Eng., 90,79-107 (1964). (27) Adamson, A. W., “Physical Chemistry of Surfaces”, 2nd ed., Interscience, New York, 1967, p 573. (28) Lemieux, R., Morrison, -J.,Can. J . Res., 25B, 212 (1947). (29) Zechmeister, L., “Progress in Chromatography”, Wiley, New York, 1950, p 189. (30) Weber, W. J., Jr., van Vliet, B. M., presented a t the 176th National Meeting of the American Chemical Society, Miami, Fla., Sept 10-15,1978, (31) Liapis, A. I., Rippin, D. W. T., Chem. Eng. Sci., 32, 619 (1977). (32) Butler, J. A. V., Ockrent, C. J . , J . Phys. Chem., 34, 2841 (1930). (33) Jain, J. S., Snoeyink, V. L., J Water Pollut. Control Fed., 45, 2463-79 (1973). (34) Radke, C. J., Prausnitz, J. M., AIChE J , 18,761-8 (1972). ( 3 5 ) Jossens, L., Prausnitz, J. M., Fritz, W., Schunder, E. U., Myers, A. L.. Chem. Enp. Sci.. 33, 1097 (1978). (36) Myers, A. L.,‘Prausnitz, J. M., AIChE J., 11, 121 (1965). ( 3 7 ) DiGiano, F. A,, Baldauf, G., Frick, B., Sontheimer, H., Chem. Eng. Sci., 33, 1667-73 (1978). (38) Manes, M., presented a t the 176th National Meeting of the American Chemical Society, Miami, Fla., Sept 10-15, 1978. (39) Hofer, L. J . E., Manes, M., Chem. Eng. Prog., 65,84 (1969). (40) Manes, M., Hofer, L. J. E., J . Phys. Chem., 73,584 (1969). (41) Wohleber, D. A,, Manes, M . , J . Phys. Chem., 75,61 (1971). (42) Wohleber, D. A,, Manes, M., J . Phys. Chem., 75,3720 (1971). (43) Rosene, M. R., Ozcan, M., Manes, M . , J . Phys. Chem., 80,2586 (1976). (44) McGuire, M. J., Suffet, I. H., Radzail, J. V., J . Am. Water Worhs A.s.soc., 70, 565 (Oct 1978). ( 4 5 ) McGuire, M. J . , Ph.D. Thesis, Drexel University, Philadelphia, 1977. (46) Keller, R. A,, Snyder. L. R., J . Chromatog. Si., 9, 346 (19711. (47)Sinanoglu, O., in ”Molecular Associations X I N Biology”, Pullman, B., Ed., Academic Press, New York, 1968, pp 427-46. (48) Sinanoglu, O., Abdulnur, S., Fed. Proc., Fed. Am. Soc. Exp. Bioi., 24, No. 2, P a r t 111, S-12 (1965). (49) Halicioglu, T., Sinanoglu, O., Ann. N.Y. Acad. Sci., 158, :308 (1969). (50) Horvath,-C., Melander, LV.,Molnar, I., J . Chromat‘ogr., 125, 129 (1976). (51) Aberhalden, E.. Foder. A , , Fermentforschung, 2, 74. 151
( 1910). (52) Cheldin, V. H., Williams, R. J., J . Am. Chem. Soc., 64, 1513 (1942). ( 5 3 ) Tiselius, A., Ed., The Svedberg, Almquist and Wiksells, Upsala, 1944, p 370. (54) Williams, R. J. P., Hagdahl, L., Tiselius, A., Ark Kemi, 7, 1 i1954). (55) Colin, H., Eon, C., Guiochon, G., J . Chromatogr., 119, 41 (1970). (56) Colin, H., Eon, C., Guiochon, G., J . Chromatogr., 122, 223 (1976). (57) Colin, H., Guiochon, G., J . Chrornatogr., 137,19 (1977). (58) Colin, H., Ward, N., Guiochon, G., J . Chromatogr., 149, 169 (1978). (59) Colin, H., Guiochon, G., J . Chromatogr., 158,183 (1978). (60) Horvath, C., Melander, W., Molnar, I., Anal. Chem., 49, 142 (1977). (61) Horvath, C., Melander, W., Am. Lab., 10, 17 (1978). (62) Franks, H. S., Evans, M. W.,J . Chem. Phys., 13(11), 507 (1945). (63) Kauzmann, W., Adu. Protein Chem., 14, 1 (1959). (64) Tanford, C., Science, 200, 1012 (1978). (65) Sourirajan, S., Matsuura, T., “Reverse Osmosis and Synthetic Membranes”, Sourirajan, S., Ed., National Research Council Canada, Ottawa, Canada, 1977, p p 5-43. (66) Barrow, G. M., J . Phys Chem., 59, 1129 (1955). (67) Gordy, W., J . Chem. Phys., 7,93-9 (1939). (68) Gordy, W., Stanford, S. C.. J . Chem. Phis., 8,170-7 (1940). (69) Gordy, W., Stanford, S. C., J . Chem. Phys., 9, 204-14 (1941). ( 7 0 ) Gordy, W., J . Chem. P h y s , 9, 215-23 (1941). (71) Kuhn, L. P., J . Am. Chem. Soc.., 74, 2492-9 (1962). (72) Kagija, T., Sumida, Y., Inoue, T,. Bull. Chem. Soc. J p n . , 41,767 (1968). ( 7 3 ) T a f t , R. W., Jr., in “Steric Effects in Organic Chemistry”, Newman, M. S., Ed., Wiley, New York, 1956, p p 556-675. (74) Hammett, L. P., “Physical Organic Chemistry”, 2nd ed., McGraw-Hill, New York, 1970, pp 347-90. (75)Pinsky, d., Mod. Plast., 34(41, 145 (1957). ( 7 6 ) Diamond, J . M., Wright. E. M., Annu. Ret’. Physiol., 31, 581-646 (1969). ~~~
~
~I
Received f o r reuieic July 20, 1978. Accepted April 12, 1979.
Emissions and Particle-Size Distributions of Minor and Trace Elements at Two Western Coal-Fired Power Plants Equipped with Cold-Side Electrostatic Precipitators John M. Ondov*, Richard C. Ragaini, and Arthur H. Biermann Lawrence Livermore Laboratory, University of California, Livermore, Calif. 94550
Concentrations and distributions according to particle size of u p to 42 elements were measured in aerosol particles collected in-stack at two western coal-fired power plants equipped with cold-side electrostatic precipitators (ESP). Elements were measured by instrumental neutron activation analysis, atomic absorption spectroscopy, and X-ray fluorescence. Particle-size distributions in filter and cascade impactor samples from both units were bimodal. Most of the particulate material from the units was emitted as large particles with mass median aerodynamic diameters of >1.6 pm. Emission rates normalized per joule of input heat strongly reflect differences in the type and efficiency of the control devices and the chemistry of t h e coal. However, the relative penetrations of many elements a t both plants were remarkably similar despite major differences in coal composition and plant design. Our results are compared with those of three other studies of similarly equipped power plants. Relative penetrations of Zn, Pb, Ba, Cr, Co, V, Rb, and S b differed significantly among the five plants.
946
Environmental Science & Technology
The National Coal Association forecasts an increase in utility coal usage from 4.0 X 10” kg in 1976 to approximately 7.7 X 10” kg in 1985. The nation’s expanding reliance on coal combustion in the production of electric power has increased the importance of evaluating the associated potential biomedical and environmental hazards. Coal combustion results in the release into the atmosphere of a number of potentially toxic substances, including naturally occurring radionuclides ( I - 5 ) , polynuclear aromatic hydrocarbons (6-8), and various inorganic chemical species (9-16), in vapor (16, 17) and condensed phases. Several investigators have studied the behavior of trace elements during coal combustion. Davison ( 2 8 ) proposed a mechanism whereby volatile species are enriched in respirable, fine particles through vaporization in the combustion zone followed by condensation on particle surfaces. Kaakinen e t al. (9) and Klein et al. ( I 1 ) demonstrated enrichment of elements in emitted particles resulting from the greater penetration of fine, highly enriched particles through emission control devices. Others ( I , 12, 14, 15, 18) observed similar
0013-936X/79/0913-0946$01 .OO/O
@
1979 American Chemical Society
enrichments by these mechanisms in small particles emitted from coal combustion. However, comprehensive studies of trace element emissions from the combustion of U S . coal have been reported for only a few utility-scale generating units. These include Unit No. 5 of the Valmont Power Station ( 9 ) , t h e T . A. Allen Steam Plant ( I I ) , and t h e Chalk Point Plant ( 2 2 ) . Most of t h e plants studied burn Eastern a n d Illinois coals, and relatively few studies have been made of plants burning low-sulfur, Western coal. Thus, in general, the specific effects on emissions of coal type, differences in t h e chemical form and physical distribution of coal constituents, and emission-control devices have not been adequately determined. Furthermore, insufficient attention has been given t o measuring particle-distribution parameters, especially in submicrometer particles. T h e emission of fine particles is of special interest because these particles often contain high concentrations of potentially toxic substances in thin, surface layers (19) and can be deposited efficiently in the pulmonary alveoli (20). In this paper, we report the results of tests on power units a t two conventional, large, western, coal-fired power plants (referred to as plants A and B),one burning low-sulfur bituminous coal, the other burning low-sulfur subbituminous coal. Cold-side electrostatic precipitators (ESP) were used to control particulate emissions a t both plants.
Experimental P l a n t D e s c r i p t i o n a n d S a m p l e Collection. At plant A, we tested a 430-MW (net electrical) coal-fired steam electric generator. T h e unit uses tangentially fired burners and a cold-side E S P with an efficiency between 99.5 and 99.8%. The unit burns -1.45 X lo5 kg of pulverized (200-mesh) western bituminous coal per hour, with ash, sulfur, and heat contents of 9.27~,0.4670, and 28660 J/g, respectively, on a dry basis (moisture content was 6.8%). Stack gases exit through a 183-m stack. Four filter and seven impactor samples were collected isokinetically in stack a t the 91-m level during a 1-week period in January 1975. Sampling times ranged from 55 min to 3 h, the stack temperature was 117 "C, stack pressure ranged from 1.12 to 1.87 mmHg (gauge), and stack-gas velocity ranged from 23.9 to 25.9 m/s. Velocity-traverse data showed t h a t the velocity profile was; flat 91 cm beyond the inside wall. Flow rates through both impactor and filter samplers ranged from 10.8 to 16.1 L/min (wet gas volume). A t plant B, the 7jO-MWe ESP-equipped unit uses opposed front- and rear-fired burners. Flue gases leaving the boiler flow through two cold-side ESPs arranged in a chevron design hefore exiting through a 91-m stack. Each precipitator is four units wide and fo'ur mechanical sections long and has a specific collecting area of 4760 cm'/m:'. When all sections are operating properly, net particle-removal efficiency of the E S P system a t full load is rated a t 97%. Samples were collected in-stack a t the 61-m level of t h e ESP-equipped unit during .July 1975 (21). T h e gross load varied from 515 to 71.5 MWe, but was constant during each test. Four of the 32 separate electrical precipitator sections were inoperative during most o f the test period. Precipitator efficiency in rernoving total suspended particles (TSP) under the test conditions was estimated a t about 97% (see below). Eight filter atid ten cascade impactor samples were collected. Additional samples, including simultaneous samples from the inlet, outlet, and plume ( 2 5 ) ,were collected during February 1976, hut are not reported here. Samples of coal, ESP-collected ash, and bottom ash were also taken a t both plants during stack fly-ash collections. At plant A, pulverimd coal samples were taken hourly, each
sample consisting of a 5-min sample from each of five feeding systems. S a m p l i n g P r o c e d u r e s . Stack emissions were sampled at each plant, using a modified EPA sampling train as described previously (22). Hourly records of plant operating data, including gross generating load, coal consumption, proximate analyses, energy-conversion factors (daily at plant A, monthly a t plant B), and status of ESP sections, were obtained a t both plants. Velocity, temperature, and pressure of the stack gas were monitored continuously during each collection. Filter samples were collected on 47-mm, 0.4-pm Nuclepore filters. Impactor samples were collected with 7 - and 11-stage University of Washington Mark I11 and Mark V source test cascade impactors, using polyethylene or polycarbonate collection substrates and 47-mm, 0.4-pm Nuclepore backup filters. Impaction substrates were coated with a vacuum grease t o improve t h e collection efficiency. Analyses. In addition to gravimetric analyses, u p to 43 elements were analyzed in stack samples, ESP-collected ash, bottom ash, and coal by instrumental neutron activation analysis (INAA) as described previously (23, 24). Cadmium a n d Be were analyzed in coal and fly ash, and P b , Cd, and Be were analyzed in filter and cascade impactor samples, all with a Perkin-Elmer Model 603 atomic absorption spectrometer equipped with a Perkin-Elmer Model 2100 heated graphite analyzer. Samples were dissolved in a mixture of perchloric, nitric, and hydrofluoric acids after ashing overnight at 450 "C. Mercury in coal was analyzed by flameless atomic absorption techniques similar to those of Murphy (25). Nickel, P b , and Cd were measured in bulk coal and fly-ash samples with energy-dispersive X-ray fluorescence analyses ( X R F ) . Coal samples for X R F were dry ashed a t 450 "C overnight, ground to 30 to 60 pm, and pressed into pellets with a n equal amount of Avicel binding agent. y rays from luvCdwere used to excite characteristic fluorescent X-rays. T h e measurement and analyses of spectra are described by Bonner et al. (26).Kesdts from each of these techniques were verified with N B S standard reference materials (SRM) 1632 (coal) and 1633 (coal fly ash), which were analyzed along with the samples, and through interlaboratory comparisons of results on S R M samples (27) and size-classified fly-ash fractions (28). Computation of C o a l Consumption a n d Atmospheric D i s c h a r g e Rates. Rates of coal consumption were computed from data provided by plant personnel. At plant A, the coal consumption rate was meawred by the plant-metering system and a t plant R it was computed from the gross electrical generating load, the known beat-to-electrical conversion factor (a monthly average), and the heat content of the coal as described previously (21 ). Input rates of constituent elements were obtained by multiplying their concentrations in coal (listed in Table I) by the rate of coal consumption. Typical rates of coal consumption a t each of the units are listed in Table 11. Rates of atmospheric discharge of minor and trace element species were computed using the measured stack concentrations and stack-gas velocities. Because the quantities of coal consumed, electric power produced, and energy-conversion factors of the units differed, comparison of the emission data is facilitated by normalizing the data to the amount of heat input into t h e boiler. T h e heat input was computed from the metered coal-flow rate and the heat content of the coal for the unit a t plant A and from the gross generating load and energy-conversion factor for the unit a t plant B. T h e energyconversion factors are listed in Table 11.
Results arid Discussion Distributions of Total Suspended Particles. Parameters used to calculate the total aerosol emitted from each of the two plants were obtained by counting particles on filter samples Volume 13, Number 8, August 1979
947
Table 1. Concentrations of Elements in Coal Burned at Two Western Power Plants, pg/g
0 2 5 0 z 0026!li 05x3 7x.5 032
+
6100
f
014
+
893 73
f
oxw 7i 4 0151 668
-
* t
-
ilOX:(121 9 2 l l 2 i ii I I if',
+
2860 267
+ +
050II?)
jOY 0062
I(125 117 133 2330 3YX
299 2 52 103 4hOO
+
+
I6
+
u;1 88 05
(lli ,141 iYi
:I I YX 519 Oiil
I
i +
+
-
. f
081 ( 5 ) Ihi lil O h (71
.
20 I41 I141 0 2 9 (15) 025
0OX
(15)
I34 T I 2 (61 I63 - 013 llji 11 251) * 1) 0 u Y ( I 1 1 3x0
5720
il5)
I131 r i i i IIUl5iii +
2 84 120 1 2
0 2 2 l l Z l I X (5) u~iii121 i1 3 3 ( 1 2 1 001215i 0 2 2 I51 0 0 2 (51 00 0 5 I 5 i I2iI i1?1
+
+
418
470 1 5 1 0025151
I154 i1 I 0 I
4
273
I04 ( 5 ) U(i5151
' 34rYr - 7 0 I51 - 0 2 i i r5) . 045 !5i - I1 1 3 I 5 1 0 32 151
234 2330 2611
ii
54 1 29411 9x6
. (I 0119 1 1 1 ) 4711 ( 1 1 )
-
+
+ +
0 5 4 115) 1 6 115) 160 ! I S ) 090iIS)
0 2 3 - 0113 2240 I 7 5 3 2 h i 3 0 26 602 : 200 2930 24X
106
12 I 12 I
. +
. *
(6) rhi
16)
!ii i:!
II ii! 0 i til I R 171
100 151 (I 160 * 0 0 1 I i l l
I16 147 io659 hi 0 (1 l h i l iIOXlr1
I65
i
. .
-
. iIilil3iji +
4x3
.
O C i 3
'
Y X
0 0 9 112)
0 0 9 ,121 u013i12! 2 i (5) * 0 t!Uh i 51
+
Mi3 (5) X? 15) Oi123t51 I?iI?i
a Number of replicate samples. Measured by atomic absorption spectroscopy. X-ray fluorescence. Sulfur analyses provided by plant personnel; all others measured by instrumental neutron activation analysis.
Table II. Typical Rates of Coal Consumption and Energy-Conversion Factors
Table 111. Size-Distribution Parameters of Total StackEmitted Aerosols Determined from Scanning Electron Microscopy Analyses of Nuclepore Filter Samples
a Number median diameter (pm). Volume median diameter (pm) determined by In VMD = In NMD f 3 In2 hg. Estimated mass median aerodynamic diameter (pm), using particle densities of 2.2 and 2.44 g/cm3 for plants A and B, respectively. d Geometric standard deviation of assumed log normal distribution.
via scanning electron microscopy (SEM) techniques (see ref 22) and are given in Table 111. The distributions were in each case bimodal, with distinct modes in sub- and supermicrometer size ranges. T h e submicrometer mode is composed of aggregates of smaller particles that may be formed in part from vapor condensation and bubble-bursting mechanisms. These particles are too small to be collected by our impactor and, ignoring wall losses and boundary effects in the impactor, are deposited totally on the backup filter. The second mode contains much larger, mostly solid, spherical particles derived 948
Environmental Science & Technology
mainly from residual mineral matter in coal (29-31). Urlich (30) observed similar particle-distribution modes and suggested that the particles in the smaller mode are largely condensed, volatile Si compounds and metal oxides. Median diameters of the smaller particles from both units are almost identical. Median diameters of the larger particles (see Table 111) strongly reflect differences in the overall collection efficiency of the control device. The smaller median diameters of the large particles emitted a t plant A indicate a greater collection efficiency of the larger particles. Optimum collection efficiencies of the plant A and plant B E S P systems are 99.7 and 97.5%, respectively. Impactor Data. In Figures 1A through E, we have plotted typical particle-size distributions of elements collected in cascade impactor samples t h a t were placed in-stack and downstream from the ESP-equipped units a t each plant. The emission factors (ng/J heat input) of several elements are plotted vs. the aerodynamic diameters of particles. Physical diameters were determined by sizing particles on impactor stages and backup filters via S E M and transformed t o aerodynamic diameters using the particle density and slip-correction factor (32). We corrected the mass of each element present on backup filters by the excess mass resulting from bounce off and reentrainment of large particles as described previously (22). Thus, we can estimate more accurately the fraction of each element emitted in submicrometer particles. Significant amounts of several elements, including V, W, Ga, Mo, Ca, Br, Ba, Se, As, Sb, U, Fe, Cr, Zn, Co, and Mn, were emitted in submicrometer particles. The fraction of other elements in submicrometer particles is generally much less, but too small to determine accurately (22). We attempted to evaluate wall losses by comparing concentrations in impactor and filter samples; because of the variation between successively collected samples, accurate comparisons could not always be made. In general, however, concentrations of elements in the flue gas, which were determined by summing the amounts on impactor stages, were typically from 10 to 60% lower than concentrations on filters in samples from the E S P a t plant A and from 12 to 40% lower in samples from the E S P at plant B. The magnitude of the discrepancy depended on the specific element, its distribution among particle sizes, and sampling time (see ref 22). Aerosol-Distribution P a r a m e t e r s of Minor a n d Trace Elements. Elemental mass median aerodynamic diameters (EMMAD) were determined from analyses of cascade impactor samples collected a t each of the units and they are listed in Table IV. These data reflect only the larger particle modes. Ranges of median EMMADs of particulate emissions from the plant A and plant B ESP units were 1.8-4.9 and 4.:3-12.1 pm, respectively. Thus, as in the case of the total aerosol (see Table HI), the EMMADs of particles from the more efficient E S P were smaller. Elements in emissions from each of the units show distinct behavior in their distributions according to particle size. At plant A, the distributions of AI, Ce, C1, Fe, Hf, K, La, Na, Sc, and T h were nearly identical. T h e EMMADs of particles containing As, Ba, Ga, I, U, V, W, and Zn were absut half those of particles with elements in the AI group. T h e EMMADs of Co, Cs, Cu, Mo, Mn, and S b were slightly smaller than those containing the AI group. If an element were distributed on only the surfaces of aerosol particles, then its mass distribution would coincide with the surface-area distribution of the aerosol. T h e calculated surface-area median aerodynamic diameters (SMAD) of elements in the AI group (about 1.8 for particles having an EMMAD of 2.5 p m and cTg of 2.3) are nearly equal to the EMMADs of elements in the As group, thus indicating surface occurrence of elements in the As group on particles containing elements in the AI group. This would occur if these elements were deposited from the vapor phase;
C .-
u)
0
c
(a UIV - rpu) aiel U O ! S S ~ U J ~ Volume 13, Number 8, August 1979
949
Table IV. Elemental Mass Median Aerodynamic Diameters of Aerosols from Two Coal-Fired Electrical Generating Units, p m
Table VI. Penetration of Elements Contained in Particles through the Boilers and ESPs at Two CoalFired Power Plants ( % ) Plant H
Plant \ 1 Icmmt
Hr ( I
cs, K b , / r
-45
1 4 - 2 ~ d
c
-33
14-24 3 0d
-25
16-27''
c a \In.
-12
2 2 -3 0
\I. Hr Le c.0 u\ t u I c . llg. K La 1 ". \ig. \d. S'. Sm I d Ill l h 1 1
-9 5
2 2 - 3 2
\*
-82
2 7 - 3 5
r.
sc
I Icrnen:
1'c"i.ildli"n~
0 029. 0 006
Prncrrarlon~'
Kr
0 I4
.\I. \ c I a n t h m d e i
09-16
Ih.II,la 1r.Ke \lg, C a . \ a h. l{l>
I R - 2 3 \ s , Ha. L a , In \lo i 1 i . C v \+ / n
-23
15-26
-1 7
1 5 - 2 0 i 2 1 - 3 i+ 1 5 - 2 3 (2 I-421'
44-63 19-34
16-29 a EMMADs estimated from up to five impactor samples. The relative standard deviatiop of successive determinationsof the EMMADs was typically about 20%, but errors among elements are typically 10%. Geometric standard deviation. Range of median values of EMMADs of up to six impactor samples for each element. Value corresponds to element underlined.
