Optics: measuring and testing – By dispersed light spectroscopy – With sample excitation
Reexamination Certificate
2001-12-05
2004-02-10
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
With sample excitation
C250S252100
Reexamination Certificate
active
06690462
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a calibration scheme for a multimetals continuous emissions monitor system (hereinafter “multimetals CEMS”). More specifically, this invention relates to a calibration scheme for continuous monitoring of mercury emissions from stationary sources by plasma emission spectrometry.
2. Description of the Prior Art
Almost exclusively, analyzers or continuous emission monitors for gaseous pollutants such as carbon monoxide, nitrogen oxides, hydrogen chloride, etc., are calibrated using commercially-available gas mixtures that contain precise, certified amounts of the pollutant species in question These mixtures are readily prepared by commercial vendors and then certified by suitable analytical methods. The certification is most often “traceable” to reference standards provided by the National Institutes of Standards and Technology (formerly the National Bureau of Standards). The continuous monitoring of hazardous air pollutant metals is an emerging technology that presents a number of unique technological challenges. Unlike the gaseous air pollutant described above, there are no sources for “standard” gas mixtures containing “known” amounts of metal pollutants. Metal elements exist primarily in the solid phase except at extremely high temperatures and therefore do not lend themselves to mixture and containment in the gaseous state. The one exception is mercury, which can exist as both a liquid and gas at room temperature. There have been some efforts in the commercial arena to prepare gaseous mixtures of mercury vapor (contained in cylinders) potentially useful in applications similar to that described in the present invention disclosure. However, those devices are most directly applicable for calibrating batch-sampling mercury analyzers exclusively dedicated to the detection of mercury. Batch-sampling mercury analyzers do not continuously measure mercury in sample gas. Rather, batch-sampling mercury analyzers accumulate a mercury sample over a period of time before measuring the sample. These calibrating batch-sampling mercury analyzers use analytical methodologies that are distinctly different from those employed by the multimetals CEMS. Thus far, the developers of this product have been able to certify the contents of the gas cylinder in terms of mercury concentration, but at present, are unable to provide certification of the mercury concentration of the delivered gas stream.
For the specific multimetals CEMS described above, a calibration method has been developed in which dry aerosols of known metal composition and concentration can be generated and used in lieu of unavailable gaseous calibration mixtures. This calibration method is briefly summarized here, but for a more detailed discussion please see Seltzer, M. D. and Meyer, G. A.
Inductively Coupled Argon Plasma Continuous Emissions Monitor for Hazardous Air Pollutant Metals
Environmental Science and Technology, Vol. 31, (1997), pp. 2665-2672, which is incorporated herein by reference. Aqueous solutions containing known amounts of dissolved metal salts, primarily metal nitrates, are delivered at a fixed rate of 1.5 mL/min into a COTS device known as an ultrasonic nebulizer. The ultrasonic nebulizer creates a fine, liquid aerosol from these solutions that is entrained by a carrier gas flow of 1.1 L/min through a desolvation system. The desolvation system consists of a heater that raises the temperature of the liquid aerosol carrier air mixture to 140° C. that effectively evaporates the liquid water and produces water vapor. Following evaporation of the water from the liquid aerosol droplets, the dissolved metal salt component of each aerosol droplet coalesces into a solid metal salt particle. The metal salt particulate/water vapor/carrier air stream then passes through a thermoelectric cooler that reduces the temperature of the mixture to 4° C. and effectively condenses and removes the water vapor component. Exiting the ultrasonic nebulizer device is a carrier air stream containing suspended metal salt particles. Through systematic characterization of the ultrasonic nebulizer's efficiency, the output of the nebulizer, in terms of micrograms of metal per minute, is precisely known. Therefore, the process described above is extremely useful for calibration of the multi metal CEMS. The validity of the calibration scheme has been established through rigorous performance testing in which the accuracy of the multimetals CEMS was verified in comparison to an EPA-approved test method, as described in Seltzer, M. D.
Performance Testing of a Multimetals Continuous Emissions Monitor,
Journal of the Air and Waste Management Association, Vol. 50 (2000), pp. 1010-1016, which is incorporated herein by reference.
The method described above for generating calibration aerosols works extremely well for virtually all metals except mercury. Since this method employs aqueous solutions of dissolved metal salts as starting materials to generate dry aerosols of the metals in question, suitable metal salts must be available for all metals. The soluble salts of mercury, in particular mercuric nitrate, thermally decompose at relatively low temperatures (ca. 100° C.) and therefore are not very stable in comparison to the salts of the other heavy metals, i.e., Pb, Cd, As, Be, Cr, etc. Also, the mercuric nitrate salt particles have a unique tendency to deposit or accumulate within the ultrasonic nebulizer and within various components of the multimetals CEMS sampling system and sampling interface. Because of their poor thermal stability, the mercuric nitrate salt particles tend to spontaneously decompose and produce elemental mercury vapor. The spurious release of the mercury vapor has the undesired effect of confounding the mercury calibration process, and later, during actual monitoring procedures, can produce “ghost” signals or “memory” effects that can adversely affect measurement accuracy.
The use of a mercury permeation device to calibrate an apparatus designed to measure airborne mercury in combustor exhaust gases is described in Baldwin, D. P. et al.
Testing of Continuous Sampling Air
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ICP and Mercury Systems as Continuous Emission Monitors at the Diagnostic Instrumentation and Analysis Laboratory,
Report IS-5144, Sep. 18-26, 2000. The technique described therein involves a permeation device to calibrate a dedicated mercury analyzer that measures mercury only, using absorption spectrometry. The method, system and apparatus of the present invention, described below, utilizes a permeation device to calibrate an instrument for mercury detection. However, the instrument of the present invention measures all metal elements, including mercury, using emission spectrometry, rather than absorption spectrometry.
Accordingly, a method for producing a mercury calibration stream is required that will provide precisely-known quantities of mercury in a thermally-stable form that will not persist in various components of the multimetals CEMS instrument after flow of the calibration stream is deliberately terminated.
SUMMARY OF THE INVENTION
A preferred embodiment of the present invention addresses the need for a simple and reliable means of generating and introducing a known concentration of mercury for calibration of analytical instrumentation used for monitoring the emission of mercury and other hazardous air pollutant metals from waste incinerators and other stationary sources of air pollutants. The calibration material should be in the same physical form, i.e., elemental vapor, as the anticipated pollutant emission from the subject stationary source in order that the amplitude and character of the response of the monitor or analyzer be similar for both the calibration material and actual pollutant species. Since extensive test evidence indicates that stationary sources including waste incinerators and coal-burning power plants emit mercury almost exclusively in the elemental vapor phase, a calibration material consisting of elemental mercury vapor is most appropriate
Evans F. L.
Foster Laura R.
Geisel Kara
Serventi Anthony J.
The United States of America as represented by the Secretary of
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