In situ combustion measurements of CO with diode-laser absorption near 23 µm

Abstract

In situ measurements of CO concentration were recorded with tunable diode-laser absorption spectroscopy techniques in both the exhaust and the immediate post-flame regions of an atmospheric-pressure flat-flame burner operating on ethylene air. Two room-temperature cw single-mode InGaAsSb/AlGaAsSb diode lasers operating near 2.3 microm were tuned over individual transitions in the CO first overtone band (v' = 2 <-- v" = 0) to record high-resolution absorption line shapes in the exhaust duct [79 cm above the burner, approximately 470 K; R(15) transition at 4311.96 cm(-1)] and the immediate postflame zone [1.5 cm above the burner, 1820-1975 K; R(30) transition at 4343.81 cm(-1)]. The CO concentration was determined from the measured absorption and the gas temperature, which was monitored with type-S thermocouples. For measurements in the exhaust duct, the noise-equivalent absorbance was approximately 3 x 10(-5) (50-kHz detection bandwidth, 50-sweep average, 0.1-s total measurement time), which corresponds to a CO detection limit of 1.5 ppm m at 470 K. Wavelength modulation spectroscopy techniques were used to improve the detection limit in the exhaust to approximately 0.1 ppm m (approximately 500-Hz detection bandwidth, 20-sweep average, 0.4-s total measurement time). For measurements in the immediate postflame zone, the measured CO concentrations in the fuel-rich flames were in good agreement with chemical equilibrium predictions. These experiments demonstrate the utility of diode-laser absorption sensors operating near 2.3 microm for in situ combustion emission monitoring and combustion diagnostics.