Coal combustion

1. Coal combustion

2. 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

3. 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. N-content) are largely independent of rank

6. Structure of Coal

7. 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

8. Combustion of Coal

9. 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

10. Devolatilisation: • It occurs as coal is heated in inert or oxidising- environments • 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

11. 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: with k = A exp(-E / RT) • "Total" volatile matter (v) is determined from proximate analyses

12. 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

13. 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

16. 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 ongas temperature and mass transfer

17. The Char: • 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

18. Char Burnout: • 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

19. 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

20. 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) • See Borman and Raglan, pp 469-470 for a full derivation for calculating kc and hD

21. Burnout Time: • Assuming that CO is formed at the surface, the global char burnout rate is: dmC/dt = • 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

22. Under diffusion-controlled conditions (kc >> hD), corresponding to high temperature and large d: • Under kinetic control (hD >> kc), associated with low temperature and small d:

23. Integrating from the initial char mass to zero, the char burnout time is obtained for the following special cases. • Constant Diameter • Constant , diffusion control • Constant , kinetic control

26. 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:

27. 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. 653-781. Eds. Bartok, W. and Sarofim, A.F., Wiley-Interscience • Solomon, P.R. and Colket, M.B. (1979), Coal devolatilization. 17th Symp. (Int.) on Combustion, p. 131

28. 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)