E.s.r. spectroscopic study on the chemistry of coal oxidation

Abstract

A better understanding of the mechanisms of pre-oxidation could lead to improved methods for prevention of coal weathering, which has a deleterious influence on liquid yield during pyrolysis. Previous Morgantown Energy Technology Center studies demonstrated that the extractable fractions of coal are more susceptible to oxidative weathering than the residue. To better understand the chemistry of coal weathering and the influence of pre-oxidation on subsequent devolatilization, in situ electron spin resonance (e.s.r.) spectroscopic studies were performed on coal, weathered coal and on the corresponding untreated and preoxidized fractions (extract and residue). In this technique, pyrolysis is performed in an e.s.r. cavity and the concentration of free radicals is followed as the sample is pyrolysed. Elemental analysis of the pre-oxidized and untreated samples indicated that H C ratio of the extract and the coal sample was reduced slightly during 48 h of oxidation at 150 °C in air whereas the H C ratio of the residue was essentially unchanged. The influence of pre-oxidation (48 h at 150 °C) was to increase the e.s.r. spin concentration of the weathered coal slightly compared to the raw coal when devolatilized at 460 °C. The e.s.r. experiments performed at 460 °C also showed that pre-oxidation significantly enhanced free radical concentration in the extract but had a lesser effect on the residue. The enhanced free radical formation of the pre-oxidized extract relative to the untreated extract implies that mild pre-oxidation affects hydrogen-rich components (extract) more than the primarily hydrogen-poor components (residue) of coal.