abatement techniques

8/12/2019 Abatement techniques

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TECHNIQUES


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IndustrialAbatementDevices

Electrostatic

precipitatorsFabric

Filters

Wet

Collectors

Additional

Oxidation ofExhaust gases

Of

report

FUELSELECTIONANDPROCESSING

Fuel flow

RESTRICTION

TheAutomotivePRoblem

Remaining

Problems

InertialCollectors

Combustion

Devices

OUT


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From the viewpoint of industry itmakes good sense to ameliorate the airpollution problem. Reducing the level ofemitted particles during the industrialprocesses helps keep plant damage at aminimum. In addition, efficient cleaningequipment which effectively removescontaminants from the air will prevent thismatter from finding its way back into the

work areas. This feature is of importance inindustries having high quality controlstandards. Also, materials recovered fromemissions may prove a valuable source fromwhich to reclaim usable material. Further, thefine dust or organic matter which might be

found in pollutants are dangerouslyconducive to fire and explosion. Efficientremoval of these contaminants can greatlyreduce this hazard. Finally, efficient cleansingof the air may enable this air to berecirculated into the work areas.


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FUEL

SELECTIONANDPROCESSING

It will be remembered that theburning of coal emits such contaminants

as carbon and particulate solids, sulfurdioxide, nitrogen dioxide, and organicpollutants such as organic acids andhydrocarbons. Burning a ton of coal

consumes about 27,000 pounds of air, andthe contaminants mentioned are released.Yet, there exist some techniques forconverting this inherently " rty " fuel toa form which is essentially cleaner.

A


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Washing Grading&

useful methods of reducing emissions

removes some

of the mineral

matter and the

finer particles

is the process of segregating the coal

according to size of the pieces. Grading is

an especially important method, because

it allows more uniform selection and

burning of equally sized pieces, which

promotes a more uniform passage of air

to support combustion in the fuel beds.

The fine coal dust which is isolated may

also be burned in special equipment.


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A new process being investigated for

converting coal into synthetic fuels

employs a technique called

magnetohydrodynamics. It contemplates

making a plasma of the coal, from which

are extracted compounds which will

allow synthesis of gasoline, crude oil, jet

fuels, pipeline gas, fuel gases, and by-

product electricity. Another by-product isa char residue which can be used as a

boiler fuel with much of the sulfur

removed, thus resulting in a fuel to help

reduce sulfur dioxide pollution.

Converting coal into

coke

another way

to make coal

a cleaner fuel

Coal is heated in the absence

of air and the volatile matter is

driven off and collected in a

condensation chamber to be

used in the manufacture of

other by-products. The end

product of the heating process

is coke, an almost pure form of

carbon. When coke is laterused as a fuel, few of the

original contaminants remain.


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One of the most widely used of the proposed

solutions to the problem of sulfur dioxide

pollution from coal lies in the use of low-

sulfur coal; it is elementary that a smaller initial

sulfur content will produce less sulfur dioxide.However, the geographical distribution of coal

presents an obstacle to this solution.

Sulfur can be removed from coal in several differentways. One method removes the pyritic forms of sulfur, which aremetallic sulfide compounds, by first crushing the coal to a finestage; denser pyrites may then be removed by gravity separationprocesses. The fine particles may also be pretreated by additionof magnetic materials and the pyrites removed by magneticseparation.

Still another technique of removing the pyrites isthrough oxidation. This is done by introducing microorganismssuch as bacteria to oxidize the pyrites. Experiments withKentucky coal indicate that up to 60% of the pyrites may beremoved from the coal after exposure to the bacteria for oneweek; absent bacteria, only 10% of the pyrites were removed.


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The problem with these techniques is that theyare not yet commercially feasible. The costsassociated with abating sulfur oxide emission bythese methods pass a considerable increase inoperating costs to the users of coal. It was notedearlier that the burning of oil and gas releases

considerably less of these contaminants thandoes the burning of coal. Thus, fuel selectionmay offer the most practical presentsolution to sulfur oxide emission from coalcombustion.


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Burning fuel in the internal combustion engine is the singlelargest source of air pollution in the United States. To reducepollution from this source, the only methods which have shownsignificant promise are those which employ either a change inthe operating conditions of the engine or oxidation of the

exhaust gases.

ThEAutomotive

PROBLEM

b


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Probably the most promising abatementdevice for cars is that which restricts fuelflow to the carburetor deceleration. Many of

the hydrocarbons emitted under decelerationresult from incomplete combustion of theextremely rich air-fuel mixture induced intothe engine. If a slow, uniform dosing of thethrottle can be accomplished during

deceleration, the air-fuel ratio will beimproved, causing less pollution fromunburned compounds in the gasoline. Onerestrictive device operates to bleed air intothe carburetor during deceleration, breaking

suction to stop fuel flow by reducing themanifold vacuum. Another device in thiscategory, the positive fuel shut-off system,interrupts the fuel flow by mechanicalmeans.

FUELFLOWRESTRICTION

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All abatement techniques which oxidize exhaustgases employ some form of afterburner. Onedevice recycles the exhaust gases to a pointbehind the carburetor so the unconsumedhydrocarbons can be reburned in the cylinders.This technique is estimated to reduce overallautomobile pollution by 30%.

In addition to the feedback method,tailpipe emissions can be reduced by either direct

flame afterburning or catalytic afterburning. Theformer uses a direct flame in a very hightemperature chamber to reburn the exhaustgases. The latter method adds a catalyticcompound to promote combustion, thus enablinguse of lower temperature combustion chambers.

Depending upon the operating conditions of theengine, devices of this type may be from 40% to90% efficient in removing the pollutants beforethe gas is expelled into the air. Both of thesemethods are costly, and the lead compounds ingasoline severely limit the life of the system.

OXIDATION

OFEXHAUSTGASES

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The ultimate solution to the automotive pollutionproblem may require abandonment of the internalcombustion engine, at least for commuter use. In theirstead, mass rapid transit systems must be developed.In the alternative, electric cars driven by batteries orsome other source of electricity may eventually beused.

The present drawback in conveyances of thiskind is lack of a commercially feasible power source. Aprototype built by Westinghouse and driven byconventional lead-acid batteries is a two-passenger vehicle with a top speed of only 25 m.p.h.

and a range of fifty miles. Ford Motor Company hasestimated that the frequent stops and starts of citydriving would reduce by one-half the range of electriccars relying on conventional batteries.

These stumbling blocks may eventually beremoved by the development of more advanced fuelcells. This is an engine-generator system in whichchemical energy is converted to electrical energy.Unlike the battery, fuel cells use hydrocarbon fuelsand have twice the efficiency of the internalcombustion engine. Although many impediments tothe practical application of these concepts remain,science continues to make impressive advances. It is tobe hoped that technology can win the race againstpollution.

REMAININGPROBLEM

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The equipmentfor cleaning

industrialexhaust may

be categorizedas follows:

TheABATEMENT

DEVICES

c

electrostaticprecipitators

Fabricfilters

wet

collectors

inertialcollectors

combustiondevices

These devices areused to separate

contaminants fromthe exhaust from

industrialprocesses.


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Basically, the function of the electrostatic

precipitator is the removal of solid or liquidparticles from the stream of exhaust gas. This isaccomplished by passing the stream of gas betweena pair of electrode plates; these consist of one plate,called the discharge electrode, which bears a highnegative electrical charge and another plate which iselectrically grounded, called the collecting electrode.

If the difference in voltage between these plates ishigh enough, a corona will surround the dischargeelectrode. Then, as the stream of gas passes throughthis corona, the gas will ionize, splitting up intocharged particles which behave like little magnets.The electrical field between the electrodes causesthese ions to migrate rapidly toward the collecting

electrode. These contaminant particles may then beremoved from the collecting electrode.


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Electrostatic precipitators are capable of very high

efficiencies- 98% on some forms of fine dust

pollutants. They are capable of handling large

quantities of exhaust gas even though the gas may be

at a high temperature. Operating and maintenance

costs are low. Among the disadvantages are the highinitial cost of the equipment, large space requirements

for installation, and the hazard of explosion if the dust

particles are in a finely divided state. The users of

these devices include cement mills, paper mills,

petroleum cracking plants, and sulfuric acid plants.


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Filtration methods of removing dusts,

mists, and fumes from gases are among themost well-known abatement techniques. Thedevice consists of a chamber containing manyfilters constructed of fine mesh materialthrough which the air to be cleaned mustpass. As collection of the particles progresses,

a filter cake-deposits of filtered matter whichbuild up on the fabric-begins to accumulate.As the cake forms, the efficiency of thecleaning process improves, because thepreviously trapped particles act as additionalfilter material. Periodically, the filters areshaken and the accumulated contaminantscollected in a hopper.


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Wet collectors, or wet scrubbers, may be defined as gas-

cleaning devices in which the separated gas, vapor, orparticulate matter is dissolved or suspended in a liquid.These devices require introduction of the gas at a veryhigh velocity. As the gas stream is forced through thescrubber, a spray of liquid (usually water) is passedthrough the gas, inducing turbulence.

The gases and vapors are absorbed by the liquid;

the mist and dust particles may impinge on the surface ofthe liquid and be carried off; or the liquid may washdown the walls of the scrubber where some particleshave adhered. Regardless of the precise mechanismwhich removes the contaminants from the gas stream,the separated materials are carried off with the liquid anddisposed of as the liquid is discarded. The primaryadvantage of the scrubber lies in the fact that it can becontinuously operated, and the waste materialscontinually separated.


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The most common form of inertial collector isthe cyclone collector. The cyclone is simple,

inexpensive, can be made of almost any kind ofmaterial, and has no moving parts. The gas to becleaned enters the device tangentially through whatare known as whirl vanes- surfaces set at angles to thegas flow which impart a rotational motion to the gasflow as it is forced around them. Centrifugal force thenthrows the particles toward the walls of the chamber.

The solid particles or liquid droplets settle through airat a slow velocity under the influence of gravity to acollecting hopper below. The swirling flow of gas,called a vortex, is reversed at the bottom of thechamber by surfaces similar to the whirl vanes and isredirected toward an outlet conduit. The smaller thediameter of the cyclone tube, the greater the efficiency

since the centrifugal acceleration increases withdecreasing radius of rotation.


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Another way of handling certain volatile contaminants is to

burn them before they can be expelled into the atmosphere.Most organic and inorganic combustibles can be incineratedto harmless carbon dioxide and water vapor. But hazardsinhere in this method of disposal. Although the effluent mayconsist of a single chemical compound, discharge from anindustrial process is more commonly a heterogeneousmixture. Vaporized combustibles usually present a fire orexplosion hazard, and combustion disposal must becarefully designed to account for the explosive propensitiesof the different compounds present.

Account must also be taken of the relativeconcentrations of compounds present in the discharge gas.Combustion temperature may be reduced to some degreeby introducing a catalyst to the combustion chamber,further decreasing the chance of explosion.


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VOLATILEORGANICCOMPOUNDS

VOCs

One of the major causes of both indoor and outdoor air

pollution are Volatile Organic Compounds (VOCs). VOCs

are found within a wide range of consumer products,

including solvent-based paints, printing inks and petrol

products. Due to the proven contribution of VOCs to airpollution (and ultimately smog) VOC control regulations are

enforced around the world to protect the environment

against them.


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Activated Carbon is a popular air pollution

control method. One of the most common

forms of carbon treatment in air pollution

control is carbon adsorption. This method

sees the use of dry chemical scrubbing

media such as carbon filters for theadsorption of fumes from the air.

ACTIVATEDCARBON


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Biofilters:

Clearing

The

Air

Biofiltration is a method of pollution

control in which process pollutants

are biologically degraded using

microorganisms. The most common

air pollution control application ofbiofilters is the microbiotic oxidation

of contaminants in the air. Biofilters

use living materials to degrade

pollutants when immobilized in the

biofilm. Applications of this method

include treatment of the off-gas from

wastewater treatment plants, after

painting and during the manufacture

of food products.


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Driving

TowardsCleaner

Emissions

Car engine emissions are

recognized as one of the leading

causes of air pollution. CatalyticOxidizers are placed in the exhaust

system of cars to reduce emissions

from the exhaust pipe. The ideal

byproducts of a cars engine are

carbon dioxide, some water andnitrogen. However in reality engines

continue to release unburned

hydrocarbons which damage the

environment. Catalytic oxidizers now

oxidize many of the unburnt

hydrocarbons from an engine,

leading to cleaner emissions from

cars.


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abatement techniques

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