Effects of hydrocarbons and water vapor on NO_x using V ₂O₅-WO₃/TiO₂ catalyst reduction in combination with nonthermal plasma

The effects of hydrocarbons and water vapor on a combined deNO_x process, which consists of nonthermal plasma oxidation and V₂O ₅-WO₃/TiO₂ selective catalytic reduction (SCR), are investigated. Test results show that the fast SCR reaction, which needs equimolar amounts of NO and NO₂, plays a dominant role in reducing NO_x under relatively low-temperature conditions, i.e., 150-200 °C. Under such low-temperature conditions, the oxidation of NO to NO ₂ induced by nonthermal plasma is useful in controlling NO ₂ fractions in NO_x for the fast SCR reaction. The role of the fast SCR reaction, however, decreases when C₃H₆ is supplied to the process as simulated hydrocarbons in diesel exhausts. Test results yielded the following conclusions: (1) C₃H₆ leads to the production of aldehydes in the nonthermal plasma reactor. (2) The NO ₂ fraction in the SCR reactor decreases as a result of aldehyde production, leading to a diminished role of the SCR reaction. This reduction in NO₂ is predominant when water vapor is not added to the test gases. (3) The decreased role of the fast SCR reaction is recovered when water vapor is present in the treated gases, and the optimal NO₂ fraction for peak deNO_x conversion shifts to higher fractions (greater than 0.5), which should be considered in treating diesel exhaust.