Redox

CemNet Training Training - Course Course CKPC01 CKPC01

Module 2.8

Cement Kiln  Process Chemistry Module 2. Cement clinker quality and composition. 2.8 Combustion – oxidation and reduction. reduction.

Cement Kiln Chemistry

2.8 Combustion

In session 1.10 of the course we talked about the stoichiometry of combustion of the fuels in a cement kiln, the quantity of combustion air that must be drawn into the kiln, and the combustion product gases that must be exhausted from the kiln. In the preceding session we talked about the need for a sharp temperature profile to achieve rapid heating to the combination temperature of the clinker. Also that the main burner must be inserted the minimum  practical distance into the kiln to minimise the cooling zone in the kiln after the clinker has passed under the flame….. …..the clinker is then rapidly cooled by falling into the cooler while still at a high temperature.

Cement Kiln Chemistry

2.8 Combustion

In session 1.10 of the course we talked about the stoichiometry of combustion of the fuels in a cement kiln, the quantity of combustion air that must be drawn into the kiln, and the combustion product gases that must be exhausted from the kiln. In the preceding session we talked about the need for a sharp temperature profile to achieve rapid heating to the combination temperature of the clinker. In order to understand how to produce that sharp temperature  profile we have to understand the dynamics of cement kiln  burners and flames….. …..even though this is not strictly “chemistry”. The design and adjustment of the burner produces the flame that can provide the sharp hot temperature profile required…… …..along with the amount of excess air and oxygen drawn into the kiln.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.8

2.8 Combustion

In session 1.10 of the course we talked about the stoichiometry of combustion of the fuels in a cement kiln, the quantity of combustion air that must be drawn into the kiln, and the combustion product gases that must be exhausted from the kiln. The cement kiln flame is generated by the combustion of hydrocarbon fuels……. …..i.e. fuels that are composed  principally of hydrogen and carbon combined in var ious hydrocarbon molecules. Oxidation of this hydrogen and carbon liberates the exothermic energy that produces the temperature for combination of the feed into the clinker minerals. C

+

O2

→

CO2

2H + ½O2

→

H2O

Cement Kiln Chemistry

2.8 Combustion

These oxidation reactions can only take place when the hydrocarbons in the fuel are brought into contact with the oxygen in the combustion air by mixing of the combustion air and fuel. C

O2

→

CO2

2H + ½O2

+

→

H2O

Once the fuel and combustion air are mixed the combustion continues through the following sequence of processes: Mixing → Ignition → Oxidation → Exhaust Once mixing is completed ignition and oxidation take place very rapidly……. …..therefore mixing is the rate determining  process……..…..provided that the combustion product gases are adequately exhausted from the flame.

Cement Kiln Chemistry

2.8 Combustion

These oxidation reactions can only take place when the hydrocarbons in the fuel are brought into contact with the oxygen in the combustion air by mixing of the combustion air and fuel. C

O2

→

CO2

2H + ½O2

+

→

H2O

Once the fuel and combustion air are mixed the combustion continues through the following sequence of processes: Mixing → Ignition → Oxidation → Exhaust Once mixing is completed ignition and oxidation take place very rapidly……. …..therefore mixing is the rate determining  process……..…..exhausting of the combustion product gases is  provided by the induced draft fan, which serves a dual purpose  by also drawing the combustion air into the kiln.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.8

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. The burner blows a jet of primary air and fuel into the kiln. Friction between the jet of  primary air and fuel and the surrounding air accelerates the surrounding secondary air. The accelerated secondary air is then pulled into the flame, expanding it. The process is momentum controlled and continues until the velocity of the jet is the same as the surrounding air.

Cement Kiln Chemistry

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. The burner blows a jet of primary air and fuel into the kiln. In a rotary kiln the entrainment of secondary air and expansion of the flame is constrained by the kiln shell and refractory lining. The availability of secondary air also constrains the entrainment into the flame. The excess momentum of the jet then pulls back exhaust gases from further up the kiln and entrains them into the flame.

Cement Kiln Chemistry

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. This phenomenon is known as “external recirculation” and is critical for characteristics of the flame. The primary air mixes with the fuel very quickly….The entrainment of the secondary air is then dependent on the ratio of the primary air/fuel jet and the momentum of the secondary air. The higher the flow rate and velocity of the primary air the quicker the mixing. Recirculation is an indication that all the secondary air has been entrained into the flame and fuel/air mixing is complete.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.8

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. This phenomenon is known as “external recirculation” and is critical for characteristics of the flame. A recirculating flame provides the short burning zone and sharp temperature  profile required for optimum clinker quality…….. ….and other non-quality related benefits. The recirculation prevents the flame expanding to impinge on the refractory. The recirculating gases  provide a cushion of cooler exhaust gas that protects the refractory. Recirculation is an indication that all the secondary air has been entrained into the flame and fuel/air mixing is complete.

Cement Kiln Chemistry

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. This phenomenon is known as “external recirculation” and is critical for characteristics of the flame. With a recirculating flame oxidising burning conditions can be guaranteed with a small excess of air above that required for combustion……. ….as little as 1~2% oxygen at the kiln inlet. Without recirculation the mixing of secondary air and fuel is inadequate and reducing burning conditions and CO can be present at 4~5% oxygen at the kiln inlet..

Cement Kiln Chemistry

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. Production of clinker in a reducing atmosphere in a cement kiln can seriously degrade the quality of clinker and the ce ment produced from the clinker. However, an industrial cement kiln is tolerant of reducing conditions without these adverse effects manifesting themselves…… ….provided that the clinker falls into the cooler at a temperature above 1250°C and is cooled in air. Cooling in air from a sufficiently high temperature negates the adverse affects of reductive  burning as the clinker reoxidises during cooling.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.8

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. The conditions that must be avoided are the clinker cooling to below 1250°C in reducing conditions in the kiln……. …..if that happens the liquid flux crystallises under reducing conditions and this leads to the serious degradation of clinker quality. In those conditions a sequence of detrimental affects on clinker quality arise, dependent on the degree of reduction.

The clinker colour changes from grey to  brown as Fe2+ replaces Mg2+ in solid solution in C4AF, which reverts to a brown colour.

Cement Kiln Chemistry

The reduction of Fe3+ to Fe2+ and the solid solution of Fe2+ in the clinker minerals is the cause of these detrimental affects.

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. The conditions that must be avoided are the clinker cooling to below 1250°C in reducing conditions in the kiln……. …..if that happens the liquid flux crystallises under reducing conditions and this leads to the serious degradation of clinker quality. In those conditions a sequence of detrimental affects on clinker quality arise dependent on the degree of reduction.

The normal grey colour of clinker arises from ..the solid solution of Mg 2+ in C4AF….. Mg2+ distorts the electron cloud of Fe 3+ in C4AF resulting in the typical grey colour of clinker and cement.

Cement Kiln Chemistry

The reduction of Fe3+ to Fe2+ and the solid solution of Fe2+ in the clinker minerals is the cause of these detrimental affects.

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. The conditions that must be avoided are the clinker cooling to below 1250°C in reducing conditions in the kiln……. …..if that happens the liquid flux crystallises under reducing conditions and this leads to the serious degradation of clinker quality. In those conditions a sequence of detrimental affects on clinker quality arise dependent on the degree of reduction.

Under more intense reducing conditions the C4AF content is depleted and C 3A content ..less Fe 3+ and therefore increased….. Fe2O3 is available to form C4AF, meaning that Al2O3 is released to form more C 3A.

The reduction of Fe3+ to Fe2+ and the solid solution of Fe2+ in the clinker minerals is the cause of these detrimental affects.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.8

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. The conditions that must be avoided are the clinker cooling to below 1250°C in reducing conditions in the kiln……. …..if that happens the liquid flux crystallises under reducing conditions and this leads to the serious degradation of clinker quality. In those conditions a sequence of detrimental affects on clinker quality arise dependent on the degree of reduction.

Under more intense reducing conditions the C4AF content is depleted and C 3A content ..setting of cement increased…..  becomes increasingly rapid and uncontrollable due to “flash” setting.

Cement Kiln Chemistry

The reduction of Fe3+ to Fe2+ and the solid solution of Fe2+ in the clinker minerals is the cause of these detrimental affects.

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. The conditions that must be avoided are the clinker cooling to below 1250°C in reducing conditions in the kiln……. …..if that happens the liquid flux crystallises under reducing conditions and this leads to the serious degradation of clinker quality. In those conditions a sequence of detrimental affects on clinker quality arise dependent on the degree of reduction.

Fe 2+

C3S is destabilised by solid solution of as FeO and the decomposition to C 2S and CaO during cooling is accelerated…. ..subsequent rapid cooling cannot stabilise C3S.

Cement Kiln Chemistry

The reduction of Fe3+ to Fe2+ and the solid solution of Fe2+ in the clinker minerals is the cause of these detrimental affects.

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. The conditions that must be avoided are the clinker cooling to below 1250°C in reducing conditions in the kiln……. …..if that happens the liquid flux crystallises under reducing conditions and this leads to the serious degradation of clinker quality. In those conditions a sequence of detrimental affects on clinker quality arise dependent on the degree of reduction. The reduction of Fe3+ to Fe2+ and the solid solution The inversion of C 2S to the hydraulically inactive of Fe2+ in the clinker γ polymorph is promoted by the incorporation of minerals is the cause of Fe2+ in C2S as FeO, with attendant dusting of the these detrimental affects. clinker.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.8

2.8 Combustion

Mixing of the combustion air and fuel in a cement kiln flame relies on jet entrainment of the secondary combustion air into the flame. However, this catalogue of potentially serious quality problems……. …..colour change from grey to brown, rapid setting due to more C 3A and low strength development due to less C 3S and hydraulically inactive C2S…... …..can all be avoided, provided that the clinker does not cool to below 1250°C in a reducing atmosphere in the cement kiln….. ….and the liquid flux does not crystallise in a reducing atmosphere.

Cement Kiln  Process Chemistry Module 2. Cement clinker quality and composition. 2.8 Combustion – oxidation and reduction.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com