600/A-98/053
COMBINED MERCURY AND SULFUR OXIDES CONTROL USING
CALCIUM-BASED SORBENTS
S. Behrooz Ghorishi
Acurex Environmental Corporation
4915 Prospectus Drive
Durham, NC 27713
and
Charles B. Sedman
National Risk Management Research Laboratory
Air Pollution Prevention and Control Division
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
Abstract
The capture of elemental mercury (Hg°) and mercuric chloride (HgCl2) by three types of calcium
(Ca)-based sorbents was examined in this bench-scale study under conditions prevalent in coal-fired
utilities. Ca-based sorbent performances were compared to that of an activated carbon. Mercury
capture of about 40% (nearly half that of the activated carbon) was achieved by two of the Ca-based
sorbents. The presence of sulfur dioxide (S02) in the simulated coal combustion flue gas enhanced
the capture of Hg° from about 10 to 40%. Increasing the temperature in the range of 65-100°C also
caused an increase in the Hg° capture by the two Ca-based sorbents. Mercuric chloride (HgCl2)
capture exhibited a totally different pattern. The presence of S02 inhibited the HgCl2 capture by Ca-
based sorbents from about 25 to less than 10%. Increasing the temperature in the studied range also
caused a decrease in HgCl2 capture. Upon further pilot-scale confirmations, the results obtained in
this bench-scale study can be used to design and manufacture more cost-effective mercury sorbents
to replace conventional sorbents already in use in mercury control.
1
-------
Introduction
Title III of the 1990 Clean Air Act Amendments (CAAA) requires the U.S. Environmental Protection
Agency (EPA) to submit a study on 189 hazardous air pollutants (HAPs). This study would include
emissions and a risk (to public health) assessment of the 189 HAPs. Among these compounds,
mercury has drawn special attention due to its increased levels in the environment and the well
documented food chain transport and bioaccumulation of this specie and its compounds such as
methyl mercury.1,2 An EPA report to Congress cites the largest emitters of mercury as coal-fired
utilities, medical waste incinerators (MWIs), municipal waste combustors (MWCs), chlor-alkali
plants, copper and lead smelters, and cement manufacturers.3 These sources are estimated to account
for over 90% of all anthropogenic mercury emissions. Utility boilers account for nearly 25% of the
total anthropogenic emissions, of which more than 90% are attributed to coal-fired utility boilers.
Mercury, a trace constituent of coal4, is readily volatilized during coal combustion.5 Mercury is
the most volatile trace metal, and major portions of it can pass through existing particulate matter
(PM) control devices.5 A sorbent reacting with this metallic species can effectively convert the
vapor to a sorbed liquid or solid phase, facilitating its removal with sorbent particles in a PM
control device. Mercury control processes which use adsorption on dry sorbents do not pose the
problem of the treatment and stabilization of a waste liquid stream and, therefore, seem very
attractive for coal combustors.
Several methods of controlling mercury emissions are in either commercial use or development for
MWCs and MWIs.6 Dry sorbent injection (DSI) of activated carbon, followed by fabric filtration (FF)
has shown consistently high (>90%) mercury removal in MWC applications. Spray drying (SD)
followed by FF, and wet scrubbing (WS) have both been successfully applied for acid gas control,
and have been found to remove substantial (60-90%) amounts of mercury in MWCs. However, all
three technologies have been less successful in removing mercury from coal-fired flue gases.7
There are primarily three reasons suspected for the observed differences in mercury capture between
2
-------
MWC and coal-fired cases: (a) the differences in the mercury components (species) present in the two
flue gases, (b) mercury species concentrations, and (c) composition of the two flue gases. On account
of the larger concentrations of hydrogen chloride (HQ) present in a MWC flue gas, mercury is
thought to exist primarily as mercuric chloride (HgCI2).8 Recent pilot plant studies on coal-fired flue
gas indicate that for some Ohio coals, a considerable portion of mercury vapor may be HgCl2.
However, the same study indicated that elemental mercury (Hg°) vapor concentration may actually
increase across a wet limestone scrubber, presumably due to the reduction of HgCl2 vapor entering
the scrubber 9 The lower concentration of HC1 in a coal-fired flue gas is believed responsible for a
portion of the mercury to exist as Hg°.
Another difference in the two types of flue gases is their total mercury concentrations. The total
mercury concentration in a MWC flue gas is typically several orders of magnitude higher than the
mercury concentration in a coal-fired flue gas. The typical mercury concentration observed in coal
combustion flue gas (2-3 ppb)10 was simulated throughout this study. The third difference between
MWC and coal-fired systems is the composition of the flue gases. Sulfur dioxide (S02) is present at
higher concentration in coal combustion flue gases and is believed to influence the capture of mercury
by different sorbents and emission control devices. The effect of S02 on mercury capture was
investigated in this study.
Pilot-scale studies have shown that, to achieve high removals of mercury in coal-fired power plants,
activated carbon to mercury (by weight) ratios of around 3000/1 were required .11,12 At an activated
carbon cost of $1,125 /kg, the material cost would be approximately $500,000 per year for a 500
MW power plant. Chang et al.u arrived at an annual cost of $100,000 to $1 million for mercury
control in a 500 MW power plant. A recent study by Chang and Offen7 estimates that removing 50%
of the mercury emitted in flue gas by U.S. power plants could range from $1 billion to $10 billion per
year. Therefore, bench-scale efforts to study process parameters and sorbent types for mercury
control in coal-fired flue gas are needed to develop effective and economic mercury capture
technology. In addition, improvement of mercury control using existing technologies for S02 and fine
PM control would appear to be prudent. Therefore this study focuses on improving the existing S02
3
-------
control sorbents for a potentially combined mercury and S02 control.
Bench-scale results from laboratories at the Air Pollution Prevention and Control Division (APPCD)
of the U.S. Environmental Protection Agency (EPA) showed that Ca-based sorbents were effective
in controlling HgCl2 under MWG operating conditions (in the absence of S02)13. It was found that
calcium oxide (CaO) and calcium hydroxide [Ca(OH)2] were effective in capturing HgCl2 at 100°C
At 140°C, however, the Ca-based sorbents were found to be less efficient in capturing HgCl2. Also,
during the Hg° capture experiments, only activated carbons exhibited significant capture at both 100
and 140°C in the absence of S02. Pilot-scale tests showed that injection of Ca-based sorbents into
a furnace reduced total mercury emissions at the outlet of the furnace.14 Stouffer et al. have shown
that, in an air toxics control pilot plant, high system HgCl2 removal can be achieved with the injection
of hydrated lime as the sorbent.15 At 93°C, removals of HgCL, from the gas were about 55 and 85%
at Ca/Hg weight ratios of5,000 and 100,000, respectively. The corresponding Hg° removals ranged
only from 10 to 20%, even at Ca/Hg weight ratios as high as 300,000.
Considering the above observations, a potential method of cost reduction in controlling mercury
emissions in coal-fired utilities (low mercury concentration) would be to utilize the cheaper Ca-based
sorbents. This paper reports results of experiments to study Hg° and HgCl2 capture by several Ca-
based sorbents and their performance compared with a lignite-coal-based activated carbon
(DARCO® FGD, Norit Americas Inc.). Hg° and HgCl2 concentrations were roughly 2 to 3 ppb in
a simulated flue gas in order to replicate conditions (as close as possible) prevalent in a coal-fired flue
gas.10 Among the Ca-based sorbents evaluated in this study were reagent grade hydrated lime
(calcium hydroxide), a mixture of fly ash and hydrated lime (advanced silicate — Advacate), and a
modified Advacate. More details on the sorbents tested in this study are given in the next section.
Capture of Hg° and HgCl2 by these sorbents was studied as a function of system temperature and SO,
present in the simulated flue gas.
4
-------
Sorbents
The capture of Hg° and HgCl, by five types of sorbents was studied in this investigation. Of the five,
the three Ca-based sorbents were hydrated lime, Advacate, and a modified Advacate. Hg° and HgCI2
capture by Clinch River Fly Ash (CRFA) and an activated carbon (FGD) was also measured for
comparison. Preparations of Advacate and modified Advacate are discussed below.
Preparation of Advacate and Modified Advacate
Advacate was prepared in a pressure hydrator at 150°C by mixing a 3/1 ratio (by weight) of CRFA
to hydrated lime. The modified Advacate was prepared by addition of a chemical agent during this
process. The entire reaction time in the pressure hydrator was 1 h. After preparation of the sorbents,
they were placed in a vacuum oven at 165°C for 24 h before use. Several batches of Advacate and
modified Advacate were prepared, and their physical characteristics were studied using nitrogen (N2)
sorption. Very similar physical characteristics were obtained for different batches of each, indicating
their reproducibility. The following subsection describes, in detail, the structural properties and
chemical compositions of the studied sorbents
Structural Properties/Chemical Compositions of the Sorbents
Information about the internal pore structure (total and incremental volume and surface area) of the
three Ca-based sorbents was determined by a Micromeritics ASAP 2600 using N2
adsorption/desorption with a Branauer-Emmett-Teller (BET) method. BET analyses on the three
Ca-based sorbents (hydrated lime, Advacate, and modified Advacate) obtained from N2 sorption are
shown in Figure 1. Of the three, hydrated lime had the lowest internal pore volume and surface area.
Advacate, containing only 25% of hydrated lime by weight, had a higher surface area than hydrated
lime. Modified Advacate had the highest surface area among the three. A bimodal pore size
distribution was seen for the three Ca-based sorbents with most of the pore diameters being
5
-------
approximately 15 to 50 nm. The total pore volume of modified Advacate was over five times that for
hydrated lime and three times the pore volume of Advacate. The internal pore structure of the studied
activated carbon (FGD) is shown in Figure 2. Bimodal pore size distribution was not observed in the
case of FGD. Unlike Ca-based sorbents, pores with diameters less than 5 nm were the significant
contributors to the total pore area and volume in FGD, causing the average pore diameter in FGD
to be considerably lower than that of Ca-based sorbents. Structural properties and chemical
composition of the studied sorbents are summarized as:
Hydrated lime: The Hydrated lime used in this research was reagent grade (Sigma Inc.)
containing 97.6% Ca(OH)2 and 1.8% calcium carbonate (CaC03). This material has a total surface
area of 13.0 m2/g and an average pore diameter of 33.4 nm.
Clinch River Fly Ash (CRFA): This material is a fly ash obtained from the Clinch River
Virginia power plant. This power plant uses a local bituminous coal. The mineral content of this fly
ash is: 5.2% CaO, 51.6% Si02,24.7% A1203, 0.5% Na2Q, 1.8% MgO, 3.3% K20, 7.8% Fe205 and
1.4% Ti02. CRFA has a total surface area of 2.3 m2/g and an average pore diameter of 8.1 nm.
Advacate: A reaction product of a 3/1 mixture of CRFA and hydrated lime. As prepared for
this study, it had a total surface area of 30.9 m2/g and an average pore diameter of 21.2 nm.
Modified Advacate: Advacate prepared with an additional chemical agent. Modified Advacate
for this study had a total surface area of 91,4 m2/g and an average pore diameter of 22.2 nm.
FGD: A trademark for an activated carbon known as "DARCO® FGD" manufactured by
Norit Americas Inc. FGD is a lignite-coal-based activated carbon manufactured specifically for the
removal of heavy metals. It has a total surface area of 575 m2/g and an average pore diameter of 3.2
nm. More information about physical characteristics and mercury capture performance of FGD can
be found elsewhere.16 Table 1 summarizes the physical properties of the studied sorbents.
Experimental Apparatus and Procedures
Figure 3 is a schematic of the experimental apparatus used to study capture of Hg° and HgCl2. Pure
HgCl2 powder in a diffusion vial was the source of HgCl2 vapor, and pure Hg° liquid in a permeation
tube was the source of Hg° vapor. The relative concentration of HgCl2 or Hg° vapor in the gas
6
-------
stream was varied by adjusting the water bath temperature. The generated HgCl2 or Hg° vapor was
carried into the main system by a nitrogen (N2) stream where it was mixed with water vapor (H20),
air, sulfur dioxide (SO2), and carbon dioxide (COz) in the manifold. The composition of the simulated
flue gas and the total system flow rate was kept constant throughout these studies as follows: 2-3 ppb
HgCl2 or Hg°, 5% HjO, 7%Oj, 10% CQ, 173 ppm SQ,, and balance of nitrogen, total system flow:
300 cm3/min
A 3-way valve placed before the manifold (Figure 3) diverted the Hg° or HgCl2 in the N2 stream away
from the manifold when desired. The first 3-way valve placed after the manifold was used to direct
flow to or away from the fixed-bed reactor. The sorbent to be tested (approximately 0.1 g) was
placed in the constant temperature reactor. A furnace kept at 850°C was added downstream of the
reactor to convert any oxidized mercury vapor to Hg°. According to thermodynamic predictions, the
only Hg specie at this temperature is Hg0.13 The presence of the furnace enabled detection of non-
adsorbed HgCl2 as Hg° by the on-line ultraviolet (UV) Hg° analyzer, thus providing actual,
continuous Hg° or HgCl2 capture data by the packed bed of sorbent. Prior to the mercury analyzer,
an ice bath served as a water trap. Quality control experiments had previously indicated no loss of
Hg° or S02 in the water trap.
It should be noted that the Hg° research apparatus is made of Teflon™. The HgCl2 apparatus is made
of quartz and avoids the use of Teflon™, which is known to adsorb I IgCl2.
The UV Hg° analyzer responded to S02 concentrations as well as to Hg°. For instance, a gas stream
consisting of 173 ppm S02 and 3 ppb Hg° produced a S02/IIg° signal ratio of 8/18. Contributions
from S02 were accounted for by placing a S02 analyzer (UV) on-line downstream of the Hg°
analyzer. The S02 analyzer was incapable of responding to mercury in the concentration range used
in this study. By subtracting the S02 signal measured by the S02 analyzer from the total response of
the mercury analyzer, the mercury concentration was obtained.
In addition to sorbents, other parameters studied in this investigation were packed bed temperature
7
-------
and the presence of S02, The two studied temperatures were 100 and 65°C. These temperatures are
typically observed in the air pollution control system of coal-fired utility boilers. The effect of S02
was studied by performing two sets of packed bed experiments — in the absence of S02 and in the
presence of 173 ppm S02. This relatively low level of S02 was selected to minimize UV interference
with Hg° detection, and to be consistent with the previous activated carbon sorption experiments,
.For the same reasons, UV monitor interference and consistency with activated carbon data, the
experiments reported here have bean limited to simulated flue gas with no nitrogen oxides, HQ, and
only 5% moisture. The effects of temperature and S02 on Hg° and HgCl2 capture by different
sorbents were studied independently. In each test, the packed bed was exposed to the simulated flue
gas for 30 minutes during which the exit concentration of mercury was continuously monitored. The
percent removal of Hg° or HgCI2 was obtained according to: percent removal= 100 (mercury to-
mercury0Ul)/mercuryijr It should be noted that each set of parameters was run in duplicate. If the
duplicates did not meet the precision goal (the data quality indicator) of ±10%, the parameters were
tested a third time.
Results and Discussions
Capture of Elemental Mercury
Figure 4 shows the effect of S02 on Hg° capture performance of the Ca-based sorbents as compared
to the activated carbon (FGD) and CRFA at 100°C. Removals presented in Figure 4 (and Figure 7)
are obtained by averaging the removal data acquired during the exposure period (30 minutes). Of the
five, FGD showed the highest capture of Hg° during the 30 minutes of exposure (constant during this
period). Both CRFA and hydrated lime exhibited insignificant capture of Hg° (approximately 5%).
Among the Ca-based sorbents, it is seen that Hg° capture increases as the total surface area and
cumulative pore volume increases (Figure 1 and Table 1). The presence of S02 significantly increased
the capture of Ca-based sorbents, especially Advacate and modified Ad vacate. The insignificant Hg°
capture by the Ca-based sorbents in the absence of S02 indicated the lack of any interaction (physical
or chemical) between the Hg° and the Ca-based sorbents. The enhancement effect of S02 at 100°C
8
-------
may indicate that the reaction of S02 and sorbents created active sulfur (S) sites for the adsorption
of Hg°, possibly through formation of Hg-S bonds (chemisorption). Conversely, the lack of
significant improvement in Hg° capture for hydrated lime with S02 present (Figure 4) indicates the
need for a fine pore structure as well as S02. If indeed, the major Hg° capture mechanism is
chemisorption by S02-generated active sites, then decreasing the system temperature should decrease
the overall rate of "active site generation and chemisorption" leading to a decrease in Hg° capture.
The effect of temperature (65 vs 100°C) on Hg° capture by Advacate and modified Ad vacate in the
presence of S02 is illustrated in Figure 5. This figure shows Hg° capture throughout the 30 minutes
of exposure. The observed higher captures at higher temperature support the chemisorption theory
of Hg° capture by the Ca-based sorbents in the presence of S02.
The capture of S02 by the five sorbents during the Hg° tests was also monitored at 100°C (Figure 6).
All three Ca-based sorbents showed higher captures of S02 than activated carbon (FGD), which was
expected because of their alkaline nature. After approximately 10 minutes of exposure to the
simulated flue gas, the S02 reaction rate (change of percent removal with time) showed diminishing
removal with increasing time. One explanation is that the reaction of S02 with Ca-based sorbents
*
may lead to pore mouth closure, thus blocking the access of S02 to the interior of the Ca-based
sorbents. This would occur within the first 10 minutes of exposure of sorbent to flue gas. One may
speculate that since all three Ca-based sorbents had the same average pore diameter (20-30 nm), they
should exhibit the same monotonicaiiy decreasing S02 capture pattern.
In summary, Hg° can be captured by previously reacted mixtures of fly ash and hydrated lime
(Advacate and modified Advacate) when S02 is present in the flue gas. Based on this observation,
one may conclude that, in terms of Hg° control, the optimum region for injection of Ca-based
sorbents is upstream of S02 control systems in which a higher concentration of S02 is present, and
flue gas temperatures are higher. In this way, both S02 and Hg° emissions may be controlled for
approximately the cost of S02 control by sorbent injection alone. Modifying sorbents to increase the
total surface area and fine pore structure increases Hg° uptake in the presence of S02 for the sorbents
9
-------
studied.
Capture of Mercuric Chloride
Figure 7 depicts the effect of S02 on HgCl2 capture performance of the three Ca-based sorbents as
compared to the activated carbon (FGD) and Clinch River Fly Ash (CRFA) at 100°C. Similar to Hg°,
FGD captured the highest fraction of incoming HgCl2 (constant removal during the exposure period),
with the three Ca-based sorbents and CRFA showing from 10 to 20% HgCI2 capture in the absence
of SOj Unlike the Hg° case, the presence of S02 inhibited the HgCl2 capture by Advacate and
modified Advacate, indicating that perhaps HgCl2 is not attracted to the sites preferred by Hg°, and
has affinity for S02 capture sites. The S02 inhibition effect may also confirm the earlier conclusion
that the presence of S02 caused a blockage of pores in Advacate and modified Advacate, and
therefore limited the access of HgCl2 to the interior structure of the sorbents. One may also attribute
the SO, inhibition effect to the competition of S02 with HgCl? (both acid gases) for the alkaline sites
located inside the pores or on the external surface of the sorbent.
Figure 8 illustrates the effect of temperature on HgCl2 capture by Advacate and modified Advacate
in the absence of S02 (optimum condition), throughout the 30 minutes of exposure. Unlike the Hg°
case, decreasing the temperature caused an increase in HgCl2 capture by these sorbents. The effect
of temperature may be explained by a physisorption mechanism through which the HgCl2 molecules
are adsorbed by the sites.
Ail interesting observation can be made by comparing Figure 8 (effect of temperature on HgCl2
capture) to Figure 5 (effect of temperature on Hg° capture). Unlike Hg°, HgCl2 capture increased
(with time) at the lowest studied temperature (65 °C). The reason may be outlined as follows.
At lower temperatures, water vapor present in the simulated flue gas may condense on the surface
of Advacate and modified Advacate. It should be noted that the homogeneous dew point of 5% water
vapor in air is below 65°C, but that the actual dew point above hygroscopic solids (such as calcium
10
-------
silicates) can be significantly higher, favoring condensation of water vapor at the solid surface. If
water vapor molecules were to condense on the sorbent sites, they could readily dissolve the
incoming HgCl2 molecules but not the insoluble Hg° molecules. As the time of exposure progresses,
an increasing number of water vapor molecules condense, thus the capture percentage of HgCl2
increases. This dissolution effect, yet to be proven for these sorbents, may be very important in
practical situations where the concentration of water vapor is likely higher than for these bench-scale
simulations.
Conclusion
The capture of elemental mercury (Hg°) and mercuric chloride (HgCl2), the mercury species
identified in coal flue gas, by three types of calcium-based sorbents differing in their internal structure,
was examined in a packed-bed, bench-scale study under simulated flue gas conditions for coal-fired
utilities. The results obtained were compared with Hg° and HgCl2 capture by an activated carbon
(FGD) under identical conditions. Tests were conducted with and without S02 to evaluate the effect
of S02 on Hg° and HgCl2 control by each of the sorbents.
The Ca-based sorbents showed insignificant removal of Hg° in the absence of S02, However, in the
presence of S02, Hg° capture was enhanced for the three Ca-based sorbents. It was postulated that
the reaction of hydrated lime with S02 would result in pore mouth closure as evidenced by the sharp
drop in the S02 removal rate after the initial 10 minutes of exposure. Despite the loss of internal
surface area, the relatively high uptake of Hg° observed for these sorbents in the presence of S02,
suggests that Hg° and S02 do not compete for the same active sites, and the sites for Hg° capture are
influenced positively by the presence of S02. Moreover, the capture of Hg° in the presence of SO,
increased with sorbent surface area and internal pore structure.
Conversely, the three Ca-based sorbents showed decreased removal of HgCl2 in the presence of S02.
In the absence of S02, roughly 25% of the incoming HgCf2 was captured. The alkaline sites in the
Ca-based sorbents were postulated to be instrumental in the capture of acidic 1 IgCl2. S02 not only
11
-------
competed for these alkaline sites but also, as mentioned, likely closed pores with subsequent
reduction in accessability of the interior of the Ca-based sorbent particles to the HgCl2 molecules.
It was hypothesized that the capture of Hg° in the presence of S02 may occur through a
chemisorption mechanism, while the nature of the adsorption of HgCl2 molecules may be explained
through a physisorption mechanism. The effect of temperature studies further confirmed this
hypothesis. Increasing the system temperature caused an increase in Hg° uptake by the sorbents in
the presence of S02. However, the increase in temperature resulted in a significant decrease in the
HgCl, uptake in the absence or presence of S02. Increased sorbent surface area and internal pore
structure had no observable effect on HgCl2 capture in the presence of S02.
With the relatively large quantities of Ca needed for S02 control at coal-fired boilers, the above
results suggest that Ca-based sorbents, modified by reaction with fly ash, can be used to control total
mercury emissions and SO, cost-effectively. The most effective Ca-based sorbents are those with
significant surface area (for S02 and HgCl2 capture) and pore volume (for Hg° capture).
Sorbents injected upstream of a fabric filter should perform as indicated by the fixed-bed reactor
simulation in this study. Confirmation of these results on a 50 cfm (0.024 m/s) pilot plant is
anticipated later this year.
Acknowledgements
The authors would like to acknowledge the logistical support of Richard E. Valentine (EPA/APPCD),
experimental assistance from Lisa Adams and Hamid Bakhteyar and sorbent development assistance
from Wojciech Jozewicz and Carl Singer (Acurex Environmental Corporation).
References
1. D.G. Langiey. "Mercury Methylation in an Aquatic Environment," J. Water Pollut. Control
Fed., 45; 44-51,(1973).
12
-------
2. G. Westoo. "Methyl Mercury a Percentage of Total Mercury in Flesh and Viscera of Salmon
and Sea Trout of Various Ages," Science, 181: 567-568, (1973).
3. U.S. Environmental Protection Agency, Mercury Study Report to Congress, Book 1 of 2,
External Review Draft, EPA/60G/P-94/002a (NTIS PB95-167334), Environmental Criteria
and Assessment Office, Cincinnati, OH, January 1995.
4. C.E. Billings; A.M. Sacco; W.R. Matson; R.M. Griffin; W.R. Coniglio, and R.A.Harley.
"Mercury Balance on a Large Pulverized Coal-fired Furnace," J. Air Pollut, Contr. Assoc.,
23:9, 773, (1973).
5. D.H. Klein; A.W.Andren, J.A. Carter, J.F. Emery; C. Feldman;W. Fulkerson, W.S. Lyon,
J.C. Ogle; Y, Talmi; RI. Van Hook, and N.Bolton. "Pathways of 37 Trace Elements Through
Coal-Fired Power Plant," Environ. Sci. & TechnoL, 9:10, 973, (1975).
6. T.G, Brna, and J.D. Kilgroe. "The Impact of Particulate Emissions Control on the Control
of Other MWC Air Emissions," J. Air & Waste Mgt. Assoc., 40(9): 1324 (1990).
7. R Chang and G R. Offen "Mercury Emission Control Technologies: An EPRI Synopsis,"
Power Engineering, November 1995.
8. B. Hall; O Lindqvist, and E.Ljungstrdm. "Mercury Chemistry in Simulated Flue Gases
Related to Waste Incineration Conditions," Environ. Sci. & TechnoL, 24: 108, (1990).
9. Redinger, K.E. Babcock & Wilcox to William Maxwell, Letter.U.S. EPA, dated Nov. 7,
1996, attachment p.5.
10. D L Laudal; M.K. Heidt, T.D. Brown; B.R. Nott, and E.P. Prestbo. "Mercury Speciation:
A Comparison Between EPA Method 29 and Other Sampling Methods," in Proceedings of
the 89th Air & Waste Management Association Annual Meeting, 96-WP64A.04, A&WMA,
Nashville, TN, (1996).
11. R. Chang; C.J. Bustard; G.Schott; T. Hunt; H.Noble, and J.Cooper. "Pilot-Scale Evaluation
of Carbon Compound Additives for the Removal of Trace Metals at Coal-Fired Utility Power
Plants," paper presented at the Second International Conference on Managing Hazardous Air
Pollutants, Washington, D C, July 13-15, 1993.
12. S.J. Miller.; D.L. Laudal; R. Chang, and P.D. Bergman. "Laboratory Scale Investigation of
Sorbents for Mercury Control," presented at the AWMA Annual Meeting, Paper no. 142,
13
-------
Cincinnati, OH, June 20-24, 1994.
13. S.V. Krishnan; B.K. Gullet, and W. Jozewicz. "Mercury Control by Injection of Activated
Carbon and Calcium-Based Sorbents," paper presented at Solid Waste Management: Thermal
Treatment & Waste-to-Energy Technologies, U.S. EPA/AEERL & AWMA, Washington,
D C., April 18-21, 1995.
14. B.K. Gullett; and K. Raghunathan. "The Effect of Sorbent Injection Technologies on
Emissions of Coal-Based, Metallic Air Toxics," In Proceedings of SO, Control Symposium,
Vol. 3, p. 63-1, EPA-600/R-95-015c (NT1S PB95-179248), February 1995.
15. M R. Stouffer; W.A. Rosenhoover, and F.P. Burke. "Investigation of Flue Gas Mercury
Measurement and Control for Coal-Fired Sources," Paper 96-WP64B.06 presented at the
89th Annual Air and Waste Management Association Meeting, Nashville, TN, June 23-28,
1996.
16. S.V. Krishnan; B.K. Gullett, and W. Jozewicz. "Sorption of Elemental Mercury by Activated
Carbons," Environ. Sci. & Tech., 28:8, 1506-1512 (1994).
Table 1. Total Surface Area and Average Pore Diameter of Sorbents
Sorbent
Total Surface Area (m2/g)
Average Pore Diameter (nm)
Hydrated Lime
13.0
33.4
Advacate
30.9
21.2
Modified Advacate
91.4
22.2
Clinch River Fly Ash (CRFA)
2.3
8.1
Activated Carbon (FGD)
575
3.2
14
-------
0.6
OS
C5 0.5
| 0,4
3
£
g 0.3
£
| 0.2
5
h
Umo
•
Advacate
©
Modified
AdvacsSe
¦»
0.12
10 20 50
Pore Diameter (nm)
Modified
Advacate
" 20
5 10 20 50
Pore Diameter (nm)
100
200
E 0.08
D
Advacate
©
Modified
Advacate
20
¦?
E 15
<
2
o 10
0-
s
c
ffi
£ c
£ 5
o
c
10 20 50 100
Pore Diameter (run)
-mA.
N
N
a
3 '•
,/ \
- V \
lime
•
Advacate
©
ModiSed
Advacate
*
¦jsmiM
10 20 50
Pore Diameter (nm)
100
200
Figure 1. Internal Pore Structure Characteristics of Lime, Advacate, and Modified Advacate
Obtained from Nitrogen Sorption
15
-------
5 10 20
Pore Diameter (nraj
10 20
Pore Diameter (ran)
^ 60
5 10 20
Poro Diameter (nm)
5 10 20
Pore Diameter (nm)
Figure 2. Internal Pore Structure Characteristics of DARCO FGD Obtained from Nitrogen Sorption
16
-------
Mercury Source
Carbon Trap
k
3-Way Valve
Water Bath
Manifold
Packed Bed Reactor
MZEEZH
On-off Valves
-H)—O—Q-
Heater
Water
Carbon
Trap
Rotameter
S02
Analyzer
By-Pass
3-Way Valves
CO.
Data Acquisition
Mercury
Analyzer
Water
Trap
Figure 3. Schematic of the Bench-Scale Packed Bed Reactor
17
-------
~ No S02 ~ 173 ppm S02
100
80
60
40
20
0
FGD
M. Advacate Advacatc
Lime
CRFA
Sorbent
ure 4. Effect of Sulfur Dioxide on Elemental Mercury Capture by the Sorhents at 100°C
18
-------
15 20
Time (min)
Adv @ 65 C
Adv(ai00°C
mod. Adv @ 65 C
mod. Adv @ 100°C
Figure 5. Effect of Temperature on Elemental Mercury Capture by Calcium-Based
Sorbents (Advacate and Modified Advacate) in the Presence of 173 ppm Sulfur Dioxide
19
-------
CRFA
Lime
-------
~ 173 ppm S02 ~ No S02
FGD M. Advacate Advacate
Sorbent
Lime
CRFA
Figure 7. Effect of Sulfur Dioxide on Mercuric Chloride Capture by the Sorbents at 100°C
21
-------
10 15 20
Time (min)
Adv @ 65 C
Adv @ 100°C
mod, Adv @ 65 C
mod. Adv @ 100°C
Figure 8. Effect of Temperature on Mercuric Chloride Capture by the Calcium-Based
Sorbents (Advacate and Modified Advacatc) in the Absence of Sulfur Dioxide
22
-------
Moix/rnr „ n-r, ,fl. TECHNICAL REPORT DATA
JN K ivl bi L,~ K I P~ I lo4 a (Please rmH In*mictvwiQPJte. WXSf2rI(r* completing}
1. REPORT NO. 2.
EPA 600/A-98/053
3. RECIPIENT'S ACCESSION NO.
4. TITLE ANO SUBTITLE
Combined Mercury and Sulfur Oxides Control Using
Calcium-Based Sorbents
S. REPORT OATE
6. PERFORMING ORGANIZATION CODE
7. AUTMOR(S)
S. Behrooz Ghorishi (A cur ex) and Charles B.
Sedman (EPA)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
A cur ex Environmental Corporation
4915 Prospectus Drive
Durham, North Carolina 27713
10. PROGRAM ELEMENT NO,
11. CONTRACT/GRANT NO.
68-D4-0005
12. SPONSORING AGENCY NAME ANO AODRESS
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; 6/96-7/97
14. SPONSORING AGENCY CODE
EPA/600/13
15. supplementary notes ^PPCD project officer is Charles B. Sedman, Mail Drop 4, 919/
541-7700. Presented at EPRI-DOE-EPA Combined Utility Air Pollutant Control Sym-
posium, 8/26-29/97, Washington, DC.
is. abstractpaper gives results of an examination of the capture of elemental mer-
cury (Hgo) and mercuric chloride (HgC12) by three types of calcium (Ca)-based sor-
bents in a bench-scale study under conditions prevalent in coal-fired utilities. Ca-
based sorbent performances were compared to that of an activated carbon. Mercury
capture of about 40% (nearly half that of the activated carbon) was achieved by two of
the Ca-based sorbents. The presence of sulfur dioxide (S02) in the simulated coal
combustion flue gas enhanced the capture of Hgo from about 10 to 40%. Increasing the
temperature in the range of 65-100 C also caused an increase in the Hgo capture by
the two Ca-based sorbents. HgC12 capture exhibited a totally different pattern. The
presence of S02 inhibited the HgC12 capture by Ca-based sorbents from about 25 to
< 10%. Increasing the temperature in the studied range also caused a decrease in
HgC12 capture. Upon further pilot-scale confirmations, the results obtained in this
bench-scale study can be used to design and manufacture more cost-effective mer-
cury sorbents to replace conventional sorbents already in use in mercury control.
17. KEY WORDS ANO DOCUMENT ANALYSIS
a. DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution Combustion
Mercury (Metal) Activated Carbon
Sulfur Dioxide
Sorbents
Calcium
Coal
Pollution Control
Stationary Sources
13 B 21B
07B
11G
21D
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
22
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
23
------- |