| Limestone-gypsum wet flue gas desulfurization system is the most popular technology for the thermal power plants. The use of desulfurization additives in slurry is the most convenience and effective method to improve the desulfurization efficiency and gypsum quality.In this paper, the sulfur dioxide absorption process, limestone dissolution process and gypsum crystallization process of limestone-gypsum wet flue gas desulfurization system are studied. The paper focuses on the effect of desulfurization additives on the three main processes and expects to develop a new desulfurization additive, which can not only enhance desulfurization efficiency but also improve gypsum quality. This new desulfurization additive can meet the enterprise's desulfurization requirement and gypsum recycling and utilization standard.A bubbling device was used to simulate the sulfur dioxide absorption process in limestone-gypsum wet flue gas desulfurization. The optimum condition for the absorption of sulfur dioxide was determined, the reaction temperature was 323 K, the stirring speed was 350 r/min. The influences of slurry concentration, limestone type and additive on sulfur dioxide absorption process were also studied. Experimental results showed that desulfurization efficiency will improve with the limestone's total content of CaCO3 and MgCO3 increased. The limestone Grude-J's total content of CaCO3 and MgCO3 was 99.3%, which can improve desulfurization efficiency to 86.4% at the pH value of 5.2.Desulfurization additives of Mg(OH)2, (NH4)2SO4, NaNO3, MgSO4, MnSO4, CuSO4, adipic acid, citric acid, sodium citrate, benzoic acid, acrylic acid, succinic acid and acetic acid were investigated. Among those additives, MnSO4, adipic acid, acrylic acid and succinic acid were the best to enhance sulfur dioxide absorption process and improve desulfurization efficiency, which respectively enhance desulfurization efficiency to 91.5%,89.5%,90.6% and 91.4% at the pH value of 5.2 and enhance desulfurization efficiency to 91.9%,91.9%,91.8% and 92.9% at the pH value of 5.6.After the study of the sulfur dioxide absorption process, the method of automatic potentiometric titration was used to study the dissolution process of limestone. The results showed that at pH value 5.5, stirring speed 600 r/min, solution concentration of Cl- 0.2 mol/L, the activation energy of the reaction was 25.76 KJ/mol; and the relationship of mass transfer coefficient KL and stirring rate co was kL=0.0205 · ?0511. And five kinds of desulfurization additives were selected which can increase the conversion rate of limestone and promote the dissolution of limestone as:NaCl, NaNO3, (NHO2SO4, Mg(OH)2, MnSO4.In the study of the gypsum crystallization process, the reaction between CaCl2 solution and Na2SO4 solution was used to determine the optimum experimental conditions of the gypsum crystallization reaction as follow:reaction temperature 323K, stirring speed 300 r/min, and the initial solution pH value 7.0. And the effect of the impurities and desulfurization additives on the gypsum crystallization process was also studied. The results showed that:sodium citrate can promote the growth of gypsum crystal, the induction time increased from 4.3 min to 7.8 min; gypsum's moisture content reduced from 38.7% to 17.9; particle size of gypsum enlarged from 57.4?m 70.5 ?m. Mg2+, Al3+ and Fe3+ can inhibit the growth of gypsum crystal. The inhibiting effect of Fe3+ was most striking.Based on the conclusions of the three processes above, the improved limestone desulfurization equipment was used to study the effect of slurry components and additives on the quality of FGD gypsum. Mg2+, Fe3+ and Mn2+ which were prevalent in limestone were easy to be adsorbed in the{110} crystal planes of gypsum crystals composed of SO42- and Ca2+, so inhibited the growth of crystal in the a axis direction and made the crystal in the acicular structure. Cl- had some distortion effect on the gypsum crystal structure.The effect of acrylic acid and manganese sulfate on the desulfurization efficiency of limestone desulfurization system was obvious, while the effect of citric acid on the quality of desulfurization gypsum was significant. Acrylic acid and manganese sulfate can improve desulfurization efficiency from 90.5% to 97.6% and 96.8%, respectively.Through the combination experiment, the formula of manganese sulfate, sodium citrate and citric acid, which can improve the desulfurization efficiency and improve the quality of the gypsum, was finally determined as lmmol/L+1mmol/L+2mmol/L. This additive can improve the desulfurization efficiency to 98.2%, reduce the water content of gypsum to 0.4% and reduce the gypsum particles below 10 ?m significantly.This paper found desulfurization additives for limestone-gypsum wet fule gas desulfurization system, which can be the technology base of improving desulfurization efficiency and gypsum quality. |
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