Development of an engineering tool for design and optimisation of biomass combustion in grate-fired boilers (original) (raw)

2000

Abstract

The present work aims at studying the interaction between the fuel bed and the furnace in a grate fired boiler using wood chips. The heat and mass transfer from the furnace, i.e. the homogeneous combustion reactions and thermal radiation to the fuel bed is simulated using a general-purpose CFD code (Fluent). The resulting release of gases, i.e. the water vapour, pyrolysis gas, CO and CO 2 , from the bed is simulated by a fuel bed model. The fuel bed model is based on experiments in a laboratory-scale cylindrical furnace using wood chips with varied moisture contents. The wood chips rest on a fixed grate through which combustion air is added. The fuel bed is heated from the top by a radiant heater kept at a constant temperature to resemble the conditions in a grate-fired boiler. During the experiment, the bed temperatures are measured at several positions above the grate and the mass loss of the bed material is recorded simultaneously. Experiments show that the mass loss and heating of a fuel bed varies with the moisture content, bed porosity and density. A fixed bed model describing the combustion process, i.e. drying, pyrolysis and char reactions, in a fuel bed is being developed. Integration of the fuel bed model as part of a general furnace model for grate-fired boilers is carried out. This preliminary study focuses on the influence of fuel moisture content on the predicted temperature and species distributions in a utility grate-fired boiler. The results are presented and discussed in this paper. This work demonstrates that the developed model has a potential as a promising engineering tool for design and optimisation of biomass combustion in grate-fired boilers.

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