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

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.