Evolution of tar in coal pyrolysis in conditions relevant to moving bed gasification

Most tars in gasification are produced in the pyrolysis step at lower temperatures than those required for gasification reactions. In updraft moving beds, the tars form through pyrolysis as the coal is relatively slowly heated. Tars are promptly carried away from the gasifier by the gas flow as part of the gas product. This study was designed in order to understand the effect of the coal bed on the tar cracking process during the pyrolytic step of a slowly heated South African inertinite-rich coal sample. A wire mesh reactor was employed to determine the intrinsic extent of tar release from the coal particles, while a fixed bed reactor enabled the measurement of the extent of tar cracking by interactions between tars and the heated coal bed. Experiments were carried out at atmospheric and high pressure, and the effect of temperature and heating rate on tar quantity and quality was studied. A significant effect of the bed on tar reduction was observed even at the lower temperatures used in these experiments. Heating rate still showed a significant effect on tar yield despite slow heating being used in all cases. High pressure caused a decrease in tar yields due to intraparticle tar cracking and repolymerization, which was more marked than the decrease in total volatiles. The effect of temperature and heating rate on tar quality was larger at atmospheric pressure. Primary tar cracking mainly took place at aliphatic bridges connecting aromatic rings. Subsequent condensation reactions led larger fused aromatic rings. Tars obtained in the fixed bed reactor were lighter than those from the wire mesh reactor.