Journal of Environmental Sciences Vol 17, No 2, pp 335-339, 2005IssN100l—0742CN-2629XArticle t:10010742(2005)020335-05CLC number: X701Document code: AAgglomeration rate and action forces between atomized particles ofagglomerator and inhaled- particles from coal combustionWEI Feng, ZHANG Jun-ying ZHENG Chu-guangNational Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail: jyzhang@ mailhostedu. cnAbstract: In order to remove efficiently haled-particles emissions from coal combustions, a new way was used to put forward the processof agglomeration and the atomization was produced by the nozzle and then sprayed into the flue before precipitation devices of powerstation boiler in order to make inhaled-particles agglomerate into bigger particles, which can be easily removed but not change existingrunning conditions of boiler. According to this idea, a model is set up to study agglomeration rate and effect forces between fly ash inhaledparticles and atomized agglomerator particles. the developed agglomeration rate was expressed by relative particle number decreasingspeed per unit volume. The result showed that viscosity force and flow resistance force give main influences on agglomeration effect ofinhaled particles, while springiness force and gravity have little effect on agglomeration effect of theirs. Factors infiuencing theagglomeration rate and effect forces are studied, including agglomerator concentration, agglomerator flux and agglomerator densityatomized- particles diameters and inhaled- particles diameter and so onKeywords: inhaled particles; agglomerator; effect forces; agglomeration ratearticulate collector. It can be operated that certain quantity of anIIntroductionagglomerator is atomized by an atomizing nozzle and forms many fineliquid particles that are sprayed into the agglomeration chamber. Thes. Q recently, hazards caused by inhaled particles of diameter smallerthan 2.5 um from coal combustion are getting a matter of environmentalagglomerator is a water solution and formed by solving a material of higlconcern( Zheng, 2002 ) In China, 70% of power stations have beenwater-solubility, high viscidity, good stability under high temperatures inadopting coal combustion operation to generate electricity, which leads to water. Because atomized liquid particles of the agglomerator have higha high inhaled-particles emission content from coal combustion. It isviscidity surfaces, they can be stick with fly ash particles only ifvery difficult for traditional removing equipments to control emissions ofatomized particles are contact with fly ash particles. Ininhaled- particles. Currently, some measures are put forward thatcomposition of liquid particles is further vaporized as time passes due toagglomeration pretreatments are set up to make inhaled-particleshigh gas temperature, which can make plenty of inhaled particles closelyagglomerate into bigger particles which can be easily removed byglomerate into bigger particles. The method is based on the theory oftraditional removing manners. Many pretreatment methods, includingsolidity agglomeration. According to the agglomeration method ,anelectrostatic agglomeration(Watanabe, 1995), magnetic agglomeration agglomeration model has been set up and studies the agglomeration effectWei, 2003), sound agglomeration Ricra, 1986),heating of an agglomerator on inhaled particles, which is important guidanceagglomeration( Lind, 1996)and light agglomeration( Di, 1996), are meaning for experimental studiesstudied. However it is difficult for these measures to use in the industryParticulatebecause of their shortcomings( Wei, 2003)providerA new effective method is put forward that agglomerators of highviscidity and high water-solubility are sprayed into the flue gas beforeprecipitators, which can not change existent removing manners and theBlowerAir preheaterrunning operations of the boilers. A model about agglomeration rate andeffect forces between agglomerators and inhaled particles was set upAgglomeratorAgglomeration rate has been developed by the expression of decreasingticle number per unit volume. Some influencing factors areAgglomerationTemperaturechamberalso studied and analyzedcontroller1 Agglomeration theory and methodAccording to the movement ways of inhaled particlesagglomeration phenomena are plotkinds: self-agglomerationFluc gas analyzerof quiet particles and agglomeration phenomena brought by particlemovement. All of the two sorts are including soft agglomeration effectand solidity agglomeration effect. Soft agglomeration is mainly generatedInduced draught fanTemperatureby vander Waals force, coulomb force, which can be easily eliminatedParticulateby chemistry effect, machine effect and other effects. Solidityagglomeration is mainly generated by Vander Waal force, coulombridge force anrhich are commonlydifficult to eliminate by chemistry effect and machine effectFig 1 Sketch of haled particles agglomerating test deviceAn agglomerating test device was designed and set up in order tosimulate full-scale flue gas flow before the removing device of power2 Agglomeration forces modestations,as shown in Fig. 1. The instrument is mainly made up of one 2.1 Hypotheses of agglomeration modelblower, one air preheated, one mixer, one particulate provider,twoFoundation of the model has its theory meaning to studytemperature controllers, one agglomeration chamber, one atomizationglomeration mechanism of fly ash inhaled particles and guide:Ie, one flueanalyzer, one induced draught fan andexperiments of fly ash agglomeration. Then, hypotheses are showed inFoundation item The National Natural Science Foundation of China( No. 50176019)and the Special Funds for Major Stale Basic Research Projects( G2002CB211602)dind authe萬方數(shù)336WEI Feng et aVol 17the following: (1 )Supposing that fly ash particlesSupposing that n is the sediment number of haled particles in eachparticles of agglomerators are of roundness. Volumes and qualities of atomized liquid particles and t is the residence time of agglomerator inwhich are uniform;(2)supposing that movements of fly ash particles are agglomeration chambercontinued in the gas flow processes; (3 )only if fly ash particles are6F.Mcontact with atomized particles, they will adhere to atomized particle(5)Sediment number of fly ash particles on each atomized particle isSo agglomeration rate n is:same;(4)taking no account of the varies of difference materials2.2 Agglomeration rate calculation(6)Based on the model put forward by literature( Watanabe, 19952.3 Effect forces between agglomerated particles( Fig. 2)and the theory of atomizing double-stream nozzle( Hou, 2002),theagglomeration model is set up. Through the third hypothesis, it is knownthat the sediment number of fly ash particles on atomized particlesAtomizedF,Fparticldepends on theirs sediment speed. Through the theory of atomizingdouble-stream nozzle, average diameter of atomized liquid particlesInhaledSobtainedparticle0.45Inhaled585√+5971000(1)particleQkFs FL, Fn, F8Through conservation of mass, the number of atomized particles perBunit time is obtained6qupwlFig 2 Effect forces between agglomerated particlesFor the same reason, the number of fly ash particles per unit volume6Q. /(TPpAgglomeration effect forces between single atomized particle andsingle haled particle are flow resistance, Vander Waal force, liquidWhere nm is the number of atomized particles per unit time, n/idge force, gravity, surface layer viscosity force and surface elasticityQ is the flux of agglomerator solution before atomizing, m /s; Pwl is force. The following is obtained by force balance of agglomeratedthe density of an agglomerator solution before atomizing, kg/m' Puz isparticlesthe density of liquid particles after atomizing kg/m'idz is thF,-F,=0diameter of atomized liquid particles, um; L is the length of blend Flow resistance F(Tan, 1998)chamber of double-flow nozzle ma g is the surface tension of anF2=0.055xpd2(up-um)2e-83agglomerator, dyn/cm;u is viscidity of an agglomerator, cP; c is the Surface elasticity force F, is( Visser, 1976):correlation coefficient; Q& is the volume flux of atomize air, m/s; n3000kthe particle number of flue gas per unit time,s" : Q is the gas qualityFI (m, m,l d,t d2flux per unit time, kg/s; d, is the inhaled particle diameter, umVander Waal force F(He, 2003)the inhaled particle density, kg/mBd. den, sediment speed of haled particles in the surface of atomized12z7+62(10liquid particles F is (Lind, 1996)according to the theory of surface-layer viscosity(Melanie, 1997)F2 td ndx△PxC,(4)surface-layer viscosity force FN is expressed by the followinghere AP is the pressure difference, AP= P- Po, Po is the pressurew =A Ly-4y/z)YAZ11)in the agglomeration chamber and P is the pressure of atomized liquid due to agglomerated particles lying in force balance condition, thenparticles; D is the number concentration of atomized liquid particlesa(y-△y(12)-1+1.71kn+1.333kn)2(13)(T)=1.0×10-5×(298Where So is the interface area of two particles; a is the visciditycoefficient of an agglomerator; u is the balance velocity of agglomeratedWhere T is the temperature of the agglomeration chamber, k =1particles and obtained by momentum conservation theoremapparent gravity(also called gravity errand )Fc:isBF22802260→Fz14z2240122220100g2200402140000.1030.40.60.8091.01.21.31.50.00.103040.60.8091.01.21.31.5Quanlity concentration, %oQuanlity concentrationFig- 3 Impact of agglomerator quality concentration on agglomeration rate and effect forces萬方數(shù)Agglomeration rate and action forces between atomized particles of agglomerator and inhaled-particles from coal combustion337GP- Gw2= 2rg(dp P, -duz Pv2)(14)ig. 3 shows the impact of agglomerator quality concentration onagglomeration rate and effect forces under the same conditions. Fig 3Then, interface area S of liquid particle and haled particles islows the flow resistance and the surface-layer viscosity force has twoquantity-level units higher than elasticity force and apparent gravityWhere E is defined as interspaced rate of agglomeration chamber; which means the main impact of gas flow and agglomerator viscosity onetween inhaled particles. Apparentgravity dliquid particle flux, kg/m': up, uw2 are the velocities of liquid particles when agglomerator concentration increases. Wheen aand haled particles, respectively, m/s; mp m, 2 are the mass of liquid concentration is lower than 0. 2%0, apparent grayand haled particles, respectively; k, kwe are their elasticity When agglomerator concentration is above 0. 2%00, apparent gravityoefficient of liquid-and holed particles respectively. Due to viscositydecreased slowly more and more. However elasticity force does nothange with agglomerator concentration. Fig 3b shows the agglomerationparticles, then hm2=0, k,=rE y Is the poisson ratio, e is therate is increasing with agglomerator concentration, while flow resistancepolar-gurmame module measure; A is the Hamaker constant; B is the is not changing and viscosity force is decreasingadsorption constant of Vander Waal fo3.2 Agglomerator flux efifect3 Results and discussionFig. 4 shows the impact of agglomerator flux on agglomeration rateand agglomeration effect forces at other same conditions. It has beenAccording to the model above, the results showed that the quantityknown that flow resistance and surface-layer viscosity force have twlevel unit of most effect forces effecting agglomeration are consistent with quantity-level units higher than elasticity force and apparent gravity andthat of result calculated by Tang et al.( Tang, 2001). Especiallythe effect forces are not changing withVander Waal force and gravitation are neglected for particles of diameter is known from Fig. 4b that agglomeration rate was increasing withmore than 1.0 um. It is discovered in the results that agglomerator flux, increasing agglomerator flux because greater agglomerator flux leads toagglomerator concentration, flue gasgreater atomized particle number of agglomerator and theen ncreasesatomized-particle diameter and inhaled particle diameter have impact on contact opportunity with inhaled particleeach effect forces, as follows3.3 Gas flux effect3.1 Agglomerator quality concentration effect6060032AFH FrF8→F2580540121551045000010.020030.040.050010020030.05Solntion flux、10-m3/sSolution flux. 10-m/sFig 4 Impact of agglomerator flux on agglomeration rate and effect forceFig 5 shows the impact of flue gas flux on agglomeration rate and 3.4 Agglomerator density effectagglomeration effect forces at the same conditions. It has been knownFig. 6 showg the impact of agglomerator density on agglomerationthat flow resistance and surface-layer viscosity force has two quantity- rate and effect forces at the same conditions. It has been known that flowlevel units higher than elasticity force and apparent gravity, which are resistance and surface-layer viscosity force has two quantity-level unitsnot changing with flue gas flux. When flue gas flux is increasinggher than elasticity force and apparent gravity. Elasticity force is notrface-layer viscosity force and flow resistance increases and changing and flow resistance increases slowly with agglomerator densitagglomeration rate decreased because great flue gas flux lead to greater When agglomerator density is increasing, both weight errand andinhaled particle numberviscosity force rapidly increase, while agglomeration rate decreased2200BFRFFt2000→F2241800云e占1600230100080080.00400050.0060.0070.0080.0040.0050.0060.0070008Gas flux, m/sGas flux. m /sFig 5 Impact of flue gas flux on agglomeration rate and effect forces萬方數(shù)338WEI Feng et alVol 173. 5 Inhaled-particle diameter effectit is finally equal to elasticity force when diameter of particles is greaterFig. 7 shows the impact of inhaled particle diameter on than 1.0 um. Fig. 7b shows that the flow resistance and agglomerationagglomeration rate and effect forces at the same conditions. Fig. 7a shows rate can not change with diameter of inhaled, while viscosity force couldthat elasticity force increased very slowly with inhaled particle diameter. changed when diameter of inhaled particles is lower than 2.5 umopparent gravity could not nearly change when diameter of inhaled viscosity force toboggans when diameter of inhaled particles is greaterArticles is lower than 1.0 Rm, while apparent can rapidly decrease until than 2.5 um until it is less than flow resistance610BF34057032828550530800l000110075085095010501150Solution density, mg/m3Solution density, mg/m 3Fig6 Impact of agglomerator density on agglomeration rate and agglomeration effect forceFg575Fe☆F25703032z565g20Agg3015540535260★青★530Particle diameter of gas, umParticle diameter of gas, m3/sig.7 Impact of fly particle dirate and effect fo11056FtFr486001602830333538404345485053555860303335384043454850535558Liquid particle diameter, umiquid particle diameter, HmFig 8 Impact of atomized liquid particle diameter on agglomeration rate and effect forces3.6 Atomized particle diameter effectliquid particle diameterthecle diameteglomeration rate and effect forces. It has been known that flow4 Conclusionsresistance and surface-layer viscosity force has two quantity-level unitsThe model has been set up to study agglomeration rate and effecthigher than elasticity force and apparent gravity. Fig. 8a shows that the forces between fly ash inhaled-particles and atomized particles ofelasticity force does hardly change and apparent gravity can increaseagglomerators. The following results are obtained (1)WhenFig. 8b shows the viscosity force and flow resistance have resemble rising agglomerator flux and concentration are rising at other same conditionstrend when atomized particles diameter is less than 40 um. Viscosityagglomeration rate are also much increasing. When fluee gas fluxforce increases more quickly when atomized particles diameter is greater agglomerator density and atomized particle diameter are risingthan 40 um. However, agglomeration rate is decreasing with atomized agglomeration rate is decreasing. (2) It has been known under all the萬方數(shù)No. 2Agglomeration rate and action forces between atomized particles of agglomerator and inhaled-particles from coal combustion339conditions that flow resistance and surface-layer viscosity force have twosilica[ J. Journal of Colloid and Interface Science, 193: 200--214quantity-level units higher than elasticity force and apparent gravityNaville M, McCarthy J F, Sarofim A F, 1983. Size fraction of submicrometer coalmeans the main impact of gas flow and agglomerator viscosity oncombustion aerosol for chemical analysis J J. Atmos Environ, 17(12on between particles. (3) Viscosity force can also rapidlyincrease with agglomerator density, flue gas flux and atomized particlesRiera F, 1986. Ultrasonic agglomeration of micron aerosols under standing waveonditions[j]. Journal of Sound Vibration, 110: 413--427diameter, it can decrease with agglomerator concentration and inhaledTan Y J, Zhu LX, 1998. 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