Numerical investigation on separate physicochemical effects of carbon dioxide on coal char combustion in O2/CO2 environments

Abstract

The thermophysical and chemical effects of CO2 on the combustion characteristics of a single char particle in O2/CO2 environments remain unclear because these effects are intercoupled during the complex process of coal char combustion. A numerical method was proposed to adjust the char particle temperature in an O2/CO2 environment back to that in an O2/N2 environment by replacing an appropriate portion of CO2 with argon because of the molar heat capacity of Ar < N2 < CO2, and to quantitatively isolate the effects of CO2 on bituminous coal char combustion in specially designed combustion environments with the continuous-film model. The numerical result shows that the char particle temperature in a 21% O2/79% CO2 environment is lower by 493 K than that in a 21% O2/79% N2 environment because of the combined effects of lower local gas temperature and endothermic char gasification reaction with CO2. The char particle temperature increases significantly with the oxygen concentration, and the char temperature in a 52.5% O2/47.5% CO2 environment is equivalent to that in air environment. Most importantly, the effects of CO2 on bituminous coal char combustion in O2/CO2 environments are quantitatively isolated with specially designed combustion environments (21% O2/79% N2, 21% O2/11% CO2/68% Ar, 52.5% O2/47.5% CO2 and 52.5% O2/30% CO2/17.5% Ar). The relative contributions of the oxygen concentration, thermal, and chemical effects on the char combustion rate are 82.1%, 11.2%, and 6.7%, respectively, at an ambient gas temperature of 1200 K for bituminous coal char of 91 µm. The results show that the oxygen concentration effect is the most important factor, followed by the thermal effect and the chemical effect of char gasification reaction with CO2.