EE-II Unit - II

8/10/2019 EE-II Unit - II

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ENVIRONMENTAL ENGINEERING IIUnit II: Air Pollution Control Methods Particulate Control Devices General Methods of

Controlling Gaseous Emission

2.0. Air Pollution Control Methods:

The treatment facilities designed by the environmental engineer are based on the

principles of self-cleansing observed in nature; but the engineered processes amplify and

optimize the operations observed in nature to handle larger volumes of pollutants and to treat

them more rapidly. Therefore, it becomes necessary to understand the atmospheric self-

cleansing processes before studying the approaches to air pollution control.

2.1 Atmospheric Self-Cleansing processes:

The atmosphere, like a stream or river, has natural built-in-self-cleansing processes.

Dispersion, gravitational settling, flocculation, absorption, rainout, and adsorption are some of

the most significant natural removal mechanisms at work in the atmosphere.

Though not literally a removal mechanism, dispersion of pollutants by wind decreases

the concentrations of pollutants at any place. Gravitational settling is one of the most important

natural mechanisms for removing particulates from the atmosphere, especially particles larger

than 20m in size. It also plays an important role in several of the other natural atmospheric

cleansing processes. In flocculation, larger particles act as receptor for smaller ones to form a

unit, and the process is repeated until a small folc is formed, which is large and heavy enough

to settle under gravity. In the natural absorption process, particulates or gaseous pollutants are

collected in rain or mist and then settle-out with moisture. This phenomenon is known as

washout or scavenging, it takes place below cloud level. Rainout is another natural cleansing

process that occurs within the clouds, when sub-micron particulates serve as condensation

nuclei around which drops of water may form. This phenomenon has resulted in increased

rainfall and for formation in urban areas. Adsorption occurs primarily in the friction layer of the

atmosphere, i.e., the layer close to the earths surface, such as soil, rocks, leaves or blades of

grass, where they are concentrated and retained.

When the various natural atmospheric cleansing mechanisms are overwhelmed by

emissions, the effects of air pollution become increasingly more evident and requirement or

need is felt to establish control procedures or to install control devices.

2.2 Approaches to Air Pollution Control:

Basically, there are four approaches available for the control of emissions discharged into

the atmosphere. They are:


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i. Dispersion of source locations: Air pollution can be controlled/checked by dispersion of

the sources of air pollutants, through allocation of land, i.e., by proper planning and

zoning of industrial areas.

ii. Dilution: By using tall stacks for industries or thermal plants, the emissions or pollutants

can be discharged at a sufficient height from the ground, where the air movement, both

horizontal and vertical, is more and chances of downward movement of air (i.e.,

inversion conditions) are less. This will help in dispersion of pollutants over a larger area

in less time, and hence dilute the concentrations of pollutants near the source.

iii. Reduction at source by process changes: This can be achieved by:

a) Substitution of raw materials; e.g., the use of low-volatile coal in place of high-

volatile coal, eliminates smoke and soot.

Substitution of fuel; e.g., desulphurization and de-ashing reduce emissions of SO2,

SPM (suspended particulate matter) and ash. Similarly, natural gas can be used in

place of coal, to minimize emissions.b) Modification of the process; e.g., in case of disposal of combustible refuse,

sanitary landfill can be used instead of incinerators.

c) Modification of the process equipment, or repair and maintenance of existing

equipment helps in reducing atmospheric pollution.

iv. Reduction at source by using control equipment: This is the most effective method for

reducing air pollution at source. Various control devices are used for controlling different

types of pollutants emitted from different sources.

Since there are only a very few devices which are effective in the control of both

particulate and gaseous contaminants, therefore, the control devices are normally

designed to control either one or the other. In terms of volume of pollutants, the control

of gaseous air pollutants is of primary importance; however, in actual practice, greater

emphasis is toward the control of particulate contaminants, perhaps because they are

more easily seen. Therefore, control of particulate from stationary sources is discussed

first; while the control of gaseous contaminants from stationary sources is discussed

after.

2.3 Control Devices for particulate contaminants

The various types of control devices or equipments used for the removal of

particulate matter from stationary sources are briefly discussed as under:

Settling Chamber:

Of all the control equipment, gravity separator, commonly known as settling chamber

is the simplest and cheapest. However, today settling chamber is not used as standalone

control equipment. It cannot fulfill the present day emission norm because of its low


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efficiency. Settling chamber or baffle chamber, a modified form of settling chamber,

however, are often used as a Pre-collector placed upstream of high efficiency collector like

fabric filter or electrostatic precipitator.

A Pre-collection is used:

When dust loading is very high or it is required to separate the large particles before

the gas enters into main collector like fabric filter or electrostatic precipitator;

In applications where glowing particles are envisaged in the gas stream, baffle

chamber is used proceeding fabric filter to prevent glowing particles to enter into the

fabric filter, and this takes care of probable damage of bags from burning;

When the dust is abrasive it is often necessary to remove the coarser portion to

avoid damage to the bag material in a fabric filter; and

When there is a possibility that dust may contain foreign objects like nails, etc. which

may cause damage to the bag material.

Gravity collector can efficiency collect particles over 100m, Baffle chamber is

slightly more efficient than settling chamber. Since collection is solely dependent on

gravitational force the size of the collector becomes very big requiring large space.

Fig 2.1 Settling Chamber

Fig 2.2 Baffle Chamber


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Working Principle: Due to inertia, dust in a gas stream has a tendency to move in the

same direction of the gas stream. If the gas velocity is reduced substantially then the

dust is acted up on by the gravity and it settles down at the bottom. This principle is

applied in settling chamber. Cross sectional area of the chamber is several times more

than the dust area thereby velocity in the chamber is reduced substantially to a fifth or

sixth of dust velocity. Given a sufficient time dust settles at the bottom of the chamber

and is collected in the hopper below.

In case of baffle chamber a baffle is placed after the entry. Dusts impinge/impact

on the baffle and the gas flow changes direction. Baffle helps in collecting larger particles

on impaction. Change of direction also causes efficient dust to fall out. After the battle,

area again expands and velocity falls drastically. Combination of all three makes the

baffle chamber a little better efficient collector than settling chamber.

Parameters Affecting Efficiency of Settling Chamber: These include

Particle diameter: Efficiency varies directly with the square for the diameter.

Larger the diameter greater the collection.

Density of the particle: Higher the density higher is the efficiency. Temperature of the gas: Gas viscosity increases with increase in temperature.

Drag force is more at higher viscosity and thereby collection is less. The total

efficiency will be reduced to 66.953% from 73.789% due to temperature

increases from 200

C to 2000

C. Gas Velocity: Lower the velocity higher is the collection. Low velocity allows the

particles to settle quickly and also the re-entrainment loss is less.

Design Criteria for Settling Chamber: The design parameters required are:

Gas Velocity in the chamber < 2m/s Residence time = 2.4 to 3.0 seconds

Overall efficiency of the settling chamber = 65 75%

Pressure drop = NegligibleMerits: Various merits of settling chambers are:

Low initial cost Simple construction (in brick or stone masonry or concrete) Low maintenance cost

Dry and continuous disposal of solid particulates

Demerits: Various demerits of settling chambers are:

Large space requirement

Comparatively larger size (>10m) particles can be collected.


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Cyclone:

Efficiency of a dust separator is greatly enhanced if gravitational force. Principle

of separating dust by centrifugal force is used in cyclone. Cyclone is the most common

type of dust collector. There is no moving part and therefore it is almost maintenance

free. In spite of its simplicity cyclone has lost its yesteryears usefulness as control

equipment because of todays stringent pollution control norms. Therefore, today cycloneis not used as stand alone control equipment. It is used as an excellent pre-collector

preceding fabric filter.

Cyclone is effective in removing much smaller particles than settling chamber can

and occupies less space to handle the same gas volume. Cyclone can collect occupies

less space to handle the same gas volume. Cyclone can collect particle diameter down to

10m very effectively. Where dust loading is very high like in pneumatic conveying or

coarse particles are required to be separated before fabric filter or electrostaticprecipitator, cyclone is effectively used.

Fig 2.2 Cyclone


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Working Principle: Fig 2.2 shows the principle of separation of dust from the gas

stream in a cyclone. Gas enters the cyclone tangentially. Tangential entry imparts a

centrifugal force to the gas and causes it to spin. Particles are thrown outwards from the

spinning gas by the centrifugal force tot eh cyclone wall. Within the cyclone, incoming

gas spirals down and at the point close to the dust discharge, reverse towards the outlet,

creating a double vortex. Dust once reaches the wall loses its inertia and thus slides

down to the lower cone of the cyclone only to be discharged.

Performance Parameters of Cyclone: The Performance of cyclone depends on the

following parameter:

Particle size: Performance increases with increase of particle size.

Cyclone diameter: Performance decreases with increase in diameter.

Cyclone body length: A higher body length means more retention time for the

gas inside the cyclone and hence better efficiency. Ratio of cyclone diameter to cyclone outlet diameter: Performance is directly

proportional to the ration D/De.

No of gas revolutions: More the number of revolutions of gas better is the

performance. This necessitates longer cyclone.

Inlet velocity: Dust removal efficiency of cyclone increases with the increase of

gas flow, i.e. gas velocity to the extent when excessive turbulence is induced in

the cyclone. A velocity beyond 26m/s causes the efficiency to fall. In a practical

cyclone velocity ranging from 12m/s to 22m/s is considered to be suitable for goodefficiency.

Particle density: Cyclone efficiency is directly proportional to particle density. Gas viscosity: Increase of viscosity causes the fluid drag to increase and thus the

performance decreases.

Gas temperature: Temperature has a direct effect on the viscosity. Viscosity

increases with temperature which increases drag force and reduces the efficiency

of cyclone.

Inlet dust concentration: With the increase of inlet dust concentration theseparation in cyclone changes from individual particle collection to collection

separation. Even for a fine dust a moderate increase in dust concentration does

have a significant effect on separation. Cyclone, thus, is very effective in collecting

conveyed dust in pneumatic conveying system having very high dust

concentration.

Design Criteria for Cyclone separator: The design parameters required are:

Gas Velocity within cyclone = 12m/s

Particle size effectively removed = > 10m

Overall efficiency of the cyclone = 75 85%


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Pressure drop = 0.45 0.5 Kpa

Merits:

It has Low initial and running cost There are no moving parts and hence practically it has no maintenance

Has moderate pressure drop Low space requirements Can handle high dust loading and Has a possible use in high temperature and pressure

Demerits:

Problem with abrasive dust; Difficulty in removing light and fluffy dusts; Tendency of outlet clogging because of tickly and hydroscopic dusts; Clogging of outlet possibility due to dew point problem; and

Low collection efficiency for smaller particles less than 10m;

Multicyclone: Mutlicyclone is resorted to when gas volume to be handled is high and

number of parallel cyclones becomes unpractical or higher efficiency is in demand.

Multicyclone consists of a number of highly efficient small cyclones and therefore it offers

a higher efficiency than an unit cyclone of same capacity. Small cyclones often called

cells are arranged in arrays. Mutlicyclone finds its application as an excellent pre-

collector before fabric filter or electrostatic precipitator. It cannot be used as a stand

alone dust control equipment since it is unable to limit the emissions as per norms.

Cells are arranged in rows (array) and mounted to a plate which can be vertical,

horizontal or stepped depending on the design. Cell diameter varies from 150mm to

250mm. material of construction is generally alloy cast iron having hardness of 430

450 Brinell. Alloy cast iron gives wear resistant property. However for applications where

severe erosion is not expected, mild steel cells are also used.

Axial entry reverse flow cyclone cell is equipped with a fixed spinner designed to

give the incoming gas a spinning effect. This creates necessary centrifugal force within

the cell. Many times outlet recovery vanes are used to recover the rotational energy of

the exit gas. It is, however, not recommended to use where sticky dust is encountered.


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Fig 2.3 Multicyclone

Fabric Filters: Fabric filters, also known as Bag filter is most widely used among the

high performing dust collectors like electrostatic precipitator and scrubber because of its

simplicity, low initial cost and its achieving a very low outlet emission.

Working Principle: The principle of fabric filtration is conceived in a simple way. Dust

laden gas is drawn by a fan through a fabric either woven or non-woven. Dust in

suspension is arrested on the surface of the fabric and clean gas exhausted through the

fan and stacked into the atmosphere. In the process of filtration a dust cake layer is

formed on the fabric and the pressure drop increases across the fabric. If the pressure

drop increases to an extent that fan is incapable of drawing gas anymore, then the

filtration process becomes ineffective. To continue the filtration process, it becomes

necessary to remove the accumulated dust and a suitable cleaning method is employed

to clean the fabric. Once cleaned, the fabric is restored for the filtration cycle. To include,

the fabric filtration consists of the following aspects.


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(1) The filtration Process

(2) Fabrics and

(3) The cleaning of fabric

Fig 2.4 Principle of fabric filtration

Filtration Process: Fabric filter often captures particles much smaller than the fabric

pore size which shows that the mechanism of capture goes beyond simple sieving.

Capture of particles on a fabric element thus follows one or combination of the following

capture mechanism:

Inertial impaction

Direct interception Sieving Electrostatic attraction and Diffusion

Fabrics materials: The heart of fabric filter is fabric. Varieties of fabrics, natural like

cotton, wool, etc. or manmade like polyester, polypropylene, acrylic, polyamides,

aramide, PTFE et., woven or felted are available. Each has their merits for use for

specific purpose. The main purpose of the fabric is to capture particulates and create

dust layer on the surface of the fabric which acts as filter media to collect particles even

smaller than the equivalent pore size of fabric itself (25-50m).


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Table 2.1 Properties of Fabric Material:

F a b r i c t y p e

M a x

i m u m

o p e r a t i n g

t e m

p ( 0 C )

M a x i m

u m

m o

i s t

t e m

p ( 0 C )

P h y s i c a

l

r e s i s t a n c e

C h e m

i c a

l

r e s i s t a n c e

R e l a t i v e p

l a c e

A p p

l i c a t i o n

D r y

h e a t

M o

i s t

h e a t

A b r a s i o n

S h a

k i n g

F l e x

i n g

H y

d r o

l y s i s

M i n e r a

l a c i

d s

O r g a n

i c a c i

d s

a l k a

l i e s

O x

i d i z i n g

a g e n t s

O r g a n

i c

s o l v e n t s

Cotton 80 80 G G F G G - P G F F E - -

Polyster 150 100 G F G E E P G G F/G G E 1

Mining, cement, iron

and steel, wood,

ceramic, plastic and

pigment

Nylon

(polyamide)110 100 G G E E E - P F G F E -

-

Polyacrylic 125 120 F F F P/F G G G G G G G 1.6Asphalt, spray dryer,

lime, plastic

polypropylene 90 90 G F E E G E E E E G G 1

Food industry (milk

powder, sugar flour),

detergent

Nomex 190 170 E E E E E F P/F E G G E 5

Asphalt, iron, cement,

lime, metal, alloy

smelting, ceramic

Teflon 260 260 E E P/F G G E E E E E E 15

Carbon black, coal

fired boilers (stoker)refuse incineration

extreme chemical

Wool 90 105 F F G F G - F F P P F - -

Glass 260 260 E E P/F P/F F G E E G E E 2.5

Coal fired boilers

(FBC, PC), electro

smelting oven, cupolas

Polyphenylene

sulphide-ryton190 150 E E G G G E G G G P E 5

Coal fired boilers

(FBC, PC), chemical,

application at elevated

chemical attackPolyimide

P84140 - E - G G G F G G F - - 6

-

Ceramic-

Nextel 3121150 1150 E E - - - E E E G E E -

-

E=Excellent, G = Good, F = Fair, P = Poor

Principle of Fabric Cleaning: As dust start accumulating on the surface of the bag the

pressure drop across the bag also starts building up until it reaches a value which is

unacceptable for the operation of the plant. The fan in the system fails to extract any more gas


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and ceases to work. Therefore, cleaning of fabric becomes necessary at regular intervals to

regenerate the fabric for continuing the filtration process.

A cleaning mechanism is considered to be most effective when:

Energy requirement is low; There remains a residual pressure drop across the fabric or in other words a residual

layer of dust so that there is no appreciable reduction in efficiency of the cleaned fabric in

next cycle of filtering. A completely cleaned fabric is most inefficient.

The required time of cleaning is minimum; and The effect of cleaning on fabric like wear, etc. is minimum. Wear may happen due to

excess or poorly distributed cleaning energy.

Cleaning, basically, is resorted to in three ways:

Mechanical shaking Reverse air and Pulse jet

Mechanical Shaking Cleaning: The concept of mechanical shaking is to give vertical and

horizontal motion to the filter elements. The filter elements in the form of bags are hanged from

a top frame and fixed at the bottom. The frame is given a forward and backward or up and

down motion through an eccentric which is connected to a motor.

Figure 2.5 shows the principles of off-line cleaning for compartmentalized design with

mechanical shaking. The figure shows three compartments with mechanical shaking, each

having one shaking mechanism. One compartment in turn is shut for cleaning while the others

are under filtration. On receipt of an impulse from the timer (T) at the solenroid of pneumatic

cylinder, the damper (D) at the outlet of the chamber, taken under cleaning, closes. At the

same time solenoid of shaking motor receives the signal and the shaking motor (M) of the same

chamber starts. The dust is shaken off and drops into the bottom hopper. When the bags are

cleaned, after a pre-set time, the damper reopens for filtration and the next compartment is

taken into the same process of cleaning.

Fig 2.4 Off-line cleaning with mechanical shaking


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Reverse Air Cleaning Bag Collapse: Bag collapse reverse air cleaning is employed for

inside-to-outside filtration like mechanical shaking. Ambient air is made to pass through the

fabric in the opposite direction to the gas flow. Thereby the bag deflates or collapses

(Figure 2.6).

Fig 2.6 Reverse air cleaning-bag collapse type

Fig 2.7 Off-line cleaning with reverse air-bag collapse type

Pulse Jet Cleaning: In Pulse Cleaning, compressed air at a pressure varying between 2-

6kg/cm2 depending on the design and size of the pulse value, is used. A sort quick pulse of

air is applied to a row of bags through a solenoid operated diaphragm value.


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Fig 2.8 Low pressure pulse jet cleaning process

Parameters Affecting Fabric Filter Performance : Fabric filter is often called as constant

emission machine because of its immunity of variation in operating parameters. However

operating data from filters at elevated temperature show performance change due to change

in physical and chemical behavior of gas and dust.

The most significant parameter is dynamic viscosity. Dynamic viscosity increases with

higher temperature and leads to a higher pressure drop. Dust deposition rate is diminished

with higher viscosity. This leads to dust suspension for fine particulates which are re-

entrained resulting into higher pressure drop.

For an individual application the gas and dust parameters can affect the performance,

such as; (i) Temperature, (ii) Dew point and moisture, (iii) Particle size distribution,

(iv) Chemical composition of dust, and (v) Operating pressure of the system

To avoid corrosion, operating temperature must be at least 10 0C 15 0 C or to form may

lead to blinding of filter element. Since the condensation often takes place during start up

and shut down care must be taken not to invite condensation.

Density of gas decreases with increasing temperature. For a pulse jet filter pulse are

generally less effective in low density air.


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For an effective capture of dusts in low gas density higher filtering velocity is needed

which in turn makes the cleaning difficult.

Hygroscopic or deliquescent dusts attract moisture and agglomerates which affects the

effectiveness of cleaning.

Possibility of corrosion is increased when presence of moisture is associated with elevated

temperature. It becomes still acute when condensed water combines with acid forming gas

components.

Particle shape and size plays important role for re-entrainment and pressure drop.

Some dusts are cohesive in lower temperature but becomes tacky at higher

temperature.

Design Criteria for bag filters: The design parameters required are:

Rate of filtration = 50 60m/hour = 50 60 m 3 / m 2 Particle size effectively removed = > 0.5m Overall efficiency of the bag filters = 97 99% Pressure drop = 1.4 to 1.6 Kpa

Merits: High collection efficiency for particles less than 10m in diameter

Simple construction and operation Normal power consumption Collection of particulates in dry form Relatively low pressure drop

Demerits:

High maintenance and fabric replacement cost

Large size of equipment Operating limits are imposed by high temperature of carrier gas and high

humidity

High construction cost There application only to process temperatures generally below 285 0C.

Electrostatic Precipitators (ESPs):

Electrostatic precipitator is used to separate dust particles from flue gases is many

industrial processes. Due to its many-fold advantages like low pressure drop, low sensitivityto high temperature and aggressive gases, high collecting efficiency well in excess of 99%,


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and low maintenance, electrostatic precipitator has gained popularity over other dust

collectors.

An electrostatic precipitator, commonly called as ESP, is basically an electrical equipment

where DC voltage is imparted through an electrode creating an electrical field around it. Dust

particles carried by the gas while passing through the electrical field is charged to saturationand electrical force causes the charged particles to migrate towards the grounded electrode of

opposite polarity where they are deposited as dust layer. By suitable rapping mechanism dust

layer is dislodged.

Working Principle: The working principle of an ESP may be studies with a single stage

tubular model as shown in fig. Dirty gas enters from the bottom and flows upwards through

the electrical field generated by high voltage imparted to an electrode called discharge or

emitting electrode. Gas around the discharge electrode is ionized and the region is filled with

negative ions in the order of 10 9 ions/cm 3 . Dust particles get charged to saturation within a

few milliseconds immediately after entering into the ionized space. Amount of saturation

charge is proportional to the surface area of the particle and the strength of the electrical

field. Typically a particle diameter of 1m gets a saturation charge equivalent to 250

electrons. After being charged the particles migrate towards another electrode that is earthed

and called collecting electrode. Movement of particles is opposed by the viscous drag of the

medium and a resultant velocity called migration or drift velocity is attained by the particles.

For a typical population of particle diameter varying between 1m to 30m, migration velocity

may vary from about 0.03 to 0.5m/s. particles on reaching collecting electrode get deposited,

discharged and adhered to. Deposited dust cake is periodically dislodged by rapping collecting

electrode.

The principles of electrostatic precipitation have four distinct phases as follows:

Ionization or corona formation Charging of particles

Migration and precipitation of particles, and

Removal of deposited dust


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Fig 2.9 Single stage tubular model precipitator

Removal of Deposited Dust: Process of electrostatic precipitator of dust is considered to be

complete only when the deposited dust layer is removed effectively without impairing

precipitator performance. The dislodging of dust is most effective when the dust is removed

from the collecting plate in the form of cake and not in disintegrated form. With disintegration,dust is again dispersed in and carried by the gas. This causes the dust to penetrate though

the precipitator if not collected in the downstream. This phenomenon is known as

re-entrainment. It is therefore, of utmost importance to remove the dust layer by very

effective means which is physically done by rapping the collecting electrode curtain in case of

a dry precipitator. In case of wet precipitator collecting curtain is irrigated or flushed with

water and thus no re-entrainment occurs whereby almost 100% deposited dust is collected in

the form of slurry.

Rapping is a mechanical process by which acceleration both in plane and normal to

collecting plate is produced. Rapping is resorted to by means of energy giving mechanical or

electro-magnetic or hydraulic hammers which rams on an anvil or anvil beam connected to a

collecting plate curtain. Rapping or water flushing is also applied to discharge electrodes for

removing deposited dust on the same.

Effectiveness of removal of dust depends on the following:

The plate behavior due to rapping blow

The dust layer response Dust layer strength


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Fall phase of dislodged dust

Process parameters affecting ESP performance: According to the ESP efficiency,

migration velocity is a critical factor on which the size of ESP depends. Besides the particular

application process, migration velocity depends on factors like temperature, moisture content,

gas velocity, dust resistivity, and particle size. Dust Resistivity: For high resistivity dust, electrical field strength is very high in the

dust layer and it can cause back corona. Force with which the dust layer is adhered to in

this case is very high and thus is difficult to dislodge by rapping.

Gas Temperature: Gas temperature has a great influence on the ESP performance

either directly or indirectly through resistivity.

Moisture Content: Moisture content directly influences the voltage current

characteristics, i.e. at higher moisture content higher voltage can be impressed into the

ESP which will create stronger electrical field. Higher field strength helps better chargingof particles and consequently higher precipitation.

Particle size: Theoretically precipitator performance increase with increased particle

size because larger particles receive charges more quickly and saturation charging.

Design Criteria for Electrostatic Precipitator (ESP): The design parameters required are:

L/H ratio of plate = > 0.8 (L = Total Length of field and H = Height of the field)

H/B ratio of plate = < 2.0 (B = Breadth of field)

Gas velocity with in ESP = 0.4 1.2m/s

Maximum collection area per field = 3000 4000m 2

Collecting electrode spacing = 300mm

Current density = 0.35 0.5 ma/m 2

Efficiency of ESP = 99.0 99.8%

Power input = DC (rectifier which converts A.C. to D.C)

Merits:

Pressure drop and hence power requirement is small compared to the other

devices; economical and simple to operate 99%+ efficiency obtainable. Very small particles can be collected, wet or dry Can handle both gases and mists for high voltages volume flow

Few moving parts; can be operated at high temperatures and pressures

Demerits:

Relatively high initial cost and large space requirement Sensitive to variable particulate loadings or flow rates

Safeguarding operating personnel from high is necessary

Collection efficiency can deteriorate gradually


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Fig 2.10 particulate scrubber

These scrubber works on the same principle of the removal of particulate matter from

the atmosphere by the rain drop in the process of rain.

Collection Mechanism in Particulate Scrubber: There are various types of collection

mechanism like inertial impaction, interception, Brownian diffusion, electrostatic force, etc.

which are essentially the same as that in fabric filter. The inertial force by far is the most

predominant collection mechanism in the liquid atomizing zone. At lower velocities

electrostatic forces may be important. Brownain diffusion occurs when particles diameter is

less than 0.1m and relative velocity between the dust particles and droplets is small.

Spray Tower: Spray tower is the simplest type of scrubber. A set of nozzle bank with nozzles isused to produce liquid droplets. Droplets are allowed to fall due to gravity that interacts with

the countercurrent gas flow coming from below. Dusts carried by the gas are intercepted by the

liquid droplets. The predominant collection mechanisms are inertial impaction and interception.

Fig 2.11 Spray tower with multiple spray header for particulate collection


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Speed of the droplets must be greater than the gas velocity so as not to be entrained by

the gas and hence the droplet size has to be sufficiently large. Droplet size of about 1mm gives

a good result. Proper nozzle is selected accordingly for producing the correct droplet size.

Slurry that is generated after collection of dust is generally taken to a slurry tank and a

portion of water is re-circulated. In practice about 30-35% liquid is re-circulated. Re-circulation

has s risk that water carries some amount of dust, which may cause erosion, and clog the

nozzles. However it is believed that solid content in the scrubbing liquid reduces surface

tension, improves wetting characteristics resulting into better collection.

Design parameters:

Velocity of gas is recommended to be 0.6 1.2m/s

Liquid gas ratio may be considered as 3.5 4.0 litre / m 3 Collection efficiency that can be achieved in spray tower is not very high. Fig shows a

relation between outlet concentration and specific water consumption at various inlet

dust burden. The curves can be utilized for designing a spray tower.

Centrifugal Spray Scrubber: Like in cyclone, the collection efficiency is enhanced over

settling chamber; scrubber efficiency is also increased from that of simple spray tower by

employing centrifugal force. Velocity for both scrubbing liquid and gas is increased. Fig shows a

typical centrifugal scrubber where liquid is introduced from a centrally located header. Gasenters tangentially at the bottom of the scrubber giving spinning motion to the gas. Water is

directed towards the periphery of the cyclone casing and care is taken so that no core of liquid

is formed. To accomplish the same a core buster disc is provided. Because of centrifugal force

of the gas dusts are thrown away towards the wall where they impact with liquid droplets only

to be captured and collected.


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Reverberatory lead

furnace

Lead

compound

0.5-2.0+ 1.1-4.5 0.05-0.09 95-98

Rotary dryer Ammonium

nitrate

Large unstable

agglomerate

- - 99+

Superphosphate den

and mixer

Fluorine

compound

Mist 0.31 0.007 97.8

Air bodying of castor

oil

Castor oil Mist 0.006 0.0013 78

Impingement Scrubber: This type of scrubber employs the principle of impingement and self-

induced spray. The dust laden gas is made to impinge on to a pool of water and pass between

the lip of venture and the surface of water as shown in fig.

Fig 2.13 self induced or impingement scrubber

This is generally used for the separation of particulate matters generated from grinding

operation, foundries, coal handling plant, mining operation, etc.


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Design parameters:

This type of scrubber is basically a medium energy scrubber. Pressure drop across the

scrubber varies between 150 and 300mm WG

Impingement velocity may be 15m/s which creates a droplet size of 300-400m It can achieve collection efficiency as high as 99% even with particle size of 0.5m

Power requirement range between 1.0 and 1.3 kW/1000m 3 /hr of gas flow

Selection of Equipment

For selecting a particular collector from various equipments available, the following

factors must be taken into consideration:

i. Particulate size, shape and density

ii. Particulate loading, in mg/m 3

iii. Efficiency required

iv. Capital and operating cost of equipmentv. Ease of maintenance and reliability

vi. Flow rate and variations in flow rate of the carrier gas

vii. Specific properties of the contaminants like composition solubility, combustibility,

reactivity, toxicity etc.

viii. Properties of the carrier gas, i.e., composition, temperature, pressure, density,

viscosity, humidity, reactivity, combustibility etc.

Venturi s Scrubber:Venture Scrubber is the most efficient particulate scrubber amongst all and most

commonly used. Very high efficiency even for particle size of 0.5m is achieved by

venture scrubber. There are applications such as for sticky and corrosive dusts where

venture scrubber is the only solution over fabric filter or electrostatic precipitator.

In venture scrubber gas is allowed to pass through a narrow throat where water

is introduced. Water is disintegrated by the high velocity gas producing liquid droplets.

Energy alone for disintegration is coming from the velocity of the gas. The narrow throatis called venture section. It is preceded by a convergent section and succeeded by a

divergent section. The throat can be rectangular or circular in cross section depending on

the design.

Design Criteria for Venturi Scrubber: The design parameters required are:

Throat Velocity = 60m/s Liquid-gas ratio = 0.87 liters / m 3 of gas Pressure drop = 1400 - 1500mm W.G.


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Gas Absorption Scrubbers: In gas absorption scrubber, scrubbing liquid is employed to

absorb more or less soluble pollutant gas (Table 2.4). Gas absorption scrubber may be of two

types, (i) Spray Tower (Fig 2.11), and (ii) Packed Bed Tower

Spray tower: Spray absorption towers are used for removal of highly soluble gases such as

hydrochloric acid (HCl), Ammonia (NH 3), Hydrofluoric acid (HF) etc. Spray tower is not very

efficient one and as such is only used where inlet concentration of the toxic gas is very low and

high efficiency is not demanded or an inexpensive solution is preferred.

Design parameters:

Recommended velocity in the tower is 0.5 to 1.0m/s Recommended liquid gas ratio is 3.5 4.0 litre/m 3

Packed Bed Tower: When sparingly soluble gases like Sulphur dioxide(SO 2), Hydrogen

Sulphide(H 2S) etc are to be removed from the gas it is necessary to increase the contact area

between gas and the scrubbing liquid. One of the most convenient way to do this is to allow the

gas and liquid to come in contact counter currently through randomly packed bed. Packing of

various designs are commercially available for the purpose. Packed bed tower is also used for

highly soluble gases to achieve high removal efficiency. Gaseous pollutants like Acetic acid,

Alkaline fume, Ammonia, Amines, Chlorine, Chromic acid, Cyanide, Hydrochloric acid,

Hydrofluoric acid, Hydrogen Sulphide, Sulphur dioxide, Sulphuric acid mist can be removed

effectively by packed bed tower to the efficiency as high as 99.5%.

Fig 2.14 Gas Absorption packed bed tower


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The characteristics of packing material shall be:

Packing must provide good contact between the solute gas and the solvent liquid Packing must be chemically inert so as to not to react with the liquid or gas Packing must allow adequate passage of both gas and liquid without any hold up or

excessive pressure drop

Packing must be light weight so as not to burden the distribution plate and thereby

overall cost

Design parameters:

Velocity of gas through the tower is recommended to be 5-8ft/s (1.5 2.5m/s). Liquid-gas ratio is recommended to be 3.5 4.0 litre / m3 of gas flow or 2-8gpm/ft2

(80-325 lpm/m2) of tower cross sectional area

Pressure drop is recommended to be 15-150mm W.G/m of packed bed height

Maximum inlet concentration 5000 ppm by volume

Choice of control equipment with respect to particle size:

Fig 2.15 Particle diameter in micron

Table 2.3 Guidelines for selection of control equipment

Operatingcondition

Gasconditions

Dust characteristics Others

F l o w v a r i a t i o n

P r e s s u r e

d r o p

t e m p

1 5 0 C

T e m p

1 5 0 - 3

5 0 C

T e m p

3 5 0 0 C

C o m

b u s t

i b l e

C o r r o s i v e

O p .

t e m p n e a r

d e w p o

i r

H i g h e r v

i s c o s i t y

h y g r o s c o p

i c

A b r a s i v e

S t i c k y

F l u f f y

B u

i l d U p

D i f f i c u

l t t o w e t

A g g

l o m e r a t i n g

E x p

l o s i v e

> 1 0 p a r t

i c l e d i a m e t e r

1 - 1

0 p a r t


< 1 0 p a r t


H i g h r e s i s t

i v i t y

E l e c t r o s t a t i c

D r y c o

l l e c t

i o n

L o w

i n i t i a l c o s t

L o w o p e r a t i n g c o s t

Settling chamber + L + + + + + + - - + + X X

Dry cyclone / + H + + + + + + - - + + X X


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mutlicyclone

Wet cyclone + H + + + + - - X X

Fabric filter H + + + + + + - + + + + -

Dry electro static

precipitator+ L + + - + + + + + + - X +

Wet electrostatic

precipitators+ L + + + + + - + X X +

Particulate scrubber

- Low energy + H + + + - + X X +

- High energy+

V

H + + + - + X + -

= suitable; + = Special attention; - = Generally to be avoided; x = unsuitable; L = Low; H =

High; VH = Very High

Parameters affecting the Scrubber performance:

Particle size: Efficiency increases with larger particle size. Gas velocity at throat: Efficiency increases with increased velocity. Suggested velocities range between 50-180m/s. Pressure drop: Efficiency increases with increased pressure drop, the increase specially

rapid for pressure drop over 10W.G (250mm W.G). Inlet dust concentration: Efficiency decreases with decrease in inlet dust

concentration and increases with the increase. Increased inlet dust concentration has

little effect on pressure drop.

Nature of dust: A soluble or hydrophilic dust is easily attached to the water droplet

while hydrophobic dust is difficult to collect and thus requires more direct contact with

the droplets.

Water flow rate: low velocity with high water rate may give desired efficiency than

high velocity and low water flow. However, very low velocity is ineffective for

atomization. At low liquid-gas ratio i.e. below 2gal/1000cft, there is an increase in the

effectiveness of water.

Spray system: Liquid velocity and number of spray nozzles affect performance. For

optimum collection efficiency injection velocity of 20ft/s (6m/s) or below appears

desirable. Method or position of liquid introduction is important in so far as it prevents

channeling of gas and liquid. Pressure drop across the venture varies with the amount

and method of water injection.

Power input: High efficiency is associated with high power input. Power requirement

increases as particle size decreases.

Scrubber design: Efficiency has little relation with scrubber design.


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6. Molecular sieves (synthetic,

silicate or zeolite molecular

sieves)

For controlling and recovering Hg, SO2 and NOx

emissions

7. Silica gel For drying and purifying gases

8. Strontium sulphate For removing iron from caustic solutions

The equipments that contain the adsorbent solid through which the effluent gas must

pass are called adsorbers. They can be of fixed, moving or fluidized beds type. The container for

a simple fixed-bed adsorption unit can be vertical or horizontal cylindrical shell fig 2.15 shows in

multiple fixed bed adsorber, in which the adsorbent (say activated carbon) is arranged on beds

or trays in layers. When the adsorbent becomes saturated with adsorbate, then the adsorber

must be regenerated or renewed. If the collected gas can be easily desorbed, then the

adsorbent can be reused; otherwise, the absorbent has to be replaced regeneration of an

adsorbent can be accomplished by the use of superheated steam or circulating hot air. It should

be noted that the bed must be cooled before reuse.

Fig 2.16 Multiple fixed-bed adsorber

The recovery of ethyl alcohol vapors from a whisky warehouse and the recovery of

methyl chloroform from a movie-film processing plant may be economical. Organic vapors that

can be controlled by adsorption processes include those discarded from the following industrial

processes: solvent extracting, dry-cleaning, metal-foil coating, paint spraying, etc. emissions


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from chemical pharmaceutical, plastics, rubber, and many more manufacturing processes can

also controlled by adsorption.

Absorption:

A mass transfer in which a gaseous pollutant is dissolved in a liquid is known as

absorption.

Control devices based on the principle of absorption attempt to transfer the pollutant from a gas

to a liquid phase. This is a mass-transfer process in which the gas dissolves in the liquid. The

dissolution may or may not be accompanied by a reaction with an ingredient of the liquid. Mass

transfer is a diffusion process where in the pollutant gas moves from points of higher

concentration to points of lower concentration. The removal of the pollutant gas takes place in

three steps:

Diffusion of the pollutant gas to the surface of the liquid Transfer across the gas-liquid interface (dissolution) Diffusion of the dissolved gas away from the interface into the liquid

Structures such as spray chambers and towers or columns are two classes of devices

employed to absorb pollutant gases.

Fig 2.17 Absorption system


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Table 2.5 Absorption liquids for various pollutant gases

Pollutant gas Absorption liquids

Sulphurdioxide Sodium hydroxide, sodium sulphite, magnesium oxide, calcium carbonate,

calcium oxide and calcium hydroxide solutions.

Oxides of nitrogen Ammonium bicarbonate, ammonium bisulphate, calcium hydroxide,

magnesium hydroxide, and sodium hydroxide.

Hydrogen sulphide Sodium hydroxide, potassium hydroxide solutions.

Hydrogen chloride Water, ammonia, calcium and magnesium hydroxide solutions.

Chlorine Solutions of sodium hydroxide, sodium sulphite, sodium thiosulphite, and

water.

Phosgene Sodium hydroxide and water

Ammonia Sulphuric acid, nitric acid

Mercaptans Sodium hypochlorite solution

Condensation:

The process by which water changes from water changes from vapour state into the

liquid or solid state. It is the reverse of evaporation. In cases where pollutants have low vapour

pressures, condensation is effective for removing a significant part of the vapour. The

condensation process works on the principle of cooling the pollutant gas stream to a

temperature below the dew point. Organic vapours, hydrocarbons including solvents are

removed by condensation process. A typical condensation process applied for industrial organic

vapours purification is shown in fig 2.18.

Fig 2.18 Direct contact condenser used as a pollution control device


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Combustion:

Combustion is chemical oxidation accompanied by the generation of light and heat.

When the gaseous pollutants are oxidizable to an inert gas. Combustion is a possible

alternative method of control. Typically, Co and hydrocarbons methane gas, organic vapours

and hydrogen gas fall into this category. Both direct flame combustion by after burners and

catalytic combustion have been used in commercial applications.

Direct flame combustion: Direct flame combustion for any combustible gaseous pollutants

can be applied, if it has a heating value of above 900 Kcal/m3. A typical direct flame

combustion process is presented in fig 2.19.

Fig 2.19 Duct Type burner

Catalytic combustion:

Some catalytic materials enable oxidation of combustible gaseous pollutants below

900K Cal/m3. Platinum or palladium compounds are used on catalysts. Catalytic combustion

process is successfully employed to printing press, varnish cooking, and asphalt oxidation

emissions. A typical catalytic converter combustion process is shown in fig 2.20.

Fig 2.20 Catalytic Combustion

Sufficient oxygen is to be supplied in the form of air for conversion into inert gases.

The reactions of combustion are represented as below:


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Reactions:

2.5 REMOVAL OF SULPHUR DIOXIDE: The major source of Sulphurdioxide are

1. Sulphuric acid plants

2. Combustion of coal or fuel oil in boilers

3. Thermal power plants

4. Petro chemical units and oil refineries

5. Metallurgical industries such as sponge iron plants, blast furnaces, pellet plants, and

non-ferrous smelting plants

(i) From Sulphuric acid plants: The sulphuric acid plants release about 7 9% of SO 2

gas. Other impurities may be absent. SO 2 is removed by double-conversion and

double absorption. The sulphur dioxide is converted into usable product of sulphuric

acid. The sulphur dioxide removal process in sulphuric acid plants is presented in fig

2.21.

Fig 2.21 Double-conversion double absorption (DCDA) system in the manufacture of

sulphuric acid

(ii) Combustion of coal and Limestone mixtures in fluidized bed combustion or limestone

coal pellets as fuel may reduce the SO 2 emissions.

(iii) Desulphurization of Fuels: These may be accomplished by 1. Coal gasification, 2.

Coal liquefaction.


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a. Coal gasification: Coal gasification means any processes that are used to convert

solid coals to gaseous, fuels for cleaner combustion. Sulphur is recovered in this

process with hydrogen complexation followed by partial oxidation with oxygen.

b.

Coal Liquefaction: Coal Liquefaction is a process for changing coal into synthetic oil.It is similar to coal gasification. Two basic approaches for liquefaction are used. One

involves using a gasifier to convert coal to carbon monoxide, hydrogen, and methane,

followed by condensation to convert the gases to oils. The second approach uses a

solvent or slurry to liquefy pulverized coal and then processes this liquid into a heavy

fuel oil. Some processes produce both a synthetic gas and synthetic oil. Hydrogen is

used to convert sulfur in the coal to hydrogen sulfide gas. Hydrogen sulfide is partially

oxidized to form elemental sulfur and water. More than 85% of the sulfur is removed

from coal by liquefaction.

(iv) Flue gas desulphurization: Flue gas desulphurization (FGD) may be carried by Wet

scrubbing process or dry scrubbing process.

Dry flue gas desulphurization offers a cheaper solution since elaborate sludge handling

is absent. In this process flue gas is desulphurized before it enters into the particulate

collector. However pre-collector is often used for the collection of dry, clean fly ash when it

is meant to reuse like it cement-making. Dry/semidry process is very much suitable for

retrofit. It is also a preferred choice for new plants as a cheaper solution.

In this system flue gas enters into an absorber where calcium hydroxide Ca(OH) 2 slurry

is injected and the reaction takes place in liquid phase. Water is evaporated in the

absorber and dry product is forced into a collector. Calcium hydroxide or slaked lime

Ca(OH) 2 reacts with sulphur dioxide (SO 2) to form calcium sulphite (CaSO 3) and water.

Calcium sulphite (CaSO 3) reacting with Oxygen (O 2) transforms into Calcium sulphate

(CaSO 4).

The advantage of this process is that, it does not produce any waste water. Plant is

fairly simple and no waste water treatment plant is necessary. Part of collected product is

put back into the system due to its high absorbent capabilities.


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Fig 2.22 Dry / Semi dry FGD process

Wet Process: Wet Process generally installed after the particulate collector, consist of

counter current spray absorber having several levels of nozzle banks for injecting lime and

water.

Collected sludge is then passed through hydrocyclone / thickener to vacuum filter /

filter press for dewatering. Absorber may be with forced oxidation arrangement for

obtaining full formation of calcium sulphate, otherwise separate arrangement may also be

accomplished.

Lime or Limestone dissolves slowly in water and react with Sulphur dioxide (SO2) to

form calcium sulphate (CaSO4) or sukphite (CaSO3) in solid particles or crystals. Solids

are removed by thickening process and dewatered through vacuum filter to produce a

moist sludge having 10 to 15% water and 90 to 50% solid. The sludge then is mixed with

dry fly ash from preceding particulate collector and lime to make a stable land fill

compound.

If there is a market for gypsum, slurry from the absorber is oxidized in a forced

oxidation tank to convert all calcium sulphite (CaSO3) to calcium sulphate (CaSO4) in the

form of gypsum crystals. This commercial grade gypsum is the raw material for wall board

manufacture. Utilization of waste material is thus possible.


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Fig 2.23 Wet FGD process

(v) Dilution using Tall Stacks: Dilution by tall stacks will have to be widely practised

until direct control methods are employed for the removal of SO 2 . It is quite common

practice to curtail operations during adverse meteorological conditions so as to keep

the ground level concentration of SO 2 within acceptable limits.

For industries where SO 2 emission is estimated as w kg/hr then the stack height (h) is

calculates as:

H = 14 (w) 0.3 (Less than 200MW coal based thermal power plants)

H = 220m (200 to 500MW coal based thermal power plants)

H = 275m (500 MW and above coal based thermal power plants)

From structural point of view the height shall not be less than 20 30 times diameter

of the stack.

2.6 CONTROL OF OXIDES OF NITROGEN:

They include absorption in solutions; adsorption and catalytic oxidation improved

design of combustion engines / furnaces, as well as their proper maintenance and operation

also help reduce pollution due to NO x .

(i) Absorption: For absorption of gases containing NO x from boilers and process industries,

aqueous solutions of magnesium hydroxide, magnesium carbonate, calcium hydroxide

and ammonia, or their combinations, are used.


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(ii) Absorption in Calcium Hydroxide Solution: The gases containing nitrogen oxides are

absorbed in calcium hydroxide solution obtained from the slaking of lime.

NO + NO 2 N 2O3

N2O3 + Ca(OH) 2 Ca(NO 2) 2 + H 2O

4NO 2 + 2Ca(OH) 2 Ca(NO 3) 2 + Ca(NO 2) 2 + 2H 2 O

2H 2SO 4 + 3Ca(Na 2) 2 2CaSO 4 + 4NO + Ca(NO 3) 2 + 2H 2 O

Fig 2.24 Removal of No x by absorption system

(iii) Adsorption of NO x : For adsorption of NO x, NO is first oxidized to NO 2 which can be

adsorbed by silica gel, activated carbon, etc. The adsorbent is regenerated by heating

and NO2 evolved is reused for HNO3 manufacture.

Natural zeolites can be employed to remove NO x levels up to 200ppm. Their capacity

for removal is, however, limited to about 2.2kg / 100kg of zeoloite.

(iv) Catalytic reduction of NO x : NO x can be reduced using methane gas. In the first

stage NO 2 is reduced to NO. In the second stage all formed NO is reduced to elemental

nitrogen (N 2). The reactions are follows:

CH4 + 4NO 2 4NO + CO 2 + 2H 2O (first stage)

CH4 + 4NO CO 2 + 2H 2O + 2N 2 (second stage)


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Fig 2.25 Catalytic reduction of NO x using methane

(v) Selective catalytic Reduction: In SCR system, ammonia is injected in the gas stream

which reacts with nitrogen oxides at reasonably high temperature above 3000C in thepresence of a catalyst like active phase of vanadium pentoxide and tungsten trioxide on

a carrier of titanium to produce nitrogen and water.

8NH 3 + 6NO 2 7N 2 + 12H 2O

4NH 3 + 6NO 5N 2 + 6H 2O

Fig 2.26 SCR Process high-dust installation in boiler plant

(vi) Combustion Modification Process: Temperature, oxygen concentration and length

of time of combustion are important parameters for deciding the NOx levels in the flue

gases. NOx can be controlled by choosing proper operating conditions for the combustionprocess.


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(a) Recirculation of Flue gases: In this method a part of the flue gas is recirculated and

used for combustion. This makes the combustion deficient in oxygen with the result

that the peak combustion temperature is reduced. As much as 90-95 percent of NOx in

the exhaust gas is reduced.

(b) Low Excess Air Utilization: Less excess air is used here. This increases the N/O

ration and reduces the formation of NOx. Better control of combustion is necessary,

otherwise fuel is left unburnt and there is a possibility of CO formation. Burners using

gas or oil use less excess air for combustion.

(c) Two stage supply of Air: Here, about 90 percent of the stoichiometric requirement of

air is supplied to the burner. The rest of the air required for combustion is supplied at a

location above the burner flame. This reduces the NO x formation due to reduced flame

temperature and slightly reduced levels of oxygen where it matters most. Reductionsin the NOx concentration by 40 percent have been observed in gas-and oil-fired

furnaces by this modification.

(d) Tangential Firing: The burners here are located tangentially around the combustion

chamber radiating heat to a concentric cooling area. The peak temperature, as a

result, are reduced, thus reducing NOx emissions.

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