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Refractory, the heat-resistant materials that line the inside of most furnaces, are an important component of any steam generator because they protect expensive and vital parts of the generator from thermal shock, caustic chemicals, erosion from slag buildup, damage from falling slag, and catalytic heat. (See "Understanding Refractory Failures," p. 48, for an in-depth look at these phenomena.) Most refractories are more heat-resistant than metals and are needed in parts of the plant where temperatures exceed 1,000F.
According to the Refractories Institute, there are a wide variety of refractory compositions, but they fall into only two categories: brick and fired shapes, and monolithic refractories typically fabricated from plastics, castables, gunning mixes, or ramming mixes. Combinations of both types also are in use.
Although refractories may look like ordinary construction bricks, they are anything but. Refractory designs can have very complex or unusual geometries, and the finished products run the size gamut. Refractory is typically produced from combinations of compounds such as alumina, fireclays, bauxite, chromite, dolomite, magnesite, silicon carbide, and zirconia.
CFBC crucibles
ACC Refractories (Nagpur, Maharashtra), which has provided the refractories of 13 of the 20 circulating fluidized-bed combustion (CFBC) plants in India, notes that CFBC plants are good test sites for analyzing refractory performance because of their high temperatures and caustic chemical environment.
CFBC boilers have had great success in Europe and Japan because of their ability to burn low-grade fuels such as peat, wood waste, and pet coke, as well as hazardous wastes, very efficiently. NOx and SO2 emissions from CFBC plants are low due to the high operating temperature of the boiler. And although the high turbulence of the fluidized bed and the long residence time of coal in the cyclone improve the combustion efficiency of these plants, the high heat and caustic chemicals that are part of the CFBC process can cause refractories to be damaged.
Most of the fuel used in India for power generation is coal, lignite, or pet coke of highly variable quality. In particular, sulfur content can be all over the lot, making it very difficult to adjust feeder concentrations of limestone needed to counter the chemical. If levels of corrosive sulfur oxide gases generated are not kept in check, they can wreak havoc on refractory.