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.