Study On The Mechanism Of Elemental Mercury Removal From Power Plant Flue Gas By Metal-based Adsorbents

Mercury is one of the most toxic elements to human body in nature,so reducing mercury emission from flue gas of coal-fired power plants is of great significance to protect the ecological environment and human health in China.Up till the present moment,Mercury emitted in coal-fired power plants is classified as zero valent mercury,oxidized mercury,and particulate mercury.Due to the fact that the majority of oxidized mercury and particulate mercury can be removed cooperatively using air cleaning equipment such as a dust collector and desulfurization tower,Hg⁰ has become a challenge in reducing flue gas mercury emissions due to its low liquid solubility,volatility,and high chemical inertia.In order to solve this dilemma,currently coal-fired power plants generally inject activated carbon at the tail of the flue to control mercury pollution,which is the most mature and widely used control method at present.However,the cost of activated carbon is high,the temperature range for application is limited,and the chemical and physical modification process is lengthy and laborious.Thus,beginning with the adsorption properties of conventional adsorbents,this study designed and investigated the synthetic method,adsorption performance,and mercury removal mechanism of nanoscale high-efficiency metal-based adsorbents.The mercury adsorption mechanism of a new adsorbent with high efficiency and wide application temperature derived from metal organic framework was explained by means of experimental exploration,adsorbent characterizations and first principle quantum chemistry method.It has significant scientific and practical implications for the development of efficient and costeffective mercury removal adsorbents.To begin with,this work discussed the adsorption and desorption properties of some commonly used mercury adsorbents,including activated carbon,montmorillonite,silica,zinc selenide,silver nanoparticles,and cobalt nanoparticles.The mercury removal test of sorbent in simulated flue gas demonstrated that the reaction temperature has an effect on the adsorbent’s adsorption capability.Experiments with adsorption/desorption at various reaction temperatures showed that the adsorption temperature has a direct effect on the location of the mercury desorption peak.According to the kinetic fitting,mercury adsorption on the surface of these adsorbents follows a pseudo-second order adsorption kinetics.Cobalt nanoparticles have high adsorption capacity in the region with higher temperature in the test temperature window,and their desorption activation energy is 62.33 kJ/mol,indicating that they have a great potential for adsorbent development.The adsorption mechanism of Hg⁰ on the exposed crystal surfaces of cobalt and silver nanoparticles was elucidated using density functional theory.The results indicated that the metal nanoparticles primary growth planes of（111）,（220）,and（200）have a strong chemical affinity for mercury.Mercury can form chemical interactions with metal atoms on the surface of metal nanoparticles due to the abundance of chemical adsorption sites,The（111）crystal plane can dominate the adsorption process of Hg⁰ because to its high adsorption site density and natural crystal growth advantages.After investigating the adsorption characteristics and mechanism of the adsorption of the aforementioned conventional adsorbents,it is clear that cobaltbased metal adsorbents give significant advantages in terms of adsorption capacity,temperature window,and cost when compared to other conventional adsorbents.To create a novel adsorbent with high affinity for gaseous elemental mercury over a broad temperature range,porous carbon with cobalt nanoparticle was synthesized via direct pyrolysis of cobalt zeolite imidazole framework in metal organic framework.Physical adsorption sites for Hg⁰ can be found in carbon-based carriers,whereas chemical adsorption sites for Hg⁰ can be found in cobalt nanoparticles.The mercury removal efficiency of the synthesized porous carbon with cobalt nanoparticle was greater than 85%in the temperature range of 30-240℃ due to the synergistic action of the two parts.To further enhance the adsorbent’s mercury fixation capacity and thermal stability,the zeolite imidazole framework was pyrolyzed in one step process with air as the reaction atmosphere,converting the generated products from porous carbon with cobalt nanoparticles to cobalt trioxide nanoparticles.The surface chemistry of the nanoparticles was altered,resulting in synthetic adsorbent with a high oxidation capacity for elemental mercury.The porous Co₃O₄ adsorbent doped with single-atom Pd exhibits superior Hg⁰ adsorption ability over a broad temperature range（30-350℃）with high gas hourly space velocity(about 555500 hr^-1).In the test window,the average Hg⁰ removal efficiency of Pd1/Co₃O₄-ZIF was greater than 98%.After 80 hours of continuous operation,the adsorbent’s mercury removal rate remains greater than 87%.Notably,the adsorbent shows excellent regeneration,which greatly increases its industrial application prospect.