Monday, November 4, 2013

Air Pollution - causes, effects and control measures

Air pollution - Air pollution may be defined as the presence of one or more contaminants like dust, mist, smoke and colour in the atmosphere that are injurious human beings, plants and animals.
  1. Rapid industrialization
  2. Fast urbanization
  3. Rapid growth in population
  4. Growth of vehicles on the roads and
  5. Activities of human beings have disturbed the natural balance of the atmosphere.
The composition of Air is given below:
Nitrogen                                     78%
Oxygen                                      21%
Argon                                        less than 1%
Carbondioxide                           0.037%
Water vapour                            Remaining
Ozone, Helium and ammonia     Trace amount

Sources of Air pollution
Sources of  air pollution are of two types.
Natural sources and Artificial sources

Natural sources of pollution are those that are caused due to natural phenomena. Ex: Volcanic eruptions, Forest fires, Biological decay, Pollen grains, Marshes, Radioactive materials.

Artificial sources are those which are created by man. Ex: Thermal power plants, Vehicular emissions, Fossil fuel burning, agricultural activities etc.

Classification of Air Pollutants
Depending on the form of pollutants present in the environment, they are classified as:
  1. Primary pollutants and
  2. Secondary pollutants
Primary pollutants are those that are directly emitted in the atmosphere in the harmful form
Ex: CO, NO, CO2, SO2 etc.

Secondary pollutants are those that are formed by reacting with other components or some basic component of the atmosphere to form new pollutants.
Ex: Oxides of Nitrogen (NO2 or NO3) react with moisture in the atmosphere to give Nitric acid

Indoor air pollutants are primary air pollutants. The most important indoor air pollutant is Radon gas.
Sources of indoor air pollutants are:
  1.  Radon gas is emitted from building materials like bricks, concrete, tiles, etc that are derived from soil containing radium
  2. Radon is also found  in natural gas and ground water and is emitted while being used.
  3. Burning fuel in the kitchen and cigarette smoke release pollutants like CO, SO2, HCHO (Formaldehyde) and BAP (Benzo-(A) pyrene).
SOURCES AND COMMON EFFECTS OF COMMON AIR POLLUTANTS
Carbonmonoxide: It is a colourless, odourless gas that is poisonous animals. It is formed by incomplete combustion of carbon containing fuels.
Source of carbonmonoxide is cigarette smoking and incomplete combustion of fossil fuels (more than 77% comes from motor vehicle exhaust)
Health effects include reduced ability of red blood cells to carry oxygen to body cells and tissues. This leads to headache and anemia. At high levels it causes coma, irreversible brain damage and death.

Nitrogen Dioxide: It is a reddish-brown irritating gas that causes photochemical smog. In the atmosphere, it gets converted into nitric acid (HNO3). It is caused by burning fossil fuels in industries and power plants.
Health effects include lung irritation and damage. Environmental effects involve acid deposition leading to damage of trees, lakes, soil and ancient monuments. NO2 can damage fabrics.

Sulphur Dioxide: It is a colourless and irritating gas that is formed by combustion of sulphur containing fossil fuels such as coal and oil. In the atmosphere it is converted into Sulphuric acid which is a major component of acid deposition.
Health effects involve breathing problems for healthy people.
Environmental effects involve reduced visibility and acid deposition on trees, lakes, soils and monuments leading to their deterioration and adverse effect on aquatic life.

Suspended Particulate Matter (SPM): Includes a variety of particles and droplets (aerosols) that can be suspended in atmosphere for short  to long periods.
Human sources for SPM include burning coal in power and industrial units, burning diesel and other fuels in vehicles, agriculture, unpaved roads, construction, etc.
Health effects include nose and throat irritation, ling damage, bronchitis, asthama, reproductive problems and cancer.
Environmental Effects include reduced visibility and acid deposition. Acid deposition may lead to damaged trees, soils and aquatic life in lakes.

Ozone is a highly reactive gas with an unpleasant odour occurring in the stratosphere where it protects mankind fro the harmful ultra-violet rays from the Sun. However on earth, it is a pollutant.
It occurs on earth due to reaction between Volatile Organic Compounds (VOCs) and Nitrogen Oxides. It moderates the climate

Photochemical smog is a browinsh smoke that frequently forms on clear, sunny days over large cities with significant amounts of automobile traffic.It is mainly due to chemical reactions among nitrogen oxides and hydrocarbons in the presence of sunlight.
Health effects include breathing problems, cough, eye, nose and throat irritation, heart diseases, reduced resistance to colds and pneumonia.
Environmental effects involve damage to plants and trees. Additionally, Smog reduces visibility.

Lead is a solid and highly toxic metal. Its compounds are emitted into the atmosphere as particulate matter.
Human Sources: Paint, Smelters (metal refineries), lead manufacture, storage batteries, leaded petrol, etc
Health effects: Lead accumulates in the body and brain leading to nervous system damage and mental retardation (especially in children), digestive and other health problems. Lead containing chemicals are known to cause cancer in test animals.
Environmental Effects: It can harm wildlife.



Hydrocarbons Lower haydrocarbons accumulate due to decay of vegetable matter.
Human effects: They are carcinogenic

Chromium: It is a solid toxic metal emitted into the atmosphere as particulate matter.
Human sources: Paint, Smelters,  Chromium manufacture, Chromium plating.
Health Effects: Perforation of nasal septum, chrome holes, etc.

CONTROL MEASURES
The atmosphere has several built-in self cleaning processes such as dispersion, gravitational settling, flocculation, absorption, rain-washout, etc to cleanse the atmosphere. However, control of contaminants at their source level is a desirable and effective method through preventive or control technologies.
Source control: Some measures that can be adopted in this direction are:
  1. Using unleaded petrol
  2. Using fuels with low sulphur and ash content
  3. Encouraging people to use public transport, walk or use a cycle as opposed to private vehicles
  4. Ensure that houses, schools,  restaurants and playgrounds are not located on busy streets
  5. Plant trees along busy streets as they remove particulates, carbon dioxide and absorb noise
  6. Industries and waste disposal sites should be situated outsdide the city preferably on the downwind of the city.
  7. Catalytic converters should be used to help control emissions of carbon monoxide and hydrocarbons
Control measures in industrial centers
  1. Emission rates should be restricted to permissible levels by each and every industry
  2. Incorporation of air pollution control equipment in design of plant layout must be made mandatory
  3. Continuous monitoring of the atmosphere for pollutants should be carried out to know the emission levels.
 EQUIPMENT USED TO CONTROL AIR POLLUTION
The following equipment is used to control air pollution:


    1. Control of SPM by gravitation 
    Control of SPM by gravitation
    Equipment used: Gravitational Settling Chamber A typical gravitational chamber is shown below.



    The dust laden gas enters at the inlet and due to the sudden increase in cross-section the particulate matter settles at the bottom and can be removed from the dust hoppers as shown
    The clean gas free from particulate matter exits  from the outlet
    IMPORTANT FACTS:
    Simple to construct and maintain
    Efficient to remove particles of diameter greater than 50 mm from gas streams
    They are used as pre-cleaners before passing gases through high efficiency collection devices
    They rely on gravitational settling and are the simplest and oldest mechanical collectors for removal of particulates from gas streams
    Flow within the chamber must be uniform without macroscopic mixing
    Dust removal system must be sealed to prevent production of turbulence due to air from leaking into chamber
    Efficiency of the equipment increases with increased residence time of the waste gas. Hence, the equipment is operated at lowest possible gas velocity
    The size of the unit depends on:
    gas velocity which should preferably be less than 0.3 m/s
    ADVANTAGES
    Low capital and energy cost
    Low maintenance and operating costs
    Low pressure drop
    Reliable
    Equipment is not subjected to abrasion due to low gas velocity
    Equipment provides incidental cooling of gas stream
    Temperature and pressure limitations depend on material of construction
    Pollutants are collected in dry state
    DISADVANTAGES
    Low particulate matter collection efficiency
    Unable to handle sticky materials
    Large size
    Trays in multiple tray settling chamber may warp under high temperatures.

    2.  Control of SPM by centrifugation
    Equipment used: Cyclonic separator
    Centrifugation is a process that involves the use of centrifugal force for sedimentation of a heterogeneous mixture with a centrifuge. It involves removal of particulates from air, gas or a liquid stream without use of filters with a vortex separation. When removing particulates from a gaseous stream, a gas cyclone is used while a hydrocyclone is used to remove particulates from a liquid stream. This method can also be used to separate fine droplets of liquid from a gaseous stream.
    A high speed rotating air flow is formed in a cylindrical or conical container called a cyclone.
    Air flows in a helical pattern from the top to a narrow bottom as show,


     Cyclones use the principle of inertia to remove particulate matter from a gas stream. Several cyclones operating in parallel is known as multicyclone. In a cyclone separator, dirty gas is fed into a chamber where a spiral vortex exists. The large particles hit the inside walls of the container and drop down into the collection hooper. The clean flue gas escapes from the top of the chamber. Cyclones can be used efficiently to remove particles of size 10 microns or more. High efficiency cyclones can remove particles of dimeter as small as 2.5 microns. They are the least expensive of all particulate collection devices. They are used as rough separators before the gas is passed through fine filtration systems. Their efficiency is between 50-99%. Cyclone separators work best on flue gases that contain large amount of big particulate matter.
    ADVANTAGES:
    Cyclones are less expensive to install or maintain as they do not contain any moving parts
    It is easy to dispose particulate matter as it is collected in the dry state
    Space requirement is very less
    DISADVANTAGES:
    They are not efficient in collecting particulate matter smaller than 10 microns
    They cannnot handle sticky material

    3.   Control of SPM by filtration
    In a fabric filter system, a stream of the polluted gas is made to pass through a fabric that filters out the particulate pollutant and allows the clear gas to pass through. The particulate matter is left in the form of a thin dust mat on the insides of the bag. This dust mat acts as a filtering medium for further removal of particulates increasing the efficiency of the filter bag to sieve more sub mi­cron particles (0.5 ┬Ám).

    A typical filter is a tubular bag which is closed at the upper end and has a hopper attached at the lower end to collect the particles when they are dislodged from the fabric. Many such bags are hung in a baghouse. For efficient filtration and a longer life the filter bags must be cleaned occasionally by a mechanical shaker to prevent too many particulate layers from building up on the inside surfaces of the bag. A typical bag house filter is shown in the figure below.

    ADVANTAGES:
    Bag filter is a high quality performance instrument to effectively control particulate emissions and its efficiency is as high as 99%
    Collection efficiency is not affected by sulphur content in fuel
    It is not sensitive to particle size distribution
    It does not require high voltage
    It can be used to collect flammable dust
    Special fiber or filter aids can be used to sub-micron level smoke and fumes
    DISADVANTAGES:
    Fabric life is reduced due to presence of highly acidic or alkaline atmospheres, especially at high temperatures
    Maximum operating temperature is 500 F
    Collection of hygroscopic materials or condensation of moisture can lead to fabric plugging, loss of cleaning efficiency and large pressure losses.
    Certain dusts may require special fabric treatments to aid in reducing leakage or to help in cake removal
    Fabric bags are prone to burning or melting at extreme temperatures.

    4.  Control of SPM by scrubbing
    A scrubber is a system used to remove harmful materials from industrial exhaust gases before they are released into the environment. The two main ways to scrub pollutants out of exhaust are:
    Dry scrubbing and
    Wet scrubbing
    In dry scrubbing, harmful components of exhausted flue gas are removed by introducing a solid substance (usually in the powdered form) in the gas stream.

    Wet scrubbing involves removal of harmful components from exhaust by spraying a liquid substance through the gas.

    Both methods work similarly and perform the same process of removing pollutants. The difference lies in the materials they use to remove the pollutant from the gas stream. By removing acidic gases from the exhaust before it is released into the atmosphere, scrubbers help in the prevent the formation of acid rain.
    Scrubbing is sometimes referred to as flue gas desulfurization.

    Scrubbing is the most effective technique for the removal of oxides of sulphur and is widely used. Scrubbers remove sulphur oxides from flue gases by passing the gases through a spray of water in a wet scrubber that contains many chemicals, mainly calcium carbonate.
    If a dry scrubber is used, the flue gas comes in contact with pulverised limestone. The chemical reaction between suphur dioxide and calcium carbonate yields calcium sulphite. The calcium sulphite either falls out of the gas stream or is removed with other particulates.
    Scrubbers are highly efficient and remove almost 98% of sulphur from flue gases. However, they are expensive to maintain and install. They are also energy intensive as the flue gas must be reheated after coming into contact with water vapour in the wet scrubber to make the gas buoyant to exit the smoke stacks.



    5.  Control of SPM by Electrostatic precipitator
    An Electrostatic precipitator is mainly used to control particulate matter. An Electrostatic precipitator uses electrostatic forces to separate dust particles from exhaust gases. A number of high-voltage, direct-current discharge electrodes are placed between grounded collecting electrodes. The contaminated gases flow through the passage formed by the discharge and collecting electrodes as shown in the figure below.



    Air borne particles receive a negative charge as they pass through the ionized field between the electrodes. These charged particles are then attracted to the oppositely charged electrode and stick to it. The collected material is then removed by rapping or vibrating the electrodes. Cleaning the electrodes is done without interrupting the air flow.
    The main components of all electrostatic precipitators are:


    • a power supply unit to supply high voltage DC power
    • ionizing section to impart a charge to the particulates in the gas stream
    • an attachment to remove the collected particulates
    • a housing to enclose the precipitator zone

    The following factors influence the collection efficiency of electrostatic precipitators:

    • Larger collection surface areas and lower gas flow rates increase efficiency of electrostatic precipitators due to increased time for the electrical activity to collect the dust particles
    • The dust particle migration velocity to the collecting electrodes can be increased by:
    • Decreasing gas velocity
    • Increasing gas temperature and
    • Increasing the voltage field

    There are two types of precipitators:

    1. Single-stage precipitators that combine an ionization and collection step also known as cottrell precipitators. It is mainly used in mineral processing operations.
    2. Low voltage, two stage precipitators that use a similar principle, but in this case, the ionization section is followed by collection plates. It is mainly used for filtration in air-conditioning systems. 

    Electrostatic precipitators may be:
    Plate precipitators in which particles are collected on flat parallel surfaces about 20 to 30 cm apart with a series of discharge electrodes spaced along the centerline of two adjacent plates. The contaminated particles pass through the passage between the plates and the particles get charged and adhere to the collection plates. The particles are eventually removed by rapping the plates and the dust is collected in the hoppers or bins placed at the base of the precipitator.
    Tubular precipitators consist of cylindrical collection electrodes with discharge electrodes located on the axis of the cylinder. The contaminated gases flow around the discharge electrode and through the inside of the cylinders. The charged particles are collected on the grounded walls of the cylinder. The collected dust is removed from the bottom of the cylinder. They are generally used for collection of mist or fog or for adhesive, sticky, radioactive or extremely toxic materials.
    Air  pollution can be reduced by adopting the following approaches.
    1. Ensuring sufficient supply of oxygen to the combustion chamber and adequate temperature so that the combustion is complete thereby eliminating much of the smoke consisting of partly burnt ashes and dust.
    2. To use mechanical devices such as scrubbers, cyclones, bag houses and electro-static precipitators in manufacturing processes. The equipment used to remove particulates from the exhaust gases of electric power and industrial plants are shown below. All methods retain hazardous materials that must be disposed safely. Wet scrubber can additionally reduce sulphur dioxide emissions.
    3. The air pollutants collected must be carefully disposed. The factory fumes are dealt with chemical treatment.

    19 comments:

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