Numerical simulation of single coal char particle combustion with the overall gas-phase reaction

The quasi-steady state heat and mass transfer around a single coal char particle with and without CO oxidation was numerically analyzed to investigate the effect of CO oxidation on the reaction process of char. With CO oxidation, the gas temperature around the particle is increased by the exothermic heat of CO oxidation, and the particle itself also experiences a temperature increase due to the heat transfer between high-temperature gas and the particle. In addition, the generation of additional CO₂ by CO oxidation promotes the CO₂ gasification of char, whereas the consumption of O₂ suppresses the char oxidation. As a result of balancing among these factors, the net reaction rate of char, which is the sum of the partial oxidation rate and CO₂ gasification rate, with CO oxidation became larger than that without CO oxidation. Under conditions of a higher temperature and larger particle size, the net reaction rate with CO oxidation was larger than that without CO oxidation, though the partial oxidation rate with CO oxidation was smaller than that without CO oxidation because of the promoted consumption of O₂ by CO oxidation. This result indicates that CO₂ gasification compensates for the decrease in the net reaction rate due to the suppression of char oxidation. Therefore, CO oxidation should greatly affect the heterogeneous reaction rates of char, especially CO₂ gasification, through changes in the temperature and compositions of the gas-phase near a coal char particle.