Combined removal of SO3 and HCl by modified Ca(OH)2 from coal-fired flue gas

SO3 removal by submicron absorbents synthesized via inhibition method: The product layer grows like parallel peaks

Injecting calcium hydroxide powder into the flue gas is an effective strategy for SO3 removal. However, commercial calcium hydroxide has several disadvantages, including large particle size, low efficiency, and unsuitability for excessive grinding. In this work, sub-micron calcium hydroxide was synthesized by an inhibition method and its performance for SO3 removal from flue gas was investigated on a pilot-scale platform (120 Nm3/h). When the concentration of sodium alginate solution was 100 mg/L, the average particle size of calcium hydroxide decreased from 13.66 µm to 0.84 µm, which improved the SO3 removal (92.1 %) and conversion of the absorbent. The results of the fixed-bed experiments indicate that the absorption kinetics of the reaction is consistent with the Bangham model. In addition, density functional theory verifies that calcium hydroxide captures SO3 by chemisorption. The AFM image shows that the calcium sulfate whiskers produced during the reaction grow like parallel peaks on the adsorbent surface. The calculations suggest that the driving force for SO3 adsorption originates from Ca-p orbital (Ca(OH)2) and O-s orbital (SO3) hybridization. This study complements the island growth mechanism for gas-solid two-phase reactions and provides an effective method for removing SO3 from flue gas in coal-fired power plants. In addition, it will provide an important reference for the development of submicron adsorbents.

Alkaline absorbents for SO2 and SO3 removal: A comprehensive review

Injection of an alkaline absorbent into the flue gas can significantly reduce SO2 and SO3 emissions. The article presents alkaline absorbents employed in industrial processes to remove SO2 and SO3 from flue gases, detailing their characteristics and applications across various process conditions. It summarizes the mechanisms and influencing factors behind SO2 and SO3 removal, outlines the impact of multi-component gases, particularly SO2, on SO3 removal in actual flue gases, and elucidates this competitive phenomenon from a theoretical standpoint. The article compares the application scenarios and efficiencies of alkaline absorbents across different processes, identifies the optimal combinations of various absorbents and processes, and proposes a synergistic approach for the removal of SO2 and SO3. The findings demonstrate that by injecting calcium- or sodium-based absorbents into dry processes, SO2 and SO3 can be removed efficiently and cost-effectively, with process optimization and absorbent modifications further enhancing the SOx removal efficiency. In the future, by blending two or more absorbents and applying them to dry processes, a synergistic removal of SO2 and SO3 can be achieved.

Experimental Investigation of Removal of SO3 from Flue Gas with Modified Fly Ash Adsorbents

The removal of nonconventional pollutants in coal-fired power plants, such as SO₃, has been receiving more and more attention. However, due to its unique nature, it is difficult to remove SO₃ effectively with the widely used wet flue gas desulfurization systems. Nowadays, dry-sorbent injection technology has become a promising method for SO₃ emission control in coal-fired power plants. The removal characteristics of SO₃ from flue gas with modified fly ash adsorbents were investigated in a fixed-bed reactor. Factors affecting the adsorption efficiency of SO₃ were studied, including modification method, modified fly ash adsorbent particle size, reaction temperature, and flue gas component. Combined with adsorbent characterization analysis, the adsorption kinetics of SO₃ by modified fly ash adsorbents were carried out with four different adsorption kinetics models. The results show that the SO₃ adsorption efficiency of the fly ash samples increases after modification; the best SO₃ removal performance of fly ash was achieved when 1.5 mol/L NaOH solution was used, with the highest SO₃ adsorption efficiency of up to 98.3%. The modified fly ash adsorbent particle size, water vapor content, and the addition of NO have little effect on the adsorption efficiency of SO₃. As the reaction temperature increases from 250 to 450 °C, the SO₃ adsorption efficiency first increases and then decreases, with an optimal reaction temperature of 350 °C. The addition of SO₂ would compete with SO₃ for adsorption and inhibit the uptake of SO₃ by the adsorbent. Adsorption kinetics data show that external mass transfer and chemical adsorption are the main critical mechanisms affecting the adsorption efficiency of the modified fly ash adsorbent in the SO₃ removal process compared to internal diffusion.