High-sensitivity in situ QCLAS-based ammonia concentration sensor for high-temperature applications

novel quantum cascade laser (QCL) absorption sensor is presented for high-sensitivity in situ measurements of ammonia ( NH_3 ) in high-temperature environments, using scanned wavelength modulation spectroscopy (WMS) with first-harmonic-normalized second-harmonic detection (scanned WMS-2 f /1 f ) to neutralize the effect of non-absorption losses in the harsh environment. The sensor utilized the sQ(9,9) transition of the fundamental symmetric stretch band of NH_3 at 10.39 m and was sinusoidally modulated at 10 kHz and scanned across the peak of the absorption feature at 50 Hz, leading to a detection bandwidth of 100 Hz. A novel technique was used to select an optimal WMS modulation depth parameter that reduced the sensor’s sensitivity to spectral interference from H_2O and CO_2 without significantly sacrificing signal-to-noise ratio. The sensor performance was validated by measuring known concentrations of NH_3 in a flowing gas cell. The sensor was then demonstrated in a laboratory-scale methane-air burner seeded with NH_3 , achieving a demonstrated detection limit of 2.8 ± 0.26 ppm NH_3 by mole at a path length of 179 cm, equivalence ratio of 0.6, pressure of 1 atm, and temperatures of up to 600 K.