A Cavity Ring-Down Spectroscopy Mercury Continuous Emission Monitor
The Sensor Research & Development Corporation (SRD) has undertaken the development of a Continuous Emissions Monitor (CEM) for mercury based on the technique of Cavity Ring-Down Spectroscopy (CRD). The project involved building an instrument for the detection of trace levels of mercury in the flue gas emissions from coal-fired power plants. The project has occurred over two phases. The first phase concentrated on the development of the ringdown cavity and the actual detection of mercury. The second phase dealt with the construction and integration of the sampling system, used to carry the sample from the flue stack to the CRD cavity, into the overall CRD instrument. The project incorporated a Pulsed Alexandrite Laser (PAL) system from Light Age Incorporated as the source to produce the desired narrow band 254 nm ultra-violet (UV) radiation. This laser system was seeded with a diode laser to bring the linewidth of the output beam from about 150 GHz to less than 60 MHz for the fundamental beam. Through a variety of non-linear optics the 761 nm fundamental beam is converted into the 254 nm beam needed for mercury detection. Detection of the mercury transition was verified by the identification of the characteristic natural isotopic structure observed at lower cavity pressures. The five characteristic peaks, due to both natural isotopic abundance and hyperfine splitting, provided a unique identifier for mercury. SRD scientists were able to detect mercury in air down below 10 parts-per-trillion by volume (pptr). This value is dependent on the pressure and temperature within the CRD cavity at the time of detection. Sulfur dioxide (SO{sub 2}) absorbs UV radiation in the same spectral region as mercury, which is a significant problem for most mercury detection equipment. However, SRD has not only been able to determine accurate mercury concentrations in the presence of SO{sub 2}, but the CRD instrument can in fact determine the SO{sub 2} concentration as well. Detection of mercury down to the low hundreds of pptr has been accomplished in the presence of SO{sub 2} at concentration levels much higher than that found in typical flue gas emissions. SRD scientists extended the interferent testing to each individual component found in flue gas. It was found that only SO{sub 2} had a significant effect on the ring-down decay curve. Upon completion of testing the components of flue gas individually a simulated flue gas stream was used to test to the CRD instrument. The result showed accurate detection of mercury down to levels below 100 pptr in a simulated flue gas stream with the concentrations of the various components above that found in a typical untreated flue gas. A sampling system was designed and integrated into the CRD instrument to carry the sample from the flue gas stack to the CRD cavity. The sampling system was constructed so that it could be placed very close to the sampling port. SRD scientists were able to couple the UV laser light into an optical fiber, which is then sent to the sampling system. This allows the laser system to be isolated from the sampling system. Initial long-term testing revealed a couple of problems related to the stability of the output frequency of the laser system. These problems have been successfully dealt with by incorporating specific software solutions into the overall data acquisition program. The project culminated in a field test conducted at the DOE/NETL pilot plant facility in Pittsburgh, Pennsylvania. The object of the test was the evaluation of a cavity ringdown spectrometer constructed for the detection of TOTAL vapor phase mercury as a continuous emission monitor (CEM). Although there is the potential for the instrument to determine the amount of speciation between neutral elemental mercury (Hg{sup (o)}) and oxidized mercury (Hg{sup (+2)}), the initial test plan was to concentrate on the measurement of the total mercury. Another added benefit is that the measurements will report the sulfur dioxide (SO 2) concentration throughout the test. This report concludes the technical work associated with Phases I & II and the field test for the Cavity Ring-Down mercury detection project.
- Research Organization:
- Sensor Research and Development Corporation
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FC26-01NT41221
- OSTI ID:
- 850501
- Country of Publication:
- United States
- Language:
- English
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A CAVITY RING-DOWN SPECTROSCOPY MERCURY CONTINUOUS EMISSION MONITOR
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