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.