Three-dimensional modeling of biomass fuel flow in a circulating fluidized bed furnace with an experimentally derived momentum exchange model

Abstract

In modeling of large-scale circulating fluidized bed (CFB) furnaces, semi-empirical 3D process models are used in furnace analysis, design and optimization as their accuracy versus computational times make them more favorable than high resolution computational fluid dynamics (CFD) in many engineering type of studies. One of the key aspects in determining the furnace performance and efficiency is the correct modeling of the fuel flow as it determines where the thermochemical reactions take place in the furnace, affecting the temperature and gaseous species distributions. In modeling, this requires the suitable approximation of the momentum exchange between the fuel and the gas phase as well as the fuel and the bed material. To model the momentum exchange with current models presented in the literature, information on the material properties, namely the fuel particle size has to be given. The determination of particle size is affected by the particle shape and for irregular, non-spherical material, such as biomass, currently there are no readily applicable methods to determine the effective momentum exchange based on the particle geometry.In this work, a new gas–fuel momentum exchange model is developed for the fuel flow, which acknowledges the effect of physical properties of the fuel, namely the shape and size. Due to impracticality of determining a representative fuel particle size, characterization data is utilized to directly obtain information on the gas–fuel momentum exchange and the particles are classified based on their elutriation behavior, rather than particle size. The new momentum exchange model is utilized with an existing semi-empirical 3D process model and applied in simulation of a utility-scale CFB furnace firing biomass. The simulation results are compared with measurements and are found to be in good agreement, which demonstrates the applicability of the modeling approach. The presented method improves the modeling of fuel flow with CFB process models, especially with irregular, non-spherical fuels such as biomass. The improved modeling of fuel flow within large-scale CFB furnaces enables faster and more reliable design and analysis of CFB furnaces.