The Intrinsic Kinetics Of Methane Steam Reforming And Reaction Performance Study In Micro-channel Reactor

Fuel cell (FC) is generally a high-efficiency,energy-saving and environmental friendly type of power generation device. It has a widely application future in both small-scale fixed power generating plant and mobile electric power sources. However, the supply of hydrogen source is crucial for the commercialization of FC. Because of the enrichment, methane is thought to be the first choice among hydrocarbon fuel reforming for hydrogen production. In the ways of methane reforming, methane steam reforming (MSR) has the advantages of low reaction temperature,high hydrogen production and low carbon monoxide concentration in the reforming products, which is a effective way to provide hydrogen source for FC.So far, the study of MSR is mainly focused on the development of catalyst with high activity,lower molar ratio of water to carbon (S/C) and deposited carbon inhibition. However, MSR is a strongly endothermal reaction, which is usually limited by heat and mass transfer and shows a lower dynamic responds and lower utilization ratio of catalyst in the catalyst bed. Therefore scientists begin to study micro-channel reactors which have better transport characteristic of heat and mass. Nevertheless, the micro-channel is easily blocked by the catalyst granule, and greater power is need to overcome the pressure resistance. Therefore catalyst coating is considered as an alternative scheme in the micro-channel reactors. Traditional methods of producing catalyst coating have disadvantages of complex technology and higher cost, and the coating granules are easily broken off owing to weak adhesive power and then block the reactive channel. So the supersonic cold gas dynamic spray (CGDS) technology was used for the preparation of the nickel based catalyst coating in this paper, over which the intrinsic kinetics experiment of MSR was carried out. And the reforming characteristic in micro-channel reactor was studied. The main works of this thesis was included as follow:(1) Catalyst coating was fabricated by cold gas dynamic spray (CGDS) on aluminum substrate and stainless steel substrate. The catalyst particle,substrate and catalyst coating were analyzed by scan electron microscopy (SEM),energy dispersion spectroscopy (EDS) and X-ray diffraction (XRD). The detected results show that the catalyst coating was effectively deposited on the aluminum substrate and stainless steel substrate. The coating has a better bonding strength with substrate, and the component of the coating is close to the catalyst particle. The nickel-based catalyst coating is single layer, the performance of coating is influenced by deposited velocity and broken property of nickel-based catalyst powder. Compared with the catalyst coating on Al substrate, the catalyst coating on stainless steel substrate has smaller deposited paricle, which is benefit for methane steam reforming.(2) The experiment of methane-steam reforming intrinsic kinetics was performed with nickel- based catalyst coating on the stainless steel substrate. By analyzing the experimental data and secreening kinetics model, the double rate kinetics model were established. The pre-exponential factor and the apparent activation energy for the kinetics model of carbon monoxide formation was 1.08×108 mol/(h.g.kPa0.89) and 178.98 kJ/mol respectively, and the pre-exponential factor and the apparent activation energy for the kinetics model of carbon dioxide formation was1.73×104 mol/(h.g.kPa2.06) and 139.00 kJ/mol respectively. The correlation indexs of the intrinsic kinetics model by F statistic were greater than 0.9 and the F statistical quantity was 10 times greater than the critical one with the confidence domain of 99%, so the model is acceptable and creditalbe.(3) Experiment and simulation were performed to study refrorming characteristic of MSR in micro-channel reactor, and then the performance of the catalyst coating after experiment was detected. The experimental results showed that smaller methane space velocity and higher reaction temperature will bring on higher methane conversion and hydrogen production. The selectivity of carbon monoxide ( S CO) is influenced by reaction temperature and the molar ratio of water to methane (S/C) chiefly. The higher temperature as well as smaller S/C, the bigger S CO. By analysing the catalyst coating after experiment, we discovered that carbon was deposited on the coating surface by methane decomposition, which is beard deposited carbon that would not make the catalyst coating deactivated. The inaccuracies between simulation results and experiment ones are mostly less than 15%, which approves that simulation method of MSR in micro-reactor with CFD software is available and the result is reliable. Furthermore, the effect of length and height of channel on reforming and resistance characteristic was investigated. The results show that smaller height of reaction channel can shorten the time which reactants diffuse from channel to reaction surface, longer length of micro-channel can extend residence time of reactants on the catalyst surface, and consequently increase the methane conversion and hydrogen production. The pressure drop between inlet and outlet increases lineally with the increasing of length, while the height influences the pressure drop significantly when the height is less than 0.4 mm. Based on the overall consideration for the methane conversion and resistance characteristic generally, the suitable height of channel should be between 0.4 mm and 0.6 mm.(4) A two-dimensional coupled model was established according to plate micro-channel reactor, in one channel of which the methane catalyst combustion was carried out to provide the heat for MSR reaction in another channel. The impact of reaction condition and channel length on the reaction characteristic was discussed. Calculation results show that the temperature of channel in both sides is almost isothermal for the good heat transfer characteristic of micro-channel. The coupling of methane catalytic combustion and steam reforming in a plate micro-reactor has made it possible to realize well matching between two sides through regulating reaction conditions. A longer length of channel can result in a higher methane conversion,a lower outlet temperature and reduce the selectivity of carbon monoxide. Finally, the subsection layout of catalyst coating in the catalyst combustion side decreases the maximum temperature in the micro-reactor with a stable methane conversion ratio and hydrogen production, which is benefit for the inhibition of carbon deposited and sintering of catalyst, and the temperature difference of reactor decreases in flow direction.