CO2, NOx and SO2 emissions from the combustion of coal with high oxygen concentration gases

The emissions of CO2, NOx and SO2 from the combustion of a high-volatile coal with N2- and CO2-based, high O2 concentration (20, 50, 80, 100%) inlet gases were investigated in an electrically heated up-flow-tube furnace at elevated gas temperatures (1123–1573 K). The fuel equivalence ratio, φ, was varied in the range of 0.4–1.6. Results showed that CO2 concentrations in flue gas were higher than 95% for the processes with O2 and CO2-based inlet gases. NOx emissions increased with φ under fuel-lean conditions, then declined dramatically after φ=0.8, and the peak values increased from about 1000 ppm for the air combustion process and 500 ppm for the O2(20%)+CO2(80%) inlet gas process to about 4500 ppm for the oxygen combustion process. When φ>1.4 the emissions decreased to the same level for different O2 concentration inlet gas processes. On the other hand, NOx emission indexes decreased monotonically with φ under both fuel-lean and fuel-rich combustion. SO2 emissions increased with φ under fuel-lean conditions, then declined slightly after φ>1.2. Temperature has a large effect on the NOx emission. Peak values of the NOx emission increased by 50–70% for the N2-based inlet gas processes and by 30–50% for the CO2-based inlet gas process from 1123 to 1573 K. However, there was only a small effect of temperature on the SO2 emission.