Theoretical And Experimental Study On Methane Wet-air Autothermal Catalytic Reforming At Low Temperature In The Micro-channel

With the fast development of MEMS technology, the research of micro-power plant has become the hot topic. Due to the micro combustion space and shorten time-resident, it is difficult to ensure the completeness of the fuel. At the same time, the heat loss is huge because of the great area-volume ratio in a micro chamber. Therefore, how to improve the combustion efficiency and stability has become a difficult problem in the micro-combustion field. It is obviously to improve combustion by using CH4 reforming during the flow and reaction process, in which small amounts of H2 from this reforming is mixed with the burning or unburned system. Meanwhile, methane autothermal reforming is more suitable for the initial segment of micro-combustion with low-temperature and lean oxygen environment in the use of heat, hydrogen production and carbon deposition reduced. Besides, the appropriate humidity ratio is not only benefit of reforming reaction, reducing carbon deposition, but also will not affect the combustion. Thus, in-depth study on reaction mechanism characteristics and influence factors of methane wet-air autothermal reforming at low temperature in micro-channel lays foundations for further development of micro-channel catalytic reforming combustion technology.This paper, aiming to the problems of combustion instability and ignition difficulty,proposes a the methane wet-air autoreforming method at low temperature in micro-channel. Based on the existing sizes of the premixing cavity structure, the macro-and micro-reaction mechanism of this technology below 973K are analysed. Then under the conditions at low temperature and lean oxygen, thermodynamic analysis of methane autothermal reforming reactivity in micro-channel has been done, and the performance between an autothermal reforming system and a non-oxygen system or a non-steam system has been compared. Besides, the process of methane wet-air autothermal reforming in the specific micro-tube and micro-swirl chamber is researched by numerical method. Meanwhile, the influences of reaction conditions, component ratio, and other factors on the H2 and carbon deposition from reforming in micro-channel are discussed. Moreover, the experimental system of methane autothermal reforming at low temperature in a micro-channel is designed and built. The effects of parameters change on H2 yield and methane conversion ratio are focused in the research. It further reveals that a methane wet-air autothermal reforming at low temperature plays an active role in the micro-combustion technology.Firstly, results from thermodynamic analysis of methane wet-air autothermal reforming technology show that at the technology is more superior to a non-oxygen system or a non-steam system when temperature is below 973K. Through studying the temperature, pressure, humidity ratio and other operating conditions changes, and the effects of these changes on the methane wet-air autothermal reforming and the H2 production and carbon deposition at low temperature, obtains the features of the reaction components, and the wet-air methane autothermal reforming can be achieved at the temperature change zone between 650K and 973K in the micro-channel. Moreover, the amount of carbon deposition tends to zero as the temperature reaches to 850K. It indicates that the reaction process can help to reduce carbon deposition production and promote wet air methane autothermal reforming at low-temperature when the reaction pressure between 0.1MPa and 0.2MPa, as well as the humidity ratio above 350g·kg-1 in the feed gas.Secondly, the 42-step elementary reaction mechanism on the nickel-based catalyst surface is used to study methane wet-air autothermal reforming in the specific micro-tube and micro-swirl chamber by 3-D numerical simulation. Through the numerical study of producing H2, reaction process and carbon deposition under a constant methane mass flow,component ratio and feed gas flow below 973K, result shows that the lowest temperature when autothermal reforming reacts in a micro-channel is 650K. The variation of each system component with temperature changes in the two structures is basically consistent with the thermodynamic analysis results. When temperature reaches to 850K, the amount of carbon deposition tends to zero. It is also obtained that the smaller inlet mass flow is, the better the completeness of system can be. Besides, increasing oxygen to methane ratio and humidity ratio are of advantage to produce H2 and reduce carbon deposition.In the investigations on H2 production and carbon deposition of methane wet-air autothermally catalytic reforming in micro-swirl chamber, it is found that the inhibiting effect of increasing oxygen to methane ratio on carbon deposition is better than that of increasing steam to methane ratio. With the increase of these two factors, production of H2 increases and distributes more widely, while the carbon deposition area varies from the exit to the entrance and the amount decreases. Heat loss to the environment is enhanced by the increase of oxygen to methane ratio and reduced by the increase of steam to methane ratio. In addition, effect of varying pressure on products is not so evident at low pressure condition; however, the area of carbon deposition is smaller at elevated pressure. Catalyst depositing on multi-surfaces is beneficial for hydrogen production, and the performance of depositing on side-wall is better than that of on upper and lower wall.Finally, the experiment of micro-channel autothermal reforming is realized with the tube-type nickel-based catalyst method which overcomes the problem of metal surface is hard to load the catalyst trough designing and building the experimental table. Through the visual experiments that the exhaust of low-temperature reforming in micro-channel does not flameout in a tube with radius of 1mm, it is confirmed that methane wet-air autothermal reforming at low temperature can promote and maintain combustion effectively in the micro-channel. The experimental results are consistent with the theoretical analysis in H2 yield, methane conversion ratio and carbon deposition ratio through researching the influence of temperature, air to methane ratio, steam to methane ratio and mass flow changes on reforming system. It also lays an important experimental foundation and provides useful reference value for the study of hydrocarbon fuel combustion technology in micro-channel.In the paper, it is more comprehensive to study the methane wet-air autothermal reforming at low temperature in a micro-channel by thermodynamic analysis, numerical simulation and experimental research when the temperature is below 973K. It can be well understood the feasibility and advantages of this method in a micro-channel field. The influence rules are connected with temperature, pressure, air to methane ratio, humidity ratio on reforming system and especially carbon deposition, and provide control parameters which promote the reforming reaction. At the same time, the autothermal reforming characteristics and the effects of structural changes on product distribution in the specific reactor are fully understood, which enrich and strengthen the research of methane wet-air autothermal reforming at low temperature in a micro-channel. This paper not only helps to improve and optimize the micro-reaction devices and to promote the research of micro-combustion technology in-depth, but also has important academic significance and engineering value to the development of a micro-power equipment.