铈基SCR催化剂对燃煤烟气中单质汞的催化氧化机制研究

Coal combustion is regarded as the largest anthropogenic source of mercury emission to the atmosphere. In recent years, concerns about mercury emission and its control have risen greatly because of the extreme toxicity, persistence, and bioaccumulation of methyl mercury transformed from emitted mercury. In 2011, both U.S. EPA and Chinese EPA released new mercury emission standards for power plants. It is urgent to control mercury emission from coal-fired power plants.A highly competitive method for controlling mercury emission from coal-fired power plants is removing it simultaneously with other coal combustion pollutants by using existing air pollution control devices such as wet flue gas desulfurization system (WFGD) and selective catalytic reduction (SCR) system. However, low mercury oxidation efficiency over commercial vanadia based SCR catalysts in low-rank coal combustion flue gas limited the industrial application of this technology. The applicant has developed a low temperature cerium based catalyst on which mercury was efficiently oxidized without HCl, indicating that mercury in low-rank coal combustion flue gas could be oxidized efficiently over this type of catalysts. Abundant experiments and theoretical calculations will be conducted to study the simultaneous NOx removal and mercury oxidation on the cerium based catalyst. High mercury oxidation efficiency will be achieved on the premise that NOx is efficiently removed. The relationship between selective catalytic reduction of NOx and mercury oxidation will be clarified. The effects of catalyst characteristics, experimental conditions and flue gas atmosphere on mercury oxidation will be investigated. Moreover, the mechanisms involved in catalyst deactivation or anti-deactivation will be elucidated. Based on the experimental results, a heterogeneous oxidation model will be established to confirm the mercury oxidation mechanism quantitatively. These knowledge is of fundamental importance in developing effective pollution control technologies and devices in which simultaneous removal of multi-pollutants from coal combustion are possible.

燃煤是最大的人为汞排放源，燃煤汞污染及其控制近年来引起了国际社会的高度关注。2011年美国EPA及我国环保部均颁布了针对燃煤电厂的汞排放标准，控制燃煤汞的排放已迫在眉睫。利用现有污染物控制设备在控制其它污染物的同时实现汞的脱除是一种极具竞争力汞排放控制方法。传统钒基SCR催化剂对低阶煤烟气中汞的氧化效率低下是该技术的瓶颈。申请者曾采用低温铈基SCR催化剂实现了无HCl条件下汞的高效氧化，有望克服商业钒基SCR催化剂的局限性。本项目基于该类催化剂，结合大量的实验和模拟计算，研究其对NOx与汞的协同控制，在保证NOx高效脱除的基础上协同实现燃煤烟气中汞的高效氧化，明确SCR脱硝与汞氧化过程间的相互关系；考察催化剂特征、工况条件、烟气气氛等因素对汞氧化的影响，建立汞氧化多相反应动力学模型，定量揭示汞的催化氧化机理，阐明催化剂中毒/抗中毒机制；以期为实现燃煤污染物的一体化脱除提供可靠的科学依据。

燃煤汞污染已成为国际社会的高度关注环境保护问题。利用选择性催化还原（SCR）设备，湿法脱硫（WFGD）装置等现有污染物控制设备在控制其它污染物的同时实现汞的脱除是一种极具竞争力汞排放控制方法。该技术的关键是如何在SCR催化剂上实现单质汞（Hg0）的高效氧化。低温铈基催化剂作为一种极具应用潜力的SCR催化剂在我们的前期研究中已被证实具有良好的Hg0氧化性能。本项目基于该类催化剂，结合大量的实验和模拟计算，主要开展了以下四方面研究：（1）铈基SCR催化剂上汞的多相催化转化机理；（2）铈基催化剂中毒/失效或抗中毒机制；（3）SCR脱硝过程与汞氧化过程间的相互作用规律；（4）铈基SCR催化剂对NO与Hg0的协同控制。主要结论如下：(1) CeO2-TiO2 (CeTi)催化剂上汞的氧化遵循Langmuir-Hinshelwood机制。200 °C时，CeTi催化剂上汞氧化反应速率常数在80-130 s-1之间；HCl的吸附平衡常数在2-3×103 m3•mol-1左右；（2）SCR气氛下HgO可被NO与NH3通过直接、间接两种机理还原成为Hg0；（3）有O2条件下SO2既可以促进Hg0的氧化，也可以抑制Hg0的氧化，取决于烟气中SO2的浓度；（4）SCR气氛可以通过诱导氧化态汞的还原降低Hg0转化效率，但是降低空塔气速（GHSV）可以缓解该抑制作用。实验周期内，Hg0氧化过程及其产物在催化剂表面的累积对NO还原无明显不利影响；(5) CuO与CeO2间的强相互作用提高了CeTi催化剂的催化性能及抗水抗硫性能。在200 °C，GHSV 为 54,000 h-1条件下，CuO-CeO2/TiO2催化剂上的NO 还原效率以及Hg0的协同氧化效率均可超过90%。以上研究结果不仅可为实现燃煤污染物的一体化脱除提供可靠的科学依据，还可为低温铈基SCR催化剂的工业应用提供坚实的技术支撑。