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

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.

Coal-Fired Gas Turbine for Locomotive Propulsion

“…It should be noted that the bed is "deep" (i.e., with a depth/diameter ratio greater than unity, as distinguished from the more conventional shallow beds indicated in Figure 4) to provide adequate internal heat-transfer surface without requiring the coil to extend down into the secondary combustion zone at the bottom of the bed. By removing the coil from the region of combustion, problems of possible tube material attack by sulfidation in locally oxygendeficient spots (22) are avoided. Corrosion due to existence of such periodically stagnant spots in coal-burning beds with a nominally oxidizing steady-state environment (i.e., excess air) has been reported (23), so it is prudent to keep the internal heatexchange surface out of the combustion zone.…”