CO Concentration Measurement With Tunable Diode Laser Absorption Spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) based on the molecular absorption of light provides a non-intrusive, sensitive and fast method for gas detection in combustion environments including a flame. As an important product of incomplete combustion of hydrocarbon fuels, CO is a key indicator of combustion efficiency and thus a good parameter for combustion control, which is significant to decrease energy consumption and promote production efficiency. In the flame, the generation of CO shows the characteristics of rapidity and variability. Therefore, a system for in situ, real-time monitoring of CO in flame is in great need.The traditional CO detection methods all involve gas sampling that may alter the gas flow field, or may result in significant time delay and secondary reactions in the probes, making it especially difficult for flame measurements. Furthermore, there are no CO-specific solid state sensors capable of tolerating temperatures greater than2000K in the flames. The primary objective of this research is to develop an optical sensor using tunable diode laser absorption spectroscopy sensor for CO measurement in the high temperature flames.In this research, a single-mode DFB laser operating near2.3μm was used to monitor CO concentration in flames. This extended near-IR laser provides a reasonably cheap, simple and robust light source compared with the mid-IR lasers to access the first overtone band of CO that is about two orders of magnitude stronger than the second overtone band covered by the telecom lasers. This would allow improved sensitivity in practical, harsh combustion environments.The absorption transition R (30) was selected according to HITRAN2008database simulation to minimize interference from water vapor. The sensor was validated in heated gas cell in a tubular furnace (～1200K) and premixed C3H8/Air flat flame (～2400K), where measured CO concentrations using both scanned-wavelength direct absorption (DA) and wavelength modulation spectroscopy (WMS) strategies were validated against calibrated mixture concentrations. The influence of background radiation and particle scattering were eliminated. With DA strategy, the noise-equivalent absorbance was estimated to be～5×10-4and a detection limit of200ppm-m was estimated with SNR=1. However, the detection limit of10ppm-m was estimated with WMS strategy.As crucial for trace gas sensing in moisture-abundant environment, water vapor absorption spectrum in the concerned wavelength region was also measured to confirm the proper line selection to avoid high-temperature water interference. Real-time, continuous monitoring of CO concentration was then demonstrated in the flat flame with varied equivalence ratio. The experiments have demonstrated the potential of infrared diode laser sensors for in situ gas sensing in harsh combustion environments.