EQUIPMENTS TO CONTROL AIR POLLUTION

EQUIPMENTSTO CONTROLAIR
POLLUTION
ARUN. S

COMMON CONTROL
DEVICES
POLLUTANTS EXAMPLES WHERE USED
Packed towers, spray chambers, venturi
scrubbers
Gases, vapours, sulphur oxides, solid
particles, liquid droplets
Asphalt and concrete batch plants, coal
burning power plants
Carbon absorbers Vapour phase VOCs, Hazardous Air
Pollutants (HAPs)
Soil remediation facilities, oil refineries,
steel mills, printers
Fabric filters or baghouses Particulate Matter (PM) Asphalt batch plants, steel mills,
foundries', fertilizer plants,
Catalytic reactors VOCs, Gases Land fills, oil refineries, printing or paint
shops
Cyclones Large PM Wood shops, pharmaceutical
manuf,acture rock crushers, cement plants
ESPs PM Power plants, steel & paper mills,
smelters, cement plants, refineries
Incinerators VOCs, gases, fumes, Haps, PM Soil contaminated with gasoline, ink and
printing, hazardous waste disposal
Biofilters VOCs, odours, H2S, marcaptans Wastewater treatment plants

PARTICULATE CONTROL
EQUIPMENT – Basic
Mechanisms
 Gravitational settling
 Centrifugal impaction
 Inertial impaction
 Direct interception
 Diffusion
 Electrostatic perception

GRAVITATIONAL SETTLING
CHAMBERS
 To remove large, abrasive particles from gas streams
 Require simple maintenance
 The particulate matter is settled by its own weight by
lowering the flue gas velocity.
 It essentially consists of a chamber in which the
velocity of the carrier gas is decreased so that
particles in the gas settle down by gravity.
 Velocity of a gas is reduced by expanding the
ducting into a chamber of a suitable dimension so
that a low gas velocity is obtained. Its efficiency can
be increased by providing plates or baffles

CYCLONE SEPARATOR
 Used for the separation of solids from fluids
 Separation based both on particle size and particle
density
 It uses centrifugal force to effect the separation
 It consists essentially a short vertical cylinder closed
at the top and by a conical bottom. The air with its
load of solid introduced tangentially at the top, solids
particles drop into the hopper and outlet air pass
through the top central portion.
 Widely used for the collecting of wood chips ,heavy
and coarse dusts

EQUIPMENTS TO CONTROL AIR POLLUTION

FABRIC FILTERS OR
BAGHOUSES
 Fabric collectors use filtration to separate dust
particulates from dusty gases.
 The most efficient and cost effective types of dust
collectors available and can achieve a collection
efficiency of more than 99% for very fine particulates.
 Dust-laden gases enter the bag house and pass
through fabric bags that act as filters. The bags can
be of woven or felted cotton, synthetic, or glass-fiber
material in either a tube or envelope shape.
 Their advantages include retention of finest particles,
collection of particulates in dry form and low
pressure drop
 As classified by cleaning method, three common
types of bag houses are:

Mechanical shaker
Reverse jet
Reverse air

ELECTROSTATIC
PRECIPITATOR
 Particulate collection device that removes
particles from a flowing gas (such as air)
using the force of an induced electrostatic
charge.
 Highly efficient filtration devices that
minimally impede the flow of gases through
the device, and can easily remove fine
particulate matter such as dust and smoke
from the air stream.
 ESP applies energy only to the particulate
matter being collected and therefore is very
efficient in its consumption of energy

FLUE GAS
DESULPHURISATION
 Used to remove SO2 from the exhaust flue gases of fossil
fuel power plants.
 Most FGD systems employ two stages: one for fly
ash removal and the other for SO2 removal.
 In wet scrubbing systems, the flue gas normally passes
first through a fly ash removal device, either an
electrostatic precipitator or a wet scrubber, and then into
the SO2 absorber.
 In dry injection or spray drying operations, the SO2 is first
reacted with the sorbent and then the flue gas passes
through a particulate control device.
 Hydrodesulfurization of fuel has been used for
treating fuel oils before use.
 Fluidized bed combustion adds lime to the fuel during
combustion. The lime reacts with the SO2 to
form sulfates which become part of the ash.

HYDRODESULPHURIZATI
ON
 HDS is a catalytic chemical process widely used
 The industrial hydrodesulphurization processes
include facilities for the capture and removal of the
resulting H2S gas which is converted into byproduct
elemental sulfur or sulfuric acid
 C2H5SH + H2 → C2H6 + H2S
 Reaction takes place in a fixed-bed reactor at
elevated temperatures ranging from 300 to 400 °C
and elevated pressures ranging from 30 to
130 atmospheres in the presence of
a catalyst consisting of an alumina base impregnated
with cobalt and molybdenum (usually called a CoMo)
catalyst or nickel and molybdenum (called NiMo)

DRY SCRUBBER
 Dry scrubbing systems are often used for the
removal of odorous and corrosive gases
 Dry scrubbing systems can be categorized
as dry sorbent injectors (DSIs) or as spray
dryer absorbers (SDAs)
 Dry sorbent injection involves the addition of
an alkaline material (usually hydrated
lime or soda ash) into the gas stream to react
with the acid gases.
 In spray dryer absorbers, the flue gases are
introduced into an absorbing tower (dryer)
where the gases are contacted with a finely
atomized alkaline slurry.

WET SCRUBBER
 The polluted gas stream is brought into contact with
the scrubbing liquid, by spraying it with the liquid or
by forcing it through a pool of liquid, so as to remove
the pollutants.
 The reaction taking place in wet scrubbing using a
CaCO3 (limestone) slurry can be expressed as:
CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) +
CO2 (gas)
 When wet scrubbing with a Mg(OH)2 (magnesium
hydroxide) slurry
Mg(OH)2 (solid) + SO2 (gas) → MgSO3 (solid) +
H2O (liquid)

 A converging/diverging section of duct.
 The converging section accelerates the gas stream
to high velocity. When the liquid stream is injected at
the throat, which is the point of maximum velocity,
the turbulence caused by the high gas velocity
atomizes the liquid into small droplets, which creates
the surface area necessary for mass transfer to take
place.
 The higher the pressure drop in the venturi, the
smaller the droplets and the higher the surface area.
 The penalty is in power consumption.
 For simultaneous removal of SO2 and fly ash, venturi
scrubbers can be used.

PACKED BED SCRUBBER
 Consists of a tower with packing material
inside in the shape of saddles, rings, or some
highly specialized shapes designed to
maximize contact area between the dirty gas
and liquid.
 Packed towers typically operate at much lower
pressure drops than venturi scrubbers and are
therefore cheaper to operate. They also
typically offer higher SO2 removal efficiency.
 The drawback is that they have a greater
tendency to plug up if particles are present in
excess in the exhaust air stream.

SPRAY TOWERS
 The simplest type of scrubber.
 It consists of a tower with spray
nozzles, which generate the droplets for
surface contact.
 Spray towers are typically used when
circulating a slurry .
 The high speed of a venturi would cause
erosion problems, while a packed tower
would plug up if it tried to circulate a
slurry.

ADSORPTION SYSTEMS
 Involves the molecular attraction of gases or
vapours onto the surface of adsorbent solids –
activated carbon or molecular sieves
 The attraction maybe physical or chemical
 After the activated carbon is saturated with
VOCs, it is treated to strip off the collected
VOCs
 VOCs – further treatment & Carbon – reused
in adsorption reactor
 Commonly used for removing gases from
contaminated soil, oil refineries, industrial paint
shops and steel mills

 Also known as a flare stack,it is an
elevated vertical conveyance found
accompanying the presence of oil
wells, gas wells,rigs, refineries, chemical
plants, natural gas plants, and landfills.
 They are used to eliminate waste gas
which is otherwise not feasible to use or
transport. They also act as safety systems
for non-waste gas and is released
via pressure relief valve when needed to
ease the strain on equipment.

THERMAL OXIDIZERS OR
INCINERATORS

 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.
 Used to destroy HAPs and VOCs from industrial air streams.
 Pollutants are destroyed via thermal combustion where they are
chemically changed to form CO2 and H2O.
 Energy recovery efficiency can reach 95%
 Different types include :
Regenerative thermal oxidizer (RTO)
Regenerative catalytic oxidizer (RCO)
Ventilation air methane thermal oxidizer (VAMTOX)
Thermal recuperative oxidizer
Catalytic oxidizer
Direct fired thermal oxidizer - afterburner

BIOFILTERS
 Uses living material to capture and biologically degrade
process pollutants.
 Common uses include processing waste water, capturing
harmful chemicals or silt from surface runoff,
and microbiotic oxidation of contaminants in air.
 The air flows through a packed bed and the pollutant
transfers into a thin biofilm on the surface of the packing
material. Microorganisms, including bacteria and fungi are
immobilized in the biofilm and degrade the pollutant.
 Examples of biofiltration include
Bioswales, Biostrips, Biobags, Bioscrubbers, and Trickling
filters Constructed wetlands and Natural wetlands, Slow
sand filters,Treatment ponds

CRYOGENIC
CONDENSERS
 Cryogenic condensation is a process that allows
recovery of volatile organic compounds (VOCs) for
reuse.
 The condensation process requires very low
temperatures so that VOCs can be condensed and
hence liquid nitrogen has emerged as a viable
substance for use in the extremely low-temperature
or cryogenic (less than -160 degrees C)
condensation process.
 Cryogenic condensation is best suited to exhaust
streams with low flowrates (below 2000 standard
ft3/min) and/or vapor concentrations above 100 parts
per million on a volumetric basis (ppmv).

SELECTIVE CATALYTIC
REDUCTION
 A means of converting nitrogen oxides, also referred
to as NOx with the aid of a catalyst (ceramic as
carrier, and oxides of base metals like V &
W) into N2, and H2O using a gaseous reductant -
anhydrous NH3, aqueous NH3 or urea, at a temp
between 630 and 720 K.
 Application- large utility & industrial boilers
(reduce NOx by 70-95%). Diesel engines & gas
turbines.
 Reactions
4NO + 4NH3 + O2 → 4N2 + 6H2O
2NO2 + 4NH3 + O2 → 3N2 + 6H2O
NO + NO2 + 2NH3 → 2N2 + 3H2O

Selective non-catalytic
reduction
 A method to lessen NOx emissions in conventional
power plants that burn biomass, waste and coal by
injecting NH3 or urea into the boiler at a location
where the flue gas is between1,400 & 2,000 °F
producing N2, CO2 & H2O.
 NH2CONH2 + H2O -> 2NH3 + CO2
4 NO + 4 NH3 + O2 -> 4 N2 + 6 H2O
 It can achieve the same efficiency of about 90 %
as SCR, but practical constraints of temperature,
time, and mixing exist.
 SNCR has an economical advantage as the cost of
the catalyst isn't there.

EXHAUST GAS
RECIRCULATION
 It is a NOx emissions reduction technique used in
petrol and diesel engines.
 Recirculates a portion of the engine's exhaust
gas back to the engine cylinders. In a gasoline
engine, this inert exhaust displaces the amount of
combustible matter in the cylinder. In a diesel engine,
the exhaust gas replaces some of the
excess oxygen in the pre-combustion mixture.
 The exhaust gas, added to the fuel, oxygen, and
combustion products, increases the specific heat
capacity of the cylinder contents, which lowers
the adiabatic flame temperature. Because NOx
formation progresses much faster at high
temperatures, EGR reduces the amount of NOx the
combustion generates

CATALYTIC CONVERTER
 A device used to reduce the toxicity of emissions from
an IC engine.
 Works by using a catalyst to stimulate a chemical
reaction in which toxic by-products of combustion are
converted to less-toxic substances.
 90% conversion of carbon monoxide, hydrocarbons, and
nitrogen oxides into less harmful gases in automobiles.
 Pt-Rh – reduction catalyst & Pt-Pd – oxidation catalyst
 Two way : 2CO + O2 → 2CO2
CxH2x+2 + [(3x+1)/2] O2 → xCO2 + (x+1) H2O
 Three way : 2NOx → xO2 + N2
2CO + O2 → 2CO2
CxH2x+2 + [(3x+1)/2]O2 → xCO2 + (x+1)H2O

SORBENT INJECTION
TECHNOLOGY
 Injecting a sorbent such as powdered activated
carbon (PAC) into the flue gas represents one of the
simplest and most mature approaches to controlling
mercury emissions from coal- fire boilers.
 The gas phase mercury in the flue gas contacts the
sorbent and attaches to its surface.
 The sorbent with the mercury attached is then
collected by the existing particulate control device,
either an electrostatic precipitator (ESP) or fabric
filter This combined material consisting of 99
percent flyash and one percent sorbent is then either
disposed of or beneficially used

 Sources of dioxins and furans include
waste incinerators, cement production,
fossil-fuel-fired combustors,
and forest fires.
 Dioxin and furan compound emissions
are calculated in
two different ways:
 As the total dioxin and furan compound
concentrations
 As the Toxic Equivalency Quotient (TEQ)
concentration

CONTROL TECHNIQUES
 Both dioxin and furan compounds are destroyed
when the gas temperatures exceeds approximately
1400°F
 These temperatures usually exist in the combustion
zones of incinerators and fossil-fuel-fired boilers.
 The formation mechanisms decrease to negligible
rates when the gas stream temperature decreases
below 400°F. The gas stream should be sufficiently
cooled prior to the air pollution control system can
eliminate this formation mechanism.
 The best way to control dioxin and furan emissions is
preventing their formation by reducing or eliminating
the chlorine in the fuel and waste material being
burned.

EQUIPMENTS TO CONTROL AIR POLLUTION

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