在双碱法烟气脱硫系统的亚硫酸盐氧化的抑制作用【最新】.doc
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1、Available online at JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 ScienceDirectJournal of Environmental Sciences 19(2007) 226231CN 11-2629/Xwwv.jese.ac.dnOxidation inhibition of sulfite in dual alkali flue gas desulfurization systemMO Jian-song, WU Zhong-biao”, CHENG Chang-jie2, GUAN Bao-hong1, Z
2、HAO Wei-rong1. Department of Environmental Engineering, Zhejiang University. Hangzhou 310027, China. E-mail: 2. Zhejiang 7ianlan Desulfurization and Dust-Removal Co., Ltd., Ilangzhou 310012, ChinaReceived 18 January 2006; revised 29 Match; accepted 21 April 2006AbstractA laboratory-scale well-mixed
3、thermostatic reactor with continuously blasting air was used to investigate the oxidation inhibition of sulfite in dual alkali flue gas desulfurization (FGD) system. The effects of operating parameters such as pH value and catalyst concentration on the oxidation were studied. Sodium thiosulfate was
4、used in the system, and was found that it significantly inhabited the sulfite oxidation. In the absence of catalyst, sodium thiosulfate at 12.67 mmol/L had an inisibition efficiency of approximately98%. While in the presence of catalyst, sodium thiosulfate at 26.72 mmol/L had an inhibition efficienc
5、y less than 85.0%. The oxidation reaction order of sulfite in the sodium thiosulfate was determined to be 1.90 and 0.55 in the absence and presence ofthe catalyst, respectively. Apparent activation energy of oxidation inhibition was calculated to be 53.9 kJ/mol. Pilot tests showed that the consumpti
6、on rate of thiosulfate agreed well with the laboratory-scale experimental results.Key words: dual alkali; sodium sulfite; oxidation inhibition; sodium thiosulfateIntroductionCs(Iv) is the concentration of S, CM is the concentration of catalyst and C02 is oxygen pressure. The parameters suchas oxygen
7、 concentration and sulfite concentration wereconfirmed consistently both in homogeneous and hetero geneous system by theoretical and experimental studies. However, there are few work have been done on the effect of the catalyst concentration and pH value on oxidation rate.Sodium thiosulfate was demo
8、nstrated to be an effective oxidation inhibitor, which is widely used in industry. Pilot tests were made on sodium thiosulfate oxidation inhabitation in the sodium sulfite/bisulfite solution for a limestone regenerated dual alkali system (Chang and Kaplan, 1983). The successful application had been
9、per formed in a magnesium-enhanced lime system at the Philips Station of Duquesne Light. TVA Shawnee Station tested sodium thiosulfate in a prototype spray tower. It was found that only 100 mg/rn3 of thiosulfate was needed to reduce the sulfate saturation and scale-free operation (Chang and Brna, 19
10、86). Chang and Bma (1986) found that the concentration of thiosulfate ion from 100 to 200 mg/rn3 could effectively avoid scaling. And it was also con cluded that the consumption rate to achieve the oxidation inhibition effect should be in the range of I to 3 mmol Na2S203/mol SO2 absorbed both in lim
11、e and limestone systems. Later sulfur instead of sodium thiosulfate as an inhibitor has been studied for commercial purpose (Owens et a!., 1988). Thiosulfate was produced by sulfur reacting with the sulfite. Because of the low solubility of sulfur, the conversion of the sulfur to thiosulfate was fbI
12、 very high. Phenolic compounds were also known asDue to the oxidation reaction between the sul fite/bisulfite and dissolved oxygen, sulfate can be produced both in the lime and limestone flue gas desulfurization (FGD) system. In the present of calcium ion, sulfate in the absorbent liquid will lead t
13、o the production of calcium sulfate, which is the main component of scaling and foul ing. Accordingly, the reliability of the FGD system will be lowered. During regeneration step of dual alkali FGD system, sodium sulfate will react with lime and produce calcium sulfate and sodium hydroxide. And beca
14、use of the high hardness of calcium sulfate, the absorbent liquid need to eliminate the hardness in a second sedimentation tank by sodium carbonate. One reason for the problem is that the boilers tend to be operated at higher levels of excess air. Another one is that the heavy metals in absorbent su
15、ch as manganese, cobalt, and copper catalyze the oxidation rate. So oxidation inhibition is necessary for dual alkali FGD system to improve the reliability.Uncatalyzed oxidation and catalyzed oxidation were investigated both in homogenous and heterogeneous system in a review (Linek and Vacek, 1981).
16、 It is found that the oxidation rate of sulfite can be expressed as:R = kC52CC(公式有误)where, R is the oxidation rate, k is the reaction constant,Preoject supported by the Hi-Tech Research and Development Program (863) of China (No. 2001AA642030-l), the Key Research Project of Zhe jiang Province (No. 2
17、004C23028) and New Century Excellent Scholar Program of Ministry of Education of the Peoples Republic of China (No. NCET-04-0549). *Corresponding author. E-mail: .Fig. I Sketch of the experimental apparatus.effective inhibitors for the oxidation in both of organicand inorganic, but it is harmful to
18、human health and hostile to the environment. They act as chain breakers by receiving excess energy from a “hot chain molecule” (Jeu and Alyen, 1933), or by scavenging free radicals, transients in a chain reaction (Huie and Neta, 1985). Wanda et at. (2003) explained the inhibition mechanism during th
19、e process of the removal of quinine. It was known from their work that the reaction transient acted as a catalyst, which transformed sulfite into sulfonates in the reaction. Most of these works focused on the oxidation and oxidation inhibition by thiosulfate in the lime or limestone slurry or in hig
20、h diluted sulfite solutions. In this paper, we studied the influence of catalyst concentration and pH value on sulfite oxidation, sulfite oxidation inhibition by thiosulfate, the optimal parameters and the consumption rate of the inhibitor in concentrated solutions.the temperature (T) was controlled
21、 at 2555C. A complexpH electrode connected to pH value measurement instru ment (PHS-25) was used. The experiments were carried out both in the absence of catalyst and in the presence of manganese sulfate. The concentration of catalyst varied in the range of l.0x1065x103 mol/L. The concentration of5
22、mmol/L was a high level for catalyst, and it was measured in the wastewater of desulfurization system in Hangzhou Banshan Power Plant. The pH value was controlled below10.0. It was found that when pH was higher than 10.2, the catalysis will change from acceleration to deceleration of the oxidation r
23、ate (Lim and Hamrick, 1984).The concentrations of sodium sulfite and sodium thiosulfate in the concentrated solution were measured analytically by means of an oxidation-reduction titration as the following: (1) a certain amounts of sulfite solution and thiosulfate was added to an excess of iodine-io
24、dide standard solution which was then back titrated with 0.01 mol/L of sodium thiosulfate in arsenous acid solution with starch as an indicator; (2) the concentration of thiosulfate was analyzed directly by formaldehyde fixation of sulfite followed by iodine-iodide oxidation and back titration of ex
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