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

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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 µm 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, ∼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 ∼3 × 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 ∼0.1 ppm m (∼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 µm for in situ combustion emission monitoring and combustion diagnostics.