Fuller Chemistry Handbook, Mr. Bokaian's Copy

FULLER - CEMENT CHEMISTRY - HANDBOOK

Chemistry Bible Rev.0; 7 Dec 00 1

CEMENT CHEMISTRYTable of Contents

1. INTRODUCTION1.1 INTRODUCTION1.2 DEFINITIONS - cement, concrete, cement types, raw materials etc.

2. COMPOSITION2.1 COMPOSITION - basic calculation/formulas - chemical shorthand

etc.2.2 MODULES2.3 MINERAL COMPOSITION

3. TYPES OF CEMENT3.1 TYPES OF CEMENT3.2 CEMENT STANDARDS3.3 CEMENT QUALITY - MAIN FACTORS

4. MANUFACTURE4.1 MANUFACTURE OF CEMENT - grey, mixed and white

cement - wet, dry and semi-dry process4.2 RAW MIX4.3 RAW MATERIALS4.4 CHEMICAL COMPOSITION AND CONTROL OF RAW MIX4.5 PHYSICAL CONTROL AND COMPOSITION OF RAW MIX4.6 BURNABILITY OF RAW MIX4.7 CLINKERISATION4.8 INFLUENCE OF THE RAW MIX ON CLINKER FORMATION

AND BURNABILITY

5.PROCESS AND KILNS5.1 TYPES OF KILNS - wet, dry & semi-dry5.2 WET KILN - main features - process5.3 DRY KILN

5.3.1 LONG KILN5.3.2 SP KILN5.3.3 ILC-E KILN5.3.4 ILC KILN5.3.5 SLC KILN5.3.6 SLC-S KILN5.3.7 SLC-I KILN

5.4 ASH ABSORPTION5.5 VOLATILE MATTER



5.6 MAIN FEATURES DURING BURNING

6. HEAT OF REACTION & HEAT TRANSFER

7.FUEL7.1 TYPES OF FUEL

7.1.2 COAL7.1.3 FUEL OIL7.1.4 GAS7.1.5 WASTE FUELS

8.COMBUSTION

9. COAL & OIL

9.1 FINENESS OF COAL9.2 DRYING OF COAL9.3 ASH CONTENT9.4 GAS CONTENT9.5 MINOR COMPONENTS9.6 REQUIREMENT FOR AIR

10. PROCESS GAS



1.1 INTRODUCTION

Cement is a material that binds together solid bodies (aggregate) by hardening

from a plastic state. Many materials act as adhesives or cement according to this

definition. The cement referred to above, which is used for civil engineering and

the construction industry, is portland cement. Portland cement is hydraulic and

develops strength primarily by the hydration of the di- and tri-calcium silicates it

contains. Hydraulic means that the paste of cement and water will harden under

water. Lime, on the other hand, will harden due to the reaction with carbon

dioxide from the air.

In 1793, an Englishman by the name of Smeaton found that a form of hydraulic

cement was produced when a mixture of limestone and clay were burned

together. He was the first producer of portland cement. Another Englishman,

Joseph Aspdin obtained the first patent for portland cement in 1824. He burned

a mixture of chalk and clay in a lime kiln to form a clinker and ground the clinker

to cement. Aspdin is regarded as the inventor of portland cement. The early

product did not have the same quality of cement currently produced today.

Two main developments have improved the quality of cement:

Addition of gypsum to regulate the setting time

Higher burning temperature to obtain higher lime containing silicates

Today, portland cement (portland cement clinker with a small addition of gypsum)

is used worldwide and is the primary construction material for roads, bridges,

runways, tunnels, and buildings. In addition, blended cements are produced

using portland cement clinker, a small amount of gypsum and another additive

such as blast furnace slag, fly ash, or other pozzolans. Some blended cement

has improved performance over ordinary portland cement with greater resistance

to alkali and sulfate attack.

1.2 DEFINITIONS - cement, concrete, cement types, raw materials



Cement: cement is a mix of cement clinker and gypsum

ground together into a powder.

Gypsum: calcium sulfate is used to regulate setting time.

Concrete: concrete is a mix of cement (13%), water (7%), fine

aggregate –6mm (32%), and coarse aggregate +6mm

(48%) which hardens to solid mass

Cement types: there are numerous types of cement for many

different applications. ASTM defines the

chemical and physical requirements

Ordinary general-purpose concrete

Rapid hardening precast concrete

Alkali resistant used with reactive aggregates

Sulfate resistant used in applications requiring resistance

against sulfate attack

White cement special architectural concrete

Low heat massive concrete like dams

Masonry cement mortar bonding brick or block

Blended cements use an addition of slag or other pozzolans to

achieve some of the properties of alkali or

sulfate resistance

Raw Materials: The common raw materials for manufacture of

Portland cement are:

Limestone / Marble / Marl

Clay / Shale

Sand

Iron ore or pyrite ash

Limestone and clay are primarily found near the plant, where as, sand and

iron ore are usually brought in because of their preferred chemical

composition.



Today, there is considerable interest by our society for the reuse of waste

materials. Some of these can be utilized in the raw mix feed to the kiln. Other

materials such as waste oils and solvents can be safely disposed of in the

burning zone of the rotary kiln because of the high temperatures.

2.1 COMPOSITION - basic calculation/formulations - chemical shorthand

The most common compounds in cement chemistry are:

Formula Name Source

SiO2 silicium dioxide quartz, sand

Al2O3 aluminium oxide

Fe2O3 iron oxide

CaO calcium oxide lime

CaCO3 calcium carbonate limestone, marble, marl

MgO magnesium oxide

MgCO3 magnesium carbonate

K2O potassium oxide

Na2O sodium oxide

H2O water

N2 nitrogen 79 volume percent in air

O2 oxygen 21 volume percent in air

CO carbon monoxide

CO2 carbon dioxide

SO2 sulfur dioxide

A typical analysis of clinker is given in the table below:



Table 1

CLINKER COMPOSITION

Clinker%

Min limit % Max limit %SiO2 22.51 17 26Al2O3 5.16 4 10Fe2O3 2.55 0.1 5CaO 64.97 62 69MgO 2.61 0 4

K2O+Na2O 1.7 0.3 2SO3 0.22 0 1.5

Insoluble residue 0.1 0 1Ignition loss 0.06 0 3

Total 99.88Free CaO 0.8 0 2.5

* In the table, the term ignition loss is defined as the loss in weight by heating the sample to a temperature of~900°C.

2.2 MODULES

Modules determine the proportioning of the raw mix. The three commonly used

modules are:

SILICATE MODULUS MSALUMINA MODULUS MALIME SATURATION FACTOR LSF

The silica modulus is defined as the ratio of silica to the sum of alumina and iron

oxide:

MS SiOAl O Fe O

2

2 3 2 3

The silica modulus for clinker is normally between 2.4 and 2.7. The amount of

melt phase in the burning zone is a function of MS, when MS is high, the amount

of melt is low. Therefore, when the MS is too high, the formation of nodules

might be too slow, resulting in less nodulization. The material becomes dusty

and impedes the kiln operation. Generally, a higher MS relates to a harder

burning mix.



The alumina modulus is defined as:

MA Al OFe O

2 3

2 3

The alumina modulus for clinker varies between 1.6 and 2.0. The temperature by

which the melt forms depends on the MA. The lowest temperature is obtained

when the MA is 1.6. The MA also affects the color of clinker and cement. The

higher the MA, the lighter the cement.

The lime saturation factor, LSF, is defined as the ratio of available lime to the

theoretical lime required by the other major oxides in the raw mix to form clinker

and is determined from the following equation:

LSF CaOSiO Al O Fe O

2 8 118 0 652 2 3 2 3. * . * . *

The LSF is usually expressed as a percentage. The LSF for clinker is in the

range of 88 and 98%. The theoretical maximum amount of lime, CaO, that can

be combined with the acidic oxides has been derived from phase diagrams and

can be calculated as follows:

CaOmax = 2.80*SiO2 + 1.18*Al2O3 + 0.65*Fe2O3

For cement, a variation of the previous equations has to be used to account for

the addition of gypsum to cement. The amount of CaO has to be reduced by the

amount bound in gypsum CaSO4.

The LSF formula for cement is then:

LSFCaO SO

SiO Al O Fe O

0 72 8 1 18 0 65

3

2 2 3 2 3

. *. * . * . *



2.3 MINERAL COMPOSITION

During burning in the cement kiln, the lime is combined with the acidic oxides to

form cementitious minerals. The main minerals are:

Table 2

CLINKER MINERALS

Formula Short

formula

Max

weight %

Min

weight %

Alite 3CaO.SiO2 C3S 70 35

Belite 2CaO.SiO2 C2S 45 20

Tricalcium aluminate 3CaO.Al2O3 C3A 18 3

Calcium alumino ferrite 4CaO.Al2O3.Fe2O3 C4AF 15 1

Free lime CaO 2 0.5

Note: The short formula designation is used throughout the cement industry.

It is possible to calculate the amount of the four main minerals by using theBOGUE formulas, however, the calculation is only approximate. The actualamount present of any mineral can be determined by microscopy or faster by X-ray diffraction. Using % wt., the BOGUE formulas for clinker are:

C3S = 4.07*CaO –7.60* SiO2 – 1.43* Fe2O3 – 6.72* Al2O3

C2S = 8.60* SiO2 + 1.08* Fe2O3 + 5.07* Al2O3 – 3.07*CaO(or C2S= 2.867*SiO2-0.7544* C3S )

C3A = 2.65*Al2O3 - 1.69* Fe2O3

C4AF = 3.04*Fe2O3

As mentioned above, other minerals are present and have to be taken intoaccount. These are free lime (CaO) , calcium sulfate (CaSO4) and periclase(MgO). The free lime should be under 2.0%. A higher amount indicates poorburning or faulty composition of the raw mix. Too high a free lime will result involume instability in the cement mortar or concrete. Calcium sulfate in cementcomes from the addition of gypsum and from clinker. In clinker, SO3 comes from



the sulfur in the fuel used and from sulfur in the raw materials (pyrites FeS2). MgOcan cause late expansion in concrete. A maximum of 6% MgO is allowed inordinary portland cement. Cements with higher MgO contents 4-5%, rapidcooling of the clinker is of more value for offsetting the adverse effects of highmagnesia contents. For other types of cement, the maximum limit varies.

The minerals in clinker are mainly found as crystals. A smaller amount is presentin the so-called glass phase. During burning, some liquid phase is formed at thehigh temperature in the burning zone (BZ). The liquid phase ranges from 20 to27% in normal clinker, however, some of the liquid does not have time to formcrystals during cooling. The liquid phase @ 1450°C is determined by thefollowing equation:

% Liquid Phase = 3.00A + 2.25F + (MgO + K2O + Na2O + SO3 )

3. TYPES OF CEMENT

The clinker from the kiln system is ground to cement with a small addition ofgypsum in order to control the time of setting. If no gypsum is present, thecement will set rapidly when water is added. The amount of gypsum added is 3to 5% by weight. The formula for gypsum is:

CaSO4,2H2O - which is the di-hydrate formCaSO4,0.5H2O - which is the hemi-hydrate form (plaster of paris)CaSO4 - which is the anhydrite form

The gypsum molecule is crystallized with two molecules of water. Gypsumoccurs in nature as gypsum rock. Other sources of gypsum are waste gypsumfrom exhaust gas desulfurisation at power plants, surplus gypsum from fertilizerfactories or cement manufacturing exhaust gas scrubbers. Other materials canbe added to clinker and gypsum to make cement. If the addition rate is small, theproduct can still be called portland cement.



The most common types of cement can be divided into three main groups:

a) PORTLAND CEMENTS:ORDINARY PORTLAND CEMENTRAPID HARDENING CEMENTSULFATE RESISTANT CEMENTLOW HEAT CEMENTWHITE CEMENT

b) COMPOSITE CEMENTSBLASTFURNACE CEMENTFLY ASH CEMENTPOZZOLAN CEMENTOTHER BLENDED CEMENTS

c) OIL WELL CEMENTS

a) PORTLAND CEMENTS are by far the most common type of cementsproduced around the world. The most widely used type isORDINARY PORTLAND CEMENT.RAPID HARDENING CEMENT is a portland cement that develops strengthfaster than ordinary portland cement. It is manufactured by grinding the clinkerand gypsum finer in the cement mill. Good clinker quality is needed and theaddition of gypsum is a little higher to maintain setting time and increase strengthdevelopment.SULFATE RESISTANT CEMENT is used where a higher resistance is requiredto sulfate-bearing waters. The cement composition has a lower content of C3A,less than 8%.LOW HEAT CEMENT is used where low heat development during hardening isrequired, i.e. in large concrete structures like dams. The rate of strengthdevelopment is lower than that of ordinary portland cement. Although, the finalstrength may be higher.WHITE CEMENT is used where a white color is wanted for a facade of buildings.



It is manufactured from raw materials with low content of iron chromium andmanganese.

b) COMPOSITE CEMENTS also known as blended cements start with Portlandcement clinker and gypsum but also have the addition of another material. Someadditions become hydraulically activated as they react with the Portland cementclinker. To mention a few:Blastfurnace slag cement is a cement made by grinding together portland cementclinker and granulated blastfurnace slag in the proper proportions.Masonry cement is a combination of portland cement clinker, limestone, and asmall addition of an air-entraining agent to produce a more workable, rapidhardening mortar than ordinary portland cement. It can also be made byintergrinding mixtures of portland cement with hydrated lime, granulated slag, orother waterproofing agents with inert fillers.Pozzolanic cements are produced by mixing together portland cement and apozzolana. A pozzolana is a material which is capable of reacting with lime inthe presence of water at ordinary temperatures to produce cementitiouscompounds.

c) OIL WELL CEMENTSOil well cements are used for cementing the steel casing of gas & oil wells to thewalls of the bore-hole and to seal porous formations. Usually, portland cementsmore coarsely ground than normal, with the addition of special retarders to allowfor slow-setting conditions are used.

Cement standards define the various types of cement. Today, there are twomain standards:

The US standard defined in ASTM C 150 The European standard EN 197-1

ASTM uses 5 main classes for Portland cement. There are also a number ofcomposite or blended cements defined by ASTM. The European standard has



three main strength classes each divided into two subgroups or 6 classes in all.

Tables 3 and 4 show some of the physical requirements defined by ASTM.

Table 3ASTM CEMENT TYPES

Compressive Strengths – for ASTM C109 Cubes

Number Type of Cement 3 days

compressive

strength

MPa

7 days

compressive

strength

MPa

28 days

compressive

strength

MPa

I Ordinary Portland cement 12 19

I A OPC air entraining 10 16

II Moderate sulfate resistant

Moderate heat of hydration

10 17

II A II + air entraining 8 14

III High early strength 24

III A III + air entraining 19

IV Low heat of hydration 7 17

V High sulfate resistance 8 15 21

Note: SI units are the standard .To convert to psi : SI * 142.23.



Table 4

ASTM CEMENT TYPES

Air Content, Fineness, Soundness and Setting Time

ASTMTYPE

Air content FinenessBlaine

Autoclaveexpansion

Settingminutes

vol% m2/kg % Initial ,min maxI Max 12 >280 <0.80 45 375

I A Min=16,max= 22 >280 <0.80 45 375II Max 12 >280 <0.80 45 375

II A Min=16,max= 22 >280 <0.80 45 375III Max 12 <0.80 45 375

III A Min=16,max= 22 <0.80 45 375IV Max 12 >280 <0.80 45 375V Max 12 >280 <0.80 45 375



Table 5

European Standard EN 197-1 Types of Cement

Cement type Designation Notation Clinker , K ,% Additive

I Portland cement I 95-100 Gypsum

Filler

II Portland slag

cement

II/A-S 80-94 S=Granulated

blastfurnace slagII/B-S 65-79

Portland silica fume

cement

II/A-D 90-94 D=Silica fume

Portland pozzolana

cement

II/A-P 80-94 P=Natural pozzolana

II/B-P 65-79

II/A-Q 80-94 Q=industrial

pozzolanaII/B-Q 65-79

Portland fly ash

cement

II/A-V 80-94 V=siliciceous fly ash

II/B-V 65-79

II/A-W 80-94 W=calcareous

fly ashII/B-W 65-79

Portland burnt shale

cement

II/A-T 80-94 T=burnt shale

II/B-T 65-79

Portland l/st cement II/A-L 80-94 L=limestone

II/B-L 65-79

Portland composite

cement

II/A-M 80-94 M=different materials

II/B-M 65.79

III Blastfurnace cement III/A 35-64

III/B 20-34

III/C 5-19

IV Pozzolanic cement IV/A 65-89

IV/B 45-64

V Composite cement V/A 40-64

V/B 20-39



All of the above European types of cement are the result of many years of workon a unification for the different standards. In European standards, the threemain strength classes are divided into two subgroups or 6 classes in all.

Table 6

European Standards for Strengths

Type Class 2 dayscompressive

strengthMPa

7 dayscompressive

strengthMPa

28 dayscompressive

strengthMPa

I,II,III,IV, & V 32.5 >=16 32.5-52.5I,II,III,IV, & V 32.5 R >=10 32.5-52.5I,II,III,IV, & V 42.5 >=10 42.5-62.5I,II,III,IV, & V 42.5 R >=20 42.5-62.5I,II,III,IV, & V 52.5 >=20 >=52.5I,II,III,IV, & V 52.5R >=30 >=52.5

For both ASTM and EN, there are a number of other physical requirements. Forthese it is necessary to reference the two standards. The standards also specifysome chemical requirements. For ASTM they are:



Table 7

ASTM Chemical Specifications

Cement type I and I A II and II A III and III A IV V

SiO2, min.% 20.0

Al2O3, max % 6.0

Fe2O3, max% 6.0

MgO, max% 6.0 6.0 6.0 6.0 6.0

SO3, max%

C3A <= 8%

3.0 3.0 3.5 2.3 2.3

SO3, max%

C3A > 8%

3.5 Not applicable 4.5 Not applicable Not applicable

Ign.loss, max% 3.0 3.0 3.0 2.5 3.0

Ins.res.max% 0.75 0.75 0.75 0.75 0.75

C3S ,max% 35

C2S,min% 40

C3A, max% 8 15 7 5

C4AF+2(C3A),max% 25

Optional requirements:

C3A max%,for

moderate sulfate

res.

8

C3A max%,for high

sulfate rest.

5

C3S+C3A,

max%,mod. heat of

hydration

58

Na2O+0.658K2O,

max%, low alkali

0.60 0.60 0.60 0.60 0.60

3.3 CEMENT QUALITY - MAIN FACTORS

Cement quality is just as important for the manufacturer of the product as it is forthe producers of concrete and concrete products, contractors and the private



consumer.

The cement manufacturers have to meet the standards, the clients requirementsand at the same time be competitive. The plants also have a role in fulfillingenvironmental requirements and assisting in the disposal of various wasteproducts.

The consumer has a range of requirements for the cement depending on thetype of concrete product, i.e. all purpose ready mix, precast, or pumped concreteto mention a few.

The end user wants a durable concrete which will stand up to heavy usage oninfrastructure, be frost resistant, withstand alkali aggregate reaction and at thesame time have a good appearance in the finished structure.

The cement quality depends on the clinker manufacturing process, the cementmilling, the fineness, and any changes to the cement after milling.

Composition can also vary within a single type of cement. Even ordinaryportland cement has subtle differences. For example, the gypsum addition rateand with limestone where up to 5% might have been added during grinding.

The cement fineness can be varied during grinding with a finer product reactingfaster. Fineness is expressed as: specific surface area (Blaine), residue andparticle size distribution.

The chemistry of the clinker is important. The clinker mineral composition has tobe considered. The main minerals in the clinker are:

C3S, C2S, C3A,C4AF

The main reactions at hydration in an idealized form are:



1) C3S + (3-x+y) H2O = CxSHy + (3-x) Ca(OH)2

2) C2S + (2-x+y) H2O = CxSHy + (2-x) Ca(OH)2

3) C3A + 13.5 H2O = ½ (C2AH8 + C4AH19)4) C3A + 6 H2O = C3AH6

5) C3A + Ca(OH)2+ 18 H2O = C3AH19

6) C3A +3 CaSO4 + 31H2O = C3A.3 CaSO4.H31

7) C3A + CaSO4 + 12H2O = C3A. CaSO4.H12

8) C4AF – reactions similar to C3A9) CaO + H2O = Ca(OH)2

10) MgO + H2O = Mg(OH)2

11) CaSO4 + 2 H2O = CaSO4,2 H2O12) CaSO4,1/2 H2O + 1 1/2 H2O = CaSO4,2 H2O

In all of the above reactions, water reacts with the hydration product mineralsincreasing the volume of the solid phase. Each reaction differs in velocity, volumechange, and in the nature of the hydration products. These reactions are thebackground for the setting time and strength development to the solid state fromthe plastic phase when water is first added to the cement.

Of prime importance is the state of the gypsum, as di-, hemi-, or anhydrite, in thecement as it first reacts with water. Depending upon that state, the gypsum andC3A reactions with water can result in normal, false or flash set.

False set is the premature stiffening of the cement paste due to most of thegypsum being either hemi-hydrate or soluble anhydrite due to overheating. Uponmixing with water, crystallization of reformed gypsum causes stiffening. Thisstiffening can be broken upon remixing without additional water. False set canhappen either by fast set of gypsum hemi-hydrate or because of a slow reactionbetween C3A with water. The slow reacting C3A can be caused by prehydrationor by carbonation with CO2. False set can be prevented by lowering the mill exittemperature, thereby, reducing the degree of gypsum dehydration, the amount ofgypsum added to the mix, or replacing part of the gypsum with a natural insolubleanhydrite.



On the other hand, flash set occurs if the C3A is more reactive than gypsum withwater. The setting is characterized by a high evolution of heat and short settingtime. Flash set can be prevented by adding more gypsum to the cement or bydehydrating the gypsum to a more reactive form, i.e. hemi-hydrate or anhydrite.

Therefore, as the cement is being ground, the mill material temperature mustcarefully be controlled. Between 90-130°C, the gypsum changes into calciumsulfate hemihydrate (plaster of paris) by releasing 1.5 molecule water:

CaSO4,2H2O CaSO4,½ H2O + 1½ H2O (a)

CaSO4,½H2O CaSO4+ ½ H2O (b)

The cement mill material temperature is controlled primarily by cooling the millwith an internal water spray in the second compartment. Additional cooling isaccomplished with air in the separator. The cement mill exit temperature shouldnot reach 130°C and is usually targeted at 110°C.

Finally, if the cement material temperature has not been controlled in the millsystem, the cement might enter the storage silo at too high a temperaturecausing dehydration of the gypsum. The rate of the transformation increaseswith temperature and with falling dew point. The change after equation (a) israpid at a temperature of 90-130°C. The water molecule released can give rise toformation of lumps in the cement and to scaling on the storage silo wall byformation of Syngenite K2SO4

. CaS04. H2O.

4. MANUFACTURE4.1 MANUFACTURE OF CEMENT - grey, mixed and white

cement - wet, dry and semidry process

The manufacture of Portland cement is divided into the three main parts:

a) Preparation of the kiln feed



b) Burning in the kilnc) Grinding of the clinker with gypsum and other additions

The description here will concentrate on the process for ordinary grey cement

with some comments on the other types of cement.

The dry process is used to make the majority of the cement produced in the

world. The wet process, however, is still used where fuel cost has allowed it. The

wet process can furthermore be justified where the raw materials are very wet

such as chalk, a soft limestone, and clay.

An intermediate solution is the semi-dry process where the raw mix is prepared

as slurry. The slurry can then be filtered to remove a portion of the water before

the burning or the slurry may be pumped directly into a dryer crusher working in

unison with the kiln.

4.2 RAW MIX

The raw mix must have a composition that will produce a clinker of the proper

analysis. The difference in the composition of the raw mix and the clinker is

threefold. First, is the change in each of the materials as they are heated up in

the kiln. The changes are due to a loss in weight mainly from the release of

carbon dioxide and water. Second, is a change due to absorption of ash from

coal used as fuel. There is also a change due to absorption of sulfur in the fuel.

Finally, there is a change due to the small dust loss in the exhaust gas. Some of

this dust is returned to the process but some might be wasted as in a bypass. In

a wet process, the dust may be discarded in order to reduce the alkali content in

the clinker.

The raw mix must therefore compensate for these changes and losses;

otherwise, the clinker will not have the correct chemical and mineralogical

composition. The way in which this is done will be explained below.

4.3 RAW MATERIALS

Many raw materials are suitable for the production of cement. In principle, as



long as they can be mixed to give the right composition of the clinker, they can

be used. There are some restrictions naturally. They must be available in large

quantities and be economically feasible. In addition, there might also be

restrictions on use due to minor components in the raw materials.

Limestone is the largest component used in producing cement. It is available as

CaCO3 in marble, limestone, chalk and marl. Limestone is sometimes found

together with magnesium carbonate. Only small amounts of MgO can be

tolerated in cement due to the risk of the expansion reaction in the concrete.

Limestone containing a large amount of magnesium carbonate is called dolomite.

In nature, limestone is found in many places mixed with clay and/or shale. Clay

and shale contain SiO2, Al2O3 and Fe2O3. In some cases a type of limestone is

found that is quite close in chemical composition to the cement composition.

When the limestone is of a higher purity than the requirement for the raw mix,

then other raw materials must be added to the mix. The amount of limestone is

calculated using the formula below:

LSF CaOSiO Al O Fe O

2 8 118 0 652 2 3 2 3. * . * . *

Sand is a mineral very rich in silica, SiO2. It is a very hard and abrasive mineral. It

is used when the mix is insufficient in silica. It will increase the MS or the silica

ratio:

MS SiOAl O Fe O

2

2 3 2 3

Iron can be used in the form of iron ore, usually an iron oxide, or as a waste

product from the fertilizer industry, such as pyrite ash. It is used to regulate or

reduce the alumina modulus, the ratio of alumina to iron oxide:

MA Al OFe O

2 3

2 3

Bauxite an alumina mineral rich in Al2O3 and can be used to increase the MA.



Fly ash, one of the waste materials from the power generation industry is also

used as a raw material. This is known as pulverized fly ash, PFA. Typically, this

is higher in SiO2 content.

The number of components used in the raw mix is typically 4-5 raw materials

depending upon the need for correction of the three main modules: LSF, MS,MA.

The physical nature of a raw material is also important. Very wet materials can

be the reason for choosing the wet or semi-dry process. Very abrasive materials

like sand and basalt are costly to crush and grind to the fine state needed in the

raw mix. The chemical variation in the raw material is also important. If there are

great variations, more homogenization will be required.

4.4 CHEMICAL COMPOSITION AND CONTROL OF RAW MIX

The chemical composition of the raw mix has to be prepared correctly to yield a

good clinker. Also, the variation in the raw mix going to the kiln has to be small to

obtain good burning conditions for the kiln and preheater. The first step in the mix

design is the determination of the chemical composition of the raw materials. It is

common to have an approximate analysis of each raw material and use this for

the calculation of the mix ratios. A sample after the raw mill is easily obtained for

analysis. An analysis can be performed quickly using X-ray fluorescent analysis,

XRF. Timely adjustments can then be made to the raw mill weighfeeders.

A typical calculation of a raw mix and the corresponding clinker can be made

using a simple spreadsheet like EXCEL with its SOLVER function, i.e. as shown

in table no.8 below.

The calculations made in the table show that 5 raw materials have been available

at this plant. This has made it possible to satisfy 4 conditions, one less than the

number of raw materials. The four conditions here are:



LSF = 94

MS = 2.75

MA = 1.90

Na2O+ 0.658*K2O = 0.64

The alkalies often have to be restricted to satisfy a requirement for low alkali

cement. Low alkali cement is needed when the aggregate contains reactive

silica, which can give an expansion in concrete.



Table 8Calculation of Raw Mix

RAWMIX NO: 1

HEAT CONSUMPTION 750COAL HEATVALUE Hi

7420

COAL ASH 9.4Bottom Coal

Limestone

Shale Fly ash Sand ash Rawmeal

Raw meal ash Clinker Setpoint

X-PLANT 980257 980576 980578 980575 980577 (LOI free) 970988

SiO2 5.52 54.84 54.58 78.41 39.39 14.47 22.14 36.28 22.28Al2O3 0.58 15.12 26.72 3.76 18.30 3.40 5.19 17.09 5.31Fe2O3 0.23 7.33 9.42 1.70 28.69 1.64 2.51 31.98 2.79CaO 51.81 0.81 1.49 5.84 3.34 43.49 66.53 8.48 65.98MgO 0.36 1.33 0.71 1.43 0.76 0.47 0.72 0.79 0.73Mn2O3 0.02 0.07 0.08 0.02 0.14 0.03 0.04 0.13 0.04

TiO2 0.03 0.83 1.56 0.17 0.83 0.19 0.29 0.78 0.29P2O5 0.02 0.04 0.17 0.02 0.12 0.03 0.05 0.11 0.05K2O 0.14 2.76 2.05 1.07 1.50 0.44 0.67 1.09 0.68Na2O 0.06 0.34 0.35 0.88 0.31 0.13 0.20 0.32 0.20SO3 0.19 8.71 0.45 0.08 4.14 0.61 0.93 2.48 0.95LOI @900 oC 40.74 7.95 2.15 6.53 2.44 34.64 0.00 0.00TOTAL 99.70 100.13 99.73 99.91 99.96 99.53 99.29 99.53 99.29Cl- 0.006 0.012 0.009 0.006 0.030 0.007 0.010 0.000 0.010C 0.03 0.07 0.68 0.17 0.17 0.085 0.130Free CaO 0.00LSF(SO3) 317 -3 1 3 0 94 94 93LSF 318 0 1 3 2 95 95 93.7 94Na2O+0.658*K2O 0.64 0.67MS 6.81 2.44 1.51 14.36 0.84 2.87 2.87 2.75 2.75MA 2.52 2.06 2.84 2.21 0.64 2.07 2.07 1.90 1.90CaO(SO3) 65.32C3S 56.87C2S 21.28C3A 9.34C4AF 8.49

Min. weight% 80.00 4.00 2.50 1.00 0.00 99.80Weight % (X) 83.11 4.09 7.00 4.14 1.47 99.80 99.05 0.95 100.00Max. weight % 90.0 10.0 7.0 5.0 5.0 100.2

The chloride content is also shown. Chloride is a volatile component and can

form coatings together with alkalies in the preheater. The chloride content has to

be restricted in preheater kiln systems to 0.015% by weight in the kiln feed.

.4.5 PHYSICAL CONTROL AND COMPOSITION OF RAW MIX

The raw mix must contain the proper fineness and be homogenized before



entering the kiln. For the old fashioned wet process and for the semi-dry process,

the kiln feed is slurry with water content of 33 to 40%. In the dry process, the kiln

feed is a dry powder with a typical moisture content of 0.5 to 1.5%. The fineness

of the raw mix is measured on sieves .The normal sieves with respective

residues used are:

90 or 0.09 mm 10-20% retained (equivalent to ASTM 170)

200 or 0.20 mm 0.5-1.5% retained (~ equivalent to ASTM 70)

Also, the composition of the residue is important. Free silica (quartz) will for

instance result in poor reactivity or burnability of the material in the burning zone.

When the coarse particles have a similar composition to the kiln feed (less quartz)

then a greater amount of residue can be tolerated.

In the wet process, the slurry moisture should be as low as possible but still be

transportable via slurry pumps. The amount of water that is evaporated from

slurry with a moisture content of 30% is 0.66 kg/kg clinker, while a moisture

content of 35% requires evaporation of 0.83 kg water/kg clinker.

4.6 BURNABILITY OF RAW MIX

The reactivity of the kiln feed for slurry or raw meal is checked by the burnability test in the

laboratory. The procedures can vary from different kiln suppliers but in principle a few small

nodules are made of the raw mix ground to a fixed sieve residue. Usually, three sets of nodules

each of different sieve residues (5, 10 and 15% residue on 0.09-mm sieve) are burned for a

given time and at a given temperature. The clinker formed is then crushed and ground

determining the amount of free CaO. The free CaO is compared to the free CaO content

expected or found on a standard raw mix and classified as hard, normal or easy burnability.

If the raw mix is hard to burn, then the raw mix may have to be ground finer or the composition

might have to be altered. The first changes would normally be made to the MS or LSF. The

burnability can be estimated from the below formula:



CaO (1400oC) = 0.35*(LSF-96)+1.58*(MS-1.6)+0.55*(A44)+0.12*(R125)where: A44= acid insoluble residue> 44 in %

R125 = total residue > 125 in %

4.7 CLINKERISATION

The reaction zones that occur as the raw mix is fed to the pyro system are:

1) Drying Zone: < 100°C, evaporation of free water

2) Preheating Zone: 100-750°C, loss of bound water in clays

3) Calcining Zone: 750-1000°C, decomposition of carbonates CaCO3, MgCO3 and

others in the calcination zone. The CO2 leaves the kiln with the exhaust gas. CaO

and MgO are formed.

4) Burning Zone: 1000-1450°C, some liquid is formed and the fusion forms clinker

minerals C2S and then C3S.

5) Cooling Zone: 1450-1300°C, the melt solidifies and the material crystallizes,

cooling zone.

For dry preheater kilns without a precalciner, the material entering the rotary kiln is 40 to 50%

calcined. When a calciner is installed, the material is 80 to 95% calcined when entering the

kiln. A calciner temperature of ~ 875°C will usually result in a calcination of approximately 90-

95%.

The advantages of a modern dry kiln with a preheater compared to a long wet kiln are:

- Smaller kiln

- Lower fuel consumption

- Less replacement of refractory due to longer lining life in the burning zone

- Better process control

- Larger production capacity

The advantages of a precalciner dry kiln compared to a preheater kiln are:

- Smaller kiln

- Better and easier process control

- Longer refractory life in the burning zone



- Two step firing with approx. 60% firing in the calciner and 40% firing in the kiln.

- Larger production capacity

4.8 INFLUENCE OF THE RAW MIX ON CLINKER FORMATION ANDBURNABILITY

The clinker formation is very important for plant operation and for cement quality.

Fine dusty clinker will be difficult to handle in the grate cooler and a large dust

circulation may start between the cooler and the burning zone. The coating in the

burning zone can become quite porous and unstable. Grinding of fine clinker

calls for a higher power consumption in the cement mill.

The two factors determining the clinker formation and the clinker size are:

a) Agglomeration and nodulization in the burning zone due to liquid

formation

b) Formation and growth of C3S crystals working against nodulization

The nodulization depends on the liquid to bind the fine particles together. The

formation is a function of particle size and the amount of liquid. In the rotary kiln,

the liquid phase will start forming around 1340oC and amounts to 20-25 %. An

increase in temperature does not increase the amount of melt substantially.

Formation of C3S starts, the rate increases and the size of the crystals increase.

The formation and growth of crystals in the burning zone eventually stops the

agglomeration. Four main characteristics of clinker formation are:

Alite size: measure of kiln burning zone temperature rise 1200-1450°C where

belite is combining w/ CaO to form alite crystals. Rapid heating is desirable

w/ alite size ranging from 15-20, whereas, slow heating results in sizes of

60 or greater.



Belite size: reflects retention time in burning zone above 1400°C. Maximum

retention time is desired w/ average crystal length of 25-40. Shorter

retention time = 5-10.

Belite color: rate of initial cooling to below 1000°C. Rapid cooling is desired

resulting in clear crystals. Slower cooling gives yellow to amber colored

crystals.

Alite Birefringence: difference between refractive indices of blue/red light

related to maximum kiln temperature which is desired, birefringence of 0.008-

0.010. A cooler burning zone yields ~ 0.002.

Figure 1

Melt Formation as a Function of Temperature



Figure 2

% Liquid as a Function of MS, MA and LSF

The raw mix chemistry has a major influence on the process. A high MS will

result in less liquid phase formation and require a higher burning temperature.

Lowering the MS will give better burning and nodulzation. A lower MA results in a

higher liquid phase at a lower temperature. A higher LSF will give more C3S

formation. Depending upon the level of LSF, a higher LSF will result in a higher

burning zone temperature and above a certain level the nodulization is impeded

and the clinker gets more dusty.

5. PROCESS AND KILNS

5.1 TYPES OF KILNS - wet, dry & semi-dry

The dry process is used predominantly today because of the lower heat

consumption and the better process control compared to the wet process. The

wet process is only used when fuel is very cheap or the raw materials are very

wet not making it economically feasible to replace it.

5.2 WET KILN



The wet kiln was for many years the standard equipment in the industry. Fuel

was cheap and the process of slurry preparation was easy. Homogenization in

silos and large slurry basins blended the slurry perfectly. The wet kiln had to

perform drying, preheating, calcination, burning and often clinker cooling in one

piece of equipment. However, the wet kiln has some limitations:

-Heavy fuel consumption

-Process control difficult

-Production over 1500 tpd clinker difficult

-High refractory cost

5.3 DRY KILN

A brief review of the FLS/FULLER dry kiln types is found in table no.9 below.

The table is included because there is a connection between cement chemistry and the

choice of kiln type. The layout of the different kiln types is shown on the figure on

enclosure 2.

5.3.1 LONG DRY KILNThe long dry kiln with a cross section for heat exchange is not installed any more. It has

been superceded by the more efficient preheater kiln systems

5.3.2 SP KILNThe Suspension Preheater type, SP, has preheater cyclones but no calciner. The number of

cyclones is 4 to 6. The material going into the kiln after the preheater has a degree of

calcination of 40 to 50 %. The last half of the calcination takes place in the kiln. This means

that the necessary amount of heat exchange in the kiln is larger than in the kiln types with a

separate calciner. The kiln has to be larger for a given production. Due to the calcination in the

kiln, the charge is fluidized by CO2 giving the material a chance to flow freely.

5.3.3 ILC-E KILNThe In-Line-Calciner using Excess Air type, ILC-E, has no tertiary air duct as all the air

passes through the kiln. A small calciner is built into the riser duct and the air for



combustion is drawn through the kiln.

5.3.4 ILC KILNThe In-Line-Calciner type, ILC, has a calciner in line with the kiln. The preheater is a

single string with the calciner built into the riser duct. Combustion air is drawn from the

grate cooler through a separate tertiary air duct. A damper in the tertiary air duct allows

for balancing the air between the calciner and the kiln.

5.3.5 SLC KILNThe Separate-Line-Calciner type, SLC, is a double string preheater with the calciner in

one string. The calciner is placed in parallel to the kiln riser duct. The combustion air for

the calciner is atmospheric air heated in the grate cooler and transported through a

tertiary air duct. The gas from the calciner and the gas from the kiln are not mixed and

pass through two separate strings of preheater.

.5.3.6 SLC-S KILNThe Separate-Line-Calciner –Special type, SLC-S, is a single string system with the

calciner placed at the side of the riser duct from the kiln. The gas stream from the calciner

is mixed with the gas from the kiln riser duct and pass through one string of preheater

cyclones. There is only one main ID fan. An adjustable restrictor at the top of the riser

duct makes the distribution of air between the calciner/tertiary air duct/cooler and the

kiln/riser duct.

5.3.7 SLC-I KILNSeparate-Line-Calciner –with In-line-calciner in kiln string type, SLC-I. The system is a

development of the SLC system. It has the two strings as the SLC system but with a

calciner also in the kiln string. The SLC-I system can be used for upgrading of a SLC

system by the installation of a small calciner in the kiln string. The firing in the two

calciners will be:

-SLC-calciner: 40-50% of total firing

-ILC-calciner: 10-15% of total firing



Table 9

Comparison of Kiln Systems

Type SLC SLC-I SLC-S ILC ILC-E SP

tertiary air Yes Yes yes Yes no no

production, mtpd 11000 9000 4500 7500 4000 3500

calciner, % fuel 55/60 55SLC:40/50ILC:10/15

55/60 55/60 15/25 5

bypass, max% of kiln gas 100 100 30 60 25 30

inferior fuel in calciner Yes Yes yes medium no

UNAX, KWh/t 12/14 11/13

FOLAX, KWh/t 22/24 22/24 22/24 22/24 18/20 17/19

min.production % 40 40 70 70 70 70

max.content Na2Oeq 1.0-1.5 1.0-1.5 1.0-1.5 1.0-1.5 1.0-1.5 1.0-1.5

in % SO3 0.8-1.2 0.8-1.2 0.8-1.2 0.8-1.2 1.0-1.6 1.0-1.6

(clinker basis) Cl 0.015 0.015 0.015 0.015 0.023 0.023

.5.4 ASH ABSORPTION

The ash from the coal used for combustion will be absorbed in the clinker. This is

shown on the calculation sheet above (table 8). In the example, a coal with a

heat value of 7420 kcal/kg and an ash content of 9.4% is used. The heat

consumption in the kiln is 750 kcal/kg. The amount of ash absorbed is then:

Ash absorption = (750/7420) * 9.4 = 0.95 % of clinker

Therefore, it is necessary to analyse the coal ash to allow for this addition to the

raw mix.

5.5 VOLATILE MATTER

Some of the chemicals in the materials going into the burning zone evaporate.

The components can come from the raw materials, fuel and waste products. The

four most important are: potassium, sodium, sulfur and chloride. There are others

but they are normally of minor importance. These minor materials, heavy metals



or certain organic compounds can have important implications on the

environment for a given plant. The plant should be aware of the different minor

constituents to prevent any problems.

The four volatile elements mentioned above evaporate in the burning zone and

condense again in the colder parts of the kiln system. The colder parts are the

outer walls causing coating and build-up. Volatiles will also condense on the raw

meal particles, as they are colder than the gas carrying the volatiles.

Some of the volatiles may escape from the kiln system partly being caught in the

filter or escaping into the atmosphere. The volatiles that do not leave the system

with the exhaust gas either remain in circulation in the kiln system or leave with

the clinker. The volatiles can accumulate in the kiln and preheater causing

problems with in build-up in the cyclones and riser duct. It is important to be able

to foresee any problems that may occur before the start up of a new plant or a

conversion of an existing plant to prevent the possibilities of plugging.

A certain portion of the volatiles in the material flowing into the burning zone of

the kiln will evaporate at the high temperature. The portion that evaporates is

defined as the evaporation factor called (epsilon). A portion of the volatile

material leaves the system with the exhaust gas. This is referred to as a valve V.An internal circulation of volatiles takes place and the circulation factor is called

K. The part of the volatile leaving the kiln with the clinker is the residual part

called R. A simple layout of a kiln system is shown below:



Figure 2

Circulation of volatile in simple kiln system

Evaporation factor = d/b= (b-c)/b=1- c/b

Valve V = e/d= (a-c)/(b-c)

Circulation factor K = b/a

Residual component R = c/a

For one unit of feed: a = 1 the material balance is:

K*(1-) + KV = 1; K = 1/(1-(1-V)) ;

R = K*(1-)

The circulation factor K is the amount of compound going to the burning zone

when feeding a unit of 1 (one) to the system. R is the amount going into the

clinker. It is possible to calculate the circulation K and the residual in clinker R

when the evaporation factor and the valve V are known. The enclosed table

and figure give evaporation factors and valves for typical cases.



Table 10

Melting and Boiling Points of Alkali Salts

COMPOUND K Na

Melting

temp.

°C

Boiling

temp.

°C

Melting

temp.

°C

Boiling

temp.

°C

-oxide Decompose 350 Sublime 1275

-carbonate 894 Decompose 850 Decompose

-sulfate 1074 1689 884 Decompose

-chloride 768 1411 801 1440

-hydroxide 360 1320 328 1390



Table 11

Typical values for and VSymbol K2O Na2O Cl SO3

Evaporation factor 0.20-0.40 0.10-0.25 0.990-0.996 0.35-0.80

Kiln valve,wet kiln,nodule

operatingVo 0.50 0.70 0.70 0.60

Kiln valve,wet kiln, dust

operatingVo 0.40 0.60 0.60 0.40

Long dry kiln Vo 0.20 0.50 0.60 0.40

Kiln valve,1-stage kiln Vo 0.55 0.80 0.60 0.35

Kiln valve,2-stage kiln Vo 0.70 0.85 0.80 0.60

Kiln valve,4-stage kiln Vo ~1 ~1 ~1 ~1

Kiln valve,precalciner kiln Vo ~1 ~1 ~1 ~1

Cyclone preheater valve 1-

stageVc 0.35 0.50 0.35 0.45


stageVc 0.20 0.45 0.20 0.30


stageVc 0.15 0.40 0.05 0.15-0.50

Dedusting cyclone valve Vc 0.60 0.70 0.50 0.55

Raw mill valve Vm 0.60 0.80 0.70 0.30

Cooling tower valve Vkt ~1 ~1 ~1 ~1

Electrostatic precipitator

valveVf 0.40 0.70 0.30 0.50-0.80

The evaporation of alkalies is larger when chloride is high. This is at times used

to increase the evaporation in the burning zone by adding CaCl2.

Sulfur is difficult to evaluate. Some sulfur in the raw mix is present free in various

organic compounds or in pyrites. Approximately, 50% of the sulphur burns off in

the top stages of the preheater tower. CaCO3 assisted by moisture catches some

of it in the rawmill. SO2 in the preheater also reacts with calcium carbonate with a



maximum around 800°C.

Sulfur in combination with alkalies behaves differently than SO2 from fuel. Excess

sulfur, sulfur not bound as alkali sulfates, can be calculated as:

Excess S = 1000*%SO3- 850*%K2O-650*%Na2O < 250 g/100 kg clinkerwhere: All percentages are calculated on clinker basis

SO3 is total from rawmix + fuel

To ensure trouble free operation of a preheater kiln the following limits apply:

Table 12

Limits for Volatiles

Raw mix burnability Easy Hard

Na2O + K2O (% clinker basis) Max 1.5 % Max 1.0 %

Cl (% on clinker basis) Max 0.023 % Max 0.023 %

SO3 (% on clinker basis) from raw mix + fuel ;Or excess sulfur under 250 g/100 kg clinker

Max 1.6 % Max 1.0%

If the natural valves are insufficient, then a kiln bypass can be installed. The

bypass will take part of the kiln gas before the preheater and transport it to a

separate cooling and dedusting system. The bypass gas has to be cooled

immediately to 350oC to avoid clogging. The cooling takes place in a swirling

chamber with atmospheric air. Some dust will be removed with the bypass gas

(2-3% with 10% bypass.)

5.6 MAIN FEATURES DURING BURNING

Chemical control during operation of the kiln system is divided into the following:

-Feed composition

-Product quality of clinker

-Emission control

-Fuel



-Preheater

The raw meal must have the correct quality with little variation. The standard

deviation for LSF should be less than 1% and corresponding less than 0.1 for MS

and MA. Large variations will result in irregular kiln operation resulting in

problems with ring formation and coating in the preheater, as well as, requiring

higher fuel consumption.

Performing a free lime analysis on an hourly basis monitors the product quality of

the clinker. The analysis can be made either on an average hourly sample or on

an hourly spot sample.

Environmental authorities stipulate emission control in many countries. The

plants have to control and continuously register plant emission of dust, SO2 and

other constituents in the exhaust gas. The results have to be reported back to

the authorities.

The type of fuel used in cement production is either pulverised coal, fuel oil,

natural gas, or waste products.

Pulverised coal is usually produced at the site in a coal mill that dries and grinds

the raw coal to a fineness of approximately 15% retained on the 0.09-mm sieve

and moisture content of 1 to 2 %. The residue and the moisture content vary

according to the type of coal. Some types of coal with high gas content have a

high tendency toward self-ignition, which has to be taken into account. Coal with

low content of volatiles like semi-anthracite has to be ground very fine to promote

ignition.

An important component in heavy fuel oil and coal is the sulfur content. The

sulfur has to be taken into account together with the alkalies. Sulfur content in

heavy fuel oil above 5% will usually cause build up problems. Fuel analysis

should be made regularly by either the supplier or the plant laboratory.



The preheater has to be kept free from coating that can clog the cyclone outlet or

increase the pressure drop in the riser duct. This can be followed by regular

sampling of the material going into the kiln and analyse for chloride, sulfur, and

alkalies.

6. HEAT OF REACTION & HEAT TRANSFER

The chemical change from raw material to clinker requires heat for two reasons.

The first is due to the heat of reaction for the transformation to clinker.

Secondly, because the clinker process is not an ideal or 100 % effective process,

heat is lost from the kiln system as:

Radiation loss from all outer surfaces

Heat loss with the gasses from the kiln

Excess hot air from the clinker cooler

Heat loss with hot clinker

Heat effects the chemical reactions, the formation of solutions and changes in

the state of aggregation such as melting or vaporization. The heat effects are

called exothermic, when a reaction is accompanied by heat evolution. When heat

is absorbed, then the reaction is endothermic.

The dissociation of CaCO3, calcium carbonate, is a typical endothermic reaction:

CaCO3 CaO + CO2 – 422 kcal/g

The double arrow signifies that the reaction can be reversed. In the preheater,

this is called recarbonation. The order of magnitude of the heat of recarbonation

is normally evaluated from the temperature profile and the temperature difference

between the lowermost and second lowermost cyclone in the preheater tower.

When planning a new plant or when making a kiln conversion it is important to

know the heat of reaction for the process. The analysis is made in the laboratory

of the equipment supplier. Basically, there are the following heat changes:



Table 13

Reactions During Heating

Temp °C Reaction Heat change

<100 Evaporation of free water Endothermic

100 - 400 Absorbed water evaporates Endothermic

400 - 750 Decomposition of clay minerals,

Kaolinite metakaolinite

Endothermic

600 - 900 Decomposition of metakaolinite to free reactive oxides Endothermic

600 - 1000 Decomposition of carbonates to free reactive oxides Endothermic

800 - 1300 Reactive oxides form intermediate or final clinker

minerals

Exothermic

1300 - 1380 Formation of clinker melt from aluminates and ferrites Endothermic

1250 - 1500 formation of aliteC3S the principal clinker mineral Endothermic

The reactions within the kiln system take place at a slightly negative pressure

and in an oxidizing atmosphere. Reduction does not normally take place in the

kiln system apart from a reducing zone in the riser duct of the preheater to

reduce the emission of NOx. The first five reactions in the table take place rapidly

with a velocity determined by the transfer of heat from the gas to the solid

material. The last two reactions are determined first by the contact rate of the

reactive components present in the solid phases and later in the burning zone by

diffusion of the reactive components in the clinker liquid phase. The total of the

heat reactions during clinker formation is endothermic. An example of the heat of

reaction is:



Table 14

Heat of reaction Kcal/kg

Evaporation of combined water 20

Decomposition of clay minerals 35

Dissociation of carbonates 475

Formation of clinker minerals -130

Combustibles in the raw mix -15

Total heat of reaction 385

The heat of reaction is the theoretical heat consumption for the kiln system.

Since the process is not ideal, heat losses exist in the system. The losses of

heat come from the following:

Hot exhaust gas

Heat loss from surfaces of the kiln system, i.e. radiation loss

Excess air from the clinker cooler

Heat lost with clinker after the cooler

Some of the loss can be utilized for drying of the raw materials and of raw coal.

Table 15

Typical Heat Consumption for Different Systems

Specific heat consumption for kiln systems kcal/kg clinker

Wet kiln 1400

Long dry kiln 1100

1-stage preheater kiln 1000



Semi-dry kiln w/ preheater & calciner 950

5-stage kiln w/ preheater & calciner 725

5-stage preheater kiln w/calciner and latest cooler type 690



The dry process is always chosen unless the raw materials have moisture

contents above 20-30%.

The efficiency of the heat exchange in a cyclone is the same as the separation

efficiency due to the rapid heat transfer between material and gas. Usually, there

is only a temperature difference of 5°C between the exit gas and material leaving

the cyclone. There is, however, a variation in efficiency between the cyclones as

we go lower down in the preheater. This is due to the change in the design of the

cyclones. At the high temperature in the lower cyclones these cyclones do not

usually have an internal vortex. The vortex or central pipe is difficult to construct

in a material that will last at the high temperatures.

7.FUEL

7.1 TYPES OF FUEL

The most common types of fuel are: coal, fuel oil and natural gas. The most

common fuel is coal with heavy fuel oil being second. Natural gas is used where

available and is an excellent fuel. Many waste products from a variety of

industrial sources are also used as a fuel source.

7.1.2 COAL

Coal is found all over the globe. Coal originates from plants, that over many

millenniums have been transformed into coal. The age of coal results in different

composition and quality. Anthracite and hard coal are old types, while lignite and

peat are younger types.

Raw coal is a combination of coal, ash and water. The carbon is the main

constituent in the coal, but there are also hydrocarbons, oxygen, nitrogen and

sulfur often as pyrites FeS2. Heating the coal in a non-oxidising atmosphere

drives out some of these constituents as gas also referred to as volatiles and tar.

The coal is then changed into coke. Younger coal has a higher gas content than

older coal. They are easier to ignite than the older coal. They are also prone to



self-ignition during storage.

Table 16

CLASSIFICATION OF COALS

Lignite Bituminous coal Anthracite

Total moisture % 40-50 5-10 0 – 3

Volatiles % 40 – 50 10 – 40 5

Hygroscopic water % 10-25 1-3 1

Ash % 5-25 10-20 5-10

Examples of commercial grades of coals

Chemical composition:

Carbon C % 56 70 78

Hydrogen H % 4 3 2

Sulphur S % 1 1 1

Nitrogen + oxygen N+O % 19 3 2

Heat value Gross Hs Kcal/kg 5120 6625 7100

Net Hi Kcal/kg 4820 6310 6900

Combustion air and gas

Combustion air required weight Kg/kg 7.1 9.2 9.9

volume Nm3/kg 5.5 7.1 7.6

Composition

of combustion

gases

Wet , 0%

oxygen

Total Nm3/kg 6.0 7.4 7.8

CO2 + SO2 vol% 17.8 17.6 18.9

H2O vol% 10 6.5 4.5

N2 vol% 72.2 75.9 76.6

Dew point °C 46 38 31

Table 17Typical Petroleum Cokes

Type %H2O %Volatiles %Fixed C %Ash %Sulfur Gross Heat

Value (kcal/kg)

Hardgrove

Green delayed 8 11 82 0.5 4 8000 60-100

Fluid 0 5 86 1.0 4 8000 25-30

Proximate analysis of coal:

The proximate analysis of a coal sample gives values for moisture, volatile

matter, ash and fixed carbon. It is performed under detailed laboratory



procedures, which can be found in other reference material. The volatile matter is

the portion of the coal when heated to 900°C without air is driven off as gases.

Fixed carbon is the residue remaining after the volatile matter is driven off. The

ash content is found by heating the sample to 800°C. The moisture in coal is

divided into free moisture and hygroscopic moisture, where the free moisture is

the moisture lost by air-drying. The volatile matter, fixed carbon, ash, and

moisture add up to 100%.

The amount and composition of ash varies from one coal to another. The amount

of ash and its composition has to be known, as it will be a part of the clinker.

Some coals have a very abrasive ash and a high wear index, which is of value

especially for vertical coal mills.

Coal dust can be dangerous and cause explosions. It can also self-ignite at room

temperature. For that reason a safety index for a particular coal is assigned

according to the characteristics of the coal.

The common safety index is the ratio of % fixed carbon to % volatiles. A high

safety index means a lower chance for a coal dust explosion. The safety index

for coal varies from 1 for high volatile lignite up to 15-16 for petcoke and

anthracite.

Ultimate analysis of coal:

In the ultimate analysis of coal, carbon, hydrogen, sulphur, nitrogen and oxygen

are determined.

Chemical analysis:

In the chemical analysis of coal, the inorganic composition is determined on the

coal ash. The values are used for the calculation of the raw mix and clinker



composition.

Heat value:

The heat value or calorific value is important for the evaluation of the coal and for

the heat economy of the kiln. The difference between the gross and net heat

value is the heat of evaporation of the water from combustion and the

evaporation of the water. The approximate calculation is:

Hnet = Hgross – 5.85(9*%H + % Water in sample) [kcal/kg]

7.1.3 FUEL OIL

Fuel oil is used for cement kilns at many plants. The fuel oil most commonly used

is a heavy fuel oil. The lighter fuel oil types are often used for lighting up kilns.

Typical analysis for oil is:

Table 18

TYPICAL FUEL OILSGas oil Light fuel oil Heavy fuel oil

Composition

C% 86.3 86.2 86.1

H% 12.8 12.4 11.8

S% 0.9 1.4 2.1

Specific gravity, kg/litre

0oC 0.880 0.905 0.960

15oC 0.870 0.895 0.960

At 2° Engler 0.880 0.865 0.880

Temperature in

°C for 2° Engler0 60 120

Specific heat Kcal/kg/oC 0.485 0.480 0.465

Calorific valueGross, Hs Kcal/kg 10875 10550 10375

Net, HI Kcal/kg 10200 9900 9750

Air for combustionKg/kg 14.4 14.2 14.0

Nm3/kg 11.1 11.0 10.8

Combustion gases

Wet, 0% O2 Nm3/kg 11.80 11.68 11.51

CO2+SO2 % 13.7 13.9 14.1

H2O % 12.0 11.8 11.4

N2 % 74.3 74.3 74.5

Dew point oC 50 50 49

Theoretical flame temperature oC 2160 2120 2120

The composition of fuel oils does not vary as much as coals. There are, however,



changes from place to place. The different organic components may vary. An

important value is the specific gravity because the calorific value is related to it.

Specific gravity changes with temperature. This is important to take into account

when a heat balance has to be made. The volume consumption of the oil has to

be converted into a weight consumption in order to calculate the heat input.

Another important value is the viscosity of the oil. For the burners at Fuller and

FLS, the viscosity has to be 2° Engler (or 18 centiStoke) for proper atomisation of

the oil. Some oils require a temperature of 120°C others have a higher

requirement up to 170°C.

The sulfur content could be quite high and should be limited to a maximum of 5%

S.

7.1.4 GAS

Natural gas is an excellent fuel source for cement kilns because it is neat and

clean as well as being easy to use. The installation is simple and the composition

of the gas has very little variation. Although, there is a risk for explosions and

special safeguards have to be installed and maintained. The table below has

typical values for natural gas:



Table 19

Examples of Typical Natural Gases Overseas

Dutch(Groningen)

Sahara North Sea(typical)

Com

posi

tion

CH4 Vol% 81.76 86.50 91.80

C2H6 Vol% 2.73 9.42 3.50

C3H8 Vol% 0.38 2.63 0.80

C4H10 Vol% 0.13 1.06 0.30

< C5 Vol% 0.16 0.09 0.33

Calorificvalue

Gross Kcal/Nm3 8500 10780 9700

Net Kcal/Nm3 7580 9750 8760

Air for combustion Kg/Nm3 10.91 13.96 12.60

Nm3/Nm3 8.44 10.80 9.75

Com

bust

ion

gase

s

wet

, 0%

oxy

gen

free)

Total Nm3/Nm3 9.20 11.52 10.60

CO2 Vol% 9.80 10.60 9.80

H2O Vol% 18.60 17.70 18.50

N2 Vol% 71.60 71.70 71.70

Dew point OC 59 58 59* The natural gas contains varying amounts of nitrogen.



Table 20Domestic Natural Gas Properties

City

Gross

Heating

Value

(Btu/ft3)

Specific

Gravity

Components of Gas (% volume)

CH4 C2H6 C3H8 C4H10 C5H12 C6H12

+

CO2 N2 Misc.

Baltimore, MD 1051 0.590 94.40 3.40 0.60 0.50 0.00 0.00 0.60 0.50

Birmingham, AL 1024 0.599 93.14 2.50 0.67 0.32 0.12 0.05 1.06 2.14

Boston, MA 1057 0.604 93.51 3.82 0.93 0.28 0.07 0.06 0.94 0.39

Columbus, OH 1028 0.597 93.54 3.58 0.66 0.22 0.06 0.03 0.85 1.11

Dallas, TX 1093 0.641 86.30 7.25 2.78 0.48 0.07 0.02 0.63 2.47

Houston, TX 1031 0.623 92.50 4.80 2.00 0.30 … … 0.27 0.13

Kansas City, MO 945 0.695 72.79 6.42 2.91 0.50 0.06 … 0.22 17.1

Los Angeles, CA 1084 0.638 86.50 8.00 1.90 0.30 0.10 0.10 0.50 2.60

Milwaukee, WI 1051 0.627 89.01 5.19 1.89 0.66 0.44 0.02 0.00 2.73 0.06 He

New York, NY 1049 0.595 94.52 3.29 0.73 0.26 0.10 0.09 0.70 0.31

Phoenix, AZ 1071 0.633 87.37 8.11 2.26 0.13 0.00 0.00 0.61 1.37

Salt Lake City, UT 1082 0.614 91.17 5.29 1.69 0.55 0.16 0.03 0.29 0.82

San Francisco, CA 1086 0.624 88.69 7.01 1.93 0.28 0.03 0.00 0.62 1.43 0.01 He

Washington, D.C. 1042 0.586 95.15 2.84 0.63 0.24 0.05 0.05 0.62 0.42

Gas is difficult to ignite with an ignition temperature of 600°C. Therefore, the kiln

lining has to be hot prior to starting the gas burner without a pilot.

It is an advantage that the content of sulfur is low, the volume of the combustion

products is high and has a high dew point. The heat loss with the exit gas is

consequently high, but this is compensated by a low requirement for primary air.

7.1.5 WASTE FUELS

A variety of waste fuels are burned at cement plants. They consist of old tires,

used lubrication oil, and used solvents, i.e. waste plastic. Proper precautions

have to be in effect to protect not only the personnel but the environment as well.



Cement plants have accepted the responsibility for disposing of these many

different wastes and in some cases are financially rewarded.Table 21

Typical Waste Fuel AnalysisType of

Waste

Heating value

(Btu/lb)

Volatiles

(%wt.)

Moisture

(%wt.)

Ash

(%wt.)

Sulfur

(%wt.)

Dry Combustible

(%wt.)

Density

(lb/ft3)

Paper 7572 84.6 10.2 6.0 0.20

Wood 8613 84.9 20.0 1.0 0.05

Rags 7652 93.6 10.0 2.5 0.13

Garbage 8484 53.3 72.0 16.0 0.52

Coated fabric:

rubber

10996 81.2 1.04 21.2 0.79 78.8 23.9

Coated felt:

vinyl

11054 80.87 1.5 11.39 0.80 88.61 10.7

Coated fabric:

vinyl

8899 81.06 1.48 6.33 0.02 93.67 10.1

Polyethylene

film

19161 99.02 0.15 1.49 0 98.51 5.7

Foam: scrap 12283 75.73 9.72 25.3 1.41 74.7 9.1

Tape: resin-

covered glass

7907 15.08 0.51 56.73 0.02 43.27 9.5

Fabric: nylon 13202 100.0 1.72 0.13 0 99.87 6.4

Vinyl scrap 11428 75.06 0.56 4.56 0.02 95.44 23.4

8.COMBUSTION

Combustion is the chemical reaction between oxygen and fuel. The main

component of coal and fuel oil is carbon, which is oxidised, or burns to carbon

dioxide CO2 with the evolution of heat.

The oxidisation of coal can start at room temperature with a coal of high reactivity

or high volatile content. Ignition of the raw coal can take place in the storage area

if the material is not stacked properly. If the temperature surpasses a certain

limit, the ignition temperature, a fire can easily start if oxygen is available. The

ignition temperature for coals varies in relation to the gas or volatile content.

Lignites and bituminous coals with volatile content over 30% have low ignition



temperatures whereas anthracite, petcoke and semi-anthracite require a high

ignition temperature.

In the burner, fuel is mixed with primary air as the two streams exit the burner

nozzle. The mixing depends on the direction of the stream and of the mixing

energy or velocity. The temperature of the primary and secondary air is also

important. The temperature of the flame can be calculated under ideal

conditions. The maximum theoretical flame temperature is found by fuel ignition

with the stoichiometric air requirement. The practical value is approximately 80

to 85% of the calculated one.

For coal, the water will evaporate first followed by gas evolution from the volatile

portion of coal. The rest of the fine and porous coal particles burn with a visible

flame due to the emission from the fine particles in the flame. In contrast, natural

gas can burn with hardly any visible flame. The fine coal particles will oxidise to

CO and later to CO2 as more oxygen becomes available. The flame front travels

with a certain velocity increasing with temperature and is also dependent on the

injected air/fuel stream for a steady combustion to be maintained. Coal with a low

content of gas like anthracite and semi-anthracite requires a high ignition

temperature, and usually a small amount of primary air is used.

The conditions for fuel oil are similar to coal. The oil droplets evaporate to gas

that ignites and burns. The flame is luminous because carbon or soot particles

are formed during the combustion. The ignition of oil is more difficult than with

coal due to the slower mixing of air and oil and to the evaporation of the oil

droplets.

The heat is transferred from the flame to the lining and the charge in the kiln by

radiation. The radiation follows:

R= e*k*( T14- T2

4) [kW/m2]e = coefficient of emission, max=1.

K is a constant



T1 and T2 are surface temperatures in °Kelvin

The coefficient of emission for coal e is close to ~1 at the coal flame temperature.

This is due to the content of particles in the coal flame. For oil, the value of e is

0.7 to 0.9 and for gas only 0.2 to 0.6.

Radiant heat is transferred quickly to the lining and to the charge surface. But the

transfer of heat by conduction into the charge from the charge surface and from

the lining is a lot slower. This is one of the determining factors for the length of

the burning zone. The heat transfer is better when the charge is turning over

rather than sliding on the lining.

9. COAL & OIL

9.1 FINENESS OF COAL

The coal has to be dried and ground in a coal mill. The coal mill can be an air-

swept ball mill with separator or a vertical mill. The coal fineness depends on the

type of coal as seen from table 22:

Table 22

Coal PropertiesType Volatiles % Ash % Hygroscopic water, % Sieve residues

of coal meal

Anthracite < 5 5 < 2 5-7% +0.09 mm

1 % + 0.2 mm

residues

increasing to

coarser values

30% + 0.09 mm

3 – 4 % + 0.2 mm

Semi-Anthracite

5 – 15 3-5 2 – 6

Low volatile bituminous coal 15 – 20 5 – 8 2 – 6

Bituminous coal 20-30 8 – 10 2 – 6

High volatile bituminous coal 30 – 40 10 – 20 2 – 6

Lignite 40 – 50 15 – 30 10 – 25

The grindability is usually given as a hardgrove index number. The relationships

between the hardgrove index and the grindability in kWh/t for different types of

grinding applications are given below. The Hardgrove index for coals can vary



from 40 to over 100 for very soft lignite or petcoke.

An important aspect for the choice between a ball mill and a vertical mill is the

wear index. Some coals have a high ash content with an abrasive constituent

resulting in a high rate of wear. In such cases, a ball mill might be chosen or a

vertical mill with stronger wear resistant parts.

Figure 3

9.2 DRYING OF COAL

Raw coal contains varying amounts of water. The raw coal has to be dried to

facilitate grinding and handling but the drying must not go beyond the limit of

safety. The coal is dried in the coal mill only to the recommended minimum

residual moisture content to reduce the risk for fires and explosions. This

HARDGROVE-GRINDABILITIES

0

5

10

15

20

25

30

35

40

40 50 60 70 80 90

Hardgrove index

spec

.grin

dabi

lity-

KW

h/t

ballmill, 5%+ 0.09 mm

ballmill, 10% + 0.09 mm

ballmill, 20% + 0.09 mm

vertical mill, 5%+0.09mm

vertical mill, 10% +0.09 mm

Vertical mill, 20% + 0.09 mm



residual moisture content is found by using the graph in figure 4 below

according to the actual hygroscopic moisture of the coal. The plant laboratory

determines the hygroscopic moisture and residual moistures at various drying

temperatures. A detailed procedure to determine surface and hygroscopic

moisture of coal meal is found in another reference. Presented here is an

overview of the program.

Free moisture, or surface moisture, is the water lost by air drying a prepared coal

sample at an ambient temperature of 30°C. Further drying of the coal sample at

various temperatures between 30 and 105°C drives off the remaining water. The

ratio of the change in sample weight between 30 and 105°C divided by the

weight at 105°C, represents the hygroscopic moisture. A graph of the residual

moistures at various drying temperatures shows the correct operating

temperature for the outlet of the coal mill.

Usually, a coal mill outlet temperature of between 60 and 80°C is correct. The

inlet temperature to the coal mill can be as high as 300 to 350°C. The inlet

temperature depends on the quantity of air through the mill, and requires the dew

point of the outlet air after the coal mill to be 20°C higher than the outlet air

temperature to avoid condensation and clogging in the transport system following

the mill. For example, to dry one kg of coal with 10% moisture 1.2 kg drying air

at 300°C is required.

9.3 ASH CONTENT

Figure 4Recommended moisture in coal dust

024681012

1 2 3 4 5 6

Hygroscopic w ater,%

Res

idua

l moi

stur

e,%



The ash content in various coals differs in quantity as well as composition. It is

necessary to know the exact composition of the ash because it combines with

the raw mix in the kiln system. The ash analysis is performed by the plant

laboratory or by the coal supplier.

The ash content of coal will consume some heat during combustion due to

chemical changes of the minerals in the coal. This reduces the flame

temperature. In coals with a high ash content, it can be difficult to obtain a

sufficiently high flame temperature for the process.

9.4 GAS CONTENT

The gas content is important for the ignition of the coal. Anthracite and some

petcoke have low gas contents and consequently are very difficult to ignite. Only

very fine grinding can compensate for the lack of gas. Sometimes a very high

gas content can result in problems with the mixing air. This can impede ignition

and proper burning which can result in a low flame temperature.

9.5 MINOR COMPONENTS

Coal also contains alkalies, sulfur and chloride. These constituents have to be

taken into consideration for the calculation of the raw mix and of the clinker

composition. These volatile components will participate in the internal circulation

of volatile components in the kiln, calciner and preheater.

9.6 REQUIREMENT FOR AIR

The combustion of fuel requires air to oxidise the carbon, hydrogen and sulfur in

the flame into the combustion products of carbon dioxide, sulfur dioxide and

water. In addition, a certain amount of excess air is used to obtain complete

combustion without formation of carbon monoxide. The reactions are:

C+ O2 CO2 + heat

4H + O2 2H2O + heat



S + O2 SO2 + heat

The water content in the coal is evaporated.

The minimum amount of oxygen required for combustion, Omin, can be calculated

by:

Omin = (0.01)*(32/12*C+16/2*H +32/16*S+32/14*N-O) [kg O/kg fuel]

Therefore the required amount of air, Lmin, is:

Lmin = (Omin / 1.429) [Nm3O/kg fuel]

Lmin = [Omin / (0.21*1.429)] [Nm3air/kg fuel]

The total volume of the combustion gas is:

Vmin= 0.0889*C +0.2231*H – 0.0263*O + 0.0333*S +0.0124*H2O [Nm3gas/kg fuel]

Where: C, H, S, N, O, H2O are in %wt. of the fuel

Combustion takes place with surplus air to ensure complete combustion. The

surplus of air is referred to as excess air and is symbolised by the Greek letter

lambda, . The excess air in the combustion air is calculated from the formula:

= 1/(1-79/21*O2/(100-CO2-O2))

For example, the amount of excess air is 1.20% when O2= 3.6 vol% and

CO2=15.4 vol%. If there is incomplete combustion CO is present and the formula

cannot be used. Incomplete combustion increases the heat consumption of the

kiln appreciably and it is dangerous for the precipitator.

The kiln exhaust will also contain CO2 from the decarbonation of carbonates in

the raw mix. For a normal raw mix, the amount of CO2 is 0.55 kg or 0.28 Nm3 per

one kg of clinker.

10. PROCESS GAS



The process gas contains combustion products from the fuel with some excess

air, as well as, products from heating the raw mix. The various components are

described further. The amount of combustion gas can be calculated from the

equation:

Vmin= 0.0889*C +0.2231*H – 0.0263*O + 0.0333*S +0.0124*H2O [Nm3gas/kg fuel]

This amount has to be increased by the excess air:

= 1/(1-79/21*O2/(100-CO2-O2))

The CO2 released by the heating of carbonates in the preheater, calciner and kiln

must also be added. The amount for a normal clinker is approximately 0.55 kg

per kg of clinker or 0.28 Nm3. Finally, there is the additional volume from

evaporation of water in the raw mix.

In most systems, the preheater gases are taken to a raw mill and/or coal mill and

used for drying the respective material. SO2 is present in the exhaust gases from

oxidation of the fuel S and decomposition of sulfates. Most of the SO2 is totally

scrubbed in the preheater by K2O, Na2O, and CaO. Due to the lower volatility, the

alkali sulfates will most likely leave the system with the clinker. However, the

calcium sulfate ends up evaporating in the burning zone leading to an internal

cycle condensing at the backend of the kiln. The exhaust gas also contains SO2

from the sulfides and pyritic sulfur present in the raw materials. They are oxidised

in the preheater and a portion escapes with the exhaust gas. By having the

exhaust gas go to a raw mill and/or conditioning tower, scrubbing of the SO2

takes place reducing the emissions.

NOx is formed by combustion in the kiln and calciner. Fuel NOx is formed when

combustion occurs below 1200°C. The amount of fuel NOx depends on:

Amount of N in fuel

Excess oxygen

Flame temperature



When combustion temperatures are higher than 1200°C, the NOx formed is

thermal NOx. This happens in the kiln-burning zone. The NOx formed is

predominantly NO, a small part (5%) of which in the colder parts of the preheater

is oxidised to NO2. Thermal NO increases with flame temperature above 1200°C,

with retention time, and with increasing free oxygen. Thermal NO produced in

the kiln-burning zone is related to:

Flame temperature

Flame shape

Type of fuel

Excess oxygen

Gas retention time in the burning zone

Load temperature

Load retention time in the burning zone

A short, hot flame, excess combustion air and high secondary air temperature

will all increase NO formation. While fuel moisture, dust insufflation, or a high

dust entrainment in secondary air will reduce the NO. A harder burning mix will

always lead towards a higher NO, however, overburning should be avoided. It is

common to express NOx as NO2 for regulatory purposes.

The NOx formed can be reduced in different ways. Ammonia can be injected in

the top cyclone. Another way is to have a reduction zone in the calciner, which is

the feature of the low-NOx calciner. In the reduction zone, the following reaction

takes place between 800-1100°C reducing the NOx emissions:

[N] + NO N2 + [O]

Another component sometimes found in the exhaust gas is CO. CO is usually

formed from incomplete combustion of the fuel. At temperatures around 680°C,

oxidation of CO to CO2 takes place in the presence of excess oxygen. CO



emissions at the stack may be contributed from poor mixing of the

fuel/combustion air or not enough combustion air in the burning zone.

Organic C found in the raw materials tend to oxidise at temperatures below

680°C with roughly 20% of the oxidation forms CO regardless of the amount of

oxygen present. The above holds true when the level of organic C in the raw

material is 1.4 g/kg clinker or more. For example, 2 g C/kg clinker with a 15%

conversion rate corresponds to about 250 ppm CO at 5% oxygen.

Documents

Fuller Chemistry Handbook, Mr. Bokaian's Copy