钒基脱硝催化剂氧化率控制研究进展

SO3是燃煤机组烟气关键污染物之一,影响机组稳定运行、大气环境质量和人类健康。笔者总结了控制商用SCR脱硝催化剂SO2氧化率的研究进展,包括孔结构调整、隔离层、降低壁厚、形态调整等物理方案,以及载体、助剂、活性组分及表面酸性优化等化学方案。由于受动力学因素影响(扩散控制),脱硝反应仅发生在催化剂表面75～100μm厚度以内(脱硝区域),而SO2氧化反应可在整个壁厚内发生(区域),因此减少参与SO2氧化反应的活性组分数量是物理方案的主要内容。在钒基脱硝催化剂中,活性组分V2O5既可以选择性催化还原NOx,也可将SO2催化氧化为SO3,调控活性组分与载体的相互作用,使NH3、SO2在催化剂表面形成竞争吸附,抑制催化剂对SO2的吸附,降低活性物种氧化还原能力,是化学方案的有效方法。物理方案较适用于板式脱硝催化剂,而化学方案对于催化剂结构无特殊要求,且化学方案较物理方案具有更好的效果。各技术方案主要基于脱硝反应属于气固非均相催化反应体系特点,从氨气、SO2等反应物扩散、吸附、氧化还原反应等多种角度出发对催化剂配方、物理结构进行优化调整,从而实现脱硝主反应活性K(NOx)与SO2氧化副反应活性K(SOx)的平衡。在商用低SO2氧化率脱硝催化剂开发中,可综合考虑扩散、吸附、氧化还原能力等多种因素对催化剂性能的影响,同时采取多种方案对催化剂进行改性,在保证催化剂脱硝性能的前提下,显著降低催化剂的SO2氧化率。在燃煤机组超低排放要求下,脱硝催化剂使用量或催化剂中活性组分含量明显增加,脱硝系统对SO2氧化率由1%升高到1.5%左右。同时考虑到超低排放要求下后端处理设备对SO3脱除的压力,新型脱硝催化剂SO2氧化率至少应降低1/3以上。 Sulfur trioxide is one of the key pollutants in the flue gas of coal-fired units,which has an important influence on the stable operation,air quality as well as human health. In this paper,the research results about control of SO2 oxidation over commercial SCR denitrification catalyst were summarized,including physical methods such as pore structure adjustment,isolation layer,wall thickness reduction and morphology adjustment,as well as chemical methods such as carrier,auxiliary agent,active component and surface acidity optimization. Due to the influence of kinetic factors (diffusion control),the denitrification reaction only occurs within the thickness of catalyst surface (denitrification area) of 75-100 μm,while the SO2 oxidation reaction can occur within the whole wall thickness (area),so reducing the number of active components involved in the SO2 oxidation reaction is the main technical content of the physical scheme. In vanadium denitration catalyst,the active component of V2O5 can selectively catalyze the reduction of NOx, can also be the catalytic oxidation of SO2 to SO3,and regulate the interaction of active component and carrier,and make the NH3 and SO2 in the catalyst surface to form competitive adsorption,inhibit the catalyst for SO2 adsorption,and reduce the redox capacity of the active species,which is an effective method of chemical solutions. The physical scheme is more suitable for plate denitrification catalyst,while the chemical scheme has no special requirements for catalyst structure,and the chemical scheme has better technical effect than the physical scheme. Based on the fact that denitrification belongs to a gas-solid heterogeneous catalytic reaction system,the catalyst formulation and physical structure were optimized and adjusted from the perspectives of the diffusion,adsorption,redox reaction of ammonia,SO2 and other