A simulation using a thermodynamic equilibrium model was performed to predict the effect of ash compositions on the behavior of air pollutants during fluidized bed sludge incineration. To investigate emission characteristics, the incineration temperature, air-fuel ratio ( λT) and contents of calcium and chlorine were chosen as major operating parameters. For the analyses of the desulfurization of SOx and the pollutant emissions by the limestone addition, the Ca/S molar ratio was changed from 0.5 up to 7.0. According to the simulation, NOx was not influenced markedly by the air-fuel ratio, though greatly influenced by operating temperature. The effect of desulfurization with limestone addition was greatly influenced by ash composition, and desulfurization occurred over the Ca/S molar ratio of 5.5. This was attributed to the fact that the reactivity of the P component in ash to the added CaO was higher than that of sulfur in fuel.