Chemically-bound n-alkyl groups in coal

Tars produced from the flash pyrolysis of three Australian coals and the liquid products obtained from the subsequent catalytic hydrotreatment of these tars have been analysed by high resolution gas chromatography. Yields of n-alkanes and, in the case of the tars, n-alkenes have been determined. For the subbituminous coal the total yields on a dry, ash-free coal basis increased from 2.4 wt% in the tar to a maximum of 5.8 wt% in the hydrotreated product, and for a brown coal from 0.6 wt% in the tar to 2.5 wt% in the hydrotreated product. Thus, in both cases, a substantial proportion of the n-alkanes appeared only after extensive hydrotreatment of the tar, and did not consist simply of hydrogenated n-alkenes. The results imply that a major fraction of the n-alkyl groups in the parent coal are chemically bound to other structures and are released and vaporized as such during pyrolysis. Subsequent hydrogenation of the tar liberates n-alkanes by cleavage of carbon-carbon bonds. Results for the bituminous coal are complicated by high gas formation in the hydrotreatment reactor, but suggest that some n-alkyl groups are also chemically bound.