$\hbox {NO}_{3}^{-}$ Reduction for $\hbox {NO}_{\rm x}$ Removal Using Wet-Type Plasma Reacto

The fundamental characteristics of flue gas cleaning for NO_x using a wet-type plasma reactor are evaluated, with attention being laid on concentrations of nitrate and ammonium ions in the liquid. The wet-type plasma reactor of the wire-cylinder configuration driven by a square-wave high voltage was used. A thin liquid film was maintained on the inner wall of the reactor. In this reactor, a discharge plasma oxidizes NO to NO₂, and NO₂ is dissolved into the liquid as NO₃^-. Continuous absorption of nitrogen oxides induces saturation and acidification of the liquid, therefore inhibiting further absorption. The effect of the addition of ammonium ions (NH₄⁺) into the liquid film on the enhancement of NO_x removal has been experimentally confirmed. In that case, the addition of NH₄⁺ was effective for NO oxidation, as well as NO_x absorption into the liquid, resulting in the increase of NO₂^- and NO₃^- concentrations in the liquid. With the presence of Fe²⁺ ions in the liquid exposed to the discharge plasma, a reduction of NO₃^- to NH₄⁺ takes place. The reduction of nitrate ions to ammonia raises the solution pH and ensures continuous NO_x absorption. These results suggest the possibility of the continuous operation of the wet-type plasma reactor for NO_x removal without excess acidification of the absorbing water. The process that is responsible for the reduction of nitrate ions into ammonium ions is of an electrochemical nature.