Download Air Pollution Examples Air Quality and Meteorology

Air Pollution Examples
for CEL 212-Environmental Engineering
(Second Semester 2012-13)
Air Quality and Meteorology
Dr. Arun Kumar
Civil Engineering (IIT Delhi)
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Courtesy: Dr. Irene Xagoraraki (U.S.A.)
April 27, 2013
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Dry Adiabatic Lapse Rate
Altitude, z (km)
Stability
For this parcel of air the
change in temperature with
altitude was:
Adiabatic
lapse rate
•
= (T2-T1)/(z2-z1)
z1
1
T2
Temperature,
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•
When any parcel of air
moves up or down, it’s
temperature will change
according to the adiabatic
lapse rate
T1
Dry adiabatic lapse rate: temperature decreases with
increased altitude
Γ=−
= (10-20)oC/(2000-1000)m
= -1 oC/100m
2
z2
2
dT
= −1.00 °C/100m = -5.4 °F / 1000ft
dz
Atmospheric (actual) lapse rate
< Г (temperature falls faster) unstable (super-adiabatic)
> Г (temperature falls slower) stable (sub-adiabatic)
= Г (same rate) neutral
T (oC)
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Example 1
Z(m)
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Unstable Conditions
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Rapid vertical mixing
takes place.
T(ºC)
10
5.11
202
1.09
∆T T2 − T1 1.09 − 5.11
=
=
= −0.0209 °C/m
∆z z 2 − z1
202 − 10
= −2.09 °C/100 m
Since lapse rate is more negative than Г, (-1.00 ºC/100 m)=> atmosphere
is unstable
-1.25 oC/100 m < -1 oC/100m
actual temperature falls faster than Г
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Unstable air encourages the
dispersion and dilution of pollutants.
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Stable Conditions
Air at a certain altitude remains
at the same elevation.
-0.5 oC/100 m > -1 oC/100m
actual temperature falls slower than Г
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Stable air discourages
the dispersion and
dilution of pollutants.
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Air at a certain altitude remains
at the same elevation.
Neutral Conditions
-1 oC/100 m = -1 oC/100m
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Neutrally stable air
discourages the dispersion
and dilution of pollutants.
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Why are these plumes so different?
neutral
Prediction for Pollutant Concentration
under
inversion layer
Above
inversion
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Point-Source Gaussian Plume Model
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Point-Source Gaussian Plume Model
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2
Point-Source Gaussian Plume Model
Effective Stack Height
•
Model Structure and Assumptions
– pollutants released from a “virtual point source”
– advective transport by wind
– dispersive transport (spreading) follows normal (Gaussian)
distribution away from trajectory
– constant emission rate
– wind speed constant with time and elevation
– pollutant is conservative (no reaction)
– terrain is flat and unobstructed
– uniform atmospheric stability
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H = h + ∆H
Where:
H = Effective stack height (m)
h = height of physical stack (m)
∆H = plume rise (m)
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Effective Stack Height (Holland’s formula) for
neutral conditions
∆H =
vs
u


T −T
−2
1.5 +  2.68 × 10 (P ) s a

 Ta

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Plume rise equation for neutral
conditions
 
d 

 
where v s = stack velocity (m/s)
d = stack diameter (m)
u = wind speed (m)
P = pressure (kPa)
Ts = stack temperature (ºK)
Ta = air temperature (ºK)
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Atmospheric Stability Categories
• How much will be % error in C(x,0,0) if one
uses Heffective(unstable) for stability class?
Think qualitatively.
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3
Vertical Dispersion
Horizontal Dispersion
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Wind Speed Correction
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• A stack in an urban area is emitting 80 g/s of NO. It
has an effective stack height of 100 m. The wind
speed is 4 m/s at 10 m. It is a clear summer day with
the sun nearly overhead.
• Estimate the ground level concentration at: a) 2 km
downwind on the centerline and b) 2 km downwind, 0.1
km off the centerline.
p
Where: ux = wind speed at elevation zx
p = empirical constant
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Example 2
1.
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Example 2
• Unless the wind speed at the virtual stack height is
known, it must be estimated from the ground wind
speed
z 
u2 = u1  2 
 z1 
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Example 2
2. Determine σy and σz
σy = 290, σz = 220
Determine stability class
Assume wind speed is 4 km at ground surface.
Description suggests strong solar radiation.
Stability class B
220
290
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Example 2
3.
Example 2
Estimate the wind speed at the effective stack
height
Note: effective stack height given – no need to
calculate using Holland’s formula
4.
Determine concentration
a. x = 2000, y = 0
C (2000,0) =
p
z 
 100 
u2 = u1  2  = 4

 10 
 z1 
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0.15
C ( 2000 , 0 ) = 6 . 43 × 10 − 5 g/m 3 = 64 . 3 µg/m
= 5.65 m/s
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 1  0 2 
 1  100  2 
80
exp − 
  exp − 
 
π (290)(220)(5.6)
 2  290  
 2  220  
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Example 2
Example 3
b. x = 2000, y = 0.1 km = 100 m
C (2000,100) =
 1  100  2 
 1  100  2 
80
exp − 
  exp − 
 
π (290)(220)(5.6)
 2  290  
 2  220  
C ( 2000 ,0 ) = 6 . 06 × 10 − 5 g/m 3 = 60 . 6 µg/m
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• If in example #2, there is another stack (downwind
distance from 1st stack =500m) with physical height
(203m). Now, calculate overall ground level
concentration at 2 km downwind on the center
line? This 2nd stack is also emitting NO at same 80 g/s
rate (all other conditions remain constant) (for stack
#2: inside diameter =1.07m; air temp:13degC;
barometric pressure =1000 milibars; stack gas
velocity=9.14m/s; stack gas temp: 149degC)
3
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Example 3 hints
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Example 4
• Question: Suppose an anemometer at a height of 10
m above ground measure wind velocity =2.5m/s.
estimate the wind speed at an elevation of 300 m in
rough terrain if atmosphere is unstable (i.e., k=0.2)?
• From stack #1, we know conc (C1)
• For stack #2, first calculate effective stack height using
Holland’s formula then calculate conc. at given
distance using approach given in Example 2 (apply
correction for x= distance of receptor from stack
#2)say we get conc. C2
• Now total conc. at receptor =Ctotal=C1+C2
• Now see if this is less than Callowable
• If not, then we need to control stack heights or source
strength
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• Answer:
z 
u2 = u1  2 
• U300/u10=(300/10)(0.2)
 z1 
• Wind velocity at 300m=(2.5)*(30)(0.2)=4.9m/s
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p
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5
CPCB minimum guideline for stack
based on SO2 emission
• CPCB minimum stack height =30m
• So Choose maximum (30m; hSO2)
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Example 5
• A 40% efficient 1000MW coal fired power plant emitts
SO2 at rate =6.47*108 microgram/s. the stack has
effective height =20m (CPCB recommended minimum
height =30m). An anemometer on a 10-m pole
measures 2.5m/s of wind and atmospheric class is C.
• Predict the ground-level concentration of SO2 4 km
directly downwind?
• What would be this concentration if stack height is
changed to 30 m?
• What is the recommended stack height based on
SO2 emission rate?
• Which stack height would you choose?
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Example 6
• Repeat Example 5 for stability classes :
B,C and D for calculating C(x,0,0) where
X=0-100m with 4 m gap. Now plot
C(x,0,0) versus distance or for different
stability classes. Use effective height
obtained from Example 6.
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