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.