The effect of chloroform extractables on tar formation in flash pyrolysis and catalytic flash pyrolysis

Abstract

The effect of chloroform extraction on pyrolysis product distributions from three hard coals and a lignite were found to differ markedly between slow and fast pyrolysis. The decline in flash pyrolysis tar yields upon extraction is considerably larger than the amount of extract removed whereas in slow pyrolysis the sum of tar plus extract remains essentially constant. Improvements observed in flash pyrolysis tar yields upon impregnation with ZnCl 2 were also reversed when the coal was extracted prior to impregnation. A wire mesh apparatus was used to confirm these results, over the heating rate range 0.5 °C/s to 1000 °C/s, suggesting that chloroform extractable materials play a key role in increasing tar yields at the higher end of the heating rate range. Furthermore, experiments in a “hot rod” reactor (5 °C/s, 1–50 bars) indicated that the routing of externally applied hydrogen to tar forming reactions is also limited in the absence of these low molecular weight materials. It would appear that, in the absence of externally applied hydrogen, chloroform extractable low molecular weight materials are instrumental in re-routing hydrogen (native to the coal) to tar forming reactions, that these materials play a key role routing externally applied hydrogen to tar forming reactions and that in flash pyrolysis they are retained in the coal up to higher temperatures, whereas in slow heating they are removed from the parent coal mass by evaporation/reaction at an earlier stage of the process.