Reduction of Sulfur Dioxide Emissions by Burning Coal Blends

Abstract

Given that sulfur contents of coals vary widely, this work investigated whether cofiring of highsulfur coals with lowsulfur coals of different ranks has any distinct advantages on lowering the sulfur dioxide emissions of the former coals, beyond those predicted based on their blending proportions. Such cofiring intends to take advantage of documented evidence in previous investigations at the author's laboratory, which demonstrated that lignite coals of lowsulfur, highcalcium, and highsodium content undergo massive bulk fragmentation during their devolatilization. This particular behavior generates a large number of smallsized char particles which, upon effective dispersion in the gas, can heterogeneously absorb the emitted sulfur dioxide gases, i.e., act as defacto sorbents, and then retain them in the ash. This study included two highand mediumsulfur bituminous coals, two lowsulfur lignite coals, and a subbituminous coal. Results showed that bituminous coals burning under substoichiometric (fuellean) conditions release most of their sulfur content in the form of SO2 gases, whereas lowranked coals only partly release their sulfur as SO2. Furthermore, the SO2 emission from coal blends is nonlinear with blend proportions, hence, beneficial synergisms that result in substantial overall reductions of SO2 can be attained. Finally, NOx emissions from coal blends did not show consistent beneficial synergisms under the implemented fuellean combustion conditions.