Influence of reactions in the boundary layer on Kraft char-​bed burning

This paper presents a model of Kraft char bed burning which considers gas-phase chemical reactions inside the boundary layer. The model was used to simulate the Institute of Paper Science and Technology char-burning experiments to study the effect of gas-phase reactions on char burning. The simulations showed that during combined oxidation and gasification, oxygen is consumed inside the boundary layer. Oxygen can be consumed to such an extent that carbon conversion involving gas-phase reactants takes place entirely through gasification reactions. In general, the effect of the gas-phase reactions is to decrease overall burning rate and increase bed temperature. Factors determining the magnitude of the decrease in burning rate are char gasification reactivity and boundary-layer mass transfer. The model predictions were found to be in reasonable quantitative agreement with experimental burning data on carbon conversion, bed temperatures, and carbon oxide formation during combined oxidation and gasification.