| Currently, more than 85% of the world's energy and chemical industry is built on the basis of three flammability mineral resources, which are oil, coal and natural gas.In the three pillars of energy, oil resources are depleted day after day, and the price is rising. Although the reserves of coal resources are rich, there is a serious pollution problem when it is used. And as the quality of clean energy and chemical raw materials, natural gas is expected to play a more important role in the modern economic society. At present the development and utilization of natural gas mainly concentrate on the methane conversion technologies. As for the C-H bond of methane molecules reaches 435 kJ/mol, the direct translation methane into chemicals is still far from the goal of industrialization, therefore, the indirect technique of translation "syngas" into important chemicals and liquid fuels becomes a hot topic in the contemporary world.A more systematic study about the novel technology of "partial oxidation of methane to syngas using lattice oxygen" was done, and the contents included the following key issues:thermodynamic theory analysis and equilibrium compositions simulation, experimental choosing and optimization of oxygen carriers, kinetics and reaction mechanism, and the exploration about a new reaction system.According to the basic principle of oxygen carrier choice, CeO2, Fe2O3, CuO and Mn2O3 were preliminary chosen as oxygen carriers and A,G°and ArH°of the four systems were calculated by means of thermodynamic analysis. Based on the principle of a system free energy minimum, the equilibrium compositions of oxides-CH4 system at various temperatures were calculated using thermodynamics software and database. The results showed that in theory the four oxides could partially oxide methane to syngas in a certain temperature range. If syngas with ideal n (H2) In (CO) ratio needed to be obtained, the higher temperature and appropriate ratio of methane and oxygen carriers needed to be controlled.The Ce-M-O (M=Fe, Mn, Cu) complex oxides in which the ratio of n (Ce)/n(M) is 1:1 were prepared by coprecipitation. The X-ray diffraction (XRD) characterization results of oxygen carriers showed that Mn2O3 and Fe2O3 could spread out in the CeO2, but CuO could not. The TG experiment of Ce-M-0 oxygen carrier reaction with methane illuminated that the capability of supplying lattice oxygen could be significantly improved when Fe2O3, CuO and Mn2O3 were incorporated into CeO2. So, if the capability of supplying lattice oxygen only was taken into account, the Ce-Fe-O oxygen carrier had the best oxygen storage capability (OSC). The activity evaluation experimental results of fixed-bed reactor showed that Ce-Cu-0 had a performance of complete oxidation methane. Although, Ce-Mn-0 could partially oxide methane to syngas at a higher temperature, there was a serious methane pyrogenation. Ce-Fe-O showed the best performance of partial oxidation methane and the ratio of n (H2)/n(CO) was about 2 in product gas. In conclusion, in terms of total lattice oxygen supply or partial oxidation methane or reducing the cost of production oxygen carrier, Ce-Fe-O complex oxides could take on a key role of oxygen carriers being used in "lattice oxygen partial oxidation of methane production of syngas technique"As Ce-Fe-O oxygen carrier has a good performance of partial oxidation of methane, the different n (Ce)/n(Fe) ratio of oxygen carrier was sequentially researched. The Ce-Fe-O-X (X=9/1,8/2,7/3,6/4,5/5,4/6,2/8) ceria-based oxygen carriers were prepared by coprecipitation, as the same, using some characterization methods, such as XRD, H2-TPR and so on and combination fixed-bed reactor and TG experimental platform, the oxygen carrier performance was evaluated.The results showed that Ce-Fe-O-7/3 oxygen carrier had a good performance.The activity of Ce-Fe-O-7/3 oxygen carrier at different sintering temperatures was studied, the results showed that 800℃was the most appropriate. In order to improve the circulation performance of oxygen carrier, the experiments of modification on Ce-Fe-O-7/3 oxygen carrier by ZrO2 doping were studied. The results showed that Ce-Fe-Zr-O(0.05) had the best circulation performance in all samples.The kinetics of Ce-Fe-Zr-O(0.05) oxygen carrier reaction with methane was studied. It was found that average apparent activation energy(Em) was 131.762kJ/mol, and when using the pure CeO2 as oxygen carrier the Em was 173.517kJ/mol, in other words, the reaction was easier when using Ce-Fe-Zr-O(0.05) as oxygen carrier than pure CeO2, the performance improvement of oxygen carrier was attributed to the form of Ce-Fe-Zr solid solutions and mutual cooperate of Ce4+, Fe3+, Zr4+ions.The reaction mechanism study revealed that there were three distinct stages which were initial stage, middle stage and final stage in the entire process. The reaction initial stage was combustion-reforming mechanism, the reaction middle stage was directly oxidation mechanism in which stage CO and H2 were the direct production of methane and this stage was a important period for obtaining a great deal of syngas, while in the final stage methane pyrogenation occupied dominance.A new method which was "partial oxidation of methane to syngas using lattice oxygen in molten salt system" was brough forward. The new system has some new contributions in reducing carbon deposition, heat distribution and enhancing reaction heat utilization efficiency. Different molten salt systems were studied; finally, the Na2CO3 and K2CO3 mixed moten salts (weight ratio was 1/1) were appropriate for the new system. When using Ce-Fe-Zr-O(0.05) as an oxygen carrier for partial oxidation methane in molten salt system, the experimental results showed that syngas could also be produced in the new system, and oxygen carrier regenerated through air could be successfully recycled. The new molten salt system has good functions of reducing carbon deposition and adjusting the ratio of n (H2)/n(CO). The new reaction system could not only avoid the catalyst hot-dot, but also could provide a new means for utilization of reaction heat.
|
PDF Full Text Request