Influence of dioxin reduction on chemical composition of sintering exhaust gas with adding urea

J Cent. South Univ(2012)19: 1359-13632 springerDOI:10.1007sl177-012-1150yInfluence of dioxin reduction on chemical composition ofsintering exhaust gas with adding ureaNG Hong-ming(龙红明, Li Jia-xin(李家新) WANG Ping(王平)School of Metallurgy and Resource, Anhui University of Technology, Ma' anshan 243002, Chinac Central South University Press and Springer-Verlag Berlin Heidelberg 2012Abstract: With the addition of urea as an inhibitor, four groups of reducing dioxin emission experiments in sintering pot wereconducted. The results show that, adding 0.05%,0 1% and 0.5%(mass fraction)urea, the emission concentrations of dioxin are 0. 287,0.258 and 0.217 ng-TEQm, respectively. The dioxin emission rates drop substantially compared to 0.777 ng-TEQ/m'free ofurea. With an increase of the urea content, the concentration of so2 emission reduces sharply. (NH4)SO4 formed by the reaction ofSO, and NH, goes into the dust and part of NH3 is released before reaction with the emission of exhaust gas. The NO, emissionpresents an increasing trend because the reaction of NH3 and O2 at high temperature produces NO. Based on the consideration offactors such as the effect of reducing dioxin emission, and the chemical composition of exhaust gas. 0.05% is the optimum addingKey words: sintering; dioxin; SO2; NO, ureadecomposition are mainly adopted worldwide to reduce1 Introductiondioxin produced in the sintering process [9-15]. Thesemethods are effective in some ways, but such problemsSintering plant is an important part of the entire as the complicated technology, the great renovation ofproduction chain in iron and steel industry. It provides a process flow and the huge investment limit theirlarge amount of low cost, stable quality sinter for the application Based on the analysis of the present research,blast furnace. In recent years, the increasingly stringent the conclusion is drawn that it is more highly effective toregulatory requirements and market requirements of develop the new technology inhabiting the release ofsaving energy as well as reducing costs put double dioxin in the sintering process than the traditional endpressure on the sintering plant. The discharged pollutants treatment. However, studye influence of newof sintering plant mainly include particles, SO, NO, inhabiting technique on the chemical composition offluoride and dioxin. Among them, dioxin [1-3] is the sintering exhaust gas, and comprehensive evaluation ofmost toxic persistent organic pollutant (POP) in the feasibility of the technology based on emissionunintentional synthesized byproducts. It is a tricyclic reduction effect and its impact on the environment arearomatic compound, composed by one or two oxygen seldom reported.atoms joining with two chloro-substituted benzene ringsIn this work, emission reduction effect and itsIt totallythe environment are focused on. fpact on thedibenzo-p-dioxin (PCDD) and polychlorinated with different ratios is melted in water. Then, the water isdibenzofuran(PCDF), which is collectively known as added to the raw materials when the mixture is blendeddioxin(PCDD/s). Dioxin detection is on ultra-trace in the mixer. With the sinterng process going on, urea islevel, lower by several magnitudes compared with other discomposed by heating, and ammonia is produced in thepollutants. More and more researches [4-8] show that drying and pre-heating zone. with a series of chemicalthe long-term hazards of dioxin are much more serious reactions, the production of dioxin is inhabited. Thethan we now realize. Therefore, the prevention of dioxin optimum proportion of adding urea to inhibit the dioxinemission is obtained with the sintering pot experimentsAt present, high efficient flittering technology, Further, the influence on the chemical composition ofhysical absorbing technology, and catalytic中国煤化工studied to ascertainFoundation item: Project(50904001)supported by the National Natural Science FProvincial Key Science Foundation for Outstanding Young TalerYHCNMH G ProGram for innovativeRecelved date: 2011-03-15; Accepted date: 2011-05-11Corresponding author: LONG Hong-ming. Assocate Professor, PhD; Tel: +86-555-2311571: E-mail: yafthm@1261360J. Cent. South Univ.(2012)19: 1359-1363whether it increases the release of other pollutants(So bed is 600 mm, the ignition time is 2 min, the ignitionand NO, )by addition of urea. This research will provide temperature is 1 100C, the negative pressure is 12 kPa,basic experiment data for developing high efficiency and and the hearth layer mass is 2 kg. A pumping pore oflow cost dioxin emissions reduction technology in about 30 mm in diameter is opened on the exhaust gasintering process.pipeline, and is kept sealed. The schematic diagram ofot is shown in Fig. 1. Each instance of tests in2 Experimenone group was repeated three times, and all data are theaverage values of them.2.1 Material properties2.2.2 Analysis methods of dioxin, ammonia and exhaustThe sintering material came from the raw materialof the sintering plant of an iron and steel company. TheDioxin sample collection started from sinteringferrous raw materials mainly included the ore powderignition and ended at the BTP(Burn Through Point),( Newman, Kara, Yankee and FMG) and metallurgyaccessories(blast furmace dust, and rolling skin). Fluxwhich is the top point of the exhaust gas temperaturemainly included lime, limestone, dolomite, and fuel curve. Samples were collected in the filter membrane andincludes coal and coke. The chemical compositions of adsorption materials. The sample was removed andraw materials are listed in Table 1. In the experiments, asaved. The isotope dilution high resolution gasfixed ratio of raw materials was adopted, as listed inchromatography-high resolution mass spectrometryTable 2. The urea used in the experiments is the (HRGC-HRMS)was employed to detect the dioxin.Thecommon industrial urea, in which nitrogen content is collected sample was added with isotope-labeled internalthan 46.3%,and particle size rangesacquired for dealing0.85-2. 80 mm. The urea was dissolved and added with filter membrane and adsorption materials, and thenith water in the raw material and granulating process.it was purified and concentrated into final analyticalThe urea mIxing ratio in four experiments is given in sample.Table 3The method of Nessler spectrophotometry was usedto detect ammonia, with precision of 0.03 mg/m. The2.2 Methodsportable flue gas analyzer Km9406E produced by British2.2. 1 Parameter of sinteringKane Company was used to analyze the contents of oSintering process occurred in the sintering pot, of CO, SO2, NO in exhaust gas. A set of data was recordedwhich the diameter is 150 mm, the height of the material automatically every 20 s.Table l Chemical compositions of raw materials(mass fraction, %.aw ICPilbara blend60960.0870018591500500630.01Kara65190.18193005Yankee0310.140.74FMG57320.620065Blast furnace dust 38 45Rolling steel skin 69.02 1.91 2.73 1.31 0.68 0.23LimeLimestone0951.1530.6800243.030.603.488082Coke powder1.630.604.3206482.44Table 2 Ratio of raw materials(mass fraction中国煤化工PB Newman Kala Yankee SFIT FMG Blast fumace RollCNMH Gte Coal Coke12.751593159311.9311,937951603.004837852352.35J Cent. South Univ(2012)19: 1359-136336lTable 3 Urea mixing ratio(mass fraction,high temperature gas synthesis reaction, urea has noobvious inhibition effect; in contrast, the HCl forming in005the high-temperature reaction may lead to the increase ofdioxin concentration from the low-temperature synthesisIgnition coverreactions [16].3.2 Analysis of Oz and CO contentsThe contents of O2 and CO in the sintering exhaustSintering potgas in the experiments are shown in Fig. 2 and Fig 3,respectively. With an increase of the urea ratio, O2Mixturepresents an increase trend while the concentration of Coslightly decreases. The phenomenon displays theHearthimprovement of the fuel utilization efficiency. Combinedwith sintering technical index, it also proves theSuctionefficiency improvement. Vertical sintering speedgradually increases from 23.08 to 26.09 mm/min, sinterutilization factor rises from 1.49 to 1.70 t/h m), butdrum strength decreases from 62. 19% to 58.54%Fig. I Schematic diagram of sintering potTherefore, in the view of the negative influence of sinterdrum strength, the suitable ratio of urea should not be3 Results and analysishigher than 0.05%3.1 Effect on dioxin reductionThe results show that in 17 kinds of homologues ofdioxin, the discharge concentration of dioxin in theexhaust gas of sintering pot without adding urea is 0.777计ng-TEQ/m; with adding 0.05%, 0. 1%, 0.5% urea, it is0. 287, 0.258 and 0. 217 ng-TEQ/m, respectively, aslisted in Table 4. The reducing effect is remarkable. The14dioxin emissions are decreased by 63. 1%,66.8% and72. 1% compared with free urea, which illustrates thaturea has a significant inhibitory effect on the formationof dioxin in sintering process. With increasing the ratio02004060080000of urea, the reduction also increases, but the increasingTime/strend gradually decreases. For instance, the urea content Fig. 2 O2 content in sintering exhaust gasof the fourth group is 10 times of the second one, but thereduction is increased by only 9.0%. The reason is thatthe function of inhibiting the formation of dioxin by ureamainly depends on the low-temperature synthesisreactions. Stable nitride forms through the reaction ofammonia and Cu, reducing the catalytic activity ofCu?, and through the reaction of ammonia and HCL,reducing the chlorine source of dioxin formation For theTable 4 Analyzing results of dioxin emissionDioxinDioxinNHSample concentration/ emission concentration/(ng-TEQ m)reduction/% (mg")200400600800100012001400Time/s0.777Fig 3CO conon in sintering exhaust gas0.287中国煤化工CNMH Gering exhaust gas is0.21772.1in Fig. 4. The concentration of soz variesobviously. In the sintering process, there are two clearJ. Cent. South Univ(2012)19: 1359-1363peaks. The first peak appears when igniting, where the and S4 samples. The amount is far lower without addingSO comes from ignition gas. The second peak emerges urea compared with adding urea With adding more urea,when wet material zone disappears in sintering material sulfate radical content still presents an increasing trendlayer and the temperature in lower part of material bed but with lower speed. Therefore it can be inferred thatrises quickly. It is the critical point to release SOz in reaction(3)occurs. Further analysis indicates that aftersintering exhaust gas. with an increase of urea adding the reaction of So2 and NH, in material layer, (NH4)SO4content, the two peaks decrease gradually especially for is formed and goes into wet material zone in the lowerthe second one, which even disappear in S3 and $4 part of sintering bed with exhaust gas. Water is sosamples. The results indicate that urea does have sufficient that (NH,)2SO4 can be intercepted andremarkable effect on SO, emission reduction.dissolved in water. With the temperature rising inmaterial layer, permeability becomes better, and(NHA)2SO is precipitated and attached to dust, and thenis blown out of material layer with high-speed exhaustgas. Before the high temperature arrives the bottom ofsintering bed. it exists in exhaust gas pipeline, avoidingdecomposition[18]目300Furthermore, NH, of exhaust gas in each group isanalyzed and the result is listed in Table 4. It can be seenfrom the result that when the adding ratios of urea are 0and 0.05%, there is no emission of ammonia in exhaustgas; when the adding ratios of urea are 0. 1% and 0.5%the emission concentrations are 0.07 and 0. 11 mg/m1200l600respectively. So, if the addition of urea is excessive, partof NH, before reaction will go right into the exhaust gasFig. 4 SO, concentration in sintcring exhaustpipeline and escape with the release of exhaust gas,which is undesired obviously. As ammonia is atemperature rising in material layer, urea and SOz may emission will undoubtedly increase the environmentreact in the way [I7burthen, leading to the secondary pollu(H2)C0-+H2O-NH4COONHzNH4COONH-=2NH3+CO(2) 3, 4 Analysis of NO, contentThe concentration of NO in sintering exhaust gas isSO2+2NH3+H2O+0.5O2=(NH4)2SO4(3) shown in Fig. 5. During middle time of sintering process,The reaction of SO, and NH3 produces (NH). the curve presents a regular trend with a platform. TheGibbs free energy increases with the temperature rising. difTerence is that the platforms of SI and S2 samples areUnder standard conditions, when the temperature is essentially coincident, while the platforms of $3 and S4above 800 K (527 C), the reaction cannot proceed to the samples present a significant rising trend, from anright. This illustrates that in sintering process, reaction(3) average of 200 to 230 and 275 mg/m, respectively. NO,mainly occurs in the middle-lower part of material layer synthesis is closely related to the temperature. When thebed during the period before the sintering BTP arrives. temperature rises in the range of 760-840C in materialDuring this period, the relatively higher negative layer, reaction(4)instantly occurs From the perspectivepressure and the slow flow speed ofexhauof the sintering process, this temperature range justthe reaction fully-reacted. It is shown that SO2 appears in sintering middle time and maintains for aconcentration varies with time, which coincides with the period of time. If O2 is sufficient, reaction(5)furtherexperiment resuitsoccurs, from which NO2 is formed. Thus, the NOThere is absolutely no(NH )SOa in sinter because concentration platform rises obviously, but the emissionwhen the temperature is higher than 1 200 oC, liquid regularity keeps nearly the same.phase forms consolidated sinter, and (NH4)2SO4 shoul4NH3+5O2-4NO+6H2O(4)be completely decomposed Besides,(NH4)SO4 may go 2NO-O, -2to dust. Sintering dust in dust catcher and dust left inexhaust fan rotor are collected and soaked in water. The中国煤化工 nd environmentalanalysis result of the chemical components in water protecC MH Gf NO, emission isshows that the sulfate radical contents of dust are 0.59% undesirable. lhus, the amount of urea addition should3.98%,4.52%, and 5.66%, respectively, in Sl, S2, S3 not be higher than 0.05%.J. 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