Removal of trace CH4 emissions by warm O2 plasma is kinetically limited: Insights from modeling and experiments

We investigate the potential of a warm O<inf>2</inf> plasma effluent for the removal of trace CH<inf>4</inf> concentrations in ambient air, using a combination of modeling and experiments. To parameterize the 0D model, rotational Raman measurements were performed to obtain both temperature and O atom concentration profiles downstream of the O<inf>2</inf> plasma. Subsequently, the model is validated by obtaining satisfactory agreement for CH<inf>4</inf> conversion and CO<inf>2</inf>/CO/NO<inf>x</inf> production with the experiments of [1]. Next, we explore the theoretical limits of the O<inf>2</inf> plasma effluent strategy by scanning the model over a broad range of effluent temperatures, mixing rates and ratios of plasma to barn air flow rate. Our model predicts the lowest energy cost for CH<inf>4</inf> conversion (at 100 ppm) to be 153 MJ/mol, obtained at a characteristic mixing time of 0.1 ms and flow rate ratio of 30 (plasma flow to barn air flow), which is still higher than that of catalytic thermal oxidation (ca. 120 MJ/mol). Based on these insights, we propose a new strategy that uses NO<inf>x</inf> produced by warm air plasma to oxidize CH<inf>4</inf> at a catalyst surface, potentially reducing the required operating temperature and broadening the range of viable catalytic materials with higher resistance to poisoning than conventional Pd-based catalysts.