4. Air Pollution

7/30/2019 4. Air Pollution

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AIR AND NOISE POLLUTION


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IV. AIR AND NOISE POLLUTION

4.1. Air Composition

4.2. Air Pollution

4.3. Legislation

4.4. Air cleaning processes4.3. Noise Pollution


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Air pollution is the appearance of air contaminants in the atmosphere

that can create a harmful environment to human health or welfare,

animal or plant life, or property.

Air supplies us with oxygen

which is essential for our bodies

to live. Air is 99.9% nitrogen,

oxygen, water vapor and inert

gases.


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a) Weather control.

b) Transmit the solar

energy.

c) Protector shield from

UV light and

meteorites.

d) Supports the birds and

insects fly.

e) Transports seeds and

spores.

f) It gases are one of the

important sources forlife.


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The atmosphere is a

complex dynamic

natural gaseoussystem that is

essential to support

life on planet Earth.


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The atmosphere is monitored by:

1) Meteorological Satellites

2) Radiosonda balloons (measures T, P, humidity)

3) Land stations (measures the atmosphere properties every hour)


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Air pollution is woven throughout the fabric of our modern life. A by-product of

the manner in which we build our cities, air pollution is waste remaining from

the ways we produce our goods, transport ourselves and our goods, and

generate the energy to heat and light the places where we live, play, and

work. A major cause of air pollution is combustion, yet combustion is essentialto life as we know it.

There are several main types of pollution and well-known effects of

pollution which are commonly discussed. These include smog, acid

rain, the greenhouse effect, and "holes" in the ozone layer. Each of

these problems has serious implications for our health and well-being

as well as for the whole environment.


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Carbon oxides (CO, CO2)

Sulfur oxides (SOx)

Nitrogen oxides (NOx)

Volatile organic compounds (CFCs)

Suspended particles (dust, pollen, smog)

Photochemical oxidants compounds (O3)

Radioactive compounds (Rd, Ir, Sr)

Heat

Noise

Air Pollutants


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Sulfur oxides (SOx) - especially sulfur dioxide (SO2). It is produced by volcanoes and in various

industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion

generates sulfur dioxide. Further oxidation of SO2, usually in the presence of a catalyst such as NO2,

forms H2SO4, and thus acid rain.

Nitrogen oxides (NOx) - especially nitrogen dioxide are emitted from high temperature

combustion. Can be seen as the brown haze dome above or plume downwind of cities. Nitrogen

dioxide is the chemical compound with the formula NO2. It is one of the several nitrogen oxides. This

reddish-brown toxic gas has a characteristic sharp, biting odor.

Carbon monoxide - is a colorless, odorless, non-irritating but very poisonous gas. It is a product by

incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhaust is a major source

of carbon monoxide.

Carbon dioxide (CO2) - a colorless, odorless, non-toxic greenhouse gas associated with ocean

acidification, emitted from sources such as combustion, cement production, and respiration.

Volatile organic compounds - VOCs are an important outdoor air pollutant. In this field they are

often divided into the separate categories of methane (CH4) and non-methane (NMVOCs). Methane

is an extremely efficient greenhouse gas which contributes to enhanced global warming. Other

hydrocarbon VOCs are also significant greenhouse gases via their role in creating ozone and in

prolonging the life of methane in the atmosphere, although the effect varies depending on local air

quality. Within the NMVOCs, the aromatic compounds benzene, toluene and xylene are suspected

carcinogens and may lead to leukemia through prolonged exposure. 1,3-butadiene is another

dangerous compound which is often associated with industrial uses.


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One type of air pollution is the release of particles into

the air from burning fuel for energy. Diesel smoke is a

good example of thisparticulate matter. The particles are

very small pieces of matter measuring about < 1 microns

or about > 10 microns.

1) Big particles (over 10 um like pollen): Remains in

the troposphere 1 or 2 days before to fall by gravity

or precipitation.2) Medium particles (between 1 and 10 um, such as

coal dust, flour): Remains several days before their

precipitation.

3) Thin particles (less than 1 um, like paint pigments,

cigarettes smoke, smog): Remains few weeks andit can climb up to the stratosphere. Also this

particles can penetrates the pulmonary barriers and

causes diseases like cancer.


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Pollution also needs to be considered inside

our homes, offices, and schools. Some of

these pollutants can be created by indoor

activities such as smoking and cooking. We

spend about 80-90% of our time insidebuildings, and so our exposure to harmful

indoor pollutants can be serious. It is therefore

important to consider both indoor and outdoor

air pollution.


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Legislation

Normas Oficiales Mexicanas Normas para evaluar la calidad del aire como medida

de proteccin a la salud de la poblacin.

Norma Oficial Mexicana NOM-020-SSA1-1993. Salud Ambiental. Criterios para evaluar la calidaddel aire ambiente con respecto al ozono (O3).

Norma Oficial Mexicana NOM-021-SSA1-1993. Salud Ambiental. Criterios para evaluar la calidad del

aire ambiente con respecto al monxido de carbono (CO).


aire ambiente con respecto al bixido de azufre (SO2).


aire ambiente con respecto al bixido de nitrgeno.

Norma Oficial Mexicana NOM-024-SSA1-1993. Salud ambiental. Criterio para evaluar la calidad del

aire ambiente, con respecto a las partculas suspendidas totales (pst). Valor permisible para la

concentracin de partculas suspendidas totales (pst) en el aire ambiente, como medida de proteccin a

la salud de la poblacin.

Norma Oficial Mexicana NOM-025-SSA1-1993. Salud Ambiental. Criterios para evaluar el valor lmite

permisible para la concentracin de material articulado. Valor lmite permisible para la concentracin de

partculas suspendidas totales PST, partculas menores de 10 micrmetros PM10 y partculas menores

de 2.5 micrmetros PM2.5 de la calidad del aire ambiente.


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Normas que establecen los mtodos de medicin para determinar la

concentracin del contaminante.

NORMA Oficial Mexicana NOM-034-SEMARNAT-1993, que establece los mtodos

de medicin para determinar la concentracin de monxido de carbono en el aire

ambiente y los procedimientos para la calibracin de los equipos de medicin.


de medicin para determinar la concentracin de partculas suspendidas totales en el

aire ambiente y el procedimiento para la calibracin de los equipos de medicin.


de medicin para determinar la concentracin de ozono en el aire ambiente y losprocedimientos para la calibracin de los equipos de medicin.


de medicin para determinar la concentracin de bixido de nitrgeno en el aire


NORMA Oficial Mexicana NOM-038-SEMARNAT-1993, que establece los mtodosde medicin para determinar la concentracin de bixido de azufre en el aire


NORMA Oficial Mexicana NOM-041-SEMARNAT-1999, que establece los lmites

mximos permisibles de emisin de gases contaminantes provenientes del escape

de los vehculos automotores en circulacin que usan gasolina como combustible.


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Air particle cleaners


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The following items are

commonly used as pollution

control devices by industry or

transportation devices. Theycan either destroy

contaminants or remove

them from an exhaust stream

before it is emitted into the

atmosphere.


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Particulate control:

Mechanical collectors (dust cyclones, multicyclones)

Electrostatic precipitators (ESP), or electrostatic air cleaner removes particles from aflowing gas (such as air) using the force of an induced electrostatic charge.

Baghouses, designed to handle heavy dust loads, a dust collector consists of a blower,

dust filter, a filter-cleaning system, and a dust receptacle or dust removal system

(distinguished from air cleaners which utilize disposable filters to remove the dust).

Particulate scrubbers, the term describes a variety of devices where the polluted gas

stream is brought into contact with the scrubbing liquid, by spraying it with the liquid, by

forcing it through a pool of liquid, or by some other contact method, so as to remove the

pollutants.

Baffle spray scrubber

Cyclonic spray scrubber

Ejector venturi scrubber

Mechanically aided scrubberSpray tower


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Dust Cyclones


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The air is drawn in through the top of a

cone shaped baffle and the larger

particles drop as they are spin around in

the cyclone filter and the air is exhausted

out the rear of the unit

The air entrance speed of a cyclone lies

between 10 and 20 m/s, the most usual

speed is approx. 16 m/s. At fluctuations

with these speeds (with lower speeds)

the separation efficiency decreases very

rapidly.

As the air comes in at an angle, it moves

down the cone in a spiral, increasing in

speed as the cone's circumference

grows smaller. This creates a vortex

much like a tornado or cyclone. Largeparticles are thrown against the sides of

the cone and drop to a bin at the bottom.

Dust Cyclones


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A fan at the top of the cyclone cone draws

lighter particles and the air up the center of

the cyclone to an exhaust tube or outlet,

usually to a filter for catching fine particles.

The height of the cone, diameter of the coneand the angle of the walls all affect the

efficiency of particle removal.

Cyclones are most efficient at high air

entrance speed, small cyclone diameter andlarge cylinder length (pencil cyclones).

The advantages of a cyclone as separator for

substance are:

Simple constructionNo moving components

Little maintenance

Low investment and functioning costs

Constant pressure drop

Saves room


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The disadvantages are:

High pressure drop (0.5 - 2.5 kPa), depending on of the construction

version

Low output for low particle diameter

Bad performances at charge

Emission of effluent at wet cyclone

Erosion sensitive and constipation danger to the entrance

Possible noise

Usually a cyclone is used for its relatively high remaining emission

because of its relatively small efficiency and used as a pre-separator to

remove the largest dust load for second dust removal installation


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Efficiency equations for Cyclones


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Electrostatic precipitators (ESP)


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Electrostatic precipitators (ESP)

Is a particle control device that uses electrical forces to move the particles

out of the flowing gas stream and onto collector plates. The particles are

given an electrical charge by forcing them to pass through a corona, a region

in which gaseous ions flow.

The electrical field that

forces the charged particles

to the walls comes fromelectrodes maintained at

high voltage in the center of

the flow lane.

Once the particles are

collected on the plates, theymust be removed from the

plates without reentraining

them into the gas stream.


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This is usually accomplished by knocking them loose from the plates,

allowing the collected layer of particles to slide down into a hopper from

which they are evacuated. Some precipitators remove the particles by

intermittent or continuous washing with water.


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(1) the plate-wire precipitator, the most common variety

(2) the flat plate precipitator

(3) the tubular precipitator

(4) the wet precipitator, which may have any of the previous mechanical configurations

(5) the two-stage precipitator.


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Plate-Wire Precipitators

Plate-wire ESPs are used in a wide variety of industrial applications,

including coal-fired boilers, cement kilns, solid waste incinerators, paper mill

recovery boilers, petroleum refining catalytic cracking units, sinter plants,

basic oxygen furnaces, open hearth furnaces, electric arc furnaces, cokeoven batteries, and glass furnaces.

In a plate-wire ESP, gas flows between parallel plates of sheet metal and

high-voltage electrodes. These electrodes are long wires weighted and

hanging between the plates or are supported there by mast-like structures

(rigid frames). Within each flow path, gas flow must pass each wire in

sequence as flows through the unit.


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Flat Plate Precipitators

A significant number of smaller

precipitators uses flat plates instead of

wires for the high-voltage electrodes. The

flat plates increase the average electricfield that can be used to collect the

particles, and they provide an increased

surface area for the collection of particles.

Corona cannot be generated on flat

plates by themselves, so corona-

generating electrodes are placed aheadof and sometimes behind the flat plate

collecting zones.

These electrodes may be sharp-pointed

needles attached to the edges of the

plates or independent corona wires.

Unlike place-wire or tubular ESPs, thisdesign operates equally well with either

negative or positive polarity. The

manufacturer has chosen to use positive

polarity to reduce ozone generation.


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Tubular Precipitators

The original ESPs were tubular like

the smokestacks they were placed on,

with the high-voltage electrode running

along the axis of the tube. Tubularprecipitators have typical applications

in sulfuric add plants, coke oven by-

product gas cleaning (tar removal),

and, recently, iron and steel sinter

plants. Such tubular units are still used

for some applications, with manytubes operating in parallel to handle

increased gas flows. The tubes may

be formed as a circular, square, or

hexagonal honeycomb with gas

flowing upwards or downwards. The

length of the tubes can be selected to

fit conditions. A tubular ESP can betightly sealed to prevent leaks of

material, especially valuable or

hazardous material.


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Two-Stage Precipitators

The previously described precipitators are all parallel in nature, i.e., the

discharge and collecting electrodes are side by side. The two-stage

precipitator invented by Penney is a series device with the dischargeelectrode, or ionizer, preceding the collector electrodes. For indoor

applications, the unit is operated with positive polarity to limit ozone

generation.


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Wet Precipitators

Any of the precipitator configurations

discussed above may be operated with wet

walls instead of dry. The water flow may be

applied intermittently or continuously to washthe collected particles into a sump for

disposal. The advantage of the wet wall

precipitator is that it has no problems with

rapping re-entrainment or with back coronas.

The disadvantage is the increased complexity

of the wash and the fact that the collectedslurry must be handled more carefully than a

dry product, adding to the expense of

disposal.


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Discharge Electrodes

Discharge electrodes are also referred

to as corona electrodes, corona wires,

cathodes, and high-voltage electrodes.

The discharge electrodes are

maintained at high voltage during

operation of the ESP. The high voltage

electrodes ionize the gas and establishthe electric field, which imparts a charge

to particles and causes precipitation

upon grounded collection plates.

Collection Electrodes

Collection electrodes are the grounded

metal plates upon which the dust collects.

ESPs are designed using many shapes of

flat collecting electrodes and some are

designed with cylindrical collection

surfaces. All plate configurations are

designed to maximize the electric field and

to minimize dust re-entrainment.

Shell

The casing is fabricated of steel that is

suitable for the application (type of

process, heat range, etc.). The shell is

reinforced to handle maximum positive or

negative pressure, support the weight of

the internals, and sustain environmental

stresses such as those imposed by wind,

snow, and earthquake. The shell and

insulator housing form a grounded steel

chamber, completely enclosing all thevoltage equipment to ensure the safety of

personnel.

Hopper

Dust hoppers are required for

temporary storage of the collected

dust. The most common hopper

design is pyramidal, converging to a

square or round discharge area.

Rappers and Vibrators

Rapping systems are incorporated in an

ESP to dislodge dust from the collecting

and discharge surfaces; their

effectiveness and reliability are essential

Gas Flow Distribution

High Voltage Equipment


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Design Factors that affects the performance

Gas Volume Flow

Specific Collection AreaMaximum Superficial Velocity

Collection Plate Spacing

Dust Resistivity

Particle Size


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Baghouses


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These devices are fabric collectors that use filtration to separate dust particulates from dusty

gases. They are one of the most efficient and cost effective types of dust collectors available

and can achieve a collection efficiency of more than 99% for very fine particulates.

Dust-laden gases enter the baghouse and pass through fabric bags that act as filters. The bags

can be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelopeshape.

The high efficiency of these collectors is due to the dust cake formed on the surfaces of the

bags.


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Baghouses, can be found in virtually every industry:

Foundry and steel operations

Pharmaceutical producers Food manufacturers

Chemical producers


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Types of baghousesAs classified by cleaning method

Mechanical shaker

In mechanical-shaker baghouses, tubular filter bags are

fastened onto a cell plate at the bottom of the baghouse

and suspended from horizontal beams at the top. Dirty

gas enters the bottom of the baghouse and passes

through the filter, and the dust collects on the inside

surface of the bags.

Cleaning a mechanical-shaker baghouse is

accomplished by shaking the top horizontal bar from

which the bags are suspended. Vibration produced by amotor-driven shaft and cam creates waves in the bags to

shake off the dust cake.

Shaker baghouses range in size from small, handshaker

devices to large, compartmentalized units. They can

operate intermittently or continuously. Intermittent units

can be used when processes operate on a batch basis-

when a batch is completed, the baghouse can becleaned.

Continuous processes use compartmentalized

baghouses; when one compartment is being cleaned,

the airflow can be diverted to other compartments.


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In reverse-air baghouses, the bags are fastened onto a

cell plate at the bottom of the baghouse and suspended

from an adjustable hanger frame at the top. Dirty gas

flow normally enters the baghouse and passes throughthe bag from the inside, and the dust collects on the

inside of the bags.

Reverse-air baghouses are compartmentalized to allow

continuous operation. Before a cleaning cycle begins,

filtration is stopped in the compartment to be cleaned.

Bags are cleaned by injecting clean air into the dust

collector in a reverse direction, which pressurizes thecompartment.

The pressure makes the bags collapse partially, causing

the dust cake to crack and fall into the hopper below. At

the end of the cleaning cycle, reverse airflow is

discontinued, and the compartment is returned to the

main stream.

The flow of the dirty gas helps maintain the shape of the

bag. However, to prevent total collapse and fabric

chafing during the cleaning cycle, rigid rings are sewn

into the bags at intervals

Reverse air


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In reverse-pulse-jet baghouses, individual bags are supported

by a metal cage, which is fastened onto a cell plate at the top of

the baghouse. Dirty gas enters from the bottom of the

baghouse and flows from outside to inside the bags. The metal

cage prevents collapse of the bag.

Bags are cleaned by a short burst of compressed air injectedthrough a common manifold over a row of bags. The

compressed air is accelerated by a venturi nozzle mounted at

the reverse-jet baghouse top of the bag. Since the duration of

the compressed-air burst is short (0.1s), it acts as a rapidly

moving air bubble, traveling through the entire length of the bag

and causing the bag surfaces to flex. This flexing of the bags

breaks the dust cake, and the dislodged dust falls into a storage

hopper below.

Reverse-pulse-jet dust collectors can be operated continuously

and cleaned without interruption of flow because the burst of

compressed air is very small compared with the total volume of

dusty air through the collector. Because of this continuous-

cleaning feature, reverse-jet dust collectors are usually not

compartmentalized.

The short cleaning cycle of reverse-jet collectors reduces

recirculation and redeposit of dust. These collectors provide

more complete cleaning and reconditioning of bags than shaker

or reverse-air cleaning methods. Also, the continuous-cleaning

feature allows them to operate at higher air-to-cloth ratios, so

the space requirements are lower.

In reverse-pulse-jet baghouses


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Scrubbers

The polluted air first enters the scrubber through a 90

b d d i i t t d b f t d l t


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bend and is intercepted by an array of water droplets.

The air is accelerated as it enters the scrubber by a

venturi plate to increase the number of collisions with

the water droplets.

Water is recirculated from the water tank to the sprayjets. Some of the particles drop out into the water tank

below at this stage. The remainder of the combined air-

water mixture travels through to the perforated baffle.

The water that passes through here produces a dam of

water which acts as another stage of filtration. Finally,

the air-water mixture enters a mist eliminator to removeany moisture present in the air stream. Sludge forms in

the collection tank below.

Application

Food powders, Brick dust, Metal grinding, Explosive dusts

The term "scrubber" has referred to pollution control

devices that use liquid to wash unwanted pollutants from

a gas stream


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Typical Scrubber Data

Cleaning efficiency: 70% of fine dust and 80% of

coarse dirt

Air velocity through the washer: 2 - 3 m/s

Air flow pressure drop resistance: 50 - 140 N/m2

Water pressure before nozzles: 100 - 170 kN/m2

Water consumption: 0.45 - 0.55 l/m3 air (depends

on the temperature of the process air)

Scrubbers are one of

the primary devicesthat control gaseous

emissions, especially

acid gases.
http://upload.wikimedia.org/wikipedia/commons/0/09/Bafflespray.jpg


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Baffle spray scrubber

In addition to using the energy providedby the spray nozzles, baffles are added

to allow the gas stream to atomize some

liquid as it passes over them.

Liquid sprays capture pollutants and

also remove collected particles from the

baffles. Adding baffles slightly increasesthe pressure drop of the system.

A number of wet-scrubber designs use

energy from both the gas stream and

liquid stream to collect pollutants. Many

of these combination devices are

available commercially.

Cyclonic spray scrubber
http://upload.wikimedia.org/wikipedia/commons/0/09/Bafflespray.jpg


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They use the features of both the dry cyclone and the

spray chamber to remove pollutants from gas streams.

Generally, the inlet gas enters the chamber tangentially,

swirls through the chamber in a corkscrew motion, and

exits. At the same time, liquid is sprayed inside thechamber. As the gas swirls around the chamber,

pollutants are removed when they impact on liquid

droplets, are thrown to the walls, and washed back down

and out.

Cyclonic spray scrubbers are more efficient

than spray towers, but not as efficient as

venturi scrubbers, in removing particulate

from the inlet gas stream. Particulates

larger than 5 m are generally collected by

impaction with 90% efficiency.



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To this end, an ejector venturi scrubber uses a

preformed spray, the difference is that only a

single nozzle is used instead of many nozzles.This nozzle operates at higher pressures and

higher injection rates than those in most spray

chambers. The high-pressure spray nozzle (up to

689 kPa or 100 psig) is aimed at the throat

section of a venturi constriction.

Ejector venturi is effective in

removing particles larger than

1.0 m in diameter. These

scrubbers are not used on sub

micrometer-sized particles unless

the particles are condensable

[Gilbert, 1977].

Mechanically aided scrubber


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Mechanically aided scrubber

In addition to using liquid sprays or the

exhaust stream, scrubbing systems can use

motors to supply energy. The motor drives a

rotor or paddles which, in turn, generate waterdroplets for gas and particle collection.

Systems designed in this manner have the

advantage of requiring less space than other

scrubbers, but their overall power

requirements tend to be higher than other

scrubbers of equivalent efficiency.

Spray tower
http://upload.wikimedia.org/wikipedia/commons/4/4d/Inducedspray.jpg


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They consist of empty cylindrical

vessels made of steel or plastic and

nozzles that spray liquid into the

vessels. The inlet gas stream usually

enters the bottom of the tower andmoves upward, while liquid is sprayed

downward from one or more levels.

This flow of inlet gas and liquid in the

opposite direction is called

countercurrent flow.

The gas velocity is kept low, from

0.3 to 1.2 m/s (1 to 4 ft/s) to prevent

excess droplets from being carried

out of the tower.
http://upload.wikimedia.org/wikipedia/commons/2/29/Crosscrtspraytow.jpg


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VOC abatement


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NOx control

Low NOx burners

Selective catalytic reduction (SCR)

Selective non-catalytic reduction (SNCR

NOx scrubbersExhaust gas recirculation

Catalytic converter (also for VOC control

Adsorption systems, such as activated carbon

Flares

Thermal oxidizers

Catalytic converters Biofilters

Absorption (scrubbing)

Cryogenic condensers

Vapor recovery systems

Acid Gas/SO2 control

Wet scrubbers

Dry scrubbers

Flue gas desulfurization

Noise Pollution


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Noise Pollution

Noise pollution (orenvironmental noise) is

displeasing human, animal or machine-

created sound that disrupts the activity or

balance of human or animal life.


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Noise pollution is a type of energy pollution

in which distracting, irritating, or damaging

sounds are freely audible. As with other

forms of energy pollution (such as heat and

light pollution), noise pollution contaminants

are not physical particles, but rather waves

that interfere with naturally-occurring waves

of a similar type in the same environment.

What are the sources

of outdoor noisepollution?

Transportation systems and motor

vehicles

Aircraft noise

Rail noise

Construction


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Car alarms

Emergency service sirensMechanical equipment

Fireworks

Compressed air horns

Grounds keeping equipment

Barking dogs

AppliancesAudio entertainment systems

Loud people


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Annoyance and aggression

Hypertension

High stress levels

Hearing loss

Sleep disturbances


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Cause animals stressInterfering with their use of sounds in communication (reproduction and

navigation)

Reduction of habitat (extinction)


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Tabla. Estaciones de la Red de Monitoreo de Quertaro y parmetros que mide

N ZONA ESTACION SO2 PST

1 Sureste Colegio Washington* X X

2 Sureste Nicols Campa X X

3 Centro Bellas Artes X X

4 Centro Flores Magn X X

5 Noroeste Conalep X X

6 Noroeste Felix Osores X X

* (Se encuentra fuera de operacin)

INEGI/ Instituto Nacional de Ecologa/ Direccin de Ecologa del estado, 2008

4. Air Pollution

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