Figure 7. Reaction rate contour plots in the gas phase of the catalytic
reactor with an integral heat exchange structure in an array of
longitudinally disposed, adjacent reaction passageways or channels which
are either catalyst-coated or catalyst-free.
The temperature contour plots in the catalytic reactor are illustrated
in Figure 8 with an integral heat exchange structure in an array of
longitudinally disposed, adjacent reaction passageways or channels which
are either catalyst-coated or catalyst-free. The catalyst in the
catalytically supported thermal combustion generally operates at a
temperature approximating the theoretical adiabatic flame temperature of
the fuel-air admixture charged to the combustion zone. The entire
catalyst may not be at these temperatures, but preferably a major
portion, or essentially all, of the catalyst surface is at such
operating temperatures. These temperatures are usually in the range of
about 1200 K to 2000 K. The temperature of the catalyst zone is
controlled by controlling the composition and initial temperature of the
fuel-air admixture, namely adiabatic flame temperature, as well as the
uniformity of the mixture. Relatively higher energy fuels can be admixed
with larger amounts of air in order to maintain the desired temperature
in a combustion zone. At the higher end of the temperature range,
shorter residence times of the gas in the combustion zone appear to be
desirable in order to lessen the chance of forming oxides of nitrogen.
The residence time is governed largely by space throughput, pressure,
and temperature, and generally is measured in milliseconds. The
residence time of the gases in the catalytic combustion zone and any
subsequent thermal combustion zone may be below about 0.08 second,
preferably below about 0.02 second. The gas space velocity under
standard temperature and pressure conditions may often be, for instance,
in the range of about 0.8 to 8 or more million cubic feet of total gas
per cubic foot of total combustion zone per hour. The total residence
time in the combustion system should be sufficient to provide
essentially complete combustion of the fuel, but not so long as to
result in the formation of oxides of nitrogen. In a preferred aspect,
the catalyst structure can be further characterized by catalyst-coated
channels that differ from the catalyst-free channels in one or more
critical structural defining elements which, in turn, take advantage of,
and expand upon, the concept of the increased tortuosity of the
catalyst-coated channels. In particular, the preferred catalyst
structure typically employs a plurality of longitudinally disposed
channels coated on at least a portion of their interior surface with
catalyst, that is, catalyst-coated channels, in heat exchange
relationship with adjacent channels not coated with catalyst or
catalyst-free channels wherein the catalyst-coated channels have an
average hydraulic diameter which is lower than the average hydraulic
diameter of the catalyst-free channels and the catalyst-coated channels
have a higher film heat transfer coefficient than the catalyst-free
channels [79, 80]. The average hydraulic diameter, which is defined
as four times the average cross-sectional area of all of the channels of
a particular type, for instance, catalyst-coated channels, in the
catalyst structure divided by the average wetted perimeter of all of the
channels of that type in the catalyst structure, is reflective of the
finding that the catalyst-free channels are most advantageously designed
to have a larger hydraulic diameter and to be less affected by changes
in configuration than the catalyst-coated channels [81, 82]. The
film heat transfer coefficient is an experimentally determined value
which correlates with, and expands upon the tortuosity of the average
catalyst-coated channel versus that of the average catalyst-free channel
in the catalyst structure [83, 84]. Further optimization of the
catalyst structure is obtained if, in addition to controlling the
average hydraulic diameter and the film heat transfer coefficient as set
forth above, the heat transfer surface area between the catalyst-coated
channels and the catalyst-free channels is controlled such that the heat
transfer surface area between the catalyst-coated channels and
catalyst-free channels divided by the total channel volume in the
catalyst structure is greater than about 0.8 per millimeter [85,
86]. A catalytic reactor thus provided with such passive substrate
cooling will be able to operate with a richer mixture of fuel and air
and at lower velocities without overheating and damaging the catalyst or
catalyst substrate [87, 88]. This, in effect, serves to raise the
maximum temperature of the catalyst. Another advantage of the
arrangement is that the reacting passages provide stable, high
temperature, continuous, and uniform ignition sources for the balance of
the unreacted mixture which then burns at the desired high temperature
just downstream of the catalytic reactor. In effect, the unit is a
hybrid of a catalytic reactor and a flame holder. Any hot surface acts
as a catalyst to some degree, hence even the non-catalyzed passages may
tend to provide some surface combustion. This effect will be minimized
by selecting a ceramic base material with minimal catalytic properties.
It may also be possible to control the boundary layer, decrease the
surface area, decrease the residence time, and perhaps even provide a
chain breaking or ignition delaying surface. The catalytic elements can
be engineered to provide the reactivity across the unit best tailored to
the fuel preparation zone characteristics, or to the requirements of the
catalytic reactor inlet pattern factor.