Low-temperature NO_x reduction processes using combined systems of pulsed corona discharge and catalysts

In this paper, we will report NO_x removal via reduction processes using two types of combined system of pulse corona discharge and catalysts: the single-stage plasma-driven catalyst (PDC) system, and the two-stage plasma-enhanced selective catalytic reduction (PE-SCR) system. Several catalysts, such as γ-alumina catalysts, mechanically mixed catalysts of γ-alumina with BaTiO₃ or TiO₂, and Co-ZSM-5 were tested. In the PDC system, which is directly activated by the discharge plasma, it was found that the use of additives was necessary to achieve NO_x removal by reduction. Removal rates of NO and NO_x were linearly increased as the molar ratio of additive to NO_x increased. The dependence of NO and NO_x removal on the gas hourly space velocity (GHSV) at a fixed specific input energy (SIE) indicates that plasma-induced surface reaction on the catalyst plays an important role in the PDC system. It was found that the optimal GHSV of the PDC system with the γ-alumina catalyst was smaller than 6000 h^-1. Mechanical mixing of γ-alumina with BaTiO₃ or TiO₂ did not enhance NO and NO_x removal and γ-alumina alone was found to be the most suitable catalyst. The dielectric constant of the catalyst only influenced the plasma intensity, not the NO_x removal. In the PE-SCR system, plasma-treated NO_x (mostly NO₂) was reduced effectively with NH₃ over the Co-ZSM-5 catalyst at a relatively low temperature of 150 °C. Under optimal conditions the energy cost and energy yield were 25 eV/molecule and 21 g-N (kWh)^-1, respectively.