Flue gas desulphurization using sodium sulfide on pilot scale

Flue gas desulphurization using sodiumsulfide on pilot scaleyu GuonangSchool of Environmental Science and Engineering, South China University of Technology, Guangghou 510640, China)Abstract: A novel technique of flue gas desulphurization( FGD) using industrial sodium sulfide as absorbent is de-scribed to remove SO, in flue gas. The FGD byproduct in this novel technique is sodium thiosulfate( Na2S,0,.5H,0Hypo )which can be used as chemical raw material. Optimal operating parameters about this technique have been determined. In order to enhance productive efficiency of sodium thiosulfate, EDTA disodium additive is added into absorptionsolution to prevent oxidation of sodium thiosulfate. Its optimal concentration is 0. 02 wt. % The pH value of absorptionsolution is set in the range of 5-6.5. Experimental results show that SO, removal efficiency averagely reach 98. 72%The highest productive efficiency of sodium thiosulfate reaches 83. 24 % The sodium thiosulfate formed during FGD cabe separated from saturated absorbent by filtration, concentration and crystallization. The filtrate after separating sodiumthiosulfate will be reused as SO, absorbent by replenishing some fresh sodium sulfideKey words: FGD; sodium sulfide; absorbents; sodium thiosulfate1 Introductionuation(46]. Meanwhile, the calcinations of limestonewill release tremendous amounts of carbon dioxide intoMost of flue gas desulphurization( FGD) technol- the atmosphere as a greenhouse gas, which is believedogies(S0, scrubbing technologies )in commercial ap- to cause global warming 71plication nowadays use calcareous compounds as soIt may be a novel FGD technique using sodiumesebents are mainly constituted sulfide to remove SO2 in flue gas. The sodium sulfideof limestone, calcium hydroxide and calcium oxide. possesses powerful absorbability for SO 2 because of itsThe limestone has plentiful resource in nature. It is strong alkalinity in water caused by hydrolysis of sodi-also convenient to be mined and conversed into calcium um sulfide. Hence, it is no doubt that FGD using sodi-hydroxide by calcinations and hydrationum sulfide could acquire higher removal efficiIn the past decade years, FGD technologies made The crucial problem to be solved is how to cona considerable progress in removal efficiency, reliabili- these different desulphurization end products into a sin-ty and costs. In particular, the limestone-gypsum gle chemical compound which could be purified or reprocess is the front-running FGD technologies. There used as useful chemical raw materialIr main types of FGD technologies, the spraThe sodium thiosulfate( Hypo, Na,S,O,.5H,O)scrubber, the packed tower, the jet bubbling reactor, perhaps is a more desirable end product, which isand the double loop reactor. Unfortunately, all widely applied in industries, agricultures, medicinesthese technologies still have some drawbacks in the and other areas. In thetime it is also much eas-process of FGD2.3. To generate an appropriate droplet ier to be handled and transported. What more impor-distribution, sprayers in a spray tower need to be per- tant is, because Hypo contains five crystal waters infectly designed to prevent being fouled. Plugging could every single molecule of sodium thiosulfate, its molecu-often occur in the packing layers of packed tower if the lar weight is as large as 248, which is 3 times largerslurry ph is not carefully controlled. Bubbling towers molecular weight than only 78 of that of sodium sul-also have similar problems. All those cases increase fide. Therefore, this desulphurization technique willthe operating risk and the final FGD costs .Moreo. bring better benefit than other desulphurization tech-ver, spray dry or semidry desulphurization technique niques. On the other hand, there is not plugging hap-eg a great deal ofpedifficult to be reused and only for landfillYH中国煤化工 fide because of itsCNMHGReceived 1 Nowember 2008Vol 8 No. 2, Jun 2010 39In this paper, we will discuss FGD using sodium designed to be a 2-meter- high packing tower made fromsulfide on pilot-scale absorption test; in particular, im- polystyrene. Its interior diameter was 300 mmpact factors about productive efficiency of sodium thio- The inner space was full with stuffing of raschig ringsulfate. The destination for this research will be to sup- Its porosity was above 90 % The concentration of so 2ply further design parameters for field-scale or commer- and other flue gases were detected on line by Flue Gascial application of this techniqueAnalyzer of Quinto KM-9106. Industrial sodium sul-Technological flowsfide was dissolved in preparation tank with a volume ofI m. The concentration of sodium sulfide was madeplish he pilot-scale absorption test has been accom- up about 1.5 mol.L'. A bit of dissolved sodium sulLtdfide was added with manoeuvering valve to the recircuGuangdong, China. The coal combustion boiler was a lation tank to maintain ph value of the absorption solugrate-fired furnace with combustion temperature ranging tion between 5.0 and 6. 5, which is detected by an ac-1 K-1 533 K. In order to meet experimental con- idometer during FGD. The absorption solution was re-ditions on pilot-scale absorption test, some flue gas a- cycled from the absorption tower to the recirculationbout 530 -600 Nm. hwas withdrawn from coal tank back and forth. Quantity of recycle solution variedcombustion boiler. The temperature of flue gas de- from 1.0 m. h"to 1. 5 m. h"adjusted by flow mecreased from 403-413 K at outlet of the boiler to 343 ter in response to quantity of the flue gas. The samples353 K at inlet of absorption tower, and the concen- of absorption solution were collected from recirculationtration of fly ash was minimized to 300 mg Nm"after tank per 5 minutes interval.dedusting with venturi scrubbers. Absorption tower wasAbsorption toweriBoilerVenturi scrubberRecirculation tankDA-Control valve; G-Gas flow meter S-SO, determiner, P--Pressure meter, O-LDfan,Fig 1 Pilot-scale flow chart of FGDsodium sulfide3 Results and discussionum polysulfide(Na,S,(x>2))would be released bythis chemical equation: Na?S,+ SO2(g)+H,03. 1 Flue gas absorptionNa2SO,+(x-1)S+H2st(g)( Fig 2). If pH valIn the former simulated experiment of FGD using ue was lower than 5. H,S vapor could also be given offsodium sulfide, the pH value in absorption solution中国煤化工 idic solution by thisshould be set at 5-6.5 to favor FGD. If the pH cherCNMHGaS03+HS↑(g),value was higher than 6.5, in particular, being set at which wouu unng adoutheavier environmental8-10, some H2s vapor generated by hydrolysis of sodi- pollution than SO,. For the SO, in flue gas, it can beabsorbed completely if ph value is higher than 5. For 96-115 mg. Nm, 191 -422 mg.Nm,1this reason, the ph value should be controlled in range 4 mg. Nm, and 295-975 mg. Nm"respectively atof5-6.5(Fig.2)concentrationg of O2, CO2, Co, no, no 2SO. rar9.5-10.8%,8.8~10%,105123mg·Nm3,325~377mg·Nm3,0mg·Nmand 0-17 mg. Nm respectively at outlet. In comparison with these data, we could draw a conclusionthat the concentrations of 02, CO,, Co, No at inletand outlet kept approximately stable, which indicatedthat these gases could not be absorbed by sodium sul-fide solution or conversed into desulphurization endproducts during FGD using sodium sulfidFig. 2 Relationship between concentrationsof SO,/H,S in flue gas and pHThe concentration of SO, decreased dramaticallyalue in absorption solutionfrom 295-975 mg. Nm"at inlet to 0-17 mg. NiThe concentrations of flue gases were detected on at outlet. The So, removal efficiency reached 94.23%line at inlet and outlet 8 times respectively on pilot-scale100 % Its average value was 98. 74 % which in-dicated that SO, could almost be absorbed by sodiumtest. The measured data were displayed on Table 1. Theof5.0~6.5of O2O2 ranged9.4%~11.2%,8.9%-10.1%in absorption solution(Table 1)Table 1 The concentrations of pollutants in flue gas before/after FGD using sodium sulfideTreatment7(02)7(CO2)c(co)c(NO)c(NO,)c(SO2)八mg·Nm3)/(嗎·Nm3)/(mg·Nm3)/(mg·Nm3) efficiency/9.510170003before9610.19.599.38S-Sbefore9.5100before108.9112before10.29.402020304038.9331before9.48.80Note: n( gas)/%-Volume percentage(%)o some pollutants in flue gases; c( gas)/(mg.Nm)-Some pollutants weight(mg)in flue ganormal cubic meter: ND-Not detected.3. 2 Desulphurization end producttration was too low, a lot of elemental sulfur powderDuring FGD using sodium sulfide, Na,$,O, among could occur in the absorption solution: 2Na2s+desulphurization end products was formed in the follow. 3502(g)= 2Na$,0,+ S. On the pilot-scale testing reaction: 2Na,S 2S0, (g)+02= 2Na, S, 0,. concentration of O, in flue gas ranged 9.5 9In this equation, the O2 concentration in flue gas should 10.8 %, which would lead to form complex desulphur-be set in a certain range to be favorably formed for so- iate$,O3, Nadium thiosulfate(8. If 0, concentration was too high, Na2s中国煤化工the Na, S,0, among desulphurization end prooductsCNMH OXidization of desul-would be oxidized to sodium sulfate by this equation phursome2Na, S, 0,+0,(g)=2Na, SO, +2S. If 0, concen- chemical additives such as EDTa disodiumvl8No.2,Jun.201041(CIoHN20, Na)were added into absorption solu- efficieney of sodium thiosolfate would reach 63. 56tion, Owing to coordination of EDTA disodium with 76. 16 and 83. 24 respectively. The additive en-these ions, the catalytic oxidization of Na,S,O, could sured that sodium thiosulfate could not be oxidized bybe hindered. Basically, there were four varieties of de- catalysis of transitional metal ions. There were greatsulphurization end products existing in the absorption concentrations gaps existing between sodium thiosulfatesolution after FGD using sodium sulfide. These were and other sulfur-bearing compounds. In addition toNa2S2O3, Na,S0,, Na, SO, and unspent Na, S. The this, the concentrations of other sulfur-bearing com-s203, Na,, and Na?S were all generallypounds nearly kept constant at lower level through thefied as reducing compounds. These compounds could FGD using sodium sulfide process. This gave us thebe determined by iodimetry. Accordingly, Na, S0, assurance of obtaining sodium thiosulfate as desulphuricould be eliminated from the total reducing compounds zation end product by filtration, concentration andby adding adequate formaldehyde to the absorption so- crystallization. The filtrate after separating sodium thio-lution, forming an additional product CH, (NaS0, )OH, sulfate would be reused as SO2 absorbent by replenis.which was used as a screening reagent. Similarly, hing some fresh sodium sulfide to keep pH value at 5Na, S could be also eliminated from the total reducing 6. 5. The main operating parameters for FGD using so-compounds by adding adequate zinc sulfate to the ab- dium sulfide are shown in Table 2.sorption solution, forming zinc sulfide precipitate, andTable 2 Main operating parameters forfiltering this precipitate subsequently. Thus, Na,O3FGD using sodium sulfideNa?SO,, and Na S in the absorption solution could be Nas EDTAmeasuredrangemeasured by subtracting the total reducing compoundstrationFlue gastration infor pH flux/from total consumption amount of Na,s(mol.L-value inalation the recir (Nmjthe recir-Nm'. peduring the total absorption process. Different experi- tank)mental systems have been accomplished under the conditions that edta disodium additive contents were0%,0.01%,0.013%and0.02% of total absorp-1.50-0.025-6.5530-6001-243-60tion solution respectively. The analytical results wereshown in Fig 34 ConclusionNasIt has been found to be highly feasibility for FGDusing sodium sulfide on pilot-scale test. Experimentalresults indicate that SO, removal efficiency averagelyreaches 98. 72 and productive efficiency of sodiumof sodium thiosulfateprobably EDTA disodium additive in absorption solu-tion. Optimal addition for the EDta disodium additive二in the absorption solution may be 0. 02 wt. %.ThepH value is set in the range of 5-6. 5 in the absorp-System 1 System 2 System 3 System 4tion solution. If the measures mentioned above areFig. 3 Measured results of varioustaken, general reaction will occur as 2Na,S +2S0,(g)ulfur-bearing compounds in the+O2=2Na$,O, The advantage for this FGD techption solutionique is taken of recycle fully and producing sodiumthiosulfate of desulphurization end product.The Edta disodium additive played a key role in Acknowledgmentsenhancing productive efficiency of sodium thiosulfateThe productive efficiency of sodium thiosulfate was onlyWe gratefully acknowledge C. L. Chakrabart44.47 without the EDTa disodium additive in sys- Dist中国煤化工 Carleton Universit,tem 1. If the additive concentrations in the absorption Canrefully and put for-solution were held at 0.01 % 0.013 and 0. 02 wardYHCNMHGt. from the system 2 to system 4, the productiveReferences[6 B Gorkem, H Oguz. Development of an active sorbent from fy ash[1]R K Srivastava, W Jozewict, C Singer. 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Spray dry absorption [9] Compilation Group of Handbooks of Analytical Methods for Chemi-cal Commercial Products. Handbooks of Analytical Methods forment[门].Fue,2002,81:18991905Chemical Commercial Products[ M]. Beijing, China: Chemical[5] J Warych, M Szymanowski. Optimum values of process parametertryh,1988272of the ' wet limestone fluesulphurization system'[JJAuthorShi Lin, male, was born in 1963 and graduated from Chengdu University of Technology in 1992. He is a spe-cialist in environmental engineering, especially in air pollution control. He has being engaged in the design,re-search and construction of flue gas desulfurization. As a researcher, He has published 28 papers and gotten 4 pa-tentswithin5years.HecanbereachedbyE-mail:Celshi@scut.edu.cnFoundation item: This research project was sponsored by National Natural Science Foundation(20877026)( cont from p.30)welding. The high efficient automatic welding technolo-Referencesgy is achieved, and the performance of joint meets the [1] Lei Yi, Zhang Lin. Microcomputer control technology of automaticrequirements of WPS, which provides the value of lowgirth welding with twin welding heads for offshore pile pipes [J]temperature impact ductility in-40 CManufacturing Automation, 2004, 26(11): 70-73[2] Gao Zhongyu. Mechanical and Electrical Control Engineering4) The capacity of wind resistance is improved byplying FCAW-S. Comparing with MMAW, the weld- [3] Song Bosheng. Theory, Algorithm and Technique of PLC Program-ing efficiency improves more than one time. And theming[ M ]. Beijing: China Machine Press, 2005offshore operation time is shortened obviously[4] Wu Shuxiong, Yin Shike. Guidebook for Wire Selection[ M].Bei-5)This technology has been complete and ma- [5] Miao zhangmu, Jiangjun, Wang Zhijian. A method for evaluatingture. It has better popularized prospects and can bringthe toughness of welding joints in jacket[ J]. China Oshore Plat-significant economic benefits to the offshore construc-form,2003,18(5):3437AuthorTang Deyu, male, born in 1961, graduated from Chongqing University and has been working in Research In-stitute of Engineering Technology of CNPC. Mr. Tang is a senior engineer in welding office. He has finished about15 scientific research items. Among them, 4 items have gained Ministry level scientific and technical advanced sec-ond prize. Mr. Tang can be reached by Tel: 022-66310709Foundation item Key science Research Item of CNPC (04B41HH中国煤化工CNMHGvo.8No.2,Jun.201043