Reactions in brown coal pyrolysis responsible for heating rate effect on tar yield

This study was carried out for the purpose of experimentally establishing t he variation with heating rate of the extent of bridge breaking relative to that of cross-linking, which is a reasonable explanation of the heating ra te 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 x 1 0(3) K s(-1), The yield of tar in the rapid pyrolysis increased with temper ature and leveled off at 923 K and 26 mol-C per 100 mol-C in the coal, whil e 723 K and 15 mol-C in the slow pyrolysis, The loss of aliphatic carbon (D elta C-al) due to aromatization was employed as the measure for the extent of bridge breaking, assuming that the loss is indispensable to supplying do natable hydrogen to cap radicals formed by cleavage of bridges connecting a romatic clusters. The extent of cross-linking was elucidated from the yield s of H2O and CO2 that are the major and plausible products of condensation among hydroxylic and carboxylic groups. The rapid pyrolysis was found to gi ve the yield of H2O smaller than that in the slow pyrolysis at every Delta C-al, indicative of the activation energy for H2O formation smaller than th at for the loss of aliphatic carbon. The larger ceiling yield of tar with h igher heating rate was thus consistent with relatively enhanced bridge brea king and suppressed cross-linking such as dehydration condensation. Unlike the yield of H2O, that of CO2 as a function of Delta C-al little depended o n the heating rate, suggesting that the CO2 formation is not responsible fo r the observed heating rate effect on the ceiling yield.