Opportunities exist to improve on-line process control in energy applications with a fast, non-destructive measurement of gas composition. Here, we demonstrate a Raman sensing system which is capable of reporting the concentrations of numerous species simultaneously with sub-percent accuracy and sampling times below one-second for process control applications in energy or chemical production. The sensor is based upon a hollow-core capillary waveguide with a 300 micron bore with reflective thin-film metal and dielectric linings. The effect of using such a waveguide in a Raman process is to integrate Raman photons along the length of the sample-filled waveguide, thus permitting the acquisition of very large Raman signals for low-density gases in a short time. The resultant integrated Raman signals can then be used for quick and accurate analysis of a gaseous mixture. The sensor is currently being tested for energy applications such as coal gasification, turbine control, well-head monitoring for exploration or production, and non-conventional gas utilization. In conjunction with an ongoing commercialization effort, the researchers have recently completed two prototype instruments suitable for hazardous area operation and testing. Here, we report pre-commercialization testing of those field prototypes for control applications in gasification or similar processes. Results will be discussed with respect to accuracy, calibration requirements, gas sampling techniques, and possible control strategies of industrial significance.
Hollow, metal-lined capillary waveguides have recently been utilized in spontaneous gas-Raman spectroscopy to improve signal strength and response time. The hollow waveguide is used to contain the sample gases, efficiently propagate a pump beam, and efficiently collect Raman scattering from those gases. Transmission losses in the waveguide may be reduced by using an azimuthally polarized pump beam instead of a linearly or radially polarized pump. This will lead to improved Raman signal strength, accuracy, and response time in waveguide-based Raman gas-composition sensors. A linearly polarized laser beam is azimuthally polarized using passive components including a spiral phase plate and an azimuthal-type linear analyzer element. Half-wave plates are then used to switch between the azimuthally polarized beam and the radially polarized beam with no change in input pump power. The collected Raman signal strength and laser throughput are improved when the azimuthally polarized pump is used. Optimization of the hollow waveguide Raman gas sensor is discussed with respect to incident pump polarization.
We previously reported the use of hollow metal and dielectric lined waveguides as gas cells used in real-time Raman spectroscopy of gas mixtures. Our team has constructed a multi-gas Raman sensor system capable of measuring molecular components in most gas mixtures with sub-percent accuracy and a sub-second sampling rate. This combination of speed and accuracy is enabled by the novel combination of optimized sample-cell collection and appropriate gas-stream configuration. Here, we discuss the new state-of-the-art in Raman process-gas analysis and share relevant testing data on our optimized system for potential industrial end-users. We conclude that a paradigm shift in technology for gas measurement applications could result from the instrumentation developed herein.
Hollow, metal-lined capillary waveguides have recently been utilized in spontaneous gas-Raman spectroscopy to
improve signal strength and response time. The hollow waveguide is used to contain the sample gases, efficiently
propagate a pump beam, and efficiently collect Raman scattering from those gases. Transmission losses in the waveguide
may be reduced by using an azimuthally polarized pump beam instead of a linearly or radially polarized pump. This will
lead to improved Raman signal strength, accuracy, and response time in waveguide-based Raman gas-composition
sensors. A linearly polarized laser beam is azimuthally polarized using passive components including a spiral phase plate
and an azimuthal-type linear analyzer element. Half-wave plates are then used to switch between the azimuthally
polarized beam and the radially polarized beam with no change in input pump power. The collected Raman signal
strength and laser-throughput are improved when the azimuthally polarized pump is used. Optimization of the hollow
waveguide Raman gas sensor is discussed with respect to incident pump polarization.
A gas composition sensor based on Raman spectroscopy using reflective metal lined capillary waveguides is tested under
field conditions for feed-forward applications in gas turbine control. The capillary waveguide enables effective use of
low powered lasers and rapid composition determination, for computation of required parameters to pre-adjust burner
control based on incoming fuel. Tests on high pressure fuel streams show sub-second time response and better than one
percent accuracy on natural gas fuel mixtures. Fuel composition and Wobbe constant values are provided at one second
intervals or faster. The sensor, designed and constructed at NETL, is packaged for Class I Division 2 operations typical
of gas turbine environments, and samples gas at up to 800 psig. Simultaneous determination of the hydrocarbons
methane, ethane, and propane plus CO, CO2, H2O, H2, N2, and O2 are realized. The capillary waveguide permits use of
miniature spectrometers and laser power of less than 100 mW. The capillary dimensions of 1 m length and 300 μm ID
also enable a full sample exchange in 0.4 s or less at 5 psig pressure differential, which allows a fast response to changes
in sample composition. Sensor operation under field operation conditions will be reported.
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