NSCET E-LEARNING PRESENTATION 3/unit 3air... · Branch/Year/Sem: B.E. Civil Engineering/III/VI . OBJECTIVE ... In this presentation • Working principle, Design and performance

NSCETE-LEARNING

PRESENTATIONLISTEN … LEARN… LEAD…

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DEPARTMENT OF CIVIL ENGINEERING

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Mr.K.Kirubakaran.,M.E.,

Assistant Professor, Civil,

Nadar Saraswathi College of Engineering & Technology,

Vadapudupatti, Annanji (po), Theni – 625531.

CE8005 - AIR POLLUTION AND CONTROL ENGINEERING

Branch/Year/Sem: B.E. Civil Engineering/III/VI

OBJECTIVE

To impart knowledge on the principle

and design of control of Indoor/

particulate/ gaseous air pollutant and its

emerging trends.

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UNIT III

CONTROL OF PARTICULATE CONTAMINANTS

Factors affecting selection of control

equipment – gas particle interaction –

working principle, design and

performance equations of gravity

separators, centrifugal separators fabric

filters, particulate scrubbers,

electrostatic precipitators – operational

considerations.

In this presentation

• Working principle, Design and performance

equations of Gravity Separators, Centrifugal

separators, Fabric filters

• Particulate Scrubbers

• Electrostatic Precipitators

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The available mechanism are

Gravitational Settling chambers

Centrifugal impaction

Inertial impaction

Direct interception

Diffusion

Electrostatic attraction

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Department of Civil Engineering, NSCET, Theni

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Inertial impaction

Inertial (அசைவற்ற)- it is the deposition of large aerosol particles on the walls of an

airway conduit (குழாய்). The impaction (இறுகிய பிசைப்பு) tends to occur where

the airway direction changes. Small particles have less inertia and are more likely to be carried

around corners and continue in the path of the airflow.

Direct interception (இசைமறிப்பு)

Direct interception occurs if the gas streamlines carrying the particles are close to the filter

elements for contact. Inertial impaction occurs when the particles have sufficient momentum

and cannot follow the gas stream, when the stream is diverted by the filter element and the

particles strike the filter.

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Direct interception

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Centrifugal collectors

Separate particulate matter (of 0.1 to 1000 m) using centrifugal force. Particles

of size (5 m to 25 m) are removed by this instrument.

Some Centrifugal collectors are

1. Cyclone collector or (Cyclone separator)

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In a cyclone separator, dirty flue gas is fed into a chamber.

The inside of the chamber creates a spiral vortex (சுழல்).

The lighter components of this gas have less inertia, so it is easier for them to be influenced by

the vortex and travel up it.

Contrarily, larger components of particulate matter have more inertia and are not as easily

influenced by the vortex.

Since these larger particles have difficulty following the high-speed spiral motion of the gas and

the vortex, the particles hit the inside walls of the container and drop down into a collection

hopper.

The cleaned flue gas escapes out the top of the chamber.

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Most cyclones are built to control and remove particulate matter that is larger than 10 micrometers

in diameter.

However, there do exist high efficiency cyclones that are designed to be effective on particles as

small as 2.5 micrometers.

As well, these separators are not effective on extremely large particulate matter. For particulates

around 200 micrometers in size, gravity settling chambers or momentum separators are a better

option.

Out of all of the particulate-control devices, cyclone separators are among the least expensive.

They are often used as a pre-treatment before the flue gas enters more effective pollution control

devices.

Therefore, cyclone separators can be seen as "rough separators" before the flue gas reaches the

fine filtration stages.

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The operating problems are

Erosion

Corrosion

Build-up

Applications

Saw mills- to remove saw dust from air

Oil refineries- to separate oil and gases

Cement industry

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Dynamic precipitator

Particles of size (5 m to 25 m) are removed by this instrument.

7 to 10 times higher than Cyclone separator.

Suited for

Ceramic, food, wood working and pharmaceutical industries.

Not suited for

Wet fibrous material

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Fabric filters

Dust collection of Particulate matter of size (up to 1 m)

The air pollution control equipment using fabric filters are known as bag houses.

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Air-to-cloth ratio (or filter velocity) is simply the amount of air going through each square foot of

filter media each minute. It is calculated by dividing the amount of airflow (CFM) by the amount

of filter media in the dust collector.

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Fabric Filters or baghouses, are air pollution control devices that work by

removing particles from the gas stream of industrial or electrical production

processes by collecting them on the surface of filter bags. Since the late 1970’s,

with the introduction of high-temperature fabrics (> 350 degrees Fahrenheit), the

baghouse has become increasingly popular, especially in the utility market. In

contrast to ESPs, the baghouse is a highly efficient particulate collection device

regardless of the incoming dust loading or particle size. Also, with stricter EPA

regulations for the removal of gases and heavy metals, the baghouse offers

adaptability as a dry collection device using absorbents.

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Baghouses have the capacity to remove 99.99% of even stubborn sub-micron

particles. As dust-laden gas enters the baghouse compartment(s), larger particles

drop out into the hopper, and smaller particles collect on the filter bags/media,

forming a layer or cake; clean gas passes through the media. When the dust layer

get thick enough to substantially restrict flow through the media (referred to as

pressure drop), cleaning is started. Cleaning can be done while a baghouse

compartment is still filtering (online) or with the compartment isolated and not

filtering (offline), depending on baghouse type and cleaning strategy.


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Gravity settling chambers

Gravity settling chambers are the oldest and simplest means of removing

suspended particles from a gas

In principle, pollutants are removed by reducing the velocity of the gas stream

sufficiently to allow particles to settle out under the influence of gravity

The simplest chamber is merely a horizontal duct in which large particles settle

out on the floor.

High-efficiency settling chambers are often fitted with baffles or deflectors to

change the gas flow direction.

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24Advantages

• Low capital cost

• Very low energy cost

• No moving parts, therefore, few maintenance requirements and low operating

costs

• Excellent reliability

• Low pressure drop through device

• Device not subject to abrasion due to low gas velocity Provide incidental

cooling of gas stream; Temperature and pressure limitations are only

dependent on the materials of construction

• Dry collection and disposal.Department of Civil Engineering, NSCET, Theni

25DisAdvantages

• Relatively low PM collection efficiencies, particularly for PM less than 50

micrometer in size

• Unable to handle sticky or tacky materials

• Trays in multiple-tray settling chamber may warp during high-temperature

operations

• The most common failure mode of settling chambers is plugging of the chamber

with collected dust. In expansion settling chambers the plugging can result from

hopper bridging or hopper discharge seal failure

• Multiple-tray settling chambers may experience plugging of the individual gas

passages. Department of Civil Engineering, NSCET, Theni

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Wet scrubbers

Wet scrubbers can remove particulate matter by capturing them in liquid

droplets. The droplets are then collected, with the liquid dissolving or absorbing the

pollutant gases. Any droplets that are in the scrubber inlet gas must be separated

from the outlet gas stream using a mist eliminator.

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Spray chamber

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A spray tower (or spray column or spray chamber) is gas-liquid contactor

used to achieve mass and heat transfer between a continuous gas phase (that

can contain dispersed solid particles) and a dispersed liquid phase.

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venturi scrubbers

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A Venturi Scrubber is a type of pollution control equipment that is used to

remove contaminated particles from gaseous exhaust streams. A type of

Wet Scrubber, the Venturi Scrubber uses Venturi technology, which has

been used for over 100 years.

Clean 400 liters of gas/min

Mist can remove by this


venturi scrubbers

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cyclonic scrubbers

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• Cyclonic spray scrubbers are an air pollution control technology. They use

the features of both the dry cyclone and the spray chamber to

remove pollutants from gas streams.

• Cyclonic scrubbers are generally low- to medium-energy devices, with

pressure drops of 4 to 25 cm (1.5 to 10 in) of water.

• Clean 200 liters of gas/min


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The “packed tower,” an air pollution treatment system that resembles the wet scrubber system used for removal

of particulates and discussed earlier, is the most common technology used for removal of gaseous (and

aerosol) pollutants.


Packed towers35


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Originally designed for recovery of valuable industrial-process

materials, electrostatic precipitators are used for air pollution control,

particularly for removing particles from waste gases at industrial facilities

and power-generating stations.

37PRINCIPLES OF ELECTROSTATIC PRECIPITATOR

Electrostatic precipitation is a method of dust collection that uses electrostatic

forces, and consists of discharge wires and collecting plates. A high voltage is

applied to the discharge wires to form an electrical field between the wires and

the collecting plates, and also ionizes the gas around the discharge wires to

supply ions. When gas that contains an aerosol (dust, mist) flows between the

collecting plates and the discharge wires, the aerosol particles in the gas are

charged by the ions. The Coulomb force caused by the electric field causes the

charged particles to be collected on the collecting plates, and the gas is

purified.


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Particle collection: The –vely charged particles get attracted towards the +vely

charged collecting electrodes and form a layer on the surface of the electrode.

Similarly the +vely charged particles are deposited on the discharging electrode. But

this is merely 10% of the total deposited material.

Removal of particles: The deposited particles forms a layer on the electrodes, and

after certain time interval they are removed from the electrodes by mechanisms

depending upon the type of ESP. Ash particles are collected in the hopper , and from

there they are transported to somewhere else.

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Documents

NSCET E-LEARNING PRESENTATION 3/unit 3air... · Branch/Year/Sem: B.E. Civil Engineering/III/VI . OBJECTIVE ... In this presentation • Working principle, Design and performance