AIR QUALITY

MANAGEMENT
Approaches in AQM
 Control of Air Pollutants-
Mechanisms Involved
Particulates
• Gravity Settling
• Centrifugal Forces
• Electrostatic Forces
• Filtration
• Scrubbing
Gaseous Pollutants
• Absorption
• Adsorption
• Combustion
• Masking
 Air Pollution Control Systems & Devices
(Particulate Control)

 Considering factors
• Particulate concentration

• Particulate Size

• Volume of flue gases

• Moisture content in flue gases

• Temperature of flue gases

 Gravitational Settling Chambers

• Operating temperature – 1000 deg.C

• Applications – precleaners for removing dry dust produced by
grinding in cement and lime kilns, grain elevators, rock crushers,
thermal coal dryers, furnaces
• Dust particle size - > 50 mm
• One of the first devices to control particulate emission
• Types – Simple expansion Chamber; Multi-tray settling chamber
• Force – Gravitational
• Once the horizontal gas velocity is reduced, large particles can be
overcome by gravity and fall into the hopper
• Typical horizontal velocity range of 0.3 to 3.0 m/s
 Gravitational Settling Chamber

 Expansion chamber
 Gravitational Settling Chamber
Howard Settling Chamber (multiple tray)
 Gravitational Settling Chamber
•Baffle Chamber
 Gravitational Settling Chambers

• Settling chambers have high collection efficiency

• for low gas velocities,
• high terminal settling velocities, and
• large ratio of chamber length to height
• A large L/H ratio ensures a long residence time and short vertical
distance for the particle to travel to be collected
• Settling chambers are characterized by low capital costs and low
pressure drop
• They can be used under temperature (~ 1000 deg.C) and pressure
extremes (~100 atm.)
Cyclones
• The suspended gas-particle mixture is fed tangentially
to the inlet at a high velocity and pressure.

• This creates a spiral flow wherein the suspended

particles are forced towards the wall due to centrifugal
force.

• These particles which collide at the wall, spiral

downwards in the gas boundary layer and are collected
at an outlet at the bottom.
 Cyclones
• Types of designs
• Flue Gas Entry points – Tangential and Swirl vane
• Gas Flow rates – 50 –50,000 m3/hr
• Pressure drop – 0.5 to 0.8 inches in water
• Operating temperatures - ~1000 deg.C
• Applications – Cement and lime kilns, furnaces, milling operations,
power drying processes
• Dust particle sizes - > 25mm
• Dust concentration - > 2000g/m3
• Cyclones are efficient in removing large particles but are not as
efficient with smaller particles. For this reason, they are used with
other particulate control devices.
Cyclones
Single-cyclone separators create a
dual vortex to separate coarse from
fine dust. The main vortex spirals
downward and carries most of the
coarser dust particles. The inner
vortex, created near the bottom of the
cyclone, spirals upward and carries
finer dust particles.
Cyclones

• Multiple-cyclone separators, also

known as multiclones, consist of a

number of small-diameter cyclones,

operating in parallel and having a

common gas inlet and outlet.

• Multi-clones operate on the same

principle as cyclones--creating a

main downward vortex and an

ascending inner vortex.

 Cyclones
• Multi-cyclones are more efficient than single cyclones because they
are longer and smaller in diameter.

• The longer length provides longer residence time while the smaller
diameter creates greater centrifugal force. These two factors result in
better separation of dust particulates.

• The pressure drop of multi-cyclone collectors is higher than that of

single-cyclone separators.
 Some Industrial Applications
• Used in many applications for pre-cleaning before sending to
precipitators of filters
• Recovery and recycling of solid particles such as food products or
catalysts
• Used in metallurgical industries as a first stage for control of
particulate matter emissions from roasters, kilns and furnaces
• Used after spray drying operations in food industry
• Recovery of process materials after crushing and grinding
operations.
 Advantages
• No moving parts, low maintenance costs
• Can operate over wide range of temperature and high pressure
• No pretreatment necessary
• Small space requirements
• Comparatively low capital and operating costs
Disadvantages
• Low collection efficiencies when particle size is small (<10
micrometers)
• Cannot handle sticky particles which have high inter-
particulate attraction
• High efficiency units experience high pressure drops
 Fabric Collectors / BAG FILTERS
• Commonly known as baghouses,
fabric collectors use filtration to
separate dust particulates from
dusty gases.
• They are one of 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
baghouse 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
 Fabric Collectors
The high efficiency of these collectors is due to the dust cake formed
on the surfaces of the bags. The fabric primarily provides a surface
on which dust particulates collect through the following four
mechanisms:
• Inertial Collection - Dust particles strike the fibers placed
perpendicular to the gas-flow direction instead of changing
direction with the gas stream.
• Interception - Particles that do not cross the fluid streamlines
come in contact with fibers because of the fiber size.
• Brownian Movement - Submicron particles are diffused,
increasing the probability of contact between the particles and
collecting surfaces.
• Electrostatic Forces - The presence of an electrostatic charge
on the particles and the filter can increase dust capture.
 Fabric Collectors

As classified by cleaning method, three common types of baghouses

are –

• Mechanical shaker
• Reverse air
• Reverse jet
Electrostatic Precipitators
• Electrostatic Precipitators use electrostatic forces to separate dust
particles from exhaust gases.
• A number of high-voltage, direct-current discharge electrodes are placed
between grounded collecting electrodes.
• The contaminated gases flow through the passage formed by the
discharge and collecting electrodes.
• The airborne particles receive a negative charge as they pass through the
ionized field between the electrodes.
• These charged particles are then attracted to a grounded or positively
charged electrode and adhere to it.
Electrostatic Precipitators
• The collected material on the electrodes is
removed by vibrating the collecting electrodes
either continuously or at a predetermined
interval.
• Cleaning a precipitator can usually be done
without interrupting the airflow.
The four main components of all
electrostatic precipitators are-
• Power supply unit, to provide high-
voltage, unidirectional current
• Ionizing section, to impart a charge to
particulates in the gas stream
• A means of removing the collected
particulates
• A housing to enclose the precipitator
zone
• The two major types of high-voltage precipitators currently used are-
• Plate
• Tubular
• Plate Precipitators –
• The majority of electrostatic
precipitators installed are the plate
type.
• Particles are collected on flat, parallel
surfaces that are 8 to 12 in. apart, with
a series of discharge electrodes
spaced along the centerline of two
adjacent plates.
• The contaminated gases pass through
the passage between the plates, and
the particles become charged and
adhere to the collection plates.
• Collected particles are usually removed
by rapping the plates and deposited in
bins or hoppers at the base of the
precipitator.
Tubular Precipitators –
• Tubular precipitators consist of
cylindrical collection electrodes with
discharge electrodes located on the
axis of the cylinder.
• The contaminated gases flow around
the discharge electrode and up through
the inside of the cylinders.
• The charged particles are collected on
the grounded walls of the cylinder.
• The collected dust is removed from the
bottom of the cylinder.
• Tubular precipitators are often used for
mist or fog collection or for adhesive,
sticky, radioactive, or extremely toxic
materials.
Particle Charging - Corona formation in ESP
In an Electrostatic Precipitator, dust particles are charged by corona current
electrons flowing between the discharge electrodes and the collection
electrodes (or plates). The electrostatic field drives the charged dust
particles to the collecting plates.

electrode
corona
Particle Removal
• Dust that has accumulated to a certain thickness on the collection electrode
is removed by one of two processes, depending on the type of collection
electrode.
• Tubes are usually cleaned by water sprays, while plates can be cleaned
either by water sprays or a process called rapping.
• Rapping is a process whereby deposited, dry particles are dislodged from
the collection plates by sending mechanical impulses, or vibrations, to the
plates.
• Precipitator plates are rapped periodically while maintaining the continuous
flue-gas cleaning process.
• Plates are rapped when the accumulated dust layer is relatively thick (0.08
to 1.27 cm or 0.03 to 0.5 in.). This allows the dust layer to fall off the plates
as large aggregate sheets and helps eliminate dust re-entrainment.
• Dislodged dust falls from the plates into the hopper.
• Advantages of ESPs
• Very high efficiencies, even for very small particles
• Can handle very large gas volumes with low pressure drop
• Dry collection of valuable material, or wet collection of fumes and mists
• Can be designed for a wide range of gas temperatures
• Low operating costs, except at very high efficiencies

• Disadvantages of ESPs
• High capital costs
• Will not control gaseous emissions
• Not very flexible, once installed, to changes in operating conditions
• Take up a lot of space
• Might not work on particulates with very high electrical resistivity
 COLLECTION EFFICIENCY OF ESP

The collection efficiency of an ESP as a function of gas flow rate and

precipitator size is given by the Deutsch-Andersen Equation

Assumptions:
•Repulsion effect is neglected
•Uniform gas velocity throughout the cross section
•Particles are fully charged by field charging.
•No hindered settling effect

Collection efficiency mathematically expressed as follows

Ƞ = 1- exp {(-Vpm × Ac)/Q}

where
ƞ=Fractional Collection Efficiency; Ac=Area of the collection electrode; Vpm=Particle
migration velocity; Q=Av=Volumetric flow rate of gas; v=gas velocity.
NUMERICAL
A tubular ESP having multiple cylinders is to be
designed to treat 10,000 m3/h of a gaseous stream
from a paper mill with a inlet gas velocity of 1m/s
into each cylinder of ESP. Assume an effective
migration velocity of 0.075m/s and diameter of
each cylinder to be 30cm. Design the ESP in terms
of number of cylinders required and length of
cylinders required to achieve an efficiency of 90%.
What additional length is required if an efficiency of
99% has to be achieved with the given conditions.
SOLUTION
Q=10,000 m3/h = 2.78m3/s
Dia. of cylinder = 30 cm = 0.3 m
Q= Velocity of gas in each cylinder × Area of cylinder × no of
cylinders
2.78 = 1 × Π/4 × (0.3)2 × n
n= 39.4 (i.e. 40 cylinders)
Ƞ = 1- exp {(-Vpm × Ac)/Q}

0.9 = 1 – exp {(- 0.075 × Π × 0.3 × L)/ (2.78/40)}

L= 2.262 m
For 99 % efficiency
L = 4.528 m
Additional Length = 4.528-2.262 = 2.266 m
 Wet Scrubbers (Used both for removal of particulates and

gaseous pollutants)

• Dust collectors that use liquid are commonly known as wet

scrubbers. In these systems, the scrubbing liquid (usually

water) comes into contact with a gas stream containing dust

particles. The greater the contact of the gas and liquid

streams, the higher the dust removal efficiency

Wet Scrubbers
There is a large variety of wet scrubbers; however, all have of three basic

operations:

• Gas-Humidification - The gas-humidification process conditions fine

particles to increase their size so they can be collected more easily.

• Gas-Liquid Contact - This is one of the most important factors

affecting collection efficiency. The particle and droplet come into

contact by four primary mechanisms.

• Gas-Liquid Separation - Regardless of the contact mechanism used,

as much liquid and dust as possible must be removed. Once contact

is made, dust particulates and water droplets combine to form

agglomerates. As the agglomerates grow larger, they settle into a

collector.
Wet Scrubbers

• The "cleaned" gases are normally passed through a mist eliminator

(demister pads) to remove water droplets from the gas stream.
• The dirty water from the scrubber system is either cleaned and
discharged or recycled to the scrubber.
• Dust is removed from the scrubber in a clarification unit or a drag chain
tank.
• In both systems solid material settles on the bottom of the tank.
• A drag chain system removes the sludge and deposits in into a
dumpster or stockpile.
 Scrubbers
In wet scrubbers, the pollutant stream can be treated by
one or more of the collection mechanisms such as
following:

1. Impaction
2. Diffusion
3. Absorption (for gaseous pollutant)
 Impaction
• dust particles which tend to follow with the streamlines cannot always follow as
they diverge around the droplet thus impact or hit the droplet.

Impaction increases
•as the diameter of the particle increases and
•as the relative velocity between the particle and droplets increases.
•liquid droplet decreases
 Diffusion
• Bumping or bombardment causes them to in a random
manner or to diffuse through the gas.
• This irregular motion can cause the particles to collide with a
droplet and be collected
 Absorption
• It is the process of dissolving gaseous
pollutants in a liquid.
• Absorption continues as long as a
concentration differential exists .

• Solubility governs the amount of

liquid required (liquid-to-gas ratio)
and the necessary contact time.
• More soluble gas requires less liquid &
less time.
Gaseous Emission Control
Gaseous Emission Control – Physical & Chemical Separation
• Absorption – Absorption is a basic chemical engineering operation and
is probably the most well established gas control technique (Wet
Scrubbers)
• Used extensively in the separation of corrosive, hazardous or noxious
pollutants from waste gases
• Absorption involves transferring pollutants from a gas phase to a
contacting solvent
• To maximize the mass transfer driving force, the absorber generally
operates in a counter current fashion
• Absorption systems design involves selecting a solvent and the design
of the absorber
Absorption
• Selection of the solvent
• Solubility is the most important consideration in the selection of a
solvent for absorption
• Higher the solubility, lower the amount of solvent required
• The solvent should also be non-volatile to prevent an excessive carry
over in the gas effluent
• Other properties of a good solvent are
• Low flammability and viscocity
• High chemical stability
• Acceptable corrosivity
• Low toxicity and pollution potential
Absorption

• Any gas-liquid contactors that promote the mass transfer across the
phase boundary can be used for absorption operation
• The most popular devices are spray towers, packed columns as well
as venturi scrubbers
• In case of packed columns, two types of packings are used
• Random packing
• Structured packing
• The packing involve use of pall rings
 Plate type scrubber
• It consists of a hollow vertical tower with
one or more plates .
• Gas comes in from the bottom and must
pass through perforations, valves, slots,
or other openings in each plate and exit
from top.
• Liquid from the top plate, flows
successively across each plate and exit at
the bottom.
• Gas passing through the openings in
each plate mixes with the liquid flowing
over the plate.
• The gas and liquid contact allows the
mass transfer or particle removal.
• These have the ability to remove
gaseous pollutants to any desired
concentration provided a sufficient
number of plates are used.
 Spray Tower
• It utilizes spray nozzles for liquid droplet atomization.
• The sprays are directed into a chamber to conduct the
gas through the atomized liquid droplets.
• These are designed for low pressure drop and high
liquid consumption.
• Applicable to the removal of gases which have high
liquid solubility.
• Many nozzles are placed at different heights.
• Theoretically, the smaller the droplets formed, the
higher the collection efficiency achieved
Condensation
• In cases where pollutants have low vapour pressures, condensation is
effective for removing a significant part of the vapour
• The condenser works by cooling the feed gas to a temperature below
the dew point of the feed gas
• Two types of condensers are normally used
• Surface condensers contact condensers

• Removal efficiencies of condensers typically range from 50

percent to more than 95 percent, depending on design and
applications.
Adsorption
• Adsorption is the binding of molecules or particles to a surface.
• In this phenomenon molecules from a gas will be attached physically
to a surface.
• The binding to the surface is usually weak and reversible.
• The most common industrial adsorbents are activated carbon, silica
gel, and alumina, because they have enormous surface areas per unit
weight.
• In selecting the adsorbent, the adsorption isotherm of the adsorbent
should be favorable
• An adsorption isotherm for a single gaseous adsorptive on a solid is
the function which relates at constant temperature the amount of
substance adsorbed at equilibrium to the pressure (or concentration)
of the adsorptive in the gas phase.
Other Methods
• Thermal destruction using thermal
combustion/incineration
• Biofiltration
• Biofiltration involves the removal of
oxidation of organic compounds from
contaminated air by beds of
compost, or soil.
• Biofiltration combines the
mechanism of adsorption, scrubbing
effect of water, and oxidation
• Soil and compost possess millions of
microbes which oxidize organic
compounds to carbondioxide and
water
Gaseous Emission Control in
Automobiles
 Catalytic convertor
HC, CO, NO
From
Engine
Exhaust
Reduction air Oxidation
Stage stage

Pt- H + NO → Pt-O +N2 Pt –O + HC/CO Pt- H +

+(NH3) CO2+H2O

N2, CO2,
H20
Released to
atmosphere

• Catalyst used is Platinum/ Palladium

• Both Oxidation and reduction takes place in two chambers
Thanks