Presented at the 17th Annual Waste Testing and Quality Assurance Symposium
August IS, 2001, Arlington, Virginia
Development and Evaluation of Mercury CEMS for Combustion Emissions Monitoring
Jeffrey V. Ryan
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 monitoring systems (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 the
fate and transport of Hg, and ultimately the 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
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-	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 arc currently used in Europe for compliance purposes, primarily in Germany. Hg
CEMS are installed at over 100 facilities, including fossil-fuel boilers and municipal waste
combustors.1 The types of pollution control devices associated with combustors have a
significant impact on Hg CEM measurement ability. A representative 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." The effects of potential
intcrferants such as carbon dioxide (C02), carbon monoxide (CO), nitrogen oxides (NOx), water
vapor, sulfur dioxide (S02), 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 U.S. 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. Further, there is
little incentive to generate the data necessary to support regulatory consideration. 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
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.
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Typically, Hg CEMS directly measure (detect) only elemental Hg. Hg OEMS measure total Hg
through the use of a conversion system that 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 significant 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). HgCI2 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 IIg 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 NOx, S02, HC1, and Cl2 can act as measurement
interferants as well as degrade the performance of concentrating devices (e.g., gold amalgams).
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As such, conditioning systems and/or techniques that remove or negate the effects of these
interfering gases are required prior to sample delivery to the detector. 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
S02-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 NOx in combination
can poison the gold surface, affecting Hg capture. 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. While no commercially
available CEMS measure the various speciated forms of Hg directly, several commercially
available total gaseous Hg CEMS have been enhanced to measure speciated Hg (the elemental
and oxidized forms) indirectly by determining the difference between elemental Hg and TGM.
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 HgCl2 and that HgCl2 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 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 is the reactivity of certain types of particulate matter (PM). PM may
possess catalytic properties that, at the conditions of Hg CEM PM filtering environments,
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elemental Hg can be oxidized across the PM surface.2-'"' 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-particulate 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.5
Hg CRM 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 Hg emissions
from coal-fired utilities, including the evaluation of viable Hg control techniques.3-4 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 the 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 control studies. This research has investigated the
quality of speciated Hg measurements including liquid-phase oxidation of Hg,5 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's OSW, evaluated the performance of
three total Hg CEMS at a cement kiln that also burned hazardous waste.6 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.,fl 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 HgCl2 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.6 In retrospect, the harshness of the kiln's emission environment
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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.9,10 Measurement performance was also evaluated following PS 12. These tests
demonstrated not only Hg CEM performance, but also that additional elements of the 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 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 the use of Hg CEMS for a variety of monitoring purposes is increasing. This includes
interest from the regulatory perspective at the federal, state, and regional levels. Of course, in
order to be considered for regulatory applications, acceptable performance must be demonstrated
first. The generation of Hg CEMS measurement performance data has been hindered by the lack
of accepted test procedures to accomplish this objective. PS 12, proposed by OSW to support
the collection of Hg CEMS performance data from HWCs for regulatory monitoring
consideration, remains in draft form. Application of PS 12 to multiple source categories was not
intended. The particular demands placed on Hg CEMS by the emissions characteristics of each
Hg emissions source category or subcategory must be considered. Furthermore, successful
demonstration of one particular category, does not convey necessarily to other categories. As a
result, a variety of measurement performance data representing these source categories are
needed.
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The Office of Air Quality Planning and Standards (OAQPS) Emission Measurement Center
(EMC) has recently initiated a study to determine the measurement performance of two
commercially available Hg CEMS at a coal-fired utility, which represents a source category
currently under regulatory consideration. Measures of performance will be recorded to
determine potential monitoring applications based on measurement performance achieved. Data
from this study, and future studies of Hg CEMS measurement performance at additional source
categories, should aid in the future crafting of a performance specification for application of Hg
CEMS at a variety of source categories. In the interim, OAQPS may consider publishing a
source-catcgory-specific performance specification.
In addition, the ETV pilot-scale tests, the availability of elemental Hg and HgCl2 gas standards,
and advancements in Hg CEMS technologies provide evidence that techniques and tools suitable
for assessing Hg CEMS measurement performance arc now available. However, further
demonstration of these new tools is necessary for them to be 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 HgCl, standard is valuable: this standard is used to assess Hg
conversion system effectiveness as well as overall 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 CEMS 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 CEMS reliability, maintenance and operational requirements, and long-term
performance. These performance elements will be the focus of future EPA Hg CEMS field
testing.
SUMMARY
Currently, at least 10 Hg CEMS 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 CEMS 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.
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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 HgCl2 arc significant
advancements. These improvements should be valuable inputs to any EPA efforts to revise PS
12 or develop other Hg CEMS performance specifications and to develop QA/QC requirements
for Hg CEMS operation for compliance assurance purposes.
In order for Hg CEMS to be considered for compliance assurance purposes, acceptable
performance will need to be demonstrated. However, opportunities for demonstrating
measurement performance to provide the necessary data have been limited and little incentive
exists to support such activities. Also, the procedures necessary for evaluating measurement
performance are viewed as less than adequate. As a result, the developmental progress of Hg
CEMS is hindered.
REFERENCES
1)	Parker, B., "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 (NTIS PB2001-100963)
3)	Laudal, D. L., T. D. Brown, and P. Chu, "Testing of a Mercury Continuous Emission
Monitor at Three Coal-Fired Electric Utilities," 92nd 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, W. P., J. V. Ryan, B.S. Ghorishi, and J. O L. Wendt, "Issues Related to Solution
Chemistry in Mercury Sampling Impingers," Journal of the Air and Waste Management
Association, 2001, 51:688-698
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
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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/proppcrf/ps-12.pdf (accessed July 2001)
8)	French, N,, S. Priebe, and W. Haas, Jr., "Statc-of-the-Art Mercury CEMS," Analytical
Chemistry News & Features, July 1, 1999, 470-475A
9)	Gibson, L. V., J. E. Dunn, R. L. Baker, W. Sigl, and I. Skegg, "Field Evaluation of a
Total Mercury Continuous Emission Monitor at a U. S. Department of Energy Mixed
Waste Incinerator," 92nd Annual Meeting and Exposition of the Air and Waste
Management Association, St. Louis, MO. June 1999
10)	Baker, Ronald L. "Arc 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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N RM m - RTP- P- TECHNICAL REPORT DATA
A^xuvirw-. nir r (Please read Instructions on the reverse before completing)
1. REPORT NO , , 2.
EPA/600/A-01/07 9
3. RECIPIENTS ACCESSION NO.
4. TITLE AND SUBTITLE
Development and Evaluation of Mercury CEMs for
Combustion Emissions Monitoring
5, REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHORS
Jeffrey V. Ryan
8. PERFORMING ORGANIZATION REPORT NO
9 PtftFORMING ORGANIZATION NAME AND ADDRESS
See Block 12
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
NA (Inhouse)
12 SPONSORING AGENCY NAME AND ADDRESS
U. S. EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
Published paper;
14. SPONSORING AGENCY CODE
EPA/600/13
is supplementary notes j^ppcD project officer is Jeffrey V. Ryan, Mail Drop 4, 919/541-
1437. For presentation at 17th Annual Waste Testing and Quality Assurance Sympo-
sium, Arlington. VA, August 12, 2001.
16 abstract The paper discusses the increased attention and focus being received by con-
tinuous emission monitors (CEMs) for mercury (Hg). Their potential use as a com-
pliance 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 advanced 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. IDENTIFIERS/OPEN ENDED TERMS
c COSATI Field/Group
Pollution
Mercury (Metal)
Emission
Monitors
Evaluation
Combustion
Pollution Control
Stationary Sources
13 B
07B
14G
21B
18 DISTRIBUTION STATEMENT
19 SECURITY CLASS (This Report)
21. NO. OF PAGES
8
20. SECURITY CLASS (This Page)
22 PRICE
EPA Form 2220-1 (Rev 4-77) PREVIOUS EDITION IS OBSOLETE	foğms/admW!aclwpt.tmi 7/8/99 pad

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