| OCR Text |
Show COMBUSTION EFFICIENCY NEAR PREDICTED LEVELS - The combustion efficiency achieved was about 50 percent and this result has revealed the need for developing a method for usefully oxidizing this unburned fuel. In fact, developing this method must, at some point, go handin- hand with the development of the tube itself if the entire concept is to be practical. Some methods have already been envisioned, including afterburning or recycling with heat recuperation. NO~ EMISSIONS VERY LOW - Emissions ot NO from the tube were almost unde~ectable -- less than 5 ppm v (air-free) at all test conditions. The same NOx levels were achieved in Alzeta's previous work with catalytic honeycomb monolith combustors, and it appears that the present radiant tube concept exhibits the same emissions performance. However, final verification of this low NOx emission characteristic will have to wait until a method of achieving complete combustion is devised. TUBE AND CATALYST MATERIALS ANALYSIS After combustion, SEM analysis of the tube interiors suggested that the following two important mechanisms affect platinum coating catalytic activity: 1) Oxidation of Pt and subsequent transport away from the coating site (Pt02 evaporates readily at high temperatures) • 2) Reaction of Pt with silica at the tube wall (the silica source being either silicon carbide or generated by the oxidation of SiC) to form low melting point platinumsilicon eutectics. It is well known (References 6 and 7) that under a wide range of temperature and pressure conditions platinum metal forms an oxide, Pt0 2 , on its surface and that this oxide will evaporate readily at high temperatures. This phenomenon is suspected as the mechanism responsible for the majority of the platinum coating loss from the catalytic radiant tube. This effect was particularly noticeable at the inlet of the tube, where the 02 content and temperature are both at their maximums. "Bare" areas were consistently noted at the inlet during the posttest inspections. A SEM picture of a "bare" inlet surface (Figure 4) shows 33 Fig. 4 - Severe etching of the platinum coating at the tube inlet the expected near-total removal of the platinum coating. Deep channels have been etched into the coating down to the SiC tube surface underneath. Although a uniform degradation of the coating would be expected, differences in the etching rate on individual crystals have been shown (Reference 8) to be due to differences in size and crystallographic orientation. Farther downstream along the tube the same etching phenomenon exists but is less severe than that noted at the tube inlet. What remains of the platinum coating is fairly well-dispersed and highly faceted in character. In spite of a large loss of material this coating retains enough catalytic activity to have resulted in a radiant tube. It is at this intermediate area that the second phenomenon affecting Fig. 5 - Low magnification of the upper middle tube wall |
