Mod 13-Kiln Volatiles.pdf

Process Engineering Training Program MODULE 13 Kiln Volatiles Section

Content

1

CETIC “Volatiles” Group- Final Progress Report

2

CETIC Sub Commission “Behavior of Volatile Material in Kiln Systems

3

Investigation into Potential Low Temperature Volatilization

4

Factors Affecting Sulphate and Alkali Cycles in Rotary Kilns

5

Alkali Volatilization- A Review of Literature Available in 1977

6

A Study in the Volatile Cycles on HOPE # 2 kiln

7

Design and Experience with Bypasses for Chloride, Sulphate, and Alkalis

8

Kiln Gas Bleed Considerations

9

Ring Formations in Cement Kilns

10

Kiln Build-Up Meeting

11

Cement Seminar- Rings, Balls, and Build-Ups

12

Ring and Buildups in Cement Kilns

HBM Process Engineering Conference Minimization of Volatile Cycles

Blue Circle Cement

PROCESS ENGINEERING TRAINING PROGRAM

Module 13 Section 1

CETIC “Volatiles” Group- Final Progress Report

Blue Circle Indusuies PLC

Internal memo 3

SEE BELOW:-

‘rot?

C P KERTON Your re::

Date

21 December 1994

Copies

Stijec

CETIC “VOLAl-IUS” GROUP - FINAL PROGRESS REPORT

Herewith a copy of an English version of the final report from the CETIC GOUP which worked from 1990-1992, initially inspired by 3lue Circle’s initiatives. Copies of various French documents exchanged within the group are available from me or from Cb.ris Hoit. The text follows the order of the French original, intended for those who took part in the work. For those who have not been following progress so closely, the logical order of reading would be to stan with Appendix 2, which is a published paper based on the initial work of the group on cycles of chlorides, sulfates and alkalis driven from the burning zone. Next comes Appendix 3, which is Tom Lowe-s’ account of some practical applications of these concepts when burning petroleum coke. Returning then to the start of the main text, there is an account of further discussions and activity on this topic leading finally to Appendix 1 which looks at capture of SO:, in colder parts of a kiln system. (Those concerned soleIy with pollution abatemerit might well start here!) Proposals for possible further work are listed and those relating to sulfur behaviour will be pursued by joint exchanges of experiences from 1995. Suggestions for further distribution within Blue Circle will be welcomed. c I& ,/ f&L > c P Kerton Patent & Information Service Eric To:

;P L Rover (2 copies) C P B ‘Turner T M Lowes C J Holt L P Evans R M Mutter J M Lawton P A Longman

ca: \?%+!\2112CETIC

Blue Circle Technical Centre TC 94 049

CEl-lC SlJB-CO-ON

“BEHAVlOUR

O F

VOLATILE

M A - IN KILNS”

FINAL. PROGFZES REPORT MAY 1994

This report is strictly confidential within the Blue Circle Group. Additional copies should not be released outside Blue Circle without reference to the Technical Cenme.

Please apply to the hformation Servicesat

tie adcks give0 below.

September 1994

SUMMARY

Together with an earlier report (ISTN 92/5) this text sets down the major findings of a group which met from 1990 to 1993. For minor components volatilised in the burning zone (alkalis and sulfates), there are several successes on production kilns in reducing volatile cycles by attention to burning zone conditions, especially in relation to chemical decomposition of C&I,.

Correct diagnosis of conditions is assisted by

improvements to permanent on-line exit gas monitors and to suitable standardisation

of

sampling procedures for dry process kiln enuy material. The full effects of dust cycles in confusing results of sampling exercises remain to be established.

Sulfur volatilisation and absorption at lower temperatures have also been considered and the conditions of temperature and atmosphere which aid reactions for SO2 capture by various materials are outlined. Various permutations of actions are possible to abate emissions and some will not succeed at all points in a production line or may require moisture addition or increased residence time to improve their effectiveness.

Proposals for possible topics to continue this work are listed. (This is an English version of the official French text).

=

CONTEXTS

Paee No

1.

Introduction

2.

Task of the Sub-Commission

3.

Cycles of minor elements generated in the burning zone

4.

“Cold” cycles of minor elements

11

3.

Trace elements

14

6.

Future work

15

Figures

Appendix 1

SO? CAPTURE Chknical IMechanisms far Sulfur Dioxide Absorption in Cement Kilns and other Industrial Abatement Plant.

Appendix 2

TEXT FRO,M INTERNATIONAL SYMPOSIUM ON GAS CL&WING AT HIGH TEMPERATURES. Behaviour of Volatile Materials in Cement Kiln Systems.

Appendix 3

PAPER PRESENTED BY T M L0WE.S 100% Pet Coke - Problems and Solutions.

1.

INTRODUCTION

The group has the following memberx-

HOLDERBANK

P BORKI

3LUE

P KERTON (Animateur)/C HOLT

CIRCLE

CIMENTS LAFARGE

M DANDINE/K 30ULOT/M TOUSSSAINT/ X DUPONT-WAVRLN

CIMENTS

B POLGE/G Bw BERGERY/ G FLAMENT

FRANCAIS

ITALCEMENTI

R TACCHINI’

C3R

c MEYERS/J PARlSIS/P RENIER/ M BRUYERE

CIMENTS

D’OBOURG

ENCI

F LAMPROYEfR SPILLAERT W VAN LOO/F ERE?JS

The foIlowing meetings too& place:-

1.

1990 Autumn

Maastricht

ENCI

2.

1991 Spring

Greenhithe

BLUE

3.

1991 Autumn

St Antoing

CBWSCF

4.

1992 Spring

Frangey

LAFARGE

5.

1992 Autumn

OrbY

HOLDERBANK

6.

1993 Spring

Salerno

ITALCEMENTI

7.

1993 Autumn

Obourg

OBOURG

CIRCLE

This year there was one meeting at Obourg on the 4 and 5 November 1993.

TC94049

2.

2

Tt=chu.id

Cease, .!%@cabm 1994

TASK OF THE SUEKO~MMISSION

This is “to produce a state-of-the-art report concerning the behaviour of volatile material in kilns.”

This work originates from

1.

The increased use of ail sorts of secondary raw materials and fuels.

2.

The trend to produce more low alkali cements.

3.

Emission regulations which are becoming more and more rigorous.

Furthermore, during the work, important implications for kiln output and cement quality have been found in connection with control of cycles. Our principal recent activities are covered under the three following headings, 3-5.

(A copy of an associated paper by T M Lowes is included at the end of this report, having been presented at the same meeting in May 1994).

3.

CYCLES OF MINOR ELEMENTS GENERATED IN THE BURNING ZONE

We have already produced a first text on “classical” knowledge for this group of cycles

(ISTN 92/5): this was distributed in 1992. Supplementary work is described here.

Given a sample of material and its chemical analysis, one might think that all would become clear, but to calculate a chemical balance, it is also necessary to know the mass flows which are involved in the calculation, and there are a number of methods of deciding upon these. Each method has its own advantages and disadvantages, making them more (or less) appropriate at different points in the burning line. It is important to note that

1.

In the past this sophistication has not generally been adopted

2.

Various flow rates for hot raw meal entering the kiln can be calculated by different means, each one giving a different “burning zone volatility”.

We have continued exchanges on these topics to improve our understanding. One important parameter in the calculations is the flow rates of entrained solid particles between preheater stages, in particular between the kiln and the preheater tower. There are not many results available for this parameter (we note that there are not always the same sets of results available to explain observations!).

As far as these “classical” cycles of minor elements are concerned, there is much activity within member companies at present

There are two fields of particular

interest:

Firstly, the characterisation modified geometry

and the behaviour of hot raw meal (including the use Of

cyclones and special linings), and secondly the influence of

combustion and heat transfer on volatalisation

- especially in the burning zone but also

in precalciners. The influence of the local atmosphere close to clinker granules in the

rc94049

4

TecbniGai

Came. SenM I994

burning zone seems to play an even greater role than had been thought in the past: there are several results where volatilisation has been significantly reduced by more or less simple means (additional oxygen) and we trust that one day there will be a somewhat deeper understanding than indicated in our previous report.

This

understanding will also help us in applying results from the mathematical analysis of cycles from CBR which has shown great variations in alkali and sulfate volatilities

in

the burning zone in different kilns (even when taking account of the major volatilisation of chloride which explains a proportion of these differences).

This year we have tried to bring together knowledge on:

The most recent studies on control of cycles and characterising

hot raw meal

on the industrial scale;

Mathematical analysis of cycles and cyclone performance (from chemical analysis of samples);

The effects of the flame and kiln atmosphere ‘on volatility,

especially when

using petroleum coke.

Study

of

free energies indicates the compounds expected and which of them are the

most stable in the prevaiiing conditions of temperature, pressure and composition of the solid liquid and gaseous phases. Mr Berard-Bergery distributed a summary of the talk which he previously gave on this aspect of thermodynamics, He notes that without a certain knowledge of this subject it is difficult to make progress, given the need to explain the apparently contradictory results obtained from operating kilns. (We must certainly bear in mind the fact that we need to consider dynamic equilibria, not only static systems.) Thermodynamics allow us to determine the direction, intensity and speed of transformations of physical systems as a function of conditions. The possibility of a reaction is known from calculating the change in (Gibbs) free energy, if operating at constant temperature and volume or of free enthalpy at constant temperature and

pressure. Examining the trend of this value as a function of temperature, the reactions of formation or decomposition of a single chemical compound can be considered, and the order of stability in a family of compounds (chlorides, fluorides, sulfates etc) can be deduced. Hence, the reactions between one element and the compound of another element can be foreseen. There are quite a number of diagrams of free enthalpy as a function of temperature available (determined in the field of metallurgy) and also information on partial vapour pressures which

influence the equilibria.

Along a burning line it can be seen that for chlorides KCI is the most stable and easily vaporises producing a chloride cycle.

For fluorides CaF2 is the most stable at all

temperatures, and does not easily vaporise. For fluorides, phenomena are therefore very different for those encountered with chlorides. For sulfates K2SO4 is by far the most stable and CaSO4 the least. For C&30, stability also depends among other things on the partial pressure of oxygen, so that the sulfate cycle brought about by the decomposition and re-combination of &SO4 will be very different depending on the oxidation/reduction conditions at different points along a burning line. Na20 is more stable than f(20 and is found in clinker combined in the aluminate phase, whilst K20 tends to form &SO, if possible. It is interesting to see that sulfur, for example in the form of SO,, can be captured by carbonates because sulfates are more stable. At low temperatures sulfites can also be obtained.

CIMENTS FRANCAIS note that it is necessary to define the conditions in which a kiln must operate to give the desired results, in the case of high fluxes of sulfur or alkali. Results from CC3 confirm these ideas and they are being applied in other Works. Mr Flament has told us that the text which he presented at the Berlin Congress gives a good summary of his experiences at CCB.

At CCB, using 100% petroleum coke (ore-calcination with separate air at around 55-60% kcal) a good kiln output is found when there is a higher and more constant oxygen level at the kiln exit (1.5 - 2.0%), a greater fuel fineness (residue 3.6% compared with 5.0%), a more stable flame shape, and a less severe burning regime - obtained by means Of an

examination and adjustment of geometry in the burner region. The geometrical aspect helps to avoid too high a dust cycle (including cooler dust) and flushes, which can both cause alkali capture and blockages (contrary to the ideas of certain plant suppliers). &SO4 nodules are found in the interstitial clinker phase, and there is a need to consider quality aspects further. There is a sulfate: alkali ratio of 3.5, which is acceptable in the stable kiln regime: thus it is preferred to use only 50% petroleum coke for production purposes, so as to reduce the possibility of entering into a potentially difficult state. Coke brings 26% of the sulfur arriving in the kiln system.

The sulfate/alkali ratio is an important parameter to take into account, but it is not the only one. It is also necessary to have a good and continuous analysis of kiln exit gases to allow combustion conditions to be followed.

It is preferable that gas analysis is

automatically corrected for oxygen level so as to indicate other changes more clearly. It is equally necessary to continuously monitor precalciner

combustion conditions by

means of supplementary analysis of relevant compounds in the gas phase.

The “intensity of combustion” in the burning zone is an important factor to understand and use to control events. The decomposition of CaSO, in a reducing atmosphere is the key mechanism. Each kiln has its appropriate oxygen level in the kiln exit gases, which must be respected for a given sulfate input. The word “volatile” can lead some people into error:

whilst KCl

and NaCl are present in the form of gaseous molecules,

thermodynamics indicates that K20 and Na,O decompose in the flame and re-combine later. It can be useful to separately tabulate the calculated volatilities

of KCl NaCl,

K$S02, NaSO,, CaSO, instead of only Cl, K,O, NaZO, SO3.

We have discussed the design and operation of by-passes, certain of which take a significant dust burden at the kiln exit, which brings about difficulties in operation. In Ciments Francais kilns the range of dust burdens is from some ZOO-1,000 kg of dust per tonne of clinker. This dust burden reduces the performance of the lowest cyclone: if the value of the dust burden is not known it can be difficult to interpret results. (It

was

noted that Weber recorded low dust burdens in all the kilns which he analysed) Other

important parameters which must be known in order to diagnose a situation are the chemical analyses of good samples of coal and of the hot material coming into the kiln at the feed chute level.

Given the above-mentioned data, the measurement of CO? level at the kiln exit over a certain period allows separate calculation of decarbonisation in the kiln and the preheater/precalciner.

The mixture of dust and new raw meal which comes into the kiln

can then be estimated from the loss on ignition. It is suggested that methods using chemical tracers to estimate the material flow, for example K,O, can be falsified if the level of dust cycle is not known.

It seems probable that the geometry of a plant has a marked effect on dust entrainment. In the older generations of Dopol preheater, material fell a long distance from the bottom cyclone into the kiln.

The kiln entry material had a low loss of

ignition which could be falsely attributed to good decarbonisation.

The more recent

Dopols have a side entry for material: a bypass can then expect to encounter a lower dust burden. The older “lateral centrifuge” entry of F L Smidth also produces a poor bypass efficiency, due to dust entrainment.

High dust recycle levels also have the

inconvenience of increasing the probability of blockage (Mr DuPont-Wavrin noted that the Berthold Company is supplying an X-ray detector to monitor material flow rates ex-cyclone).

It is useful to calculate the effect of dust cycles on thermal performance. A heat and mass balance for each preheater stage allows the effect of dip tube geometry changes to be observed. Opinions vary as to the appropriate choice for different stages, not to mention the use or removal of cyclone exit flaps. A variety of experiences have been reported regarding Hasle Vortex Finders. At ENCI, excellent results have been recorded for over 3 years, whilst at CBR the tubes were lost in 3 months- Ciments Francais have observed the same range of lifetimes. There is consensus on the advantages of dense ceramic Hasle units in the kiln feed chute

(with

a minimum of exposed refractory cement?,” when they are set up with a good arrangement of air cannons. Some peopie have doubts as to their sensitivity to thermal shock in other regions of kilns during heating and cooling.

ITALCEMENTI

has described similar experience with a F L Smidth chloride bypass at

Picton, also used to assist the production of low alkali clinker. Here the high dust burden (some 1,000 kg per tonne) causes efficiency of a dust bypass.

the so-called “gas bypass” to have tbe

Tests are in hand at Nazareth with a purifier which

removes SO, by injection of raw meal and water (a Monsanto design). At Colaferro two geometrically identical preheater kilns produce respectively some 1,900 and 1,150 tonnes a day. The higher output kiln is fired with a mixture of coke and coal and produces

build-up

problems, whilst

the

other

operates

satisfactorily

with

100%

petroleum coke. The only difference that has been noted in combustion conditions is a higher secondary air temperature due to the use of IKN plates in the Fuller cooler.

Italcementi manages to use 100% petroleum coke on the Lepol process, even with 10%

over-grate firing, with emissions of SO, - except during build-up losses - having only pyritic material as origin. The residue is 10% as for coal. On the dry process it is necessary to drop the residue to 4% (or 5% for a coal/coke mixture). The precalciner gas does not contain any SO2 when coke is used in the burning zone. The sulfur leaves with the clinker, partly during occasional flushes.

In the past coke or anthracite was introduced into long granule-fed kilns. This helped formation of a good burning zone, but nowadays it is found that there is also a high SO2 emission.

This coke is now added to the main burner, a procedure which operates

satisfactorily if the burning zone is controlled via NO, monitoring to avoid the problems which can be caused by the sudden arrival in the burning zone of build-ups detached from internal cruciforms.

In the same way, LAFARGE has continued with a major programme of geometric "centralisation"

of burners, noting oxygen levels (typically some 3%) and SO, levels ex-

73.i~ re-sort Ls soicrlr confidemial w.rhio the Blue Ctie Group.

kiln as a function of flame momentum. Several Works

keep the centralising

mechanisms

on the kiln platform so that alignment can be corrected if there are changes after some weeks of operation. In such circumstances 100% coke can be used (3-5% S) on the dry process with a 5-10%

residue.

Automatic kiln entry material sampling systems are generally installed with a view to assuring safe operation (Pfaff) la Nouvelle.

and Lafarge gave an account of an in-depth study at Port

The company was particularly

interested in the impact of sampling

techniques on results. Here, there is a kiln fed at some 100 tonnes an hour (50% of the heat energy coming from petroleum coke) with 3% oxygen ex-kiln. The levels of all volatiles in the collected dust go down as a function of sample suction rate, reaching a plateau. The dust Is really a mixture of fine material (high in volatiles and easy to collect) and coarse material. Although the nominal isokinetic aspiration rate for the probe was some 30 litres per second, it was not aligned with the gas fIow direction and higher suction rates were therefore needed to obtain a representative sample. Lafarge express the hope that a standard method can be written up, suitable for use throughout the world. They have currently only two or three competent sampling teams in their French group. It was noted that it is a bad practice to make use of large probes and low capacity pumps to reduce blockages as far as possible. CBR reported on SO2 levels in the kiln system at Antoing. The level of some thousands ppm ex-preheater drops to 600 ppm at the stack. This loss is split

20% to the crusher,

20% to the mill and 60% to inleaking air. It was reported that at Rekingen the raw mill is run at a reduced throughput, in order to allow SO2 capture t o continue throughout the operating day. A few supplementary results from the simplified mathematical model have been distributed, with its application to the analyses of balance samples from various member companies, so that volatilisation,

entrainment and capture coefficients can be

calculated together with the performance of some cyclones.

ThLs remrr Ls snicdy codid~riaf w-if&in tie Blue CLv!e Grarw.

A significant range of values was noted, all calculated on the same basis. K20 is a good tracer to determine raw meal

entrainment by gases. For the calculated entrainment

values in the document distributed, one must consider the position and methods used to collect the kiln exit dust samples - with a probe in the kiln, in the riser duct or exbypass. Nevertheless we consider that it will be very useful to extend the tabIe of results already obtained by sending further analyses to CBR to gather a common table describing volatilities, volatilisation)

An example of a Blue Circle kiln (poor flame with high

is the only one to have been added this year, and the model remains to

be more widely used.

At BLUE CIRCLE some thermodynamic data lead us to think that in a typical kiln gas there is at llOO*C a sufficient reduction potential produced from 2,000 ppm of CO to reduce CaSO, with a consequently much higher voiatility.

A separate paper from Blue

Circle is appended, giving an account of UK experience with use of petroleum coke.

A paper from Blue Circle regarding the design of a bypass for a new Works with high chloride raw materials was discussed.

There was also an expected high content of

alkalis in the clinker, which could perhaps be reduced by the addition of even more chloride. The performance of existing by-pass systems indicates that despite the fact that (according to suppliers) there is a possible dust loss of some 200 to 250 g/Nm-’

in the gas extracted from the system it will be best to calculate with a

nominal level of 400

In such circumstances there will be a need for a raw meal

preparation system with a significantly higher throughput than normal.

Several remarks were made: there are examples of precalciners

blocked by sulfur; the

handling of by-pass dust rich in CaCl, is much more difficult; is the fuel penalty per percent of by-pass closer to 5% than the 1% used in this example? (This latter figure was supplied by the company which intends constructing the proposed Works.)

The animateur was invited at short notice to give a paper during an International Symposium of the Cleaning of Gases at High Temperatures in December 1993, which is

This remrr Ls sm’cr(;l cmfidemiai withiu tie Blue Cri-cle Group.

appended.

Several specialist workers in this field have encountered problems of

blockages and build-ups which provoke their interest. (Note that data on the equilibrium CO-CaSO~-CaS is given in "Sulphur Capture in Fluidised Bed Boilers:

the Effect of

Reductive Decomposition of CS04”, by A Lyngfelt and B Leckner, Chemical Engineering Journal, Volume 40. pages 59-69, 1989.)

4.

“COLD” CYCLES OF MINOR ELEMENTS LEMENTS

Information on the effect of internal cycles on emissions to the exterior has been exchanged, avoiding (if possible) examination of equipment for capture of such emissions which is left to the "Environment"

sub-commission, liaising with its animateur.

topic typically concerns cold cycles of SO2

This

formed at low temperatures in the

preheater, and capture of SO, emissions in long kilns and in the Lepol process involving cycles which originated in the burning zone.

For this topic it seems that most information ‘Environment”

sub-commission:

has already been treated by the

it remains to define more precisely the chemical

reactions which are involved and the domains in which these are the most (or the least) effective. We are interested in establishing information about chemical efficiency of absorption of SO2 as a function of conditions of atmosphere, humidity, temperature, residence time, particle size, chemical composition etc.

The animateur noted the classification

of absorption mechanisms given in two USA

papers: these concern tests carried out Davenport (Steuch) and at Lone-Star (Sheth)

The document sponsored by the British Pollution Inspectorate is also available. This reviews published information on removal of trace gases. It covers a range of both chemical species

and reactive materials, The sections relating to SO2 capture appear

to provide a useful framework within which the cement industry’s experience can be classified, see Appendix. Other industries

are interested in the possible future use Of

sorbents with increased reactivity (cement kiln dust?) and in “regenerable” agents

such

as calcium disilicate.

The reports of HOLDERBANK

to the Environment Sub-Commission have indicated that

emissions of some 1,000 to 1500 mg/Nm3 of SO2 were reduced to close to 350 mg/Nm3 during the operation of the raw mill.

The same effect could be obtained in direct

operation by the addition of Ca(OH)2 to the raw meal. This provides removal of 50%

SO2 at a stoichiometric level of 5; Polysius would suggest 80% removal at a ratio of 8. Not all users seem to have taken account of the need to use superstoichiometric quantities

of sorbents, which sometimes may react with only some 10% efficiency.

Work carried out in the UK by Lodge-Cottrell desulphurisation,

in the field of the electric power station

has shown an efficiency of 25% for dry lime injection, rising to 50%

in the presence of moisture, and also that sodium based reagents had a genuine action which was almost double that of calcium based sorbents (and that these could be introduced as solutions by means of simple nozzles).

It was mentioned that Rekingen Works had modified its raw mill throughput so that it operated 24 hours daily, thus capturing SO 3 to conform to emission regulations. The Santa Cruz Works of Lonestar

may do the same. In this class of activity, CIMENTS

FRANCAIS works on the basis of 50 to 75% absorption.

ENCI gave an account of experience with operation at different oxygen levels to reduce SO2 emissions from its two-stage preheater kiln. The degree of sulfation of clinker at Maastricht is 125%. This, along with other causes, produces an emission which must be reduced to comply with new regulations During 3 weeks the oxygen level at the kiln exit was altered from 1.5, 2.0, 2.5, 1.3, 2.0%. The results for SO2 level in the emitted gases and SO3 in the raw meal and clinker are in agreement, showing that emissions can be reduced. If the effects on quality and on kiln operation are acceptable, they intend to buy new fans to guarantee sufficient oxygen level at maximum kiln output (with fuzzy control). The re-installation of Magotteaux stirrers in the kiln has once again given positive results after the last stop.

At OBOURG

there is an emission problem similar to that at ENCI, which can be

resolved by a kiln exit oxygen level of 2 to 3 % but this could give a too high a chain temperature and an unacceptable reduction in output. Oxygen has been added (1,000 m3/h either by the primary air channel or beneath the flame) gaining 3 to 4 tonnes per hour of clinker at an acceptable chain temperature. The cost of oxygen is some 3 Belgian Francs per cubic metre for a permanent irstailation, but this could be

This reqorr is snict(y confidential within the Blue Ckfe Group.

offset if cheaper (higher sulfur

fuels can be used. It seems that at least a quarter, and

nore usually about a half of the SO2 disappears between the kiln and the stack, no doubt by capture on dust. Given the large volume of gas produced by this wet process kiln and its moist fuels, a new fan would be proportionately much more costly than for ENCI.

At Obourg it seems likely that the longer kiln can satisfy emission limits through control of excess air levels. For the other kiln, another method of reduction of peaks of SO, is being studied; NaHCOS injection in the exit gas duct at the upper end of the kiln. The trial installation from Solvay (about 400 kg/h of Na.I-IC03 powder) was leased for longer trials. Chemical efficiency is 100%.

As already noted ITALCEMENTI

is looking at a Monsanto system involving a water/meal

scrubber for SO,, with a cost expected to be only 20% for that of an “Untervaz” system-

LAFARGE has studied sulfur behaviour on a semi-wet Lepol grate. In the hot chamber there is an excellent capture of sulfur coming from the kiln, but there is also decomposition of pyritic sulfur on the grate. This starts at the transition from the cold to the hot chamber (500-600'C)

and is completed by the middle of the chamber.

TC9-4049

5.

15

Tecfioical Centre. Senrember 1994

TRACE ELEMENTS

lTALCEMENTl has presented a summary of results from 30 kilns, seeking to determine the amounts of 16 metals In stack dusts, and also of 5 inorganic micropollutants. Vibo works (precalciner),

At

the raw meal is dosed with CaF2 to influence the

decomposition of strontium sulfate and so limit the undesired effects brought about by SrO during alite formadon. On the Lepol process CaF2 also provides a less dusty clinker and a reduced need for kiln system cleaning. No changes were noted in the behaviour of other halogens or of alkalis.

OBOURG has provided, a list of balances over 2 years. BLUE CIRCLE has shown a table for retention of various elements in a number of kilns as a percentage of the quantity brought in by the kiln feed (including recycled dust). An examination of Blue Circle’s conclusions regarding behaviour of trace elements considered elements as being either non-volatile or partly volatile. 3-5 results were available for each element for the wet, semi-wet and dry processes. The percentage of the input found in stack dust was very low for the non-volatile elements (As, V and sometimes Cr - unless there was enrichment from refractories), thus reflecting tie good efficiency of de-dusting and the varied additional contributions

from fuels. The highest proportions escaping with stack

dust were for cadmium, lead and thallium in semi-wet kilns. The levels noted were influenced by the rate of removal of intemediate

dusts produced in the kiln processes.

For the dry process there was less enrichment of cadmium and lead.

We have specifically looked at German work previously published in ZKG, where the need to examine the chemical combination of elements is underlined. Mercury,

for

example, must always be oxidised in a kiln system, so that the vapour pressure of the uncombined element is not to be considered.

We have also received a supplement to the bibliographical list established in Autumn 1991, which concerns cold and trace element cycles. The published literature makes it quite evident that cement kiIns are reputed to be potential origins of emissions of SO,,

Tl, Pb - and perhaps Cd and Hg, if these latter are found in the region. There is also a certain interest in the upgrading of kiln dust,

TIC re,vrc is suictiv wnfide!zu*al’

witi tie BILE C’ric!e

G~OLT.

TC94049

17

Techical

Cam-e, ?&xBder i99d

FUTURE WORK

The Plenary session has asked for a new sub-commission to be set up in 1994/1995 to follow the topics left from the existing group, with the addition of a study of corrosion and the wider application of the mathematical model already developed. We must avoid repetition of work already done, and coverage of subjects which the Maintenance Group and the new Working Party on Flames are currently examining.

(For corrosion,

depending on the activity of the Maintenance Sub-Commission, we might envisage a search for methods of reducing its effects, as well as the identification of compounds which have a strongly corrosive action).

Our

group

is convinced that there remains much to be gained for the companies of

CETIC in bringing together the specialists who are concerned with Chemical phenomena within kiln systems

and avoiding the study of topics which are more or less “legal”.

All are invited to consider a new division of chemical and engineering work with regard to cycles, transport, clinkering, emissions, interaction with refractories, combustion, sampling (gas and solid), etc etc.

A review by the Sub-Committee has produced a list of ideas, (see following page), these points generally relate to factors which have an impact on quality, cycles, transport, clinkering of emissions, refractory attack, combustion, sampiing of gas and solids etc. Some of them are more relevant to groups in the Technical Commission. These ideas remain for immediate discussion.

(NOTE :

It was subsequently agreed to concentrate initially on topics related to the

behaviour of sulfur).

C P Kerton Technical Centre May 1994.

LIST OF TOPICS

Environment: Factors effecting organic emissions and their control.

Process: Water injection in planetary coolers. CO SO2, CaS04 balances.

Control of kiln build-ups and their chemical composition.

Effects of secondary firing on volatilisation/condensation

in Lepol grates and

their control. Methods of successful use of even higher levels of S in fuels.

IMethods

allowing the retention of SO3 in clinker.

Control of chloride volatilisation.

Applications of CO, analysers.

Determination of dust cycles in kilns and heat and mass balances for each preheater

stage.

Optimum fineness of petroleum coke for kiln and precalciner burners.

Effects of volatile materials on the long term and short term stability of kilns and their consequences for static and dynamic conuol strategies

(allowing early pro-active responses t o change).

Control strategies (anticipatory control) to restore operation of a disturbed kiln (effect on throughput, environment, quality etc).

Bypass control.

Effects of geometry on cyclone operation.

Effects of volatile cycles and dust cycles on cyclone dip tubes and on various refractories.

Control of sulfur cycles at low temperature.

Effects of heating and cooling on refractories and linings; success with rapid regimes.

Effects of parameters other than CO on sulfur volatilisation.

Correlations of SO2 signals with other process parameters.

Effects of injecting and additional fuel at different points in the process (for example solids at the kiln feed chute). Distribution of trace elements throughout kiln systems, and methods of control.

Interaction of volatiles and refractories.

Influence of SO3 on refractor+ life - both direct (chemical) and indirect (perhaps a lower BZT).

Correlations between SOS and free lime in operating kilns.

Effects of V and Ni (coming from coke) on refractories. Correct regulation for burners for different levels of coke fineness and coke mixtures.

Maintenance: Corrosion in colder zones of kilns (in relation to Cl and S)

Workplace hygiene and safety aspects from the point of view of volatiles

Product/Quality Treatment and use of dust rich in volatiles

Methods for the internal use and/or upgrading of dusts which cannot be dumped.

Effects of V and Ni (coming from coke).

Effects of SO3

and of sulfate/alkali ratio on clinker quality at different

levels of free-lime (is there an optimum level?)

Effects of marginally reducing conditions on quality.

Effects of halogens on cement behaviour (standards, etc).

Effect of clinker size grading on quality.

cix/tla 13.9.94 &4:x94.049

RT lo9 pot 1b.Col)

:A -.L---

-L- -Ii---.1; -_ I. 1

Tempercrure “t 0

2 0 0

coo 6 0 0

6 0 0

I0co 1209 ISGO I~0 la^OO

-/. i .I. . zuqo 5-l. :uq,sa iForrrn.e -157

I

j-2

oxides.

.-- l..,*--l---*-l.,-~l k

; .-...--

. f ( .*--

* . . .

“““I ***-I---*!~i---I “-i-y”---I~--l--

i;‘: I- J , ,

0 L (I

LUU

4uu

: Ill1 III I Ill1 1000 1200 -Pm 7600 Eta BOO

Free cncrg diagram for sulfates.

-20

I

I

/ I

I

i

I

!

8

j

i

T2mgrz!lLii2, Y

fret energy diasram ior chiorid:s.

j :

;

i

j

,

1 1

I

-no -

Tempcraturc,=C

Fret cncrgy diagram for fluorides.

\ \ ..

j

-

!

L

I i

-

-

t \ ‘,

450 400

350 3 0 0 zso 2 0 0 7.50 1 0 0 ‘s 0 0

L

20

30

SO

40

60

7 0 Dhbor

3-90

- -43.447

+

8 0

90

1 0 0

R-t

- .a65

110

1 2 0

13*



4.042* ~&bor lSOp

3.1

Ensmble

5.3

4.2

2.1

Cl:

2.1

2.3

0.7

0.8

450 400

350 3 0 0 zso 2 0 0 7.50 1 0 0 ‘s 0 0

L

20

30

SO

40

60

7 0 Dhbor

3-90

- -43.447

+

8 0

90

1 0 0

R-t

- .a65

110

1 2 0

13*



4.042* ~&bor lSOp

3.1

Ensmble

5.3

4.2

2.1

Cl:

2.1

2.3

0.7

0.8

ENCI RELATION MG/M'.SO~

I

X

- %

02

1. General Information is presented here against the background of equipment/processes encountered outside the cement industry, where acid gas abatement may already be practised - and where ideas for transfer to our industry may originate. Our most frequent needs are to improve SC+ capture by calcium compounds in raw mills and/or compensate for the absence of this absorption when mills are not running. Whilst there is a 50 to 75% reduction in SC& levels in a number of cement Works when hydrated lime is suitably added, some sites need to understand why they record drops of only 20 to 40% and there also is an interest in better understanding the possibilities for using of alkaline sorbents. This note aims to provide suitable background information as an aid to better understanding when the complications and costs of the “Untervaz Solution” may have to be accepted. 2. Sorbent Injection 2.1 Basic Description of Technology and Principal Variations Sorbent injection is used primarily in pollution abatement as a means of reducing emissions of sulphur dioxide (SO-,> and other acid gases, such as HCI and HF. Material is usually injected into the gas stream as a fine powder, where it reacts with the acid gases, generating a dry product for collection in dust arresment equipment. Dry injection methods are particularly suitable for small boiler and incinerator plant or retrofit applications where the capital expenditure for other systems is prohibitively expensive. The efficiency of S&- removal is 40 - 80%, depending on the sorbenc used (most commonly calcium and sodium compounds). The sorbent can be injected at various points in the plant, according to the temperarure and conditions at which it is most reactive. The most common systems for boiler plant are: Furnace injection of calcium based compounds Heat exchanger injection of hydrated (slaked) lime (Ca(OH),, Post furnace injection of Ca(OHk at relatively high humidity Post furnace injection of sodium based compounds. In a cement process, we may see: “Classic” SG- capture in the lower stages of the preheater Hydrated lime injection at top of preheater or in conditioning tower (probably as slurry) Return of calcined meal to cooler parts of system (preferabiy with moisture) Wafer injection at suitable points to increase possibility of reactions with raw feed Hydrated lime addition to raw mill Injection of alkali compounds or solutions to exhaust gas duct SG- capture by limestone in raw mill in the presence of moisture. (Some different possibilities for injection are envisaged for Lepol systems.) 2.2 Principle of Operation The reaction between sulphur dioxide and dry sorbent is a heterogeneous one . (Reactions with moist sorbents are discussed later in Section 3.2). SC& molecules diffuse through the gas stream and are adsorbed on to the sorbent surface before diffusing into internal pores where chemical reaction occurs.

These mechanisms are exemplified in the classical kiln/preheater sulphate cycles. Dry calcium sorbents react with sulphur dioxide as follows: The first stage is calcination: CaC03 + CaO +CGCa(OH)z + CaO + Hz0 (These reactions occur at temperatures greater than 760 and 570°C respectively. Dolomitic limestone starts decomposition at a lower temperature.) The second stage is sulphation: CaO + SG_ + Y.Q + CaSO, With excess oxygen, complete oxidation to sulphate occurs at temperatures ahove 800°C. Below this temperature a mixture of sulphate, sulphite and sulphide is formed. (The optimum temperature range for direct reaction with hydrated lime is listed as 130 - 180 “C.) Sodium compounds react with SO, as follows: 2NaHCO,-Na&O,

+CG- + Hz0

Na&O, + sa* + ‘ha- -, lN&-so, +co, Sodium bicarbonate decomposes to sodium carbonate, which then reacts with S%- to form sodium sulphate. The these reactions are significant at temperatures above 130 - 180°C; at the lower temperatures in this range the favoured SO, reaction is directly with the carbonate, but at higher temperatures the thermal decomposition and sulphation reactions occur simultaneously. It is thought that the good performance of this sorbent may be explained by the fia that a shrinking core of bicarbonate is continuously decomposed, providing moisture at a fresh reaction interface for S%arriving through a permeable outer shell of sulphate A similar scheme is seen for potassium. 23 Selection and design considerations Dry sorbent injection is usually one of the cheapest abatement options for SG- removal, particularly for small or retrofit plant. as the capital cost is low. The choice of sorbent is a prime consideration and depends very much on availability. Calcium compounds used for dry injection are primarily naturally occurring limestone or dolomite or hydrated compounds derived from these raw materials. Reactivity is dependent on pre-treatment as well as natural properties. The sodium sorbents of interest are sodium bicarbonate (NaHCO,) and sodium sesquicarbonate (NaHCO,.Npl_C0,.2H,0). These occur naturally as nacholite and trona respectively. In making a choice, process economics are highly dependent on delivered price of reagent and rate of use, despite the initial lower cost and comparative simplicity of operation of the dry post-furnace injection processes. There are three temperature windows for calcium sorbent injection in boiler and incinerator systems, which also broadly apply to cement kilns. (Note that there is a temperature zone where neither group of reactions is very effective, especially in dry conditions.) Calcium sorbents can be injected directly into the furnace, where at temperatures of 1100 1250°C the calcination and sulphation reactions can occur, Calcium hydroxide Ca(OHX_ will react with SO, at about 550°C and hence can be injected before the heat exchanger.

At high levels of humidity calcium hydroxide will react with SO, at even lower temperatures (5 - 15°C above the saturation temperamre of the gas, as discussed in Section 3.2). water can be injected with the calcium sorbent into the duct between the heat exchanger and particulate abatement equipment or elsewhere Sodium sorbents are also injected in boiler plant to the duct between the heat exchanger and particulate abatement device, where the temperature is in or above the range 130 - 180°C. It is possible to inject sorbents at several points in the plant and to combine this technology with other abatement options. (No accounts of multiple injection systems are known for the cement industry). Handling problems may be encountered and not all sources of lime are equally effective in reacting (at a given fineness) in the time available at the point in the process where introduction is feasible. Reactivity increases as the surface area of the sorbent increases (particle size decreases), up to 40 m3/gm. The calcium:suiphur ratio is generally set at 2, but can be as high as 6, particularly for low sulphur coals where the mass of sorbent is still comparatively low. The reaction efficiency for limestone ranges from 40 - 50% at calcium:sulphur ratios of 2 - 4. Dolomites give greater conversion efficiencies, and this is attributed to the more open structure of the sorbent material which enables greater diffusion of gas into the pores of the sorbent. Conversion efficiencies of 70 - 80% are achieved with sorbents of hydrated lime Ca(OH), at 8OO”C, and sodium compounds also yield conversion efficiencies of 70 - 80%. (Use of a fabric Nter for particulate abatement is claimed to enhance SO, abatement efficiency by ca. 10% because the sorbent collected on the filter bags continues to react-with SO, during particle filtering. Dry sorbent injection to the filter bags after a cleaning cycle, is claims as an altemativelsupplementary method to enhance reaction.) Superstoichiometric quantities of reagent are usually needed because efficiency of reaction is low, and recycle may also be required. Efficiency may even be insufficient for elimination of either very high or very low SC& concenrrations at a realistic stoichiometric ratio. Use of “conditioned” (ca. 10% moist) hydrated-lime has been claimed to give improvements due to (a) breakage of particles when brought into contact with hot gases (so generating more surface area) and (b) cooling, which increases reaction efficiency. There are no known reports from the cement industry on this point, or on reactivity of different sources of limestone or dolomite. Limestone is the cheapest sorbent material currently in use. Lime (calcium hydroxide) is about 5 6 times more expensive than limestone and trona/nacholite are generally ten times more expensive than limestone. Some studies suggest N&HC03 becomes still more efficient at higher temperatures ( e.g., up to 815 deg C); it costs 2 to 4 times as much as hydrated lime. although consumption may be lower and there may be less residue to dispose of. (There is little or nothing known about the use. of alkali compounds at relatively high temperatures in the cement industry.) Combustion systems using high sulphur coals yield the most promising results where SO, levels are 2000 - 4000 ppm. At SC& concentrations < 1000 ppm the reaction is diffusion limited and it may be more difficult to achieve desired levels of efficiency. (In general, this technology is considered to be less efficient than the “wet” methods described later.) In boiler systems, the added sorbent and its interaction with the fly ash, can cause fouling of surfaces. Also, higher particulate loadings, decreased particle size and increased electrical resistiviry of the particles can impair the performance of collection devices. Handling and disposal of larger quantities of solid waste with properties different from fly ash or conventional scrubber sludge can be difficult and increase costs. For example, sodium salts are soluble in water and hence disposal requirements are more stringent.

3.1 Future Developments Research is continuing to enhance knowledge of-the appropriate mechanisms acting in these injection processes, with a view to developing alternative, moreeffective sorbents. For example, alkali metal additives in limestone enhance SO, abatement efficiency and early indications are that lime-containing waste materials, such as carbide mud and sugar mill mud, react faster and have a greater sorption capacity. The use of regenerable sorbents such as calcium silicates is another possibility. 3. Spray Dryers 3.1 Basic Description of Technology and Principal Variations

Spray drying is a standard chemical engineering operation used to produce dry powders of controlled particle size, density and moisture content Spray dryers are used in pollution abatement for the control of acidic species in a flue gas stream. Droplets of reagent are contacted with the flue gas in a reaction chamber - probably a modified conditioning tower in a cement Works. Liquid is continuously evaporating from the droplets in the chamber during the neutralisation reaction and the dry reaction product can be collected at the base of the chamber or in the dust abatement plant. A complete system consists of the spray dryer (atomiser and reaction chamber), associated slurry/ liquid handling equipment, a particulate collection devise and soiids recycling equipment. There are three types of atomiser in general use: rotary, two fluid or spray nozzles The reaction chamber can be a tower or dust, and the flow of the droplets and flue gas stream are usually co-current. Lime slurries are most often used, but sodium carbonate/bicarbonate solutions are also acceptable. 32 Principle of operation a) Lime spray driers

The atomiser generates dropiets of lime slurry which are injected into the flue gas stream in the reaction chamber. In the capture of sulphur dioxide, the chemical reactions which occur involve water and are believed to be: Liquid phase:

CaC03 + Sa- + %H,O -. CaS03.%Hz0 + Ca-

Gas/liquid phase:

Ca(OH)l

+.Sa- + Hz0 - CaSO,.‘/iHIO

+ lXH,O

Sa- is absorbed in the aqueous phase of freshly atomised droplets forming suiphurous acid, where the reaction of SO-, with lime or limestone proceeds rapidly, forming calcium sulphite which may later be oxidised and form gypsum in the presence of oxygen and water. As the droplets pass through the chamber, water evaporates to yield a porous particle which has a dry surface but a wet interior. Sa- diffuses into the wet sore of the particle and the reaction continues. The reaction of SO, with lime in the absence of any moisture is slow. Consequently, in order to extend the reactivity of the lime in the unit, the temperature near the exit is maintained just above the saturation point of the gas. As mentioned earlier (Section 2.2), these reactions can be involved in sorbent injection in cooler parts of a cement production line, for example when Ca(OH)z is injected to the preheater or Sa- reacts with limestone in the raw mill. In the absence of water, however, the reaction rate will be very slow - for example at the top of a preheater tower. Water injection to the preheater at Santa Cruz (without

adding any extra lime) was reported to allow 10-20% reduction in SO, levels. Failure to compare humidity levels and/or use fresh lime may account for several differences in experience of SGcapture in kiln systems. b) Spray driers using sodium salts

Dry S&- reacts with sodium carbonate/bicarbonate from low temperatures: and hence the requirement to enhance the reactivity by stringent control of temperature and humidity is not necessary. 3.3 Selection and Design Considerations The principal design parameters for spray dryers are droplet size and distribution, and inlet and outlet temperature. Multiple atomisers are used in order to achieve an even distribution of droplets in the reaction chamber, and the droplet size has to be such that the rate of evaporation is fast enough to prevent formation of scale in the reaction chamber as droplets/particles strike and stick to the walls, but slow enough to enable the reaction to occur. High inlet temperatures enable more water or lime to be injected, and low outlet temperatures (slightly above the saturation point of the gas) optimise the abatement efficiency of the spray dryer. Fine sprays and concentrated reagents have shorter drying and reaction times. Water evaporates from concentrated reagents rapidly and hence the neutralisation reaction occurs mainly between the acid gas and the porous particle. Fine droplets ( < 100 pm) are used with size tailored to the residence time of the flue gas and droplets in the chamber or duct. The residence time in a chamber is usually in the range 5 - 10 secs, with droplet size < 100 pm. For injection of slurry into a duct, reaction and drying times of 1 - 2 secs are typical. Residence times and evaporative heat available in an existing cement plant conditioning tower or gas duct system may limit the amount of SO, which can be scrubbed. The choice of sorbent will depend on its cost and availability: sodium salts give better “once through” efficiencies, but lime has a cost advantage over trone/nacholite and the calcium based reaaion product is insoluble in water which renders disposal easier, should this be necessary. Spray dryers have been successfully used in Europe for controlling emissions of acid gases, primarily for combustion plant and incinerators, using a lime sorbent which is recycled to improve its utilisation and achieving abatement efftciencies of > 99% and > 90% for HCl and SG- respectively. Efficiency can be enhanced by increasing the stoichiomeuic ratio for specific conditions of temperature and humidity, but the gains are limited by sorbent utiiisation, sorbent solubility and waste disposal costs. The Ca:S stoichiometric ratio is typicaiIy in the range 1 - 1.5 and liquid:gas ratios in the range 0.027 - 0.04 l/m3. Spray dryers offer several advantages over wet scrubbing, especially the fact that a dry product is formed which is easier to handle and dispose of than a liquid effluent. The capital cost, maintenance cost and energy requirements for the spray dryer system are lower than for wet scrubbing plant although reagent costs are higher. The particulate collection device can influence the operating conditions of the spray dryer. Acid gas removal can continue in a fabric filter but care has to be taken to prevent blinding of the bags. Electrostatic precipitators, however, can operate at temperatures nearer to the saturation point of the gas, hence the spray dryer outlet temperature can be lower which improves its abatement efficiency. The dry product from the spray dryer (hydrated &SO,) can be used for landfill, processed to yield anhydrite or pelletised to yield synthetic aggregate.

3.4 Scrubbers “Absorption” is a process which involves mass transfer between a soluble gas and a solvent in a contacting device; chemical reaction may or may not occur In process design, both the chemistry of the system and the physical structure of the equipment must be considered. Unfortunately, water alone is not effective at removing SO, from a gas stream because (unlike HCI) it is not very soiubie: an alkaline solution is needed The driving force for gas removal is the difference between the partial pressure of the soluble gas in the mixture and the vapour pressure of the solute gas in the liquid film in contact with the gas. Mass transfer occurs by molecular diffusion across the interface and the rate determining step can be in either the gas or the absorbent phase. When the gas is very soluble or reacts chemically with a reagent in the sorbent, the process is “gas phase controlled” Trace gas removal systems can be categorised by the solubility of the gas and by the reactivity of the system. SO-, is classed as “moderately soluble” in water (I-IF and HCI are very soluble), and sodium sulphite and-alkaline compounds are used with some success as additional reagents. Efficiencies of SC& removal of some 99% are attained in appropriate circumstances. There are no known accounts of the use of sodium sulphite solution for SO-, capture in the cement industry: usage in power stations generally appears to be associated with systems which treat the resultant chemical products to regenerate the solution of sulphite sorbent, or systems in which the sulphite is used alongside other reagents, providing an initial capture of sulphur as an alkaline compound for subsquent displacement reaction to form a more readily disposeable or saleableby-product. Gas absorbers which attain gas/liquid contact by bubbling dirty gas through a liquid are suitable for absorption processes which are “liquid phase controlled and those which involve spraying liquid through the gas stream are suitable for processes which are “gas phase controlled”. As absorption is a rate process, the concentration gradient (driving force for the reaction) and the (high) surface area of contact between the liquid and gaseous phase are crucial design parameters. The surface area is determined by the packing material or droplet size and this is usually achieved using packing materials which are coated with liquid or by droplet/bubble formation. The absorber design also has to provide a means for renewing the liquid absorbent so that a high driving force for mass transfer is maintainedGas and liquid flow rates and pressure drop across the absorber influence the driving force, the efficiency and in some cases the surface area (droplet formation). A good gas absorber design removes as much pollutant as possible in as small a space as possible. The choice of equipment depends on the abatement efficiency required, the energy and reagent requirements and the properties of the dirty gas stream.

c. P. KERTON, Blue Circle Te&nicai Centre, Greenhithe, &lay 1994 (Updated, July 1994).

- I r’

n

l-EMPERATURE C O N T .~R O L WATER

FLUE GAS BYPASS AROUND DRYER I- - - - - - - - - - - - - 1 I

I I

FLUE GAS ~no:.i POlLEn

+ C

J--J-

SPRAY DRYER WASTE RECYCLE

4-J

FABRIC FILTEH CATCH RECYCLE

L

E A N FLUE GAS

TO ASId DISPOSAL BIN

rBALL cc: M I L L _STLAKER

DETEN.TlON SLAKEFI NOT SHOWN a-l;.-,, FEED PUMP

APPENDIX 2

TEXT FROM INTERNATIONAL SYMPOSIUM ON GAS CLEANING AT HIGH TEMPERATURES Behaviour

of Volatile Materials in

Cement Kiln Systems

GAS CLEANING AT H I G H TEMPERATURES Edited

by

.

? q v

snlls o f riinny clicrnicnl (‘l’trc: usit ccnIc11I iidilslry convcnliun is loltuwrtt iii cxprcssinfi I’ atialyscs ill Icrrris of cIxitlcs. c.g.. CaO, S O , , hl,O,. clc., u s u a l l y 018 a ‘loss [ICC” b a s i s . i.e., allcr allowing, fnr ihc Ins5 ill wciglrl cvcnlually crpcricflcut rluc l o tlcslruclion o f clrbonalcs. clc.. during Iml lrcnlillcnl.)

“\‘01,A~1‘11.1~~S5”

ANI) hllXTIIANIShlS

01; VOI.A’1‘II,ISA-I’ION

‘1.11~ pritlcitd volnlil~ ~ICIIIC~IS arc K, N;I, Ct. S. I n Ihc cast of’ r a w nnlcrints, ccrlain sutrur cor~~t~~ur~d~ (rulfirtcs or ort:anics) c:ul rratlily clcco~~rt~sclvolnlilisc Indow Goo”C, bul IllOSl vdalitc co~ntmunds in raw inalcriids o n l y c v a p o r a l c pnrlially and a l higtlcr Icnitxralurcs as 111~ reed pa5scs low;rrtls llrc k i l n hrrnilig xonc. ‘I’hc rcsitluc rcmins i n lhc producl. cilllcr i n s o l i d ~cd~~iou ill IIIC plincitd phscs ol IIIC clinker o r as cliscrcic cocrqm~ncls. Whilst alnml all fcctl cllloritlr. w i l l cvatwmlc, lcs.scr aniounls 0r ollicr c o m p o u n d s do s o , wliilsl in corrlr:i’tl. liitd vcrlalilrs arc :ilii~o’;l ;ilw;iys ciilircly cvnpor;ilcd during cmiiIiIislion. lfv;rtKrrarrd volarilc~ rravcl hack up 11rc kiln wilt, 111c comlm~lion irmrt;;rnic comt~ountl’; (lilrcr;rlirrg I:~lwl Iical): i)

ii)

iii)

on llic Ucctl.

li)rmitlg lhc his

Or

n rccirculaliiig

eases and cordcrrsc a

inlcrnal votnlilc

s

load

n ;I lirrc rlrrsl r>r rcllrlc which i s fin;llty Irappcd i n lhc g a s clrxning s y s l c m o r r a w m i l l arrtl hx~mcs p a r t or an cxlcrilal vnlalilc c y c t c . a s 111151 i s parlly o r d$ly rl’lurncd ir1 tlrc sysic111 011 coltlcr strrl:rccs

ill lhc syslcrii. linming

llic Ixrsis

Or

hiltl~ups.

I’rcssurrs 10 cxploil cvcr more marginal rcscrvcs 0r r a w ni3lcrinls a n d fuels give rise l o irrcrcxirrg I;rmili;rrily willr llrc drm o f vohlilc spccics 011 process pcrlormarrcc. Wtrcri corrtlcrrsctt volalilcs rclurrr low;lrtls Ihc b u r n i n g zorrc, clctxriding o n Ihc overall clrcmicnl cr~nrliliorls and hrnint: ccrndiliorrs, llrry form a range or votalilc cnmpunds wlricll tlrcrr0clvcs cvapw;llc parli;rlly nri0 llrc cyctc only lids an utuilibriuni when ltx loiat quaillily Icarirrt: llrc syslcm (itI clirrkcr ;md mm-rclirrrrcd tlusl) equals ltral cnlcrirrc lhc s y s l c m . Alk;rlix

;md ~IIII~IICS ctrl~.rirrt! llrc trrrrrrirrp forrc irr trraclirc I:rrt:cly I’rlrrri :I \qr;lr;llc mr~llcr~ t’ll;lrI,,,,F ,w, yc:,r c;mcctl try I~ruln.:~tcr Irlcsk:rl;c 1’reun r~vc’r ‘)I) II) Icv\ 01:rn ill. 1~151 lirlrc ln~~rrs Il:rvinI: :II\~I I:rllr~n loom :~r~nmtl 4,511 Jk‘r yc:tr 111 :rlr~url IIIII. ( ‘ I hi.11. lvcrt’ :rl\o m:ijfn l::rin’; i n \I*rlt\ 1311wtl Ily rinl:\ :III~I Irr~3L;tw:1y\ ;II lhc kiln t’rllry rr.:ll.)

l3TECl~S

OF

CONDENSATION

Cast Y: A prccalcincr kiln ran well will1 a Cl lcvcl in kiln cnlry maIcrial of 3 IO 4% (aboui 0 . 5 % less 1har1 111~ K,O Icvcl) provided n o I r a c c of C O w a s indicald. (‘l’hcrc w a s aboul I . I % S O , i n I h c k i l n cnIry mc;ll i n Ihis siIuaIior1.) IC C O was tlc~cc~cd, Ihcrc w a s abnu1 2% S O , and SX K,O i n IIIC 1101 k i l n inlc1 l&cl. accompaniul hy b u i l d - u p s based o n SpurriIc (2Caz(Si0,).CaC0,) and c u b i c K C I cryslals. ( I I i s gcncrally rccogniscd lhal regular k i l n opcralion helps IO minimisc Ihc phcnonicnon 0r ccmcnlaliori by Ihc Crcczingl Ihawing Or cliloridc-linsul tlclx,siIs.)

The clrcc1 w h i c h rccirculaling vcd;IIilcs cxcrt o n b u i l d - u p rnrm:rIion a~ 111~ k i l n ( g a s ) cxi1 dcl~ntls ori coniposilion (governing Icmpccr;ilnrc or liquid lirrm;rIion :mcl Ihus lhc posilion and h;trclncss or Iiiriltl~ul1s as well as Ilic surRcc Icnsirm and viscnsily Or Ihc liquid condc~is:ilc) ‘I’hc p l a n 1 p$omclry and IllC a n d o n 1hc quanlily ( w h i c h gnvcrns ralc O r CurmaIion). ll~rougl~pul a l s o play a par1 i n m a k i n g clrccls mnrc or less prnnounccd. I n cxlrcmc cases. Cnr qrrcnching a n d scparalc dc-tluslirr~ l o par1 of 111~ k i l n P,aSCS ;ITC ‘bird” O F ‘by-passed’ rcmnvc volnlilcs . i n c u r r i n g linancial pcnallics i n plan1 cosl, complcxily a n d r0Ci use.

Cnsr In: T o c x n m i n c IIIC rcnsibiliry oC p r o d u c i n g a sulraic-rich clinker

I n

wiIhou1 insInlling

a

lhc pasl. s c v c r a l empirical

limils have been l~roposcd

rOr

conccnlralions

d

volnlilcs

~cmpcraturc in a WCI prnccsr k i l n , a considcrablc dab-bank c l I t.-6urcrncnls 1115 been buill up. The USC o f lnwrr a s h rucl lowcrcd c l i n k e r K,O l c v c l b y O.lS%. dcspilc 111~ inIroduclior1 of n IiIIlc mnrc p&ssium IO lhc s y s t e m ( a n cxlra 0 . I 16 o n clinker). T h e dusl - rclurncd IO lhc kiln - had bccomc more rich in alkalis, so lhal Ihe proportion or K,O broughl in by solid fuel fell lrom 2 4 % I O IX% while ihai brough1 i n b y dust rcIIIrn rose from 18% I O 2 9 % . This s~rp.gcs~s 1ha1 KzO incotporaIion in clinker no1 only dcpcnds on 1hc quan1ily inlroduccd 11u1 also . a n d almvc all clsc _ o n IIIC type of malcrinl w h i c h b r i n g s i1 i n a n d perhaps o n 111~ pnsiIimr whcrc i1 is injccIctl. On screening 111~ clinker al 20 mm. chcmic~l analysis showed a K,O coriccnlralinr~ snmc IO% Irighcr i n Ihc cn;~rsc f r a c t i o n . The clinker a l k a l i COIIICIII has s~~cccssli~lly been rctlucccl i n lrials b y c a l c i u m chloride atldilinri nl l h c Ilamc.

Cnsc 6: A hil:h chlnridc coal ( - 0. I5 % Cl) can only bc ~isctl as a mix will1 anolhcr coal lo avoid Imild-ullc wilh Iyliically 2 % Cl a1 Ihc ImI1nm Or cyclone 4 (lhc lowcsl in lhc prchcalcr ~nwcr). hn;rlyTis of Inriltl~ul~s along IIIC kiln indica1ul chlnridc lc,vclr up IO 30% (al zero loss nn ignirinn) in 111~ coaIing Iron1 lhc base of cyclone 2 and 20% a1 SO 111 inlo Ihc kiln (tlcspilc iit hs’i ih:m 5 % lrvcl i n Imllr lmi :md rold p;nI’i 0r Ilic k i l n ) .

C:1w 7:

Atldinl; a sccontl ln~~hc;l~cr c y c l n n c s~agc IO a l o n g d r y process k i l n ( a varinrIt of 1hc I:igutc I prnccss, will1 a higher k i l n IcngIh/dian~cIcr ralio a n d a single cyclone slagc nhnvc i1) yicldcd vnrimrs build-up problems. To resnlvc ~hcsc, IIIC kiln gas cxi1 0, lcvcl was iricrcA5rccl rrorll 0.5 In I .5 %, solid rd rcsiduc al 90 microns was rcduccd lo below 25 % and scvcral cmr~l~r~ss~d air “blaslcrs” wcrc inslallctl lo dislodge malcrial rrom lhe lower regions oC 111~ prchcnlcr. T h c s c acrions improved IIIC siIuaIior1 and sul~scqI~cnIly addiIional mcasurcs wcrc Iakcn: rcl~/l;iory slirrcrs wcrc addul l o I h c l i n i n g near the k i l n b a c k end, hurncr a i r vcltXiIy w a s incrcawd. a n d a “non-slick” lining was inslallcd in 1hc kiln cxi1 gas duel and cycl011c dip-lubes. ‘I’hcsc Crrm mntlc bcllcr ouIliu1 ralcs pnssihlc wilhoul b u i l d - u p s . IIc~cnIly. a Iiil;hcr snll’ur rd hlcntl 113s been nscd, :~ccomp;mictl b y s l a g (S - I X) :Imong lhc r;~w niix coml~~ncnIs. I’rchcaIcr blockage problems rccurrcd, bu1 b u i l d - u p s c;m be avoidctl if the SO, l c v c l i n snmplcs t a k e n Irnm the k i l n cnlry m a l c r i a l i s kcp1 b e l o w 2.5% b y lirnilirig lucl S conlcnl and sl;ig use i n lhc r a w m i x , provided lhal i n addilion l h c oxygen lcvcl at IIIC kiln back cm1 is kcp~ consis~cn~ly a1 or above 2%. Cnmpui~r c0nir01 or ihc kiln Iiclpr I n achicvc succc’i’i. rctlucinl: lhc vari:rhilily ol l h c 0, s i g n a l .

cxw Ii:

I ;~li~n:ih~ry tl:11:1 on minor clcnic111s cr;~n~plc, suITur vnl;rIiliIy i n ;I s~;~rrtlarrI rcginrc 0, OUI I~lls in ihc prcscncc Or 0,; r~~~~rihclcss a l I2W”C. The volalilily 0r m i n o r c l c m c n l s powdcrrtl m a l c r i a l lhan rOr grarIulcs.

conlirm clvccls crlrxcrvr.tl ill ln:tclicc. I:rlr (70% N,, 3 0 % C O ) i s close I O 100% a1 0% ihc CrrCd Or 0, is much ICSS al 1400°C lhan i n l h c laboralory i s a l s o m u c h grcalcr r0r

Cast Y: A prccalcincr kiln ran well will1 a Cl lcvcl in kiln cnlry maIcrial of 3 IO 4% (aboui 0 . 5 % less 1har1 111~ K,O Icvcl) provided n o I r a c c of C O w a s indicald. (‘l’hcrc w a s aboul I . I % S O , i n I h c k i l n cnIry mc;ll i n Ihis siIuaIior1.) IC C O was tlc~cc~cd, Ihcrc w a s abnu1 2% S O , and SX K,O i n IIIC 1101 k i l n inlc1 l&cl. accompaniul hy b u i l d - u p s based o n SpurriIc (2Caz(Si0,).CaC0,) and c u b i c K C I cryslals. ( I I i s gcncrally rccogniscd lhal regular k i l n opcralion helps IO minimisc Ihc phcnonicnon 0r ccmcnlaliori by Ihc Crcczingl Ihawing Or cliloridc-linsul tlclx,siIs.)

1m1 Inr some weeks will1 I h c objccIivc or reducing b u r n i n g ronc b y - p a s s . ICSIS WL volaIili.~~tinn b y p l a y i n g o n process paramcIcrs a n d prnducing a IighIly mincraliscd clinker will1 higher volalilc rclcnlion. D u r i n g changcovcr ~hcrc w a s some I c n d c n c y l o rnrrn sol1 build-ups in Ihc prchca~cr, bu1 wiIh ihc new regime csIablishcd ihcsc movctl lowards lhc kiln fcul chu~c wiIhou1 c a u s i n g m a j o r problems for k i l n opcniion. (EvidcnIly lhcrc a r c phcrIon1cna of boll1 shorI-Icrm a n d l o n g - I c r m sIabiliIy: o n c e a slablc b u r n i n g zone volaIili.saIion i s csIablishcd, in lakes Iimc f o r sIablc condiIions I O arrive higher u p the syslcm and in Ihc Iargc masses or malcrial in lhc build-ups and coalings already in cxislcncc.) The apparcn1 b u r n i n g ~nnc IcmpcraIurc w a s rcducul b y almu~ 1 2 0 ° C . while K,O volalilily droplwl IrIlm 70% III 60% i n 111~ b u r n i n g ~onc a n d IhaI ol S O , Iron1 RO% Iowartls lhc ranl:c SO% I O GO%, provitlcd lhnl k i l n cxi1 oxygen l c v c l w a s kcp1 a b o v e 2 % . Thcrc wcrc in~provcn~cnIs in kiln outpul raIc and rucl consumpIion and lhc cxpcrimcnlal Works adoplcd ccrlain Or Ihcsc changes during normal nl~ralinn rOr scvcral years. TCSIS wcrc c;Irricd 0111 invnlving variotrs N O , lcvcls ( t o Cnsc I I: IcmpcralrIrc) a s well as chcmic~lly rcrhlcing rh0C contliliruIs. so, :

K,O a n d Na,O :

indicalc

llamc

‘I’hc r a l i o or S O , i n Slngc I V I n S O , i n r a w iri~l varied Iypically Iron1 I.R IO 2.7 lnr 111~ higher lcvcls ol NO., and was 3 . 0 [or a IOW 0, Icvcl. The clinker S O , c~nlcnl fell. In a parallel manner. for K,O 1hc ralin of l h c c o n l c n l i n S~ap,c I V IO Iha i n r a w meal varictl from 3.X IO 4 . 4 a n d rnr NazO rrom I .6 IO 2.0.

In gcncral, rultrcing cnndiIions a lower clinker SO,.

incrcascd SO, lcvcl a1 Slagc IV by a

hClOr 0r

2, also giving

CXSC 1 2 : S O , 11~s l~ccn tmmit~rcd ;II lhc kilt1 Iwck cd IO dclcrwiw hrc:~l rules for avoitlinl: hltKk:igc Icndcncics. ‘I’hc S O , sil:n:rl i s ncpisy :~nd dillicull l o inlcrltrcl willroul a krtowlAl:c or IhC hiSlWy Or IllC SySlCltl. C.G.. a rcccn~ brcak;rway of sulfalc b u i l d - u p m a l c r i a l a r r i v i n g i n IIIC lmrning ~nnc c a n give a h i g h S O , s i g n a l a1 111~ k i l n back end dcspilc tlrc prc~cncc ol Icr (,I k i l n >I,,,,F ,w, yc:,r c;mcctl try I~ruln.:~tcr Irlcsk:rl;c 1’reun r~vc’r ‘)I) II) Icv\ 01:rn ill. 1~151 lirlrc ln~~rrs Il:rvinI: :II\~I I:rllr~n loom :~r~nmtl 4,511 Jk‘r yc:tr 111 :rlr~url IIIII. ( ‘ I hi.11. lvcrt’ :rl\o m:ijfn l::rin’; i n \I*rlt\ 1311wtl Ily rinl:\ :III~I Irr~3L;tw:1y\ ;II lhc kiln t’rllry rr.:ll.)

l3TECl~S

OF

CONDENSATION

The clrcc1 w h i c h rccirculaling vcd;IIilcs cxcrt o n b u i l d - u p rnrm:rIion a~ 111~ k i l n ( g a s ) cxi1 dcl~ntls ori coniposilion (governing Icmpccr;ilnrc or liquid lirrm;rIion :mcl Ihus lhc posilion and h;trclncss or Iiiriltl~ul1s as well as Ilic surRcc Icnsirm and viscnsily Or Ihc liquid condc~is:ilc) ‘I’hc p l a n 1 p$omclry and IllC a n d o n 1hc quanlily ( w h i c h gnvcrns ralc O r CurmaIion). ll~rougl~pul a l s o play a par1 i n m a k i n g clrccls mnrc or less prnnounccd. I n cxlrcmc cases. Cnr qrrcnching a n d scparalc dc-tluslirr~ l o par1 of 111~ k i l n P,aSCS ;ITC ‘bird” O F ‘by-passed’ rcmnvc volnlilcs . i n c u r r i n g linancial pcnallics i n plan1 cosl, complcxily a n d r0Ci use.

.“c]%:,qv%3

!:J’(~OS)%3)Z’r,>r

alI1

JO ?Illl,Zl

01 ,~",~'"~,I 'I\l.,ll"dlllO.3

:SMfl[,oJ

I\laJ7JJ!p

Al,'

15,ll!

a\Il

\I!

II! aSl!i,d

t,',l,aA?p

W!J

UlOllOq

Wt!Al!J3~l!

II! &OR,,

uv s! 2~311.1.

JW!?l,"J',

thl-,l,l\lq

2110

Z?:~\!l~.!l,Wll

311,

I,! \\1!,,,

,31!,\1!~.)

,N,"F/S!!?

xJno3

(lllthl!

ll,!J

1117111.75

a\(lJO S,i,l?!Jl!,?

JO JJl\ll!ll

,C!lW"d XII JO JOICXpII!

\l"!l\!,SllJ.W.,

1 11 > II

l,,,l!,,,s

=

11

Ill! SC p;r

JlpIO JJC aJXI.,,)

R'I - 2'1 %

S’b - 5’1

s-i - S’Z

in

(OZ1!,-.J ‘OS

‘t??) S!,l!J,V

p"l! ilJX3Jd ~,l!!l.,ll!lll J;ll,lO J" IlO!,!S"dUlO~ al,, 1," JO ,aAJ, J31,$, ,! \,,!M JII!JXiO 0, T,,q!SsOd "'1 i(,!lll ,,

X3,NI!

Oh!N IWO.,,) $'I, .I. lllt,l!l (1~3 I'!l".,, lililiI~~~~,~IiI~~~,~~,,~~

AIll!

,TJ\:,,

,+A!,'!,>), Sth-,3,!lll,

Sth-p,!lN, lJ"s '0s "0 ,,Xl!I,

X,!SC.,)

anp II! sthl-p,!nq pxcq.aiciwqlc~

273111Os3J a,,, ,'!ll,'!A!pll! J\ll"S

ST! 'thl-pI!"',

(,,lAOlllaJ

SSa2OJd

l',!Z, XII ll! ti,ql!J2,Ol JW?I,T)Jd JOJ Jaytl!,:,

8110!11!1111W110.7 k&,.X,C 0, J;r,IaJt, 0, k,\l.?(111.7) 11" ,3 '7A["'" '.a.? 'll,!ry l! \I! 5,q!~~!lll,w!

:(~plllS JnJ lS.7JW! ,l?!J"dS JO %l!Sq SW!K,XJX.,) W3lll:J %l!AW,,OJ X,1 \I! Il.,+, d,,I!Jlt,~l JW!.71,2Jd l! JO Sdl!lS J7h\O, X,1 II! p.,ll!J;l,"l X, Ill!.7 1,2!',"\ S;l,!ll!,nA JO S,,",,\!J,,,l.),,,,r 'll0!11!~!~1!! I! S! 71.71,1

ll[I

S&l-,l,!no '(~J\llCJX,\Wl

',,%?lS

'Slll~Il,'lJl, "II\!JJlltk DJF. ~~,'hllC~:, ,Xl,J!],,3,~! JJI! SpWlOdlll~~ W,',lllO2 Ja\IlO pill!

.“c]%:,qv%3

!:J’(~OS)%3)Z’r,>r

::,‘(“oS)‘ry?(

:('O>r3'('0!S)'V~~) >\!JJlltk DJF. ~~,'hllC~:, ,Xl,J!],,3,~! JJI! SpWlOdlll~~ W,',lllO2 Ja\IlO pill! i! - "OS*>('"OSIQZ - a,!u!rtq%a, p,,l! p?l,S!,qC,S;, 7q hl!\ll l!!Jq!,!ll,,a JO SS;rU~~!l,l ~,,'!1',,c, UK, ,,,",a', ,,JAa ," 93llCJ

alI1

\IO!iCJl\tJJlJO3

UPJ

'!lI.I,

!alqw~~,,~

:SNl~l~~S 'SI~UllOJ3

:('pos)'y!\\,>J ,"!J~ll!\\l \I, 1," p?I.,X,Y? ,"I,

rrmyr3 sC t~o~wslr~pi~o~ wJ!p '9.2 '~JU~JOJ spltn~11110~ Mall pill! St,OJp ~JW!JX,lllJI ,CllJ~lll! 'SWaJJll! S3ll!Il!O.,

?I,1 SV 'StIrI-pl!nq arr!l!lr! - sm!~lrls I10 sl!scxl.,p lIl!rn Sl! _ ,wC Ufl!Sal[pC xne.2 ,fXIl !Sl1O!IX!~J ,,!"!"'~\I~ aY,OAOJd UCJ pXtJ >,,I 110 Sl!S"h~ '(Z aJn3!:,) Dew,* S,(1,11 p!ll,\!I JO \,"!,l!t\,JOJ 3\l!?.W,It! 'aJlll~J;lt,lll;ll

4c02(g)

+ 4 =2(g)

Beactions 2, 3 and 4 will only take place under reducing conditions and these will be discussed later. Of the two remaining reactions laboratory tests 4 on the adsorption of SO2 on cement raw meal have shorn that considerable adsorption of SO2 occurs in the temperatare range 600 to

goo’c according to the reverse of equstion (5).

6) 4 CaO(s)

+ 4

a2fg) 9 3 C=s4(s) + C=Scs)

From thermodynamic data we can obtain values of enthalpy, entmpy and Gibbs free energy for the reaction at different temperatures. Then using the Qan't Hoff isotherm the value of Kp (the equilbrium constant) can be found. DG

=

-2.303

%T Log

P

K -

(2)

F'rom the stoichiometry and assuming the actitivities equal to unity, it can be seen that Kp depends solely upon the partial pressure of &J ;,J/*L; 4.) & t h e From this knowledge we can obtain a curve (Figure =2* extent to which #e reactior6progreases to the right hand sic with _~._. --.---increasing temperature. Th2.s curve show that, at a kiln back end temperature of 1050°C all the SO2 would be in the gas phase vhere as over the precalciner temperature range (85O'C - 95O'C)

there would be

between 2% to 1446 dissociation to fern S02. This is within the 3096 low temperature volatillsatlon allowed for in the report STH 81/13. This equilibrium is for a static sitcation and does not consider the removal of SO2 from the system or the rate at which the reaction proceeds.

In the kiln/preheater system the compounds are constantly

being removed and replenished and so the equilibrium is also dependant on reaction rates and residence time of the compounds. This

analysis does shou however that the bulk of the SO2 will be generated from the decomposition of CaSo4 at a temperature greater than the typical temperature of the precalciner and so a bypaes at this Mgher temperature would reduce much of the So2 available for reforming CaSO

4

CaS in the precalciner. 3.2 Effect of Reducing Conditions on the CaS/CaSO, Equilibrium -t If sulphur containing limestone is roasted in air all the salphur is converted to C&O4 by the reverse of reaction (1)4. 7)

CaO

+

So2

+

8

02

+ Cas4

Reduction in the level of oxygen causes reaction (6) to be favoured which has bee,n found to be the major reaction in cement kiln exhaust gases. Further reduction of the level of oxygen causes reactions (2) and (4) to be favoured. The extreme case of reducing conditions is given by equation (3). If CaS is present a further reaction can occur with carbon dioxide5 according to the equation. 8) CaS

+ 3 co2

= cao

-+ so2

+

3co

!I'he temperature dependance of these reactions have been studied by Turkdogen and Olsson5 who obtained expressions for the equilibrium constants

10gpm2

(p~o/pco2)3

lo~W2

(pC02/pcO)

= - (20,000/T) + 9.27 =

-

(‘9617/T)

+

8.021

From these expressions the salphate/sulphide

(3) (4)

equilibrium diagrams can

be dravn vhich are shown in Figure 4, 5 and 6. These curves show that adjustment of the ol;ygen potential of the kiln atmosphere will adjust the level of sulphur retentio:n at higher temperatures.

and that this effect is more sensitive

At the relatively lov temperatures, 95O'C and

ii) the presence of potassium containing minerals very much enhances the decomposition of CaYO

4

when compared to the effect of analogz

non potassium co&ainiry~ minerals.

These obseroatione are valid

for temperatures below 12OO'C and at atmospheric pressure. The presence of minerals which enhance evolution of s02, by the same mechanism will obviously retard its recapture. This could mean that SO2 is lost from the system via the stack in greater amounts than that predicted by STN 61/13.

3*5

Effect of the presence of orwic sulphur contaminoua..compou.nds Another form in which sulphux can enter the kiln syetem is in the form of vegetable matter. !&is vi11 evolve SO2 at very much lover temperatures than mineral based

. If this was the mjor form of sulphnr in the rav % materials, this uould mean that almost all the raw material sulphnr would be lost via the stack causing the bypass tc be very much oversized. The effects illustrated in sections 3.4 and j.5 can be investigated by eqerimerit and would ahow up as enhanced volatility at lair temperatures. Experiments carried out on the Oxford raw material have not shown this fn be the case.

3.6 Effect of sulphur in the fuel: Sulphur in the fuel will be present in the kiln and precalciner gases a8 From the discussions of their reaction of CaO and SO2 (section j.1)

=2it was stated that considera't)le amonnts of sulphur vould be absorbed at

600 to 900'~.

In fact the peak rate of absorption for SO2 is at 850°C5,

Other factors affecting the amount of sulphur absorption are the concentration of CaO and S02, the surface area of the CaO and the . exposure time. In a kiln and preheater system the time for vhich CaO i8 exposed to So2 at a temperature where absorption can take place 18 very short. HoweV8r the fineness of the CaO particles means that an enormous surface area of CaO is presented to the gas stream. This is generally the overiding factor and the reaction vi11 tend to reach equilibrium very quickly.

Hence in the stage IV preheater cyclone8

there is almost 100% absorption of SO2 by the feed. With a precalciner the reaction is complicated by two streams of gas and material, at different temperatures combining. The precalciner gas stream will contain So2 from the fuel. The precalciner _.. material stream carries‘bffsolid of increasing CaO content (see ?ig. 8). Eouever there will be little recombination of CaO with SO2 in this stream, due to the gas temperature of approxinrately llOO°C. (see Figure 4). This will combine with gas and material streams from the kiln in the riser duct from the kiln.

(Por the RSP ONOQA system shorn in Figure

8, a mixing chamber is used),,

This material stream will also contain

CaO and So2 from the kiln fuel and rav materials. The gas temperatare is also around llOO°C and so there &ill also be very little combination CaO with S02.

As the gas and material progress up the riser duct the

endothermic production of CaO from CaCO streams.

3

coolstthe gas and material

This makes absorption of SC2 by the time more favourable.

The absorption should have reached a maximum at the point uhere the gas and materials streams separate. be close to the equilibrium