Figure 1. Methanol mole fraction 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 hydrogen mole fraction contour maps are illustrated in Figure 2 for
the integrated combustion-reforming reactor in which combustion chambers
are in direct thermal contact to reaction chambers for an endothermic
steam reforming reaction. Control of the relative proportion of
homogenous and heterogeneous combustion can be achieved by manipulation
of the jet design [33, 34]. Either homogeneous or heterogeneous
combustion can be increased as needed depending upon the application. As
an example, a microchannel combustor that does not include an
endothermic reaction may be enhanced via jet design by promoting
homogenous combustion to reduce methanol or carbon monoxide emissions or
to provide a hot gas stream for subsequent use in a unit operation. The
distribution of jet orifices may depend on the intended use of the
device. Hydrogen burns immediately, thus, to avoid hot spots, the jets
should be spaced more evenly over the combustion chamber. Methane, which
burns more slowly, preferably has jets loaded near the front of the
combustion chamber. When the fuel is syngas, the distribution of jets is
intermediate. The endothermic and exothermic reaction chambers
preferably contain catalysts [35, 36]. Catalysts suitable for
catalyzing a selected exothermic or endothermic reaction are well known
to chemists and chemical engineers. Catalysts, especially an endothermic
catalyst, can be a porous catalyst. Pore sizes in the range of about 0.6
to 600 microns enable molecules to diffuse molecularly through the
materials under most gas phase catalysis conditions. The porous material
can itself be a catalyst, but more preferably the porous material
comprises a metal, ceramic or composite support having a layer or layers
of a catalyst material or materials deposited thereon. The porosity can
be geometrically regular as in a honeycomb or parallel pore structure,
or porosity may be geometrically tortuous or random. The support of the
porous material is a foam metal, foam ceramic, metal felt, or metal
screen. The porous structures could be oriented in either a flow-by or
flow-through orientation. The catalyst could also take the form of a
metal gauze that is parallel to the direction of flow in a flow-by
configuration. Alternatively, the catalyst support could also be formed
from a dense metal shim or foil. A porous catalyst layer could be coated
on the dense metal to provide sufficient active surface sites for
reaction. An active catalyst metal or metal oxide could then be
wash-coated either sequentially or concurrently to form the active
catalyst structure. The dense metal foil or shim would form an insert
structure that would be placed inside the reactor after bonding or
forming the microchannel structure. Preferably, the catalyst inserts
contact the wall or walls that are adjacent both the endothermic and
exothermic reaction chambers. The porous catalyst could alternatively be
affixed to the reactor wall through a coating process. The coating may
contain a first porous layer to increase the number of active sites.
Preferably, the pore diameter ranges from tens of nanometers to tens of
microns. An active metal or metal oxide catalyst can then be
sequentially or concurrently wash-coated on the first porous coating.
The average pore size of the catalyst layers is preferably smaller than
the average pore size of the support. Diffusion within these small pores
in the catalyst layers is typically Knudsen in nature for gas phase
systems, whereby the molecules collide with the walls of the pores more
frequently than with other gas phase molecules. The catalyst, which is
not necessarily porous, could also be applied by other methods such as
wash coating. On metal surfaces, it is preferred to first apply a buffer
layer by chemical vapor deposition and thermal oxidation, which improves
adhesion of subsequent wash coats. The methanation catalyst can be a
single material or a mixture of materials such as a methanation catalyst
powder between methanation catalyst felts. A small reforming chamber,
such as having a diameter of 0.8 mm or less provides superior results by
enhancing uniformity of conditions such as reducing hot and cold spots
and reducing channeling through a powdered catalyst. In this case, the
reforming chamber is defined at one edge by the methanation catalyst.
Where the catalysts are powders, there will not be a sharp delineation
between reforming and methanation zones and some methanation catalyst
powder will intermix with the reforming catalyst powder so that some
methanation catalyst is present within the reforming chamber. The
reforming chamber volume is defined by the volume where there is a
significant amount of reforming catalyst such that a reforming reaction
could take place under normal operating conditions. Methanol and water
can be injected at room temperature into an inlet tube where they will
be vaporized by heat conducted through the tube from the methanation and
reforming catalysts. In practice, fuel processing systems may be
significantly more complex. For example, heat from a combustor can also
be used to supply heat for other processes such as steam generation that
can be utilized for a steam reformer, autothermal reactor and water gas
shift reactor. Heat loss is a function of surface area, lowering surface
area for the same amount of heat reduces heat loss and puts thermal
energy exactly where it is needed. Therefore, in some preferred designs,
surface area is minimized. For example, in some environments,
cylindrical reforming and combustion channels can perform better than
planar geometries. Heat from the combustion chamber transfers through
the thermally conductive wall into the endothermic reaction chamber and
along the length of the endothermic reaction chamber where less than 8
percent of total heat flux into the endothermic reaction chamber is
perpendicular to length.