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
COMMONLY USED METHODS FOR AIR POLLUTION
CONTROL
PARTICULATE

Cyclones
Electrostatic Precipitators
Fabric Filter

Wet Scrubbers


GASES

Adsorption Towers

Thermal Incernation

Catalytic Combustion
SOx CONTROL
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
GENERAL METHODS FOR CONTROL OF SO2
EMISSIONS (contd.)
Use Flue Gas Desulfurization Systems
Use Alternative Energy Sources, such as Hydro-Power
or Nuclear-Power
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
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
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
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.
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
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
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.
NOx CONTROL
BACKGROUND ON NITROGEN OXIDES
There are seven known oxides of nitrogen :

NO

NO2

NO3

N2 O

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
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
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.
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
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
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
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 %
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.
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.
CARBON MONOXIDE
CONTROL
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.
FORMATION OF CARBON MONOXIDE

Due to insufficient oxygen

Factors affecting Carbon monoxide formation:

fuel-air ratio

degree of mixing

temperature
PARTICULATE MATTER
CONTROL
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
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



PARTICULATE COLLECTION MECHANISM

Gravity Settling

Centrifugal Impaction

Inertial Impaction

Direct Interception

Diffusion

Electrostatic Effects
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
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
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.
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.
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
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
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
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.

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.
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.
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.
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.
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.
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.
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.
HYDROCARBON CONTROL
GENERAL METHODS FOR CONTROL OF
HYDROCARBON EMISSIONS
Incineration or after burning

Direct flame incineration

Thermal incineration

Catalytic incineration
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.
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.
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.
GASES
AIR POLLUTION CONTROL FOR GASES

Adsorption Towers

Thermal Incernation

Catalytic Combustion
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.
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.
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.