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FIGURE 3.16 Schematic of apparatus to evaluate CFCC filters in chemical stream. Apparatus was designed and is operated by Oak Ridge National Laboratory.

After 1000 h in this 290°C (554°F) chlorosilane stream the silicon carbide-based CFCC had retained 85% of its original strength.

Corrosion tests were conducted to see if oxide-based CFCCs would survive as a material for constructing high-temperature heat exchangers. One heat exchanger, used to recover heat immediately downstream from an afterburner-equipped waste incinerator, needed materials that would withstand not only the heat but the corrosive exhaust gases from the incinerator. The incinerator feed contained heavy metals, alkali metals, and transition metals. Immediately downstream from the incinerator is an afterburner. CFCC samples were held in the middle of the stream of hot, corrosive gases immediately behind the afterburner. Fifteen test samples were mounted on three fixtures, five per fixture. Each sample was 15 cm in length. During the first 58 days of testing, the CFCC samples were exposed to 925-980°C (1690-1800°F) for a total of 560 h. The balance of the time was used in 37 cycles between this operating temperature and 370-450°C (700°-842°F), mimicking normal daily shutdown. Four cycles of longer duration also occurred. An additional 13 days exposure at 980-1040°C (1800-1900°F) completed the testing. Twenty additional cycles occurred during the final 13 days.

After exposure, the CFCC samples were cut into C-rings and their strength measured at room temperature and compared to pretest strength. The alumina and zirconia matrix CFCC specimens retained 73 and 75% of their strength, respectively.

Continuous fiber-reinforced ceramic composites have also been exposed to coal gas and ash to estimate their performance in a coal-fired hot-air heat exchanger. The exchanger tubes must perform at 1150-1260°C (2100-2300°F) during exposure to coal ash and flue gas. The ash contains high levels of alkali condensed out of the flue gas. A CFCC comprised of a silicon carbide matrix reinforced with silicon carbide fiber, produced by CVI, was chosen due to its excellent retention of physical properties in oxidizing atmospheres. Since it is susceptible to attack by alkali, previous testing predicted that this CFCC would perform satisfactorily if alkali is scrubbed from the flue gas and temperatures do not exceed 1150°C (2100°F). The current metal capability would be defined as suitable for an environment of low alkali content at 982°C (1800°F).

Foster Wheeler Corporation engineers inserted a horizontal probe into a direct-fired coal-burning boiler at Gallatin station, operated by the Tennessee Valley Authority (TVA). Placed onto the probes were four samples of CFCC tube measuring 2 in. inner diameter x 3 in. long x | in. thick. The environment included high levels of alkali and a temperature of 1750°F. After 8000 h of exposure, the CFCC sample were removed, inspected, and tested. The specimen had thick layers of coal ash on them, which was easily removed. There was no evidence of corrosion when analyzed both visually and by metallographic techniques. There was no measurable thinning of the CFCC specimens. There was no loss of physical strength.

Another application-oriented corrosion evaluation of CFCCs concerned their evaluation in steam crackers used to crack ethylene. Several CFCCs were evaluated in conditions simulating a steam cracker environment. The flexure bar-shaped test

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