Process analysis of SO3 removal by Ca(OH)2 particles from flue gas

Abstract

SO3 in coal-fired flue gas will cause equipment corrosion and the emission of sulfuric acid mist. The performance of injecting the alkaline adsorbent into the flue gas to remove SO3 is affected by the adsorption of SO2 in the flue gas. Taking Ca(OH)2 as a typical alkaline adsorbent, a model describing the adsorption of SO3 and SO2 by Ca(OH)2 particles was established, based on the combination of the product-island-growth model and the grain model. The parameters in the model were determined by the experimental data of adsorption in the fixed-bed reactor. The adsorption process of a single Ca(OH)2 particle was studied by numerical simulation. The adsorption of SO3 and SO2 was found to involve two stages: (1) In the early stage when there existed unreacted grain surface in the particle, SO3 and SO2 mainly reacted with the left fresh grain surface. The SO3 adsorption was controlled by internal pore diffusion and surface reaction, in which internal pore diffusion played a major role. The SO3 adsorption showed a shrinking core mode. The SO2 adsorption was controlled by internal pore diffusion and surface reaction and occurred throughout the entire particle. The SO3 adsorption selectivity at the outer surface of the particle was the highest, and decreased with a decreasing dimensionless particle radius R/R0. (2) In the later stage when the grain surface was completely reacted, the SO3 adsorption was controlled by internal pore diffusion and product-layer diffusion, in which product-layer diffusion played a major role. The SO2 adsorption was controlled by product-layer diffusion. The product-layer diffusion coefficient in the SO3 adsorption process was significantly higher than that of the SO2 adsorption, so SO3 was still significantly adsorbed in the later stage. The adsorption process of Ca(OH)2 under different particle structure parameters showed that decreasing the particle radius and increasing the particle porosity and specific surface area increased the SO3 removal efficiency as well as the total conversion of Ca(OH)2. Decreasing the particle radius and the specific surface area and increasing the particle porosity increased the SO3 adsorption selectivity by Ca(OH)2. When preparing industrial adsorbents, the optimization of the particle porous structure should be given attention.