Modelling the co-firing of coal and biomass in a 10 kW_th oxy-fuel fluidized bed

The oxy-fuel co-firing of solid fuels (such as coal, biomass, and solid waste) in fluidized beds is one of the most promising technologies for the industrial application of CO₂ capture and waste disposal. However, both the practical experimentation and numerical simulations for elucidating co-firing in an oxy-fuel fluidized bed are still limited. In this study, a multiphase particle-in-cell scheme based on a 3D Eulerian–Lagrangian model was further developed following our previous research on oxy-fuel co-firing in a micro fluidized bed (Powder Technol. 2020, 373, 522–534). The refined JL 4-step mechanism for the CO-CO₂ homogeneous reactions, the heterogeneous reactions of char oxidation and gasification, the heterogeneous reactions of NO and N₂O formation from char-N, and the self-desulfurization effect were comprehensively considered. The improvement of the models was verified through the continuous operation of 10 kW_th oxy-fuel fluidized bed tests (Fuel 2021,286,119,312; Energy Fuels, 2020, 34, 7373–7387), and the effects of the biomass blending ratio (M_b) on co-firing characteristics were discussed. It was found that the improvements could enhance the adaptability of the models to the oxy-fuel atmosphere, and the accurate prediction of NO, N₂O, SO₂. With an increase in M_b, the main reaction zone expanded or moved up along the riser height, and the volume of the high-temperature area increased, which promoted the burnout of particles and CO₂ emission when M_b is 50%. The high volatility of biomass increased O₂ consumption and CO concentration at the upper part of the riser, reduced N₂O formation, and had a significant impact on NO reduction. The low sulfur content and high Ca/S ratio of the biomass considerably reduced the SO₂ concentration. The simulation results also provided helpful information for the design and operation control of oxy-fuel co-firing of coal and biomass in a fluidized bed, such as the oxidant supply in different areas and grades, appropriate increase in the riser height, and reasonable adoption of M_b.