2002 Volume 35 Issue 8 Pages 778-785
Suppression of the cross-linking reaction at low temperature is a key factor to increase total volatiles during the pyrolysis of brown coals. To do so, it is essential to clarify the mechanism and kinetics of such cross-linking reactions. We prepared coal samples having similar monomer units but different macro-molecular structures through liquid phase oxidation of an Australian brown coal. The oxidized coal samples were heated from 303 to 1173 K at the rate of 20 K/min in a TG-MS analyzer and were pyrolyzed. It was found that the formation rates of inorganic gases, H2O, CO and CO2, were significantly dependent on the amount of oxygen functional groups in the coals. Oxidized coals having a large amount of COOH groups decomposed at lower temperatures and distinct peaks of the formation rates of inorganic gases appeared at around 673 K. We proposed six kinds of cross-linking reactions based on the types of hydrogen bonding formed between the functional groups, and estimated the change in the extent of each reaction with increasing temperature from the formation rates of the inorganic gases measured. The hydrogen bonded COOH–COOH and COOH–OH decomposed at 500–600 K and 600–700 K, respectively, to form the inorganic gases and cross-linked products such as anhydrides, ether, and ester. These cross-linked products decomposed further into CO and CO2 at temperatures above 800 K. Based on the above analysis and the 13C-NMR measurement of the pyrolyzed chars, it was clarified that the number of cross-links was equal to the number of H2O molecules formed and that the CO and CO2 formation rates were closely related to the H2O formation rate.