Reactions in Brown Coal Pyrolysis Responsible for Heating Rate Effect on Tar Yield

Abstract

This study was carried out for the purpose of experimentally establishing the variation with heating rate of the extent of bridge breaking relative to that of cross-linking, which is a reasonable explanation of the heating rate effect on the tar yield in pyrolysis of low rank coals. A brown coal was pyrolyzed slowly at a heating rate of 0.167 K s-1 and rapidly at 2−3 × 103 K s-1. The yield of tar in the rapid pyrolysis increased with temperature and leveled off at 923 K and 26 mol-C per 100 mol-C in the coal, while 723 K and 15 mol-C in the slow pyrolysis. The loss of aliphatic carbon (ΔCal) due to aromatization was employed as the measure for the extent of bridge breaking, assuming that the loss is indispensable to supplying donatable hydrogen to cap radicals formed by cleavage of bridges connecting aromatic clusters. The extent of cross-linking was elucidated from the yields of H2O and CO2 that are the major and plausible products of condensation among hydroxylic and carboxylic groups. The rapid pyrolysis was found to give the yield of H2O smaller than that in the slow pyrolysis at every ΔCal, indicative of the activation energy for H2O formation smaller than that for the loss of aliphatic carbon. The larger ceiling yield of tar with higher heating rate was thus consistent with relatively enhanced bridge breaking and suppressed cross-linking such as dehydration condensation. Unlike the yield of H2O, that of CO2 as a function of ΔCal little depended on the heating rate, suggesting that the CO2 formation is not responsible for the observed heating rate effect on the ceiling yield.