CHARACTERIZATION OF ACTIVATED CARBONS' PHYSICAL AND
CHEMICAL PROPERTIES IN RELATION TO THEIR MERCURY
ADSORPTION
Y. H. Li* C. W. Lee, andB. K. Guilett
U.S. Environmental Protection Agency
Office of Research and Development
National Risk Management Research Laboratory
Air Pollution Prevention and Control Division (MD-65)
Research Triangle Park, NC 27711
Introduction
Mercury (Hg) has been identified by the U.S.
Environmental Protection Agency (EPA) as a toxic
pollutant of great environmental health concern that is
emitted from coal-fired power plants. Regulations
governing Hg emissions from coal-fired power plants are
to be issued by EPA by December 15, 2004 [1], One
potential method for controlling Hg emissions from coal-
fired utility boilers is the injection of an activated carbon
sorbent upstream of a particulate control device such as an
electrostatic precipitator or a bag filter. The control of Hg
emissions is strongly dependent on the form of Hg emitted
(oxidized vs. elemental). A major portion of the Hg
emitted from many coal-fired power plants is in the
elemental form (Hg°). It is known that Hg° is more
difficult to capture than its oxidized forms from
combustion flue gases, due to its higher volatility and
insolubility in water [2],
Many studies have been carried out to evaluate the
effectiveness of Hg° adsorption by activated carbons [3-7].
It has been shown that carbon sorbent type and its
associated properties are the most important factors
influencing Hg° adsorption [6], Different activated
carbons have been examined in bench-, pilot-, and full-
scale tests; however, correlations between carbon physical
and chemical properties and Hg° adsorption have not been
established. The mechanism involved in Hg° adsorption is
not well understood. A previous study [8] of Hg°
adsorption by activated carbons performed at room
temperature suggests that carbon surface moisture has a
strong enhancement effect to promote Hg° adsorption.
Results of the temperature-programmed desorption (TPD)
measurement of adsorbed Hg and X-ray absorption fine
'structure (XAFS) spectroscopy analysis of surface Hg
bonding suggest that chemisorption of Hg° is a dominant
process over physisorption for the moisture-containing
samples [8]. Hg° bonding on the carbon surface appears to
be associated with oxygen [8]. It was also suggested that
* Supported by ORISE Postdoctoral Research Program.
surface oxygen complexes are most likely to provide the
active sites for Hg° bonding. Electron transfer processes
are likely to be involved during the chemisorption of Hg°.
However, it is not clear what particular surface functional
groups are participating in Hg° adsorption due to the lack
of chemical characteristic information of the carbons
tested. Characterization of the carbon oxygen surface
groups in relation to the carbon's Hg° adsorption capacity
could provide important mechanistic information on Hg°
adsorption. The objective of this study was to characterize
the physical and chemical properties of the carbons used
for Hg° adsorption, in order to understand the role of
oxygen surface functional groups on the mechanism of
Hg° adsorption by activated carbons.
Experimental
Sample preparation
The samples tested were two commercially available,
bituminous-coal-based activated carbons (WPL, BPL,
Calgon Carbon Corporation, Pittsburgh, PA). The samples
were washed with de-ionized (DI) water and air dried at
383 K, and designated as -AR. Acid treatments were also
performed to the above two carbons in order to modify the
surface chemical characteristics of the carbons. They were
treated in a 6 N nitric acid (HN03) solution at room
temperature for 5 hrs, then washed with DI water and dried
at 383 K in an oven overnight, and designated as -HN03.
The two -AR samples were air oxidized at 693 K. The -AR
sample was first heated in a quartz tube reactor under a
helium (He) atmosphere to 1200 K for 2 hrs to remove the
surface oxygen complexes present originally on the carbon
surfaces, and cooled to 693 K under a He atmosphere.
Then, an air flow (55 cm3/min, standard temperature and
pressure, STP) was introduced into the reactor for 10 hrs.
The gas flow was switched back to He and cooled to room
temperature. The weight loss of the sample was
determined to be about 25% for both the BPL and WPL
carbons during air oxidation.
Characterization of samples
BET (Brunauer-Emmett-Teller) and DR (Dubinin-
Radushkevich) surface areas of the carbons were measured

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Table 1	Pore structure characterization results
Sample
SHET(m2/g)
Sc02 (m2/g)
V,(cm3/g)
Vmicr„(cmJ/g)
L (nm)
BPL-AR
1136
976
0.58
0.37
1.5
BPL-1200
1246
1013
0.62
0.38
1.5
BPL-HNO,
1088
929
0.57
0.35
1.4
BPL-693
1656
970
0.96
0.37
1.5
WPL-AR
1057
825
0.62
0.31
1.4
WPL-1200
1064
802
0.62
0.30
1.4
WPL-HNO,
931
805
0.54
0.31
1.4
WPL-693
1407
919
0.83
0.35
1.5
by nitrogen (N2) adsorption at 77 K with P/P0 up to 0.99,
and carbon dioxide (0O2) adsorption at 273 K with P/P0 up
to about 0,03, respectively, using a volumetric adsorption
apparatus (ASAP 2400, Micromeritics). The total pore
volume was evaluated from the N2 adsorption isotherm at
P/P0 = 0.99, and the micropore volume was estimated from
C02 adsorption at 273 K using the DR equation.
Temperature-programmed desorption (TPD) of the carbon
samples was carried out by heating the sample to 1200 K
under a He flow of 55 cnrVmin (STP), and at a heating rate
of 10 K/min. About 200 mg of sample was placed in a
quartz reactor on a fine frit of 25 mm diameter. The
reactor exit was connected to a quadrupole mass
spectrometer (Dyeor, Model M200) set up for continuous
measurement of evolving gases. The responses of carbon
monoxide (CO), C02, and water (H20) of the spectrometer
were calibrated periodically using the weighted amounts of
calcium oxalate monohydrate (CaC2CVH20). The
temperature of the carbon was measured by a
thermocouple positioned into the carbon bed.
Base-acid titration of the carbons based on Boehm's
method [9] was performed to measure the oxygen
functionalities of the samples. About 0.8 g of carbon was
placed in 50 mL of the following 0.05 N solutions: sodium
bicarbonate (NaHC03), sodium carbonate (Na2C03),
sodium hydroxide (NaOH), and hydrochloric acid (HC1).
The vials were sealed and shaken for 24 hrs, and then
filtered. A total of 10 mL each of the filtrate was pipetted,
and the excess of base and acid of the filtrate was titrated
with HC1 and NaOH, respectively. The specific surface
oxygen functional groups were estimated by using the data
measured from the base titration and the TPD experiment,
based on the following assumptions: NaHCO, titrates
carboxyl groups only; NaOH titrates carboxyi, lactone, and
phenol groups; C02 is a decomposition product of
carboxyl and lactone groups, and CO is a decomposition
product of phenol and carbonyl groups [10].
Mercury adsorption
Hg° adsorption experiments were performed at a
temperature of 398 K under a N2 atmosphere, and in a
fixed-bed reactor surrounded by a temperature-controlled
electrical furnace. The detailed experimental setup and
procedures were described elsewhere [8]. Hg° adsorption
experiments were also performed on the heat-treated
samples (designated as -1200). The sample was heated at
a constant heating rate (10 K/min) to 1200 K under a N2
atmosphere, and then held for 2 hrs. The reactor was then
cooled to the Hg° adsorption temperature (398 K) under a
N2 flow, before the adsorption experiment was started.
Therefore, re-oxidation due to exposure of the carbon
sample to the atmosphere could be avoided. A Hg°
concentration of 55±2 ppb (450 figWm3) in Nj at a total
flow rate of 370 mL/min (STP) was generated and used for
all the Hg° adsorption experiments.
Results and Discussions
Sample characterization
The characteristics of the samples' pore structures are
shown in Table 1. It can be seen from the table that both
BPL-AR and WPL-AR are microporous carbons. Heat
treatment at 1200 K under N2 did not significantly change
their measured pore structures. A decrease in surface area
and pore volume was observed for the HN03-treated
samples, possibly as a result of the destruction of some of
the thin pore walls and blocking of the pore entrances by
oxygen functional groups [11]. As expected, air oxidation
treatment of the carbons performed at 693 K increased the
surface area and pore volume of the samples significantly,
as the results of carbon burn off lead to pore development.
The base-acid titration and the TPD results are
summarized in Table 2. The oxygen surface complex
concentration, [O] (mmol/lOOg), was calculated from the
measured total amounts of C02, CO, and H20 thermally
desorbed from the carbon surface during the TPD
experiment. As shown in Table 2, significant differences
in the base-acid titration values and the C02. and CO
decomposition values between the BPL-AR and the WPL-
AR samples were observed. The oxygen content of the
WPL-AR (293 mmol/lOOg) measured by TPD is much
higher than that (106 mmol/lOOg) of the BPL-AR sample.
In general, the acidity and basicity of the carbon can be
estimated approximately by its NaOH and HC1 titration
values, respectively. It can be seen that the amount of
basicity (53 meq/lOOg) is about double that of the acidity
(25 meq/lOOg) for the BPL-AR. This indicates that BPL-
AR is a basic carbon as reported previously in the

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Tabic 2
Base-acid neutralization and TPD experiment results

Base-acid neutralization
TPD
Sample
NaHCOi
NazCOj
NaOH
HCI
[CO,]
[CO]
[O]
CO/CO,


(meq/lOOg)


(mmol/lOOg)


BPL-AR
2
4
25
53
11
67
106
6.1
BPL-HNO;
29
50
81
17
75
262
442
3.5
BPL-693
37
67
148
32
38
397
489
10.4
WPL-AR
7
16
51
42
46
158
293
3.4
WPL-HNO-,
13
30
80
7
77
235
413
3.1
W PI.-691
42
72
176
19
60
475
609
7.9
Table 3	Surface acidic functional groups of samples
Samples
Carboxyl
Lactone
(mmol/lOOg)
Phenol
Carbonyl
BPL-AR
2
9
14
53
BPL-HN03
29
46
6
256
BPL-693
37
1
110
287
WPL-AR
7
39
5
153
WPL-UNO,
13
64
3
232
WPL-693
42
18
116
359
literature [12], On the other hand, the basicity of the
WPL-AR is lower than its acidity.
The effects of HN03 treatment and air oxidation on surface
chemistry are pronounced. The total oxygen concentration
increased significantly after the two samples were treated
with a 1INO, solution. Similar increases in oxygen
concentration were also observed after the samples were
air oxidized at a temperature of 693 K. More acidic
groups were formed, resulting in high NaOH titration
values and low HCI titration values, for both the HNOr
treated and the air-oxidized samples. It can be seen from
Table 2 that the amount of surface acidic groups, titrated
by NaOH, is almost equal to that of the CO, desorbed from
the TPD experiment for the two HNOrtreated samples, as
well as for the WPL-AR sample. For the air-oxidized
samples, it was found that high NaOH titration values and
more CO-forming complexes were obtained, which results
in a higher CO/C02 ratio as shown in Table 2. Although
significant amounts of CO were measured from the TPD
experiments for the I INO ,-treated samples and for the
WPL-AR samples, their C0/C02 ratios remain low (3.1-
3.5). These may be due to the C02-forming complexes
produced through oxidation of the carbon during HN03
treatment remained on the surface, which decomposed to
yield C02 later during the TPD experiment to produce a
low C0/C02 ratio. However, some of the C02-forming
complexes produced during air oxidation at 693 K may not
be stable, resulting in a higher C0/C02 ratio.
Both the HNO3 treatment and the air oxidation at 693 K
were found to increase the total oxygen content of the
treated samples. However, the surface functional groups
formed from these two treatment processes are different,
resulting in different chemical characteristics of these two
treated samples. Table 3 shows the different acidic groups
estimated from the base-acid titration and from the TPD
experiment. It can be seen from the table that more
lactone (39 mmol/lOOg) and carbonyl (153 mmol/lOOg)
groups are present in the WPL-AR sample compared to
those (9 mmol/lOOg and 53 mmol/lOOg, respectively) in
the BPL-AR sample. However, much more phenol groups
(14 mmol/lOOg) are present in the BPL-AR sample than
those (5 mmol/lOOg) in the WPL-AR. For the HNO,-
treated samples, almost all the acidic functional groups
measured had increased, except for the phenol groups. On
the other hand, more phenol groups and relatively less
lactone groups were measured after the samples were air
oxidized at 693 K, and more carboxyl groups were also
found.
Mercury adsorption
Table 4 summarizes Hg° adsorption capacities of the
samples. It was found that the Hg° adsorption capacity of
the BPL-AR is very low (40 (J.g/g), which is in agreement
with that reported in the literature for the same sample
measured at the similar experimental conditions [13], As
shown in Table 4, there are no Hg° uptakes of both the
BPL and WPL after they were heat-treated at a
temperature of 1200 K under a N2 atmosphere. The
breakthrough curve obtained matches that of the blank
experiment when only sand particles and quartz wool were
present in the reactor. It can be assumed that most of the
oxygen surface complexes are removed after the carbons
are heat treated at 1200 K under a N2 atmosphere. The
elimination of Hg° adsorption capacity of these two
samples after heat treatment provides further evidence that
oxygen surface complexes are the most likely active sites

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for Hg° capture [8], It is noted that the surface area and
pore volume of the heat-treated samples are still preserved.
A comparison of the Hg° adsorption capacity and the
characteristics of the surface functional groups between
BPL-AR and WPL-AR suggests that Hg° adsorption
capacity seems to have a strong dependence on the
carbon's chemical characteristics. Both lactone and
carbonyl groups appear to be the possible active sites for
Hg° adsorption. It is well known that both lactone and
car'oonyl arc the reducible functional groups on carbon
surfaces f 14], Results obtained from this study suggest
that Hg° adsorption on carbon surfaces seems to follow the
oxidation-reduction mechanism. Upon treatment with a
HNO; solution, Hg° adsorption capacity of the BPL-HNO;
shows a sharp increase and reaches the highest value
(1,520 ng/g). However, the capacity of another HNO3-
treated sample, WPL-IINOi, is lower than that of its
untreated counterpart (925 ftg/g); but it is still maintaining
a high value (700 jig/g). A 12% reduction in BET surface
area was observed for the HNOr,-treated WPL carbon. The
difference in Hg° adsorption capacity between WPI.-HNO;
and WPL-AR may be explained by the reduction in total
surface area of the WPL-HNO3 sample.
Table 4 Mercury adsorption capacity and the
phenol/carbonyl
Sample
Hg" (ng/g)
Phenol/carbonvl
BPL-AR
40
0.26
BPL-1200
0
n.a.
BPL-HNO,
1520
0.02
BPL-693
40
0.38
WPL-AR
925
0.03
WPL-1200
0
n.a.
WPL-HNO-,
700
0.01
WPL-693
20
0.32
The Hg° adsorption capacities of the two air-oxidized
samples were found to be reduced to very low values (20-
40 ng/g) as a result of air oxidation. It can be seen from
Table 3 that the most distinguishing feature of the surface
chemical characteristics associated with the air-oxidized
samples is their high phenol groups concentrations (110-
116 mmoI/lOOg) and their CO/CC)2 ratio shown in Table 2.
These two parameters seem to correlate with the Hg°
adsorption capacity. The samples with a low CO/C02
ratio arid phenol group concentration tend to give a high
Hg° adsorption capacity, indicating that phenol groups
may prohibit Hg° oxidation reaction or they affect the
equilibrium concentrations of the lactone or carbonyl
groups. It has been shown that the apparent ability of the
carbon surfaces to catalyze both oxidation and reduction
reactions can be explained by equilibrium considerations
[10], The resonance-stabilized structures of the surface
functional groups (e.g., quinonoid complexes) could
behave as an electrode [10, 15] as shown by:
C6Ii,02 + 2H" + 2e' -> C6H,(OH)2	(1)
The potential (Eft) of the electrode can be expressed as
*. = 5,-^1"	(2)
where E„ is the characteristic constant potential, and a is
the activity (~ concentrationxactivity coefficient). As
shown in Equation (2), higher hydroquinone or phenol
[C6H4(OI I)2] concentration could significantly reduce the
potential of quinhydrone electrodes (E/,) and result in a
lower potential difference (AE), which would be required
for a given oxidation-reduction reaction system. It was
found that the carbonyl groups on the carbon surface are
also associated or chelated with the phenolic hydroxyl
groups through hydrogen bonding [14]. It is interesting to
see if reaction (1) would proceed forward during Hg°
adsorption. The last column of Table 4 shows the ratios of
phenol and carbonyl groups of the samples evaluated. It
appears that a high Hg° adsorption capacity is associated
with a low phenol/carbonyl ratio, which would give a
higher potential of the quinhydrone electrode (E/,)
according to Equation (2). This suggests that the
mechanism of Hg° adsorption involves an electron transfer
process, and the carbon surface possibly acts as an
electrode for Hg° oxidation. However, the kinetics and
mechanism of such reactions may be more complex, and
other surface functional groups, such as lactones, could
also participate in the electron transfer process [10].
Further studies to investigate carbon electrochemistry
would be useful for gaining more mechanistic insights of
Hg° adsorption.
Conclusion
The physical and chemical properties, and Hg° adsorption
capacity of two as-received and treated activated carbons
were characterized. Heat treatment of the samples,
performed at a temperature of 1200 K under N2, did not
cause significant changes in their pore structure
characteristics. A decrease in surface area and pore
volume was observed after the samples were treated with a
HNO-, solution. Air oxidation of the samples, performed at
a temperature of 693 K, significantly increased both the
surface area and the pore volume.
Significant differences in chemical characteristics were
found in the two as-received samples. The total oxygen
content of the WPL-AR, measured by the TPD
experiment, is much higher than that of the BPL-AR
sample. More lactone and carbonyl groups are present in
the WPL-AR sample compared to those present in the
BPL-AR. Significantly more phenol groups are present in
the BPL-AR sample than those present in the WPL-AR.
The total oxygen concentration increased significantly

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after the samples were treated with a HNO:, solution. Air
oxidation of the samples ai 693 K also raised the total
oxygen concentration of the samples. It was found that all
the functional groups measured were higher, except for the
phenol groups, as a result of HNO3 treatment. More
phenol groups and relatively less lactone groups are
formed upon air oxidation at 693 K, significantly more
carboxyi groups are also created.
The results of Hg° adsorption of the as-received and
oxidized samples suggest that Hg° adsorption capacity
tends to have a strong dependence on the sample's
chemical characteristics. No Hg° uptakes were measured
for the samples after they were heat-treated at 1200 K
under a N2 gas atmosphere, although significant surface
area and pore volumes of the samples are still largely
preserved after the heat treatment. Such observation
provides further support that oxygen surface complexes are
the possible active sites for Hg° capture. Both lactone and
carbonyl groups are the likely active sites for Hg°
adsorption, and Hg° adsorption seems to follow the
oxidation-reduction mechanism. It was found that the
CO/CO? ratio measured by the TPD experiment, and
concentration of the phenol groups estimated from titration
appear to be correlated with the sample's Hg° adsorption
capacity. The samples, which have a lower C0/C02 ratio
and phenol group concentration tend to have a higher Hg°
adsorption capacity, indicating that pheno! groups present
in carbon prohibit Hg° adsorption. It is also found that a
high Hg° adsorption capacity is associated with a low ratio
of the phenol/carbonyl groups. The results of this study
suggest that the mechanism of Hg° adsorption involves an
electron transfer process, and the carbon surface may act
as an electrode for Hg° oxidation.
References
1.	U.S. EPA, Mercury, www.epa.gov/mercury,
Washington, DC.
2.	Chang R, Offen OR. Power Eng 1995; 99: 51-57.
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Technol 1994; 28: 1506-1512.
4.	Huang HS, Wu JM, Livengood CD. Hazardous Waste
& Hazardous Materials 1996; 13: 107-119.
5.	Korpiel JA, Vidic RD. Environ Sci Technol 1997; 31:
2319-2325.
6.	Carey TR, Hargrove OW Jr, Richardson CF, Chang R,
Meserole FB. J. Air & Waste Manage. Assoc. 1998;
48, 1166-1174.
7.	Rostam-Abadi M, Chen SG, Hsi HC, Rood M, Chang
R, Carey T, Hargrove B, Richardson C, Rosenhoover
W, Meserole F. Presented at the EPRI-DQE-EPA
Combined Utility Air Pollutant Control Symposium,
Washington, DC, 1997.
8.	Li YH, Lee CW, Gullett BK. Carbon 2001, accepted.
9.	Boehm HP. Advan. Catalysis 1966; 16: 179-274.
10.	Leon y Leon CA, Radovic LR. In Thrower PA, editor,
Chemistry and Physics of Carbon, Vol. 24, New York:
Marcel Dekker, 1994:213-310.
11.	Moreno-Castilla C, Carrasco-Martin F, Maldonado-
Hodar FJ, Rivera-Utrilla J. Carbon 1998; 36, 145-151.
12.	Barton SS, Evans MJB, Hallop E, MacDonald JAF.
Carbon 1997; 35, 1361-1366.
13.	Vidic RD, Chang MT. J. Air & Waste Manage. Assoc.
1998; 48, 247-255.
14.	Bansal RC, Donnet JB, Stocckli, F. Active Carbon.
New York and Basel: Marcel Dekker, 1988.
15.	Puri BR. In Walker PL Jr, editor. Chemistry and
Physics of Carbon, Vol. 6, New York: Marcel Dekker,
1970: 191-282.
Acknowledgement
This research was supported in part by an appointment (Y.
H. Li) to the Postdoctoral Research Program at the
National Risk Management Research Laboratory
administered by the Oak Ridge Institute for Science and
Education through interagency agreement DW89938167
between the U.S. Department of Energy and the U.S.
Environmental Protection Agency.

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Mt,Mdt _TJT-T3-T3 RQ7 TECHNICAL REPORT DATA
In ruvl. xvA—' It-i " " «J» l (Please readIrtumctions on the reverse before completing
1. REPORT NO. 2.
EPA/600/A-01/075
3. BE
4. TITLE AND SUBTITLE
Characterization of Activated Carbons' Physical and
Chemical Properties in Relation to Their Mercury
Adsorption
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Y.H.Li, C.W.Lee, B.K. Gullett
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
See Block 12
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
EPA/DoE IAG DW89938167
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Published paper; 1/00-1/01
14. SPONSORING AGENCY CODE
EPA/600/13
15. supplementary notes A PPCD pr0ject officer is C.W.Lee, Mail Drop 65, 919/541-7663.
For presentation at Carbon '01, Lexington, KY, 7/14-19/01.
16. abstract The paper gives results of a characterization of the physical and chemical
properties of the activated carbons used for elemental mercury (Hgo) adsorption, in
order to understand the role of oxygen surface functional groups on the mechanism
of Hgo adsorption by activated carbons. (NOTE: Hgo is known to be more difficult
to capture than its oxidized forms from combustion flue gases, due to its higher
volatility and insolubility in water.) Many studies have been carried out to evaluate
the effectiveness of Hgo adsorption by activated carbons. Carbon sorbent type and
its associated properties have been shown to be the most important factors influen-
cing Hgo sorption. Different activated carbons have been examined in bench-, pilot-,
and full-scale tests; however, correlations between carbon physical and chemical
properties and Hgo adsorption have not been established. The mechanism involved
in Hgo adsorption is not well understood. A previous study of Hgo adsorption by
activated carbons performed at room temperature suggests that carbon surface moi-
sture has a strong enhancement effect to promote Hgo adsorption. Results of the
temperature-prog rammed desorption (TPD) measurment of adsorbed Hg and X-ray
absorption fine structure (XAFS) spectroscopy analysis of surface mercury bonding
suggest that chemisorption of Hgo is dominant over physisorption.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. descriptors
b.IDENTIFIERS/OPEN ENDED TERMS
c. cosati Field/Group
Pollution
Mercury (Metal)
Activated Carbon
Adsorption
Oxygen
Flue Gases
Pollution Control
Stationary Sources
13	B
07B
UG
14	G
2 IB
18. distribution statement
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
5
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)

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