Influence of particle size on the dominant surface reaction in char conversion: A simplified model for DEM-CFD

For large particles, gas-phase reactions within the boundary layer determine whether O 2 or C O 2 acts as the primary oxidizer at the surface of char particles. Such large particles (in the multi centimetre range) are particularly relevant for applications such as municipal solid waste (MSW) incineration. This study presents a simplified model to investigate the dominant heterogeneous surface reactions and their kinetics during char conversion of a single spherical particle. Two heterogeneous reactions—direct oxidation ( C + 1 2 O 2 → C O ), and the Boudouard reaction ( C + C O 2 → 2 C O ), are incorporated alongside the homogeneous oxidation of C O ( C O + 1 2 O 2 → C O 2 ) within the gas-phase. The governing conservation equations are solved numerically in spherical coordinates using Arrhenius-type kinetics over a broad range of particle sizes and temperatures under oxidizing conditions. The simulations reveal clear transitions between kinetically controlled and diffusion-controlled regimes. For particle sizes larger than 0.1 m and temperatures exceeding 1200 K, the Boudouard reaction dominates carbon conversion, while oxygen is consumed within the boundary layer and does not reach the particle surface. The basic model has been further simplified to allow for integration into an unresolved Discrete Element Method − Computational Fluid Dynamics (DEM-CFD) framework in the form of a correlation with reasonable computational cost.