| Energy and environment are key issues in the process of development of human society. In China, the coal-dominated energy structure will not change in the coming decades, and the coal-burning has become the primary anthropogenic emissions of mercury to atmosphere. Gaseous elemental mercury has attracted worldwide attention because it is volatile and insoluble in water and not readily removed by existing air pollution control devices. Some mandatory implementation (administrative rules and regulations) have been taken to control the mercury emissions from coal-fired power plants in some developed countries (such as USA). However, the sifting of effective sorbents and researches for mercury control in coal-fired power plants are still weak in China. And China has become the world’s largest emitter of mercury. To strengthen the control of mercury from coal-burning has become very important.This paper summarized the control methods for mercury emission from coal burning worldwide, focusing on the control researches in post-combustion flue gas. The source of the bamboo charcoal, preparation methods, nature of the bamboo charcoal and the application status of bamboo charcoal are systematically reviewed. The mercury removal using activated carbon has been proven to be an effective method, but with the restriction of high cost. A new idea was proposed in this paper that the bamboo charcoal was used as an alternative to activated carbon in the removal of elemental mercury.In order to study the physical and chemical properties of bamboo charcoal, elemental analysis, Fourier transform infrared spectroscopy (FTIR), BET surface area and pore size distribution analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM) were used to characterize the bamboo charcoal. The effect of granularity, W/F, the initial mercury concentration, adsorption temperature, oxygen concentration, HCl on gas-phase Hg0adsorption by bamboo charcoal was evaluated on a fixed bed reactor using an online gas-mercury analyzer.To obtain cost-effective sorbents for mercury removal, some oxidizing reagents and alkaline reagent were used to modify BC. The mercury removal performance of the resultant sorbents was studied in a fixed-bed reactor. The effect of adsorption temperature, reaction time, and flue gas composition for the mercury removal was studied. BET surface area and pore size distribution analysis, X-ray photoelectron spectroscopy (XPS) were used to characterize the sorbents. The results show that the oxidative modification for BC could improve the mercury removal performance of the sorbents, while the promotion effect of alkaline modification was slight. The modification using potassium permanganate, nitric acid and hydrogen peroxide were found to functionalize the BC. Especially, the C=O content was significantly increased after modification using hydrogen peroxide and nitric acid. An increase of π-electron content was observed from the KMnO4-modified BC, this could explain the sorbent’s strong chemisorption capacity.ZnCl2and KI were used to prepare the halogenated sorbents, and the removal experiments for gaseous elemental mercury were carried out in bench-scaled reactor. The effects of specific surface area, pore size distribution and other physical properties and adsorption temperature on the adsorption of mercury were studied. The impacts of SOx, NOx in the flue gas on the mercury removal by sorbents were studied.The composition and microstructure of the sorbents before and after experiments were studied using elemental analysis, XPS analysis, thermal gravimetric analysis to reveal the transformation of surface functional groups in the process of modification and mercury removal. The results show that mercury removal by modified BC depends mainly on physical adsorption, its chemical adsorption capacity is very weak at140℃. ZnCl2and KI modified BC have strong chemical adsorption capacity for Hg0at140℃and180℃. XPS test can confirm the oxidation of elemental mercury on the surface of ZnCl2and KI modified BC after adsorption of mercury. The KI modified BC has the ability of anti-SO2/NO-poison, its mercury removal efficiency decreased slightly with the presence of1000ppm SO2or10ppm NO.The suitable kinetic model based on experiments was proposed, according to the mass balance. Some parameters were obtained as reference for further predictions compared with the experimental results. At the same time, the calculated desorption energy of desorption experimental data were obtained through desorption model. The experiments showed that the chemical adsorption ability of BCZ sample was stronger than the BCZA. This possibly due to the decomposition of functional groups (lactone, carbonyl, chlorine, etc) during the activation process. The heterogeneous mercury removal process by sorbents is a complex multi-step process, in which multiple factors play key role as rate-control-step. The results of TPD experiment and desorption model clarified that the mercury removal by unmodified bamboo charcoal at20℃was mainly through physical adsorption of mercury, and mainly by chemisorption at140℃; the mercury removal by zinc chloride modified BC were both by chemisorption at20℃and140℃. This conclusion was consistent with previous experimental phenomena and experimental results. |
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