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1、Experimental study on the absorption behaviorsof gas phase bivalent mercury in Ca-based wetflue gas desulfurization slurry systemAbstractiGas phase oxidation and catalytic oxidation of element mercury (IIg) to bivalent mercury (Hg2)were proposed to improve the mercury removal efficiency in the wet f

2、lue gas desulfurization (WFGD) system. However, the re-emission of Hg, generated by the reduction of absorbed Hg2, would lead to a damping of the total mercury removal efficiency. In this paper, the absorption and reduction behaviors of bivalent mercury in the Ca-based WFGD slurry were evaluated in

3、our purpose-built device. According to our experimental results, the slurry chemistry (such as CaS3 content, SO42 , Cl and pH value) had a strong influence on the reduction of absorbed bivalent mercury. And the inlet concentrations of SO2 and 2 contribute little to the mercury absorption. Within the

4、 typical pH value range of 4.5-5.5, about 70% of inlet bivalent mercury was converted to Hg. The re-emission of Hg would be greatly retarded with the increase of SO42 due to the formation of HgS4 or Hg32S4. Moreover, it was found that Cl would also inhibit the reduction of bivalent mercury through t

5、he ligands reactions between Cl and Hgz .1. IntroductionMercury, due to its persistence, bio-accumulation and neurological toxicity, had received considerable attention from environmental engineers and environmental protecting institutions 1. According to the conservative estimation of Environmental

6、 Protection Agency (EPA), the exposure reference dose of 0.1 fig (mercury)/kg (body weight)/day was justified to protect against harmful neurological effects during fetal development and early childhood 2.Coal combustion was considered as the largest source of anthropogenic mercury emission. Mercury

7、 existed in the flue gas mainly in three forms: element mercury (HgO), oxidized mercury (Hg2一）and particle-bound mercury (Hg (P) 3. In the combustion zone, the mercury in the coal was first evaporated and turned to the elemental form (Hg), which was then partly oxidized to oxidized mercury (Hg2 ) by

8、 some flue gas components such as IIC1, S2, NOx and fly ash as passing through the down stream of the combustion zone. Some mercury was associated to fly ash or particulates and turned to particle-bound mercury (Hg (P), which was easily removed by dust collection 4, 5. Since the mercury in the flue

9、gas was mainly in element form (Hg) and was hardly captured by flue gas cleaning equipment, the majority of mercury was emitted.Active carbon injection method on mercury capture from coal derived flue gases had been widely investigated 6, 7. However, the major drawback of its application was the rat

10、her high operating cost of using activated carbon. Many efforts had been done to improve the economy of mercury removal process such as the use of inexpensive powdered carbon sorbents 8-10. Another way was to remove mercury and S2 simultaneously in the WFGD scrubber, which had been received many res

11、earch interests nowadays 11-13. Since mercury in flue gas mainly existed in element form, its solubility in the aqueous solution was very low. Tn general, the mercury removal efficiency by wet scrubbing varied from 40 to 80% depending on the coal type and combustion conditions. Thus, most of researc

12、hes were focused on the oxidation technology of Hg to Hg2+ by catalytic oxidation 14-18, photochemical oxidation 19-21 and gas-phase oxidation 22-25 to enhance its solubility, thereby improved the removal efficiency. However, little work had been conducted to identify the absorption of bivalency mer

13、cury and its reduction by the scrubber slurry. Some formerresearchers had found the Ilg2 reduction and re-emitted to the flue gas during their researches 26-28. However, detail information was not given due to the lack of information about the slurrys chemical and physical properties as well as the

14、complexity of the reactions. This drawback made it difficult to predict possible consequent re-emission and removal efficiency of mercury in FGD system.The experimental work in this paper was carried on by assuming there was 100% mercury oxidation by the oxidation technology in gas phase. The main o

15、bjective of the investigation was to examine the effects of slurry conditions (CaS3 content, pll value, SO42 , Cl , and slurry temperature) of the typical Ca-based FGD system on the bivalent mercury removal and its reduction behaviors. Further more, the contributions of main gas phase components (SO

16、2 and O2) to bivalent mercury adsorption were evaluated as well.2. Experimental and test methods2.1. Experimental apparatus and materials2.1.1. Experimental apparatusIn order to investigate the Hg2+ absorption and its reduction by the Ca-based WFGD slurry, a purpose-built device was designed, as sho

17、wn in Fig.Sampling*lcmpcralurc control device FIr. 1. The lab-device for Hg2* absorbing experiment.The system is composed of three parts: the Hg2+ generation oven, the flue gas mixing oven and the Hg2f bubbling absorbing reactor. The bubbling reactor was of a volume of 400 mL. A ceramic prilling spr

18、ay was attached to the end of the stem for generating tine bubbles and increasing the mixing of gas, liquid and solid phases.The IIg2f generation oven was made of glass and located in a silicon oil bath to maintain a desired temperature of 60。(1 A Teflon stir-bar was immersed inside the bath and the

19、 silicon oil was stirred to keep a uniform temperature.The flue gas mixing oven, which was also made of glass, was located in a different silicon oil bath. After passing through the flue gas mixing oven, the Hg2+-containted simulated flue gas and the diluent gas were well mixed and heated up to the

20、desired temperature. By adjusting the flow rate of the Hg2f-containted gas and the diluent gas, the designed flowrate and mercury concentration were both obtained. All the connectors and lines were made of glass or Teflon.At the beginning of each experiment, 300mL of absorption slurry was added to t

21、he Hg2+ bubbling reactor and then heated up to the predetermined temperature. 2 L/min of synthetic gas containing80|xg/m3 was introduced intoSI1iiithe Hg2+bubbling reactor. By determining the Hg24 and Hg concentration of the outlet simulated flue gas, the effect of different parameters on absorption

22、 and its reduction by the slurry were evaluated. The exhaust gas was decontaminated by passing through two washing-bottles and one adsorbent column. The bottles contained 200mL 4% (w/v) KMn4 with 10% (v/v)H2S4 solution and the adsorbent column contained 200mL C impregnated AC. After that the gas was

23、 directly emitted to outdoor.2.1.2. MaterialsCalcium hydroxide, concentrated sulfuric acid, concentrated nitric acid, potassium chloride and potassium dichromate were supplied by Sinopharm Chemical Reagent Co.Xtd. Concentrated hydrochloric acid was bought from Hangzhou Chemical Reagent Co., Ltd. All

24、 the chemicals mentioned above were of analytical grade. Mercuric chloride and calcium sulfite were bought from Shanghai Shenbo Chemical Co., Ltd. and the purities were higher than 99.9%. The water used in our tests was ultrapure water and supplied by the Zhejiang University.2.2. Test methods2.2.1.

25、Hg2+ and Hg measurementThe Hg2+ and Hg concentration were determined using the Ontario Hydro method. Flue gas passed though four impingers containing 20mL absorption solution. The first and second impingers contained Imol/L KC1 with 5% (v/v) IIC1 solution, which were used to absorb Ilg2 . Ilgwas abs

26、orbed by the 0.05% (w/v) KMn4 with 5% (v/v) H2S4 solution in the third and fourth impingers. After absorbing, several drops of 10% (w/v)were added to theabsorption solution until slightly purple color was stably obtained. All samples were then analyzed for mercury concentration using cold-atomic flu

27、orescence spectroscopy (AFS-230E,Bei jing Kechuanghaiguang Instrument Co., Ltd.).0.05% NaBH4 with 1% NaOH solution was selected as the reduction reagent in our experiment according to the instrument introduction. The maximum measurement error during our study was within 2%.2.2.2. CalculationIn this

28、article, the Hg2 removal efficiency ( ti Hg2 ), the Hg2+ reduction rate (T| Hg ) and the total mercury removal efficiency ( r Hg(tot) were calculated as follows:#(*)- ilOO(2)C3)where *n IIg2 is the IIg2 removal efficiency, Tillg0 the Hg2 reductionrate by the slurry, irillg(tot) the total mercury rem

29、oval efficiency, cllg2 out the Hg2+concentration in the outlet gas, cllg2+in the Ilg2 concentration in the inlet gas, and cHgout is the Hg concentration in the outlet gas.3. Results and discussion3.1. Effect of slurry chemistry3.1.1. Effect of CaS03 contentIn the Ca-based WFGD slurry, CaS3 was used

30、as the main S2 absorbent during its daily operation. Its effect on IIg2+ absorption and reduction was shown in Fig. 2.1009080706050AO3020100Fig. 2. EfFea of CaS3 content on Hg2+ absorption and reduction. Experimental conditions: gas flow ratc=2L/min: Hg2*=80p,g/m3; pH value=5.0; slurry volume-300 mL

31、: gas temperature* 110 C: slurry temperature=40 C.It could be seen that the CaS3 content in the slurry had neglected influence on Hg2 absorption, but had significant effect on the Ilg2 reduction. When the CaS3 content was low (2 +(8)3.1.2. Effect of CaS04 contentCaS4 was also the main component of C

32、a-based WFGD slurry. Thus, the effect of CaS4 content had been also investigated and the results were shown in Fig. 3.From this figure, it could be found that the increasing of the CaS4 had little influence on both bivalent mercury absorption and its reduction in the slurry. Though the solubility of

33、 CaS3 was lower than CaS4, the adding of CaS4 would slightly lower the SO32 concentration. It might inhibit the formation of HgS3 by reaction of the aqueous Hg2 with SO32 in Eq. (5)， which was reported to be unstable and immediately decomposed to Ilg and SO3 30,31. The S032 in the slurry was much mo

34、re than the absorbed Hg2+ and the slight decrease of SO32 due to the adding of CaS4 could be ignored to the formation of HgS3 for its instant reaction. The reduction of Hgf to Hg6.5The pH of the slurryAs shown in Fig. 4, within the pH value range (4.5-5.0) of the typical Ca-based FGD system, about 7

35、0% of total divalent mercury was converted to Hg and re-emitted to gas phase. As a result, the total mercury removal efficiency was only about 20%. About 30% of the total bivalent mercury was converted to Hgwhen the pH value was higher than 5.5 or lower than 4.0. The S(IV) in solution existed mainly

36、 in the form of SO2 II2O under the low pH value (5.0, the S(IV) in the slurry was mainly in the form of SO32 . The re-emission of Hg to the gas-phase mainly came from the decomposed of the HgS03 by the reaction of SO32 with Hg2 in the slurry according to the formercould be performed through Eq. (7)

37、or Eq. (8). Though the lower of SO32 might inhibit the Eq. (7) reduction, the reduction could go through Eq. (8) and did not affect the Hg2 reduction process. So the later simulated Ca-based FGD slurry was prepared by CaS3，not the mixture of CaSO；? and CaS4, for the convenience of the experiment.3.1

38、.3. Effect of initial pH value and temperatureFig. 4 showed the effect of the initial pH value on Hg2+ absorption and reduction in the slurry.o/JAOU-sealaAoluaa:200000000 98765432130354045505560Temperature (C )As shown in Fig. 5, the temperature had no effect on Hg2+ absorption during low temperatur

39、e range (30-40 C), but the Hg2* reduction was enhanced when the temperature was higher than 40 C. This result was consistent with the results of the previous study 32, 34. At temperatures below 45 C, the reaction Eq. (6) between S03 and HgSO was promoted, inhabiting the decomposition ofHgS03.researc

40、hers Eqs. (5)-(8) 31,32. But when the pH value was in the rang of 4.5-5.0, the S(IV) was mainly in the form of HS03 . Both HgS03 and HgS03H+ were formed by the reaction of Hg2+ with HSO3 in the solution. HgS3H+ would also decompose to form Hg according to the previous work 33. And the decomposition

41、of HgS3H+was kinetically significant and its rate constant was much higher than that of IIgS3, thereby accelerating the reduction reaction.The effect of slurry temperature on Ilg2 absorption and reduction by CaS3 was studied in our experiment and the result was shown in Fig. 5.sergpeoPAOUlK)gog20goo

42、9876I543QOOOQOOOO 91876514321HgS03 + S32- Hg(S3)22-3.1.4. Effect of SO/ concentration(9)0.0000.0050.0100.0150.0200.0250.030The conccntraiion of SO.2 (mol/L),.i.It was found that the S4 concentration had significant effect on Hg reduction. When the concentration of SO42 increased from 0 to 0.01 mol/L

43、, the HgT reduction ratio decreased from 45% to 10%. When the S4 concentration further increased, the value of Hg2 reduction ratio remained stable. As a result, 85% of total mercury was removed by the slurry.The oxidized mercury was considered to be absorbed by the solid particles presenting in the

44、slurry after the S2-CaO reaction 29. And it might have two possible ways to prevent Hg2* reduction: (i) it was more difficult to reduce bivalent mercury to element mercury after Hg2 adsorbed and wrapped by slurry particles and (ii) the Hg* reacted with SO42 to form a stable compound, HgSCU.The IIgS4 was greatly hydrolyzed in an acidic solution to form Hg302S4, which was readily in dilute mineral acids and existed in the solution, as un-dissociated molecules 35. The mercury in the WFGD byproduct was reported to be stable 36, which would partially support this conclusion. The1一 # |In