CFD simulation of a 4 MW biomass grate furnace using an Eulerian fixed-bed model: Validation of in-bed and freeboard results

Biomass combustion in grate-fired systems plays a key role in the renewable generation of thermal energy. This study employs an in-house Eulerian bed model, based on the porous media approach, to perform CFD simulations of a 4 MW grate-fired biomass furnace under two distinct operating conditions. The numerical results are validated against temperature and species measurements from the literature. The model predicts a solid fuel combustion pattern characteristic of high-moisture fuels, with the reaction front propagating from the grate upward to the surface of the bed. Maximum temperatures of around 1200 degrees C are observed near the grate, along with significant horizontal temperature gradients in the bottom char layer, influenced by primary airflow entering through the grate slots. Temperature profiles and contours confirm the presence of a wet fuel layer covering the char, consistent with observations from the experimental study. The freeboard temperature profiles are predicted with high accuracy, though discrepancies are noted in gas composition predictions, with CO and CH4 concentrations underpredicted and CO2 levels overestimated. When the furnace operates with 50 % excess air and higher secondary and tertiary airflows, it achieves 1.5 % higher combustion efficiency compared to operation with 15% excess air and lower secondary and tertiary air ratios. In the latter case, the secondary oxidation flame is less intense, leading to higher peak temperatures in the post-combustion zone and increased unburned emissions. The developed model provides valuable insights for improving thermal efficiency and combustion stability in biomass furnaces.