of a coating, the use of gas derived from coal gasification is likely to have a significant impact on the choice of turbine blade coatings. The complex chemical reactions that occur at high temperatures, and the susceptibility of these reactions to small chemical changes in the coating and gaseous environment, suggest that significant effort will be necessary to develop and evaluate coatings for turbines used in coal gasification-based power systems. There are a large number of commercial coatings available, and a number of different application methods that influence the coating behavior, but there is no one coating that is resistant to all types of high-temperature attack. It has been suggested that in systems using coal-derived fuel, coatings on advanced superalloys and the alloys themselves will need to form chromia rather than alumina scales for increased corrosion resistance (Bannister et al., 1994). In the case of the substrate (blade) materials, this constraint may limit the availability of suitable high-strength alloys.

Recently, the use of thermal barrier coatings (TBCs) has proven extremely useful in extending the temperature capabilities of existing superalloys. TBCs are ceramic coatings applied over metal substrates to insulate them from high temperatures. They consist of a layer of stabilized zirconium oxide that is 0.12 to 0.38 mm (0.005 to 0.015 inches) thick applied over a bond coat composed of an oxidation-resistant metal coating. Although TBCs themselves are expected to be only minimally corroded by the more aggressive environment in coal-fueled turbines, both the substrate and the bond coat may be adversely affected.

The development of alternative turbine materials with higher-temperature capability than existing superalloys—notably monolithic ceramics and ceramic matrix composites—is being addressed in the ATS program. The potential improvements in high-temperature corrosion resistance of ceramic materials compared to state-of-the-art superalloys is of interest for turbines using coal-derived fuels.

Heat Exchangers

In terms of materials behavior, the critical requirements for the ceramic heat exchanger for EFCC power generation systems (see Chapter 7) are

  • to maximize operating temperatures for the proposed duty cycle, notably combinations of high-temperature and pressure;
  • to resist fouling and alkali corrosion, with emphasis on the latter for low-rank coals; and
  • to avoid catastrophic failure.

Although advanced ceramics offer excellent high-temperature properties, such as high strength, corrosion and erosion resistance, and refractoriness, they are subject to brittle fracture due to critical flaws. High-velocity fragments from a failed ceramic tube have the potential to initiate rapid sequential failure of the array of ceramic tubes in the heat exchanger. The current proprietary tube design permits ''graceful" rather than catastrophic failure.

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