Effects of heating rate and ion-exchangeable cations on the pyrolysis yields from a Victorian brown coal

A Victorian low-rank coal (Loy Yang) was acid-washed and ion-exchanged with Na and Ca to prepare the H-form, Na-form, and Ca-form coal samples. Two more H-form samples were also prepared by rewashing the Na-form and Ca-form samples with acid. These coal samples were pyrolyzed in a wire-mesh reactor where the secondary reactions of the evolved volatiles were minimized. The ion-exchanged coal samples were found to give very different tar yields from those of the raw coal samples. While the tar yields from the pyrolysis of the raw and H-form coal samples were observed to be very sensitive to changes in heating rate, the tar yields from the Ca-form and Na-form samples showed little heating rate sensitivity. Unlike higher rank coals studied previously, the tar yields from the pyrolysis of the raw coal and the H-form coal samples at 600 degrees C were found to increase much more than the corresponding increases in the total volatile yields as the heating rate was increased from 1 to 2000 K s(-1), Reexchanging Na in the Na-form sample and Ca in the Ca-form sample with H confirmed the effects of Na and Ca but also suggested that the irreversible structural changes taking place during ion-exchange, possibly including the loss of humic materials and the physical reconfiguration of the macromolecular network, should also be considered to ascertain the effects of ion-exchangeable cations during pyrolysis. The heating rate sensitivity of pyrolysis yields is believed to be at least partly related to the presence of carboxyl/carboxylate groups and other bulky substitution groups in the coal as well as the rapid pressure buildup within the particles. The major roles of ion-exchangeable cations during pyrolysis are also discussed in the paper.