Ash formation from the combustion of coals with maceral concentrates at various pressures

Two coal samples of different lithotypes and maceral-enriched fractions wereprepared (+63-90 pm) from samples of the same bituminous coal seam. Densityseparations at a specific gravity of 2.0 were applied for the prepared coal samples inorder to remove the majority of the excluded mineral matter in the final coal samples.These coal samples were then pyrolysed or combusted in drop-tube firrnaces at0.1 and 1.5 MPa to produce char and ash samples at 1573K. The results indicate thatthere are two main factors influencing char and ash formation under the experimentalconditions, i.e. pressure and coal petrographic property. In agreement with ourprevious results, chars prepared at high pressure are more porous and ash particlesare finer. The structure of the porous char particles at 0.1 and 1.5 MPa are verydifferent, as evidenced by SEM surjace observation. The porous char particlesprepared at 1.5 MPa have a ‘Ifoam” structure, while those at atmospheric pressureare mostly balloon-like structures. The vitrinite-rich coal produces a higherproportion of porous char particles during pyrolysis. Although the mineral particlesin the vitrinite-rich coal sample are coarser than those in the inertinite-rich coalsample, the ash particles produced from the vitrinite-rich coal afler combustion aremuch finer. The results suggest that the extent of coalescence of included mineralmatter of the inernite-rich coal sample is more significant than that of the vitriniterichcoal sample. Char particles from the vihl’nite-rich coal sample are more porouscompared to those from the inernite-rich coal sample, leading to more intensive charfragmentation so that finer ash particles form during combustion. At 1.5 MPa, ashparticles produced from both coals during combustion are finer than at atmosphericpressure, due to the char particle produced at 1.5 MPa being more porous. However,the sensitivity of the effect ofpressure is less for the vitrinite-rich coal sample.