Modelling of sulfur retention in circulating fluidized bed combustors

The effects of operating conditions linear gas velocity, Ca/S molar ratio, type of coal and limestone, etc.) on sulfur retention efficiency in a circulating fluidized bed combustor were studied. The operating conditions affect the sulfur retention differently. To explain this, a mathematical model was developed. To determine axial voidage profiles, the model uses a modified exponential decay hydrodynamic model, which divides the bed into a dense region at the bottom of the bed and a dilute region above. In the dilute region a core-annulus structure with solid dispersion from core to annulus is considered. The SO2 retention rate is considered to depend on the bed height through the SO2 profiles generated by the coal devolatilization and char combustion rates, which change with the bed height. The model gives good predictions of the effect of the operating variables on sulfur retention in a CFB pilot plant of 20 cm i.d. and 6.5 m height, burning two different lignites and an anthracite with four types of Limestone.