certain inorganic aerosols like NaCl of CsI produce pulses with marked diminuation or disappearance of one or both shoulders. Of possible practical significance were certain biological aerosols like yeast (Saccharomyces cereuisiae) and a mold (Aspergillus niger) which produced unique waveforms. Presence of these aerosols in the air might be detected by an FID equipped with a suitable pattern recognition system. The electrodes of the FID described in this paper form a parallel plate capacitor with the flame centered between the plates. As a particle is consumed in the flame and the resultant charge carriers are separated by the field, there effectively occurs a transient localized change in the permittivity of the medium between the electrodes. The result of such an “upset” event is a transient fluctuation in the field strength and also in the steady-state charge accumulated at each electrode or plate. During or following this process, the created charge carriers then arrive at their respective electrodes. The resultant output current pulse would be the algebraic sum of these two effects. If one takes a typical mobility of positive ions in ambient air as 1cmz/V-s ( 1 1 ) and allows for the effect of the elevated temperatures within the FID enclosure upon the mean speed, then at 500 Vdc and a flame-to-plate distance of approximately 7-8 mm, a time-of-flight of less than 3 ms is reasonable. This is consistent with the duration of the FID pulse. If part of the FID current pulse were induced by the changing electric field, the resultant flow of charge toward or away from the collection electrode might be distributed relatively evenly across the electrode surface. On the other hand, the arrival of gaseous charge carriers a t the electrode surface might not be distributed as evenly. Thus, it would not be unreasonable to expect the flux rate of charge carriers impinging
on the surface of the collection electrode to be greatest in an area directly across from the flame. An experiment to distinguish these two effects resulted in the development of a novel flame ionization detector (12). Literature Cited (1) Ohline, R. W., Anal. Chem., 37 (1),93 (1965). (2) Ohline, R. W., Thall, E., Oey, P. H., ibid., 41 (21,302 (1969). (3) Frostling, H., Lindgren, P. H., J. Gas Chromatogr., 4, 243
(1966). (4) Crider, W. L., Strong, A.A.,Reu. Sci. Instrum., 38,1772 (1967). ( 5 ) Bolton, H. C., McWilliam, I. G., Anal. Chem., 44 (9), 1575 (1972). (6) Altpeter, L. L., Jr., Pilney, J. P., Rust, L. W., Senechal, A. J., Overland, D. L., “Recent Developments Regarding the Use of a Flame Ionization Detector as an Aerosol Monitor”, in Symposium on Fine Particles, Minneapolis, Minn., May 2&30,1975; Academic Press, New York, N.Y., in press. (7) Berglund, R. N., Liu, B.Y.H., Environ. Sci. Technol., 7 (2), 147 (1973). (8) Breed, R. S., Murray, E.G.D., Smith, N. R., “Bergey’s Manual of Determinative Bacteriology”, 7th ed., Williams and Wilkins, Baltimore, Md., 1957. (9) Dean, J. A., “Flame Photometry”, McGraw-Hill, New York, N.Y., 1960. (10) Calcote, H. F., “Ion Production and Removal in Flames”, in Eighth Symposium (International) on Combustion, Pasadena, Calif., Aug. 29-Sept. 2, 1960. (11) Von Engel, A., “Ionized Gases”, 2nd ed., Oxford-at-the-Clarendon Press, 1965. (12) Altpeter, L., Jr., Loucks, S. R., Overland, D. L. “Evaluation of the Performance of Multiple-Electrode Flame Ionization Detector (MFID) As an Ambient Aerosol Monitor”, in Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1-5, 1976. Received for review May 22, 1975. Accepted April 2, 1976.
Fractional Collection Efficiency of Electrostatic Precipitator for Open Hearth Furnace Trace Metal Emissions Robert B. Jacko”, David W. Neuendorf, and Francois Faure Environmental Engineering, School of Civil Engineering, Purdue University, W. Lafayette, Ind. 47907
Simultaneous inlet and outlet particulate sampling was performed on an electrostatic precipitator controlling a group of open hearth steelmaking furnaces in Northwest Indiana. Both standard EPA particulate emission tests and in-situ aerodynamic particle size tests resulted in the quantification of the overall and fractional particulate collection efficiency of the precipitator. Metal fractional collection efficiencies were also determined for Cd, Pb, Zn, Cu, Fe, and Ni, and a minimum in collection efficiency was found at 5-7 (r aerodynamic diameter. This efficiency decrement appears to be specific for precipitators as it was not observed for baghouses or wet scrubbers. Agglomeration of fine charged particulate within the precipitator and in downstream ductwork is proposed as a possible cause of observed collection efficiency behavior.
The open hearth steelmaking process produces particulate emissions containing a number of metals in varying concentrations ( I ). Electrostatic precipitators are often relied upon for control of these emissions, although little is known concerning the fractional collection efficiency of trace metals in these devices. Such factors as particle size segregation and varying electrical resistivity of the metals can be expected to have an effect on the collection efficiency. The research de1002
Environmental Science & Technology
scribed in this paper characterized the total particulate and trace metal fractional collection efficiency of an electrostatic precipitator controlling a battery of seven open hearth furnaces. Emission tests following standard EPA particulate sampling procedures were performed simultaneously at the inlet duct and outlet stack of the precipitator. Particle size distributions were measured with the Andersen in-stack sampler. Gravimetric and atomic absorption analysis of the resulting samples was used to determine the overall and fractional collection efficiency for total particulate and the trace metals Cd, Pb, Zn, Cu, Fe, and Ni. Facility Description The electrostatic precipitator (ESP) serves a group of seven basic open hearth furnaces. Figure 1is an end elevation of the ESP building, including the original (capped) stacks, mixing chamber, and ESP exhaust stack. A manifold collects gases from all seven furnaces, depositing them in an 18 X 20 X 32 f t mixing chamber. Ten 8 X 7 f t rectangular ducts carry gases from the mixing chamber to the precipitator itself. A second manifold directs gases from the five ESP chambers to a 22-ft diam exhaust stack. The furnaces were operated according to a scraphot metal practice ( 2 )common among basic open hearth facilities. Since
particulate emissions from an individual furnace vary considerably in quantity and composition as the cycle or “heat” progresses ( I ) , direct input to the ESP would result in undesirable extremes in grain loading, particle resistivity, etc., a t different points in the precipitator. Thus, the mixing chamber is used to provide a homogeneous mixture of particulate at the E S P inlet. The ESP itself contains 1360 flat collection plates, spaced approximately 10 in. apart. There is a total of 490 000 ft2 of collecting surface, for a ratio of collecting area to gas flow of approximately 304 ft2/1000 ACFM. Procedure
Overall collection efficiency was determined by sampling simultaneously the precipitator inlet and outlet, using EPA Method 5 ( 3 ) .The EPA sampling trains were modified by the use of 5% nitric acid in the first two impingers to ensure solution of any Cd, Pb, Zn, Cu, Fe, and Ni particulates which escape the filter. The impinger contents were not evaporated to dryness to determine condensable particulate fraction, since the formation of nitrates would bias the determination. The probe backwash, glass fiber filter, and impinger contents were 4
E.S.P. EXHAUST /-4TAcK
ORIGINAL STACKS
f
(7)
analyzed separately for Cd, Pb, Zn, Cu, Fe, and Ni, using a Perkin-Elmer atomic absorption spectrometer equipped with a deuterium background corrector. Note that probe backwash sample retrieval procedure is critical when analyzing for trace metals ( 4 ) . Precipitator inlet testing was performed in one of the 8 X 7 ft rectangular ducts between the mixing chamber and the ESP. Since the sampling ports were in a turbulent location due to the proximity of a 90’ bend and other flow disturbances, 32 traverse points were selected according to EPA Method 1 ( 3 ) .ESP outlet samples wer? taken from the 22-ft diam exhaust stack. Examination of a number of stack velocity profiles on two diameters revealed that the velocity was sufficiently stable to justify traversing a single diameter. A typical velocity profile across the stack diameter which was traversed is shown in Figure 2, along with the 18 traverse points selected on this diameter. Fractional collection efficiency was determined by sampling simultaneously, with Andersen fractionating heads ( 5 ) attached to the nozzle end of the probes, a t the same inlet and outlet locations used for the overall particulate samples. Each Andersen sample was taken a t a constant isokinetic sample flow rate at a single point near the average velocity point in the duct. For all inlet and outlet samples (Method 5 and Andersen), isokinetic sampling conditions were maintained. The isokinetic variation ranged from 89.4 to 103%,averaging 96.8%.A value of 100 f 10% is generally considered acceptable. Results and Discussion
MIXING CHAMBER
ARTH
Figure 1. End elevation of ESP facility at Northern Indiana open hearth plant
:I
ON
0.6
I
,
’.
I
TRAVERSE POINTS
I
I
I
I
Figure 2. Velocity profile along diameter of ESP outlet stack
The particulate concentrations and mass flow rates for the grain loading samples are presented along with various sampling conditions in Table I. Note that the electrostatic precipitator handles an average of 1.6 X lo6 ACFM with an outlet particulate concentration of 8 X grains/SCF. The total inlet mass flow rate (lb/h) in Table I was calculated from the grain loading (gr/SCF) measured at the mixing chamber duct and the volumetric flow rate (SCFM) measured at the exhaust stack. The various overall metal collection efficiencies in Table I1 were calculated by applying Equation 1 to the particulate concentrations in Table I and the metal concentrations obtained by atomic absorption analysis.
where qrn = collection efficiency for metal, % C I = inlet particulate concentration, gr/SCF CO = outlet particulate concentration, gr/SCF M I = inlet particulate metal concentration, % M o = outlet particulate metal concentration, % Metal emission factors were also calculated for the precipitator-controlled furnaces and are presented in Table 111. Note that the average emission factor for zinc is 26% higher than lb/ton that for iron (3.4 X lo-* lb/ton for zinc vs. 2.7 X for iron). This is the result of the variation of precipitator efficiency among the metals. The 99.5% collection efficiency for iron allows only 0.5% of the ESP iron input to escape. In contrast the 97.8% efficiency for zinc allows 2.2% of the ESP zinc input to escape. Average lead emissions (5.2 X 10-3 lb/ ton) likewise appear to be relatively significant due to the 98.5% collection efficiency for lead. Figure 3 presents the total aerodynamic particulate size distributions for the ESP inlet and outlet. The mass median diameter (50th percentile or geometric mean size) of both the inlet and outlet particulate is relatively small owing to the characteristic nature of metallic fume which predominates Volume 10, Number 10, October 1976
1003
Table I. Stack Sampling Conditions for Grain Loading Tests of Open Hearth Electrostatic Precipitator Run no.
Time
Stack teomp, F
Moisture vol fraction
Volumetric gas flow, A C F M
Volumetric gas flow, Std Cond, SCFM
Grain loading, grains/SCF
Mass emission rate, Ib/h
Stack velocity, ft/s
% lsokinetic
Precipitator outlet stacka 611 1/74 2:25-5:30 p.m. 6/12/74 11:55-1:55 p.m. 6/12/74 5:OO-7:lO p.m.
1
435
0.057
1 620 000
937 000
0.0117
93.7
71.1
103
2
460
0.049
1 590 000
897 000
0.0045
34.7
69.9
101
3
460
0.056
1 630 000 915 000 chamber ductb
0.0083
65.1
71.3
101
Mixing
611 1/74 2:25-5:30 p.m. 6112/74 11:55-1:55 p.m. 6/12/74 5:OO-7:lO p.m.
1
463
0.037
168 000
93 500
0.340
2730C
49.5
94.2
2
520
0.057
159 000
83 400
0.375
2880C
46.7
92.2
3
520
0.040
162 000
85 000
0.331
2590C
47.6
89.4
aHandles seven furnaces. bHandles approximately one-tenth of total gases. cTotai, a i l 10 mixing chamber ducts.
Table II. Overall Metal Collection Efficiencies of Open Hearth Electrostatic Precipitator
Run no.
1 2 3 Av
Cd, %
98.7 97.2 98.0 98.1
Pb, %
98.1 98.8 98.6 98.5
Zn, %
97.7 97.8 97.9 97.8
Cu, %
Fe,
97.1 98.7 96.1 97.3
99.7 99.0 99.7 99.5
%r
l5
Totala particulate, %
Ni, %
98.5 98.4 99.3 98.7
96.6 98.8 97.5 97.6
aTotal sample including trace metals.
t
O
Y
0
I
61
0
4
5
2 31 I
/ /-
Table 111. Trace Metal Emission Factors for Electrostatic Precipitator-Controlled Open Hearth Furnaces ( L b metal emission/ton steel produced) X l o 4 Run
Cd
Pb
Zn
Cu
Fe
Ni
Totala particulate
1 2 3
1.1 1.2 0.81
Av
1.0
79 38 38 52
540 190 280 340
5.9 4.0 6.4 5.4
120 590 89 270
1.1 1.1 1.8 1.3
4600 1600 2800 3000
X: OUTLET
CUMULdTlVE PERCENT (LESS THAN STATED DIAMETER)
aTotal sample including trace metals.
Figure 3. ESP inlet and outlet total particulate size distribution
in the furnace emissions. From Figure 3, the particulate matter leaving the ESP has a mass median diameter of 0.8 w . Calculation of the fractional collection efficiencies is based on an interpolation technique. Figure 4 is an example of the graphs used in this technique. Since the effective cutoff diameters of a cascade impactor are a function of flow rate and temperature, the effective stagewise cutoff diameters of upstream and downstream samples vary. A t each selected aerodynamic size, the corresponding sample mass collected is used in Equation 2 to compute the collection efficiency.
where qrn = collection efficiency for metal m at a given particle
size Go = grams collected at given particle size in outlet sample GI = grams collected a t given particle size in inlet sample 1004
Environmental Science & Technology
Vo = volume of outlet gas sample, SCF V I = volume of inlet gas sample, SCF Figure 5 illustrates the results of the above calculations for the metals Cd, Pb, Zn, Cu, Fe, and Ni. Note that in every case a minimum in collection efficiency occurs in the 5 - 7 - p range. The average total particulate E S P efficiency passes through a minimum of 96% at 6-paerodynamic diameter, with values for zinc of 91%, lead 94%, nickel 95%, cadmium 94%, iron 98%, and copper 98%. The fraction of particulate not accounted for by the various metal analyses, about 50% by mass, exhibited an efficiency minimum very near that of the overall particulate. The location of the minimum varies from 5 (Cd and Ni) to 7 y (CU). The EPA Method 5 results tend to support the Andersen test results. The magnitude of the minimum (from Andersen data) appears to correspond t o the overall metal collection efficiency (obtained from Method 5 data). For example, iron, with the least severe efficiency minimum of 98% at 6 p , has the
‘-1 01 PRECIPITATOR XI
INLET
PRECIPITATOR OUTLET
nt
I
20 40 60 MASS COLLECTED PER PLATE, GRAMS
0
80
Example of graphs used for calculation of ESP fractional collection efficiency
Figure 4.
t
so]
0
6: IRON 7: NICKEL
;
’
; ’
;
;
;
; ; ; ;
6 B IO AERODYNAMIC DIAMETER, MICRONS
I I2
Figure 5. ESP total particulate and metal collection efficiencies as function of aerodynamic particle diameter
house and scrubber data exhibit no such behavior for the outlet particulate. The apparent minimum in ESP collection efficiency could be caused either by reentrainment of large particulate flocs during rapping or by agglomeration of fine suspended particulate within the E S P and in downstream ductwork. Measurements of adhesion force as a function of particle size (8) show that charged metal particulate has a marked increase in adhesion force as particle size decreases below 6-10 p. The particulate matter measured here consisted of more than 50 wt % metal species. This trend in adhesion force, coupled with particle collisions due to Brownian motion and mechanical turbulence, could result in agglomeration of fine particles both within the ESP and in the downstream ductwork. Those charged particles agglomerating downstream from the ESP would cause an apparent minimum in collection efficiency in the 6-prange, as was observed in this study. The baghouse and scrubber (7), lacking the charged outlet particles of the ESP, exhibited no collection efficiency minimum.
Conclusions The following conclusions can be stated regarding the total particulate and trace metal fractional collection efficiency of an electrostatic precipitator controlling open hearth furnace emissions. A dip to approximately 96% in electrostatic precipitator total particulate collection efficiency has been observed at aerodynamic diameters between 5-7 p. Agglomeration of the charged particles leaving the ESP is postulated as the mechanism responsible for this collection efficiency behavior. The magnitude of the fractional efficiency minimum varied from 91 to 98% for the metals Cd, Pb, Zn, Cu, Fe, and Ni. Overall metal collection efficiency varied from 97.3 to 99.5% for the various metals. Total particulate collection efficiency was 97.6%. Metal emission factors for precipitator-controlled open hearth furnaces ranged from 1.0 X lb/ton steel for cadlb/ton steel for zinc. The total particulate mium to 3.4 X emission factor was 0.30 lb/ton steel. The fractional collection efficiency decrements observed in this study did not appear in tests of control equipment other than electrostatic precipitators in the limited comparisons which were made. Thus, it is possible that the decrements are a phenomenon unique to precipitators. Literature Cited
highest overall collection efficiency of 99.5%. Zinc, with a significant efficiency minimum of 91% at 6 p , has a low overall collection efficiency of 97.8%. This generalization does not hold in all cases, but it is true for the metals that are present in the highest concentration (Zn, Fe, and Pb). Observations of particulate collection efficiency minima have been made for precipitators controlling coal-fired power plants on both a laboratory and a commercial scale (6). More recent data reported by the EPA (7) show a marked similarity to that reported here. Though insufficient data were reported by the EPA for calculation of fractional efficiencies, plots of percent of mass collected vs. aerodynamic diameter similar to Figure 4 could be made for the EPA data. The EPA data are the results of tests performed on a precipitator-controlled coal-fired power plant, baghouse-controlled electric ferroalloy plant, and a scrubber-controlled cotton gin. Note that the electrostatic precipitator has a peak in outlet emissions between 2-and 4-paerodynamic diameter which would produce a collection efficiency minimum in this size range. The bag-
(1) Jacko, R. B., Loop, J. P., “A Parametric Study of an Open Hearth
Furnace Particulate Emissions”, Proceedings, 68th Annual Meeting of APCA, Boston, Mass., 1975. (2) Mudd, S. W., “Basic Open Hearth Steelmaking”, pp 255-338, Metallurgical Society of AIME, New York, N.Y., 1964. (3) EPA, Fed. Reg., 36 (247), 24882-91 (Dec. 1971). (4) Jacko, R. B., Neuendorf, D. W., Yost, K. J., J. Air Pollut. Control Assoc., 25 (10) (Oct. 1975). ( 5 ) American Conference of Governmental Industrial Hygienists (ACGIH), “Air Sampling Instruments”, 4th ed., pp L16-17, Cincinnati, Ohio, 1972. (6) Dalmon, J., Lowe, H. J., “Experimental Investigations into the Performance of Electrostatic Precipitators for P.F. Power Stations”, “La Physique des Forces Electrostatiques et leurs Application”, Grenoble 1960, Paris 1961, Colloques Internationaux, 102, pp 363-79. (7) Lee, R. E., Jr., Crist, H. L., Riley, A. E., MacLeod, K. E.,Enuiron. Sei. Technol., 9 (7), 643-47 (July 1975). (8) Penney, G. W., J. Air Pollut. Control Assoc., 25 (2), 113-17 (Feb. 1975). Received for review November 18,1975. Accepted April 2,1976. Research supported by the National Science Foundation, Research Applied to National Needs ( R A N N ) ,under Grant GI-35106.
Volume 10, Number 10, October 1976 1005
