Comparison of soot oxidation by NO2 only and plasma-treated gas containing NO2, O2, and hydrocarbons

NO₂ is an effective soot oxidizer operating at lower temperatures than O₂. The effect of pure NO₂ on soot oxidation was evaluated and compared with the gas treated by plasma, which initially consisted of NO, O₂, and hydrocarbons. The cutout of a commercial DPF was used and the pressure difference across the DPF was monitored for an hour. The concentration of NO/NO₂, CO, CO₂ at the outlet of the DPF was measured as a function of time. CO and CO₂ concentration was measured periodically by gas chromatography. The experiment was performed at 230, 250, 300, 350°C. When NO₂ only was used as an oxidizing agent, there was a close relationship between the decrease of the pressure difference across the DPF, the CO and CO₂ concentration at the outlet of the DPF, and the back conversion of NO₂ to NO. The CO and CO₂ concentration was not detectable under 250°C; the pressure differences and the change of NO and NO₂ concentration across the DPF showed the NO₂-carbon reaction occurred only slightly at 250°C. The oxidation rate increased as the temperature went up at the given conditions. NO₂ gas was reduced to NO while oxidizing soot and the soot was oxidized to CO and CO₂ simultaneously. In the case of the plasma-treated gas, the soot oxidation rate was much higher than that for pure NO₂ at the same temperatures. A decrease in the pressure difference across the DPF was observed under 250°C and more NO₂ was detected in the outlet of the DPF than for the NO₂ only case showing the higher decrease rate of the pressure difference across the DPF.