【作者】 罗光前；

【导师】 徐明厚； 姚洪； Xu Zhenghe；

【作者基本信息】 华中科技大学 ， 热能工程， 2009， 博士

【摘要】 燃煤电站是全球最大的人为汞排放源,其汞排放与控制已经成为愈来愈受关切的热点。本文在燃煤电厂汞排放形态及分布、煤中汞的形态识别和燃烧前脱除、煤燃烧过程中汞的形态、气相单质汞脱除等方面进行了研究,促进煤燃烧过程中汞污染控制技术的发展。用Ontario Hydro法对2台配备ESP的200MW燃煤电站锅炉汞排放形态和分布进行测试分析。汞平衡计算得,输出汞与输入汞质量比在113%到125%之间,表明结果可信。大部分输出的汞在飞灰和排放的烟气中,只有不到2%的汞保留在锅炉灰渣和省煤器底灰中。因为ESP除尘效率高达99%以上,所以几乎100%的颗粒态汞都被除掉了。超过总汞量55%的汞以气态汞的形式排放入大气,其中超过65%是氧化态汞。提高低NOx燃烧器锅炉负荷可以提高ESP入口的颗粒态汞比例,从而降低ESP出口烟气中的汞浓度。传统燃烧器比低NOx燃烧器锅炉向大气中排放的氯少而汞多。低NOx燃烧器的飞灰汞含量比传统燃烧器高,这归因于较高的烟气氯含量、飞灰含碳量和比表面积。通过程序升温热解、浮沉分离、硝酸浸提、选择性连续浸提识别煤中汞的赋存形态,同时考察了它们对煤中汞的脱除效率。首次建立了煤程序升温热解过程中汞释放的温度区间和汞形态的对应关系:＜150℃是Hg⁰,150-250℃是HgCl₂/有机结合态Hg,250-400℃是HgS/硅酸盐结合态汞,400-600℃是黄铁矿结合态汞。煤中汞的赋存形态影响各种燃前脱汞技术的效率。对于黄铁矿结合态汞含量高的煤种,浮沉分离然后低温热解是相对经济和环境友好的一种方法。煤的快速升温热解实验表明,提高温度,加快汞的脱除速率和提高汞的脱除率。降低压力热解可以提高汞脱除速率。通过化学热力学平衡计算、化学动力学计算和管式炉煤燃烧实验,研究了影响煤燃烧过程中汞形态转化的因素。化学热力学平衡计算结果表明,Cl₂对汞的氧化能力比HCl强得多。H₂O的存在会抑制汞的氧化。汞氧化生成的产物主要是HgCl₂,也可能存在少量HgO和HgSO₄。化学动力学计算结果表明,HgCl₂的生成速率受温度影响很大,在900K左右达到了峰值,而当温度高于1200K或低于700K时很低。敏感性分析表明:在793K时,HgCl₂主要通过反应HgCl+HOCl=HgCl₂+OH生成。当温度升到900K时,除了上述反应外,反应Hg+Cl₂=HgCl+Cl也变得敏感。但是,该温度下反应Hg+Cl+M=HgCl+M的逆反应导致HgCl的分解而抑制了HgCl₂的生成。1100K时,反应Hg+Cl+M=HgCl+M和反应HgCl+HCl=HgCl₂+H的逆反应对抑制HgCl₂生成起主要作用。通过石英管式炉的煤燃烧实验研究了影响汞氧化的因素。较强的氧化性气氛有利于汞氧化物的生成。燃烧温度影响汞的氧化,在燃烧温度900℃达到汞的最大氧化率。延长降温段停留时间可以略微促进汞的氧化,但效果不明显。不同密度段的煤燃烧汞氧化率依次为中密度段＞高密度段＞低密度段。Na对Hg氧化有较强抑制作用。热水洗煤和氨水洗煤都可以得到很好的脱氯效果。氯含量的降低大大降低了汞的氧化率。煤中添加CaCl₂可以促进汞的氧化反应,但是促进程度对煤种的依赖性高。吸附剂吸附是有效控制燃煤烟气汞污染,尤其是单质汞排放的有效途径。合成了一种新型可再生吸附剂载银纳米碳管（Ag-CNT）,并测试其作为吸附剂脱除或作为汞阱检测燃煤电厂烟气中的单质汞的性能。实验表明,Ag-CNT在从室温到150℃的温度范围内能彻底的捕集汞,可加热再生,汞吸附容量高,记忆效应极小。应用Ag-CNT制作汞阱用做CVAFS测汞仪的汞富集单元时,检测荧光结果对CO₂,O₂,SO₂和NO_x不敏感,标定获得标准曲线线性相关性高。实验表明Ag-CNT值得进一步测试其作为商用汞阱吸附剂的潜力。成功对高岭土、沸石、石灰石进行浸溴改性,并对吸附剂的汞吸附性能分别在固定床和沉降炉反应器上进行了测试。浸溴处理可以提高三种吸附剂的汞吸附效率。未改性吸附剂汞吸附效率随温度升高而降低,表现出明显的物理吸附。而浸溴改性吸附剂汞吸附效率随着温度升高而升高,表现出化学吸附特性。更多还原

【Abstract】 Worldwide,coal-fired power plants are known to be the largest anthropogenic source of mercury emissions.Mercury emission is a significant environmental concern due to its toxicity and high volatility.Identification of mercury occurrences in coal and mercury removal before coal utilization,mercury oxidation during coal combustion,elemental mercury capture by sorbents,and mercury speciation and distribution in full scale power plants have been studied,providing useful information for selecting/developing mercury emission control technologies.Two 200MW utilities without any differences besides burner type,low-NOx burners versus conventional burners,were studied on emission and behavior of mercury by Ontario-Hydro Method.The out/in mercury balance ranged from 113%to 125%was obtained in this study.Major mercury outgoing was fly ash in hopper and flue gas in stack as well as negligible minor amount of mercury＜2%was remained in boiler slag and economizer bottom ash.Nearly 100%of Hg_p could be removed by ESP due to its＞99% high dust removal efficiency.More than 55%of total mercury was released into the atmosphere as vapor phase,of which＞65%of total mercury was in oxidized form. Increasing operation load of boiler with low-NOx burners leads to higher Hg_p proportion in flue gas at ESP inlet and lower mercury concentration of flue gas at ESP outlet.More mercury and less chlorine were released into the atmosphere from boiler with conventional burners than with low-NOx burners.There was a weak correlation among mercury species, HCl and Cl₂.The fraction of elemental mercury decreased slightly with increasing concentration of HCl and Cl₂.Higher mercury contents were detected in fly ash from boiler with low-NOx burner than from conventional one due to higher carbon content and specific surface area.Mercury emission control technology in the combustion flue gas of coal-fired power plants has been under intensive development,but none has been implemented commercially because of the high cost.Treatment of coal before combustion provides an alternative option for Hg emission control.Samples of one anthracite,one lignite and three bituminous coals were investigated to determine the feasibility of precombustion Hg emission control. Density separation,temperature-programmed pyrolysis,HNO₃ extraction and sequential selective extraction（SSE） were used to determine the occurrence and species of mercury in the different types of coal.The results showed that the characteristic temperature range for Hg release is＜150℃for Hg⁰,150-250℃for HgCl₂/organic-bound Hg,250-400℃for HgS/silicate-bound Hg,and 400-600℃for pyrite-bound Hg.The mercury removal of different technologies are dependent on mercury occurrence in coal.HNO₃ extraction and sink-float were found to be effective for the removal of pyrite bound Hg and then the coal can be pyrolysed at 400℃to remove other forms of Hg.Experiments of rapid pyrolysis of coal showed that higher temperature led to higher rate and efficiency of mercury removal. In addition,decreasing pyrolysis pressure can increase mercury removal rate.The effects of different factors on mercury oxidation were investigated by thermodynamic calculations,kinetic calculations,and tests of coal combustion in a horizontal electrically heated tube furnace.Thermodynamic calculations suggested that HCl is a very weak oxidizer of mercury,while Cl₂ is a very strong one.The presence of H₂O may inhabit mercury oxidation.The dominating product of mercury oxidation was HgCl₂, as well as minor amount of HgO and HgSO₄.Kinetic calculations showed that HgCl,Cl₂, and HOCl formation is important in producing the oxidized mercury during combustion of coal.A suitable temperature for Hg oxidization when Cl₂ is the oxidization resource is 700-1200 K.Results of batch combustion of coal in the tube furnace showed that many factors could influence mercury oxidation.Stronger oxidizing atmosphere was propitious to mercury transformation to oxidized form.The highest degree of mercury oxidation was attained at the combustion temperature of 900℃.Extending residence time in the cooling zone can slightly promote mercury oxidation.Mercury oxidation factions during combustion of density fractionated coal are high＞medium＞light density.Na has obvious inhabitation effect on mercury oxidation via completion reaction with chlorine species. Coal leaching by hot water and ammonia got a high chlorine removal fraction,resulting in a lower oxidized mercury fraction.By cofiring CaCl₂ with coal,more elemental mercury was converted to oxidized gaseous species.Capture by sorbents is effective method to control mercury emission form coal combustion,especially for elemental mercury.A simple method combining wet-chemistry and thermal reduction was adopted to synthesize silver on the surface of carbon nano tube （CNT）.The synthesized Ag-CNT was tested as sorbents for the removal of of elemental mercury from flue gases of coal-fired power plants and as a mercury trap for elementary mercury analysis.A complete capture of mercury by Ag-CNT was achieved up to a capture temperature of 150℃.The captured mercury could be quickly and completely released by simple heating at 330℃,to restore its mercury adsorption capacity.Silver on Ag-CNT were shown to be the main active component for mercury capture via amalgamation mechanism in contrast to simple physical adsorption on undoped CNT.Compared with silver-coated quartz beads（Ag-Beads） and gold-coated quartz beads（Au-Beads）,which is conventionally used as mercury trap for mercury measurements,Ag-CNT showed a much higher mercury capture capacity and a minimal memory effect.With Ag-CNT as mercury preconcentration trap,calibration results showed a satisfactory linear coefficient of≥0.9998 between known amounts of standard mercury and their corresponding fluorescence signals of a Cold Vapor Atomic Fluorescence Spectrophotometry（CVAFS）.The presence of SO₂,NO_x,CO₂ or O₂ showed a negligible impact on the mercury capture performance of Ag-CNT.A fixed bed and and drop tube reactor were used to test the performances of mercury capture by kaolin, zeolite,lime stone and their corresponding bromine impregnated sorbents.Bromination can obviously promote mercury adsorption efficiency.Capture efficiency of origin sorbents decreased with increasing temperature,exhibiting a physical adsorption.In contrast,capture efficiency of brominated sorbents increased with increasing temperature,exhibiting a chemisorption.更多还原