EIA 5 Industrial Air Pollutants Summer 2014

8/12/2019 EIA 5 Industrial Air Pollutants Summer 2014

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Industrial Air Pollutants (Gaseous,

Noise, Electromagnetic) andAbatement Techniques

Dr. Shahid AmjadInstitute of Business Management

(IoBM)


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The Pressure Gradient Force and Other Effects on Wind

Within the atmosphere, there are several forces thatimpact the speed and direction of winds. The most

important though is the Earths gravitational force. Asgravity compresses the Earths atmosphere, it creates airpressure- the driving force of wind. Without gravity, therewould be no atmosphere or air pressure and thus, no wind.

The force actually responsible for causing the movement of

air though is the pressure gradient force. Differences in airpressure and the pressure gradient force are caused by theunequal heating of the Earths surface when incoming solarradiation concentrates at the equator. Because of theenergy surplus at low latitudes for example, the air there iswarmer than that at the poles.

Warm air is less dense and has a lower barometric pressurethan the cold air at high latitudes. These differences inbarometric pressure are what create the pressure gradientforce and wind as air constantly moves between areas ofhigh and low pressure.


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Industrial Air Quality There are big industrial estates, and small cottage industries that

generate hazardous and toxic chemicals and Air pollutants.

The fertilizer plants, textile industry, glass industry, steel plants etc.

These industries contribute SO2NO2, smoke, volatile organiccompounds, chlorine gas, particulate matter, ammonia, CO, CO

2

,phenol, cyanide into the atmosphere.

The human health is affected by different-sized airborne particulatematter. Larger particles PM10are trapped in the nose and throat,whereas smaller particles (PM2.5) penetrate the lungs and are

associated with a range of respiratory symptoms.

Traces of metals such as lead, iron, and copper are particulates aswell as plant pollen.


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Primary/Secondary Air Pollutant

Sources

Secondary air pollutants derive from reactions that occurbetween primary pollutants and other atmospheric chemicalsOzone PANs (peroxyacetyl nitrates) Acids: sulfuric and nitric


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There are six pollutants which have the maincontribution in creating air pollution. They are

primary pollutants like carbon monoxide (CO),

sulphur dioxide (SO2),

nitrogen oxide (NO),

lead (Pb), and

particulate matters (PM),

Primary pollutants react with water molecules to

form Secondary pollutants such as weak Nitricacid, carbonic acid, sulphuric acid etc, and groundlevel ozone (O3).


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Control Devices And Equipments (End-of-the

Pipe Cleaning)

The strategy for this method is to remove theparticulate and gaseous pollutants after they

are formed.

The cleaning devices can be broadly classifiedinto 2 groups

(i) Particulate Control and

(ii) Gaseous Pollutant Control.


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Particulate Control

Particulate Matters (PMs) are common pollutantspresent in the emission of an industry. They are

formed in wide range of sizes, diameter varyingfrom 0.0002 to 500 micron (1 micron = 106m).

PM Control Devices can be broadly divided into 3categories, viz.,

Internal Separators, Wet Collection Devices and

Electrostatic Precipitators.

Internal separators. Three types of devices arecommonly in use as internal separators. They are(i) Gravitational settling chambers, (ii) Cycloneseparators and (iii) Bag filters or fabric filters.


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Gravitational settling chambers are generally

used to collect larger particles of size greater

than 50 micron. A stream of dust-laden gas atless than 3 m/s is passed into the settling

chamber where the velocity of the gas is

further reduced.

As a result, the dust particles settle down and

collected through a hopper at the bottom.

The collection efficiency can be increased by

installing a series of settling chambers in

parallel.


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Cyclone separator utilizes centrifugal force to separatethe particulate matters from the carrier gas. The particleladen gas receives a rotating motion as it enters thecyclone cylinder. A centrifugal force is developed due tothe vortex formed which throws the particles towardsthe wall. These particles get collected at the conicalbottom of the cyclone cylinder.

High efficient cyclones are designed. A series of cyclones,

called the multiclone, are used in many industries toincrease the efficiency of collection.

The settling efficiency of cyclone separator is higher thanthat of gravity separator because the centrifugal force isadded to the gravitational force to settle the particles.

An ordinary conventional cyclone can have an efficiencyof 95-99% collection for particles greater than 40 microndiameter. At the other end, for particles of size greaterthan 5 micron, the collection efficiency can be 50-80%with cyclones of very high efficiency


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Bag filters are devices by which gas is purifiedthrough various filtering cloths (cotton, wool,chemical filters, fibre glass, etc.) or fabric filters

(filters made of granular materials like ceramics,porous plastics, etc).

The dusts are collected in a hopper at the bottom.

Bag filters have high efficiency but the filters are

required to be cleaned and changed in regularintervals. The Bag filters are attached toMechanical shakers.

Many filters made of clothes, plastics, etc. cannot

work at high temperatures. However though bag filters are highly efficient,

these are more expensive and require frequentmaintenance.


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Wet collection devices.

Inthe wet collection methods, various types ofscrubbers are used by which the particulate matters

are brought into contact with the scrubbing liquid,usually water, and the wetted particles settle down.

This is similar to the natural process of cleaning of theatmosphere by the water cycle. Wet collections have

the advantage of collecting. not only particulates but also gaseous impurities by

suitable choice of the scrubbing liquid (which canabsorb the gases). But wet scrubbers have the

problems of corrosion. The scrubbed water (slurry) is required to be treated

before disposal. In this system, air pollution problem ispassed on to the water environment.


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The collection efficiency of spray towers can

be increased by the use of centrifugal

scrubbers in which the water spraying nozzles

are fitted inside a conventional dry cyclone.

The water spray acts on the particles in the

outer vortex and the particulate matters arecollected at the bottom as in dry cyclones but

in the form of slurry


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Packed beds and plate columns

are well-known absorbers in chemical

industries. These can also be used as wetscrubbers. These are distinguished by their

simple design and operation, stability in

service, low hydraulic resistance and lowenergy consumption.

In a simple packed bed scrubber, the polluted

air stream moving upwards comes in contactwith the scrubbing liquid stream moving

downward over the packing (Figure 5.5).


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Venturi scrubber

followed by cyclone separator, the venturiscrubber offers a high performance collection offine particles (2-3 micron). The polluted gasstream is accelerated by the passage through anarrow throttle (velocity of the order of 60-120

m/sec). Scrubbing liquid is injected at this throttle

through low pressure nozzles. The gas-liquidmixture then goes to the cyclone separator.

The separated slurry is removed from the bottom(Figure 5.6). Venturi scrubbers are particularlysuitable for sticky and flammable particles.


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Electrostatic precipitation. The gaseous stream ispassed through a strong electric field where theparticulate matters get electrically charged and areattracted towards the electrode of opposite charge. In

a typical Electrostatic Precipitator (ESP), the positiveelectrode (grounded) is the collecting electrode andthe negative electrode is suspended at the top.

A high voltage DC current through the electrodes

produces a corona. The gas close to the negativeelectrode is ionized. The electrons produced in theionization process move towards the positively chargedgrounded surface.

During this passage, the particulate matters also get

negatively charged by the electrons and get collectedon the positive surface (Figure 5.7).

ESPs are considered as highly efficient particulatecollecting systems and are widely used inindustries.


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Gaseous Pollutant Control

Two techniques are generally followed for the

control of gaseous pollutants. They are(i) Sorption of the pollutant (absorption in a

liquid or adsorption on a solid surface

(ii) Chemical Alterations (the pollutant isconverted to innocuous substances).


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Absorption by a liquid The pollutant is brought

into contact with an appropriate liquid or solution

by counter current methods whereby the gaseous

impurity is transferred from the gaseous phase to

the liquid phase.

The gas absorption devices are similar to the wet

collection devices for particulate control(scrubbers, spray towers, Venturi scrubber, etc.)

with minor modifications wherever necessary.

The different absorbent liquids or solutions usedfor various pollutant gases are shown in Table 5.1.


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Adsorption on solid surface. Inthis method, the pollutedair is passed through a column of absorbent solid, usuallyactivated carbon, activated alumina, silica gel and organicpolymeric compound (molecular sieve), etc. whereby thepollutant in the air stream is held on the solid surface. Thismethod, though not very common, is useful for odourremoval.

The following lists some adsorbents and their common use.

Activated carbon : Used for odour removal, purification ofindustrial gases, hydrocarbons, pesticides etc.

Silica gel: Used for dehydration of gases

Activated alumina: Used for dehydration of gases and for

removal of HF in aluminum smelter Molecular sieves: Wide application, selective adsorption of

gases like NH3, SO2, etc.


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CONSTRUCTION OF HIGH STACKS/CHIMNEYS

With all the control devices, it may not be always possible toeffectively remove the pollutants from the emission. Sometimesthe available technology for complete pollutant removal may becost-prohibitive.

The next best approach for control of pollution is to allow thepollutant to be diluted to the maximum and dispersed to minimizeits adverse effect. This can be done by releasing the emissions at ahigh altitude with long stacks. The height and the diameter of thestack should be designed to keep the ground level concentration

within the permissible limits. The concentration of the pollutant at the ground level decreases

exponentially with the height of the stack and is found to bemaximum at a distance of about 5 to 10 times the stack height atnormal meteorological condition.

The coal used in thermal power plants contains sulphur, weak acid

rain can be except in small pockets. Installation of desulphurization unit requires heavy capital

investment. The sulphur dioxide control thermal power plants ismostly done by maintaining long stack heights


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Plume rise depends on momentum and

buoyancy. Buoyancy factor is due to the

temperature difference of the stack gases and

surrounding air and the momentum due to

the molecular weight of the exhaust gases

against air.


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A W K th t


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Ans, We Know that

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Exercise

A coal power plant has a physical stack height

(chimney) height of 500 m, with an inside

diameter of 10 m. The stack gas velocity is 20

m.sec-1 . The stack gas temperature is 200oC

(473 K) and the ambient air temperature is10oC (283 K). The atmospheric pressure is 1

bar; average wind speed is 5m. sec-1 .

Calculate the effective stack height. Ans = 1150.74 m


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Exercise

A power plant which runs on coal burning,

consumes 50,000 tons of coal per day. TheSulphur content of coal is 10 percent.

Calculate the rate of emission of Sulphur

dioxide in grams. sec-1 into the atmosphere.

(At wt of Sulphur = 32, Oxygen = 16)

Ans= 115,740 gms. sec-1


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Exercise

Before the installation of an Electrostatic

Precipitator (ESP), the Stack gas of a power

plant contained 6.0 g particulates per m3 of

gas. The gas flow rate is 350m3/minute. The

new ESP can remove 2500 kg particulates/day.i. Calculate the emission rate of particulates

before and after pollution control in Kg/day.

ii. What is the efficiency of the new ESPiii. Will the new system meet the particulate

emission standard of 0.7 gms/m3


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Solution: Data

a) w=6gms/m3; flow rate Q=350m3; m=2500 kg/day.

350x6=2100gms/min or 2100x10-3 kg/min.

2100x10-3x24x60 = 3024 kg/day (Before).New ESP 2500 kg/day

3024-2500=524 kg/day

b) n=Quantity of dust collection x100

Quantity of dust in flowing gas

2500 x100 =82.7%

3024

c) Total particulates at source=6gms

ESP efficiency= 82.7%

Particulates removed 6x0.827 = 4.962gms.

Particulates remain 6.0 - 4.962 = 1.038 gms/m3

So the new ESP system cannot meet emission standard of 0.7 gms/m3

C S d


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Case Study A survey was undertaken to monitor the levels of atmosphere Suspended

Particulate matter in the industrial area of Korangi using a high volume

sampler. The PMmonitoring was done in December and subsequently as afollow up, the process of particulates monitoring was also repeated inmonth of July.

During the December observations, the fiber glass filter paper used had aninitial weight of 7.02345 gms. After 24 hrs of sampling using a high Volumeair sampler, the filter paper weighed 8.56432 gms. The volume of air flow(measured using a rotameter) at the start and at the end of theexperiment was recorded to be 1.2 m3/min and 0.8 m3/min. respectively.

Similarly, in monsoonal month of July. The initial and final weight of thefiberglass filter paper was 8.00234 and 8.16012 gms. The air flow at thestart and at the end of the experiment was recorded to be 2.2 m3/min and1.8 m3/min. The sampling time was 24 hrs. respectively.

Calculate the concentration of suspended matter in the atmosphere inug/m3at the Korangi location in December and July accordingly. Explainvariation in SPM during July and December months. Does the air qualitymeet the NEQs Standards.


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P d


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Procedure1. you have the data from a 24-hour run of a Mini Volume

SPM sampler. The data include:

Weight of the filter before sampling Weight of the filter after sampling

Total time the sampler was running

Average flow rate of the sampler

2. Using the data provided, perform the step-by-stepcalculation of ambient particulate concentration.

3. Compare and explain variation in the results of particulate

matter during summer and winter to the NationalEnvironmental Air Quality Standard (NEQs)/Air QualityIndex (AQI)


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Solution

Calculate the weight of particulate matter Final

and Initial weight of the Filter paper in g(multiply weight in gm by 1000000.)

Calculate the average air flow (from the

rotameter data) (Start +End data/2) air Volumesampled

Calculate the sampling duration in 24 hrs (i.e total

air flow duration of sampling. (Av flow x 60 x 24). Total Suspended Matter Weight of Particulate

matter/Volume Air flow sampled).


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Guideline for Control of Air Pollution Pollution of air in the atmosphere cannot be fully abated but can be

controlled if certain measures are taken.

Develop peoples awareness in all spheres including the

governments. Free burning of wood, coal, wastes of forest, agriculture waste and

many other solid matters must be discouraged in the open spaces.

Vehicular emissions must me checked periodically.

Use of natural gas (CNG) or low sulphur containing fuels should be

encouraged in place of petrol and diesel. Industry must adopt waste gas treatment facility before releasing it

into the atmosphere.

Lead free petrol should be introduced to control lead pollution.

All sorts of waste materials must be disposed properly.

Greater emphasis should be put on the use of renewable energysources such as hydroelectric, solar and wind energies.

Use of chlorofluorocarbons and other toxic materials must bereduced or substituted.


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EIA 5 Industrial Air Pollutants Summer 2014