Study On Diffusion Mechanism In Ca-based Sorbent Delsulfurization Production Layer

Recent study on the release control of SO₂ has become the most important focus in the study of clean coal combustion. Ca-based sorbent (limestone) has been widely applied in dry desulfurization technology, and still, its desulfurization efficiency and calcium utilizable efficiency are both low. In order to attain the theoretical supports for enhancing the efficiency of sulfur removal, the main research contents in this paper include: the main causes of micro-pore structure in sorbent particle, the fractal property of micro-pore structure in sorbent particle, the adhesion and fusion rule of grain aggregates, the reaction gas kinetics diffusion rule inside pores, the solid-state ion mass transfer law inside compact grain, the gas-solid mass diffusion's coupling control model. The details and results are as follows:(1) The technical improvement of DTU-2A thermal gravity analyzer (TGA) has been made to explore the influence of different CFB calcination conditions on sorbent's microstructure property. The TGA experiments show that, high concentration of CO₂ is liable to lead to carbonation, H2O steam promote limestone decomposition, high temperature accelerates limestone decomposition and causes sintering and aggravated micro-pore structure, and the impurities reduce the final conversion rate of limestone decomposition.(2) Fractal dimension is an important parameter for representing micro-particle structure properties. Comparison experiments show, both image analysis technology and mercury intrusion method can efficiently survey particle sample's fractal dimension. Also, more impurities or higher temperature lead to lower fractal dimension because of sintering. A determination method of multi-fractal spectrum about limestone calcination particle morphology is presented in this chapter.(3) The adhesion equation in three-dimension space was found to simulate the CaO grain's aggregation and fusion process in computer. The results were compared with experiment data from mercury injection apparatus. During limestone high-temperature calcinations, the appearance of grain fusion caused the decrease of fractal dimension, porosity, specific surface area. The concept of grains aggregate is presented in this chapter. According to this accept, the product layer is divided into two layers, one is the product layer of gas diffusion mechanism inside particle, and the other one is the product layer of solid-state ion exchange mechanism inside grain.(4) The experiment results on mercury injection apparatus show that, the size of pores in the CaO particle is 5-2000nm, their Knudsen values are high in the rang of K n>0.1. The diffusion of gaseous reaction SO₂ molecular inside particle layer occur rarefied gas effect. The fractal structure inside particle product layer leads to the decrease of gas diffusion capacity. The coefficient of gas diffusion inside particle product layer has been calculated out in this part, and also the concentration of reaction gas has been analyzed along the direction of diameter in the particle.(5) Through the energy spectrum analysis of scanning electron microscopy, ion diffusion flow was studied inside the product layer of grain. The Ca²⁺ ion diffusion model of growth outward is the main form of the mass diffusion inside the grain product layer, which is neutral diffusion, the driving force for diffusion comes from defect drift in the surface of grain. Using the compensation effect of conductivity in CaSO₄ product layer, it is obtained that the characteristic temperature of Arrhenius is 600 K, characteristic conductivity is 10-5.722S. The activity energy of solid-state ion diffusion inside grain product layer is 223.98KJ/mol by a different-heat analysis. The coefficient of solid-state ion diffusion inside compact grain product layer has been calculated out in this chapter.(6) The coupling control equation of sulfur reaction mass flow has been found, which results compared with the data from TGA experiments. It is shown that, the diffusion process of the sulfur reaction is coupled of two processes, one is gas diffusion process, and the other is solid-state ion diffusion process. The one of two processes, which diffusion rate is slower, controls the whole desulphurization process. The control effect of solid-state ion diffusion to the whole sulfur process is extremely important, especially during the beginning of reaction, which had been ignored by people in the past.