Optimization and mechanism of wet desulfurization with fly ash based on response surface

To investigate the desulfurization effect and mechanism of fly ash as a desulfurized, single factor experiment and response surface method were used to analyze the parameters of ash slurry solid-liquid mass ratio , SO₂ volume fraction, and gas flow rate, and the reaction mechanism of fly ash wet desulfurization was investigated by X-ray fluorescence spectroscopy and scanning electron microscopy. The results show that within a certain range, increasing the solid-liquid ratio can increase the penetration time, the total adsorption capacity of SO₂, and the adsorption capacity of SO₂ per unit of fly ash slurry by increasing the pH. However, when the solid-liquid ratio exceeds 1∶1, the penetration time and adsorption capacity of SO₂ will decrease. With the increase in SO₂ volume fraction, the adsorption capacity of fly ash slurry to SO₂ first increased and then decreased. When the SO₂ volume fraction was 750×10⁻⁶, the maximum adsorption capacity was 64.35 mg, High SO₂ volume fraction can significantly reduce the penetration time. With the increase of the gas flow rate, the penetration time, the total adsorption amount of SO₂, and the adsorption amount of SO₂ per unit mass of fly ash slurry decreased. The results of response surface analysis are shown by denoting the three factors of solid-liquid ratio, SO₂ concentration and gas flow rate as A, B and C, respectively, and their interaction terms as AB, AC and BC, respectively: the primary and secondary order of the factors affecting the penetration time were C, B, A, BC, AC, and AB. The primary and secondary order of the factors affecting the total adsorption capacity of SO₂ is B, A, C, AB, BC and AC. The primary and secondary order of factors affecting the adsorption of SO₂ per unit mass of fly ash slurry is A, B, C, AB, AC and BC. The optimum desulphurisation process conditions are as follows: the solid/liquid ratio is 0.87, the SO₂ volume fraction is 472×10⁻⁶ and the gas flow rate is 1 500 mL/min. XRF and SEM analysis of fly ash before and after desulfurization show that the sulfur oxide content of fly ash after desulfurization increases significantly, and the original surface plate Ca(OH)₂ forms CaSO₄·2H₂O and CaO·Al₂O₃·3CaSO₄·32H₂O in bulk and rod. The possible desulfurization mechanism is that fly ash dissolves a large amount of Ca²⁺ and OH⁻ in water. SiO₂ and Al₂O₃ react with Ca(OH)₂ under strong alkaline conditions to form cementitious materials such as hydrated calcium silicate and hydrated calcium aluminate (C−S−H and C−A−H gels).When the sulfur-containing flue gas passes into the fly ash slurry, SO₂ is transferred to the liquid phase, forming H⁺, \mathrmHSO_3^- and \mathrmSO_3^2- , and H⁺ reacts with the fly ash slurry, leaching Ca²⁺, Fe³⁺ and other elements contained in the fly ash and catalyzing oxidation of SO₂ gas dissolved in the fly ash slurry to produce H₂SO₄ and CaSO₄. After combining with water molecules in the slurry, CaSO₄ was precipitated in the form of CaSO₄·2H₂O.