Abstract
In spite of the potential advantages of atmospheric fluidized bed systems, experience has proved that, under certain environments and operating conditions, a given material employed for internal components could lead to catastrophic events. In this study, an attempt is made to establish material selection and operational criteria that optimize performance and availability based on theoretical considerations of the bed hydrodynamics, thermodynamics and combustion process. The theoretical results may indicate that, for high-volatile coals with particle diameters (d c ) of 1-3 mm and sand particle size (d s ) of 0.674 mm, a considerable proportion of alkali chlorides may be transferred into the freeboard region of fluidized bed combustors as vapor phase, at bed temperatures (T b ) s ) ≤ 0.2 m and fluidizing velocity (U o ) b > 890 °C and XSA > 30 %, a high oxidation rate of the in-bed tubes may be present. Nevertheless, for these higher T b values and XSA < 10 %, corrosion attack of metallic components, via sulfidation, would occur since the excessive gas-phase combustion within the bed induced a local oxygen depletion.
Highlights
Industrial power plants utilizing fluidized bed combustors (FBC) have been developed and operated worldwide
The modeling of the bed hydrodynamics and the kinetics of devolatilization, coupled with a calculation of the theoretical oxygen required for the volatiles within an evolution region in the bed, has satisfactorily predicted the combustion of coal volatiles for a fluidized bed combustor
For highvolatile coals with d^ < 5 mm, the evolution of volatiles in the bed is mechanistically controlled by both diffusion and chemical kinetics, while the mixing process of volatiles and oxygen in the bed region is the determining step for the gas-phase combustion
Summary
Industrial power plants utilizing fluidized bed combustors (FBC) have been developed and operated worldwide. In this context, a number of studies^ ^^ have reported that the occurrence of undesirable high temperatures (up to 200-300 °C in excess of the bed temperature) due to the volatiles combustion which escaped unburnt from the bed has led to serious problems in prototypes and commercial installations of such FBC systems (i.e. damage of materials of construction due to hot spots, fouling and corrosion of the heat transfer surfaces by softened ash, high C O and NG^ emission levels, etc.). The extent of external corrosion on heat transfer surfaces in the FBC may be predicted under various operating and fluidizing conditions
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