High-temperature dual-species (CO/NH₃) detection using calibration-free scanned-wavelength-modulation spectroscopy at 2.3 μm

A dual-species sensor based on scanned-wavelength-modulation spectroscopy (scanned-WMS) with the 2f/1f detection is developed for calibration-free CO and NH₃ detection in high-temperature environments. A 2.3 μm distributed feedback (DFB) diode laser was used to exploit the CO and NH₃ lines centred at 4300.7 cm⁻¹ and 4301.13 cm⁻¹, respectively. The DFB laser was scanned across the target absorption lines at 100 Hz with fast sinusoidal modulation at 5 kHz. The spectral fitting strategy was employed to least-square fit the simulated and measured WMS-2f/1f spectra. The integrated absorbance of target transitions inferred from the scanned-WMS-2f/1f spectral fitting was used to determine the species concentration. The performance of the developed sensor was validated by simultaneously measuring species concentrations under various temperatures (300–700 K) and at 1 atm in a high-temperature gas cell. The noise equivalent absorption (NEA) coefficient of 3.5 × 10^-6 cm⁻¹ for CO and 4.9 × 10^-7 cm⁻¹ for NH₃ was achieved at 700 K with an absorption path length of 20 cm. The developed dual-species sensor has the potential to be applied in the exhaust of practical combustion systems such as NH₃ monitoring for NO_x abatement in selective catalytic reduction (SCR) aftertreatment and CO sensing for optimizing combustor efficiency.