水蒸气和CO_2对等离子体—催化还原NO_x的影响

Influences of Water Vapor and CO_2 on NO_x Removal by Plasma-facilitated Selective Catalytic Reduction

等离子体协助选择性催化还原(PF-SCR)可以促进富氧环境下NOx的有效脱除。为研究实际尾气中含有的水蒸气和CO2对PF-SCR脱除NOx系统的影响,以C2H4作为还原剂,Ag/γ-Al2O3为催化剂,考察了3种(N2/O2/NOx/C2H4,N2/O2/NOx/C2H4/H2O(气)和N2/O2/NOx/C2H4/H2O(气)/CO2)配气条件下PF-SCR脱除NOx效果的变化。试验结果表明,较低水蒸气含量(φ(H2O)=1%)对PF-SCR还原NOx具有明显的抑制作用,其原因除了Ag/γ-Al2O3的抗湿性能较差外,水蒸气对等离子体反应器中C2H4和NO氧化的抑制以及由此导致的活性中间产物生成抑制也是一个重要因素。另一方面,水蒸气的存在也抑制了PF-SCR过程中副产物CO的大量生成,这可能是由于PF-SCR对C2H4的氧化减弱以及等离子体氧化生成的中间产物减少造成的。有水蒸气存在时,体系中的CO对PF-SCR还原NO影响不大。

Plasma facilitated Selective Catalytic Reduction （PF-SCR） was considered as a promising way to realize NOx control in order to meet increasingly strict standard. Earlier study has proved that plasma raised NOx removal rate significantly after combined with catalysts, and high N2 selectivity was obtained. But the influences of water vapor and CO2 , which existed largely in real flue gas and exhaust gas, were not considered and analyzed detailedly in the process of PF-SCR in view of intermediate products. In this paper, three mixture systems including N2/O2/ NOx/C2 H4 , N2/O2/NOx/C2 H4/H2O （ g ） and N2/O2/NOx/C2 H4/H2O （ g ）/CO2 were treated respectively by PF- SCR over Ag/γ-Al2O3 in the presence of 500 ppm NO, 6800 ppm C2H4 , 8% O2, 10%CO2 and about 1%H2O（g）. N2 was balance gas; Plasma was produced by dielectric barrier discharge in a line-cylinder structure reactor which was filled with glass beads. Results showed that NOx removal was suppressed remarkably by water vapor in PF-SCR system in contrast with no water vapor containing. It＇s attributed to the cooperative results of water suppression to plasma oxidation and suppression to catalysts. In addition to poor hydrothermal stability of Ag/γ-Al2O3, water vapor with lower concentration suppressed the intermediates formation like C2 H2 , aldehydes and N-containing organics in plasma oxidation stage, which further deteriorated NOx removal over catalyst. GC spectrums also testified the conclusion above. As the main byproduct, CO formation was suppressed because of decrease of C2 H4 oxidation and intermediates production. CO2 didn＇t seem to show obvious influence on NOx removal by PF-SCR system in the presence of vapor.