Corrosion in atmospheric fluidized bed combustors—The reactions of CaSO4 with Cr, Ni, Co, Fe, and several alloys

Abstract

When coal is burned in the presence of limestone in an atmospheric fluidized-bed combustor (AFBC), the sulfur emission can be kept below acceptable EPA levels. Calcining of the limestone produces CaO, which then forms solid CaSO4 by a reaction with the SO2 produced during coal combustion. The internal components (e.g., heat exchanger tubes) of the bed, however, become coated with a compact layer of CaSO4, CaO, and ash during combustion. It has been suggested that the presence of the sulfate on these hot metal surfaces is the cause of observed instances of accelerated oxidation-sulfidation. This paper presents results which support the above suggestion. The reactions between Cr, Ni, Co, Fe, alloy 800, 2.25 Cr-1 Mo, 9 Cr-1 Mo steels, or 304 stainless steel with CaSO4 were studied using differential thermal and thermogravimetric analyses. The reaction products were analyzed using X-ray diffraction, optical microscopy, and in some instances, X-ray energy dispersive analyses. The chromium-calcium sulfate reaction is the only case studied in which a sulfide is not formed. In that case, CaCr2O4 is the reaction product. In all other cases, the reactions are oxidation-sulfidation processes.