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This paper presents a computational fluid dynamic analysis of a small-scale commercial biomass pellet boiler. Combustion of the particles in the bed plays a key role in the analysis of this kind of systems, and a stand-alone code was used in this work to provide the inlet conditions for the CFD analysis. Model predictions were compared with the experimental gas temperature and species concentration measurements, which were in good agreement. On the basis of the CFD analysis it can be concluded that the interaction of the particles in the bed and the poor mixing of the gases in the furnace are the key factors leading to the high emission levels that are typical of small-scale systems.

A mathematical model describing the thermal degradation of densified wood particles is presented. The influence of structural changes on the heat transfer properties of wood in its different stages is studied. The model uses a specific discretization scheme and combines intraparticle processes such as drying, pyrolysis and char heterogeneous reactions with extraparticle processes such as heat and species diffusional and convective transport. The model also accounts for particle shrinkage. The model is used to compare the predicted data to data on the mass loss dynamic and internal temperature of several particles from the relevant literature, and a good agreement is found.