Abstract
A numerical model is proposed to perform CFD simulations of biomass boilers working in different operating conditions and analyse the results with low computational effort. The model is based on steady fluxes that represent the biomass thermal conversion stages through the conservation of mass, energy, and chemical species in the packed bed region. The conversion reactions are combined with heat and mass transfer submodels that release the combustion products to the gas flow. The gas flow is calculated through classical finite volume techniques to model the transport and reaction phenomena. The overall process is calculated in a steady state with a fast, efficient, and reasonably accurate method, which allows the results to converge without long computation times. The modelling is applied to the simulation of a 30 kW domestic boiler, and the results are compared with experimental tests with reasonably good results for such a simple model. The model is also applied to study the effect of air enrichment in boiler performance and gas emissions. The boiler operation is simulated using different oxygen concentrations that range from 21% to 90% in the feeding air, and parameters such as the heat transferred, fume temperatures, and emissions of CO, CO2, and NOx are analysed. The results show that with a moderated air enrichment of 40% oxygen, the energy performance can be increased by 8%, CO emissions are noticeably reduced, and NOx remains practically stable.
Highlights
The need to reduce the use of fossil fuels in the recent decades has led to the investigation of alternative fuels and combustion systems
The development of computational technology has allowed for research of the combustion phenomena through simulation models, which provide the opportunity to analyse the behaviour of combustion systems and reduce costs [1,2]
Some overviews on the modelling of biomass combustion were presented by Chaney et al [3], Karin and Naser [4], and Khodaei et al [5] presented in-depth reviews about different bed combustion approaches and submodels
Summary
The need to reduce the use of fossil fuels in the recent decades has led to the investigation of alternative fuels and combustion systems. Previous works [9,10,11] have modelled the advance of biomass in a grate though several discrete columns whose properties change with the column position All these works calculate the bed conversion out of the computational domain. A more accurate approach is to introduce the packed bed in the boiler CFD domain to use the local properties of each point in the conversion calculations through user defined functions (UDF). Collazo et al [15] and Gómez et al [16] proposed three-dimensional models that implement the bed with the gas flow through the use of user defined variables to account for the combustion stages and heat and mass exchanges. The thermal conversion of biomass is modelled through reactive fluxes that introduce in the packed bed the source terms of mass, energy, and species representing the products of the combustion stages. This same boiler is simulated to analyse the effect of oxygen enrichment in the energy performance and contaminant emissions
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