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Show level. At this particular point, a critical mass transfer rate (i.e., gas and air molecules reaching the surface) is achieved, and sufficient heat is generated by catalytic combustion to oxidize the platinum and cause it to leave the surface. Eventually, the platinum coating etches away to expose the silicon carbide wall to the combustion gases. Once these "pockets" in the platinum coating have been created, a reducing atmosphere can come into existence locally and allow the formation of low melting point platinum-silicon eutectics. The precise effect of these eutectics on catalytic activity is not yet known, but it appears that these eutectics may, at the least, cover and deactivate sites that would otherwise be active. At the higher end of the reactant flow rate range, increased convective cooling stabilizes the platinum and prevents its loss. Figure 10 also shows the influence of stoichiometry on combustion stability. (The shape of the family of theoretical air curves is arbitrary.) The test data suggested that the range of reactant flow rate in which the loss threshold temperature was exceeded narrows 'as the stoichiometry becomes leaner. This may be due to greater convective cooling associated with leaner conditions. All of this suggests the need for a platinum catalyst with a higher melting point. If a means to increase the melting point and thereby decrease the potential for oxidation could be found, then the system could be run at temperatures above the present threshold and result in a higher and more desirable tube temperature. Exploratory research has indicated that the following issues must be addressed before the long-term viability of this concept can be determined: • Best tube (e.g., silicon carbide, other ceramic, or metal) and catalyst (platinum or metal oxide) combination • Catalyst coating durability over thousands of operational hours • Optimal reactant flow rate and stoichiometry for prevention of catalyst loss • For silicon carbide tubes, controlling or eliminating platinum-silicon generation by the tube itself It is recommended that these issues be addressed in a second phase 36 of research, Proof of Concept. work in this phase will be specifical~y directed towards answering questlons of a materials-related nature. phase 2 will be considered successfully completed when the radiant condition can be sustained in the tube for an indefinite period of time under laboratory conditions. Upon completion of Phase 2, the third phase, Prototype system Development, will begin. Hardware and systems-related concerns will be addressed during Phase 3 by building and testing a prototype furnace equipped with catalytic radiant tubes. The furnace will be a boxtype furnace, as depicted in Figure 2. Nominal design and performance goals of the prototype furnace are as follows: • Operating temperature: 12000 C (2200 0 F) • Heat inpu~: 45 kW (150 x 10 Btu/hr) • Number of tubes: 10 to 20 • Heat flux densitY2per tube: 47.3 to 94.6 kW/~ (30.5 to 61 W/in ) • Thermal efficiency: 50 to 75 percent (depends on furnace use temperature) • Combustion efficiency: near 100 percent • NOx emissions: less than 10 ppm v (air-free) Prototype ignition, heat recuperation and combustibles consumption methods will be incorporated into the furnace design. Upon completion of the prototype system development, several preproduction prototype furnaces should be built and field-tested prior to market introduction. REFERENCES 1. J. P. Kesselring and W. V. Krill, Acurex Corporation Draft Final Report, EPA Contract 68-02-3122, October 1982. 2. J. P. Kesselring, W. V. Krill, E. K. Chu, R. M. Kendall, Mechanical Engineering, 1980, 102 (8), 28. 3. J. P. Kesselring, W. V. Krill, M. J. Angwin, H. L. Atkins, EPA Report EPA-600/9-80-035, August 1980. 4. J. P. Kesselring, W. V. Krill, R. M. Kendall, ASME Paper 79-HT- 54, August 1979. |