Transcript Document

Dispersion of Air Pollutants
The dispersion of air pollutants is primarily determined by
atmospheric conditions.
If conditions are superadiabatic a great deal of vertical air
movement will result and dispersion is enhanced.
Subadiabatic conditions produce the opposite
characteristics. IN an inversion, for example, vertical air
movement is almost non-existent and little dispersion will
occur.
Effect of atmospheric Stability on plume Dispersion
Plume Rise
As you can see from the preceding figure the height a plume rises is very
important.
No theoretical model has been developed to predict plume rise, but
several good empirical models have been developed. One of those is
presented below:
)h = 2.6 [F/( u S)]1/3
)h = plume rise above stack, m
F = [gVsd2(Ts – Ta)] / [4 (Ta + 273)]
u = average wind speed, m/sec
S = [g/(Ta + 273)] [ ()T/)z) + 0.01]
)T/)z = prevailing lapse rate, oC/m
Vs = stack gas exit velocity, m/s
d = stack diameter, m
Ta = temperature of atmosphere, oC
Ts = temperature of stack gas, oC
F = buoyancy flux, m4sec-3
Example
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 exit
elevation is 28oC and the temperature of the emission is 167oC. The
atmosphere is at neutral stability. What is the expected rise of the plume?
)h = 2.6 [F/( u S)]1/3
At neutral stability, )T/)z = 0.01 oC/m
F = [9.8 x 3 x 22 x (167 – 28)] / [ 4 (28 + 273)] = 13.6
S = [9.8 / (28 + 273)] ( 0.01 + 0.01) = 6.51 x 10-4
)h = 2.6 [13.6 / ( 6 x 6.51 x 10-4)]1/3 = 40 m
Dispersion Modeling
Dispersion is the process of spreading out the emission over a large area
thereby reducing the concentration of the pollutants.
Plume dispersion is in two dimensions: horizontal and vertical. It is
assumed that the greatest concentration of the pollutants is on the plume
centerline in the direction of the prevailing wind. The further the away
from the centerline the lower the concentration.
The spread of the plume is approximated by Gaussian probability curve
C(x,y,z) = [Q/(2 B u Fy
Fz)] exp[ -1/2 [(y / Fy)2 + (z / Fz)2]
C(x, y, z) = concentration at some point in coordinate space, kg,m3
Q = source strength, kg/sec
Fy,Fz
= standard deviation of the dispersion in the y and z directions
y = distance crosswind horizontally, m
z = distance crosswind vertically, m
z is in the vertical direction, y is in the horizontal crosswind direction, and x is the downwind direction
C(x,y,z) = [Q/(2 B u Fy
Fz)] exp[ -1/2 [(y / Fy)2 + (z / Fz)2]]
The degree of dispersion is controlled by the standard deviations in the equation.
When F is large the spread is great, so the concentration is low (the mass is
spread out over a larger area.)
Dispersion is dependent on both atmospheric stability and distance from the
source
The values for the standard deviations for this equation can be found using
tables which have been prepared for that purpose.
To use the table you first must estimate the atmospheric stability
Consider this figure:
A plume emitted from a stack has an
effective height H (you have to calculate
)h). The centerline of the plume, z, is
then H and the dispersion equation
becomes:
C(x,y,z) = [Q/(2 B u Fy
Fz)] exp[ -1/2 [(y / Fy)2 + (z - H / Fz)2]
This equation and the one presented previously hold as long as the ground does not
influence the diffusion (the plume hits the ground)
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
the ground.
One way of accounting for this influence is to assume all pollutants are
reflected by the ground.
A new equation can be written based on
this assumption:
C(x,y,z) = [Q/(2 B u Fy
Fz)] exp[ -1/2 (y / Fy)2] x {exp[-1/2[(z + H) / Fz
exp [ -1/2 [(z – H)/Fz]]2}
Example
A source emits 0.01 kg/sec of a Sox on a sunny summer afternoon with an
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
stack.
A sunny afternoon would give
curve B
Now from the figures:
Fy = 36 m
Fz = 20 m
Now x = 200 m, y = 0, z = 0 m, and:
C(x,y,z) = [Q/(2 B u Fy
Fz)] exp[ -1/2 (y / Fy)2] x {exp[-1/2[(z + H) / F
exp [ -1/2 [(z – H)/Fz]]2}
C(200,0,0) = [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]2] = exp[ -1/2 x [(0 + 20)/20]2]}}
= 6.7 x 10-7 kg/m3 = 670 :g/m3
Special Conditions
If the measurement is taken at ground level and the plume is emitted at
ground level (Z = 0, H = 0):
C(x,y,z) = [Q/(2 B u Fy
Fz)] exp[ -1/2 (y / Fy)2 ]
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
= 0, and y = 0), the equation is even further simplified to:
C(x,y,z) = [Q/(2 B u Fy
Fz)]
Control of Pollution from Automobiles
Important points requiring control:
Evaporation loss from fuel
tank
Evaporation of HC’s from
carburetor
Emission of unburned gas
and partially oxidized HC’s
from crankcase
NOx., HC’s, and CO in the
exhaust
Control of the potential emission points
Evaporation form the
gas tank can be
eliminated by use of gas
tank caps that prevent
vapor escaping
Crankcase emissions have been eliminated by recycling
crankcase gases into the intake manifold and the
installation of the positive crankcase ventilation valve
(PCV).
Losses from carburetors
can be reduced by using
activated carbon
canisters that adsorb
vapors emitted when the
engine is turned off and
hot gasoline in the
carburetor vaporizes.
The vapors are purged
from the canister by air
when the car is restarted
and burned in the engine
Exhaust Emissions
60% of the HC’s and almost all of the NOx, CO, and lead come
from the exhaust.
The quantity of emissions changes with the operating
conditions of the vehicle.
When the car is accelerating the combustion is efficient (low CO and HC), but
high amounts of NOx are produced.
When the car is decelerating there are low amounts of NOx produced but high
amounts of HC’s due to partially burned fuel.
This makes it difficult to determine how much pollution a particular engine
design produces. The EPA has developed a standard test to make this
determination. The test includes a cold start, cruising with a simulated load,
and a hot start.
Exhaust emission control techniques
Tuning the engine to burn fuel efficiently
Installation of catalytic reactors
Engine modifications
Engine Tuning
A well tuned engine is the first line of defense for
controlling automobile emissions
Catalytic Converters
Oxidize CO and HC’s to CO2 and H2O
Most common catalyst - platinum
Problems:
Fouled by some gasoline additives like lead (this is why
lead has been eliminated from gasoline)
Sulfur in gasoline converted to particulate SO3
Redesign of internal combustion engines
Cylinder configuration
Fuel injectors