Dual Comb Spectroscopy of High Temperature Environments

A frequency comb is a laser whose output is composed of many narrow linewidth, precisely spaced optical frequencies. Robust designs and dual comb interference techniques have recently enabled tooth-by-tooth resolved absorption spectroscopy in a compact and flexible package. In this work, we present both the first mobilization of a dual comb spectrometer in harsh environments and the first broadband, high-resolution investigation of high temperature water vapor lineshapes. The instrument was deployed to the basement of a 16MW natural gas turbine facility at CU Boulder to simultaneously measure H2O and CO2 concentrations and gas temperature in the exhaust stream. This revealed errors in the two most common spectroscopic databases used for high temperature studies, HITRAN2012 and HITEMP2010. These databases are accurate at room temperature, though their accuracy diminishes at elevated temperatures. We used the dual comb spectrometer at NIST Boulder to compare, for the first time, spectra of pure and air-broadened water at temperatures up to 1000⁰C with models generated by both databases. Their data is almost entirely extracted using Voigt lineshape profiles while advanced profiles that fix problems with the Voigt have not been broadly tested at elevated temperatures. We fit the spectra from the above comparison with Voigt, Rautian, and quadratic speed dependent Voigt lineshapes to examine their effect on 232 extracted temperature-scaling exponents. Only the quadratic speed dependent Voigt produced values that validated external computations while only requiring a single exponent to fit each transition up to 1000⁰C. We then utilized these fits to generate a new database of spectral parameters to increase high temperature accuracy of spectroscopic measurements. The database contains line centers, linestrengths, self-broadening coefficients, temperature-scaling exponents, and speed dependence parameters for 625 water vapor absorption transitions. Finally, we deployed the mobile spectrometer to Colorado School of the Mines to measure water mole fraction and temperature in the core of an entrained flow gasifier. We used measurements of the water vapor temperature up to 1400⁰C and 15atm using argon as the entrainment gas to elucidate char gasification kinetics models. Without the newly created databases, measurements in this argon rich environment would have been impossible