Release of alkali salts and coal volatiles affecting internal components in fluidized bed combustion systems

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) < 840 °C, excess air (XSA) ≤ 20 %, static bed height (H_s) ≤ 0.2 m and fluidizing velocity (U_o) < 1 m/s. Under these operating conditions, a high alkali deposition may be expected to occur in heat exchange tubes located above the bed. Conversely, when the combustors operate at T_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.