Effect of Pressure on Ash Formation from Combustion of Coals with Maceral Concentrates

Two coal samples of different lithotypes and maceral-enriched fractions were prepared (+63- 90 μm) from samples of the same bituminous coal seam. Density separations at a specific gravity of 2.0 were applied for the prepared coal samples in order 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 furnaces at 0.1 and 1.5 MPa to produce char and ash samples at 1573K. The results indicate that there are two main factors influencing char and ash formation under the experimental conditions, ie, pressure and coal petrographic property. In agreement with our previous results, char prepared at high pressure are more porous and ash particles are finer. The structure of the porous char particles at 0.1 and 1.5 MPa are very different, evidenced by SEM surface observation. The porous char particles prepared at 1.5 MPa have a 'foam' structure while those at atmospheric pressure are mostly balloon-like structures. The vitrinite-rich coal produces a higher proportion of porous char particles during pyrolysis. Although the mineral particles in the vitrinite-rich coal sample are coarser than those in the inertinite-rich coal sample, the ash particles produced from the vitrinite-rich coal after combustion are much finer. The results suggest that the extent of coalescence of included mineral matter of the inernite-rich coal sample is more significant than that of the vitriniterich coal sample. Char particles from the vitrinite-rich coal sample are more porous compared to those from the inernite-rich coal sample, leading to more intensive char fragmentation so that finer ash particles form during combustion. At 1.5 MPa, ash particles produced from both coals during combustion are finer than at atmospheric pressure, due to the char particle produced at 1.5 MPa being more porous. However, the sensitivity of the effect of pressure is less for the vitrinite-rich coal sample.