On the use of single-film models to describe the oxy-fuel combustion of pulverized coal char

Abstract

Computational fluid dynamic (CFD) simulations traditionally rely on the computational efficiency of single-film global kinetic oxidation models to predict char particle temperatures and char conversion rates in pulverized coal boilers. In oxy-fuel combustion with flue gas recirculation (FGR), as is commonly employed, char combustion occurs in the presence of elevated CO2 levels and, frequently, elevated water vapor levels (when employing wet FGR). Furthermore, local oxygen concentrations can be quite high in the vicinity of oxygen injection lances. The suitability of existing approaches to modeling char combustion under these conditions has been unclear. In particular, our previous work has shown that both boundary layer conversion of CO and gasification reactions of steam and CO2 need to be included to give reasonable agreement with the experimental measurements, for particles over 60μm in size. In this paper, we report on the development and application of an extended single-film reaction model that includes both oxidation and gasification reactions. We have systematically interrogated the performance of the model in comparison to experimental data for two US coals (a Powder River Basin subbituminous coal and a low-sulfur, high-volatile bituminous coal) for a variety of model assumptions. While the extended single-film model does not give perfect agreement with the data, reasonably good agreement is achieved for high-temperature environments with 12–36vol.% O2 and 16vol.% H2O in either N2 or CO2 diluent. The analysis shows that, to achieve such agreement with the data while maintaining reasonable values for activation energy of the reactions, incorporation of both steam and CO2 gasification reactions is required.