MERCURY OEMs: TECHNOLOGY UPDATE
Jeffrey V. Ryan and James D. Kilgroe
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
Air Pollution Prevention and Control Division
Research Triangle Park, North Carolina 27711
ABSTRACT
Continuous emission monitors (CEMs) for mercury
(Hg) are receiving increased attention and focus. Their
potential use as a compliance assurance tool is of
particular interest While Hg CEMs are currently used
in Europe for compliance purposes, use of Hg CEMs in
the United States (U.S.) has focused on combustion
research and Hg control technology evaluation
applications. Hg CEMs are now receiving increased
attention as compliance assurance tools. Several
programs exist to evaluate Hg CEM measurement
performance. It is through these efforts that
application-specific measurement issues are
investigated. Collectively, these efforts have served to
advance the state-of-the-art of the technology as
evidenced by the number and types of CEMs now
available and the various applications in use.
INTRODUCTION AND BACKGROUND
Anthropogenic releases of Hg to the environment have
become a serious global and national concern due to the
toxicity of Hg in its organic form. Combustion of fossil
fuels, municipal and medical waste, as well as hazardous
waste collectively represents a significant contribution to
the Hg released in the U.S. These combustion processes
emit Hg in a number of inorganic forms that can be
converted, by naturally occurring biological processes, into
the highly toxic methyl Hg species. Understanding
combustion source emissions is a necessary step in
understanding fate and transport of Hg, and ultimately risk
to human health and the environment. Hg CEMs arc
valuable tools that can aid in understanding the
contributions from these sources as well as potentially
provide assurance of compliance with established emission
limits. In addition, Hg CEMs can provide a number of
other potential benefits, including:
-	Real-time emission data
-	Greater understanding of process variability and
operation
-	Operational data for system optimization and
process control
-	Evaluation of Hg control strategies
-	Potentially less reliance on waste feed
characterization (i.e., for incinerators)
-	Greater public assurance
These approaches have largely driven the advancement of
the Hg CEM technologies in the U.S., despite the lack of a
clear regulatory incentive.
Hg CEMs are currently used in Europe for compliance
purposes, primarily in Germany. Hg CEMs arc installed at
over 100 facilities, including fossil-fuel boilers and
municipal waste combustors,' The types of pollution
control devices associated with combustors have a
significant impact on Hg CEM measurement ability. An
ordinary German site has eight separate devices to control
emissions: two electrostatic precipitators (ESPs), two
scrubbers, a spray dryer, a carbon adsorber, a catalytic
oxidizer, and a baghouse.1 The effects of potential
interferants such as carbon dioxide (C02), carbon
monoxide (CO), nitrogen oxides (NOJ, water vapor, sulfur
dioxide (SOj), ammonia (NH3), hydrochloric acid (HC1),
chlorine (Cl2), hydrocarbons, and particulate are
minimized, if not eliminated. After passing through the
control devices, particularly ESPs, baghouses, and wet
scrubbers, most, if not all, of the Hg remaining in the flue
gas is in the elemental phase.' Measuring elemental Hg is
much less difficult than measuring the other forms of Hg
associated with combustion processes.
Extrapolating European Hg CEM measurement
performance to U.S. applications is difficult due to the
diversity in combustion sources and pollution control
device availability and configuration. As a result, the
measurement environment is likely to be much more severe
and diverse as well. In order for Hg CEMs to be
considered for regulatory compliance assurance, acceptable
performance will need to be demonstrated. Hg CEMs are
not likely to be required unless sufficient performance data
are available to justify the promulgation of a CEM-based
standard. It is this lack of demonstrated performance that
caused EPA's Office of Solid Waste (OSW) to propose the
use of total Hg CEMs for compliance assurance only as an
option in the Phase I Maximum Achievable Control
Technology (MACT) rule for Hazardous Waste
Combustors (HWCs).2 Without a mandatory requirement
for Hg CEMs, Hg CEM vendors and potentially regulated
facilities appear to be reluctant to invest in their further
development As a result, few opportunities exist to
demonstrate CEM performance, and those demonstrations
that have been conducted have not been sufficiently robust
to fully support a Hg CEM-based standard. As a result, the
developmental progress of Hg CEMs in the U.S. has been
hindered.2

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This paper describes the current state-of-the-art of Hg
CEM technologies, as well as issues associated with
specific measurement applications. This paper does not
address regulatory issues or direction.
HOW Hg CEMS WORK
Mercury CEMs are similar to most combustion process
CEMs in that the emission sample typically must be
extracted from the stack and then transferred to the
analyzer for detection. However, Hg monitoring is
complicated by the fact that Hg exists in different forms
(particulate-bound, oxidized, elemental) and that
quantitative transport of all these forms is difficult.
Typically, Hg CEMs directly measure (detect) only
elemental Hg. Hg CEMs measure total Hg through the use
of a conversion system that converts (reduces) the
nonelemental or oxidized Hg to elemental Hg for detection.
Mercuric chloride is considered to be the primary oxidized
form of Hg. Although particulate-bound Hg can also be
reduced to the gaseous elemental form, particulate sample
delivery issues make this impractical. As a result, for most
commercially available CEMs, the total Hg measured is in
fact total gaseous Hg (TGM).
Nonelemental Hg is commonly converted using a liquid
reducing agent (e.g., stannous chloride). This technique is
least preferable, though more established. The use of wet
chemical reagents is considered to be a significant
limitation to Hg CEM use. The wet chemicals typically
possess corrosive properties and require frequent
replenishment. The spent reagents commonly possess
hazardous properties that result in waste disposal concerns.
In addition, the reducing ability of reagents such as
stannous chloride is affected by high levels of SO;.3
In addition to the more established wet chemistry
conversion methods, dry conversion methods are now
available. These techniques use high temperature catalysts
or thermal reduction units to not only convert nonelemental
Hg to the reduced form, but also condition the sample for
analysis by removing selective interferants. This approach
does much to minimize the size of the conversion system
as well as maintenance requirements.
Because the particulate form is difficult to transfer and is
also often a measurement interferant, the particulate is
typically filtered out and remains unmeasured. This could
potentially impart a negative bias to the total Hg
measurement. This bias could be further amplified as
certain types of particulate may actually capture gas-phase
Hg. This may not be a significant issue for sources where
particulate-bound Hg is not present in appreciable
quantities, but may be significant for high particulate-
emitting sources (e.g., sources with minimal particulate
control). Therefore, the ability to measure the particulate
component is important and should not be ignored.
Similarly, there are known complications with the
quantitative transfer of mercuric chloride (HgCl2). HgCl2
is water soluble and reactive with many surfaces. Losses
due to adsorption are a major concern. As a result, recent
emphasis has been placed on locating the nonelemental Hg
conversion system as close as possible to the source so that
the less reactive elemental form is transferred from the
source to the detection unit.
In general, Hg CEMs can be distinguished by their Hg
measurement detection principle. Detection systems
include: cold-vapor atomic absorption spectrometry
(CVAAS); cold-vapor atomic fluorescence spectrometry
(CVAFS); in-situ ultraviolet differential optical absorption
spectroscopy (UVDOAS); and atomic emission
spectrometry (AES).
Most Hg CEM systems employ CVAAS or CVAFS as the
detection technique. These detection techniques are
susceptible to measurement interferences resulting from the
presence of common combustion process emissions. Gases
such as NO,, S02, HC1, and Cl2 can act as measurement
interferants as well as degrade the performance of
concentrating devices (e.g., gold amalgams). As such,
conditioning systems and/or techniques that remove or
negate the effects of these interfering gases prior to sample
delivery to the detector are required. S02 is a major
spectral interferant with most CVAA detection systems.
The effects of S02 are commonly negated through the use
of a gold trap. The sample gas is directed through a gold
trap, where the Hg forms an amalgam with the gold
surface. Once the trap is loaded, it is heated and flushed
with a SO,-free carrier gas to the detector. The trapping
also serves to improve measurement sensitivity by
concentrating the sample. A trapping device is required of
CVAFS systems to achieve optimum sensitivity: not
because of the concentrating aspect, but because the carrier
gas will enable maximum sensitivity. Oxygen and
nitrogen, present in combustion flue gases, have spectral
quenching effects that suppress measurement sensitivity.
Conditioning of the sample gas prior to reaching the gold
trap is often required. In addition, HC1 and NO, in
combination can poison the gold surface, preventing the
amalgam with Hg. Removal of both or either of these
constituents is required.
An alternative to the Hg measurement approach is AES.
With this technique, Hg is ionized by a high energy source
(e.g., plasma) and the emission energy detected. A major
advantage of this technique is that all forms of Hg,
including particulate-bound Hg, are capable of being
ionized and detected. Another advantage of AES is that
the ionization source and detector can be located directly at
the source, avoiding sample delivery issues. In addition,
AES is less susceptible to spectral interferences from
common flue gas constituents as compounds are ionized to
their elemental form prior to detection.
Speciated Hg measurements are becoming increasingly
important with respect to characterizing combustion
process emissions and evaluating Hg control strategies.

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While there are no commercially available CEMs that
directly measure the various speciated forms of Hg, several
commercially available total gaseous Hg CEMs have been
enhanced to indirectly measure speciated Hg (the elemental
and oxidized forms) by determining the difference between
elemental Hg and total gaseous Hg. This difference is
recognized as the oxidized form. Separate Hg
measurements are made before and after the conversion
step in order to calculate the oxidized form. This indirect
speciation method is referred to as "speciation by
difference." Based on the current understanding that the
oxidized species of primary interest is HgCI, and that
HgClj is the dominant form of oxidized Hg present, the
"speciation by difference" technique is considered an
acceptable approach to obtaining speciated Hg
measurements.2
The key to performing the speciated Hg measurement is
being able to perform reliable elemental Hg measurements.
The oxidized form must be removed without affecting the
true elemental component. This is often accomplished
using a liquid reagent of some sort to quantitatively remove
the water-soluble oxidized Hg forms and allow the
insoluble elemental Hg to pass through, unretained. These
reagents may also serve to neutralize the effects of
measurement interferants. The greatest concern is the
reliability of the speciated Hg measurement Measurement
artifacts exist that bias the speciation, primarily by over-
reporting the level of the oxidized species. The largest
cause of this bias comes from the reactivity of certain types
of particulate matter (PM), PM may possess catalytic
properties that, at the conditions of Hg CEM PM filtering
environments, elemental Hg can be oxidized across the PM
surface.1* This is not an issue from a TGM measurement
standpoint (unless transport of oxidized Hg is an issue).
However, it may have major implications when measuring
environments possessing high PM loadings. This bias is
minimized in low PM loading environments, consistent
with post-paniculate control measurement locations.4
Another potentially significant source of speciated Hg
measurement bias takes place in the liquid phase. In
combustion flue gas environments where Cl2 is present,
under certain conditions the Cl2 may react in the liquid
phase to oxidize elemental Hg.5 There is evidence that this
problem can be mitigated by modifying the liquid reagent.
Hg CEM APPLICATIONS AND PERFORMANCE
EVALUATION ACTIVITIES
Hg CEMs in the U.S. have been used primarily to support
combustion research objectives and characterize the
emissions from various combustion sources. These have
largely been independent efforts. More recently,
collaborative efforts have been used to further knowledge
of Hg emissions from coal-fired utilities, including Hg
emission control.
The Department of Energy (DOE) and EPA, in conjunction
with the University of North Dakota (UND), have
conducted a number of laboratory studies and field tests
evaluating the measurement performance of select Hg
CEMs to support research characterizing the Hg emissions
from coal-fired utilities, including the evaluation of viable
Hg control techniques.M These tests have done much to
investigate measurement issues specific to this combustion
source category, particularly with respect to the quality of
speciated Hg measurements. This research has investigated
alternative sample conditioning and Hg conversion
systems, the catalytic effects of PM, and quality of
reference method (RM) measurements used for
comparative purposes.
Similarly, the EPA's Office of Research and Development,
National Risk Management Research Laboratory
(NRMRL) has conducted research examining the
measurement performance of select Hg CEMs in support of
fundamental Hg oontrol studies. This research has
investigated the quality of speciated Hg measurements
including liquid-phase oxidation of Hg,® sample
conditioning approaches, and the development and
evaluation of tools necessary for the conduct of field
performance testing. Quality Assurance/Quality Control
(QA/QC) tools such as elemental and oxidized Hg gas
standards have been investigated.
A number of tests have been conducted specifically to
evaluate Hg CEMs as a compliance assurance tool. The
first such test, sponsored by EPA/OSW, evaluated the
performance of three total Hg CEMs at a cement kiln that
also burned hazardous waste.* Measurement performance
was evaluated following Draft Performance Specification
12 (PS 12) entitled "Specifications and Test Procedures for
Total Mercury Continuous Monitoring Systems in
Stationary Sources.M At the time, this was a relatively
new test procedure and had yet to be implemented. In fact,
the guidance called for elemental Hg and HgCI, gas
standards that had yet to be developed and proven. The
tests were only marginally successful. None of the Hg
CEMs met the performance test requirements. OSW
concluded that Hg CEMs would not be considered as a
compliance tool for HWCs.' In retrospect, the harshness of
the kiln's emission environment was concluded as a major
cause of the test program's lack of success.6-8 The cement
kiln chosen lacked acid gas control and had relatively high
PM loading, resulting in severe interferences and
operational difficulties.
The DOE Mixed Waste Focus Area (MWFA) has
sponsored several tests determining the measurement
performance of a single total Hg CEM under hazardous
waste incineration conditions.*'10 Measurement
performance was also evaluated following PS 12. These
tests demonstrated not only Hg CEM performance, but also
that PS 12 test procedures could be implemented. A
prototype elemental Hg compressed gas standard was used
for the first time. While these tests have been relatively
successful, they are still limited in scope and application.
Recently, the EPA's Environmental Technology

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Verification (ETV) Program, in collaboration with
NRMRL, has completed testing of four commercially
available Hg CEMs from three vendors using the unique
capabilities of NRMRL's pilot-scale combustion test
facilty. These tests examined the measurement
performance of both total and speciated Hg CEMs under
two distinct and diverse combustion conditions. Coal and
chlorinated waste combustion conditions were simulated.
These verification tests used PS 12 as guidance, but also
considered specific measurement issues of interest and
innovative approaches that better examined these issues.
The pilot-scale tests were unique in that specific
measurement issues were investigated as variables. The
pilot-scale combustion facility enabled independent control
of Hg concentration and species. As a result, the total Hg
measurement could be challenged by the distribution of
oxidized and elemental Hg. Interference flue gas
constituents were also independently examined. The ETV
testing made use of several new QA/QC tools. Newly
developed elemental Hg compressed gas standards were
used to determine Hg CEM calibration drift and system
bias. As a result, not only were Hg CEMs evaluated, but
improved techniques for evaluating Hg CEMs were
demonstrated. Performance data for the participating Hg
CEMs are not yet available.
FUTURE DIRECTIONS AND NEEDS
Interest in Hg CEMs and their use is increasing. The ETV
Advanced Monitoring Center program and recent ETV Hg
CEM testing have done much to determine level of interest
as well as interested parties. It is as a result of this interest
that additional ETV field verification testing is currently
being considered. There is considerable interest at the
State and Regional regulatory level. Verification testing at
coal-fired utilities as well as municipal waste combustors
are among the source(s) being considered. Increased
participation from the vendors for the field verifications is
likely.
Hg CEMs, both total and spcciating, will be integral
components of the DOE/EPA Hg control technology
evaluations for coal-fired utilities. Hg CEMs will be
installed at pollution control inlet and outlet locations to
evaluate and optimize control technology performance.
This will also afford opportunities to evaluate Hg CEM
performance. Specifically, issues such as field durability,
long-term performance, and maintenance requirements can
be investigated.
Revisions to PS 12 to reflect current testing capabilities are
possible. The ETV pilot-scale tests, availability of new
QA/QC tools such as elemental Hg and HgCl, gas
standards, and advancements in Hg CEM technologies
provide evidence that valid techniques suitable for
assessing Hg CEM measurement performance are now
available. However, these new tools must be finalized and
accepted. While the stability of the elemental Hg
compressed gas standard has been confirmed, techniques
for establishing the standard's true concentration have not.
As a result, quantitative use of the standard is limited.
Similarly, acceptance of a HgCl2 standard is imperative as
this standard is used to assess Hg conversion system
effectiveness as well as sampling system delivery
efficiency and reactivity, parameters not challenged by an
elemental Hg gas standard. This is particularly relevant in
measurement applications where oxidized Hg may be the
predominant Hg form present.
Additional Hg CEM research and measurement
performance data are still needed to truly demonstrate the
viability of the technology under all potential applications.
As a process control monitor or as a tool to evaluate Hg
control strategies, there are still measurement obstacles to
be overcome, particularly with respect to speciated
measurements. Sampling at pollution control inlet
locations presents unique measurement challenges. When
considering Hg CEMs as a potential compliance assurance
tool, the obstacles do not appear to be technological as
much as lack of performance demonstration. Data are
needed that demonstrate not only measurement abilities,
but also CEM reliability, maintenance and operational
requirements, and long term performance. Performance
data will be a focus of future EPA Hg CEM field testing.
SUMMARY
Currently, at least 10 Hg CEM vendors exist. Half of them
offer speciating versions. Total Hg CEMs appear to be a
more mature technology than has been widely perceived in
the past. The units are becoming simpler to operate and
maintain. The techniques employed to reduce oxidized
species to the detectable elemental form are less reliant on
wet chemical approaches. In addition, techniques for
managing potential interferants are also more advanced.
Moreover, several Hg CEM vendors have developed
QA/QC capabilities to perform their own instrument
calibration drift and system bias checks from internal
elemental Hg gas sources. These capabilities are needed
for routine daily operational performance verification.
Hg CEMs for both total and speciated Hg measurements
are now becoming an integral component of EPA's and
DOE's Hg combustion research programs. It is through
these research programs that the techniques and tools
necessary for evaluating measurement performance have
been improved. The development of gas standards for
elemental Hg and HgCI, are significant advancements.
These improvements may be valuable inputs to any EPA
efforts to revise PS 12 and develop QA/QC requirements
for Hg CEM operation for compliance assurance purposes.
In order for Hg CEMs to be considered for compliance
assurance purposes, acceptable performance will need to be
demonstrated. This is complicated because of the diversity
and complexity of measurement environments resulting
from multiple combustion sources and variation among
pollution control device configurations. As a result, a need
to demonstrate measurement performance under multiple
conditions exists. This need has contributed to the lack of

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performance demonstration opportunities. However,	9)
demonstrating performance under realistic extremes could
reduce this need. For those sources with pollution control
device configurations consistent with European
configurations, acceptable measurement performance
should not be an issue,
REFERENCES	10)
1)	Parker, Barrett, "European Mercury Monitoring
Trip Report," U.S. Environmental Protection
Agency, Office of Air Quality Planning and
Standards, Emission Measurement Center,
Research Triangle Park, NC, October 10,2000
2)	Hedges, S., J. Ryan, and R. Stevens, "Workshop
on Source Emission and Ambient Air Monitoring
of Mercury," September 13-14, 1999,
Bloomington, MN, U.S. Environmental
Protection Agency, National Risk Management
Research Laboratory, Cincinnati, OH, June 2000,
EPA/625/R-00/002
3)	Laudal, Dennis L., et al., "Testing of a Mercury
Continuous Emission Monitor at Three Coal-
Fired Electric Utilities," 92"i Annual Meeting
and Exposition of the Air and Waste
Management Association, St. Louis, MO, June
1999
4)	Electric Power Research Institute, "Evaluation of
Flue Gas Mercury Speciation Methods," Final
Report TR-108988, Palo Alto, CA, December
1997
5)	Linak, William P., et al., "Issues Related to
Solution Chemistry in Mercury Sampling
Impingers," Journal of the Air and Waste
Management Association, in press
6)	U. S. Environmental Protection Agency, "Draft
Mercury Continuous Emissions Monitor System
Demonstration, Volume I: Holnam, Inc.,
Hazardous Waste Burning Kiln, Holly Hill, SC,"
Office of Solid Waste and Emergency Response,
Washington, DC, March 1998
7)	U. S. Environmental Protection Agency, "Draft
Performance Specification 12 - Specifications
and Test Procedures for Total Mercury
Continuous Monitoring Systems in Stationary
Sources," Office of Air Quality Planning and
Standards, Emission Measurement Center,
Research Triangle Park, NC,
http://www.epa.gov/ttn/emc/propperf7ps-12.pdf
8)	French, N., S. Priebe, and W. Haas, Jr., "State-of-
thc-Art Mercury CEMS," Analytical Chemistry
News & Features, July 1, 1999
Gibson, L. V., et al., "Field Evaluation of a Total
Mercury Continuous Emission at a U. S.
Department of Energy Mixed Waste Incinerator,"
92°d Annual Meeting and Exposition of the Air
and Waste Management Association, St. Louis,
MO, June 1999
Baker, Ronald L. "Are We Ready for Meeting
Continuous Emission Monitoring Requirements
for Total Mercury Combustion Sources?" 93rd
Annual Meeting and Exposition of the Air and
Waste Management Association, Salt Lake City,
UT, June 2000

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MDmdi T-.-T- r-, „ KQ1 TECHNICAL REPORT DATA
IN ruVlxtJ-'"" X\1 Jr ir~ DoJ. (Please read Instructions on the reverse before completing)
1. REPORT NO. 2.
EPA/600/A-01/036
3. RECIPIENT'S ACCESSION NO.
4. title and subtitle
Mercury CEMs: Technology Update
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHORIS)
Jeffrey V. Ryan and James D. Kilgroe
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
See Block 12
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
NA (lnhouse)
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 ANO PERIOD COVEREO
Published paper; 2/96-1/01
14. SPONSORING AGENCY CODE
EPA/600/13
t5. supplementary notes ^PPCD project officer is Jeffrey V. Ryan, Mail Drop 04, 919/
541-1437. Presented at Ninth Annual North American Conference, Miami, FL,
5/6-9/01.
i6. abstract pap6r reviews the technologies involved with continuous emission
monitors (CEMs) for mercury (Hg) which are receiving increased attention and fo-
cus. Their potential use as a compliance assurance tool is of particular interest.
While Hg CEMs are currently used in Europe for compliance purposes, use of Hg
CEMs in the United States (U.S.) has focused on combustion research and Hg con-
trol technology evaluation applications. Hg CEMs are now receiving increased at-
tention as compliance assurance tools. Several programs exist to evaluate Hg CEM
measurement performance. It is through these efforts that application-specific
measurement issues are investigated. Collectively, these efforts have served to
advance the state-of-the-art of the technology as evidenced by the number and
types of CEMs now available and the various applications in use.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b. IDENTI F1ERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution
Mercury (Metal)
Emission
Monitors
Measurement
Combustion
Pollution Control
Stationary Sources
Compliance Assurance
13	B
07B
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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