Coal combustionCoal combustion

The most common example of solid fuel combustion The most common example of solid fuel combustion is pulverised coal combustionis pulverised coal combustion

Related applications are: fluidised-bed combustion, Related applications are: fluidised-bed combustion, coal gasification, biomass combustion and coal gasification, biomass combustion and gasification, and the burning of refuse and woodgasification, and the burning of refuse and wood

These processes all involve the initial These processes all involve the initial liberation of liberation of volatile material (devolatilisation),volatile material (devolatilisation), which reacts in the which reacts in the gas phase, followed by the subsequent gas phase, followed by the subsequent burnout of the burnout of the remaining charremaining char with any inert material remaining as with any inert material remaining as ashash

Coal ClassificationCoal Classification Coals are classified based on their rank from Coals are classified based on their rank from lignite lignite

(lowest rank) through (lowest rank) through subbituminous, bituminoussubbituminous, bituminous and and anthracites anthracites (highest rank)(highest rank)

The ASTM system is based on the fraction of fixed The ASTM system is based on the fraction of fixed carbon (ie. combustible material in the coal) and on carbon (ie. combustible material in the coal) and on heating valueheating value

Large deviations in behaviour still exist within a Large deviations in behaviour still exist within a given rank and many properties of coal (e.g. N-given rank and many properties of coal (e.g. N-content) are largely independent of rankcontent) are largely independent of rank

Structure of CoalStructure of Coal

Although the Although the structure of coalstructure of coal is very random, is very random, it is it is highly planar and layeredhighly planar and layered with a pore with a pore volume of approx. 8-20%volume of approx. 8-20%

A fragment of a hypothetical coal molecule is A fragment of a hypothetical coal molecule is shown in the previous slideshown in the previous slide

In addition to aromatic and aliphatic carbon, In addition to aromatic and aliphatic carbon, coals contain coals contain C, H, 0, SC, H, 0, S and and N N atoms in a atoms in a number of different structural groupsnumber of different structural groups

Coals also contain moisture which may be Coals also contain moisture which may be free free waterwater or or water which is physically boundwater which is physically bound within the coal matrixwithin the coal matrix

Coals contain a diverse range of Coals contain a diverse range of mineral mineral matter as ashmatter as ash

Combustion of CoalCombustion of Coal

Outline for Combustion of CoalOutline for Combustion of Coal:: DevolatilisationDevolatilisation Devolatilisation ModelsDevolatilisation Models Particle Heat-up during DevolatilisationParticle Heat-up during Devolatilisation The CharThe Char Char BurnoutChar Burnout Global Reaction RateGlobal Reaction Rate Burnout TimeBurnout Time Char Surface TemperatureChar Surface Temperature

Devolatilisation:Devolatilisation: It occurs as coal is heated in inert or oxidising- It occurs as coal is heated in inert or oxidising-

environmentsenvironments MoistureMoisture is is evolved earlyevolved early during heating during heating At At higher temphigher temp, , gases and heavy tars (volatiles) are gases and heavy tars (volatiles) are

emittedemitted Particle may soften and become plasticParticle may soften and become plastic Extent of pyrolysis varies from a few percent up to Extent of pyrolysis varies from a few percent up to

70-80%70-80% Both the pyrolysis time and the extent of pyrolysis Both the pyrolysis time and the extent of pyrolysis

depends on particle size, coal type and pyrolysis depends on particle size, coal type and pyrolysis temperaturetemperature

Devolatilisation Models:Devolatilisation Models: Smoot (1991) discusses a number of empirical and Smoot (1991) discusses a number of empirical and

semi-empirical models for predicting coal semi-empirical models for predicting coal devolatilisation ratesdevolatilisation rates

Badzioch and HawksjeyBadzioch and Hawksjey (1970) proposed a (1970) proposed a simple simple first-order modelfirst-order model

Postulate that the devolatilisation rate is proportional to Postulate that the devolatilisation rate is proportional to the amount of volatile material remaining in the coal:the amount of volatile material remaining in the coal:

with with k = A exp(-E / RT)k = A exp(-E / RT)

"Total" volatile matter (v"Total" volatile matter (v)) is determined from is determined from proximate analysesproximate analyses

( )d

kdt

The Badzioch and Hawksley model is too simple to The Badzioch and Hawksley model is too simple to accurately describe many of the experimental accurately describe many of the experimental observationsobservations

A A more complicated modelmore complicated model is is proposed by Kobayashiproposed by Kobayashi et al (1977) in which the pyrolysis process is et al (1977) in which the pyrolysis process is modelled as a pair of parallel, first-order, irreversible modelled as a pair of parallel, first-order, irreversible reactions:reactions:

C C →→ (1- Y (1- Y11)S)S11 + Y + Y1111, rate constant k, rate constant k11

C C →→ (1 - Y (1 - Y22)S)S22 + Y + Y2 2 2 2 , rate constant k, rate constant k22

with rate equations given by:with rate equations given by:dC/dt= - (kdC/dt= - (k11 + k + k22) C) Cdd/dt= (Y/dt= (Y11kk11 +Y +Y22kk22) C) CC = coal, S = solid, C = coal, S = solid, = volatiles = volatiles

Experimental & calculated weight loss for pyrolysis Experimental & calculated weight loss for pyrolysis of a lignite & bituminous coal (Kobayashi et al., of a lignite & bituminous coal (Kobayashi et al., 1977)1977)

YY11= 0.3, E= 0.3, E11= 25 kcal/mol, A= 25 kcal/mol, A11= 2x105 s= 2x105 s-1-1

YY22= 1.0, E= 1.0, E22= 30 kcal/mol, A= 30 kcal/mol, A22= 1.3x107 s= 1.3x107 s-1-1

Particle heat-up during devolatilisation:Particle heat-up during devolatilisation: Coal reaction processes are dependent on the particle Coal reaction processes are dependent on the particle

heating rateheating rate (order 10 (order 1044 K/s) and K/s) and maximum particle maximum particle temperaturetemperature

DevolatilisationDevolatilisation is initiated at about is initiated at about 30030000CC Convective heating is initially retarded by flow of Convective heating is initially retarded by flow of

volatiles out of particlevolatiles out of particle However, reaction of the volatiles heats the However, reaction of the volatiles heats the

surrounding gas and increases the heating ratesurrounding gas and increases the heating rate Therefore, the Therefore, the devolatilisation rate dependsdevolatilisation rate depends on on gas gas

temperature and mass transfertemperature and mass transfer

The Char:The Char: The residual material (char) is enriched in carbon but The residual material (char) is enriched in carbon but

depleted in oxygen and hydrogendepleted in oxygen and hydrogen CharChar has some N & S and has some N & S and retains most of the mineral retains most of the mineral

mattermatter Particles may have cracks and holes caused by Particles may have cracks and holes caused by

escaping gases and may have swelled to a larger sizeescaping gases and may have swelled to a larger size Char particles have Char particles have high porosities (high porosities (0.7)0.7) and high and high

specific surface areas (specific surface areas (100 m100 m22/g)/g) Properties of char depend strongly on pyrolysis Properties of char depend strongly on pyrolysis

conditionsconditions

Char Burnout:Char Burnout: Reaction between char and oxygen is heterogeneous Reaction between char and oxygen is heterogeneous

and occurs at the gas-solid interfaceand occurs at the gas-solid interface Primary product of surface reactionPrimary product of surface reaction is is COCO CO reacts in the gas-phase to form COCO reacts in the gas-phase to form CO22 (highly (highly

exothermic)exothermic) Char burnout is generally much slower than Char burnout is generally much slower than

devolatilisationdevolatilisation Overall char burning rate depends on the Overall char burning rate depends on the chemical chemical

raterate of the carbon-oxygen reaction at the surface and of the carbon-oxygen reaction at the surface and also on the also on the rate of Orate of O22 diffusion diffusion through the boundary through the boundary layer and poreslayer and pores

Char reactivity varies with coal type, temperature, Char reactivity varies with coal type, temperature, pressure, char characteristics (size, surface area, etc) pressure, char characteristics (size, surface area, etc) and Oand O22 conc. conc.

Burnout rates of the char are generally determined Burnout rates of the char are generally determined using a global reaction rate which is found using a global reaction rate which is found empirically for a given coal and range of conditionsempirically for a given coal and range of conditions

The global reaction rate is expressed in terms of The global reaction rate is expressed in terms of rate rate of mass loss per unit external areaof mass loss per unit external area

Progress is being made in the use of intrinsic Progress is being made in the use of intrinsic (elementary) reactions rates but we still some way to (elementary) reactions rates but we still some way to gogo

Global reaction rate:Global reaction rate: C + 1/2 OC + 1/2 O22 CO CO

d md mCC/dt = -2 MW/dt = -2 MWCC/MW/MWO2O2 A App k kcc ( (ρρ(O2)(O2)ss))nn

where Awhere APP = external particle surface area; k = external particle surface area; kcc = rate = rate constant; constant; O2(s)O2(s) = oxygen partial density at surface of = oxygen partial density at surface of particle; and n = order of reactionparticle; and n = order of reaction

For n = 1, and eliminating the unknown For n = 1, and eliminating the unknown ρρ(O2)(O2)ss::

d md mCC/dt = -2 MW/dt = -2 MWCC/MW/MWO2O2 A App k kee ( (ρρ(O2)(O2)(())))

where where kkee is an effective rate constant and is a function of is an effective rate constant and is a function of the the kinetics (kkinetics (kcc)) and and diffusion (hdiffusion (hDD))

See Borman and Raglan, pp 469-470 for a full derivation See Borman and Raglan, pp 469-470 for a full derivation for calculating kfor calculating kc c and hand hDD

D ce

D c

h kk

h k

Burnout Time:Burnout Time: Assuming that CO is formed at the surface, the global Assuming that CO is formed at the surface, the global

char burnout rate is:char burnout rate is:

dmdmCC/dt = /dt = Consider the limiting cases of: (i) the burning at Consider the limiting cases of: (i) the burning at

constant diameterconstant diameter (with decreasing density) or (ii) at (with decreasing density) or (ii) at constant densityconstant density (with decreasing diameter) (with decreasing diameter)

For For constant diameterconstant diameter, this equation is integrated , this equation is integrated directlydirectly

For For constant density,constant density, d = (6m d = (6mCC//))1/31/3

2

2 12

16 e Od k

Under Under diffusion-controlled conditionsdiffusion-controlled conditions ( (kkcc >> h >> hDD), ),

corresponding to corresponding to high temperaturehigh temperature and and large d:large d:

Under Under kinetic controlkinetic control ( (hhDD >> k >> kcc), associated with ), associated with low low

temperaturetemperature and and small dsmall d::

2

1/36 12

216

C CAB O

C

dm mD

dt

2

2/36 12

16C C

c OC

dm mk

dt

Integrating from the initial char mass to zero, the char Integrating from the initial char mass to zero, the char burnout time is obtained for the following special burnout time is obtained for the following special cases.cases.

Constant Diameter Constant Diameter

Constant Constant , diffusion control, diffusion control

Constant Constant , kinetic control, kinetic control

24.5

iC iC

e O

dt

k

2

2

6iC i

CAB O

dt

D

21.5

C iC

c O

dt

k

Char Surface Temperature:Char Surface Temperature: In an oxidising environment, the char surface In an oxidising environment, the char surface

temperature is typically hotter than the gas temperature is typically hotter than the gas temperaturetemperature

Particle temperature is strongly coupled to the Particle temperature is strongly coupled to the burning rateburning rate

A steady-state energy balance, equating A steady-state energy balance, equating heat heat generation by reactiongeneration by reaction at the surface with at the surface with heat heat loss by convection and radiationloss by convection and radiation (neglecting (neglecting conduction), results in:conduction), results in:

4 4( ) ( )CC p p g p p b

dmH hA T T A T T

dt

ReferencesReferences Badzioch, S. and Hawksley, P.G.W. (1970), Kinetics Badzioch, S. and Hawksley, P.G.W. (1970), Kinetics

of thermal decomposition of pulverized coal particles, of thermal decomposition of pulverized coal particles, Ind. Eng. Chem. Process DesInd. Eng. Chem. Process Des. Dev., 9, p. 521. Dev., 9, p. 521

Kobayashi, H., Howard, J.B. and Sarofim, A.F. Kobayashi, H., Howard, J.B. and Sarofim, A.F. (1977), Coal devolatilization at high temp., 16(1977), Coal devolatilization at high temp., 16 th th

Symp. (Int.) on Combustion, p. 411 Symp. (Int.) on Combustion, p. 411 Smoot, L.D. (1991), Coal and Char Combustion, in Smoot, L.D. (1991), Coal and Char Combustion, in

""Fossil Fuel Combustion: A Source BookFossil Fuel Combustion: A Source Book". pp. 653-". pp. 653-781. Eds. Bartok, W. and Sarofim, A.F., Wiley-781. Eds. Bartok, W. and Sarofim, A.F., Wiley-InterscienceInterscience

Solomon, P.R. and Colket, M.B. (1979), Coal Solomon, P.R. and Colket, M.B. (1979), Coal devolatilization. 17devolatilization. 17thth Symp. (Int.) on Combustion, p. Symp. (Int.) on Combustion, p. 131131

Further ReadingFurther Reading Turns, S.R. (1996), "An Introduction to combustion Turns, S.R. (1996), "An Introduction to combustion

Concepts and applications", McGraw-Hill, Chapter Concepts and applications", McGraw-Hill, Chapter 14 (1st Edition)14 (1st Edition)

Borman, G.L. and Ragland, K.W. (1998), Borman, G.L. and Ragland, K.W. (1998), "Combustion engineering", WCB/McGraw-Hill, pp "Combustion engineering", WCB/McGraw-Hill, pp 47-57 and Chapter 1447-57 and Chapter 14

Bartok, W. and Sarofim, A.F. (1991), "Fossil fuel Bartok, W. and Sarofim, A.F. (1991), "Fossil fuel combustion: A source Book", Wiley-Interscience, combustion: A source Book", Wiley-Interscience, Chapter 10 (particularly pp 660-694)Chapter 10 (particularly pp 660-694)