HEAPS AS BIOREACTORS FOR COAL BIOPROCESSING

The long time required for the microbial removal of pyritic sulfur from coal does not become an economic issue ff the coal is treated in heaps over which bacteria and nutrients are trickled. Correct heap design is dictated by the variation of the inherent process rate with coal particle size, the ability of water to trickle through heaps of fine coal without waterlogging owing to capillary forces, and the ability of oxygen and carbon dioxide to diffuse into the heap. A theoretical analysis of oxygen penetration combined with experimental measurements of water holdup suggests an optimum coal particle size in the range 6 x 30 mesh. This is confirmed by experiments on 4 in x 3 ft heaps of Illinois #6 coal ground to 30 x 100 and 12 x 30 mesh. The former plugs with biomass and ferric precipitates, whereas the latter shows good depyritization rate with minimal gradients through the heap.