A resistance-in-series kinetic model for the low temperature reaction of sulfur dioxide with limestone is presented. The resistances considered are the gas-phase transport of sulfur dioxide, the liquid-phase diffusion of both the sulfur species and the calcium species and the solid-phase dissolution of limestone. The model uses film theory to predict the liquid concentrations of the dissolved species and assumes an instantaneous reaction between the sulfur species and calcium species.
The kinetic model incorporates three rate equations for the removal of sulfur dioxide. When the rate of removal is limited by the diffusion of sulfur dioxide across the gas film surrounding the limestone particle, a gas-phase controlled rate equation is used. When the diffusion of the reacting species through the liquid film covering the limestone particle is the predominant resistance, a liquid-phase controlled rate equation is used. When the rate is limited by the dissolution of limestone, a solid- phase controlled rate equation is used.
The kinetic model is incorporated into a flow model for the fixed-bed Limestone Emission Control (LEC) system. The LEC system employs a fixed-bed of standard quarry-sized limestone to remove sulfur dioxide from coal-fired boiler flue gases. The flow modeling equations for the fixed-bed LEC system, which include simultaneous heat and mass transfer as applied to water-phase evaporation and condensation, are also presented.
The combined kinetic and flow model is subjected to a parametric study and the modeling predictions are compared with experimental results. A parametric study on the rate variables indicate that the modeling results are very sensitive to the values chosen for the solubility and specific surface area of the limestone. The modeling predictions for the water evaporation temperature and gas-phase saturation humidity are in excellent quantitative agreement with the psychrometric chart. The modeling predictions for the limestone bed temperature and sulfur dioxide removal efficiency are in excellent qualitative agreement with pilot plant experimental results.