Relation of the chemical structure of coal to its hydropyrolysis

Abstract

The hydropyrolysis of Illinois No. 6 coal has been studied in a batch reactor, in which the reactions were initiated by explosion of H2O2 mixtures. The ratio of H2 to O2 was kept at 8, while the total pressure of the gas mixture was changed to vary the reaction temperature. The heating rate was ≈ 50 000 °C s−1, and the reaction time was < 50 ms. The conversion of the feed coal increased from 19% at 620 °C to 81%at ⪢ 1500 °C. At conversions < 50%, the gaseous product consisted of mainly CH4 and CO in almost equal proportions, and at conversions ⪢ 60% the concentration of CO increased. Comparison with results from a large flow reactor revealed that comparable conversions were obtained in the present batch reactor, although product distributions were markedly different from each other. The dissimilar product distribution is attributed to different reacting media: preburning of H2 and O2 in the flow reactor versus in situ burning of the mixture in the batch reactor. The H/C ratios of solid residues after the hydropyrolysis decreased linearly as the conversion increased, revealing that the portions of coal having high H/C ratios were preferentially gasified. This observation was substantiated by a high H/C ratio, 1.74 of the first portion of coal gasified, and by a sharp decrease in H/C ratio in subsequent gasified portions. These data indicated that aliphatic side chains (or linkages) and single-ring aromatic clusters in the feed coal were gasified first, followed by larger aromatic clusters. Semi-quantitative determination of the distribution of different aromatic clusters showed good agreement with current structural information on coal. Thus, the effects of reaction variables were explained in terms of the structural features of coal, and the ratelimiting steps in the hydropyrolysis process were identified.