1. TECHNIQUES WITHOUT USING EMISSION CONTROL DEVICES
Process Change
Wind, Geothermal, Hydroelectric, or Solar Unit instead of Fossil fired Unit.
Change in Fuel
e.g. Use of Low Sulfur Fuel, instead of High Sulfur fuel.
Good Operating Practices
Good Housekeeping
Maintenance
Plant Shutdown

2. COMMONLY USED METHODS FOR AIR POLLUTION CONTROL
PARTICULATE
Cyclones
Electrostatic Precipitators
Fabric Filter
Wet Scrubbers
GASES
Adsorption Towers
Thermal Incernation
Catalytic Combustion

3. SOx CONTROL

4. GENERAL METHODS FOR CONTROL OF SO2 EMISSIONS
Change to Low Sulfur Fuel
Natural Gas
Liquefied Natural Gas
Low Sulfur Oil
Low Sulfur Coal
Use Desulfurized Coal and Oil Increase Effective Stack Height
Build Tall Stacks
Redistribution of Stack Gas Velocity Profile
Modification of Plume Buoyancy

5. GENERAL METHODS FOR CONTROL OF SO2 EMISSIONS (contd.)
Use Flue Gas Desulfurization Systems
Use Alternative Energy Sources, such as Hydro-Power
or Nuclear-Power

6. FLUE GAS DESULFURIZATION
SO2 scrubbing, or Flue Gas Desulfurization processes can be classified as:
Throwaway or Regenerative, depending upon whether the recovered sulfur is discarded or recycled.
Wet or Dry, depending upon whether the scrubber is a liquid or a solid.
Flue Gas Desulfurization Processes
The major flue gas desulfurization ( FGD ), processes are :
Limestone Scrubbing
Lime Scrubbing
Dual Alkali Processes
Lime Spray Drying
Wellman-Lord Process

7. LIMESTONE SCRUBBING
Limestone slurry is sprayed on the incoming flue gas. The sulfur dioxide gets absorbed The limestone and the sulfur dioxide react as follows :
CaCO3 + H2O + 2SO2 ----> Ca+2 + 2HSO3-+ CO2
CaCO3 + 2HSO3-+ Ca > 2CaSO3 + CO2 + H2O

8. CaO + H2O -----> Ca(OH)2 SO2 + H2O <----> H2SO3
LIME SCRUBBING
The equipment and the processes are similar to those in limestone scrubbing Lime Scrubbing offers better utilization of the reagent. The operation is more flexible. The major disadvantage is the high cost of lime compared to limestone.
The reactions occurring during lime scrubbing are :
CaO + H2O -----> Ca(OH)2
SO2 + H2O <----> H2SO3
H2SO3 + Ca(OH) > CaSO3.2 H2O
CaSO3.2 H2O + (1/2)O > CaSO4.2 H2O

9. DUAL ALKALI SYSTEM
Lime and Limestone scrubbing lead to deposits inside spray tower.
The deposits can lead to plugging of the nozzles through which the scrubbing slurry is sprayed.
The Dual Alkali system uses two regents to remove the sulfur dioxide.
Sodium sulfite / Sodium hydroxide are used for the absorption of sulfur dioxide inside the spray chamber.
The resulting sodium salts are soluble in water,so no deposits are formed.
The spray water is treated with lime or limestone, along with make-up sodium hydroxide or sodium carbonate.
The sulfite / sulfate ions are precipitated, and the sodium hydroxide is regenerated.

10. LIME - SPRAY DRYING Lime Slurry is sprayed into the chamber
The sulfur dioxide is absorbed by the slurry
The liquid-to-gas ratio is maintained such that the spray dries before it reaches the bottom of the chamber
The dry solids are carried out with the gas, and are collected in fabric filtration unit
This system needs lower maintenance, lower capital costs, and lower energy usage

11. This process consists of the following subprocesses:
WELLMAN - LORD PROCESS
This process consists of the following subprocesses:
Flue gas pre-treatment.
Sulfur dioxide absorption by sodium sulfite
Purge treatment
Sodium sulfite regeneration.
The concentrated sulfur dioxide stream is processed to a marketable product.
The flue gas is pre - treated to remove the particulate. The sodium sulfite neutralizes the sulfur dioxide :
Na2SO3 + SO2 + H2O -----> 2NaHSO3

12. WELLMAN - LORD PROCESS (contd.)
Some of the Na2SO3 reacts with O2 and the SO3 present in the flue gas to form Na2SO4 and NaHSO3.
Sodium sulfate does not help in the removal of sulfur dioxide, and is removed. Part of the bisulfate stream is chilled to precipitate the remaining bisulfate. The remaining bisulfate stream is evaporated to release the sulfur dioxide, and regenerate the bisulfite.

13. NOx CONTROL

14. BACKGROUND ON NITROGEN OXIDES
There are seven known oxides of nitrogen : NO
NO2
NO3
N2O
N2O3
N2O4
N2O5
NO and NO2 are the most common of the seven oxides listed above. NOx released from stationary sources is of two types

15. GENERAL METHODS FOR CONTROL OF NOx EMISSIONS
NOx control can be achieved by:
Fuel Denitrogenation
Combustion Modification
Modification of operating conditions
Tail-end control equipment
Selective Catalytic Reduction
Selective Non - Catalytic Reduction
Electron Beam Radiation
Staged Combustion

16. FUEL DENITROGENATION
One approach of fuel denitrogenation is to remove a large part of the nitrogen contained in the fuels. Nitrogen is removed from liquid fuels by mixing the fuels with hydrogen gas, heating the mixture and using a catalyst to cause nitrogen in the fuel and gaseous hydrogen to unite. This produces ammonia and cleaner fuel.
This technology can reduce the nitrogen contained in both naturally occurring and synthetic fuels.

17. COMBUSTION MODIFICATION
Combustion control uses one of the following strategies:
Reduce peak temperatures of the flame zone. The methods are :
increase the rate of flame cooling
decrease the adiabatic flame temperature by dilution
Reduce residence time in the flame zone. For this we,
change the shape of the flame zone
Reduce Oxygen concentration in the flame one. This can be accomplished by:
decreasing the excess air
controlled mixing of fuel and air
using a fuel rich primary flame zone

18. MODIFICATION OF OPERATING CONDITIONS
The operating conditions can be modified to achieve significant reductions in the rate of thermal NOx production. the various methods are:
Low-excess firing
Off-stoichiometric combustion ( staged combustion )
Flue gas recirculation
Reduced air preheat
Reduced firing rates
Water Injection

19. TAIL-END CONTROL PROCESSES
Combustion modification and modification of operating conditions provide significant reductions in NOx, but not enough to meet regulations.
For further reduction in emissions, tail-end control equipment is required.
Some of the control processes are:
Selective Catalytic Reduction
Selective Non-catalytic Reduction
Electron Beam Radiation
Staged Combustion

20. SELECTIVE CATALYTIC REDUCTION ( SCR )
In this process, the nitrogen oxides in the flue gases are reduced to nitrogen
During this process, only the NOx species are reduced
NH3 is used as a reducing gas
The catalyst is a combination of titanium and vanadium oxides. The reactions are given below :
4 NO + 4 NH3 + O > 4N2 + 6H2O
2NO2 + 4 NH3+ O > 3N2 + 6H2O
Selective catalytic reduction catalyst is best at around 300 too 400 oC.
Typical efficiencies are around 80 %

21. ELECTRON BEAM RADIATION
This treatment process is under development, and is not widely used. Work is underway to determine the feasibility of electron beam radiation for neutralizing hazardous wastes and air toxics.
Irradiation of flue gases containing NOx or SOx produce nitrate and sulfate ions.
The addition of water and ammonia produces NH4NO3, and (NH4)2SO4
The solids are removed from the gas, and are sold as fertilizers.

22. STAGED COMBUSTION PRINCIPLE
Initially, less air is supplied to bring about incomplete combustion
Nitrogen is not oxidized. Carbon particles and CO are released.
In the second stage, more air is supplied to complete the combustion of carbon and carbon monoxide.
30% to 50% reductions in NOx emissions are achieved.

23. CARBON MONOXIDE CONTROL

24. GENERAL METHODS FOR CONTROL OF CO EMISSIONS
Control carbon monoxide formation.
Note : CO & NOx control strategies are in conflict.
Stationary Sources
Proper Design
Installation
Operation
Maintenance
Process Industries
Burn in furnaces or waste heat boilers.

25. FORMATION OF CARBON MONOXIDE
Due to insufficient oxygen
Factors affecting Carbon monoxide formation:
fuel-air ratio
degree of mixing
temperature

26. PARTICULATE MATTER CONTROL

27. GENERAL METHODS FOR CONTROL OF PARTICULATE EMISSIONS
Five Basic Types of Dust Collectors :
Gravity and Momentum collectors
settling chambers, louvers, baffle chambers
Centrifugal Collectors
cyclones
mechanical centrifugal collectors
Fabric Filters
baghouses
fabric collectors

28. GENERAL METHODS FOR CONTROL OF PARTICULATE EMISSIONS (contd.)
Electrostatic Precipitators
tubular
plate
wet
dry
Wet Collectors
spray towers
impingement scrubbers
wet cyclones
peaked towers
mobile bed scrubbers

29. PARTICULATE COLLECTION MECHANISM
Gravity Settling
Centrifugal Impaction
Inertial Impaction
Direct Interception
Diffusion
Electrostatic Effects

30. INDUSTRIAL SOURCES OF PARTICULATE EMISSIONS
Iron & Steel Mills, the blast furnaces, steel making furnaces.
Petroleum Refineries, the catalyst regenerators, air-blown asphalt stills, and sludge burners.
Portland cement industry
Asphalt batching plants
Production of sulfuric acid
Production of phosphoric acid
Soap and Synthetic detergent manufacturing
Glass & glass fiber industry
Instant coffee plants

31. EFFECTS OF PARTICULATE EMISSIONS
Primary Effects
Reduction of visibility
size distribution and refractive index of the particles
direct absorption of light by particles
direct light scattering by particles
150 micro g / m3 concentration ~ average visibility of 5 miles
( satisfactory for air and ground transportation )
Soiling of nuisance
increase cost of building maintenance, cleaning of furnishings, and households
threshold limit is micro g / m3 ( dust )
levels of micro g / m3 considered as nuisance

32. CYCLONES
Principle
The particles are removed by the application of a centrifugal force. The polluted gas stream is forced into a vortex. the motion of the gas exerts a centrifugal force on the particles, and they get deposited on the inner surface of the cyclones
Construction and Operation
The gas enters through the inlet, and is forced into a spiral.
At the bottom, the gas reverses direction and flows upwards.
To prevent particles in the incoming stream from contaminating the clean gas, a vortex finder is provided to separate them. the cleaned gas flows out through the vortex finder. .

33. CYCLONES (contd.) Advantages of Cyclones
Cyclones have a lost capital cost
Reasonable high efficiency for specially designed cyclones.
They can be used under almost any operating condition.
Cyclones can be constructed of a wide variety of materials.
There are no moving parts, so there are no maintenance requirements.
Disadvantages of Cyclones
They can be used for small particles
High pressure drops contribute to increased costs of operation.

34. Advantages of Fabric Filters
Principle
The filters retain particles larger than the mesh size
Air and most of the smaller particles flow through. Some of the smaller particles are retained due to interception and diffusion.
The retained particles cause a reduction in the mesh size.
The primary collection is on the layer of previously deposited particles
Advantages of Fabric Filters
Very high collection efficiency
They can operate over a wide range of volumetric flow rates
The pressure drops are reasonably low.
Fabric Filter houses are modular in design, and can be pre-assembled at the factory

35. FABRIC FILTERS (contd.)
Disadvantages of Fabric Filters
Fabric Filters require a large floor area.
The fabric is damaged at high temperature.
Ordinary fabrics cannot handle corrosive gases.
Fabric Filters cannot handle moist gas streams
A fabric filtration unit is a potential fire hazard

36. ELECTROSTATIC PRECIPITATOR
Principle
The particles in a polluted gas stream are charged by passing them through an electric field.
The charged particles are led through collector plates
The collector plates carry charges opposite to that on the particles
The particles are attracted to these collector plates and are thus removed from the gas steam
Construction and Operation of Electrostatic Precipitator
Charging Electrodes in the form of thin wires are placed in the path of the influent gas.
The charging electrodes generate a strong electric field, which charges the particles as they flow through it.
The collector plates get deposited with the particles. the particles are occasionally removed either by rapping or by washing the collector plates.

37. ELECTROSTATIC PRECIPITATOR (contd.)
Advantages of Electrostatic Precipitators
Electrostatic precipitators are capable very high efficiency, generally of the order of %.
Since the electrostatic precipitators act on the particles and not on the air, they can handle higher loads with lower pressure drops.
They can operate at higher temperatures.
The operating costs are generally low.
Disadvantages of Electrostatic Precipitators
The initial capital costs are high.
Although they can be designed for a variety of operating conditions, they are not very flexible to changes in the operating conditions, once installed.
Particulate with high resistivity may go uncollected.

38. Construction and Operation
WET SCRUBBERS
Principle
Wet scrubbers are used for removal of particles which have a diameter of the order of 0.2 mm or higher.
Wet scrubbers work by spraying a stream of fine liquid droplets on the incoming stream.
The droplets capture the particles
The liquid is subsequently removed for treatment.
Construction and Operation
A wet scrubber consists of a rectangular or circular chamber in which nozzles are mounted.
The nozzles spray a stream of droplets on the incoming gas stream
The droplets contact the particulate matter, and the particles get sorbed.
The droplet size has to be optimized.

39. WET SCRUBBERS (contd.) Construction and Operation (contd.)
Smaller droplets provide better cleaning, but are more difficult to remove from the cleaned stream.
The polluted spray is collected.
Particles are settled out or otherwise removed from the liquid.
The liquid is recycled.
Wet scrubbers are also used for the removal of gases from the air streams.

40. Disadvantages of Wet Scrubbers
WET SCRUBBERS (contd.)
Advantages of Wet Scrubbers
Wet Scrubbers can handle incoming streams at high temperature, thus removing the need for temperature control equipment.
Wet scrubbers can handle high particle loading.
Loading fluctuations do not affect the removal efficiency.
They can handle explosive gases with little risk.
Gas adsorption and dust collection are handled in one unit.
Corrosive gases and dusts are neutralized.
Disadvantages of Wet Scrubbers
High potential for corrosive problems
Effluent scrubbing liquid poses a water pollution problem.

41. CYCLONE SPRAY CHAMBERS
These scrubbers combine a cyclone with a spray nozzle.
The added centrifugal force permits good separation of the droplets, hence a smaller droplet size can be used.
Cyclone spray chambers provide up to 95% removal of particles > 5 micron.

42. ORIFICE SCRUBBERS
The gas is impacted onto a layer of the scrubbing liquid.
The gas passes through the liquid, thus removing almost all the particulate matter, and a large portion of the probable gases.
After coming out of the liquid, the gas is passed through baffles to remove the liquid droplets.

43. IMPINGEMENT SCRUBBERS
In Impingement scrubbers, the gas impacts a layer of liquid/froth through a perforated tray.
Passing through this layer removes the particulate matter.
The wet gas stream is then passed through a mist collector.

44. VENTURI SCRUBBERS
The dirty gas is led in to the chamber at high inlet velocities.
At the inlet throat, liquid at low pressure is added to the gas stream
This increases the relative velocity between the gas and the droplets, thus increasing the efficiency of removal.
Efficiencies of the range of 95% for particles larger than 0.2 mm have been obtained.

45. HYDROCARBON CONTROL

46. GENERAL METHODS FOR CONTROL OF HYDROCARBON EMISSIONS
Incineration or after burning
Direct flame incineration
Thermal incineration
Catalytic incineration

47. CxHy + ( x + y/4 ) O2 x CO2 + (y/2) H2O
VOC INCINERATORS
Principle
VOC incinerators thermally oxidize the effluent stream, in the presence of excess air.
The complete oxidation of the VOC results in the formation of carbon monoxide and water. The reaction proceeds as follows:
CxHy + ( x + y/4 ) O2 x CO2 + (y/2) H2O
Operation
The most important parameters in the design and operation of an incineration system are what are called the
' three T's ' Temperature, Turbulence, and residence Time.

48. VOC INCINERATORS (contd.)
Temperature
The reaction kinetics are very sensitive to temperature
The higher the temperature, the faster the reaction
Timing
A certain time has to be provided for the reaction to proceed
Turbulence
Turbulence promotes mixing between the VOC's and oxygen
Proper mixing helps the reaction to proceed to completion in the given time.

49. VOC INCINERATORS (contd.)
The various methods for incineration are:
Elevated fires, for concentrated streams
Direct thermal oxidation, for dilute streams
Catalytic oxidation, for dilute streams.

50. GASES

51. AIR POLLUTION CONTROL FOR GASES
Adsorption Towers
Thermal Incernation
Catalytic Combustion

52. Principle ADSORPTION TOWERS
Adsorption towers use adsorbents to remove the impurities from the gas stream.
The impurities bind either physically or chemically to the adsorbing material.
The impurities can be recovered by regenerating the adsorbent.
Adsorption towers can remove low concentrations of impurities from the flue gas stream.

53. ADSORPTION TOWERS (contd.)
Construction and Operation
Adsorption towers consist of cylinders packed with the adsorbent.
The adsorbent is supported on a heavy screen
Since adsorption is temperature dependent, the flue gas is temperature conditioned.
Vapor monitors are provided to detect for large concentrations in the effluent. Large concentrations of the pollutant in the effluent indicate that the adsorbent needs to be regenerated.
Advantages of Adsorption Towers
Very low concentrations of pollutants can be removed.
Energy consumption is low.
Do not need much maintenance.
Economically valuable material can be recovered during regeneration.

54. ADSORPTION TOWERS (contd.)
Disadvantages of adsorption Towers
Operation is not continuous.
They can only be used for specific pollutants.
Extensive temperature pre-conditioning equipment to be installed.
Despite regeneration, the capacity of the adsorbent decreases with use.