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Show In selecting a catalyst to be used to coat the interior of these tubes the following criteria were considered: o Combustion activity o Durability at temperature o Cost Previous work by Alzeta personnel on catalytic combustion demonstrated that platinum metal has the highest activity, but that it also had poor durability in its pure form and a high cost (about $7.50/gm for the acid precursor). A better choice in terms of cost and durability is one of the metal oxides proven to have catalytic properties, such as chromium oxide (Cr203). However, while Cr203 is more attractive than platinum from economic (costing only $0.09/gm) and durability standpoints, its required ignition temperature is higher: 980 C (1800 0 F) vs. 5400C (lOOOoF) for platinum. In any case, both platinum and metal oxide catalyst precursor materials were procured for the combustion experiments. TUBE PREPARATION The coating techniques used in applying a catalytic material onto the interior surface of the silicon carbide tube were evaluated in the course of this study in terms of the following characteristics: • Proposed coating procedures -how the coating precursor is prepared and what physical method should be used to apply this coating. • Coating adhesion how well the coating adheres to the SiC substrate under the temperature and flow conditions used. • Coating continuity -- how close the coating technique came to the conceptual optimum: 100 percent coverage of the SiC substrate with an oxide catalyst or 100 percent dispersion on the SiC substrate with a platinum catalyst. • Coating cost -- platinum metal was the coating of choice but efforts were made to use a less expensive, but still catalytic, metal oxide coating (e.g., Cr203' NiO). Platinum coatings were produced by pouring a 50 weight percent chloroplatinic acid (CPA) aqueous solution into the SiC tube. The CPA coating was then decomposed to 31 pl a tinum by careful heating using a propane torch. Four coa ts of CPA were required to obtain a visually shiny, well-dispersed coating. Cr203 was applied to the tube using a slmilar method. COMBUS~ION TEST APPARATUS A diagram of the combustion test apparatus is shown in Figure 3. This Compressed air (5-10 p.igl nov llOV Electric air p,.eheaters Soleno id .hut-off valve Pressure regulator Fig. 3 - Flow and instrumentation diagram of test apparatus apparatus provided steady, metered quantities of natural gas and air to the catalytic tube. To ensure complete mixing, the natural gas was mixed with the preheated air at a point about 10 pipe diameters upstream of the tube entrance. All data were recorded manually during the test runs. TESTING In general, the test parameters included variations in catalyst used, fuel flow rate and reactant stoichiometry. Specifically, the independent variables were as follows: • Catalyst type -- platinum or metal oxide • Tube length -- varied from 55.9 cm to 152.4 cm (22 inches to 60 inches) to investigate the effect on combustion efficiency. • Combustion air preheat -- the reactant temperature measured at the air/ fuel mixing point, just upstream of the tube entrance. • Firing rate -- fuel gas input. • Stoichiometry -- percentage of theoretical air for complete |
