Pyrolysis of a Victorian brown coal and gasification of nascent char in CO2 atmosphere in a wire-mesh reactor

Temperature-programmed pyrolysis of a Victorian brown coal was performed under flow of atmospheric He or CO2 in a wire-mesh reactor, in which extent of the secondary reactions of volatiles was minimised. Over the ranges of heating rate, holding temperature and holding time of 0.5-1000degreesC s(-1), 500-900degreesC, and 0-120 s, respectively, changing the atmosphere from He to CO2 influenced neither the yield nor composition of tar. Even under heating at 1000degreesC s(-1), the tar evolution was completed before temperature reached 600 degreesC. Below this temperature, CO, behaved as an inert gas in the pore systems of pyrolysing particles. After completion of the tar evolution above 600degreesC, CO2 participated in the formation of light gases from the nascent char. Initial CO2 gasification of the nascent char occurred at a considerably high rate simultaneously with its thermal cracking. The char yield decreased by 11 wt%-daf in the course of heating the char in CO2 from 700 to 900degreesC at 1000degreesC s(-1). It was estimated that about a half of the decrease was caused by the CO2 gasification at an average rate over 20 wt%-daf-coal s(-1). Rate of such rapid CO2 gasification seemed to strongly depend on the rate of thermal cracking, i.e. concentration of radicals formed by the thermal cracking. This hypothesis was consistent with the observed heating rate effects on the rates of the thermal cracking and CO2 gasification of the char. (C) 2003 Elsevier Ltd. All rights reserved.