硫化氢预处理和过渡金属掺杂强化磁性铁基尖晶石低温吸附气态零价汞的机制研究

The control of Hg0 emission from coal-fired plants is preferred as a co-benefit of currently available control devices in coal-fired plants. Now, it mainly falls into one of the two routes: One is the catalytic oxidation of Hg0 by the selective catalytic reduction (SCR) catalyst with HCl in the flue gas as the oxidant to Hg2+, which is then removed from the flue gas in the flue gas desulfurization (FGD) unit; The other is the conversion of Hg0 to Hgp by the capture of sorbents (for example brominated activated carbon) upstream of the ESP, which is then removed from the flue gas with the fly ash in the ESP unit. However, there is a major disadvantage for the two methods that gaseous Hg0 is not captured for the centralized control or converted to low toxic Hg species. Hg0 in the flue gas is converted to HgCl2 in the desulfurization gypsum or to HgBr2 in the fly ash, whose toxicities are much higher than that of Hg0. Therefore, the control of Hg0 emission from coal-fired plants by the two methods may increase the exposure risk of local residents to mercury. .Now, many researchers focus on the centralized control of Hg0 emission from the flue gas. To meet the stricter emission standards for the ultra-low emission, wet electrostatic precipitators (WESPs) are gradually installed downstream of the FGD in coal-fired plants of China. Magnetic sorbents may be injected into the flue gas downstream of the FGD to capture gaseous Hg0. Then, they are removed from the flue gas by the WESP. As more than 99% of the fly ash has been removed from the flue gas by the ESP, the mixture collected by the WESP mainly contained spent magnetic sorbents and small amounts of ultrafine particulates and desulfurization gypsum. Therefore, it is practical to recover spent magnetic sorbents from the mixture collected by the WESP for the recycling and achieving the centralized control of Hg pollution..In our previous study, magnetic Fe-based spinels were developed to capture gaseous Hg0 from the flue gas. However, the temperature window of the sorbents for gaseous Hg0 capture did not match the temperature range of the flue gas downstream of the FGD (i.e., 40-80 oC). In this proposal, Fe-based spinels will be modified with H2S pretreatment and doping with transition metals to improve their abilities for capturing gaseous elemental mercury from the flue gas at low temperatures. According to the reaction mechanism and kinetics of the chemical adsorption of gaseous elemental mercury on modified Fe-based spinels, the structure-activity relationship among H2S pretreatment and the doping with transition metals, the physicochemical properties of Fe-based spinels, and their ability for capturing gaseous elemental mercury from the flue gas at low temperatures will be built. Based on the structure-activity relationship, Fe-based spinels with excellent performance for recovering gaseous elemental mercury from the flue gas at low temperatures will be devised with the targeted modification of H2S pretreatment and doping with transition metals. Furthermore, natural magnetic iron ore for example magnetite, titanomagnetite and ilmenite will be collected and modified with H2S pretreatment according to the structure-activity relationship to improve its ability for recovering gaseous elemental mercury from the flue gas at low temperatures. . In summary, this proposal provides a new idea for the control of elemental mercury emission from coal-fired plants, which will promote the transformation and upgrading of the control of Hg0 emission from valence state regulation to the centralized control.

目前燃煤烟气汞排放控制技术未能实现汞污染的集中控制，而是将气态零价汞转化为毒性更大的二价汞分布在飞灰或脱硫石膏中，极易引起二次污染。本课题创新性地提出利用可循环使用的磁性吸附剂与湿静电除尘器协同集中控制燃煤烟气汞污染排放的新思路。在青年基金的支持下，申请者曾经深入研究磁性铁基尖晶石对气态零价汞的化学吸附，但铁基尖晶石吸附气态零价汞的温度窗口与湿静电除尘器前的烟温范围（40-80 ℃）不匹配。为此本课题拟通过硫化氢预处理和过渡金属掺杂等手段对铁基尖晶石进行改性，提高其低温化学吸附气态零价汞的性能，使之与湿静电除尘器相结合集中控制燃煤烟气汞污染。与此同时，拟采集我国各地的磁性铁矿石，结合其表征结果，通过硫化氢预处理等手段提高天然磁性铁矿石低温吸附气态零价汞的性能，降低集中控制汞污染排放的磁性吸附剂的成本。本课题的批准实施将能有力推动我国燃煤电厂汞污染排放控制从形态调控向集中控制的转型升级。

旨在全球范围内控制和减少汞排放的国际公约《水俣公约》已经正式生效。燃煤是我国汞污染排放的第一大人为源，控制燃煤烟气汞污染排放势在必行。目前燃煤烟气汞排放控制技术未能实现汞污染的集中控制，而是将气态零价汞转化为毒性更大的二价汞分布在飞灰或脱硫石膏中，极易引起二次污染。. 本课题创新性地提出利用可再生的磁性吸附剂与湿静电除尘器协同集中控制燃煤烟气汞污染排放的新思路。针对磁性铁基尖晶石在低温吸附气态汞性能较差的问题，提出利用气态硫化氢与磁性铁基尖晶石表面反应在其表面镀上一层金属硫化物，提高其低温吸附气态零价汞性能。吸附在吸附剂表面的汞可以热脱附为超高浓度的气态零价汞，冷凝后集中控制。汞脱附后的吸附剂可通过再硫化进行再生。. 研究发现气态零价汞在硫化铁基尖晶石表面的吸附主要遵循Mars-Maessen机制，即气态零价汞首先物理吸附在吸附剂表面，随后其被吸附剂表面的活性硫S22-氧化成稳定的硫化汞。基于气态零价汞在硫化铁基尖晶石表面吸附机制，建立了气态零价汞的吸附动力学模型。通过动力学研究建立了硫化铁基尖晶石、其物理化学性质及其吸附气态零价汞性能三者之间的构效关系（即硫化铁基尖晶石吸附气态零价汞的容量等于其表面气态零价汞物理吸附位数量和表面活性硫数量两者的较低值，吸附气态零价汞的速率与气态零价汞物理吸附位数量和表面活性硫数量的乘积成正比）。依据建立的构效关系，通过过渡金属掺杂（如Ti4+等）和多酸（磷钨酸和磷钼酸等）嫁接等手段进一步提高硫化铁基尖晶石吸附气态零价汞的性能，开发出了能与湿静电除尘器协同集中控制燃煤烟气汞污染排放的可再生磁性吸附剂。为了进一步降低吸附剂的成本，通过对天然钛磁铁矿进行硫化，显著提高了其低温吸附气态零价汞的性能，也能实现燃煤电厂气态零价汞的集中控制。. 综上所述，本项目形成的科研成果将能有力推动我国燃煤电厂汞污染排放控制从形态调控向集中控制的转型升级，为《水俣公约》的履约做好技术储备。