controlling-filters air pollution

8/10/2019 Controlling-Filters Air Pollution

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Controlling air pollutioncourtesy:Green force engineers Pvt. ltd.

Introduction

Clean air, an essential component of a healthful environment, is a mixture of many different gases. Two gasespredominate: nitrogen, which makes up 78 percent of the volume of clean dry air, and oxygen, which makes up2 percent. !rgon, an inert element, accounts for almost percent of clean dry air, and the remainder includesvery small or trace concentrations of car"on dioxide, methane, hydrogen, helium, o#one, and other gases. $n the%arth&s atmosphere, water vapour is also a significant component "ut the most varia"le one, ranging from '.'to ( percent "y volume) its concentration in air varies daily and seasonally, as well as geographically.

!ir is considered to "e polluted when it contains certain su"stances in concentrations high enough and fordurations long enough to cause harm or undesira"le effects. These include adverse effects on human health,property, and atmospheric visi"ility. The atmosphere is suscepti"le to pollution from natural sources as well asfrom human activities. *ome natural phenomena, such as volcanic eruptions and forest fires, may have not onlylocal and regional effects "ut also long+lasting glo"al ones. evertheless, only pollution caused "y human

activities, such as industry and transportation, is su"-ect to mitigation and control.

ost air contaminants originate from com"ustion processes. $n the iddle !ges the "urning of coal for fuelcaused recurrent air+pollution pro"lems in /ondon and in other large %uropean cities. 0eginning in the 1thcentury, in the wake of the $ndustrial evolution, increasing use of fossil fuels intensified the severity andfre3uency of air+pollution episodes. The advent of mo"ile sources of air pollution++i.e., gasoline+powered highwayvehicles++had a tremendous impact on air 3uality pro"lems in cities. $t was not until the middle of the 2'th


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century, however, that meaningful and lasting attempts were made to regulate or limit emissions of air pollutantsfrom stationary and mo"ile sources and to control air 3uality on "oth regional and local scales.

The focus of air+pollution regulation in industriali#ed countries was initially on protecting am"ient or outdoor air3uality. This involved the control of a small num"er of specific criteria pollutants known to contri"ute to ur"an

smog and chronic pu"lic health pro"lems. Toward the end of the 2'th century, the ha#ardous effects of traceamounts of many other air pollutants were recogni#ed, and emission regulations were implemented. /ong+termand far+reaching effects of certain su"stances on atmospheric chemistry and climate were also o"served at thattime, and cooperative international efforts were "egun to mitigate their glo"al effects.

Techniques for controlling air pollution

Control of particulates

!ir"orne particles can "e removed from a polluted airstream "y a variety of physical processes. Common types ofe3uipment for collecting fine particulates include cyclones, scrubbers, electrostatic precipitators, andbaghouse filters. 4nce collected, particulates adhere to each other, forming agglomerates that can readily "eremoved from the e3uipment and disposed of, usually in a landfill.

0ecause each air+pollution control pro-ect is uni3ue, it is usually not possi"le to decide in advance what the "esttype of particle collection device 5or com"ination of devices6 will "e) control systems must "e designed on acase+"y+case "asis. $mportant particulate characteristics that influence the selection of collection devices includecorrosivity, reactivity, shape, density, and especially si#e and si#e distri"ution 5the range of different particle si#esin the airstream6. 4ther design factors include airstream characteristics 5e.g., pressure, temperature, and

viscosity6, flow rate, removal efficiency re3uirements, and allowa"le resistance to airflow. $n general, cyclonecollectors are often used to control industrial dust emissions and as precleaners for other kinds of collectiondevices. et scru""ers are usually applied in the control of flamma"le or explosive dusts or mists from suchsources as industrial and chemical processing facilities and ha#ardous+waste incinerators) they can handle hotairstreams and sticky particles. %lectrostatic


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Types of Air Pollution Control Devices

1. Electrostatic Precipitator

2. Wet Scrubber

3. Cyclone

4. aghouse

Electrostatic Precipitator

%lectrostatic precipitation is a commonly used method for removing fine particulates from airstreams. $n anelectrostatic precipitator, particles suspended in the airstream are given an electric charge as they enter the unitand are then removed "y the influence of an electric field. The precipitation unit comprises "affles for distri"utingairflow, discharge and collection electrodes, a dust clean+out system, and collection hoppers. ! high C voltage5as much as '',''' volts6 is applied to the discharge electrodes to charge the particles, which then are

attracted to oppositely charged collection electrodes, on which they "ecome trapped.


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9articles that stick to the collection plates are removed periodically when the plates are shaken, or rapped.apping is a mechanical techni3ue for separating the trapped particles from the plates, which typically "ecomecovered with a ;+mm 5'.2+inch6 layer of dust. appers are either of the impulse 5single+"low6 or vi"rating type.The dislodged particles are collected in a hopper at the "ottom of the unit and removed for disposal. !nelectrostatic precipitator can remove particulates as small as m 5'.''''( inch6 with an efficiency

exceeding 11 percent. The effectiveness of electrostatic precipitators in removing fly ash from the com"ustion

gases of fossil+fuel furnaces accounts for their high fre3uency of use at power stations.$n a typical unit thecollection electrodes comprise a group of large rectangular metal plates suspended vertically and parallel to eachother inside a "oxlike structure. There are often hundreds of plates having a com"ined surface area of tens ofthousands of s3uare metres. ows of discharge electrode wires hang "etween the collection plates. The wires aregiven a negative electric charge, whereas the plates are grounded and thus "ecome positively charged.


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WetScrubber

evices called wet scru""ers trap suspendedparticles "y direct contact with a spray of wateror other li3uid. $n effect, a scru""er washesthe particulates out of the dirty airstream asthey collide with and are entrained "y thecountless tiny droplets in the spray.

*everal configurations of wet scru""ers are inuse. $n a spray+tower scru""er, an upward+flowing airstream is washed "y water sprayeddownward from a series of no##les. The wateris recirculated after it is sufficiently cleaned toprevent clogging of the no##les. *pray+towerscru""ers can remove 1' percent ofparticulates larger than a"out 8 m

5'.'''< inch6.

$n orifice scru""ers and wet+impingementscru""ers, the air and droplet mixture collideswith a solid surface. Collision with a surface

atomi#es the droplets, reducing droplet si#eand there"y increasing total surface contactarea. These devices have the advantage oflower water+recirculation rates, and they offerremoval efficiencies of a"out 1' percent for


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particles larger than 2 m 5'.''''8 inch6.

=enturi scru""ers are the most efficient of the

wet collectors, achieving efficiencies of morethan 18 percent for particles larger than '.>m 5'.''''2 inch6 in diameter. *cru""er

efficiency depends on the relative velocity"etween the droplets and the particulates.=enturi scru""ers achieve high relativevelocities "y in-ecting water into the throat of aventuri channel++a constriction in the flowpath++through which particulate+laden air ispassing at high speed.

Cyclone


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! cyclone removes particulates "y causing the dirtyairstream to flow in a spiral path inside a cylindrical

cham"er. irty air enters the cham"er from a tangentialdirection at the outer wall of the device, forming avortex as it swirls within the cham"er. The largerparticulates, "ecause of their greater inertia, moveoutward and are forced against the cham"er wall.*lowed "y friction with the wall surface, they then slidedown the wall into a conical dust hopper at the "ottomof the cyclone. The cleaned air swirls upward in anarrower spiral through an inner cylinder and emergesfrom an outlet at the top. !ccumulated particulate dustis periodically removed from the hopper for disposal.

Cyclones are "est at removing relatively coarseparticulates. They can routinely achieve efficiencies of1' percent for particles larger than a"out 2' m

5'.'''8 inch6. 0y themselves, however, cyclones are notsufficient to meet stringent air 3uality standards. Theyare typically used as precleaners and are followed "ymore efficient air+cleaning e3uipment such aselectrostatic precipitators and "aghouses.

ag !ouse


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4ne of the most efficient devices for removing suspended particulates is an assem"ly of fa"ric filter "ags,commonly called a "aghouse. ! typical "aghouse comprises an array of long, narrow "ags++each a"out 2> cm5' inches6 in diameter++that are suspended upside down in a large enclosure. ust+laden air is "lown upward

through the "ottom of the enclosure "y fans. 9articulates are trapped inside the filter "ags, while the clean airpasses through the fa"ric and exits at the top of the "aghouse.

0ag house !rrangement

! fa"ric+filter dust collector can remove very nearly '' percent of particles as small as m 5'.''''( inch6

and a significant fraction of particles as small as '.' m 5'.''''''( inch6. ?a"ric filters, however, offer

relatively high resistance to airflow, and they are expensive to operate and maintain. !dditionally, to prolong theuseful life of the filter fa"ric, the air to "e cleaned must "e cooled 5usually "elow 7' @?B6 "efore it ispassed through the unit) cooling coils needed for this purpose add to the expense. 5Certain filter fa"rics++e.g.,


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those made of ceramic or mineral materials++can operate at higher temperatures.6

*everal compartments of filter "ags are often used at a single "aghouse installation. This arrangement allowsindividual compartments to "e cleaned while others remain in service. The "ags are cleaned "y mechanicalshakers or "y reversing the flow of air, and the loosened particulates are collected and removed for disposal.

controlling-filters air pollution

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