This document discusses various methods for controlling air pollution from industrial sources. It describes controlling pollutants at their source through prevention, trapping, or altering pollutants before release. Common control methods for particulate pollutants include filtration, electrostatic precipitation, and wet scrubbers. Gaseous pollutants can be controlled through combustion, absorption, adsorption, or closed collection systems. The best approach is to prevent pollution at the source, but using equipment to destroy, alter or trap pollutants before emission is also effective.

1. Air Pollution Control
V. A. SASTRY,
Department of Chemical Engineering,
Indian Institute of Technology,
Madras.
INTRODUCTION
Regardless of the air pollution problem to be at-
tacked there are two fundamental approaches to con-
trol, (1) Contro! of the pollutant at the source so that
excessive amounts are not emitted to the atmosphere
and (2) Control by natural dilution of the pollutant in
atmosphere.
Control of the pollutant at the source may bo
accomplished by (1) preventing the pollutant from
coming into existence, (2) trapping, destroying or alter-
ing the pollutant that is emitted before it enters the
atmosphere. The best method would be to prevent
the pollution from coming into existence or, if this is
impossible, to keep the quantity to a minimum. De-
pending upon circumstances this may be achieved by
raw material change, process changes, operational
changes, modification of process equipment and more
efficient operation of existing equipment. If the
polhitan to cannot, be prevented from forming, equip-
ment which destroy, alter or trap the pollutant have to
be used.
The common methods used for reducing a pollu-
tant to tolerable levels before it is emitted from the
^tack include (1) destruction of the pollutants by use of
fire or catalytic burners (applicable only to those wastes
that arc combustible), (2) masking the pollutant (e.g.
odour masking by substances which give stronger
odour), (3) counteracting the pollutant (e.g. when two
antagonistic odour are intermingled, both odours are
diminished) and (4) collecting the pollutant from waste
stroam using collection equipment such as bag filters
cyclone scparaters. Scrubbers, electrostatic precipi-
tators, etc.
The best method of controlling air pollution is to
confine the contaminant at its source. If this is not
possible, the second alternative is to control the harm-
ful concentration of pollutants by natural dilution be-
fore it can reach the receptor. Methods of attempting
to accomplish natural dilution include (3) use of tall
stacks, (2) Community planning in which coming the
use of the air is adopted (air zoning) and (3) control
of the process technique according to meteorological
conditions. In employing tall stacks it is hoped that
the discharge is high enough to disperse the pollutants
into atmosphere without reaching the ground. Air
zoning involves community planning to prevent harmful
ground concentration from occurring within disigoated
areas. In the third method, manufacturing methods
are curtailed or completely shut down during period^
of adverse meteorological conditions.
CONTROL EQUIPMENT FOR PARTICULATE
EMISSIONS
Dust collection in general is based on the size,
shape, hygroscopic and electrical properties of the dust
particles. Dust particles evolving from known sources
and confined to well defined gas streams can be remov-
ed from a carrier gas by various collection device-.
These devices use one of the following mechanisms
(R 7):
1. Gravity Settling: The horizontal carrier gas velo-
city is reduced sufficiently to allow the particles to
settle by force of gravity.
2. Intertial Forces: By suddenly changing the direc-
tion of the gas flow, the greater momentum of
the particles causes them to depart from the gas
stream flow lines.
3. Filtration: Dust-laden gases pass through a porous
medium upon which dust particles are trapped,
leaving a cleaner gas to be discharged.
4. Electrostatic precipitation: Electrically charged
particles are attracted to objects- of an opposite-
charge.
5. Particles Conditioning: By causing intimate con-
tact of dust particles and water, a heavier water-
particle agglemarate is formed. This can be more
easily separated from, the gas stream by one of the
collection mechanisms
22 Industrial Safety C,./onic!f

2. Some of the equipment used for dust removal are
•ribcd briefly.
SETTLING CHAMBERS
Settling chamber is a type of dust collector which
in its relativity simple form consists only of an en-
largement of dust, where the gas velocity is decreased
to allow bigger particles to settle by gravity. This is
usually made as a rectangular chamber and is often
equipped with one or several intermediate walls to
change the direction of flow and thus also makes use
of the inertial forces.
CYCLONE SEPARATORS
In its simplest form, a cyclone collector consists
of a cylindrical shell fitted with a tangential inlet
through which the dust-laden gas enters, an axial exit
pipe for discharging the cleansed gas, a conical base,
and a hopper to facilitate the collection and removal
of dust. Dust-laden gas is swirled in the cylindrical and
conical section by admitting it tangentially at the peri-
phory. The gas proceeds downwards into the conical
section, forms another spiral upward within the down-
ward spiral and thence travels to the outlet. Particles,
which are thrown from the rotating streamlines and are
able to reach the walls of the cyclone, slide down to
the hopper. The collecting efficiency of a cyclone de-
pends, apart from the diameter, height and dimensions
of central pipe.
For certain applications where a high collecting
efficiency is desired and large gas volumes are involv-
ed, it has been proved to be economical to build to-
gether a large number of small diameter (about 150
mm) cyclones, to form a so-called multi-cyclone. In a
multi-cyclone, the two features of having a small dia-
meter to increase the centrifugal force, and a large
cross sectional area to maintain a low pressure drop,
are combined. The small diamensions of the cyclones
in a multi-cyclone permit them to be made of cast iron,
which makes them comparatively more suitable for col-
lection of abrasive dust.
The cyclone is the most universal equipment
avai'able for dust collection, but it cannot be used for
very fine fractions. For collecting dust particles of less
than 5 micron diameter at an efficiency of more than
90%, fabric filter, wet separators or electrostatic pre-
cipitators have to be used.
FIBROUS AND CLOTH FILTERS
Filteration is one of the oldest method of remov-
ing particulate matter from gases. Two types of filters
are in use. Fibrous or deep-bed filters, and cloth fil-
ters. In the deep-bed filters, a fibrous medium acts as
the separator and the collection takes ph in the in-
terestics of the bed. The efficiency of fibrous filters may
be improved by coating the fibrous with a viscous fluid,
such as a high flash point, low volatile oil. The re-
sulting unit is called a viscous filter.
Cloth filters a. _ used in the form of tubular bags
or as cloth envelopes pulled over a wire screen frame
like a pillow case. The most commonly used bag type
filter consists of cylindrical bags which are hung in a
frame work equipped with an automatic shaking device
for cleaning the bags. The open lower ends of the
bag, are connected to a dust hopper where also the
inlet of dusting air is located. The gas passes upwards
through the bags and the dust is collected on the in-
side of the same. The accumulation of dust increases
the air resistance of the filter and therefore it is neces-
sary to clean the bags regularly. Bag filters require
large space and investment and maintenance cost is
high. A relatively recent development in bag filters is
the self-cleaning reverse jot filter.
A wide variety of filter cloths like cotton, fabrics,
wool fabrics, synthetic fabrics, etc. are available com-
mercially. The greatest problem inherent in cloth filters
is rapture of cloth. The most extensive use of cloth
filters is in metallurgical industries, food and chemical
process industries in connection with grinding and dry-
ing operations. Maximum continuous operating tempe-
ratures reported for various filters and their chemical
resistance data are given in Tables 1 and 2 respec-
tively.
Table 1 : Maximum operating Temperatures Reported for various
Fabric Filter Media
Fabric Maximum onerating Temperature
* (.C)
Cotton
Wool
Vinyon
Nylon
Orion
Silicone covered glass cloth
Abestes
Dccron
80-90
100-1.15
90
90-110
120-175
250-350
350
175
•October-December, 19S7 23

3. Table 2 : Chemical Resistance of various filter Fabrics
Chemical Resistance
Fabric
Acid Alkali
•C'olton • • • . • • Poor Fiarlygood
Wool Good Poor
Vinyon Good Poor
Nylon . • • • • • Poor Good
Asbestec . . . . . • Poor Good
Orion Good Poor
The heat and chemical resistance of filter fabrics
.such as these used in bag filters have improved stea-
dily in the past decade through the use of such syn-
thetic materials as glass fibre.
ELECTROSTATIC PRECIPITATORS
When gas containing an aerosol is passed between
two electrodes that are electrically insulated from each
other and between which there is considerable differ-
ence in electrical potential, aerosol particles precipitate
on the low-potential electrode. Electrostatic precipita-
tion requires a discharge electrode (usually negative) of
small cross-sectional area such as a wire and a collect-
ing electrode (usually positive and at ground potential)
of large surface area such as a plate or a tube. Basi-
cally an electrostatic precipitator has four principal
parts:,. (1) a source of high voltage, (2) high voltage
ionising electrodes and collecting electrodes, (3) a
means for disposal of the collected material and (4) an
outer housing to form an enclosure around the elec-
trode. There are four steps involved in electrostatic pre-
cipitation: (1) electrically charging the particles by
ionisation, (2) transporting the charged particles by the
force exerted upon them in the electric field to a col-
lecting surface, (3) neutralising the electrically charged
particles precipitated on the collecting surface and
(4) removing the precipitated particles from the collect-
ing surface.
There are two broad classes of electrostatic preci-
pitators: (1) one-stage precipitators and (2) two-state
precipitators. The one-stage precipitators like wire-in-
tube type or wire-in-plate type combine ionisation arid
collection in a single step. In the two-stage electro-
static precipitator, there is a preionising step followed
by collection. It is generally unsuitable for dealing with
heavy dust concentration. Thus, it finds its principal
application in air conditioning plants.
Electrostatic precipitators find their use where'
(J) very high efficiencies are required for the removal
of fine materials, (2) volume of gases are very large,
(3) water availability and disposal are problems and
(4) valuable dry material is to be recovered.
The design factors of electrostatic precipitators
have been discussed by Schmidt and Flodin. Electro-
static precipitators are now being used in our country
for pollution control in cement plants, chemical indus-
tries, refineries, carbon black industry, etc.
The efficiency of electrostatic precipitators in col-
lecting fly ash in thermal power plants varies from 98
to 99.9%. In cement industries in India, the capital
plus running cost of electrostatic precipitators would
work out to approximately Rs. 4/- per tonne of dust
removed annually.
WET SCRUBBERS
In a scrubber, gas cleaning is done by injecting
water into a high velocity turbulent gas stream. The
high velocity area is created by either a ventury sec-
tion, an orifice plate or sprays. This turbulence serves
to break up the water into very fine droplets and to
trap the solid particles within the droplets. The final
collection is made by the separation of the water spray
from the gas stream,. The scrubbers in common use
in air pollution control include: (1) gravity spray tower,
(2) YClUUfi Scrubber, (3) disintegrator, (4) wet-type
dynamic precipitator, (5) wet impinger scrubber,
(6) collector with self induced sprays, (7) wet contri-
fugal scrubbers and (8) cyclone spray chambers.
A wet separator can, in practice be used for
cleaning operation for contaminants in any state (solid,
liquid or gaseous), at temperature upto 300°C or even
above. Generally wet scrubbers find use where (1) fine
particles must be removed at high efficiency, (2) cool-
ing is desired and moisture addition is not objection-
able, (3) gaseous contaminants as well as particulates
are involved, (4) gases to be treated are combustible.
(5) volumes arc relatively low and (6) large variation
in process flows must be accommodated.
SELECTION OF DUST COLLECTING
EQUIPMENT
The selection of a dust collector in an industry
involves many considerations. Some are subject to
scientific rationale and others are gained by experience.
Successful selection requires careful balancing and eva-
luation of the following factors:
24 Industrial Safety C,./onic!f

4. 1 . Particle Characteristics: Size distribution, shape,
density, stickiness, hygroscopity, electrical proper-
ties.
2. Carrier Gas Properties: Temperature, moisture
content, corrosiveness, flammability.
3. Process Factors: Clas (low rate, particle concentra-
tion, allowable pressure drop, continuous or inter-
mittent operation, desired efficiency, ultimate w;
disposal.
4. Economic Considerations: Installation cost, opera-
tion cost, maintenance cost.
The suggested minimum particle size ranges for
•different collecting equipment are shown in Table 3.
Table 3 : Ranges of minimum particle size for different collections
Type of Collection
Minimum Particle Size,
(m)
Settling chamber 100-200
Inertia! collector 50-200
Centrifugal collector 40-60
Cyclone (Small diameter) .. 20-30
Filter 0.5—2.0
Wet collector 1.0—2.0
Electrostatic precipitators 0.001—1.0
Through improved technology, good charging pro-
cedure and incorporation of the appropriate cleaning
equipment it is possible to reduce considerable air pol-
lution from different industries. Maintaining air pollu-
tion control equipment at designed efficiency requires
constant attention, ll is not unusual to find electro-
static precipitators that appear to be operating pro-
perly but are actually performing at 5-10% below de-
sign efficiency because the operating conditions have
changed from the conditions used to design the equip-
ment.
CONTROL METHODS FOR GASEOUS
POLLUTANTS
The control of gaseous pollutants from stack gases
depends on their properties. The methods of control
include:
(1) combustion, (2) absorption, (3) adsorption,
(4) closed collection and recovery systems and
(5) masking and counter action (odours).
COMBUSTION
Combustion processes like flame combustion or
catalytic combustion can be utilised to greatest advant-
age when the gases or vapour to be controlled are
organic in nature. Equipment employing the principle
of flame combustion include (1) fume and vapour in-
cinerators, (2) after-burners and (3) flares, either steam
injection or venturi flare. The use of after-burners on
incinerators has been met with varying success depend-
ing on the kind of after-burner used and the type of
incinerator. Flare design should provide for smokeless
combustion of gases of variable composition and a
wide raage of flow rates. Venturi flares mix air with
the gas in the proper ratio prior to ignition to achicve
smokeless burning. Steam injection flares mix stream
with the stack gases as they reach the stack.
When the concentration of combustible portion of
gas stream is below flammable range and when lower
operating temperatures are desired, catalytic combus-
tion processes are used. Catalytic combustion process
is used with success for the control of effluent gases,
fumes and odours from refineries burning waste crack-
ing gases, phenolic-resin curing ovens, paint and ena-
mel baking ovens, coffee roasting processes, foundry
core baking ovens and chemical plants discharging
maleicand pathalic anhydrides. Gases and fumes con-
taining excessive amounts of particulate matter reduce
the effectiveness of catalytic combustion units due to
coating that forms on the catalyst.
ABSORPTION
In this process, effluent gases arc passed through
absorbers (scrubbers) which contain liquid absorbents
that remove one or more of the pollutants in the gas
stream. The efficiency of this process depends on
(1) amount of surface contact between gas liquid,
(2) contact time, (3) concentration of absorbing
medium, and (4) speed of reaction between the absor-
bent and the gases. Absorbents are being used to re-
move sulphur dioxide, hydrogen sulphide, sulphur trio-
xides and fluorides and oxides of nitrogen. The absor-
bents may be either reactive or non-reactive with the
pollutant removed by them. Some of the reactive ab-
sorbents arc regenerative (i.e. they may be treated and "
reused), while others are of non-regenc. „tive type.
The equipment using the principle of absorption
for the removal of gaseous pollutants include (1) pack-
ed vver, (2) plate tower, (3) bubble-cap plate tower,
(4) spray tower and (5) liquid jet scrubber absorbers.
Selective chromatographic absorption of gases on small
pellets may offer much higher rates than those achiev-
ed in packed towers.
•October-December, 19S7 25

5. The absorbents commonly used for different gases
is given in Table 4.
Table 4 : The Common Absorbing Solutions used for Removing
Different Gaseous Pollutants from Gas Streams
Gaseous Pollutant Common Absorbent used in Solution
Form
Sulphur dioxide
Hydrogen sulphide
Hydrogen fluoride
Oxides of nitrogen
Dimethylaniline, jniKtu.ro of xylidine
and water ( 1 : 1 ) ammonium sulphite,
basic aluminum sulphate, cthanol
amines (monoclhenol amine, dicthano!
ajnine, methyl dictlutnol amine or
iryothanol amine), sodium sulphite,
ammonium sulphite and bisulphite,
water, alkaline water, a suspension
of calcium hydroixde, calcium sulphite
calcium sulphate, barium thionates
and sulphates.
Sodium hydroxide and phenol mix
(mole ratio 3 : 2) tripotassium phos-
phate, sodium alajnine or potassium
dimethyl glycine, otluiriolamines, soda
ash solution containing suspended
iron oxide or hybroxide, soda ash
alone, sodium thioaisonate, ammo-
niacal liquor from coke ovens.
Water, sodium hydroxide.
Water, aquous nitric acid.
Packed tower consists of a vertical shell, filled
with a suitable packing material and liquid flows over
the surface of the packing in this films. The efficiency
of packing towers are being improved in recent years
by use of new kinds of packing materials. Plate tower
consist of a vertical shell in which are mounted a large
number of equally spaced circular perforated plates:
gases and vapours bubble upward through the liquid
seal above each plate. Bubble-cap plate tower consists
of a vertical shell in which are mounted a large num-
ber of equally spaced circular bubble-cap plates. In
spray tower the absorbing liquid is sprayed through the
gas. By applying centrifugal force and the liquid spray
to the gas path at the same time, maximum contact
of gas and liquid is possible. In the liquid jet scrubber,
the absorbing liquid enters the equipment under pres-
sure through the top and vapours and gases are let in
the upper side. Pratt and Rutherford have described the
design and operation of a spray scrubber used to re-
duce the hydrogen sulphide from a rayon plant.
ABSORPTION
In this process the effluent gases are passed
through absorbers which contain solids of porous struc-
ture. The commonly used absorbers include active car-
bon, silica gel, activated alumina, lithium chloride, acti-
vated bauxite, etc. Active carbon appears to be the
absorbent most suitable for recovering organic solvent
vapours. The steps necessary for effective removal of
gaseous pollutants by absorbents are: (1) contact of
the gaseous or vaporous pollutant with the solid ab-
sorbent, (2) separation (deserption) of the absorbed
gaseous pollutant from the solid absor- ;nt by regene-
ration or replacement of the absorbent and (3) recovery
of the gases for the final disposal. The efficiency ol" re-
moval of gases by absorbents depends on (1) the phy-
sical and chemical characteristics of the absorbent in-
cluding the surface area per gram of absorbent and
(2) the concentration and nature of gas to be absorbed.
Desorption is accomplished by raising the temperature
of the granular bed above the uoiling temperature of
the pollutant by superheated steam, submerged heating
elements or combustion gases. Desorption may also be
performed by reducing the pressure. The absorbents
commonly used for removal of different gases are
:,'n in Table 5.
Table 5 : The Common Absorbents used for Removing Different
Gaseous Pollutants from Gas Streams
Gaseous
Pollutant
Adsorbents used in solid form
Sulphur dioxide
Hydrogen sulphide
Hydrogen fluride
Oxides of nitrogen
Organic solvent vapours Active carbon.
Pulverised limestone or dolomite, alka-
lisod alumina (aluminium oxide plus
sodium oxide)
Iron oxide
lumpline stone, porous sodium fluo-
ride pellets
Silica gel
CLOSED CIRCUIT AND RECOVERY SYSTEMS
Gases like sulphur dioxide, oxides of nitrogen and
hydrocarbons can be recovered from the waste gas
streams if they are present in sufficient concentrations.
For example, where the concentration is of the order
of 5 to 10% sulphur dioxide, as in smelter gases, the
sulphur content may be recovered economically. The
most usual method at smelters is to use the sulphur
dioxide stream as the raw material for the manufac-
ture of sulphuric acid. Similarly the vapour-recovery
methods used in refiners are useful when the concen-
tration of hydrocarbons in the effluent stream is high
and relatively uncontaminated.
Oxides of nitrogen from waste gas streams in a
nitric acid plant are recovered using-commercial zeo-
lite. Oxides of nitrogen absorbed in the bed are re-
covered as enriched oxides of nitrogen and nitric acid
by regenerating the bed at elevated temperature with
hot air or steam.
In the allcalised alumina sorption process, oxides
of sulphur in the stack gas are absorbed on spheres
26 Industrial Safety C,./onic!f

6. (I.b 111111) of aikalised alumina (a mixture of alumi-
nium oxide and sodium oxide) in a bed suspended
in the stream. The oxides are t! removed f r o m
the spheres by reaction with a reducing gas containing
hydrogen and carbon monoxide, producing carbon dio-
xide and hydrogen sulphide. The hydrogen sulphide is
converted to elemental sulphur, which can be sold, and
the regenerated aikalised alumina is recycled. The pro-
cess would remove about 90% of the oxides of sul-
phur in the stack gas. On a 800 MVV power plant
burning coal of 3% sulphur content, it would produce
about 180 tons of sulphur per clay.
In another process known as wet lime process for
removing sulphur oxides from power plants, pulverised
limestone is injected into the boiler furnace, where the
heat drives off carbon dioxide, converting the calcium
carbonate to the reactive oxide form. The oxide then
reacts with the sulphur oxides to form solid sulphites
and sulphates. Some of the conversion takes place be-
fore the stack gas reaches the water scrubber, but most
of it takes place in the scrubber after the reactants
dissolve in the water. The resulting solids, as vvei! as
the fly ash removed in the scrubber, go to the settling
pond, and water from the settling pond is recycled to
the scrubber.
MASKING AND COUNTERACTION OF
ODOROUS GASES
Odour masking and odour counteraction are be-
coming extremely popular in odour control, because of
their effectiveness and comparatively low cost. Odour
masking is based on the principle that when two
odours are mixed, the stronger one will predominate.
Thus, when a sufficient amount of a pleasant odour
is mixed, with unpleasant one, the latter will become
unnoticeable by using perfumes like odonel, putrifac-
tive odours are masked.
Odour counteraction, on the other hand, is based
on the principle that certain pairs of odours, in ap-
propriate relative concentrations, are antagonistic. Thus,
when two odours are mixed the noticeability of each is
greatly diminished. Selection of the proper counter-
actant is more difficult than the selection of a mask-
ing agent. The application usually consists of spraying
on, over or about the odoriferous area by means of
calibrated atomising nozzles.
Odour masking on a commercial scale is a relative-
ly new development with the following possible applica-
tion routes; (1) spraying, vaporising or atomising the
selected odorant into air, (2) adding to a process
wherever possible, (3) adding to scrubbing liquors and
(4) spreading or floating on contaminated surfaces
without dilution.
By using perfumes like nitrobenseno, citronelia,
synthetic ro< pinotar, alpha cinnamic aldehyde,
cucalyptour citriedora, votivar oil, jasmine oil, etc.,
pleasant smells were imparted to leathers during pro-
cessing itself.
The methods for source control of odorous gases
include; (1) change of composition of process material
or removal of causative impurities, (2) drawing the
odorous air from working atmosphere by exhau;. --2S
and diluting and relatively clean air, (3) masking,
counteraction or sorptions of odorous gases in a suit-
able solvent or by absorption using active carbon, (4)
removal of odour bearing dusts by cyclone separate:
and (5) combustion of odorous compounds to odourless
non-objectionable products.
AIR POLLUTION FROM AUTOMOBILES
The three main types of automotive vehicles being
used in our country are (1) passenger cars powered by
four stroke gasoline engines, (2) motor cycles, scooters
and autorickshaws powered mostly by small two
stroke gasoline engine and (3) large buses and
trucks powered mostly by four stroke diesel engines.
Emissions from gasoline powered vehicles are generally
classified as (1) exhaust emissions, (2) crank-case-
emissions and (3) evaporative emissions. The amount
of pollutants, that an automobile emits depends on a -
number of factors, including the design and operation
(idle, acceleration, etc.). Of the hydrocarbons emitted
by a car with no controls, the exhaust gases account
for roughly 65%, evaporation from the fuel tank and
carburettor for roughly 15'% and blowby or crank-case
emission (gases that escape around the piston rings)
for about 20%. Carbon monoxide nitrogen oxides and
lead compounds are emitted almost exclusively in the
exhaust gases. Effect of engine operating conditions on
the- composition of auto exhaust is shown in Table 6.;
Table 6 : Effect of Engine Operating Conditions on the Composition
of Auio Exhaust
Idle Accelert - Cruising Decelera-
tion lion
Air-fuel ratio
Exhaust Analysis
CO %
No, ml/m3
Hydro carbons,
ml/m8
Unburn! Fuel
/^supplied fuel
(1 ml/pi3
—1 ppni)
11:1-12.5:1 11:1-13:113:1-15:1 11:1-12.5:1
4-6
10-50
0-6 1-4 - 2-4
100 -40000 1000-3000 10-50
500-1000 50-500 200-3C0 4000-1200
4-6 2-4 2-4 20-60
•October-December, 19S7
6

7. Diesel-powered vehicles create relatively minor poi-
Itilioit piobtnir. umipaicil to gasoline powwul which." .
The diesel engine exhausts only about a tenth of tire
amount of carbon monoxide exhausted by a gasoline
engine, although its hydrocarbon emissions may ap-
proach those of the gasoline engine Blowby is negli-
gible in the diesel, since the cylinders contain only air
on the compression stroke. Evaporative emissions arc
also low because the diesel uses a closed injection fuel
system and because the fuel is less volatile than gaso-
line. The major problems of diesel engine are smoke
and odour.
EXHAUST EMISSIONS
The important exhaust emissions from a gasoline
engine are carbon monoxide, unburnt hydrocarbons,
nitrogen oxides and particulates containing lead com-
pounds. These emissions vary with air-fuel ratio, spark
timings and the engine operating conditions.
To meet the exhaust emission standards for car-
bon monoxide and hydrocarbons, the automobile manu-
facturers have used two basic methods. The first is to
inject air into the exhaust manifold near the exhaust
valves, where exhaust gas temperature is highest, thus
inducing further oxidation of unoxidise or partially-
oxidised substances. The second basic method is to de-
sign cylinders and adjust the fuel-air ratio, spark tim-
ing and other variables to reduce the amounts of
hydrocarbons and carbon monoxide is the exhaust to
the point where air injection is not required.
Devices and methods to control hydrocarbon emis-
sions fall into three classes: (1) devices that modify
engine operating concisions such as intake manifold
vacuum breakers, carburation mixture improvers, th
tie retarders, etc. (2) devices that 'troat' exhaust gases
such as afterburners, catalytic converters, absorbers and
adsorbers and filters, (3) use of modified or alternate •
fuels.
CRANK CASE EMISSIONS
Crank case emissions consist of engine blowby
which leak past the piston mainly during the compres-
sion stroke, and of oil vapours generated into the
crank ease. The quantity of blowby depends on engine
design and condition and operating .conditions. Worn
out piston rings and cylinder liner may greatly in-
crease blowby. These gases mainly contain hydro-
carbons and aacount nearly 25% of the total hydro-
carbons emissions from a passenger car.
Emissions of hydrocarbons from the crank case
til atitoimibiH'?; I'itu Ik* liiigolv climimttpii by nosiiivr
crank case ventilation (PCV) system. These system^
recycle crank case ventilation air and blowly gases
the engine intake instead of venting them to the
atmosphere.
EVAPORATIVE EMISSIONS
Through a short term experiment ' ^termination
of Indian Im:'!.ute of Petroleum it has been estimates
that an average Indian passenger car would emi:
20 Kg of hydrocarbons through evaporation annually.
. . controlling evaporation of fuel from the carbure -
tor and fuel system, are being developed that store
fuel vapours in the crank case or in charcoal canister
that absorb hydrocarbons, for recycling to the engine
Evaporative emissions mig; also be dealt with b>
changing the properties of gasoline such us reducing
the volatility of. fuel and replacing the C, and - elo-
finic hydrocarbons in the fuel with the less-reacac
C< and C5 paraffine hydrocarbons. Mechanical -
can also be used to control evaporative emissions.
The panel on Electrically Powered Vehicles n
USA estimated that the systems used now to cenircl
carbon monoxide and hydrocarbon in autoe.hu'.:>;s - i -
$25 to $50 to the cost of the car. The p^ne'. s-1J
that it should become commercially feasible in the n o :
decade to reduce emissions from automobiles using in-
internal combustion es... ;e to 500 ml/'m3
(500 ppm
hydrocarbons, 0.5% carbon monoxide and 250 ml; sf
(250 ppm) nitrogen oxide. The systems used, t..e pin;"
estimated, might add $50 to $300 to the cost of the
car produced in 1975-1989.
CONTROL OF HYDROCARBON EMISSIONS
FROM AUTOMOBILES
Devices and methods ? control hydro, -boa emis-
sions fall into three classes.
1. Devices that modify engine operating cona -
tions.
2. Devices that treat exhaust gases.
3. Use of modified or alternate fuels.
Devices proposed for modifying engine operating
conditions, usually called induction devices have as th:!:
goal improvement of combustion during all or a por-
tion of the driving cycle. They may be generally classi-
fied as follows:
1. Fuel cut off during declaration.
2. Intake manifold vacuum breakers.
3 . Exhaust system vacuum breakers.
28 Industrial Safety C,./onic!f

8. 4. Throttle retarders.
5. Vacuum control throttle openers.
6. Carburetion mixture improvers.
Improved carburettors involving heating of the fuel
or fuelnair mixture to vaporise the fuel completely or
alternately, mechanical disporsion of fuel droplets to a
lire and stable aerosel are in use but they do not
marketly reduce the hydrocarbon content of exhaust.
The advantages of. devices to remove hydro-
carbons directly from exhaust gas is that the same de-
vice may be used for all phases of the operating cycle,
although the physical dimensions and the chemical
composition of the exhaust gas will vary from one phase
to another. The various devices proposed may be
classified as;
1. Afterburners
2. Catalytic converters
3. Liquid washing devices (absorbers)
4. Absorbers (porous solids)
5. Miscellaneous filters, condensers, and air
dilution devices.
The principle of after burners involves the igni-
tion and burning of the hydrocarbons in exhaust gas.
Two of the inherent problems of the after burners,
flame maintenence and difficulty of low temperature
ignition, are overcome by the catalytic convertor. The
most vexing problem faced by those working on the
catalyst problem is over coming, catalyst susceptibility
to lead compounds formed from the tetraethyl lead
used as an antiknock additive in fuels. Lead is a noto-
rious catalyst poison.
The liquids proposed for washing out pollutants
from exhaust gas include water, solutions of inorganic
substances such as potassium permanganate, dichro-
mate or perorcides and various organic solvents includ-
ing fuel oil. So far no system using this method is
commercialised.
The use of antiknock agents other than tetraethyl
lead has been tried. The compound, methyl cycle
pentadienyl manganese tricarbonyl is under test.
CONTROL OF OXIDES OF NITROGEN
Several methods for reducing the nitric oxide con-
tent of auto exhaust have been studied. The most ap-
pealing of these is catalytic decomposition of nitric
oxide between the exhaust valve and the end of the
tail pipe. Nitric oxide is not stable at atmospheric
temperature. The only reason it is present in exhaust
gases is that it forms at the high temperatures in the
engine cylinder and is quenched so rapidly as it leaves,
the cylinder that it does not have sufficient time to
decompose. It will, of course, eventually decompose at
atmospheric temperature, but the reaction rate under
these conditions is extremely slow. An obvious attack
is to maintain exhaust gases at a high temperature for
sufficient length of time to promote decomposition at
greater than atmospheric temperatures of a catalyst
could be found to further accelarate the reaction, it
could be incorporated in a suitable device that could
be installed in the automobile exhaust system.
Carbon monoxide remains in the exhaust if the
oxidation of Co to CO- is not complete. Generally this
is due to a lack of sufficient oxygen. After burners,
catalytic reactors etc. are used for CO oxidation, the
catalytic reactor or catalytic converter, can operate
either on rich or lean mixtures and operates at lower
temperatures than the thermal reactor. A catalytic de-
vice consists of the active catalyst deposited on a sup-
port system and place in a can that looks about the
muffler. General Motors has evaluated about 800 mate-
rials as possible catalyst. Platinum and Palladium are
possibilities for the oxidising catalyst.
For control of pollutants in diesel exhaust a
variety of after burners, both catalytic and direct flame,
have been used to reduce hydrocarbons, aldehydes,
carbon monoxide, smoke, hydrogen and other combus-
tibles. The biggest problem here is the low temperature
and low combustible concentration of the exhaust. Both
factors limits the effectiveness of any practical device.
The solutions to the automobile exhaust is not yet
found. It is apparent that the most probable solution
will be complete oxidation of exhaust hydrocarbons,
either catalytically or by direct flame, or the decom-
position of nitric oxide, or both.
SMOKE CONTROL FROM DIESEL ENGINES
The following remedial measure have been con-
sidered to reduce smoke and considerable success has.
been achieved.
1. Good maintenance of injective system.
2. Improved combustion process brought about
by
(a) Carburation of a lighter supplementary
fuel
(b) Fumigation of a part of the diesel fuel.
3. Modification of the combustion chamber de-
sign.
4. Derating the engine.
5. Use of smoke supprosent additives like
barium based and manganese based additives..
•October-December, 19S7 29

9. ip—ssmemt of Emissions from Industries
The emission from industries are usually assessed
^ following methods (a) material balance, (b) using
• emission factors and (c) carrying out stack sampling,
r The first two methods give the theoritically possible
r emission and the third one measures the actual emis-
sions coming out of any industry.
From input and output quantities following mate-
rial balance calculations, the emissions can be assess-
ed. Emisision factor is a statistical average of the
mass of pollutant emitted from each source of pollu-
tion per unit quantity of material handled, processed
or burnt. By using emission factor for the specific
process, one can calculate the total emission of diffe-
rent pollutants by knowing the quantity of material
manufactured, processed or burnt.
The purpose of stack sampling is to determine
the actual quantity and types of pollutants that are
contained in the gases emitted from a source. The
purpose of stack sampling survey is (a) to provide
basic data for the design of air pollution control
equipment, (b) to check the performance of control
equipment, to determine the compliance or otherwise
of emissions with emission standards or norms and
(d) to determine the emission factors for use in the
compilation of emission inventories.
The dust in a gas steam is usually collected in a
filtering media which allows the gas to pass through
and retains the dust upt'o a certain minimum size.
The dust can also be collected through impingement
by bubbling through water. The selection of trapping
device depends on many parameters, namely, the tem-
perature and pressure encountered, the moisture con-
tent of the gas, the physical and chemical properties
of the dust and the gas stream to be sampled. The
different types of trapping media used in collection of
dust samples from stack gases bubblers and their
characteristics are shown in Table 12.
Table 12 : Characteristics of Trapping Media used in (he Collec-
tion of Dust samples from Gaseous Streams
Trapping medium Characteristics of the medium
Alundum thimble Resistant to temperature upto 540°C
and high moisture contents; suitable for
high dust loading.
Paper thimble Suitable for temperature upto 120°C,
low moisture contents and high dust
loading.
Fibre glass filters Suitable for high dust loading; higlv
collection efficiency.
Membrane filters ±tigh collection efficiency; low resis-
tance to gas flow.
Bubblers Dust not suitable in water; resistant to
corrosion.
REFERENCES
1. Desai, H. B. "Air pollution control technology in petroleum,
refineries" Proc. Symp. Air. Pol. Control• Techniques, CLI,
CPHERI & SOCLEN Bombay (Sep. 1973).
2. Sinha, J. K. "Pollution from cement industry" Proc. 3rd
Cement Industry Operation Seminar, New Delhi (1973).
3. Engineer, N. B. and Doshi, V. C. "Air pollution in cement
industry" Proc. Symp. Air. pollution control Techniques,
CPHERI & SOCLEN, Bombay (Sep. 1973).
4. "Report of the sub-committee appointed by the panel on
cement industry" Cement Industry Assn. Bombay (1973).
5. Mathura, H. B., Ja, G. S. and Bakshi, R. K. "Control of
particulate emissions from iron and steel industry" Proc.
Symp. Air pollution Control Techniques, CLI, CPHERI &
SCCLEN, Boirbay (Sep. 1973).
6. Anon "Industrial plants and stations show progress in pollu-
tion control" power, 114, 27 (1970).
J uty-September, 1987 31

10. Pollution Problems from Different Industries
DR. C. A. SASTRY,
Professor and Head,
Centre for Bio-Sciences &
Bio-Technology, I.I.T.
Madras.
Introduction
Even though there are many ditfcrent sources
-which contribute to air pollution, industries contri-
bute a major share. There are a number of indus-
tries like cement factories, petroleum refineries, iron
& steel industry, non-ferrous metal industries, thermal
power plants, fertilizer industry, inorganic and organic
chemical industries, and pulp and paper industries etc.
which are responsible lor air pollution. Industrial
sources generate a range of air pollutants specific to
the process involved.
Air pollution sources are divided for convenience
into two classes, (a) specific and (b) multiple sources.
Specific sources are largely industrial in nature, thus
permitting their potential to pollute a community
atmosphere to be readily assayed on an-industry-by-
industry (source-by-source) basis. They are fixed and
commonly occupy a limited area relative to the com-
munity. Multiple sources are those which cannot be
assayed practically on a source-by-source basis
e.g. combustion of fuels in stationary sources,
combustion of fuel for power production, for trans-
portation and domestic purposes, etc. incineration of
solid wastes, evaporation of petroleum products and
odour sources come under multiple sources.
Information on emissions associated with different
industries is given in Table 1.
Petroleum Refineries
Depending on the size and complexity of the re-
finery, the number and type of units could vary con-
siderably from one to another. Some of the common
processes that one would come across in a medium
sized refinery, are high vacuum distilation unit for
preparation of cracking and bitumen food stocks,
catalytic cracking, thermal cracking, catalytic reform-
ing, asphalt blowing and acid/caustic treating. Modern
refineries have hydrosulphurisors. The pollutants
commonly found in petroleum refineries include sul-
phur dioxide, hydrocarbons, carbon monoxide,
odorous materials, particulate matter.
Information on potential sources of pollutants
from petroleum refining is given in Table 2.
The characteristics of substances found in refinery
emissions depend upon the types of crude processed
and the complexities of the refineries. In general,
the estimated daily emissions (without rigourous con-
trols, from a refinery processing 10,000 tonnes of
crude per day is shown (1) in Table 3).
Table 1 : Air Pollution Problems from some Typical Industrial and other sources
Sources
Besides smoke, sulphur dioxide, oxides of nitrogen and fly-ash, the
following specific pollutants may also be found
Fertiliser indus try and aluminium manufacturing plants
Heavy chemical industry like acid plants, synthetic fibre, etc.
Lead casting and melting, pigments, etc.
Tanneries and leather industry
Cement industry
Paints, pigments and dye industry
Carbon black manufacture
Coal tar industry
Paper and paper products
Refinery and pelro-chejnical industry
Metallurgical industry
Electrolytic manufacture of chlorine
Coal burning (power plants)
Vehicle emission
(a) Petrol
(b) Diesel
Hydrogen fluoride, ammonia, fluorides, fertiliser dust and sulphuric
acid mist.
Acid fumes.
Tin, lead, etc. fumes and oxides solvents and thinners.
Mercaptans and sulphides
Cement and lime dust
Nitrobenzene and aniline, thinners, solvents and base material
Polynuclear hydrocarbons, carbon soot and hydrogen sulphide
Polynuclear hydrocarbons and aerosols of tar
Hydrogen sulphide and mercaptans
Hydrogen sulphide, hydrocarbons, odours of mercaptans
Metallic fumes, dust
Chlorine
Soot
Hydrocarbons, H C H O
Hydrocarbons, H C H O
J uty-September, 1987 25

11. Table 2 : Potential Sources of Pollutants in a Petroleum Refinery
; of emission Potential source
Hydrocarbons
Sulphur oxides
Carbon monoxide
Nitrogen oxides
Particulate matter
Odours
Aldehydes
Ammonia
Air blowing, barometric condensers, blind changing, blow-down system, boilers, catalyst regeneratorss
compressors, cooling towers, decoking operations, flare, heaters, incinerators, loading facilities, pro-
cessing vessels, pumps, sampling operations, tanks turn around operations, vacuum jets, effluent-
handling equipment.
Boilers, catalyst regenerators, decoking operations, flares, heaters, incinerators, treaters, acid sludge
disposal.
Catalyst regenerators, compressor engines, coking operations, incinerators.
Boilers, catalyst regenerators, compressor engines, flares.
Boilers, catalyst regenerators coking operations, heaters, incinerators.
Air blowing, barometric condensers, drains process vessels, steam blowing tanks, treators, effluent
handling equipment.
Catalyst regenerators, compressor engines.
Catalyst regenerators.
However with adequate controls the levels of the
above emissions could be brought down to reasonable
values. In the Esso refinery at Bombay, reductions
have been achieved as given (1) in Table 4.
Table 3 : Estimated Daily Emissions from A Refinery Processing
10 kt/d
(without rigorous controls)
Pollutant Estimated range of emissions
t/d
Carbon monoxide
Sulphur dioxide
Sulphur trioxide
Hydrocarbons
Particulate matter
Oxides of nitrogen
Ammonia, aldehydes, organic
acids and aerosols
40—120
30—90
less than 2
30—60
3—10
1—3
less than 1
Table 4 : Quantity of Pollutants Emitted from the Esso Refinery
(with controls)
Pollutant Range of emission?, t/d
Carbon monoxide
Sulphur dioxide
Hydrocarbons
Particulate matter
20—30
10—20
5—10
0.5—1.0
Cement Industries
Portland cement is manufactured from a suitable
mixture of limestone and clay, or from marls which
are first crushed and ground, either in the dry state
or with water. The raw mixture is thereafter burnt
at a sintering temperature and the clinker thus ob-
tained is ground to a fine powder with the addition
of gypsum to give cement. Cement is packed in jute
bags and despatched in this form in railway wagons
or trucks. Alternately, it is also despatched in bulk
as loose cement. Thus by the very nature of the
above processes, there is considerable generation of
dust, size of which ranges from 1 tolOO m and above.
The prevailing environmental conditions in our
country has been studied by Central Mining Institute,
Dhanbad and the concentration of air borne dust at
different operations in ten cement factories reported
(2). The findings of these studies are shown in Table
5.
Table 5 : Air Borne Dust Concentrations at Different Locations
in a Cement Factory
Operation/
location
Concentration of air borne dust
particles per mi-
Minimum Maximum Average
(1) (2) (3) (4)
Lime stone crushing 957 6,905 2,367
At the kiln firing end 110 1,596 580
At clinker cooler area 430 6,430 1,394
Around cemunt mill 146 3,267 1,214
Packing of cement 1,024 8,480 3,330
Loading of cement into wagon 3,670 18,020 6,723
Around coal crushing plant 771 4,180 1,843
Around coal drier 1,920 3,385 1,609
Around coal mill 325 4,000 1,769
General atmosphere within fac-
tory area 145 950 567
In front of office 35 918 181
The concentration of air borne dust at limestone
crushing, cement packing machines, around' wagons
during cement loading, and at the coal grinding and
coal drier areas was rather high, whereas the same
around the kiln firing end, clinker cooler and cement
mills was within the permissible limits.3
The question of fixing dust emission standards
for cement kilns has been engaging the attention of
various countries. In India a recommendation was
made4
that dust emission should be restricted to 200-
.300 mg/m3
in wet processing plants and to 300-340
mg/m3
in dry and semi dry process plants.
26 Industrial Safety C,./onic!f
- j

12. and Steel Industry
The steel industry is one of the major sources
air pollution. During the operation o an ffiXegf^.-
cJ steel plant many harmful materials are emitted
which are in the form of fumes, dust, smoke and
gases. The processes with high potential for air pollu-
tion are (1) metarial handling, (2) coke making and
(3) steel making. Raw materials used include, (a)
iron ore, (b) coal and (c ) lime and dolomite. AH
the materials handling operations like unloading of
raw materials, coal handling and washing are gene-
rally carried out in open air. The emissions from
stocking and handling of raw materials can be re-
duced to a great extent by the correct use of grabs,
covered tipplers and conveyors.5
Production of metal-
lurgical coke is essential for blast furnance operation,
as coke helps the reduction of iron ore and pig iron.
The very nature of coking process results in the emis-
sion of pollutants like smoke, grit and dust. The
rate of emission is highest during the charging opera-
tion, the actual quantity varying widely from plant to
plant depending upon the condition of the oven, the
type of coal used and the mode of operation at
each plant.
Refining of steel by oxygen generates copious
fumes containing very fine iron oxide particles. The
most commonly employed processes of steel-making
use either an open hearth furnace, oxygen-converter
furnace or an electric arc furnace. Fumes from
oxygen-converter furnace are more intense compared
to those from other furnaces. Oxygen converter fur-
naces using top blown oxygen process give out 8 to
12 kg of fumes per tonne of steel produced. The
potential sources of pollutants in iron and steel in-
dustry are shown in Table 6.
Table 6 : Potential Sources of Pollutants in iron and steel Industry
Pollutant Major source
Dust or particulates
Sulphur dioxide
Carbon monoxide
Acid fumes
Oxide fujnes
Oil and solvent fumes
•Odour
Heat
Material handling dolomitic plant, LD
converters, electric smelting furnaces,
electric are furnace, grinding equipment,
etc.
All stack gases from furnaces and boilers
LD Converters, electric furnaces and
other furnaces
Pickling tanks, acid regeneration plant and
battery room
Electric furnances, LD converters
Oil storage tanks, cold mills, painting
chambers of the maintenance shop
Pickling tanks, coke ovens, etc.
Furnaces, boilers, confined work areas,
work space near machines.
Gases such as sulphur dioxide, oxides of nitrogen
•etc. are also emitted from some of the above pro-
cesses. Various kinds of dust collectors and gas
cleaning equipment are being employed in steel mills
to suit different operations.
Non-Ferrous Metal Industries
The non-ferrous metal industries such as copper,
lead and aluminium are also major sources of air
pollution.
Copper sulphide is the major ore used for pro-
duction of copper. The ore is crushed, slurry is made
treated in flotation cells and the rich ore is sent to
the smelter. The rich ore is normally roasted in a
multiple hearth furnace to remove moisture, to burn
part of contained sulphur and to preheat the material
before changing into reverberatory furnace. The
emissions from this industry include dust and sulphur
dioxide of about 2 — 8% in the flue gases.
Lead is produced from lead sulphide and its
manufacturing method is more or less identical to
copper production. The major emissions are dust,
fumes and sulphur dioxide. During sintering the sul-
phur dioxide ranges between 1.5 to 5% in the emis-
sion. Dust load varies from 2 — 15 g/m3
during
sintering, 5 — 15 g/m3
in blast furnace waste gases
and 3 — 20g/m3
in reverberatory furnace gas.
The aluminium metal is produced by electrolytic
reduction of alumina- Gases like CO, C02, HF,
CF4, SO;>, are liberated along with dust, alumina, etc.
The carbon monoxide is burnt to carbon dioxide.
The major pollutants emitted are gaseous fluorine com-
pounds, sulphur dioxide and fluoride particulates.
Fertilizer Industry
The major fertilisers made in India include phos-
phatic fertilisers, urea ammonium sulphate, ammonium
nitrate, nitrophosphates and combination of some of
these. The raw materials required (sulphuric acid,
nitric acid, phosphoric acid, ammonia, etc.) are made
in the industry itself. The potential sources of pollu-
tants and the type of emissions from a fertiliser in-
dustry is shown in Table 7.
Table 7 : Sources of Pollutants from Fertiliser Industry
Potential source Type of emission
Sulphuric acid plant
Nitric acid plant
Phosphoric acid plant
Ammonia plant
Urea plant
Nitrophosphate plant
Ammonium nitrate
SO2, S 0 3 and acid mist
Oxides of nitrogen
Fluorides, phospheric acid mist
S02 , oxides of nitrogen, ammonia
Urea dust, ammonia
Fertiliser dust, ammonia, fluorides
Dust, ammonia
In the urea plant, there is possibility for leakage
of ammonia from various places. The various loca-
J uty-September, 1987 27

13. lions from which ammonia normally leaks in a total
recycle process include ammonia charge pumps, re-
covered solution charge pumps and recovery tower.
In the gasification section, the gas produced contains
46% carbon monoxide which is toxic. There is like-
lihood of carbon monoxide pollution of atmosphere if
any leaks develop in the system. Normally these leaks
are very nominal and the air samples do not indicate
presence of carbon monoxide.
Thermal Power Plants
Thermal power plants utilise fuel to produce
steam for power generation. The combustion of fuels
produce significant amount of air pollutant. The
types of pollutant depend on the nature of fuel used.
If coal is used, fly ash, sulphur dioxide, oxides of.
nitrogen are the major pollutants. In the case of fuel
oil sulphur dioxide, and oxides of nitrogen are major
pollutants emitted to the atmosphere. The amount
of fly ash and sulphur dioxide released depend on the
sulphur and ash content of the fuel used. Data on
particulate emission from coal fired boilers without air
pollution control is given6
in Table 8.
Table 8 : Particulate Emission from Coal Fired Boilers
(without rigorous pollution control)
Particulate in kg per t of
Types of furnaces coal burnt
Pulverised
General
Dry bottom
Wet bottom without fly ash reinjection
Wet bottom with fly ash reinjection
Cyclone
Spreader stoker
Without fly ash reinjection
with fly ash reinjection
All other stockers
7.3 A
7.8 A
6.OA
10.8A
0.9A
6. OA
9.1A
2.3 A
Note : A is multiplication factor representing % ash in coal
values represent mass of particulates reaching control
equipment used on this type of furnace; they are not
emissions.
The three major air pollutants from power station
are thus: particulate matter (fly ash and soot), sul-
phur oxides (SO^ and SOH) and oxides for nitrogen
(NO and NOu). Besides, these, there is possibility
of emission of carbon monoxide and unburnt carbon;
but in the modern thermal power stations with auto-
matic combustion control system, the formation of
these products is eliminated. Another pollutant from
coal fired stations is coal dust emission from the coal
handling plant.
Chemical Industries
The nature and quantity of air pollutants let out
by chemical industry will depend on number of factors
such as raw materials used, products made, processes
adopted and types of equipment used. Almost all the
pollutants are traced in the stack emissions from diffe-
rent chemical industries. The predominant ones are
oxides of sulphur and nitrogen, hydrogen sulphide and
fluoride, hydrocarbons and carbon monoxide (organo
chemical industries), mercury and chlorine gas (chlor-
alkali plants) and particulate matter.
The sources of different pollutants from chemical
industries are shown in Table 9.
Nature and quantity of pollutants discharged into
atmosphere by chemical industries depend upon raw
material, products, processes and equipment use.
Table 9 : Pollutants from Different Chemical Industries
Pollutant Source
Sulphur dioxide
Hydrogen sulphide
Oxides of nitrogen
Hydrogen fluoride
Carbon monoxide
Mercury and chlorine
Hydrocarbons
Particulates
Sulphuric acid plant, CS., plant, oil
refineries etc.
Viscose rayon, oil refinery, CS» plant,
dye manufacture, tanneries.
Nitric acid manufacture, explosive in-
dustry, automobiles.
Fertilisers, chemical, aluminium indus-
try.
Oil refinery, furnaces, automobiles.
Chloroalkali industries.
Organic chemical industry, refineries,
automobiles.
Mine quarries, pottery and ceramic,
power station, cement.
Absorbents and adsorbents like magnesium oxide
slurry, lime slurry, soda ash, ammonia alkalised
alumina, activated carbon, monoethanolamine are used
for removal of sulphur dioxide from stack gases from
chemical industries. Hydrogen sulphide is removed
by adsorption on iron oxides, absorption in liquid
caustic soda, combustion, catalytic conversion to sul-
phur or scrubbers. Oxides of nitrogen are removed
by adsorption, burning and catalytic combustion.
Sulphuric Acid Plants
Sulphuric acid is produced by burning sulphur
to sulphur dioxide, which is converted to sulphur
trioxide over vanadium pentoxide catalyst. The sul-
phur trioxide is then absorbed in towers with circulat-
ing sulphuric acid to yield 98.5% commercial grade
acid.
In sulphuric acid plants usually pollutants dis-
charged are S02 and acid mist. There will be occa-
sional gas leaks. In normally well operated plants gas
leaks can be avoided to a great extent. Frequent
fertilizers. The estimated production of P2O5 and
28 Industrial Safety C,./onic!f

14. downs of plant, due to power failure, low vol-
effcct not only the performance of the pant
also increase SOL, emission. Material fatigue on
tings and converter can cause leaks due to failure-
Normally the conversion efficiency of SCX to SO:i
by catalyst is 98 - -98.5%. By use of quench air
system, SO- discharge in the system can be reduced.
An absorption tower has to be operated with 98 to
99'i sulphuric acid. Any acid concentration beyond
this range of circulating acid strength induces thick
curdy white stack emissions. By proper control
absorption tower operation, acid mist can be controll-
ed.
A break-through in the process technology of
manufacturiing sulphuric acid was achieved in 70's.
Double catalyst, Double Absorption (DC DA) has
gradually replaced the earlier single context technology
on account of pollution control measure. The pro-
duction of sulphuric acid during 1985 in the country
is about 5 million tonnes. Capacity of more than
1.5 million tonnes of H2 S04 per year has been esta-
blished in recent years with DCDA system. AH new
plants in India will be based on DCDA processes
since it is not only economically viable but profitable
to use DCDA system for sulphuric acid plants with
capacity above 100 tonnes per day.
Super Phosphate Plants
Sulphuric acid production in India in recent years
is closely following the growth rate of phosphatic
sulphuric acid for the next three years is given in
Table 10.
Table 10 : Estimated Demand of Sulphuric Acid and P a Of r
('000 metric tonnes)
PaOs
HgSOt
1986-87
1676
5172
1987-8
1917
5416
1988-89
2209*
5548
1989-90
2247*
5898
* more nitro phosphates expected.
In a super phosphate plant rock phosphate is
ground in closed circuit grinding mills, to 80 — 85%
through 100 mesh. By using dust collector bags parti-
culates from this section can be controlled to per-
missable limits. Rock phosphate used contains
usually 3 — 4% fluorine of which about 25% is re-
leased during mixing operation with acid while the
remaining 75% is retained in single super phosphate.
At mixer exis the fluorine concentration will be about
5500 — 6000 mg/NM.3
Among the gaseous pollutants, SOs has done
more harm to the global environment than any other
single chemical present in the stack emissions of in-
dustrialised nations. An official estimate puts a figure
of about 5 million tonnes of SO. emissions in India
from all sources including power plants during 1985.
If converted to sulphuric acid this would mean an
acid of about 7.5 million tonnes.
A comparison of standards laid down by IS! and
central pollution control board for gaseous pollutants
for sulphuric acid and super phosphate plants is given
in Table 11.
Table 11 : Comparison of Standards Laid Down by ISI and Central Pollution Control— Board
ISI 8635-1977 MINAS* (1983-84)
1. Fluorine (as F2 )
(a) Phosphoric Acid plants
Existing
New
(b) S S P Plants
Existing
New
(c) T S P Plants
Existing
New
1.50 kg/T of P205
0.65 kg/T of P2Q5
0.50 kg/T of product
0.10 kg/T of product
0.3 kg/T of product
0.075 kg/T of product
2. Particulate matter when emitted through stacks
4.
(a) S S P Plants
(b) T S P Plants
Sulphur dioxide
(a) plans upto 200 TPD
(b) Plants above 200 T P D
(c) New plants upto 200 TPD
(d) New plants above 200 TPD
Sulphur trioxide
(a) Existing plants
(b) New plants upto 200 TPD
(W) New plants above 200 TPD
500 pig/NM3
4 kg/T of product
16 kg/T of 100% H2S04
12 kg/T of 1 0 0 % H 2 S 0 4
12 kg/T of 100% H2S04
4 kg/T of 100 % H2S04
5 kg/T of 100% H2S04
5 kg/T of 100% H2S04
0.5 kg/T of 100% H2S04
No standard
No standard
25 mg/NM3 as Total F or 0.12 kg fluoride/T of
product or 0.20 kgF/T of rock phosphate percent.
No standard
No standard
150 mg/NM3 of particulate matter for granulation
mixing grinding.
Single conversion of 10 kg/T of 100% H2S04
D C D A 4 kg/T of 100% H2SQ4
50 mg/NM3 or 0.01 kg/T of 100% H2SQ4
*Minijnu»n National Standards by Central Pollution Control Board.
July-September, 1987 29

15. Paper Industry
Odorous and particulates pollutants are emitted
from diiferent stages of pulp and paper making.
During digestion, some cellulose is demethylated which
reacts with sulfide to yield mercaptans and methyl
sulphide. Hydrogen sulphide may also be produced.
Build-up of head pressure in the digester is inter-
mittently relieved to the atmosphere, thereby contri-
buting small volume of volatile and turpentine com-
pounds. It is reported that Euclyptus pulping gives
out very small quantities of isopropyl mercaptan.
Digestion parameters like pressure, temperature, nature
•of wood, time and concentration of cooking materials
influence the quantities of pollutants discharged.
•Odorous noncondensable gases escape from blow heat
recovery system, unless collected and treated. From
pulp washing, some occluded volatile sulphur com-
pounds are lost from the residual black liquor and
usually exhausted through roof vents above washers.
•Considerable quantities of methyl mercaptans, methyl
sulphide and hydrogen sulphide leave multiple effect
•evaporators through barometric leg of the jet condenser
from chemical recovery section hydrogen sulphide,
methyl mercaptan, sulphur containing compounds and
non sulphur organic compounds are released in small
•concentration into the atmosphere usually recovery
furnances are provided with electrostatic precipitator.
Magnitude of loss of sulphur compounds from recovery
furnaces is estimated as "sulphidity". Emissions
from this source include hydrogen sulphide and mer-
captans. The dust concentration from the stack of
recovery boilers vary from 600 — 2000 mg/NM,3
sulphur dioxide concentration 60 — 150 mg/NM3
and
hydrogen sujphide 10:— 110 mg/NM.3
Mercaptans
in digester gas (intermittent discharge) vary from
200 — 2500 mg/NM.3
Methods used for control of
air pollution include black liquor oxidation, combus-
tion, chlorine oxidation, oxidation by air or ozone,
scrubbing, stripping, absorption etc. Ventury scrubber
and principle collectors used for collection of salt coke
from refurnace effluent gases.
Textile Industry
Emissions from textile processes excluding steam
.generation fall into four general categories (a) oil and
acid mi'st, (b) solvent vapours, (c) odours and (d)
dust and lint.
Oil mists are produced when textile materials
containing oils, plasticizer and other materials that
can voltalize or be thermally degraded into vol-.—
substances which are subjected to heat. Volan_e
matter is driven oil and is condensed on cooling into
a blue haze of droplets, most of which are in
range of 0.1 to 1 micron diameter. The most com-
mon source of oil mists in the textile industry is ix
tenter frame, because of the higher operating tempera-
ture which range from 125 — 150°C. Compound
in tenter exhausts are partially oxidised and; therefore,
more odourous and corrosive. Other processes pro-
ing oil mists include heat setting and drying Te.i_-
rizers produce the cleanest oil mists, consisting mainiy
of spinning oils.
Plastilizers are driven off from all high tempera-
ture processes involving vinyl, such as extrusion coat-
ing, tentering and calendering.
Acid mists are produced during the carbonizing
of wool and during some types of spray dyeing. Or-
ganic solvent vapours are released during and after all
solvent processing operations. Solvent dyeing and
printing and the application of finishes from solvent
solution create problems.
Odours are often associated with oil mists and
solvent vapours. In other cases odorant is present
mainly in vapour phase. The most common odour
problem of this type are the carrier odours from
aqueous polyester dyeing and processes subsequent
to it. Resin finishing also produces odours, chiefly
of formaldehyde. Other sources of odours are sulphur
dyeing on cotton, reducing or stripping dyes with
hydrosulphide, bonding, laminating, black coating,
bleaching with chlorine dioxide etc.
Dust and fly ash are produced during processing
of natural fibres and synthetic staple prior to and
during spinning, napping and carpet shearing.
To a lesser extent, most other textile processes
produce lint, which, while it is not a major pollutant
by itself, complicates abatement processes for other
pollutants.
Air pollution abatement technique in textile in-
dustry include (a) those that destroy the pollutants,
(b) those that collect the pollutant in a revolatively
concentrated dry form and (c) those that wash the
pollutants from exhaust gases into water or some
other collecting fluild.
30 Industrial Safety C,./onic!f