Modeling of single-particle char combustion under O₂/H₂O conditions: Effects of temperature and steam concentration

In this study, the single-particle char-O₂/H₂O combustion kinetic model was modified based on the one-dimensional unsteady mass and heat transfer equation coupled with the Maxwell–Stefan multicomponent diffusion coefficient. Numerical simulations of char-O₂/H₂O combustion at different reaction temperatures and H₂O concentrations were performed. The simulation results show that the average temperature of a char particle decreases with increasing H₂O concentration, and the particle temperature is higher in the inner layer than in the outer layer. Without H₂O, O₂ consumption in the boundary layer accounts for 50% of the total O₂ consumption at different temperatures. Higher temperatures and H₂O concentrations promote competition for O₂ by homogeneous reactions in the boundary layer. Char-O₂/H₂O combustion is simultaneously controlled by O₂ external diffusion and internal diffusion. Control by O₂ internal diffusion is hardly affected by the H₂O concentration. The contribution of the gasification reaction to carbon consumption increases with increasing temperature and H₂O concentration. Increasing the H₂O concentration three-fold from 5% increases the contribution of the gasification reaction to carbon consumption 1.96-fold at 1373 K, but only 1.25-fold at 1673 K.