1. Introduction
The availability of hydrogen is the fundamental condition for use of fuel cells in mobile and stationary applications. As the use of fuel cells is becoming more frequent, for example, in automobiles it makes sense to restrict the operation of the energy generating units of the automobile to one energy source, such as methanol, gasoline, or diesel fuel rather than feeding each energy generating unit from a different source of energy, such as one for the Otto carburetor engine for driving, diesel for the heating system, and methanol for the fuel cell for air conditioning and current supply [1, 2]. For this reason, attempts have been made to utilize the customary fuels for the production of the hydrogen needed for the fuel cell [3, 4]. It is a well-established process in industry to reform higher hydrocarbons or alcohols to hydrogen [5, 6]. However, when applying this reforming process to obtain hydrogen for fuel cells, the equipment known to date still is rather big and, therefore, ill-suited for employment in mobile installations [7, 8]. The cause of another problem in producing hydrogen for fuel cells by way of reforming higher hydrocarbons or alcohols is the complicated nature of the chemical processes that occur in reforming and the consequential difficulty of conducting the reaction [9, 10]. Known aggregates for reforming hydrocarbons or alcohols, therefore, comprise expensive means of control and regulation to handle the complicated reaction processes and thus are not suited for use in mobile installations, such as automobiles.
It is, therefore, necessary to provide an improved process and apparatus for reforming higher hydrocarbons or alcohols, such as gasoline, diesel fuel, methanol, or methane, that will facilitate hydrogen production for a fuel cell in mobile equipment, especially vehicles. The provision and utilization of a microreactor network with its microreactors and microchannels permit high selectivity in influencing the various partial reactions which are intricately interconnected in reforming hydrocarbons or alcohols [11, 12]. The small dimensions of the reaction spaces in the microreactors make it easier to regulate and keep under control the reactions taking place and, therefore, reduce the necessary expenditure for mechanical equipment [13, 14]. It is another advantage that the microreactor network is particularly well suited as a means for producing hydrogen for non-industrial applications [15, 16] because the space requirement of the apparatus has been reduced considerably in comparison with known industrial installations [17, 18]. Apart from application in mobile equipment, the hydrogen obtained from reforming also may be put to use, for example, in fuel cells for housing energy supply systems [19, 20]. The use of microreactors for in-situ and on-demand chemical production is gaining increasing importance [21, 22] as the field of micro-reaction engineering matures from the stage of being regarded as a theoretical concept to a technology with significant industrial applications.
Various research groups have successfully developed microreactors for chemical processing applications such as partial oxidation of ammonia, nitration and chemical detection [23, 24]. One of the objectives of the research efforts is to demonstrate a working micro-reaction system for use as a sustained source of hydrogen fuel for proton exchange membrane fuel cells through catalytic steam reforming of methanol [25, 26]. The complete reformer-fuel cell unit is proposed as an alternative to conventional portable sources of electricity such as batteries for laptop computers and mobile phones due to its ability to provide an uninterrupted supply of electricity as long as a supply of methanol and water can be provided. Though considerable work already exists in the literature on the catalytic steam reforming of methanol for production of hydrogen using conventional reactors [27, 28], the use of microreactors for in-situ methanol reforming is a relatively new idea. Literature on the macro-scale steam reforming of methanol includes analysis of the reaction thermodynamics for prediction of optimum reactor temperature and feed compositions, catalyst characterization studies, and experimental studies on macro-scale pilot reactors [29, 30]. Results obtained in the study of methanol reforming in these conventional reactors form a good background for the development of prototype microreactors for this purpose [31, 32]. Silicon is considered a good material for fabrication of microreactors due to the high strength of the silicon-silicon bonds which results in the chemical inertness and thermal stability of silicon. Well established silicon micromachining techniques commonly used in the microelectronics industry facilitate easy fabrication of microchannels and other desired features on silicon substrates thus making silicon the preferred material for prototype microreactor fabrication.
The present study is focused primarily upon the exothermic and endothermic reaction characteristics and operation methods of integrated combustion-reforming reactors. The endothermic reaction is implemented in heat exchange with a heating medium. The endothermic reaction is implemented in heat exchange with an exothermic reaction. The exothermic reaction is implemented in heat exchange with a coolant. The design is described in the example of a process for the production of hydrogen from methanol and a compact reactor that is used in this case for simultaneous implementation of endothermic steam reforming and exothermic catalytic combustion, without being limited thereto. Such processes are essentially based on two catalytic reactions. First, a methanol-containing feedstock is sent into a process for catalytic steam reforming. This reaction is endothermic. The necessary heat for the reaction is supplied by catalytic combustion. The synthesis gas-containing reaction products of catalytic steam reforming are sent as feedstock into a process for further synthesis. Both the steam reforming and catalytic combustion reactions are implemented in compact reactors. Such compact reactors have several plates with flow channels, through which the respective gaseous and liquid media are sent. The media on the individual plates are in indirect heat exchange with one another and vary from plate to plate. The present study aims to provide a fundamental understanding of the exothermic and endothermic reaction characteristics and operation methods of integrated combustion-reforming reactors. Particular emphasis is placed upon the simultaneous implementation of the endothermic steam reforming and the heat-supplying exothermic catalytic combustion such that the thermal stability of the reaction system is increased.