TECHNOLOGY FOR AIR POLLUTION
CONTROL
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
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 makeup 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

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
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
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.
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
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
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
FORMATION OF CARBON MONOXIDE

Due to insufficient oxygen

Factors affecting Carbon monoxide formation:

Fuel-air ratio

Degree of mixing

Temperature
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.
CARBON DIOXIDE CONTROL
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
SOURCES OF CO2 EMISSIONS IN THE U.S.
(x-axis units are teragrams of CO2 equivalent)
Source: USEPA
CO2 EMISSIONS FROM FOSSIL FUEL
COMBUSTION BY SECTOR AND FUEL TYPE
(y-axis units are teragrams of CO2 equivalent)
Source: USEPA
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
CONTROL OF MERCURY EMISSIONS
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).

Types of Sources
Source: Seingeur, 2004 and Mason and Sheu, 2002.
Worldwide Distribution of Emissions
Source: Presentation by J. Pacyna and J. Munthe at mercury workshop in Brussels,
March 29-30, 2004
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 coalfired combustor using a suction pipe, or "thief," and injects the resulting
sorbent into the flue gas to capture the mercury.

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
Overall collection η
Ci
Co
inlet concentration
outlet concentration
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.
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.
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.
Fabric Filters
ΔP
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
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
Darcy’s equation
ΔPf
Pressure drop N/m2
ΔPp
Pressure 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
A
cloth area m2
Dust Layer
L
t
ρL
Dust loading kg/m3
time of operation min
Bulk density of the particulate layer kg/m3
ΔP = ΔPf + ΔPp
Filter Drag S = ΔP/V
Areal dust density W = LVt
S= k1+k2W
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.
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
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

Cunningham correction factor
where,
T = absolute temperature (°k)
dp = diameter of particle (μm)
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.)
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.
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)
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.
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
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


o
Timing

o
The reaction kinetics are very sensitive to temperature
The higher the temperature, the faster the reaction
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.
Xi volume of i component in the mixture
Xm volume of mixture
LELi LEL of i component
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.
INITIAL PLUME RADIUS
Initial Standard Deviation:
Where,
M = mass of liquid
ρ = density of the plume
σ = R/2.15
Adjustment – 1
where,
w = initial width of the plume
h = initial width of the plume
Adjustment -2
where,
H = length scale of the spill
PROBLEM

Estimate the net cloth area for a shaker bag house that
must filter 40,000 cfm of air with 10 grams of flour dust
per cubic foot of air. Also specify the number of
components to be used and calculate the total number
of bags required if each bag is 8 feet long and 0.5 feet in
diameter. The maximum filtering velocity for flour dust is
2.5 ft/min.
SOLUTION
Step 1:
Calculate total area and number of components required.
A = Q/V
 Step 2:
Calculate the area of each bag.
A = Π(d)l
 Step 3:
Calculate the total number of bags required.
Number of bags required = Total area / Area per bag
= 1270 bags
