CHANGES IN COAL STRUCTURE ACCOMPANYING THE FORMATION OF REGENERATED HUMIC ACIDS DURING AIR OXIDATION

Dry-phase oxidation of two subbituminous coals and a lignite was carried out in a fluidized bed reactor at 200-degrees-C, at different oxygen partial pressures and reaction times up to 4 h. The formation and evolution of various oxygenated functional groups (ester, carboxyl, ketone and hydroxyl) was investigated by FT-i.r. spectroscopy using a curve-resolving procedure and acetylation of coat samples. The development of alkali solubility of the coal as a function of reaction time was studied and the average molecular weight of regenerated humic acids (RHA) extracted from the oxidized coal was determined. The lower molecular weight of RHA extracted from highly oxidized coal samples suggests that besides the build-up of various oxygenated functional groups, the development of alkali solubility also results from concomitant 'depolymerization' of the coal network. The latter may occur through oxidation of aliphatic cross-links and formation of ester groups hydrolysable in basic solution. The FT-i.r. results were correlated with the development of alkali solubility. Some oxidation pathways are proposed.