CFD modeling of oxy-coal combustion in circulating fluidized bed

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 SO2 emission. The effects of combustion atmospheres, including air (21% O2/79% N2) and oxygen/recycled flue gas (O2/RFG) atmosphere with different O2 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 O2/RFG modes with different O2 concentrations referred to above, about 70% volume fraction of CO2 were achieved in the wet flue gas with more than 25% volume fraction of H2O. The rates of char gasification by CO2 and H2O in 30% O2/70% RFG atmosphere were obviously higher than that in air. However, compared to air combustion, CO concentration in the dense zone in 30% O2/70% RFG atmosphere was lower because high concentration of H2O 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 O2 inlet concentration gaseous pollutant SO2 was enriched.

► CFD simulation experiments are carried out for oxy-coal combustion in a CFB.
► About 70% CO2 and 25% H2O in the flue gas are detected with wet recycle.
► Increase of O2 concentration reduces solid volume fraction in the dilute zone.
► Flue gas wet recycle enhances coal gasification by H2O.
► As inlet O2 concentration increases from 21% to 40%, SO2 is enriched.