Fuel Processing Technology 85 (2004) 487 – 499
www.elsevier.com/locate/fuproc
Mercury emissions from a 100-MW wall-fired boiler
as measured by semicontinuous mercury monitor and
Ontario Hydro Method
Shawn Kellie a, Yufeng Duan a, Yan Cao a, Paul Chu b, Arun Mehta b,
Ron Carty c, Kunlei Liu a, Wei-Ping Pan a,*, John T. Riley a
a
Combustion Laboratory, Western Kentucky University, Bowling Green, KY 42101, USA
b
Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, CA 94304, USA
c
Illinois Clean Coal Institute, Suite 2000 Coal Development Park, Carterville, IL 62918, USA
Abstract
Western Kentucky University (WKU) recently established a mobile laboratory for monitoring
mercury emissions (MMEML). The lab contains facilities to perform both continuous emissions
monitoring and the Ontario Hydro Method for mercury analysis. Among the instruments available
in the lab are a semicontinuous mercury emissions monitor (SCEM), pretreatment and speciation
unit for the SCEM, and an atomic absorption spectrometer with automated sampler. The MMEML
was recently utilized at a power plant site that had a 100-MW, wall-fired combustor with low-NOx
burners. At this site, a comparison between OHM and SCEM data was possible for testing
locations before and after the ESP. OHM and SCEM produced analogous results for the
measurement of total mercury, but differ in their measurement of mercury speciation. Testing by
OHM also showed that vapor-phase mercury decreases as temperature decreases and as fly ash is
removed. Our results suggest that the removal of vapor-phase mercury by fly ash is mostly the
removal of oxidized mercury.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Ontario Hydro Method; MMEML; SCEM
1. Introduction
In 1997, The EPA issued a Mercury Study Report to Congress, which estimated that
anthropogenic sources in the US emitted 158 tons of mercury into the atmosphere in
* Corresponding author. Fax: +1-270-745-5361.
E-mail address: [email protected] (W.-P. Pan).
0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.fuproc.2003.11.004
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S. Kellie et al. / Fuel Processing Technology 85 (2004) 487–499
1994 – 1995. The report estimates that approximately 33% of these anthropogenic
sources are coal-fired combustion sources [1]. Because of the danger to human health
posed by mercury, Congress and the EPA are determined to regulate mercury emissions.
The EPA has set a target date of 2004 for new mercury regulations. At the same time
that the EPA seeks to lower total mercury emissions, there is a resurgence in the
construction of new coal boilers [2]. Therefore, the need to control and measure mercury
emissions in a cost-effective manner has become an issue of importance to both the coal
industry and regulators.
In flue gas, mercury exists in three primary forms, elemental mercury (Hg0),
oxidized mercury, and particle associated mercury. Most oxidized mercury in flue
gas is in the mercury(II) state (Hg2 +). Oxidized mercury is soluble and has a
tendency to associate with particulate matter. Therefore, emissions of oxidized
mercury may be efficiently controlled by air emission and particulate controlling
apparatus such as a flue gas desulfurization (FGD) scrubber system, electrostatic
precipitator (ESP), and activated carbon injection systems (ACI). On the other hand,
elemental mercury is extremely volatile and insoluble. Elemental mercury has a high
vapor pressure at typical air emission and particular control device-operating temperatures. Therefore, effective collection by particulate matter control devices is highly
variable. In addition, elemental mercury is not captured by FGD and any kind of PCD
systems. While elemental mercury may be removed by some chemically treated
activated carbon or selective absorbents, they are more difficult to collect and treat.
Therefore, elemental mercury emissions are harder to reduce than oxidized mercury
emissions.
Studies indicate that the distribution of Hg species in coal-fired flue gas is strongly
dependent on the type of coal (e.g., bituminous, subbituminous, or lignite), the operating
conditions of the combustion system (in terms of unburned carbon in the ash), and
temperature and residence time in the particulate control device [3– 11]. The variability
in the distribution of vapor-phase mercury species in coal-fired flue gas may depend
upon the coal’s chloride concentration. Higher concentrations of ionic mercury are
obtained in utility flue gas when the combusted coal has a high chloride content (0.1 –
0.3 wt.%) [12 – 15]. Additional studies including ones conducted at Western Kentucky
University (WKU) have suggested that calcium may play a role in mercury speciation
[16,17]. Furthermore, other components of the air pollutant control systems such as FGD
and selective catalytic reduction (SCR) systems have also been shown to affect both the
speciation of mercury in the stack and the amount of mercury removed in the air
pollutant control equipment.
To help study the OHM and SCEM techniques and the effects of flue gas temperature
on mercury in flue gas, the Western Kentucky University built a mobile mercury
emissions monitoring lab (MMEML). The MMEML contains the facilities to collect and
analyze Ontario Hydro Method samples on site. The MMEML also has a semicontinuous mercury emissions monitor (SCEM), PSA’s Sir Galahad, and two PSA
speciation/pretreatment units. WKU’s SCEM setup allows for two points in a combustor
to be monitored and speciated simultaneously. WKU’s MMEML—described in detail in
the following section—has been moved to a 100-MW boiler with wall-fired low-NOx
burners.
S. Kellie et al. / Fuel Processing Technology 85 (2004) 487–499
489
2. Experimental
2.1. Mobile mercury emissions monitoring lab
Western Kentucky University (WKU) designed a Mobile Mercury Emissions Monitoring Lab (MMEML). The MMEML is shown in Fig. 1. It was built from a 53-ft tractortrailer. To limit sample contamination problems, the trailer was divided into three rooms: an
OHM preparation room, an analysis room, and a storage room. The lab is both heated and
air-conditioned to minimize instrumentation problems due to temperature fluctuations. The
preparation room is a clean room for the preparation of OHM solutions and sample trains. It
is also the area were the OHM sample trains are disassembled. The preparation room has a
functioning sink. The analysis room contains a Leeman Hydra Prep, a Leeman Hydra AA,
and their standards. The storage room contains areas for storage of the mercury probes, gas
analysis equipment, and spare glassware. The storage area also has tie-downs for the PSA
Analytical Mercury Semicontinuous Emission Monitor (SCEM) system. The storage area
has a separate door to the outside from the other two areas of the lab; this facilitates the
movement of equipment and helps reduce possible contamination of the samples and
equipment in the other two rooms.
2.2. Ontario Hydro Method
The Ontario Hydro Method (OHM) is the standard—but unadopted—method of
measuring and speciating mercury in flue gas. A diagram of the OHM sampling train is
shown in Fig. 2. OHM has two possible configurations based on EPA Methods 5 and 17 out-
Fig. 1. Mobile Mercury Emissions Monitoring Lab (MMEML) Floor Plan.
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Fig. 2. Ontario Hydro Method impinger train.
of-stack filtration and in-stack filtration, respectively. A standard Method 5 configuration is
shown in Fig. 3. The EPA Method 17 configuration was used at the sampling point after the
electrostatic precipitator. Due to the high volume of fly ash immediately before the ESP
region, a modified sampling train (EPA Method 5) with both in-stack and out-of-stack
filtration was used.
2.3. Semicontinuous emissions monitoring
The semicontinuous emissions monitor (SCEM) used in this study is the Sir Galahad
II manufactured by PS Analytical. It uses a gold trap to collect the mercury from the
Fig. 3. EPA Method 5 configuration.
S. Kellie et al. / Fuel Processing Technology 85 (2004) 487–499
491
flue gas before analysis with an atomic fluorescence detector. The Sir Galahad system
also has a Hg vapor generator capable of supplying a constant stream of Hg vapor
(about 14 l/min) for calibration purposes. Another important feature of the Sir Galahad
system is its stream selection box. The selection box allows the Sir Galahad software to
differentiate between different streams for the measuring of different points or different
mercury species.
Without the aid of a pretreatment system, the Sir Galahad is unable to speciate
mercury. The pretreatment system, Model S235C400 manufactured by PS Analytical,
splits the incoming flue gas into two streams. One stream passes through a KCl solution,
which removes oxidized mercury, thereby allowing only elemental mercury to reach the
detector. The other stream passes through a stannous chloride solution, which reduces
oxidized mercury to Hg0, thus facilitating the measurement of total mercury. Both
solutions also serve the dual purpose of removing acidic gases that could damage the
gold detector.
Our lab owns two pretreatment systems, which along with the Sir Galahad’s stream
selection box allow us the ability to monitor and speciate mercury at two locations at
once. At our current project, we monitor the flue gas before and after the ESP. A
diagram of our monitoring arrangement can be seen in Fig. 4. (All sample lines shown
in the diagram are heated at 200 jC to avoid the loss of mercury and the condensation
of acidic gases.)
Fig. 4. SCEM and pretreatment system configuration.
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2.4. Hydra AA
The OHM solutions were analyzed using a Leeman Labs Hydra AA. The Hydra AA is
a cold vapor atomic absorption (CVAA) instrument dedicated to mercury analysis. It has a
detection limit of 1 ppt. Additionally, to ensure maximum reproducibility and to allow the
rapid processing of samples, the lab has a Hydra Prep, which automates the sample
digestion process. A diagram of the Hydra AA is shown in Fig. 5.
2.5. Leco Advanced Mercury Analyzer 254
Ash samples collected from the dust collector, ESP and/or OHM are analyzed using the
Leco Advanced Mercury Analyzer 254 (AMA 254.) Coal samples are also analyzed using
the AMA 254. The AMA 254 is CVAA instrument. In addition to performing basic
CVAA, the AMA 254 has a gold amalgamate trap to pre-concentrate the mercury. The
AMA 254 has a detection limit of 0.01 ng and a detection range of 0.05 – 600 ng. The
AMA 254 conforms to EPA Method 7473.
Fig. 5. Hydra AA.
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493
Fig. 6. The 100-MW boiler and mercury testing locations.
2.6. Testing locations
The results discussed in this paper were obtained in a 100-MW boiler with wall-fired
low-NOx burners. The boiler is in a commercial plant and was operated normally
throughout the duration of testing. The load carried by the plant was constant during the
course of each individual test. A diagram of the boiler is shown in Fig. 6. The concentration
of mercury in the flue gas was measured at three locations: immediately prior to the air
preheater, immediately before the dust collector and electrostatic precipitator (ESP), and
immediately after the ESP in the duct leading to the stack. All three locations are shown in
Table 1
Analytical values for coals used in this study on a dry basis
Coal sample
Hg ppb
% Ash
Cl ppm
% Fixed C
%S
Coal 1
Coal 2
120
100
9.6
10.1
1010
1450
55
60
1.35
1.76
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Fig. 6. OHM was used to measure and speciate the mercury at all three locations. The
SCEM and pretreatment systems were used only at the testing locations immediately before
and after the ESP. Two coals were used during the course of this study are shown in Table 1.
3. Results and discussion
3.1. Comparison of SCEM and OHM results
As mentioned earlier, mercury measurements were made at two locations, before and
after the ESP, using both OHM and SCEM. The total vapor mercury measurement results are
shown in Fig. 7. For both coals and locations, SCEM and OHM showed good agreement
with each other. The measurements for total vapor-phase mercury were within 2300 ng/N m3
of each other, and neither method produced consistently higher results than the other.
When the data for oxidized vapor-phase mercury was examined, it showed that OHM
measured more oxidized mercury in all four cases. (Please note the term vapor-phase
mercury excludes particle-bound mercury.) As seen in Fig. 8, the difference between the two
methods was as high as 4900 ng/N m3 for coal 1 before the ESP and as low as 110 ng/N m3
for coal 2 after the ESP. Because OHM produced consistently higher levels of oxidized
mercury than SCEM, it suggests something in the methods maybe responsible.
Because of the trend observed for oxidized mercury, an analogous trend might be
expected in element vapor-phase mercury data. As shown in Fig. 9, there is a large
disagreement between the two methods. For coal 2 after the ESP, OHM measures three
times the quantity of elemental vapor-phase mercury, as does SCEM. For coal 1 before the
ESP, SCEM measures a third more mercury than OHM. The only trend evident in
Fig. 7. Total vapor-phase mercury before and after the ESP for coals 1 and 2.
S. Kellie et al. / Fuel Processing Technology 85 (2004) 487–499
495
Fig. 8. Oxidized vapor-phase mercury before and after ESP for coals 1 and 2.
comparing the elemental mercury data between the two methods is that the OHM
measured higher levels of elemental mercury than SCEM after the ESP. The exact
opposite was true before the SCEM. A large quantity of the fly ash is removed as the
flue gas passes through the ESP; therefore, it is logical to assume that fly ash may play a
role in the differing measurement of elemental mercury at these locations. A possible
explanation would be before the ESP, high levels of fly ash on the OHM filter convert
Fig. 9. Elemental vapor-phase mercury before and after ESP for coals 1 and 2.
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Fig. 10. Total vapor-phase mercury as measured by OHM at three locations.
elemental mercury to oxidized mercury. This explanation is supported by the presence
1575 ng/N m3 for coal 1 and 2100 ng/N m3 for coal 2 of particle bound vapor-phase
mercury before the ESP compared to quantities below detection after the ESP. Other
researchers have reported biases caused by the fly ash accumulated on the filter in the
OHM method. To determine what this mechanism maybe or to determine if the methods
contain a bias, further research is required.
3.2. Testing location and temperature effects on mercury measurement
Based on previous research linking temperature and fly ash concentration to mercury
speciation, we predicted that vapor-phase mercury concentrations would fall as the flue gas
Fig. 11. Percentage of total vapor-phase mercury in elemental form.
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497
Table 2
Ash samples from the dust collector and ESP
Ash sample
Hg ppb (dry basis)
Loss on ignition
Coal
Coal
Coal
Coal
900
4100
600
3100
5.7
6.4
4.0
5.7
1
1
2
2
dust collector
ESP
dust collector
ESP
moved out of the combustor. The results shown in Fig. 10 suggest that this prediction was
correct. The measurements in Fig. 10 were taken with OHM and show a decrease in total
vapor-phase mercury concentration with the decrease in temperature from the air preheater at
a temperature of 750 –298 jF before the ESP. At the exit to the ESP, the flue gas temperature
is 259 jF; therefore, there is only a slight change in temperature occurring in the ESP. Any
changes in mercury flue gas concentration occurring in the ESP are more likely the result of
factors other than temperature, probably the fly ash in ESP acting as a filter for mercury.
The effect of location on vapor-phase mercury speciation can be seen in Fig. 11. The
percentage of vapor-phase mercury in elemental form increases with the decrease of
temperature and the decrease of fly ash. This trend suggests that the majority of oxidized
vapor-phase mercury is removed in the fly ash and that only elemental mercury remains in
the vapor phase.
3.3. Ash mercury concentrations
Ash was collected during the beginning and end of every OHM sampling period. The ash
was collected from sampling ports in the dust collector and the ESP. Both sampling ports
were blown clean every half-hour by routine operation of the plant; therefore, the ash
collected will correspond to the coal being burnt during the sampling period. The mercury
concentration of the ash and other factors can be seen in Table 2.
For both coals 1 and 2, the highest concentration of mercury was found in the ash from
the ESP. The difference in mercury bound fly ash may be higher LOI exhibited by the ESP
fly ash compared to that collected from the dust collector, which has been linked with high
levels of mercury retention in other studies [18 –20].
4. Conclusions
1. Western Kentucky University (WKU) recently established a mobile mercury emissions
monitoring lab (MMEML). The lab is capable of collecting Ontario Hydro Samples and
analyzing them on site. The lab has a PSA Sir Galahad SCEM with two pretreatment
systems; therefore, it is capable of simultaneously performing analysis at two points.
2. OHM and SCEM produce analogous results for the measurement of total mercury, but
differ in their measurement of mercury speciation.
3. Vapor-phase mercury decreases as temperature decreases and as fly ash is removed.
4. The removal of vapor-phase mercury by fly ash is mostly the removal of oxidized
mercury.
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Acknowledgements
This paper was prepared by Western Kentucky University research group with support,
in part, by grants made possible by the Illinois Department of Commerce and Community
Affairs through the Office of Coal Development and the Illinois Clean Coal Institute and
Electric Power Research Institute. Neither Western Kentucky University nor the Illinois
Department of Commerce and Community Affairs, Office of Coal Development, the
Illinois Clean Coal Institute, nor any person acting on behalf of either (A) makes any
warrant of representation, express or implied, with respect to the accuracy, completeness,
or usefulness of the information contained in this paper, or that the use of any information,
apparatus, method, or process disclosed in this paper may not infringe privately owned
rights, or (B) assumes any liabilities with respect to the use of, or for damages resulting
from the use of, any information, apparatus, method or process disclosed in this paper.
Reference herein to any specific commercial product, process, or service by trade name,
trademark, manufacturer, or otherwise, does not necessarily state or reflect those of the
Illinois Department of Commerce and Community Affairs, Office of Coal Development,
or the Illinois Clean Coal Institute.
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