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2. UNIT KEJURUTERAAN ALAM SEKITARUNIT KEJURUTERAAN ALAM SEKITAR
JABATAN KEJURUTERAAN AWAMJABATAN KEJURUTERAAN AWAM
POLITEKNIK SULTAN IDRIS SHAH
CHAPTER 4CHAPTER 4
AIR POLLUTION CONTROL ANDAIR POLLUTION CONTROL AND
TECHNOLOGIESTECHNOLOGIES
2

3. Upon completion of this course,
student should be able to :
 Identify air pollution control for particle pollutant
 Choose air pollution control device for particle
pollutant
 Understand air pollution control method for gas
pollutant
 Describe adsorption method for control gas pollutant
 Describe absorption method for control gas pollutant
 Describe condensation method for control gas
pollutant

4. Technology for Air Pollution
Control

5. Techniques Without Using Emissions
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

6. Commonly Used Methods For Air
Pollution Control
PARTICULATE
• Cyclones
• Electrostatic Precipitators
• Fabric Filter
• Wet Scrubbers
GASES
• Adsorption Towers
• Thermal Incernation
• Catalytic Combustion

7. SOx CONTROL

8. 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

9. General Methods for Control of SO2
Emissions (contd.)
 Use Flue Gas Desulfurization Systems
 Use Alternative Energy Sources, such as
Hydro-Power or Nuclear-Power

10. Flue Gas Desulfurization
 SO2 scrubbing, or Flue Gas Desulfurization processes
can be classified as:
• Throw away 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

11. 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+2
----> 2CaSO3 + CO2 + H2O

12. Limestone Scrubbing

13. Lime Scrubbing

14. 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)2 -----> CaSO3.2 H2O
CaSO3.2 H2O + (1/2)O2 -----> CaSO4.2 H2O

15. 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.

16. 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

17. 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

18. Wellman – Lord Process
(contd.)
 Some of the Na2SO3 reacts with O2 and the SO3 present in the flue
gas to form Na2SO4and 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.

19. Wellman – Lord Process
Schematic process flow diagram – SO2 scrubbing and recovery
system

20. NOX CONTROL

21. 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

22. 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

23. Fuel Denitrogenation
o 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.

24. 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

25. 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:
(reduce NOx)
• Low-excess firing
• Off-stoichiometric combustion ( staged combustion )
• Flue gas recirculation
• Reduced air preheat
• Reduced firing rates
• Water Injection

26. Tail-end Control Processes
o 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

27. Selective Catalytic Reduction
(SCR)
Schematic process flow diagram – NOX control

28. 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 + O2-----> 4N2 + 6H2O
2NO2+ 4 NH3+ O2-----> 3N2 + 6H2O
• Selective catalytic reduction catalyst is best at around 300 too 400 o
C.
• Typical efficiencies are around 80 %

29. Catalytic Combustion

30. Catalytic Emission Control

31. 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.

32. 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.

33. Staged Combustion

34. CARBON MONOXIDE CONTROL

35. Formation Of Carbon
Monoxide
• Due to insufficient oxygen
• Factors affecting Carbon monoxide formation:
• Fuel-air ratio
• Degree of mixing
• Temperature

36. 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.

37. CARBON DIOXIDE CONTROL

38. Sources of Carbon Dioxide
Human-Related Sources
 Combustion of fossil fuels: Coal, Oil, and Natural
Gas in power plants, automobiles, and industrial
facilities
 Use of petroleum-based products
 Industrial processes: Iron and steel production,
cement, lime, and aluminum manufactures
Natural Sources
 Volcanic eruptions
 Ocean-atmosphere exchange
 Plant photosynthesis

39. Sources of CO2 Emissions in the
U.S.
Source: USEPA(x-axis units are teragrams of CO2
equivalent)

40. CO2 Emissions from Fossil Fuel Combustion
by Sector and Fuel Type
Source: USEPA(y-axis units are teragrams of CO2
equivalent)

41. General Methods For Control of CO2
Emissions
 Reducing energy consumption, increasing the
efficiency of energy conversion
 Switching to less carbon intensive fuels
 Increasing the use of renewable sources
 Sequestering CO2 through biological, chemical,
or physical processes

42. CONTROL OF MERCURY EMISSIONS

43. Mercury Emissions
 Mercury exists in trace amounts in
 Fossil fuels such as Coal, Oil, and Natural Gas
 Vegetation
 Waste products
 Mercury is released to the atmosphere through
combustion or natural processes
 It creates both human and environmental risks
 Fish consumption is the primary pathway for human
and wildlife exposure
 United states is the first country in the world to
regulate mercury emissions from coal-fired power
plants (March 15, 2005).

44. Source: Seingeur, 2004 and Mason and Sheu,
2002.
Source: Presentation by J. Pacyna and J. Munthe at mercury workshop in
Brussels, March 29-30, 2004
Types of Sources
Worldwide Distribution of Emissions

45. Control Technologies for Mercury
Emissions
 Currently installed control devices for SO2, NOX,and particulates, in a
power plant, remove some of the mercury before releasing from the
stack
 Activated Carbon Injection:
Particles of activated carbon are injected into the exit gas flow, downstream
of the boiler. The mercury attaches to the carbon particles and is
removed in a particle control device
 Thief process for the removal of mercury from flue gas:
It is a process which extracts partially burned coal from a pulverized coal-
fired combustor using a suction pipe, or "thief," and injects the resulting
sorbent into the flue gas to capture the mercury.

46. PARTICULATE MATTER CONTROL

47. 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

48. 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

49. 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

50. Particulate Collection
Mechanism
• Gravity Settling
• Centrifugal Impaction
• Inertial Impaction
• Direct Interception
• Diffusion
• Electrostatic Effects

52. 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

53. 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 200 - 250 micro g / m3
( dust )
• levels of 400 - 500 micro g / m3
considered as nuisance

54. 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
• centrifugal force to capture, recover or remove large and
high-volume dust from industrial applications.
Overall collection η
Ci inlet concentration
Co outlet concentration

56. Cyclones (contd.)
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.

57. 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.

58. 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.

59. Design of Fabric Filters
 The equation for fabric filters is based on
Darcy’s law for flow through porous media.
 Fabric filtration can be represented by the
following equation:
S = Ke + Ksw
Where,
S = filter drag, N-min/m3
Ke = extrapolated clean filter drag, N-min/m3
Ks = slope constant. Varies with the dust, gas and fabric, N-min/kg-m
W= Areal dust density = LVt, where
L = dust loading (g/m3), V = velocity (m/s)
 Both Ke and Ks are determined empirically from
pilot tests.

60. Fabric Filters
Δ P Total pressure drop
Δ Pf Pressure drop due to the fabric
Δ Pp Pressure drop due to the particulate layer
Δ Ps Pressure drop due to the bag house structure

61. 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

62. 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

63. ΔPf Pressure drop N/m2
ΔPpPressure drop N/m2
Df Depth of filter in the direction of flow (m)
Dp Depth of particulate layer in the direction of flow (m)
μ Gas viscosity kg/m-s
V superficial filtering velocity m/min
Kf, Kp Permeability (filter & particulate layer m2
)
60 Conversion factor δ/min
V = Q/A
Q volumetric gas flow rate m3
/min
2
Darcy’s equation

64. L Dust loading kg/m3
t time of operation min
ρL Bulk density of the particulate layer kg/m3
ΔP = ΔPf + ΔPp
Filter Drag S = ΔP/V
Areal dust density W = LVt
S= k +k W
Dust Layer

65. 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.

66. Design of Electrostatic Precipitators
 The efficiency of removal of particles by an
Electrostatic Precipitator is given by
η = fractional collection efficiency
w = drift velocity, m/min.
A = available collection area, m2
Q = volumetric flow rate m3
/min

67. Migration velocity
Where,
q = charge (columbos)
Ep = collection field intensity (volts/m)
r = particle radius (m)
μ = dynamic viscosity of gas (Pa-S)
c = cunningham correction factor

68.  Cunningham correction factor
where,
T = absolute temperature (°k)
dp = diameter of particle (μm)

69. ELECTROSTATIC PRECIPITATOR
(contd.)

Advantages of Electrostatic Precipitators
• Electrostatic precipitators are capable very high efficiency,
generally of the order of 99.5-99.9%.
• 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.

70. 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.

72. Wet Scrubbers (contd.)
o 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.

73. Scrubber
 Efficiency
where,
k = Scrubber coefficient (m3
of gas/ m3
of liquid)
R = Liquid-to-gas flow rate (QL/QG)
ψ = internal impaction parameter
 Internal impaction parameter
where,
c = cunningham correction factor
ρp = particle density (kg/m3
)
Vg = speed of gas at throat (m/sec)
dp = diameter of particle (m)
dd = diameter of droplet (m)
μ = dynamic viscosity of gas, (Pa-S)

74. 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.

75. 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.

76. 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.

77. 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.

78. 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.

79. Venturi Scrubber
Absolute Pressure Drop
Δp = pressure drop ( cm of water)
ug = gas velocity (cm/s)
Qt = liquid volume flow rate
Qg = gas volume flow rate

80. HYDROCARBON CONTROL

81. General Methods For Control Of
Hydrocarbon Emissions
 Incineration or after burning
• Direct flame incineration
• Thermal incineration
• Catalytic incineration

82. 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.

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

84. 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.

85. Xi volume of i component in the mixture
Xm volume of mixture
LELi LEL of i component

86. GASES

87. Air Pollution Control For Gases
• Adsorption Towers
• Thermal Incernation
• Catalytic Combustion

88. Adsorption Towers
 Principle
• 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.

89. 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.

90. 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.

91. The End!!!