NO_x Removal By Synergistic Effects Of Plasmas And Catalysts In The Presence Of Excess Oxygen

The NOx removal by the combination of dielectric barrier discharge (DBD) plasmas and catalysts in the presence of excess oxygen have been investigated. The main results presented in this dissertation were summarized as follows:1. NO, removal by pure dielectric barrier discharges in various systems was studied at atmosphere pressure. In NO/N2 system, NO decomposition to N2 and O2 is the major process. In NO/C2H4/N2 system, NO was also mainly reduced to N2, but with less NO2 formation. In NO/O2/N2 system, with increasing O2 concentration NOx conversion was decreased due to oxidation of part of NO to NO2. When O2 and C2H4 coexist in the system (NO/C2H4/O2/N2), the majority of NO was oxidized to NO2. The above results show that NO, can not be effectively removed by the plasma itself.2. In the presence of excess oxygen and higher reaction temperature(>300-350℃,NO/O2/N2), decomposition of NOx by the combination of plasmas and CuZSM-5 catalysts using one-stage plasma-over-catalyst (POC) reactor at the atmospheric presence was investigated. The experiments indicate that NOx removal efficiency is markedly decreased by the rising DBD input power. It was found that significant amount of NOx could be generated from N2 and O2 on the catalyst surface at temperatures higher than 300℃. NOx can be generated very likely via reactions between the plasma-produced N atom and catalyst-activated O2 molecules. This also partially explains why the plasma-catalytic removal of NOx from oxygen-rich-exhaust gases degrades at higher temperatures and indicates that the plasma-catalytic removal of air pollutants should be conducted in relatively low temperatures.3. In the NO/C2H4/O2/N2 system, the combination of DBDs and various catalysts, including metal ion-exchange zeolites and metal oxides supported on NaZSM-5, shows different synergistic effects for NO, removal in the one-stage POC reactor. The experiments demonstrate that the better synergistic effects between plasmas and catalysts can be obtained only for the catalysts with high activity at low temperatures. Among all the catalysts tested, Cu(165)ZSM-5 was chosen for the systematic study. Significant synergistic effects betweenDBD plasmas and CuZSM-5 catalysts for C2H4 selective reduction of NOx have been observed in a one-stage POC reactor operated at 250℃. With a reacting gas mixture of 530 ppm NO, 650 ppm C2H4, 5.8% O2 in N2 and GHSV = 12000 h-1, the pure catalytic, pure plasma-induced and plasma-catalytic (in the POC reactor) NOx conversion percentages are 39%, 1.5% and 79%, respectively. The NOx conversion percentage obtained in one stage POC reactor (79%) is markedly higher than that by a two-stage plasma-followed-by-catalyst (PFC) system (52%).4. The characteristics of DBDs and catalysts combined systems using the one-stage POC reactor were investigated by Lissajous figures, voltage-current waveforms and the temperature programmed desorption (TPD), in-situ optical emission spectroscopy (200-900 run). With increasing reaction temperature the discharge energy deposited into the reactor was increased. It was also found that energy consumption per converted NO molecule was increased with the increasing diameter of the plasma reactor. TPD of NO and O2 indicate that it is certain the discharge plasma alters some properties of a catalyst such as adsorption, desorption, and induces surface reaction on catalyst. The in-situ optical emission spectra showed that some short-lived active species formed from plasma-induced or plasma-over-catalyst-induced processes may be responsible to the observed synergistic effects between plasmas and catalysts in the one-stage POC system.