| Ammonia-based flue gas desulfurization (AFGD) has been increasingly used in recent years because of high desulfurization efficiency, lower cost, recyclable ammonium sulfite, etc. Ammonium sulfite byproduct in AFGD is easily decomposed into SO2and then results in secondary environmental pollution, while stable ammonium sulfate can be used as fertilizer in agriculture. Over the last few decades, forced oxidation technology has been utilized for oxidizing ammonium sulfite generated in FGD process. However, this technology exists some drawbacks, such as high energy consumption and low oxidation efficiency. Non-thermal plasma technology as one of the advanced oxidation processes (AOPs) has been widely used in the contaminations treatment, because active species (·O,·OH,·HO2,O3, etc) produced by plasma have the strong oxidizing. In this paper, oxidation of ammonium sulfite produced in flue gas desulfurization process was investigated in a needle-to-plate negative DC corona discharge reactor, and the oxidation efficiency, oxidation mechanism and influencing factors of ammonium sulfite oxidation was examined and analyzed. The main results are as follows:(1) A small advection needle-to-plate negative DC corona discharge reactor for oxidation of ammonium sulfite was designed. The oxidation efficiencies of ammonium sulfite under different experimental conditions were investigated and the oxidizing mechanism was also deduced. The results reveal that the oxidation efficiency of ammonium sulfite increases with the absolute value of discharge voltage; while the oxidation efficiency increases first and then decreases with the flow rate of the ammonium sulfite solution and the initial pH; the initial concentration of ammonium sulfite increases, the oxidation efficiency decreases. The hydroxyl radicals play a key role in the plasma-induced oxidation of ammonium sulfite.(2) A large-scale advection needle-to-plate negative DC corona discharge reactor for oxidation of ammonium sulfite was designed. Firstly, experimental parameters were optimized with a single needle-plate reactor, and the parameters are as follows:the initial concentration of ammonium sulfite is0.5mol/L. the discharge voltage is-13.6kV, the initial pH is7.0and gas supply type is parallel to the needle cathode. The oxidation rate of ammonium sulfite reached the highest value of93.8%under the optimal parameters. Application of the optimal solution and gas parameters in a five parallel needle-to-the plate reactors, the oxidation efficiency and energy consumption of19.5L ammonium sulfite solution with an initial concentration of0.5mol/L reached42.0%and53.7Wh/mol under applied voltage of-11.4kV; in addition, when the initial concentration of ammonium sulfite increased to1.0mol/L. the oxidation efficiency and energy consumption decreased to17.0%and72.9Wh/mol. respectively.(3) A vertical needle-to-plate negative DC corona discharge reactor for oxidation of ammonium sulfite was designed. Factors influencing the oxidation efficiencies of ammonium sulfite were investigated. The results are as follows:The effects of solution flow rate and concentration of ammonium sulfite on the discharge characteristics of needle electrodes could be ignored. When the concentration of ammonium sulfite solution was equal to and higher than0.3mol/L, the spontaneous oxidation of ammonium sulfite by air could be ignored too. When the initial concentration of ammonium sulfite increased, the oxidation efficiency of ammonium sulfite decreased, while the oxidation rate increased first and then decreased. Before and after the discharge, the total molar concentration of sulfur changed little, which means the sulfur balance. The oxidation efficiency and energy consumption of0.4mol/L ammonium sulfite reached80.3%and72.9Wh/mol within60min. |
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