Ten samples, from different FBC boiler systems burning petroleum coke, were chosen to study the development, structure and composition of deposits formed by agglomeration at various locations in the boilers. The work focused on examination of the samples by scanning electron microscopy (SEM) and dispersive X-ray analysis (EDX). Chemical analysis and other techniques were also employed. The results obtained have not brought to light any evidence of the participation of the liquid phase or of vanadium or alkaline metal compounds. The CaSO4 of the deposits is high (80 to 100%) and the agglomeration results from the prolonged sintering of CaSO4 particles, until a strong 3-dimensional framework is formed, in which other, unrelated particles may be trapped, without contributing to cohesion. While CaO is still available, “chemical sintering” associated with its conversion to CaSO4 appears to be important, but sintering also occurs by a slower mass transfer mechanism and continues after the depletion of CaO. Deposits formed in regions only reached by fly ash (convection section), and also in-bed deposits, grow from particles <50 μm, mostly very small ones, < 10 μm. Where the bed ash can collect (e.g., J-valves), the deposits grow by the sintering together of larger particles, 100–300 μm, which themselves appear to be conglomerates of smaller particles sintered together.

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