CFD modeling of oxy-coal combustion in circulating fluidized bed

Abstract

Based on the Computational Fluid Dynamics (CFD) method, the previously established and validated 2-dimensional model was employed to predict the oxy-coal combustion processes in a 50 kW circulating fluidized bed (CFB) at Southeast University, China. The simulated processes of coal conversion in our CFB combustion system included particle drying, dry coal devolatilization, volatile combustion, char combustion, char gasification and SO 2 emission. The effects of combustion atmospheres, including air (21% O 2/79% N 2) and oxygen/recycled flue gas (O 2/RFG) atmosphere with different O 2 concentration (from 21% to 40%), were investigated on distributions of solid volume fraction, temperature and gas concentration in the riser. During combustion of Xuzhou bituminous coal in O 2/RFG modes with different O 2 concentrations referred to above, about 70% volume fraction of CO 2 were achieved in the wet flue gas with more than 25% volume fraction of H 2O. The rates of char gasification by CO 2 and H 2O in 30% O 2/70% RFG atmosphere were obviously higher than that in air. However, compared to air combustion, CO concentration in the dense zone in 30% O 2/70% RFG atmosphere was lower because high concentration of H 2O accelerated CO combustion. Both indirect and direct desulfurization mechanisms were considered for sulfur retention by limestone and the corresponding overall reaction rates were calculated statistically. With increase in O 2 inlet concentration gaseous pollutant SO 2 was enriched.