【作者】 李攀；

【导师】 刘振宇； 刘清雅；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2012， 博士

【摘要】 煤炭是我国最丰富的化石能源，我国以煤炭为主要一次能源的格局在未来相当长时期内不会发生改变。煤炭在我国的主要利用方式是直接燃烧，用量占煤炭总消耗量的80%以上。煤炭的大量燃烧已经导致我国的很多地区呈现严重的煤烟型大气污染，NOx（氮氧化物，简称硝）是主要污染物之一，其对人体健康和生态环境均构成了巨大的威胁。烟气脱硝的技术很多，但以NH₃为还原剂的选择性催化还原（SCR）法在世界上应用最多、研究最广、技术最成熟且最有成效。针对目前以V₂O₅/TiO₂催化剂为代表的中温脱硝技术（～400℃）所存在的问题，前人研究开发了以活性焦为载体的V₂O₅/AC催化剂，发现其具有较高的低温（～200℃）脱硝活性和抗SO₂毒化性能，应用前景广阔，研究范畴涉及制备条件、反应行为、SO₂和H₂O的影响、反应机理和动力学等。然而到目前为止，人们在活性焦性质对中毒物硫铵盐稳定性的影响、SO₂影响催化剂脱硝的机理、SCR反应选择性、催化剂的热稳定性及挥发性重金属Hg对催化剂脱硝行为的影响等方面还缺乏认识。本论文针对上述问题进行了深入研究，得到以下主要结论：（1）脱硝过程中产生的中毒物NH₄HSO₄首先沉积在AC表面的强吸附位上，随着其量的增加再逐渐沉积在弱吸附位上。在惰性气氛中，NH₄HSO₄在AC表面的程序升温分解过程为两步反应机理：从170℃起生成NH₃和H₂SO₄，后者在较高的温度下被AC还原为SO₂气体。NO的存在和V₂O₅的担载不影响该两步反应机理。（2）V₂O₅/AC催化剂的低温抗SO₂毒化能力源于上述AC对H₂SO₄较强的低温还原能力，从而恢复其孔结构。在本文的实验条件范围内，AC的比表面积和孔体积对H₂SO₄的还原行为没有影响；AC中的含N和含O官能团有利于H₂SO₄的还原；AC中的某些矿物质会和H₂SO₄（或NH₄HSO₄）反应生成金属硫酸盐；V₂O₅的存在和气氛中的NO不利于H₂SO₄的还原。（3）温度低于175℃时，NH₄HSO₄抑制AC的脱硝活性，且抑制作用随NH₄HSO₄量增加而加剧；高于175℃时，NH₄HSO₄促进AC的脱硝活性，且促进作用随NH₄HSO₄量增加而增大。在本文实验条件范围内，AC的比表面积和孔体积对其脱硝活性几乎没有影响，NH₄HSO₄对AC脱硝活性的影响与其比表面积和孔体积大小无关；NH₄HSO₄对化学性质不同的AC的脱硝活性影响略有不同。（4）SO₂对催化剂脱硝的影响具有多重作用：一方面，SO₂与O₂+H₂O+NH₃原位生成的硫铵盐对脱硝具有促进作用，另一方面，硫酸（或硫铵盐）堵孔、SO₂与NH₃竞争V₂O₅表面活性位，两者对脱硝具有抑制作用。在本文的实验条件下，SO₂与NH₃的竞争吸附所表现的抑制作用显著于硫酸（或硫铵盐）对脱硝的作用，因此总体表现为SO₂抑制脱硝。（5）在150-250℃之间，V₂O₅/AC脱硝生成的副产物N₂O很少，浓度在10ppm以下。N₂O源于AC和NH₃共同还原NO的反应。V₂O₅的担载量对N₂O的生成没有显著影响，SO₂和H₂O略微促进了N₂O的生成。V₂O₅的担载量对SCR反应的选择性（即对主产物N₂的选择性）有显著促进作用，0.91wt%的V₂O₅就足以使N₂的选择性保持在95%以上。（6）V₂O₅担载量和反应温度均促进AC的氧化。SO₂也促进AC的氧化，尤其是在SO₂通入反应器初期。该促进作用源于SO₂和O₂及H₂O在催化剂表面生成的硫酸与AC中的含碳氧官能团发生的反应。AC表面的某些含碳氧官能团不能与氧气发生反应，却能被硫酸氧化。（7）无论V₂O₅/AC表面是否吸附有汞，升高反应温度均促进其脱硝活性。汞对催化剂的脱硝有抑制作用，该作用随汞含量和反应温度升高而加剧。H₂O和SO₂加剧了汞对V₂O₅/AC脱硝的毒化作用。汞吸附对NH₃在催化剂表面的吸附和氧化行为没有影响，其对脱硝的毒化作用可能源于抑制了吸附的NH₃和NO之间的反应。更多还原

【Abstract】 Coal is the richest fossil energy in China. The dominant role of coal inthe structure of primary energy in China will do not change for a long time inthe future. Combustion is the traditional mode of coal utilization in China,which takes up more than80%of the total consumption. Combustion of suchgreat amount of coal has already led to serious coal-burning air pollution inour country. NOx is one of the main pollutants in flue gases, which hasbrought tremendous threat on human health and ecological environment.Among various technologies developed, selective catalytic reduction（SCR） of NOXwith NH₃is recognized to be the most widespread and effectivetechnology for NOXabatement from flue gases of stationary sources. Aimingat the problems of V₂O₅/TiO₂based mid-temperature catalysts （usable ataround400oC）, activated coke supported V₂O₅（V₂O₅/AC） catalyst wasinvented, which has a high low-temperature （～200oC） SCR catalytic activityand resistance to SO₂poisoning. Thus, this catalyst was studied extensively inthe past decade including preparation techniques, reaction behaviors, effects ofSO₂and H₂O, reaction mechanism and kinetics etc. So far, however, effect ofAC’s properties on stability of ammonium sulfates, mechanism of SO₂ influence on SCR behavior, SCR selectivity to N₂, thermal stability of catalyst,and effect of Hg on SCR activity are not well understood, which are studiedsystematacially in this paper.The major conclusions obtained are as follows:（1） NH₄HSO₄tends to anchor at strong adsorption sites of AC initially atlower loadings and then progressively at weak adsorption sites with theincrease of NH₄HSO₄. Decomposition of NH₄HSO₄on AC follows two-stepreaction mechanism: formation of NH₃and H₂SO₄（leading to release of NH₃）starting at about170oC, and reduction of H₂SO₄（or SO₂-4） by AC to form SO₂at higher temperatures. The presence of NO and V₂O₅do not affect thistwo-step reaction mechanism.（2） The reduction of H₂SO₄to SO₂by AC support is responsible for theV₂O₅/AC catalyst’s low-temperature resistance to SO₂poisoning since it freesthe pores of AC. In the experimental conditions, the BET surface area andpore volume of AC do not affect the reduction behavior of H₂SO₄, regardlessof the presence of NO. The oxygen and nitrogen containing functional groupsin AC benefit the reduction of H₂SO₄, while the minerals in AC areunfavorable to the reduction of H₂SO₄. V₂O₅loaded on AC and NO in the gasboth inhibit reduction of H₂SO₄to SO₂.（3） NH₄HSO₄inhibits the SCR activity of AC below175oC, and theinhibiting effect aggravates with the increase of NH₄HSO₄; NH₄HSO₄promotes the SCR activity above175oC, and the promoting effect increases with the increase of NH₄HSO₄. In the experimental conditions, the BETsurface area and pore volume of AC do not affect its SCR activity, and alsohave no relationship with the effect of NH₄HSO₄on SCR activity.（4） The effect of on-line gaseous SO₂on SCR activity of V₂O₅/ACcatalyst is different from that of pre-loaded NH₄HSO₄. SO₂has multi-role onSCR activity of V₂O₅/AC: on one hand, the sulfate species formed by SO₂andO₂+H₂O+NH₃can promote the catalytic activity; on the other hand, blockingthe pores of catalyst by sulfate species and competitive adsorption of SO₂withNH₃on V₂O₅surface both inhibit the catalytic activity. In the experimentalconditions, the inhibiting effect by competitive adsorption of SO₂with NH₃isobviously larger than the effect by sulfate species, thus SO₂inhibits SCRactivity of V₂O₅/AC.（5） The N₂O formation over V₂O₅/AC catalyst during SCR of NO by NH₃is generally low, less than10ppm in the temperature range of150-250oC.N₂O is formed from reduction of NO by both AC and NH₃. V₂O₅contributeslittle to the N₂O formation while SO₂+H₂O slightly accelerate N₂O formation.SCR selectivity to N₂is obviously promoted by V₂O₅and0.91wt%V₂O₅issufficient to yield a N₂selectivity of higher than95%.（6） Oxidation of the AC support to CO₂occurs in the SCR process and itsrate increases with an increase in V₂O₅loading and reaction temperature. SO₂in the gas promotes AC oxidation to CO₂due to formation of sulfuric acid onthe surface, which reacts with the carbon-and-oxygen containing functional groups on the AC. Some of the carbon-and-oxygen containing functionalgroups do not react with O₂under the conditions used, but can be oxidized bysulfuric acid.（7） SCR activity of V₂O₅/AC catalyst increases with the increasingtemperature, regardless of the adsorption of Hg on its surface. Hg adsorptioninhibits the catalytic activity, and the inhibiting effect aggravates with theincreasing Hg adsorption quantity and reaction temperature. The presence ofH₂O+SO₂strengthens the inhibiting effect of Hg on V₂O₅/AC. The adsorptionand oxidation behaviors of NH₃over V₂O₅/AC are not influenced by Hgadsorption.更多还原