Air Pollutants & Control Technicques

8/2/2019 Air Pollutants & Control Technicques

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AIR POLLUTANTS &

CONTROL

TECHNICQUES

Joni HermanaRachmat BoedisantosoJurusan Teknik Lingkungan FTSP ITSKampus Sukolilo, Surabaya 60111

TEKNOLOGI PENGENDALIAN

PENCEMAR UDARA

Semest er Ganj i l 2010-2011


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Particulate matter control technology typicallyincludes one of five main particulate controltechniques, there are many different designtypes:

1.Gravity settling chamber

2.Mechanical collectors;

3.Particulate wet scrubbers;

4.Electrostatic precipitators; and

5.Fabric filters

I NTRODUCTI ON


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This category of control devices relies upon gravitysettling to remove particles from the gas stream.

Gravity settling chambers are used only for very largeparticles in the upper end of the super coarse size

range (approximately 75 micrometers and larger).

The stringent control requirements adopted in the late1960s through early 1970s have resulted in a sharp

decline in the use of this type of collector.

There are very few gravity settling chambers still incommercial use.

Gravit y Set t ling Chambers


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This category of control devicesuse the inertia of the particlesfor collection.

The particulate-laden gas streamis forced to spin in a cyclonicmanner. The mass of theparticles causes them to movetoward the outside of the

vortex. Most of the large-diameter particles enter ahopper below the cyclonictubes while the gas stream

turns and exits the tube.

Mechanical Collectors


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Small-diameter cyclone tube isshown in Figure 2.

Vanes located on the inlet ofeach of the tubes create thespinning movement of the gas

stream. Due to the limited gashandling capacity of each tube,large numbers of tubes aremounted in parallel in a singlecollect

Small-diameter multi-cyclones,such as the one shown in Figure2 are capable of removingparticles having diameters down

to 5 micrometers



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Some mechanical collectors are speciallydesigned to provide high-efficiencyparticulate matter collection down to aparticle size of one micrometer.

A typical application of a conventional

multi-cyclone collector is shown in Figure 3.In this example, the multi-cyclone islocated after a small, wood-fired boiler andis used as a upstream collector for thefabric filter.

Mechanical collectors are used wheneverthe particle size distributions generated bythe process are relatively large (greaterthan 5 micrometers) and/or the controlefficiency requirements are in the low-to-moderate range of 50 to 90%.



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Part iculate Wet Scrubbers

There are a number of major categories ofparticulate wet scrubbers

Venturis Impingement and Sieve Plates

Spray Towers Mechanically Aided Condensation Growth Packed Beds

Ejector Mobile Bed Caternary Grid Froth Tower Oriented Fiber Pad

Wetted Mist Eliminators


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Ventur i Scrubbers

Particulate matter, which acceleratesas it enters the throat, is driveninto the slow moving, large water

droplets that are introduced nearthe high velocity point at the inletof the venturi throat. Theadjustable dampers in the unit

illustrated are used to adjust theopen cross-sectional area andthereby affect the speed of theparticles entrained in the inlet gasstream.



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I mpingement Plat eScrubbers

These scrubbers usually have oneto three horizontal plates, eachof which has a large number ofsmall holes. The gas streamaccelerating through the holes

atomizes some water dropletsin the water layer above theplate. Particles impact intothese water droplets



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Spray Tow er Scrubbers

This is the simplest type ofparticulate wet scrubber incommercial service. Sets ofspray nozzles located near thetop of the scrubber vesselgenerate water droplets that

impact with particles in thegas stream as the gas streammoves upwards.



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Wet Scrubbing Systems

Each particulate wet scrubber vessel ispart of a large, and sometimes complex,wet scrubbing system.

The figure to the right illustrates aventuri scrubber in a scrubbing system.

The evaporative cooler, located beforethe venturi scrubber in the system, coolsthe gas stream, which serves thefollowing purpose:

It protects the construction materials of

the venturi throat. It helps to homogeneously andheterogeneously nucleate vapor phasematerial emitted from the process before itreaches the scrubbing system. It prevents the water droplets from

evaporating and inhibiting inertialimpaction.



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Advant ages of Scrubbers

One of the main advantages of particulate wetscrubbers is that they are often able tosimultaneously collect particulate matter and gaseouspollutants.

Also, wet scrubbers can often be used on sources thathave potentially explosive gases or particulate matter.

They are compact and can often be retrofitted intoexisting plants with very limited space.



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Disadvant ages of Scrubbers

One of the main disadvantages of particulate wetscrubbers is that they require make-up water toreplace the water vaporized into the gas stream andlost to purge liquid and sludge removed from thescrubber system.

Wet scrubbers generate a waste stream that must betreated properly.



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Elect rostat ic Precipit ators

There are three main styles of electrostatic precipitators:

negatively charged dry precipitators;(1) negatively charged wetted-wall precipitators, and;(2) positively charged two-stage precipitators.

The negatively charged dry precipitators are the type mostfrequently used on large applications such as coal-firedboilers, cement kilns, and Kraft pulp mills.

Wetted-wall precipitators (sometimes called wet precipitators)

are often used to collect mist and/or solid material that ismoderately sticky.

The positively charged two-stage precipitators are used only

for the removal of mists.


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The figure to the rightshows the scale of atypical electrostatic

precipitator used at acoal-fired boiler.

Typically these units aredivided into a number ofseparately energizedareas that are termedfields.



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Advant ages of ESPs

Electrostatic precipitators can have very high efficienciesdue to the strong electrical forces applied to the small

particles. These types of collectors can be used whenthe gas stream is notexplosive and does notcontainentrained droplets or other sticky material.

The efficiency of an ESP is usually at a minimum in therange of 0.1 to 0.5 micrometers



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Disadvant ages of ESPs

The composition of the particulate matter is very importantbecause it influences the electrical conductivity within thedust layers on the collection plate.

Resistivity, an important concept associated withelectrostatic precipitators, is a measure of the ability ofthe particulate matter to conduct electricity and isexpressed in units of ohm-cm.

As the resistivity increases, the ability of the particulatematter to conduct electricity decreases. Precipitators canbe designed to work in any resistivity range; however,they usually work best when the resistivity is in the

moderate range (108 to 1010 ohms-cm).



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Fabric filters collect particulate matter on the surfaces offilter bags.

Most of the particles are captured by inertial impaction,interception, Brownian diffusion, and sieving on already

collected particles that have formed a dust layer on thebags.

The fabric material itself can capture particles that have

penetrated through the dust layers.

Electrostatic attraction may contribute to particle capturein the dust layer and in the fabric itself.

Fabr ic Fil t ers


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Types of Fabr ic Fil t ers

Areverse-air-typefabricfilter is used mainly for

large industrial sources.In this type of unit, theparticle-laden gasstream enters from thebottom and passes into

the inside of the bags.

Fabr ic Fil t ers


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Types of Fabr ic Fil t ers

Another common type of fabricfilter is the pulse jet. In this

type of unit, the bags aresupported on metal wirecages that are suspendedfrom the top of the unit.Particulate-laden gas flows

around the outsideof thebags, and a dust cakeaccumulates on the exteriorsurfaces

Fabr ic Fil t ers


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Advant ages and Disadvant ages of Fabric Fil t ers

Fabric filters are used in a wide variety of applicationswhere high efficiency particulate collection is needed.The control efficiencies usually range from 99% togreater than 99.5% depending on the characteristicsof the particulate matter and the fabric filter design

Fabr ic Fil t ers


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Fabr ic Fil t ers

Disadvant ages of Fabric Fil t ers

Fabric filters are not used when the gas stream generatedby the process equipment includes corrosive materials thatcould chemically attack the filter media. Fabric filters arealso not used when there are sticky or wet particles in the

gas stream.

These materials accumulate on the filter media surface andblock gas movement.

Fabric filters must be designed carefully if there arepotentially combustible or explosive particulate matter,gases, or vapors in the gas stream being treated. If theseconditions are severe, alternative control techniques, such

as wet scrubbers, are often used.


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There are numerous exceptions to the generalapplicability information presented in the previous slidedue to site-specific process conditions and unique

particulate matter control systems. Nevertheless, thischart provides a general indication of the uses andlimitations of many commercially available particulatematter control systems.

General Appl icabil it y ofPart iculate Cont rol Syst ems


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General Appl icabil it y ofPart iculate Cont rol Syst ems

Particulate matter control systems are often selected based on the generalcriteria listed below:


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Air Pollutants & Control Technicques

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