Effects of low temperature air oxidation (weathering) reactions on the pyrolysis mass spectra of US coals

Abstract

Two high volatile bituminous coals (Upper Freeport and Hiawatha seams), a subbituminous coal (Adaville seam) and a lignite (Anderson seam) were used to investigate the effect of low temperature air oxidation (‘weathering’) on the yield and composition of vacuum pyrolysis products. Fresh coal samples were exposed to air at 80 and 100°C for up to 10 days under controlled laboratory conditions. Curie-point pyrolysis mass spectrometry combined with computerized data analysis was applied to study the weathering induced changes in a series of samples weathered for various lengths of time. It was found that the abundance of small oxygen-containing molecules such as carbon monoxide, carbon dioxide and aliphatic carboxylic acids increased in the pyrolysate of all samples, whereas phenols and dihydroxybenzenes showed decreased yields, especially in the two lowest rank coals. Besides phenols, alkylnaphthalenes and alkyltetralins showed decreased abundances in the pyrolysis mass spectra of the two high volatile bituminous coals. An attempt is made to explain the difference in pyrolysis patterns observed before and after weathering experiments in terms of underlying structural changes.