氯化镁调质烟气促进汞催化氧化的试验与机理研究

发布时间：2017-12-28 14:27

本文关键词：氯化镁调质烟气促进汞催化氧化的试验与机理研究　出处：《山东大学》2015年硕士论文　论文类型：学位论文

【摘要】：汞具有剧毒性、高挥发性和生物富集性,是一种全球性污染元素。燃煤汞排放被认为是大气中汞污染的主要来源,对环境造成的污染问题越来越受到世界各国的广泛关注。烟气中的汞主要有3种形态：单质汞(Hg0)、氧化态汞(Hg2+)和颗粒态汞(Hgp),Hg2+易溶于水,绝大部分可通过湿法脱硫设备得到脱除,Hgp易于被除尘设备所捕获,单质汞由于具有极大的化学稳定性和较差的水溶性而不易被现有污染物控制设备所脱除。利用SCR催化剂对汞的催化氧化作用,结合湿法脱硫设备联合脱汞投资及运行成本低、适合现有电厂的改造,是一种经济可行且极具竞争力的燃煤电厂汞污染控制方法。利用常规污染物控制设备联合脱汞的关键在于提高SCR催化剂对单质汞的氧化效率。由于SCR催化剂对单质汞的氧化效率受烟气中氯含量的制约,煤中氯含量高,烟气中HCl含量就越高,SCR催化剂对汞的氧化效率越高,因而对于低氯煤,SCR催化剂对汞的氧化受到限制。脱硫废水中富含氯化镁,氯化镁在高温下可分解产生氯化氢(HCl)气体,本文提出将脱硫废水中的主要成分氯化镁喷射在SCR脱硝装置的前置烟道中,利用高温烟气促进氯化镁分解,产生氯化氢,从而促进SCR催化剂对单质汞的氧化。首先,研究了商业钒钛基SCR催化剂对单质汞的氧化性能,进而对SCR催化剂氧化单质汞的机理进行了探讨。试验结果表明,HCl可以明显提高SCR催化剂对单质汞的氧化作用,汞的氧化效率随着温度的升高而升高,350℃时氧化效率可达到80%左右,继续升高温度,氧化效率变化不大,可见催化剂对脱汞和脱硝的温度窗口相一致。O2可以显著提高SCR催化剂对单质汞的吸附能力,HCl和Hg0都可以吸附在催化剂表面,但HCl的吸附弱于单质汞的吸附,SCR催化剂催化Hg0氧化的反应机理主要是Eley-Rideal机理,即催化剂表面吸附态的Hg与气相中的HCl或弱吸附态的HCl反应,实现汞的氧化。其次,在固定床试验台上研究了商业SCR催化剂添加氯化镁对单质汞形态转化的影响,主要考察氯化镁添加量、温度以及气氛对汞形态转化的影响。试验结果表明,氯化镁在高温下分解产生HCl和活性氯,可以促进单质汞的均相氧化。用氯化镁调质SCR催化剂可以大幅度提高其对单质汞的氧化效率,在350℃,基本烟气条件下,氯化镁添加量从0wt%到1wt%范围内,汞的氧化效率随添加量增加从22.3%迅速提高至83.4%。温度升高有利于单质汞向氧化态汞转化,其中在350℃达到峰值,是汞催化氧化的最佳温度。O2和NO可以促进添加氯化镁后催化剂对单质汞的氧化,SO2和H2O抑制单质汞的氧化,添加氯化镁后对催化剂的脱硝效率没有明显的影响。机理试验表明氯化镁分解产生HCl和活性氯,在催化剂的作用下可以达到对汞的高效氧化。最后,量子化学密度泛函理论是计算晶体结构和表面反应机理的精确方法,本文利用密度泛函理论结合周期性平板模型研究氯化镁产生的活性氯促进SCR催化剂氧化单质汞的机理。模拟结果表明,Cl在V2O5(001)表面为化学吸附,Cl与V2O5表面的O有强烈的成键作用,对V2O5起到改性的作用,增加了催化剂表面的活性位。Hg0在吸附Cl后的V2O5(001)表面可以稳定地生成HgCl,一部分HgCl吸附在催化剂表面,一部分HgCl从催化剂表面脱附,两种情况都有利于单质汞的氧化。
[Abstract]:Mercury is a global pollution element, which is highly toxic, highly volatile and bioenriched. Mercury emission from coal burning is considered to be the main source of mercury pollution in the atmosphere, and the pollution problem caused by the environment has attracted more and more attention from all over the world. Mercury in flue gas mainly has 3 forms: elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate mercury (Hgp), Hg2+ soluble in water, most can be removed by wet desulfurization equipment, Hgp can easily be captured by the elemental mercury removal equipment, because of its great chemical stability and the poor water solubility and not easy to be the removal of existing pollutant control equipment. Using SCR catalyst to catalyze the oxidation of mercury, combined with the combined mercury removal from wet desulphurization equipment and low cost of operation, is suitable for the transformation of existing power plants. It is an economically viable and competitive way of mercury pollution control in coal-fired power plants. The key of combined dehhl removal by conventional pollutant control equipment is to improve the oxidation efficiency of SCR catalyst for Hg. Because the oxidation efficiency of SCR catalyst to elemental mercury is restricted by chlorine content in flue gas, the content of chlorine in coal is high, the content of HCl in flue gas is higher, and the oxidation efficiency of SCR catalyst to Hg is higher. Therefore, for SCR of low chlorine coal, the oxidation of Hg is limited. Rich in magnesium chloride desulfurization wastewater, magnesium chloride can be decomposed to produce hydrogen chloride at high temperature (HCl) gas, this paper put forward the main components of magnesium chloride injection in SCR desulfurization wastewater in flue pre denitrification device, promote the decomposition of magnesium chloride by high temperature flue gas, hydrogen chloride is produced, so as to promote the oxidation of elemental mercury catalyst SCR the. First, the oxidation performance of commercial vanadium and titanium based SCR catalyst for the oxidation of Hg was studied, and the mechanism of SCR catalyst for the oxidation of Hg was discussed. The test results show that HCl can significantly improve the oxidation effect of SCR catalyst on elemental mercury, mercury oxidation efficiency increases with the increase of temperature at 350 DEG C, the oxidation efficiency can reach about 80%, with increasing of temperature, the oxidation efficiency changed little, visible catalyst on the temperature window for mercury removal and denitrification are consistent. O2 can significantly improve the adsorption ability of SCR catalyst for the mercury, HCl and Hg0 can be adsorbed on the catalyst surface, but the adsorption of HCl on the weak adsorption of elemental mercury, the reaction mechanism of Hg0 catalytic oxidation of SCR catalyst is the main mechanism of Eley-Rideal, Hg and HCl reaction gas adsorption state phase HCl or weak adsorption state, realize the oxidation of mercury. Secondly, the influence of commercial SCR catalyst added with magnesium chloride on the transformation of elemental mercury was studied on a fixed bed test bench. The effects of magnesium chloride addition, temperature and atmosphere on mercury transformation were investigated. The results show that the decomposition of magnesium chloride at high temperature produces HCl and active chlorine, which can promote the homogeneous phase oxidation of the elemental mercury. The SCR catalyst with magnesium chloride quenched and tempered can significantly improve the oxidation efficiency of elemental mercury. At 350, the oxidation efficiency of mercury increased from 22.3% to 83.4% when the amount of magnesium chloride increased from 0wt% to 1wt%. The increase of temperature is beneficial to the conversion of the elemental mercury to the oxidized mercury, which reaches the peak at 350, which is the best temperature for the catalytic oxidation of mercury. O2 and NO can promote the oxidation of elemental mercury after adding magnesium chloride, and SO2 and H2O inhibit the oxidation of elemental mercury. The addition of magnesium chloride has no obvious effect on the denitration efficiency of the catalysts. The mechanism test shows that magnesium chloride is decomposed to produce HCl and active chlorine, and the high efficient oxidation of mercury can be achieved under the action of the catalyst. Finally, quantum chemical density functional theory is an accurate method to calculate crystal structure and surface reaction mechanism. In this paper, the theory of density functional theory combined with periodic slab model is used to study the active chlorine produced by magnesium chloride to promote the mechanism of SCR catalyst to oxidize elemental mercury. The simulation results show that Cl is chemisorbed on the surface of V2O5 (001), and Cl has strong bonding effect with O on the surface of V2O5, which plays a modified role in V2O5 and increases the active site on the catalyst surface. Hg0 can stably generate HgCl on the V2O5 (001) surface after Cl adsorption. A part of HgCl adsorbed on the surface of the catalyst. A part of HgCl is adsorbed on the catalyst surface. The two conditions are beneficial to the oxidation of elemental mercury.
【学位授予单位】：山东大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ132.2;X773

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