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.

AVAILABILITY

* Petcoke can be made available to cement industry byPetro Chemical Industry as it is as by product andimposses disposal problem.

* Petroleum coke (Petcoke) produced around 2.0 milliontonnes per year.

It can be made available at :

* Reliance Petroleum Limited, Jamnagar, Gujarat.

* Indian Oil Corporation (Assam Unit)

* Hindustan Petroleum

PROPERTIES OF PETCOKE

* Petroleum coke is a waste-product of oil refining crackingprocesses.

* When oil is subjected to extreme cracking and practicallyall the liquid phases are driven off, a residual product remainsconsisting mainly of carbon.

* There are two commonly used processes for coke manufacturingnamely ‘delayed coking’ and ‘fluid coking’.

* Although there are three to four categories of petcoke, the ‘delayed coke’ is mostly used as a fuel.

* In comparison to coal, pet-coke is categoriesed as a materialwith low volatile, very low ash content, high fixed carboncontent and high sulphur content.

* Depending on the type of crude oil, the sulphut content mayvary between 0.7% to 8%.

* However, mostly the sulphur content is high, typicallyin the range of 2 to 8%.

* A typical range of analysis for the delayed type pet-cokeis as under mentioned :-

Moisture, % - 6 to 12Ash content, % - 0.2 to 2Volatiles, % - 5 to 14Sulphur, % - 2 to 8Calorific value, K.cal/kg - 7500 to 8000Hardgrove Index - 35 to 70Bulk density, Kg/m3 - 0.77 to 0.88Lump size, mm - 0 to 75Fixed carbon, % - 88 to 90

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 basisof sulphur content in pet-coke and clinker.

* This is because of the fact that higher concentrations ofsulphur cause coating formations in cyclones and kiln inlet areas resulting into operational disturbances.

* In extreme cases, kiln has to be stopped for cleaning of thesebuild-ups.

* Consequently, the availability and hence productivity of thekiln 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 oxidesto sulphur tri oxide should be between 0.8 to 1.2).

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 causeclogging.

* 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.

GRINDING

* Pet-coke being highly abrasive and hard, is difficult togrind.

* The grinding system is designed to minimize the wear ofmill 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.

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 590-620oC as compared to the ignition temp.ofcoal 400-500oC.

* In this case, the particles need a higher ignition temperature anda longer time to burn as the rate of chemical reaction is very slow.

* An important factor influencing the rate of combustion is thespecific surface area of the fired pet-coke. So, the fineness ofpet-coke is increased to improve its burning behaviour.

* Based on the experience of other plants, pet-coke fired incement kiln and calciner is of fineness of 4% to 6% residueon 90 micron sieve.

Use in Kiln

* If the plants have single channel burners for kiln then thesewill have to be replaced by specially designed Multi-Fuelburners.

* Such a burner will ensure the complete mixing of pet-cokeand air, burn out time of individual particles and creatingthe required flame impulse.

* Burner must be suitable to handle highly abrasive material.

* A little quantity of support fuel like coal with high volatilematter or oil or gas can be used to ensure ignition.

* Several plants have successfully use pet-coke up to 100%in main firing.

Use in Precalciner

* The desired conditions for the proper combustion ofpet-coke in calciner are high temperature, high oxygencontents and high temperatue. These conditions can beachieved by the following modifications:

* Design of ‘Hot spot’ in ‘In line Calciner’ by shiftingupstream the meal entry point by about 1.5 - 2 m. A bettersolution may be the provision of a precombustion chamber.

* The precalciner is designed for gas residence time of aminimum 3.5 seconds.

* Creation of adequate turbulence in the calciner.

Advantages of use of Pet-coke

The various advantages of using pet-coke as an alternate fuel areas highlighted here :

* As compared to use of coal which has higher volatile contentsthan pet-coke, requirement of combustion air is less. Similarly,the generation of the flue gas is lower which results into savingin energy consumption of fans handling it. Alternatively, thereduction in gas volume can be used to increase the productionfrom the kiln provided there are margins in other equipmentin the system.

* As the pet-coke is a high heat value fuel, the temp.of the flameincreases and hence the heat transfer in the kiln improves.

* If the alkali contents in the system are high then somesulphur rich raw materials or high sulphur coals areused which may cost more. However, the pet-cokefulfills 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-cokeis used.

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 quantityof its use is limited by the molar ratio of oxides of alkaliesto 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 maintainedbelow 1.4%.

* As the grinding capacity of the coal mills lowers by 50% to 80%, there should be adequate coal grinding capacity afteruse of the pet-coke.

* If at a certain level of use of pet-coke, system is not capableof its proper burning (as indicated by the CO contents higherthan 0.2%), then its quantity cannot be increased further.

* Temperature of the exhaust gas increases if pet-coke is introduced with raw meal resulting into increased gas volumeand increased specific heat consumption. In that situation,the benefits should be worked out considering reduction inkiln production and increased fuel consumption.

* Pet-coke has some toxic contents like vanadium, nickel etc.,which go along with the dust. Hence, plants which disposethe return dust have to ensure that the concentration of thesecompounds do not exceed the limits defined by the environmental laws.

* When cost of pet-coke is more than cost of coal or fuel beingused, there is no economy of using it.

Properties of Pet-coke

The various properties of the pet-coke proposed to be used are givenas follows:

Parameter Unit ValueMoisture content (avg.) % 12Size 50 mm sieve 100% passingBulk density t/m3 0.9Calorific value net K.Cal./kg 8000Volatile matter % 9 - 11Ash content % 0.2Hard groove index 50 - 55Sulphur % 6Carbon % 89.05Hydrogen % 3.5 - 4.0

Substitution level

* The substitution level of existing fuel by pet-coke is decidedon the basis of following criteria:

Criteria Limiting value

SO3 / Alkali molar ratio 0.8 - 1.0

Volatile matter incoal mix, % 20 (min.)

* Calculation was carried out for different additions of pet-cokein existing fuel. The results are presented as under mentioned:

S.No Pet-coke SO3/alkali SO3 in Volatile Ashuse, % molar ratio clinker, matter in absorption,

(M R) in % coal mi %clinker (VM), %

1 -- 0.529 0.700 23.00 1.8062 15 0.645 0.849 21.70 1.6283 20 0.762 1.003 20.40 1.4504 25 0.821 1.080 19.75 1.3605 24 0.798 1.049 20.01 1.396

* From the above results it is observed that for use ofpet-coke up to 25% (SN 4) to M R and SO3 in clinkerremain within limits.

* But for use of pet-coke upto 25%, the V M contentlowers than minimum requirement.

* Hence the criteria for deciding optimum substitution level is V M content.

* Another calculation was done for substitution by pet-cokeup to 24% wherein the V M value comes in.

EFFECT OF SULPHUR ON QUALITY CLINKER

* The cement broadly consist of following minerals CaO, SiO2,Al2O3 and Fe2O3 beside these major components it alsocontains minor constituents (ie.) MgO, Alkalies and Sulphur.

* As sulphur reacts before C2S and C3S formation it will consumepart of lime available. Remaining lime will now react to formC4AF, C3A, C3S and C2S. Lime consumed by SO3 is :

CaO (56) + SO3 (80) = CaSO4(136)

CaO / SO3 = 56 / 80 = 0.7

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%

Process implications

The expected implications on process due to use of pet-coke aremaintained 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 maintaineabove 20%.

* The higher sulphur content in the fuel will increase the SO2emission in exhaust gases which will be within acceptabelrange of prevalent environmental laws. However the corrosionwill increase in the transport ducts and equipments, particularlyin corners.

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 oftotal fuel.

* The quality of clinker will be maintained same except thatSO3 content will increase.

* The total sulphur input in fuel is limited to avoid problemsof build ups as 1.23% and 1.60 % sulphur for substitutionof 10% and 17% respectively.

* In burning zone high alumina bricks are not suitable withpet-coke and basic bricks may be required.

IMPACT ON PROCESS AND QUALITY OF CEMENT

* An Indian cement companies encountering operationaldifficulties after the introduction of petcoke firing in theircement kiln.

* These problems are due to poor nodulisation of the clinkerand loss of cement mill output in grinding this clinker.

* A review of the underlying reasons for this are presented below.

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 beground more finely in the coal mills in order to ignite and burn in the kiln flame.

* This requirement for fine grinding can become a capacityconstraint on the amount of petcoke that can be preparedfor firing in the kiln.

* The low volatiles content and resultant longer combustiontime also means that modification to the kiln burner is oftenneeded.

* In precalciners, carry over of unburnt or burning coke intothe bottom stages of the preheater can occur, resulting inelevated temperatures and potential blockage problems.

* Steps need to be taken to increase the petcoke’s residencetime in the precalciner to ensure complete combustion andseparate 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 sulphurfrom the coke will be retained in the clinker as sulphates,meaning the clinker sulphate content can approach or exceedtwo percent.

* International standards limit total sulphate in cement to three or 3.5 percent, a high clinker sulphate limits the amountof additional sulphate that is added, at the cement grindingstage.

* Addition of sulphate as gypsum is required to retard thehydration and setting of cement.

* If insufficient gypsum can be added then the cement may beinadequately retarded leading to rapid setting and loss ofworkability of the cement.

* Unfortunately sulphate bound in the clinker is not often as effective as gypsum added at the cement grinding stage inretarding the cement due to sulphate in the clinker being ‘dead-burnt’ and unreactive.

* High sulphur content can also mean operational problemswith the kiln.

* Petcoke firing usually results in the sulphates in the kilnbeing in excess of the alkalies with that excess being presentin combintion with CaO as anhydrite CaSO4 or double saltswith CaSO4 in combination with K2SO4 such as calciumlangbeinite.

* In the calcining zone and kiln inlet this calcium sulphatecan form intermediate compounds, such as sulphate spurritewhich can lead to ring formations and restrictions at the backofr the kiln.

* In the worst cases the hot feed is dammed behind these ringsand falls through the kiln inlet presenting a hazard and demanding a kiln rate reduction.

* The nodulisation problems are also related to the sulphurcontent of the pet coke.

* High sulphate content is one of the primary causes of dustyclinker and can present major problems with dust recirculationfrom the grate cooler to the kiln in the secondary combustionair.

* In the worst cases this recirculating dust absorbs a significantquantity of the thermal energy provided in the fuel and henceinadequate energy is available to combine the clinker intothe clinker minerals.

* The excess sulphate in the clinker adds to the volume of flux in the burning zone and also reduces the viscosity and surfacetension of these liquid phases fluxing the kiln.

* Two effects result from this:

(1) the reduced viscosity means that formation of aliteis promoted and there is a tendency for large crystalsof alite to be formed, and

(2) the reduced surface tension means the clinker particlesare 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 greaterproportion of dust.

* Explanation of a problem is the first step in identifyinga potential solution.

* In this case as the problem is increased viscosity and reducedsurface tension of the flux, adjusting the mix to reduce thisviscosity and increase the surface tension of the clinker liquidphase should solve the problem.

* One way to do this by increasing the alkali content of theclinker, 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 producelow alkali clinker and this is likely to be impractical inmany instances.

* Silica is reported to markedly increase the viscosity of theflux and therefore lifting the silica modules of the kiln feedmay solve the problem.

* Alumina is alse reported to do this is to a lesser extent.

* Alse calcium fluoride promotes the crystallisation of alitefrom the clinker liquid which serves to stiffen the flux andwould have the same effect in addition to mineralisingthe kiln.

i) MOLAR BALANCING (WITH OUT PETCOKE)

Molar balancing with 100% coal (Ind. Coal = 93.0%, Imp. Coal = 7.0%) and it is0.985, which is normal.

MOLAR BALANCING

BASE

IND. COAL = 93.0 % NCV= 5733 K.Cal/Kg. CoalIMP. COAL = 07.0% Heat Cons. = 735 K.Cal/Kg. Clk

KILN FEED VENTSO3 =0.19%K2O = 0.33%Na2O = 0.09%Cl- = 0.012%

KILN SYSTEM

COAL CLINKER

SO3 = 2.00 SO3 = 0.58K2O = 0.06 K2O = 0.71Na2O = 0.06 Na2O = 0.17Cl- = 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------------------------------------- = 0.98502K2O/94 + Na2O/62 – 2XCl/71

a) MOLAR BALANCING

Molar balancing with 20% Pet coke and 80% Coal.

BASE

COAL = 80 % NCV= 6052 K.Cal/Kg. CoalPET COKE = 20 % Heat Cons. = 735 K.Cal/Kg. Cl

KILN FEED VENTSO3 =0.19 %K2O = 0.33 %Na2O = 0.12 %Cl- = 0.012%

KILN SYSTEM

COAL+PET COKE CLINKER

SO3 = 4.85 SO3 = 0.65K2O = 0.481 K2O = 0.69Na2O = 0.481 Na2O = 0.15Cl- = 0.004 Cl- = 0.0119

Section Qty. SO3 K2O Na2O Cl- (MT)

Kiln Feed 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

Formula for molar balancing :-SO3/80

------------------------------------- = 1.4282K2O/94 + Na2O/62 – 2XCl/71

The value of molar balancing comes to 1.4282, which is higher than recommended value of 1.0. whichindicates abnormal and hard deposit at kiln inlet/kiln riser duct.

COST ECONOMICS

20% of mixed coal is replaced by Pet Coke and cost difference (landed and moisture free) istabulated below :-

S.No.

Type of coal NCV(Kcal/Kg)

Ash(%)

Moisture(%)

Landed cost(Moisture free)

Rs. / MT1. Imported coal 5800 15 8 2867/-2. Pet Coke 8100 2 6 2580/-

DescriptionImported Coal 80% Imp. coal :

20% Pet CokeNCV 5800 6260Heat Cons. (Kcal/Kg.) 735 735Cost Rs. / Ton of Fuel 2867 2810% fuel consumption 13 12.5% Ash 11 8A. Fuel cost Rs. /Ton of clinker 372.75 351.25B. Additional Cost for :1. Extra power consumption i.e. @ 2.5 Unit/Ton Rs.

4.20 per 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. 10.00 per ton of clinker= Rs. 36,000/- per day (i.e.) Rs.10.8 Lacs per month.

Conclusions

* 100% Pet-coke not possible without modifcation and substantial investment.

* 20-25% petcoke is possible in dry process kiln without anymodification and investment.

* It is concluded that use of pet-coke as a partial substituefuel is feasible in most cases. The critical factor to decidelevel of substitution by pet-coke is the molar ratio of oxidesof alkalies to oxides of sulphur in clinker. A detailed studyis recommended to modify the existing process and equipment to suit the properties of proposed quality ofpet-coke.

* In view of company of production of cement and attractivepayback periods, the use of pet-coke is likely to increaserapidly.

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.