Air pollution control

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

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

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:


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.


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 A b s o r b e n t 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 f luride

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 f luo-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


( I . b 111111) of aikalised alumina (a mixture of alumi-nium oxide and sodium oxide) in a bed suspended in t h e s t r e a m . 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


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.

E X H A U S T 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-reaca\c 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 declarat ion. 2. Intake manifold vacuum breakers. 3 . Exhaust system vacuum breakers.


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..


ip—ssmemt of Emissions f rom Industr ies

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 fo r 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 f low.

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, C P H E R I & S O C L E N 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 pollut ion in cement industry" Proc. Symp. Air. pollution control Techniques, CPHERI & SOCLEN, Bombay (Sep. 1973).

4. "Repor t 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

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


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 d u s t 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.


- j

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 S 0 2 , 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-


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 bot tom with fly ash reinjection

Cyclone

Spreader stoker Without f ly ash reinjection with fly ash reinjection

All other stockers

7 .3 A 7 .8 A 6.OA

10.8A 0 .9A

6. OA 9 . 1 A

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


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 H 2 S0 4 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 Phospha te P lan ts 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 O f r ('000 metric tonnes)

PaO s 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 F 2 ) (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 P 2 0 5 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 T P D (b) Plants above 200 T P D (c) New plants upto 200 T P D (d) New plants above 200 T P D

Sulphur trioxide (a) Existing plants (b) New plants upto 200 T P D (W) New plants above 200 T P D

500 pig/NM3 4 kg/T of product

16 kg/T of 100% H 2 S 0 4 12 kg/T of 1 0 0 % H 2 S 0 4

12 kg/T of 100% H 2 S 0 4 4 kg/T of 100 % H 2 S 0 4

5 kg/T of 100% H 2 S 0 4 5 kg/T of 100% H 2 S 0 4

0 . 5 kg/T of 100% H 2 S 0 4

No standard No standard

25 mg/NM3 as Total F or 0 .12 kg f luoride/T of product or 0 .20 kgF/T of rock phosphate percent.

No standard No s tandard

150 mg/NM3 of particulate matter for granulation mixing grinding.

Single conversion of 10 kg/T of 100% H 2 S 0 4 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

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.


Environment

Air pollution control