Laboratory Evaluation of a Mercury CEM Using Atomic Absorption with

system using a deuterium (D2) lamp was developed and tested in the laboratory. ... Hg analyzer for use in a Hg continuous emission monitor. (CEM) was ...
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Environ. Sci. Technol. 2002, 36, 1767-1773

Laboratory Evaluation of a Mercury CEM Using Atomic Absorption with a Deuterium Background Correction System GLENN A. NORTON* AND DAVID E. ECKELS 276 Metals Development Building, Ames Laboratory, Iowa State University, Ames, Iowa 50011

A prototype mercury (Hg) analyzer based on atomic absorption (AA) with a broadband background correction system using a deuterium (D2) lamp was developed and tested in the laboratory. Initial tests were performed using a small commercially available AA Hg detector operated in series with an optical breadboard version of the D2 background correction system. Based on encouraging results obtained with that system while using streams of elemental Hg and SO2 (an interfering gas), a compact prototype Hg analyzer for use in a Hg continuous emission monitor (CEM) was built. In laboratory tests performed in the absence of interfering gases, the analyzer could detect 0.5 µg/m3 or less of elemental Hg. The noise in the D2 channel of the analyzer was a limiting factor with respect to instrument sensitivity when background-corrected absorbance values were used. Tests with gas streams containing 9-26 µg/ m3 of elemental Hg and 0.02-0.44% SO2 indicated that the D2 background correction approach did a good job at subtracting the large interfering signal due to SO2. However, a correction factor had to be applied to the absorbance readings from the D2 channel of the analyzer in order to obtain acceptable accuracy for the background-subtracted Hg measurements.

Introduction In the 1990 Amendments to the Clean Air Act, 189 compounds or elements are listed as hazardous air pollutants. Among those air toxics, mercury (Hg) is one element of potential concern related to coal-based power generation. This is particularly true in view of the toxicity of Hg and the estimates that coal-fired power generation is responsible for the annual release of roughly 50 tons of Hg into the atmosphere (1). In recent years, there has been a great deal of interest in measuring Hg emissions from coal combustion effluent streams. This interest has stemmed not only from research needs (e.g., assessing the effectiveness of Hg abatement technologies for process gas streams) but also from the possible regulation of Hg emissions from coal-fired power plants. In fact, the U.S. Environmental Protection Agency recently decided that Hg emissions would be regulated from such power plants (2). Manual methods for determining Hg in process gas streams are tedious, time-consuming, and prone to sample contamination. Consequently, considerable effort has been * Corresponding author phone: (515)294-1035; fax: (515)294-3091; e-mail: [email protected]. 10.1021/es0113899 CCC: $22.00 Published on Web 03/03/2002

 2002 American Chemical Society

devoted to the development of Hg continuous emission monitors (CEMs) for process gas analyses. In addition to monitoring coal combustion streams, the National Energy Technology Laboratory is interested in performing online Hg determinations in coal gasification streams. In this regard, the Ames Laboratory has been developing an integrated sampling and analytical system suitable for determining Hg in coal gasification effluents. One of the thrusts of the work has been to develop an inexpensive Hg CEM for monitoring Hg in those gas streams. In earlier work, both cold vapor atomic absorption (CVAA) and cold vapor atomic fluorescence (CVAF) were considered for use in our Hg CEM for coal gasification streams. However, during laboratory testing with a CVAF Hg detector, it was observed that many of the gases that would be present in coal gasification effluents caused severe quenching of the fluorescence signal (3). This precluded direct analysis of gasifier streams by CVAF. When using CVAF, a gold cartridge is typically used to collect the Hg. The gold cartridge is subsequently heated to thermally desorb the Hg from the gold. The Hg is desorbed into a high purity argon carrier gas stream, which then passes into the Hg detector. Using goldbased Hg collectors avoids most interfering gases and preconcentrates the Hg prior to the thermal evolution and detection steps. Early in the project, there were concerns about possible problems associated with using gold cartridges in some process gas streams (e.g., coal combustion effluents) due to poisoning of the gold cartridges. Gas conditioning approaches are now available to circumvent those problems (4). However, at the time our work began, suitable gas conditioning systems were not yet available. Therefore, CVAA was selected for our CEM because it is well developed, sensitive, inexpensive, and can analyze a gas stream directly for Hg. However, because of spectral interferences from other gases, some sort of background correction technique is needed in order to perform continuous, online, real time Hg analyses in process gas streams using CVAA. The use of CVAA for determining Hg in process gas streams has been around for some time. In one early study, CVAA was used to continuously and directly (no preconcentration onto noble metals) determine Hg in stack gas after first reducing the Hg with SnCl2 (5). No background correction was used because it was believed that the levels of interfering gases did not cause significant interferences in their particular gas matrix. In another study, Hg emissions from an oil shale retort were measured using a continuous, online Hg monitor based on Zeeman-modulated CVAA (6). In other work, minimal testing was performed using CVAA for the determination of Hg in effluents at a coal-fired plant (7). A catalytic converter was used prior to the detector to convert all of the gaseous oxidized Hg to elemental Hg, since Hg must be in the elemental form in order to be detected by CVAA. More recently, a number of Hg CEMs have undergone development, and many of those systems are commercially available. Summaries of those CEMs have been presented elsewhere (8, 9). Despite the commercial availability of Hg CEMs, a substantial amount of innovative work is necessary before reliable, accurate, and low-maintenance Hg CEMs are available for routine use by the electric utility industry (1). Some of the Hg CEMs employ CVAA for Hg detection, and Zeeman modulation is used for performing background corrections in some of those CVAA systems. Although that background correction method appears to be very promising, we chose to investigate an alternate approach for performing the background corrections. Specifically, we investigated the effectiveness of using a broadband spectral correction VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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approach employing a deuterium (D2) lamp as the broadband radiation source. In this regard, a prototype Hg analyzer using CVAA with a D2-based background correction system was built and tested in the laboratory. No Hg CEMs currently employ this background correction method. Both the Zeeman modulation and broadband correction approaches are commonly used in laboratory CVAA instruments. Therefore, we decided to explore the merits of using the later background correction method for the difficult task of monitoring Hg in process gas streams. This paper discusses the results of our work to date. Although efforts focused on applications to coal gasification streams, the work is also applicable to other process gas streams, including those from coal combustion and waste incineration. Comparisons of this technology with other available technologies are beyond the scope of this paper. Instead, the purpose of this paper is to share results obtained thus far with the prototype analyzer with the scientific community. Although considerable gas stream conditioning is required prior to analysis of coal gasification streams, this paper focuses only on the Hg detection system. In addition, this work is intended only as an initial evaluation of the technology to help assess its overall potential.

Experimental Section Principles of Operation. In the determination of Hg by CVAA, free neutral ground-state atoms (i.e., elemental Hg vapor) absorb light energy that is characteristic of that particular element. Light at the characteristic energy (wavelength of 254 nm is normally used) with a given incident intensity will be absorbed by the analyte in the sample cell and produce a decrease in the transmitted light intensity. As the concentration of elemental Hg vapor in the sample cell increases, the amount of light (at 254 nm) transmitted through the sample cell decreases. The incident and transmitted light intensities are measured by the detector and are used to calculate the sample “absorbance”. The absorbance is linear with changing analyte concentration and is reported in “absorbance units” or “AU.” At low absorbance values, “milliabsorbance units”, or “mAU,” are usually used. For our application, the spectral interferences of primary concern are associated with water vapor, aromatic hydrocarbons, and sulfur-containing gases. For accurate Hg measurements, either background corrections must be made in order to correct for the presence of interfering gases, or else those gases must be eliminated prior to the CVAA detection cell. Based on the anticipated H2S levels (0.1-0.5%) in raw coal gasification effluents, the interference from H2S could produce an absorbance value equivalent to that observed for a 10 µg/m3 Hg stream. This is of concern because Hg concentrations in coal gasifier streams are anticipated to be in the 1-10 µg/m3 range. Catalytic oxidation of the gas stream will be used to remove heavy tars and light hydrocarbons prior to analysis. Oxidation of the sample gases will convert the H2S to SO2, which absorbs light at 254 nm 20-30 times more strongly than H2S does. Although the presence of SO2 rather than H2S is desirable from the standpoint of reduced memory effects, it is undesirable from an interference standpoint. Nonetheless, gas stream oxidation may be the most desirable approach in view of the tars and other gasifier components that must be addressed. Because CVAA detects only the elemental form of Hg, the oxidized gases are passed through a pyrolyzer to convert the oxidized Hg vapors to elemental Hg. The gas stream is then passed through a Nafion-based drying system prior to entering the Hg detector. Based on this gas conditioning approach, the assumption was made that hydrocarbons and moisture will be absent from the gas streams entering the Hg detection system and that SO2 will be the primary analytical interference of concern. 1768

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The Ames Laboratory Hg analyzer uses two ultraviolet (UV) lamps. One is a Hg lamp and the other is a D2 lamp. The principle of operation for the background corrections is based on the fact that the Hg UV lamp emits mostly in a very narrow spectral line at 254 nm, while the D2 lamp emits in a relatively broad band over the same spectral region. The Hg channel of the analyzer responds to both Hg and interfering gases at 254 nm, while the D2 channel of the analyzer responds only to gases which absorb over a much broader spectral region. The absorption region for Hg is so narrow compared to the broad band associated with the D2 lamp that Hg should not be detected with the D2 system. Therefore, the Hg content of a gas stream can theoretically be determined by subtracting the signal from the D2 channel (absorbance due to interfering gases only) from that of the Hg channel (absorbance from both Hg and interfering gases). This provides a background-subtracted absorbance reading that theoretically corresponds to only the elemental Hg in the gas stream. The background-corrected absorbance readings are then compared to a Hg calibration curve (Hg concentration versus absorbance reading) prepared while using gas streams that do not contain any interfering gases. Optical Breadboard Setup. The initial design of the D2 background correction module was formulated, assembled on an optical breadboard, and tested using a single photodiode to assess the optical throughput. The optical system was later modified to include a reference beam measurement of the D2 lamp emission using a second photodiode. The baseline drift and noise were examined for the optical breadboard setup using a dual photodiode system. A variety of tests was performed using dry gases at room temperature. First, tests were performed with a Thermo Separation Products Model 3200 Mercury Monitor (a CVAA Hg detector no longer in production) and a homemade (optical breadboard system) D2 module operating in series, such that the sample gases exiting the Hg detector immediately entered the detection cell in the D2 module. For these tests, SO2 from a compressed gas cylinder was used as an interfering gas since it is anticipated that gas stream oxidation will be used to eliminate hydrocarbon interferences prior to Hg analysis in actual process gas streams. A stream of 2% SO2 (in a N2 balance) was blended with air to give a final SO2 concentration of about 0.05%. Streams of elemental Hg vapor were added to the SO2 stream by using a VICI Metronics elemental Hg permeation tube in a VICI Metronics Model 340 Dynacalibrator. The Hg concentration in the final blended gas stream was roughly 20 µg/m3. The absorbance readings from the Hg detector and the D2 module were monitored to assess the general performance of the system. In other tests with the Hg detector and D2 module operating in series, the accuracy of determining low levels of Hg in dry gas streams containing high levels of SO2 was investigated. This represented a worst case scenario. Gas streams containing 2 µg/m3 of elemental Hg and 0.3% SO2 were used with total gas flow rates of about 1000 mL/min. The absorbance signal from SO2 in the Hg detector was about 100 times higher than the absorbance signal from the Hg. Tests were also performed using 4 µg/m3 elemental Hg and 0.4% SO2 at a total gas flow rate of 500 mL/min. In those tests, the absorbance signal from SO2 in the Hg detector was about 50 times higher than the absorbance signal from the Hg. A calibration curve (Hg concentration versus absorbance reading) was prepared with the Hg detector while using Hg concentrations of 0.1-5 µg/m3 in the absence of SO2. For the tests involving Hg blended with SO2, the background absorbance reading (in mAU) from the D2 module was used to correct the absorbance readings from the Hg detector to give a background-corrected absorbance value. These background-corrected absorbance values were then compared

to the calibration curve noted above to obtain the Hg concentration in the gas stream entering the analyzer. Another series of tests were performed using Hg streams blended with SO2. For these tests, nominal Hg concentrations ranged from 2.5 to 12 µg/m3 and SO2 concentrations ranged from 0.02 to 0.5%. When using those Hg and SO2 concentrations, the absorbance due to SO2 in the Hg detector was 10-30 times higher than the absorbance due to Hg. Tests were performed by first zeroing the instruments (Hg detector and D2 module) while using air or nitrogen. Next, various concentrations of SO2 were passed into the analyzers without any Hg in the gas streams. Individual absorbance readings from the Hg detector and D2 module were recorded. Then, Hg was added to the SO2 streams, and the absorbance readings were recorded again. Finally, the Hg was removed from the gas stream and the absorbance readings were taken for a second time in the absence of Hg (i.e., using only the SO2 streams). Because of the need for a correction factor on the absorbance readings from the D2 module (see results), a D2 correction factor was calculated for each SO2 concentration by taking the ratio of the absorbance reading from the Hg detector to that of the absorbance reading from the D2 module while only SO2 streams were being used. The background-corrected absorbance readings (i.e., the absorbance due only to the Hg in the gas stream) for the gases containing both Hg and SO2 were then calculated as follows:

net absorbance (mAU) ) (absorbanceHg) (absorbanced2)(D2 correction factor) In that equation, “absorbancehg” is the absorbance measured with the Hg detector and “absorbanced2” is the absorbance measured with the D2 module. The net absorbance reading could then be compared to the Hg calibration curve (obtained in the absence of interfering gases) in order to obtain the Hg concentration. Prototype Hg Analyzer. Although good results were obtained when a Hg detector and a breadboard D2 module were used in series to analyze Hg streams containing high concentrations of SO2, the Hg detector was a unit that has been discontinued by the vendor. In addition, the Hg detector and breadboard D2 module were operated as separate units. It is desirable to have a Hg CEM that incorporates both a Hg detector and a D2 correction system into a single, compact, fully integrated Hg analyzer. Therefore, a prototype Hg analyzer using a CVAA Hg detector with a D2-based broadband background correction system was constructed and tested. DMK Engineering (Rancho Palos Verdes, CA) was responsible for designing and building the prototype Hg analyzer for Ames Laboratory. When going from an optical breadboard setup to a fully integrated analyzer, numerous changes were made in the electrical/optical design of the system. The Hg analyzer weighs about 20 kg and is housed in a 66 × 24 × 46-cm case equipped with a carrying handle. The case opens up like a suitcase to provide easy access to all optical and electrical components. The analyzer has a 10-cm quartz cell (volume of about 18 cm3) that can be heated to 30-225 °C. A thumbwheel on the instrument cover allows for operator selection of the cell temperature. The analyzer contains a temperature-controlled, low-pressure Hg lamp for measuring total absorbance at 254 nm and a 40-W D2 lamp for measuring the background in that region. A single beam splitter with a shutter mechanism sends the beams from the Hg and D2 lamps (switching back and forth between the two light sources) through the sample cell to the photodiode detector. A reference beam is used to reduce baseline noise and drift by compensating for fluctuations in lamp intensity and other instrumental variables. This is accomplished by sending a portion of the beams from the

FIGURE 1. Schematic diagram of the prototype Hg analyzer. Hg and D2 lamps to a reference photodiode detector. A simplified schematic diagram of the interior of the analyzer is shown in Figure 1. A small panel mounted in the instrument case displays a reading about every 3 s, and up to 48 h of data can be stored in nonvolatile internal memory. Those data can later be downloaded onto a laptop computer or a PC. By using the panel display, the instrument can be zeroed, data acquisition into the internal memory buffer can be initiated, and one of two data display modes can be selected. The analyzer can also be operated with an external laptop computer or a PC through a DB-9 connector. With an external computer hooked up to the analyzer, one of six data display modes can be selected on the computer screen. In addition, there are diagnostic tools, adjustments to data processing parameters, instrument zeroing capabilities, and optical alignment features. Data smoothing is accomplished with an adjustable digital filter. With the digital filter, there is a tradeoff between signal smoothing and instrument response time. In other words, as the signal is smoothed to a greater degree, the instrument takes longer to respond and reach a steady reading. The Hg analyzer was tested to assess its overall performance as well as its strengths and limitations. Air streams containing known concentrations of elemental Hg were generated using a VICI Metronics Model 340 Dynacalibrator in conjunction with VICI Metronics elemental Hg permeation tubes. These gases were then blended with either N2 or SO2 and were subsequently passed directly into the analyzer via unheated Teflon lines. As with the tests with the breadboard optical setup, tests were performed only with dry gases. Also, unless otherwise noted, all tests were performed at room temperature. While using only the Hg channel of the analyzer (i.e., readings from the D2 module in the analyzer were not being used), calibration curves were obtained for elemental Hg streams (no SO2) containing Hg concentrations of 0.5-50 µg/m3. These tests were performed to assess detection limits and the linearity of the calibration curves. In addition, levels of baseline noise and drift were assessed for both the Hg and D2 channels in the analyzer. Because of the observation that a D2 correction factor was needed in the tests with the breadboard optical setup, a variety of tests were performed with the prototype analyzer while using various concentrations of SO2 without any Hg present. In one series of tests, 2.0% SO2 (in a N2 balance) was blended with air and then analyzed with the Hg CEM. Total gas flow rates of 500, 700, 1000, and 1500 mL/min were used, while the flow rate of the 2% SO2 was varied from 50 to 300 mL/min. For each total gas flow rate, the air and SO2 flow rates were adjusted to give a variety of SO2 concentrations while keeping the total gas flow rate constant. This provided SO2 concentrations of 0.1-0.8% in the final gas stream. The absorbance readings from the Hg detector and D2 module in the analyzer were both measured and recorded. For each total gas flow rate and SO2 concentration, the ratio of the VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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absorbance value from the Hg detector to that of the absorbance value from the D2 channel was calculated. This ratio is the D2 correction factor. The D2 correction factor was also calculated when passing a stream of 1000 ppm SO2 (in N2) directly into the analyzer at flow rates of 100, 250, 500, 750, and 1000 mL/min. This approach eliminated uncertainties in the concentration of SO2 as the total gas flow rates changed, since no blending of gases was being performed. The tests with 1000 ppm SO2 were performed solely to see if there were any flow rate effects on the D2 correction factor. Limited testing was also performed after making some optical modifications in the instrument in an attempt to eliminate the D2 correction factor. In those tests, gas streams containing SO2 concentrations of 0.1-0.6% (in air) were passed into the analyzer at a total gas flow rate of 1000 mL/ min. The D2 correction factor (ratio of the absorbance reading from the Hg detector to the absorbance reading from the D2 module) was determined at each SO2 concentration to determine if the modified optics affected the need for the correction factor. Similarly, tests were performed using streams of Hg and SO2 with the modified optics to see how effectively the D2 module corrected for high levels of SO2 in those gas streams. For tests involving both Hg and SO2, Hg concentrations of 9-26 µg/m3 were used with total gas flow rates ranging from 360 to 1020 mL/min. The Hg streams were blended with 2.0% SO2 (in N2), and the SO2 flow rate was adjusted to provide SO2 concentrations of 0.02-0.44% in the final blended gas stream. For these tests, the absorbance at 254 nm due to SO2 in the Hg channel of the analyzer was 15-35 times higher than the absorbance due to Hg. For each SO2 flow rate, corresponding Hg calibration curves were prepared in the absence of SO2 by blending Hg streams from the calibrator with N2 from a compressed gas cylinder. For each Hg concentration, the absorbance value was measured with the Hg channel of the analyzer. Those absorbance values were then plotted as a function of the Hg concentration in order to obtain the calibration curves.

Results and Discussion Optical Breadboard Setup. Tests with a single photodiode indicated that using photodiodes with the D2 lamp should not add significant analytical uncertainties for determining Hg concentrations on the order of 1 µg/m3. Also, the signal strength from the photodiode was very good. After adding a reference beam and a second photodiode, excellent results were obtained with respect to baseline noise and drift. The amount of baseline drift over a period of 8 min was nearly zero, and the standard deviation (noise) of the baseline absorbance signal was only 24 × 10-6. Calculations indicated that an absorbance standard deviation of less than 100 × 10-6 for background measurements would have no significant impact on the accuracy of the Hg determination. Tests on the ratio of the photodiode signals measured simultaneously from the sample and reference beams indicated that this ratio was very stable for our purposes. When initially operating the D2 module and Hg detector in series, the D2 module exhibited a noise level of about 0.03 mAU and a long term drift of about 2 mAU/h. Smaller iris sizes and alternate data processing methods were tested as a means to reduce the drift. After reducing the iris size in the optical setup, the drift was reduced to about 1 mAU/h. With further testing, it was discovered that the drift was primarily from the reference channel of the D2 system. After careful realignment of the optics, the long-term drift in the D2 channel was reduced to only 0.1 mAU/h. A reading of 0.1 mAU is roughly equivalent to a 0.5 µg/m3 Hg stream. The drift for the Hg channel was also about 0.1 mAU/h, while the noise level was about 0.1 mAU. Thus, the lamp, power supply, and 1770

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FIGURE 2. Background-corrected absorbance data from the breadboard optical setup for varying concentrations of Hg and SO2. measuring electronics had sufficient stability for our purposes. When gas streams containing 0.05% SO2 and 20 µg/m3 of Hg were used, the D2 module gave the same reading for SO2 alone (no Hg) as it did for the SO2/Hg blend. This is of critical importance and indicates that the D2 module was operating as expected, since the D2 module was sensitive to SO2 but not Hg. However, when using only SO2 streams (no Hg), the absorbance reading from the D2 module was about 20% larger than the absorbance reading from the Hg detector. This did not appear to be caused by the electronics or optics. The deviation from the expected values may be caused by complexities (structure) in the SO2 absorption spectrum in the wavelength region being used for analysis. This necessitates the use of a correction factor for the D2 absorbance values so that they match the absorbance values from the Hg detector when using SO2 streams in the absence of Hg. Otherwise, the absorbance values from the D2 module will not serve as an adequate background subtraction method for SO2. While using streams of SO2 with no Hg, the ratio of the absorbance reading from the Hg detector to the absorbance reading from the D2 module did not vary by more than a few percent over the range of test conditions used. It was anticipated that the correction factor would be accurate as long as SO2 is the interfering gas with the greatest absorbance, which is expected to be the case after gas stream oxidation. For the tests involving Hg concentrations of 2 and 4 µg/ m3 in the presence of high levels of SO2, background-corrected Hg concentrations of 3.5 and 5 µg/m3 were obtained, respectively. In view of the low Hg concentrations involved and the fact that the interfering gas (i.e., SO2) gave absorbance values 50-100 times higher than the Hg did, the results obtained in this worst-case scenario were encouraging. In Figure 2, background-corrected absorbance values for the tests involving Hg concentrations of 2.5-12 µg/m3 in the presence of varying levels (0.02-0.5%) of SO2 are shown. As noted earlier, the net absorbance is the total absorbance measured by the Hg detector minus the absorbance measured in the gas cell illuminated by the D2 light source (i.e., the background). Also, as discussed earlier, each data point had its own D2 correction factor applied. The backgroundcorrected concentrations for Hg in the presence of SO2 were typically within 5% of the values obtained (experimentally measured) for a given Hg concentration in the absence of SO2. These data demonstrate that using a D2 lamp is a viable approach for measuring the absorbance of interfering gas species and that low levels of Hg can be measured in gas streams containing high levels of SO2 using this background correction method.

Prototype Hg Analyzer. In the Hg channel of the prototype analyzer, the baseline noise was typically about 0.02 mAU and the baseline drift varied from 0.1 to 0.4 mAU/h. Excellent calibration curves were obtained with the Hg channel of the detector when using Hg concentrations of 0.5-50 µg/m3 in the absence of SO2. For the entire calibration range, the linear regression coefficient was 0.999. For Hg concentrations in the range of 0-10 µg/m3 the linear regression coefficient was 0.993. Based on the absorbance readings from the Hg channel of the analyzer, the detection limit is estimated to be between 0.1 and 0.5 µg/m3 while using the 10-cm sample cell. This estimate is based on the signal-to-noise ratio (about 4:1) of the signal observed for a 0.5 µg/m3 Hg stream in the absence of any interfering gases and on the assumption that the peak height for the signal decreases linearly with decreasing Hg concentration below 0.5 µg/m3. However, the uncertainty limits for this very low Hg concentration of 0.5 µg/m3 may have been as high as (50%. A precise detection limit cannot be determined without first narrowing the uncertainty limits associated with the 0.5 µg/m3 Hg stream. The detection limit could probably be reduced considerably by increasing the length of the sample cell to 100 cm. However, it is anticipated that the detection limit with only a 10-cm sample cell can be improved (e.g., down to 0.05 µg/m3) by making several electrical/optical design modifications. For the D2 channel of the analyzer, the noise levels were typically 0.2-0.3 mAU, and the baseline drift was typically about 0.3 mAU/h. Thus, the D2 channel was substantially noisier than the Hg channel, which is the opposite of what had been anticipated. The noise in the D2 channel is currently one of the limiting factors affecting the detection limit and the accuracy of the analyzer. It may be possible to obtain a quieter lamp/power supply combination than the one currently used in the instrument. Also, some of the problem could possibly be related to the peripheral electronics, which can also be improved upon if needed. In addition, since noise and drift are affected by lamp and optics alignments, the possibility cannot be ruled out that more precise alignment is necessary. When streams of Hg and air were blended to provide a final Hg concentration of 50 µg/m3, the signal from the D2 channel of the analyzer was unaffected (i.e., any effect of the Hg on the D2 baseline was well within the baseline noise). Since this Hg concentration is much higher than those anticipated for gaseous effluents from coal-based power generation, it is clear that the D2 baseline will not be affected by Hg in the gas streams to be sampled. This is critical to the successful application of the D2-based background correction approach used in our analyzer. In the tests where 2% SO2 was blended with air to give varying SO2 concentrations and varying total gas flow rates, the absorbance value from the D2 channel was significantly higher than the absorbance value observed in the Hg channel. Therefore, a correction factor must be applied to the absorbance values from the D2 channel of the analyzer. This is in agreement with the observations made with the breadboard optical design and may be due to the relatively complex SO2 absorption spectrum in the wavelength region being used for analysis. In these tests, the Hg/D2 absorbance ratio (i.e., the D2 correction factor) was not affected by the total gas flow rate for a given SO2 concentration. Similarly, when a 1000 ppm SO2 stream entered the analyzer at flow rates of 100-1000 mL/min, no flow rate effects on the D2 correction factor were observed. Although the D2 correction factor was not affected by the total gas flow rate while using 2% SO2 blended with air, the correction factor was affected slightly by the SO2 concentration. This is shown in Figure 3. The Hg/D2 absorbance ratio increased from 0.82 to 0.87 as the SO2 concentration was increased from 0.1 to 0.8%,

FIGURE 3. Effects of SO2 concentration on the Hg/D2 ratio using the prototype Hg analyzer.

FIGURE 4. Effects of SO2 concentration on the absorbance values from the Hg and D2 channels of the analyzer (using modified optics). respectively. Although the effect of SO2 on the Hg/D2 ratio was small, these small differences can have a considerable impact on the background-corrected absorbance readings when using blends of Hg and SO2 in the concentration ranges of interest. Despite the current need for a D2 correction factor, it is hoped that this need can be diminished or even eliminated through modifications in the optics design. An attempt was made to reduce the need for a D2 correction factor by modifying the optics in the analyzer. In particular, different band-pass filters were used to slightly shift the wavelength range being used for analysis. After making the optical modifications, tests with the analyzer indicated that the D2 correction factor still increased slightly as the SO2 concentration in the gas stream increased. Whereas the correction factor with the initial optics was about 0.8, the correction factor with the modified optics was about 1.1. However, the actual value of the correction factor is not the primary issue. Rather, the most important issue is obtaining a D2 correction factor that does not change as the SO2 concentration varies. As noted above, installation of the new optical components did not help in this respect. Therefore, in the tests performed with Hg concentrations of 9-26 µg/ m3 in the presence of 0.02-0.44% SO2, a fixed D2 correction factor was still inadequate for accurately performing the background corrections over the entire range of SO2 concentrations used. In fact, results were nearly identical to those obtained under the same experimental conditions but while using the original optics. With the modified optics installed, gas streams containing SO2 concentrations of 0.1-0.6% were analyzed. No Hg was present in those tests. The absorbance values from both the Hg and D2 channels of the analyzer were recorded as the SO2 concentration was varied. Those data are plotted in Figure 4. As can be seen, the responses (i.e., absorbance values) observed for each channel of the analyzer diverged more and more from one another as the SO2 concentration increased above about 0.2%. Although not displayed on the plots, the absorbance values from both the Hg and D2 channels fell short of the theoretical values that were expected based on the changes in SO2 concentrations. In other words, VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Uncorrected absorbance readings obtained with the prototype analyzer for various Hg concentrations with and without SO2 present.

FIGURE 6. Corrected absorbance readings using sliding D2 correction factor. the slopes of the curves in Figure 4 were not equal to “1.0”. When the SO2 concentration was tripled from the starting concentration of 0.1%, the absorbance values increased by slightly less than a factor of 3 for each channel of the analyzer. Also, the absorbance values from the Hg channel fell short of the “theoretical” values by a slightly greater amount than the absorbance values from the D2 channel of the analyzer did. No more attempts were made to reduce the need for a D2 correction factor. However, it may still be possible to do so with other optical modifications and/or through modifications in the electronics (including the photodiodes). Results of the tests with the blends of Hg and SO2 (using the original optics) are shown in Figures 5 and 6. In Figure 5, the raw (uncorrected) absorbance values observed for various Hg concentrations with and without SO2 present are shown. As can be seen, there are three linear curves for the data collected in the presence of SO2. This is due to the details of the experimental design. The SO2 was blended with air and Hg to give a variety of SO2 and Hg concentrations. The three linear curves observed for the data collected in the presence of SO2 reflect the fact that three different starting SO2 concentrations (shown by the three left-most data points with SO2) were used with Hg concentrations of 9-10 µg/m3. Those curves are vertically offset from one another since greater SO2 levels give greater absorbance values. After obtaining data for a given starting SO2 concentration, the SO2 and Hg flow rates were held constant, while the air flow was decreased incrementally, thereby increasing both the SO2 and Hg concentrations simultaneously. The increasing concentrations of SO2 and Hg resulted in increasing absorbance readings. In short, each linear curve showing data collected in the presence of SO2 reflects increasing concentrations of SO2 and Hg as the total gas flow rate was decreased while keeping the SO2 and Hg flow rates fixed. Because the absorbances due to Hg were minimal compared to the absorbances from SO2, those curves essentially reflect the increasing SO2 concentrations (from a given starting SO2 level) as the Hg concentrations also increased. The slopes of the 1772

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FIGURE 7. Background-corrected Hg concentration obtained with the CEM for a gas stream containing 9 µg/m3 Hg and 0.2% SO2. lines representing data collected with SO2 in the gas stream were 3.4, 5.7, and 11, while the slope of the line representing data collected in the absence of SO2 was 0.28. The severity of the strong SO2 interference is very evident, and it is clearly impossible to accurately analyze such gas streams for Hg directly by CVAA without some sort of background correction. In Figure 6, the background-corrected absorbance values are shown and compared with a calibration curve obtained for Hg in the absence of SO2. Looking at the data as a whole, the D2 background correction approach did a good job at correcting for the SO2 interference. For all of the data, the absorbance readings due to SO2 were 15-35 times higher than the absorbance readings due to Hg. Yet, with the background correction applied (SO2 contribution to the total absorbance signal subtracted out), the absorbance readings were generally (85% of all cases) within 10% of the theoretical absorbance values (i.e., values representing only the Hg contribution to the overall absorbance reading). The background-corrected absorbance values were within 5% of the theoretical absorbance values in about 50% of all cases. For the data shown in Figure 6, it should be noted that a “sliding” D2 correction factor was applied in order to effectively correct for the SO2 interference. Since the D2 correction factor changes slightly as the SO2 concentration varies, the D2 correction factors were bracketed along with the SO2 concentrations in order to provide a “sliding” correction factor that was dependent on the SO2 concentration. In other words, for different SO2 concentrations, slightly different D2 correction factors were used. Initially, a fixed D2 correction factor (calculated while using 0.1% SO2 without Hg) was applied to all of the D2 absorbance values, regardless of the SO2 concentration. This worked very well when SO2 concentrations were at or below about 0.2%. At those SO2 concentrations, the background-corrected absorbance values were in excellent agreement with absorbance values obtained from the Hg channel of the analyzer when using Hg streams in the absence of SO2. As the SO2 concentration increased above about 0.2%, the error in the background-corrected data increased with increasing SO2 concentration. As the SO2 concentration approached 0.5%, using the fixed D2 correction factor could no longer accurately correct for the SO2 interference. This necessitated using a “sliding” D2 correction factor. In Figure 7, the results are shown for a 9 µg/m3 Hg stream containing 0.2% SO2. The data not only are corrected for the SO2 interference using the D2 data but also are automatically correlated with a Hg calibration curve (prepared in the absence of SO2) in order to relate the background-corrected absorbance values to actual Hg concentrations. To obtain this readout, a calibration coefficient is simply input into the analyzer. The D2 background correction did an excellent job at correcting for the SO2 interference and correlating the corrected absorbance readings to an actual Hg concentration. The background-corrected absorbance values are slightly noisier than the absorbance values taken from only the Hg

channel while analyzing Hg streams in the absence of SO2. This is to be expected since absorbance readings from the D2 channel are currently significantly noisier than those from the Hg channel. The long-term signal stability was not investigated in view of some of the technological problems (i.e., the need for a variable D2 correction factor) encountered with the background correction approach. Also, it should be noted that any steps taken to resolve those problems could significantly affect (either positively or negatively) the longterm signal stability. Once those problems are remedied, issues important to field testing (including signal stability) will need to be investigated. In minimal testing with a heated sample cell, a cyclic baseline was observed due to the cycling of the temperature controller. However, this problem can probably be corrected through minor refinements in the design of the analyzer. Also, tests indicated that the problem can be avoided by indirectly heating the sample cell. This is accomplished by turning off the cell heater, insulating the cell, and heating the inlet gas line with heating tape that is controlled by an external temperature controller. Thus, the baseline problem observed while using the cell heater can easily be eliminated through design refinements if it is determined that there are advantages to using a heated sample cell. Aside from the cyclic nature of the baseline observed with a heated sample cell, the instrument sensitivity decreased due to defocusing of the optics when the cell was heated. This problem became more and more pronounced as the sample cell temperature was increased from 30 to 225 °C. If a heated sample cell is considered essential, then this phenomenon would need to be studied further. Although the use of a heated sample cell was investigated, using a heated sample cell may not offer many advantages over a cell at room temperature. The Hg CEM uses only a 10-cm sample cell yet shows very high sensitivity and rapid response (rapid rise and fall times) when using a cell at room temperature. When using streams of Hg in air, signal rise and fall times were not significantly affected by heating the cell. Therefore, from the standpoint of reducing any wall effects with Hg, there would not appear to be any significant benefits to using a heated sample cell. Results obtained to date with the prototype Hg CEM employing a D2-based background correction approach are encouraging. However, additional development is needed before testing of the Hg CEM in the field is warranted. In particular, efforts are needed to eliminate the need for a correction factor on the absorbance data from the D2 channel of the analyzer or at least to minimize the importance of the correction factor. The reason for the effects of varying SO2 concentrations on the D2 correction factor requires further investigation. Additional work should also be performed to

lower the detection limit and increase the baseline stability. Since this analyzer is the first of its kind, it is likely that the analyzer can be improved substantially through modifications in the optical/electrical design as well as upgrades in the optical/electrical components. In the sample conditioning scheme involving catalytic oxidation of the sample gases upstream from the analyzer, the possible reoxidation of the Hg downstream from the pyrolytic Hg conversion unit (to convert gaseous oxidized Hg to elemental Hg) is an area of potential concern that requires thorough investigation. Testing the analyzer while using a more complex gas matrix that more closely resembles an actual process gas stream would also be of interest. However, those tests are not warranted until some of the problems observed in this work (e.g., the changing D2 correction factor with changing SO2 concentrations) have been resolved. This work demonstrated the general technical feasibility of the analytical approach, and some technical issues were identified that need to be resolved. However, no additional development of the analyzer is planned at this time.

Acknowledgments This work was supported by funds provided to Ames Laboratory through the National Energy Technology Laboratory of the U.S. Department of Energy. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82.

Literature Cited (1) Brown, T. D.; Smith, D. N.; Hargis, R. A., Jr.; O’Dowd, W. J. J. Air Waste Management Assoc. 1999, June Issue, 1-97. (2) Johnson, J. Chem. Eng. News 2000, 79(1), 18-19. (3) Norton, G. A.; Eckels, D. E. Presented at the EPRI/DOE International Conference on Managing Hazardous and Particulate Air Pollutants; Toronto, Ontario, Canada, August 1518, 1995. (4) Laudal, D. personal communication, December 1998. (5) Iwasaki, Y.; Nakaura, H.; Tanikawa, N. Taiki Osen Gakkaishi 1988, 23(5), 293-298. (6) Hodgson, A. T.; Pollard, M. J.; Brown, N. J. Atmos. Environ. 1984, 18(2), 247-253. (7) Statnick, R.; Grote, R.; Steiber, R. Environ. Sci. Technol. 1976, 10(6), 595-596. (8) French, N. B.; Priebe, S. J.; Haas, W. J., Jr. Anal. Chem. (News Features) 1999, 71(13), 470A-475A. (9) Laudal, D. L.; French, N. B. Proc. Air Quality II Conf.; McLean, VA, Sept. 19-21, 2000.

Received for review October 29, 2001. Revised manuscript received January 30, 2002. Accepted January 31, 2002. ES0113899

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