CFD modelling of bed shrinkage and channelling in fixed-bed combustion

Combustion of fixed fuel beds in grate furnaces is common within production of heat and power from solid fuels. Available theoretical and experimental experience provides a solid base of knowledge on how a conversion model of a fuel bed, using Computational Fluid Dynamics (CFD), needs to be structured and solved. Most existing models, however, handle the conversion in one single dimension of constant bed properties; when observing a burning fuel bed in a grate furnace it becomes apparent that the fuel bed is neither homogeneous nor uni-dimensional. In this study, a two-dimensional model of the combustion of fixed fuel beds has been developed for the purpose of studying the influence of heterogeneous fuel-bed properties on the conversion. In the model, the available experience from fuel-bed modelling by means of the sub-models for fixed-bed conversion was structured into a fluid-flow scale and into a fuel-particle scale, in which new formulations describing the shrinkage of the fuel bed on a multi-particle scale was introduced. Both available and new sub-models were introduced into a pre-existing CFD-platform, in which the framework for simulating fluid flow in porous media was used to solve also the conversion related processes acting within the particle scales as well as within the multi-particle scales. The complete model was validated with good correspondence between available measurements of temperature and species concentration in a wood-char combustor. In addition, the modelled shrinkage was found to well describe the observed shrinkage of the fuel bed in a combustion experiment. Results of model simulations by using heterogeneous bed porosity show that a porous passage through the bed risks causing channelling in the fuel bed – a phenomenon common in modern grate furnaces and suspected to cause increased emissions of nitric oxides and unburned carbon compounds. The channelling tendency could, however, to a large extent be reduced by grates of higher flow resistance. The natural porosity increase attributable to the packing of particles onto a wall was shown to concentrate combustion disturbances close to the surface of the grate. Thus, larger changes in the porosity than caused by natural fuel packing against a wall are needed to give rise to channels that emerge through the fuel bed.