Numerical And Experimental Study Of Transient Performance Of Methane Catalytic Auto-Thermal Reforming In Micro-channel

With the development of micro-electromechanical systems, micro-miniaturization of system and application become a hot research issue in today's society. Micro-burner, because of its small size, hydrocarbon fuel is difficult to combust stably and efficiently. Studies have shown that the hydrogen produced by methane reforming under the condition of catalyst, mixed with the fuel mixture can improve the micro- combustion. The methane autothermal reforming couples the high endothermic steam reforming and the methane partial oxidation reforming, which has low energy consumption, becomes a hot research issue. At present, the micro-scale catalytic reforming researches are mostly concentrated in the characteristics of methane autothermal reforming under the steady state, and the reports of the transient performance of the reaction is few; In addition, the experimental study of methane autothermal reforming, the catalyst is almost in the form of porous media filled in the reactor, while the catalyst coated on the inner surface of the reactor is rare.In this thesis, the transient performance of CH4/O2/H2O catalytic reforming based the detailed reaction mechanism in Ni/Al2O3 catalyst in micro-straight tube reactor has been simulated by CFD and analyzed completely. The paper focused on the hydrogen production and the methane conversion efficiency changes with time and also the time that the reaction reaching stability, which affected by the reactor catalytic wall temperature, initial temperature of reaction mixture, the raw mixture composition, the mixture mass flow rate, the reactor inlet size and other factors.The numerical results show that in the autothermal reforming, the hydrogen concentration at the outlet of the micro-reactor increased first and then decreased by the time. The hot catalytic wall temperature and the raw gases initial temperature both have a major impact on the reaction characteristics of methane autothermal reforming. Either increase the catalytic wall temperature or enhance the raw gases initial temperature can also make a favorable impact on methane autothermal reforming. Raising the temperature will not only enhance the hydrogen yield and methane conversion, but also accelerate the reaction to achieve stability. Varying the inlet gas temperatures did not produce significant difference in hydrogen yield and CH4 conversion, when the wall temperature is higher; it only affected the initial stage of the reaction. But when the catalytic wall temperature is lower, increasing the inlet gas temperatures can promote the hydrogen yield and CH4 conversion more obviously. In addition, the time of the reaction reaching the steady state is shortened while increasing the temperature, either the wall temperature or the gas inlet temperature. Besides this, at the same temperature increasing, raising the wall temperature is better than raising the gas inlet temperature in improving hydrogen yield and CH4 conversion. Therefore, since the catalyst does not loss its activity, we must try to enhance the catalytic wall temperature.The time of hydrogen yield reaching the maximum change a little while varying the mixture components when the wall temperature is high, but vary slightly larger with the mixture flow rate. Keep O2/CH4 mole rate constant, the hydrogen yield at the reactor outlet increased and the time of hydrogen yield reaching a steady state is shortened while increasing the H2O/CH4 mole rate. When keep H2O/CH4 mole rate constant, the time of hydrogen yield reaching a steady state was extended and the hydrogen yield at the reactor outlet decreased while increasing the O2/CH4. The optimal condition for high hydrogen yield is at CH4/O2/H2O of 1:0.5:3.5.Under this condition, the hydrogen yield can achieve stability in time 90ms, and the volume fraction of hydrogen in the stable condition was 54%.At the same catalyst temperature, changing the reactor diameter will affect the heat transfer and molecular diffusion. In the range this paper researched, whether the raw gases flow rate and the catalyst load change along with the proportion of the reactor diameter, the smaller the diameter, the easilier the reaction stabilized.In the high temperature environments of the electric heating, nickel-based catalyst coated on the inner wall of the reactor, the impact of the catalytic wall temperature, air/CH4 molar ratio, H2O/CH4 molar ratio and the methane volume flow rate on the hydrogen production and the reforming conversion efficiency have been studied.The experiment results show that catalytic temperature have important influence on methane conversion and hydrogen yield, with the increasing of catalyst temperature, methane conversion and hydrogen production showed a tendency to increase. The hydrogen yield decreased with the air/CH4 increasing, but increased with the H2O/CH4 increasing. Increasing methane volume fraction in the mixed gases, the hydrogen yield and methane conversion efficiency was increased firstly and then decreased.