From particle to boundary layer: A multiscale kinetic model unlocks O₂ competition mechanism of char heterogeneous and CO/H₂ homogeneous combustion in a pressurized O₂/H₂O atmosphere

Pressurized char-O₂/H₂O combustion offers a promising strategy for efficient carbon conversion and mitigating CO₂ emissions, yet the dynamic competition for O₂ consumption between char heterogeneous and CO/H₂ homogeneous combustion pathways remains poorly understood. This study develops a multiscale kinetic model using the finite volume method (FVM) to simulate single-particle char combustion in a multi-component system under pressurized O₂/H₂O conditions, uniquely integrating dynamic char structure evolution and boundary layer chemical reactions. Validated against experimental data, the model quantifies the effects of pressure, H₂O concentration, and temperature, as well as their coupling effect, on the O₂ competition mechanism. Key findings reveal that increasing H₂O concentration from 0 % to 40 % reduces the char center temperature by 220.8 K. Radial non-uniformity in heterogeneous reaction rates within the char shows pore diffusion limits. Higher H₂O reduces the peak char-O₂ combustion rate but boosts the peak CO-O₂ rate. When the temperature and H₂O concentration change, H₂ combustion is the primary pathway for O₂ competition, while when the pressure changes, CO combustion is the main path of competition for O₂·H₂O modifies the O₂ competition mechanism through a cascade effect encompassing “surface adsorption competition → gasification product generation → boundary layer combustion”. Pressure changes the spatial distribution of reaction products through the molecular diffusion volume-dependent selective diffusion inhibition effect. Temperature determines the competition weights of CO and H₂ combustion through different activation energies of homogeneous reactions. Collectively, these factors drive the dynamic transition of the primary competing pathway between H₂ and CO combustion.