Reaction behaviors of a single coal char particle affected by oxygen and steam under oxy-fuel combustion

Abstract

The reaction behaviors of coal char particles tend to become more complex with the presence of gasification reaction under mixtures of O2/CO2/H2O, which is an urgent demand for the better understandings of pulverized coal oxy-fuel combustion with wet recycle. In this paper, numerical simulations are adopted to evaluate the individual effects of O2 and steam concentration on the reaction behaviors of a single coal char particle. The results reveal that, as the steam content increases from 0% to 10%, both the peak and surface temperatures increase by 600 K and 30 K, respectively. As the steam level continues to increase, the peak temperature increases slightly and the surface temperature is hardly changed. The formation of H2 can be promoted by the addition of steam, while that of CO is attributed to the increased O2 level. The sensibility of O2 and steam content on the homogeneous reactions follows R1 (CO + 0.5O2 + H2O → CO2 + H2O) > R3 (CO2 + H2 → CO + H2O) > R2 (CO + H2O → CO2 + H2), and the H2O molecules in R1 play the roles of both reactant and catalyst. The reaction of char-CO2 affected by the char-H2O is dependent on steam level. Strengthening the char reaction with H2O can benefit the char reaction with CO2 at below 20% steam level, while adverse to the char-CO2 reaction at above 20% steam level. The increase in O2 content promotes the char-CO2 reaction but slightly inhibits the char reaction with H2O. Furthermore, the carbon consumption rate can be improved by the increase in O2 level because both oxidation and gasification reactions are enhanced. As the steam concentration increases, the carbon consumption rate evidently decreases at first, but slightly increases thereafter because of the enhanced char-H2O reaction.