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Pet coke in cement clinker

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Petcoke in Cement Clinker

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Pet coke in cement clinker

  1. 1. What is PET-COKE * Petroleum coke is a by-product / undesirable product of oil refining cracking process. * The most attractive thing for use of Pet-coke is the high calorific value having lower cost as well as lower handling cost per unit of heat content.
  2. 2. AVAILABILITY * Petcoke can be made available to cement industry by Petro Chemical Industry as it is as by product and imposses disposal problem. * Petroleum coke (Petcoke) produced around 2.0 million tonnes per year. It can be made available at : * Reliance Petroleum Limited, Jamnagar, Gujarat. * Indian Oil Corporation (Assam Unit) * Hindustan Petroleum
  3. 3. PROPERTIES OF PETCOKE * Petroleum coke is a waste-product of oil refining cracking processes. * When oil is subjected to extreme cracking and practically all the liquid phases are driven off, a residual product remains consisting mainly of carbon. * There are two commonly used processes for coke manufacturing namely ‘delayed coking’ and ‘fluid coking’. * Although there are three to four categories of petcoke, the ‘delayed coke’ is mostly used as a fuel.
  4. 4. * In comparison to coal, pet-coke is categoriesed as a material with low volatile, very low ash content, high fixed carbon content and high sulphur content. * Depending on the type of crude oil, the sulphut content may vary between 0.7% to 8%. * However, mostly the sulphur content is high, typically in the range of 2 to 8%.
  5. 5. * A typical range of analysis for the delayed type pet-coke is as under mentioned :- Moisture, % - 6 to 12 Ash content, % - 0.2 to 2 Volatiles, % - 5 to 14 Sulphur, % - 2 to 8 Calorific value, K.cal/kg - 7500 to 8000 Hardgrove Index - 35 to 70 Bulk density, Kg/m3 - 0.77 to 0.88 Lump size, mm - 0 to 75 Fixed carbon, % - 88 to 90
  6. 6. Utilisation of Pet-coke in Cement Manufacture Extent of Use. * The level of substitution of pet-coke in an existing kiln (having no by-pass arrangement) is decided on the basis of sulphur content in pet-coke and clinker. * This is because of the fact that higher concentrations of sulphur cause coating formations in cyclones and kiln inlet areas resulting into operational disturbances. * In extreme cases, kiln has to be stopped for cleaning of these build-ups. * Consequently, the availability and hence productivity of the kiln is adversely affected. * The decision of extent of sulphur input to kiln depends on the type of kiln and raw mix design (molar ratio of alkali oxides to sulphur tri oxide should be between 0.8 to 1.2).
  7. 7. Mode of use. * Pet-coke can be used by two methods. * One is to mix it with the raw meal. * In this method the reducing atmosphere is created. * This may result into change in temperature profile and increase in preheater outlet temperature and uncontrolled CO formation. * In preheater cyclones, the reducing atmosphere may cause clogging. * Another method which is being used widely, is to blend pet-coke with the fuel firing in calciner and kiln. In this method of use, it must be ensured that the pet-coke is burnt properly. * It is also possible to introduce pet-coke along with the raw meal and simultaneously fire along with fuel in calciner and kiln.
  8. 8. GRINDING * Pet-coke being highly abrasive and hard, is difficult to grind. * The grinding system is designed to minimize the wear of mill components, ducting, transport system etc., * In most cases, the target for the fineness of ground pet-coke from the mill is 4 to 6% residue on 90 micron sieve.
  9. 9. COMBUSTION CHARACTERISTICS * Due to lower volatile content in pet-coke, it is difficult to ignite, burn and maintain a proper shape of flame. The ignition temp. Of pet-coke is 590-620o C as compared to the ignition temp.of coal 400-500o C. * In this case, the particles need a higher ignition temperature and a longer time to burn as the rate of chemical reaction is very slow. * An important factor influencing the rate of combustion is the specific surface area of the fired pet-coke. So, the fineness of pet-coke is increased to improve its burning behaviour. * Based on the experience of other plants, pet-coke fired in cement kiln and calciner is of fineness of 4% to 6% residue on 90 micron sieve.
  10. 10. Use in Kiln * If the plants have single channel burners for kiln then these will have to be replaced by specially designed Multi-Fuel burners. * Such a burner will ensure the complete mixing of pet-coke and air, burn out time of individual particles and creating the required flame impulse. * Burner must be suitable to handle highly abrasive material. * A little quantity of support fuel like coal with high volatile matter or oil or gas can be used to ensure ignition. * Several plants have successfully use pet-coke up to 100% in main firing.
  11. 11. Use in Precalciner * The desired conditions for the proper combustion of pet-coke in calciner are high temperature, high oxygen contents and high temperatue. These conditions can be achieved by the following modifications: * Design of ‘Hot spot’ in ‘In line Calciner’ by shifting upstream the meal entry point by about 1.5 - 2 m. A better solution may be the provision of a precombustion chamber. * The precalciner is designed for gas residence time of a minimum 3.5 seconds. * Creation of adequate turbulence in the calciner.
  12. 12. Advantages of use of Pet-coke The various advantages of using pet-coke as an alternate fuel are as highlighted here : * As compared to use of coal which has higher volatile contents than pet-coke, requirement of combustion air is less. Similarly, the generation of the flue gas is lower which results into saving in energy consumption of fans handling it. Alternatively, the reduction in gas volume can be used to increase the production from the kiln provided there are margins in other equipment in the system. * As the pet-coke is a high heat value fuel, the temp.of the flame increases and hence the heat transfer in the kiln improves.
  13. 13. * If the alkali contents in the system are high then some sulphur rich raw materials or high sulphur coals are used which may cost more. However, the pet-coke fulfills the sulphur requirements at lower cost. * Pet-coke is mostly available at lower price than coal. Hence, the specific cost of fuel lowers when pet-coke is used.
  14. 14. Limitations of use of Pet-coke A few of the limitations of the use of pet-coke are as under mentioned:- * Since the sulphur content in the pet-coke is high, the quantity of its use is limited by the molar ratio of oxides of alkalies to sulphur in hot meal entering the kiln which must be maintained as 1 to 1.2. On the basis of experience of the users, the total sulphur content in the clinker is maintained below 1.4%. * As the grinding capacity of the coal mills lowers by 50% to 80%, there should be adequate coal grinding capacity after use of the pet-coke. * If at a certain level of use of pet-coke, system is not capable of its proper burning (as indicated by the CO contents higher than 0.2%), then its quantity cannot be increased further.
  15. 15. * Temperature of the exhaust gas increases if pet-coke is introduced with raw meal resulting into increased gas volume and increased specific heat consumption. In that situation, the benefits should be worked out considering reduction in kiln production and increased fuel consumption. * Pet-coke has some toxic contents like vanadium, nickel etc., which go along with the dust. Hence, plants which dispose the return dust have to ensure that the concentration of these compounds do not exceed the limits defined by the environmental laws. * When cost of pet-coke is more than cost of coal or fuel being used, there is no economy of using it.
  16. 16. Properties of Pet-coke The various properties of the pet-coke proposed to be used are given as follows: Parameter Unit Value Moisture content (avg.) % 12 Size 50 mm sieve 100% passing Bulk density t/m3 0.9 Calorific value net K.Cal./kg 8000 Volatile matter % 9 - 11 Ash content % 0.2 Hard groove index 50 - 55 Sulphur % 6 Carbon % 89.05 Hydrogen % 3.5 - 4.0
  17. 17. Substitution level * The substitution level of existing fuel by pet-coke is decided on the basis of following criteria: Criteria Limiting value SO3 / Alkali molar ratio 0.8 - 1.0 Volatile matter incoal mix, % 20 (min.)
  18. 18. * Calculation was carried out for different additions of pet-coke in existing fuel. The results are presented as under mentioned: S.No Pet-coke SO3/alkali SO3 in Volatile Ash use, % molar ratio clinker, matter in absorption, (M R) in % coal mi % clinker (VM), % 1 -- 0.529 0.700 23.00 1.806 2 15 0.645 0.849 21.70 1.628 3 20 0.762 1.003 20.40 1.450 4 25 0.821 1.080 19.75 1.360 5 24 0.798 1.049 20.01 1.396
  19. 19. * From the above results it is observed that for use of pet-coke up to 25% (SN 4) to M R and SO3 in clinker remain within limits. * But for use of pet-coke upto 25%, the V M content lowers than minimum requirement. * Hence the criteria for deciding optimum substitution level is V M content. * Another calculation was done for substitution by pet-coke up to 24% wherein the V M value comes in.
  20. 20. EFFECT OF SULPHUR ON QUALITY CLINKER * The cement broadly consist of following minerals CaO, SiO2, Al2O3 and Fe2O3 beside these major components it also contains minor constituents (ie.) MgO, Alkalies and Sulphur. * As sulphur reacts before C2S and C3S formation it will consume part of lime available. Remaining lime will now react to form C4AF, C3A, C3S and C2S. Lime consumed by SO3 is : CaO (56) + SO3 (80) = CaSO4(136) CaO / SO3 = 56 / 80 = 0.7
  21. 21. In case study it was observed that : Case-1 - with 100% import coal - SO3 in clinker was 0.4 Case-2 - with 8% Petcoke - SO3 in clinker was 0.55 Case-3 - with 25% Petcoke - SO3 in clinker was 0.9% Case-4 - with 50% Petcoke - SO3 in clinker is 1.2%
  22. 22. Process implications The expected implications on process due to use of pet-coke are maintained as following: * The specific fuel consumption was likely to lower slightly. * Moisture in pet-coke was within the reasonable limits. * The volatile matter in the blended fuel was to be maintaine above 20%. * The higher sulphur content in the fuel will increase the SO2 emission in exhaust gases which will be within acceptabel range of prevalent environmental laws. However the corrosion will increase in the transport ducts and equipments, particularly in corners.
  23. 23. Quality implications The implications on the quality were as under mentioned : * Reduction in ash absorption in the clinker from 1.93 to 1.69% and 1.53% for substitution of 10% and 17% respectively of total fuel. * The quality of clinker will be maintained same except that SO3 content will increase. * The total sulphur input in fuel is limited to avoid problems of build ups as 1.23% and 1.60 % sulphur for substitution of 10% and 17% respectively. * In burning zone high alumina bricks are not suitable with pet-coke and basic bricks may be required.
  24. 24. IMPACT ON PROCESS AND QUALITY OF CEMENT * An Indian cement companies encountering operational difficulties after the introduction of petcoke firing in their cement kiln. * These problems are due to poor nodulisation of the clinker and loss of cement mill output in grinding this clinker. * A review of the underlying reasons for this are presented below.
  25. 25. Factors inherent in pet-coke. * (1) Its low volatiles content and (2) Its high sulphur content. * Low volatile content of the coke means that it needs to be ground more finely in the coal mills in order to ignite and burn in the kiln flame. * This requirement for fine grinding can become a capacity constraint on the amount of petcoke that can be prepared for firing in the kiln. * The low volatiles content and resultant longer combustion time also means that modification to the kiln burner is often needed.
  26. 26. * In precalciners, carry over of unburnt or burning coke into the bottom stages of the preheater can occur, resulting in elevated temperatures and potential blockage problems. * Steps need to be taken to increase the petcoke’s residence time in the precalciner to ensure complete combustion and separate calciner combustion chambers or ‘down draft’ calciners are introduced to achieve this. * High sulphut content brings with it a number of problems. * In modern kilns with preheaters virtually all the sulphur from the coke will be retained in the clinker as sulphates, meaning the clinker sulphate content can approach or exceed two percent.
  27. 27. * International standards limit total sulphate in cement to three or 3.5 percent, a high clinker sulphate limits the amount of additional sulphate that is added, at the cement grinding stage. * Addition of sulphate as gypsum is required to retard the hydration and setting of cement. * If insufficient gypsum can be added then the cement may be inadequately retarded leading to rapid setting and loss of workability of the cement. * Unfortunately sulphate bound in the clinker is not often as effective as gypsum added at the cement grinding stage in retarding the cement due to sulphate in the clinker being ‘dead-burnt’ and unreactive.
  28. 28. * High sulphur content can also mean operational problems with the kiln. * Petcoke firing usually results in the sulphates in the kiln being in excess of the alkalies with that excess being present in combintion with CaO as anhydrite CaSO4 or double salts with CaSO4 in combination with K2SO4 such as calcium langbeinite. * In the calcining zone and kiln inlet this calcium sulphate can form intermediate compounds, such as sulphate spurrite which can lead to ring formations and restrictions at the back ofr the kiln. * In the worst cases the hot feed is dammed behind these rings and falls through the kiln inlet presenting a hazard and demanding a kiln rate reduction.
  29. 29. * The nodulisation problems are also related to the sulphur content of the pet coke. * High sulphate content is one of the primary causes of dusty clinker and can present major problems with dust recirculation from the grate cooler to the kiln in the secondary combustion air. * In the worst cases this recirculating dust absorbs a significant quantity of the thermal energy provided in the fuel and hence inadequate energy is available to combine the clinker into the clinker minerals. * The excess sulphate in the clinker adds to the volume of flux in the burning zone and also reduces the viscosity and surface tension of these liquid phases fluxing the kiln.
  30. 30. * Two effects result from this: (1) the reduced viscosity means that formation of alite is promoted and there is a tendency for large crystals of alite to be formed, and (2) the reduced surface tension means the clinker particles are more easily ‘wetted’ and the particle adhesion forces in the clinker are reduced. This reduction in the adhesion forces is why the clinker structure is loosened resulting in a finer particle size distribution and greater proportion of dust.
  31. 31. * Explanation of a problem is the first step in identifying a potential solution. * In this case as the problem is increased viscosity and reduced surface tension of the flux, adjusting the mix to reduce this viscosity and increase the surface tension of the clinker liquid phase should solve the problem. * One way to do this by increasing the alkali content of the clinker, perhaps by the addition of feldspar to the raw mix, as this has the required effect on the flux viscosity. * However, many producers go to great lengths to produce low alkali clinker and this is likely to be impractical in many instances.
  32. 32. * Silica is reported to markedly increase the viscosity of the flux and therefore lifting the silica modules of the kiln feed may solve the problem. * Alumina is alse reported to do this is to a lesser extent. * Alse calcium fluoride promotes the crystallisation of alite from the clinker liquid which serves to stiffen the flux and would have the same effect in addition to mineralising the kiln.
  33. 33. i) MOLARBALANCING (WITH OUTPETCOKE) Molar balancing with 100% coal (Ind. Coal = 93.0%, Imp. Coal = 7.0%) and it is 0.985, which is normal. MOLARBALANCING BASE IND. COAL = 93.0 % NCV= 5733 K.Cal/Kg. Coal IMP. COAL = 07.0% Heat Cons. = 735 K.Cal/Kg. Clk KILNFEED VENT SO3 =0.19% K2O = 0.33% Na2O = 0.09% Cl- = 0.012% KILNSYSTEM COAL CLINKER SO3 = 2.00 SO3 = 0.58 K2O = 0.06 K2O= 0.71 Na2O = 0.06 Na2O= 0.17 Cl- = 0.005 Cl- = 0.017 Section Qty. SO3 K2O Na2O Cl- (MT) Kiln Feed 4960 9.424 16.368 4.464 0.5952 Fuel 459 9.180 0.2754 0.2754 0.0228 Total 18.604 16.6434 4.7394 0.6180 Formula for molar balancing :- SO3/80 ------------------------------------- = 0.98502 K2O/94 + Na2O/62 – 2XCl/71
  34. 34. a) MOLARBALANCING Molar balancingwith20%Pet coke and80%Coal. BASE COAL =80% NCV=6052K.Cal/Kg. Coal PETCOKE=20% Heat Cons. =735K.Cal/Kg. Cl KILNFEED VENT SO3 =0.19% K2O =0.33% Na2O =0.12% Cl- =0.012% KILNSYSTEM COAL+PETCOKE CLINKER SO3 =4.85 SO3 =0.65 K2O =0.481 K2O=0.69 Na2O =0.481 Na2O=0.15 Cl- =0.004 Cl- =0.0119 Section Qty. SO3 K2O Na2O Cl- (MT) KilnFeed 4960 9.424 16.368 5.952 0.5952 Fuel 414 20.079 0.1991 0.1991 0.0166 Total 29.503 16.5670 6.1512 0.6118 Formulafor molar balancing:- SO3/80 ------------------------------------- =1.4282 K2O/94+Na2O/62–2XCl/71 The value of molar balancing comes to 1.4282, which is higher than recommended value of 1.0. which indicates abnormal andharddeposit at kilninlet/kilnriser duct.
  35. 35. COSTECONOMICS 20% of mixed coal is replaced by Pet Coke and cost difference (landed and moisture free) is tabulatedbelow:- S. No. Type of coal NCV (Kcal/Kg) Ash (%) Moisture (%) Landedcost (Moisture free) Rs. / MT 1. Importedcoal 5800 15 8 2867/- 2. Pet Coke 8100 2 6 2580/- Description ImportedCoal 80%Imp. coal : 20%Pet Coke NCV 5800 6260 Heat Cons. (Kcal/Kg.) 735 735 Cost Rs. / Tonof Fuel 2867 2810 %fuel consumption 13 12.5 %Ash 11 8 A. Fuel cost Rs. /Tonof clinker 372.75 351.25 B. Additional Cost for : 1. Extra power consumption i.e. @ 2.5 Unit/Ton Rs. 4.20per Unit. 2. Misc. additional cost for additional wear & tear of mill liner etc., --- --- 10.50 1.00 Total (A+B) 372.75 362.75 Saving = Rs. 372.75- Rs. 362.75 = Rs. 10.00per tonof clinker = Rs. 36,000/- per day (i.e.) Rs.10.8Lacs per month.
  36. 36. Conclusions * 100% Pet-coke not possible without modifcation and substantial investment. * 20-25% petcoke is possible in dry process kiln without any modification and investment. * It is concluded that use of pet-coke as a partial substitue fuel is feasible in most cases. The critical factor to decide level of substitution by pet-coke is the molar ratio of oxides of alkalies to oxides of sulphur in clinker. A detailed study is recommended to modify the existing process and equipment to suit the properties of proposed quality of pet-coke. * In view of company of production of cement and attractive payback periods, the use of pet-coke is likely to increase rapidly.
  37. 37. The CMA Technical Committee has been convinced of several advantages including socio-economic benefits from use of Petcoke as cheaper, energy-economic and environment- friendly substitute of non-renewable fuel like coal. CMA had therefore proposed to the Bureau of Indian Standards (BIS) to revise the relevant standards to increase the permissible SO3 content up to 3.5% for all cements and up to 4% when petcoke is used as a fuel.

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