THE EFFECT OF THE COMPOSITION OF FLUIDIZED-BED COMBUSTOR BED MATERIALS ON THE EROSION CORROSION OF CARBON-STEEL

The relative amounts of SiO2, CaO, CaCO3, CaSO4, and alkali compounds in the bed materials of circulating fluidized bed combustors determine the metal wastage of heat exchanger tubing alloys exposed to the particles. The weight ratio Of SiO2 to CaO and to CaCO3 is especially important in determining the erosivity of the bed materials. It was found that increasing this ratio increased the metal wastage. A sharp transition ratio occurred, on either side of which the metal wastages were linear. The slope of the linear curve was low for the lower sand content mixtures and higher for the higher sand content mixtures. The transition ratio required a higher CaO content for higher velocity particles. The transition ratio of SiO2 to CaCO3 is related to the formation or lack of formation of a protective layer on the metal surface. The transition ratio of SiO2 to CaO is related to the formation of a thick protective layer or a thin, less protective layer on the metal surface. In the thick layer, CaO mechanically mixed with iron oxide to form an intimate mixture whose CaO content decreased inward from the gas-scale layer surface.