活性焦干法烟气脱硫过程中单质汞的脱除机理及产物分布规律研究

Coal combustion from industrial is the largest anthropogenic emissions of toxic heavy metal mercury, and Hg0 is the most difficult to remove because of its relatively low water solubility and high vapor pressure. It is of great significance for the pollutants emission control from coal-fired flue gas that activated cokes could remove elemental mercury at the same time in the process of dry flue gas desulfurization. Coexistence atmospheres take advantage in the diffusion coefficients, the gas concentrations and so on, thus preferentially reacting with material further to change the mercury removal mechanisms and products distribution. Carrying out the research on mercury removal mechanisms and products distribution under the coexistence atmospheres provides a new idea for mercury emission control through regulating the flue gas parameters to improve the mercury removal activity on the activated cokes as well as optimizing the mercury removal products distribution further to make it migrate to the activated cokes surface. This project intends to evaluate the mercury removal performance over different activated cokes. The surface properties characteristic parameters of activated cokes that affect Hg0 removal performance are revealed, and the relationship between surface properties and mercury removal performance is established. By characterizing the physical and chemical properties of activated cokes, the characteristic flue gas parameters that influence Hg0 removal paths and products species are identified, and the elemental mercury removal mechanisms are clarified. On the basis of the principle of the element mass conservation, the equilibrium distribution laws of mercury removal products under various flue gas parameters are built. This result will provide theoretical guidance for directional control of activated coke preparation, meanwhile provide technical reserves for mercury emissions control from coal-fired flue gas.

工业燃煤是我国大气有害重金属汞的最大人为排放源。其中，单质汞挥发性强且难溶于水，是最难控制的汞形态。活性焦干法烟气脱硫过程中同时实现单质汞的脱除，对燃煤烟气污染物的排放控制有重要意义。SO2、NO等共存气氛在扩散系数、气体浓度等方面有优势，会优先与材料反应，改变脱汞机理和产物分布。研究共存气氛下单质汞的脱除机理及产物分布，为调控烟气参数提高活性焦的脱汞活性、优化产物分布使其向活性焦表面迁移进而为实现汞排放控制提供新思路。本项目拟对不同活性焦的脱汞性能进行评价，揭示影响脱汞活性的活性焦表面性质特征参数，建立表面性质和脱汞活性的构-效关系；通过对活性焦理化性质表征，获得影响脱汞反应路径及产物类型的特征烟气参数，阐明单质汞的脱除机理；根据元素质量守恒原理，构建脱汞产物在不同烟气参数下的平衡分布规律。本研究将为定向调控活性焦制备提供理论指导，同时为燃煤烟气汞的排放控制提供技术储备。

实际燃煤电厂排放的烟气组分复杂，气体种类较多，并且各气体组分性质和浓度差别巨大，会对活性焦脱汞性能产生影响。本项目基于燃煤烟气的特征，从金属氧化物负载型活性焦表面性质和脱汞活性的构-效关系、单质汞与烟气组分在活性焦上的相互作用机制和脱除机理，以及脱汞产物的固、气相分布三个层面开展研究，取得了系列特色成果：（1）构建了活性焦表面性质和脱汞活性的构-效关系。低温范围内（< 200 oC），改性活性焦对汞的吸附包括物理吸附和化学吸附过程。1nm 附近最可几孔径在孔径分布的占比高，有利于增强汞的物理吸附。负载型活性焦活性组分的晶型结构为CuO，以孤立态和聚集态键合于载体上，可提高活性焦对汞的化学吸附作用，化学吸附反应主要发生在Oβ位点，脱汞产物为HgO。（2）揭示了复杂烟气组分共存时单质汞的脱除机理。O2能够提高CuO/AC的脱汞效率，当O2浓度达到7%时，汞的去除效率是关于O2浓度的零级反应；SO2气氛会与物理吸附态汞产生竞争吸附，或与CuO结合使其畸变，进而显著降低活性焦的脱汞性能，即使存在O2气氛，SO2也会抑制化学吸附态脱汞产物的生成，并且抑制部分汞的物理吸附；NO的吸附产物与汞间的直接氧化反应是促进汞吸附的主要原因，并不是通过再生表面活性氧，然而高浓度NO会一定程度抑制产物HgO的生成；HCl促进材料脱汞性能提高的本质原因是化学吸附生成活性Cl原子，进而会将Hg0氧化成HgCl2。此外，探明了烟气组分在活性焦上的吸附产物与汞结合能力强弱：HCl>NO>O2。（3）阐明了脱汞产物在气、固相分布规律。模拟烟气中O2可同时增强活性焦对Hg0的物理吸附和化学吸附，但对物理吸附作用影响更强。NO气氛会促进物理吸附态汞转变为强化学吸附态汞。存在HCl气氛时，会从CuO/AC表面逃逸出二价汞，但由于活性焦载体大的比表面，脱汞产物仍然主要为化学吸附态HgCl2。