Petcoke in Cement Clinker

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. 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. 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. * 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. * 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. 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. 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. 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. 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. 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. 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. 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. * 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. 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. * 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. 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. 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. * 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. * 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. 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. 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. 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. 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. 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. 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. * 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. * 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. * 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. * 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. * 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. * 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. * 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. 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. 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. 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. 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. 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.