environmental engg air pollution

7/30/2019 environmental engg air pollution

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Air Pollution

Sources, Effects & Control


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Highlights Introduction Clean air composition

Definition of Air Pollution, History & Episodes

Sources of Air pollution

Based on activity

Based on shape of entry of pollutants

Factors affecting air pollution

Classification of air pollutants


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Effects of Air Pollutants Major effects: (a) Eyes, (b) Respiratory system

Suspended Particulate Matter (SPM)

Solid form (ex. dust, smoke, fume etc.)

+

liquid form (ex. Mist, fog)

TSPM Total SPM

RSPM Respirable SPM


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Effects of Air Pollutants Major effects: (a) Eyes, (b) Respiratory system

SPM >10 m Settle in

Cilia of the nose

RSPM orPM 10


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Effects of Air Pollutants For residential areas, max annual conc.

TSPM 140 g/m3;

RSPM - 60 g/m3

National Ambient Air Quality Standards of

India (http://moef.nic.in/modules/rules-and-

regulations/air-pollution/)


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Effects of Air Pollutants Eye problems From gaseous & PM

When contact with internal mucous

line

Pollutant effect

Aldehyde Irritation to eyes, skinAmmonia Corrosive to mucous membrane

Arsenic Damage to skin

Cadmium Damage to kidney and liver

Chlorine Irritation to eyes & throat

Lead Deposition in lungs

Nickel Lung cancer & respiratory system


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Effects of Air Pollutants Carbon Monoxide:

Diffuses into blood

CO + Haemoglobin Carboxyhaemoglobin

CoHB - Absorbs CO2 200 times more than

that of O2

Ambient air quality standard - 10 ppm

Dangerous at 750 ppm


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Effects of Air Pollutants Sulphur Dioxide:

Cause irritation (Throat ~ 8-12 ppm)

Reduction in visibility & Respiratory diseases

Dangerous to life ~ 400-500 ppm for few minCoal induced pollution:

From burning coal smoke, flyash, sulfur

compounds Occurs in cold climate when calm meteorological

conditions prevail


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Oxidants:

O3 cause eye irritation ~ >0.1 ppm

Photochemical smog O3, PAN, PBN, aldehyde

Effects of Air Pollutants

Photochemical Smog:

Air stagnation, abundant sunlight, highconcentrations of hydrocarbons and nitrogenoxides in the atmosphere

By the interaction of some HCs & Oxidants

Produce Peroxy acetyl nitrate (PAN)


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Effects of Air Pollutants

Oxides of Nitrogen:

NO & NO2

Haemo lobin has 300, 000 times more affinit

towards NO2 than O2

Cause lung cancer

Dangerous level ~ 150 ppm


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Effects of Air PollutantsDiseases:

Lung cancer destruction of lung tissues

Chronic Bronchitis reduction of

Bronchioles diameter

Bronchial Asthma Narrowing of air ways

Emphysema Diminish the ability of lungs to

exchange O2 and CO2


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Air Pollution

ontro ev ces


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Control by Source Correction Change in raw materials

Low sulphur fuel in place of high sulphurfuel

Removal of non essential ingredients

Use of exhaust hoods and ducts

Recovery

Equipment modification/replacement Replace old equipment by new equipment

Proper maintenance of equipment


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Selection of Air Pollution Control Devices(1) Carrier gas characteristics

Pressure

Dew point Density

Viscosity

Temperature etc

(2) Operational factors

Head room Floor space

Service requirement etc


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Selection of Air Pollution Control Devices

(3) Process factors

Allowable pressure drop

Gas flow rate Collection efficiency requirement etc

(4) Particulate characteristics Shape & Size

Density

Stickiness Corrosiveness & Toxicity

Electrical conductivity etc


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Particulates Collection Devices Classified into three groups

Based on Collection /Operation mode

Internal Separators

(a) Gravity Settling Chamber, (b) Cyclone

Wet Collection Devices

Electrostatic Precipitators


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1. Gravity Settling Chamber Enclosed chamber where the velocity of the

dust laden gas considerably reduced

Particles settle by gravity

Horizontal gas velocity - to allow stream line

flow

Gas velocities ~ 0.30 to 3 m/s

Particles coarser than 40 microns settle

Internal separators Wet collectors Electrostatic precipitators


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Gravity Settling Chamber



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2. Cyclone Separators Separation - Transforming the inlet gas velocity into

a double vortex

Gas spirals down the inner surface & spirals up at thecentral portion

Efficiency increases with

Inlet velocity of the gas (no agglomeration)

Diameter & density of the dust particle

Dust concentration in the gas

Smoothness of the inner wall of the cyclone



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Cyclone Separators



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Cyclone Separators Advantages:

No moving parts

Withstand harsh conditions

Operate in a wide range of conditions

Disadvantages:

Moderately efficient

High operating cost

Pressure drop problems



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Wet Collectors

Separation - By wetting the particle with liquid

Mechanism:Liquid droplet diffusion/condensation/impinging the

wetted or un-wetted articles on a collectin surface

Common wet collection devices

Cyclonic scrubbers

Spray chambers

Venturi scrubbers

Packed towers



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Water

Gas InflowGas Outflow

Mechanism of Wet Collectors

Wastewater & Particulates

Note: Direction of gas & water flow may be different based on type



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1. Cyclonic Scrubbers Also called wet cyclones

Particulates separated by centrifugal force &impingement of water at the entrance

Moisture elimination section consists of zi -

zag plates

Water requirement ~ 2 to 50 lit / 40 lit of

gas Gas flow rate ~ 2000 lit/min

Removal particles 5 m



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Cyclonic Scrubbers



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2. Venturi Scrubbers

Clean about 4000 lit of gas/min

Consists venturi throat through which carrier gaspushes at a velocity of 3400 to 12600 m/min

The scrubbing liquid, usually water is added in thedirection of flow of gas at the throat at the rateof ~ 0.3-1.5 lit/lit of gas

Efficiency can be as high as 99%



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Venturi Scrubbers



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3. Spray Chambers Fine water spray washes the gas



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Packed



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Advantages:

- Handle flammable and explosive dust

- Gas absorption and dust collection- Handle mists & Cooling of hot gases

- Handle corrosive ases and dust

Wet Collectors

Disadvantages:

- High corrosion potential

- Liquid waste stream treatment

- Freezing protection needed

- No recycling of particulate

- High energy costs



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Electrostatic Precipitator Can be applied to a great variety of problems

Efficiencies as high as 99.99%

Capacities up to 2,00,000 lit/min Temperatures up to 6000 C

Pressure drop is very low ~ 6 to 10 mm of water

Dirty gas is allowed to pass through narrow, vertical gaspassages formed by parallel rows of grounded collecting

electrodes

Electrically insulated high voltage wires are spaced precisely

on the centre lines of each passage thereby causing dirt gas

to pass between the high voltage wires and the grounded

plates



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Operational principle

Ionization of gas

Dust charging dust particles get negatively chargedbecause the negatively charged ions collide with them

driven by electrical forces to the positively chargedgrounded plate and held to them goes on accumulatingto form a thick layer

As the thickness of the dust layer increases more than 6mm, electrical attraction becomes weak efficiency of theESP comes down a sharp rap causes the dust layer toshear away agglomerates are collected in hoppers



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Gas Outflow

Working Mechanism of ESP

Particulates

Electric Plates

Gas Inflow



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Electrostatic Precipitator



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ESP Advantages & Disadvantages

Advantages:

- High efficiencies for small particles

- Large gas volumes with low pressure drops- Dry collection of valuable materials

- Wet collection of fumes or mists

-

Disadvantages:

- High capital costs

- No control of gaseous emissions- Inflexible to changing operating conditions

- Large space requirements

- Resistivity problems



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Air Pollution Control Gas & Vapor

SOX, NOX, VOCs, CO

Absorption

Thermal incineration

Catalytic incineration Condensation


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Spray towers

Plate towers Packed towers

Absorption Units

en ur scru ers

Principle

Transfer of pollutants from gas phase to

the liquid phase

Diffusion and dissolution


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Adsorption Units

Pollutants adsorbed in the surface pores of

the adsorbents Activated carbon

Silica gel

Molecular sieves Ex.: Dehydrated

zeolites


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Desulfurization (Coal cleaning)

Low sulfur fuel

Flue as desulfurization FGD

SOx Control Methods

- dry or wet processes(activated carbon adsorption/Water absorption)

- Regenerative processes

FGD - flue gas react with lime and thereby removesi.e. wet limestone-gypsum process)


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Thermal NOx vs. fuel NOx

combustion modifications

- low NOx burners

- flue gas recirculation

NOx Control Methods

- staged combustion Selective Catalytic Reduction (SCR)

Selective Non-catalytic Reduction (SNCR)

* Chemical reaction (through catalyst) degrade NOx into N2and H2O (ammonia-based selective catalytic reduction)


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Meteorology of Air Pollution If hot air released into the atmosphere,

- Air tends to expand and rise- Equal to surrounding air temp. &

No heat transfer between air parcel &

atmosphere

Dry Adiabatic Lapse Rate (DALR) ~ -1

C/100 m Ambient (or) Environmental Lapse Rate (ELR)


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If ELR > DALR Super adiabatic lapse rate

ELR < DALR Sub adiabatic lapse rate

Meteorology of Air Pollution

Lapse rate if zero calledIsothermallayer

Inversion Surface of earths temperature

cooler than temperature at high altitudes


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Plume Behavior


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Stack Height & Mixing Depth

Dispersion of pollutant

Function of stability of atmosphereStack height

Stack height Determines GLC of pollutant

Effective stack height(He) = H + H

H Physical stack height; H Plume rise

Mixing Depth Ht. available for mixing


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Stack Height & Mixing Depth


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h = (Vs D/u) [ 1.5+2.68 X 10-3 P D {( Ts Ta )/Ts}

h = rise of plume above the stack in m

v stack as velocit m sec

Hollands Equation

D = inside exit diameter of stack in mu = wind in m/sec

P = atmospheric pressure in millibars

Ts = stack gas temperature in 0KTa = air temperature in

0K


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Briggs Equation( )

( )mstackofheighthscalinreleaseheatQ

u

Qhh

H

H

=

=

+=

/

09.04.1284.04

1

For hot releases106 cal/s or more

( )

( )mdiameterexitstackd

smvelocityaffluxV u

dVh

=

=

=

/

3

0

0

For less hotreleases,

( )mriseplumeh =


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Modelling of Dispersion of Pollutants

Dispersion models - Quantify transport &

dispersion in atmosphere

Gaussian Dispersion Model Follow Gaussian

Assumptions:

Pollutants released at a steady state

Wind speed is constant

Major distribution of pollutant along x-axis


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Gaussian Dispersion Model


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Concentration alon lume centre line

Concentration of pollutant at ground level,

2

2

1

,0,0,

2

=Z

H

zy

Hxe

u

QC

Refer: http://www.ajdesigner.com/phpdispersion/ground_level_equation.php

S k H i h I di P i


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Stack Height Indian Practice

Values obtained using the above equations shall be subject tothe following minimum values

Chimneys for industries in general (except TPP) -30m For TPP up to 500 MW capacity -220m

For TPP >500 MW capacity -275m

-

For boilers generating steam @ >30t/h - 30m

For boilers of intermediate capacity - 9 to 30m

For DG sets, minimum stack height shall be 1.5 to 3.5 m more than theheight of the building and shall be worked out as

H=H + 0.2 KVA

Where H= height of buildingH = stack height

KVA is the capacity of generator.

environmental engg air pollution

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