CFD Modeling of Small Scale Pellet Boilers: An Overview with Preliminary Results from a Simplified Approach

The increasing global energy demand and mounting pressures for CO2 mitigation call for increased efficient utilisation of biomass, particularly for heating domestic and commercial buildings. The authors of the present paper are investigating the combustion performance and NOx and particulate emissions of a 50 kW biomass pellet boiler manufactured by a UK manufacturer. The boiler has a number of adjustable parameters such as the ratio of air flow split between the primary and secondary supplies, the orientation, height, direction and number of the secondary inlets. The optimisation of these parameters provides opportunities to improve the combustion efficiency and to minimize emissions. When used carefully in conjunction with experiments, CFD (Computational Fluid Dynamics) modelling provides a useful tool, enabling trends in the performance of multiple configurations of the boiler to be rapidly assessed, thus saving time and cost. However, modelling combustion and emissions of a small-scale biomass boiler is not trivial and appropriate models that can be coupled with a CFD code need to be developed. This paper reviews previous approaches to modelling combustion with CFD codes, which are specifically relevant to simulating a packed-bed of wood pellets in a small-scale biomass pellet boiler. In the first part it considers how different researchers have modelled the heterogeneous solid phase and coupled this with the gas phase. The essential components of the models are then overviewed. Importantly, for the optimisation 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 a 50kW packed-bed biomass pellet boiler.