Nitrogen Dioxide Absorption and Sulfite Oxidation in Aqueous Sulfite

Removal of NO from flue gas may be achieved by its oxidation to NO 2 followed by absorption in sulfite solution in existing scrubbers for desulfurization. Rates of NO 2 absorption and sulfite oxidation were measured in a highly characterized stirred-cell contactor. The results were interpreted by the theory of mass transfer with fast reaction in the boundary layer. The reactions between NO 2 and sulfite (SO 3 2 - ), bisulfite (HSO 3 - ), and thiosulfate (S 2 O 3 2 - ) were first order in NO 2 concentration, and their respective rate constants at 55 °C were 11.2 × 10 5, 2.8 × 10 4, and 5.4 × 10 3 M - 1 s - 1 . The NO 2 hydrolysis reaction was second order in NO 2 concentration, with a rate constant of 1.6 × 10 7 M - 1 s - 1 at 55 °C. NO 2 absorption initiates sulfite oxidation in the presence of oxygen, and this study quantified the effect of sulfite and oxygen concentration, the rate of NO 2 absorption, and the presence of thiosulfate and gas-phase SO 2 on the rate of sulfite oxidation. With typical conditions for limestone slurry, 10 mM total dissolved S(IV) and pH 4−5, the estimated NO 2 removal was less than 50%. Therefore, an acceptable level of NO 2 removal by a conventional limestone slurry scrubber is not probable. However, NO 2 absorption is feasible in sodium sulfite inhibited by thiosulfate. NO 2 may also be an effective catalyst for sulfite oxidation in limestone slurry scrubbing.