As the regulations on an exhaust gas emitted from the exhaust system of an engine such as an internal combustion engine, a boiler, or a gas turbine are strengthened, various improvements with regard to the engine, such as modifying the fuel composition, recirculating the exhaust gas back into the engine, and improving the combustion process itself, have been made. On the other hand, there has been an attempt to remove harmful components such as nitrogen oxides (NOX) in the exhaust gas emitted from the engine.
Patent Document 1 discloses a method including the following processes. First, NOX in an exhaust gas is oxidized to NO2 by applying low-temperature non-equilibrium plasma to the exhaust gas. Then, the exhaust gas is introduced into a reducing agent solution to remove the NO2 from the exhaust gas.
Patent Document 2 discloses a method including the following processes. First, an exhaust gas is introduced into a first plasma reactor in which an adsorbent adsorbs target components to be treated, including NOX, in the exhaust gas. Then, a nitrogen gas with a low oxygen concentration is introduced into the first plasma reactor to generate electric discharge, and non-thermal plasma of the nitrogen gas is applied to the adsorbent. Thus, the target components are desorbed from the adsorbent, so that the adsorbent is recycled. Subsequently, the nitrogen gas containing the desorbed target components is introduced into a second plasma reactor in which non-thermal plasma is applied, thereby reducing NOX to N2.    Patent Document 1: JP 2000-117049 A    Patent Document 2: WO 2005/037412 A1