Figure 3. Chemical enthalpy profiles along the length of 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 sensible enthalpy profiles along the length of the integrated
combustion-reforming reactor are illustrated in Figure 4 in which
combustion chambers are in direct thermal contact to reaction chambers
for an endothermic steam reforming reaction. The present design can
provide numerous advantages. First, light-weight and compact energy
sources can be obtained [37, 38]. Further, the rapid heat and mass
transfer in a small device can enable the use of extremely active
catalysts, catalyst which are active at low temperature, and catalysts
with high throughput per volume [39, 40]. It is also possible to
control process conditions, such as operating temperature, very
precisely, so that high performance can be attained. The fuel combustion
and steam reforming processes can be stably and efficiently operated at
lower temperatures, without the need for energy input to sustain or even
to start the micro-combustion process. In some instances, the
micro-combustor is started with hydrogen or vapors such as methanol.
Heat losses can be effectively controlled and reduced. Another advantage
is that the simplicity of the design and the materials used enable mass
production at competitive costs. Advantages can also include higher
conversions, lower carbon monoxide selectivity, and simplicity of
design. Another advantage that results from small size is better control
of heating and uniformity of temperature in a reaction zone. Another
advantage is an extremely fast response time, that is, a change in fluid
flow can result in a nearly instantaneous change in temperature.
Further, the micro-combustor or micro-reformer can be part of an
efficient integrated system, which can reform lower hydrocarbons and
even higher hydrocarbons that require higher processing temperature,
such as butane. Carbon dioxide selectivity over carbon monoxide, a
poison to fuel cells, of the steam reforming process is high, so that it
is possible to avoid or reduce requirements for removing carbon monoxide
after reforming and before supplying the gas to the fuel cell, thereby
greatly simplifying the overall system and reducing system size. Since
catalytic combustion is used, stable low temperature performance is
easily attained for the combustor to provide uninterrupted operational
heat for vaporizers and steam reformer units so they may operate in a
steady optimum manner. The low temperature operation and manufacturing
made possible by the design allows a greater choice of insulating
materials, enables greater use of materials with dissimilar thermal
expansion coefficients, and enables manufacture on semiconductor chips.
The combustors and reformers can be made from plastic. There are
numerous advantages of manufacturing in plastic including low weight and
less required insulation. Additional advantages include: more design
options, cheap high-volume manufacturing, eliminating the need for
expensive manufacturing machines compared to the equipment used in
silicon processing, and inexpensive materials. A thermal cycle is
heating a device up to operational temperature, operating the device at
an operating temperature and observing the results, and cooling the
device to about room temperature. Thermal efficiency is calculated by
dividing the lower heating value of the hydrogen in the reformats stream
by the total heating value of the methanol fed the reformer plus the
heating value of the fuel fed to the combustor. Volume of a combustor,
combustion chamber, reformer chamber or reformer, unless otherwise
indicated, refers to the internal volume where reaction substantially
occurs but not adjacent material. Where a catalyst is present, the
volume includes at least the catalyst volume and catalyst void fraction.
Volume of a device, unless otherwise indicated, refers to the combustor
and reformer volume and the volume of any intervening and integral
components such as heat exchangers, preheaters, vaporization chambers,
and recuperators. By directing the flow in this manner, a temperature
gradient is established between the center and the outer edges of the
catalyst bed with the highest temperature at the center of the thermally
conductive transverse separator plate located between the two reactor
chambers; thus, minimizing heat loss through the reactor walls. That a
fuel combusts on the combustion catalyst means contacting a fuel with a
solid catalyst, including within a porous catalyst or over a catalyst
coating. The efficiency could be substantially improved by removing the
thermocouple and by use of improved insulator materials such as
metallized polyimide, and it is believed that with these improvements
the devices can be thermally efficient. The steam may be introduced by
direct injection of steam and by saturation of the feedstock by contact
of the latter with a stream of heated water. The amount of steam
introduced is preferably such as to give a steam ratio in the range 2 to
3.5 moles of steam per gram atom of methanol carbon in the feedstock.