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

    b1

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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

    b2

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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

    b3

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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