Coal combustion

The most common example of solid fuel combustion
is pulverised coal combustion

Related applications are: fluidised-bed combustion,
coal gasification, biomass combustion and
gasification, and the burning of refuse and wood

These processes all involve the initial liberation of
volatile material (devolatilisation), which reacts in the
gas phase, followed by the subsequent burnout of the
remaining char with any inert material remaining as
ash
Coal Classification

Coals are classified based on their rank from lignite
(lowest rank) through subbituminous, bituminous and
anthracites (highest rank)

The ASTM system is based on the fraction of fixed
carbon (ie. combustible material in the coal) and on
heating value

Large deviations in behaviour still exist within a
given rank and many properties of coal (e.g. Ncontent) are largely independent of rank
Structure of Coal

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Although the structure of coal is very random,
it is highly planar and layered with a pore
volume of approx. 8-20%
A fragment of a hypothetical coal molecule is
shown in the previous slide
In addition to aromatic and aliphatic carbon,
coals contain C, H, 0, S and N atoms in a
number of different structural groups
Coals also contain moisture which may be free
water or water which is physically bound
within the coal matrix
Coals contain a diverse range of mineral
matter as ash
Combustion of Coal
Outline for Combustion of Coal:
 Devolatilisation
 Devolatilisation Models
 Particle Heat-up during Devolatilisation
 The Char
 Char Burnout
 Global Reaction Rate
 Burnout Time
 Char Surface Temperature
Devolatilisation:

It occurs as coal is heated in inert or oxidisingenvironments

Moisture is evolved early during heating

At higher temp, gases and heavy tars (volatiles) are
emitted

Particle may soften and become plastic

Extent of pyrolysis varies from a few percent up to
70-80%

Both the pyrolysis time and the extent of pyrolysis
depends on particle size, coal type and pyrolysis
temperature
Devolatilisation Models:

Smoot (1991) discusses a number of empirical and
semi-empirical models for predicting coal
devolatilisation rates

Badzioch and Hawksjey (1970) proposed a simple
first-order model

Postulate that the devolatilisation rate is proportional to
the amount of volatile material remaining in the coal:
d
 k (    )
dt

with k = A exp(-E / RT)
"Total" volatile matter (v) is determined from
proximate analyses


The Badzioch and Hawksley model is too simple to
accurately describe many of the experimental
observations
A more complicated model is proposed by Kobayashi
et al (1977) in which the pyrolysis process is
modelled as a pair of parallel, first-order, irreversible
reactions:
C → (1- Y1)S1 + Y11, rate constant k1
C → (1 - Y2)S2 + Y2 2 , rate constant k2
with rate equations given by:
dC/dt= - (k1 + k2) C
d/dt= (Y1k1 +Y2k2) C
C = coal, S = solid,  = volatiles

Experimental & calculated weight loss for pyrolysis
of a lignite & bituminous coal (Kobayashi et al.,
1977)
Y1= 0.3, E1= 25 kcal/mol, A1= 2x105 s-1
Y2= 1.0, E2= 30 kcal/mol, A2= 1.3x107 s-1
Particle heat-up during devolatilisation:

Coal reaction processes are dependent on the particle
heating rate (order 104 K/s) and maximum particle
temperature

Devolatilisation is initiated at about 3000C

Convective heating is initially retarded by flow of
volatiles out of particle

However, reaction of the volatiles heats the
surrounding gas and increases the heating rate

Therefore, the devolatilisation rate depends on gas
temperature and mass transfer
The Char:
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The residual material (char) is enriched in carbon but
depleted in oxygen and hydrogen
Char has some N & S and retains most of the mineral
matter
Particles may have cracks and holes caused by
escaping gases and may have swelled to a larger size
Char particles have high porosities (0.7) and high
specific surface areas (100 m2/g)
Properties of char depend strongly on pyrolysis
conditions
Char Burnout:

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Reaction between char and oxygen is heterogeneous
and occurs at the gas-solid interface
Primary product of surface reaction is CO
CO reacts in the gas-phase to form CO2 (highly
exothermic)
Char burnout is generally much slower than
devolatilisation
Overall char burning rate depends on the chemical
rate of the carbon-oxygen reaction at the surface and
also on the rate of O2 diffusion through the boundary
layer and pores
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Char reactivity varies with coal type, temperature,
pressure, char characteristics (size, surface area, etc)
and O2 conc.
Burnout rates of the char are generally determined
using a global reaction rate which is found
empirically for a given coal and range of conditions
The global reaction rate is expressed in terms of rate
of mass loss per unit external area
Progress is being made in the use of intrinsic
(elementary) reactions rates but we still some way to
go
Global reaction rate:
C + 1/2 O2  CO
d mC/dt = -2 MWC/MWO2 Ap kc (ρ(O2)s)n
where AP = external particle surface area; kc = rate
constant; O2(s) = oxygen partial density at surface of
particle; and n = order of reaction
 For n = 1, and eliminating the unknown ρ(O2)s:
d mC/dt = -2 MWC/MWO2 Ap ke (ρ(O2)())
where ke is an effective rate constant and is a function of
the kinetics (kc) and diffusion (hD)
hDk c
ke 
hD  k c
 See Borman and Raglan, pp 469-470 for a full derivation
for calculating kc and hD

Burnout Time:

Assuming that CO is formed at the surface, the global
char burnout rate is:
dmC/dt =
 12 
d   k eO2   
 16 
2

Consider the limiting cases of: (i) the burning at
constant diameter (with decreasing density) or (ii) at
constant density (with decreasing diameter)

For constant diameter, this equation is integrated
directly

For constant density, d = (6mC/)1/3

Under diffusion-controlled conditions (kc >> hD),
corresponding to high temperature and large d:
1/ 3
 6 mC 
dmC
 2 

dt

 C 

 12 
  DABO2
 16 
Under kinetic control (hD >> kc), associated with low
temperature and small d:
 6 mC 
dmC
  

dt

C 

2/3
 12 
  k c O 2
 16 


Integrating from the initial char mass to zero, the char
burnout time is obtained for the following special
cases.
Constant Diameter
tC 

4.5 k eO2
Constant , diffusion control
tC 

Ci di
Ci di 2
6 D ABO2
Constant , kinetic control
tC 
C di
1.5 k c O2
Char Surface Temperature:
 In an oxidising environment, the char surface
temperature is typically hotter than the gas
temperature
 Particle temperature is strongly coupled to the
burning rate
 A steady-state energy balance, equating heat
generation by reaction at the surface with heat
loss by convection and radiation (neglecting
conduction), results in:
dmC

HC  hA p (Tp  Tg )  A p (Tp 4  Tb 4 )
dt
References
 Badzioch, S. and Hawksley, P.G.W. (1970), Kinetics
of thermal decomposition of pulverized coal particles,
Ind. Eng. Chem. Process Des. Dev., 9, p. 521
 Kobayashi, H., Howard, J.B. and Sarofim, A.F.
(1977), Coal devolatilization at high temp., 16th
Symp. (Int.) on Combustion, p. 411
 Smoot, L.D. (1991), Coal and Char Combustion, in
"Fossil Fuel Combustion: A Source Book". pp. 653781. Eds. Bartok, W. and Sarofim, A.F., WileyInterscience
 Solomon, P.R. and Colket, M.B. (1979), Coal
devolatilization. 17th Symp. (Int.) on Combustion, p.
131
Further Reading
 Turns, S.R. (1996), "An Introduction to combustion
Concepts and applications", McGraw-Hill, Chapter
14 (1st Edition)
 Borman, G.L. and Ragland, K.W. (1998),
"Combustion engineering", WCB/McGraw-Hill, pp
47-57 and Chapter 14
 Bartok, W. and Sarofim, A.F. (1991), "Fossil fuel
combustion: A source Book", Wiley-Interscience,
Chapter 10 (particularly pp 660-694)