【作者】 漆波；

【导师】 李隆键；

【作者基本信息】 重庆大学 ， 工程热物理， 2009， 博士

【摘要】 燃料电池是高效、节能和环保的发电方式,在可移动及小规模电源系统中具有广泛的应用前景,其氢源制备是燃料电池商业化发展的关键。甲烷资源丰富且容易获得,是碳氢燃料重整制氢中首选的燃料。在甲烷重整制氢方式中,甲烷蒸汽重整具有反应温度相对较低、产氢率较高和产物中一氧化碳含量低等优点,是解决燃料电池氢源的一有效途径。目前,对甲烷蒸汽重整制氢的研究主要集中在高活性催化剂的开发上。甲烷蒸汽重整反应是强吸热过程,该反应在固定床反应器中因受热质传输的限制,存在动态响应慢、催化剂利用率低等问题,人们转而研究具有良好传热传质性能的微通道反应器。但微通道尺度小,采用颗粒催化剂容易堵塞通道且压降较大,催化剂的利用率低,所以催化剂涂层的制备是微通道反应器发展的关键因素之一。传统的催化剂涂层制备方法工艺复杂、成本较高,存在涂层附着力不强、容易脱落及堵塞通道等问题。因此,本文采用冷喷涂技术制备了可用于甲烷蒸汽重整的镍基催化剂涂层,对催化剂涂层上甲烷蒸汽重整的本征动力学及其在微通道反应器内的反应性能进行了研究,本文的主要工作如下:(1)创新性地采用冷气体动力喷涂技术在铝基板和不锈钢基板上制备了镍基催化剂涂层,利用扫描电镜(SEM)、能谱分析(EDS)和X射线衍射分析(XRD)对催化剂颗粒、基板及催化剂涂层进行了表征分析。结果表明:喷涂颗粒在铝基板和不锈钢基板上实现了有效的沉积,催化剂涂层与基板结合紧密,其组成和催化剂成分相近,催化剂涂层中没有新的相生成;冷喷涂技术制备的镍基催化剂涂层基本是单层沉积,对于易碎、不规则、氧化物聚合体的镍基催化剂颗粒来说涂层质量优劣不仅和颗粒沉积速度有关,还和其破碎特性有关;与铝基板上沉积的催化剂涂层相比,在不锈钢基板上沉积的催化剂颗粒更小,这对催化剂涂层上的甲烷蒸汽重整反应是有益的。(2)在冷喷涂技术制备的镍基催化剂涂层上首次进行了甲烷蒸汽重整的本征动力学实验研究,得到了双速率方程的本征动力学模型。甲烷生成一氧化碳的反应速率指前因子为1.08×108 mol/(h.g.kPa0.89),活化能为178.98kJ/mol;甲烷生成二氧化碳的反应速率指前因子为1.73×104 mol/(h.g.kPa2.06),活化能为139.00kJ/mol。F统计检验所得本征动力学模型的复相关指数大于0.9,且F统计量大于置信域为99%的临界F统计量的10倍以上,结果表明此模型是显著的、可信的。(3)对甲烷蒸汽重整在微通道反应器中的反应性能进行了实验测试和数值计算,并对实验后的催化剂涂层进行了表征分析。研究表明:较小的甲烷空速或较高的反应温度可提高甲烷转化率和产氢率,较高的反应温度或较低的水碳摩尔比时产物中一氧化碳的选择性较高;催化剂涂层表面因甲烷裂解而产生了积碳,且是以不会造成催化剂失活的胡须状积碳为主;数值计算结果与实验结果的相对误差总体在15%以内,因此采用CFD的有限速率模型对微通道中的甲烷蒸汽重整进行数值分析的方法是可行的。然后首次考察了微通道结构参数对甲烷蒸汽重整性能和流动阻力特性的影响。结果表明:较小的通道高度能够减小反应物在通道中的扩散时间,较长的反应通道长度能够增加反应物与催化剂的接触时间,甲烷转化率和产氢率提高;通道长度线性地影响微通道进出口压降的大小,而在通道高度小于0.4mm时,通道高度显著地影响进出口压降的大小;综合考虑微通道的重整性能和进出口压降等因素,通道高度在0.4~0.6mm之间较为合适。(4)针对甲烷催化燃烧为甲烷蒸汽重整反应供热的平板型微通道反应器建立了二维的耦合模型,分析了反应条件、通道长度对反应器重整性能的影响,并通过甲烷催化燃烧侧催化剂的分段布置对反应器的温度场进行了优化。结果表明:微通道反应器具有良好的传热性能,除进口附近外,隔板两侧的反应通道近似等温;通过调整反应条件可实现平板微通道反应器中甲烷催化燃烧和甲烷蒸汽重整两侧热量的良好匹配;反应通道长度增大,重整侧甲烷转化率和产氢率提高,反应器最高温度和出口温度降低;最后通过对甲烷催化燃烧侧催化剂的分段布置,在甲烷转化率和产氢率基本不变的情况下,降低了反应器的最高温度,有利于减少催化剂烧结及积碳的发生,且反应器在流动方向的温差减小。更多还原

【Abstract】 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.更多还原