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Environmental Pollution Control(ETZC362)

Dr. Jegatha Nambi Krishnan

Department of Chemical Engineering

18thJan. 2014

etzc362

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BITS Pilani, K K Birla Goa Campus18thJan. 2014 (Course Code: etzc 362)

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Meteorological aspects of air pollutant dispersion

Temperature lapse rates and stability

Wind velocity and turbulence

Lecture-3

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Meteorological aspects of AirPollutant Dispersion

Air pollutants emitted from anthropogenic sources

must first be transported and diluted in the

atmosphere before these undergo various physical

and photochemical transformations and ultimately

reach their receptors.

Otherwise, the pollutant concentrations reach

dangerous levels near the source of emission

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Hence, it is important that we understand the

natural processes that are responsible for their

dispersion

Effective dispersion of pollutants in the atmosphere

depends primarily on degree of stabilityof

atmosphere and on its turbulent structure

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The important meteorological parameters thatinfluence air pollution can be classified into primaryand secondary parameters.

Primary

Wind Direction and speed Temperature Atmospheric stability Mixing height

Secondary Precipitation Humidity Solar radiation Visibility


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Lapse Rate ():

The Lapse Rate is the rate at which temperature

changes with height in the atmosphere

Environmental Lapse Rate:

The environmental lapse rate is the cooling orwarming as measured by a weather balloon


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The degree of stability of the atmosphere in turn depends on

the rate of change of ambient temperature with altitude. The

relationship between the temperature and the altitude can be

obtained by considering air to be an ideal gas

The change of pressure in the vertical direction may be

represented by the relation


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gz

p

The change of pressure in the vertical direction may be

represented by the relation

Where,p= atmospheric pressure, z= altitude,= atmospheric density

Temperature Lapse Rates andStability

.. (1)

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Also, the perfect gas relation

Where, R= gas constant, T= absolute temperature

By assuming isothermal atmosphere and integrating the

above Eqn.


RTp

RT

pg

dz

dp

.. (2)

.. (3)

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Where, pois the pressure at ground level (1.013 bars)

The above equation gives an exponentially decreasing

pressure with altitude which is a typical characteristic of

compressible fluids such as air

zRT

gpp exp0 .. (4)

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Temperature

Altitude

Troposphere

Polytropic

n= 1.23

Isothermal n=1Stratosphere

25 km

12 km

220 K

288 K


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However, this model of the atmosphere may still

be used by assuming a series of air layers, each

layer having a uniform but different temperature.


0p

pTT o

nn /1

.. (5)

Substituting Eq. (5) for p in Eq. (3) and differentiating, we get

n

n

dz

dT 1

R

g.. (6)

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The environmental lapse rateis not always

constant particularly in the lower troposphere,

a zone from the ground level up to an altitude of

about 2 km where there is a considerable variation

from the normal rate.


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This is due to the character of the underlying

surface and the rate of radiation emanating from it.

On sunny days, because of strong solar heating

from the ground, the environmental lapse rate

often exceeds 10o

C/1000 meters


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The change of temperature with height has a profound

influence on the upward lift of the air pollutants discharged

into the atmosphere and hence on their ultimate dispersion

and dilution.

The adiabatic decrease in temperature with altitude is

especially important in the vertical movement of airpollutants and can be explained by utilizing the concept of

an air parcel, which is pictured as a little sphere of air.

Adiabatic Lapse Rate

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The lapse rate of a parcel of dry air as it moves upward in ahydrostatically stable environment and expands slowly tolower environmental pressure without exchange of heat, isknown as the adiabatic lapse rate.

Adiabatic Lapse Rate

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It is the rate of change of temperature with heightas a parcel of dry airrises orsinks without exchange of heat with the surroundings (adiabatic heat exchange).

As the air parcel risesin the atmosphere, it goes through a region of decreasingpressure and expands to accommodate the decreasing pressure. As it expands, itdoes work on the surroundings. Since the process is usually rapid, there is noheat transfer between the air parcel and the surrounding air.

For the case of an adiabatic processthe first law of thermodynamics yields,

dUdW

This means that the internal energy decreases thereby decreasing thetemperature. The decrease in temperature of a rising, expanding air parcel is animportant feature of vertical air motion.

.. (7)

Dry Adiabatic Lapse Rate

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The work doneon the surroundings is equal topdVand change in internal energybecomes equal to CvdT, where Cv is the specific heat at constant volume. Bysubstitutingthese terms (in) equation(7) becomes,

0 dTCpdV V

0 dTCpdv v

.. (8)

.. (9)

Equation(8) may be written for per unit mass of the air parcelas,


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Where vis the specific volume and Cvis the specific heat per unit mass

For a perfect gas, equation (9) becomes (work out),

vdpdTRCv )(The variation of temperature with altitude can be written as a product of two

terms:

dz

dp

dp

dT

dz

dT.

After rearranging equation (10), we obtain

RC

v

dp

dT

v

.. (10)

.. (11)

.. (12)

Substituting Equation (12) and (3) into equation (11), we get (show)

RC

g

dz

dT

v .. (13)


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Nothing that Cv+R=Cp, the specific heat at constant pressure per unit mass of airparcel, we can write equation (13) as

pC

g

dz

dT

The above expression gives the temperature lapse rate for dry parcel of airmovingupwardsadiabaticallyand is known as dry adiabatic lapse rate.

When equation (14) is evaluated for dry air, the lapse rate is found to be

meters

C

dz

dT o

adia 1000

86.9

Thus, the dry adiabatic lapse rate, (gama) is given by

km

C

dz

dT o

adia 1

10

.. (14)

.. (15)

.. (16)


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Adding this term to the right hand side of equation (8) andproceeding further we get the lapse rate in a saturatedcondition:

dz

dw

CC

g

dz

dT

pp

Above equation is referred to as the wet adiabatic lapse rateequation although the process is not strictly adiabatic.

.. (17)


Dry adiabatic lapse rate

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Dry adiabatic lapse rateVs

Wet adiabatic lapse rate

The temperature decreases upon increase inaltitude is less for moist air than that for dry air

The wet adiabatic lapse rate is not a constant,independent of altitude.

In warm tropical air the wet adiabatic lapse rateis approximately one-third of the dry adiabaticlapse rate

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Atmospheric Stability

The tendency of the atmosphere to resist or enhance vertical motion istermed stability. It is related to both wind speedand the change of airtemperature with height (lapse rate).

A comparison of the adiabatic lapse ratewith the environmental lapserategives an idea of the stabilityof the atmosphere.

If an air parcel is displaced from its original height it can:

Returnto its original height - Stable

Accelerateupward because it is buoyant - Unstable

Stayat the place to which it was displaced - Neutral

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When the envand the adiaare exactly same, a rising parcel of air will have the samepressure, temperature and density as those of the surroundings and would experienceno buoyant force. Such an atmosphere is said to be neutrally stablewhere a displacedmass of air neither tends to return to its original position nor tends to continue itsdisplacement


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When the env is less than the adia, a rising air parcel becomes cooler and moredensethan its surroundings and tends to fall backto its original position. Such anatmosphere condition is called stableand the lapse rateis said to be subadiabatic.Under stable conditions there is very little vertical mixing and pollutants can onlydisperse very slowly


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When the env is greater than the adia, the atmosphere is said to be superadiabatic.Hence, a rising parcel of air, cooling at the adiabatic rate, will be warmer and lessdense than the surrounding environment. As a result, it becomes more buoyantandtends to continue its upward motion. Since vertical motion is enhanced by buoyancy,such an atmosphere is called unstable. In the unstable atmosphere the air fromdifferent altitude mixes thoroughly.

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Absolute instability

If the environmental lapse rate is greater than the dryadiabatic lapse rate

Absolute stability

If the environmental lapse rate is less than the wet adiabaticlapse rate

Conditional stabilityIf the environmental lapse rate is between the dry adiabaticlapse rate and wet adiabatic lapse rate

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Atmospheric inversions influence the dispersion of pollutants by restricting vertical mixing.There are several ways by which inversion layers can be formed

Subsidence Inversion

This is usually associated with subtropical anticyclonewhere the air is warmed by compressionas it descends in a high pressure system and achieves temperature higher than that of the airunderneath. If the temperature increase is sufficient, an inversion will result. The subsidence iscaused by air flowing down to replace air which has flowed out of the high pressure region

Radiational Inversion

This results from the normal diurnal cooling cycle. After sunset, the ground cools quickly byradiation heat transfer, and the lowest layer of air in contact with the surface loses sensible heatthrough conduction and small scale mixing. Consequently, a temperature inversion is set up

between the cool low-level air and the warmer air above, in the first few hundred meters abovethe surface.

The extreme caseof a stableatmosphere, called an inversion, occurs when temperature increaseswith altitude. Such a lapse rate is known as negative lapse rate. Under these conditions, theatmosphere is very stableand practically no mixing of pollutants takes place

Inversion

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Valleys and low lying areas are particularly affected by this type of nocturnalinversions because denser, colder air tends to sink down beneath the warmer air. Thenext day sunlight destroys the inversion as the Earth is warmed and the air previouslystratified by inversion is overturned by convective currents. The temperature profilefor a combined radiation and subsidence inversion is illustrated below

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Advective InversionIs formed when warm air moves over a cold surface or cold air. The inversion can beground-based in the former case, or elevated in the latter case.

An example of an elevated advective inversion occurs when a hill range forces awarm land breeze to flow at high levels and a cool sea breeze flows at low levels in

the opposite direction

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Differential solar heating of the Earths surface produces pressure andtemperature gradient. As a result, the atmosphere is practically incontinuous motion with air movements being always turbulent.

Thermal Turbulence: The motion of air near the surface of the Earth isretarded by friction, which varies with surface roughness. The planetary

boundary layer, in which friction is significant, extends to about 1 kmabove the Earthssurface. The wind velocity profile within the layer is notonly influenced by the surface roughness but also by the time of day.During the day, solar heating causes thermal turbulence or eddies andthese eddies set up convective currents so that turbulent mixing is

increased. This results in a more flat velocity profile in the day than that atnight.

Thermal turbulence also depends on the thermal stability of theatmosphere. It is maximum on a clear sunny day in the afternoon andminimum at night or in the early morning.

Wind Velocity and Turbulence

d l f l d

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Wind Velocity profiles duringday and night

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BITS Pilani, K K Birla Goa Campus

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18thJan. 2014 (Course Code: etzc 362)

It is produced by shearing stress generated by air movement over the Earthssurface-the greater the surface roughness, the greater the turbulence. For smooth surfaces, theair velocity profile becomes very steep near the ground. For rougher surfaces such asthose in urban areas more mechanical turbulenceis generated and the velocity profilebecomes less steep and reaches deeper into the atmosphere

The mean wind speed variation with altitudein the planetary boundary layer can berepresented by a simple empirical power law such as,

11 z

z

u

u

Where u is the wind speed at altitude z, u1 is the wind speed at altitude z1 and theexponent varies between 0.14 and 0.40 depending on the roughness of the groundsurface as well as on the temperature stability of the atmosphere. The exponent isobserved to increase with increasing stability or with increasing surface roughness.

Adiabatic lapse rate: 7/1

Mechanical Turbulence

Effect of terrain roughness on the

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Effect of terrain roughness on thewind-speed profile

Values along curves represent percentages of gradient wind value

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Atmospheric turbulence is characterized by different sizes ofeddies. These eddiesare primarily responsible for diluting andtransportingthe pollutants injected into the atmosphere.

If the size of the eddy is largerthan the size of plume or a puff

then the plume will be transported down-wind by the eddywith little dilution. Molecular diffusion will ultimatelydissipatethe plume or the puff.

If the eddy is smallerthan the plume or the puff, the plume or

the puff will disperse uniformlyas the eddy entrains fresh airat its boundary.

When eddies are of comparable size: in such cases the eddywill both distort and disperse the puff.

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The smoke trail or plumefrom a tall stack located on

flat terrain has been found to exhibit a characteristic

shape that is dependent on the stability of the

atmosphere. The six classical plumes are described

below, along with the corresponding temperatureprofiles.

Plume Behavior

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Behavior of plume emittedfrom elevated sourcedepends on atmos.instability & windturbulence.

Some common types ofPlume Behavior are looping,coning, fanning,fumigation, lofting,trapping etc

Looping:i. Super adiabatic conditionsii. moderate wind speediii. hot summer afternooniv. large scale eddies present

Eddies carry emission to groundlevel for short time period &cause high conc. of pollutant nearstack.

Plume = form ofeffluent in water oremissions in air

Plume (Emission) Behavior

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i. When, when lapse rate neutral

ii. Moderate to high wind speed

iii. cloudy atmos

iv. any time of day or night

Coning

Emission carrieddownwindfar-away before

reaching to ground

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i. When, lapse rate is stableii. light wind speediii. early morning , all seasons

Stable lapse rate suppressvertical mixing but not thehorizontal mixing

plume travel parallel to ground~ 10-20 km downwind

favorable meteorological

condition because the plumedoes not contribute groundpollution

Fanning

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Fumigation

i. when lapse rate changes from stable tounstable, i.e. inversion break

ii. light wind speediii. clean sky, summeriv. early morning when sun rise

Stable layer of air liesabove a short height ofplume release point andunstable layer lies belowrelease point

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Lofting

i. When lapse rate changesfrom stable to unstable,

ii. Inverse of fumigation

unstable layer of air lies aboveplume release point & stablelayer lies below

pollutant dispersed upwarddirection since the top ofinversion layer acts as shied fordownward movement

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Trapping

i. When lapse rate changes from stable to unstableto stable,

ii. two inversion layers

Emitted pollutant lies between two inversion layers

Aerodynamic Effects of Structures

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Deflection and disturbance of the wind field by a structure has a majorinfluence on the distribution of pollutant from a stack on or close to thestructure.

The flow around an isolated building is characterized by the formation of acavity behind the building and a wake further downstream.

Wind flow field around a sharp edged building

Cavity

Wake

Aerodynamic Effects of Structuresand Terrain

Aerodynamic Effects of Structures

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When the chimneystack is situated on or adjacent to the building, the plume may be

entrained in the characteristic flow pattern. An empirical rule-of-thumb for suchstacks is that the height of the stack should be at least two and a half times the heightof the surrounding building. If the stack height is much belowthis value, the plumeis trappedin the wake zone of the building.

Wind

Wind

Effect of building on plume behavio

Aerodynamic Effects of Structuresand Terrain

A plume emitting from a stack located in a deep valleycan be

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p g p ycarried to the ground when the wind blows over a cliff. Thiscondition known as downwash occurs unless the stackextends well above the rim of the valley.

Behavior of the plume in a valley

Cliff

If the velocity of the plume through the stack is about equal tol th th i d l it th l t i b th

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or less than the wind velocity, the plume may not riseabove thestack level and may even be carried downward on the back-sideof the stack.

Vs

V

VVs

Downwash of pollutants in the separated region

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An Ideal Plume

Dispersion of Pollutants

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E ti ti f d

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Estimation of yand z

Based on the experimental observations and

atmospheric stability, Pasquill and Gifford have

devised a method for calculating these values.

Pz

y

Bx

Ax

903.0


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The correlations are subjected to the followinglimitations.

These concentrations estimated from the use of thesecharts and equations should correspond to a samplingtime of 10 min.

yand z are based on a surface corresponding to anopen level country and probably underestimate the

plume dispersion in an urban area.

The uncertainties associated with the estimates of yandzwill increase with distance from the source.


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If the time interval employed in the sampling is

other than 10 min, the following correction need

to be applied which is valid till 2 hrs.

q

AAt

tCC

2

1

12

Where, C is the concentration and t2is the sampling timeperiod in minutes and t1is 0 min and q has a value between0.17 and 0.20.


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Average wind speed u

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Average wind speed u

11

2

z

H

u

u

u1is usually the meteorological value of the velocitymeasured at Z1= 10m. Alpha is 0.25 for unstable and0.5 for stable conditions.


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Dispersion EquationDiff t F

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Different Forms

2

2

2exp

zzy

H

u

Q

General Equation Plume with Reflection for Stack Height H

2

2

2

2

2

2

2222zzyzy

HzHzy

u

QHzyxC

expexpexp);,,(

Ground Level Concentration Stack at Height H

2

2

2

2

220

zyzy

HyuQHyxC

expexp);,,(

Ground Level Center Line Concentration Stack at Height H

);0,0,( HxC

Ground Level Center Line Ground Point Source

)0;0,0,(xC

z=0

zyu

Q


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Calculation ofEff ti St k H i ht

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Effective Stack Height

H = hs+ h, where

h is the stack rise.Stack rise is dependant on stack characteristics,Meteorology, and physico-chemical nature of effluent.

* Carson-Moses Equation:

s

h

s

s

u

Q

u

dVh

21

6220290 ..

* Holland Formula:

dV

Q

u

dVh

s

h

s

s 0096051 ..

* Concawe Formula:

6940

4440

714.

.

.s

h

u

Qh

asph

TTCmQ

Ts= stack gas temperature, K

Ta= ambient temperature,K

kJ/s

m= gas mass flow rate, kg/s

Vs= stack gas exit velocity, m/s

us= wind speed at exit, m/s

d= stack exit diameter, m

All Equations assuming that no stack tipdownwash occurs: i.e. when

Vs 1.5 us


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Briggs Formula

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Neutral or unstable condition (A,B,C,D)

mx

mHFx

T

TTrgVF

U

xFH

f

sf

a

asss

f

305Hfor674

305Hfor16.2

6.1

s

4.0

s

6.04.0

2

3/23/1

Xf= down wind distance to final plume rise in m.U = wind speed at stack tip m/sF = Buoyancy flux parameter in m4/s3.

Briggs Formula


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For stable conditions (E and F)

mC

Z

T

T

gS

US

F

H

a

a

/01.0

4.2

3/1

S= Stability parameter


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Example

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Example

2

2

2exp)150;0,0,500(

zzy

H

u

QC

NOxemission from a stack at a rate of 100 g/s from an urban stack ofphysical height 100m and a plume rise of 50m. What is the ground-levelconcentration at a distance of 500 m from the stack and along the center-line on a clear sunny day? Wind speed is measured at 2 m/s at 10m.

H = hs+ h

2

2

2

2

2

2

2exp

2exp

2exp

2);,,(

zzyzy

HzHzy

u

QHzyxC

Centerline y = 0 Ground Level z = 0

hs= 100; h = 50H = 150

Equation for emission from a stack with Reflection


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Example -contd.

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Example contd.

Dispersion Coefficients Calculation

1. Graphical (from Fig 3.19 and 3.20)

Two options to compute yand z-

y~ 120 m

For x = 500 m

z~ 120 m

From Table 3-2, ClearStronginsolationStability class = A or B


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Example -contd

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Example contd.

2. Empirical Equations (from Equations 3.57 and 3.58 and tables 3.2 and 3.3)

2

26

)105(2

150exp

)105)(109)(2(

10*100)150;0,0,500(

C

y: From Table 3.3,

y=109 m

Z= 105 m

Data: Q = 100 g/s H = 150 m u =2 m/s y= 109 m z= 105 m

= 501 g/m3of NOX

x in km : 0.5 km


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Example 2

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Example 2

2

2

2

2

2exp

2exp)150;0,100,500(

yzzy

yH

u

QC

NOXemission from a stack Estimate NOXconcentration at ground level,100 m crosswind, 500 m from the stack

2

2

2

2

2

2

2exp

2exp

2exp

2);,,(

zzyzy

HzHzy

u

QHzyxC

)1132

100(exp*501)150;0,100,500(

2

2

C = 338 g/m3of NOX

NAAQS standard for NO2= 100 g/m3 for annual average

et zc362-l3

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