(10) Bonner, N. A., Bazan, F., Camp, D. C., “Elemental Analysis of Air Filter Samples Using X-Ray Fluorescence”,Lawrence Livermore Laboratory, Rept. UCRL-51361,1975. (11) Wedepohl, K. H., “Origin and Distribution of the Elements”, p 999, L. H. Ahrens, Ed., Pergamon Press, London, England, 1968. (12) Gordon, G. E., Zoller, W. H., Gladney, E. S., “AbnormallyEnriched Trace Elements in the AtmosDhere”,7th Annual Conference on Trace Substances in Environmental Health, University of Missouri, Columbia, Mo., 1973. (13) Stafford, R. G.. Ettinper. H. J.. Atmos. Environ., 6. 353 (1972). (14) Ruddy, R. H., Washington State University, Pullman, Wash., private communication,June 1975. (15) Ng, Y. C., Burton, C. A., Thompson, S. E., Tandy, R. K., Kretner, H. K., Pratt, M. W., “Handbook for Estimating the Maximum InL
ternal Dose from Radionuclides Released to the Biosphere”, Lawrence Livermore Laboratory, Rept. UCRL-50163, Part IV, 1968. (16) Thompson, S.E., Lawrence Livermore Laboratory, private communication, December 1975. (17) Idaho Emission Inventory, 1971, Department of Health and Welfare, State of Idaho, 1971. Received for review March 17,1976. Accepted March 11,1977. Work performed under the auspices of the U.S. Energy Research and Development Administration under contract No. W-7405-Eng-48. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Energy Research & Development Administration to the exclusion of others that may be suitable.
Fractionator for Size Classification of Aerosolized Solid Particulate Matter Andrew R. McFarland” and Russell W. Bertch Civil Engineering Department, Texas A & M University, College Station, Tex. 77843
Gerald L. Fisher and Bruce A. Prentice Radiobiology Laboratory, University of California, Davis, Calif. 95616
System Description A system is developed for collecting kilogram quantities of size-fractionated particulate matter which can be subsequently used for biologic testing purposes. The apparatus consists of a series of two cyclones and a centripeter and yields four size fractions. When used over a 12-day period to classify fly ash from stack gas passed through the electrostatic precipitator of a coal-fired power plant, the collected fractions have volume median diameters of 20,6.3,3.2, and 2.2 wm with geometric standard deviations of approximately 1.8. A total of 8.08 kg of fly ash is collected. The increasing national demand for electrical power generation requires further assessment of present pollution abatement technologies and the potential health hazards associated with energy technologies. Particulate matter from coal combustion has received special emphasis in recent years. To evaluate the health hazards of coal combustion and to develop new and improved pollution abatement technologies, relatively large quantities of size-classified particulates are required. Although cascade impactors are routinely used for aerodynamic size classification in emission testing, these instruments are designed to classify milligram quantities of particulates. T o achieve our goals for biologic testing, we have developed a stack-sampling system capable of classifying kilogram quantities of aerodynamically size-fractionated fly ash. The particle fractionator is designed to provide fly ash in volume-size classes for biological testing, namely, with median sizes of diameters: 2 , 3 , 6 , and 20 wm. The 2-pm fraction represents the size region suggested of most interest for pulmonary deposition after inhalation in humans ( I , 2). The 3- and 6-wm size classes will predominately be deposited in the nasopharyngeal region, i.e., the upper regions of the respiratory tract where most particles will be removed by ciliary action, swallowed, and ingested. Although the largest size class (20 wm) is probably not of concern in inhalation exposure due to the short atmospheric residence time, these particles may ultimately be ingested after foliar deposition on food crops.
The apparatus which was developed to fractionate and collect fly ash is shown schematically in Figure 1.Basically it consists of a set of two cyclones in series followed by a centripeter-an arrangement which permits separation of the particulate matter into four size fractions. With reference to Figure 1, stack gas is drawn through the inlet probe into the heated enclosure which contains the fractionation devices. Heating to about 100 “C is necessary to prevent condensation of moisture due to the high dew point associated with power plant stack gases. The first cyclone through which the gas flows was designed from use of the data and model of Ancel ( 3 )to have a cut size of 10 pm for unit density particles. Cut size is defined as the diameter of a spherical particle for which the probability of collection is 50%.After passing through the first cyclone, the gas flows through the second cyclone which has a design cut point of 7 pm and thence into the centripeter. The centripeter principle, developed almost simultaneously by Hounam and Sherwood ( 4 ) and Conner ( 5 ) ,is illustrated in Figure 2. Aerosol is drawn through an acceleration nozzle wherein it achieves a velocity on the order of 10-100 m/s. A
r: - STACK G A S FLOW
Flgure 1. Schematic diagram of fractionation system
Volume 11, Number 8, August 1977
781
collection nozzle is placed at a distance of approximately one diameter of the acceleration nozzle downstream from the exit plane of the acceleration nozzle. Typically the entrance size of the collection nozzle is 1.4 times as large as the acceleration nozzle diameter. Only a small portion (2-10%) of the flow which effluxes from the acceleration nozzle is drawn into the collection nozzle with the remainder being diverted around it. Large particles, due to the high inertia associated with the acceleration jet exit-plane velocity, are driven into the collection nozzle and carried away from the classification zone by the small flow of transport gas. Small particles contained within the 2-10% transport air are also drawn along with the large particles. The majority of the total gas flow and the small particles are diverted around the collection nozzle and passed out of the collection system. Particle cut size for the centripeter was calculated from the expression (6):
+
where C = Cuningham's correction = 1 2.46 X/D,; X = gas mean free path length, m; D , = particle diameter, m; 1.1 = gas viscosity, kg/m.s; dj = acceleration jet diameter, m; p p = particle density, kg/m3; and V , = particle velocity a t acceleration jet exit plane, m/s. Since a total air flow rate of approximately 0.85 m3/min and particle cut size of from 3 to 5 pm were desired, it was necessary to design the prototype system with multiple jets operating in parallel. Two versions have been employed, the first with 25 jets each 0.5 cm diameter and the second which also has 25 jets with each having a size of 0.65 cm. Results presented herein are those obtained with the latter design. The critical design parameters which were used in the centripeter system are given in Table I. Gas containing the larger particles (Figure 1)is processed through a fiberglass fabric filter and then drawn through a flow meter and control valve. Similarly, the small particle stream is drawn into a fiberglass fabric filter, through a control valve, and then is joined with the gas from the large particle fabric filter. The combined stream goes through a blower and thence is discharged to the atmosphere. Filtered, high-pressure building supply air (100 psig) is pulsed a t approximately 2-min intervals for cleaning of the fabric filters. The particles which are removed in the cleaning cycle are collected in dust hoppers. With respect to the ability of the fabric filters to collect the fly ash particles, Billings and Wilder (7) reported that for six applications of reverse-jet fabric filters on fly ash, the minimum efficiency noted was over 99.9%. Peterson and Whitby (8) reported that the fractional efficiency of a fabric filter media loaded with NBS fly ash was greater than 9% over the size range of 0.06-1.0 pm. In application, the fly ash fractionator system was mounted on the breeching after the electrostatic precipitator of a 750-MW coal-fired electric power plant and operated for a period of 12 days at a flow rate of 0.85 m3/min. Fuel combusted contained approximately 0.3% sulfur, 22% ash, and 10% moisture. The probe was located along the vertical centerline of the 10-m-high rectangularly shaped breeching at a position 2.5 m below the top of the ducting. Operating temperature was maintained at 92-102 "C which precluded condensation of the 73 "C dew point stack gas. Methods
The analysis of system performance consisted of determining for each size fraction the total mass collected over the operating period, the size distributions of representative fly ash samples, and the basic average particle densities. Size distribution data were obtained by first ultrasonically dispersing a 0.3-g fly ash sample in a methanol-electrolyte 782
Environmental Science & Technology
solution for a 2-min period. An aliquot of the dispersion was then analyzed with a Coulter Counter Model T system. Each size distribution determination represents the composite of duplicate dispersions and analyses. As a backup a Particle Data Inc., Celloscope was employed to analyze the finest size fraction. With a saline solution used as the electrolyte, the apparatus yielded data identical to that of the Coulter approach. The basic particle density in the four sized fractions was determined in triplicate utilizing a gravimetric, liquid displacement technique. After dispersing the fly ash in l-propanol and degassing in vacuo, the suspension and tared container were weighed completely immersed in 1-propanol. The displaced liquid volume was calculated from this gravimetric determination.
Results The quantities of material collected during the 12-day period in each fraction are given in Table I1 and show that the total accumulation of post-precipitator fly ash was 680 g/day with 9.9% in the smallest size fraction. Size distribution of the material drawn through the probe into the system is shown in Figure 3. The volume median diameter is 13 pm with 90% of the distribution associated with particles of size less than 37 pm and 10% with sizes less than 2.5 pm. The resulting volume size distributions of the four fly ash fractions are shown in Figure 4. Assuming that the distributions are approximately logarithmic normal, the statistical
Total
Air Flow Streamline and Small P a r t i c l e 7 Trajectory
j;p
Acceleration Nozzle
Large Particle Trajectory .Collection Nozzle
I
1
Large Particles and Transpor I Gas
Figure 2. Centripeter concept-single
Table 1. Centripeter Design Parameters Number of jets Acceleration nozzle diameter Collection nozzle diameter Gap between acceleration and collection nozzles Acceleration nozzle Reynolds number Collection nozzle Reynolds number Total flow rate through system Large particle transport flow rate Particle cut size (aerodynamic diam)
Small Particle and Main Gas Flow
jet
25
0.65 cm 0.91 cm 0.65 cm 3900 280 850 Llmin 85 L/min 5.2 urn
Table II. Material Collected by System Mass collected, kga
%
First cyclone
5.38
66.6
Second cyclone Centripeter-large particles Centripeter-small particles
1.31 0.59 0.80
16.2
Fraction
a
7.3 9.9
Twelveday collection period.
P a r t i c l e Diameter, D p , Micrometers
Figure 3. Size distribution of fly ash drawn into system
P a r t i c l e Diameter, Dp, Micrometers
Figure 4. Size distribution of fly ash fractions
Table 111. Characteristics of Fly Ash Fractions Mean particle densitis Fraction
First cyclone
Second cyclone Centripeter-large particles Centripeter-small particles a Mean of two determinations. determinations.
glCm
1.8526 2.186b 2.358c
VOI
median diam, SD
0.019 0.023 0.015
2.445b 0.014
ma 20.0
Geometrlc SD
3.2
1.8 1.8 1.8
2.2
1.9
6.3
Mean of three determinations. Mean of four
description of each fraction may be represented as shown in Table 111. Volume median sizes of the fractions range from 20 pm for the first cyclone to 2.2 pm for the small particles from the centripeter. Photomicrographs of the collected fly ash fractions (Figure 5 ) show the quality of size separation which is achieved by the system. The basic performance of a particle fractionation stage can be represented by its fractional efficiency, that is, the efficiency as a function of particle diameter. For the three fractionation stages employed in the system, these relationships were determined through knowledge of the total mass of each fraction and the corresponding Coulter Counter mass frequency size data. The computational scheme for efficiency was to compare the fraction of mass in each size interval which was removed by the collection stage with that which entered the stage. To convert the Coulter Counter particle size to a more general size parameter, the aerodynamic equivalent size, the following expression was employed:
DZ = p p D j (2) where subscript a refers to the aerodynamic size parameter. The values of particle density appropriate to each of the size fractions were determined by a liquid displacement technique and are shown in Table 111.Note that the density of the fly ash varied from 1.85to 2.45 g/cm3 depending upon the size fraction. This is consistent with the results of Fisher et al. (9), who reported on the presence of plerospheres (hollow spheres packed with smaller spheres) and cenospheres in hopper fly ash. Based upon microscopic analysis of the four fly ash fractions, we have quantitated the relative abundance of morphologic particle types (IO).From this analysis it appears that vesicular particles are predominantly associated with the coarser fractions and solid spheres predominate in the finer fractions. It therefore appears that the variation in particle density may be explained primarily on the basis of the relative abundance of vesicular particles and solid spheres within the fly ash fractions. Note that the apparent reduction in slope of the centripeter fractional efficiency (Figure 6) for the smallest particles reflects the presence of the small particles which are carried with the transport air in the fractionation zone. This causes introduction of up to 10%fine particles in the coarse fraction. Summary Through the use of a specially developed particle fractionation system, it has been possible to collect kilogram quantities of size-fractionated particulate matter from an air pollution emission source. These quantities are compatible with the needs of biologic testing programs. Also, since the material is fractionated directly from the stack gas rather than from reaerosolized samples of bulk powders (e.g., laboratory classification of electrostatic precipitator hopper fly ash), it should provide a more representative biological challenge in health-hazard evaluation studies. Aerodynamic cut sizes of the fractionation stages for the system which was developed were 10,6.6, and 5.2 pm; however, other values for the cut sizes can easily be obtained through a combination of operating conditions and design parameters. When employed in the collection of fly ash from a power plant, the system provided kilogram quantities of material with volume median sizes of 20,6.3,3.2, and 2.2 pm. Geometric standard deviations of the fractions were approximately 1.8. Aside from the obvious requirement for biological testing of large quantities of aerodynamically sized particulates, the fractionator has utility in laboratory development and evaluation of pollution abatement instrumentation. Another
Volume 11, Number 8, August 1977 783
terial is a blend of fly ashes collected by electrostatic precipitators and mechanical collectors at five electric power plants. Since this standard is a blend of ashes which were collected by the particle emission abatement technologies at the various plants, the particle size is relatively large (11lm-mass median Stoke’s diameter) ( I I ) , and the collected material is not characteristic of that which escapes the control systems. By use of the system described here, fly ash was collected and fractionated in situ after the ESP. This material is representative of fly ash that is released from the stack of the power plant, and the fractionation provides sufficient quantities of fly ash size fractions of environmental and biomedical importance for analytical technique development and testing. Furthermore, with additional fractionation of the size-classified material, such as by employing a spiral-duct aerosol centrifuge (12), essentially aerodynamically monodisperse particles may be obtained. The availability of monodisperse particles derived from actual air pollutants will provide the opportunity to evaluate important particle properties such as aerodynamic shape factors, morphologicheterogeneity (IO), and chemical composition (IO) within well-defined size classes. Such studies are ultimately required to provide specifications for the design of control technologies for specific components of particulates within size classes.
1 .
First
Cut
Third
Cut
Second Cui
Fourth Cut
,
Impm
Acknowledgment
SCOI€
Figure 5. Photomicrographs of collected samples
98
I
I
I
I I Ill1
I
I
I I l l
95 -
-
90 -
-
+ E
L i t e r a t u r e Cited
-
-
a
n 702 ; 0
-
50.-5 0
;
i 3
c
-
-
-
- 300 c
0 .c
-
0
0
e
A 0
l0-
First Cyclone Second Cyclone Centripeter
5-
2
I.o
I
I
I
I I I l l
5.0
I
IO
I
I
-
I I I l l
50
100
important application of this system involves the collection of sufficient quantities of materials for particulate standards to be used in the development and testing of analytical techniques. In this regard, a hopper fly ash standard certified for concentrations of 12 trace elements is available from the National Bureau of Standards (NBS-SRM 1633). This ma-
784
We thank Calvin Parnell, Texas A & M University, for performing the Coulter Counter analysis; Charles Nash, University of California, Davis, for assistance with the density determining technique; Connie King, Texas A & M University, for her computational efforts; and Otto Raabe, University of California, Davis, for his advice in the review of this manuscript.
Environmental Science & Technology
(1) Stuart, B. O.,Arch. Intern. Med., 131,60 (1973).
(2) Yeh, H. C., Phalen, R. F., Raabe, 0. G., Environ. HealthPerspect., 15, 147 (1976). (3) Ancel, J., MS thesis, University of Notre Dame, Notre Dame, Ind., 1973. (4) Hounam, R. F., Sherwood, R. J., A m . Znd. Hyg. Assoc. J., 26,122 (1965). (5) Conner, W. D., J . Air Pollut. Control Assoc., 16,35 (1966). (6) McFarland, A. R., Stevens, R. K., Wedding, J. B., “The EPA Dichotomous Sampler”, Texas A & M University, Civil Engineering Department, Publ. 8110/04/ARM, 1976. (7) Billings, C. E., Wilder, J., “Fabric Filter Systems Study, Volume I, Handbook of Fabric Filter Technology”, GCA Corp., Bedford, Mass., Final Report EPA Contract CAP 22-69-38, 1970. (8) Peterson, C. M., Whitby, K. T., A m . Soc. Heat. Refrig. Air Cond. Eng. J., 7,42 (1965). (9) Fisher, G. L., Chang, D.P.Y., Brummer, M., Science, 192, 553 (1976). (10) Fisher, G. L., Prentice, B. A,, Silberman, D., Ondov, J. M., Ragaini, R. C., Bierman, A. R., McFarland, A. R., Pawley, J. B., “Size-Dependence of the Physical and Chemical Properties of Coal Fly Ash”, Div. Fuel Chem. Symp. Properties of Coal Ash, ACS, Montreal, Canada, May 1977, in press. (11) Fisher, G. L., Prentice, B. A., unpublished data. (12) Raabe, 0. G., Boyd, H. A,, Kanapilly, G. M., Wilkinson, C. J., Newton, G. J., Health Phys., 28,655 (1975).
Received for review July 26,1976. Accepted March 11,1977.Research sponsored by the U.S. Energy Research and Deuelopment Administration through the Radiobiology Laboratory, Uniuersity of California, Davis.
