Temperature and concentration measurements in a high-pressure gasifier enabled by cepstral analysis of dual frequency comb spectroscopy

• Published in: Proceedings of the Combustion Institute 2021, Vol.38 (1), p.1561-1569
• Main Authors: Schroeder, Paul J., Makowiecki, Amanda S., Kelley, Madison A., Cole, Ryan K., Malarich, Nathan A., Wright, Robert J., Porter, Jason M., Rieker, Gregory B.
• Format: Article
• Language: English
• Subjects: Absorption spectroscopy; Cepstral analysis; Dual-comb spectroscopy; Gasification; Modified free-induction decay spectroscopy
• ISSN: 1540-7489; 1873-2704
• DOI: 10.1016/j.proci.2020.06.011

High-pressure gasification processes are important for conversion of solid materials into gaseous fuels and other chemicals. Laser absorption diagnostics are an important means to study these processes, but are challenging to implement due to the extreme temperatures and pressures present in the system. Here, we combine broadband high-resolution dual-comb spectroscopy with an advanced spectral absorption database and a new means for baseline-free absorption spectroscopy analysis to enable measurements of temperature and water vapor concentration in the core of an entrained flow gasifier operating at up to 1700 K and 15 bar. The dual-comb spectrometer measures the absorption of water vapor from 6800 to 7150 cm−1 with a point spacing of 0.0067 cm−1. The bandwidth is helpful for resolving the complex, congested absorption fingerprint of water vapor that is used to determine the species concentrations and temperature. We interpret the spectrum with absorption models based on a database measured under carefully controlled high-temperature conditions with the dual-comb spectrometer. The database includes the pressure broadening, shift, and temperature dependence of these parameters for water vapor in argon, which is the gasifier bath gas. Finally, fitting the absorption model to the data is enabled by modified free induction decay analysis, which is an approach for quantitatively obtaining species and temperature information without determining the baseline intensity of the spectrometer. The baseline-free approach is crucial to success in this environment, where there are no non-absorbing regions of the spectrum to anchor the normalization of the laser intensity as in traditional direct absorption spectroscopy. We demonstrate good agreement with temperatures measured on the reactor core via optical pyrometry, and show that water vapor concentrations in the reactor core did not reach the expected system set points during some experiments.