Detection limit of CO concentration measurement in lpg/air flame flue gas using tunable diode laser absorption spectroscopy

In a combustion reaction of hydrocarbon fuel, carbon monoxide (CO) is a gas species that is closely related to air pollution generation and combustion efficiency. It has a trade-off with nitrogen oxide and increases rapidly in case of incomplete combustion or in fuel-rich (Φ > 1) environments. Therefore, it is essential to measure CO concentration in order to optimize the combustion condition. In the case of a steel annealing system, the combustion environment is maintained in a deoxidation atmosphere to prevent the formation of an oxide layer on the steel sheet surface. However, it is difficult to measure the CO concentration in a combustion furnace in real-time because of the harsh environment in the furnace. Tunable diode laser absorption spectroscopy, which has the advantages of non-invasiveness, fast response, and in situ measurement-based optical measurement, is highly attractive for measuring the concentration of a certain gas species in a combustion environment. In this study, a combustion system of a partially premixed flamed burner was designed to control the equivalence ratio for fuel-rich conditions. CO concentration was measured using a distributed feedback laser with a wavenumber of 4300.7 cm⁻¹ in the mid-infrared region. The results showed that the CO concentration measured at an equivalence ratio of 1.15 to 1.50 was 0.495% to 6.139%. The detection limit in the combustion environment was analyzed at a path length of 190 cm and an internal temperature of 733 K. The ranges of the peak absorbance were derived as 0.064 and 0.787, which were within the theoretical bounds of 10⁻³ and 0.80 when the equivalence ratio was varied from 1.15 to 1.50.