The MAERZ® Parallel Flow Regenerative Lime Kiln
The MAERZ® Parallel Flow Regenerative Lime Kiln 1. Limestone, Lime and Dolomite Lime is one of the key elements in life. This natural raw material is involved in the production of the majority of modern products. The production of iron and steel, gold, silver, copper and plastics as well as many chemical products and foodstuffs, just to mention a few, requires lime and, to a lesser extent, dolomite. The most important fields of application for lime and dolomitic lime are: • Iron and steel • Non-ferrous metals • Building industry • Pulp and paper • Chemical industry • PCC - Precipitated calcium carbonate • Sugar • Glass • Flue gas desulphurisation • Agriculture • Soil stabilisation • Water treatment • Sewage treatment. World wide more than 120 million tons per year of lime and dolomitic lime are produced. The iron and steel industry is the primary consumer with an annual demand of approx. 40 million tons.
tive heat transfer to the core. A temperature of 900 °C has to be reached in the core at least for a short period of time since the atmosphere inside the material is pure CO2. The stone surface must be heated to greater than 900 °C to maintain the required temperature gradient and overcome the insulating effect of the calcined material on the stone surface. When producing soft-burnt lime the surface temperature must not exceed 1100 to 1150 °C as otherwise re-crystallisation of the CaO will occur and result in lower reactivity and thus reduced slaking properties of the burnt product. A certain retention or residence time is required to transfer heat from the combustion gases to the surface of the stone and then from the surface to the core of the stone. Larger stones require longer time to calcinate than smaller ones. In principal, calcining at higher temperatures reduces the retention time needed. However, too high temperatures will adversely affect the reactivity of the product. The relation between burning temperature and retention time required for different stone sizes is shown in the following table. Stone size Calcining temperature [mm] [°C] 50 1200 1000 100 1200 1000
Approx. residence time [hours] 0.7 2.1 2.9 8.3
High quality limestone contains 97 to 99% CaCO3. It requires approximately 1.75 tons of limestone to produce one ton of lime. High quality dolomite contains 40 to 43% MgCO3 and 57 to 60% CaCO3. It requires approximately 2 tons of dolomitic stone to produce one ton of dolomitic lime.
Throughout this paper, the word “lime” is used interchangeably to mean “high calcium lime” or “dolomitic lime”.
The calcination or burning of limestone and dolomite is a simple chemical process. When heated the carbonate decomposes according to its respective equation.
Two types of kilns are primarily used to calcine limestone and dolomite in today’s lime industry: • Rotary kilns, and • Vertical shaft kilns.
CaCO3 + approx. 3180 kJ (760 kcal) = CaO + CO2 CaMg(CO3)2 + approx. 3050 kJ (725 kcal) = CaO.MgO + 2 CO2 The decomposition temperature depends on the partial pressure of the carbon dioxide present in the process atmosphere. In a combustion gas atmosphere of normal pressure and 25% CO2, the dissociation of limestone commences at 810 °C. In an atmosphere of 100% CO2, the initial dissociation temperature would be 900 °C. Dolomite decomposes in two stages starting at approx. 550 °C for the MgCO3 portion and approx. 810 °C for CaCO3. In order to fully calcine the stone and to have no residual core, heat supplied to the stone surface must penetrate via conduc-
2. Lime Production Equipment
Rotary kilns, with or without preheater, usually process grain sizes between 6 and 50 mm. The heat balance of this type of kilns is characterised by rather high losses with the off-gases and through the kiln shell. Typical figures for off-gas losses are in the range of 20 to 25% and for kiln shell losses 15 to 20% of the total heat requirement. Only approx. 60% of the fuel energy introduced into preheater type kilns is used for the calcining process itself. For all types of vertical single shaft kilns there is an imbalance between the heat available from the burning zone and the heat required in the preheating zone. Even with an ideal calcination process (having an excess air factor of 1.0) a waste gas tempe3
rature of 100°C may only be achieved with limestone containing less than 88% CaCO3. However, lime produced from such low quality limestone has only a restricted field of application. In practice limestones with much higher carbonate content are processed resulting in higher waste gas temperature which is the consequence of excess available heat in the preheating zone. The question now is: How can the surplus heat available in the calcining zone of the kiln be utilised to minimise heat consumption and how do the modern kiln types match this aspect. An almost perfect solution for this problem is offered by the Maerz® Parallel Flow Regenerative Lime Kiln (PFR-Kiln).
3. The PFR-Kiln Two main types of vertical shaft kilns exist. The single shaft counter flow heating kiln and the multiple shaft parallel flow heating kiln. The standard PFR-Kiln is a two-shaft kiln defined by alternating burning and non-burning shaft operation. There are two key characteristics of the PFR-Kiln: 1) the parallel flow of hot gases and stone in the burning zone, and 2) the regenerative preheating of all combustion air in the process. The kiln is ideally suited to produce soft-burnt, high reactive lime and dolomitic lime because of the conditions created by the parallel flow of the stone and the combustion gases in the burning shaft. Additionally, the regenerative process provides the lowest heat consumption of all modern kilns available today. The difference in the temperature profile of conventional single shaft kilns and PFR-Kilns is depicted in Fig. 1. The curves show the temperatures of the material, of the air and of the combustion gases flowing through the kiln.
Fig. 1a: Temperature Profile in a Counter Flow Kiln
Fig. 1 compares parallel flow heating with counter flow heating. In single shaft kilns usually counter flow heating is applied, a typical temperature profile is shown in Fig. 1a. The green line shows the temperature of the material. The blue line shows the temperature of the cooling air and the red line the temperature of the combustion gas and kiln off-gas. As the amount of cooling air is not sufficient for complete combustion of the fuel additional air has to be introduced via the lateral burners. As in this type of kiln the fuel is introduced at the lower end of the burning zone (where the material is already calcined) the temperature in this area is significantly higher than required for production of high reactive lime. In parallel flow kilns the fuel is introduced at the upper end of the burning zone and the combustion gases travel parallel to the material. Fig. 1b shows a typical temperature profile where the green line represents the material, the blue line in the preheating zone the combustion air, the blue line in the cooling zone the cooling air and the red line the combustion gas and kiln off-gas. As the fuel is injected at the upper end of the burning zone where the material can absorb most of the heat rele-
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Fig. 1b: Temperature Profile in a Maerz PFR-Kiln
ased by the fuel the temperature in the burning zone is typically 950°C in average. Because of this, parallel flow heating is the best solution for the production of soft-burnt, reactive lime and dolomitic lime.
reactivity of burnt lime. Generally shorter and hotter flames reduce the reactivity of the burnt product.
The second important characteristic of the PFR-Kiln is the regenerative preheating a part of the combustion air. In kilns with counter flow heating, the combustion air is preheated in the cooling zone by the sensible heat contained in the calcined lime. The amount of preheating is limited, however, by the enthalpy of the lime. In the counter flow heating process there is a surplus of usable sensible heat contained in the off-gas that is not recovered prior to being exhausted. Some single shaft kiln designs therefore have incorporated recuperators in an effort to recover this waste heat, but such heat exchangers are susceptible to disruptions caused by dust contained in the hot off-gases.
4. The Operating Principle of the Maerz PFR-Kiln Fig. 2 shows the basic operating principle of the PFR-Kiln and illustrates the two phases of gas flow. Two shafts, designated 1 and 2, contain the material to be calcined. The stone charging system, the reversal traps for fuel, combustion air, and off-gas, and the lime discharge system have been omitted from this diagram. The shafts are either alternately or simultaneously charged with stone depending on kiln capacity. Lime is discharged continuously at the bottom of both shafts.
In the parallel flow regenerative kiln the combustion air is preheated in an ideal manner. The regenerative process requires two connected shafts. Each shaft is subject to two distinct modes of operation, burning and non-burning. One shaft operates in the burning mode and simultaneously, the second shaft operates in the non-burning or exhaust mode. Each shaft spends an equal amount of time in both the burning and non-burning modes of operation. In burning mode, a shaft is characterised by the parallel flow of combustion gases and raw stone, whereas, in non-burning mode a shaft is characterised by the counter-current flow of off-gases and raw stone. The combustion gases exit the burning shaft through a crossover channel into the non-burning shaft. The alternating burning / non-burning shaft sequence serves as a regenerative preheating process. Heat is transferred to the raw stone from the off-gases during the nonburning mode and then reclaimed by the combustion air from the raw stone during the burning mode. The stone preheating zone acts as a regenerator with the stone charge as chequers. This kind of regenerator is completely insensitive to dust-laden or corroding gases and, at the same time, shows excellent heat transfer characteristics. The regenerative preheating of the combustion air makes the thermal efficiency of the kiln practically independent from the excess combustion air factor. This considerably simplifies the setting of the correct length of the flame to produce the desired quality of soft-burnt lime. A larger quantity of excess air produces a shorter flame and less excess air produces a longer flame. The length of the flame is one of the key factors to control the
Fig. 2: Operating Principle of the Maerz PFR-Kiln 5
Fuel is supplied to only one of the two shafts. In Fig. 2 it is supplied to shaft 1 thus shaft 1 is designated the burning shaft and shaft 2 is designated the non-burning shaft. The fuel is introduced through multiple lance tubes that vertically extend to the bottom of the preheating zone. The lower end of the lance tubes marks the changeover to burning zone from the preheating zone. Fuel is injected through these lances and evenly distributed over the cross sectional area of the shaft. Combustion air is introduced under pressure at the top of the preheating zone above the stone bed. The complete system is pressurised. The combustion air is preheated by the stone in the regenerator (preheating zone) prior to mixing with the fuel. The air/fuel flame is in direct contact with the calcining material as it passes through the burning zone from top to bottom (parallel flow heating). The off-gases leave the burning shaft and enter the non-burning shaft through the crossover channel, travelling up in counter flow to the stone. The off-gases transfer heat to the stone bed in the non-burning shaft and even calcine it to a small degree. The off-gases then regenerate the stone bed in the preheating zone in preparation for the next burning cycle on that particular shaft. Each shaft cycles through the burning and non-burning mode at intervals of approximately 12 minutes. The changeover from burning to non-burning is called “reversal period”. During each reversal period a measured amount of stone is charged to the kiln. Calcined product is discharged from both shafts continuously throughout the burning cycle by discharge tables into a pressurised hopper. Cooling air is continuously introduced at the bottom of both shafts to reduce the temperature of the product prior to being discharged into the lime storage hopper. During reversal periods, when the kiln is depressurised, the product is discharged from the storage hopper onto vibrating feeders and conveyor belts. The excellent thermal conception of the PFR-Kiln can be satisfactorily proven by means of the heat balance. The sum of effective heat, i.e. heat required for dissociation, and of the heat losses provides the thermal requirement of the kiln.
Fig. 3a: Gas Flow in the Rectangular Maerz PFR-Kiln 6
The heat losses consist of: • the loss through the kiln wall equal to approximately 170 kJ (40 kcal)/kg of lime, • the sensible heat of the discharged burnt lime equal to approximately 80 kJ (20 kcal)/kg of lime at a discharge temperature of 100 °C, and • the sensible heat contained in the off-gases equal to approximately 290 kJ (70 kcal)/kg of lime at a discharge temperature of 100 °C. Because the kiln has no moving shell as a rotary kiln it can be well insulated and the loss through the walls can be kept to a minimum by using the appropriate insulating refractory lining. The refractory lining installed has been determined to provide the lowest heat loss for the money invested. Additional insulation to further reduce the wall losses would be too costly for the corresponding savings. A sufficient amount of cooling air is used to reduce the temperature of the calcined lime in the cooling zone. The heated air is subsequently used in the process thereby improving the kiln efficiency. Although it is theoretically possible to reduce the off-gas temperature below 100 °C, operating below this value is not advisable because of condensation and corrosion problems when operating in the range of the gases’ dew point. Considering these design criteria for heat losses of the kiln when producing lime with 96% CaO the total thermal requirement is approx. 3500 kJ (840 kcal)/kg or 3.02 million Btu per ton of burnt lime. PFR-Kilns are typically designed with two shafts of either rectangular or circular cross sectional shape. The shafts are connected by a crossover channel at the bottom of the burning zone. The crossover channel serves as the transport duct to allow the hot gases to exit the burning shaft and enter the non-burning shaft.
Fig. 3b: Gas Flow in the Circular Maerz PFR-Kiln
The simplest design is to lengthwise place two shafts with rectangular cross section side by side in such a manner that the kiln gases can flow directly from one shaft to the other (Fig. 3a). A disadvantage of this design occurs at larger kiln capacities (and consequently larger shaft cross sections) where the hot gases have the tendency to concentrate on the crossoverchannel side of the shafts and the gas distribution is not uniform. Therefore at larger capacities, kilns of circular cross section are proposed. These kilns have circular connecting channels, as illustrated in Fig. 3b. The off-gases exit the burning shaft and enter the non-burning shaft radially around the complete shaft perimeter thereby guaranteeing an absolute even heat distribution which is a key factor for a high quality of the burnt lime.
5. Kiln Components and Equipment Fig. 4 shows the general arrangement of the PFR-Kiln and illustrates various components and equipment related to this kiln type. 5.1 Kiln Shafts In the early days of PFR-Kiln design, two-shaft PFR-Kilns used stone sized between 40 mm and 120 mm. When the requirements were for high output using stone less than 40 mm in size, three shafts were used. Small stone size creates a greater pressure drop in the shaft and increases the pressure inside the kiln. When three shafts were used, the off-gases of the burning shaft were distributed into two exhaust shafts thereby reducing the gas speed by one half and the pressure drop by approximately three fourths. Technical development and experience has allowed the use of two-shaft kilns for almost all applications and as such has eliminated the need for three-shaft kilns. The PFR-Kiln operates under pressure therefore the steel shell must be sealed air tight. All openings at the top of the kiln for charging stone and the bottom of the shafts for discharging lime are sealed by hydraulically operated traps. 5.2 Refractory Lining The preheating and cooling zones of the kiln are lined with an abrasion resistant wear lining backed by insulating firebricks. The wear lining in the burning zone is made of high quality magnesite bricks with an insulating secondary lining. The working lining has a thickness of 250 mm and is backed by an insulating lining made from light fireclay bricks and calcium silicate boards.
Fig. 4: The Maerz PFR-Kiln
The arrangement of the brickwork is simple as shown in Fig. 5. There are no burner bridges or other devices in the shafts that would hinder the free flow of stone and calcined product as it passes through the kiln. In the case of rectangular shafts, standard brick shapes can be used to a large extent with a minimal number of special shapes required. This provides low cost
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Fig. 6: Rotary Piston Blowers control of the required volumes of combustion and cooling air. The variable control can be either manual or automatic and it is used to provide the correct air flows at any kiln output and to meet all product quality requirements.
Fig. 5: Refractory Lining lining and a simple inventory of spare bricks for repairs. Circular shafts are not as simple and require somewhat more special shapes. However, due to the static nature of the kiln and the constantly improving refractory materials, a long life with minimal maintenance can be expected. 5.3 Air Blowers The kiln system is pressurised up to typically 40 kPa (400 mbar). Rotary piston (Roots type) blowers produce combustion air as well as lime and lance cooling air. Rotary piston blowers supply practically constant volumes of air, independent from the pressure drop created by the resistance of the stone column. Depending upon the kiln capacity, a varying number of blowers is installed to supply combustion air and cooling air. One combustion air blower and one lime cooling air blower are driven with variable speed with the remaining blowers driven at fixed speed. The variable speed blowers provide the accurate
Fig. 7: Burner Lance System in Maerz Lime Kilns 8
From the combustion air blowers an air duct leads to the top of the kiln, from the cooling air blowers a similar duct leads to the discharge device. The combustion air is fed to the kiln above the stone charge, and the cooling air enters the lime charge through the discharge devices. The blowers are installed in a room designed to minimise the sound emissions (Fig. 6). All blowers are provided with inlet and outlet silencers. During the reversal period all air flow to the kiln must be stopped and the kiln de-pressurised. Bypass valves are provided so that the blowers can remain in operation during this time. 5.4 Firing Equipment The main requirement to produce a high, uniform quality product, i.e. quicklime or burnt dolomite, is to achieve a uniform distribution of the fuel over the entire shaft cross sectional area. This is accomplished by installing vertically suspended burner lances inside the stone charge as shown in Figure 7.
Fig. 8: Gas Firing System The PFR-Kiln can be fired with virtually all kinds of gaseous, liquid and pulverised solid fuels as described in the following chapters. 5.4.1 Natural Gas Even distribution of gaseous fuels is accomplished by feeding gas into a main ring duct (Figure 8). From the main ring duct it is distributed to the individual burner lances. Each burner lance contains a limiting nozzle to control the actual gas flow and the distribution between the lances.
The burner lances are in direct contact with hot gas and hot stone therefore cooling air is required to cool the lances and to prevent dust from entering into the lances in the non-burning shaft during the exhaust gas cycle. Positive pressure, Rootstype blowers supply air to cool and purge the lances thereby maximising lance life. 5.4.2 Fuel Oil Vertical burner lances similar to those used for gas firing, however, consisting of two concentric pipes are used for oil firing (Figure 9).
Fig. 9: Fuel Oil Firing System 9
Cooling air passes through the space between the inner and outer pipes. Steam or compressed air is used to atomise the fuel oil and to purge the lances to avoid clogging and/or coking at the lance tip. Individual dosing pumps or control valves assure constant and even oil flow to each burner lance. 5.4.3 Coal In many areas, coal, lignite or petcoke are less expensive and more readily available than natural gas and fuel oil. A system similar to the gaseous fuel design was developed to allow pulverised lignite, coal and petcoke be injected through the burner lances. Air is used as the carrier gas and lance cooling medium. Figure 10 shows the basic scheme of the Maerz Pulverised Solid Fuel firing system (PSF). Since its introduction in 1980 the PSF system has been proven in more than 40 installations. Pulverised fuel is stored in a bin to allow batch discharge of the fuel into the weigh hopper beneath. The outlet cone of the storage bin is equipped with fluidising devices operated with compressed air or inert gas such as nitrogen or carbon dioxide. The required weight of coal for one burning cycle is fed from the bin into the weigh hopper during reversal periods when the flow of coal and combustion air to the kiln is stopped. The bottom of the weigh hopper is designed with evenly spaced outlets around the circumference of the hopper leading to rotary valve feeders that discharge into conveying pipes. Contrary to the firing systems for gaseous and liquid fuels the system for solid fuels operates in two steps. In the first step the required amount of coal for one burning cycle is fed into the weigh hopper. In the second step the coal is conveyed from the weigh hopper via rotary dosing valves to the burner lances.
Fig. 10: Maerz PSF – Pulverised Solid Fuel Firing System 10
5.4.4 LPG (Propane, Butane) Propane and butane (LPG), as well as LVN (Light Virgin Naphtha), are excellent fuels for lime kilns. They are used in specific cases where available at favourable prices and where high purity lime is required. In some cases LPG is used in combination with other fuels, such as sulphur containing coal, to keep the overall sulphur content of the product at acceptable levels. The firing system used for LPG on Maerz lime kilns is very similar to that used for natural gas. In most cases LPG is vaporised before being fed to the burner lances via the main ring duct and nozzles. Occasionally, liquid gas is fed directly to the burner lances by dosing pumps. 5.4.5 Coke Oven Gas and Low Calorific Value Gas Steadily rising fuel prices coupled with the requirement to reduce the use of non-renewable fossil fuels has created the desire to use low calorific value gases known as lean gases. These LCV gases are typically off-gases from pyrolytic processes and the manufacture of iron and steel. Converter gas, Corex gas, blast furnace gas or a mixture of all are successfully used on Maerz PFR-Kilns. These “waste gases” are produced at low pressure so their pressure must be boosted by means of rotary compressors or Roots type blowers before being used. The same procedure applies to coke oven gas. 5.4.6 Wood The earliest lime kilns used wood as the standard fuel. Wood was substituted first by coal and coke and later by fuel oil and natural gas. In recent years the use of wood waste, readily available in certain areas, has gained importance. Maerz has developed a firing system, similar to the one for coal, where wood
waste (specifically sawdust and grinding dust from the furniture industry) is injected through the burner lances. The particle size should be less than 3 mm before being fed to the kiln. 5.4.7 Waste Oil The recycling of waste oil, primarily used lubricants, has become an important and necessary routine in our industrialised world. There are several Maerz lime kilns using “converted oil” as fuel. Many waste fuels contain elevated levels of impurities that can result in the production of toxic gases or the discharge of heavy metals. Special attention must be paid to environmental issues and the use of waste fuels may be restricted by environmental regulations. 5.4.8 Simultaneous Use of Fuels Besides the use of single fuels as described above the Maerz PFR-Kiln may also be operated using two fuels simultaneously in order to optimise overall fuel costs. Typical combinations of fuels are coal dust and natural gas, fuel oil and natural gas as well as coal dust and fuel oil. It is also common practice to design the kiln for a single fuel in the first stage and only later upgrade the firing system for the use of two types of fuel. 5.5 Hydraulic Equipment The kiln operation requires the alternating burning and nonburning shaft procedure. The kiln must be opened, closed, sealed, pressurised, fired, and de-pressurised. These actions require the use of hydraulically operated, movable parts. These moveable parts include: • reversal traps for combustion air and off-gas • shaft closing traps • discharge tables • discharge traps • traps at the weigh hopper • relief valves in the air ducts • reversal valves for fuel and purging media • stone level indicators.
The non-compressible feature of hydraulic oils has the advantage of producing strong moving force with small construction elements. Operation is safe, reliable and requires minimum service. The hydraulic system consists of a power unit (as shown in Fig. 11) comprising an oil reservoir, pumps and filters, as well as cylinders and control valve stands. 5.6 Electric, Measuring and Control Equipment 5.6.1 Motor Control Centre The Motor Control Centre (MCC) generally is of conventional relay technology design. The main switches, current, voltage and protection elements as well as the transformer for control voltage are all installed in the entry section. The other sections house the control, switch and protection elements for the individual drives as well as the frequency converters for the blower drives. The last section of the cabinet comprises all electric apparatus that must be emergency operated in case of power failure. The provision and scope of the emergency section depends on the type of fuel used and customer requirements. For each drive the operation modes “Local - Off - Auto” may be selected with a key-operated switch on local control panels on the kiln or at the control panel depending on customer request. Furthermore the individual drives may also be equipped with local isolators. 5.6.2 Control Panel and Remote I/O Stations The control panel houses the following instruments: • PLC • Input/Output module cards (if not located in remote I/O stations) • Control power supply • Interface relays • Transmitters (if not located in remote I/O stations) • Uninterruptible Power Supply (UPS) for the PLC • Data bus interfaces for the visualisation station and remote I/Os. All digital and analogue signals from the field or the power cabinet are transmitted to the system via PLC input/output module cards installed in the control panel or the remote I/O stations. Measuring signals for temperatures and pressures are transmitted as analogue signals. Signals from limit switches and other position indicator devices are digitally transmitted. 5.6.3 Operator Control Station The control station consists of a visualisation industrial PC with monitor, keyboard and printers. All kiln operation commands such as kiln start, conveyor start, etc. can be given via the PC. In Fig. 12 a graphic display of the kiln and its operating parameters is shown.
Fig. 11: Hydraulic Power Unit 11
Fig. 12: Graphic Display of the Maerz PFR-Kiln
Process data and limit value inputs are handled via the visualisation PC. Also historical data may be charted from the station’s database. The operator interface system is programmed to provide the following information: • Indication of system operating conditions in a process flow chart • Input/output of process data and measured values • Output of alarm messages • Output of short term trends • Storage of data on hard disk for long term trends • Output of production reports • Print function of all charts/graphs/pictures and reports. 5.6.4 Process Parameters Calculation Module All process parameters are calculated in a program module according to the input data. The kiln operator can for example make the following selections: • Production rate (t/d) • Amount of stone per cycle (kg) • Heat consumption (kJ/kg lime) • Excess combustion air factor (-) • Cooling air volume (m3n/kg lime)
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From these data the following process parameters are calculated among others: • Number of cycles per day (cycles/d) • Combustion or burning time (sec) • Fuel quantity per cycle (kg or m3n) • Fuel flow (kg or m3n/hr) • Combustion air flow (m3n/hr) • Cooling air flow (m3n/hr). 5.6.5 Production Report Module The following reports may be displayed: • Cycle report • Day report. The cycle report provides specific data on date and time such as stone charge weight in each shaft, fuel per cycle, heat consumption, crossover channel temperature and actual combustion air factor. The day report is a summary of the cycle reports giving an overview over the most important operating parameters. 5.6.6 Operation without Human Intervention In a number of cases Maerz kilns are operated without continuous personnel interaction during night shifts and weekends.
Should any serious problem occur, the kiln is automatically stopped and the appropriate person is electronically notified. The operator could be elsewhere in the plant or even off site. The person notified can analyse the problem via modem and if corrected can attempt to restart the kiln. If the problem cannot be solved through the control system then other appropriate actions can be taken. Upon request Maerz may design such systems adapted to customer needs. 5.7 Charging Equipment of the Kiln A kiln with such a high thermal efficiency demands constant material throughput. During each reversal a weighed amount of stone is fed to the kiln. The number of charges per hour and the duration of the heating cycle are regulated to control the kiln output. Typical operation demands a reversal cycle approximately every 12 minutes. Local site conditions determine the method of transporting the stone to the top of the kiln. The various charging methods comprise: • Standard conveyor belt • Skip hoist • Vertical conveyor belt. A stone hopper on top of the kiln receives the limestone. Depending on local conditions this hopper may also serve as weigh hopper.
In its simplest design the stone hopper distributes the stone charge to the two kiln shafts by having two traps on its bottom that open alternatively. The stone slides through the open trap via a chute into the corresponding shaft. The shafts are opened for charging and sealed by hydraulic traps for operation. The stone charging system described above is a simple system used in many applications. Some operations, such as those requiring a wide range of stone gradation, demand a more sophisticated charging system. The purpose of the sophistication is to ensure a uniformly distributed stone size across the kiln cross section. This is critical when a wide range of top to bottom size is desired or when small stone is used. Fig. 13 shows a system that uses rotating buckets in place of the charge chutes. The purpose is to improve the distribution of the stone prior to charging into the shaft as the buckets are located directly above the shafts. Additional designs include removable distribution cones located in the top of the shafts. These cones serve to control the distribution of the varying stone size inside the shaft and are successfully used in Finelime Kilns. 5.8 Reversal Device Kiln reversal is the periodic transfer or swapping of the burning and non-burning shafts. This requires devices to control the flow of fuel, combustion air and off-gas. The fuel flow is swapped between shafts by on/off valves. Double-acting hydraulic cylinders insure the correct trap position for the flow of combustion air and off-gas in each shaft. Fig. 14 shows the position of the traps, the burning shaft on the left and the non-burning or off-gas shaft on the right. In the diagram on the left side the combustion air trap is open allowing the flow of combustion air to the top of the shaft. At the same time, the right side of the diagram shows the off-gas trap open allowing the flow of off-gases from the non-burning shaft to the stack. These traps are in the opposite position after a reversal. The reversal is controlled automatically by the kiln control system. Cooling air flows continuously into the bottom of both shafts. Distribution of the cooling air between the burning and the non-burning shaft is obtained by butterfly control valves.
Fig. 13: Stone Charging Device 13
Fig. 14: Reversal Device
Fig. 15: Discharge Device 5.9 Discharge Device The calcined lime or dolomitic lime material is continuously discharged from both shafts. Kilns with rectangular shafts use reciprocating tables while kilns with circular shafts use two crosswise operating tables. All types of discharge tables are hydraulically operated. The rate of discharge is automatically regulated by the stone level control system located in the preheating zone. Fig. 15 shows the arrangement of the discharge device of a double-shaft kiln with circular cross section. A small hopper is situated underneath each discharge table to collect the lime discharged from the tables during the 12-minute burning period. The hoppers are sealed by airtight, hydraulically operated traps. During each reversal period the traps open, lime drops into the pressure-free receiving hopper and is then discharged by vibrating feeders. A roof-like saddle or a steel cone is constructed above the discharge table to maintain lime flow and discharge. 5.10 Environmental Control Equipment Two main issues must be considered when looking at environmental protection and operation of PFR-Kilns. • Noise emissions • Emissions into the atmosphere. 5.10.1 Noise Protection Noise emissions are mainly generated in three places: • charging of stone into the kiln • discharging of product from the kiln • air blowers. 14
Stones falling from buckets, conveyors, and chutes into metal hoppers create excessive noise. Therefore, the upper part of the kiln where the stone is dumped and charged must be completely enclosed to control the noise that escapes into the surrounding area. In addition chutes, buckets and hoppers are rubber lined. Most of the lime discharge device is located within the concrete foundation structure at the bottom of the kiln and thus is less critical for noise emissions. The Roots-type blowers that supply combustion and cooling air operate at high noise levels. To reduce the noise emissions to acceptable levels each blower is equipped with inlet suction and pressure discharge silencers. Additionally, all blowers are located in a special building made from concrete or concrete masonry block. The building is equipped with a silencer located in the inlet suction channel that supplies all air to the inside of the building. All doors to the building are sound proof. 5.10.2 Off-gas Emissions Off-gas containing particles of carbonate and calcined material exit the non-burning shaft. The particulate matter is usually controlled by bag house filters, an example being shown in Fig. 16. The dust content in the raw gases exiting the kiln is usually around 5 g/m3n. The bag house filter reduces the final emission level to less than 20 mg/m3n with the final level depending on local regulations.
Emissions of carbon monoxide, sulphur dioxide, heavy metals, etc. depend to a large extent on the type of stone and fuel used. NOx formation is inherently low in the PFR-Kilns because there is no free flame generated. The flame is produced within the stone bed, completely surrounded by material of lower temperature. As the heat is released it is immediately transferred thereby minimising the peak flame temperature and consequently the formation of nitrous oxides.
6. Kiln Operation 6.1 Principles and Control Philosophy The PFR-Kiln operates in cycles: during each 10 to 15 minute cycle, fuel and combustion air are fed into one shaft - the burning shaft - while the other shaft serves as the preheating and off-gas shaft - the non-burning shaft. During kiln reversal the fuel flow is stopped and combustion and cooling air are vented to the open. In this phase the kiln is de-pressurised, stone is charged into the kiln and calcined product is discharged from the collecting hoppers underneath the discharge tables. When the reversal period ends the shafts switch roles, the burning shaft is now the non-burning shaft and the non-burning shaft becomes the burning shaft. The high thermal efficiency of the Maerz PFR-Kiln requires the accurate control of multiple operating parameters such as: • Weight of charged stone • Stone level in the kiln shafts • Combustion and cooling air flow rate • Fuel flow rate, i.e. heat input • Temperature and pressure inside the kiln • Discharge speed of calcined product. The batch process of charging stone allows accurate weighing of each charge added to the preheating zone of the shafts. Level control is usually done by either a mechanical level probe connected to an electronic instrument or by gamma ray instrumentation. Continuous measuring of ambient air temperature and pressure allows the setting of a constant amount of air under local conditions thereby eliminating the influence of outside air temperature and barometric pressure fluctuations. Constant air flow is maintained with variable speed blower motors.
Fig. 16: Bag House Filter Plant
Calorimetric equipment may be used to measure the heat value of gaseous fuel. Although the heat value of natural gas is rather constant it can vary significantly with coke oven gas, converter gas, electric furnace gas, or a mixture thereof. In this case continuous measuring of the heat value and subsequent control of the heat input is critical. Liquid fuels typically do not vary in heat value. The calorific value of solid fuels may vary but continuous measuring is a problem. Regular heat value checks help to maintain constant heat input.
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An important objective for kiln operation is to control the temperature in the calcining zone in a consistent, uniform manner. Temperature within this zone can vary due the stone grain size, the chemical analysis of the stone, variations in the amount and distribution of air flow, and variations of the heat value of the fuel. Fuel input has to be controlled as a function of the calcining temperature. As the temperature in the crossover channel is an excellent indicator for the calcining temperature an accurate measurement of this temperature by optical pyrometer is required. 6.2 Grain Size of the Stone A narrow range of grain size is ideal for any kiln, but, due to the crushing properties of stone, a widely varying grain size is the typical situation in the quarry. The PFR-Kiln is able to calcine a wide range of top to bottom stone size because of its sophisticated charging system. The ideal range is 2:1, but operation using 4:1 is still permissible. The top to bottom size range is not the only criteria though as the shape of the grain also plays a role. The minimum stone size for the standard type PFR-Kiln is approximately 25 mm with a typical maximum stone size of 125 mm. Upon customer request the maximum size may be as high as 180 mm provided the burning zone as well as the feeding and discharge equipment have been adequately designed for it. 6.3 Quality of the Stone As for all types of vertical shaft kilns the use of hard, nondecrepitating, high purity limestone is an ideal condition for trouble-free operation of the PFR-Kiln. Nevertheless, due to the fact that the shafts of the PFR-Kiln are virtually a pipe without any devices which could obstruct the free flow of limestone and lime the movement of the material column is slow and uniform minimising abrasion and formation of fines. This means that also soft limestone can be calcined in the PFRKiln. In case the limestone has a tendency to decrepitate during the calcining process an increased percentage of fines will be generated. The installation of so called air cannons in the crossover channel area where dust particles could stick to the refractory lining facilitates the calcination of soft and decrepitating stone. High quality limestone and dolomite with consistent chemical properties is often not available or is scarce. Varying contents of carbonates and impurities can result in the production of overburnt or underburnt product with inconsistent values for residual CO2 and loss on ignition. For such cases a fully automatic temperature control system of the Maerz PFR-Kiln may be implemented to adjust the heat input to maintain uniform quality of the calcined product.
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6.4 Excess Combustion Air Excess air has a considerable influence on fuel consumption in the typical counter flow shaft kiln. But this is not the case in the parallel flow regenerative kiln where the excess air factor has hardly any effect. The same amount of heat is recovered in the stone of the non-burning shaft regardless of the introduced excess combustion air. Therefore the air volume can be adjusted to produce a short or long flame and adapt the burning zone temperature to produce the desired product. Lime cooling air does not take part in the combustion and dilutes the combustion gases thereby making the CO2 content in the off-gas of PFR-Kilns lower than in a conventional single shaft kiln.
7. Performance, Product Quality, Energy Consumption, Maintenance 7.1 Kiln Capacity The trend in today’s market is to focus on large capacity kilns. PFR-Kilns with a daily output of 600 tons have been in operation for years with up to 1000 tpd available. Small capacity kilns are restricted by economic factors. The relation between the cost to install a large kiln and a small kiln is not linear. It is generally recognised and accepted that the investment costs per ton of burnt lime are higher on small kilns than they are on a larger kiln. Even so, under certain conditions, PFR-Kilns with a daily output of 50-75 tons have been proven economical. The output of a PFR-Kiln can be varied within a wide range: it is quite possible to operate the kiln at only one half of the nominal capacity without considerable influence on the specific fuel and power consumption. 7.2 Product Quality 7.2.1 Residual CO2 The PFR-Kiln allows the production of lime and dolomitic lime with residual CO2 figures as low as 0.5%, in certain cases even lower. The steel industry, the biggest consumer of lime and dolomitic lime, generally asks for residual CO2 contents of less than 2%. 7.2.2 Reactivity The parallel flow of material and combustion gases during the calcining process is the ideal condition to produce high reactive lime and dolomitic lime as required for most applications. For special applications such as the production of porous concrete, lime with medium or low reactivity is required. By adapting operating parameters, such as excess air ratio and heat input, medium burnt lime can be produced in the PFR-Kiln with adequate quality of the raw stone. The production of hard burnt lime, however, is in general not possible in this type of kiln.
7.3 Fuel and Electric Energy Requirement The PFR-Kiln has the highest heat efficiency of all modern lime kilns. Energy efficiencies of 85% and higher have been achieved. Typical heat consumption (based on the net calorific value of the fuel) is in the range of 3350 to 3600 kJ (800 to 860 kcal)/kg or 2.88 to 3.09 million Btu per ton of lime depending on chemical analysis and grain size of the stone and the type of fuel. Electric energy consumption depends on the stone size, the fuel used and the kiln’s elevation above sea level. Consumption figures range between 25 and 35 kWh/ton of product. 7.4 Maintenance Work Long term operating experience suggests the following refractory repair intervals for normal kiln operation:
8. Modernisation and Revamping 8.1 Technical Progress The first Maerz PFR-Kilns were built more than 35 years ago and are still operating. Despite the tremendous technical development occurred since then the basic and unique principle of the PFR-Kiln has remained unchanged. In fact studies published in the literature have come to the conclusion that the thermal efficiency of this kiln type cannot sensibly be improved. On the other hand extensive tests in the Maerz laboratory, theoretical investigations and, most important of all, the fruitful co-operation with kiln operators for so many years have resulted in vast experience and subsequently in essential improvements in kiln design and operation.
3 to 4 years 6 to 8 years 9 to 12 years
8.2 Revamping of Kilns The most important factors which make modernising of a Maerz Kiln desirable and interesting are:
Using this expected life and normal repairs and maintenance, the average figure for refractory consumption is less than 0.3 kg per ton of lime produced.
• Environmental issues The installation of modern bag house filters reduces dust emissions to meet the environmental regulations set by local authorities. Better control of the combustion process results in lower CO and NOx emissions.
• Crossover channel area: • Burning zone: • Preheating and cooling zones:
7.5 Supervision and Maintenance Personnel As a standard recommendation one operator per shift is required to supervise operation of one or more kilns. His main work place is the control room. He can, however, perform duties outside the control room as the kiln will be shut down automatically in case of any serious trouble. For maintenance and repair work a mechanic as well as an electrician should be available.
• Improvement of kiln operation Kiln operation can be improved by an increased degree of automation and centralised control. • Increased availability and safety Improved and automatic control of kiln operation increase availability and operational safety. • Improved quality of product Stricter operational control improves product quality and consistency. An increase in the number of burner lances leads to more uniform distribution of the fuel and thus further improved product quality. • Flexibility in fuel application Flexibility in the use of different fuels, separately or combined, depending on availability, market price and product requirement increase competitiveness (Fig. 17). • Capacity increase Through adaptation of the reversal sequence it is possible to charge the kiln during the burning period. The shortening of the reversal period results in an increase of kiln output. • Enhanced range of stone grain size The use of broader grain size range through improved charging systems will increase the quarry yield and thus reduce raw material costs.
Fig. 17: Modernised Fuel Oil Firing System 17
A narrow grain size range of the stone is desirable in shaft kiln operation. On the other side a high quarry yield and thus the use of a max. allowable grain size range is imperative to reduce production costs. To achieve this goal Maerz has designed the so-called “Sandwich Charging System” for its PFR-Kilns. The successive charging of stone in layers of different size reduces the pressure drop in the stone column compared to charging a mixture of the two stone fractions. At the same time quality of the calcined product is improved. 8.3 Conversion of Single Shaft Kilns Under specific conditions it is desirable and feasible to convert existing single shaft kilns into PFR-Kilns. The reasons for conversion include: • There is insufficient area to construct a completely new installation on site. • Existing facilities such as the stone charging equipment, the discharge arrangement, the dust collecting equipment and the kiln foundation structure must be re-used. On certain occasions even the kiln shells have been re-used. Sufficient height between charging and discharging equipment as well as an appropriate distance between the shaft kilns to be
converted into PFR-Kilns are essential pre-conditions for any conversion. Furthermore the existing foundation structure must have enough bearing capacity to support the new kiln structure. • Capital expenditure for conversion is lower than for installation of a new kiln. The above factors may make the conversion of existing kilns into PFR-Kilns an attractive solution. To adapt the new kiln system to the existing installation requires substantially more engineering and design work from the kiln constructor. Conversions carried out by Maerz to date comprise the following alternatives: • Two existing coke fired, single shaft kilns have been connected by a new crossover channel and thus converted to a PFR-Kiln with gas firing. • A newly built shaft has been added to an existing single shaft kiln to form a PFR-Kiln. • The two shafts required by a PFR-Kiln have been installed within the shell of one existing shaft kiln.
9. Specially Designed Maerz PFR-Kilns 9.1 Maerz Finelime Kiln Typical stone sizes processed in conventional vertical shaft kilns are larger than 30 mm. Stone sizes of 6 to 30 mm can be used in rotary kilns, however with lower thermal efficiency. Maerz has developed a special type PFR-Kiln to burn stone sized between approx. 10 and 40 mm at thermal efficiencies equal to or greater than conventional PFR-Kilns. The primary features of the Maerz Finelime Kiln (Fig. 18) are: • New stone charging system – the system allows each shaft to be charged simultaneously and provides control of the smaller fraction to be charged to the outside of the shaft cross section. • Adapted fuel injection system – uses a greater number of burner lances for increased uniformity of heat input into the burning zone. • Inner shape of kiln – the Maerz Finelime Kiln has strictly a circular design. A kiln with rectangular or semi-circular cross section would neither allow sufficiently uniform charging of stone nor uniform flow of air and combustion gases. 9.2 Maerz MG-PFR-Kiln One of the key factors for lime producers today is to increase the yield of their quarry operations. An important factor in this is the ability to use the maximum grain size range in the calcining kilns. In many cases only one kiln type is available and it normally accepts only a limited size range. Maerz has developed a special type of PFR-Kiln that is able to process top to bottom grain size of 6:1, e.g. 20 to 120mm.
Fig. 18: 300 tpd Maerz Finelime Kiln 18
10. References
11. Summary
Since 1966 more than 350 Maerz PFR-Kilns have been built around the world (Fig. 19). Of these, approx. 150 kilns are gas fired, approx. 130 kilns fuel oil fired, approx. 20 kilns solid fuel fired and approx. 50 kilns are fired with a combination of gaseous, liquid and solid fuels.
Since its introduction into lime industry in the late fifties the Maerz PFR-Kiln has become a piece of standard equipment in the lime and dolomite industry. Its unbeatable thermal efficiency together with the flexibility in operation with virtually all types of fuel and raw material have made him an excellent choice whenever the installation of new calcining equipment is an issue.
Gaseous fuels used include: • Natural gas • LPG (propane, butane) • Coke oven gas • Electric furnace gas Liquid fuels used are: • Heavy fuel oil • Light fuel oil (Diesel oil) • Waste oil Solid fuels used are: • Bituminous coal • Lignite • Petroleum coke • Wood. Maerz PFR-Kilns have so far been built with daily capacities ranging between 100 and 600 metric tons of calcined product. The kilns can be operated between 50% and 100% of their nominal capacity. The following number of kilns have been built according to daily capacity: 100 to 150 metric tons per day: 160 to 250 metric tons per day: 260 to 350 metric tons per day: 360 to 450 metric tons per day: 460 to 600 metric tons per day:
approx. 80 approx. 90 approx. 110 approx. 40 approx. 30
Bibliography R.S. Boynton, “Chemistry and Technology of Lime and Limestone”, John Wiley & Sons, 1980 J.A.H. Oates, “Lime and Limestone”, Wiley – VCH, 1998 E. Schiele / L.W. Berens, “Kalk”, Verlag Stahleisen, 1972
Fig. 17: Maerz Lime and Dolomite Kilns World Wide 19
Maerz Ofenbau AG Richard Wagner-Strasse 28 CH-8027 Zurich Switzerland
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