【作者】 谭增强；

【导师】 邱建荣； 向军；

【作者基本信息】 华中科技大学 ， 热能工程， 2012， 博士

【摘要】 能源与环境是制约人类社会发展的关键问题,以煤为主的能源结构在未来几十年不会改变,煤在燃烧过程中向环境排放了大量的汞。汞由于其生物累积性和易挥发等特性对人类造成巨大的毒害,已引起世界范围内的关注。美国等发达国家已将燃煤火电厂的汞排放控制列入法规强制执行,中国已经成为世界上最大的汞排放国,加强对燃煤污染物汞的控制研究是十分必要的。本文总结了国内外燃煤污染物汞的排放控制方法,着重叙述了燃烧后烟气中的汞脱除方法；系统综述了竹炭的来源、制备方法、竹炭的性质以及竹炭的应用现状。活性炭除汞被证明是一种有效的汞脱除方法,但是存在材料成本高的限制,寻求新型的汞吸附剂是一个研究热点,本文提出采用生物质竹炭用于燃煤烟气中单质汞脱除的新思路。为了研究竹炭的物理化学特性,采用元素分析、傅里叶变换红外(FTIR)、比表面积及孔径分布分析、X-射线衍射(XRD)、扫描电镜(SEM)等测试手段对竹炭进行了表征测试。在小型燃煤烟气汞脱除实验台上和模拟烟气气氛下研究了低温下竹炭对汞吸附性能,重点考察了粒度、W/F(吸附剂用量/气体流量)、初始汞浓度、吸附温度、氧气浓度、HCl对竹炭脱汞的影响。为了得到廉价高效的脱汞吸附剂,采用氧化试剂和碱性试剂对竹炭进行改性,在固定床实验台架上系统地研究了气体单质汞在吸附剂表面进行的吸附及氧化行为,并重点考察吸附温度、反应时间以及烟气成分等因素对脱汞性能的影响。采用比表面积及孔径分布分析、X射线光电子能谱(XPS)等分析测试手段对吸附剂进行表征以探讨其中的机理。结果表明氧化改性提高了吸附剂的脱汞性能,而碱性改性对吸附剂的脱汞性能影响较小。高锰酸钾、硝酸、双氧水改性使竹炭表面官能化,特别是双氧水、硝酸改性使竹炭的C=O含量显著增加,高锰酸钾增加了竹炭的π电子含量,这是氧化改性竹炭具有较强化学吸附脱汞能力的原因。采用ZnCl2、KI制备卤素改性的吸附剂,并在小型实验台上开展气态单质汞的脱除实验,重点考察吸附剂的比表面、孔隙分布等物理特性和亲汞官能团、吸附温度等因素对吸附剂脱汞性能的影响,并探讨了烟气中SOx、NOx等酸性气体对竹炭脱汞的影响。通过元素分析、XPS分析、热重分析、比表面积及孔径分布分析等测试手段分析脱汞前后的吸附剂的组成和微观组成变化,以揭示表面官能团在改性过程中的转化规律和其在脱汞过程中的作用机理。结果表明未改性竹炭对汞的吸附主要靠物理吸附,在140℃时对Hg0的化学吸附能力极弱。而ZnCl2、KI改性的竹炭在140℃和180℃下对Hg0都具有极强的化学吸附能力。XPS测试可以确认ZnCl2和KI浸渍改性的竹炭吸汞后在其表面生成氧化态的汞。KI改性的竹炭材料具有一定的抗SO2/NO毒害的能力,当有2600ppm S02或300ppm的NO存在时,KI改性的竹炭捕获汞的量只略微降低。在实验基础上提出适合实际的模型,根据竹炭填充吸附床内气相以及吸附剂孔内质量平衡,利用平衡关系式建立竹炭吸附汞过程的动力学模型,并与试验结果进行对比得到吸附过程的一些参数,为进一步的预测提供参考。同时,采用脱附模型对脱附实验数据进行拟合计算得到脱附能等参数。实验发现载氯竹炭样品的化学吸附脱汞的能力要远强于活化竹炭样品,主要是由于活化过程中内酯基、羰基、氯等亲汞官能团及元素受热分解所致。动力学模型计算得发现竹炭基吸附剂对汞的非均相脱除过程是一个复杂的多步骤过程,其中多因素起到速率控制的作用。TPD实验结果和脱附模型计算推断未改性的竹炭在20℃时主要靠物理吸附脱汞,在140℃时主要靠化学吸附脱汞；氯化锌改性的竹炭在20℃和140℃时均靠化学吸附脱汞,这与前面的实验现象和实验结论一致。更多还原

【Abstract】 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.更多还原