The document discusses various strategies for air pollution prevention and control, with a focus on strategies for controlling nitrogen oxides (NOx) and sulfur dioxide (SO2) emissions. It describes several methods for NOx control including fuel denitrogenation, combustion modification, selective catalytic reduction, and selective non-catalytic reduction. For SO2 control it discusses flue gas desulfurization systems like limestone scrubbing, lime scrubbing, and spray drying, as well as changing to lower sulfur fuels. The document provides details on the chemistry and processes involved in different pollution control methods.

1. AIR POLLUTION PREVENTION
AND CONTROL
Mr. William Kitagwa
Department of Environmental Health
School of Science and Technology
University of Kabianga

2. Pollution Prevention Strategies
• Pollution prevention [vs. control] offers important economic
benefits and at the same time allows continued protection of the
environment.
• While most pollution control strategies cost money, pollution
prevention has saved many firms thousands of dollars in treatment
and disposal costs.
• More importantly, pollution prevention should be viewed as a
means to increase company productivity.
• By reducing the amount of raw materials that are wasted and
disposed of; manufacturing processes become more efficient,
resulting in cost savings to the company.

3. • Pollution prevention should be the first consideration in
planning for processes that emit air contaminants.
• Undertaking pollution prevention practices may reduce
air emissions enough to allow a business or industry to
avoid classification as a major air emission source.

4. What is Pollution Prevention?
• Pollution prevention is the elimination or prevention of wastes (air
emissions, water discharges, or solid/hazardous waste) at the
source. In other words, pollution prevention is eliminating wastes
before they are generated.
• Pollution prevention approaches can be applied to all pollution
generating activity: hazardous and nonhazardous, regulated and
unregulated. Pollution prevention does not include practices that
create new risks of concern.

5. Pollution Prevention Act
In 1990, the US Congress established federal policy on
pollution prevention by passing the Pollution
Prevention Act. The Act states:
1. pollution should be prevented or reduced at the source
whenever feasible (i.e., source reduction),
2. pollution that cannot be prevented should be recycled
in an environmentally safe manner whenever feasible,

6. 3. pollution that cannot be prevented or recycled should
be treated in an environmentally safe manner
whenever feasible, and
4. disposal or other release into the environment should
be employed only as last resort and should be
conducted in an environmentally safe manner.

7. The Pollution Prevention Act defines pollution prevention as
source reduction. Recycling, energy recovery, treatment and
disposal are not considered pollution prevention under the
Act.

8. SOURCE REDUCTION
• Product Changes
• Designing and producing a product that has less environmental impact
• Changing the composition of a product so that less hazardous
chemicals are used in, and result from, production
• Using recycled materials in the product
• Reusing the generated scrap and excess raw materials back in the
process
• Minimizing product filler and packaging
• Producing goods and packaging reusable by the consumer
• Producing more durable products

9. 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

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

11. • Input Material Changes
• Material substitution Using a less hazardous or
toxic solvent for cleaning or as coating
• Purchasing raw materials that are free of trace
quantities of hazardous or toxic impurities

12. Equipment and Process Modifications
•Changing the production process or flow of materials through the
process.
•Replacing or modifying the process equipment, piping or layout.
•Using automation.
•Changing process operating conditions such as flow rates,
temperatures, pressures and residence times.
•Implementing new technologies

13. Good Operating Practices
• Instituting management and personnel programs such as
employee training or employee incentive programs that
encourage employees to reduce waste.
• Performing good material handling and inventory control
practices that reduce loss of materials due to mishandling,
expired shelf life, or improper storage.
• Preventing loss of materials from equipment leaks and spills.
• Segregating hazardous waste from non-hazardous waste to
reduce the volume of hazardous waste disposed.

14. • Using standard operating procedures for process operation
and maintenance tasks
• Performing preventative maintenance checks to avoid
unexpected problems with equipment.
• Turning off equipment when not in use.
• Improving or increasing insulation on heating or cooling
lines.
• Environmentally Sound Reuse and Recycling

15. Control of Gaseous Pollutants
• Absorption
• Adsorption
• Oxidation
• Reduction

16. Absorption
Primary application: inorganic gases
Example: SO2
Mass transfer from gas to liquid
Contaminant is dissolved in liquid
Liquid must be treated

19. Adsorption
Primary application: organic gases
Example: trichloroethylene
Mass transfer from gas to solid
Contaminant is ‘bound’ to solid
Adsorbent may be regenerated

20. Common Adsorbents
Activated carbon
Silica gel
Activated alumina
Zeolites (molecular sieves)

24. Oxidation
• Thermal Oxidation
• Catalytic Oxidation

25. • A thermal oxidizer (or thermal oxidiser) is a process unit for air
pollution control in many chemical plants that decomposes
hazardous gases at a high temperature and releases them into the
atmosphere.
• Thermal Oxidizers are typically used to destroy Hazardous Air
Pollutants (HAPs) and Volatile Organic Compounds (VOCs) from
industrial air streams.
• These pollutants are generally hydrocarbon based and when
destroyed via thermal combustion they are chemically changed to
form CO and H O.

26. Thermal Oxidation
Application: organic gases
Autogenous gases = 7 MJ/kg (heat value)
Operating temperatures: 700 - 1300 oC
Efficiency = 95 - 99%
By-products must not be more hazardous
Heat recovery is economical necessity

28. • Catalytic oxidation is a relatively recently applied alternative for
the treatment of VOCs in air streams resulting from remedial
operations.
• The addition of a catalyst accelerates the rate of oxidation by
adsorbing the oxygen and the contaminant on the catalyst
surface where they react to form carbon dioxide, water, and
hydrochloric gas.
• The catalyst enables the oxidation reaction to occur at much
lower temperatures than required by a conventional thermal
oxidation
Catalytic Oxidation

29. Catalytic Oxidation
Application: organic gases
Non-autogenous gases < 7 MJ/kg
Operating temperatures: 250 - 425 oC
Efficiency = 90 - 98%
Catalyst may be poisoned
Heat recovery is not normal

30. Selective Catalytic Reduction
(SCR)
Application: NOx control
Ammonia is reducing agent injected into exhaust
NOx is reduced to N2 in a separate reactor containing
catalyst
Reactions:
4NO + 4NH3 + O2 --> 4N2 + 6H2O
2NO2 + 4NH3 + O2 --> 3N2 + 6H2O

31. Control of Particulate
Pollutants
• Spray chamber
• Cyclone
• Bag house
• Venturi
• Electrostatic Precipitator (ESP)

32. Spray Chamber

33. Spray Chamber
Primary collection mechanism:
Inertial impaction of particle into water
droplet
Efficiency:
< 1% for < 1 um diameter
>90% for > 5 um diameter
Pressure drop: 0.5 to 1.5 cm of H2O
Water droplet size range: 50 - 200 um

34. Spray Chamber
Applications:
1. Sticky, wet corrosive or liquid particles
Examples: chrome plating bath
paint booth over spray
2. Explosive or combustible particles
3. Simultaneous particle/gas removal

35. Cyclone

36. Cyclone
(Multi-clones for high gas volumes)
Primary collection mechanism:
Centrifugal force carries particle to wall
Efficiency:
<50% for <1 um diameter
>95% for >5 um diameter

37. Cyclone
(Multi-clones for high gas volumes)
Pressure drop: 8-12 cm of H2O
Applications:
1. Dry particles
Examples: fly ash pre-cleaner
saw dust
2. Liquid particles
Examples: following venturi

38. Bag House

39. Bag House
Particle Collection Mechanisms
Screening Impaction
+
-
Electrostatic

40. Bag House
Efficiency:
>99.5% for <1 um diameter
>99.8% for >5 um diameter
Fabric filter materials:
1. Natural fibers (cotton & wool)
Temperature limit: 80 oC
2. Synthetics (acetates, acrylics, etc.)
Temperature limit: 90 oC
3. Fiberglass
Temperature limit: 260 oC

41. Bag House
Bag dimensions:
15 to 30 cm diameter
~10 m in length
Pressure drop: 10-15 cm of H2O
Cleaning:
1. Shaker
2. Reverse air
3. Pulse jet

42. Bag House
Applications:
Dry collection
Fly ash
Grain dust
Fertilizer
May be combined with dry adsorption media
to control gaseous emission (e.g. SO2)

43. Venturi

44. Venturi
Primary collection mechanism:
Inertial impaction of particle into water droplet
Water droplet size: 50 to 100 um
Water drop and collected particle are removed by
cyclone

45. Venturi
Efficiency:
>98% for >1 um diameter
>99.9% for > 5 um diameter
Very high pressure drop: 60 to 120 cm of H2O
Liquid/gas ratios: 1.4 - 32 gal/1000 ft3 of gas

46. Venturi
Applications:
Phosphoric acid mist
Open hearth steel (metal fume)
Ferro-silicon furnace

47. Electrostatic Precipitator (ESP)

48. ESP Tube (a) and Plate (b) collectors

49. ESP Collection Mechanism

50. Electrostatic Precipitator (ESP)
Efficiency:
>95% for >1 um diameter
>99.5% for > 5 um diameter
Pressure drop: 0.5 to 1.5 cm of H2O
Voltage: 20 to 100 kV dc
Plate spacing: 30 cm
Plate dimensions: 10-12 m high x 8-10 m long
Gas velocity: 1 to 1.5 m/s
Cleaning: rapping plates

51. Electrostatic Precipitator (ESP)
Applications (non-explosive):
1. Fly ash
2. Cement dust
3. Iron/steel sinter

52. Flue Gas Desulfurization
(FGD)
Predominant Processes (all non-regenerative):
1. Limestone wet scrubbing
2. Lime wet scrubbing
3. Lime spray drying
Typical scrubbers: venturi, packed bed and
plate towers and spray towers

53. Flue Gas Desulfurization
(FGD)
Spray dryer systems include a spray dryer
absorber and a particle-collection system
(either a bag house or an ESP)
In 1990 the average design efficiency for new
and retrofit systems was 82% and 76%
respectively

54. Flue Gas Desulfurization
(FGD)
Overall reactions:
Limestone: SO2 + CaCO3 --> CaSO3 + CO2
Lime: SO2 + Ca(OH)2 --> CaSO3 + H2O

55. SOX CONTROL

56. 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

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

58. 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

59. Limestone Scrubbing

60. 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

61. Lime Scrubbing

62. 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.
 .

63. Dual Alkali System con`t
 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.

64. 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

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

66. Wellman – Lord Process
• This process consists of the following sub
processes:
 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

67. 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.

68. NOX CONTROL

69. 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

70. 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

71. 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.

72. 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

73. Catalytic Combustion

74. Catalytic Emission Control

75. 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

76. 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

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

78. 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 oC.
 Typical efficiencies are around 80 %

79. Selective Non-catalytic Reduction (SNR)
• At higher temperatures (900-1000oC), NH3
will reduce NOX to nitrogen without a
catalyst.
• At NH3 : NOX molar ratios 1:1 to 2:1, about
40-60%reduction is obtained.
• SNR is cheaper than SCR in terms of
operation cost and capital cost.

80. 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.

81. Staged Combustion

82. 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.