Modeling of Volatiles Combustion and Alkali Deposition in a Fluidized Bed Coal Combustor

Abstract

A simplified kinetic model, coupled with the bed hydrodynamics and a volatile evolution region within the bed, was formulated to predict the extent of gas-phase combustion in a laboratory-scale fluidized bed coal combustor (FBC). A close examination has also been made to highlight the relevance of the reducing/oxidizing environment (computed with the present theoretical model) in relation to FBC materials exposed to fireside corrosion at high temperature, under various operating conditions. The model results revealed that, for high-volatile coals with particle diameters (d c ) of 1-3 mm and sand particle size (d s ) of 0.674 mm, over one third of the original coal volatiles may burn in the freeboard region at bed temperature (T b ) ≤ 850 °C and excess air (XSA) ≤ 10 %. These values, together with the computed equilibrium conversion of alkali chlorides to sulfates, may suggest that sodium and potassium salts present in the vapor phase are likely to accelerate hot corrosion of heat exchange tubes above the bed when an FBC operates at T b ≤ 840 °C, XSA ≤ 20 %, d c 890 °C and XSA > 30 %, high oxidation rates may be present for the in-bed tubes. At these higher T b values and XSA < 10 %, a sulfidation mechanism presumably influences the extent of corrosion on the metallic components within the bed.