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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
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