In-situ CO measurement of gas and oil combustion flame using near infrared tunable diode laser with direct and modulated absorption signals

An in-situ optical system consisting of a near infrared single-mode distributed-feedback laser and a range-extended InGaAsSb detector is reported for CO measurement in gas and industrial oil combustion flames. To minimize interference from the other major gas species in the flame, the individual R(30) transition in the first overtone band was selected from the HITRAN/HITEMP database. The average CO volume concentration along the optical path was retrieved using two different signals: the direct absorption signal and the secondary harmonic signal of wavelength modulated absorption spectroscopy. The measurement limit was approximately 300 ppm when the direct absorption signal was used and can be improved to 30 ppm by using the secondary modulated harmonic signal. An electrically heated tubular gas cell with precisely controlled temperature and CO concentration was used for calibration. The measurement system accuracy was evaluated using a laboratory premixed C3H8/air flat flame. Practical measurements of an industrial oil flame were carried out under different fuel/air equivalence ratios. The results indicate that the system is capable of fast in-situ CO measurement of different flames and that the interference from water vapor is negligible. The maximum relative difference between the measured CO concentration and the theoretically calculated value was less than 200 ppm in a stable gas flame. For industrial oil flames, the oscillation behavior of the CO concentration and the effect of the equivalence ratio on CO formation were captured clearly.