Numerical study of biomass combustion of a grate-firing furnace with emphasis on gas-phase combustion modeling

Abstract

As a renewable energy resource, biomass is slowly becoming a substitute for traditional fossil fuels due to global warming concerns and limited availability of fossil fuel resources. Direct combustion of biomass in grate-firing furnaces is the most widespread established technology to produce power and heat from biomass fuel. This is driven by low investment cost of grate-firing furnaces and their capability of burning biomass over a wide range of particles size, ash and moisture contents. Nevertheless, relatively small scale grate-firing biomass furnaces suffer from generating high level of emissions (e.g., CO and NOx) due mainly to their smaller volume, and consequently poor mixing between fuel and injected air, and also shorter residence time of combustion. Hence, further improvements need to be developed and implemented in their design in order to maximize thermal efficiency and further reduce pollutant emissions. Such improvements require comprehensive understanding of biomass conversion as well as gas-phase combustion of volatiles in the free room above the bed, known as a freeboard. The present research aims to develop a reliable gas-phase combustion scheme suitable for modeling biomass combustion in grate-firing furnaces by means of computational fluid dynamics (CFD). The study investigates the influence of chemical mechanisms and turbulence models on temperature field and species concentrations predicted by CFD platform. Alongside the aforementioned parameters, a substantial effort is devoted to understand the role of the combustion model in modeling gas-phase biomass combustion, which resulted in the development of a novel combustion approach. The performance of the proposed combustion model is examined under different flow-field conditions relevant to practical biomass furnaces. The results of this research clearly reveal superior performance of the proposed combustion model compared to currently available models used for the simulation of biomass combustion. Particularly, the performance of the new model is more highlighted in the predicting minor species such as CO and NO.