燃煤烟气SCR脱硝体系中氧化态汞的还原过程及机理研究

Concerns about global mercury pollution due to excessive mercury emission from coal combustion have been surged in recent years. As the key technology for simultaneous control of elemental mercury (Hg0) and other flue gas pollutants, Hg0 oxidation over selective catalytic reduction (SCR) catalysts has been widely studied. However, reduction of oxidized mercury (Hg2+), which is the inverse process to Hg0 oxidation, has mostly been neglected thus far. We recently have demonstrated that reduction of Hg2+ could be predominant over Hg0 oxidation under some SCR conditions, i.e., lower Hg2+ to Hg0 ratio was observed downstream of SCR catalyst comparing to that upstream of the SCR catalyst. This phenomenon not only overturns the knowledge about enhanced Hg0 oxidation over SCR catalysts, but also related to whether Hg0 oxidation technologies such as oxidizing agent addition would facilitate Hg0 oxidation efficiently. To avoid the neglect of Hg2+ reduction and overcome the shortages of existing studies, this project will focus on Hg2+ reduction over SCR catalysts. Abundant experiments and theoretical calculations will be systematically conducted to study the existence, reaction pathways, micro-mechanisms and kinetics of Hg2+ reduction over SCR catalysts. The relationship between multiple redox processes on SCR catalysts will also be clarified. Such new knowledge will help to precisely predict Hg0 oxidation efficiencies through SCR catalysts, and to guarantee the effectiveness of different Hg0 oxidation technologies. The ultimate goal is to provide scientific basis and technical support for an efficient removal of mercury from coal combustion flue gas.

燃煤汞污染及其防治近年来得到了国际社会的高度关注。选择性催化还原（SCR）催化剂上单质汞的氧化作为燃煤烟气中汞与其它污染物协同控制技术的关键已得到广泛研究，但作为汞氧化过程的逆过程，氧化态汞的还原过程及机理却鲜有报道。申请者的前期研究发现氧化态汞的还原过程在某些SCR条件下可以占据主导作用，经过SCR催化剂后烟气中氧化态汞的比例不升反降。该现象的发现不仅颠覆了人们关于SCR催化剂促进汞氧化的认识，还事关添加氧化剂等汞氧化技术是否能起到汞氧化作用的重大技术问题。本项目基于SCR催化剂上氧化态汞还原这一重要过程，针对现有研究对该过程的忽视和认识的不足，首次系统研究氧化态汞还原过程的存在条件、实现途径、微观机理及动力学，从本质上阐明SCR催化剂上多种氧化还原过程间的相互作用规律，以期准确预测汞氧化效率，确保多种汞氧化技术的实施效果，为我国防治大气汞污染和履行国际汞公约提供科学依据与技术支撑。

随着《水俣公约》的正式生效，燃煤汞污染及其防治近年来得到了国际社会的高度关注。作为汞污染最为严重的国家之一，我国面临环境保护与履行国际汞公约的双重压力。选择性催化还原(SCR)催化剂表面单质汞(Hg0)的氧化作为燃煤烟气中汞与其他污染物协同控制技术的关键已得到广泛研究，但作为Hg0氧化过程的逆过程，氧化态汞(Hg2+)的还原过程及机理却鲜有报道。本项目围绕SCR催化剂上Hg2+还原这一重要过程，针对现有研究对该过程的忽视与认识不足，系统研究了Hg2+还原过程的存在条件、实现途径及微观机理，主要结论如下：(1) NO与NH3的共存气氛会显著抑制SCR催化剂的Hg0氧化效率，这归因于氧化态汞的还原，且NO: NH3 = 1时的抑制作用最为显著；(2) SCR脱硝体系中Hg2+的还原主要归因于SCR反应中的大量H质子转移，H质子会攻击HgO/HgCl2的边缘O/Cl原子，实现Hg2+的还原；(3) SCR气氛下HgO/HgCl2的还原过程是自发的，且NH3/NO可能对还原过程的发生途径产生影响；HgO/HgCl2还原过程的主要速控步骤为Hg-O/Cl键的断裂；(4) 阻碍H质子向HgO/HgCl2中的O/Cl原子的转移是阻止Hg2+还原的根本途径，降低烟气中NH3浓度或增加O2浓度、调控过程温度等是抑制Hg2+还原过程的有效手段。以上研究结果能为我国防治大气汞污染与履行国际汞公约提供科学依据与技术支撑。