Comparison between in-/injected-plasma catalysis for enhancing hydrocarbon selective catalytic reduction of NOx at low temperatures

Plasma-assisted hydrocarbon selective catalytic reduction (HC-SCR) of NOx was investigated by a packed Ag/γ-Al2O3 catalyst in a dielectric barrier discharge reactor at low temperatures. The effect of plasma discharge on NOx removal was investigated with two methods for comparison, in- and injected-plasma catalysis systems. In-plasma catalysis comprised a plasma-catalytic discharge with the entire feed gas, and injected-plasma catalysis incorporated a part of feed gas consisting of dodecane pretreated with the plasma-catalytic discharge and mixed with the remaining gas before passing through the catalyst-alone stage. Here, using the surface response method with temperature and specific energy input (SEI) as independent variables, the results revealed that energy delivery through plasma gained more NOx removal efficiency than that by thermal catalyst process; a set of SEI and temperature is needed to obtain a similar level of NOx removal. There is a significant synergy between plasma and catalyst in in-plasma catalysis at low temperatures (≤250 °C) that increased NOx removal efficiency up to Δη = 50% (SEI = 137 J/L; T = 200 °C). In-plasma catalysis is superior to injected-plasma catalysis for enhancing the NOx removal and widening the temperature window of effective NOx removal owing to the plasma-catalytic interface in-plasma catalysis supplies. Interestingly, the results also revealed that the plasma in the catalyst for both methods did not always improve the NOx removal efficiency. The plasma had a negative effect on NOx removal at sufficient temperature and SEI. As a result, the in-plasma catalysis presented a high NOx removal efficiency; it can be higher than 70% at temperatures of 200–250 °C. However, injected-plasma catalysis has potential applications in circumstantial high-throughput gas due to low plasma energy consumption by adjusting the entire feed to plasma-treated gas ratio.