Experimental Studies on the Emission of Gaseous Pollutants in an Oxy-Fuel-Fluidized Bed with the Cofiring of Coal and Biomass Waste Fuels

The cofiring of coal and biomass waste is an important technological direction in oxy-fuel combustion for both CO2 capture and waste disposal. The emission of pollutants during cofiring in an oxy-fuel-fluidized bed is a complex process, and practical knowledge of this process is still very limited. In this work, experimental studies on the emission of gaseous pollutants in a 10 kWth oxy-fuel-fluidized bed (combustion temperatures T1 = 800 and 850 °C and inlet O2 concentration = 30%) were carried out. The effects of the biomass blending mass ratio (Mb = 0, 10, 20, 30, 50, 70, and 100%), fuel property (fuel volatility and fuel O/N, Ca/S, and K2/S molar ratios), and excess oxygen ratio (α = 1.10, 1.25, and 1.40) on gaseous pollutants CO, CH4, NO, NOx (including NO and NO2), N2O, and SO2 were systematically investigated. The results show that both CO and CH4 increase with increasing Mb, and an increase in α leads to a significant decrease in CO and a slight change in CH4. The emissions of NO and NOx decrease with increasing Mb because of the dilution of fuel-N and the enhancement of reduction reactions. The generation rate of N2O is much higher than that of NOx, and it decreases with increasing Mb. The total conversion rate of fuel-N to nitrogen oxides is lower than 50% when cofiring coal and biomass, which is promoted by α but not by Mb. In addition, the ratio of the NOx generation rate to the N2O generation rate is larger at higher fuel O/N molar ratios, and the NOx emissions per unit calorific value decrease rapidly as the fuel volatility increases. Furthermore, the SO2 emissions per unit calorific value decrease with increasing Mb and Ca/S and K2/S molar ratios in fuel. An increase in α promotes the generation and emission of SO2.