【作者】 张国杰；

【导师】 张永发； 谢克昌；

【作者基本信息】 太原理工大学 ， 化学工程与技术， 2012， 博士

【摘要】 CO2和CH4是主要的温室气体,同时它们也是一种宝贵的资源。随着全球变暖,CO2排放问题正在引起国际社会的关注。以气化煤气和焦炉煤气为源头的“双气头”多联产系统,不仅可以产生人类必须的能源和化工产品,而且可以综合提高能源的转化效率,对环境温和,是煤炭洁净转化高效利用和解决温室气体排放的重要途径,在整个系统中,最关键的技术是焦炉煤气(CH4)和气化煤气(CO2)重整转化制合成气,其中重整催化剂是重中之重。因此,开发新的催化剂材料并通过对其改性,为工业化获得廉价高活性催化剂,始终是材料科学和催化学科领域的热点研究课题。炭材料因廉价、丰富的孔结构和高的比表面积近年来受到了广‘泛的关注。炭材料的种类众多,结构和官能团丰富,因而可以通过改性来改变炭材料的结构和官能团的组成来调控催化剂的活性。同传统的负载型催化剂相比较,炭材料种类的丰富性及性能的可变性,不仅为新型炭材料的制备提供了新的途径,也为重整催化剂的开发提供了新的机会。本文通过实验研究、现代仪器分析和反应机理分析等对炭材料催化二氧化碳重整甲烷进行了系统研究,考察了高温炭材料的“催化”本质、重整过程中产生的积炭以及压力对重整转化的影响；初步探讨了重整反应的机理和动力学,得到的主要结论如下：1、高温炭材料对C02-CH4重整反应具有明显的催化作用,催化的实质在于：①炭催化剂表面含有丰富的含氧官能团,含氧官能团中含有活性的极性氧,极性氧具有较强的活性,可以以偶极作用力与甲烷的氢键的形式缔合,促进CH4和CO2的活化转化；②过程发育的比表面积和孔结构有助于CH4和C02的吸附和解离活化；③炭材料催化剂中的灰分的促进作用；④炭材料含氧官能团的活性氧、比表面、微孔和灰分的协同作用。2、在非催化条件下,C02-CH4重整反应主要是先进行甲烷的裂解反应,然后裂解产生的积炭和二氧化碳发生气化反应；在高温炭材料催化条件下,C02-CH4重整转化反应分为两步：首先是甲烷和二氧化碳被活化；然后两者发生反应生成一氧化碳和氢。其中甲烷的活化是一个链反应过程,其链引发是关键步骤。即CH4与炭材料表面活性点发生碰撞获得能量而活化。炭材料表面的活性点是表面的官能团,其能降低CH4脱H的活化能。催化活性主要取决于含氧官能团中氧的极性,所含氧物种的不同使其表面的电核性质也不一致,从而使其表现出不同的催化性能。采用化学滴定和XPS分析证实了炭材料表面的酸酐和内酯结构是炭材料催化剂的主要活性中心。3、首次采用炭材料催化剂对焦炉煤气-CO2重整进行了研究,发现不同炭材料催化活性有所差别,但在不同炭材料上作用下重整转化表现出相同的趋势：初始阶段,催化活性逐渐下降,CH4转化率明显下降,C02转化率略有下降；稳定阶段,催化剂保持较好的活性和稳定性,甲烷和二氧化碳转化率基本恒定。提高重整反应温度和增加重整反应停留时间,都可以提高重整反应的转化率,当温度从700℃升高到1050℃时,CH4转化率从小于5%提高到97.3%；同时可以通过调节入口气CH4／CO2的比来调节出口气CO／H2比,以更好的满足后续合成工业对原料气的要求。长达600min寿命考察,催化剂未发现明显失活,炭材料催化剂显示出较好的催化活性和稳定性。4、C02-CH4重整过程中根据温度的不同产生的积炭可分为两类：易气化炭Ⅰ(1000℃以下产生的积炭)和难气化炭Ⅱ(1000℃以上产生的积炭)。易气化炭Ⅰ易与CO2发生气化反应,难气化炭Ⅱ趋于石墨化,活性较差,难与CO：发生气化消炭反应；由此可以通过控制重整反应的温度(低于1000℃),进行控制积炭的生成速率。5、根据Hume-Rothery理论,采用具有面心立方晶格的两种金属对炭材料进行改性,开发出了一种高活性和抗积碳的Cu-Co／炭材料催化剂；该催化剂降低了CH4和C02活化重整反应能垒,使CH4和C02活化比单炭材料催化更容易(同样转化率,反应温度降低200℃)。6、根据建立的传热传质模型,研制出了一种新型高温高压反应器(1200℃,12MPa),该反应器具有高温区不承压,承压区不高温的特点。并在该反应器上考察了压力对C02-CH4重整反应的影响,由于压力效应导致催化剂表面的甲烷解离脱H和CO2的吸附发生变化,进而改变CHx*在炭材料上沉积速率,不利于炭材料催化剂的活性和稳定性7、炭催化剂能够提供一种活性物质,这种活性物质能有效的改变二氧化碳重整甲烷反应的历程,从而改变二氧化碳重整甲烷反应的活化能。在炭材料催化二氧化碳重整甲烷反应体系内同时发生甲烷裂解、二氧化碳气化和二氧化碳重整甲烷反应；由平推流模型求得甲烷非催化和炭材料催化裂解的表观活化能分别为154.02kJ／mol和56.42kJ／mol；重整过程中,C02过量时,不仅产生的积碳被CO2气化反应消耗,而且部分炭材料在反应过程中也被消耗,采用总包一级未反应芯收缩模型对炭材料消耗动力学参数进行了计算,得到C02-炭催化剂气化、甲烷裂解和CH4-C02直接重整三者的活化能顺序依次为：C02-炭催化剂气化>CH4-C02重整>甲烷裂解。8、对经典的Eley-Ridea (ER)和Langmuir-Hinshaelwood (LH)两种机理模型进行了分析和比较,结合实验研究数据,提出了炭催化C02-CH4重整的两种反应机制：1)开始阶段CH4被O-M活化生成CHxO*,然后生成CO和H2；同时C02被活化生成CO和O*。稳定阶段：C02活化生成CO和O-M,CH4被O-M和*活化生成CHx*、OH-M和H*,然后生成H20和H2。2)CH4和C02同时被不同的活性物质活化,活化后形成CHx*、CO-M和H*,然后快速生成产物。根据提出的机理,建立了动力学模型,在700-850℃的温度区间范围内,求得到了重整反应的活化能E=128.3kJ/mol及反应气吸附平衡活化能ECH4=52.2kJ/mol, ECO2=17.5kJ/mol。更多还原

【Abstract】 CO2and CH4are not only the main greenhouse gas, but also are an invaluable resource. With global warming, CO2emissions are attracting the attention of the The double gas head of multi-generation system taking gasification gas and coke oven gas as the gas resource, not only can produce energy and chemical products, but also can improve energy conversion efficiency. It is an important way to improve using efficiency of clean coal conversion and address greenhouse gas emissions. In the conversion system, the most critical technology is the syngas production from the reforming of coke oven gas (CH4) and gasified gas (CO2), in which reforming catalyst is the most important. Therefore, developing new catalyst materials, to obtain cheap and high-activity catalyst for the industrialization through modification, which are always being a hot research topic of materials science and catalytic fields.Carbonaceous catalysts have been concerned widespread because of cheap, rich pore structure and large specific surface area in recent years. Carbonaceous catalysts has many types, its structure and functional groups, so we can modify and change the structure of the carbonaceous catalysts and the composition of functional groups to regulate the activity of carbonaceous catalysts. Comparing with the traditional supported catalyst, it has species richness and the variability of the performance, not only provides a new way for the preparation of new carbonaceous catalysts, and also provides new opportunities for the development of reforming catalysts. In this paper, we systematically study the carbon dioxide reforming of methane under carbonaceous catalysts by means of experiment, modern instrumental analysis and reaction mechanism analysis. We have investigated factors influencing the process of reforming reaction, and provided the data for the design and scale of the pilot. Main conclusions are as follows:1. the carbon had apparent catalyzation to CO2-CH4reforming, the substance of Carbon catalytic is:①Carbonaceous catalysts surface has a rich oxygen functional groups, which is contribute to the dissociation of methan, and promote the conversion of CH4.②Development of surface area and pore structure increases the active sites, increases CH4reaction probability and catalytic activity.③The high catalytic efficiency and long life characteristics of carbon indicate, the different of oxygen species of carbon surface with metal oxides is the oxygen functional groups of the carbon catalyst generated by CO2or H2O gasification in reforming process, which is useful for extension catalyst catalytic of carbon.2. In the non-catalytic CO2-CH4reforming reaction, the first step reaction is methane decomposition forming the coke, and then the coke reacts with carbon dioxide. In the cabon-catalytic CO2-CH4reforming reaction, the first step methane and carbon dioxide are activation; then CO2-CH4reforming occurred. The methane decomposition is chain branching reactions, the chain initiation (methane activation) is a key step, that is to say, CH4collision with the active site of carbon materials surface get the energy and activated. The containing oxygen substances or oxygen functional groups of carbon materials surface is the catalytic active center (site), which can reduce the activation energy of CH4dissociation. The catalytic activity depends on the nucleophilic and electrophilic of oxygen species, the surface electro-nuclear nature of oxygen species is different due to different the metal species in carbon, the results of catalytic performance is inconsistent. The surface of the number of oxygen functional groups was determined by titration method, combined with quantum chemical and XPS analysis, that the oxygen functional groups of anhydride and the lactone structure in carbon materials surface is considered the catalytic active center.3. Carbons catalyst was the first time used to study the CO2reforming coke oven gas. Experimental results showed that the coal char was an effective catalyst for production of syngas, and addition of CO2did not enhance the CH4reforming to H2. It was also found that the product gas ratio of H2/CO is strongly influenced by the feed ratio of CO2/CH4, which rang from0.2to1.1. The modified coal char catalyst has more active than coal char catalyst I and II in CO2reforming of CH4. The conversion of methane can be divided into two stages. In the first stage, the conversion of CH4gradually decreased. In the second stage, the conversion of methane maintained nearly constant. The conversion of CO2decreased slightly during the overall reactions in CO2-CH4reforming. The coal char catalyst is a highly promising catalyst for the CO2reforming of methane to syngas.4. It has been found that the carbonaceous catalyst is an efficient catalyst on methane decomposition and CH4-CO2reforming. The trend of methane decomposition at lower temperature is similar to that at higher temperature. The methane conversion is high during initial of stage of the reaction, and then decays to a relatively fixed value after about30min. With the temperature increasing, the methane decomposition rate increases quickly. The reaction temperature has significant influence on methane decompositon, whereas, the carbon deposition does not affect methane decompositon significantly. Different types of carbon deposition were formed at different methane decompositon reaction temperature. The carbon deposition Type Ⅰ generated at900℃has minor effect on CH4-CO2reforming and it easily reacts with carbon dioxide but the carbon deposition Type II generated at1000℃and1100℃clearly inhibited CH4-CO2reforming and it is difficult to react with carbon dioxide. The results of XRD showed that the some graphite structure was found in carbon deposition Type Ⅱ. It indicated that carbon deposition gradually tends to be graphitization, with the increase of temperature reforming reaction. 5. Development of high performance catalysts for the production of synthesis gas from CO2-CH4reforming. This catalyst with high standard quality and steady performance, has reached the international advanced levels. It shows good activity and stability for CO2-CH4reforming at900℃for within1440min-on-stream, with conversion of CH4and CO2being above90%. Therefore, it is a promising candidate for utilizing CO2-CH4reforming to produce syngas applications.6. A small high pressure reactor system was built with1200℃,12MPa. The catalytic activity and stability of catalysts were closely related to the reaction pressure. It was observed that the use of higher pressure substantially decreased CO2and CH4conversion and increased catalyst deactivation during CO2reforming of CH4, compared to runs at0.5MPa for carbonaceous catalysts. Deactivation was related to carbon formation. Some process adjustment methods, such as increasing the reaction temperature, prolonging reaction residence time, and increasing CO2and CH4molar ratio, which could improve catalysts activity and stability. Besides, the positive effect of surface oxygen containing groups (C-O) on catalyst activity had been demonstrated over carbonaceous catalysts. The basic function of the carbonaceous materials surface area also seemed to increase H-abstraction of methane and CO2adsorption.7. The carbon catalyst is not only as catalyst, also can take part in a chemical (physical) reaction. Carbon catalyst can provide an active substance, which can change the course of the carbon dioxide reforming of methane, and chang the activation energy of CO2-CH4reforming reaction. The carbon balance analysis showed that three reactions occur simultaneously in the CO2-CH4reforming reaction system over carbonaceous catalyst, methane decomposition, carbon dioxide gasification and carbon dioxide reforming of methane. The plug flow reactor model is used to describe chemical reactions in continuous, flowing systems. The apparent activation energy of non-catalytic and catalytic methane decomposition is154.02kJ/mol and56.42kJ/mol, respectively. From the material balance analysis of C atom, it can be found that the mass of carbonaceous catalyst reduce during the carbon dioxide reforming reaction of methane. It indicated that the gasification of carbonaceous materials by CO2takes place during the synthesis process. The consumption kinetics parameters of carbonaceous materials were determined by an overall unreacted shrinking core model. The apparent activation energy of consumption carbonaceous materials was more than230kJ/mol during CO2-CH4reforming. The apparent activation energy order of carbon dioxide gasification, methane decomposition and CO2-CH4reforming is:CO2gasification> CO2-CH4reforming> methane decomposition.8. By analyzing and comparing the mechanism model of Eley-Ridea and Langmuir-Hinshaelwood, we propose two possible mechanisms. The reforming reaction dynamic model was established based on the experimental data. We investigate the carbon dioxide reforming of methane dynamics in the range of 700-850℃, and obtain kinetic parameters (activation energy and frequency factor). These parameters are then used to predict reaction progress under various temperature ranges and conditions. By comparison, the result agreed with experimental data更多还原