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 carri ed out in a fluidized bed reactor at 200-degrees-C, at different oxyge n partial pressures and reaction times up to 4 h. The formation and ev olution of various oxygenated functional groups (ester, carboxyl, keto ne and hydroxyl) was investigated by FT-i.r. spectroscopy using a curv e-resolving procedure and acetylation of coat samples. The development of alkali solubility of the coal as a function of reaction time was s tudied and the average molecular weight of regenerated humic acids (RH A) extracted from the oxidized coal was determined. The lower molecula r weight of RHA extracted from highly oxidized coal samples suggests t hat besides the build-up of various oxygenated functional groups, the development of alkali solubility also results from concomitant 'depoly merization' of the coal network. The latter may occur through oxidatio n of aliphatic cross-links and formation of ester groups hydrolysable in basic solution. The FT-i.r. results were correlated with the develo pment of alkali solubility. Some oxidation pathways are proposed.