Hot-Corrosion Reactions of Calcium Sulfate with Cr, Ni, Co, Fe, and Several Alloys

摘要

When coal is burned in the presence of limestone in an atmospheric fluidized-bed combustor (AFBC), the sulfur emission can be kept below acceptable Environmental Protection Agency levels. Calcining the limestone produces CaO, which then forms solid CaSO sub 4 by a reaction with the SO sub 2 produced during coal combustion. The internal components (e.g., heat exchanger tubes) of the bed, however, become coated with a compact layer of CaSO sub 4 , CaO, and ash during combustion. The presence of the sulfate on these hot metal surfaces is a suggested cause of observed instances of accelerated oxidation-sulfidation. This paper presents results that support this suggestion. The reactions between Cr, Ni, Co, Fe, alloy 800, 2 1/4 Cr-1 Mo and 9 Cr-1 Mo steels, and type 304 stainless steel with CaSO sub 4 were studied by differential thermal and thermogravimetric analysis of metal-sulfate powders. The reaction products were analyzed by x-ray diffraction, optical microscopy, and in some instances x-ray energy-dispersive analysis. The chromium-calcium sulfate reaction is the only case studied in which a sulfide is not formed. In that case, CaCr sub 2 O sub 4 is the reaction product and may serve as a means for corrosion reduction in an AFBC. In all other cases, the reactions are oxidation-sulfidation processes. (ERA citation 08:038424)