dr. arun kumarweb.iitd.ernet.in/~arunku/files/cvl212_y17/airpollution_intro.pdf · dr. arun kumar...

Air Pollution-Introductionfor CVL212-Environmental Engineering

(Second Semester 2017-18)

Dr. Arun Kumar Civil Engineering (IIT Delhi)

arunku@civil.iitd.ac.in

Courtesy: Dr. Irene Xagoraraki (U.S.A.)

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Plumes

neutral

under inversion layer

Above inversion

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Prediction for Pollutant Concentration

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Point-Source Gaussian Plume Model

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Point-Source Gaussian Plume Model

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Point-Source Gaussian Plume Model

• 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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Effective Stack Height

Where:

H = Effective stack height (m)

h = height of physical stack (m)

∆H = plume rise (m)

HhH ∆+=

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Effective Stack Height (Holland’s formula) for

neutral conditions

where vs = stack velocity (m/s)

d = stack diameter (m)

u = wind speed (m)

P = pressure (kPa)

Ts = stack temperature (ºK)

Ta = air temperature (ºK)

( )

−×+=∆

−d

T

TTP

u

vH

a

ass 21068.25.1

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• How much will be % error in C(x,0,0) if one uses Heffective(unstable) for stability class? Think qualitatively.

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Atmospheric Stability Categories

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Horizontal Dispersion

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Vertical Dispersion

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Wind Speed Correction

• Unless the wind speed at the virtual stack height is known, it must be estimated from the ground wind speed

Where: ux = wind speed at elevation zx

p = empirical constant

p

z

zuu

=

1

212

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Example 2

• 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.

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1. Determine stability class

Assume wind speed is 4 km at ground surface. Description suggests strong solar radiation.

Stability class B

Example 2

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2. Determine σy and σz

σy = 290, σz = 220

290

220

Example 2

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3. Estimate the wind speed at the effective stack height

Note: effective stack height given – no need to

calculate using Holland’s formula

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4. Determine concentration

a. x = 2000, y = 0

−

−=

22

220

100

2

1exp

290

0

2

1exp

)6.5)(220)(290(

80)0,2000(

πC

33 µg/m g/m 3.641043.6)0,2000( 5=×=

−C

Example 2

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b. x = 2000, y = 0.1 km = 100 m

−

−=

22

220

100

2

1exp

290

100

2

1exp

)6.5)(220)(290(

80)100,2000(

πC

33 µg/m g/m 6.601006.6)0,2000( 5=×=

−C

Example 2

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Example 3

• 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)

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Example 3 hints

• 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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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)?

• Answer:

• U300/u10=(300/10)(0.2)

• Wind velocity at 300m=(2.5)*(30)(0.2)=4.9m/s

p

z

zuu

=

1

212

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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.

appendix

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Dry Adiabatic Lapse Rate

Temperature, T (oC)

Altitu

de

, z (

km

)

Adiabatic lapse rate

1

2

= (T2-T1)/(z2-z1)

When any parcel of air moves up or down, it’s

temperature will change according to the adiabatic

lapse rate

For this parcel of air the

change in temperature with

altitude was:

T1T2

z1

z2= (10-20)oC/(2000-1000)m

= -1 oC/100m

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Stability

• Dry adiabatic lapse rate: temperature decreases with increased altitude

• Atmospheric (actual) lapse rate

< Г (temperature falls faster) unstable (super-adiabatic)

> Г (temperature falls slower) stable (sub-adiabatic)

= Г (same rate) neutral

ft 1000F mC/100 /4500.1 °=°−=−=Γ .- dz

dT

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Example 1

Z(m) T(ºC)

10 5.11

202 1.09

C/m °−=−

−=

−

−=

∆

∆0209.0

10202

11.509.1

12

12

zz

TT

z

T

m C/100 °−= 09.2

Since lapse rate is more negative than Г, (-1.00 ºC/100 m)=> atmosphereis unstable

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Unstable Conditions Rapid vertical mixing

takes place.

-1.25 oC/100 m < -1 oC/100m Unstable air encourages the dispersion and dilution of pollutants.

actual temperature falls faster than Г

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Stable Conditions Air at a certain altitude remains

at the same elevation.

-0.5 oC/100 m > -1 oC/100m

Stable air discourages

the dispersion and dilution of pollutants.

actual temperature falls slower than Г

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Neutral Conditions Air at a certain altitude remains

at the same elevation.

Neutrally stable air discourages the dispersion

and dilution of pollutants.

-1 oC/100 m = -1 oC/100m