Nitrogen dioxide absorption in aqueous sodium sulfite

The Clean Air Act of 1990 requires additional reduction of acid gases, sulfur dioxide, and nitrogen oxides released into the atmosphere from coal-fired electric power plants. In the case of older existing power plants, a possible retrofit strategy is to oxidize nitric oxide (NO, the major constituent of NO{dollar}sb{lcub}rm X{rcub}{dollar} in flue gas) to nitrogen dioxide (NO{dollar}sb2{dollar}) by the addition of methanol or other hydrocarbons into the duct at an optimum temperature regime. NO{dollar}sb2{dollar} can then be removed by either modifying existing SO{dollar}sb2{dollar} control equipment or by adding a limestone (CaCO{dollar}sb3{dollar}) slurry scrubbing process. Limestone reacts with SO{dollar}sb2{dollar} to from CaSO{dollar}sb3{dollar}, and the free sulfite (SO{dollar}{lcub}sb3sp{lcub}={rcub}{rcub}{dollar}) in the solution is reactive toward NO{dollar}sb2{dollar}. The focus of this research is to study the reaction between NO{dollar}sb2{dollar} and aqueous sulfite at elevated temperature and in the presence of gas phase O{dollar}sb2.{dollar}; The removal of NO{dollar}sb2{dollar} by limestone slurry scrubbing involves the reaction between NO{dollar}sb2{dollar} and SO{dollar}{lcub}sb3sp{lcub}={rcub}{rcub},{dollar} bisulfite (HSO{dollar}{lcub}sb3sp{lcub}-{rcub}{rcub}{dollar}) and water. The reactions between NO{dollar}sb2{dollar} and SO{dollar}{lcub}sb3sp{lcub}={rcub}{rcub}{dollar}/HSO{dollar}{lcub}sb3sp{lcub}-{rcub}{rcub}{dollar} are first order in both reactants, while the NO{dollar}sb2{dollar}-water reaction is second order in NO{dollar}sb2{dollar} concentration. The rate constants of the above reactions and the NO{dollar}sb2{dollar}-thiosulfate (S{dollar}sb2{dollar}O{dollar}{lcub}sb3sp{lcub}={rcub}{rcub}{dollar}) reaction were determined at 55{dollar}spcirc{dollar}C. SO{dollar}{lcub}sb3sp{lcub}={rcub}{rcub}{dollar} was found to be the most reactive toward NO{dollar}sb2,{dollar} while the contribution of chemical reaction still dominated in the absorption of NO{dollar}sb2{dollar} into water. The effect of gas phase SO{dollar}sb2{dollar} and O{dollar}sb2{dollar}, and liquid phase additives such as S{dollar}sb2{dollar}O{dollar}{lcub}sb3sp{lcub}={rcub}{rcub},{dollar} Ca{dollar}sp{lcub}++{rcub}{dollar}, Mg{dollar}sp{lcub}++{rcub}{dollar}, and Cl{dollar}sp{lcub}-{rcub}{dollar} on NO{dollar}sb2{dollar} absorption was also investigated.; The absorption of NO{dollar}sb2{dollar} catalyzes free radical reactions that lead to sulfite oxidation. A semi-empirical model was proposed to relate the rate of sulfite oxidation to the rate of NO{dollar}sb2{dollar} absorption. Thiosulfate inhibits sulfite oxidation by providing an alternative route for the termination of the free radical reactions, and a fundamental model was derived to quantify the effect of S{dollar}sb2{dollar}O{dollar}{lcub}sb3sp{lcub}={rcub}{rcub}{dollar} on sulfite oxidation.; The absorption of NO{dollar}sb2{dollar} into aqueous bisulfide (HS{dollar}sp{lcub}-{rcub}{dollar}) was studied in an attempt to discover alternative scrubbing technologies. The reaction between NO{dollar}sb2{dollar} and HS{dollar}sp{lcub}-{rcub}{dollar} is twice as fast as the NO{dollar}sb2{dollar}-SO{dollar}{lcub}sb3sp{lcub}={rcub}{rcub}{dollar} reaction at 55{dollar}spcirc{dollar}C. A semi-empirical model was proposed to relate NO{dollar}sb2{dollar} absorption to HS{dollar}sp{lcub}-{rcub}{dollar} oxidation.; This study has shown that acceptable level of NO{dollar}sb2{dollar} removal by a conventional limestone slurry scrubber is not probable. However, aqueous scrubbing of NO{dollar}sb2{dollar} by Na{dollar}sb2{dollar}SO{dollar}sb3{dollar} and Na{dollar}sb2{dollar}S solutions are viable options. Furthermore, significant reduction in hold tank liquid depth and/or oxidizing air stoichiometry is possible by NO{dollar}sb2{dollar} injection.