Reduction of Sulfur Dioxide Emissions by Burning Coal Blends

Given that sulfur contents of coals vary widely, this work investigated whether cofiring of high-sulfur coals with low-sulfur 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 low-sulfur, high-calcium, and high-sodium content undergo massive bulk fragmentation during their devolatilization. This particular behavior generates a large number of small-sized 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 high- and medium-sulfur bituminous coals, two low-sulfur lignite coals, and a sub-bituminous coal. Results showed that bituminous coals burning under substoichiometric (fuel-lean) conditions release most of their sulfur content in the form of SO₂ gases, whereas low-ranked 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, NO_x emissions from coal blends did not show consistent beneficial synergisms under the implemented fuel-lean combustion conditions.