EEM 402 Lect 3 Air Pollution Control Methods Summer 2014

8/12/2019 EEM 402 Lect 3 Air Pollution Control Methods Summer 2014

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Air Pollution Prevention

Management and ControlStrategies

Dr. Shahid Amjad

Institute of Business Management

(IoBM)


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Air Pollution Effects

Adversely affects human health

Damages crops and forests

Highly corrosive


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Industrial Emission

Emission from an industry may pass into the airenvironment through the stack/chimney (stack emission).

It can also be fugitive in nature (fugitive emission), that is, itcan come out from various leakage points or from an opensystem and not from any dedicated duct.

While the impact of stack emission can be felt at longdistances, the fugitive emissions are confined to the work

environment and the areas of close vicinity of the plant. Good house keeping, maintenance of joints, hoods, etc.

and small measures of trapping emissions at the sources ofgeneration (like water spraying at transfer points of coalhandling) can greatly reduce fugitive emissions.

Since fugitive emissions are difficult to measure, regulatoryagencies often tend to ignore such emissions in theirauthorizations, consents, etc. Nevertheless, control offugitive emissions is very important, at least for protectionof workers' health.


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Episodic emissions always draw public attention.These emissions occur in upset conditions, oftenresulting in release of toxic gases and thus causing

loss to life and property. The Bhopal gas tragedy is probably the worst

incident of episodic emissions experienced so far.Preventive maintenance, close monitoring, etc. are

some of the measures the management shouldtake to prevent episodic emissions.

Once such an emission occurs, immediatemitigative measures are required for which

contingent action plans must be ready. On-site and off-site disaster management plans are

requirements under the law for major hazardousinstallations.


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Gaseous Pollutant Control Techniques

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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Adsorption on solid surface. Inthis method, the pollutedair is passed through a column of absorbent solid, usuallyactivated carbon, activated alumina, silica gel and organic

polymeric 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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Air Pollution Control Strategies

Air pollution control methods can, in general, be

grouped into the following categories:(i) Pollution control at source,

(ii) Installation of control devices and equipments

(End-of-the pipe cleaning),

(iii) Plantation of trees and creation of green belt,

(iv) Construction of high stacks and chimneys.

All these methods, however, propose good

maintenance and house keeping in the factory.


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POLLUTION CONTROL AT SOURCE

The ideal approach to pollution control is to

prevent or minimize the formation ofpollutants at the source itself, i.e., in themanufacturing process.

This should be done at an early stage ofprocess design and development. Anymodifications at the later stage may becomecostly.

Source correction methods include: rawmaterial changes, process changes andequipment modification or replacement.


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Raw Material and Process Changes

Pollutants have their origin in the raw materials. They are

either present in the raw materials as such or are byproductsof those materials. Thus use of purer grade raw materialsmay be beneficial in producing less pollutants.

Use of low-sulphur fuels instead of high-sulphur ones inpower generation produces less oxides of sulphur as

pollutant. Desulphurization of the fuel is an option availablefor cleaner production, but economics of this often preventsthis option.

Nevertheless there is no escape from this if a power plant isto be established using high-sulphur coal in an area.

Ore handling operations in steel plants causes dust pollution.Much of this nuisance can be removed by using dustsuppressors or by replacing ordinary ores with pelletedsintered ones.


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Equipment Modification or Replacement

Loss of hydrocarbons by evaporation in the

storage tanks of refineries can be controlled,to a very large extent, by designing the tanks

with floating roof covers or by pressurizing the

tanks.


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Particulate Control

Particulate Matters (PMs) are common pollutants presentin the emission of an industry. They are formed in widerange of sizes, diameter varying from 0.0002 to 500 micron

(1 micron = 106m). Their life-time in the atmosphere depends on the settling

range. Larger size particles settle faster than the smallerones. Different devices are available for the collection ofparticulate matters from the gaseous stream.

These can be broadly divided into 3 categories, viz., InternalSeparators, Wet Collection Devices and ElectrostaticPrecipitators.

Internal separators. Three types of devices are commonlyin use as internal separators. They are (i) Gravitationalsettling chambers, (ii) Cyclone separators and (iii) Bag filtersor fabric filters. and wet scrubbers.


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Gravitational settling chambers are generallyused to collect larger particles of size greaterthan 50 micron. A stream of dust-laden gas ispassed into the settling chamber where thevelocity of the gas is suddenly reduced.

As a result, the dust particles settle down and

collected through a hopper at the bottom. There are various types of settling chambers.

Figure 5.1 shows a horizontal flow typesettling chamber.

The collection efficiency can be increased byinstalling a series of settling chambers inparallel.


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Cyclone separator utilizes centrifugal force to separate theparticulate matters from the carrier gas. The particle laden gasreceives a rotating motion as it enters the cyclone cylinder. Acentrifugal force is developed due to the vortex formed which throws

the particles towards the wall. These particles get collected at theconical bottom of the cyclone cylinder.

The spinning gas stream spirals downward to the bottom of the cone.At this point the flow reverses to form an inner vortex and finallyleaves the cylinder through the outlet pipe situated at the top (Figure5.2).

High efficient cyclones are designed. A series of cyclones, called themulticlone, are used in many industries to increase the efficiency ofcollection.

The settling efficiency of cyclone separator is higher than that ofgravity separator because the centrifugal force is added to thegravitational force to settle the particles.

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


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Bag filters are devices by which gas is purified throughvarious filtering cloths (cotton, wool, chemical filters, fibreglass, etc.) or fabric filters (filters made of granular materialslike ceramics, porous plastics, etc).

A typical bag house consists of a tubular bag or an envelopemounted in such a manner that the particle laden gaspassing through the filter bags get deposited on theinside/outside surface of the bags which are dusted of byshaking in intervals.

The dusts are collected in a hopper at the bottom. Thecollection of the dust particles on the surface of the filterstake place

Bag filters have high efficiency but the filters are required to

be cleaned and changed in regular intervals. Many filtersmade of clothes, plastics, etc. cannot work at hightemperatures. Figure 5.3 shows the arrangement of a typicalbag house.

However though bag filters are highly efficient, these are

more expensive and require frequent maintenance.


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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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PLANTATION OF TREES AND CREATION OF GREEN BELT

Adequate greenery in industrial establishment helps in creatingbetter environment in many ways.

It provides a sylvan surrounding to improve the aesthetical

conditions which, in turn, improve the working condition of theworkers.

Tall trees attract birds to roost and also provide shelter to othersmall creatures like squirrel, and other reptiles, etc. Thusbiodiversity is restored.

properly designed green belt of adequate width acts as a filter of airpollutants for outside. Fugitive emissions are mainly controlled bythe green belt. Plantation of pollution indicating species at strategiclocations can indicate the air pollution status of the area. These areplant species which are sensitive to specific air pollutants. Suchspecies serve as 'bioindicators'.

Green belt acts as a noise barrier for outside. Treated wastewater of an industry is always recommended for

maximum utilization within the premises. If the wastewater isutilized for irrigation of the green.belt and other plantations within,the objective is partially achieved.


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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 acidrain can be except in small pockets.

Installation of desulphurization unit requires heavy capitalinvestment. The sulphur dioxide control thermal power plants ismostly done by maintaining long stack heights


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ExerciseBefore the installation of an Electrostatic

Precipitator (ESP) the stack of a power plantcontained 6.0 gms particulates per m3of gas. TheGas flow rate is 300 m3/minute. The newPrecipitator can remove 2591 kg particulates/day.

What is the emission rate of particulates beforeand after pollution control in Kg/day?

What is the efficiency (%)of the ESP.? Will the new system meet the emission

particulate standards of 500g/m3


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Solution: Data

a) w=6gms/m3; flow rate Q=300m3; m=2591 kg/day.

300x6=1800gms/min or 1800x10-3 kg/min.

1800x10-3x24x60 = 2592kg/day (Before).New ESP 2591 kg/day

2592-2591=1 kg/day

b) n=Quantity of dust collection x100

Quantity of dust in flowing gas

2591 x100 =99.96%

2592

c) Total particulates at source=6gms

ESP efficiency= 99.96%

Particulates removed 6x0.9996 = 5.9976gms.

Particulates remain 6.05.9976 = 0.0024gms/m(2400g)

So the new ESP system cannot meet emission standard of 500g/m3


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EEM 402 Lect 3 Air Pollution Control Methods Summer 2014

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