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AIR POLLUTION

LEVELS OF AIR POLLUTION OF PUBLIC HEALTH CONCERN MICRO SCALE :

Pollution in a confined area e.g. indoors . This is caused byinadequate ventilation products used in construction materials

radioactive materials etc MESO SCALE :

Pollution in outdoor ambient air . Examples are industrial e.g.emission from factories and mobile sources e.g. emission fromvehicular traffic.

MACROSCALE : This involves transport of ambient air pollutants over large

distances with global impact e.g. haze caused by bushburning , acid rain , ozone pollution

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EFFECTS OF AIR POLLUTANTS

EFFECTS ON MATERIALS : Five mechanisms responsible for air pollution are :

Abrasion Deposition and removal Direct chemical attack

Electrochemical corrosion. Factors that influence deterioration are :

Moisture , Temperature Sunlight and position of the material (e .g upper or lower , horizontal or vertical )

EFFECTS ON VEGETATION : Deposits and attack on leaves Ozone attack on palisade cells leading condensation of chloroplasts and collapse of

cell walls. Inhibition of plant growth due to exposure to N02 , S02 etc leading to necrosis (loss of

protoplasm) Transfer of effect of air pollution damage to animals e . g grazing animals eating grass

with high levels of fluoride may cause mottles in teeth and eventual loss of teeth.

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AIR POLLUTION( contd.)

EFFECTS ON HEALTH

Pollution from smoking and exposure to air pollutantconcentrations e .g motor car exhaust gases

Inhalation and retention of particles Chronic respiratory disease Air pollution substances e .g V0 ,Pb ,NO2 photochemical,

oxidants, sulfur oxides (S0x) total suspended solids etc. AIR POLLUTION EPISODE

This is a disaster caused by air pollution with the followingcharacteristics : A large number of pollution sources A restricted air volume Failure of officials to recognize that anything is wrong The presence of water droplets of the right size Many people get sick and many die

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ORIGIN AND FATE OF AIR

POLLUTANTS

Carbon Monoxide :

Caused by incomplete combustion from Natural sourcesexample anaerobic decomposition of carbonaceous

material by soil microorganisms Anthropogenic sources (human activities ) e .g. motor vehicles, fossil

fuel burning for heat and electricity, industrial processes, solid wastemanagement( incineration), bush burning etc.

Hazardous Air Pollutants (HAPs) :

Sources : Fuel combustion , Metal processing

Petroleum, natural gas production, and refining Surface coating processes , Waste treatment and disposal Agricultural and chemical production , polymers and resins.

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POLLUTANTS (contd.)

Lead : From volcanic activity (atmospheric source) , Smelters and refining processes Discharge from gasoline combustion in motor vehicle

Nitrogen Dioxide (NO2) : Sources ;

Bacteria action in soil , Anthropogenic sources from combustion processes

Photochemical Oxidants : Caused by atmospheric chemical reactions e.g. ozone Formation due to nitrogen dioxide photolytic cycle. Hydrocarbons modify this

cycle to form free radicals by reaction with atomic oxygen

In the presence of abundant sunshine, the reaction leads to the formation ofa thick / dense cloud known as smog.

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POLLUTANTS (contd.)

Photochemical oxidation are known as secondary pollutants

Sulfur Oxides :

May be primary and secondary pollutants

Primary pollutants sources : Power plants industry , ocean , volcanoes etc emit S03 S04

Secondary pollutants : Catalytic oxidation or photochemical oxidation of SO2 to H2SO4 i.e. acid

rain

Particulates :

Soil dust

Smoke from forest fires Sea salt ,

Anthropogenic sources e.g. fossil fuel burning and industrialprocesses.

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POLLUTANTS (contd.)

OTHER EFFECTS :

ACID RAIN - Unpolluted rainwater is mainly acidic (pH 5.6) containing

dissolved carbon dioxide(CO2)

Polluted rain : made up of chemical reactions in theatmosphere which converts SO2 N0x VOCs to acidiccompounds and oxidants.

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POLLUTANTS (contd.)

OZONE DEPLETION

Presence of ozone provides a barrier to ultraviolet (UV)radiation.

Too much UV will cause skin cancer

Causes:

Anthropogenic e.g. reaction of CFC with ozone

Natural: volcanic eruptions and weather e.g. strongcold winds prevent replenishment of ozone-rich air.

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OZONE

PHOTO REACTIONS OF OZONE

OZONE DESTRUCTION BY CFC

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ORIGIN AND FATE OF

AIR POLLUTANTS (contd.)

GREEN HOUSE EFFECT

This is caused by CO2 which allows shortwave radiationfrom the sun to heat the ground surface .

It then restricts( traps) the loss of heat by radiation from theground surface .

Other gases contributing to the greenhouse effect areMethane (CH4) , nitrous oxide (NO2) and CFCs .

Other causes of reduction in CO2 removal are deforestation

(sink) burning timber etc.

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AIR POLLUTION CONTROL

STATIONARY SOURCES

Gaseous Pollutants

Absorption : This involves the transfer of the pollutant from agas phase to a liquid phase (dissolution) .

Equipment used include spray chambers , towers or columns ,scrubbers.

The solubility of the pollutant in the liquid must be relativelyhigh

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AIR POLLUTION CONTROL (contd.)

GAS ABSORPTION DEVICES

SPRAY TOWER

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GAS ABSORPTION DEVICES( contd.)

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ADSORPTION:

A mass transfer process in which the gas is bonded to asolid .

The gas known as adsorbatepenetrates the pores of the

solid (adsorbent) . The bond may be physical or chemical

The reaction is a surface phenomenon

Examples of adsorbent are activated carbon, molecular

sieve, silica gel, activated alumina etc.

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ADSORPTION SYSTEM

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COMBUSTION:

Applied to oxidizable pollutants to an inert gas. Examples : CO and hydrocarbons

Method : Direct flame by after burners and catalytic combustion

DIRECT FLAME INCINERATION

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FLUE GAS DESULFURIZATION (FGD) This method is used to remove sulfur oxides from the gas stream. Two types :

Regenerative : The reagent used to remove sulfur oxides isrecovered and reused.

Non-regenerative : The reagent used to remove sulfur oxides from the

waste stream is discarded. Chemical reagent used :lime (CaO) caustic soda (Na0H), soda ash

(Na2CO3), Ammonia (NH3) . Types of devices used :

Absorbers (wet scrubbing) : Venturi scrubber absorbers ,

static packed absorbers , moving bed absorbers , tray towers , and spray towers.

Spray dryer based FGO :

Make up spray dryer and particulate collector

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CONTROL TECHNOLOGIES FOR NITROGEN OXIDES

Prevention : This method employs temperature zone toreduce N0x formation

Alternative methods : Minimizing operating temperatures Fuel switching Low excess air Flue gas recirculation Lean combustion Stage combustion Low N0x burners Secondary combustion Water / stream injection

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CONTROL TECHNOLOGIES FOR NITROGEN OXIDES(contd.)

Post-combustion : This involve conversion of N0x to nitrogengas in three processes :

Selective catalytic reduction (SCR) vanadium titanium ,zeolite

Selective noncatalytic reduction (SNCR) : ammonia or urea

Non selective catalytic reduction (NSCR) : Three waycatalyst

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PARTICULATE POLLUTANTS

Cyclones :used to remove particulates with sizes greaterthan 10ym

Filters : for high efficiency removal of particles smaller than5ym

Types of filter Deep bed filter

The baghouse

Liquid scrubbing : used for collection of wet ,corrosive or veryhot particulate matter .examples : talc dust , foundry dust ,

furnace fumes , phosphoric acid mist etc. Electrostatic Precipitation (ESP) : This is used for dry

collection of particulates from hot gas stream using electriccurrent potential (30-75V)

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Reverse Flow Cyclone

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The Bag house Shaker cleaned baghouse Pulse Jet Cleaned baghouse

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Venturi Scrubber

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Electrostatic Precipitator

Wire tube Wire and Plate

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Particle charging and collection in an

Electrostatic Precipitator (ESP)

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AIR POLLUTION CONTROL OF MOBILE SOURCES

WASTE MINIMIZATION STRATEGIES : Minimization of pollutants production

Water conservation : energy consumption and water supply

Building smaller automobiles to reduce fuel consumption

use of alternative sources of energy viz solar ,wind . biofueletc.

use of alternative propellant for spray cans.

use of alternative materials as refrigerants to replace CFCs.

Reduced used of solvents and replacement of solventbased paints by water based paints

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