A kinetic study on char oxidation in mixtures of O2, CO2 and H2O

Abstract

Flue gas is usually recycled to a boiler during oxy-fuel combustion. Elevated levels of CO2 and steam in the recycled flue gas may affect coal combustion. To investigate the effect of H2O and CO2 on char oxidation, two kinds of Chinese coals, including YQ anthracite and SH lignite, were combusted in the mixtures of O2, CO2 and H2O atmosphere. The char conversion rates in different atmospheres were measured at the temperature of 1273 K. Further, H218O isotopic tracing method was adopted to clarify the reaction path of H2O. The results show that the shrinking core model is applicable to predict both combustion and gasification reactions in TGA. For both YQ and SH chars, char-O2 reaction is independent but char overall oxidation rate is promoted by the increase of H2O partial pressure. The active sites for char-O2 and char-H2O reactions are partly independent and partly at the same active sites for YQ anthracite, but they share common active sites for SH lignite. Char-H2O reaction is inhibited by char-O2 reaction but the 18O in H2O is still transferred to CO2 even in oxygen-rich atmosphere. The possible reaction may be C + H218O + 16O2 = C16O18O + H216O. However, CO2 has little influence on YQ char oxidation, but it has a slight promotion on SH char oxidation in low oxygen atmosphere at 1273 K. Also, the active sites for char-O2 and char-CO2 reactions are partly independent and partly at the same active sites for SH lignite, though the active site for char-CO2 reactions is few. Finally, a reaction kinetic model is purposed to predict char consumption rate in the mixtures of O2, CO2 and H2O.