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
19. Adsorption
Primary application: organic gases
Example: trichloroethylene
Mass transfer from gas to solid
Contaminant is ‘bound’ to solid
Adsorbent may be regenerated
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
27.
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
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
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
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
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
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
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
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
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
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.
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
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
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.
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.