【作者】 丁峰；

【导师】 张军营； 郑楚光；

【作者基本信息】 华中科技大学 ， 热能工程， 2012， 博士

【摘要】 煤炭作为我国的主要能源这一现状在很长一段时间内难以改变,大量的煤炭消耗带来了严重的环境问题。汞因为具备挥发性、持久性和生物积累性,被认为是一种全球性的污染物,近年来受到极大的关注。我国是世界上汞排放量最大的国家之一,而燃煤电厂是大气中汞的主要排放源之一,因此必须对其进行控制。吸附剂喷入法是一种,极具应用前景的脱汞技术,其能够有效利用现有的除尘设备实现烟气中汞的脱除。然而现有的活性炭类吸附剂在应用过程中存在两个主要问题：其一是活性炭价格较高导致燃煤电厂脱汞成本太高；其二是活性炭的喷入会增加飞灰中的碳含量,从而影响到飞灰在水泥工业中的应用,由此不仅会造成资源的浪费还会导致新的环境问题。因此研究和开发新型的低成本高效率的汞吸附剂具有重要意义。本文利用廉价的天然矿物材料作为活性炭的替代品,合成制备出了一系列的汞吸附剂,并对其脱汞性能进行了综合评价,深入探讨了矿物吸附剂的脱汞机理。首先选取了凹凸棒石、膨润土、丝光沸石和蛭石四种天然的硅酸盐矿物作为天然吸附剂,利用热活化法对吸附剂进行了活化处理,采用N2吸附-脱附、X-射线衍射(XRD)和X-射线荧光探针(XRF)等方法对吸附剂进行表征,在模拟烟气条件(N2、O2和CO2)下,利用VM3000测汞仪在固定床实验台架上对吸附剂的脱汞效果进行了测试。结果表明,热活化并未增强吸附剂的脱汞能力,而升高温度有利于吸附剂对Hg0的脱除。四种天然吸附剂中膨润土的脱汞效果最好,而蛭石的脱汞效果最差,天然矿物吸附剂对汞的脱除主要基于化学吸附机理。消解实验结果显示丝光沸石和凹凸棒石能够氧化Hg0,其原因与吸附剂中的钛元素有关。在对四种天然硅酸盐矿物脱汞能力的研究基础上,选取了凹凸棒石、膨润土和丝光沸石三种天然矿物作为吸附剂材料,选用了单质硫、交联硫化钠、巯基乙酸钙、二巯基吡啶、二巯基吡啶N氧化物、3-巯丙基-三甲氧基硅烷、半胱氨酸、胱氨酸、氯化铜、氯酸钠、溴化钾和碘化钾等各种试剂作为改性剂,制备出了一系列的化学改性吸附剂,并同时制备了Al柱撑蒙脱石吸附剂。研究结果表明,不同的吸附剂材料对改性剂的负载量不同,丝光沸石对有机改性剂的负载量最小,而层状结构有利于负载有机改性剂；单质硫改性吸附剂的脱汞效果要优于有机硫改性吸附剂,温度从70℃升高到120℃有利于吸附剂对汞的脱除；有机螯合改性吸附剂能够有效脱除Hg0,而单一的巯基基团并不能有效吸附Hg0;丝光沸石对氯化铜的负载能力较差,导致其改性后脱汞效果要弱于膨润土和凹凸棒石；在120℃时氯酸钠改性凹凸棒石表现出超过90%的汞脱除率,天然凹凸棒石中的铁氧化物能够消弱氯酸钠中的C1-O键从而促进其对汞的氧化；碘化钾改性吸附剂在干燥过程中会因碘化钾的分解而产生单质12,从而有效促进吸附剂对Hg0的脱除：三种天然矿物吸附剂对溴化钾的负载能力较差,导致溴化钾改性吸附剂无法有效脱除Hg0;A1柱撑蒙脱石极大改善了蒙脱石的物理特性,其比表面积由原来的61 m2/g增加到了209 m2/g,但脱汞效果并不明显；总体看来,平均汞脱除率在80%以上的吸附剂有：硫改性膨润土(S-Ben)、氯化铜改性凹凸棒石(Cu-Atp)、氯化铜改性膨润土(Cu-Ben)、氯酸钠改性凹凸棒石(Cl-Atp)和三种碘化钾改性吸附剂(I-Atp、I-Ben和I-Mor)。通过对比分析,选取了Cu-Atp作为最合适的汞吸附剂,结合VM3000测汞仪和安大略法两种测汞方式,在N2气氛下利用固定床实验台架对影响吸附剂脱汞效果的各因素进行了研究,发现S02和H20会略微抑制吸附剂对汞的脱除,NO则并无明显影响,不同形态的铜化合物及溶剂均会影响吸附剂的脱汞效果,在150℃下加热会导致吸附剂中氯化铜的水解,从而降低吸附剂的脱汞效果。分别利用准一级和准二级反应动力学模型对实验结果进行模拟研究,显示实验结果与动力学模型的模拟结果相吻合。随后为了改善吸附剂在高温下的脱汞效果,合成了锰掺杂改性吸附剂(Mn-Cu-Atp),实验结果显示,在150℃下该吸附剂对Hg0的脱除率能够长时间保持在80%,说明Mn的掺杂有效改善了Cu-Atp吸附剂在高温下的脱汞能力。更多还原

【Abstract】 Coal is still and will be the mainly energy source in a long time for China. Resultant huge amount of coal consumption will result in serious environmental problem. China is one of the largest mercury emissions countries in the word and coal-fired power plants are considered as the largest anthropogenic sources of mercury pollution. Therefore the mercury emissions control of coal-fired power plants is reasonable and necessary. Sorbent injection is a promising mercury emissions control technique. It could well use the existing particulate matter controller and effectively removal mercury from flue gas. However, the application of activated carbons as the current mercury sorbents is limited by following two reasons. On one hand, the high price of activated carbons and a huge amount of sorbent demanded by coal-fired power plants result in a huge cost for mercury control. On the other hand, the application of carbon-based sorbents increases the carbon content in fly ash and affects the utilization of fly ash in cement. This is not only a waste of resource but also a cause of new environmental problems. Therefore, it is important to find out efficient and cost-effective mercury sorbents. In this paper, natural mineral materials were applied as alternative to activated carbons due to their low cost. Various of sorbents were synthesized and their mercury removal performances were evaluated. The mercury removal mechanism of mineral sorbents was also deeply discussed.Four types of natural minerals, bentonite (Ben), mordenite (Mor), attapulgite (Atp), and vermiculite (Ben), were selected as raw sorbents and thermal treatment were conducted. Characteristics of these sorbents were analyzed by an X-ray diffractometer (XRD), an accelerated surface area and porosimeter (ASAP) using the N2 isotherm adsorption/desorption method, and an X-ray fluorescence spectrometry (XRF). The sorbents performances on mercury removal were evaluated on a lab-scale fixed-bed system under a simulated flue gas condition (N2, O2, and CO2) and the mercury concentration was detected continuously using a VM3000 online mercury analyzer. The results showed that thermal treatment could not enhance mercury removal abilities of sorbents and increasing temperature benefited the mercury removal. Bentonite exhibited the best mercury removal performance in the four natural sorbents and a poor mercury removal performance was observed for the vermiculite. The Hg0 removal by natural mineral sorbents was mainly based on the chemosorption. The reason that the natural mordenite and attapulgite could oxidize elemental mercury should relate to the existence of active titanium.According to the experimental results of four silicate minerals, three minerals, Atp, Ben, and Mor, were selected as raw sorbents, and several chemical promoters were employed to enhance mercury removal abilities of these raw sorbents. The chemical promoters are element sulfur, sodium polysulfide, calcium thioglycolate,2-mercaptopydine, 2-pyridinethiol-l-oxide, (3-mercaptopropyl)trimethoxysilane, cysteine, cystine, copper chloride, sodium chlorate, potassium bromide, and potassium iodide. The Al-pillared montmorillonite was also synthesized. The results showed that the loading of promoters in the sorbents varied with the different sorbents materials. In general, the contents of organic promoters in Mor were the lowest and layer structure benefited organic promoters loading; The element S-modified sorbents performed well than the organic S-modified sorbents in mercury removal and increasing temperature promoted mercury removal for element S-modified sorbents; Single sulfhydryl group could not adsorbed Hg0, whereas the existing of different types of radical were propitious to mercury removal based on chelation; The mercury removal efficiency of Cu-Mor was lower than that of Cu-Atp and Cu-Ben dut to the poor capacity in ionic copper adsorption for Mor; Cl-Atp showed an average Hg0 removal efficiency more than 90%at 120℃. Iron oxide in natural Atp could promoted Hg0 oxidation by weakening the Cl-O bond in the chloride; For KI-modified sorbents, I2 was generated from the oxidation of KI during the drying process and it effectively promoted the Hg0 removal; The three natural minerals presented poor adsorption abilities for bromine, which resulted in the disappointing mercury removal efficiencies; The physical characteristics of montmorillonite was improved after Al-pillared modification and the value of BET surface area increased from 61 m2/g to 209 m2/g. However, the mercury removal efficiency of the sorbent was still poor. Generally, the sorbents with more than 80%average Hg0 removal efficiency are element S-modified Ben (S-Ben), CuCl2-modified Atp (Cu-Atp), CuCl2-modified Ben (Cu-Ben), NaClO3-modified Atp (Cl-Atp), and three Kl-modified sorbents (I-Atp, I-Ben, and I-Mor)。 After the comparison of the different sorbents, Cu-Atp was selected as the most promising sorbent for further study. Two methods (using a VM3000 online mercury analyzer, and Ontario Hydro Method) were applied for mercury detecting. The experiment was conducted on a lab-scale fixed-bed system under a nitrogen gas condition and the various factors affecting mercury removal efficiency were studied. SO2 and H2O slightly inhibited mercury removal respectively. There was insignificant effect on mercury removal efficiency in the presence of NO. Both the forms of copper compounds and the solvents used in sorbent synthesis process could affect mercury removal efficiency of sorbents. The copper chloride in Cu-Atp would be hydrolyzed when Cu-Atp was heated at 150℃, and it could reduce the sorbent ability of mercury removal. The kinetic model simulation results were in good agreement with the experimental results. To improve the mercury removal ability of Cu-Atp at high temperature, Mn-doped Cu-Atp was synthesized. The experimental results showed that the mercury removal efficiency of Mn-doped Cu-Atp could retain at 80%in a long time at 150℃. It means that Mn doping effectively improved the sustained mercury removal ability of Cu-Atp.更多还原