Emissions of NOx and SO2 from Coals of Various Ranks, Bagasse, and Coal-Bagasse Blends Burning in O2/N2 and O2/CO2 Environments

Oxy-coal combustion is a viable technology, for new and existing coal-fired power plants, as it facilitates carbon capture and, thereby, can mitigate climate change. Pulverized coals of various ranks, biomass, and their blends were burned to assess the evolution of combustion effluent gases, such as NOx, SO2, and CO, under a variety of background gas compositions. The fuels were burned in an electrically heated laboratory drop-tube furnace in O-2/N-2 and O-2/CO2 environments with oxygen mole fractions of 20%, 40%, 60%, 80%, and 100%, at a furnace temperature of 1400 K. The fuel mass flow rate was kept constant in most cases, and combustion was fuel-lean. Results showed that in the case of four coals studied, NOx emissions in O-2/CO2 environments were lower than those in O-2/N-2 environments by amounts that ranged from 19 to 43% at the same oxygen concentration. In the case of bagasse and coal/bagasse blends, the corresponding NOx reductions ranged from 22 to 39%. NOx emissions were found to increase with increasing oxygen mole fraction until similar to 50% O-2 was reached; thereafter, they monotonically decreased with increasing oxygen concentration. NOx emissions from the various fuels burned did not clearly reflect their nitrogen content (0.2-1.4%), except when large content differences were present. SO2 emissions from all fuels remained largely unaffected by the replacement of the N-2 diluent gas with CO2, whereas they typically increased with increasing sulfur content of the fuels (0.07-1.4%) and decreased with increasing calcium content of the fuels (0.28-2.7%). Under the conditions of this work, 20-50% of the fuel-nitrogen was converted to NOx. The amount of fuel-sulfur converted to SO2 varied widely, depending on the fuel and, in the case of the bituminous coal, also depending on the O-2 mole fraction. Blending the sub-bituminous coal with bagasse reduced its SO2 yields, whereas blending the bituminous coal with bagasse reduced both its SO2 and NOx yields. CO emissions were generally very low in all cases. The emission trends were interpreted on the basis of separate combustion observations.