Abstract
The formation and subsequent burnout behaviour of coal slurry fuel chars has been studied using a suspended single droplet technique and solid sampling techniques in large turbulent diffusion flames. Both these techniques yielded comparable results on the structure of carbonaceous residues from a range of coal slurry fuels containing coals of different rank, size and concentration. The structure and burnout behaviour of all the fuels tested was dependant on coal rank, particle size, and concentration, although these variables did not always produce the same differences in behaviour in coal-oil and coal-water chars. Mid-rank coals, produced swollen well-fused chars which were substantially hollow and incorporated large surface blowholes. Such residues showed little tendency to fragment during combustion. Low and high rank coals produced less swollen chars which were comprised of loosely agglomerated coal particles causing more extensive internal structure and smaller surface blowholes. The furnace results of this and other studies indicate that the latter type of char structure is more conducive to rapid to rapid burnout. Small concentrations of emulsified water in coal-oil slurries were observed to increase the intensity of ebullition during devolatilisation although no evidence was obtained from any of the experimental techniques to suggest that this caused droplet shattering. However, water addition did increase the rate of combustion of coal-oil fuels. A theoretical model of the behaviour of coal slurry fuel chars during combustion has been presented which explains the experimental results in terms of the internal surface area of the char and the availability of this area to the gaseous reactants. It has been concluded that the rate of combustion of coal slurry chars of the size encountered in flames is largely controlled by the rate of internal diffusion of gaseous reactants and reaction within the porous char structure. The different reactivities of coal slurry fuels of different coal ranks has therefore been explained in terms of the various char structures which were observed The beneficial effects of water on the combustion of coal-oil fuels has also been explained within this theory since the addition of water could increase the porosity of the chars. However this has not been confirmed experimentally.