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Control of Specific Gaseous Pollutants

Dr.Vandana

1ET ZC362: Environmental Pollution ControlBITS Pilani

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Learning Objectives

Dry and wet techniques for desulphurization of flue gases

Control of NOx, CO, Hydrocarbons and pollutants from mobile sources

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Two classes of tech. by which gaseous pollutants may be

removed from an effluent gas

1.Sorption of pollutant

Absorption

Adsorption

2. Chemical alteration of the pollutant

combustioncatalytic treatment

Summary of Lecture 7

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Absorption : Involves the transfer of pollutant from gas phase to liquid phaseacross the interface. A two-resistance theory is used to explain this process, the

interface offers no resistance to mass transfer and the mass transfer rate between

the two phases is controlled by the rates of diffusion through the phases on each

side of the interface.

Adsorption : Surface phenomenon by which gas or liquid molecules are

captured & adhere to the surface of a solid. The molecules which are adsorbed on

the surface is called adsorbate and the substance on which an adsorbate is

adsorbed is called adsorbent.

eg. molecular sieves, activated C etc.

Two types: Physical adsorption and chemical adsorption

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Adsorption techniques are widely used in the field of odour control and also

used for collecting valuable organic substances that can not be picked by

scrubbing.

The rate of adsorption depends on the concentration of the material aroundthe adsorbent, the surface area of the adsorbent, the pore volume of the

adsorbent, and properties like temperature, molecular polarity and the

chemical nature of the adsorbent surface.

Commonly used adsorbents in air pollution control are activated C, activated

alumina, silica gel and molecular sieves.

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Removal of Pollutants by Adsorption

Removal of pollutants by adsorption can be carried out in a batchwise or

continuous manner of operation.

Eg; Fixed bed absorber.

Some time before sending the waste gas to the adsorber , it is filtered to

prevent bed contamination by soot, resin droplets & large particulates.

Molecular sieves are normally used for the removal of gaseous pollutants.

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Combustion/ Thermal Oxidation

Thermal oxidation ( flaring/ incineration ) is the process of oxidizing

combustible materials in presence of air at a high temperature for sufficient time

to complete combustion to CO2and water vap.

For complete combustion, the O2 must come into intimate contact with the

combustible material through adequate turbulence at sufficiently hightemperature and have a sufficiently long residence time.

Time, temp; and turbulence have important roles in combustion and they are

often called the three Tsof combustion. Normal ranges: Temp: 375 825 oC, residence time: 0.2 0.5 sec,

gas velocity: 4.5

7.5 m/s.

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Three methods of combustion

1. Direct combustion ( Flaring)

2. Thermal incineration (Flame combustion)

3. Catalytic Oxidation

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Control of sp.gaseous pollutants

The sulphur oxides, the oxides of N2, CO2and hydrocarbons are the important

gaseous air pollutants because of their known harmful effects and their

presence in the atmosphere.

Three basic procedures for controlling SO2 emissions from stationary combustionsources:

1. The extraction of sulphur from fuels

2. Sulphur reduction within the combustion chamber

3. Treatment of flue gases.

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Hydrodesulphurization of Coal:

Useful for removing both organic and inorganic forms of S.

Solvent: Anthracene

1-2% H2 to avoid polymerization

Temp: 4500

CGasification of Coal

C + H2O CO + H2

S is converted to H2S and which is separated by either absorption or adsorption.

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For absorption, Na2CO3 or ethanolamine is used to scrub the

gases, followed by the regeneration of the reagent with the

production of elemental S.

The absorption of H2S takes place in a 15 20 % aq. solution of

the amine and a temperature of 30-40

0

C.

The solvent is regenerated and H2S is converted to elemental S by Claus

process.

1/3 H2S + 1/2 O2 1/3 SO2+ 1/3H2O

1/2 SO2+ 2/3 H2S S + 2/3 H2O

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Adsorption of H2S

Adsorbent : FeO in fluidized bed

Temp: 4000C

FeO is regenerated by roasting it in air at 8000C

Generated SO2used in H2SO4plant

Sulphur reduction during combustion

Dry limestone technique

CaCO3 CaO + CO2

CaO + SO2 CaSO3

CaO + SO2+ 1/2 O2 CaSO4

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Treatment of flue gases

Dry process are

a) adsorption of SO2 by metal oxide to form sulphites or sulphates &

then regeneration of oxide & recovery of S

b) adsorption on activated C and then followed by regeneration &conversion of SO2to H2SO4

Wet process are

a) Lime Limestone scrubbing

b) Magnesium oxide scrubbing

c) WelmanLord process

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Adsorption of SO2by metal oxides

Aluminium sodium oxide ( alkalized alumina) and manganese oxide are the widely

used adsorbents, but oxides of Co and Cu are also active.

Alkalyzed Alumina Process

Na2O.Al2O3+ SO2+ 1/2O2 Na2SO4+ Al2O3

Na2SO4+ Al2O3+ 4H2 Na2O.Al2O3 + H2S + 3H2O

Manganese oxide process (90% removal)

MnOx.yH2O + SO2+ 1/2 (2-x)O2 MnSO4+ y H2O

MnSO

4

+2NH

3

+2H

2

O+(y-1)H

2

O+1/2(x-1)O

2

(

4

)

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Adsorption on activated carbon

Attractive method for continuous removal of SO2because of the high surface area andlow cost of activated C.

1. The Reinluft Process

Uses Cheap semicoke of peat, carbonized under vacuum at 6000C, as the adsorbent.

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This uses a specially developed activated C having high resistivity against ignition and

high SO2 adsorption capacity. The adsorbent can be regenerated either thermally or

by washing with water.

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Westvaco Process

This process utilizes fluidized beds of high efficiency activated C and uses H2S toreduce H2SO4to sulphur.

Flue gas is contacted with activated C in the adsorber unit where the C acts as a catalyst

in the oxidation of SO2to SO3.

O2, H2O

SO3 H2SO4

The spent C is fed to a S generator and it is contacted with H2S to form S.

H2SO4+ 3 H2S 4S + 4 H2O

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Adsorption of SO2by Wet method

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Hydrated lime can also be used in this process.

2Ca(OH)2+ 2 SO2+ O2 CaSO3+ CaSO4+ 2H2O

Various types of Scrubbers employed commercially :

Spray towers, venturies, packed beds, and turbulent contact absorbers.

The scrubbing systems can be designed for 80 95 % SO2removal.

Advantage: Relatively simple, low cost and the easy availability of limestone

Disadvantage: Sludge disposal

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Magnesium Oxide Scrubbing

The flue gas is scrubbed with a slurry of MgO which absorbs SO2 and yields

magnesium sulphite and sulphate.

MgO + SO2 MgSO3

MgSO3+ SO2+ H2O

Mg(HSO3)2+ MgO

MgSO3+ O2 MgSO4

heatMgSO3 MgO + SO2

MgSO4+ C MgO + SO2+ CO2

Mg(HSO3)2

2 MgSO3+ H2O

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Welman-Lord Process

Saturated sloution of Sodium sulphite absorb

SO2from flue gases; sulphite is converted to

bisulphite.

Na2SO3+ SO2+ H2O 2 NaHSO3

Bisulphite decomposes into sodium sulphite

around 1100C, releases conc.SO2and steam.

2NaHSO3 Na2SO3+ SO2+ H2O

Degree of desulphurization : 90 %

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SO2 oxidation leads to the formation of sodium sulphate which can not be

regenerated and therefore it is purged from the system.

An equivalent amount of NaOH is addded to maintain the sodium balance.

Deg.of desulphurization 90 %

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Other flue Gas Scrubbing processes

1. Stone& Webster Ionics Process

Reaction is based upon the reaction of caustic soda solution with SO2to form

sodium sulphite and bisulphite.

2 N a O H + C O 2 N a 2 C O 3 + H 2 O

N a 2 C O 3 + S O 2N a 2 S O 3 + C O 2

N a 2 S O 3 + 1 / 2 O 2 N a 2 S O 4

N a 2 S O 3 + S O 2 + H 2 O2 N a H S O 3

NaHSO3/ Na2SO3is treated with dil.H2SO4to get sod.sulphate and Conc.SO2

gas.

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Na2SO3+H2SO4 Na2SO4+SO2+

2NaHSO3+H2SO4 Na2SO4+2SO2+

The caustic soda is returned to the absorber and acid is used in the desorption

tower.

Electrolytic regeneration cell converts Na2SO4into caustic soda and H2SO4.The overall reaction is

Na2SO4+3H2O2a

2. The Atomics International molten salt process

Unconventional scrubbing process which uses a mixture of lithium, sodium and

potassium carbonates in a melt at about 4250C.

SO2and SO3react with metal carbonates to yield corresponding sulphites and

sulphates. .

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M2CO3+ SO2 M2SO3+ CO2

M2CO3+ SO3 M2SO4 + CO2M- alkali metal ions

The mixture is regenerated in a two stage process. First stage, the sulphateand sulphite are reduced to sulphide by using producer gas ( CO + H2).

M2SO3+ 2CO + H2 M2S + 2 CO2+ H2O

M2SO4+ 2CO + 2H2 M2S + 2CO2+ 2 H2O

The reduced melt is then reacted with steam and CO2 to produce H2S and

M2CO3

M2S + H2O + CO2 M2CO3+ H2S

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Metal Smelting Operations - Sources of SO2emissions

The conc. of SO2emissions from smelting is high enough to produce either

H2SO4or S.

Liquid SO2can be produced using theASARCOprocess by absorbing SO2

in dimethylamine.

Byproduct H2SO4manufacture

The manufacture of H2SO4 from smelting involves gas conditioning,

drying, catalytic conversion, and absorption. The gas conditioning step removes impurities like As, Cl-, F-, fumes of

metal sulphide or oxide, mercury etc. This is necessary to avoid rapid

deactivation of the catalyst.

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Double contact, double absorption (DCDA) method

This method provides higher plantefficiencies (99.5%) and SO2emissions of

below 500 ppm.

The gases from the converter, after

conversion of (90%) SO2to SO3passed at

an intermediate stage to an absorber toremove the SO3.

The gases are reheated and returned to the

converter for further conversion.

This then pass through additional catalyst,

cooled and flow through a secondabsorber and then to the atmosphere.

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Absorption in Dimethylaniline- ASARCO Process

ASARCO- American Smelting& Refining Co. The waste gases containing SO2 are absorbed in

dimethyaniline in the lower stages of an absorber.

The desulphurized gas containing dimethylaniline

is scrubbed with Na2CO3 solution and then with

dil.H2SO4 to remove traces of SO2 and

dimethylaniline. The SO2 rich liquor is fed to a steam distillation

column which strips SO2 from the liquor and

dimethylaniline is recycled.

SO2 is dried with 98% H2SO4 in a drying tower,

liquified and sent to storage for further processing.

The tail gas leaving the stack after scrubbing

contains around 500ppm SO2.

This process is economical only for SO2 conc.

Above 2%.

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Absorption in Ammonia

Aq.ammonia is used in the COMINCO (Consolidated mining and

smelting Co.) process for reducing the SO2conc.

A 30% solution of aq.ammonia absorbs SO2 , producing ammonium

bisulphite.

SO2 is then stripped from this solution by adding 93% H2SO4 in astripping column.

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Around 2000 PPM of SO2is released through tail gases (limit: 500 PPM)

DCDA (double contact and double absorption) process reduces the SO2emissions.

Here 90% of SO2 is converted to SO3. The gases are re-heated, passed through

additional catalyst, cooled, flow through a second absorber and then to the

atmosphere. Here microporous molecular sieves such as alumino-silicate zeolites

are used as adsorbents.

The sodium (Welman) and magnesium (Chemico) scrubbing systems have been

used to reduce the SO2emission to less than 300 PPM

Ammonium scrubbing process is also used and here the ammonium sulphite-

bisulphite formed is reacted with nitric or orthophosphoric acid to produce the

ammonium salt and SO2is recovered for further processing.

Contact process is used to manufacture Oleum and

Sulfuric acid. The methods involved in controlling SO2

emissions in this process.

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Petroleum Refining

Sulphur is found in the combined form in crude oil; H2S and thiophenes are

the most common S compounds , but it may also be present in the form of

mercaptans and sulphides.

Desulphurization is carried out , not only for the sake of pollution control but

also for improving the product quality and minimizing corrosion problems in

the processing plant. Removal of H2S and mercaptans can be carried out by washing the oil with

aq.soda solution and complex S compounds have to be broken down

catalytically .

Hydrodesulphurization is the most common method in which H2reacts with

organic S compounds forming H2S which is collected and can be convertedto S.

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Control of NOx emission

Two main sources of N2which contribute to the formation of nitrogen oxides

during combustion are atmospheric nitrogen and the other is the bound nitrogen

in the fuel.

NOxare formed in the combustion processes according to the reactions

1/2N2+1/2O2

NO+1/2O2

The primary oxide is NO, which rapidly converts by reaction with O2 or

ozone to NO2. NOx emissions from stationary sources can be reduced by

1. minimizing the residence time

2. minimizing temp.

3. minimizing the availability of O2for reaction with N2.

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The methods for the removal of NOx can be grouped into two categories

1. Scrubbing - two ways a) Absorption by liquids

b) Adsorption by solids

2. Catalytic decomposition and reduction : Involves decomposition of nitric

oxide into N2and O2or reaction with another gas such as CO.

Absorption by liquids

The processes which have been proposed for controlling NOx emissions from

power plants are

1. Treatment with lime slurry2. Scrubbing with Magnesium hydroxide liquor

3. Absorption in H2SO4

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All these processes are complex and require attainment of equimolarconcentrations of NO and NO2 in the gas, since the absorption of the

combined oxide N2O3is most favourable.

In Magnesium hydroxide scrubbing process, the oxides of nitrogen are

absorbed by magnesium hydroxide liquor in an absorption tower.

The resulting magnesium nitrate/nitrite solution is taken to a pressure reactorand nitrite is converted to nitrate.

The byproduct NO is oxidized to NO2 and the liquid leaving the pressure

reactor, consisting of Mg(NO3)2/ Mg(OH)2 is sent to a settling chamber. Here

the nitrate is separated from the hydroxide which is recycled to the absorption

tower. Part of the NO2is sent to the absorber to maintain equimolar conc. Of NO and

NO2while the rest of NO2is used for HNO3production.

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Absorption of NOxby mag.hydroxide

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Other types of scrubbing techniques developed

1. Two stage absorption, first in water and then in NaOH, yielding nitrite and nitrate salts.

2. Absorption in various types of ammonical solutions such as ammonium bicarbonate

and ammonium bisulphite : Ammonium bisulphite can be used as a scrubbing liquor when

NOxreact quickly at normal pressure and temp. to form ammonium sulphate which can be

used a fertilizer.

3. Absorption with an aq.suspension of lime: Aq.suspension of calcium hydroxide as

absorbing medium results in reduction of NOx levels to 200 ppm. The calcium nitrite

can be converted to calcium nitrate by treating withH2SO4. During the oxidation of

nitrite to nitrate, NO is evolved.

2H2SO4+ 3 Ca(NO2)2 2 CaSO4+ 4NO + Ca(NO3)2+ 2 H2O

The evolved NO is recycled to the HNO3plant and calcium nitrate can be used as a

fertilizer.

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Adsorption by solids

The commonly used adsorbents are activated C, silica gel, molecular sieves, ion

exchange resins and metal oxides like manganese oxide and alkalised ferric

oxides. These adsorbents shows some capacity for oxidising NO to NO2and for

adsorbing nitrogen dioxide.

The main disadvantage of using molecular sieves as adsorbents is the

simultaneous adsorption of water vapour in the tail gas which decreases theefficiency of the bed.

The most suitable adsorbent is the one which can be regenerated and at the

same time which does not react with water vapour or with CO2and the most

preferred adsorbent is ferrous salt.

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Catalytic Decomposition The decomposition of NO ( both homogeneous and heterogeneous) is slow

and therefore we can use variety of catalysts like Al2O3, Cr2O3, Fe2O3, ZrO3

and Co3O4for the decomposition.

At reasonable temp. no catalyst has been found with sufficient activity.

Catalytic Reduction

Catalytic reduction is an attractive method for controlling nitrogen oxide

emissions.

Catalytic decolourizers are used to reduce NOxto NO, but NO reduction to N2is

limited.

The decolourization product NO could be reduced to molecular N2 in the

presence of reducing agents like H2, natural gas and CO or we can use the most

effective catalysts like Pd and Pt (catalytic abatement process).

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The noble metal catalysts used in HNO3 plants, for application to flue gasemission control, are expensive and subject to S poisoning in the case of coal

and oil fired equipment and this led to the study of the reduction reaction over

different types of commercial catalysts.

There are two types of reduction process, selective and non-selective.

The added reactant reduces NOx in selective reduction, and in non-selective

reduction, the excessive oxygen must be consumed first and the former one is

preferred because it minimizes the amount of reactant required. Selective

reduction can be carried out with H2, CO or NH3as the reactant gas.

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Typical reactions are

2 NO + 2 H2 N2+ 2H2O

2NO + 2CO N2+ 2CO2

6NO + 4NH3 5N2+ 6H2O

In non-selective reduction, there are two types of reactions. First type involves

the reaction of the fuel with oxygen and NO2, the latter being reduced to NO.

CH4+ 2O2 CO2+ 2 H2O

CH4+ 4NO2 CO2+ 2H2O + 4 NO

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The second reaction reduces the NO but the reduction does not take placeuntil all the oxygen in the flue gas has been reacted with the fuel and

reducing conditions have been achieved.

CH4+ 4NO CO2+ 2H2O + 2N2

CO control

CO is an intermediate product of chemical reaction between fossil fuels and

oxygen.

There are two reasons for the formation of CO

1. Because of insufficient quantity of O2.

2. Due to poor turbulance of the fuel and air in the reaction chamber or due to

dissociation of CO2to CO in high temp zones.

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The control of CO formation is not easy because the control strategies forCO and NOxare in conflict.

The most practical method of reducing the CO emisions from stationary

combustion sources is by proper design, installation, operation and

maintenance of the combustion equipment.

CO and H2are widely used in the chemical industry for the manufacture of

methanol,ammonia and various organic acids and aldehydes.

The emissions from the chemical and petroleum industry are minor because

of good design codes and careful operating practices.

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Control of Hydrocarbons

Four techniques are there to control hydrocarbon emissions from stationary

sources.

1. Incineration

2. Adsorption

3. Absorption4. Condensation

Adsorption: The polluted gas stream is passed through one or several adsorbers

operated in parallel.

Operating temp: between 30 and 600C

Preferred adsorbent: granular activated C The adsorbed vapours are removed by passing steam through the system.

The mixture of steam and hydrocarbons are liquefied in a mixture and cooled

down to ambient temp. in a cooler.

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The hydrocarbons are separated from the water and recovered for further use.

The most effective way of removing hydrocarbons from a polluted stream is by

contact with a liquid or a series of different liquids, in one or more absorption

towers.

If the hydrocarbons have sufficiently high solubility water is a suitable solvent .

Condensation : - Direct Contact condenser (counter current flow)- Surface type condenser (shell and tube) In direct contact condensers, a stream of water or other cooling liquid is in

contact with the vapour to be condensed.

The polluted stream enters the condenser at the bottom and the cooling liquid is

introduced at the top.

The gaseous stream leaving the condenser at the top contains non condensable

gases and the condensate is withdrawn from the bottom of the condenser.

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Spray chambers, cyclone scrubbers and venturi scrubbers can be used as direct

contact condenser.

Vertical condenser

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The polluted gas stream enters at the top

and flows through the tube passes and

the cold liquid enters at the bottom on

the shell side and leaves at the top.

The condensed liquid is separated fromthe non-condensable vapours at the

bottom of the condenser.

Vertical type surface condenser

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Biological Oxidation

Biological oxidation is an important method in industry for hydrocarbon and

odour control.

In this method, microorganisms are employed to metabolize pollutants in gas

streams. This process takes place at ambient temp., consume very little energy and

produce no nitrogen oxides.

The oxidation reactions are carried out in biofilters and the end products are

CO2, H2O and microbial mass.

If the conc. Of hydrocarbons is below 1.0 mg/m3biofilters are most effective.

Biofilter consists of a bed of packing material on which a microbial film is

attached. Activated sludge from municipal waste water treatment plants are also

used.

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The polluted gas is introduced at the bottom of the filter and the pollutantsdiffuse into the biofilm and there they are metabolised by aerobic bacteria.

Mobile Sources

The principal pollutants emitted from mobile sources are CO, NOx and

hydrocarbons. The major source of air pollution from automobiles is the exhaust pipe which

accounts for about 70% of the pollution and crank case emissions about 20 % and

evaporations from fuel tank and carburettor account for the remaining.

Control methods applied to one pollutant influence the out put of other pollutants.

Eg; The amount of CO formed during the combustion period is related to the ratioof air and fuel in the cylinder. If there is excess of fuel, then the products of

combustion contain high level of CO and hydrocarbons and low levels of NOx.

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The control of pollutant emissions from automobiles is based on the following

approaches:

1. Reduction of the amount of pollutants formed during combustion by suitable

modification of the internal combustion engine.

2. Development of exhaust system reactors that will complete the combustionprocess and change potential pollutants into more acceptable materials.

3. Development of substitute fuels for petrol that will produce low levels of

pollutants upon combustion.

4. Replacement of internal combustion engine with low pollution producing

engines.

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Exhaust system reactors , both thermal and catalytic can be used to reduce CO

and hydrocarbon emissions.

By changing the fuel either by mixing petrol with other substances or by

substituting another fuel with petrol.

Alternative fuels are there and the known petrol substitutes are methane or natural

gas, hydrogen and methanol. Replacement of IC engines with other types of engines like steam, electric and gas

turbine engines is a good solution to the emission control problem.

The control of hydrocarbon emissions from automobiles is more complicated than

control of CO emissions evaporation contribute to the pollution problem.

The use of a canister filled with activated C to adsorb the hydrocarbon vapourscan reduce the evaporation losses from fuel tank and carburettor. The adsorbed

vapours are desorbed and returned to the carburettor and burned in the engine.

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The control of NOxemission from automobiles

1. Exhaust gas recirculation (EGR)

In EGR, portion of the exhaust gas (inert), is continuously recirculated through the engine

so that the burning air-fuel mixture is diluted.

This decreases the O2conc. In the burning mixture thereby lowering the combustiontemperature.

The exhaust gas is regulated by an EGR valve.

2. Catalytic reduction.

Dual catalyst system in which NO is reduced to N2 and hydrocarbons and CO are

oxidized to CO2and H2O simultaneously.

The exhaust gases are passed over a reductive catalyst and here NO reacts with CO to

form N2and CO2.

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Air is injected and the mixture is passed over an oxidative catalyst and the remaininghydrocarbons and CO are oxidised to CO2and H2O.

In reduction catalyst chamber, ammonia may form which will be converted to NO or

N2O in the oxidising catalyst chamber.

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Briefly describe the two techniques used in Control of

hydrocarbon emissions from stationary sources.

Incineration:

If the waste gas contains sufficient combustible material, then incineration may be

the simplest route. Normally it is smokeless and odorless unless high molecular

weight hydrocarbons are used. Most efficient in destroying diluted gas streams.

Waste gas is preheated over an auxiliary fuel fired burner and passed into acombustion chamber where a temperature of 500-800 OC is maintained. Gas stream

is kept at this temperature for a residence time of 0.3 to 0.7s to allow complete

oxidation Gas stream is introduced in such geometry and at a velocity that promote

turbulence and thorough mixing with a burning fuel. High operating cost of fuel is

reduced by recovering the waste heat. It emits NOX, which can be reduced by

adopting catalytic incineration.

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Adsorption:

Hydrocarbons (HC) are passed through one or several absorbers in

parallel Operating temperature is kept between 30 -60OC. Adsorbent:

granular activated carbon (particle diameter: 2-4 mm). Adsorbed vapors

are removed by passing steam through the system. The mixture is

liquefied in a condenser and cooled down to a ambient temperature. HC

are separated from water and used again. Capital cost is high, but low

maintenance .

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Write the name, source and control of air pollutants from

automobiles?

Air Pollutants from automobiles:

Principal Pollutants from automobiles: CO, NOx, HC

Sources: Exhaust pipe, Crankcase emissions and Evaporations from fuel tank/

Carburettor. Exhaust emissions differs with the type of engine (petrol or diesel),

because diesel engine works with compression ignition and petrol engine works

with spark ignition.Control of Pollutant emission is possible by

1. Modification or replacement of internal combustion Engine

2. Developing the exhaust system

3. Developing the fuel alternatives