Models for Wet Scrubbing of SO2 and NOx

University dissertation from Chemical Engineering II, Chemical Center, P.O. Box 124, S-221 00 Lund, Sweden

Abstract: Combustion of fossil fuels is the main source for emissions of sulfur dioxide and nitrogen oxides. Emission regulations and the growing environmental awareness will make great demands upon cost-effective deSO2 and deNOx techniques in the future. Many of the techniques available today have been on the market for a long time, however, most of them still have a potential for improvement. To be able to optimize existing techniques it is crucial to increase the understanding of the processes taking place within the scrubber, i.e. chemistry, mass transfer and fluid dynamcis. Mathemathical modeling is an important tool for increasing the understanding. This research work was divided into two different parts: the first part focused on identifying suitable absorbents for wet NOx removal and the second part focused on deriving a model for absorption of SO2 into a limestone slurry, wet flue gas desulfurization. The ability to absorb NOx were tested in a bubbler for the most common oxidizing agents and EDTA. Further experiments were done with the most promising absorbents, NaClO2 and KMnO4, in a packed column where the chemistry was studied in more detail. The absorption process was modeled and rate constants describing the absorption were estimated. A model based on the penetration theory was derived to calculate the absorption of SO2 into a limestone slurry droplet. The model includes instantaneous acid-base reactions as well as reactions with finite rates, e.g. limestone dissolution, CO2 hydrolysis, etc. The model was used to quantify the extent of spatial variations in mass transfer within a spray scrubber and the impact of the reactions with finite rate on SO2 mass transfer. Due to the significance of limestone dissolution a separate model taking into account the impact of the residence time distribution of a continuous system on the particle size distribution was derived. The model was verified by dissolution experiments in a continuous stirred tank reactor

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