Carbonaceous Catalysts For Syngas Production From Carbon Dioxide And Methane

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...