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 0.5-1000 °C s ^-1, 500-900 °C, and 0-120 s, respectively, changing the atmosphere from He to CO₂ influenced neither the yield nor composition of tar. Even under heating at 1000 °C s^-1, the tar evolution was completed before temperature reached 600 °C. Below this temperature, CO₂ behaved as an inert gas in the pore systems of pyrolysing particles. After completion of the tar evolution (above 600 °C), 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 11wt%-daf in the course of heating the char in CO₂ from 700 to 900 °C at 1000 °C s^-1. It was estimated that about a half of the decrease was caused by the CO₂ gasification at an average rate over 20wt%-daf-coals^-1. Rate of such rapid CO ₂ 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.