Industrial emission control techniques

1. Industrial Emission Control Techniques
And Equipments
B. Padma S. Rao
Scientist
Air Pollution Control Division, NEERI, Nagpur-440 020
Introduction
The problem of air pollution can be prevented by reducing the formation of
the pollutants or minimizing their emissions at the source itself. In the case of
industrial pollutants, this can often be achieved by investigating various
approaches at an early stage of process design and development, and selecting
those methods, which do not contribute to air pollution or have the minimum air
pollution potential. These are known as source correction methods. Application
of these methods to existing plants is difficult, but still some of these correction
methods could be applied without severely upsetting the economy of the
operation. Control of the pollutant at the source can be accomplished in several
ways through raw material changes, operational changes, modification or
replacement of process equipment, and by more effective operation of existing
equipment.
When source correction methods cannot achieve the desired goal of air
pollution control, use is made of effluent gas cleaning techniques. These involve
many of the chemical engineering unit operations and at present form the main
part of pollution control technology. Many advanced techniques have been
developed recently like bio filters & plasma technology but its cost effectivity is
still under study. A brief analysis of various techniques and equipments used for
industrial emission control is described in the following section.
Source Correction Techniques
These are further classified as :
• Raw Material Changes e.g. Ciean fuel, sintered etc.
• Process changes e.g. Petroleum refinery

2. • Equipment Modification/Replacement e.g. Floating roof tanks
• Proper Operation and Maintenance
Emissions Cleaning Techniques :
The emissions from industries are mainly particulates & gases. The cleaning
techniques are divided into :
• Gaseous/Odour Pollutant Control Method
- Absorption (wet and dry)
- Chemical alteration usually through combustion and catalytic
conversion
- Incineration
(Basic mechanism is diffusion of the gas either to the surface of the
absorbing liquid or adsorbing solid/catalyst or to the reaction zone of a
chemical reaction)
• Particulate Emission Control Method
- Choice of collection device depends on
* Physico chemical characteristics
* Size and concentration
* Volume handled
* Temperature and humidity etc.
(Basic mechanism relies on diffusion of particles from gas stream to a
collecting surface etc.)
The important performance of these techniques is collection efficiency
which differs based on collection mechanism and is given by :

3. Collection Efficiency yiy
p">»
u
c
2
o
£<d
£
O•*-»
o
0)
O
100
75
50
25
0
100 x weight of material collected
Total amount entering the collection
10 20 30 40 50
Particle diameter, jjm
Fractional efficiency curve
However for each fraction of the material vis-a-vis, each size of the material the
fractional efficiency curve is plotted for all control methods. From this figure, the
efficiency in each size range can be computed by the equation.
, „. . weight of fraction i collected A n n
Fractional efficiency r), = x100
m.
Where, mi is the weight of material in range i
The overall efficiency is then calculated for n number of size fractions as
n
I m n.
i=1 1 1
%Overall collection efficiency hT =1
M
Where, M is total amount entering the collection

4. Baffled Plate Scrubber
• Gas stream passes through a flooded perforated plate for scrubbing
The efficiencies of dust collectors can be obtained at a glance for
preliminary selection of a suitable control device for specific applications.
A comparitive cost analysis and environmental implications are taken into
account for final selection.
Type of Collector % EfficiencyType of Collector
at 10pm at 5pm at 2pm at 1pm
Inertial collector 30 16 7 3
Medium-efficiency cyclone 45 27 14 8
High-efficiency cyclone 87 73 46 27
Low-resistance cellular cyclone 62 42 21 13
Tubular cyclone 98 89 77 40
Irrigated cyclone 97 87 60 42
Self-inducted spray deduster 98 93 75 40
Spray tower 97 94 87 55
Wet impingement scrubber >99 97 92 80
Disintegrator 99 98 95 91
Venturi scrubber-medium energy >99.9 99.9 99 97
Venturi scrubber-high energy >99.9 99.9 99.5 98.5
Electrostatic precipitator >99.5 >99.5 >99.5 >99.5
Irrigated electrostatic precipitator >99.5 >99.5 >99.5 >99.5
Shaker-type fabric filter >99.9 99.6 99.6 f 9
Pulse-jet fabric filter >99.5 99.6 99.6 99.6

5. Guide to Selection of Dust Control Equipment
Factors Cyclones Wet
washers
low-
energy
Wet
washers
high-
energy
Dry
electros-
tatic
precipit-
ators
Wet
electros-
tatic
precipit-
ators
Aggre-
gate
filters
Fabric
filters
Fibrous
filters
DUST PROPERTIES
High inlet
burden
Care Care Care Beware
Erosive Care Care
Sticky Beware Care Beware Beware Care
Light fluffy Beware Care
Difficult to
wet
Care Care Care
Pyrophoric Care Care Beware Care
Resisitivity
problem
Beware Care
GAS CONDITIONS
Constant
pressure
drop
Care Care Care
Varying flow Care Care Care Care Care Care
Explosive,
combustible
Care Beware Beware Care Care Care
Corrosive Care Care Care Care Care Care Care Care
Suitable for
high
pressure
Care Care Care
OTHER FACTORS
Minimum
ancillary
equipment
Care Care Care Care Care Care
On-line
regeneration
Care Care Care Beware
Key :
Indicates that the type of plant can generally cope with the process requirement,
if well designed
• care indicates that special attention is required in plant design and operation
to prevent problems
• beware indicates that this process condition could lead to severe operational
difficulties alternatives that avoid the problem are normally sought.
For all gas cleaning problems associated with explosive or combustible materials,
competent advice should be sought.
r^ A r-

6. Techniques for Dry Collectors Particulate Matter Control
The basic mechanism & equipment needed for particulate control are as under:
Basic Mechanism Equipments
Gravitational settling Settling chambers
Centrifugal Impaction Cyclone and Multicyclone
Inertial Impaction Battled settlers
Direct Interception Fabric filters
Diffusion Scrubbers
Electrostatic precipitation ESP
Plasma technology combined ESP & bag house ESP
Gravitational Settling Chambers
The technique is generally used to remove large abrasive particles (>50 |itn) from
gas stream
>- Chamber
't 7
Y
Dust
Typical Gravity Settling Chamber
It provides low pressure drop to the flue gas stream and is simple for
maintenance. However it give low efficiency for smaller particles and hence are
mostly used as precleaners
In the settling chamber the gas steam with its entrained particles is
allowed to flow at a low velocity allowing sufficient time for particles to settle down
and maintain near-laminar flow condition. Since these equipments work under
the principle of gravity only larger size particles can be removed (> 50pm).
246

7. Cyclone Separators
These utilize centrifugal force generated by a spinning gas stream to
separate the particulate matter from the flue gas. This unit works on the principle
that the centrifugal force is much greater than the gravity and hence these
cyclones can remove, smaller particles of size (> 10pm).
Salient Features
• Utilises less space to handle large volume of gas
• Inertial separating force is the radial component of the simple centrifugal
force and is a function of tangential velocity
• The performance of a cyclone depends on the separation factor (5-2500).
High separation factor takes into account high velocity and pressure drop
• Most satisfactory expression for cyclone performance is the Lapple
correlation which provides collection efficiency as a function of cut size
diameter dpc i.e. size of the particles collected with 50% efficiency
• The operating range pressure drop is 25-30 cm
• The main advantage of this unit is its compactness and the principal
drawback is its tendency towards plugging from solid buildup
0
i H
Generalized design configuration of a cyclone

8. Reverse flow cyclone separator
248

9. Efficiency curve
P a r t ic 1« - size r a t i o <Jp /d
Lapple's correlation for cyclone efficiency

10. FABRIC FILTERS
These are highly efficient dry particulate collectors working on the principle
mechanisms of impaction, interception and diffusion. These are as follows :
Inertial Impaction
Direct Interception
Diffusion
change in particle trajectory around the fibre
provides particle deposition
Interception of particle with the fabric provides
particle deposition
for Sub-micron sized (0.001-0.5 pm). The zigzag
brownian motion causes the particles to impinge
and adhere to the surface of the fiber
The salient features of these units are :
• These are high efficiency type but has a draw back of working in lower
temperature range
• Periodic maintenance and cleaning is necessary for their efficient
utilization
• These units can be categorized as woven fabric or felt cloth; cotton is least
expensive fiber
• The choice of fabric depends on operating temperature and abbrasiveness
of particles
• They can collect fine particles in dry form and provides low pressure drop
but have high contribution cost
— C l t o n go*
hotter mtehanKm
Fill«r baefs
D u s t y g a s i n - —
Hopper
— Lorq«f portictt
s«porotion by
c«n»ri'fugol action
Dutt outltl
Typical fabric filter systems

11. Electrostatic Precipitator
This unit is mostly used in power plants, cement and paper mills.
Electrostatic, precipitation is a physical process in which particles suspended in
gas stream are electrically charged and get separated from the gas stream under
electric field.
An electrostatic precipitator consists of a positively charged collecting
surface and a high voltage discharge electrode wire suspended from an
insulation on the top and held in position by weight at the bottom. At a very high
DC voltage (50 KV) the corona discharge occurs close to the negative electrode
setting up an electric field between the emitter and the grounded surface.
Particle laden gas enters bottom and flows upward. The negative charged
particles pass through the grounded positive electrode and deposit there/get
collected. Periodically the collected particles are removed by rapping or vibrating
the collection plate.
• The collection efficiency is a function of gas flow rate and precipitation
size
• The particle migration velocity is another important term. It is a
function of electric field strength, gas viscosity, dielectric and resistivity
property of the dust
• The unique feature of this unit is the separating force is applied directly
to the particles without the necessity of accelerating the gas

12. A typical wire and plate precipitator
Wet Scrubber
These are used for simultaneous removal of particulates and gaseous
pollutants. These have problems of corrosion and liquid waste disposal. The
principle for particulate collection is by scrubbing in contact with liquid usually
water. The contact with liquid can be achieved through impacting targets i.e.
wetted surface (packed bed) and individual droplets (spray scrubber).
The basic collection mechanism is inertial impaction and interception for
particles of diameter greater than 3pm size while diffusion for particles of dia less
than 3pm size.
Various types of scrubber are spray towers, centrifugal scrubbers and
packed bed/plate columns these are used for collecting particles greater than
3pm size whereas the venturi scrubbers are used for collecting particles less than
3pm size.

13. Packed Bed and Plate Columns
• In the counter current packed bed scrubber, the packing provides a
target which allows the gas steam to follow a curved path through the
pore spaces while the particles carried by the gas stream are captured
by inertial impaction. Fine particles are collected by diffusion
• The packing can be of raschig rings, saddle coke, broken stone etc.
• These can be used for particles that are insoluble in the scrubbing
liquid. Otherwise it will encounter plugging problems which can be
reduced by low density packing material or by backwashing
The schematic of Packed Bed and Plate Columns
Spray Scrubber
• water spray is introduced through nozzles
• polluted gas flows upwards and particle collection results by inertial
impaction and interception of droplets
• optimum droplet size/dia as found experimentally should be > 600pm
t
(0.6 mm)

14. • nozzle producing spray with droplet size < 1 mm is satisfactory
• efficiency of spray tower is a complex function of droplet size, gas
velocity, liquid : gas ratio & droplet trajectory
The schematic of Spray Scrubber
Centrifugal Scrubber
• Simplest type is the one wherein banks of nozzles are inserted inside a
conventional dry cyclone
• The spray acts on the particles in the outer vertex and the droplets
loaded with particles are thrown outward against the wet inner wall of
the cyclone
• The particle cut diameter i.e. with >50% efficiency is between 2-3pm
with droplet size of 100pm
Clean q<K
t
254

15. Venturi Scrubber
• Offers high performance collection for fine particles usually smaller than 2-3 pm
• Suitable for sticky, flammable and corrosive particles
• Gas stream velocity ranges from 60-120 m/s (high)
• In the throat, the droplet accelerates and due to velocity difference between the
particles and droplets, the particles get collected
• Particle collection continues in diverging section also
• The gas-liquid mixture is diverted to a cyclone seperator wherein the particles are
further collected
• Mechanism of collection are:
- Inertial impaction
- Diffusion
- Condensation and agglomeration
• Used in many applications viz. fertilizer, paint, paper industry
Clton 9<rt ou<
i
SlMffy ou*
Schematic of venturi scrubber
Control of Gaseous Emissions
Various methods for the gaseous emission control are :
• Absorption
• Adsorption
• Combustion
• Catalytic oxidation

16. Absorption by liquids
These are used for controlling concentration of gaseous pollutants before
being discharged into the air. It involves transfer of pollutants from gas to liquid
phase across the interface in response to concentration gradient with the
concentration decreasing in the direction of mass transfer. The rate of mass
transfer is controlled by diffusion through the phases on each side of the
interface. The selection of scrubbing medium depends on scrubbing liquids with:
o High gas solubility
o Low vapour pressure
o Easy regeneration and recovery
o Low cost
o Low viscosity
o Non flammable and non toxic
o Chemical stability
o High selectivity between solute species
Adsorption on solids
These are used for odour control and for recovery of valuable organic substances
that cannot be scrubbed. The desorption process makes the adsorbent inactive
until it is regenerated. The rate of adsorption depends on :
o Concentration of material around the adsorbent
o Surface area of adsorbent
o Pore volume of adsorbent
o Temperature and chemical nature of adsorbent
Combustion
Combustion methods are used for controlling odours, toxic/reactive
hydrocarbons. The products of combustion are mostly harmless. However
higher molecular weight compound can cause smoke. The normal incineration
temp, range is 375-825 C with residence time of 0.2-0.5 sec and velocity of 4.5-
7.5 m/sec. Combustion methods include :
o Direct combustion (flares)
o Thermal incineration (after burners)
Catalytic oxidation
It is used when thermal incineration is not economical. In this technique,
combustion takes place on a catalyst usually platinum, palladium and their alloys.
These catalyst are arranged to provide maximum surface area and are coated in
suitable elements such as metal ribbons, ceramic rods or alumina pellets
256

17. f lame
Polluted
stream
Steam injection type flare
Fud go
Sc roll
E * hau5?
P o M u ' o n i
gOS
1
( T c n g c n t i r . l
» n l r y )
' l i l .
/ / / / / / / /
Flo Combustion chamber
i Minna
s / ss sys ss s J / / / s
H e t f a c t o r y l i n e d st««l «*all
Thermal incinerator

18. b I O V r
» o U v j ' e n t 4 .v.
Ochcot bur r«?i
Schematic of catalytic oxidation units
The selection of suitable control device for a specific application requires
consideration of particle size, its concentration, desired efficiency of collection,
space available and maintenance cost. A preliminary selection of these devices
can be made based on the secondary data available. This has to be followed by
comparative cost analysis which must include operating and maintenance cost
also. After a general selection, a detailed engineering design is made which is
followed by actual cost, operating efficiency and pressure drop estimation. Due
weightage to the environmental implication are given before taking final decision.
Alternatively a physical simulation and control technique through treatability
studies would provide better option for final selection. This would not only
provide better control but also lead to realistic cost estimation for techno-
economic control.
References :
1. Danielson, "Air Pollution Engineering Manual", 1990
2. C.S. Rao, "Environmental Pollution Control Engineering", 1995
3. G.J. Celenza "Designing air pollution control systems", Chemical Engineering
Progress, Vol. 66(11), (1970), p.p. 31
258