The Corrosive Environment in the Fluidized-Bed Heat-Exchanger for CCGT Service

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Abstract
Corrosion and combustion diagnostic data were gathered in Battelle’s 0.6m diameter coal-fired atmospheric fluidized-bed combustor (AFBC). Corrosion probes, constructed from ring specimens of candidate heat-exchanger alloys, were exposed to the fluidized-bed environment during three different combustion experiments (50%, 20% and 0% excess air). An in-situ oxygen probe was used to monitor the oxygen partial pressure at the exposure locations. Two different mechanisms of material degradation were identified, i.e., both corrosion and erosion. An adherent deposit of bed material formed on all areas of the corrosion probes. The corrosion behavior of the alloys beneath the deposit, and the corresponding corrosion product morphologies, appeared to correlate well with predictions based on the oxygen partial pressure measurements from the exposure location. The results suggest the oxygen probe may be a useful diagnostic tool for locating regions with high corrosive potentials. However, the upstream faces of the corrosion probes were subjected to enhanced mechanical damage, and this periodic removal of both the deposit and corrosion products resulted in significantly more metal degradation. Also, this corrosion/erosion process may locally deplete the alloy in chromium, leaving it susceptible to severe sulfidation and/or accelerated oxidation. It was suggested that these locations would be the first to experience heat-exchanger tube failure, and the coupled corrosion/erosion process would be the failure mechanism.

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