Investigation of factors affecting channelling in fixed-bed solid fuel combustion using CFD

Channelling is an undesirable phenomenon in fixed-bed combustion. It is characterised by an uneven air distribution, and thus fuel conversion, throughout the fuel bed. To investigate factors that influence channelling, an unsteady, two-dimensional numerical model capable of predicting solid fuel combustion under fixed-bed conditions is presented. Biomass is the focus of this study, but the model can be readily applied to other solid fuels, such as coal and municipal solid waste. The model includes drying, pyrolysis, and heterogeneous char reactions, and incorporates bed shrinkage processes comprised of both continuous shrinkage and abrupt collapses. It is also capable of representing spatial non-uniformities which may occur throughout a bed, arising from irregular packing and non-homogeneous fuel composition. The overall model is validated by means of two different data sources: the first for ignition rates and the second for species profiles through a biomass fuel bed. The validated model is then applied to investigate factors affecting channelling in a randomly packed bed containing a high-porosity passage. The influence of flow resistance through the grate and bed height are compared with previous observations. Additional factors investigated include flue gas recirculation and initial moisture content of the fuel. Predictions show that increasing the flow resistance of the grate improves the gas distribution and reduces channelling because it inhibits flow from deviating towards the relatively porous passage. For deeper beds, however, the effectiveness of grate resistance is diminished because the gas then has more residence time within the bed to track towards the passage. Increasing the initial moisture content from 0% to 30% has a weak influence; nonetheless, wetter fuels show a propensity to amplify channelling. The impact of flue gas recirculation on channelling appears to be insignificant, although its benefits, such as reduced peak temperature, are apparent.