Inductively Coupled Argon Plasma Continuous Emissions Monitor for

Environ. Sci. Technol. 1997, 31, 2665-2672

Inductively Coupled Argon Plasma Continuous Emissions Monitor for Hazardous Air Pollutant Metals MICHAEL D. SELTZER* Research and Technology Group, Code 4B2300D, Naval Air Warfare Center Weapons Division, China Lake, California 93555-6001 GERHARD A. MEYER Thermo Jarrell Ash Corporation, Franklin, Massachusetts 02038

The design and operation of instrumentation capable of continuous, real-time detection and monitoring of hazardous air pollutant metals in the effluent of boilers, incinerators, and furnaces is reported. A commercially available inductively coupled argon plasma spectrometer, modified for introduction of sample air, provides sensitivity for several metals comparable to that of EPA-approved manual methods, with a measurement frequency of 30-60 measurements/h. Achievable detection limits for the present list of hazardous air pollutant metals range from 0.1 to 20 µg/dry standard cubic meter; a capability unmatched by competing CEM technologies. To ensure that a representative sample is obtained at all times, air is isokinetically extracted from a stack or duct by an automated system. An innovative sampling interface combines continuous sample extraction at the relatively high and often variable flow rates required for isokinetic conditions with periodic introduction of aliquots of sample air and reference air at the constant and low flow rates required for injection into the argon plasma.

Introduction The promulgation of regulations governing the destruction of hazardous wastes in boilers and industrial furnaces (BIFs) in 1991 has created an impetus for the development of instrumentation and methodology capable of continuous monitoring of metal concentrations in flue gas emissions (1). The current list of hazardous air pollutant (HAP) metals includes Ag, As, Ba, Be, Cd, Co, Cr, Hg, Mn, Ni, Pb, Sb, Se, and Tl. Recently, more definitive rules have been proposed that restrict metal emissions from hazardous waste combustors including incinerators, cement kilns, and light-weight aggregate kilns (2). These proposed guidelines, known as the MACT (maximum achievable control technology) rules, offer significant operating incentives for incorporating multimetals continuous emissions monitors (CEMs), pending their future commercial availability. From the time the rules described above were promulgated or proposed to the present, no commercial CEMs for metals have existed. The assurance of continuous compliance with present and future regulations regarding airborne metal emissions will likely be achieved only in conjunction with the advent of several new technologies being developed for metals CEM applications (3-9). The advantages offered by a metals CEM include continuous * Corresponding author telephone: 760-939-1608; fax; 760-9391617; e-mail: mike [email protected].

S0013-936X(97)00084-9 CCC: $14.00

 1997 American Chemical Society

analysis of flue gases for metal concentrations, unprecedented rapid response, and nearly instantaneous indication of compliance status. These previously nonexistent capabilities ensure that effective remedial action can take place even prior to onset of noncompliance. Among these emerging technologies, the multimetals CEM system described in the present paper, henceforth referred to as the TraceAIR system, has demonstrated the most success in achieving performance objectives considered essential for hazardous waste applications (10). The TraceAIR system provides sensitive and simultaneous multi-element detection of entrained metal aerosols and vapors by direct injection of a stream of sample air into an inductively coupled argon plasma (ICAP) spectrometer. Direct analysis of metals in a sample airstream eliminates the need for the time-consuming and labor-intensive sample collection and digestion procedures that are necessary components of conventional stack testing methods. The introduction of sample air, extracted from the stack of a Navy pyrotechnics incinerator, directly into an inductively coupled air plasma was successfully demonstrated in 1992 and permitted rapid and sensitive detection of copper, barium, and strontium emissions (3). This prototype system represented the first automated multimetals CEM, and although it was limited in terms of the number of metals that could be detected simultaneously, its potential gained considerable recognition (10), and much of its innovative methodology is extant in the present CEM. Introduction of sample air was straightforward and presented no particular difficulties due to compatibility of the sample air with the gas used to sustain the plasma. Although the air plasma proved adequate for monitoring metals such as the pyrotechnic components listed above, an argon plasma is required to provide adequate excitation of a range of metals as diverse as those included among the HAP metals. Introduction of sample air into an argon plasma is complicated by differences in both the electrical and thermal properties of argon and air. Particular attention must be paid to selecting the proper torch geometry, gas flow rates, and applied power to achieve satisfactory interaction and energy transfer between the argon plasma and HAP metals entrained into the air-dominated analytical region in the center of the plasma. Optimizing this interaction results in a reasonably robust plasma and an attendant level of analytical performance approaching that which is achievable in pure argon for aqueous samples. A robust plasma in the present case, is one in which the sample matrix, i.e., concomitant moisture content and molecular composition, does not adversely affect the plasma’s ability to adequately excite airborne metals. A number of feasibility studies concerning the introduction of sample air into an argon plasma have been reported (4, 11, 12). On the basis of both theoretical considerations and empirical observations regarding the presence of air in the argon plasma and the advantages of viewing the mixed-gas plasma axially, independent predictions of detection sensitivity for a wide range of HAP metals were made (11, 12). As a result of the present investigation, these predictions have since been validated. Conventional manual methods for measuring airborne metal concentrations require that a truly representative stream of air be extracted from a stack or duct to permit subsequent capture of metal aerosols, particulates, and vapors using a sampling train consisting of a filter and a series of liquidfilled impingers (13). This is typically accomplished by extracting the sample air under isokinetic conditions that minimize perturbations to the stack air flow and the extracted slipstream that might otherwise affect the analytical results.

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Similar sample extraction protocols are employed by the TraceAIR system. However, volumetric sampling rates required to achieve strictly isokinetic extraction greatly exceed the flow rates used for analytical injection of sample air into the argon plasma. Additionally, to maintain strictly isokinetic extraction under conditions where stack velocity, temperature, and pressure are fluctuating, the volumetric sampling rate must be continuously adjusted in accordance with these fluctuations, while at the same time, a constant sample injection rate must be observed. It is a challenge therefore, to accommodate the mismatch between the high and often variable volumetric sampling rates required for isokinetic extraction and the relatively low yet constant sample injection flow rates that are optimum for analytical measurement in the plasma. One solution to this problem is the incorporation of a sampling interface similar to that described in ref 3 and in later sections of this paper. Ideally, a continuous emissions monitor should provide frequent measurements in which all target species are analyzed simultaneously. The CEM should also provide sufficient sensitivity to permit reasonably precise measurement of all species at concentrations well below regulatory limits. Speed and sensitivity appear to be mutually-exclusive goals for several prototype multimetals CEMs. For example, laser-assisted spark spectrometry (LASS) (6, 7) is potentially the most rapid method demonstrated to date but is inherently “sample-starved” due to the minute volume of air that is analyzed during each laser pulse. A similar compromise exists for the multimetals CEM approach that employs X-ray fluorescence spectrometry for elemental analysis (8, 9). This technique is intrinsically very sensitive yet requires a minimum of 15-30 min for sample collection/preconcentration and is incapable of detecting beryllium, an important air toxic. In stark contrast to these methods, the TraceAIR ICAP-based multimetals CEM has the potential to achieve both high speed and sensitivity simultaneously. Its sensitivity advantage arises from efficient use of the available sample, i.e., time-integrated atomic emission as opposed to pulsed, point sampling as is done in LASS, and the selection of only the most sensitive HAP metal emission lines, many of which are not adequately excited in less robust sources. The ultimate sensitivity achieved by the ICAP approach is only limited by the volume of sample air that can be injected into the argon plasma per unit time, a process for which optimization has yet to be fully explored. The TraceAIR system offers a measurement frequency of 30-60 measurements per hour and can be configured to automatically adjust its measurement frequency according to the compliance status of detected metal emissions. It is important to note that, by incorporating a multichannel direct-reader spectrometer, all metals are analyzed simultaneously within a single 1-2-min measurement cycle. Consequently, the TraceAIR system could have a considerable advantage over techniques that are potentially more rapid but can only measure a few metals at a time. The TraceAIR system integrates the capabilities of a state of the art ICAP as an elemental analyzer, sample extraction methods that emulate those employed in EPA-approved conventional methods, and sound analytical practices including spectral background correction, correction for spectral interferences, and blank subtraction. As a result, analytical performance, rivaling that of conventional reference methods, is achieved with the added advantage of nearly-instantaneous response. The successful demonstraction of the TraceAIR system’s capabilities has established this prototype multimetals CEM as the most promising solution to date to the problem of on-line compliance monitoring for industrialscale combustors and other stationary sources of hazardous air pollutant metals (14, 15). The present paper discusses the design, operation, and results of preliminary field evaluation of the TraceAIR ICAP-based multimetals CEM.

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TABLE 1. Analytical Wavelengths element

wavelength

element

wavelength

aluminum antimony arsenic barium beryllium cadmium chromium cobalt lead

308.215 206.838a 189.042a 493.409 313.042 226.502 267.716 228.616 220.353a

iron manganese mercury nickel selenium silver thallium uranium

292.969 257.610 184.950b 231.604a 196.026a 328.068 190.864a 367.007

a

Second order.

b

Mercury 253.652 line was used previously.

Instrumentation The TraceAIR multimetals CEM consists of five subsystems: an elemental analyzer (ICAP), an automated isokinetic extraction system, a sampling interface, a calibration apparatus, and a computer controller. Operating in a highlyintegrated manner, these components are responsible for automated, continuous sample extraction and transport from the stack to the analyzer and subsequent measurement of entrained metal aerosols, particulates, and vapors. Elemental Analyzer. At the heart of the TraceAIR system is a Trace 61E ICAP (Thermo Jarrell Ash, Franklin, MA) featuring a 27-MHz direct coupled RF generator, axial viewing of the plasma, and a simultaneous multichannel direct-reader vacuum spectrometer. A number of modifications have been made to the ICAP instrument to permit the introduction of sample stream air. A low-flow, Fassel-type torch has been substituted for the torch supplied by the manufacturer for conventional argon operation. The critical diameters of the torch tubing are as follows: outer tube 18 × 20 mm; intermediate tube 15 × 17 mm; and injector 1.5 × 4 mm. The plasma is sustained using 1350-W applied RF power with outer and auxiliary argon flows of 16 and 0.8 L/min, respectively. The carrier gas flow through the plasma torch injector consists of 0.43 L/min sample air and 0.18 L/min argon. The 0.18 L/min argon flow added to the sample air enhances excitation conditions relative to those obtained using only pure air in the central channel of the plasma. The 0.43 L/min sample air flow rate represents a compromise between optimum sample throughput and sample residence time in the plasma (see discussion of sampling interface). The 0.43 L/min sample air flow and the 0.18 L/min argon flow are combined, prior to plasma injection, in a glass mixing chamber, located adjacent to the plasma torch. Table 1 lists the wavelengths of the detector channels installed in the direct-reader vacuum spectrometer. Atomic emission is detected using first-order lines except where noted. These wavelengths were selected on the basis of expected sensitivity. The spectrometer is capable of simultaneous analysis of metals on up to 61 individual wavelengths permitting selection of primary and secondary emission lines for each metal of interest. In a mixed-gas plasma such as the one described here, the presence of numerous molecular emission bands of nitrogen and nitrogen oxides gives rise to a very complex background spectrum relative to that observed in pure argon. Spectral background correction is achieved by measuring background emission intensity at selected offpeak wavelengths for each element. This is accomplished by scanning a galvanometer-driven, quartz refractor plate (spectrum shifter) located behind the entrance slit of the spectrometer. The maximum allowable signal integration time is determined by the duration of steady-state sample introduction achieved when an aliquot of sample air is pneumatically transported from the sampling interface and injected into the plasma (see discussion of sampling interface). Spectral interferences from concomitant metals such as iron and aluminum have the potential for generating sig-

FIGURE 1. Block diagram of TraceAIR multimetals CEM system including detail of heated probe assembly. nificant errors due to coincidence of emission lines with those of the target elements. The present system is configured to allow simultaneous monitoring of iron and aluminum in stack gases so that appropriate corrections can be made to eliminate the effects of spectral interferences arising from these elements. Corrections are made actively during each stack air measurement by applying appropriate interfering element coefficients determined previously for each affected HAP metal. Isokinetic Extraction System. The TraceAIR system employs a classic isokinetic sampling apparatus to ensure extraction of a representative sample airstream. Stack velocity is obtained indirectly by measuring the differential pressure across a pair of S-type pitot tubes located at the end of the heated sampling probe (Clean Air Engineering, Palatine, IL), as illustrated in Figure 1. The pitot tubes are connected by an umbilical to digital micromanometer (Furness Controls, Charlotte, NC) that measures and records the differential pressure readings. The sampling probe is placed in the stack, and the sampling nozzle is oriented to directly intercept air flow, usually along the centerline of the stack, although stack traverses are possible. Also located at the end of the sampling probe is a thermocouple for measuring stack temperature. The thermocouple is also connected to the micromanometer by means of the umbilical and a temperature transmitter. Stack absolute pressure is obtained from an in-stack transducer. Differential pressure, stack temperature, and absolute pressure values are periodically downloaded from the micromanometer to the computer controller. Stack moisture content is obtained by independent means and is provided to the computer through a data input file. A compiled BASIC program, subservient to the CEM operating software, calculates the stack velocity in the manner described in EPA Test Method 2 (16). The same program then calculates the appropriate volumetric sampling rate, in standard liters per minute, required to achieve 100% isokinetic extraction under the present stack conditions through a sampling nozzle of known diameter, using eq 1. Accordingly, a proportional voltage from a digital to analog converter is used to generate a set point for a mass flow controller mounted on the downstream side of a diaphragm pump (Figure 1). The volumetric sampling rate is recalculated prior to each measurement cycle, and the flow is adjusted accordingly. In this manner, continuous isokinetic extraction can be achieved despite inadvertent fluctuations in stack velocity or temperature:

Rvs )

60000VsTstdPabsAn(1 - Bw) TsPstd

(1)

where Rvs is the volumetric sampling rate (standard L/min),

FIGURE 2. Schematic drawing of sampling interface illustrating sequential filling and purging of sample loop. Vs is the stack gas velocity (m/s), Tstd is the standard temperature (294 K), Rabs is the absolute stack pressure (kPa), An is the cross-sectional area of nozzle (m2), Bw is the mole fraction of stack gas water content, Ts is the stack temperature (K), and Pstd is the standard pressure (101.3 kPa). The sampling probe is connected to the sampling interface using a 1.0 cm i.d., heated Teflon sampling line (Technical Heaters, San Fernando, CA) thermostatted to 135-150 °C. The heated probe and sample are required to maintain the extracted stack air above its dewpoint, thereby preserving the integrity of the sample airstream. It is advantageous in terms of sample transport efficiency to select a sampling nozzle of appropriate diameter such that the volumetric sampling rate required for 100% isokinetic extraction and the associated linear velocity through the heated sample line are maximized. Sampling Interface. As stated earlier, the purpose of the sampling interface is to accommodate the mismatch in sample air flow rates required for both optimum collection and plasma introduction. This is accomplished using the hardware described below, which allows acquisition of an aliquot of sample air at high and even variable flow rates and immediate injection of that aliquot of sample air into the plasma at a low but constant flow rate. The sampling interface used in the present CEM system is similar to a design described in ref 3 with the exception of a number of design improvements. Figure 2 shows an operational schematic of the sampling interface. The present design employs a 12-m sample loop constructed from 1.1 cm i.d. copper tubing. Each of the threeway solenoid valves used in the previous design has been replaced with a complementary pair of two-way valves, one normally open and one normally closed. The present valves are also much less restrictive to air flow. An additional modification involves the placement of a thermocouple near the end of the sample loop for measuring sample air temperature; a process essential to achieving accurate correction of measurements to standard conditions. Stack air is transported by suction from the stack and through the interface by a diaphragm pump located downstream. When the solenoid valves are in the de-energized state as illustrated in Figure 2a, stack air enters the upstream end of the sample loop, traverses the length of the loop, and exits the interface through an exhaust port so that at any instant a representative aliquot of stack air is available in the sample loop for transfer to the plasma spectrometer. This phase of operation is referred to as “sample loop filling”. Simultaneously, a 0.43 L/min flow of clean air is introduced directly into the glass mixing chamber adjacent to the plasma torch for mixture with argon and subsequent injection into


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the plasma to allow an instrument blank measurement to be obtained. (The instrument blank is subtracted from all subsequent stack air measurements and is used primarily to correct for small numerical offsets.) To initiate measurement of a stack air sample, the solenoid valves in the sampling interface are energized upon computer command and opened or closed as illustrated in Figure 2b. The 0.43 L/min flow of clean air previously introduced directly into the mixing chamber is diverted to the upstream end of the sample loop where it acts as a moving piston to propel the resident stack air through the glass mixing chamber and into the plasma. This phase of operation is referred to as “sample loop purging”. Energizing the solenoid valves also results in diversion of the flow of stack air directly to the exhaust port of the sampling interface, bypassing the sample loop. Approximately 15 s after the solenoid valves are energized, introduction of sample air into the plasma attains a steady state that lasts for approximately 45 s. It is during this interval that all signal integrations are made. After signal integrations have been completed, the solenoid valves are de-energized, the 0.43 L/min flow of clean air is diverted back to the mixing chamber, and stack air is allowed to fill the sample loop. Following a 15-s delay to allow the sample loop to be thoroughly flushed with fresh stack air and the mixing chamber and plasma to be purged of sample air, the process is repeated ad infinitum. Additional time delays may be inserted in order to reduce the sampling and measurement frequency. It is important to note that, while a dramatic reduction in sample air velocity in the sample loop occurs at the moment the loop begins to purge, attendant losses in both calibration and sample aerosols at this instant are appear to be negligible. The entire length of the sample loop, all solenoid valves, and the aerosol transfer tube used to deliver sample air to the mixing chamber and plasma are heat traced to prevent condensation of moisture in the sample path. The sampling interface is housed in an aluminum enclosure and located directly beneath the plasma torch box. After the unused portion of the stack airstream exits the sampling interface, excess moisture is condensed and removed from stack gas to protect the diaphragm pump and the mass flow controller by a water-cooled helical heat exchanger positioned immediately downstream from the sampling interface and ICAP (Figure 1). Calibration Apparatus. Standard metal aerosols are not available as calibration gases from commercial sources. Consequently, metal aerosols of known concentration must be generated as needed for calibration purposes. For the present application, an ultrasonic nebulizer (Cetac Model 5500AT, Omaha, NE) is used to produce dry aerosol particles from aqueous solutions containing the metals of interest. The ultrasonic nebulizer employs a desolvation system that separates and removes most of the moisture from the generated aerosols. The apparatus provides a stable flow of metal-containing aerosols of submicron size. By carefully optimizing the operating conditions for the aerosol generator, very reproducible output efficiency can be obtained, and airborne metal aerosols of known concentration can thereby be produced. Aqueous solutions are introduced using an uptake rate of 1.5 mL/min that is regulated by a peristaltic pump. As the liquid stream impinges upon the ultrasonic transducer, a fine mist is generated. The mist is heated to approximately 140 °C to evaporate moisture and subsequently cooled to approximately 4 °C to condense the moisture and effectively remove it from the 1.1 L/min air carrier stream used to transport the residual metal aerosols to the CEM. At the input port of the sampling interface, the carrier flow containing the aerosols is the mixed with ambient dilution air. A mass flow controller, calibrated at standard conditions, i.e., 21 °C and 101.3 kPa, is used to establish a total flow rate of 15 standard L/min of calibration air through the sampling

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interface. Since the mass per unit time output of the aerosol generator is known, the resulting net aerosol concentration after dilution can be established by knowing the total flow rate of the diluted aerosol mixture or calibration air. The output efficiency of the aerosol generator was determined in the laboratory by absolute mass analyses involving replicate capture of aerosols on cellulose ester filters followed by nitric acid digestion of the filters and spectrochemical analysis using ICAP atomic emission spectrometry. This method proved reliable, and a relative standard deviation of 5% was achieved for individual elements in experiments in which five replicate samples were collected. Under these conditions, output efficiencies of 25 ( 1% were routinely obtained. To generate metal aerosols of a particular concentration, an aqueous solution of appropriate concentration is used. For example, a 10 µg/mL aqueous solution of lead nitrate introduced under these conditions described above can be used to generate an airborne lead aerosol with a final concentration, after mixing and subsequent dilution, of 250 µg/dry standard m3. This process was found to be highly repeatable and was therefore adopted as the basis of a field calibration scheme for the multimetals CEM. Linear correlation coefficients of 0.999 or greater are typical for TraceAIR calibration. A unique advantage of the present calibration system is that, with the exception of being transported through a long length of sample line as are sample aerosols, the calibration aerosols follow an identical path through the sampling interface prior to introduction into the plasma; therefore, it is reasonable to assume that both sample and calibration aerosols are introduced with similar efficiency. Although this system is typically calibrated with metal aerosols containing concentrations as high as 500 µg/dry standard m3, highly linear response to concentrations in excess of 5000 µg/dry standard m3 is routinely observed. Computer Controller. All instrument and sampling functions are fully automated using a 486 personal computer. Thermospec software, in conjunction with user-written software in Thermo Jarrell Ash command language, are used to control all aspects of data acquisition, signal measurement, and sequencing of sample introduction. Data output are available in a number of useful formats. Airborne metal concentration data are printed out in tabular form and, graphically, in trend chart format. Provision has been made to trigger audible alarms and illuminate status warning lights in response to limit check failures. Additionally, a process control circuit has been configured to automatically trigger a cessation of incinerator waste feed at the onset of noncompliant flue gas emissions. As described in an earlier section concerning isokinetic extraction, several functions are automated by means of a compiled BASIC program that controls operation of a serial interface for communication with the digital micromanometer; analog-to-digital converters for acquisition of temperature, pressure, and oxygen data; and a digital-to-analog converter for generating set point voltages for a mass flow controller. Additionally, the BASIC program communicates with a number of data files for the purpose of acquiring user-provided parameter values and passing these and other parameters along to the CEM operating software.

System Operation The TraceAIR system is presently mounted in a 20 ft long × 8 ft wide air-conditioned instrument trailer. Only an electrical power connection and a source of argon gas are required for operation of this entirely self-contained system. After the sampling probe has been mounted in an access port on the stack to be monitored, heated sample lines of appropriate length are connected between the probe and the CEM. All heated portions of the sampling system are allowed to equilibrate to a preset temperature, usually well above the dewpoint of the stack air. The plasma is then ignited and

allowed a 20-30-min period of stabilization. Following stabilization and prior to the initiation of monitoring, the CEM is calibrated. Calibration is achieved by introducing a series of aqueous multielement solutions into the ultrasonic nebulizer as described above and initiating data collection. The resulting metal aerosols are introduced through the sampling interface by remotely activating the solenoid valves. Typically, three standard aerosol concentrations are generated for each element of interest in addition to a calibration blank that is generated by introducing pure deionized water into the aerosol generator system. Linear least-squares calculations are automatically carried out to establish analytical working curves. The routine achievement of correlation coefficients of 0.999 or better for the calibration of most metals attests to the precise and reliable operation of sampling interface and the metal aerosol calibration apparatus and the efficient throughput of the calibration aerosols. Molecular species are known to have a pronounced thermodynamic effects on plasma excitation (4). When the present system is calibrated using ambient air as a calibration diluent or matrix, it cannot be assumed that the response of the ICAP will be identical in the presence of stack air that typically includes elevated levels of H2O and CO2. We have recently documented a linear relationship between the percent of CO2 in the sample air and the degree of attendant degradation in plasma excitation of the target metals (see later discussion) and have found it preferable to normalize our data accordingly rather than try to calibrate under “stack” conditions. The reason is simple; CO2 perturbs excitation rather uniformly over a wide range of elements while spectral interferences arising from the presence of CO2 in the plasma (see later discussion) vary in magnitude from element to element. Calibrating under stack conditions may offer a general solution to the excitation problem, but it is not compatible with our attempts to effectively correct for molecular spectral interferences using a method that is presently under development. At the initiation of air monitoring, a spectrometer alignment is performed resulting in the acquisition of a spectral profile of the argon reference emission line at 430.010 nm. Slight wavelength deviations from the reference value are noted, and the spectrum shifter is automatically compensated to correct the error prior to subsequent measurements. Automatic profiling of the spectrometer is then repeated as part of each ensuing measurement cycle. Following the spectrometer alignment is the measurement of an instrument blank using clean air. Quantitation. Measurement of atomic emission intensity on each spectrometer channel includes automatic spectral background correction. Raw emission data are also corrected for spectral interferences due to concomitant metal species in the stack gas and then undergo a blank subtraction using emission data obtained during measurements made on clean reference air. The net result is metal concentration expressed in micrograms per actual cubic meter. The use of a mass flow controller to establish the total air flow rate during calibration ensures that valid calibration is achieved independent of ambient air temperature and pressure. However, in order to normalize concentration data acquired under actual sample conditions, to dry standard conditions, the actual conditions of the air exiting the sample loop (temperature, pressure, and moisture content) must be taken into account. As described above, sample air pressure and an estimate of moisture content are accessed from a user-provided data file. Sample loop air temperature is automatically acquired using a thermocouple, signal conditioner, and an analog to digital converter. A simple calculation (eq 2) is automatically carried out using these parameters to normalize the data to dry standard conditions. Provision is also made for acquiring data from an instream

TABLE 2. Detection Limits: TraceAIR CEM System vs EPA Test Method 29 and Proposed MACT Rule Limits EPA Method 29 proposed MACT CEM stack air HAP detection limit emission limits detection limit 3 3 metal (µg/dry standard m ) (µg/dry standard m ) (µg/dry standard m3) Ag As Ba Be Cd Co Cr Hg Mn Ni Pb Sb Se Tl a

1.1 8 0.5 0.1 0.25 0.4 0.5 10 0.1 0.8 2.5 7 10 20

2.6 0.4 0.8 0.08 0.03 0.3 0.3 0.56 0.3 5.4 0.3 1.1 0.8 0.3

210/60a 210/60 270/62 210/60 50/50

270/62 210/62

Existing/new incinerators.

oxygen analyzer to permit further normalization of data to 7% oxygen for reporting purposes:

Cm(std) ) Cm(actual)

TlPstd

(2)

TstdPl(1 - Bw)

where Cm(std) is the metal concentration normalized to dry standard conditions, Cm(actual) is the metal concentration measured at actual conditions, Tl is the sample loop temperature (K), Tstd is the standard temperature (294 K), Pl is the sample loop pressure (kPa), Pstd is the standard pressure (101.3 kPa), and Bw is the mole fraction of stack gas water content.

CEM System Performance The primary performance objectives for the multimetals CEM system described above are sensitivity and rapid response. The need for accuracy and precision are implicit. In ICAP atomic emission spectrometry, sensitivity varies from metal to metal as a consequence of the intrinsic emission intensity of each metal in the plasma and the complexity and intensity of the plasma background intensity at a given analytical wavelength. Therefore, it is reasonable to expect detection limits for some of the more sensitive metals that are 1-2 orders of magnitude superior to those of the less sensitive metals. The detection limit is defined as that concentration of analyte that produces an emission intensity that is equivalent to three times the limiting noise level. An assessment of limiting noise can most easily be obtained by making seven replicate measurements of a stack air matrix containing negligible metal concentrations. The detection limit is then approximated by multiplying the standard deviation of the seven replicate measurements by 3. Table 2 lists TraceAIR system stack air detection limits, expressed in micrograms per dry standard cubic meter of air, obtained for selected metals and corresponding in-stack detection limits typically obtained using EPA Method 29. For comparison purposes, the proposed MACT emission limits are listed for selected metals (2). Although detection limit values do not represent practical levels for quantitation, they are nevertheless valuable figures of merit for optimizing instrument performance and making quantitative comparisons between different instruments or techniques.

Initial Field Evaluation A preliminary assessment of field performance of the TraceAIR system was a necessary step in identifying which aspects of this complex system worked well and at the same time identifying various problems that became apparent only under real-world conditions. The initial field evaluation took


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TABLE 3. Average TraceAIR System Concentration Valuesa vs Reference Method element

low spike

medium spike

high spike

As(CEM) As(RM) Be(CEM) Be(RM) Cd(CEM) Cd(RM) Cr(CEM) Cr(RM) Hg(CEM)b Hg(RM) Pb(CEM) Pb(RM) Sb(CEM) Sb(RM)

21.2 17.0 9.7 12.6 7.2 10.4 6.7 18.2 1.1c 1.8 5.8 16.7 20.2 13.7

45.9 66.6 51.3 52.1 40.9 45.1 16.3 56.6 6.9c 61.5 21.7 53.7 64.0 60.2

307 386 401 492 284 441 61.8 180 26.4 329 146 442 378 390

a All concentrations expressed in µg/dry standard m3. b 253.652 nm Hg detection. c Average value is less than the 15 µg/dry standard m3 Hg detection limit at 253.652 nm.

place at the U.S. EPA Research Center in Research Triangle Park, NC, in April 1996. As part of a joint EPA/DOE sponsored technology demonstration, a rotary kiln research incinerator was operated as a test bed for side-by-side comparison of prototype CEM systems. CEM performance was evaluated with due consideration to the performance specifications proposed in ref 2. For an assessment of relative accuracy, acidified aqueous solutions containing a range of target metals was nebulized into the incinerator afterburner. Replicate SW-846 Test Method 0060 runs were carried out on each of several days to evaluate the performance of the CEMs under low, medium, and high metal spike conditions. To ensure that the evaluation was done under challenging conditions, fly ash was also introduced into the incinerator at rates of 25-50 mg/dry standard m3. Conditions in the exhaust duct from which sample air was extracted were as follows: temperature, 232 °C; pressure, -0.5 kPa; moisture, 6%; oxygen, 16%; carbon dioxide, 7%; flow velocity, 8.1 m/s. A sampling nozzle with a diameter of 6.35 mm (0.25 in.) was available at the time of the test. The 9 L/min volumetric sampling rate required to achieve isokinetic extraction through this nozzle size was later determined to be not optimum for efficient transportation of airborne particulates through 23 m of heated sample line. Table 3 provides a summary of the results of comparative reference method measurements and representative CEM data, comprised of individual measurements made at 4-min intervals during the course of numerous reference method runs. Listed in Table 3 are composite results, each representing the averages of a series of reference method runs and corresponding CEM measurements conducted at each of three metal spiking levels. The results are presented in this format for the sake of brevity and to illustrate general performance at the various metal spiking levels. A more detailed tabulation of these results is available elsewhere (14). All concentrations are expressed in micrograms per dry standard cubic meter. Preliminary measurements in the absence of any metal introduction into the incinerator revealed the presence of spectral interferences arising from light emitted by the stable CN radical, a nascent molecular species formed in the plasma as a result of chemical reactions between carbon and nitrogen; the combustion product CO2 being the primary source of carbon. Spectral interferences of significant magnitude were noted for arsenic, antimony, lead, mercury, and selenium. In some instances, the effects of the spectral interference became much less pronounced at higher metal spiking levels because of the decreasing contribution of the interference to the total signal.

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Attempts were made to estimate the extent of the interferences, and many of the values shown in Table 3 reflect measurements corrected for spectral interferences after the fact. Work is presently under way to develop an active method of correction for spectral interferences due to molecular emission, similar to that used for interferences arising from concomitant metals. We have determined, subsequent to these tests, that injection into the argon plasma of air containing carbon dioxide at typical flue gas concentrations causes significant suppression of plasma excitation. An attendant loss in emission intensity, as much as 30-40% relative to that achieved in ambient air, has been observed for virtually all of the HAP metals. At the time of these tests, calibration was carried out using ambient air as a metal aerosol diluent, and considering the extent of the excitation suppression associated with carbon dioxide levels present in the stack air during subsequent measurements, it is reasonable to ascribe some of the discrepancies shown in Table 3 to this phenomenon. Poor agreement between mercury CEM values and reference method values shown in Table 3 can be attributed to several factors including spectral interferences, calibration problems, and generally unsatisfactory detection of mercury emission at 253.6 nm. During the calibration for mercury, erratic behavior was noted for the generation of mercurycontaining aerosols, suggesting the occurrence of an appreciable aerosol memory effect within the calibration apparatus or sampling interface, or both. Multipoint linear calibration could not be achieved for mercury under these circumstances, and as an alternative, a simple two-point standardization procedure was adopted that yielded an inordinately steep calibration slope for mercury. In the absence of multipoint calibration, the validity of the resulting calibration slope could not easily be confirmed. We have since determined that mercury-containing calibration aerosols are reliably generated only when additional metal salts are present in the starting solution, and as a result, linear calibration is now routinely achieved for mercury with linearity comparable to that of the other HAP metals. The 253.6-nm emission line has since been abandoned in favor of the more analytically favorable mercury line at 184.950 nm. Following the tests during which the results shown in Table 3 were obtained, the sample lines totaling 75 ft in length were rinsed with dilute nitric acid in an attempt to recover residual particulate matter and condensed metal vapors. The rinsate was digested and subsequently analyzed by ICAP atomic emission spectrometry to estimate the extent of possible sample losses in the lines. Sufficient amounts of Pb, As, Be, Cd, and Cr were found to account for an approximately 1015% sample loss for these elements. Deposition losses of particulate matter in the sample line can be attributed to using a less than optimum volumetric sampling rate at which linear velocity was not sufficient to promote efficient transport of the particulates through the entire length of the sample line. Losses in the sample line and elsewhere in the system may have been exacerbated by the heavy particulate loading caused by the deliberate introduction of fly ash into the flue gases. This is a condition not accounted for in the present calibration scheme. Figures 3-5 illustrate the capability of the TraceAIR system to obtain frequent, repetitive, and consistent measurements of metal concentrations in extracted stack air over a severalhour period and the rapid response of the TraceAIR system to abrupt changes in the introduction of metal spikes into the incinerator afterburner. Figure 3 shows antimony CEM data recorded during a series of reference method runs made under medium-level spiking conditions. The antimony concentrations obtained using the reference method are depicted by the solid lines in Figure 3 and are within reasonable agreement with the corresponding CEM values.

FIGURE 3. Comparative measurement data for antimony. CEM data points are plotted alongside solid lines representing average values determined by reference method. Reference method values are also shown numerically.

FIGURE 5. CEM data recorded for beryllium and yttrium over a 3-h period during which metal spiking levels were varied. Measurements were made at 4-min intervals. environment has been successfully demonstrated. However, the modest degradation of analytical performance by such factors as spectral interferences, suppressed plasma excitation, and inefficient sample transport has highlighted the need for further system optimization and refinement. Efforts are presently underway to rectify these problems, and the strategies adopted for this purpose have already shown significant promise in subsequent field evaluations and will be the subject of future papers. Despite less than perfect performance in its maiden field installation, the TraceAIR multimetals CEM system has distinguished itself as a promising solution to the need for fast and sensitive measurement of metal emissions in the flue gases of hazardous waste combustors. It is fully anticipated that, as a result of iterative testing and refinement, this system will adequately fulfill the requirements of the demanding application for which it was designed.

FIGURE 4. CEM data recorded simultaneously for lead and arsenic showing high degree of correlation. Measurements were made at 4-min intervals. Figure 4 shows arsenic and lead data, recorded over a 3-h period with measurements taken at approximately 4-min intervals. There is a high degree of correlation evident between the two metals that is reasonable since metal aerosols were generated by nebulizing a homogeneous aqueous solution of target metals into the afterburner. Although reference method measurements indicate that the airborne concentrations of lead and arsenic were nearly identical, as they were in solution, the lead concentrations depicted in Figure 4 are suppressed relative to arsenic but appear nevertheless to be in consistent proportion with arsenic. Figure 5 shows beryllium and yttrium data recorded over a period of time as the beryllium spiking level was abruptly increased while the yttrium spiking was held constant. The rapid response of the TraceAIR system is evident in Figure 5. Following a brief interval of high beryllium spiking, all metal introduction was terminated, and CEM data rapidly returned to baseline levels. The sudden decay in detected metal emissions following termination of metal spike feed clearly demonstrates that there was no appreciable hysteresis introduced by the 75 ft length of heated sample line, despite the apparent deposition of particulate matter noted above.

Conclusions The design, operation, and preliminary evaluation of a multimetals continuous emissions monitor, exhibiting a number of desirable performance capabilities, has been described. The ability of the TraceAIR system to provide meaningful results while operating in a real-world stack

Acknowledgments The authors wish to acknowledge the U.S. Army Demil Technology Office and the U.S. Army Armaments Research, Development and Engineering Center, for generous and sustained sponsorship of the present effort.

Literature Cited (1) Burning of Hazardous Waste in Boilers and Industrial Furnaces. 40 CFR 266, subpart H. (2) Revised Standards for Hazardous Waste Combustors, Proposed Rules. Fed. Regist. 1996, 61 (77), 17357-17536. (3) Seltzer, M. D.; Green, R. B. Process Control Qual. 1994, 6, 37. (4) Trassy, C. C.; Diemiaszonek, R. C. J. Anal. At. Spectrom. 1995, 9, 661. (5) Gomes, A. M.; Sarrette, J. P.; Madon, L.; Almi, A. Spectrochim. Acta Part B 1996, 51, 1695. (6) Peng, L. W.; Flower, W. L.; Hencken, K. R.; Johnsen, H. A.; Renzi, R. F.; Bergan French, N. Process Control Qual. 1995, 7, 39. (7) Zhang, H.; Singh, J. P.; Yueh, F. Y.; Cook, R. L. Appl. Spectrosc. 1995, 49, 1617. (8) Cooper, J. A. Fuel Process. Technol. 1994, 39, 251. (9) Ghorishi, S. B.; Wentworth, W. E., Jr.; Goldman, C. G.; Waterland, L. R. Report on Testing the Performance of Real-Time Incinerator Emission Monitors. National Risk Management Research Laboratory, U.S. EPA: Cincinatti, OH, 1996. (10) Final Report for Airborne Metals CEMs Prepared for U.S. EPA Office of Solid Waste. Energy and Environmental Research Corporation: Durham, NC, March 25, 1994. (11) Seltzer, M. D. Process Control Qual. 1995, 7, 71. (12) Meyer, G. A. Process Control Qual. 1994, 6, 187. (13) EPA Test Method 29sDetermination of Metals Emissions from Stationary Sources. 40 CFR 60, Appendix A; U.S. Government Printing Office: Washington, D.C. (14) Haas, W. Performance Testing of Multi-Metal Continuous Emissions Monitors. Draft Report; Ames Laboratory, U.S. Department of Energy: Dec 8, 1996.


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(15) Joklik, R. Status of Multimetal CEMS Development; EPRI Workshop, Dec 12-13, 1996, Research Triangle Park, NC. (16) EPA Test Method 2sDetermination of Stack Gas Velocity and Volumetric Flow Rate (Type S Pitot Tube). 40 CFR 60, Appendix A; U.S. Government Printing Office: Washington, DC.

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Received for review January 31, 1997. Accepted May 11, 1997.X ES970084K X

Abstract published in Advance ACS Abstracts, July 1, 1997.

Inductively Coupled Argon Plasma Continuous Emissions Monitor for

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