Elemental Composition of Atmospheric Fine Particles Emitted from

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Elemental Composition of Atmospheric Fine Particles

Emitted

Electric

Power

from Plant

Coal Equipped

Burned

in

with

a

a

Modern Flue-Gas

Desulfurization System J. M. ONDOV, A. H. BIERMANN, R. E. HEFT, and R. F. KOSZYKOWSKI Environmental Sciences Division, Lawrence Livermore Laboratory, P.O. Box 5507, L-453, Livermore, CA 94550 Improved control devices now frequently installed on conventional c o a l - u t i l i t y boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal u t i l i t y boiler. Particulate samples were collected in situ on membrane f i l t e r s and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0.07 to 20 μm; with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. I t i s now w e l l documented t h a t l a r g e numbers o f p a r t i c l e s a r e e m i t t e d f r o m c o a l - f i r e d power p l a n t s i n d i s t i n c t d i s t r i b u t i o n s w i t h m o d a l d i a m e t e r s l e s s t h a n 1 ym ( 1 - 5 ) . W h i l e m a j o r components o f t h o s e a e r o s o l s a r e t h o u g h t t o b e o x i d e s o f S i , A l , Na, and Ca ( 5 - 6 ) , t h e c o n c e n t r a t i o n s o f most o f t h e o t h e r e l e m e n t s have n o t been a c c u r a t e l y d e t e r m i n e d . As p r e d i c t e d by m o d e l s o f g a s - t o - p a r t i c l e d e p o s i t i o n ( 2 , 7 - 9 ) , e x p e r i m e n t a l d a t a (8-11) i n d i c a t e t h a t the c o n c e n t r a t i o n s o f vapor-deposited species i n these very f i n e p a r t i c l e s are o f t e n

0097-6156/81/0167-0173$05.00,/0 © 1981 American Chemical Society

Macias and Hopke; Atmospheric Aerosol ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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10- t o 1 0 0 - f o l d g r e a t e r t h a n t h o s e o f t h e l a r g e - p a r t i c l e r e g i o n of the d i s t r i b u t i o n . T h e o r e t i c a l l y , a t l e a s t some o f t h e p a r t i c l e s may be h o m o g e n e o u s l y c o n d e n s e d and c o n t a i n a s i n g l e c h e m i c a l component. A l t h o u g h t h e r e l e a s e o f l a r g e numbers o f t h e s e h i g h l y c h e m i c a l l y e n r i c h e d p a r t i c l e s may h a v e p r o f o u n d h e a l t h and e n v i r o n m e n t a l consequences, the p u b l i s h e d l i t e r a t u r e c o n t a i n s v i r t u a l l y no d a t a c h a r a c t e r i z i n g t h e d i s t r i b u t i o n , p h y s i c a l p r o p e r t i e s , c h e m i c a l c o m p o s i t i o n , and s o u r c e t e r m s o f a b r o a d range of important primary p o l l u t a n t s e m i t t e d i n submicrometer particles. I n p r e v i o u s s t u d i e s we f o u n d t h a t i m p r o v e d e m i s s i o n - c o n t r o l d e v i c e s ( s u c h as h o t - s i d e e l e c t r o s t a t i c p r e c i p i t a t e r s and w e t - s c r u b b e r s y s t e m s ) now b e i n g i n s t a l l e d on modern p u l v e r i z e d c o a l - f i r e d power p l a n t s m o d i f y t h e q u a n t i t y , c h e m i c a l c o m p o s i t i o n , and d i s t r i b u t i o n c h a r a c t e r i s t i c s o f f i n e a e r o s o l emissions (12,13). Such m o d i f i c a t i o n s must be u n d e r s t o o d t o a d e q u a t e l y a s s e s s human h e a l t h and e n v i r o n m e n t a l h a z a r d s , and t o a p p o r t i o n the c o n t r i b u t i o n s of sources to urban p o l l u t a n t inventories. I n t h i s work, we u s e a U n i v e r s i t y o f W a s h i n g t o n low p r e s s u r e i m p a c t o r ( L P I ) and i n s t r u m e n t a l n e u t r o n a c t i v a t i o n a n a l y s i s (INAA) t o d e t e r m i n e t h e e l e m e n t a l c o m p o s i t i o n o f a e r o s o l s f r o m a two 430 MWe c o a l - u t i l i t y b o i l e r s , ranging i n d i a m e t e r f r o m l e s s t h a n 0.07 t o a b o u t 10 ym, and t o i n v e s t i g a t e t h e m o d i f i c a t i o n o f t h e a e r o s o l by a modern f l u e - g a s d e s u l f u r i z a t i o n system. A p r e l i m i n a r y a c c o u n t o f t h e work i s presented here. Experimental

Measurements

Plant description. Two n e a r l y i d e n t i c a l 430-Mw(e), western, conventional p u l v e r i z e d - c o a l - u t i l i t y b o i l e r s ( r e f e r r e d t o as p l a n t s A and D) were t e s t e d . B o t h u n i t s u s e t a n g e n t i a l l y f i r e d b u r n e r s and b u r n l o w - s u l f u r 200-mesh c o a l o f h e a t c o n t e n t a p p r o x i m a t e l y 27 000 J / g . Both u n i t s are equipped w i t h c o l d - s i d e e l e c t r o s t a t i c p r e c i p i t a t o r s (ESP) o f d e s i g n e f f i c i e n c y o f 99.5% o r g r e a t e r , and a modern f l u e - g a s d e s u l f u r i z a t i o n (FGD) system c o n s i s t i n g of f o u r v e r t i c l e spray towers. E a c h a d s o r b e r t o w e r c o n t a i n s a s e r i e s o f s p r a y n o z z l e s and a m i s t e l i m i n a t o r as shown i n F i g u r e 1. A f t e r c o n t a c t i n g the f l u e g a s , t h e s c r u b b i n g s o l u t i o n ( a 6% by w e i g h t c a l c i u m o x i d e - w a t e r s l u r r y ) c o l l e c t s i n t h e b o t t o m o f t h e tower where i t is continuously s t i r r e d . F r e s h l i m e s l u r r y i s added a t the b o t t o m o f t h e tower where i t m i x e s w i t h t h e l i q u i d i n j e c t e d i n t o the spray s e c t i o n . T h i s mixed l i q u i d i s c o n t i n u o u s l y r e c y c l e d t o t h e s p r a y s e c t i o n , and f u n c t i o n s as t h e a d s o r b i n g a g e n t . O v e r a l l r e d u c t i o n i n t h e S O 2 c o n c e n t r a t i o n i s d e s i g n e d t o be 80% when 10% o f t h e f l u e gas i s b y p a s s e d . The s y s t e m i s d e s i g n e d t o l i m i t the e m i s s i o n of t o t a l suspended p a r t i c l e s


ONDOV

E T

A L .

Fine

Particles Emitted

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Coal

1

Mist eliminator spray (cooling tower blowdown)

To thickener

Overflow from thickener

Figure 1. Flow schematic of a spray-tower adsorber. The scrubbing solution is contacted with hot flue gas, collected in the bottom, and continuously recycled and contacted. Suspended solids and pH of liquid in the recycle loop of Plant D spray towers ranged from 5.2 to 8.7%, and from 5.2 to 6.80%, respectively.


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( T S P ) t o 0.032 g/m , p r o v i d e d not exceed t h a t l e v e l .

the i n l e t

AEROSOL

TSP c o n c e n t r a t i o n d o e s

Sampling. F o u r a e r o s o l s a m p l e s were c o l l e c t e d i s o k i n e t i c a l l y on J u l y 26, 1979, a t p o r t s on t h e i n l e t d u c t o f t h e ESP a t P l a n t A; two w e r e t a k e n w i t h 62-mm f l u o r o p o r e f i l t e r s and two were t a k e n w i t h t h e U n i v e r s i t y o f W a s h i n g t o n MKV C a s c a d e impactor (14). A t p l a n t D, s a m p l e s were c o l l e c t e d o v e r a 6-day p e r i o d a t p o r t s b o t h i n t h e o u t l e t d u c t o f t h e ESP ( i . e . , u p s t r e a m o f t h e FGD s y s t e m ) a n d a t t h e 91-m l e v e l o f t h e s t a c k . E l e v e n f l u o r o p o r e f i l t e r , 1 MKV, a n d 4 M K I I I i m p a c t o r s a m p l e s a t e a c h l o c a t i o n , g i v i n g a t o t a l o f 22 f i l t e r , a n d 8 M K I I I samples. A s i n g l e MKV sample was a l s o c o l l e c t e d i n - s t a c k a t r e d u c e d p r e s s u r e a t p l a n t D d u r i n g t h e 6-day p e r i o d . P o l y c a r b o n a t e m a t e r i a l c o a t e d w i t h a p i e z o n L vacuum g r e a s e a n d 62 o r 47-mm-diam, 1-pm p o r e F l u o r o p o r e f i l t e r s were u s e d a s b a c k - u p f i l t e r s i n t h e MKV i m p a c t o r . The pH a n d s o l i d s c o n t e n t o f l i q u i d i n t h e r e c y c l e l o o p o f . e a c h s p r a y t o w e r a n d o f t h e l i m e s l u r r y were m o n i t o r e d a n d r e c o r d e d by p l a n t p e r s o n n e l a t 4-h i n t e r v a l s d u r i n g t h e e n t i r e t e s t p e r i o d . T h e pH a n d s o l i d s c o n t e n t o f t h e r e c y c l e d l i q u i d r a n g e d f r o m 5.2 t o 6.8 a n d 5.2 t o 8.7%, r e s p e c t i v e l y ; f o r t h e l i m e s l u r r y , t h e r a n g e s were 4.8 t o 5.9, a n d 12.2 t o 12.8%, respectively. O p e r a t i o n o f t h e U n i v e r s i t y o f W a s h i n g t o n MKV I m p a c t o r . The MKV i s a m u l t i c i r c u l a r j e t i m p a c t o r s i m i l a r i n d e s i g n t o t h e M K I I I , b u t i t h a s 11 i m p a c t o r s t a g e s a n d may b e o p e r a t e d a s a h i g h - p r e s s u r e d r o p i m p a c t o r ( 1 5 ) , w i t h an o u t - b o a r d b a c k - u p f i l t e r holder. The o r i f i c e p l a t e s o f t h e l a s t f o u r stages o f t h e MKV a r e q u i t e s i m i l a r t o t h o s e u s e d i n t h e U n i v e r s i t y o f W a s h i n g t o n MKIV, w h i c h was d e s i g n e d s p e c i f i c a l l y f o r l o w p r e s s u r e o p e r a t i o n . W i t h a p r e c y c l o n e , t h e u n i t c a n p r o v i d e 13 d i s c r e t e p a r t i c l e s i z e f r a c t i o n s , w i t h up t o 6 o f t h e f r a c t i o n s in the h i g h l y r e s p i r a b l e submicrometer p a r t i c l e - d i a m e t e r - r a n g e . U n l i k e t h e MKIV, t h e MKV c a n be i n s e r t e d i n t o t h e d u c t t h r o u g h s t a n d a r d 4 - i n i . d . sampling p o r t s , so t h a t t h e a e r o s o l i s s i z e d in s i t u without d i l u t i o n o r heating. A rotatable joint ( d e s c r i b e d i n R e f e r e n c e 11) p l a c e d b e t w e e n t h e p r o b e and t h e sampler a l l o w s us t o t u r n t h e sampler i n t o t h e gas flow so t h a t a s t r a i g h t , r a t h e r t h a n c u r v e d ( " g o o s e n e c k " ) , n o z z l e may be used. In low-pressure operation, the absolute pressure o f gas in the l a s t stage i s monitored throughout the r u n . T h e o r e t i c a l e f f i c i e n c y c u r v e s f o r e a c h s t a g e may b e c a l c u l a t e d i f t h e a b s o l u t e p r e s s u r e o f t h e gas i n each o f t h e s t a g e s i s known. P i l a t et_ a K (JL6>) h a v e e x t e n s i v e l y m e a s u r e d t h e p r e s s u r e s on e a c h s t a g e o f t h e i r MKIV i m p a c t o r a s a f u n c t i o n of g a s f l o w , and have c o n s t r u c t e d an e m p i r i c a l r e l a t i o n s h i p f o r c a l c u l a t i n g the pressures of preceeding stages, given the p r e s s u r e o f t h e f i n a l s t a g e and t h e o v e r a l l f l o w r a t e o f gas through the impactor. P i l a t and Mummey ( 1 5 ) u s e d t h i s e m p i r i c a l


9.

ONDOV ET A L .

Fine Particles

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r e l a t i o n s h i p t o prepare a set o f t h e o r e t i c a l c a l i b r a t i o n curves s p e c i f i c a l l y f o r t h e f i n a l s t a g e p r e s s u r e s and f l o w r a t e s o f o u r MKV u n i t . Because o f the c o n s t r a i n t s o f i s o k i n e t i c sampling, and t h e somewhat h i g h - p r e s s u r e - d r o p - f i l t e r r e q u i r e d f o r o u r c h e m i c a l a n a l y s e s , we were u n a b l e t o d u p l i c a t e t h e f i n a l s t a g e p r e s s u r e s and sample f l o w r a t e s c a l l e d f o r i n t h e t h e o r e t i c a l c u r v e s , s o we d i d n o t u s e them i n t h e s e e x p e r i m e n t s . A n a l y t i c a l Techniques. Numerous f a c t o r s ( e . g . i n a d e q u a t e t h e o r y , p a r t i c l e b o u n c e , c r o s s s e n s i t i v i t y , and u n r e p r o d u c i b l e gas l e a k a g e ) p r e c l u d e d a c c u r a t e p r e d i c t i o n o f t h e t r u e d i s t r i b u t i o n s o f p a r t i c l e s c o l l e c t e d on t h e l o w - p r e s s u r e i m p a c t o r s t a g e s . T h e r e f o r e we c h o s e t o v e r i f y t h e o p e r a t i o n o f t h e i m p a c t o r b y s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM). Particles f r o m a 1 0 % p o r t i o n o f e a c h o f t h e s u b s t r a t e s t h e f i r s t MKV i m p a c t o r sample o b t a i n e d a t p l a n t A were d i s p e r s e d i n t o hexane by s o n i c a t i o n , f i l t e r e d on t o N u c l e p o r e f i l t e r s , and s i z e d f r o m s c a n n i n g e l e c t r o n m i c r o g r a p h s by c o u n t i n g p a r t i c l e s i n d i s c r e t e s i z e r a n g e s . A Q u a n t i m e t image a n a l y z e r i n t e r f a c e d t o t h e SEM s i z e d t h e p a r t i c l e s when t h e c o n c e n t r a t i o n was l o w enough t o resolve individual particles. We were n o t a b l e t o o b t a i n t h e d i s t r i b u t i o n s o f p a r t i c l e s on t h e b a c k - u p f i l t e r s b e c a u s e t h e p a r t i c l e s a r e s m a l l , and t h e F l u o r o p o r e f i l t e r i s f i b r o u s . A l l i m p a c t o r and f i l t e r s a m p l e s were a n a l y z e d f o r up t o 4 5 e l e m e n t s by i n s t r u m e n t a l n e u t r o n a c t i v a t i o n a n a l y s i s ( I N A A ) a s d e s c r i b e d by H e f t ( 1 7 ) . Samples w e r e i r r a d i a t e d s i m u l t a n e o u s l y w i t h s t a n d a r d f l u x m o n i t o r s i n t h e 3-MW L i v e r m o r e p o o l r e a c t o r . The x - r a y s p e c t r a o f t h e r a d i o a c t i v e s p e c i e s w e r e t a k e n w i t h l a r g e - v o l u m e , h i g h - r e s o l u t i o n G e ( L i ) s p e c t r o m e t e r systems. The s p e c t r a l d a t a were t r a n s f e r r e d t o a CDC 7600 c o m p u t e r a n d a n a l y z e d w i t h t h e GAMANAL code ( 1 8 ) , w h i c h i n c o r p o r a t e s a b a c k g r o u n d - s m o o t h i n g r o u t i n e and f i t s t h e p e a k s w i t h G a u s s i a n and exponential functions. A c t i v a t i o n a n a l y s i s w i t h t h e r m a l n e u t r o n s i s t h e method o f c h o i c e because o f t h e i n h e r e n t l y h i g h accuracy o f t h e t e c h n i q u e . Because m a t r i x e f f e c t s are v i r t u a l l y i n s i g n i f i c a n t over t h e energy range o f n u c l e a r photons used i n t h e a n a l y s i s , t h e a c c u r a c y o f t h e r e s u l t s i s l i m i t e d o n l y by t h e c o u n t i n g s t a t i s t i c s , t h e r e p r o d u c i b i l i t y o f t h e s o l i d a n g l e i m p i n g i n g on t h e d e t e c t o r , and t h e a c c u r a c y o f t h e s t a n d a r d s . This i s r o u t i n e l y w i t h i n 5 t o 1 0 % f o r most e l e m e n t s d e t e r m i n e d . In c o m p a r i n g t h e mass o f a s i n g l e e l e m e n t i n d i f f e r e n t s a m p l e s o r on s u c e s s i v e i m p a c t o r s t a g e s , however, u n c e r t a i n i t y i n t h e s t a n d a r d may be n e g l e c t e d . F u r t h e r , b e c a u s e o f t h e r a t h e r l a r g e d i s t a n c e between t h e sample and t h e d e t e c t o r , e r r o r s i n p o s i t i o n i n g t h e s a m p l e a r e a l s o s m a l l , a n d t h e l i m i t i n g u n c e r t a i n t y becomes t h a t of the c o u n t i n g s t a t i s t i c s . The o n e - s i g m a u n c e r t a i n t y f o r Sc a n d As i s o f t e n 2 a n d 5%, r e s p e c t i v e l y , o f t h e t o a l c o u n t . This h i g h degree o f p r e c i s i o n permits i n t e r p r e t a t i o n o f f i n e s t r u c t u r e i n t h e d a t a t h a t m i g h t o t h e r w i s e be m i s s e d by o t h e r t e c h n i q u e s .


ATMOSPHERIC AEROSOL

178 R e s u l t s and D i s c u s s i o n

Low-Pressure Impactor Data. Table I l i s t s the count medians and g e o m e t r i c s t a n d a r d d e v i a t i o n s f r o m l o g - n o r m a l f i t s o f t h e d i s t r i b u t i o n s o b t a i n e d by SEM a n a l y s e s o f sample MKV-1. L i s t e d a l s o a r e t h e c o r r e s p o n d i n g mass m e d i a n s c a l c u l a t e d f r o m t h e c o u n t distributions. The c o u n t - m e d i a n d i a m e t e r s o f 6 o f t h e s t a g e s a r e b e l o w 1 Mm, and t h e l a s t 4 a r e w e l l b e l o w t h e s i z e r a n g e o b t a i n a b l e by i n e r t i a l i m p a c t i o n a t n e a r - a t m o s p h e r i c p r e s s u r e . F u r t h e r , we o p e r a t e d t h e i m p a c t o r a t a somewhat h i g h e r f i n a l s t a g e p r e s s u r e t h a n o p t i m a l (415 r a t h e r t h a n 345 mm H g ) , t h e r e f o r e , i t i s p o s s i b l e t o o b t a i n even s m a l l e r - s i z e c u t s . A t t h i s t i m e , we h a v e n o t t h o r o u g h l y v e r i f i e d o u r SEM t e c h n i q u e s f o r s i z i n g a g g r e g a t e d p a r t i c l e s s m a l l e r t h a n 1 ym. P a r t i c l e s of the accumulation r e g i o n a r e a g g r e g a t e s formed by c o a g u l a t i o n o f much s m a l l e r particles. Thus i n s o n i c a l l y d i s p e r s i n g them, some b i a s t o w a r d s s m a l l e r s i z e s m i g h t be e x p e c t e d i f t h e a g g r e g a t e s a r e b r o k e n up. In p r a c t i c e , however, i t i s g e n e r a l l y d i f f i c u l t t o d i s p e r s e submicrometer p a r t i c l e s ( e s p e c i a l l y p a r t i c l e s s m a l l e r than 1 ym), b e c a u s e t h e r e l a t i v e s u r f a c e f o r c e s a r e much s t r o n g e r between p a r t i c l e s o f s m a l l e r s i z e . T h u s , i t i s j u s t as l i k e l y t h a t p a r t i c l e s w i l l d i s p e r s e more n e a r l y i n t h e manner i n w h i c h t h e y came i n t o t h e i m p a c t o r , o r t h a t t h e SEM d i s t r i b u t i o n s a r e b i a s e d towards l a r g e r s i z e s . I d e a l l y , the impactor w i l l s i z e p a r t i c l e s i n t o d i s c r e t e , narrow i n t e r v a l s . T h i s should reduce t h e e f f e c t s o f b i a s by s o n i c d i s p e r s i o n . T h u s , we b e l i e v e t h a t the s i z e d i s t r i b u t i o n s are q u a l i t a t i v e l y c o r r e c t , c u t c a u t i o n must be e x e r c i s e d i n u s i n g t h e d a t a . We a l s o r e c o g n i z e t h a t l i q u i d ( o r v o l a t i l e s o l i d ) a e r o s o l s , s u c h as s u l f u r i c a c i d m i s t s , c a n n o t be s i z e d by t h e s e t e c h n i q u e s . I n F i g u r e 2 we show t h e amount o f A l , F e , Sc, V, U, and Se i n p a r t i c l e s per l o g - s i z e - i n t e r v a l of each impactor s t a g e , per m o f gas p l o t t e d a g a i n s t t h e mass m e d i a n d i a m e t e r s (mmd) o f T a b l e I . N o t e t h a t i n c h o o s i n g t h e mmd and l o g - s i z e i n t e r v a l f o r t h e f i l t e r , we assumed t h a t t h e s u b m i c r o m e t e r d i s t r i b u t i o n i s l o g - n o r m a l and t h a t a l l o f t h e mass on t h e f i l t e r i s c o n t a i n e d i n p a r t i c l e s o f d i a m e t e r s between 0.01 and 0.07 ym. These d a t a suggest t h a t the impactor i n t e r v a l s n i c e l y b r a c k e t the a c c u m u l a t i o n mode t h a t o c c u r r s a t 0.11 ym. The s u b m i c r o m e t e r mode o f a l l t h e 35 e l e m e n t s d e t e r m i n e d by INAA, o c c u r e d a t 0.11 ym e x c e p t f o r t h o s e o f C r ( F i g u r e 3) and Mn ( n o t shown) w h i c h o c c u r r e d a t 0.14 ym. Particles in this s i z e r a n g e a r e so h i g h l y c o n c e n t r a t e d ( l O ^ - p a r t i c l e s / c m a t 0.07 ym) t h a t t h e y c o a g u l a t e v e r y r a p i d l y . T h e r e f o r e , the s h i f t i n t h e modes o f C r and Mn may c o r r e s p o n d t o a t e m p o r a l i n c r e m e n t s e p a r a t i n g t h e f o r m a t i o n o f C r and Mn a e r o s o l s f r o m t h e other elements. A l t e r n a t i v e l y h o w e v e r , t h i s may i n d i c a t e t h a t C r and Mn a e r o s o l s a r e f o r m e d by a d i f f e r e n t mechanism t h a n a r e t h e other elements. More c a r e f u l a n a l y s i s o f t h e modes o f t h e i n d i v i d u a l e l e m e n t s may p r o v i d e i n s i g h t s i n t o t h e t r a c e 3

3

3


9.

ONDOV

I0

E T

A L .

Fine

r

b

Particles Emitted

-

10-

T T T 1

J

T

I ,| T

(b)

10

-

J

10

-

I

/

Coal

, M

1

-

Al

_

10'

-|

.

1 1 l|

I

i l l

2 1

(c)

n

10

1

10"

.,1 > , n'

179

M

10

K

10«

~

from

/

Sc _

r

1

l ,

,1

>

I

ill

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i i i .

Figure 2. Concentration vs. size curves of Fe, Al, Sc, V, U, and Se in aerosol particles collected upstream of an electrostatic precipitator of a coal utility boiler.

0.1

1.0

10

Mass median particle diameter (um)

Figure 3. Profiles of concentration vs. particle size of Cr in particles collected upstream of an electrostatic precipitator of a coal utility boiler.


ATMOSPHERIC AEROSOL

180

T a b l e I . L o g - n o r m a l d i s t r i b u t i o n p a r a m e t e r s and s e l e c t e d a n a l y s e s o f p a r t i c l e s on s t a g e s o f i m p a c t o r sample M K V - l . a

CMD Stage

b

d

MMD

(ym)

T o t a l mass (Um)

Elemental mass

(mg)

Al,

mg

Sc,

e

yg

A s , yg

1

4.07

1.62

8.13

255

21+1

3080+20

1.3+0.3

2

2.70

1.48

3.79

558

39+2

6250+50

1.8+0.4

3

2.86

1.36

3.81

130

11.3+0.7

2270+20

1.4+0.4

4

1.87

1.43

2.73

35.2

2.9+0.4

686+5

0.72+0.07

5

1.14

1.48

1.82

28.0

2.6+0.1

639+4

0.75+0.04

6

0.76

1.58

1.43

16.5

1.8+0.3

371+3

0.45+0.4

7

0.50

1.46

0.77

—

0.22+0.03

53.6+0.5

0.133+0.05

—

0.053+0.005

13.5+0.3

0.088+0.002

—

0.034+0.003

8.7+0.3

0.104+0.004

0.14

—

0.041+0.004

9.1+0.4

0.167+0.003

0.11

—

0.103+0.007

21.1+0.4

0.436+0.006

0.146+0.009

41.3+0.7

1.03+0.02

8

0.184

1.66

0.40

9

0.15

1.44

0.22

10

0.10

1.36

11

0.077

1.42

12

4.54

a

T h e i m p a c t o r was o p e r a t e d a t 7.84 s l p m ; t h e f i n a l s t a g e p r e s s u r e was 415 mm Hg a n d s t a c k g a s t e m p e r a t u r e , 1 1 7 ° C . ^Count median d i a m e t e r . G e o m e t r i c s t a n d a r d d e v i a t i o n . ^Mass m e d i a n d i a m e t e r . T h e u n c e r t a i n t i e s r e p o r t e d a r e t h e w e i g h t e d means o f t h e o n e - s i g m a u n c e r t a i n t i e s ( f r o m c o u n t i n g s t a t i s t i c s ) o f the m u l t i p l e photopeaks used i n the a n a l y s i s as d e s c r i b e d by H e f t ( 1 7 ) . c

e

e l e m e n t c o m b u s t i o n c h e m i s t r y ; t h a t i s t o s a y , i t may be p o s s i b l e t o d i s c r i m i n a t e b e t w e e n l i k e l y c h o i c e s o f c h e m i c a l forms i n which the elements condense. Chemical enrichment of a e r o s o l s . I n F i g u r e 4 we p l o t t h e r e l a t i v e c o n c e n t r a t i o n s o f W, V, U, and As i n p a r t i c l e s c o l l e c t e d i n s t a c k and a t t h e ESP o u t l e t l o c a t i o n w i t h t h e 8-stage impactors vs e q u i v a l e n t aerodynamic d i a m e t e r (d5Q). H e r e t h e r e l a t i v e c o n c e n t r a t i o n s a r e e x p r e s s e d as t h e e n r i c h m e n t f a c t o r (EF) r a t h e r than the weight/weight c o n c e n t r a t i o n ( i . e . , Mg/g) b e c a u s e we c a n n o t i n e v e r y c a s e a c c u r a t e l y w e i g h t h e p a r t i c l e s on t h e s t a g e s . The E F s a r e a p p r o x i m a t e l y p r o p o r t i o n a l t o t h e r e l a t i v e mass c o n c e n t r a t i o n s , and h a v e t h e added i n t e r p r e t i v e v a l u e o f s h o w i n g t h e c h e m i c a l enhancement w i t h


ONDOV

E T

A L .

Fine

Particles Emitted from

Coal

Figure 4. Enrichment factors (relative to Sc) vs. particle size curves for aerosols collected up- and downstream of the flue-gas desulfurization system show considerable concentration enhancement in the submicrometer size region.


1

182

ATMOSPHERIC AEROSOL

respect to c o a l . The E F s a r e g i v e n by t a k i n g t h e r a t i o s o f t h e e l e m e n t s t o t h a t o f Sc f o r t h e a e r o s o l p a r t i c l e s o f e a c h s t a g e and d i v i d i n g by t h e r a t i o o f t h e same e l e m e n t s i n p u l v e r i z e d coal. In t h i s d a t a s e t , the c o n c e n t r a t i o n s of elements i n submicrometer p a r t i c l e s are c l e a r l y enhanced d u r i n g passage t h r o u g h t h e FGD s y s t e m . The c o n c e n t r a t i o n s o f W, ( F i g u r e 4 a ) , C r , and Mn ( F i g u r e 5 a and b) show enhancement i n b o t h s u b m i c r o m e t e r - and s u p e r - m i c r o m e t e r - d i a m e t e r p a r t i c l e s . S e l e n i u m ( F i g u r e 6) was e n h a n c e d on p a r t i c l e s o f a l l s i z e s . These f i n d i n g s are c o n s i s t e n t w i t h our e a r l i e r study of h i g h - e n e r g y V e n t u r i wet s c r u b b e r s y s t e m s ( 1 2 ) . I t appears t h a t h i g h l o c a l a c i d i t y o c c u r s when t h e a e r o s o l i z e d l i q u i d d r o p l e t s c o n t a c t t h e f l u e g a s , as e v i d e n c e d by c o r r o s i o n p r o b l e m s t h a t often plague these d e v i c e s . N o t e t h a t w h i l e t h e pH o f t h e l i q u i d i n the bottom of the s c r u b b e r i s m a i n t a i n e d a t about 5.6, l i q u i d a e r o s o l s e x t r a c t e d from the upper r e g i o n s o f the towers show much l o w e r pH v a l u e s . These c o n d i t i o n s a c c e l e r a t e the d i s s o l u t i o n o f m i n o r and t r a c e e l e m e n t s on a e r o s o l p a r t i c l e s u r f a c e s , thereby e n r i c h i n g the c o n c e n t r a t i o n s of the element i n s c r u b b e r s o l u t i o n s . The b u l k o f t h e s c r u b b i n g s o l u t i o n i s c o n t i n u o u s l y i n j e c t e d i n t o t h e s p r a y s e c t i o n s by t h e r e c y c l e pumps, a l l o w i n g c o n s i d e r a b l e b u i l d u p o f t h e l e a c h e d substances i n the s c r u b b i n g s o l u t i o n . On c o n t a c t w i t h t h e h o t f l u e g a s , t h e w a t e r e v a p o r a t e s f r o m some o f t h e a e r o s o l i z e d d r o p l e t s . The e n r i c h m e n t may o c c u r r by c o a g u l a t i o n o f t h e l i q u i d d r o p l e t s w i t h f i n e p a r t i c l e s , f o l l o w e d by e v a p o r a t i o n o f t h e l i q u i d t o l e a v e a more h i g h l y c h e m i c a l l y e n r i c h e d p a r t i c l e o f a somehwat l a r g e r , yet s t i l l quite small s i z e . However, e v a p o r a t i o n o f w a t e r f r o m the a e r o s o l i z e d d r o p l e t s to form h i g h l y e n r i c h e d submicrometer p a r t i c l e s f r o m t h e r e s i d u e , may occur. F i n a l l y , c o r r o s i o n of i n t e r n a l metal surfaces, c o n s t i t u e n t s o f t h e a b s o r b i n g and w a t e r - c o n d i t i o n i n g a g e n t s , and condensation o f v a p o r components f u r t h e r c o n t r i b u t e t o t h e c o n c e n t r a t i o n s o f C r and Mn, Ca and P, and Se and S i n t h e s c r u b b i n g s o l u t i o n and lead to t h e i r enrichment i n f i n e a e r o s o l emissions. E s t i m a t e s o f the r e l a t i v e c o n c e n t a t i o n s o f s e l e c t e d elements i n f i n e p a r t i c l e s c o l l e c t e d upstream from the FGD s y s t e m a r e l i s t e d i n T a b l e I I . The v a l u e s l i s t e d a r e some o f the h i g h e s t c o n c e n t r a t i o n s y e t r e p o r t e d i n the l i t e r a t u r e f o r f i n e c o a l - d e r i v e d a e r o s o l s , e s p e c i a l l y t h o s e o f V, C r , and Zn, which l i e i n the t e n t h - p e r c e n t range. A d d i t i o n a l enhancements i n t h e c o n c e n t r a t i o n o f t h e s e e l e m e n t s r e s u l t i n g f r o m FGD were a b o u t a f a c t o r o f 2 f o r V, U, W, and A s , a f a c t o r o f 4 f o r C r and Mn, and a f a c t o r o f 15 f o r Se. I f , as p r e d i c t e d by v a p o r - d e p o s i t i o n m o d e l s , t h e mass o f t h e r e c o n s t i t u e n t s r e s i d e s i n 0.02-ym-thick s u r f a c e l a y e r s ( H ) ) , then the s u r f a c e c o n c e n t r a t i o n w o u l d be y e t a n o t h e r 5 t i m e s g r e a t e r , y i e l d i n g c o n c e n t r a t i o n s o f V, Zn, C r , and Se i n t h e r a n g e o f 1 t o 5%.


9.

ONDOV ET A L .

Fine

Particles Emitted

from

Coal

183

Figure 5. Enrichment factor vs. particle size curves for Cr and Mn show chemical enhancement of both large and smaller particles.

Figure 6. Enrichment factor vs. particle size curves for Se in aerosol particles sampled upstream of the Plant A coldside electrostatic precipitator ( ) and downstream of the Plant D flue-gas desulfurization system ( ). Aerodynamic diameters of equivalent unit density spheres were obtained by adjusting the mmds of Table I for slip and particle density (assumed to be 2.4 g/cm for the particles collected). 3

Equivalent aerodynamic diameter (jum)


ATMOSPHERIC

184

AEROSOL

Conclusions We h a v e o p e r a t e d t h e U n i v e r s i t y o f W a s h i n g t o n MKV impactor as a l o w - p r e s s u r e i m p a c t o r t o p r o v i d e f o r c h e m i c a l a n a l y s i s , four d i s c r e t e l y s i z e d f l y - a s h f r a c t i o n s i n the sub-half-micrometer- diameter a e r o s o l accumulation r e g i o n . Instrumental neutron a c t i v a t i o n a n a l y s i s provided the s e n s i t i v i t y t o d e t e r m i n e a c c u r a t e l y t h e c o n c e n t r a t i o n s o f 28 m a j o r , m i n o r , and t r a c e e l e m e n t s w i t h s u f f i c i e n t p r e c i s i o n t o r e v e a l f i n e s t r u c t u r e i n the elemental d i s t r i b u t i o n s that might be m i s s e d by t e c h n i q u e s o f l e s s e r a c c u r a c y and p r e c i s i o n . We h a v e f u r t h e r a p p l i e d t h e s e t e c h n i q u e s to investigate t h e c h e m i c a l m o d i f i c a t i o n o f a e r o s o l s by a modern f l u e - g a s d e s u l f u r i z a t i o n system. T h i s s t u d y c o n f i r m s o u r e a r l i e r work w i t h a h i g h - e n e r g y V e n t u r i wet s c r u b b e r s y s t e m , i n w h i c h we observed h i g h c h e m i c a l enrichment o f a e r o s o l s from e v a p o r a t i v e processes. I n g e n e r a l , we f e e l t h a t t h e r e a r e f a r t o o few s t u d i e s o f the c o m p o s i t i o n o f p a r t i c l e s i n the submicrometer r e g i o n . More s t u d i e s a r e n e e d e d t o a d e q u a t e l y c h a r a c t e r i z e t h e human h e a l t h and e n v i r o n m e n t a l h a z a r d s a s s o c i a t e d w i t h u t i l i t y c o a l c o m b u s t i o n , and t o more a c c u r a t e l y d e t e r m i n e t h e i r c o n t r i b u t i o n to urban p o l l u t a n t i n v e n t o r i e s .

T a b l e I I . E s t i m a t e s o f the c o n c e n t r a t i o n s o f s e l e c t e d elements i n submicrometer-diameter p a r t i c l e s c o l l e c t e d upstream of a c o l d - s i d e e l e c t r o s t a t i c p r e c i p i t a t o r , ppm.

Element

Filter

V

1900

1080

1170

Zn

1450

720

550

Stage

11

Stage

Cr

806

445

2400

As

230

140

130

Se

600

350

420

Mo

130

80

130

Co

160

20

66

Mn

160

80

20

U

107

68

68

W

76

49

63

Ni

-

-

Sb

82

47


780 48

10

9.

ONDOV ET AL.

Fine Particles Emitted from Coal

185

REFERENCES 1. 2.

3. 4.

5.

6.

7. 8. 9. 10. 11. 12. 13.

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RECEIVED March 25, 1981.


Elemental Composition of Atmospheric Fine Particles Emitted from

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