Utilization of multiple harmonics of wavelength modulation absorption spectroscopy for practical gas sensing [electronic resource]
Keywords: Mechanical Engineering.

To meet more rigorous criteria for environmental-unfriendly emissions and to increase energy efficiency, in-situ real-time sensors are needed to optimize the performance of next-generation energy systems. The emergence of high-quality (narrow linewidth, fast tuning capability) tunable diode lasers (TDLs) has enabled the use of wavelength modulation spectroscopy (WMS) for harsh industrial applications. Compared to conventional direct absorption measurements, WMS has the advantage of 10-100 times better detection sensitivity, avoids the need to obtain a zero-absorption baseline, and provides much better isolation from the beam steering, non-absorption transmission loss (e.g., light scattering) or mechanical vibrations. Many models have been developed to interpret the measured WMS signal into absolute absorption. However, most of these models are limited to specific applications by a wide variety of assumptions and approximation most of which deal with the simultaneous intensity and wavelength modulation of injection-current-modulated diode lasers. In this dissertation, two generalized approaches to analyze the WMS absorption signal were developed that account for non-ideal simultaneous intensity modulation of laser output when injection current variation is used for wavelength modulation. The first approach is ideal for wavelength-fixed WMS (the laser mean wavelength is fixed) analysis and the second approach is ideal for wavelength-scanned (the laser mean wavelength is scanned) WMS analysis, and both of them can be used for arbitrary modulation depth, or laser architectures even when severe non-linear intensity modulation occurs simultaneously with wavelength modulation. These new interpretations of WMS absorption signals provide the potential for extended and improved use of WMS for practical gas sensing in a much wider array of applications. The first approach built on earlier work in our laboratory. The analysis of calibration-free, 1f-normalized, WMS-2f absorption signals was extended to higher harmonics (for example 3f, 4f...) using traditional Fourier analysis. The new approach and procedure developed also accounts for non-ideal wavelength-tuning of the injection-current tuned laser as well as etalon interference from the optical components in the laser line-of-sight (LOS). This approach was validated using measurements of the CO transition of R (11) in the 1st overtone band near 2.3µm in a laboratory cell at room temperature for a range of CO mole fractions (0.21-2.8%) and pressures (5-20atm). For high-pressure gas sensing, wavelength modulation spectroscopy with higher-order harmonic detection (WMS-nf, n> 2) was found to have less influence from the WMS background signals when the selected modulation depth was near the optimal modulation depth for the WMS-2f signal. This WMS approach was then used for measurements in a pilot-scale entrained-flow coal gasifier at the University of Utah. Even though the particulate scattering reduced the laser transmission as much as 99.997%, and pressure broadening at the 18atm (~250psig) operating pressure blended the absorption transitions, successful in-situ rapid-time-resolved 1f-normalized WMS-2f absorption measurements for gas temperature and H2O mole fraction were made. Based on lessons learned during the gasifier measurements at Utah and a desire to eventually develop real-time sensors for long-term monitoring, a second approach for WMS analysis was developed that differs from previous WMS analysis strategies in two significant ways: (1) the measured laser intensity without absorption is used to simulate the transmitted laser intensity with absorption and (2) digital lock-in and low-pass filter software is used to expand both simulated and measured transmitted laser intensities into harmonics of the modulation frequency, WMS-nf (n=1,2,3, ...), avoiding the need for an analytic model of intensity modulation or Fourier expansion of the simulated WMS harmonics. The new method was demonstrated and validated with WMS of H2O dilute in air (1atm, 296K, near 1392nm). WMS-nf harmonics for n=1 to 6 are extracted and the simulations and measurements are found in good agreement for the entire WMS lineshape. This new analysis scheme was applied to monitor the synthesis gas output from an engineering-scale transport reactor coal gasifier at the National Carbon Capture Center. There the pressures ranged up to 15 atm (~220psig) and temperatures up to 650K. Continuous monitoring of moisture level in the gasifier output with 2s time resolution was performed by the TDL sensor for more than 500 hours, including the periods of burner ignition, combustion heating with a propane flame, coal combustion, coal gasification, and reactor shut-down via coal-feed termination. In addition, a novel and rapid approach to determine the collisional linewidth via the WMS signals at different harmonics at the modulation frequency is presented. The peak values of the WMS-nf absorption spectrum near the transition line center are used to infer the absorption lineshape, which is exploited here to extract collision-broadening halfwidth from the ratio of WMS-4f/WMS-2f (or other even harmonics) signals when the mean laser wavelength is tuned to line center. Measurement of the absorption linewidth enables quantitative WMS measurements without the need for a collision-broadening database. Alternatively, when collision-broadened spectral data are available, a WMS-based pressure sensor can be realized, and a demonstration using the 4fpeak/2fpeak ratio gives less than 0.7% difference for the pressure for cell measurements from 100 torr to 753 torr. These new WMS analysis schemes have been validated in near commercial environments and illustrate the potential of their use to develop practical TDL sensors for a wide variety of industrial applications.