Sensitive detection of CO2 concentration and temperature for hot gases using quantum-cascade laser absorption spectroscopy near 4.2 μm

Abstract

Mid-infrared quantum-cascade laser (QCL) absorption spectroscopy of CO2 near 4.2 mu m has been developed for measurement of temperature and concentration in hot gases. With stronger absorption line-strengths than transitions near 1.5, 2.0, and 2.7 mu m used previously, the fundamental band (00(0)1-00(0)0) of CO2 near 4.2 mu m provides greatly enhanced sensitivity and accuracy to sense CO2 in high-temperature gases. Line R(74) and line R(96) are chosen as optimum pair for sensitive temperature measurements due to their high-temperature sensitivity, equal signal-to-noise ratio (SNR), weak interference of H2O transitions, as well as relatively strong line-strengths in high temperature and weak absorption in room temperature. The high-resolution absorption spectrum of the far wings of the R-branch (R56-R100) in the fundamental vibrational band of CO2 is measured in a heated cell over the range 2,384-2,396 cm(-1) at different temperatures from 700 to 1,200 K. Taking three factors into consideration, including SNR, concentration detectability, and uncertainty sensitivity, the absorption line R(74) is selected to calculate CO2 concentration. The tunable QCL absorption sensor is validated in mixtures of CO2 and N-2 in a static cell for temperature range of 700-1,200 K, achieving an accuracy of +/- 6 K for temperature and +/- 5 % for concentration measurements.