ANALYSIS OF SIMULTANEOUS DIFFUSION AND CHEMICAL REACTION IN THE CALCIUM-OXIDE/SULFUR-DIOXIDE SYSTEM

There has been significant interest in the reaction between calcium oxide and sulfur dioxide because of its application in the reduction of gaseous sulfur emissions resulting from coal combustion by dry scrubbing and in fluidized bed combustion. This study includes a theoretical and experimental effort to study this reaction.; A model is proposed which considers the important diffusion and reaction steps that are important in the sulfation of calcined limestone. The model describes the reacting solid as a sphere made up of randomly-oriented open pores. The macroscopic properties of the solid are obtained by integrating over the entire pore size distribution. The evolution of the pore size distribution is followed by use of a population balance.; The predictions of this model are compared to kinetic data obtained using a thermogravimetric analyzer (TGA). Data obtained by the author and others show excellent agreement with the model. The predicted penetration of the sulfur into the reacting pellet is in qualitative agreement with results obtained with a scanning electron microscope. From estimates of the surface reaction rate constants it was determined that the rate controlling step was identical in the presence or absence of oxygen.; Finally the results of experiments to study the effect of temperature on the reaction between sulfur dioxide and calcium oxide in lignite ash and limestone are presented. Four different lignite ashes and two different limestones were studied. Similar trends of decreasing total conversion with increasing temperature were observed, but it was not clear that these trends have the same causes.