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Gas Cleaning Efficiency Requirements For DifferentPollutantsWesley C. L. Hemeon

a

aDirector , Hemeon Associates , Pittsburgh , Pa , USA

Published online: 19 Mar 2012.

To cite this article:Wesley C. L. Hemeon (1962) Gas Cleaning Efficiency Requirements For Different Pollutants, Journal

of the Air Pollution Control Association, 12:3, 105-108, DOI: 10.1080/00022470.1962.10468053

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G a s l e a n i n g E F F IC IE N C Y R E Q U I R E M E N T S F o rD i f f e r e n t P O L L U T A N T S

WESLEY C. L. HEMEON Direc tor Hemeon Associates Pittsburgh Pa.

I he shotgun approach to theabatement of air pollution, apparentin control equipment engineering aswell as in public administration, tendsto be frustrating and ineffective orexcessively wasteful. It usually re-sults in installation of corrective equip-ment that either does not result innoticeable alleviation of the problem,or costs very much more than theproblem demands when considered inrelation to the needs of people offendedthereby.The offenses of the shotgun approachmay be illustrated by a consideration ofthe air pollution due to apartment-house incinerators of large cities. Theoffensive qualities of incinerator opera-tion may potentially include the follow-ing three (and unrelated) effects:(./) malodors; 2) obscuring hazedue to fine particulate matter in at-mospheric suspension; 8 ) objectionabledeposition of flakes of paper ash onpeople's premises.The sho tgun app roach fails to considerwhether all three of these are practicallyobjectionable and demands the neutral-ization of all of them for a perfectcontrol of all emissions. If they arenot all important, the cost of resultingequipment may be excessive in theseterms.If careful investigation were toestablish, for example, that the paperash nuisance were of dominant im-portance, then an attack on thisparticular quality could result in thedesign of controls at a fraction of thecost of the more perfect shotgun solu-tions.Another characteristic of this faultyapproach to air pollution problemsis in its failure adequately to considerthe quantitative aspects of some objec-tionable emission in relation to theaffected in hab itan ts. A common de-scription of gas cleaning equipment citesweight collection efficiency, and if thecollection efficiency is in the 90's,some quirk of the adult mind automati-

* Presented at the 54th Annual Meetingof APCA, Commodore Hotel, June 11-15,1 9 6 1 New York, New York.

cally concludes that this is good,perhapsa subconscious reaction resulting fromschool-day experience with scales ofscholastic performance. A moment'sreflection of course indicates in fact howmeaningless such percentagefiguresare.Similarly, the common basis forspecifying gas cleanliness in units ofweight concentration or weight rateof emission ignores the relationshipbetween emission rates and objection-able conditions in the neighborhood,the limitations of which will be betterunderstood from the later discussion.Knowledge is now available wherebythe particular needs of a communitycan be identified and from a considera-tion of the permissible ground con-centration, engineering specificationscan be derived for the performance ofgas cleaning equipment, required heightof stack, or other control measures.We shall illustrate these principles interms of the following common specificnuisance pollutants which encompassthe vast majority of objectionable airpollution effects: smoke andhaze,dust-fall, incinerator paper ash, metal cor-rosion, malodors.Dust Weight Emission Rates

A description of weight rate of emis-sion of dust is subject to some seriousshortcomings. As everyone knows, thequantitative knowledge describing di-luting power of the atmosphere, onwhich subject so much knowledge hasdeveloped in the past 15 to 20 years,provides the basis for calculating ingiven atmosph eric conditions, concentra-tion at ground level resulting from anygiven rate of emission. Note , however,that when the units of emission are inweight per unit time, it is possible onlyto compute ground-level concentrationsin terms of weight per unit volume (e.g.,milligrams per cubic meter) which infact have limited practical application,since these are not the units ordinarilyemployed in describing intensity of par-ticulate matter in the community.By way of introduction to latersubjects, the logic of consistent unitsfor concentration and rate of emission

is in the following ta bulatio n, comparingthem with the familiar units employedfor g a s concentrations:

PollutantConcentrationUnitsGroundLevel

Gas (e.g., SO,)Dust

Ft3gas/ft3air(ppm)Mgms/M 3airCorresponding Stack Gas DataConcentration Emission RateFt3gas/ft3flue gas(ppm)Mgms/ft3flue gas

Ft3gas/hrMgms dust/hr

Concentrations of a gaseous con-taminant, usually expressed as partsper million in the ambient ground levelatmosphere, demand the same kindof units to express stack gas concentra-tions which, when multiplied by thestack gas flow rate, give the emissionrate in cubic feet per hour.Similarly a s to dust, on the second line,ground level concentrations when ex-pressed in concentration units of weightper unit volume, e.g., milligrams percubic meter of air, require concentrationunits and units describing emissionrate in the corresponding units shownin the third and fourth columns. Thelatter are those customarily employedfor the description of stack dust con-centrations and stack dust emissionrates.Note that such stack gas data do notpermit calculation of ground levelconcentrations of particulate matterin the more common units, COHsper 1000 feet, for smoke, etc., or tonsper square mile per month, for dustfall.Stack Gas Smoke and Haze Units

While we have previously discussed1the practical value in measurementof smoke concentration in stack gasesby the filter paper technique and ex-pression of the results in the units,COHs/ft, it is important to our presentthesis to repeat those principles here.The common description of smoke andMarch 1962 / Volume 12 No. 3 105

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haze ground concentration in the unitsCOHs per foot of air, it will now beseen from the preceding discussion ofweight concentration, demands thesame kind of units for a descriptionof smoke concentration in stack gasesand consistent units for total rate ofemission of smoke. These relationshipsare illustrated in the following extensionof the preceding tabulatio n:

PollutantConcentrationUnitsGroundLevel

Smoke, haze COHs/ft air Corresponding Stack DataConcentration Emission Rate

COHs/ft flue gas COHs-ft2/hr

The peculiar units COH-ft2/hr derivefrom the multiplication of COHs/ft(smoke concentration) and ft3/hr(stackgas flow rate), as explained in thepreviously cited reference. xampleSmokeandHaze

In its application to the question ofgas cleaning efficiency specifications,the following example is given:A flue gas stack emission flows ata rate of 550,000 cfm, the smoke con-centration in which has been meas-ured at an average intensity of 4.5COHs per foot (4500 COHs per 1000feet).The mass rate of emission then is550,000 X 4.5 = 2.48 X 106

COH-ft2/minSuppose further t ha t the stack height,the X-distance of interest, and the ap-propriate diffusion parameters (Sutton'sC v, C, n,andu amount to a Meteorolog-ical Ventilation Rate* of 1.5 X 109cu ft/min.The ground concentration thenequals:

Mass R ate of Emission. Met Vent Rate

_ 2.48 X 106 COH-ft2/min1.5 X 109 ft3/min

- 1.6 X 10 ~3 COHs/ft= i.ecoHs/ioooftThe required efficiency of any gascleaning equipment, additional stackheight, or other corrective means,can now be determined by referenceto standards appropriate to a particularsituation. If for example it werefelt that ground level concentrationshould not exceed 0.2 COH/1000 ft, an*It will be noted th at Met Vent R ate ismerely the ratio

Mass rate of emission of any substanceCalculated ground concentration

efficiency of around 80% would bespecified.Stack Gas Dusffall

We had been concerned for manyyears with the problem of measuringdustfall in stack gases. The importanceof such a technique is pointed up by thereminder that the dustfall nuisanceis probably the most common andobjectionable nuisance w ith which peoplecontend. The solution to what hadoriginally been a vexing and elusiveproblem appeared with the successfuldevelopment of the channel elutriator.2While originally conceived as a toolfor the rating of dust collectors of acommon class, it has an equally im-portant function for the measurementof dustfall, based on its ability to meas-ure the spectrum of settling velocitiesin a dust mixture.Moreover, as will be apparent fromthe following discussion, in the ratingof du st collectors the device is applicablenot only for describing the percentweight efficiency but also the dustfallcollection efficiency. A review of theunits describing ground level dustfalland stack dustfall emission rate, follow-ing the logic in the preceding example,lookslike this:

PollutantConcentrationUnits-GroundLevel

Dustfall Tons/mile2/monthLbs/ft2/hr/ Corresponding Stack Data

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TableIExampleDataandCalculationofDustfall Intensityin aStackGasand Dustfall Emission Rate

SettlingVelocityRange, ft/min a)>2010-207-104-72-4V2-2 v

VMedianSettlingVelocity, ft/min b)20158. 55. 531.250. 5

MeasuredWt % EachFraction c)5. 31.55. 89. 616.034.727.1

C-QWt RateEach Fraction,lbs/mina d)4.51.34.98.213.629.523.0

C-Q-VDustfallEmission Ratein EachVelocity Range,lb-ft/min2 e)90194245413712

Based on, separately measured, total lbs/min = 85; 286 lb-ft/min2.This may be converted to the units,

tons per square mile per month, bymultiplying it by 0.60 X 109which re-sults in the calculated dustfall rate of14.3 tons per square mile per month.

This is the hourly rate, indicatingthat it applies only when the wind is in aparticular direction. Suppose that ananalysis of wind direction frequencyshows that the wind blows toward thislocation four percent of the total time,then the average dustfall rate is

14.3 X 0.04 = 0.6 ton per squaremile per month.

If the monthly measured rate at thislocation be, for example, 30 tons persquare mile per month, it is concludedthat the particular dustfall emissionaccounts for two percent of the totaldustfall.

The collection efficiency required of adust collector to reduce this contributioncan now be specified on an engineeringbasis and by reference to the circum-stances of a particular locality. Re-ferring to Table I, it can be seen that thedustfall contributed by particles havingsettling velocity less than two feet perminute* is 37 + 12, i.e., 49/286 = 17%.Therefore if the dust collector removedall larger particles, the dustfall col-lection efficiency would be 83%. Theweight collection efficiency requiredfor this result is found by reference tothe corresponding figures in the thirdcolumn (34.7 + 27.1) and indicatesthat a dust collector having a weightefficiency of only 38% would reducethe dustfall contribution to 17%.Incinerator PaperAsh

Waste incinerators of the type usedin large city apartment houses may beresponsible as indicated in the intro-duction, for two separate kinds ofparticulate nuisance (in addition to thatof malodors): CO haze and smoke;{2 paper ash flakes. The first of these*The settling velocity, Ut,of sphericalparticles, sp gr = Z, in air at 70F,corresponds to particle diameter, dmmicrons, as follows:

dm = 10.WUt/Z

would logically be measured and ratedby reference to the COH units pre-viously discussed. The second, how-ever, requires separate consideration.

While deposition of paper ash isessentially a dustfall nuisance, it isnot adequately described by the con-ventional measurement of dustfall inweight units, because the magnitudeof the paper ash nuisance is out of allproportion to the weight of suchparticles. These considerations sig-nify the need for a quite different meansfor measurement of paper ash dustfall.With this single modification, all of theprinciples discussed in the precedingsection on dustfall will still apply.

We have found that incinerator stackgas sampling with the channel elutriatoris an effective means for capturing themin a systematic manner and unfractured.Adhesive-coated glass slides on theelutriator plates serve conveniently toreceive them. Evaluation of the quan-tity is most logically determined by alight-measurement means because of theoptical nature of the nuisance effect.Percent light transmission and sub-sequent transformation according tothe well-known principle embodied inthe quantity log Io/I appears to be asatisfactory means for this purpose.One could thus develop and employ asystem of measurements analogous toCOH units.

Suppose such optical density unitsfor paper ash flakes were thus describedas PAF units. Unlike the case ofdustfall, they would be related onlyto time since area cancels out in theratio Io/I just as in COH units forstained filter paper. The relationshipbetween concentrations and emissionrates would thus appear as in the follow-ing tabulation:

PollutantConcentrationUnitsGround Level

Paper ash flakes PAFs/hrCorresponding Stack DataConcentration Emission Rate

Corrosion PotentialThe same principles have been inter-

estingly applied by us in the measure-ment of corrosion potential, a termwhich, it will be noted, deemphasizesthe chemistry of materials causing thecorrosion and emphasizes as before, theobjectionable effect. Selected speci-mens of metal, glass and other materialsare mounted in an enclosure which iscontinuously ventilated with dilutedstack gases. At the end of the exposureperiod, the degree of corrosion is meas-ured, usually by determination of re-duced light reflectance, and the rela-tionship represented once again by logIo/I applied to a description of the corro-sion effect in terms of reduced capacityfor reflection of light. The resultingnumbers which for convenience we shallterm COR units, appear in the tabula-tion below as units of concentration andprovide the basis for description of corro-sion potential emission rate. (The ab-sence of area in the concentration unitshas the same explanation as in the pre-ceding case.)

PollutantConcentrationUnitsGround Level

Corrosion potential CORs/hrCorresponding Stack Data -Concentration Emission Rate

CORs/hr COR-ft3/hr2

PAFs/hr PAFs-ft3/hr2

A specification for gas cleaning equip-ment can now be framed on exactly thesame basis as the other pollution effectsto insure any desired degree of accom-plishment relative thereto in the neigh-borhood.Odors

The handling of odor problems on aperfectly quantitative basis follows thesame principles as apply to the previ-ously discussed pollutants, but subjectto an interesting difference in detail per-taining to a description of odor concen-tration.

The chemistry of an odor, like thechemistry of corrosion potential, is ofno particular interest. In fact, its con-sideration would only confuse the issue.It follows that lack of knowledge of itschemistry prevents a description of odorconcentration directly. This, however,is no bar to a quantitative t reatment.The approach employed in this case is todeal entirely with ratios, as illustratedby the tabulation of units below, inwhich Ca represents concentration ofodor in the ground level atmosphere,Cg represents concentration of odorin the stack gas, and Ct represents theconcentration corresponding to thethreshold of perception as determinedby quantitative subjective measure-ments, according to techniques outlined

arch1962 / Volume12 No. 3 1 7

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Table IISummary of a System of Consistent Units for Expressing GroundConcentrations an d Stack Gas Con centrations Respectively for Various A irPollution Qu alities; Whereb y Gas C leaning Performance C an Be Related toGround Level Concentrations

PollutantGas (e.g., SO2)DustSmoke, hazeDustfallPaper ash(incinerator)CorrosionpotentialOdors

elsewhere.3

PollutantOdors

ConcentrationUnitsGround LevelFt3gas/ft3air (ppm)Mgms/M3airCOHs/ft airTons/mile2/hrLbs/ft 2/h rPAFs/hrCORs/hrCa/C t

ConcentrationUnitsGroundLevelCa/Ct

- CorrespondingStack DataConcentration Emission Ratec c t Ft/hr(dilution air)

. Corresponding Stack Gas DataConcentration Emission Ra teFt3gas/ft3flue gas Ft3gas/hr(ppm)Mgms/ft3flue gas Mgm s/hrCOHs/ft flue gas COHs-ftVhrLbs/ft2/hr Lbs-ft/hr2PAFs/hr PAFs-fF/hT1CORs/hr CORs-ft3/hr2Cg/Ct F t3 /h r

ment methods appropriate to each de-mand the same kind of measurement atground level as in stack gases. The airpollution qualities discussed and thesystems of consistent units are summa-rized in Table II . The approach out-lined here permits statements of qu anti-tative relationships between emissionquantities and concentrations at placesof habitation and can provide a basis fordevelopment of sound policy for abate-The subjective methods of measurementprovide a direct indication of the ratioCg/Ct which, when multiplied by thetotal flue gas rate of emission, results inthe units cubic feet of air per hour asshown in the last column.This quantity, a description of thetotal rat e offlowof air that would be re-quired to dilute all the odor to thethreshold level, can be applied to anyquantitative engineering analysis of anymeans of odor abatement whether it beby scrubbers or by employment of tallstacks to elevate the source aboveground level. In the case of the latte rwhich refers to the diluting capacity ofthe atmosphere, the kind of calculationsillustrated in the following can be ap-plied to a qua ntita tive conclusion. Asin all the preceding examples, the ratioof mass rate of emission to Meteoro-logical Ventilation Rate prevailing inspecified weather conditions, stackheight and distance, results in the re-sulting concentration at the location ofinterest:

Mass Ra te of Emission ft3/h rM et Vent Rate ft3/h r

In this case the "concentration" ap-pears as a dimensionless ratio whichdescribes the number of times the actualodor concentration bears to the thresh-old, i.e.,Ca/Ct.Conclusion

We have indicated in this paper theneed for sorting out the different typesof air pollution nuisances that mayafflict a community and how measure-

ment of the particular air pollutionnuisances that are objectionable in anycommunity.It is sometimes suggested that a primerequiremen t for imp ortan t progress in airpollution abatement is in the discoveryof cheaper methods and equipment forthe cleaning flue gases. We disagreewith the implications of this viewpoint.We believe the real need is for more dis-criminating appraisal of the characterof air pollution in a community for thedesign of a pin-pointed attack on theparticular problems found to be of prin-cipal significance.Such progress as has been ma de in th eLos Angeles area (and scientifically theprogress is quite satisfactory) has de-rived from adherence t o th is principle ofdiscrimination and a quantitative ap-praisal of their particu lar problem. Itis hoped that the ideas presented in thispaper will stimulate equally good andconstructive scientific efforts for attackon the important problems of the East.REFERENCES1. W. C. L. Hemeon, G. F. Haines, Jr.,and Harold M. Ide, "Determination ofHaze and Smoke Concentrations byFilter Paper Samplers,''Air Repair3 :1, 22-29 (August 1953).2. W. C. L. Hemeon, G. F. Haines, Jr.,and S. D. Puntureri, "Rating of DustCollectors According to Dust SettlingVelocities,'' J. Air Poll.Control Assoc.1 1 :6 , 264-67 (June 1961).3. Hemeon Associates, "Odor ControlResearch and Engineering" (June1958).

1962 ANNUAL MEETINGM A Y 2 0 - 2 4

BOARD OF DIRECTORSHEARS ACTION REQUESTS

At its meeting on November 14, 1961,in Pittsburgh, the Board of Directorsreceived and discussed the followingletter from Raymond Smith, Chief ofAir Pollution Control, City of Phila-delphia, who was acting in his capacityin New York as Chairman of the Con-trol Officials' Conference Committee."Being duly assembled at their forumon Tuesday June 13, 1961, the controlofficials present did resolve by unani-mous vote that the chairman of saidforum (Raymond Smith) should conveyto the Board of Directors of the AirPollution Control A ssociation the follow-ing requests for action:

1) That the designers and manufac-turers of diesel operated vehiclesbe requested by the Air PollutionControl Association, through itsBoard of Directors, to ener-getically undertake a programfor the abatement of odors charac-teristically associated with dieselengine operation.

2) That the Board of Directors ofthe Air Pollution Control Asso-ciation establish a more effectiveand expeditious means of trans-mitting information between theBoard and control officials onmatters of specific concern tocontrol officials.3) Th at the Bo ard of Directors of theAir Pollution Control Association

formally report to the member-ship each year on the manner inwhich each objective of the As-sociation, as listed in Article IIIof the By-Laws, has been metduring the past year with specialreference to sub-items (a), (b),(c), (e), (f), and (h) of ArticleIII ,Section 1.4) Tha t the Executive Secretaryadvise, in writing, all controlofficials who are members of theAir Pollution Control Associationof the action taken by the Boardon the disposition of each of the

above three requests for actionprior to the 1962 annual meetingof the Association.The following actions were taken bythe Board on each paragraph of thelett er: S. Smith Griswold has requestedin his capacity as Chairman of TA-10Vehicular Exhaust Committee to pre-pare an appropriate statement for theBoard on diesel smoke and fumes. Noaction was taken on paragraph2. Withrespect to paragraph 3 the ExecutiveSecretary is to prepare a report annuallyfor the President. For item 4, this

note informing the member of the aboveactions is to be pub lished in the JOURNAL. 8 Journal of the Air Pollution Control Association