Dispersion of Air PollutantsDispersion of Air PollutantsThe dispersion of air pollutants is primarily determined by The dispersion of air pollutants is primarily determined by atmospheric conditions.atmospheric conditions.

If conditions are superadiabatic a great deal of vertical air If conditions are superadiabatic a great deal of vertical air movement will result and dispersion is enhanced.movement will result and dispersion is enhanced.

Subadiabatic conditions produce the opposite Subadiabatic conditions produce the opposite characteristics. IN an inversion, for example, vertical air characteristics. IN an inversion, for example, vertical air movement is almost non-existent and little dispersion will movement is almost non-existent and little dispersion will occur.occur.

Effect of atmospheric Stability on plume DispersionEffect of atmospheric Stability on plume Dispersion

Plume RisePlume RiseAs you can see from the preceding figure the height a plume rises is very As you can see from the preceding figure the height a plume rises is very important.important.

No theoretical model has been developed to predict plume rise, but No theoretical model has been developed to predict plume rise, but several good empirical models have been developed. One of those is several good empirical models have been developed. One of those is presented below:presented below:

h = 2.6 [F/( u S)]h = 2.6 [F/( u S)]1/31/3

F = [gVF = [gVssdd22(T(Tss – T – Taa)] / [4 (T)] / [4 (Taa + 273)] + 273)]

S = [g/(TS = [g/(Taa + 273)] [ ( + 273)] [ (T/T/z) + 0.01]z) + 0.01]

h = plume rise above stack, mh = plume rise above stack, m

u = average wind speed, m/secu = average wind speed, m/sec

T/T/z = prevailing lapse rate, z = prevailing lapse rate, ooC/mC/m

VVss = stack gas exit velocity, m/s = stack gas exit velocity, m/s

d = stack diameter, md = stack diameter, m

TTaa = temperature of atmosphere, = temperature of atmosphere, ooCC

TTss = temperature of stack gas, = temperature of stack gas, ooCC

F = buoyancy flux, mF = buoyancy flux, m44secsec-3-3

ExampleExample

A stack has an emission exiting at 3 m/sec through a stack with a diameter of A stack has an emission exiting at 3 m/sec through a stack with a diameter of 2 m. The average wind speed is 6 m/sec. The air temperature at the stack 2 m. The average wind speed is 6 m/sec. The air temperature at the stack exit elevation is 28exit elevation is 28ooC and the temperature of the emission is 167C and the temperature of the emission is 167ooC. The C. The atmosphere is at neutral stability. What is the expected rise of the plume?atmosphere is at neutral stability. What is the expected rise of the plume?

At neutral stability, At neutral stability, T/T/z = 0.01 z = 0.01 ooC/mC/m

F = [9.8 x 3 x 2F = [9.8 x 3 x 222 x (167 – 28)] / [ 4 (28 + 273)] = 13.6 x (167 – 28)] / [ 4 (28 + 273)] = 13.6

S = [9.8 / (28 + 273)] ( 0.01 + 0.01) = 6.51 x 10S = [9.8 / (28 + 273)] ( 0.01 + 0.01) = 6.51 x 10 -4-4

h = 2.6 [13.6 / ( 6 x 6.51 x 10h = 2.6 [13.6 / ( 6 x 6.51 x 10-4-4)])]1/31/3 = 40 m = 40 m

h = 2.6 [F/( u S)]h = 2.6 [F/( u S)]1/31/3

Dispersion ModelingDispersion ModelingDispersion is the process of spreading out the emission over a large area Dispersion is the process of spreading out the emission over a large area thereby reducing the concentration of the pollutants.thereby reducing the concentration of the pollutants.

Plume dispersion is in two dimensions: horizontal and vertical. It is Plume dispersion is in two dimensions: horizontal and vertical. It is assumed that the greatest concentration of the pollutants is on the plume assumed that the greatest concentration of the pollutants is on the plume centerline in the direction of the prevailing wind. The further the away centerline in the direction of the prevailing wind. The further the away from the centerline the lower the concentration.from the centerline the lower the concentration.

The spread of the plume is approximated by Gaussian probability curveThe spread of the plume is approximated by Gaussian probability curve

CC(x,y,z)(x,y,z) = [Q/(2 = [Q/(2 u u yyzz)] exp[ -1/2 [(y / )] exp[ -1/2 [(y / yy))22 + (z / + (z / zz))22]]]]

CC(x, y, z)(x, y, z) = concentration at some point in coordinate space, kg,m = concentration at some point in coordinate space, kg,m33

Q = source strength, kg/secQ = source strength, kg/sec

y,y,= standard deviation of the dispersion in the y and z directions= standard deviation of the dispersion in the y and z directions

y = distance crosswind horizontally, my = distance crosswind horizontally, m

z = distance crosswind vertically, mz = distance crosswind vertically, m

z is in the vertical direction, y is in the horizontal crosswind direction, and x is the downwind directionz is in the vertical direction, y is in the horizontal crosswind direction, and x is the downwind direction

CC(x,y,z)(x,y,z) = [Q/(2 = [Q/(2 u u yyzz)] exp[ -1/2 [(y / )] exp[ -1/2 [(y / yy))22 + (z / + (z / zz))22]]]]

The degree of dispersion is controlled by the standard deviations in the The degree of dispersion is controlled by the standard deviations in the equation. When equation. When is large the spread is great, so the concentration is low (the is large the spread is great, so the concentration is low (the mass is spread out over a larger area.)mass is spread out over a larger area.)

Dispersion is dependent on both atmospheric stability and distance from the Dispersion is dependent on both atmospheric stability and distance from the sourcesource

The values for the standard deviations for this equation can be found using The values for the standard deviations for this equation can be found using tables which have been prepared for that purpose.tables which have been prepared for that purpose.

To use the table you first must estimate the atmospheric stability To use the table you first must estimate the atmospheric stability

Consider this figure:Consider this figure:

A plume emitted from a stack has an A plume emitted from a stack has an effective height H (you have to calculate effective height H (you have to calculate h). The centerline of the plume, z, is h). The centerline of the plume, z, is then H and the dispersion equation then H and the dispersion equation becomes:becomes:

CC(x,y,z)(x,y,z) = [Q/(2 = [Q/(2 u u yyzz)] exp[ -1/2 [(y / )] exp[ -1/2 [(y / yy))22 + (z - H / + (z - H / zz))22]]]]

This equation and the one presented previously hold as long as the ground does not This equation and the one presented previously hold as long as the ground does not influence the diffusion (the plume hits the ground)influence the diffusion (the plume hits the ground)

Since most pollutants are not absorbed by the ground, and they can not diffuse Since most pollutants are not absorbed by the ground, and they can not diffuse into the ground the equations above will not work when there is influence from into the ground the equations above will not work when there is influence from the ground.the ground.

One way of accounting for this influence is to assume all pollutants are One way of accounting for this influence is to assume all pollutants are reflected by the ground.reflected by the ground.

A new equation can be written based on A new equation can be written based on this assumption:this assumption:

CC(x,y,z)(x,y,z) = [Q/(2 = [Q/(2 u u yyzz)] exp[ -1/2 (y / )] exp[ -1/2 (y / yy))22] x {exp[-1/2[(z + H) / ] x {exp[-1/2[(z + H) / zz))22] + ] +

exp [ -1/2 [(z – H)/exp [ -1/2 [(z – H)/zz]]]]22}}

ExampleExample

A source emits 0.01 kg/sec of a SoA source emits 0.01 kg/sec of a Soxx on a sunny summer afternoon with an on a sunny summer afternoon with an

average wind speed of 4 m/sec. The effective stack height has been determined average wind speed of 4 m/sec. The effective stack height has been determined to be 20 m. Find the ground level concentration 200 m downwind from the to be 20 m. Find the ground level concentration 200 m downwind from the stack.stack.

A sunny afternoon would give A sunny afternoon would give curve Bcurve B

Now from the figures:Now from the figures:

yy = 36 m = 36 m zz = 20 m = 20 m

Now x = 200 m, y = 0, z = 0 m, and:Now x = 200 m, y = 0, z = 0 m, and:

CC(200,0,0)(200,0,0) = [0.01 / (2 x 3.14 x 4 x 36 x 20)] x { exp[ –1/2(0/36)] x = [0.01 / (2 x 3.14 x 4 x 36 x 20)] x { exp[ –1/2(0/36)] x

{exp[ -1/2 x [(0 – 20)/20]{exp[ -1/2 x [(0 – 20)/20]22] = exp[ -1/2 x [(0 + 20)/20]] = exp[ -1/2 x [(0 + 20)/20]22]}}]}}

= 6.7 x 10= 6.7 x 10-7-7 kg/m kg/m33 = 670 = 670 g/mg/m33

CC(x,y,z)(x,y,z) = [Q/(2 = [Q/(2 u u yyzz)] exp[ -1/2 (y / )] exp[ -1/2 (y / yy))22] x {exp[-1/2[(z + H) / ] x {exp[-1/2[(z + H) / zz))22] + ] +

exp [ -1/2 [(z – H)/exp [ -1/2 [(z – H)/zz]]]]22}}

Special ConditionsSpecial Conditions

If the measurement is taken at ground level and the plume is emitted at If the measurement is taken at ground level and the plume is emitted at ground level (Z = 0, H = 0):ground level (Z = 0, H = 0):

CC(x,y,z)(x,y,z) = [Q/(2 = [Q/(2 u u yyzz)] exp[ -1/2 (y / )] exp[ -1/2 (y / yy))22 ] ]

If the emission is at ground level and the pollutant is measured at If the emission is at ground level and the pollutant is measured at ground level on the center line in the direction of the wind (H = 0, z ground level on the center line in the direction of the wind (H = 0, z = 0, and y = 0), the equation is even further simplified to:= 0, and y = 0), the equation is even further simplified to:

CC(x,y,z)(x,y,z) = [Q/(2 = [Q/(2 u u yyzz)] )]

Control of Pollution from AutomobilesControl of Pollution from Automobiles

Important points requiring control:Important points requiring control:

Evaporation loss from fuel Evaporation loss from fuel tanktank

Evaporation of HC’s from Evaporation of HC’s from carburetorcarburetor

Emission of unburned gas Emission of unburned gas and partially oxidized HC’s and partially oxidized HC’s from crankcasefrom crankcase

NONOxx., HC’s, and CO in the ., HC’s, and CO in the

exhaustexhaust

Control of the potential emission pointsControl of the potential emission points

Evaporation form the Evaporation form the gas tank can be gas tank can be eliminated by use of gas eliminated by use of gas tank caps that prevent tank caps that prevent vapor escapingvapor escaping

Losses from carburetors Losses from carburetors can be reduced by using can be reduced by using activated carbon activated carbon canisters that adsorb canisters that adsorb vapors emitted when the vapors emitted when the engine is turned off and engine is turned off and hot gasoline in the hot gasoline in the carburetor vaporizes. carburetor vaporizes. The vapors are purged The vapors are purged from the canister by air from the canister by air when the car is restarted when the car is restarted and burned in the engineand burned in the engine

Crankcase emissions have been eliminated by recycling Crankcase emissions have been eliminated by recycling crankcase gases into the intake manifold and the crankcase gases into the intake manifold and the installation of the positive crankcase ventilation valve installation of the positive crankcase ventilation valve (PCV).(PCV).

Exhaust EmissionsExhaust Emissions

60% of the HC’s and almost all of the NO60% of the HC’s and almost all of the NOxx, CO, and lead come , CO, and lead come

from the exhaust.from the exhaust.

The quantity of emissions changes with the operating The quantity of emissions changes with the operating conditions of the vehicle.conditions of the vehicle.

When the car is accelerating the combustion is efficient (low CO and HC), but When the car is accelerating the combustion is efficient (low CO and HC), but high amounts of NOhigh amounts of NOxx are produced. are produced.

When the car is decelerating there are low amounts of NOWhen the car is decelerating there are low amounts of NOxx produced but high produced but high

amounts of HC’s due to partially burned fuel.amounts of HC’s due to partially burned fuel.

This makes it difficult to determine how much pollution a particular engine This makes it difficult to determine how much pollution a particular engine design produces. The EPA has developed a standard test to make this design produces. The EPA has developed a standard test to make this determination. The test includes a cold start, cruising with a simulated load, determination. The test includes a cold start, cruising with a simulated load, and a hot start.and a hot start.

Exhaust emission control techniquesExhaust emission control techniques

Tuning the engine to burn fuel efficientlyTuning the engine to burn fuel efficiently

Installation of catalytic reactorsInstallation of catalytic reactors

Engine modificationsEngine modifications

Engine TuningEngine Tuning

A well tuned engine is the first line of defense for A well tuned engine is the first line of defense for controlling automobile emissionscontrolling automobile emissions

Catalytic ConvertersCatalytic Converters

Oxidize CO and HC’s to COOxidize CO and HC’s to CO22 and H and H22OO

Most common catalyst - platinumMost common catalyst - platinum

Problems:Problems:

Fouled by some gasoline additives like lead (this is why Fouled by some gasoline additives like lead (this is why lead has been eliminated from gasoline)lead has been eliminated from gasoline)

Sulfur in gasoline converted to particulate SOSulfur in gasoline converted to particulate SO33

Redesign of internal combustion enginesRedesign of internal combustion engines

Cylinder configurationCylinder configuration

Fuel injectorsFuel injectors