Effects of water and molecular hydrogen on heat treatment of Turkish low-rank coals

Two low-rank Turkish coals, Tuncbilek (TB) subbituminous and Goynuk (GN) lignite, were subjected to low-severity heat treatment with or without added water, under N-2 Or H-2 atmospheres at 285-330 degrees C, both as original coals and after demineralization with HCl(aq)/HF(aq) treatment. The samples processed in H-2-H2O combination seemed to be more dissociated or decomposed than those processed in N-2-H2O or under H-2 without water. Gas analyses and spectroscopy of samples clearly indicated an effect of water in enhancing decarboxylation reactions and that oxygen rejection from the coals was mainly due to CO2 formation. Water also enhanced the cleavage of aryl-ether bonds. These cleavage reactions are more pronounced in GN coal than in TB coal, probably due to a higher concentration of activated ether structures in the former coal. The disruption of noncovalent interactions is considered mainly responsible for the extensive depolymerization of TB coal. The role of Ha in these heat treatment conditions is considered to be predominantly hydrogenation of polyaromatic sites with the help of mineral matter and scavenging radicals, which arose from cleavage of weak aliphatic ether bonds. The coal samples treated in H-2-H2O donated more hydrogen to anthracene during their co-pyrolysis, attributed mainly to a reduced tendency of the treated samples to undergo radical-generating reactions.