MERCURY CEMs: 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 are 

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 are 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.’ The effects of potential 

interferants such as carbon dioxide (CO,), carbon 

monoxide (CO), nitrogen oxides (NO,), water vapor, sulfur 

dioxide (SO,), ammonia (NH,), hydrochloric acid (HCI), 

chlorine (Cl,), 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 diflicult 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).~ 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.’ 
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,.’ 

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 particulateemitting 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 (HgCl,) . HgCI, 

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,, SO,, HCl, and Cl, 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. SO2 is a major 

spectral interferant with most CVAA detection systems. 

The effects of SO, 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 Sol-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, HCl 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. 
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 HgCl, and that 

HgCl, is the dominant form of oxidized Hg present, the 

“speciation by difference” technique is considered an 

acceptable approach to obtaining speciated Hg 

measurements? 

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 overreporting 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.” 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.’ 

Another potentially significant source of speciated Hg 

measurement bias takes place in the liquid phase. In 

combustion flue gas environments where Cl, is present, 

under certain conditions the Cl, may react in the liquid 

phase to oxidize elemental Hg.’ 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 

r 

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. 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 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,’ 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.6 Measurement performance 

was evaluated following Draft Performance Specification 

I2 (PS 12) entitled “Specifications and Test Procedures for 

Total Mercury Continuous Monitoring Systems in 

Stationary Sources. ” 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.‘*” 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 
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. 

As a result, quantitative use of the standard is limited. 

Similarly, acceptance of a HgCI, 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 

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 speciating, 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 HgCI, gas 

standards, and advancements in Hg CEM technologies 

provide evidence that valid techniques suitable for 

i 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. 

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 
performance demonstration opportunities. However, 

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. 

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Mercury Continuous Emission at a U. S. 

Department of Energy Mixed Waste Incinerator,” 

92” Annual Meeting and Exposition of the Air 

and Waste Management Association, St. Louis, 

MO, June 1999 

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News & Features, July I, 1999