Removal of Nitrogen Oxides Using Hydrocarbon Selective Catalytic Reduction Coupled with Plasma

Low-temperature conversion of nitrogen oxides using plasma-assisted hydrocarbon selective catalytic reduction of (HC-SCR) was investigated. Plasma was created in the catalyst-packed bed so that it could directly interact with the catalyst. The effect of the reaction temperature, the shape of catalyst, the concentration of n-heptane as a reducing agent, the oxygen content, the water vapor content and the energy density on NOx removal was examined. NOx conversion efficiencies achieved with the plasma-cata- lytic hybrid process at a temperature of 250 ℃ and an specific energy input (SIE) of 42 J L -1 were 83% and 69% for one-dimen- sional Ag catalyst (Ag (nanowire)/γ-Al2O3) and spherical Ag catalyst (Ag (sphere)/γ-Al2O3), respectively, whereas that ob- tained with the catalyst-alone was considerably lower (about 30%) even with Ag (nanowire)/γ-Al2O3 under the same condition. The enhanced catalytic activity towards NOx conversion in the presence of plasma can be explained by the formation of more reactive NO2 species and partially oxidized hydrocarbon intermediates from the oxidation of NO and n-heptane under plasma discharge. Increasing the SIE tended to improve NOx conversion efficiency, and so did the increase in the n-heptane concen- tration; however, a further increase in the n-heptane concentration beyond C1/NOx ratio of 5 did not improve the NOx conversion efficiency any more. The increase in the humidity affected negatively the NOx conversion efficiency, resulting in lowering the NOx conversion efficiency at the higher water vapor content, because water molecules competed with NOx species for the same active site. The NOx conversion efficiency increased with increasing the oxygen content from 3 to 15%, in particular at low SIE values, because the formation of NO2 and partially oxidized hydrocarbon intermediates was facilitated.