Biomass/coal co-firing in a 600 MW opposed wall-fired boiler: Impact of large non-spherical biomass particles on motion dynamics and boiler performance

This study investigates the complex multiphase flow dynamics of large, irregularly shaped biomass particles intended for co-firing in the heat and power sector. Existing simulation tools, primarily developed for nearly spherical, tiny particles like pulverized coal, often fail to capture these differences. To address this gap, a cylindrical aerodynamic model (CAM) is developed. It solves the coupled translational and rotational motion of non-spherical particles, by integrating drag, lift and torque coefficients derived from particle-resolved direct numerical simulations over a wide range of particle and flow conditions. This model is used in simulations of wheat straw (thermal input of 8 %) co-firing with pulverized coal in a 600 MW opposed wall-fired boiler. The wheat straw particles are predominantly cylindrical with an average length of 20 mm and diameter of 1.5 mm (equi-volume diameter of 4 mm), constituting 80 % of the total particle mass. Simulation results are compared with those obtained using conventional particle tracking models such as the spherical drag model (SDM) and shape factor drag model (SFDM), as well as with field measurements. The findings reveal that even with shape factor corrections, the accuracy of SFDM declines significantly as the particle equi-volume diameter exceeds 3 mm, leading to substantial discrepancies in unburnt char content compared to experimental data. For straw particles in this boiler, simulations with the CAM more accurately predict particle trajectories and replicate fly ash carbon content and boiler exit oxygen levels in closer agreement with the experimental data than the conventional particle models. This new particle tracking model becomes increasingly critical in effectiveness as biomass particle sizes and the share of biomass co-fired increase. Therefore, it is highly recommended for use in design, retrofit, optimization, or trouble-shooting in such scenarios.