【作者】 张安超；

【导师】 向军； 孙路石；

【作者基本信息】 华中科技大学 ， 热能工程， 2010， 博士

【摘要】 未来的几十年煤炭仍将是我国能源消费的主体,由煤炭燃烧而产生的环境污染问题也必将存在。燃煤电站是人为汞排放污染的主要来源,已对生态环境和人类健康造成了极大危害。目前,中国已经成为亚洲乃至全球人为汞排放最多的国家之一。美国、加拿大等西方发达国家已将燃煤电站汞排放控制列入日程,而我国在这方面的研究与电厂试验还比较薄弱。因此,加快开展优良吸附剂的研制和实验工作是非常必要的,其意义重大而深远。本文详细地介绍了国内外燃煤电站汞排放减少的方法,着重叙述了近年来广泛研究的固体吸附剂及其脱汞性能；系统综述了壳聚糖(CTS)的改性方法及其在重金属脱除领域的应用；深入了解了量子化学理论在单质汞(Hg0)的氧化与吸附和壳聚糖／贵金属吸附剂吸附重金属方面的研究；分析了改性壳聚糖吸附剂脱除燃煤烟气中Hg0的可行性。论文制备了五种改性壳聚糖吸附剂(铜模板改性壳聚糖吸附剂、硅基改性壳聚糖吸附剂、碘(溴)和酸改性壳聚糖吸附剂、碘和酸改性膨润土／壳聚糖吸附剂和银改性膨润土／壳聚糖吸附剂),采用N2吸附-脱附、傅里叶变换红外(FTIR)、热重分析(TGA)、X-射线衍射(XRD)、X-射线荧光探针(XRF)和扫描电镜(SEM)等方法对吸附剂进行表征。结果表明：改性后,壳聚糖、硅基改性壳聚糖和膨润土／壳聚糖的比表面积均发生不同程度的减小。FTIR分析显示铜模板吸附剂改性中N原子参与了Cu2+的配位；壳聚糖中的氨基与硫酸和碘发生了化学反应,形成了具有磺酸根和碘两种活性位的良好吸附剂。TGA表明经硫酸和碘化钾改性后,吸附剂的热稳定性发生降低；而负载膨润土后,吸附剂的热稳定性大大提高。XRD分析发现经盐酸和硫酸溶液浸渍后,其结晶度降低；铜模板改性壳聚糖吸附剂、碘和酸改性壳聚糖吸附剂的晶相也发生不同程度的变化,这说明改性破坏了壳聚糖分子内的氢键,进而影响了其晶体结构；在碘和硫酸改性的吸附剂中,发现了单质碘(I2)的存在,进一步说明碘化钾、硫酸和壳聚糖之间发生了化学反应；XPS结果表明碘和硫酸改性的壳聚糖吸附剂中,N原子提供电子,S原子则接受电子。XRF表明硅烷化有利于硅基改性壳聚糖吸附剂引入更多的含硫含氯官能团。采用VM3000在线测汞仪,在固定床实验台架上进行了N2氛围下吸附剂的筛选实验。结果表明：未经任何处理的壳聚糖原样对Hg0仅有较弱的物理吸附；铜模板CTS吸附剂因空缺Cu2+而具有“记忆功能”,增加了该类吸附剂的脱汞能力。对于硅基改性CTS吸附剂,氧气的加入使其脱汞效率有明显提高。随着硫酸含量的增加,碘和硫酸改性壳聚糖吸附剂的脱汞效率逐渐提高,但硫酸的含量并不是越多越好；不论是矿物吸附剂还是壳聚糖吸附剂,碘改性吸附剂的脱汞效果均比溴改性吸附剂的脱汞效果好。银改性膨润土／壳聚糖吸附剂在较高温度下的脱汞效果较差,然而该吸附剂在较低温度下的脱汞效果良好。模拟烟气下的脱汞实验研究表明,SO2对吸附剂的脱汞有稍许抑制作用；而HCl对吸附剂的脱汞有稍许促进作用；H2O和NO对碘和酸改性膨润土／壳聚糖与银改性膨润土／壳聚糖吸附剂均有促进作用,但这两种气体对银改性吸附剂的促进作用更大。根据碘和酸改性CTS吸附剂的特征,采用密度泛函理论(DFT)系统地开展了几种可能活性位吸附Hg0的量子化学研究。计算发现,CTS的1#、2#和3#位对Hg0的吸附能力非常弱；CTS的氨基对H+和HI的吸附能力较强,其吸附能分别约为991和74kJ/mol;而CTS对I2仅有较弱的物理吸附；以上改性复合物对Hg0均没有吸附能力。壳聚糖对H+和I2同时吸附的能力非常强,其吸附能约为1020 kJ/mol;吸附剂CTS-H+-I2的最高占据分子轨道(HOMO)主要由I1的p轨道组成；吸附Hg0后,Hg0的d轨道对整体的HOMO有较大贡献。CTS-H+-I2对Hg0具有较强的吸附能力,其吸附能约为127kJ/mol。单原子贵金属(Ag、Au、Pd和Pt)在壳聚糖三个活性位的吸附及其复合后吸附Hg0的量子化学研究表明,Ag和Au在CTS 2#位的吸附能最大,Pd在CTS 3#位的吸附能最大,而Pt在CTS 1#位的吸附能最大。除Pt外,各吸附位对同一种贵金属的吸附能力相差不多；同一贵金属改性的CTS吸附剂在不同吸附位之间对Hg0的吸附能相差亦不多。壳聚糖对贵金属及贵金属改性壳聚糖对Hg0吸附能的大小顺序均为：Pt>Pd > Au> Ag。贵金属负载沸石M-T3对Hg0吸附能的大小顺序为：Au-T3> Cu-T3>Ag-T3；对于单吸附质的吸附系统,吸附能、贵金属与吸附质的键长和Mulliken电荷之间有明显的线性关系。NO的出现会促进Hg0的吸附,而SO2则会抑制其吸附,这和实验得到的结论是一致的。更多还原

【Abstract】 Most of fuel consumed in China will be coal for several decades in future. The serious environmental pollution caused by coal combustion will be inevitable. As the largest source of anthropogenic mercury emissions, mercury emitted from coal-fired power plants has been identified as a hazardous pollutant to both human health and environment. At present, China is one of the largest source regions that release mercury into the atmosphere. Some measures and rules have been taken to control the mercury emissions from coal-fired power plants in some western developed countries, such as American, and Canada. However, the sifting of effective sorbents and researches of in situ full-scale demonstration for mercury control in coal-fired power plants are still weak in China. Therefore, it is necessary and important to find out the effective sorbents on mercury capture and to understand the mechanism of mercury removal.In this paper, several measures of mercury removal from coal-fired power plant were presented in detail. The solid sorbents researched extensively and their performances on mercury removal were emphatically introduced. The method of chitosan (CTS) modification and their applications on heavy metals removal were systematically summarized. The quantum chemistry theory studies on oxidation/adsorption of elemental mercury (Hg0) and the mechanism of CTS sorbents for heavy metals adsorption were deeply discussed. The possibility of modified chitosan sorbent on Hg0 capture from flue gases was analyzed.Five types of modified chitosan sorbents were synthesized using several advantages of chitosan. There are copper-templated CTS sorbent, silicon-modified CTS sorbent, iodine (or bromide) and acid modified CTS sorbent, iodine and acid modified bentonite (B)/CTS sorbent, and silver-modified B/CTS sorbent. The characterization of these sorbents was analyzed using N2 adsorption and desorption method, Fourier transform infra-red spectroscopy (FTIR), X-ray diffraction (XRD), Thermogravimetric analysis (TGA), X-ray fluorescence (XRF), and Scanning electron microscope (SEM), et al. The results show that the surface areas of CTS, silicon-modified CTS, and B/CTS sorbent decrease after modification. The FTIR spectra demonstrate that the reaction of amide of CTS and Cu2+ occurs in the process of copper-templated CTS sorbent preparation. And the chemical reaction of iodine and sulfate ion with the amide of CTS occurs. TGA indicates that the thermal stability of CTS decreases after modifying by H2SO4, and potassium iodide, while it can be increased greatly when bentonite supported on the chitosan. XRD spectra show that the area under amorphous decreases after modification. This is caused by the fact that inter- and extra-molecular hydrogen bonds are destroyed due to the presence of H2SO4. Furthermore, the peak of I2 is observed in the iodine and H2SO4 modified CTS sorbent, which implies the occurrence of the interaction between H2SO4, KI and chitosan. XPS analysis reveals that nitrogen atom provides electron pair, and sulfur atom inclines to accept the electron. More active sites, such as S and Cl, can be easily obtained after silanization in silicon-modified CTS sorbent.Adsorption experiments of vapor-phase elemental mercury (Hg0) were studied using these sorbents in a laboratory-scale fixed-bed reactor with nitrogen as gas sources. VM3000 online mercury analyzer was applied to detect the inlet and outlet Hg0 concentrations. The results show that the parent CTS has no effect on Hg0 removal. The copper-templated CTS sorbents, which have memory function for Cu2+ vacancy, can adsorb the metal ion or vapor-phase metal which has similar radius as copper ion. These changes can improve the Hg0 and Hg2+ capture efficiency greatly. For the silicon-modified CTS sorbent, mercury removal efficiency improves with the presence of O2. For the iodine or bromide-modified CTS sorbents, mercury removal efficiency of CTS sorbents could be significantly promoted when appropriate amount of H2SO4 was added. Generally, the iodine and acid modified sorbents demonstrate higher mercury capture efficiency than that of bromine and acid modified sorbents. Silver-modified B/CTS sorbents exhibit somewhat poor mercury removal efficiency at higher reaction temperature, while they show higher mercury capture performance at room temperature. The results obtained under simulated flue gases show that presence of SO2 has negative effect on Hg0 removal. Somehow positive effect occurs when HC1 is added. For iodine and acid modified B/CTS and silver-modified B/CTS sorbents, both H2O and NO can enhance the adsorption of Hg0. Compared to iodine and acid modified B/CTS sorbents, silver-modified B/CTS sorbents show a more significant change caused by H2O and NO.According to the possible adsorption sites of iodine and acid modified CTS sorbent, the adsorption of Hg0 on modified CTS sorbent was investigated systematically by Density Functional Theory (DFT). The results show that the adsorption abilities in the sites 1#,2#, and 3# of parent CTS for Hg0 are very small. It is found that the amide of CTS exhibits higher adsorption abilities for H+ and HI, and their adsorption energies are 991 and 74kJ/mol, respectively. While for I2, the physical adsorption phenomenon is found. These sorbents modified by H+, HI, or I2 all have little effect on Hg0 removal. CTS reveals an excellent adsorption performance for H+ and I2 simultaneously and the adsorption energy is 1020 kJ/mol. The main contribution to the highest occupied molecular orbital (HOMO) of CTS-H+-I2 complex is from the p orbital of I atom near the amide of CTS. There is a significant contribution from the d orbital of Hg0 atom in CTS-H+-I2-Hg complex after Hg0 adsorption. The complex of CTS-H+-I2 displays a big binding energy of 127kJ/mol for Hg0 adsorption.Furthermore, theoretical explorations of single noble metal (Ag, Au, Pd, and Pt) on the three sites of CTS, respectively, and Hg0 adsorption on the single noble metal-CTS complexes were also conducted by DFT method. The results show that among the three sites of CTS, the site 2# of CTS exhibits the biggest adsorption energies for Ag and Au atoms, respectively. While for Pt and Pd atoms, the sites are 1# and 3#, respectively. It has been found that, in each site, the adsorption energies of noble metal on CTS are almost the same except Pt atom. Similarly, the adsorption energies of Hg0 on noble metal-CTS complexes are also the same. It has been found that, in each site, the adsorption energies of Hg0 on noble metal-CTS complexes occur in the following order:Pt> Pd> Au> Ag, which is also consistent with order of the adsorption energies of noble metal on CTS. It has been found that the adsorption energies of Hg0 on three M-T3 clusters occur in the following order:Au-T3> Cu-T3> Ag-T3. For the individual adsorption system, a clear correlation between adsorption energy and M-Hg distance (and Mulliken charges) is found. Hg0 adsorption energy is substantially increased upon NO being preadsorbed on M-T3 surface, however, a significant weakening of the Hg0 bonding to the sites takes place in the vicinity of a bound SO2 molecule. These phenomena are in agreement with the experimental results.更多还原