Advanced structural ceramics with increased temperature capability and improved toughness are under development in a number of government/industry programs, including the ATS program (see above), the Integrated High-Performance Turbine Energy Technology program, including the U.S. Air Force, Navy, Army, Advanced Research Projects Agency, and the National Aeronautics and Space Administration (NASA), and the NASA Enabling Propulsion Materials program. Materials developed in these and other programs for high-temperature gas turbine applications may offer the higher operating temperatures and improved brittle fracture characteristics required for the ceramic heat exchanger in EFCC power generation systems. Since the proposed ceramic heat exchanger involves no moving parts, it is significantly less susceptible to deterioration from ash deposition or corrosion than are rotating components in the gas path of a turbine (LaHaye and Bary, 1994). However, the ash deposition and corrosion problems encountered using pulverized coal and the high-pressure cycles encountered in EFCC applications are unlikely to be addressed in materials development programs that are not targeted at coal-based technologies. In the view of the committee, the DOE coal materials program should focus on such issues specific to coal-based systems.
Current materials development and testing of the ceramic heat exchanger for EFCC systems is being conducted by Hague International (Orozco and Vandervort, 1993; Vandervort et al., 1993; Orozco, 1993; LaHaye and Bary, 1994). Activities are focusing on pressure and environmental testing. Over 2 million hours of successful operation of low-pressure ceramic heat exchanger units in corrosive high-temperature industrial environments has already been demonstrated. A series of tests is planned to demonstrate that a complete ceramic heat exchanger can contain pressures up to 1.21 Mpa (175 psia), endure at least 100 hours of operation under static and dynamic loadings, and meet thermal performance requirements. During these tests, the combustor will be fired with natural gas for operational simplicity. Subsequent testing with a coal-fired combustor will verify the ability of the slag screen to protect the ceramic heat exchanger from coal ash.
Ceramic materials demonstrate superior corrosion resistance compared to conventional metals and superalloys but can be severely degraded by alkali metals in coal combustion products. In particular, nonoxide ceramics such as silicon carbide (SiC) corrode in an oxidizing environment. The corrosion process is affected by the material processing technique, grain size, and impurity content. Hague International has conducted a series of corrosion tests on 46-cm (18-inch) long, 2.5-cm (1-inch) diameter tubular coupons of candidate heat exchanger materials, notably, an alumina matrix composite, reaction-bonded SiC, mullite (orthorhombic aluminum silicate, Al6Si2O13), and monolithic alumina (Al2O3). Preliminary results indicate that mullite shows the highest temperature capability and good corrosion resistance. After 300 hours at 1090 °C (2000 °F) with brief excursions to 1480 °C (2700 °F), little corrosion was observed.