Chapter 6 - Air Pollution 2

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<ul><li><p>CHAPTER 6B</p><p>Air Pollution: </p><p>1. Meteorology and Dispersion Modeling</p><p>2. Air Pollution Control2. Air Pollution Control</p></li><li><p>Learning Objectives</p><p>At the end of this lesson the students should </p><p>be able to;</p><p> Understand apply the meteorological </p><p>aspects and dispersion of air pollutantaspects and dispersion of air pollutant</p><p> Engineering assessment and application </p><p>of various methods in controlling air </p><p>pollution</p></li><li><p>Air Quality and Meteorology</p><p> Air quality depends on </p><p> wind</p><p> sunlight</p><p> temperature</p><p> precipitation and humidity precipitation and humidity </p><p> Energy from the sun and earths rotation drives atmospheric circulation</p><p> Circulation, and the resulting interactions with water and temperature differences produce the climate and weather we observe</p></li><li><p>Air Quality and Meteorology</p><p> Somewhat less observable issue relates to mixing</p><p> Easy to understand how wind and turbulence produce mixingturbulence produce mixing</p><p> Inherent mixing property that derives from pressure, volume, temperature relationships</p><p> Lapse rate change in temperature with height (altitude)</p></li><li><p>Air Quality and MeteorologyFactors controlling air quality</p><p>1. Downwind distance</p><p>The air pollution will disperse as the downwind distance increases. The</p><p>further away the distance will have lower air pollution concentrations.</p><p>2. Wind speed and direction (air mixing)Wind speed also contributes to how quickly pollutants are carried away</p><p>from their original source. However, strong winds don't always dispersefrom their original source. However, strong winds don't always disperse</p><p>the pollutants. They can transport pollutants to a larger area, such as</p><p>the smoke from open burning or forest fires.</p><p>3. Atmospheric stabilityOnce pollutants are emitted into the air, the weather (atmospheric</p><p>stability) largely determines how well they disperse. Turbulence mixes</p><p>pollutants into the surrounding air. For example, during a hot summer</p><p>day, the air near the surface can be much warmer than the air above.</p><p>Sometimes large volumes of this warm air will rise to great heights.</p><p>This results in vigorous mixing.</p></li><li><p>Air Pollution Occurrences</p><p> The most obvious factor influencing air pollution is the quantity of contaminants emitted into the atmosphere. </p><p> However, when air pollution episodes take place, they are not generally the result of a drastic increase in the output of pollutants; instead, they occur because of changes in certain atmospheric conditions. changes in certain atmospheric conditions. </p><p> Two of the most important atmospheric conditions affecting the dispersion of pollutants are:</p><p> (1) the strength of the wind and </p><p> (2) the stability of the air. </p></li><li><p>Air Mixing</p><p> The direct effect of wind speed is to influence the </p><p>concentration of pollutants. </p><p> Atmospheric stability determines the extent to which </p><p>vertical motions will mix the pollution with cleaner air </p><p>above the surface layers. above the surface layers. </p><p> The vertical distance between Earth's surface and the </p><p>height to which convectional movements extend is called </p><p>the mixing depth. </p><p> Generally, the greater the mixing depth, the better the air </p><p>quality. </p></li><li><p>Stability</p><p> Dry adiabatic lapse rate temperature decreases due to lower pressure (ideal gas law)</p><p>ft 1000F mC/100 /4500.1 === .- dz</p><p>dT</p><p> Ambient (actual) lapse rate</p><p>&lt; (temperature falls faster) unstable or superadiabatic</p><p>&gt; (temperature falls slower) stable or subadiabatic</p><p>= (same rate) neutral</p></li><li><p>Neutral Conditions</p></li><li><p>Unstable Conditions</p></li><li><p>Stable Conditions</p></li><li><p>Example</p><p>Z(m) T(C)</p><p>2 -3.05</p><p>318 -6.21</p><p>( )C/m =</p><p>=</p><p>=</p><p>0100.0</p><p>05.321.612 TTT ( ) C/m =</p><p>=</p><p>=</p><p>0100.02318</p><p>05.321.6</p><p>12</p><p>12</p><p>zz</p><p>TT</p><p>z</p><p>T</p><p>m C/100 = 00.1</p><p>Since lapse rate = , atmosphere is neutral</p></li><li><p>Example</p><p>Z(m) T(C)</p><p>10 5.11</p><p>202 1.09</p><p>C/m =</p><p>=</p><p>=</p><p>0209.011.509.112 TTT C/m =</p><p>=</p><p>=</p><p>0209.010202</p><p>11.509.1</p><p>12</p><p>12</p><p>zz</p><p>TT</p><p>z</p><p>T</p><p>m C/100 = 09.2</p><p>Since lapse rate is more negative than , </p><p>(-1.00 C/100 m), atmosphere is unstable</p></li><li><p>Example</p><p>Z(m) T(C)</p><p>18 14.03</p><p>286 12.56</p><p>C/m =</p><p>=</p><p>=</p><p>0055.003.1456.1212 TTT C/m =</p><p>=</p><p>=</p><p>0055.018286</p><p>03.1456.12</p><p>12</p><p>12</p><p>zz</p><p>TT</p><p>z</p><p>T</p><p>m C/100 = 55.0</p><p>Since lapse rate more positive than , </p><p>atmosphere is stable</p></li><li><p>Temperature Inversions</p><p> Extreme case of stability when lapse rate </p><p>is actually positive, i.e. temperature </p><p>increases with altitude</p><p> Resulting temperature inversion prevents </p><p>nearly all upward mixingnearly all upward mixing</p></li><li><p>Why are these plumes so different?</p></li><li><p>Effect of Lapse Rate on Plumes</p></li><li><p>UNSTABLE</p><p></p><p>STABLE</p></li><li><p>Inversion</p></li><li><p>Point Source Gaussian Plume </p><p>Model</p></li><li><p>Point Source Gaussian Plume </p><p>Model</p></li><li><p>Point Source Gaussian Plume </p><p>Model</p><p> Model Structure and Assumptions</p><p> pollutants released from a virtual point </p><p>source</p><p> advective transport by wind advective transport by wind</p><p> dispersive transport (spreading) follows </p><p>normal (Gaussian) distribution away from </p><p>trajectory</p><p> constant emission rate</p></li><li><p>Point Source Gaussian Plume </p><p>Model</p><p> Model Structure and Assumptions (cont)</p><p> wind speed constant with time and elevation</p><p> pollutant is conservative (no reaction)</p><p> pollutant is reflected by ground pollutant is reflected by ground</p><p> terrain is flat and unobstructed</p><p> uniform atmospheric stability</p></li><li><p>Point Source Gaussian Plume </p><p>Model</p><p>Where = downwind concentration at </p><p>( )</p><p>=</p><p>22</p><p>2</p><p>1exp</p><p>2</p><p>1exp,0,,</p><p>zyzy s</p><p>H</p><p>s</p><p>y</p><p>uss</p><p>EHyx</p><p>Where = downwind concentration at </p><p>ground level (g/m3)</p><p>E = emission rate of pollutant (g/s)</p><p>sy,sz = plume standard deviations (m)</p><p>u = wind speed (m/s)</p><p>x, y, z, H = distances (m)</p></li><li><p>Point Source Gaussian Plume </p><p>Model Effective Stack Height</p><p>where</p><p>HhH +=</p><p>where</p><p>H = Effective stack height (m)</p><p>h = height of physical stack (m)</p><p>H = plume rise (m)</p></li><li><p>Point Source Gaussian Plume </p><p>Model Effective Stack Height</p><p> Hollands formula</p><p>( )</p><p> += d</p><p>T</p><p>TTP</p><p>u</p><p>vH</p><p>a</p><p>ass 21068.25.1</p><p>where vs = stack velocity (m/s)</p><p>d = stack diameter (m)</p><p>u = wind speed (m)</p><p>P = pressure (kPa)</p><p>Ts = stack temperature (K)</p><p>Ta = air temperature (K)</p></li><li><p>Point Source Gaussian Plume </p><p>Model Stability Categories </p></li><li><p>Point Source Gaussian Plume </p><p>Model Horizontal Dispersion</p><p> OR use Eq. 11-15 </p><p>and Table 11-7</p><p>Use Fig. 11-18</p></li><li><p>Point Source Gaussian Plume </p><p>Model Vertical Dispersion</p><p> OR use Eq. 11-16 </p><p>and Table 11-7</p><p>Use Fig. 11-19</p></li><li><p>Point Source Gaussian Plume </p><p>Model Wind Speed Correction</p><p> Unless the wind speed at the virtual stack </p><p>height is known, it must be estimated from the </p><p>ground wind speed</p><p>where ux = wind speed at p</p><p>z where ux = wind speed at elexation zx</p><p>p = empirical constant</p><p>p</p><p>z</p><p>zuu </p><p>=</p><p>1</p><p>212</p></li><li><p>Example</p><p> A stack in an urban area is emitting 80 g/s </p><p>of NO. It has an effective stack height of </p><p>100 m. The wind speed is 4 m/s at 10 m. </p><p>It is a clear summer day with the sun It is a clear summer day with the sun </p><p>nearly overhead. Estimate the ground </p><p>level concentration at a) 2 km downwind </p><p>on the centerline and b) 2 km downwind, </p><p>0.1 km off the centerline. </p></li><li><p>Example</p><p>1. Determine stability class</p><p>Assume wind speed is 4 km at ground surface. Description suggests strong solar radiation.</p><p>Stability class BStability class B</p></li><li><p>Example</p><p>2. Estimate the wind speed at the effective stack </p><p>height</p><p>Note: effective stack height given no need to </p><p>calculate using Hollands formula</p><p>m/s 65.510</p><p>1004</p><p>15.0</p><p>1</p><p>212 =</p><p>=</p><p>=</p><p>p</p><p>z</p><p>zuu</p></li><li><p>Example3. Determine y and z</p><p>y = 290</p><p>z = 220</p><p>290</p><p>220</p></li><li><p>Example</p><p>4. Determine concentration using Eq 11-12</p><p>a. x = 2000, y = 0</p><p>=22</p><p>220</p><p>100</p><p>2</p><p>1exp</p><p>290</p><p>0</p><p>2</p><p>1exp</p><p>)6.5)(220)(290(</p><p>80)0,2000(</p><p>C</p><p>33 g/m g/m 3.641043.6)0,2000( 5 == C</p></li><li><p>Example</p><p>b. x = 2000, y = 0.1 km = 100 m</p><p>=22</p><p>220</p><p>100</p><p>2</p><p>1exp</p><p>290</p><p>100</p><p>2</p><p>1exp</p><p>)6.5)(220)(290(</p><p>80)100,2000(</p><p>C</p><p>33 g/m g/m 6.601006.6)0,2000( 5 == C</p></li><li><p>2. Air Pollution Control</p></li><li><p>Adsorption</p><p> Adsorption :</p><p> Control of principal polluting gas such as </p><p>sulfur oxides, nitrogen oxides, CO2 and </p><p>hydrocarbonshydrocarbons</p><p> Passing stream of effluent gas through solid </p><p>porous material (adsorbent). The surface of </p><p>porous material attract and hold the gas by </p><p>physical or chemical adsorption</p></li><li><p>Adsorption</p></li><li><p>Absorption</p><p> Absorption</p><p> Absorption also known as scrubbing bringing </p><p>contaminated gas (absorbate or solute) into </p><p>contact with liquid absorbent (solvent)contact with liquid absorbent (solvent)</p><p> One or more of the constituents of the effluent </p><p>gas are removed, treated or modified by the </p><p>liquid absorbent</p><p> The amount of gas absorbed will depend on </p><p>the properties of both gas and solvent</p></li><li><p>Absorption</p></li><li><p>Combustion</p><p>Particulates are </p><p>burned down by </p><p>having four basic </p><p>elements : oxygen, elements : oxygen, </p><p>temperature </p><p>(650oC), turbulence </p><p>(for mixing of </p><p>oxygen) and time</p></li><li><p>Cyclone</p><p> Dust laden gas </p><p>enters tangentially</p><p> Under influence of </p><p>centrifugal force centrifugal force </p><p>generated by </p><p>spinning gas, solid </p><p>particles thrown onto </p><p>walls and slide down </p><p>the walls into the </p><p>hopper</p></li><li><p>Filtration</p><p> Fabric filter system, </p><p>particulate laden gas </p><p>passed thru a woven </p><p>filter fabrics</p><p> Particulates are </p><p>trappedtrapped</p><p> Fabric must be </p><p>cleaned regularly to </p><p>remove trapped </p><p>particulates material</p><p> If not cleaned filter </p><p>can explode due to </p><p>build up of pressure</p></li><li><p>Electrostatic Precipitator</p><p> Low voltage two </p><p>staged units or High </p><p>Voltage single stage </p><p>unit</p><p> Particulate are given Particulate are given </p><p>negative charge and </p><p>attached themselves </p><p>to positive electrodes </p><p>and collected there</p><p> Extremely efficient up </p><p>to 99% removal</p></li><li><p>Liquid Scrubber</p><p>In wet cyclone scrubber, high pressure spray nozzle </p><p>generate fine spray that intercepts the small particles </p><p>entrained in the swirling gases. The particulate matter </p><p>thrown onto the wall by centrifugal force then drained into </p><p>collection sump</p></li><li><p>Sulfur Dioxide Control</p><p></p></li><li><p>Catalytic Converter</p></li><li><p>Catalytic Converter</p><p> A catalytic converter is a vehicle emissions control </p><p>device which converts toxic byproducts of combustion in </p><p>the exhaust of an internal combustion engine to less toxic </p><p>substances by way of catalyzed chemical reactions. </p><p> The specific reactions vary with the type of catalyst </p><p>installed. installed. </p><p> Most present-day vehicles that run on gasoline are fitted </p><p>with a three-way converter, so named because it </p><p>converts the three main pollutants in automobile exhaust: </p><p>carbon monoxide, unburned hydrocarbon and oxides of </p><p>nitrogen. </p><p> The first two undergo catalytic combustion and the last is </p><p>reduced back to nitrogen.</p></li><li><p>ENDEND</p></li></ul>