COMPARISON OF THERMAL BREAKDOWN IN COAL PYROLYSIS AND LIQUEFACTION

The courses of the thermal breakdown reactions in coal pyrolysis and liquefaction were compared using similar coal and maceral concentrate samples in parallel experiments. A wire-mesh pyrolysis and a flowing-solvent liquefaction reactor were used, both reactors providing for rapid removal of products from the reaction zone in order to suppress secondary reactions; product structures were therefore expected to be relatively free of reactor-related effects. An attempt was made to distinguish between the effects of density, solvent power and donor ability of media surrounding the particles during thermal breakdown. Products were characterized by size exclusion chromatography, u.v.-fluorescence and FT-i.r. spectroscopy. The results suggest that the sensitivity of pyrolysis yields to changes in heating rate is related to competing rates of volatile release and resolidification within the pyrolysing mass: the presence of locally available hydrogen - e.g. from hydrogen donor species - appears to shift this competition in favour of greater product release and slower resolidification. This explains the absence of a direct effect of heating rate on liquefaction yields at high tetralin:coal ratios. Broad similarity was found between liquefaction yields in hexadecane and yields from pyrolysis experiments, where effects of sample resolidification similar to those in 'dry' pyrolysis were observed. Molecular mass distributions extending to higher levels were observed in liquefaction extracts than in pyrolysis tars; however, it remains possible that aromatic nuclei with ranges of cluster sizes similar to those available in the original coal may be found in both tar and extract molecules.