Study of low temperature coal oxidation by 13C CP/MAS n.m.r.

Abstract

Dry phase oxidation of a subbituminous coal with air was carried out in a fluidized bed reactor at 175, 200, 250 and 275 °C. Coal samples were withdrawn during the reaction at fixed intervals up to 12 h. Reaction pathways and chemical changes occurring in coal structure during oxidation were studied by 13C n.m.r. spectroscopy, elemental analysis and weights variation. By coupling 13C n.m.r. and carbon mass balance data it was possible to detect semiquantitatively the fate of aliphatic and aromatic carbon. The results show that, at each temperature, aliphatic carbon is more reactive than aromatic carbon, and that aromatic carbon does not seem to be affected by oxidation ⩽ 200 °C. A decrease in the aromatic carbon of 15% at 250 °C, and 20% at 275 °C was detected after 12 h of oxidation. The mass balance data and elemental analyses were used to examine the effect of reaction temperature on the relative importance of reaction pathways which lead to oxygen fixation or coal combustion. The weight losses at 250 and 275 °C are mainly due to combustion of the aliphatic structure.