【作者】 陈明明；

【导师】 段钰锋；

【作者基本信息】 东南大学 ， 动力工程及工程热物理， 2017， 硕士

【摘要】 我国是煤炭资源丰富的能源消费大国,燃煤电厂已成为最大的固定汞排放源,因此汞污染和排放控制已成为研究热点。活性炭喷射(ACI)应用来脱除燃煤烟气中的汞是目前认为最具商业化应用前景,但成本昂贵。燃煤飞灰价格低廉,来源广泛,且具有潜在脱汞能力,成为研究热点。有关飞灰吸附烟气汞的影响研究已有报道,但汞在吸附剂表面的形态研究甚少。本课题旨在研究廉价高效的脱汞吸附剂,首先对溴素改性后的活性炭和飞灰在模拟烟气中进行汞吸附的影响因素实验,然后采用程序升温脱附(Temperature Programmed Desorption,TPD)方法,研究不同烟气组分及模拟烟气条件下,汞在改性活性炭及飞灰表面的吸附形态与脱附特性,深入探讨汞吸附的机理。首先,选取四种燃煤电厂ESP飞灰(FA)作为汞吸附剂,利用NH4Br溶液浸渍搅拌改性(NH4Br-FA,NBFA)。采用N2吸附/脱附、X射线荧光光谱法(XRF)、扫描电子显微镜(SEM)等手段对样品进行表征。在固定床汞吸附实验装置上,对改性前后飞灰样进行模拟烟气条件下Hg0吸附实验研究。结果表明:负载到飞灰中未燃碳(UBC)表面的溴元素,会增加其邻位的活性,使其更容易吸附Hg0，促进了飞灰对Hg0的吸附及氧化作用,使飞灰吸附Hg0能力大大增强。烟气中酸性气体的存在能极大地促进Hg0氧化,被氧化的Hg0中有75%的气态Hg2+,其余的Hg0被氧化吸附在飞灰表面。UBC含量对改性飞灰脱除Hg0的影响很大,含量越高,吸附氧化作用越强;含量越低,催化氧化作用越强。低UBC含量的飞灰表面结构形态对Hg0的催化氧化作用具有一定促进作用。其次,在固定床进行了烟气组分O2,SO2,NO对两种溴素改性飞灰的汞吸附和汞氧化能力的影响研究。选取飞灰中主要的无机金属氧化物,利用NH4Br溶液进行浸渍改性,研究飞灰的无机组分在模拟烟气气氛下对Hg0脱除的影响。利用TPD方法对飞灰吸附样表面的汞形态进行探究,获得了单烟气组分与无机金属氧化物对飞灰吸附Hg0的反应机理。结果表明:溴素对飞灰中Fe2O3和TiO2的影响主要在于增加它们的晶格氧量,同时烟气中的O2不断补充消耗的晶格氧,促进了它们对Hg0啲催化氧化。烟气组分NO对NBFA吸附氧化Hg0起着重要作用,在飞灰表面形成Hg2(NO3)2和HgBr2,部分Hg2(NO3)2在150℃反应温度下分解生成Hg(NO3)2和HgO。UBC含量低的飞灰活性位点少,表面分解的物质会脱离飞灰表面活性位点,促进Hg2+比例的增加;SO2对飞灰脱除Hg0起抑制作用,主要原因在于Hg0对碳质材料活性位点形成竞争吸附。UBC成分对Hg0的吸附氧化机制主要遵循Langmuir-Hinshelwood机理,而有溴素存在的无机金属氧化物对Hg0的作用机制主要遵循Mars-Masessen机理。因此飞灰对汞吸附氧化脱除的过程伴随着多种机理的协同作用。最后,考察了一种原始活性炭(Fresh Activated Carbon,FAC)在N2气氛,溴素改性活性炭(NH4Br-AC,NBAC)分别在N2,O2,SO2,NO及模拟烟气气氛下,对气态汞的吸附性能。利用N2吸附/脱附、傅里叶变换红外光谱(FTIR)和扫描电子显微镜/X射线能谱仪(SEM-EDX)对活性炭进行表征。对纯汞化合物与Si02的混合物、FAC以及NBAC吸附Hg0后的产物分别进行TPD实验,研究其表面的汞吸附形态。结果表明,AC脱除Hg0机理是AC表面的有机物与Hg0反应生成Hg-OM(Hg Organic Matter),部分含氧官能团与元素汞结合生成HgO,但是效率很低,穿透率高达94%。溴素对FAC的改性增加了活性炭表面溴素邻位点的活性,极大地促进了对Hg0的吸附能力,且在表面形成了 HgBr2。单组分O2与NO可促进NBAC对Hg0的吸附,而S02起抑制作用。02主要在NBAC表面促进Hg0氧化生成HgO。NO效果最为显著,在活性炭表面生成了Hg2(N03)2。SO2与Hg0对活性炭表面的酯基存在竞争吸附,降低了对Hg0的吸附率。溴素浸渍改性活性炭对Hg0脱除的机理与溴素改性飞灰中的UBC对Hg0脱除的机理相似,且受烟气组分中的NO的影响较大,都遵循Langmuir-Hinshelwood机理。更多还原

【Abstract】 Coal combustion is one of the largest anthropogenic sources of atmospheric mercury emission,and coal-fired power plants occupy the main part in China,which is the largest fixed source of mercury emissions.Therefore,mercury control technology in coal fired power plant has been a hot topic.Activated carbon injection(ACI)is deemed to be one of the most commercial promising mercury removal technologies for coal-fired power plants but costly.ESP fly ash has attracted wide attention of scholars due to its inexpensive and mercury removal capability potentially characteristics.However,the experimental study of the factors on mercury adsorption in flue gases and the mercury speciation on the adsorbents surface are seldom.The purpose of this study was to investigate a kinds of cheap and efficient mercury removal adsorbent.The effects of bromine modified activated carbon(AC)and fly ash(FA)on the adsorption of mercury in flue gas were discussed.And then the temperature programmed desorption(TPD)was applied to study the adsorption mechanism and desorption characteristics of mercury species on the surface of the adsorbed bromine modified activated carbon and fly ash.Firstly,four kinds of fly alises(FA)from coal-fired power plants ESP were selected as the mercury removal adsorbents and impregnated with 1%NH4Br solution(NH4Br-FA,NBFA).The samples were characterized by N2 adsorption/desorption,X-ray fluorescence spectrometry(XRF)and scanning electron microscopy(SEM).Experimental study on the Hg0 adsorption by original and modified fly ash under simulated flue gas conditions were conducted on a fixed bed reactor.The results showed that the active sites were enhanced when bromine was impregnated on the surface of UBC in fly ashes,which promoted the adsorption and oxidation of Hg0 in N2 and simulated flue gas.Acid gases in simulated flue gas oxidized Hg0 obviously so that 75%of Hg0 was oxidized and released from the surface of fly ash samples and others adsorbed.The content of UBC in NBFA has a great influence on mercury removal.The higher amount of UBC,the stronger adsorption is,and the lower content is,the stronger catalytic oxidation is.Secondly,the effect of single component such as O2,SO2 and NO on mercury capture and adsorption under simulated flue gas were investigated in a fixed-bed reactor.The modification method on several metallic oxidizations contained in fly ash were the same with fly ash.Then TPD was applied to identify the mercury species of adsorption products.The results showed that the effect of bromine on Fe2O3 and TiO2 in fly ash was mainly to increase the lattice oxygen content and promote the oxidization of Hg0,forming HgO,and O2 in the flue gas replenishes the consumption of lattice oxygen continuously.NO could react with adsorbed Hg0 to form Hg2(NO3)2 on the surface of fly ash,then some would decompose to Hg(NO3)2 and HgO.The amount of UBC in the fly ash affected the amount of active sites,which had a great impact on mercury removal.Lower content of UBC in the flyash,higher percent of Hg2+ at the outlet of fixed-bed reactor was detected.The adsorption and oxidation of Hg0 by UBC followed the Langmuir-Hinshelwood mechanism,while metallic oxidization containing bromine catalic oxidized Hg0 mainly following the Mars-Masessen mechanism.Therefore,the process of adsorption and oxidation of Hg0 by NBFA is accompanied with two kinds of synergies mechanisms.At last,the Hg0 adsorption properties of NH4Br-AC in the presence of N2,O2,SO2,NO and simulated flue gas were investigated.The FAC and NBAC were characterized by N2 adsorption/desorption,Fourier transform infrared spectroscopy(FTIR),and scanning electron microscopy/X-ray energy dispersive spectrometer(SEM-EDX),which provided the scientific basis for the experimental results analysis.Then TPD were carried out on pure mercury compounds,the mercury adsorption products of FAC and NH4Br-AC,respectively,to study the mercury species on the surface of the samples.The experimental results showed that the Hg-OM(Hg Organic Matter)was formed by the reaction between Hg0 and organic matter which existed on the surface of AC.Though some Hg0 could be oxidized by the functional group.to form HgO,the efficiency is very low and the penetration rate was up to 94%.The embedding of bromine atom increased the activity of its neighboring site which in turn increases the adsorption capacity of Hg0,which explained why the modification promoted the mercury removal perfectly.TPD results indicated that large amount of HgBr2 was formed on the surface of NBAC.The single flue gas component O2 and NO promoted the adsorption of mercury because of Hg2(NO3)2 formed on the surface of NBAC.SO2 competed with Hg0 on the NBAC ester group,which inhibited Hg0 adsorption.Additionally,the mechanism of Hg0 removal by NBAC was similar to the UBC contained in NBFA,which was mainly affected by NO in the flue gas and followed Langmuir-Hinshelwood mechanism.更多还原