摘要
砷是有毒的类金属元素，在煤炭的利用过程中，其排放造成的环境污染，对人体健康及生态带来了危害。煤作为我国重要的一次能源，为电力、冶金等领域提供了动力，但它的燃烧却成为砷排放的主要来源之一。在煤电行业，砷的控制技术还不成熟，投入实践任重而道远。根据国内外研究近况，燃煤锅炉中砷的脱除方法可以分为燃烧前脱砷、燃烧中脱砷和燃烧后脱砷三个方面。虽然提高原煤的入洗率、优化燃烧技术可以在一定程度上减少砷的排放，但是并不能满足未来日益严格的重金属砷排放控制标准，需要采取燃烧后的吸附技术进一步减少砷的污染。现阶段针对砷的吸附剂设计方式主要是通过实验反复测验，该方式需要消耗大量的时间和成本，本文通过研究砷在分子筛中的吸附规律，为主动设计吸附剂提供指导。 砷在煤的燃烧过程中的形态各异，其理化性质存在差异。掌握砷在燃煤电站不同阶段的形态及能量，有利于提出合理有效的砷控制技术。燃煤电站的烟气在锅炉燃烧区、水平烟道区、脱硝装置和排出烟囱后的环境温度，分别取1200℃、800℃、370℃和25℃。本文针对以上温度，利用密度泛函理论，通过吉布斯自由能判定，得到了砷在上述温度下的存在形态及能量，发现砷在高温条件下多以As3+存在，而随着温度降低至25℃，多以As5+存在；砷的形态随温度的变化存在差异，在高温条件下，砷多以稳定的脱水化合物（AsOOH、As4O6和As2O3）存在，这些脱水化合物易与烟气中的H2O、O2反应；当温度降低至370℃，砷多以As4O6存在；当温度降低至25℃，稳定存在的砷酸成为砷的主要形态。
分子筛对砷的吸附规律可以为砷污染控制技术的应用提供指导。基于As4O6是脱硝装置中的主要存在形态，本文结合蒙特卡洛方法和密度泛函理论研究了As4O6分子在不同孔径的分子筛中吸附能的变化规律，发现其在孔径为7Å的分子筛中吸附最优；同时，理论计算发现分子筛对As4O6分子的吸附量随自身有效孔隙率的增大而增大。基于上述研究，本文理论分析了As4O6分子在不同Si/Al的H-ZSM-5分子筛中的吸附情况，同时结合实验发现As4O6在H-ZSM-5中的吸附量随Si/Al比的减小而增大；为了进一步提高H-ZSM-5分子筛对As4O6的吸附能力，本研究通过掺杂金属氧化物对Si/Al=12.5的分子筛改性，实验发现添加Cu以后可以有效地提高分子筛对砷的吸附能力；此外，利用蒙特卡洛模拟研究了烟气中SO2，NO，H2O，CO2和N2对H-ZSM-5吸附剂脱砷的影响，发现当烟气中存在SO2和NO时，会优先占据吸附位，降低砷的吸附量。
关键词：砷；吸附；分子筛；密度泛函理论；蒙特卡洛方法
ABSTRACT
Arsenic is a toxic semi-metal element.During utilization of coal,the emission of arsenic has contributed significantly to the severe pollution for environment and human health.Coal has maintained to be the major primary energy in our country.It has provided energy for power,metallurgy,etc.But combustion of extensive coal has caused the emission of arsenic.At presen
t,there is no mature technology for arsenic pollution control of industrial coal-fired boiler.According to the current research progress and present problems,arsenic pollution control of industrial coal-fired boiler may be divided into three categories.They are pre-combustion arsenic capture, during-combustion arsenic capture,post-combustion arsenic capture,respectively. Though improving rinse effect of coal and optimizing boiler combustion will reduce the emission of arsenic to some extent,yet it is far not enough to meet the increasingly stringent emission requirements.Therefore,the further developing plan of arsenic pollution control of industrial coal-fired boiler is expected,we must take measures to reduce pollution.The technique of adsorption as a simple and viable technique to remove arsenic is becoming more and more promising.Zeolites are widely used as sorbents in the field of adsorption for air pollutant.Their well-defined pore structure and excellent properties make them highly active in a large variety of the adsorption process. Nowadays,the design of sorbent is based on the trial-and-error experiments.There is no doubt that we must take measures to rationally design sorbent with advanced method.In this study,theoretical Grand Canonical Monte Carlo method and Density Functional Theory calculations are applied to predict microscopic behavior of the zeolite and arsenic.
The existing forms of arsenic and its chemical structures in different stages of the boiler were calcula
明基p50
ted by DFT.The different forms of arsenic have differently physical and chemical characteristics.The reasonable controlling technology of arsenic must build on a rational knowledge of the arsenic.Gibbs energy minimum principle was applied to determine the stationary structure of arsenic at four typical temperatures.The temperature is various in different stages of the boiler.The temperature at1200°C is the typical temperature in the combustion area of the coal-fired boiler.The temperature at 800°C is the temperature of flue gas in the horizontal flue.The temperature at370°C is the temperature of flue gas in the NOx removal reactor.The room temperature was
studied as well to obtain the stationary structure in air after being emitted from the stack. The results show that trivalent arsenic molecules are thermodynamically stable at high temperatures and pentavalent species are stable at low temperatures.The arsenic species vary with the temperature.At high temperatures,dehydrated compounds are the major species.These compounds will be hydrated and oxidized by O2when the temperature declines,as implied by the reaction path study.In the area of the NOx remove reactor, trivalent As4O6is the major species and pentavalent arsenic acid will be produced. Arsenic acid becomes the most thermodynamically stable species at25°C.
Acquiring regularity of absorption for arsenic in zeolite can offer theoretic basis and the practice guide for arsenic pollution control of industrial coal-fired boiler.Based on the discussion above,in the
area of the NOx removal reactor,trivalent As4O6is the major species.In this part,GCMC method and DFT were used to study the adsorption of arsenic in zeolites with various Si/Al ratios and pore sizes.The results show that pores with diameter of7Åare optimal for As4O6adsorption in zeolites.High porosity can also promote the adsorption of As4O6in a specific amount of zeolite.Al substitutions are beneficial for As4O6adsorption.Experiments were used to test the adsorption of As4O6using H-ZSM-5zeolites with different Si/Al ratios.The comparison between experimental and theoretical results has led to an excellent match.In order to further improve the ability of zeolites for As4O6adsorption,metal oxide is doped into H-ZSM-5(Si/Al=12.5)zeolite.Test results showed that zeolite doped by Cu demonstrated excellent property for the adsorption of As4O6.The influences of possible compositions in flue gas including SO2,NO,H2O,CO2and N2were also tested using GCMC method.SO2and NO can cause notable drop of As4O6adsorption.
Keywords:arsenic;adsorption;zeolite;Density Functional Theory;Grand Canonical Monte Carlo method
目录
中文摘要............................................................................................................................................I 英
文摘要...........................................................................................................................................II 符号说明.........................................................................................................................................VI 1绪论.. (1)
1.1研究背景 (1)
1.2砷控制技术及现状 (3)
1.3脱砷吸附剂研究现状 (5)
1.3.1飞灰 (6)
1.3.2活性炭 (6)
1.3.3金属氧化物 (7)
1.3.4分子筛 (7)
1.4本文研究目的、内容及意义 (8)
2实验系统及模拟方法 (11)
2.1吸附剂制备 (11)
2.1.1吸附剂的制备方法 (11)
2.1.2化学试剂与仪器 (13)
2.2砷吸附剂性能测试系统 (13)
灯影组2.3吸附剂表征手段 (16)
2.3.1扫描电子显微镜（SEM） (16)
2.3.2X射线晶体衍射（XRD） (16)
2.3.3吸附剂比表面积测试（BET） (17)
2.3.4高分辨透射电子显微镜（HR-TEM） (17)
2.4理论计算方法 (17)
2.4.1吸附理论 (17)
2.4.2计算方法 (19)混凝土仿钢纤维
3砷在烟气中的形态的密度泛函分析 (22)
3.1引言 (22)
3.2计算方法 (22)
3.3烟气中不同阶段的砷的存在形态及能量 (23)
3.4含水条件下的砷的转化 (27)
3.5含氧条件下的砷的转化 (29)
3.6本章小结 (31)
4分子筛吸附砷规律研究 (32)
4.1引言 (32)
4.2计算方法及实验 (32)
赫伯特西蒙4.3物理结构对分子筛吸附砷的影响 (34)
49未知天命4.4硅铝比对分子筛吸附砷的影响 (37)
4.5掺杂金属对分子筛吸附砷的影响 (39)
4.6烟气组分对分子筛吸附砷的影响 (44)
4.7本章小结 (46)
5全文工作总结及展望 (48)
5.1全文总结 (48)
5.2主要创新 (48)
5.3未来工作展望 (49)
致谢 (50)
参考文献 (51)
品牌诉求
附录 (57)
作者在攻读硕士期间发表的论文目录 (57)
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