【作者】 胡长兴；

【导师】 骆仲泱； 周劲松； 岑可法；

【作者基本信息】 浙江大学 ， 环境工程， 2007， 博士

【摘要】 燃煤汞排放作为主要的环境污染问题之一，其危害已引起国内外研究者及相关政府部门的关注，并积极开展了这方面的控制研究工作。本文依托国家863计划及国家自然科学基金资助项目，围绕当前燃煤汞污染及其控制方法的新兴研究，对我国燃煤电站汞排放污染情况及活性炭喷射烟气除汞过程中的稳定吸附机理问题开展研究。通过对中国典型燃煤电站锅炉汞排放特性研究，深入准确地了解我国燃煤电站汞排放的现状。针对活性炭喷射这一目前主要的烟气汞控制技术，着重研究了汞在活性炭吸附产物中的稳定性，吸附机理以及增强稳定的途径和方法，并提出相应的燃煤电站汞排放估算模型和烟道活性炭喷射汞吸附模型，对于推动我国燃煤汞污染排放控制机理研究及控制技术的工程应用具有重要的现实和未来意义。首先利用大型燃煤锅炉汞排放分析测试平台对国内6套典型燃煤锅炉进行了系统的汞排放测试研究。研究表明当电站尾部只布置静电除尘（Electrostatic Precipitator，ESP）时，燃烧生成的80％以上的汞以气态形式排入大气；当ESP和湿法脱硫（Wet Flue Gas Desulfurization，WFGD）共同布置时，气态汞比例降低到了25％以下；当ESP、WFGD和选择性催化还原脱硝设备（Selective Catalytic Reduction，SCR）共同布置时，气态汞的比例甚至降低到了7％以下。通过模型估算，每100MW机组总汞排放量在40～50Kg／年左右，气态汞排放量受尾部污染物控制设备的影响较大。2005年全国电站燃煤锅炉总汞排放量约为193.644吨，其中气态汞排放量约为147.014吨，约占总汞排放量的75.92％；固态汞约为30.023吨。针对活性炭喷射汞吸附方法可能形成汞再次释放而引起二次污染的问题，着重研究了水环境，受热环境以及自然堆积环境下活性炭吸附汞的稳定性问题。首先通过实验证明了毒性特性浸出程序（TCLP，Toxicity Characteristic Leaching Procedure）是一种可靠的实验室加速评估方法，可作为评估燃煤汞排放活性炭汞吸附产物是否稳定的参考方法。研究表明在水环境下活性炭吸附的汞比较稳定。但当活性炭经过强氧化性的浸渍改性后，汞的吸附稳定性有所下降。受热环境下的稳定性研究表明，随着受热温度的增加和时间的延长均不利于汞在活性炭吸附剂中的稳定；尤其在较高温度段活性炭中汞变得相当不稳定，易造成二次污染。另外研究表明以Hg²⁺汞源吸附的活性炭中汞稳定性要比以Hg⁰汞源的差。飞灰中汞及活性炭吸附的汞在自然环境下较稳定，对周围环境影响不大。在研究增强活性炭汞吸附稳定的方法和途径之前，首先利用实验的手段研究了活性炭汞吸附机理。从吸附机理实验来看，活性炭对Hg⁰的吸附不是简单的直接的物理吸附过程，而是复杂的化学吸附过程。如果孔表面不存在任何有关的化学元素组分（如Cl元素等），在N₂气氛下活性炭不会通过孔隙间的范德华力作用以物理形式吸附Hg⁰，也不会以C-Hg直接化学结合的形式吸附Hg⁰。而活性炭对Hg²⁺的吸附具有化学和物理吸附的双重特征。研究表明在模拟烟气下活性炭对Hg⁰的吸附过程中主要以这样的方式进行：在活性炭表面C催化作用下模拟烟气中的Hg⁰被酸性气氛氧化成Hg²⁺并同时被活性炭孔表面吸附位所吸附。而活性炭孔及表面物理结构特性可能在最终完成化学吸附及吸附容量方面起着比较大的作用。酸性气氛在吸附过程中氧化作用可通过预先在活性炭注入或增加具有氧化性的化学元素来代替，这为获得或制造高吸附效力和高稳定性的吸附剂提供了直接的实验依据。根据活性炭汞吸附机理研究及汞化合物特性，主要研究了以S为基本元素的增强活性炭汞吸附稳定的改性途径和方法。以不同改性方式，从不同的角度将S元素引入活性炭，并进行了吸附能力和稳定性实验。从对汞的吸附能力来看，各种改性方式的按优到差的顺序是：600℃高温渗硫＞200℃高温渗硫＞（NH₄）₂S浸渍＞SO₂分子源自由基簇射＞S（CCl₂）浸渍＞Na₂S浸渍。TCLP实验结果表明各种硫改性活性炭吸附的汞在非强酸强碱的水环境下非常稳定。对各改性活性炭进行的热稳定性实验表明，改性渗硫温度越低似乎活性炭中汞越不稳定。YK-AC-600℃-S-Hg和MZ-AC-600℃-S-Hg在热稳定实验的低温区段表现了稳定性增强的趋势。（NH₄）₂S浸渍改性后活性炭吸附汞的稳定性加强。综合考虑吸附能力及稳定性，在各种硫改性的方法中（NH₄）₂S浸渍改性活性炭为最佳改性手段。结合实际燃煤电站的尾部烟道结构，在活性炭吸附机理研究的基础上，建立了以化学吸附理论为基础的燃煤烟道活性炭喷射的汞吸附模型。模拟研究活性炭喷射量，活性炭颗粒直径，烟气温度、烟气流速、烟气汞含量、燃煤烟道尾部长度等因素对活性炭吸附汞的影响。模型的建立可以用来指导燃煤烟道活性炭喷射汞吸附控制装置的设计，预测喷射除汞装置的脱除效率和吸附量计算等。更多还原

【Abstract】 This paper gives a systematic and comprehensive research on the mercury （Hg） emission and control of coal-fired power plant in China with the support of the National High-tech Research and Development Program （863） and the National Natural Science Foundation. It includes characteristics of mercury emission from the coal-fired boiler in Chinese power plant, stabilization of mercury on the activated carbon’s surface, adsorption mechanism of gaseous mercury on the activated carbon （AC） and modification the AC surface to enhance adsorbability of mercury, etc.It firstly investigated the Hg emission characteristics of six typical large coal-fired boilers using the advanced sampling equipments and analytical instruments in China. Through a study of the distribution of mercury content in combustion products of the six boilers, it found that the ratio of Hg_gas is above 80% of the total Hg with the electrostatic precipitator （ESP）; the ratio of Hg_gas is reduced below 25% with the ESP and wet flue gas desulfurization （WFGD）; especially, with the ESP, WFGD and Selective Catalytic Reduction （SCR）, the ratio of Hg_gas is lower than 5%. According to the mercury emissions estimation model, the total amount of mercury emission of every 100MW is about 40～50 Kg/year. The ratio of Hg_gas to Hg_solid is affected by the equipped air pollution control device in the tail flue. The total amount of mercury emission from power plant was approximately 193.644 tons, which included 147.014 tons of Hg_gas and 46.630 tons of Hg_solid in China in 2005.The stability of mercury on the AC’s surface was studied in the different environments, such as solution environment, heated environment and natural deposition environment. According to the experiment, TCLP is a reliable laboratory accelerated assessment method, which can act as a reference method to evaluate the stability of mercury on the AC. Mercury appeared to be stable on AC in the solution environment according to leaching tests. However, the stability of mercury adsorbed on the oxidation treated AC was not better than that of untreated carbon on the whole. Mercury leaching showed strong pH dependence. Compared with the neutral pH test, lower and higher pH leaching tests for mercury were more effective for removing Hg. As was the result of mercury thermal desorption in air condition, there was much more mercury released from AC when heated either longer time or higher temperature for both kinds of mercury. In addition, it seemed that Hg°was bonded more firmly on AC than Hg²⁺. According to the natural deposited experiment results, the Hg was stable in both fly ash and AC, and would not significantly affect the environment.Using the experimental solution, the adsorption mechanism of Hg on AC was investigated before the research of enhancing Hg adsorption stability of AC. Thinking of adsorption mechanism experimental phenomena, it can conclude that the adsorption mechanism of Hg⁰ on AC is not simple physical adsorption, but complicated chemical adsorption. If there are no any chemical elements existing on the surface, AC will not adsorb the Hg°through Van der Waals force and chemical force in N₂ ambience. According to the experiment, sorption of Hg²⁺ on AC showed the both chemical and physical characteristics. Based on the experimental results and analysis, the AC adsorption mechanism of Hg°in a simulated flue gas is that the Hg°is oxidized to Hg²⁺ by acidic atmosphere with the help of C catalysis on the AC’s surface, and at the same time Hg²⁺ is adsorbed by active adsorption sites. It believes that the physical structure characteristic of AC’s surface may work on adsorption capacity. Moreover, the oxidation of pre-impregnated oxidative chemical elements on the surface of AC can be instead of that of acidic atmosphere during the adsorption process. It provides direct experimental evidence for the acquisition or manufacture of adsorbent with high efficiency and high stability.Several types of S were introduced into the ACs through different modification. Compared the adsorption capacity of different modified ACs, the modified methods were arranged in order of efficacy as follows: 600℃-S-deposition > 200℃-S-deposition > （NH4）₂S-dipping > free radical with SO₂ source > S （CCl₂）-dipping > Na₂S-dipping. As the experimental results of TCLP, the Hg is quite stable on all modified ACs in general water environment. YK-AC-600℃-S-Hg and MZ-AC-600℃-S-Hg showed the enhanced trend of Hg stability on surface in the low temperature environment. （NH4）₂S dipping modification enhanced the stability of Hg on the ACs. Considering adsorption capacity and stability, （NH₄）₂S dipping is the best way among various sulfur modification methods.Based on the result of Hg adsorption mechanism research and the tail flue structure of coal-fired power plant, the chemical adsorption model of AC injection in flue for coal-fired mercury control was established. It can simulate and study the effect of injection amount, particle diameter, flue-gas temperature, flue-gas velocity, mercury concentration, and flue length on the mercury adsorption of AC.更多还原