Atmospheric Disperssion

Transcription

1 Atmospheric Disperssion Dispersion is the process by which contaminants move through the air and a plume spreads over a large area, thus reducing the concentration of the pollutants it contains. Atmospheric Disperssion The plume spreads both horizontally and vertically. If it is a gaseous plume, the motion of the molecules follows the laws of gaseous diffusion. 1

2 Gaussian Dispersion Model Wind Rose 2

3 Dispersion Model Assumptions The predominant force is the wind. The greatest concentration of the pollutant molecules is along the plume centerline. The process is a steady state process. Dispersion Model Construction Plume travels horizontally in x direction Plume disperses horizontally (y) and vertically (z) Concentration inside the plume follows Gaussian Distribution Concentration (C (x,y,z) ) is proportional to: Source strength (Q) Inverse of wind speed (1/U) Normalized Gaussian distribution function in the y and z directions that is dependent on weather conditions 3

4 Plume Dispersion Coordinate System Gaussian Dispersion Model At ground level, we have z=0, thus 4

5 Gaussian Dispersion Model The greatest value of the ground level concentration in any direction, and this is the concentration along the plume centerline; that is, for y = 0. We have Finally, for a source of emission at ground level, H = 0, and the ground level concentration of pollutant downwind along the plume centerline is given by Maximum ground level conc. For a release above ground level the maximum downwind ground level concentration Occurs along the plume centerline when the following condition is satisfied: z H 2 5

6 Key to Stability Categories Horizontal Dispersion Coefficients 6

7 Vertical Dispersion Coefficients Effective Stack Height Carson and Moses Equation Superadiabatic Stability Nuetral Stability Subadiabatic Stability V s = stack gas exit speed (in m/s ), d = stack diameter (in m), and Q h = heat emission rate from the stack (in kj/s). 7

8 Effective Stack Height Holland Formula H Where v d u s 2 ( ) v s = stack velocity, m/s d = stack diameter, m u = wind speed, m/s P = pressure, kpa T s = stack temperature, K T a = air temperature, K P T s T T s a d Example It has been estimated that the emission of SO 2 from a colafired power plant is 1,656.2 g/s. At 3 km downwind on an overcast summer afternoon, what is the centerline concentration of SO 2 if the wind speed is 4.50 m/s? (Note: centerline implies y =0) Stack parameter: Height = m Diameter = 1.20 m Exit velocity = 10.0 m/s Temperature 315 o C Atmospheric conditions: Pressure = 95.0 kpa Temperature = 25.0 o C 8

9 Air Pollution Control Source Correction Changing or eliminating a process that produces a polluting air effluent Elimination of lead from gasoline removal of sulfur from coal and oil before the fuel is burned Controls: raw material substitution, and equipment modification to meet emission standards Abatement : devices and methods for decreasing the quantity of pollutant reaching the atmosphere, once it has been generated by the source. 9

10 COLLECTION OF POLLUTANTS Collection of pollutants for treatment is the most serious problem in air pollution control. COOLING The exhaust gases to be treated are sometimes too hot for the control equipment, and must first be cooled. 10

11 Air Pollution Control Technologies Control of Particulate Emission Settling Cyclone separation Wet scrubbing Baghouse filtration Electrostatic precipitation Control of Vapor phase Emissions Wet scrubbing Activated carbon adsorption Incineration Cyclone Control particulates Used as precleaners >90% efficiency for > 5m In expensive and maintenance free 11

12 Cyclones Fabric Filters known as baghouses Control particulates efficient and cost effective 99% efficient for very fine particulates (<1m). 12

13 5/12/2011 Baghouses Wet Collectors The spray tower or scrubber Remove larger particles effectively can remove both gases and particulate matter. A venturi scrubber is a frequently used high energy wet collector. 100% efficient in removing particles >5 pm 13

14 Wet collectors Electrostatic Precipitators 14

15 Electrostatic Precipitators CONTROL OF GASEOUS POLLUTANTS Wet scrubbers: can remove pollutants by dissolving them in the scrubber solution. Ex. SO2 and NO2 in power plant. Packed scrubbers, spray towers packed with glass platelets or glass frit, more efficient. Ex. removal of fluoride from aluminum smelter exhaust gases. 15

16 Activated Carbon Adsorber Removal of organic compounds with an adsorbent like activated charcoal. Incinerator Incineration, or flaring, is used when an organic pollutant can be oxidized to CO 2 and water, or in oxidizing H 2 S to SO 2. Catalytic combustion is a variant of incineration in which the reaction is facilitated energetically and carried out at a lower temperature by surface catalysis, 16

17 Effectiveness of Technologies 17