Accurate modelling of biomass char particle oxidation with a novel external heat and mass transfer correlation and an appropriate CO/CO2 ratio

Biomass char oxidation is strongly influenced by the gas product distribution of the reaction and by external heat and mass transfer around the particle. If these processes are not properly described, particle scale models may reproduce conversion trends through compensating errors, while predicting inaccurate temperatures and gas compositions. In this work, biomass char oxidation is investigated by coupling a volumetric single particle model with new external heat and mass transfer correlations developed for short cylinders in crossflow. First, numerical simulations were performed for heated cylinders with aspect ratios from 1 to 10 and particle Reynolds numbers up to 50. Based on these simulations, new Nusselt and Sherwood correlations were derived as a function of cylinder aspect ratio and implemented in the particle model. The influence of the external transport description, CO/CO 2 ratio, diffusion factor and char composition was then assessed. Model predictions were validated against literature data for particle conversion and center temperature under different gas temperatures, oxygen partial pressures and flow velocities. The results show that the external transport correlation and the CO/CO 2 ratio are the dominant modelling parameters, whereas diffusion factor and char composition have only minor effects. Using a CO/CO 2 correlation representative of biomass char together with the new short cylinder correlation yields accurate predictions of both conversion and particle temperature, while approaches based on infinite cylinder correlations or CO/CO 2 ratios derived for coal may reproduce conversion trends but lead to less physically consistent temperatures and gas product distributions.