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.