Selectivity of transition metal catalysts in promoting the oxidation of solid sulfites in flue gas desulfurization

Abstract

Wet flue gas desulfurization (WFGD) has been widely used to reduce the harmful effects of sulfur dioxide during coal combustion, of which sulfite is a byproduct. Recycling this desulfurization byproduct requires the oxidation of sulfite, and transition metals might considerably promote the sulfite oxidation rate. We compared the oxidation rates of four sulfites—sodium sulfite, ammonium sulfite, magnesium sulfite, and calcium sulfite—and found that transition metals have selective catalytic performance. Although each transition metal was confined to specific sulfites, Co2+ and Mn2+ were the most effective catalysts. In addition, the sensitivity levels of the sulfite oxidation rates to various transition metals were classified. The orders of the reactions of the four sulfites catalyzed by Co2+ and Mn2+ were calculated. Although both magnesium sulfite and calcium sulfite exist as solids, the oxidation rate of magnesium sulfite was considerably sensitive to Co2+. The same results were observed for sodium sulfite and ammonium sulfite. However, the oxidation rate of calcium sulfite was sensitive only to Mn2+. The properties of the two prepared solid samples, magnesium sulfite and calcium sulfite, were compared using X-ray diffraction, scanning electron microscopy, and particle size analysis. The position-occupying hypothesis was applied to determine the selective catalytic behaviors of transition metals for the oxidation of various sulfites by comparing the ionic potentials of the metals. The results provide an effective theoretical basis for catalyzing the sulfite oxidation in WFGD.