Alkali influences on sulfur capture for North Dakota lignite combustion

Pilot-scale testing was performed at the University of North Dakota Energy and Environmental Research Center (EERC) to determine the levels and mechanisms of sulfur capture on fly ash. Core samples of lignite having low-, medium-, and high-sulfur levels were sampled and analyzed from a potential lignite mine area in western North Dakota. Computer-controlled scanning electron microscopy (CCSEM) analysis revealed that sulfur levels were strongly associated with pyrite content; however, organic sulfur was also significant. Combustion tests of the three lignites revealed SO2 in the flue gas at levels ranging between 700 and 2300 ppm. Sulfur levels were 2–5% in the fly ash. Higher-sulfur retention or capture was evident in the medium-and low-sulfur lignite, which also had greater quantities of alkali inorganics that were organically bound in the lignite. Sulfur capture was mostly influenced by the presence of sodium and, secondarily, by calcium as oxides or hydroxides in the flue gas or on surfaces of porous char particles. Gas–particle interactions that led to sulfur capture most likely involved the heterogeneous condensation of sulfur on fine alkali particles or on fine alkali-rich fly ash particles in cooler condensing zones of the combustion system. No direct correlation is evident between sulfur quantity in the coal and capture levels. Regression analysis indicated that an earlier model developed for alkali-rich coal gave an adequate representation of the sulfur capture and emissions expected from low-rank coals; however, an improved model was devised.