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

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 NO
₂
, and NO
₂
is dissolved into the liquid as NO
₃
^-
. Continuous absorption of nitrogen oxides induces saturation and acidification of the liquid, therefore inhibiting further absorption. The effect of the addition of ammonium ions (NH
₄
⁺
) into the liquid film on the enhancement of NO
_x
removal has been experimentally confirmed. In that case, the addition of NH
₄
⁺
was effective for NO oxidation, as well as NO
_x
absorption into the liquid, resulting in the increase of NO
₂
^-
and NO
₃
^-
concentrations in the liquid. With the presence of Fe
²⁺
ions in the liquid exposed to the discharge plasma, a reduction of NO
₃
^-
to NH
₄
⁺
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.