CURRENT RESEARCH Performance of Flame Ionization Detector as Atmospheric Aerosol Monitor. 1. Conventional Model Lawrence L. Altpeter, Jr.*, T. S. Hermann’, J. P. Pilney, A. J. Senecha12,and D. L. Overland3 North Star Division, Midwest Research Institute, Minneapolis, Minn. 55406
With a flame ionization detector designed for ambient aerosol monitoring, the output signals from single aerosol particles were examined in a series of high-speed waveform studies which show that additional information regarding particle characterization may be available from the details of the output current pulse. Analytical parameters of this detector were evaluated with high-speed data taken from a variety of aerosols under controlled conditions of size, number concentration, and composition. Repeatability of the flame ionization detector was estimated to be 5-15%. For single component aerosols such as NaCl, the response of the detector varied with the 3/2 power of the particle size throughout the tested size range of 0.5-10 pm. Size limit of detection was 0.15-0.2 pm, with evidence of ways to improve this limit significantly. There was evidence of limited selectivity when the detector was used with parallel plate electrode geometry and a stoichiometric premix air-hydrogen flame. The ideal atmospheric aerosol monitor would perform on a real time and continuous basis while obtaining analytical data on particle characteristics such as aerodynamic size, number concentration, and composition. At present, no such monitor exists. Although the flame ionization detector (FID) can detect single particles on a real time and continuous basis (1-5), there has been relatively little interest in its use as an aerosol monitor for several reasons. First, the response of the FID varies with particle composition which means that accurate size information is not possible when the device is challenged with typical heterogeneous aerosols. Second, the coincidence of two or.more particles in the flame cannot easily be deciphered so that the FID is count-rate limited. With appropriate sample-conditioning, the count-rate limitation can be minimized or eliminated. However, the variation of response with particle composition has remained a drawback of the FID. We wish to report the results of high-speed waveform studies in which the output current pulses of the FID, produced by single particles, were examined on an expanded time base. If a high-speed amplifier is connected to the collection electrode of an FID, then the output signal, produced by a single particle as it is consumed in the flame, can be observed as a pulse, to 10-6 A in amplitude and from a fraction of a millisecond to several milliseconds in duration. If parallel plate electrode geometry is employed, this current pulse is Present address, Carnegie-Mellon Institute, Pittsburgh, Pa. 15213. Present address, Hennepin Technical Center, Eden Prairie, Minn. 55343. Present address, Digital Systems, Division of Detection Sciences, Golden Valley, Minn. 55424.
observed to possess an irregular pattern of modulation. The modulation pattern is repeatable for a given aerosol species. The signal amplitude is repeatable for a given particle size. Further, during preliminary studies at this laboratory, for certain aerosols the waveform of the current pulse might serve as an indicator of specie. Because relatively little detailed data have been reported for the FID, a general performance study was undertaken to evaluate analytical parameters such as precision, response, sensitivity, and size limit of detection. A variety of aerosols were examined under controlled conditions of size, number concentration, and composition.
Experimental Apparatus. FID Detectors. The FID employed throughout this study is shown schematically in Figure 1. Significant features of this configuration included plane parallel plate electrodes, a premixed stoichiometric air-hydrogen flame, a ground-side amplifier, and removal of combustion products of evacuation. Details of the working configuration have been presented elsewhere (6). The polarizing and collection electrodes were mounted on opposite walls of a Lava Stone chamber with a conical burner centered between the elecEXHAUST
+
RF
r
SAMPLE IN
Schematic illustration of conventional FID aerosol monitor (parallel plate) Figure 1.
Volume 10, Number 10, October 1976
997
trodes. A polarizing voltage of 500 Vdc was applied throughout most of this study. Hydrogen is premixed with a stoichiometric amount of the sample-laden air in a stainless steel tee below the burner. Secondary air entered the enclosed space of the FID chamber at the base of the rear wall (not shown). Exhaust products were removed from the chamber by means of an aspirator (not shown). The latter was required to develop sample air flow and to direct hydrogen and secondary air into and through the FID. To minimize electronic interferences from pickup, the burner was enclosed in a Faraday-like cage composed of two compartments; it was fabricated from panels of copper-laminated epoxy board which were soldered together. One compartment of the cage accepted the FID housing. The other compartment contained the solid-state amplifier which was connected to the collecting electrode of the FID with a minimum length lead. Discrete elements for the amplifier were also located in this compartment. With the indicated structural features and 500 Vdc applied, the rms background current from the FID was approximately 50 PA. Electrodes have been extended in the vertical direction to eliminate the problem of blow-by. The electrode width was chosen as a compromise between electrode size and uniformity of the electric field in the combustion zone. Use of a ground side collection electrode simplifies signal processing. Experimental System. The FID was centered within an atmospheric simulation chamber which had a volume of 3.68 m3 (128ft3).The chamber was made from plywood sheets, 2.5 cm thick (1in.) and 1.22 by 2.44 m (4 by 8 ft). The inner walls were treated with two coats of sealer and three coats of epoxy paint, with sufficient time allowed for complete curing. The polarizing voltage for the FID was fed to the detector for an external power supply through a BNC cable. Power for the amplifier was supplied by a 15-Vdc supply located outside the chamber. The output signal was carried out of the chamber through a BNC cable to an oscilloscope and a Honeywell Visicorder oscillograph, which were connected in parallel. The oscilloscope provided a convenient means for visually monitoring the FID output, while the Visicorder provided a permanent record on command. Further details regarding equipment are listed in Table I. A 100 CFM exhaust blower was positioned downstream from the simulation chamber to draw the aerosol into the chamber. For most experimental work a purge or filling time equivalent to at least 5-7 chamber volumes was necessary. The exhaust blower permitted the chamber to be maintained a t a slight negative pressure. Intake air passed through an absolute filter, located upstream from the chamber. Aerosol samples were delivered to the filtered intake air from the aerosol generator by a 3.7-cm (I1/!in.) line at 85 l./min. (3 CFM). Vigilance was necessary to preclude the possibility of unburned hydrogen accumulating in the chamber. A solenoidactuated ball valve could be placed in the hydrogen line to be driven by a signal from a thermocouple placed in the exhaust line. If the flame went out for any reason, the resultant drop in temperature would actuate the ball valve and shut off the hydrogen. Aerosol Sample Preparation. Atmospheric aerosols can be divided into three general categories: inorganic, organic, and biological. Sodium chloride was used extensively as an example of an inorganic aerosol. Monodisperse latex particles served as an organic aerosol. For biological aerosols a set of commonly occurring microorganisms was prepared. Inorganic and organic aerosols were prepared with deionized and distilled water which was doubly filtered through Wattman 40 paper and Millipore 0.45-pm paper. Test cells of biological aerosols were removed from their nutrient broth 998
Environmental Science & Technology
Table 1. Experimental Equipment and Specifications Aerosol generator
OsciIloscope
Oscillograph
FID power supply FID amplifier
Chamber exhaust blower Chamber intake filter Rotameters (H2, air) Rotameter (sample)
Berglund-Liu monodisperse aerosol generator, Model 3050, Thermo-Systems, Inc., equipped with 5-,lo-, and 20-pm orifices for liquid solution samples and 100-pm orifice for suspensions of large particle size. Filtered air supply required Techtronix Type 535 operated typically at ordinate sensitivities of 5-200 mV/cm and time base of 0.2-1 ms/cm Honeywell Model 1806 fiber optics CRT visicorder operated with ordinate and abscissa scales identical to those of oscilloscope John Fluke Precision DC Supply, Model 301E, 0 4 6 0 0 Vdc1300 mA Analog Devices, Model 48K current feedback amplifier, 15-MHz bandwidth (at unity gain), open loop gain of lo5 (500 52 dc load), settling time of 500 ns to 0.01 % (at unity gain), input impedance of 10” R and 3.5 pf, powered with Analog Devices 15-Vdc power supply Dayton, Shaded Pole, 115 Vac, 126 W, rated at 100 CFM Flanders, Model 7ClO-L-N2C2 absolute filter rated at 600 CFM maximum Brooks, tube size R-2-15D CO equipped with needle valves Brooks-Mite Model 2051, 2 SCFH (sample line normally open)
by centrifugation and then resuspended in tap water which was dechlorinated, sterilized, and doubly filtered as described above. Biological aerosols were measured promptly to minimize the extent of lysis. Aerosols were generated with a Berglund-Liu Model 3050 monodisperse aerosol generator (Table I). In the Model 3050 the liquid solution is fed by a syringe pump at a predetermined rate through a small orifice which is in contact with a vibrating piezoelectric ceramic. The would-be liquid jet is thereby converted into a stream of droplets of uniform size. The droplet stream is injected axially along the center of a turbulent air jet which disperses the droplets and prevents coagulation. A Kr-85 radioactive source neutralizes the electrostatic charge on the particles incurred during the spraying process. Using a set of three interchangeable platinum orifices (5,10, and 20 pm diameter) and different solution concentrations, uniform particles in the 0.6-40-pm diameter range can be generated [gg = 1.06 (7)].The goal of these studies was involved with the examination of the output signals from single particles in the FID. In evaluating the performance of the FID, it was important that the aerosol be as monodisperse as possible, i.e., that its particle size be well defined. Results
Ion Collection Efficiency. If the FID signal were produced wholly, or in part, by gaseous chemiions or attached charge carriers, then it was desirable that the voltage applied to the electrodes of the FID should be high enough to overcome ion recombination and to ensure, thereby, highest efficiency of ion collection. This could be determined by gradually increasing the applied voltage and noting at what level the ion current could no longer be increased. For particle sensing, this amounted to measuring the number of coulombs produced by the current pulse from each particle as it combusted in the flame. One gathered a statistically reliable set of measure-
ments for each voltage and then noted when the mean value of coulombs (or area under the curve) no longer increased. The results of such a study are shown in Figure 2 for a 4-pm NaCl particle. The collection efficiency reaches a plateau a t 80-100 Vdc. Coulombs per pulse were measured in terms of area under the curve with a planimeter. The mean rate of mass input to the flame was 125 ng/s. In a partial study performed from 500 to 100 Vdc with an aerosol of S. marcescens, a straight line was obtained. The results shown in Figure 2 suggested that this model of the FID should be operated at voltages not less than approximately 100 Vdc to be assured of optimum sensitivity, precision, and reliability. The duration of the waveform varied inversely with the applied voltage or field strength for all tested aerosols. It should have been expected, if the duration of the output signal were related a t all to the mobility of the charge carriers, in accordance with the general relationship u = bE, where v is the drift velocity (cm/s), b is the mobility (cm2/V-s),and E is the field strength (V/cm). Charge Carriers per FID Current Pulse and FID Ionization Efficiency. Toward the end of these studies, a Keithley Model 427 high-speed current amplifier became available which had a selectable volts/ampere gain setting. A voltage-to-current transduction factor of 1mV per 25 pA was determined for the ordinate of the FID output. With this information, it was possible to calculate coulombs per pulse and the number of charge carriers per pulse since:
Q = JIdt
= ne
(1)
where Q is in coulombs, I is the current (amperes), t is time (s), n is the number of carriers, and e is the number of couThe number lombs per singly charged carrier (1.6 X of coulombs per pulse produced by a 4-pm NaCl particle, Q, was 2.3 X the number of charge carriers, n, was 1.4 X lo7. Calculating the number of molecules in a 4-pm NaCl particle, one can then estimate an ionization efficiency for this size particle in the flame: Ionization efficiency = n/n, (2) = 1.4 X 107/7.5 X lo1' = 1.9 x 10-5 where n is the number of charge carriers generated from a particle of no molecules. This result can be compared with a similar calculation performed with a gaseous sample (1ppm isobutane in nitrogen at 3.9 cc/s) in which an ionization effiwas calculated. The mean rates of mass ciency of 4.5 X input to the flame in each case were 125 and 14 ng/s, respectively. Note that spherical particles and unipositive charge carriers were assumed. Almost all of the output waveform data displayed in the following sections are presented with a millivolt ordinate as taken from the oscillographic readout. Use of the transduction figure mentioned above will convert these units to amperes with reasonable accuracy. FID Response, Sensitivity, and Detection Limits. The response of the FID to particles in the size range 0.5-10 wm was measured for selected inorganic, organic, and biological aerosols. Solution and suspension samples were prepared as described above and dispersed from the aerosol generator. The FID was centered in the atmospheric simulation chamber. Approximately 20 output signals were recorded for each particle size of a given aerosol specie. Output signals were measured with a planimeter. The mean area under the curve was taken as a measure of detector response. Sodium chloride served as an example of an inorganic aerosol. Measurements were performed on two separate days for each of the following particle sizes: 0.5, 1.0,2.0,4.0,8.0,and
10 gm. A straight line regression analysis of the data transformed to a log-log format yielded the equation:
+ 1.54 loglo x
loglo y = 0.723
(3)
with a correlation coefficient of 0.996. Latex particles were tested as an example of an organic aerosol in three sizes: 0.8,3.5, and 9.5 pm. When plotted on the log-log format described for NaC1, the result was a straight line nearly parallel to that of NaCl which was fitted approximately by the equation: loglo y = 0.99
+ 1.59 loglo x
(4)
By extrapolating to the 2 noise limit, the size limits of detection for NaCl and latex were 0.2 and 0.15 pm, respectively. The value for NaCl agreed well with that found by Crider and Strong ( 4 ) . Several microbiological aerosols were selected to cover the size range 0.9-5.5 pm. These included S. marcescens, P. vulgaris, B. cereus, and S. cerevisiae. The results for microbiological aerosols are summarized in Table 11. Among six common, linearly transformable functions, a best least-squares fit was obtained with the exponential function: y = 14.8 eo.04x
(5)
While the difference in response for the microbiological aerosols is not completely understood, it should be recalled that these are multicomponent particles, composed of 70-800h water, to which the FID does not respond. The slope of the response curve or the sensitivity was nearly 3/2 in the cases of the organic and inorganic aerosols (Equations 3 and 4). FID Precision. The 4-pm sodium chloride aerosol was selected for a precision study. The usual techniques of sample preparation and aerosol generation were employed. A set of a t least 20 output waveforms was measured each day by planimeter for a total of six days. Precision data are listed in Table 111. These data were subjected to an analysis of variance to separate analytical and sampling errors. Relative standard deviations of 16.8 and 2%, respectively, were obtained. The small sampling error suggests that good control was achieved over the sample generation and sample transfer systems. Built 6Or
UNDER
THE CURVE (Crn2) 20
0
H P P H
5
100
T
4
I
I
I
I
200
300
400
500
VOLTS (DC)
Figure 2. Ion collection efficiency of FID for particles (4-km NaCI)
Table II. Summary of Microbiological Mass Response Data from FID Mlcroblologlcal aerosol
Mean vol ( 8 ) , lrm3
S. marcescens P. vulgaris 6.cereus S. cerevisiae
4.1 f 3.3 15.0 f 7.7 90.1 f 74.5
0.4 & 0.4
Mean area under curve, crn2
15.1 19.0 23.0
420.0
Re1 SD, %
15 18 20 25
Volume 10, Number 10, October 1976 999
into the relatively large analytical error (16.8%)were certain undefined sources of error which included electronic settings and planimeter data reduction. It would seem reasonable, therefore, to assume that a representative figure for the precision of the FID alone should lie in the range of 5-15% with the experimental arrangements employed in these studies. A spherical 4-pm particle of NaCl has a mass of approximately 73 Pg. FID Selectivity. In the following section, real FID output data are presented for a number of aerosols. These data, photographed from the original Xerox copy, consist of one or more oscillographic recordings in which each recording corresponds to the pulse produced by a single particle. The oscilloscope was set to unblank and record only signals above the noise level; thus, leading edges of the waveforms are missing. Superimposed on the waveform data is a calibrated grid in which each horizontal line is separated from the next by 1 cm and each vertical hash mark is separated from its neighbors by 1 cm. Waveforms do not necessarily begin at the first vertical hash mark. The ordinate and abscissa for each waveform are mV/cm and s/cm, respectively. Specific values are listed in the caption of each figure along with particle dimensions in parentheses. The NaCl data (Figure 3) reveal a sharp leading peak with one shoulder. Several extraneous smaller peaks can be seen,
occurring perhaps due to memory effects from the experimental system. Data for CsI (Figure 4) show a single peak with no shoulders. An inorganic aerosol with a divalent metal ion, barium chloride, produces an output waveform consisting of a leading peak followed by two shoulders (Figure 5). Other multivalent cation salts produced data like that of BaC12. Data for latex, the only organic aerosol tested, reveal the more commonly observed type of waveform, namely, that of a leading spike with two trailing shoulders (Figure 6). All but two of the biological aerosols that were tested produced FID waveforms much like Serratia marcescens (Figure 7), Le., a leading spike followed by two shoulders. Unique waveforms were obtained from a yeast (Saccaromyces cereuisiae) and a mold (Aspergillus niger, as shown in Figures 8 and 9). Data for Aspergillus niger produced the largest signal of all species tested. Other Electrode Geometries. Other electrode geometries which were tested include: the concentric cylinder, the wire loop, a pair of plane parallel collecting electrodes with the polarizing electrode in the orifice of the burner tip, and a pair of parallel, horizontal wires. For the first two geometries listed, the polarizing electrode was a 5-mil Nichrome wire centered in the orifice of the burner tip. In almost every case, the structural details of the waveform have been lost, although in the case of the cylindrical electrode, there was a consider-
Table 111. Analysis of Variance of FID Data from 4-pm NaCl Aerosol Day
1 2 3 4 5
6
Mean area under curve, cm2
46.2 44.7
Re1 SD, % Variance
45.5
64.0 47.6 49.0
44.9 50.0 47.6
100.0 68.9
Analytical
Sampling
16.8
2.2
38.4
2L Figure 4. FID output signals from 4-pm C,I aerosol. Oscillograph sen-
sitivity, 200 mV/cm; time base, 1 ms/cm
b
i
I-----
-
,-
Figure 3. FID output signals from 4-pm NaCl aerosol. Oscillograph sensitivity, 100 mV/cm; time base, 1 ms/cm 1000
i
Environmental Science & Technology
Figure 5. FID output signals from 4-pm BaCI, aerosol. Oscillograph sensitivity, 20 mV/cm; time base, 1 ms/cm
--
“3
’& : Figure 8. FID output signals from S.C. aerosol (4-5 pm approximately). Oscillograph sensitivity, 200 mV/cm; time base 1, ms/cm e
*
-t
*
--,
I. s.
Figure 6. FID output signals from 0.8-pm latex aerosol. Oscillographic sensitivity, 20 mV/cm; time base, 1 ms/cm
+L..+
.i.-
,-,
-.-,
. ,
i L ’*P
Figure 7. FID output from S.m. aerosol (0.5-1 .O by 1.O Wm). Oscillograph sensitivity, 20 mV/cm; time base, 1 ms/cm
Figure 9. FID output signals from A.n. aerosol. Oscillograph sensltivity, 500 mV/cm; time base, 1 m d c m
able improvement in signal-to-noise. An aerosol of S . marcescens, e.g., produced single particle signal amplitudes greater than 400 mV compared to that of approximately 40 mV from the parallel plate geometry.
where A is a neutral atom produced in the flame following the dissociation of molecule AB,K1 is the appropriate equilibrium constant, and h is a term which includes KI1r2 and the appropriate conversion factors to relate the mass of A in [ A ]to the size of the particle from which it originated. The same line of reasoning might be extended to latex, which also displayed a three-halves power relationship between particle size and detector response. A monovalent species such as CHO+ (10):
Discussion The FID can perform as an extremely sensitive particle detector. Thus, with parallel plate geometry and a stoichiometric air-hydrogen flame, the size limit of detection for NaCl corresponds to a mass of 10 fg approximately (assuming spherical particle shape). Use of a fuel-rich flame or other electrode geometries such as the cylindrical electrode may further reduce this limit. The response of the parallel plate FID was related to the three-halves power of the particle size. In the case of alkali metals such as sodium, it is possible to explain such a slope if it can be assumed that the ionization process in the flame is an equilibrium process (9). Thus: A =A+
+ e-
(6)
[ A + ] = [e-] = (K1[A])1/2 = kd,4~3’*
(8)
CH
+ 0 = CHO+ + e -
(9)
could be invoked. I t should be recalled, however, that only three points were available for the latex response data. The biological aerosols tested in this study displayed a different response. It is not clear what effect the presence of 70-80% water would have upon the particle integrity during the vaporization and excitation processes. Of particular interest for any ambient aerosol monitor would be the selectivity of the device, the extent to which the output signal could be uniquely related to particle specie(s). Of the electrode geometries tested in the FID, only from the parallel plate electrodes were signals obtained with discrete information which might serve potentially as a “fingerprint” for identification. These signals were composed, generally, of a leading spike with one or two trailing shoulders. Organic and biologic aerosols produce pulses with two shoulders while Volume 10, Number 10, October 1976
1001
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
