Development of an Improved Semi-Global Intrinsic Char Oxidation Kinetics Model

摘要

Several semi-global intrinsic char kinetics models have been proposed to describe high-temperature char combustion phenomena with greater detail than is captured in traditional apparent kinetics models, typically formulated as power laws. A critical component of such models is prediction of the CO_2/CO production ratio, which has a strong impact on both the combustion heat release and the consumption rate of carbon. Some existing intrinsic char kinetics models have been calibrated to describe the temperature-dependence of the CO_2/CO production ratio. However, experimental studies have consistently demonstrated an oxygen-dependence of the CO_2/CO production ratio, and none of the existing intrinsic char kinetics models have been validated against this oxygen-dependence. The most extensive previous study of the CO_2/CO production ratio was that of Tognotti et al., whose data interpretation relied on the assumption of kinetic-limited char oxidation and negligible conversion of CO to CO_2 in the particle boundary layer. Our simulation of the Tognotti experiments, using a detailed model of the porous char particle and the surrounding boundary layer, has confirmed the validity of these assumptions. In addition, we have employed steady-state analysis to demonstrate the limitations of the existing 3-step models and to assess more sophisticated models to adequately capture both the temperature-and oxygen-dependence of the CO_2/CO production ratio. A suitable 5-step heterogeneous reaction mechanism has been developed and its rate parameters fit to match CO_2/CO production data, global reaction orders, and activation energies reported in the literature.