Figure 5. Temperature contour maps in the integrated
combustion-reforming reactor in which combustion chambers are in direct
thermal contact to reaction chambers for an endothermic steam-methanol
reforming reaction.
The steam mole fraction contour maps are illustrated in Figure 6 for the
integrated combustion-reforming reactor in which combustion chambers are
in direct thermal contact to reaction chambers for an endothermic steam
reforming reaction. For efficient operation of the steam reforming
reaction, large surface areas are required to transfer the heat from the
combusted gases to the tubes [41, 42]. In reformers presently used
for steam reforming small diameter reaction tubes are clustered closely
together in the furnace so that heat transfer from the combusting gases
in the reactor into the catalyst packed tubes is optimized [43, 44].
The use of a plurality of tubes to accomplish heat transfer contributes
to the large size and high cost of the reformer. In fuel cell systems in
which the reformer and the fuel cell are fully integrated, namely the
combustion gases for the reforming reaction are derived from the fuel
cell exhaust, the shell side heat transfer coefficient between the hot
gas and the tube is characteristically low and hence, the rate of
reaction is limited primarily by the rate of heat transfer. This problem
is particularly severe at the reactor entrance as the rate of the
endothermic reaction is very high, and thus, the amount of heat required
is very high while the shell side heat transfer coefficient is often low
as the mechanical design of typical reactors often allows the gases in
the shell to be relatively stagnant near the tube entrances. This leads
to a drop in the overall efficiency as a large portion of each reactor
tube operates at an undesirably low temperature. Thus, in order to
effect complete conversion, the reformer must be relatively large and
expensive. It is the primary object of the present study to provide a
novel process and apparatus for the production of hydrogen by steam
reforming of methanol that can be accomplished with a thermally
efficient reformer of reduced size and cost which can be integrated with
a fuel cell power system or used as a standalone hydrogen generator.
Production of hydrogen by steam reforming of methanol or other
hydrocarbon fuels is accomplished in a reformer of substantially reduced
size by superheating a gaseous mixture of water and methanol to a
temperature of about 548 K and then passing the superheated gaseous
mixture over a catalyst bed contained in a reformer. At least a
substantial portion of the heat for the endothermic steam-methanol
reforming reaction is provided by the sensible heat in the superheated
steam-methanol stream augmented by heat transferred through the tube
wall depending on the overall system considerations. The concept of
providing a substantial portion of the heat for the endothermic
reforming reaction by sensible heat in the superheated steam-methanol
stream is referred to hence forth as direct heating. Direct heating is
of considerable advantage as it largely overcomes the problems
encountered with reaction rates being limited by the rate of heat
transfer through the tube wall especially near the reformer entrance and
thus, for a given conversion, the reactor may be smaller, more efficient
and less expensive. High steam-to-methanol ratios are required for
direct heated reformers. The relatively large amounts of steam passed
through the bed continuously clean the catalyst by removing ethanol and
suppressing production of carbon monoxide and retard catalyst poisoning
thereby enhancing catalyst stability. Direct heated reformers are
particularly suitable for integration with fuel cells as the heat and
fuel values contained in exhaust stream from each component can be
utilized in the other. Hydrogen contained in the exhaust gas from the
fuel cell anode may be burned to superheat the methanol-water mixture
being fed to the reformer. Once the reactor is warmed up, the entire
fuel requirements for the system are provided by the methanol being fed
to the reformer. In order to accommodate the endothermicity of the
reforming reaction, at least a major portion of the heat required for
reforming is provided to the reformer as sensible heat contained in the
superheated gases. Thus, when methanol and steam vapors contact a
catalyst such as a combination of zinc oxide and copper oxide at a
temperature of about 548 K at atmospheric or higher pressure, methanol
decomposes to carbon monoxide and hydrogen and the carbon monoxide and
steam react according to the well-known water gas shift reaction to form
carbon dioxide and hydrogen. High exhaust gas temperatures may be
indicative of unreacted fuel entering the exhaust chamber while low
exhaust gas temperatures are indicative of a low fuel feed rate, or an
unreactive catalyst bed. The efficiency decreased because the thermal
losses as a percent of the total amount of power fed to the device
increases as the size is reduced. In reactor configurations comprising
solid catalyst particles disposed as a bed outside a plurality of heat
transfer tubes, the layout of such heat transfer tubes is of critical
importance, since it would be desirable to achieve a uniform temperature
distribution across the radial direction of the reactor.