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Air Pollution Control Methods and Equipment

(Control of Gaseous Pollutants)

ET ZC362: Environmental Pollution Control BITS Pilani1

Dr.Vandana

Lecture 7

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Learning Objectives

Control of Gaseous Emissions

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There are 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

Control of Gaseous Emissions

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Absorption by liquids

This tech. is widely used for controlling/ removing the

gaseous pollutants from an effluent gas

It involves the transfer of pollutant from gas phase to

liquid phase across the interface .

A two-resistance theory is used to explain this process.

According to this theory, 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 thephases on each side of the interface.

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For dilute solutions, the equilibrium relation between pA and cA can be

expressed in terms of Henryslaw.

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Henrys Law : Definition

At a constant temp, the amount of gas that dissolves in a liquid is directly

proportional to the partial pressure of that gas in equilibrium with that

liquid.

Mathematically we can express Henrys as

pA= KcA

Where K is the Henrys constant, p is partial pressure and c is

concentration.

OrpA= K1

xA, x is mole fraction.

K depends on nature of solute, solvent and temperature.

Smaller value of K1represents higher solubility.

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Selection of solvents

Effectiveness of absorption process highly depends on

Gas solubility in the solvent

Solvent vapor pressure (low VP is favorable)

Corrosiveness of solvent (non corrosive is favorable)Solvent regeneration (easy to regenerate)

Cost of solvent (low cost)

Operating temperature (low temp.)

Viscosity of solvent(low viscosity)

Chemical stability

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Adsorption by Solids

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Two types of adsorption

1. Physical adsorption ( Physisorption)

The attractive forces holding the molecules at the surface is physical.

The gaseous material condenses upon the surface of the solid, accompanied by

an evolution of heat.

The adsorbed material can be removed by reducing the pressure or by

increasing the temperature and so it is reversible.

2. Chemical adsorption ( Chemisorption)

Result of a chemical interaction between the solid and the adsorbed material.

The molecules are held to the solid surface by chemical bonds.

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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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Adsorbent Uses

Activated Carbon Removal of odours and trace of impurities

from gases, purification of Industrial gases

and hydrocarbons, solvent recovery.

Activated Alumina Dehydration of gases and liquids.

Silica Gel Dehydration and purification of gases

Molecular sieves Selective adsorption of CO2, NH3, C2H2, H2S and

SO2.

Adsorbents and their uses

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Adsorption Steps

Diffusion of the gas molecules into the pores of the solid.

Actual adsorption on the active sites in the pores.

Diffusion of the pollutant from the bulk gas phase to the external

surface of the solid ( similar to the diffusion of the gas to the gas-

liquid interface in absorption.)

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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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The adsorber is normally regenerated when the break-through point is reached.

The removal of pollutants from the adsorbent and thereby renewing it for further

use is called regeneration.

Regeneration is often enabled by using parallel beds, one of which removes

pollutant while the other is being regenerated.

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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: 375825 oC, residence time: 0.20.5 sec,

gas velocity: 4.57.5 m/s.

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Three methods of combustion

1. Direct combustion ( Flaring)

2. Thermal incineration (Flame combustion)

3. Catalytic Oxidation

1. Flaring

Highly combustible streams with high heating values can be eliminated by

flaring.

This method is not good, if the gas streams contain excessive amounts of

inorganic pollutants ( like S, F, Cl2etc).

The gas streams have to be pretreated before flaring.

Unsaturated hydrocarbons ( aromatics, olefins etc) produce smoke and we can

use some designs in such a way that it allows them to burn smokelessly.

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The waste gas is preheated and passed into a combustion chamber where a temp.of

500-8000C is maintained.

Thermal Incineration

Preheating of the gas stream is required, if the combustible gases are diluted with

inert gases.

Thermal incineration is the most flexible technique for destroying diluted gas

streams.

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Catalytic Oxidation

Catalytic oxidizers or catalytic incinerators, operate similarly to thermal

oxidizers. The primary difference is that the gas, after passing through the flame

area, passes through a catalyst bed.

The temp. requirement is much lower than the thermal ignition. Waste gas is

typically heated by auxiliary burners to 320 4300C before entering the catalyst

bed. The max. design temp of the catalyst exhaust is 5406750C.

Generally catalysts are oxides of precious metals such as Pt & Pd. The potential

poisons of the catalyst are S, Si, P, As.

To maintain catalyst activity and to achieve complete combustion 1% excess O2

is allowed

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Control of Specific Gaseous Pollutants

Chapter 6

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Learning Objectives

Application of air pollution control equipment and strategies for the control

of major gaseous pollutant emissions.

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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.

Introduction

Control of SO2Emission

The main source of SO2

emissions are the fossil fuel burning .

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Three basic procedures for controlling SO2emissions from stationary combustion

sources:

1. The extraction of sulphur from fuels

2. Sulphur reduction within the combustion chamber

3. Treatment of flue gases.

Most convenient method of solving SO2 problem is to disperse SO2

containing waste gas from stacks at sufficient height, but due to more

stringent HSE policy, sometimes this method is not acceptable

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1. Extraction of S from fuels

S in coal is present in both inorganic and organic forms.

All inorganic S exists in the coal as FeS2, i.e; in the form of pyrites andmarcasites and organic S in the form of cystin, thiols, sulphides and some

cyclic compounds.

Washing can reduce the pyritic S content by 30 % and the organic S can be

removed only by chemical processing.

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Hydrodesulphurization of Coal

This process can remove both the inorganic as well as organic forms of Sulphur.

In this process, finely ground coal is slurried with anthracene oil ( a small

amount of H2 is added to avoid repolymerization and the slurry is heated at a

high temperature in order to dissolve the coal.

The ash residue containing pyritic S and other minerals is eliminated by pressure

filtration.

The filtrate is sent to a flash evaporator ( light fractions is removed) andsubsequent operation in a distillation unit recovers the solvent.

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2. Gasification of coal

Partial oxidation of coal in the presence of O2and steam to yield CO and

H2.

C + H2O CO + H2

O2added to burn some coal to attain reaction temperature. S is converted to H2S and which is separated by either absorption or

adsorption.

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 30-400C and the reaction is shown below.

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Then the solvent is regenerated and H2S is converted to elemental S by Claus

process.

1/3H2S+1/2O2 1/3SO2+1/3H2

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

The S is condensed and sent to a S storage tank.

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Claus Process

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The absorption process are simple but require cooling of the gas that involvesconsiderable heat losses.

Dry methods can be carried out at high temperatures and one such processes

involves the adsorption of H2S on ferric oxide in a fluidized bed at 4000C.

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Adsorption of H2S

For adsorption

Adsorbent : FeO in fluidized bed

Temp: 4000C

FeO is regenerated by roasting it in air at 8000C

Generated SO2used in H2SO4plant

Iron oxide is often used for H2S removal. It can remove H2S by forming insoluble

iron sulfides. The chemical reactions involved in this process are

Fe2O3+ 3H2S Fe2S3+ 3H2O

Fe2S3+ 3/2O2 Fe2O3+3S

Iron oxide is often used in a form called ironsponge for adsorption processes.

Iron sponge is iron oxide-impregnated wood chips. Iron oxides of the forms Fe2O3and Fe3O4are present in iron sponge. It can be regenerated after it is saturated,

but it has been found that the activity is reduced by about one-third after each

regeneration cycle . Iron sponge has removal rates as high as 2,500 mg H2S/g

Fe2O3.

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The most effective method for the removal of S from the fuel oil is the

hydrodesulphurization.

Treats fuel oil with H2over a catalyst at temp. of 300- 500oC and pressures

of 40- 150 atm.

H2reacts with S compounds forming H2S which is collected and used for

production of S.

The catalyst used is Co-Mb based.

Drawback

Cost of desulphurization is high.

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Fuel Oil Gasification

Involves catalytic steam cracking where the hydrocarbon reacts with steam

over Ni catalyst at temp. of 700- 10000C and at atmospheric pressure to form

CO, CH4and H2.

The S in the fuel is converted into H2S which is then separated by one of the

known processes, e.g., ethanolamine.

H2S is finally converted to S in a Claus system plant.

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Sulphur reduction during combustion

Limestone is used in the combustion chamber and it is calcinated to CaO by the

heat of combustion.

This reacts with SO2in the flue gas to form sulphites and sulphates.

C a C O3

C a O + C O 2

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

C a O + S O 2 + 1 / 2 O 2 C a S O 4

The solid reaction products, unreacted materials and fly ash are removed by eitherdry collectors or by wet scrubbing.

Dry limestone technique

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Disadvantages:

S reduction is 50 % only.

CaSO4is unstable at high temperature.

Limestone does not react completely with SO2

Fluidized bed combustion process

Limestone and crushed coal together form the fluidized bed and air isused as the fluidizing medium.

Operating temp: 700- 1000 0C

Degree of desulphurization is 90 %, more than twice as high as that in

the dry limestone tech.

Drawback Requires novel design of boilers and additional installations for the preparation

of limestone, collection of solid particles carried off from the fluidized bed and

regeneration of CaSO4.

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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.

Metal oxides on support exhibit an enhanced reactivity compared with that of

bulk oxides.

Na (or K), Sr, Cr, and Cu impregnated oxides supported on alumina are rated as

superior catalysts.

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Alkalyzed Alumina Process

The dust free flu gas is fed to a reactor wherein the adsorbent, a porous form of

sodium aluminate (Na2O.Al2O3) , adsorbs SO2at a temp. of 3150C. During this

process, the SO2 and O2 in the flue gas react with the adsorbent.

a2O.Al2O3+SO2+1/2O2Na2S4

The spent material is in contact with a reducing gas ( H2) in a regenerator at

about 6800C to produce H2S.

a2SO4+Al2O3+4H2Na2

.l2

3

The sodium aluminate pellets are recycled and the H2S gas is sent to a Claus unit for

S production

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Major problem is related to maintenance of the granular strength of the sorbent.

Exposure of the sorbent to continued sorption-regeneration cycle results is

uneconomically high attrition rates.

Drawback

Manganese oxide process (90% removal)

The powdered adsorbent is fed into an entrainment reactor and in that, the

activated MnO reacts with SO2to produce MnSO4.

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

Where x is in bet. 1.5 & 1.8 and y is in bet. 0.1 &1.

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The regeneration of the spent sorbent is carried out by reacting the MnSO4with air and NH3 to produce ammonium sulphate.

MnOx is filtered out and & filtrate is passed through crystallizer to separate

out ammonium sulphate which is used for fertilizer.

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Other dry removal systems

1) Cat-Ox process produces sulphur ic acid

2) Shell copper oxide process produces elemental sulphur.

Cat-Ox process Fly ash is removed from the flue gas by a hightemperture electrostatic precipitator, SO2 is catalytically oxidized to SO3

and recovered as sulphuric acid.

The catalyst is Cat-ox A ( from Monsanto company).

Shell copper oxide process- The copper oxide is impregnated onto an

inert base (alumina). SO2 reacts with copper oxide at about 4000C toform copper sulphate.

SO2+1/2 O2+ CuO CuSO4

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CuSO4 is reduced in a hydrogen rich gas

CuSO4+ 2H2 Cu + SO2+ 2 H2O

The concentrated SO2 is then sent to Claus sulphur recovery

unit.

Attractive method for continuous removal of SO2 because of

the high surface area and low cost of activated C.

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The Reinluft Process Uses Cheap semicoke of peat, carbonized under vacuum at

6000C, as the adsorbent

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Developed an improved tech. which use harder, less combustible

activated C and the processes is called BF process. 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. .

Draw back

Instability of C in the presence of flue gas O2which leads to C combustion.

Attrition of C and corrosion.

Recirculation of very large amounts of C and cost of heat and heat reducing

agent is high.

BF process

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Westvaco Process

This process utilizes fluidized beds of high efficiency activated C and uses H2S to

reduce 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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A fraction of the sulphur is recovered by vaporization by hotinert gases in a sulphur stripper and is recondensed as a

molten product.

The remaining S reacts with hydrogen in a hydrogen sulphide

generator to form H2S. The regenerated C is recycled to the adsorber.