Description, applications and numerical modelling of bubbling fluidized bed combustion in waste-to-energy plants

The use of the fluidized bed combustor (FBC) has increased. It began in the 20th century as coal combustion and gasification, which then developed into catalytic reactions. Only recently, the application field has been extended to the incineration of biomass and pretreated waste, for either power generation or waste disposal. The success of fluidized bed combustion is due to high combustion efficiency, great flexibility when it comes to the heating value of the fuel and reduction in pollutants emitted with the flue gas. The purpose of this paper is to extend the computational fluid dynamics (CFD) modeling, which has recently proved to be an effective means of analysis and optimization of energy-conversion processes, even to fluidized bed combustion of refuse-derived fuel (RDF). The increase in computer power over recent years has made this goal attainable. This overview is divided into three main parts. In the first one, after a brief introduction to the fluidized bed hydrodynamics, the phenomena taking place during fluidized bed combustion of solid fuels are explained, with particular interest in drying, devolatilization and char oxidation. Differences in combustion between conventional and alternative fuels are highlighted so as to fully characterize the RDF behaviour in the bed. In the second part, a review of the state of the art of combustion modelling is presented: not only one-dimensional but also multidimensional models, applied to the fluidized bed and conventional combustors, are analysed in order to emphasize what has already been done and what remains to be done. From the investigation it emerges that multi-dimensional models of fluidized bed furnaces are still lacking in literature: up to now, several bed hydrodynamics models have been achieved but none of them takes chemical reactions into account. In the last section, a numerical model of a bubbling FBC fed by RDF is presented. The combustor is divided into two regions: the bed and the freeboard. The calculation of mass and energy fluxes entering from the bed into the freeboard provides the boundary conditions for the subsequent CFD analysis. The freeboard model, implemented by means of the commercial code FLUENT 6.1, is mainly concerned with employing the twomixture-fraction-pdf approach to track both the gases coming from the bed and the solid fuel particles that do not burn in the bed but above it. The excess air is injected through four series of nozzles; furthermore, the heat transfer between the flue gases and the internals is taken into account. The freeboard operation was simulated consistent with two conditions representing the combustor minimum and maximum load. The comparison between the predicted and the experimental data are in agreement: the reliability of the model results proves that CFD modelling is a powerful method to give insight into the behaviour of a full-scale FBC fed by a non-conventional fuel. The minimum load condition is found to reduce combustion efficiency and heat transfer from the flue gas to the boiler tubes.