Product Review
State-of-the-Art
Mercury CEMs ercury is one of the most studied pollutants because of its toxicity and multiple exposure pathways. Of the various mercury species, methyl mercury is the most toxic. It bioaccumulates in fish and other foods; humans and animals are exposed primarily through fish consumption. Mercury is both difficult :o control and challenging to measure. It is highly volatile, soluble in water in its oxidized mercury form, and able to adsorb onto surfaces. In their 1995 report to the U.S. Congress (1), the U.S. Environmental Protecion Agency (EPA) estimated that 240 tons per year of mercury were emitted into the atmosphere from 1990 to 1993. Combusion sources were the largest contributors, especially medical and municipal waste ncine Mercury emissions from hazardous waste incinerators (HWIs) are regulated :>y EPA; the current approach for limiting hose emissions uses mercury feed rate imitation and control of facility operating conditions. Incinerator operators are required to measure and limit the rate of mercury feed and to operate the facility within a range of conditions known to adequately limit mercury in the stack-gas emissions. The range of the waste feed 'ate and the other facility operating condiions under which an incinerator can peroral with acceptably low mercury emissions is established during an EPA trial comprehensive test During those ests which tvoically last one or two
M
Nina Bergan French
New technologies for continuous monitoring of mercury emissions from hazardous and mixed-waste incineration.
Sky+ Ltd.
Stephen J . Priebe Idaho National Engineering gnd Environmental Laboratory
William J . H a a s , Jr. Ames Laboratory, Iowa State University
470 A Analytical Chemistry News & Features, July 1, 1999
Table 1 : Summary of commercially available mercury CEMs. Product
Hg-Mat 2
MERCEM
HM-1400
Manufacturer
Seefelder Messtechnik (Germany)
Perkin-Elmer
Verewa (owned by Durag) Semtech (Sweden)
Opsis (Sweden)
U.S. or European distributor
EcoChem Analytics 22605 Valerio St. West Hills, CA 91307 818-347-4369
Aldora Technologies League City, TX 281-338-9888
Durag 1970 Christensen Ave. West St. Paul, MN 55118 651-451-1710
Opsis 1165 Linda Vista Dr. Ste. 112 San Marcos, CA 92033 760-752-3005
URL/e-mail
www.ecochem-analytics, com/
www.aldora-cems. com/
www.durag.com/
Approximate cost (U.S. dollars)
$60-80,000
>$80,000 with probe
$70,000
$45-50,000
$90-140,000
Technology
CVAAS; chemical conver- CVAAS; gold-trap presion using ascorbic acid concentration; wet conversion using SnCI2
CVAAS; converts Hg to HgCI2; measures Hg in one beam, uses activated carbon to remove Hg, and measures background in second beam
Differential CVAAS; thermal cracking to convert oxidized Hg to elemental Hg; Zeeman background correction
In situ DOAS; Xe lamp
Lab/field performance data
TUV certified; MDL of 10-15 ug/dscm
TUV certified; MDL of 100 ppm can be a problem
Special features
Auto calibration of zero point; Hg gas generator for calibration
Avoids S 0 2 interference; high sample flow rates minimize HgCI2 memory effects
Thermally desorbs particle-bound Hg; Hg gas generator for calibration
Sensitivity to 0.3 ug/ dscm; automatic zero point calibration; callbration cell
In situ; measures organics and inorganics, including HCI and Cl 2 ; automated calibration; calibration checks
Reader service number
410
411
412
413
414
weeks, the performance of the HWI and its associated air pollution control equipment is measured. Measuring mercury in the waste feed is costly and difficult, especially for heterogeneous waste. Representative sampling is nearly impossible, and mercury is notorious for not being evenly distributed in the waste. Even worse, the sampling and analysis in-
Hg 2 0 0 0
Semtech Metallurgy Ideon, Lund Sweden +46-468182550
www.opsis.se
Refill wet reagent weekly; check on pumps and lines weekly
Complex system; difficulties with photometer
crease the risk of personnel being exposed to hazardous chemicals and, in the case of mixed waste, to radionuclides. Nevertheless, this approach to mercury monitoring is the best that can be done without continuous emission monitors (CEMs). The mercury emissions are measured by collecting samples and sending them to a laboratory for analysis by EPA Method 0060
A R 6 0 2 Z/600
Cannot measure HgCI2
SW-846. Unfortunately, such analyses typically have turnaround times of several weeks and provide Uttle data. The regulations require periodic trial burns (every three to five years, depending on the size and type of facility) to verify that the incinerator continues to meet emissions standards. The feed and facility control approach described above is useful, but it only pro-
Analytical Chemistry News & Features, July 1, 1999 471 A
Product
Review
Table 2: Summary of m e n:ury CEMs under development. Developer ADA Technologies ADA Technologies 304 Inverness Way South 304 Inverness Way South Ste. 365 Ste. 365 Englewood, CO 80112 Englewood, CO 80112 303-792-5615 303-792-5615
Tennessee Valley Authority TVA CEB 1C Muscle Shoals, AL 35662 256-386-3539
Sensor R&D 5 Godfrey Dr. Orono, ME 04473 207-866-0100
URL/e-mail
www.adatech.com/
www.adatech.com/
[email protected] [email protected] Technology
CVAAS; Zeeman background correction; thermal cracking to convert oxidized Hg to elemental Hg
Cavity ringdown spectrometry
CVAAS after particulate removal with dry, gas-phase conversion method
Gold film on SAW oscillator
Projected cost (U.S. dollars)
$50-75,000
Not known
$20-40,000
$2-4000
Lab/field performance data
MDL of 1 ug/dscm; field tests detected 5-7 ug/dscm in the presence of 300 ppm S 0 2 and 250 ppm N0 2 ; no data for high particulate matter case
Not available
Field data limited to elemental mercury; MDL of 1.0 ug/m3
Laboratory MDL of 5 ug/ dscm
Maintenance requirements
High-temperature ovens for cracking
Unknown
Gold filter and H 2 gas (for dry thermal conversion) will require quarterly service
If in situ, simple and low maintenance
Disadvantages
Still to be resolved: controlling temperature of quarter wave plate; memory effect of 5 min sample switching
S 0 2 levels of >300 ppm appear to interfere
Special features
Simpler than wet chemistry systems; tolerates high S 0 2 levels (up to 2000 ppm); MDL could go down to 0.1 ug/dscm
High sensitivities (to 200 ppq)
vides emissions data during short, widely spaced test periods. The presumption is that emissions are also under control during the three tofiveyears between tests, but data to confirm this presumption are not available. Clearly, the current regulatory approach does not provide continuous assurance of compliance with emissions standards. By contrast, a CEM provides real-time emissions data and, therefore, continuous assurance of compliance for regulators and the public. CEMs might also allow increased waste throughput and improved mercury emissions control. Commercial mercury CEMs are available and widely used in Europe and Asia. Germany has a standardized CEM test applicable to mercury CEMs. Instruments that pass this test are officially designated "TUV-approved" in the newsletter of the German BMU (Federal Ministry for the Environment). Mercury CEMs have seen very limited service in the United States, because they are not required by regulations and the benefits of voluntary use have not been demonstrated. A1997 mercury CEM vali472 A
Unknown durability in combustion effluent; needs field demonstration of MDL for total mercury No wet reagent
dation test in the United States was inconclusive, and it could not be repeated in time to mandate mercury CEMs in the new EPA rule for maximum achievable control technology for hazardous waste combustors (MACT rule) (2). Instead, the MACT rule will rely on conventional waste-feed characterization and feed-rate limitations to establish compliance. However, EPA recognizes the benefit of mercury CEMs, and the new rule provides for the voluntary of mercury CEMs. Regional EPA regulators have the authority to allow the use of mercury CEMs on a site-by-site basis in lieu of some waste-feed characterization Details will be negotiated on a site-by-site basis This Product Review considers the costs, benefits, and risks of using mercury CEMs. Table 1 summarizes the characteristics offivecommercially available mercury CEM systems. All these systems are TUV-certified in Germany. In fact, all are European technologies with distributors in the United States. Six additional mercury CEM systems under development are listed in Table 2. Table 3 compares key characteristics of mercury
Analytical Chemistry News & Features, July 1, 1999
Potentially small and inexpensive
CEMs with those of multimetal CEMs. The latter aim to measure all six metals governed by the MACT rule: mercury, cadmium, chromium, beryllium, lead, and arsenic. This review is not intended to be comprehensive; readers should contact the companies or instrument developers for further information. CEM approaches
In flue gas from incinerators, mercury can exist in two fundamental chemical forms: elemental mercury or oxidized (speciated) mercury (HgCl2 or HgO). Oxidized mercury is soluble in water, which means that a facility with a wet scrubber as part of its air pollution control equipment can remove most of it, leaving primarily elemental mercury in its effluent. If there is chlorine in the waste feed, any oxidized mercury in the effluent will tend to be HgCl2, otherwise it is HgO (3) Based on thermodynamics, it is unlikely that form of methyl mercury [(CH )Hg+] the most toxic of all mercury species will be found in incinerator emissions Because of regulations in the United States and in Europe most CEMs are designed to measure total mercurv that is
Table 2: Summary of CEMs under development PS Analytical Orpington, Kent U.K. +44-1689891211
EEI P.O. Box 6 Pluckemin, NJ 07978 609-243-3212
[email protected] [email protected] CVAFS
Microwave plasma AES
$25,000
$50-75,000
Tested at the University of North Dakota in 1997; did not operate properly during the test
In lab tests, MDL of 0.03 mg/ dscm; field test measured 0.71.4 mg/dscm
Semiannual filter replacements
Measures total mercury; developing methods to measure total ionic mercury
elemental mercury plus oxidized species. However, most analytical techniques measure only elemental mercury; so the CEM must convert oxidized mercury to the elemental form. A CEM that measures only elemental mercury might be adequate for compliance monitoring if it can be determined that little or no oxidized mercury exists in the stack gas. This will likely be the case for facilities where the predominant form of oxidized mercury is mercuric chloride and where the mercuric chloride is removed in a wet scrubber. For example, the Opsis mercury CEM measures only elemental but is simple and durable and avoids sample extraction and mercury conversion There are two types of instruments sometimes referred to as "speciating" mercury CEMs. The first uses a "difference" measurement (total minus elemental mercury) to infer how much oxidized mercury is present. They perform no speciation, however, so it is incorrect to call them speciating CEMs. The second type, true speciating mercury CEMs, would measure the concentra-
tion of each individual mercury species. No current instrument has this capability and, from a regulatory viewpoint, there is no need for real-time true mercury speciation of combustion flue gas other than for research on control and fate and transport. Mercury CEMs can be categorized by how they separate mercury signals from those of interfering species (e.g., S02), how they reduce (convert) oxidized mercury to elemental mercury (when the measurement technique can measure only elemental mercury), and how they in easure the Separation and conversion techniques
Two approaches are used to separate mercury from interfering species, such as S02. The most common uses amalgamation in a gold trap to physically separate (trap) elemental mercury vapor from the gas stream. The other approach, which we call "virtual separation", separates mercury signals from signals arising from interferants. Amalgamation also can be used to preconcentrate mercury, enabling the measurement of low concentrations. The mercury is collected over a period of time and then rapidly desorbed, periodically delivering a mercury pulse to the measurement device. S0 2 interference can be avoided if an inert gas is used for the desorption step. Such separation and preconcentration steps satisfy EPA's requirement for continuous sampling if two gold traps are used alternately, one sampling while the other is desorbed. The Tennessee Valley Authority has a CEM with a gold filter, which is used to create a mercury-free flue gas. When the signal from that background gas is subtracted from the total signal, interference effects are removed. It is well known that gold adsorbs elemental mercury; however, according to
some recent observations at ADA Technologies, gold may adsorb both elemental mercury and mercuric chloride. Other, unpublished research by Dennis Laudal of the University of North Dakota Energy & Environment Research Center, indicates that mercury does not easily desorb from the gold trap when both HC1 and NO are present. These observations are significant and worthy of further investigation. Examples of virtual separation techniques are Zeeman background correction and differential optical absorption spectroscopy (DOAS) (4). Zeeman background correction is used in some mercury cold vapor atomic absorption spectroscopy (CVAAS) systems to correct for interferants, such as S02. A magnetic field splits the Hg° absorption line at 253.7 nm into components with slightly different wavelengths and different light polarizations. These differences distinguish Hg° absorption from the background absorption (which is affected by S0 ) The magnetic field can be aoolied to either the source lamo or the absomtion cell It is either constant from a permanent magnet or alternating from an electromagnet driven by alternating DOAS is based on the absorption of light by gaseous constituents in a light path. It has the capability to simultaneously measure Hg° and other species, such as S02, NO, N02, C02, CO, NH3, HC1, HF, and 0 3 . This allows DOAs to accurately separate the mercury signal from the signals of interfering species, such as S02. In those techniques that only measure elemental mercury (e.g., CVAAS and Cold Vapor Atomic Fluorescence Spectroscopy [CVAFS]), oxidized mercury must first be reduced to elemental mercury. The traditional conversion technique is wet chemistry using ascorbic acid, stannous chloride, or sodium borohydride. However, maintenance requirements are a consideration in these systems. Reagents need to be replenished at least once a month, and pumps and lines need to be checked daily. Developmental techniques for conversion include dry chemistry or thermal cracking. Thermal cracking is similar to the roasting technique used to produce mercury from its natural cinnabar ore
Analytical Chemistry News & Features, July 1, 1999 4 7 3 A
Product
Review
Measurement techniques
CVAAS is the most popular measurement technique for mercury. In CVAAS, a llght is passed through an optical cell containing an extracted sample of gas. The amount of light absorbed at 253.7 nm is proportional to the amount of elemental mercury vapor in the sample. The name "cold vapor" in CVAAS distinguishes it from other atomic absorption techniques, because no flame or furnace is needed to vaporize mercury at flue gas temperatures. The Opsis mercury DOAS CEM uses a high-pressure xenon discharge lamp as a continuum light source. DOAS avoids S02 interference better than CVAAS without separation. Using long optical paths, DOAS has been used to measure the various species mentioned above in ambient air. In CEM applications, in situ measurements are obtained using a cross-stack light path. High spectral resolution is required for DOAS mercury measurements because of the narrow width of the mercury line at 253.6 nm and the strong interference from 0 2 absorption lines in the same region (6). DOAS CEMs typically use rapid data acquisition (100 spectra/s) and data averaging to minimize the effects of turbulence or variations in the particulate content of the gas stream, which may occur during the data averaging period. In CVAFS, light is emitted from gaseous atoms that have been excited to higher energy states by the absorption of light. CVAFS measures the light emitted to determine the concentration of the emitting atoms in the excitation volume. The atoms are usually generated by the vaporization of sample material in a furnace or flame, and the excitation light is provided by a pulsed hollow cathode lamp, whose emission is specific to the analyte of concern. For mercury, the excitation wavelength is 253.6 nm. Mercury has appreciable vapor pressure at room temperature so no furnace or flame is required for vaporization CVAFS is used in the PS Analyttcal CEM, which is currently under development. Originally designed to monitor total mercury in ambient air, this instrument uses a gold filter for mercury separation and preconcentration and CVAFS for the mercury measurement. Cavity ringdown spectroscopy, which 474 A
is being evaluated by ADA Technologies, offers the potential for monitoring mercury to ppq levels, but it appears that S0 2 concentrations greater than about 30 ppm may cause interference problems. Currently, the technique is better-suited for ambient mercury monitoring than for stack monitoring. The EEI mercury CEM is based on microwave plasma-induced atomic emission spectroscopy (AES). A CEM ussng AES does not
mercury vapor, the film conductivity (resistivity) and the mass change as a function of mercury absorption. The SAW device monitors the film changes and outputs a frequency that indicates the mercury concentration. In laboratory experiments under a dry nitrogen atmosphere, this sensor has demonstrated sensitivity to
