Emissions of SO2, NOx, CO2, and HCl from Co-firing of coals with raw and torrefied biomass fuels

This work examined acid gas emissions of sulfur dioxide, nitrogen oxide, carbon dioxide, and hydrogen chloride from co-firing biomass (corn straw and rice husk) with either a high-sulfur bituminous coal or a low-sulfur sub-bituminous coal. Pulverized neat coals, neat biomass, either raw or torrefied, and 50-50 wt% blends thereof were introduced to a laboratory-scale electrically-heated drop-tube furnace (DTF), operated at a gas temperature of 1350 K, and experienced high heating rates. Emissions from the combustion of the fuels in air were measured at the furnace effluent. Coal particles were in the range of 75-90 µm and biomass particles in the range of 90-150 µm. Results revealed that blending of both coals with raw and torrefied biomass drastically reduced the coal's SO2 and NOx emission yields to values that were below those predicted by linear interpolation of the corresponding emission yields of the two neat fuels. The SO2 emission yields from torrefied biomass were lower than those of their raw biomass precursors due to their lower sulfur contents. Similarly to the emission yields, the SO2 emission factors (based on the energy content of each sample) from the blends of coal with torrefied biomass were also lower than the blends of coal with raw biomass. NOx emission yields from neat torrefied biomass were mildly higher than those from raw biomass, as the latter had higher nitrogen content per unit mass. There was no discernible trend in NOx emissions from the blends based on their nitrogen contents. HCl emission from torrefied corn straw was lower than that from its raw precursor, as the former had a lower chlorine content. The HCl emission yields from the blends of corn straw with coal were much higher than those from neat coal combustion. Finally, the HCl emission yield from blends of the high-sulfur coal with corn straw were higher than those from the blends of the same biomass with the low-sulfur coal.