Plasma Catalytic Removal Of Nitrogen Oxides And Benzene At Low Temperature

Nitrogen oxides (NO_x) and benzene are the typical gaseous pollutants difficult to remove for conventional technologies. In this paper, plasma catalytic technology was employed to study on selective reduction of NO_x with hydrocarbons at low temperature and complete-oxidation removal of dilute benzene from air. The results were summarized as follows.1. Selective reduction of NO_x with hydrocarbons at low temperature using plasma catalytic technology(1) The selective reduction of NO_x by C₂H₂ was investigated using a "one - stage" plasma catalytic reactor composed of dielectric barrier discharge (DBD) plasmas and H-MOR catalysts. Significant synergistic effects of DBD plasma and H-MOR catalysts for C₂H₂ selective reduction of NO_x at low temperatures (100 - 200℃) and across a wide range of O₂ content (0 - 15%) were achieved. With a reactant gas mixture of 500 ppm NO, 500 ppm C₂H₂, 10% O₂ in N₂, GHSV = 12000 h^-1 and input energy density of 125 J L^-1, at 100℃, NO_x conversions were 3.3, 11.6 and 66.7% for the plasma alone, catalyst alone and plasma catalytic cases, respectively; at 200℃, NO_x conversions were 3.8, 54.0 and 91.4% for the above three cases, respectively. Based upon the results of on-line MS examination, for HC-SCR of NO_x at low temperature, strong MS signals of HCN byproduct were observed in the catalysts alone system. By contrast, almost no HCN was detected in plasma catalytic system. The reason for the striking difference is that, far more NO₂ and H₂O could be produced in the plasma catalytic system comparing those in the catalyst alone system, so HCN rapidly consumed by reacting with H₂O and NO₂.(2) The MS experimental results show that plasma could speed up the formation of NO_y, (y≥2) species and oxidation of C₂H₂ on H-MOR catalysts. Also, the reaction ability of the surface species formed on the pretreatment of H-MOR catalysts in a NO/C₂H₂/O₂ gas mixture with NO, O₂, NO + O₂ was investigated. It was suggested that the surface species possess a selective reductive property for NO in co-existence of NO and O₂.2. Complete-oxidation removal of dilute benzene from air using plasma catalytic technology (1) A cycled "Storage-Discharge" (CSD) plasma catalytic technology was used for diluted benzene removal from air. Various catalysts were investigated in simulated air containing low-concentration benzene with 50% RH in terms of their performances in benzene storage and plasma catalytic oxidation of adsorbed benzene. Ag/HZSM-5 catalysts exhibit not only preferential adsorption of benzene in humid air in a storage stage, but also almost complete oxidation of adsorbed benzene in a discharge stage. The excellent water resistance of Ag/HZSM-5 catalysts in humid air was due to the hydrophobicity of high silica HZSM-5 carrier. Complete oxidation of adsorbed benzene to CO₂ and H₂O (Sco₂～100%) was enhanced by silver. Based upon the results of catalyst characterization, it may be concluded that, with increasing silver loading, the adsorption of benzene on the catalysts become stronger and stronger, which results in poor carbon balance for high Ag-loading case.(2) At the same conditions, the breakthrough time of benzene on Ag/HZSM-5 catalysts was three times as long as HZSM-5 catalysts. Long breakthrough time of Ag/HZSM-5 catalysts towards benzene may lead to lowering energy cost of this plasma catalytic process. Using the CSD plasma catalytic technology for remedying simulated air containing 4.7 ppm benzene with 50% RH and 600mL min^-1 flow rate, the energy cost was as low as 3.7×10^-3 kWh m^-3-air (Ag/HZSM-5 catalysts, P = 4.7 W). This extremely low energy cost to remove low-concentration pollutants from air undoubtedly makes the environmental applications of plasma catalytic technique practical.