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 e ect 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 CO 2 by CO oxidation promotes the CO 2 gasi cation of char, whereas the consumption of O 2 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 CO 2 gasi cation 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 O 2 by CO oxidation. This result indicates that CO 2 gasi cation compensates for the decrease in the net reaction rate due to the suppression of char oxidation. Therefore, CO oxidation should greatly a ect the heterogeneous reaction rates of char, especially CO 2 gasi cation, through changes in the temperature and compositions of the gas-phase near a coal char particle.
A new concept for modeling the overlap between devolatilization and char oxidation was extended to various coal types. Quasi-steady mass transfer analysis around a single coal particle with devolatilization was performed, and the overlap between devolatilization and char oxidation was modeled with a semi-parallel reaction model. The semi-parallel reaction model can capture the differences in the char oxidation rates of each coal by a single fitting parameter called the inhibition factor. The combustion simulation of the single coal particle indicated the superiority of the semi-parallel reaction model over the previously reported modeling concepts because the semi-parallel reaction model could describe the decrease in char oxidation rate during devolatilization, which is not represented by other reaction models. Although the inhibition factor is not correlated with the proximate analysis of the volatile matter, it is correlated with the temperature at which the rate-controlling step of char oxidation shifts. This is because the proximate analysis does not include information on char reactivity. Therefore, it is suggested that the inhibition effect of devolatilization on char oxidation strongly correlates with the parameter that describes the sensitivity of char oxidation to the mass transfer processes of the oxidant.
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