CFD modelling of bed shrinkage and channelling in fixed-bed combustion
Artikel i vetenskaplig tidskrift, 2011

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.

Solid fuels

Grate furnaces

Fixed bed





Sven Hermansson

Chalmers, Energi och miljö, Energiteknik

Henrik Thunman

Chalmers, Energi och miljö, Energiteknik

Combustion and Flame

0010-2180 (ISSN) 15562921 (eISSN)

Vol. 158 5 988-999







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