A Comparison of Combustion of Coal Chars in O2/CO2 and O2/N2 Mixtures - Isothermal TGA Studies

Abstract

Two chars prepared from parent coals of a high volatile bituminous coal and an anthracite coal were subjected to isothermal combustion tests in a thermogravimetric analyser (TGA). The chars were burned in mixtures of O2/CO2 and O2/N2 with O2 concentrations of 3%, 6%, 10%, 21% and 30% and combustion temperature ranging from 723 K to 1163 K. Experimental results show that replacing the inert nitrogen gas in the oxidizer with CO2 has little influence on the measured reactivity of coal chars under conditions of the experiments. The measured reactivity of each char was found to be approximately first order (0.88 – 0.90) to the concentration of O2 in the O2/CO2 mixtures. As char combustion tests were conducted under relatively high temperatures conditions, internal and external effectiveness factors were estimated in order to derive combustion kinetics of both chars. The values of activation energy for both chars, obtained by taking account of both the internal and external effectiveness factors (i.e. with correction), were very much in line with the literature data. The activation energy of the bituminous coal char combustion in O2/CO2 was found to be 96323 (without correction) – 108893 (with correction) kJ/kmol within the temperature range of 723 – 873 K, whereas the activation energy of the anthracite char was found to be 125716 (without correction) – 140486 (with correction) kJ/kmol within the temperature range of 773 – 973 K.