An overview of CFD modelling of small-scale fixed-bed biomass pellet boilers with preliminary results from a simplified approach

The increasing global energy demand and mounting pressures for CO₂ mitigation call for increased efficient utilization of biomass, particularly for heating domestic and commercial buildings. The authors of the present paper are investigating the optimization of the combustion performance and NOₓ emissions of a 50kW biomass pellet boiler fabricated by a UK manufacturer. The boiler has a number of adjustable parameters including the ratio of air flow split between the primary and secondary supplies, the orientation, height, direction and number of the secondary inlets. The optimization of these parameters provides opportunities to improve both the combustion efficiency and NOₓ emissions. When used carefully in conjunction with experiments, Computational Fluid Dynamics (CFD) modelling is a useful tool for rapidly and at minimum cost examining the combustion performance and emissions from a boiler with multiple variable parameters. However, modelling combustion and emissions of a small-scale biomass pellet boiler is not trivial and appropriate fixed-bed models that can be coupled with the CFD code are required. This paper reviews previous approaches specifically relevant to simulating fixed-bed biomass boilers. In the first part it considers approaches to modelling the heterogeneous solid phase and coupling this with the gas phase. The essential components of the sub-models are then overviewed. Importantly, for the optimization process a model is required that has a good balance between accuracy in predicting physical trends, with low computational run time. Finally, a simple combustion model written within a FLUENT user defined function is described, together with the preliminary modelling results obtained for modelling the combustion of wood pellets in a 50kW fixed-bed biomass pellet boiler.