1、Prof. Jiakuan YangHuazhong University of Science and TechnologyQuestions for the Last Lecture1. Please give examples of primary particulates and secondary particulates.2. For particles too large or too small, how to modify the Stokes law.AnswerPrimary particles are found in the atmosphere in the for

2、m in which they were emitted.Fly ashZnO FumeCoal dustCement dustAnswer(3) secondary particlesSecondary particles are formed in the atmosphere from gaseous pollutants which are called as the precursors.Secondary particlesTobacco smokesulfuric MistBlack smoke of carCxHySO2CxHyprecursorsParticles too l

3、arge for Stokes Law2134fluiddparttCgDVwhen Rp 0.3, pdRC24When 0.3 Rp 10007 . 014. 0124ppdRRCParticles too Small for Stokes LawDAgDVpart1182182CgDVpartControl of Primary ParticulatesMost of fine particles in the atmosphere are secondary particles, 1m.They are difficult to be collected by a control de

4、vice. The best way to control secondary particles is process change.Control of Primary ParticulatesThe control of primary particles is a major part of air pollution control engineering.For the particulates in size range 1 m 70 m. The control device can be designed on the basis of Stokes law.The natu

5、re of Particulate Pollutants Types of Particles collection devices Gravity Settlers Centrifugal Separators Electrostatic Precipitators (ESP) Summary Types of Particles collection deviceswall collection devicecyclone separator( (壁式收集設(shè)備）壁式收集設(shè)備）electrostatic precipitatorsgravity settlers( (重力沉降式室重力沉降式室

6、) )( (旋風(fēng)除塵器）旋風(fēng)除塵器）( (靜電除塵器）靜電除塵器） Types of Particles collection devicesscrubbersurface filterdividing collection device( (表面式過濾器）表面式過濾器）( (縱深式過濾器）縱深式過濾器）（分離式收集設(shè)備）（分離式收集設(shè)備）( (氣體洗滌器）氣體洗滌器）depth filterThe general idea of wall collection deviceDriving the particles to wallParticles adhere to each other

7、to agglomerateRemoved from the wall2.Gravity settlers(1) Schematic of typical gravity settlerSome baffles: inlet baffles and out baffles reducing the velocity of gas in gravity setter.Gas flow inInlet ductoutlet ductchamberGas flow outLHWCross-sectional area (WH) is much larger than that of inlet du

8、ct, or that of outlet duct.(2) Block flow modelAssumptions Average horizontal gas velocity in chamberWHQVavg The horizontal velocity of the gas in the chamber everywhere = Vavg(2) Block flow model The horizontal component of the velocity of the particles =Vavg The vertical component of the velocity

9、is equal to Vt If a particle settles to the floor, it stays there and is not re-entrained.collection efficiency(2) Block flow model The time the gas will take to pass through the chamber in the flow direction. During that time, the particle fall down a distance by gravity.avgVLt Vertical Setting Dis

10、tance(VSH) = t Vt =avgtVLV (2) Block flow modelVertical Setting Distance(VSH) = t Vt =avgtVLV If VSDH, the particles will be captured on the bottom wall of chamber.collection efficiency(2) Block flow model Efficiency= Fractional captured =avgtHVLVcollection efficiencySubstituting Stokes law equation

11、 in the above equation.182avgpartHVLgDD2(3) Mixed flow modelTotally mixing in the z direction but not in the x direction.18exp12avgpartmixedHVLgD)exp(1blowflowmixedAssumptions, Page 252Mixed flow model and Block flow model Diameter of the particles, Collection efficiency, % mixedblockShortcoming of

12、Gravity settlersThis type of device would be useful for collecting particles with diameters of perhaps 100m, but not for smaller particles of air pollution interest.100 15100 515 This Lecture and the Next Lectureq This Lecture:v Chapter 9 Page 249280 q The Next Lecture:v Chapter 9 page 280314Exerciser Page 315 9.7 9.10DISCUSSIONTopics about Air pollution:q Introducing yourselfq Interesting ne