Study of Oxy-fuel Coal Combustion in a 0.1 MWth Circulating Fluidized Bed at High Oxygen Concentrations

The combustion and emission characteristics under three kinds of firing modes [O-2/N-2, O-2/CO2, and O-2/recycled flue gas (RFG)] at high oxygen concentrations were investigated in a 0.1 MWth circulating fluidized-bed combustion system. The 0.1 MWth circulating fluidized-bed combustion unit was built in 2010 and retrofitted to use RFG in 2012, which was specially designed for combustion at high oxygen concentrations (up to 55%). The experimental results showed that CO2 concentrations in the flue gas reached a maximum of 93% when the overall oxygen concentration was in the range of 46.4-54.6% during the combustion. Under the O-2/RFG firing mode, the CO concentration dropped from 803.4 to below 100 mg/MJ with the increase of the concentration of oxygen in the flue gas from 1.1 to 7.5%. The fuel N to NO conversion was in the range of 3.5-4.6% under the O-2/RFG firing mode, which was much less than that observed under the O-2/N-2 mode (8.1-8.4%) and O-2/CO2 mode (6.5-7.6%). The limestone utilization in the O-2/RFG firing mode was found to be much lower than that for both O-2/N-2 and O-2/CO2 modes. Sulfur capture efficiencies were 70.2 and 93.7% under O-2/RFG and O-2/N-2 firing mode conditions, respectively, with a Ca/S molar ratio of 2.2. An increase in the Ca/S molar ratio to 3 resulted in sulfur capture efficiencies of 83.9% under the O-2/RFG firing mode.