Continuous Emission Monitors for Mercury Measurement

Automated on-line mercury (Hg) analyzers are being developed to measure levels of Hg in flue gas streams derived from combustion devices. The analyzers are based on well-established techniques, including cold-vapor atomic absorption spectroscopy (CVAAS), cold-vapor atomic fluorescence spectroscopy (CVAFS), and atomic emission spectroscopy (AES), as well as on the emerging technology of chemical microsensors. The analyzers can be used to directly measure Hg⁰ in fossil fuel combustion flue gas on a continuous or semicontinuous basis. The analyzers can also be equipped with converters for reducing Hg²⁺ forms to Hg⁰ for determining total Hg, and the Hg²⁺ concentration can be determined by difference. Although costly to purchase, install, and maintain, on-line continuous emission analyzers offer several advantages:

• An analyzer can be used for feedback process control of Hg control systems, thus maximizing removal efficiency.

• A properly designed analyzer requires minimal operator input.

• An analyzer can provide information on the temporal variations of Hg emissions for a process that may be variable in its emission characteristics.

On-line Hg emission analyzers can be categorized as either extractive or in situ. Extractive analyzers are usually located remote to the sample extraction point; therefore, a flue gas sample is removed, transported, and conditioned before it actually enters the Hg analyzer. In situ Hg emission analyzers are mounted on the stack or duct and do not require sample transport or gas conditioning. All on-line analyzers use elaborate calibration systems.

Several on-line Hg analyzers have recently been developed primarily for measuring total Hg emissions from waste incinerators. Application of these analyzers to coal combustion flue gas is difficult because Hg emissions from coal combustion are much lower than those from waste incinerators, and the presence of acid gases and other flue gas components causes interference with the Hg measurement techniques.

During the past 4 years, side-by-side evaluations have been done using several analyzers. Two of these tests were sponsored by the U.S. Environmental Protection Agency (EPA) [Richards et al. "Performance Tests of Mercury Continuous Emissions Monitors at the U.S. EPA Incineration Research Facility," June 1998; Burns et al. "Joint EPA/DOE Demonstration Program for Total Mercury Continuous Emissions Monitors," June 1997]. One test was conducted at an incinerator in Arkansas, the other at a cement kiln in South Carolina. The third side-by-side comparison was done at the coal-fired pilot plant at the Energy & Environmental Research Center (EERC) and was sponsored by EPRI and the U.S. Department of Energy (DOE) [EPRI. "A State-of-the-Art Review of Flue Gas Mercury Speciation Methods," Nov. 1996; EPRI. "Evaluation of Flue Gas Mercury Speciation Methods," Dec. 1997]. The instruments used for each test are shown in the table. The results from the two EPA tests were not very successful in that the operational problems occurred for all the instruments, and they were not able to meet the relative accuracy criteria of 20% based on wet-chemistry measurements. Operation problems involved plugging of the instruments or corrosion due to high levels of acid gases.

*Company is no longer in operation.

The tests conducted at the EERC were not meant to be a formal evaluation of Hg continuous emission monitors (CEMs). The CEMs were used primarily to determine the variability in Hg concentration with time. The CEMs were used in conjunction with several wet-chemistry methods that were undergoing evaluation as to their ability to speciate Hg. The instruments were fairly successful in measuring total Hg, compared to the wet-chemistry methods. The figures on the following page show the results of the tests. As can be seen, typically the CEMs were about ±20% of the wet-chemistry methods.

Based on the results of limited pilot-scale testing of Hg CEMs at the EERC, the following conclusions can be drawn:

• The Perkin-Elmer MERCEM, PS Analytical Sir Galahad, and Semtech Hg 2000 Hg analyzers have the capability of measuring total vapor-phase Hg in coal combustion flue gas within ±20% of the wet-chemistry methods.

• A pretreatment cell must be used to remove acid gases (HCl and SO₂) from the flue gas prior to measuring the Hg concentration with the PS Analytical and Semtech Hg analyzers for coals with high chlorine and sulfur contents.

Recommended further research to fully develop Hg CEMs for utility flue gas applications includes the following:

• Although several analyzers have real potential for measuring total Hg, additional work is necessary to develop a robust conversion system that will allow CEMs to reliably speciate Hg in combustion flue gas.

• The analyzers must be rigorously tested in pilot-scale and longer-term field tests to resolve questions regarding accuracy, precision, reliability, and maintainability before Hg CEMs can be used routinely for Hg measurement in utility flue gas applications.

For more information, please contact Dennis Laudal, EERC Research Manager, at (701) 777-5138 or at dlaudal@eerc.und.nodak.edu.

CATM DIRECTOR'S MESSAGE Steven A. Benson, Ph.D.

Mercury: Fish Consumption

Mercury, specifically methylmercury, has been identified as a neurotoxin and has the highest potential health effects on the developing fetus and children. Fish consumption is the dominant pathway for human and wildlife exposure to Hg. Concern over potential human health risks associated with chemically contaminated fish and shellfish has led many states to issue consumption advisories and bans in an effort to limit exposures to certain organic compounds and metals that may become concentrated in the tissues of fish. The consumption of fish is highly variable across the U.S. population. The inclusion of fish in diets varies with geographic location, seasons of the year, ethnicity, and personal food preferences.

In December 1997, the Mercury Study Report to Congress provided an assessment of the magnitude of U.S. Hg emissions by source, the health and environmental implications of those emissions, and the availability and cost of control technologies. The report did not quantify the risk from Hg exposure because of uncertainties in a number of areas, with one being actual human fish consumption patterns. Volume IV of the Mercury Study Report to Congress provides a compilation of information regarding fish consumption among the general U.S. population. The results show a wide variation in fish consumption rates depending upon region.

Jacobs and others (Risk Analysis 1998, 18 (3), 283-291) in a recent analysis of the U.S. Department of Agriculture (USDA) Continuing Survey of Food Intake by Individuals (CSFII) estimated food consumption rates for three fish habitats: freshwater/estuarine fish, marine fish, and all fish. The estimated fish consumption rates for all fish for the U.S. population was 15.65 grams/person/day (gpd), with 4.71 gpd from freshwater/estuarine sources and 10.94 gpd from marine sources. The average consumption rate for women aged 18-44 years from all sources was found to be 14.25 gpd.

Ebert and others (North American Journal of Fisheries Management 1993, 13, 737-745) reviewed fish consumption surveys and found the mean rates of fish consumption ranged from 2 to 31 gpd on the basis of surveys of anglers from selected states and river systems and the general U.S. population. Ebert and others surveyed 2500 licensed resident anglers in Maine. That survey indicated an annual average consumption rate of freshwater river fish of 3.7 gpd. Another recent study by Sorenson and others (Environ. Sci. Technol. 1990, 24, 1716-1727) of 1000 randomly selected New Jersey residents found the consumption rates (mean) for all adults to be 50.2 gpd and for women aged 18-40 years to be 41 gpd.

Rates of fish consumption are highly variable depending upon the region of the U.S. survey. Additionally, the variation among selected populations has also been shown to have high variability in fish consumption. Therefore, fish consumption surveys must be conducted that consider specific regions and populations in order to establish water quality regulations and impact of fish consumption on human health.

For more information, contact Steve Benson, CATM Director, at (701) 777-5177 or at sbenson@eerc.und.nodak.edu.

TECHNICAL FOCUS

Speciating Mercury in Combustion Flue Gas Using Cryogenic Trapping

In recent years, research efforts directed at Hg from combustion sources has determined that elemental and oxidized forms of Hg (primarily mercuric chloride [HgCl₂]) behave very differently within combustion and emission control systems. Elemental Hg (Hg⁰) is volatile, relatively inert, and virtually insoluble. As a result, it passes through conventional scrubbers and particulate control devices, contributing to the global Hg emissions inventory. On the other hand, the oxidized forms of Hg are soluble and can be collected in existing scrubbers. Theoretically speaking, most of the Hg in flue gas should be in the HgCl₂ form. However, because of kinetic limitations, this is not what has been observed. Results of previous research indicate that the partitioning of Hg in combustion systems between oxidized and elemental forms is a function of several parameters including feed coal, type of combustion system, temperatures, and other flue gas constituents (NO_x, SO_x, O₂, CO₂, HCl, etc.). A major limitation to date has been the lack of reliable sampling and analytical techniques that can be used to differentiate between the different possible Hg species.

To address this need, research at the EERC has been aimed at developing a technique for the speciation of HgCl₂ from combustion flue gases. The technique being explored is based on the thermodynamic differences between combustion flue gases, Hg⁰, and HgCl₂. Both Hg⁰ and HgCl₂ are stable at flue gas temperatures and volatile at higher temperatures. Taking advantage of this and the fact that the volatility is a function of temperature, a unique cryogenic trap was designed to sample flue gases for Hg compounds.

Since the volatility is temperature-sensitive, the cryogenic trap was designed with ease of temperature control. Cooling is accomplished with a liquid nitrogen source and a cryogenic valve, while heating is accomplished with an insulated heater element. This flexible design allows a range of adsorption (sampling) and desorption temperatures to be tested. The figure below shows a schematic of the sampling system.

The sampling system was constructed and connected to a bench-scale flue gas simulator complete with Hg⁰ and HgCl₂ vapor generators. The detection system used for testing included a thermal conversion unit for the conversion of HgCl₂ to Hg⁰, a moisture trap, and an atomic fluorescence detector. The basic operation of the system is as follows:

• Cool the trap to the desired temperature.

• Switch the valve to direct the sample through the trap.

• Wait a specified amount of time (typically 1 to 5 minutes).

• Switch the valve to direct the argon gas flow through the trap to the detection system.

• Switch the system from cooling to heating.

Ideally, the system is designed to sample Hg species from flue gases without trapping the flue gases themselves. Additionally, in order to produce a reliable system, the trap must not collect these combustion flue gases since they interfere with the detection of Hg by atomic fluorescence, by quenching fluorescence.

With the use of the bench-scale flue gas simulator, the thermal profiles of Hg⁰, HgCl₂, and combustion flue gases were determined for the sampling system. The combustion flue gases included in the study were oxygen, carbon dioxide, sulfur dioxide, nitrous oxide, nitric oxide, hydrochloric acid, and water vapor. Initial tests were carried out with Hg⁰ to optimize flows for the system. Following this, tests were carried out to determine the collection efficiency of the trap for each of the combustion flue gases. Collection temperatures used for the combustion flue gases (excluding water vapor) were -150, -100, and -50C. To detect these species, a novel detector was constructed by introducing a constant source of Hg into the detector along with the desorption stream. Consequently, these compounds could be detected by quenching the Hg fluorescence signal. The result of these tests is that none of the flue gases are trapped at -50C.

In a similar set of tests, using direct detection, the thermal profiles for Hg⁰ and HgCl₂ were also determined. A more rigorous set of temperatures was used for these tests which showed that Hg⁰ was efficiently trapped at -125C and was not trapped at -75C. The tests with HgCl₂ showed that the trap efficiently collected HgCl₂ at -25C as shown by the thermal profiles in the figure below.

Following the bench-scale tests, the system was moved to the pilot plant for testing. The cryogenic sampling unit was operated during combustion of two different lignite coals as well as natural gas. The results of the pilot-scale testing showed that cryogenic trapping is an efficient method for sampling HgCl₂ from flue gas stack streams.

Future plans include research on other oxidized forms of Hg, the validation of the method, and the development of high-temperature particulate removal methods for sampling prior to the stack. As this phase begins, the EERC is seeking industrial partners for the commercialization of the method.

For more information, please contact Jeff Thompson, EERC Research Associate, at (701) 777-5245 or at jthompson@eerc.und.nodak.edu; or John Pavlish, CATM Associate Director, at (701) 777-5268 or at jpavlish@eerc.und.nodak.edu.

Questions About Mercury

Questions are often asked about Hg from combustion devices and how it affects the general population. The following article addresses both the certainties and uncertainties surrounding some of those questions.

Is Hg emission from combustion devices considered to be an air toxic?

Hg is an air toxic based on the list of 188 compounds that EPA has deemed to be air toxics; however, data suggest that breathing Hg in the air has no health effects on humans or wildlife. Hg is mainly a water issue: air is simply the vehicle by which Hg is transported to the water.

Does coal contain large amounts of Hg?

The answer is clearly no. The concentration of Hg in the flue gas from coal-fired power plants is very low. By comparison, the concentration to which municipal solid waste incinerators are required by EPA to reduce their Hg emissions (80 g/Nm³) is an order of magnitude greater than the concentration typically contained in flue gases produced by coal-fired power plants (8 g/Nm³).

Is all Hg the same?

Again, clearly the answer is no. Hg is often identified as either reactive Hg (Hg that has been oxidized) or elemental Hg (Hg⁰). When Hg⁰ is emitted from a source, it does not generally deposit locally or regionally, but becomes part of the global Hg burden. In fact, Hg can stay in the atmosphere for up to 3 years and reacts very slowly, if at all. Reactive Hg, on the other hand, is water-soluble and will deposit fairly quickly. It can also react with organic compounds in the water and soil to form methylmercury compounds that are toxic and bioaccumulate in fish.

What type of Hg is emitted by combustion devices?

This varies greatly across the country. Some coals, when burned in a power plant, generate Hg emissions that are nearly 100% Hg⁰, while others generate nearly 100% oxidized Hg. Although there are a number of exceptions, western coals appear to generate a higher percentage of Hg⁰ than do eastern coals. By comparison, waste-to-energy plants generally have a much higher proportion of oxidized Hg.

Can Hg from combustion devices be reduced in a cost-effective manner?

Most emission controls considered today are generally below $1000/lb-pollutant removed. However, because of technology limitations, the cost of Hg control for coal-fired power plants is significantly higher, $20,000-$50,000/lb-Hg removed. Simply switching to natural gas is not the answer. Coal is a huge source of energy in this country. Hg emissions could be controlled by implementing radical changes in power plant operation: by reducing the temperature at air pollution control devices, small amounts of carbon may reduce the amount of Hg emitted. However, in addition to the cost of the carbon, there would also be a great reduction in the overall efficiency of the power plant. The recently issued EPA Mercury Reports to Congress stated admittedly that there are no cost-effective control strategies for coal-fired combustion systems. The primary problem is, ironically, that the Hg concentration in the gas being emitted from coal-fired power plants is so low that it is extremely difficult to reduce.

Is all of the Hg in the fuel emitted to the atmosphere when it is combusted in a plant?

Not always. It depends on the fuel being burned and on the type of pollution equipment installed. Some fuels produce a fly ash that can sorb much of the Hg and thus be collected in an ESP or baghouse. Additionally, the oxidized form of Hg is efficiently captured in wet scrubbers; therefore, if a plant has a wet scrubber to remove SO₂, it will also remove most of the oxidized Hg that is generated.

Does chronic exposure to low levels of Hg have adverse health effects?

Very high levels of Hg, such as those which occurred in the Minamata Bay incident in Japan, can have very severe health effects. But the question of what health effects, if any, can be attributed to the very low levels of Hg typically found in the environment is much more difficult to answer. Several major studies have been and are currently being conducted on the effects of Hg on populations that consume large amounts of fish. Because methylmercury compounds bioaccumulate in fish tissue, eating fish is the major source of Hg for humans. Many of these studies have come to different conclusions. In the Mercury Reports to Congress, EPA did conclude that Hg in the environment is a problem that needs to be addressed.

A number of lakes have Hg warnings; would you eat fish from these lakes?

Sensitive populations such as women of child-bearing age and children need to take precautions. The "reference maximum dosage" proposed in the December 1997 EPA Mercury Report to Congress is 0.1 g/kg body weight/day, which is five times lower than the dosage established by the Food and Drug Administration (FDA). When fish are eaten from lakes with posted warnings, consideration should be given to the level of Hg in the fish, the quantity of fish consumed, and the persons body weight. This information can be used to ensure that the Hg dosage does not exceed the reference maximum dosage using either the FDA or the proposed EPA criteria.

For more information, please contact Dennis Laudal, EERC Research Manager, at (701) 777-5138 or at dlaudal@eerc.und.nodak.edu; John Pavlish, CATM Associate Director, at (701) 777-5268 or at jpavlish@ eerc.und.nodak.edu; or Steve Benson, CATM Director, at (701) 777-517or at sbenson@eerc.und.nodak.edu.

Air Quality Experts to Meet for International Conference

Experts from industry, government, environmental groups, and the research community will meet in December for a four-day international conference to discuss scientific issues that will directly affect state and national regulatory policies.

Entitled "Air Quality: Mercury, Trace Elements, and Particulate Matter," the conference is organized and sponsored by the EERC, EPA through the EERCs CATM program, and the U.S. DOEs Federal Energy Technology Center (FETC). The event will take place Dec. 1-4, 1998, at the McLean Hilton at Tysons Corner in McLean, Va., northwest of Washington, D.C.

This conference will provide up-to-date information on how human health and ecosystems can be impacted by hazardous air pollutants (HAPs) and fine airborne particles. Key issues covered will include potential health risks, air pollution control technologies, research needs, and regulatory policies. The topics will be presented and discussed in forums with representatives from state and federal agencies, industry, environmental groups, and the research community.

"There are important scientific questions related to the sources of these pollutants, their form and how they behave in the environment, our ability to accurately measure them, and the development of technologies to prevent and control them," says Steve Benson, CATM director. "The answers to these questions will directly affect state and national regulatory policies."

EPA has identified 188 chemicals, chemical compounds, fibers and types of emissions as HAPs that either cause or are suspected of causing cancer, birth defects, and other health problems, as well as adverse environmental impacts. HAPs include forms of mercury (Hg), arsenic, lead, selenium, cyanide, nickel, and chromium. While some HAPs occur naturally in the environment, most come from vehicle exhaust and emissions from factories, refineries, waste incinerators and power plants.

Hg--which is toxic to humans, other mammals, and birds--has been singled out for special study by EPA. Hg can remain in the atmosphere up to a year, be transported thousands of miles from its source, and does not degrade in the environment. It is widely accepted that exposure to Hg is harmful to the nervous system, although questions remain about the amount of Hg and duration of time required to cause negative health effects.

When elemental Hg naturally transforms to methylmercury, it can accumulate in fish populations, leading to high Hg concentrations that make eating the fish hazardous. High levels of Hg affect the human nervous system, kidneys, and developing fetuses. It is estimated that 75% of Hg in the atmosphere is the result of air pollution, with 90% of that coming from combustion sources, such as the burning of waste and fossil fuels.

Another pollutant affecting air quality is known as particulate matter, a mixture of solid and liquid particles suspended in the air. These particles degrade visibility and cause respiratory problems in humans. Much of the particulate matter in the atmosphere is produced by human activities, such as energy use, agriculture, wood stoves, and industrial processes. However, it also comes from natural sources, including sea spray, wind-blown dust, volcanic eruptions, and forest fires.

Although individual particles are often invisible to the human eye, when present in high concentrations, they become visible as haze or smog and are a major source of visibility reduction in many regions of the United States. EPA is especially concerned about fine particles known as PM_2.5 (microscopic particles that are 2.5 m or smaller). Because they can be inhaled deeply into the lungs, PM_2.5 can cause respiratory illnesses that lead to premature death. Children, the elderly, asthmatics, and people with heart and lung disease are especially susceptible to illnesses caused by fine-particle pollution.

Currently, EPA and numerous state regulatory agencies are either considering, have proposed, or have implemented regulations intended to reduce HAPs that include Hg, trace elements, and particulate matter. Last year, EPA issued new air quality standards for ozone and particulate matter, which it said would reduce regional haze and provide increased public health protection from these pollutants. In August, the Air Resources Board of the California EPA identified particulate emissions from diesel engines as a toxic air contaminant, citing them as a significant contributor to photochemical smog and fine-particle pollution.

EPAs proposed revisions to national air quality standards for PM_2.5 could cost industry and agriculture billions of dollars, cause job loss, and put many heavily populated areas out of compliance. Environmental and health organizations say that without more stringent standards, thousands will continue to die prematurely each year, and visibility will be affected in once-pristine areas.

Under the 1990 Clean Air Act Amendments, Congress authorized EPA to study HAPs and Hg emissions from electric utility power plants. Based on these recently completed studies, EPA is assessing the impact on the environment and public health in an effort to determine whether the pollutants should be regulated.

On-line registration materials can be obtained from the EERCs Web site at http://www.eerc.und.nodak.edu/regist.html. Requests for information and completed registration forms can be faxed or e-mailed to Anne Fiala, EERC administrative manager, at (701) 777-5181 or at afiala@eerc.und.nodak.edu.

This article was contributed by Pat Miller, EERC Communications Coordinator.

University of North Dakota
PO Box 9018
Grand Forks, ND, USA 58202-9018
Phone: (701) 777-5000 Fax: (701) 777-5181
e-mail: sbenson@eerc.und.nodak.edu
World Wide Web Server Address:http://www.eerc.und.nodak.edu

This newsletter is funded wholly or in part by the U.S. Environmental Protection Agency

through its Office of Research and Development under assistance agreements CR821518,

CR823173, and R824854. It has not been subjected to EPA review and therefore does not

necessarily reflect the views of the EPA, and no official endorsement should be inferred.

Phone: (701) 777-5161 Fax: (701) 777-5181

E-mail: cwixo@eerc.und.nodak.edu

We welcome your comments.

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