This document discusses air pollution and methods for controlling it. It begins by providing background on how Earth's atmosphere has changed over time and how human activities now contribute significantly to air pollution. The main sources of air pollutants are described as internal combustion engines, fuel combustion at power plants and industrial facilities. The document then classifies primary and secondary pollutants and discusses criteria pollutants. It focuses on methods for reducing emissions from fossil fuel combustion, including pre-combustion, combustion and post-combustion controls. Specific technologies like scrubbers, electrostatic precipitators and baghouses are explained for controlling particulate matter, SOx and NOx emissions.

1. 1
Air pollution: Introduction
 Since Earth’s atmosphere was first formed, its composition undoubtedly has undergone great
changes. The “normal” composition of air today is not likely the same as it was when the first
primitive living cells inhabited this planet. Earth’s earliest atmosphere probably contained almost
no free oxygen. The oxygen in today’s atmosphere is probably the result of several million of
years of photosynthesis.
 When environmental changes occur more rapidly than a species’ ability to adapt, the species
oftentimes either does not thrive or does not survive. Human contributions to environmental
changes in recent history, e.g., global warming, have come relatively quickly compared to the
natural rate of change, and Earth’s and its inhabitants’ natural adaptation capabilities might not be
adequate to meet this challenge.
 Air pollution has been around for a long time. Natural phenomena such as volcanoes,
windstorms, forest fires, and decaying organic matter contribute substantial amounts of air
pollutants. Plants and trees also emit organic vapors and particles. For the most part, Earth,
which has a well-balanced natural “cleansing” system, is able to keep up with natural pollution.

2. 2
Air pollution has bedeviled humanity since the first person discovered fire.
Humans did not significantly affect the environment until relatively recent times. This is due to two
reasons: (1) the human population has been large for only a small part of recorded history, and (2)
the bulk of human-made produced air pollution is intimately related to industrialization. In fact,
humans did not begin to alter the
environment until they began to live in communities.
Primary sources of air pollutants:
 Internal combustion engines of cars and other motor vehicles.
 Fuel burned at stationary sources such as power-generating plants.
 Industrial processes.
 Climatic condition
The control of pollutants rarely takes place prior to public outcry, even though the technology for
controlling pollutants may be available. Early recognition of pollutants as health hazards have not
resulted in pollution reduction; traditionally, only when personal survival is at stake has effective
action been taken.

3. 3
Air pollution: Classification of pollutants
Primary pollutants: Emitted directly into the atmosphere.
Secondary pollutants: Created by various physical processes and chemical reactions that take
place in the atmosphere.
Creation processes: Combustion (Automobile and power plants), evaporation (volatile substances,
e.g, gasoline, paints, cleaning fluids), grinding (dust formation during land plowing) , abrasion
(asbestos fibers)
Primary pollutants:
Combustion of pure hydrocarbon fuel.
CH4 + 2O2  mostly (CO2 + 2H2O) + traces of [CO + (HC)]
Incomplete combustion due to
lower temperature, lesser
oxygen, not enough time
Air (N2 + O2) + Heat  Thermal NOx
Fuel impurities lead to emission of additional NOx (fuel NOx), SOx, lead, mercury, particulate
matter, ash (unburnable)
Secondary pollutant:
Volatile organic compounds (VOC) + NOx + Senlight  Photochemical smog (O3 + etc)

4. 4
Air pollution: Criteria pollutants
 Carbon Monoxide (CO)
 Sulfur dioxide (SO2)
 Nitrogen dioxide (NO2)
 Ground level Ozone (O3)
 Small Particulates (PM 10 and PM 2.5)
 Lead (Pb)

5. Air pollution:
Type of sources of air pollutants:
 Mobile sources (Highway
vehicles, Rails, Aircrafts, Farm
Vehicles, Ships etc.)
 Stationary Sources(Electric
power plants, Industrial
energy systems etc.)
5

6. Stationary source emissions:
 Fossil fuel combustions. (Major Contributor)
 VOC evaporation.
 Grinding.
 Forest fires etc.
(NOx, SOx and particulate matter.)
How to reduce fossil fuel emissions ?
Reduce the consumption of fuels.
6

7. Approaches to reduce fossil fuel consumption:
 Improve conversion efficiency from fuel to energy.
(Improved design of power plant; combined cycle power
plants)
 Efficient usage of (electrical) energy. (Improvements in
electrical motors and motor controls, better lighting systems,
efficient manufacturing process; reduce per capita electricity
demands)
 Replacement of fossil fuels for energy sources. (Alternate
energy sources i.e solar, wind, hydro, geothermal, nuclear
etc.) 7

8. Ways to reduce emissions from fossil fuel:
 Pre-combustion controls.
(Reduce the emission potential of fuel itself)
 Combustion control.
(Improve the combustion process itself)
 Post-combustion controls.
(Capture and treat the emissions after they have formed
but before they are released to air)
8

9. 9
Modern coal fired plant:(NOx, SOx, CO2 and particulate matter
Post combustion
controls
Electrostatic
precipitator:
Particulate
control
Scrubber: SOx
control
Emissions controls are costly and uses part of the power generated

10. 10
Pre-combustion control (of sulfur content) in a
coal plant:
 Fuel switching.
(Use low-sulfur coal; SO2 emission can be reduced to
30-90% depending on the sulfur content)
 Coal cleaning.
(Chemical or Physical treatment)
Chemical: When the sulfur is bound to the organic molecules of
coal.
Physical: When the sulfur is in the form of inorganic pyrite
(FeS2) which has 3.6 times higher specific gravity higher than
coal

11. 11
Combustion control (of sulfur content) in a coal
plant: Fluidized Bed Combustion (FBC)
Crushed coal +
Limestone
CaCO3 (limestone) + SO2  CaSO3 + CO2
CaSO3 + O2  2 CaSO4
Solid CaSO4 falls to the bottom of the furnace
and removed.
Advantages:
 Fluidized particles are direct contact with the boiler tubes.
Efficient heat transfer by conduction. Boiler operating
temperature reduces to 800 C from 1400 C.
 Low operating temperature reduces NOx emission.
 Less sensitive to coal quality. Coal with higher ash content
can be burned, since lower combustion temperature is
below the melting point of ash.

12. 12
Integrated Gasification Combined Cycle:
C + H2O  CO + H2 (syngas)

13. 13
Controlling NOx emissions:
 Low excess air technique.
(Air is controlled at the minimum amount required for
complete combustion)
 Low NOx burner technology.
(Staged combustion process that delays mixing the fuel
and air in the boiler)
 Selective Catalytic Reduction (SCR)
(4 NO + 4 NH3 + O2  4 N2 + 6 H2O
2 NO2 + 4 NH3 + O2  3 N2 + 6 H2O)

14. 14
Flue gas desulfurization (Scrubbers):
CaCO3+ SO2 + 2 H2O
CaSO3 .2H2O+ CO2
CaO + SO2 + 2 H2O 
CaSO3 .2H2O
About 90% of the SO2
can be captured from
the flue gas.

15. 15
Post-combustion Particulate control:
Centrifugal or cyclone collector
Particle collection process.
 Efficiency is above 90% for particles larger than 5 mmand drops
off rapidly for smaller particle sizes.
 Longer overall length means more number of turns. Smaller the
inlet size, greater the inlet velocity. Increase in efficiency.
Reverse flow top inlet
cyclone.
Removal efficiency depends on
 Size of particle
 Cyclone dimensions

16. 16
Post-combustion Particulate control:
Electrostatic precipitator (ESP)
Negatively charged discharge
electrodes (~ 100 kV)
Grounded collector
plates
Gas flow
Corona Generation
Particles in the gas stream acquire negative charge as
they pass through the corona and are then attracted to the
grounded collecting plates

17. 17
Electrostatic precipitator (ESP):
Deutsch- Anderson equation
Collection efficiency of the precipitator under ideal conditions
)/exp(1 QwA
A: total area of collection plates
Q: volumetric flow rate of gas through the precipitator
w: effective drift velocity or the terminal speed

18. 18
Post-combustion Particulate control: Baghouses
 Dust bearing gases are passed through fabric
filter bags, which are suspended upside down
in a large chamber.
 Filtration accomplished by the fabric and by
the dust itself.
 1 mm or smaller particles can be removed.
Highly efficient.
 Large and expensive.
 Corrosive chemicals in flue gas may harm
them.