CFD Analysis of a Bubbling Fluidized Bed Combustor Based on Co-Firing Biomass & Coal

Abstract

Coal is one of the greatest sources of energy used for power generation across the world, providing almost 40% of the total energy produced. However, coal burning is a major source of CO2 emission, which adversely affects the climate. As a result, techniques such as co-firing coal with biomass have become increasingly popular as an alternative. Through the utilization of biomass, in the form of agricultural residues such as cotton stalks, wheat straw, rice straw, sugarcane bagasse and rice husks, not only can heat and power be produced efficiently, but there is also the potential to improve rural income and energy security through the substitution of coal, oil and natural gas. The global demand for electric energy is expected to increase from about 14 billion tonnes per coal equivalent (TCE) to19 billion TCE by 2020. In India too, the demand for electric energy is expected to rise dramatically. Power generation based on biomass holds considerable promise in agricultural states such as Punjab, which has huge biomass resources from the crop production system and agricultural industries. With the availability of 15million tons of paddy straw and 5 million tonnes of other agricultural residues such as cotton and mustard stalk, the state has the potential to generate 2000 MW of power through biomass. Fluidized bed combustion (FBC) is considered an established technology for burning coal, biomass and waste fuels for power generation. The present study was performed in two stages. In first stage operational and monitoring data were collected from a real power plant FBC boiler of 45TPH capacity. In the next stage three dimensional computational fluid dynamics (CFD) analysis was performed using Fluent 6.3 CFD code on this large scale commercial FBC boiler. The boiler was fuelled by both coal and biomass. Coal was fed from the bottom through 16 ports, uniformly distributed in the four zones of the bed and biomass was fed from four ports from a height of 1·8 m above the bed. Discrete phase modeling (DPM) approach has been used to determine particle residence time to predict the flow behaviour of fuel particles. Discrete phase modeling of char burnout and devolatilization has been also performed to study the effect on combustion efficiency by using both of the fuels. The standard k–ε two-phase turbulence model was used to describe the gas–solids flow in the fluidized bed combustor and the combustion analysis was done through a non-premixed (NPM) approach in a species model. Temperature profiles and mass fractions of CO2 and O2 were obtained over the entire combustion domain and validated with experimental data.