大气污染控制工程ppt英文课件.ppt

* §8 The nature of particulate pollutants particulate pollutants are not uniform—in a wide variety of sizes ,shapes, chemical compositions. 8.1 primary and secondary particles basic properties: 1. Particle diameter The particles produced by the industry are not round shape (spherical). Therefore, volume equivalent diameter is used. However, the volume equivalent diameter is difficult to be determined. Thus, in practice, Martin diameter (the length of a particle in given direction) and Stokes diameter are often used. (1) Martin diameter Dp (2) Stokes diameter If the settling velocity of particle in the air is known, the particle diameter can be calculated. The above formula is quit useful. We will talk it later. 2. Particle formation and definition Dust—created mechanically by crushing, grinding, sieving, powder loading and discharging (1~400 micron meter). Fly ash—non combustible remains of fuel or waste from the combustion zone(0.1-10μm). Fume—created by combustion, evaporation, or condensation processes( 0.01~1μm). Mist or fog—is the result of liquid droplets formed by condensation after appropriate nucleation from a super saturated atmosphere (1~100μm). Aerosol—systems of gases in which a number of particles is suspended. The mixture of gas and particles is aerosol. Primary particle—its size can be further reduced, and also can become loosely by Van der Waals force , or by electrostatic attractions. Secondary particle—formed from gaseous pollutant, most from hydrocarbons, nitrogen. Lung damaging dust—respirable（呼吸性） particle (0.1~5μm). We often call as PM5. But nowadays, PM10, and PM2.5 are often used. 8.2.1 Stokes’ Law Drag force Buoyant force Gravity force 8.2 Settling velocity and drag force There are three forces acting on the particle: drag force, buoyant force and gravity force. The drag force can be described by Stokes’ Law Where, Dp is the particle diameter, V is the relative velocity the gas to the particle, and μ the viscosity of th