Abstract

Fourier transform infrared absorption spectroscopy has been used for the determination of CO and CO₂ gas concentrations in a high-temperature cell. The gas mixtures analyzed consisted of CO, CO₂, and nitrogen; among the samples, the concentration of CO was varied between 0.5 and 4.7% and the CO₂ ranged between 0.7 and 4.9%. The temperature of the gas cell was varied between 295 and 1250 K, while the pressure was maintained at atmospheric. Throughout this temperature range, 123 absorption spectra were recorded in the gas cell at a nominal instrument resolution of 0.25 cm⁻¹. The absorption lines used for the concentration analysis consisted of 22 <i>P</i>-branch CO vibrational-rotational lines from the fundamental absorption band, and 19 <i>R</i>-branch CO₂ vibrational-rotational lines from the <i>v</i>₃ fundamental absorption band. All of the peak heights used for the concentration calculations were first numerically corrected for photometric errors resulting from the finite resolution of the FT-IR instrument. The corrected peak heights were assumed to follow the Bouguer-Lambert law at a constant furnace temperature. Fifty-one of the spectra were used to determine the temperature dependence of the line strength for each of the 41 lines. The experimentally obtained line strengths were then used to determine the gas concentrations of all 123 spectra. The calculated concentrations were compared to NDIR instrument measurements of the gas composition exiting the flow-through high-temperature gas cell. Comparison of the NDIR measured gas concentrations with the calculated concentrations from absorption spectra yielded an average accuracy of 3.6% for the CO spectra and 4.9% for the CO₂ spectra.

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