Methane To Restore The Basic Research Of The Zinc Oxide Syngas And Metal Zinc In The Molten Salt System

In order to meet the needs of the conversion of methane,the preparation of metallic zinc or hydrogen and the utilization of solar energy in the engineering applications, this thesis addresses the exploration of an unconventional route for potentially cost effective and cleanly technology by partial oxidation of methane with zinc oxide to co-production metallic Zn and synthesis gas.By combining the high temperature molten salt technology and the catalytic reaction process into one reactor,molten salt can dynamic supply the energy needs to molten salt reactor because molten carbonates have superior heat storage and transfer properties to avoid the loss heat capacity,also enhance the stability and security of the reaction process effectively,which is considered as the key of the one of the most promising routes for methane conversion,is expected to significantly solve the problems including energy,resource and environment etc.Molten carbonates,as the key of the reaction system,based on the special physical chemistry ability at high temperatures(typically above 983K),which directly affect the foundation of the reaction system and the control of the reaction process.At present,alkali molten carbonates had wide applications in nonflame combustion,energy conversion,material preparation and molten carbonate fuel cell(MCFC).Related researches also gather the theories and techniques of Chemical Engineering,Metallurgy,Energy,Catalysis and Materials,and are forming a new field in molten salt science.The present researches started with the basic relationships among the composition and property of molten salt with the aid of the advanced characterization methods of materials and the high temperature molten salt reaction measurements in our laboratory. Most of the attentions were focused on the choice of molten salt,reaction system foundation and reaction process,which classified the present researches into four parts:1.Thermodynamic analysisFirstly,this thesis presents the analysis of the thermodynamic of the system by equilibrium model.Based on Gibbs free energy minimization,using the HSC Chemistry(?)5.1 thermodynamics software,theâ–³_rG°,â–³_rH°,K_p°,C_p°,and equilibrium composition of the significant intermediate reactions in the system were investigated. The analysis shows that the appropriate molten salt system should be Na₂CO₃/K₂CO₃ mixture,and that Li₂CO₃ is unadopted due to the strong reaction between CH₄ and Li₂CO₃.For the equilibrium compositions of CH₄+ZnO with alkali molten carbonates system, synthesis gas concentration increases with the temperature,the appropriate reaction temperature is around 1200K,and at the same time,the ratio of H₂/CO is about two when the reactant ratio of CH₄/ZnO is 1:1.This system should be operated with a higher ZnO-to-CH₄ ratio to attain maximum methane conversion and decrease the formation of carbon.The maximum conversion values are 100%and 80%for CH₄ and ZnO respectively in stoichiometric ratio reaction within the temperature range of 1073～1223K under 0.1MPa pressure.2.The reduction behavior of methane in alkali molten carbonatesThis reaction system must take into account the reaction between CH₄ and alkali molten carbonate since CH₄ has a higher reductive capability.Thermodynamic and experimental investigations were performed to quantify the product component of the reaction between methane and alkali carbonates(Li₂CO₃,Na₂CO₃,K₂CO₃)at high temperatures,besides the reaction mechanism was discussed.The results showed that the CO and CO₂ contents of the product gases increased with the increasing of reaction temperature,while decreased with the reaction time.The reaction activity order was Li₂CO₃＞＞Na₂CO₃＞K₂CO₃,which the theoretical and experimental investigations results were uniform.The selectivity of CO in the mixed molten salts system was higher than a single salt system,whereas the product gases contents of CO and CO₂, the conversion of CH₄ lost rapidly with reaction time,so the mixed salts system was more suited as the reaction medium of the system proposed.The reaction mechanism was given that the product of H₂ was mainly from the CH₄ cracking,the CO and CO₂ from the reaction between CH₄ cracking carbon and molten alkali carbonates.3.The reaction of CH₄+ZnO with molten salt systemThe results of experiment in the fixed bed reactor showed that a small quantity of carbonates Na₂CO₃/K₂CO₃ had the catalytic effect to CH₄ and ZnO reaction,which catalysis mechanism was similar to the coal-CO₂ gasification mechanism.The reaction process and mechanism could be described using shrink the nuclear model,which mathematical expression was primary suggested in this thesis.Follow that,the experimental investigations had been carried out in a molten salt three phase flowing-bed reactor at the temperature range of 1123～1223K.The GC analysis results of exhaust revealed that the gases compositions were H₂,CO and CH₄, in which CO₂ could not be detected.Both the CH₄ conversion and the ratio of H₂/CO increased with the increasing of reaction temperature,the ratio of H₂/CO was 2 at around temperatures 1198K.After the reaction completed the product of metallic zinc was successfully collected in the molten salt,which was characterized by X-ray diffraction(XRD),Electron Probe Microanalysis(EPMA),SEM and EDS.The results showed that metallic Zn and synthesis gas could be produced from ZnO+CH₄ in molten carbonates,and that metallic zinc and synthesis gas were attained separately,thus this process could solve the problems including reactor plugging and products separating.The reaction of zinc oxide with methane in the absence and presence of CO₂ were theoretically and experimentally investigated.In the absence of CO₂ at 1193 K,the reduction of ZnO was accompanied with methane cracking,and metallic zinc,CO,and H₂ were the main reaction products,in which ZnO was a donor of oxygen.In the presence of CO₂,ZnO played as a catalyst in the CO₂ reforming of methane and produced syngas with the average H₂/CO ratio of 0.88 at 1193 K,which was close to the total reaction theoretic value of 1.It was also found that higher temperature favored high CH₄ and CO₂ conversions.XRD technique was used to characterize the ZnO species.The result showed that there were no differences in the peak profiles of the XRD patterns of the ZnO powder obtained before and after passing the CH₄/CO₂ mixed gases for 6 h at 1193 K.It was suggested that ZnO functioned as a catalyst according to the redox cycle and metallic zinc played the role of intermediate product in this process.Based on the above research background,it could be concluded that ZnO simultaneity acted as the oxygen donor and catalyst in a molten salt reaction system, the eutectic mixture of K₂CO₃ and Na₂CO₃ acted as the reaction medium,the thermal storage and the catalyst in this process was feasible.4.New thermochemical cyclic system in molten salt using solar process heatUtilizing solar energy massive and high-efficient hydrogen production can be realized with carbon dioxide emission removed or mitigated.This will provide a solid base for the future sustainable energy system.A novel thermochemical cyclic system for hydrogen and synthesis gas production using solar process heat is proposed in this study.In this technology,a whole cyclic process is divided into two steps.In the first step,solar energy is absorbed,stored and transferred by alkali molten carbonate,methane reaction with metallic oxide(M_xO_y)to form synthesis gas and metal in the molten salt medium;In the other step,M_xO_y and H₂ are produced from water-splitting with metal.Thus M_xO_y from this step is recycled to the first step.The results show that only ZnO and SnO₂ are feasible for this reaction system in theory,synthesis gas concentration increases with the temperature.For large scale metallic zinc production,a 100MW solar energy system can satisfy the energy requirements for producing 5.32kg liquid zinc per second,at the same time the solar energy of 3.6×10⁴kJ can be transformed to chemical energy.Our results were not only developed a new route for industrialized methane conversion and metallic zinc preparation,and helpful to deepen our understanding of the reaction process so that farther exploit the application in a molten salt field,but also showed a potential new way for hydrogen production using solar energy process heat.