【作者】 何胜；

【导师】 倪明江； 周劲松； 程乐鸣；

【作者基本信息】 浙江大学 ， 工程热物理， 2009， 博士

【摘要】 燃煤电站烟气汞排放是环境中主要的汞污染源之一,已经得到越来越广泛的关注。在各种燃煤烟气汞排放控制技术中,利用现有污染物脱除装置实现汞排放控制是一种经济可行的方法。由于不同形态汞的物理化学特性不同,现有污染物脱除装置的汞脱除效率受汞形态分布的影响较大。因而如何尽可能的实现汞形态转化,把气态元素汞高效转化成氧化态汞,对于利用燃煤电站现有污染物脱除装置实现烟气汞排放控制是关键所在。本文依托国家自然科学基金、高等学校博士点基金和国家“863”计划项目,对燃煤烟气汞的催化氧化进行了系统的试验和机理研究。本文的研究结果为燃煤烟气汞的非均相汞形态转化及其反应机理的研究打下坚实基础,有助于发展适合我国国情的燃煤烟气汞排放控制技术。目前,燃煤电站烟气脱硝主要采用选择性催化还原(Selective Catalytic Reduction,SCR)技术。本文试验研究了钒系SCR催化剂和烟气组分对燃煤烟气中汞形态转化的影响。通过试验研究表明,SCR汞形态转化反应跟催化剂表面的活性中心位置密切相关,活性组分V205负载量的增大在催化剂表面形成更多的V活性中心位,促进汞形态的转化；在试验条件下,SCR催化剂活性随温度升高而增强；烟气中的HCl提供汞形态转化反应所必需的活性Cl原子,起最重要的促进作用；SO2对反应影响不大；随着烟气中H20的加入,汞形态转化率有所降低；NH3与汞形态转化反应组分在催化剂表面发生竞争吸附,抑制了反应的发生；空速增大不利于汞的形态转化。显然,SCR汞形态转化反应是由催化剂、烟气组分(主要是HCl)和Hg三者共同参与的复杂非均相氧化反应。本文采用机理试验和表面分析相结合的方法研究其反应机理。在钒系SCR催化剂的汞吸附特性试验中发现,钒系SCR催化剂对Hg0具有化学吸附作用,吸附后形成Hg…O-V。催化剂负载的V205含量越大,吸附能力越强。O2使得催化剂表面的部分H-O-V物种转化成O=V物种,促进Hg0在催化剂表面的吸附作用；而NH3和Hg在V活性中心位形成竞争吸附,对Hg的吸附起抑制作用。SCR催化剂经过HCl吸附处理后,其对Hg0的吸附行为发生很大变化；同时发现反应生成Hg2+。试验发现,往Hg0饱和吸附的催化剂中通入HCl,导致Hg0重新从催化剂表面脱附下来。这表明两者在活性位上形成竞争吸附,相对于Hg0,HCl在活性中心位上的吸附为强吸附。催化剂表面分析进一步发现,HCl吸附到SCR催化剂表面跟V205形成钒氯中间产物,产生活性C1。SCR催化剂表面发生的汞形态转化反应可以采用Langmuir-Hinshelwood反应机理来解释,即HCl和Hg0首先分别吸附到活性中心位上,然后相邻的吸附态HCl和吸附态Hg0反应完成汞形态转化,分布在SCR催化剂表面的活性组分V2O5为反应提供所需的活性中心位。基于试验研究的结果,建立简化的SCR汞形态转化反应的Langmuir-Hinshelwood模型。利用最小二乘法进行动力学参数的估计,结果表明NH3吸附平衡常数最大,HCl次之,Hg0最小。根据得到的动力学参数进行不同条件下汞形态转化过程的模拟计算,研究分析HCl浓度、氨氮比、初始NO浓度、HCl、Hgo和NH3的吸附平衡常数等对汞形态转化反应的影响。结果表明,喷氨与不喷氨条件下汞形态转化反应速率变化趋势有所区别。喷氨时,SCR催化剂可分为两个区域,入口区域主要发生脱硝反应,后面区域则主要发生汞形态转化反应。氨氮比越大,反应初期NH3的抑制作用持续时间越长,越不利于汞形态转化反应的进行。NO初始浓度的增大不利于汞形态转化反应的进行。HCl吸附平衡常数对反应速率起最大的促进作用,Hg0吸附平衡常数次之,而反应速率与NH3吸附平衡常数之间呈负相关性。通过以上研究发现,HCl是SCR汞形态转化反应中不可或缺的反应组分,当反应系统中HCl浓度较低时,汞形态转化效率较低。这意味着实际工业应用中,SCR的汞形态转化效率受到煤中氯含量的限制。结合中国煤种大部分是特低氯煤的特性,因而有必要开发对HCl依赖性较弱的新型催化剂。采用浸渍法制备锰系催化剂(MnOx/AL2O3),通过BET、XRD、SEM等催化剂表征,发现制备的催化剂具有比表面积大、锰分散度好等特点；通过TPR和XPS研究表明其表面负载的锰主要以MnO2的形式存在。纯氮气环境下锰系催化剂对汞具有强化学吸附作用,在150℃时吸附速率最大,达2.15μg/g·h。对汞吸附后的形态分析表明,催化剂表面的汞以HgO的形式存在,其吸附行为符合Mars-Maessen机理。汞氧化活性试验表明,该催化剂的反应温度窗口较宽,在低温低氯条件下仍具有较高的汞氧化能力。研究进一步发现除了HCl,烟气中的NO和SO2均可在O2存在的条件下促进汞形态的转化。H2O的存在抑制了反应的发生。催化机理研究表明,该催化剂可有效地吸附NO和SO2,在O2存在的条件下将其转换成NO3-和SO42-,同时与吸附生成的HgO反应实现汞形态的转化。催化剂表面的晶格氧在反应中起到关键作用,消耗的晶格氧从烟气中不断得到补充。锰系催化剂在汞催化氧化方面优异的性能使其在燃煤锅炉烟气汞形态转化方面具有较好的应用前景,特别是当锅炉燃用低氯煤种时。更多还原

【Abstract】 Mercury (Hg) emissions from coal-fired power plants are considered to be the largest anthropogenic source of Hg emissions to the atmosphere and have received increased attention. Among the technologies of mercury reduction in coal-fired power plants, the combination of catalytic oxidation from Hg0 to Hg2+ followed by WFGD is a promising and economical strategy to remove Hg0. Relative to Hg0, the Hg2+ compounds in coal flue gases are less volatile and weakly to strongly soluble in water and can, therefore, be captured and removed in conventional air pollutant control devices (APCDs). Thus, if mercury control targets are to be met, methods oxidizing Hg0 to Hg2+ in the flue gas from coal-fired power plants must be developed. With the support of the National High-tech Research and Debelopment Program (863) and the Specialized Research Fund for the Doctoral Program of Higher Education of China, the specific goal was to investigate the potential catalytic oxidation of Hg0 in coal-fired flue gas by different kinds of catalysts.Selective catalytic reduction (SCR) has been a well-developed, commonly used in large scale and commercialized technology for controlling NOx emissions from coal-fired power plants. The vanadia-based (V2O5/TiO2) SCR catalyst was synthesized by an impregnation method. The mercury speciation transformation across the SCR catalyst was evaluated using a bench-scale SCR reactor system. Results showed that the active component V2O5 in the SCR catalyst promoted the mercury oxidation by impacting the pool of vanadium active sites, which are critical for mercury oxidation. The activities of SCR catalyst for mercury oxidation were higher at higher temperature. HCl was important for the mercury speciation transformation by providing the active Cl, which was responsible for the mercury oxidation. NH3 inhibited the mercury oxidation due to the competition for the active sites on the catalyst surface. Larger space velocity was negative for the mercury oxidation.The mercury speciation tramsformation among elemental mercury and simulated flue gas across SCR system should be regarded as the heterogeneous oxidation. The reaction mechanism was studied by bench-scale experiments and various surface analytic technologies. It was observed that Hg is weakly adsorbed (Hg…O-V) onto the catalyst surface in N2 environment, which was confired by XPS analysis. The ability of Hg adsorption increased with VOx loading in the vanadia based catalyst. O2 prompted the transformation of H-O-V species to O=V species, which is responsible for the adsorption of Hg. However, NH3 inhibited the Hg adsorption due to the conpetive adsorption on the vanadium active sites. The monomeric vanadyl sites were found to be active for Hg adsorption.Experimental results showed that the Hg removal behavior is changed by passing HCl through the SCR catalyst first, and then passing Hg vapor without HCl through the catalyst. Simutaneously, mercury oxidation was observed when pro-exposure of the SCR catalyst to HCl, followed by passing through Hg0/N2 in the absence of gas-phase HCl. At testing conditions, Hg0 was found to desorb from the catalyst surface by adding HCl to the gas steam, which implies that HCl adsorption onto the SCR catalyst is strong relative to the mercury. Surface analysis verified the absorption of HCl onto the SCR catalysts forming vanadium-chlorine intermedia, in which the chlorine was reactive. Furthermore, the detailed Langmuir-Hinshelwood mechanism was proposed to explain the mercury oxidation on the SCR catalyst, where reactive Cl generated from adsorbed HCl reacts with adjacent Hg0.Based on the experimental results, a simplified Langmuir-Hinshelwood model of mercury speciation transrformation over the SCR catalyst was developed. The experimental data were fit by the model and the kinetic parameters were determined by the least-squares method. The effects of HCl concentration, NH3/NOx and adsorption equilibrium constants on mercury oxidation were evaluated. Results showed that the balance adsorption constant of HCl and Hg were much lower than that of NH3. Results of reaction analysis showed that the SCR catalysts can be envisioned as having two distinct zones. In the zone, which is near the entrance to the SCR, NH3 is the predominant adsorbed species and NOx reduction is dominating. When the NH3 is exhausted, HCl adsorption becomes the dominant, and mercury oxidation takes place. Larger NH3/NO means longer inhibition time. Promotion effects of KHCl, KHg on reaction rate were found. However, there is a negative correction between KNH3 and reaction rate.It was observed that HCl is the most critical flue gas component that causes conversion of Hg0 to Hg2+ under SCR reaction conditions. The activity of Hg0 oxidation is low when there is no HCl in flue gas or the HCl concentration is low. It suggests that the Hg0 oxidation activity of the SCR system is affected by the chlorine content in the coal. It is imperative to develop new Hg0 oxidation catalyst which performance is not sensitive to the HCl.Manganese oxide catalysts supported on alumina (MnOx/Al2O3) were synthesized by an impregnation method for Hg0 oxidation in simulated coal-fired flue gas. The catalysts were characterized by BET, XRD and SEM. The catalysts has large surface ares and highly dispersed manganese oxides can be obtained. The highly dispersed manganese oxides uniformly distributed on support surface mainly as Mn4+, which was confirmed by TPR and XPS analysis. MnOx/Al2O3 were efftive for adsorption of Hg0, and the rate of adsorption reached 2.15μg/g·h at 150℃. XPS analyses on the surface of catalysts after the removal of Hg0 suggest that adsorbed Hg0 oxidatively transformaed to HgO by surface lattice oxygen, consistent with the Mars-Maessen mechanism.Test results showed that the Hg0 oxidation activity of MnOx/Al2O3 was high in low chlorine contained flue gas, and the temperature window was relative wide. Both HCl, SO2 and NO enhanced Hg oxidation in experimental flue gas, while H2O inhibited Hg oxidation due to the competitive adsorption for active sites. Besides mercury chloride, other oxidized mercury species may be formed by the MnOx/Al2O3. NO and SO2 can be transformed to nitrite species and sulfate species in the presence of O2 on the MnOx/Al2O3. Mercury oxidation over the MnOx/Al2O3 could occur between adsorbed Hg0 and reactive species adsorbed at an adjacent site via a Langmuir-Hinshelwood mechanism. The consumed lattice oxygen was compensated by the O2 in flue gas. The catalytic performances of MnOx/Al2O3 on the oxidation of Hg0 appeared to be promising in the control of mercury emissions from coal-fired boilers, especially when firing the low rank coals.更多还原