Download - Chapter 6 - Air Pollution 2

CHAPTER 6B

Air Pollution:

1. Meteorology and Dispersion Modeling

2. Air Pollution Control2. Air Pollution Control

Learning Objectives

At the end of this lesson the students should

be able to;

• Understand apply the meteorological

aspects and dispersion of air pollutantaspects and dispersion of air pollutant

• Engineering assessment and application

of various methods in controlling air

pollution

Air Quality and Meteorology

• Air quality depends on

– wind

– sunlight

– temperature

– precipitation and humidity – precipitation and humidity

– Energy from the sun and earth’s rotation drives atmospheric circulation

• Circulation, and the resulting interactions with water and temperature differences produce the climate and weather we observe

Air Quality and Meteorology

• Somewhat less observable issue relates to mixing

• Easy to understand how wind and turbulence produce mixingturbulence produce mixing

• “Inherent” mixing property that derives from pressure, volume, temperature relationships

• Lapse rate – change in temperature with height (altitude)

Air Quality and MeteorologyFactors controlling air quality

1. Downwind distance

The air pollution will disperse as the downwind distance increases. The

further away the distance will have lower air pollution concentrations.

2. Wind speed and direction (air mixing)Wind speed also contributes to how quickly pollutants are carried away

from their original source. However, strong winds don't always dispersefrom their original source. However, strong winds don't always disperse

the pollutants. They can transport pollutants to a larger area, such as

the smoke from open burning or forest fires.

3. Atmospheric stabilityOnce pollutants are emitted into the air, the weather (atmospheric

stability) largely determines how well they disperse. Turbulence mixes

pollutants into the surrounding air. For example, during a hot summer

day, the air near the surface can be much warmer than the air above.

Sometimes large volumes of this warm air will rise to great heights.

This results in vigorous mixing.

Air Pollution Occurrences

• The most obvious factor influencing air pollution is the quantity of contaminants emitted into the atmosphere.

• However, when air pollution episodes take place, they are not generally the result of a drastic increase in the output of pollutants; instead, they occur because of changes in certain atmospheric conditions. changes in certain atmospheric conditions.

• Two of the most important atmospheric conditions affecting the dispersion of pollutants are:

– (1) the strength of the wind and

– (2) the stability of the air.

Air Mixing

• The direct effect of wind speed is to influence the

concentration of pollutants.

• Atmospheric stability determines the extent to which

vertical motions will mix the pollution with cleaner air

above the surface layers. above the surface layers.

• The vertical distance between Earth's surface and the

height to which convectional movements extend is called

the mixing depth.

• Generally, the greater the mixing depth, the better the air

quality.

Stability

• Dry adiabatic lapse rate – temperature decreases due to lower pressure (ideal gas law)

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

dT

• Ambient (actual) lapse rate

< Г (temperature falls faster) unstable or superadiabatic

> Г (temperature falls slower) stable or subadiabatic

= Г (same rate) neutral

Neutral Conditions

Unstable Conditions

Stable Conditions

Example

Z(m) T(ºC)

2 -3.05

318 -6.21

( )C/m °−=

−−−=

−=

∆0100.0

05.321.612 TTT ( )C/m °−=

−−−−

=−−

=∆∆

0100.02318

05.321.6

12

12

zz

TT

z

T

m C/100 °−= 00.1

Since lapse rate = Г, atmosphere is neutral

Example

Z(m) T(ºC)

10 5.11

202 1.09

C/m °−=−

=−

=∆

0209.011.509.112 TTT

C/m °−=−−

=−−

=∆∆

0209.010202

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), atmosphere is unstable

Example

Z(m) T(ºC)

18 14.03

286 12.56

C/m °−=−

=−

=∆

0055.003.1456.1212 TTT

C/m °−=−−

=−−

=∆∆

0055.018286

03.1456.12

12

12

zz

TT

z

T

m C/100 °−= 55.0

Since lapse rate more positive than Г,

atmosphere is stable

Temperature Inversions

• Extreme case of stability when lapse rate

is actually positive, i.e. temperature

increases with altitude

• Resulting temperature inversion prevents

nearly all upward mixingnearly all upward mixing

Why are these plumes so different?

Effect of Lapse Rate on Plumes

UNSTABLE

www.u.arizona.edu/ic/nats1011/lectures

STABLE

Inversion

Point Source Gaussian Plume

Model


Model

• Model Structure and Assumptions

– pollutants released from a “virtual point

source”

– advective transport by wind– advective transport by wind

– dispersive transport (spreading) follows

normal (Gaussian) distribution away from

trajectory

– constant emission rate


Model

• Model Structure and Assumptions (cont)

– wind speed constant with time and elevation

– pollutant is conservative (no reaction)

– pollutant is “reflected by ground”– pollutant is “reflected by ground”

– terrain is flat and unobstructed

– uniform atmospheric stability


Model

Where χ = downwind concentration at

( )

−

−

=

22

2

1exp

2

1exp,0,,

zyzy s

H

s

y

uss

EHyx

πχ

Where χ = downwind concentration at

ground level (g/m3)

E = emission rate of pollutant (g/s)

sy,sz = plume standard deviations (m)

u = wind speed (m/s)

x, y, z, H = distances (m)


Model – Effective Stack Height

where

HhH ∆+=

where

H = Effective stack height (m)

h = height of physical stack (m)

∆H = plume rise (m)


Model – Effective Stack Height

• Holland’s formula

( )

−×+=∆ − d

T

TTP

u

vH

a

ass 21068.25.1

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)


Model – Stability Categories


Model – Horizontal Dispersion

• OR use Eq. 11-15

and Table 11-7

Use Fig. 11-18


Model – Vertical Dispersion

• OR use Eq. 11-16

and Table 11-7

Use Fig. 11-19


Model – 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 p

z where ux = wind speed at

elexation zx

p = empirical constant

p

z

zuu

=

1

212

Example

• 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 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.

Example

1. Determine stability class

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

Stability class BStability class B

Example

2. Estimate the wind speed at the effective stack

height

Note: effective stack height given – no need to

calculate using Holland’s formula

m/s 65.510

1004

15.0

1

212 =

=

=

p

z

zuu

Example3. Determine σy and σz

σy = 290

σz = 220

290

220

Example

4. Determine concentration using Eq 11-12

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

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

2. Air Pollution Control

Adsorption

• Adsorption :

– Control of principal polluting gas such as

sulfur oxides, nitrogen oxides, CO2 and

hydrocarbonshydrocarbons

– Passing stream of effluent gas through solid

porous material (adsorbent). The surface of

porous material attract and hold the gas by

physical or chemical adsorption

Adsorption

Absorption

• Absorption

– Absorption also known as scrubbing bringing

contaminated gas (absorbate or solute) into

contact with liquid absorbent (solvent)contact with liquid absorbent (solvent)

– One or more of the constituents of the effluent

gas are removed, treated or modified by the

liquid absorbent

– The amount of gas absorbed will depend on

the properties of both gas and solvent

Absorption

Combustion

Particulates are

burned down by

having four basic

elements : oxygen, elements : oxygen,

temperature

(650oC), turbulence

(for mixing of

oxygen) and time

Cyclone

• Dust laden gas

enters tangentially

• Under influence of

centrifugal force centrifugal force

generated by

spinning gas, solid

particles thrown onto

walls and slide down

the walls into the

hopper

Filtration

• Fabric filter system,

particulate laden gas

passed thru a woven

filter fabrics

• Particulates are

trappedtrapped

• Fabric must be

cleaned regularly to

remove trapped

particulates material

• If not cleaned filter

can explode due to

build up of pressure

Electrostatic Precipitator

• Low voltage two

staged units or High

Voltage single stage

unit

• Particulate are given • Particulate are given

negative charge and

attached themselves

to positive electrodes

and collected there

• Extremely efficient up

to 99% removal

Liquid Scrubber

In wet cyclone scrubber, high pressure spray nozzle

generate fine spray that intercepts the small particles

entrained in the swirling gases. The particulate matter

thrown onto the wall by centrifugal force then drained into

collection sump

Sulfur Dioxide Control

http://www.apt.lanl.gov/projects/cctc/factsheets/puair/adflugasdemo.html

Catalytic Converter

Catalytic Converter

• A catalytic converter is a vehicle emissions control

device which converts toxic byproducts of combustion in

the exhaust of an internal combustion engine to less toxic

substances by way of catalyzed chemical reactions.

• The specific reactions vary with the type of catalyst

installed. installed.

• Most present-day vehicles that run on gasoline are fitted

with a “three-way” converter, so named because it

converts the three main pollutants in automobile exhaust:

carbon monoxide, unburned hydrocarbon and oxides of

nitrogen.

• The first two undergo catalytic combustion and the last is

reduced back to nitrogen.

ENDEND

Download - Chapter 6 - Air Pollution 2

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