A Preliminary Study On The Conversion Of Semi-dry FGD Ashes Based On Its Characteristics

As the government attached more and more importance to the industrial SO₂emissionscontrol, calcium-based flue gas desulfurization （FGD）, especially the semi-dry FGD, hasbeen widely used in coal-fired power and sintering plants. However, the disposal problemcomes up as a large amount of desulfurization by-products. Due to the complexcomposition and calcium sulfite, the comprehensive utilization of semi-dry FGD ashes isdifficult. Therefore, they are mainly abandoned or stacked up which will occupy a largeamount of lands and cause severe waste of sulphur resources and second pollution. It issignificant to utilization the ashes for the development of semi-dry FGD technology andenvironment protection.In this thesis, the physical and chemical properties of industrial NID semi-dry FGDashes from certain factory were systematically analyzed. Based on the results, thedirection for the comprehensive utilization was proposed and a primary research on theoxidation transformation was conducted. The main contents of this work consist of thefollowing4parts:（1） The physical and chemical properties of the FGD ashes were characterized byX-ray diffraction （XRD）, Laser particle size analyzer, Thermogravimetric-DifferentialThermal analyzer （TG-DTA）. The results shown that the average particle diameter andthe half-content diameter of the ashes were13.01μm and12.78μm, respectively. Theingredients of the ashes are CaSO₃1/2H₂O, Ca（OH）₂, CaSO₄2H₂O, CaCO₃. Thecalcium sulfite in FGD ashes start to decompose at675°C under N2gas atmosphere andproduce SO₂.（2） An simple approach to detect the contents of Ca（OH）₂was established. Theefficiency of the desulfurization process was evaluated based on the amounts of theCa（OH）₂and sulfur. The direction of comprehensive utilization was also proposed. It wasfound that the systematic error of the Ca（OH）₂contents measured by acid-base titration method is±0.5%, the recovery of the added index was between97.4%¹06.0%. Theseresult showed that this method is valid. The content of CaSO₃was about26%±5%which was not suit for resource utilization. The content of Ca（OH）₂was about26%±6%,the efficiency of desulfurizer could be further improved. The contents of the total Ca andS were about49%±1%and27%±3%, respectively.（3） The stability of CaSO₃in FGD ashes under different conditions was investigated.The results indicated that the oxidation of FGD ashes started at450°C in air, at the sametime, a part of CaSO₃decomposed into SO₂due to the incomplete oxidation. CaSO₃cannot sustain for a long time, which would be oxidized into CaSO₄slowly in wet air.CaSO₃was easy to decompose into SO₂when pH was below4.（4） The feasibility of the conversion of FGD ashes was studied. The parameters such asmixing time, solid liquid ratio, the dosage of H₂O₂on the oxidation of CaSO₃in the FGDashes were optimized in the process of the FGD ashes, where H₂O₂was adopted asoxidant. The conversion of the CaSO₄into CaCO₃via adding NH₄HCO₃was also studied.The optimization conditions as followers: the mole ratio H₂O₂/CaSO₃is1.4, thesolid-liquid ratio is5:1, mixing2h., the alkalinity of the FGD ashes reduced from41.5%to29.2%. At the same time, the oxidation ratio of CaSO₃amounts was more than97%.The results of the CaSO₄conversion shown that at CaSO₄:NH₄HCO₃:NH₃·H₂O of1:1.1:1.1, the production ratio of CaCO₃is98%.