 Typical Design Features
3.1.3.1
Typical Design Features
♦ Grate plates with round holes ♦ Two to three grates, depending on size ♦ Grate slope 0° or 3° or both, depending on supplier ♦ Mechanical excenter drives for reciprocating grate ♦ Chamber aeration ♦ Fan pressure 45 mbar (first) to 25 mbar (last) ♦ Smaller compartments at inlet, larger towards outlet ♦ Clinker riddling extraction with hoppers, gates and dragchain (some earlier designs: internal drag chain without hoppers) ♦ Hammer crusher at cooler discharge World’s largest kilns (10'000 t/d in Thailand) are equipped with conventional grate coolers from CPAG with 4 grates. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.3 Conventional Grate Coolers (1980’s) / 3.1.3.2 Strengths and Weaknesses of Conventional Grate Coolers
3.1.3.2
Strengths and Weaknesses of Conventional Grate Coolers
Strenghts
Weaknesses
•
Lower clinker end temperature due to • higher amount of cooling air
Waste air handling system (dedusting, fan) required
•
Possibility of adjusting cooling air and • grate speed provides higher flexibility
More complex cooler requires higher capital investment
•
Optimization possibilities during operation
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•
Higher power consumption than planetary or tube cooler
•
Uneven clinker discharge / segregation leads to several problems
•
Red river
•
Snowmen
•
Air breakthrough (bubbling, geyser)
•
Reduced plate life
•
Excessive clinker fall through between gaps
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Causes and mechanism of those problems are further explained in the next paragraph. Figure 7
Conventional grate coolers: Design features
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.4 Typical Grate Cooler Problems
3.1.4
Typical Grate Cooler Problems
Most grate coolers show a tendency to one or more of the system inherent problems, and in many cases there is no real cure. Investigations of the causes lead to the development of the modern cooler technology. ♦ Segregation: Due to its physical properties, the clinker is lifted by the kiln rotation before it is discharged into the cooler. Installation of the grate axis offset from the cooler axis should compensate for this effect. However, since discharge behavior of finer and coarser clinker particles differ from each other, the clinker fractions are not evenly distributed across the grate. Fines are discharged later and are thus found predominantly on the rising side of the kiln shell (Fig. 8a). ♦ Thin clinker bed in recuperation zone: With a conventional grate cooler with chamber aeration, the clinker bed thickness is limited directly by the installed cooling fan pressure and indirectly by the quality of compartment seals and distribution of the clinker across the width. In order to avoid overheated plates, the operator will set the bed not higher than allowed to guarantee airflow through the plate carrying the clinker with the highest bed resistance. Thin bed operation leads to unfavorably high air to clinker ratio and poor heat exchange on the sides with consequently low recuperation efficiency. ♦ Red river: The infamous red river is one of the most feared problems with grate coolers. Due to segregation, fine clinker has always its preferred side (see above). Different bed resistance on either side and only one air chamber across the entire width often cause fluidization of the fine clinker laying on top. This fluidized clinker does no longer follow the speed of the grate, but shoots much faster towards the cooler discharge end. Because the residence time of that fine clinker is much reduced, it does not follow the general cooling curve and © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 forms a red hot layer on top of the regularly cooled, already black clinker. Hence the term "red river". It is not the missed heat recuperation, but the red hot material being in touch with cooler walls, plates and side seals in the colder area where such temperatures should normally not occur. Premature destruction of those pieces results in poor availability, high maintenance and ultimately in loss of production and sales revenues. ♦ Snowman: The sticky consistence of the hot clinker leaving the kiln combined with the compaction at the drop point often leads to formation of solid clinker mountains on the grate. Not permeable for cooling air, they grow larger and disturb the flow pattern of the clinker in this anyway critical inlet area. ♦ Air breaking through: Due to the different resistance of the clinker bed and the fear of overheated plates, too much air is put on the first grate compared to the clinker bed. The result is air shooting through the bed, hardly taking any heat and thus not contributing to the heat exchange. In addition to that, the clinker is mixed which can be seen by the bubbling action, and the layered clinker bed (colder clinker below, hotter on top) is destroyed thus disturbing the cross flow heat exchange pattern. The results are low recuperation and too much heat going to the aftercooling zone. ♦ Figure 8a: Segregation at cooler inlet
Figure 8c
Red River
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Figure 8b: Clinker bed depth effect on cooling
Figure 8d
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Snowman
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Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.5 Modern Grate Coolers (1990’s)
3.1.5
Modern Grate Coolers (1990’s)
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.5 Modern Grate Coolers (1990’s) / 3.1.5.1 Design Features
3.1.5.1
Design Features
The successful clinker cooler has:
1) À Correct allocation of cooling air to clinker Á Sustainable gap widths in the entire cooler All new or redesigned clinker coolers are aiming at the above two goals: ♦ Modern grate plates, designed to cope with high temperature differences ♦ Inclined inlet section without moving rows ♦ Pattern of zones for individually adjustable aeration in recuperation zone ♦ Modern plates for a tight grate in the after cooling zone ♦ New, improved side seal plate design for tight gaps and low wear ♦ Careful undergrate compartment sealing ♦ Adequate seal air system with correct control ♦ Wider and shorter coolers; lower number of grates ♦ Improved and wear protected moving grate support and guidance ♦ Hydraulic grate drive with optimized control system ♦ Cooling air fans with inlet vane control and inlet nozzle for measuring flow ♦ Roller crusher Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.5 Modern Grate Coolers (1990’s) / 3.1.5.2 Strengths and Weaknesses of Modern Grate Coolers
3.1.5.2
Strengths and Weaknesses of Modern Grate Coolers
Strenghts
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Weaknesses
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"Holderbank" - Cement Course 2000 Strenghts
Weaknesses
•
More constant heat recuperation → improved, smoother kiln operation
•
More complicated mechanical installation (varies with supplier)
•
Cooler inlet: improved clinker distribution across grate width
•
Higher secondary air temp. increases wear of nose ring and burner refractories
•
Higher actual (m3/h) tertiary air flow can increase dust entertainment at take off point
•
Teething problems with new designs -> design changes still in progress
•
Elimination / control of red river
•
Significantly reduced grate riddlings (clinker fall through)
•
Higher waste air temperature (valuable for drying)
•
Lower heat consumption due to higher heat recuperation (cooler efficiency)
•
Reduced power consumption due to less waste air
•
Lower civil cost due to more compact cooler
•
Lower investment due to smaller waste air system
•
Reduced cost for maintenance
Figure 9
Modern Grate Coolers: Design features
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers
3.1.6
Design Highlights of Modern Grate Coolers
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.1 Modern Grate Plates
3.1.6.1
Modern Grate Plates
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"Holderbank" - Cement Course 2000 In the mid 1980's, the first modern grate plates were installed in grate coolers by IKN and CPAG. They were designed for the following targets: ♦ Allow for lower air/clinker ratio in the recuperation zone for higher recuperation ♦ Improve clinker distribution across the grate width ♦ Assure that all grate plates are always sufficiently cooled by air The above targets were reached using the following ideas: •
Higher built-in pressure drop Similar to the effect of thick bed operation, a higher pressure drop across the plate reduces the relative influence of variations in permeability of the clinker bed.
•
No more fine clinker falling through Fine clinker falling through means loss of heat and thermal stress on the drag chain. For forced aeration (below) it is mandatory that no material can fall in the air ducts where it would cut off the air supply.
•
Forced (direct) aeration via air ducts In order to ensure that all plates get enough air, to allow individual allocation of air to different areas and to avoid that air escapes through gaps, groups of plates are supplied with air directly via a special duct system
•
Tight gaps between plates and plates/casing Not only through the grate surface, but also through gaps between plates within the same row as well as from one row to the next, fine clinker can fall through. Those gaps have to be sealed as well, e.g. by interlinked steps in the plate sides (Fuller, Polysius) or by bolting them together as packages (IKN).
The modern grate plates are the basis of modern cooler technology. Problems experienced with the first generation of modern grate plates lead to several detail modifications: ♦ Cracks in corners of air outlet openings → Solution: modified shape ♦ Plastic deformation caused premature failure with many designs → Solution: thermally flexible plates built from two or more pieces ♦ Preferred plate internal airflow left plates locally uncooled → Solution: plate internal guide vanes, optimized air channelling ♦ ♦ Modern grate cooler, as the IKN Pendulum Cooler, use also Pneumatic Hopper Drains (PHD) to withdraw the fine clinker fall through. Figure 10
Modern grate plates
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Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.2 Air Ducts
3.1.6.2
Air Ducts
The concept of forced aeration, i.e. the idea to bring the air directly to the grate plates requires a flexible air connection between the (stationary) fan and the moving rows. Initially, the most obvious and simple approach was chosen: flexible hoses or bellows. IKN, CPAG, Polysius and Fuller used this solution at the beginning. However, experience showed that those hoses were sensitive to design (geometry), installation and material qualities. While many coolers operated without any problem, others showed frequent rupture of those hoses, very often causing severe plate damage and consequently kiln downtime. Meanwhile, all suppliers developed new solutions. Only KHD avoided these problems by using telescopic ducts from the beginning. The individual suppliers are now using the following standard solutions: ♦ Telescopic air connector (BMH-CPAG, KHD) ♦ Ball and socket type air connector (FLS, Fuller) ♦ Gate type air connector (Polysius) ♦ Open air beam (IKN) Figure 11
Forced (direct) aeration to moving rows: Flexible ducts
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Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.3 Aeration Concept
3.1.6.3
Aeration Concept
It was soon recognized that only a few (6 to 8) rows of direct and individual aeration are not sufficient to improve clinker distribution or to eliminate/control red river formation. The number of rows with direct aeration was gradually increased and soon the suppliers started to equip the entire recuperation zone or even the entire cooler with direct aeration. Indeed, this improved the control possibilities, but created the following new drawbacks: ♦ Complicated and expensive equipment ♦ More parameters to control ♦ Difficult access underneath grate ♦ High number of potential problem areas (flexible hoses!) Ways had to be found to reduce the number of air ducts to the individually aerated cooler zones. There are two ways to achieve this: ♦ Reduce number of individually aerated zones ♦ Modify the air duct system Today, the following different solutions with varying degrees of experience are presently available from the suppliers: ♦ No moving rows requiring flexible air connectors in inlet section ♦ Longitudinal structural beams designed as air ducts ♦ Short air ducts from one moving row to the next (“Air bridge“) ♦ Direct aeration for fixed rows only (“hybrid aeration“) ♦ Full chamber aeration with modern grate plates Figure 12
Aeration patterns
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.4 Seal Air (Confining Air)
3.1.6.4
Seal Air (Confining Air)
When direct plate aeration was introduced, the significance of the seal air or confining air was not properly investigated. It was expected that direct individual aeration of the plates alone would be enough to get the desired improvement due to better air to clinker allocation. If the cooler grates were tight and had no or very narrow gaps between moving and fixed rows or between grate and cooler casing, this would indeed be true. However, real grates have large gaps, which is one of the reasons why direct aeration was introduced. The effect of insufficient seal air pressure for direct aerated grates can be explained as follows: ♦ High resistance in clinker bed (bed thickness, kiln upset, granulometry) ♦ Cooling air sneaks around plate edge to undergrate compartment instead ♦ Clinker dust carried in this air → abrasion / wear ♦ Gap becomes larger → seal air can escape → more “sneak“ air ♦ Stops for repair reduce availability and increase operating cost Today it is generally accepted that partition, sealing and pressurizing of the undergrate compartments is even more important than with chamber aerated coolers. Ideally, the partition of the undergrate compartments should repeat the pattern of the individually aerated grate zones of the grate itself. Since this would lead to very complicated and expensive designs with difficult access, simpler solutions had to be found. One of the most common countermeasures is, to install larger seal air fans. It was interesting to observe the installed cooling air to be gradually increased with each new project. This did not only lead to larger waste air systems but also to higher cooling fan motor power which partially offset the savings expected from modern coolers. The suppliers have proposed the following improvements: ♦ Larger seal air fans ♦ Seal air branched off from cooling air fans ♦ Seal air from booster fan using air from cooling air fans ♦ Undergrate pressure controlled by cooling air fan pressure ♦ Careful sealing of undergrate compartments ♦ No more moving rows in hot inlet zone Figure 13
Seal air systems
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Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.5 Side Seal Systems
3.1.6.5
Side Seal Systems
Extremely serious wear problems occurred along the side seal plates on each side of the grate. Excessive fall through along the sides and shockingly short lifetime of the side seal plates, mainly in the recuperation zone, were the result. The main reasons for this problem can be listed as follows: ♦ The same seal element used for lateral and longitudinal movement ♦ Side seal plates fixed to cooler casing ♦ Entire thermal expansion to be compensated by (cold) gap on each side ♦ Side plates used for lateral guidance of the grate (older designs) ♦ More lateral thermal expansion of wider grates for large units The following new solutions have been developed and are now part of the contemporary standards: ♦ Entirely new side seal plate concepts ♦ Side seal plates bolted to cross beams of fixed rows (no longer to cooler casing) ♦ Joints for thermal lateral expansion and mechanical longitudinal movement between moving rows and casing separated ♦ Center grate guide for large coolers Figure 14
Side seal designs
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Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.7 Clinker Crushers
3.1.7
Clinker Crushers
All kiln systems produce larger than normal clinker lumps more or less frequently. Large balls of material enter the cooler when coating drops during kiln upsets. Such large clinker masses can only be cooled superficially and contain a lot of heat. Before being discharged to the clinker conveyor, they must at least be crushed to smaller particles. All clinker coolers, regardless of the type, are equipped with a clinker crusher. Traditionally, this is a hammer crusher which has proven to be reliable. In order to cool large clinker lumps, they must be crushed within the cooler. In reality, this means installing the crusher before the last grate. Early trials with hammer crushers were not successful, however. Based on the idea and experience with roller grate bottoms in shaft kilns (and shaft coolers), CPAG developed the roller crusher to be used as intermediate crusher in a step cooler. The advantages of the roller crusher make it also superior at the cooler outlet. Hydraulic or electric drives as well as different combinations of reversing rollers are available from various suppliers. Compared to the hammer crusher, the roller crusher is rated as follows:
Strengths
Weaknesses
•
low speed
•
higher initial investment
•
low wear
•
chokes easier
•
low dust generation
•
more difficult to design
•
equalization of material rushes
•
suitable for high temperatures
•
lower power consumption
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Figure 15a
Hammer crusher
Figure 15b
Roller crusher
Figure 16
Heat and air balance of a modern Grate cooler
Figure 17
Optimization
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Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.8 Cooler control
3.1.8
Cooler control
One of the advantages of the reciprocating grate cooler is its high flexibility, due to operating variables adjustable independently from kiln operation. Usually three main variables are controlled automatically. a) Grate speed In order to prevent the clinker bed resistance from exceeding the pressure capabilities of the cooling fans (which would mean too little cooling air and danger of heat damage), the bed resistance on the grate should be kept constant. To do this, each grate section drive is controlled by the undergrate pressure of the first or second compartment in each grate section. An increase in pressure indicates an increase in bed resistance (either more material in the cooler or finer material). The reaction is an increase of the grate speed, causing the bed to become thinner. If the undergrate pressure decreases, the drive slows down and the bed becomes thicker. Another possibility is to control only the first grate by the undergrate pressure, and to keep the speed of the following grates proportional to the speed of the first grate. More sophisticated control systems use the weighted average of several undergrate pressures to control first grate speed. In many cases, however, control systems amplify fluctuations from the kiln instead of smoothening them. Increasing the bandwidth of the control system has shown good results in several cases. b) Airflow This control is complementary to the grate speed control. It maintains a constant volume of cooling air entering the cooler independently from the grate underpressure. Each cooling fan is equipped with a piezometer sensor which will recognize an increase or decrease of the airflow and cause the cooling fan damper to close or open (in case of inlet vane damper control) or the fan motor speed to decrease or increase (in case of variable speed fan drives). During normal conditions the cooling fans operate at about 2/3 to 3/4 of their maximum performance so that enough spare capacity is left to cope with eventual kiln rushes. Together, grate speed and air flow control will on one hand ensure a sufficient cooling air supply to the cooler and, on the other hand, tend to provide more uniform combustion air temperature to the kiln. © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 c) Hood draft The third component of the cooler control system is the hood draft control. An automatically controlled grate cooler can improve the whole kiln operation and allows the operator to concentrate on other problems. The kiln hood pressure is used to regulate the cooler vent air fan speed to maintain a constant pre-set draft. As the draft tends to become positive, the cooler vent fan speed is increased. This takes more air from the cooler and maintains the draft setpoint. As with the other controls, reaction in the opposite direction is just as important. Coolers with radiation walls (IKN) allow hood draft control by one of the first cooling air fans. Figure 18
Cooler control
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.9 Cooler Dedusting
3.1.9
Cooler Dedusting
While dedusting of kiln exhaust gas can be commonly solved by using one type of dust collector only (electrostatic precipitator), the choice of the most adequate system for dedusting clinker cooler vent air raises quite often many discussions. This choice problem is basically a result of the special and fluctuating conditions of the vent air to be dedusted: normal operation
kiln upset
airflow (actual volume)
%
100
up to 150
air temperature
°C
200 - 250
up to 450
air dew point
°C
5 - 20
5 - 20
g/Nm3
5 - 15
25 - 35
dust load
The dust particle size distribution can vary in a wide range depending on the burning conditions in the kiln. Dimensioning of the dedusting equipment must take into account the worst conditions, in order to © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 maintain the required clean gas dust content even at kiln upset condition. The types of dust collectors for this application are compared below. Today's trend is: ♦ multiclones will no longer be tolerated in new and many existing plants ♦ gravel bed filters have proved to be inefficient and expensive ♦ use of electrostatic precipitators is possible without restriction ♦ bag filters with cooling of the vent air in a heat exchanger are often used nowadays
Type of collector
Strengths
Weaknesses
multiclone
simple low investment cost low space requirement not sensitive to temperature peaks
poor efficiency for particles < 20 µm efficiency sensitive to gas flow fluctuation comparatively high pressure loss high operating cost
electrostatic precipitator
low pressure loss low operating cost low maintenance cost
big unit required or use of pulse generator -> high investment cost possibly water injection required
gravel bed filter
not sensitive to temperature peaks
highest investment cost highest pressure loss high operating cost
bag filter
high efficiency relatively low investment cost
no bags for temperatures up to 450°C Õ precooling required high pressure loss high operating cost high maintenance cost
Figure 19
Grate cooler dedusting
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Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.10 Developments
3.1.10 Developments Air recirculating (Duotherm) cooler A patent has been taken out in 1970 by the "Société des Ciments Français" concerning the recirculation of the vent air after sending it through a heat exchanger. The first application of the unconventional system has been realized in 1970 at the Beaucaire plant of the above mentioned company, on a 1500 t/d Fuller cooler. Initial experience gained with this installation was very satisfactory. Only few installations using this principle have been realized, e.g. in the Ulco plant. The main advantages and disadvantages of this system are:
Strengths
Weaknesses
•
no dust emission at all
•
•
simple
•
low investment cost
possible wear of fan blades (preventative measures necessary)
•
heat recovery possible (at various temperature levels)
•
•
maintenance and operating costs higher than conventional cooler dedusting system with EP
extension possible by adding further heat exchange units
Modern cooler technology and problems in some cases have pushed this idea in the background. However, it might be reactivated if it can be combined with modern cooler systems.
Dual pass cooler A completely new principle of cooling in a grate cooler has been introduced by Polysius in 1994: the dual pass cooler or REPOL-ZS. © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 This cooler can be considered a two-grate cooler with intermediate crusher where grate 1 and 2 are identical. The hot, 1400°C clinker from the kiln is fed on top of a layer of colder clinker already laying on the cooler grate. At the end of the grate, the now cold lower clinker layer is extracted via a special system consisting of reciprocating bars and a hopper. The upper layer which has reached about 500°C passes a roller crusher and is then returned to a intermediate hopper below the kiln from where it is fed onto the empty grate to pass the cooling air a second time, this time below the fresh hot clinker. One 1400 t/d unit is in operation in Germany using Jet-Ring technology. With less than 1.6 Nm3/kg cooling air, extremely low clinker temperatures have been reported. The crucial problems of this solution are intermediate transport and storage. In spite of the compact size, high cooling degree with low air flow and low plate temperatures, this cooler will only be successful if the intermediate temperature level can be increased and the heat losses reduced.
Figure 20a
Non venting cooler
Figure 20b
Dual pass cooler (Polysius)
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.2 The Cross Bar Cooler
3.2
The Cross Bar Cooler
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.2 The Cross Bar Cooler / 3.2.1 © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 Principle
3.2.1
Principle
F.L.Smidth and Fuller developed together the new SF (Smidth - Fuller) Cross Bar Cooler representing a completely new concept. The basic idea was to develop a cooler in which conveying of clinker and air distribution systems are separated. The SF cooler has a clinker conveying device installed above an entirely fixed grate. In addition the cooler should be less complicated, more efficient and easier to operate than other grate coolers on the market. Sealing air is eliminated and the distribution of air is optimized for all modes of operation The thermal behavior of the SF cooler (e.g. heat balance, recuperation) is similar to the other grate coolers. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.2 The Cross Bar Cooler / 3.2.2 Main features
3.2.2
Main features
•
One inclined fixed grate.
•
Clinker conveying by cross bars, separate from air distribution.
•
No thermal stress of grate.
•
Minimum wear on grateplates due to a dead layer of clinker (50 mm) protecting the grate surface. The thickness is given by the space between the cross bars and the grate. (Anticipated service life time at least 5 years)
•
Dynamic flow control unit (mechanical flow regulator) for each grate plate. The mechanical flow regulator maintains a constant airflow through the grate and clinker bed, irrespective of the clinker bed height, particle size distribution, temperature, etc.
•
No fall through of clinker to the undergrate compartment. → Eliminating undergrate clinker transport resulting in low installation height for new plants.
•
Easy cooler operation by elimination of sealing air and automatic control of air distribution.
•
Modularized cooler concept → short delivery and installation time.
•
Different drive speeds across the cooler possible. → Additional control of clinker distribution.
•
Fewer and less expensive wear parts (easy to replace).
•
Easy visual inspection of undergrate compartment (clean undergrate, windows).
•
Sustainably high thermal cooler efficiency throughout the lifetime of the cooler. → Reduced system heat consumption.
Figure 21a:
SF Cross Bar Cooler
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Figure 21b:
SF cooler grate with cross bars
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.2 The Cross Bar Cooler / 3.2.3 Strengths and Weaknesses
3.2.3
Strengths and Weaknesses
Strengths
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"Holderbank" Strengths - Cement Course 2000 •
No clinker fall through (no hoppers, no dragchain).
•
The grate is protected from overheating.
•
Very high availability is expected.
•
Wear and tear affects only the conveying system and not the air distribution system.
•
For each plate, the cooling air is individually controlled.
•
The amount of cooling air is about 1.6 to 1.8 Nm3/kg.
•
Reduced height and maintenance required since the undergrate clinker transport can be dropped.
•
Time for installation is short due to modular concept.
Weaknesses •
The clinker bed seems to be influenced by the conveying reciprocating cross bar, resulting in disturbed clinker layers.
•
In case of fine clinker and coating drops, air breakthroughs can occur.
•
The performance of the mechanical flow regulator (amount of cooling air) and its distribution is yet to be assessed.
•
Airflow through the fixed grate at the cooler inlet (CIS) can generate dust and dust cycle.
Remark: So far, no SF Cross Bar Cooler is in use within the “Holderbank” group and therefore no first hand experience is available. Worldwide, there are only three SF cross bar coolers installed. Two of a capacity of 450 t/d and one of 2000 t/d. (as of January 1999)
Figure 22a:
Cross Bars: Easy to replace wear parts
Figure 22b:
Mechanical flow regulator
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Figure 22c:
Modular concept: One module
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.3 The Travelling Grate Cooler
3.3
The Travelling Grate Cooler
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.3 The Travelling Grate Cooler / 3.3.1 Principle
3.3.1
Principle
The traveling grate cooler (Recupol) was originally developed by Polysius for use in combination with grate preheater (Lepol) kilns. Using the same principle and similar technology, it uses the same wear parts. The following main components can be distinguished: •
Casing with kiln hood and connections for air at different temperature levels
•
Inlet with water cooled chute (2nd generation) and pulsator
•
Traveling grate with return carrying idlers and drive system
•
Aeration system with fans, undergrate compartments
•
Riddling extraction system with chutes, flap gates, hoppers and transport
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Clinker crusher
♦ Material transport The clinker is carried by a horizontal traveling grate which works like a stationary caterpillar chain with perforated chain plates. In contrast to the reciprocating grate cooler, the clinker does not tumble over plate edges, but remains as undisturbed layered bed from inlet to discharge. ♦ Heat exchange Heat exchange takes place, like for the reciprocating grate according to the cross current principle. Because the layers remain, it should be even better, at least theoretically. ♦ Cooling air Ambient air is blown by a number of cooling air fans to underneath of the travelling grate plates carrying the clinker. Pressure and flow criteria of cooling air are basically as for the reciprocating grate cooler. ♦ Water cooled inlet chute In order to achieve rapid cooling in the inlet section, but also to protect the travelling grate from the highest clinker temperatures, Recupol coolers were equipped with a water cooled inlet chute. ♦
Key figures / KPI Specific grate loading: 25 - 30 t/d m2 (design) Largest units: 3000 t/d (Lägerdorf kiln 10)
♦ Figure 23
Travelling grate cooler
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.3 The Travelling Grate Cooler / 3.3.2 Strengths and Weaknesses
3.3.2
Strengths and Weaknesses
Travelling grate cooler compared to reciprocating coolers: Strengths
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"Holderbank" - Cement Course 2000 Strengths
Weaknesses
•
Possibility of replacing grate plates during operation (on the returning part)
•
•
Undisturbed, layered clinker bed is better for optimum heat exchange
Larger machine for the same grate area equipment requiring more space and higher civil cost
•
Lower specific grate loadings adding further to overall size
•
More expensive to build than a reciprocating grate cooler
•
The absence of clinker movement (see above) was often considered a disadvantage because of cases where a solid (fritted) layer on top of the clinker bed made it impermeable for air. For this reason, pulsators were installed for first cooling fans.
•
Much higher maintenance requirement with ageing equipment
•
Heat loss via cooling water for inlet chute
Due to the mentioned weaknesses, Polysius eventually decided to develop their own reciprocating grate cooler (Repol) around 1980: Figure 24
Travelling grate cooler: Design details
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS
4.
ROTATING COOLERS
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler
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"Holderbank" - Cement Course 2000 4.1
The Rotary Cooler or Tube Cooler
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler / 4.1.1 Principle
4.1.1
Principle
The rotary cooler consists mainly of a rotating cylinder, similar to a rotary kiln. The clinker is fed through the inlet chute and is then cooled by air while being transported towards the outlet end. Cooling is performed in countercurrent flow. The tube is equipped with internal lifters which improve the heat transfer. About 2/3 (66%) of the cooler length is lined with refractory bricks. The rotary cooler is of simple design and is the oldest type of clinker coolers. It was seldom used for modern, large kiln systems. Therefore comparatively little design and operating experience is nowadays available for rotary coolers above 2000 t/d. However, the application of rotary coolers still offers certain advantages. Presently units up to 4500 t/d (dimensions dia 6.3/6.0 x 80 m) are in operation. It will be interesting to follow the future development of large rotary coolers. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler / 4.1.2 Design Features
4.1.2
Design Features
♦ Arrangement of the rotary cooler is normally in the extension of the kiln axis; in many cases the reverse manner (underneath the kiln) has been applied. ♦ The diameter of the cooler is similar to that of a corresponding suspension preheater kiln. Likewise the rotating speed is in the same range as for the kiln (max. 3 rpm). Length/diameter ratio: L/D ~ 10. Many cooler tubes are designed with an extension in diameter in order to reduce air velocity. ♦ The inclination is comparatively high (in the order of 5%). ♦ Like for all rotating coolers, the internal heat transfer equipment is an important part of the rotary cooler. Its task is to generate additional area by scattering the clinker without generating too much dust. Basically a similar design may be applied as in a planetary cooler tube (see next chapter) however the following differences must be considered: •
The clinker falling heights are larger. Wear protection of shell and lining is essential.
•
At a comparative length position the clinker in a rotary cooler is hotter than in a planetary cooler.
Figure 25
Rotary cooler
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The following zones can typically be distinguished in a rotary cooler (simplified): A
Lined inlet zone
B
Lined crushing teeth zone (metallic teeth)
C
Lined cast lifter zone, lining protected by wearing plates (at least in the second half)
D
Cast lifter zone, shell protected by wearing plates (having air gap, giving also insulating effect)
E
Sheet metal zone with wearing plates
Construction materials have to be selected according to the high temperature and wear requirements. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler / 4.1.3 Cooling performance
4.1.3
Cooling performance
Depending on the design and the shape of the lifters clinker outlet temperature usually tends to be high. In many cases it is necessary to enhance the cooling by injecting water into the tube (up to 60 g/kg clinker) in order to reach reasonably low clinker temperatures of 100° to 150°C. The cooling efficiency (heat recuperation) is equal or even slightly better than on a planetary cooler. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler / 4.1.4 Strengths / Weaknesses
4.1.4
Strengths / Weaknesses
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"Holderbank" - Cement Course 2000 Strengths
Weaknesses
•
Simplicity of cooler design, robust piece of equipment.
•
Not recommended for large units (above 2000 t/d)
•
No special mechanical problems comparable to a rotary kiln.
•
•
No control loops.
•
Formation of build-ups ("snowmen") in the inlet chute. A water-cooled chute or a dislodging device is required in such case.
Easy commissioning.
•
•
No waste air and therefore no dedusting equipment required
Clinker outlet temperatures tend to be high and therefore water injection is usually required.
•
Electrical energy consumption up to 5 kWh/t lower compared to grate cooler.
•
•
Rotational speed can be adjusted and therefore upset kiln conditions can be handled easier than with a planetary cooler.
Due to large falling height wear protection in the tube must be reinforced (compared to a planetary cooler).
•
High kiln foundations are required.
•
Cooler inlet seal can contribute to additional false air inlet.
•
Suitable for AS type precalcining system tertiary (extraction of hot air is possible).
Figure 26
Internal transfer equipment for rotary and planetary coolers
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler
4.2
The Planetary Cooler
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler / 4.2.1 Principle
4.2.1
Principle
The planetary cooler is based on the same cooling principle as the rotary cooler in the preceding chapter. However, the essential difference of a planetary cooler is the number of individual cooling tubes. The flow of clinker is subdivided into 9 to 11 (usually 10) cooling tubes which are installed around the kiln circumference at the kiln outlet (see Fig. 15). Therefore the planetary tubes follow the © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 kiln rotation. Because of their connection to the kiln rotation, planetary coolers do not need a separate drive. This fact already illustrates one main advantage of the planetary cooler: its simplicity in operation. Strictly speaking the cooling of clinker does not only start in the cooling tubes but already in the kiln. In the case of a planetary cooler the kiln burner pipe is always inserted into the rotary kiln so that a cooling zone behind the flame of 1.5 to 2.5 kiln diameters is created. This zone is called the "kiln internal cooling" zone and must be considered as an integral part of any planetary cooler. In this zone the temperature of the clinker drops from 1450° to 1200 - 1300°C. This temperature reduction is important for the protection of the inlet opening, the elbow and the first section of the cooling tubes. After this first cooling in the kiln internal cooling zone the clinker falls into the elbows when they reach their lowest point of kiln rotation. The hot clinker is then cooled by air in counterflow (the amount of air equals the amount of secondary air). The air is heated up to approx. 700°C. The clinker reaches final temperatures which are typically in the range of 140° to 240°C. A considerable amount of heat is also transferred to ambient by radiation and convection since approx. 75% of the cooler shell is not insulated. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler / 4.2.2 Historical
4.2.2
Historical
Planetary coolers have been used since 1920. When large kiln units and grate coolers were developed planetary coolers were abandoned for many years. But about 1966 planetary coolers of large capacities were introduced. At that stage serious mechanical problems occurred on these first large planetary coolers. As a consequence a lot of work had to be done in order to improve the mechanical design of planetary coolers. As a result of extensive computer calculations and operating experience the planetary cooler became a mechanically reliable piece of equipment. In the late 1970's, the design had reached a high standard and a considerable level of perfection. Units of up to 5000 t/d were envisaged. With the demand for permanently larger units using precalciner technology with separate tertiary air dusts, the boom period of the planetary coolers came to an end. Figure 27
Planetary cooler
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler / 4.2.3 Design features
4.2.3
Design features
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"Holderbank" - Cement Course 2000 Planetary coolers in the late 1970's had the following design features: ♦ Shell extension: The kiln shell is extended beyond the cooling tube outlets and is supported by an additional roller station. ♦ Fixation of cooling tubes: Fixed support of cooling tubes near inlet and loose support near outlet end. With larger coolers, the cooling tubes can consist of two separate sections requiring three supports. In that case two fixed supports are located near inlet and near outlet and a loose support is located at the interconnection point in the middle. ♦ Design of cooler supports: The kiln shell is reinforced (high thickness) where the cooler support structure for the cooler is welded on. The support structure (base and brackets) itself is of heavy design consisting of reinforcement ribs and box beams. ♦ Cooler length: Length/diameter ratio of tubes is approx. 10:1 ♦ Inlet openings: The inlet openings to the cooler elbows weaken the kiln shell and high mechanical and thermal stresses occur in that zone. The openings are made of oval shape and the kiln shell is considerably reinforced in its thickness (up to 140 mm in large kilns) in order to compensate for the weakening. In some cases a diagonal retaining bar (made of high heat resistant steel) is incorporated in the opening in order to avoid that large lumps can enter the cooler. ♦ Kiln-to-elbow joint: This joint is designed in a manner that no forces due to thermal expansion and deformation are transmitted from elbow to kiln. ♦ Elbow: In order to prevent that clinker is falling back into the kiln while the opening is on top position, the position of the cooling tube is displaced back against the direction of rotation. The elbow design must avoid excessive dust backspillage and wear. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler / 4.2.4 Internal heat transfer equipment (see Fig. 26)
4.2.4
Internal heat transfer equipment (see Fig. 26)
Cooling performance depends strongly on efficient lifters of solid and durable design. Since high heat resistant metallic lifers are available on the market also the high temperature zones can be adequately equipped. Special high temperature alloys can be used for this purpose. They can withstand maximum temperatures of up to 1150°C. These alloys are usually characterized by a high chromium content of approx. 30% Cr. Other elements as Ni or Mo can occur in various proportions. Fig. 26 shows a typical arrangement of heat transfer internals. Breaking teeth are applied in the hottest zone. They are able to crush large lumps of clinker and create also a tumbling effect, which improves the heat transfer. They are of heavy design and mounted on separate supports. The first rows of lifters must be carefully selected regarding design and material. Their functioning is very important since they also protect the following lifters from overheating. Figure 28a
Temperature profile in planetary cooler
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Figure 28b
Water cooling for planetary coolers
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 5. VERTICAL COOLERS
5.
VERTICAL COOLERS
Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 5. VERTICAL COOLERS / 5.1 The Gravity Cooler (G Cooler)
5.1
The Gravity Cooler (G - Cooler)
The Claudius Peters Company have developed the “g-cooler”. The letter "g" stands for gravity since clinker movement is performed by gravity. This cooler is designed as an after cooler and can therefore only be used in connection with a primary cooler such as a short grate cooler or a planetary cooler. The installation together with a grate cooler is shown in Fig. 29. An intermediary crusher reduces the clinker size to 20 - 30 mm. The material of approx. 400°C is then filled by a drag chain into a vertical shaft. Cooling is performed by horizontal rows of tubes which are cooled by internal air flow. The heat is therefore exchanged indirectly and the air remains dust-free. The clinker slowly drops down (at a speed of 20 – 30 mm/s) and reaches final temperatures of approx. 100°C at the discharge. © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 There is no dedusting equipment required for the cooling air. However, the system according to Fig. 29 as a whole is usually not free from dusty waste air. In case of a suspension preheater kiln system there is still some waste air required on the grate cooler since the kiln cannot take all the hot air produced during the first cooling step. In addition, a marginal amount of dusty air is produced by the g-cooler itself (top and discharge). The application of this cooler type is often considered for kiln extension projects. If an existing grate cooler (or a planetary cooler) has to be operated at higher capacity the new clinker outlet temperature can become too high. In this case the clinker temperature can be reduced by a g-cooler used as an aftercooler. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 5. VERTICAL COOLERS / 5.2 The Shaft Cooler
5.2
The Shaft Cooler
A shaft cooler can be operated waste-air-free and theoretically offers an ideal countercurrent heat exchange and thus high recuperating efficiency. Based on the idea the first large shaft cooler was designed and constructed on a 3000 t/d kiln in 1973. The experience gained in the plant shows that it is possible to operate such equipment but some serious disadvantages have to be taken into account: ♦ All depends of the clinker granulometry! Theoretically, an extremely uniform clinker granulometry having no fines and no coarse material would be required. This is hardly achievable in a cement kiln. Therefore, fluctuations occur. ♦ High cooling air quantity (= secondary air) of 1.05 Nm3/kg cli is required but even so the clinker exit temperature of 350°C is very high. ♦ High power consumption (10 kWh/t) For the above reasons, the technical realization is not yet solved. The shaft cooler so far is not a reasonable alternative to the conventional clinker coolers. Figure 29
Gravity cooler (g-cooler, CPAG)
Figure 30
Shaft cooler
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Figure 31
Claudius Peters CPAG: Combi Cooler
Figure 32
FLS: Coolax Grate Cooler
Figure 33
Fuller: Controlled Flow Grate (CFG) Cooler
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Figure 34
IKN: Pendulum Cooler
Figure 35
KHD: Pyrostep Cooler
Figure 36
Polysius: Repol RS Cooler
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C02 - Internal Kiln Fittings
C02 - Internal Kiln Fittings
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems
Kiln Chain Systems A. Obrist PT 96/ 14036 / E 1. INTRODUCTION 2. Functions of a Kiln Chain System 2.1 Heat Exchange 2.2 Cleaning of the Kiln Shell 2.3 Transport of Material 2.4 Reduce Dust Emissions 3. Individual Zones of a Chain System 3.1 Free Zone of the Kiln Inlet 3.2 Dust Curtain Zone 3.3 Plastic Zone 3.4 Granular Zone (Preheating Zone) 3.5 Heat Resistant Zone 1.6 Main Characteristic Data of the Individual Chain System Zones 4. Arrangement of Chains 4.1 Straight Curtain 4.2 Spiral Zone 1.3 Multiple Spiral Curtain 1.4 Triangular Curtain (Z-Curtain) 1.5 Garlands 1.6 Festoons 1.7 Spiral Garlands 1.8 Thermochains 5. Types of Chain Links 5.1 Round Links 5.2 Long Links 5.3 Oval Links 5.4 Other Types of Chain Links 6. Chain Material 6.1 Mild Steel Chains © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 6.2 Heat Resistant Alloy Chains 7. Chain Hangers 8. Main Characteristic Data of Chain Systems 9. ANNEXES 10. LITERATURE 11. Test Questions
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"Holderbank" - Cement Course 2000 Summary: A kiln chain system has four main functions: ♦ It helps to increase the heat exchange between gas and raw meal ♦ It keeps the kiln shell (lining surface) clean ♦ It assists the transport of material through the kiln tube ♦ It helps to reduce the dust emission A properly designed chain system must respect the changing properties of material passing through the kiln tube. In a wet process kiln the material is fed as a liquid slurry and changes it properties subsequently in several steps inside the chain system to dry preheated granules. In accordance with the changing material properties different arrangement of chains (straight curtains, spirals, garlands, etc.) have to be used for individual parts of the system to satisfy the specific requirements. Also the chain densities and the height of the free tunnel below the chains have to be selected carefully in order to reach the maximum efficiency. The chain links can have different shapes (round, long, oval etc.), preferably round links. The chemical composition of the chains' alloy and its physical treatment (hardening) strongly influence the life time of the system. Different types of chain hangers can be used (single or multiple hangers, with or without shackles etc.). They have to guarantee a sufficient stability, to enable an easy installation and they should as far as possible assist the function of the chains. NOMENCLATURE Just a few symbols and names are to be explained before starting this lecture, the other ones will be explained in the respective chapters. Figure:
Dis
Diameter inside kiln steel shell
DIL
Diameter inside kiln lining
hfr
Theoretical free height under the chains (see attached sketch), expressed in mm or as % of DIL
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"Holderbank" - Cement Course 2000 expressed in mm or as % of DIL density of chains m2/m3
is calculated for individual parts (zones) of the system as the total surface area of chains in the respective zone divided by the volume inside lining of this zone
density of chains kg/m3
similar to the above mentioned density, but concerns the weight of chains instead of their surface
Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 1. INTRODUCTION
1.
INTRODUCTION
Wet process kilns cannot be successfully operated without internal kiln fittings, among which the kiln chains are the most typical and most frequently used ones. The number of existing wet process kilns is still high (~33% in "Holderbank" Group) and a conversion from wet to dry process is very expensive. By improving the existing chain systems or, where necessary, by installing a completely new chain system, the kiln operation can be upgraded considerably with relatively moderate investment costs. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM
2.
FUNCTIONS OF A KILN CHAIN SYSTEM
The kiln chain system has 4 main functions: Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM / 2.1 Heat Exchange
2.1
Heat Exchange
The heat exchange between hot gases and the raw material depends on the surface area exposed to the hot medium. In the parts of kiln where no chains are installed, this surface area consists of the surface of the material layer on the kiln bottom and of the surface of the remaining part of the kiln shell (resp. lining). By installing the chains a large additional surface area can be gained, exceeding that one mentioned above several times (up to 10 times and more) in the respective part of the kiln. By improving the heat exchange the specific heat demand is reduced and the kiln output is increased. In Fig. 1 different positions of a chain during one kiln rotation are shown. In position 1 the chain is exposed to the stream of hot gases and thus heated up. The cooling of the chain (which passes its heat to the layer of material) starts in position 2, continues in position 3 and ends in position 4. Figure 1:
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Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM / 2.2 Cleaning of the Kiln Shell
2.2
Cleaning of the Kiln Shell
In the upper part of the kiln the characteristics of the wet, sticky raw material favors the formation of mud coating and mud rings. This would reduce the free kiln cross sectional area and thus obstruct the flow of material and gases. Growing mud rings make the kiln operation difficult. It is one of the main functions of the chain system to keep the internal kiln shell surface clean, free of coating or rings. Due to the kiln rotation the chains slide on the kiln shell (resp. lining) and destroy the rings and the coating. The sliding movement of a chain cleaning the kiln shell is shown in Fig. 1 (position 3). Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM / 2.3 Transport of Material
2.3
Transport of Material
The properties of material in different parts of the kiln differ considerably. In some sections of the upper part of the kiln, where the material is sticky and plastic, its transporting is more difficult than in other sections. As a regular flow of material is of an eminent importance for a smooth kiln operation, it is necessary to install material flow assisting devices in some sections. Some special arrangements of chains can help to draw the material through the critical sections. This can be achieved by chains moving in the desired direction (garlands) or by a screw shaped arrangement of the chain fastening points. Other arrangements of chains can be an obstruction to the flow of material and should therefore never be used in the critical sections. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM / 2.4 Reduce Dust Emissions
2.4
Reduce Dust Emissions
The gases leaving the kiln contain a certain amount of dust consisting mainly of partly calcined, hot raw material. The dust load of gases depends on the properties of the raw material and on the specific conditions of the kiln operation. Dust loss should be kept small, it means a loss of heat and material. The kiln chain system, mainly its upper part, can help to reduce the dust emission. Dust particles carried by the stream of gases stick to the wet surface of chains and later when these chains are emerged into the layer of material, this dust is passed over to the slurry. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 3.
INDIVIDUAL ZONES OF A CHAIN SYSTEM
The material passing the chain system changes subsequently its properties - it loses water and is heated up. According to the different material properties the total chain system can be divided into several zones. These zones are: Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.1 Free Zone of the Kiln Inlet
3.1
Free Zone of the Kiln Inlet
This short zone is considered to be a part of the chain system in spite of the fact that no chains are installed here. A sufficient amount of slurry should be accumulated in this zone in order to guarantee a constant and regular flow into the lower parts of the system. Good results have been obtained with the zone length of 1 to 1.5 kiln diameters. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.2 Dust Curtain Zone
3.2
Dust Curtain Zone
The dust curtain zone is relatively short, its length does not exceed 0.5 DIL under normal conditions. The material entering this zone still has the relatively good flow properties of the kiln feed (slurry). When leaving this zone, the material has a lower water content and becomes more "plastic", essentially due to the inter-mixing of the dust previously retained by the chains in this zone. In order to achieve a good dust catching efficiency, the density of chains must be high (some 8 to 15 m2/m3) and the free height below the chains should be 18 - 27% of DIL. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.3 Plastic Zone
3.3
Plastic Zone
The length of this zone depends on properties of raw material, slurry moisture, characteristics of the kiln operation etc. and can vary in a wide range (approx. between 1.5 and 4 DIL). The material in this zone is plastic and sticky, still relatively cold and wet and because of these properties it favors mud coating and mud ring formation. The transport of material through this zone is the most difficult one among all the zones of the chain system. Due to the material properties mentioned above the chains in this zone must have a good shell cleaning and material transporting efficiency. The density of chains should be relatively low, some 5 to 8 m2/m3. As to achieve a big free tunnel under the chains, the free height hfr should be approx. 30% or, if garland chains are installed in this zone, some 40%. Heavier (thick wire) chains should be installed. In order to be sure that the zone of plastic material will always stay inside the zone of chains which can treat it successfully, the respective arrangement of chains should be slightly extended in the downstream direction as to obtain a sufficient safety. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.4 Granular Zone (Preheating Zone)
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"Holderbank" - Cement Course 2000 3.4
Granular Zone (Preheating Zone)
The recommendable length of this zone depends on the desired material temperature and the rest water content at its discharge end. Good results have been achieved with a zone length between 2 and 4,5 DIL. The material entering this zone is not plastic any more, it forms granules which are easy to be transported and do not favor a mud ring formation. The granules should be dried and heated up in this zone. The chains should heat the material gently without unnecessary dust generation, they should enable a good heat exchange. Lighter (thin wire) chains should therefore be installed. A chain density of approx. 6 to 10 m2/m3 and a free height of approx. 25 to 30% can be recommended. This zone is sometimes divided into an upper and a lower part. Both parts have the same (or at least a similar) arrangement, but the lower part has a higher density of chains than the upper one. The damming effect of the lower part, caused by the thicker layer of chains on the kiln bottom, helps to increase the material retention time and improves the heat exchange. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.5 Heat Resistant Zone
3.5
Heat Resistant Zone
This zone is relatively short, its length does not exceed 1,5 DIL. The material, dry and hot granules, can easily be transported. A very gentle treatment of the material is required in order to keep the dust creation as low as possible. The main function of the chains is to protect the upstream part of the system against heat radiation and too high a gas temperature. Chains made of heat resistant steel should be installed in this zone. Lighter (thin wire) chains should be preferred. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.6 Main Characteristic Data of the Individual Chain System Zones
3.6
Main Characteristic Data of the Individual Chain System Zones
Table 1 Zone Free
Dust
Plastic
Curtain
Lower
Resistant
1 to 3
≤ 1,5
≤ 1,5
1.5 to 4
% DIL
18 - 27
40 / 30
25 to 30
Density
m2/m3
8 to 15
5 to 8
6 to 10
Moisture
%
DIL
hfr
Material temp. °C Chain temp.
°C
Gas temp.
°C
Heat
Upper
≤ 0.5
Length
≤ 1,5
Granular
30 to 40
15 - 25
20
100
109
Application range:
As long as there is a minimum temperature difference of a few degrees and the diameter range is over 1 m, above limitation does not affect the calculation. A mathematical transformation of the basic equation reveals that the free convection heat transfer does not at all depend on the characteristical dimension! It follows: 1
1
c ⋅ λ2 ⋅ g ⋅ δ 2 3 ∆T 3 (W / m 2K ) α =0.13 p T η 0 At ∆T = 0 the free convection becomes zero (which is different from the behavior of the radiation heat transfer!). The numerical values for cp, λ, ρ, η must be taken from tables for air at the average temperature between surface and ambient (use SI-units only). Note that the density ρ depends also on the barometric pressure and therefore the result will depend on the altitude above sea level (∝ ∼ p2/3). As a general guideline the convective heat transfer drops by about 8% per 1000 m of altitude. By using numerical approximations for the properties of air the following relationship has been developed (dimensional equation): α ≅ 1.4 · (ρ0 · ρ · ∆T)1/3 © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 ρo
(kg/m3)=
density at ambient temperature
ρ
(kg/m3)=
density at average temperature
Though its simplicity the latter formula covers the temperature range from 0...500°C with an accuracy of better than 1%! This is more than enough for practical purposes. At sea level and at 20°C ambient it follows
∆T α ≅1.6 ∆T 1+ 2 ⋅T 0
1 3
(W / m 2K )
Above relationship is also an excellent numerical approximation of the curve for v = O in Fig. 10 (see paragraph 4.10.3), which is actually based on complete computer calculations out of properties for air. Table Temp.ϑ
Properties of Air at Pressure = 1 bar ρ
°C
kg/m
-180 -160
cp 3
β
λ 3
η 3
ν 6
α 6
2
Pr 6
2
kJ/kg K
10 /K
10 W/mK
10 kg/ms
10 m /s
10 m /s
1
3.8515
1.071
11.701
9.0
6.44
1.67
2.18
0.77
3.1258
1.036
9.320
10.9
7.85
2.51
3.37
0.75
-140
2.6391
1.021
7.758
12.7
9.20
3.48
4.71
0.74
-120
2.2867
1.014
6.659
14.6
10.49
4.587
6.30
0.73
-100
2.0186
1.011
5.846
16.4
11.72
5.806
8.04
0.72
-80
1.8073
1.009
5.219
18.16
12.89
7.132
9.96
0.72
-60
1.6364
1.007
4.719
19.83
14.02
8.567
12.0
0.71
-40
1.4952
1.006
4.304
21.45
15.09
10.09
14.3
0.71
-20
1.3765
1.006
3.962
23.01
16.15
11.73
16.6
0.71
0
1.2754
1.006
3.671
24.54
17.10
13.41
19.1
0.70
20
1.1881
1.007
3.419
26.03
17.98
15.13
21.8
0.70
40
1.1120
1.008
3.200
27.49
18.81
16.92
24.5
0.69
60
1.0452
1.009
3.007
28.94
19.73
18.88
27.4
0.69
80
0.9859
1.010
2.836
30.38
20.73
21.02
30.5
0.69
100
0.9329
1.012
2.684
31.81
21.60
23.15
33.7
0.69
120
0.8854
1.014
2.547
33.23
22.43
25.33
37.0
0.68
140
0.8425
1.017
2.423
34.66
23.19
27.53
40.5
0.68
160
0.8036
1.020
2.311
36.07
24.01
29.88
44.0
0.68
180
0.7681
1.023
2.209
37.49
24.91
32.43
47.7
0.68
200
0.7356
1.026
2.115
38.91
25.70
34.94
51.6
0.68
250
0.6653
1.035
1.912
42.43
27.40
41.18
61.6
0.67
300
0.6072
1.046
1.745
45.91
29.20
48.09
72.3
0.67
350
0.5585
1.057
1.605
49.31
30.90
55.33
83.5
0.66
400
0.5170
1.069
1.485
52.57
32.55
62.95
95.1
0.66
450
0.4813
1.081
1.383
55.64
34.00
70.64
107
0.66
500
0.4502
1.093
1.293
58.48
35.50
78.86
119
0.66
600
0.3986
1.116
1.145
63.5
38.30
96.08
143
0.67
700
0.3577
1.137
1.027
67.8
40.87
114.3
166
0.69
800
0.3243
1.155
0.932
71.3
43.32
133.6
190
0.70
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"Holderbank" Course 2000 800 0.3243 - Cement 1.155 0.932 71.3
43.32
133.6
190
0.70
900
0.2967
1.171
0.852
74.3
45.65
153.9
214
0.72
1000
0.2734
1.185
0.786
76.8
47.88
175.1
237
0.7
↓ (actual cp, not average)
Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.4 Convective Heat Transfer / 6.4.2 Forced Convection
6.4.2
Forced Convection
Forced convection occurs at comparatively high wind velocity and dominates the convective heat transfer, i.e. the free convection is suppressed. The calculation of forced convection is depending on many factors, such as: ♦ Wind velocity ♦ Direction of the wind ♦ Velocity distribution and flow obstacles ♦ Uniformity of wind ♦ Reynolds number (depends on kiln diameter). There are a few more influence factors than in case of the free convection. Generally speaking the calculation of forced convection heat transfer contains more possible sources of error than free convection. As a simplification, we will assume a cylinder in a non disturbed flow of a constant velocity v (at 90° against kiln axis). Fig. 14
For air the following formula apply: Nu
=
0.0239 · Re 0.805
for Re =
40’000...400’000
Nu
=
0.00672 · Re 0.905
for Re >
400’000
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"Holderbank" - Cement Course 2000
vD v ⋅ D ⋅ ρ = =Re ynoldsNumber ν η αD Nu= =Nusselt Number λ
Re=
The properties η, λ, ϑ have to be taken at average air temperature. There are other formulas in use which can give different results, the above formulas are preferred due to their simplicity. In any case there is always a incertitude from the mode of calculation itself. Two main factors determine the ∝-value: ♦ Velocity v ♦ Average temperature (between surface and ambient) In addition the ∝ does also depend on the diameter D. If the equation for high Reynolds number (Re > 400’000) is solved for ∝, the following relationship is obtained: ∝
∼ D-0.095 ↓ proportional
This means that the ∝ value does not much depend on D! Therefore it is possible to work with constant values within a certain diameter range. This actually the base of Fig. 10 (see paragraph 4.10.3) which is calculated for a common range from 3...4...6 m diameter. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.4 Convective Heat Transfer / 6.4.3 Free Convection Plus Forced Convection
6.4.3
Free Convection Plus Forced Convection
If the convection is clearly dominated either by free or forced convection the final determination of the representative ∝ does not give any problem, since the higher value has to be taken. If the two values are of the same order they must be combined with an appropriate method. It would be certainly wrong to add the two values. A better approach is the square addition:
α tot = α 2 free + α 2 forced It is also valid if either the free convection or the forced convection dominates. Fig. 10 is based on the above method. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.5 Effect of Thermal Improvements
6.5
Effect of Thermal Improvements
According to the actual condition of an existing kiln system (heat consumption, heat balance, other operating data) we can envisage an optimization campaign. From the thermal point of view we can take certain measures in order to reduce the specific heat consumption. For example: ♦ Better insulation of rotary kiln or preheater/precalciner in order to reduce radiation losses (except the sintering zone). © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 ♦ Improvement of the cooler efficiency (optimization of grate cooler operation or installation of highly efficient internal equipment in planetary or rotary cooler). ♦ Reducing of false air inleaks at kiln seals or at preheater. ♦ Reducing of internal dust circulations in cooler, kiln or preheater (improves the internal counter-current heat transfer). ♦ Modification of raw mix in order to decrease the sintering temperature which in turn will also reduce heat losses. A further effect of such a measure can be the reduction of internal dust circulations due to better clinker granulometry (see above). ♦ Modification of raw mix in order to decrease the heat required for decarbonation, e.g. by making use of non-carbonatic CaO raw material sources. This possibility, however, is very rare and often not feasible. (Note the basic difference to the measures which tend to decrease the sintering temperature or increase the proportion of melting phase!) The above measures are just a few typical examples. When one goes into such items, an important phenomenon will appear soon: The so called „loss multiplication“ factor for thermal losses (or savings). What does this mean? If a saving (or loss) in the high temperature zone in a kiln of say 100 kJ occurs the possible gain in fuel heat consumption will not be 100 kJ but rather 130 to 150 kJ. That means that the primary saving (in terms of heat balance item) will be multiplied by a factor of up to 1.5. At the first glance the above principle seems to be contradicious because it would violate the principle of heat balance or the energy law. However, what really happens is a differential change of more than only one heat balance item. To illustrate this fact we start from an example where we have reduced the shell radiation losses by 100 kJ/kg cli. The following differential balance situation occurs: Fig. 15
The corresponding multiplication factor for the above case is: multiplication factor
=
- 140 kJ/kg - 100 kJ/kg
=
1.40
The above fact does result from the thermal behavior of the system and can be verified by simulation models (not by a simple balance only).
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"Holderbank" - Cement Course 2000 A factor in the 1.4-range is quite typical for the situation in the high temperature zone (above 800°C) of a cement kiln. The main effect of a change in this zone will be a corresponding change at the exhaust gas, but also other minor effects will occur (e.g. at cooler losses). A “differential balance“ of heat can still be applied according to:
Input Change
Output Change
fuel
radiation
=
- 100 kJ/kg
exhaust
=
- 40 kJ/kg
total
=
- 140 kJ/kg
total
=
=
- 140 kJ/kg
- 140 kJ/kg
The principle of loss multiplication does not only apply for radiation in the high temperature zone but also for the heat which is recuperated in the clinker cooler. Regarding the false air inleaks the corresponding deterioration of heat consumption is often under-estimated. If false air inleaks into the high temperature zone it does not only cause a heat loss because this air must be heated up to the exhaust gas temperature of the kiln system! It actually causes much more losses than what would result from such a simplified calculation approach. As a rule of thumb we may consider the following two main effects in order to come to a realistic result: ♦ Heating up of false air at the temperature of the high temperature zone which can be set approximately to 800°C (end of HT-zone). ♦ Multiplication of the above heat requirement by a loss multiplication factor. The above calculation is a rough approach. By more sophisticated simulation models we find e.g. that a false air inleak into the high temperature zone of 0.08 Nm3/kg cli can cause an additional heat consumption of 100 kJ/kg cli. In contrast, the inleak in the low temperature zone (e.g. air lift on SP-kiln) is much less critical. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.6 Heat Transfer in Preheaters and Coolers and Improvement Potential
6.6
Heat Transfer in Preheaters and Coolers and Improvement Potential
Normally we are considering a cement kiln as an equipment for burning cement clinker. As an essential feature we must be able to generate a high sintering temperature in the order of 1450°C (material temperature). But a kiln system is of course much more than a generator of sintering temperature. It is also a system of heat exchangers which allows for burning at low heat consumption. Generally speaking we will find two essential heat-exchanging systems on any cement kiln: a) raw meal preheater b) clinker cooler Low heat consumption is only possible if the above two „heat exchangers“ perform optimally. How can we get optimum heat exchange performance? Form the basic theory it is known that even in © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 case of an exchanger which is perfectly insulated against ambient temperature influence, three important conditions are required for optimum heat exchange: 1) Optimum heat transfer rate (here: from gas to solid) → high specific contact or surface area, high ∝ (W/m2C). 2) The two heat exchanging streams must flow in counter-current manner, or at least in an arrangement which has similar characteristics to a counter-current system (e.g. overall arrangement of a 4-stage cyclone preheater). 3) If we want to recover heat from a „flow 1“ completely into a „flow 2“ the „flow 2“ must have at least the same heat equivalence as „flow 1“: (flow 2) x (cp2) ≥ (flow 1) x (cp1)
[kW/C] or [kJ/kg cli C]
flow: [kg/s] or [kg/kg cli] cp: [kJ/kg C] In the above equation the cp values are considered as constant (approximation). Graphically this means that the heat characteristic curves of the two heat flows must be in a certain relationship as shown here:
What are the practical consequences for a cement kiln? We consider: A) Raw meal preheater B) Clinker cooler A)
Raw meal preheater
1)
Heat transfer rate: Optimum heat transfer rate and optimum specific surface (small particles) and distribution has been almost achieved in the cyclone suspension preheater. After every cyclone stage material and gas come to almost complete temperature approach and there is usually little to improve on that. Improvement are rather to be done where the heat exchange takes place in the rotary part itself. Especially on wet and long dry kilns the kiln internal fittings are essential for good heat exchange.
∗
∗
2) Counter-current principle: ∗ Counter-current flow in its proper sense does not exist in case of a cyclone suspension preheater. Instead, we have usually four co-current heat exchanging units, but the overall © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 arrangement acts as counter-current system. To reach an ideal state one would have to apply an infinite number of cyclone stages. Practically the common arrangement of 4 or 5 stages can be considered as sufficient. ∗ True counter-current preheaters are shaft preheaters or preheating in long rotary kilns. Such preheaters would theoretically be ideal. Practically they are less efficient because of distribution problems and backmixing effect (internal material circulations) and comparatively high losses to outside (in case of a rotary kiln). 3) Equivalence of heat flow characteristics: ∗ A general feature of any raw meal preheater is the surplus of heat input by the hot gases. After the calcining step the hot gas has a temperature of approx. 850°C and the specific quantity is always above ∼ 1.3 Nm3/kg cli even in case of an optimum kiln system (4-stage SP kiln). This amount of heat is too much, it cannot be used completely for preheating of raw meal (∼ 1.56 kg RM/kg cli) up to calcining temperature (∼ 800°C). Therefore a certain amount of waste heat will occur even in an ideal case. Theoretically we should not exceed ∼ 1 Nm3/kg cli for ideal recuperation. Practically this cannot be realized, not even on modern kiln systems which produce comparatively little exhaust gas. ∗ Graphically we have the following heat situation in a raw meal preheater: Fig. 16
Above diagram is simplified but typical for any preheater. Because of the „heat surplus“ of the exhaust gas it is not possible to achieve an ideal recuperation even at perfect counter-current heat transfer (e.g. infinite number of cyclone stages). The exhaust gas will always give a certain residual heat content. Practically this means that all kind of improvements on the preheater have a limited potential. In the example of a 4-stage preheater kiln we can add a fifth stage which causes a reduction of about 100 kJ/kg cli in fuel heat consumption. More than 5 stages will bring only marginal economical point of view. *) Even at 5 stages we may check if the necessary investment and the (possible) increase of pressure drop can be justified by the local cost structure. *)
Instead of constructing more than 5 conventional cyclone stages one would prefer here „non-conventional“ preheaters, such as the cross-suspension-preheater (two strings with cross flow of raw meals). B)
Clinker Coolers
1) ∗
Heat transfer rate: On a planetary or a rotary cooler we have the possibility to increase the heat transfer rate by
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"Holderbank" - Cement Course 2000 installation of efficient internal equipment (tumblers, lifters) which increase the active heat transfer area by better moving and scattering of the clinker. If a cooler has worn out internal equipment or equipment of inadequate design we may realize a considerable potential for improvements. Improvements may also result from a more uniform clinker granulometry (less internal dust circulations). ∗ On a grate cooler we find quite a different situation. The real problem is not the heat transfer rate between a piece of clinker and the cooling air but rather the uniform air distribution through the clinker bed. Also here we may realize a considerable improvement (thick bed operation, mechanical modifications at inlet zone etc.). 2) Counter-current principle: ∗ There is an obvious difference between planetary/rotary coolers and the grate cooler: - planetary / rotary → counter-current flow - grate cooler → almost cross-current flow ∗ The grate cooler has a nearly cross-current performance and has therefore, from its principle, a limited heat recovery potential. Improvements are possible when air recirculation (of hot waste air) to the first grate section is applied in order to increase the heat content of the secondary air. Another quite different measure is the increasing of the bed thickness in order to come more towards a counter-current-like exchange (similar to a shaft cooler). The disadvantage is an increase of the cooling air pressure. 3) Equivalence of heat characteristic ∗ On a clinker cooler we would theoretically require approx. 0.77 Nm3/kg cli of cooling air in order to transfer the clinker heat completely to the secondary air (under perfect counter-current heat exchange). ∗ The practical figures are usually above 0.8 Nm3/kg cli. Compared to a raw meal preheater we have an inverse situation: The heat from the hot clinker could theoretically be completely recovered (under ideal conditions)! Of course, we know that the common, practical figures are often below 70%. The latter fact illustrates that from the basic principle there is still a considerable heat potential which is not used for reducing the kiln heat consumption. ∗ In this context it is also important to note that the practical efficiency of any type of clinker cooler increases with higher cooling air quantity. As a consequence we should draw as much cooling air as possible through the cooler and therefore avoid or reduce excessive primary air quantities or false air inleaks at the kiln hood or kiln seals. The improvement does not come from the heat transfer proper but rather from the improved „heat characteristic“ (air/clinker ratio). Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 7. TEST QUESTIONS
7.
TEST QUESTIONS
1) Give an example where it can be worthwhile to execute a complete heat balance on a kiln. 2) Summarize all important measurement points which are needed for doing a complete heat balance on a suspension preheater kiln. 3) What is the usual value (or range) for the specific heat (kJ/Nm3 C) of exit gas of an SP-kiln at 350°C? 4) What is the sensible heat content of 1 kg clinker at 1450°C, expressed as kJ/kg cli? 5) What is the standard value (or range) for heat of formation (kJ/kg cli) for clinker burning? Which heat effects are included in above value? © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 6) Determine the heat transfer coefficient (W/m2C) for the total heat transfer by radiation plus free convection. The temperature of the kiln shell section is 200°C (ambient = 20°C, ε = 0.9). 7) When has the forced convection heat transfer (instead of free convection) to be considered? How is the above value affected in case of smaller dimensions (say 1 m instead of 5 m diameter)? 8) If the shell losses in the calcining zone can be reduced by 50 kJ/kg cli, what will be the approximate saving of fuel heat (kJ/kg cli)? 9) What is the approximate fuel heat which can be saved through the installation of a fifth cyclone stage on a suspension preheater kiln? What would be the approximate amount of false air reduction (Nm3/kg cli into the high temperature zone) in order to achieve a similar fuel heat saving? Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 8. LITERATURE
8.
LITERATURE
1) Zur Strassen, H. Der theoretische Wärmebedarf des Zementbrandes ZKG 10 (1957), Vol. 1, p. 1-12 1) Jakob, M. Heat transfer, Vol. I (1949), p. 529 2) Hilpert, R. Wärmeabgabe von geheizten Drähten und Rohren im Luftstrom Forsch.-Ing.-Wes., Vol. 4 (1939), p. 215-224 3) Gygi, H. Thermodynamics of the cement kiln, third industrial symposium on the chemistry of cement 4) Eigen, H. Beitrag zur Thermodynamik des Drehofens Tonindustrie-Zeitung 82 (1958), No. 16, p. 337-341 5) Frankenberger, R. Beitrag zur Berechnung des Wärmeübergangs in Zementdrehöfen Dissertation, Technische Universität Clausthal (1969) 6) Kühle, W. Untersuchung über die äussere Wärmeabgabe von Drehöfen durch Strahlung und Konvektion Zement-Kalk-Gips, Vol. 6, 1970, p. 263 7) VDZ Unterlagen „Wärmetechnische Berechnungen“ Verein Deutscher Zementwerke E.V., Düsseldorf 8) VDI-Wärematlas Berechnungsblätter für den Wäremübergang VDI-Verlag GmbH, Düsseldorf 9) Barin, I. and Knacke, O. Thermochemical properties of inorganic substances Springer Verlag, Berlin, Heidelberg, New York 10)
Elkajer, P. (FLS) Die Bestimmung des Wärmeverbrauches mit vierstufigem Zyklonvorwärmer durch Aufstellung
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"Holderbank" - Cement Course 2000 eines mathematischen Modelles Zement-Kalk-Gips, Vol. 2, 1980 11)
Gardeik, H.O. Berechnung des Wandwärmeverlustes von Drehöfen und Mühlen Zement-Kalk-Gips, Vol. 2, 1980
12)
Rother, W. Ausführung von Rohmehl-Wärmetauschern unter Berücksichtigung heutiger Kostenfaktoren Zement-Kalk-Gips, Vol. 2, 1982, p. 66 ff.
Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 9. SYMBOLS AND UNITS
9.
SYMBOLS AND UNITS
A
m2
area
CR
W/m2K4
radiation constant
cP
kJ/kg C
specific heat (at const. pressure),
or kJ/Nm3 C
specific heat capacity
CV
kJ/kg
calorific value
D
m
diameter
g
m/s2
gravity constant
h
kJ/kg
heat content (specific)
or kJ/Nm3 or kJ/kg cli L
m
length
m
kg
mass
or kg/kg
specific mass
mf
kg/h
mass flow
Qf
kW
heat flow (1 kW = 1 kJ/s)
t
C
temperature (Centigrade)
T
K
temperature (Kelvin)
v
m/s
velocity
w
kg/kg
water content
∝
W/m2K
heat transfer coefficient
ε
-
emissivity (for radiation)
λ
W/m C
heat conductivity
ρ
kg/m3
density
Greek Letters
Dimensionless Numbers Nu © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 Nu
Nusselt number (for heat transfer)
Pr
Prandtl number
R
kiln feed (raw meal) / clinker-ratio
Re
Reynolds number
Indices conv
convection
rad
radiation
tot
total
o
ambient condition or zero condition
Conversion Factors Length
1 inch
0.0254 m
1 ft
0.3048 m
Area
1 sq. ft
0.092903 m2
Volume, Volume Flow
1 cu.ft
0.028316 m3
1 cu.ft/min
1.699 m3/h (actual m3)
1 lb.
0.45359 kg
1 short ton (USA)
907.185 kg
Mass
Pressure Energy
Temperature Conversion Heat Flow
Specific Heat Heat Transfer Coeffic.
Standard Conditions for Gases
Nm 3 =act .m 3 ×
1 bar
105 N/m2
1 atm.
1.013 bar
1 kJ
1000 J
1 MJ
1000 kJ
1 kWh
3600 kJ
1 kcal
4.187 kJ
1 BTU
1.055 kJ
C=
5/9(F - 32)
K=
273.15 + C
1 kW
1000 W = 1 kJ/s
1 kcal/h
1.163 W
1 BTU/h
0.29307 W
1 kcal/kg C
4.187 kJ/kg C =
4187 J/kg C
1 BTU/lb F
1 kcal/kg C
4.187 kJ/kg C
1 kcal/m2h C
1.163 W/m2 C
1 BTU/ft2h F
5.678 W/m2C
Standard Conditions
0°C and 1 atm. (1.013 bar)
=
2.73.15 p(bar ) × 273.16 + t (c ) 1.013bar
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C10 - Main Fans
C10 - Main Fans
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"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C10 - Main Fans / Main Fans
Main Fans Authors: W. Zeller, Th. Richner, D. Brassel PT 99/14500/E 1. Design and efficiency of fan impellers 1.1 Fan impeller types 1.2 Selection criteria 1.3 Fan applications in the cement industry 2. Fan performance curves 2.1 System Resistance Curve 2.2 Fan curves 2.2.1
Fan equations
2.2.2
Adjusting fan performance curves
3. Flow control 3.1 Damper control 3.2 Radial inlet vane 3.3 Speed control 3.3.1
Hydraulic transmission with fixed speed motor
3.3.2
Speed-controlled electric motors
4. Possible problems with fans 4.1 Vibrations 4.1.1
Variable speed operation
4.1.2
Thermal effects
4.1.3
Hot shutdowns
4.2 Material build-up 4.2.1
Kiln exhaust fan build-up
4.2.2
Recommendations against build-up
4.3 Erosion 4.3.1
Erosion types
4.3.2
Improper duct connection
4.3.3
Effects of impeller speed and wheel inlet velocity
4.4 Wear protection 4.4.1
Protection of parts subjected to abrasion
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"Holderbank" - Cement Course 2000 4.4.2
Deflection of abrasive particles
4.4.3
Liner materials
4.5 Bearings 5. Fan Capacity Adjustment 5.1 Fan capacity too low 5.2 Fan capacity too high 6. Troubleshooting 7. Start-up of fans 8. Fan impeller arrangement and connections 8.1 Assembly 8.1.1
Overhung assembly (Fig. 8.1)
8.1.2
Center hung assembly (Fig. 8.2)
8.2 Foundations 8.3 Connections 8.3.1
Inlet connections
8.3.2
Outlet connections
9. Information Sources
Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 1. DESIGN AND EFFICIENCY OF FAN IMPELLERS
1.
DESIGN AND EFFICIENCY OF FAN IMPELLERS
Fans are essential components of the cement manufacturing process and merit therefore particular considerations with respect to •
design and efficiency of the impeller
•
fan size and operating point
•
flow control
•
reliability
Main fans in a Cement Plant are found as •
Kiln ID Fan
•
Raw Mill Fan
•
Cooler Exhaust Gas Fan
•
Kiln Dedusting Fan
Altogether these fans consume between 30 and 50% of the plants total electrical energy. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 1. DESIGN AND EFFICIENCY OF FAN IMPELLERS / 1.1 Fan impeller types
1.1
Fan impeller types
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"Holderbank" - Cement Course 2000 There are four basic blade forms used in industrial induced draft service: •
backward airfoil blades
•
backward curved blades
•
backward inclined blades
•
radial straight blades
Fig. 1.1 shows design and efficiency of these impellers.
Fig. 1.1:
Design and efficiency of impellers
TYPE
backward airfoil blades
backward curved blades
© Holderbank Management & Consulting, 2000 Query:
EFFICIENCY η
> 84 %
up to 82 %
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APPLICATION
for clean gas applications (dust < 50 g/m3)
for gas with a dust concentration < 100 g/m3
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"Holderbank" - Cement Course 2000
Backward inclined blades
up to 80 %
for gas with a dustload up to 100 g/m3
radial straight blades
60 - 75 %
for gas with a high dustload (dust > 100 g/m3)
Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 1. DESIGN AND EFFICIENCY OF FAN IMPELLERS / 1.2 Selection criteria
1.2
Selection criteria
It is of great importance that critical process equipment, such as fans, is selected on the basis of proven ability in order to provide maximum reliability rather than on an efficiency rating. In situations where more than one blade form will meet a performance requirement, it then becomes necessary to select the one form that will be most overall cost-effective. For the selection process the supplier should provide the operating and service manual for the equipment type being considered. The operating and service report should include all occurrences that require fan maintenance. To assist in fan type selection, there are at least four important points to be considered. ♦ Fan efficiency: Because many of the higher efficiency fans can only achieve their stated efficiency within a narrow operating range, a true energy evaluation must consider the actual operating point and alternate © Holderbank Management & Consulting, 2000 Query:
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"Holderbank" - Cement Course 2000 operating points on a time basis. Many systems include a built-in safety factor, which results in reduced efficiency when operated at constant speed with damper regulation. ♦ Continuous operation: Any type of equipment will require maintenance. In blade form selection, blade build-up (cp. section 4.2) and erosion (cp. section 4.3) have the greatest effect on fan operation. Build-up on the wheel results in reduced performance. There is an increased tendency for material to build up on blades as the blade angle is tilted back from radial. This build-up can accumulate to the degree that it restricts and alters air passages, reducing both efficiency and performance. ♦ Mechanical design: All fan rotors are subject to centrifugal force. Depending on blade form and angle, different types of stress occur in a blade. The radial Blade is in tension, while bending and tensile stresses act on the backward inclined/Airfoil design. Bending stresses are more subject to fatigue stresses. ♦ Equipment costs Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 1. DESIGN AND EFFICIENCY OF FAN IMPELLERS / 1.3 Fan applications in the cement industry
1.3
Fan applications in the cement industry
The following table shows an overview of fan applications in the cement industry. Location
Dust load [g/Nm3]
Coal Mill < 0.15 Filter exhaust Separator / cyclone < 100 exhaust
Blades mainly used
Max. Temp [°C]
max. speed [rpm]
Flow regulation
Rotor protection
Stator protection
Efficiency
F/C/A
150
1800
VC/ILD
(WP)
-
70 - 85
R/F/C
150
1200
ILD
HSWP
WP( HSWP)
55 - 75
[%]
Raw Mill Filter exhaust
< 0.15
F/C/A*
300
1800
VC/ILD/VS
-
-
70 - 85
Separator / cyclone exhaust
< 100
F/C
300
1200
ILD/VS
(HW/HWSP)
WP
65 - 75
Filter exhaust
< 0.15
F/C/A
200
1800
VC/ILD
-
-
75 - 85
Separator / cyclone exhaust
< 100
F/C
200
1200
ILD
(WP/HSWP)
WP
65 - 75
Preheater exhaust
< 100
R/F/C
450
1200
ILD/VS
(WP)
-
55 - 75
Kiln line filter exhaust
< 0.15
F/C/A*
350
1200
VC/ILD/VS
-
-
70 - 85
Recirculation fan
< 20
R/F
450
750
ILD/VS
HSWP
WP
60 - 70
Kiln line filter exhaust
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