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

Abstract

A novel quantum cascade laser (QCL) absorption sensor is presented for high-sensitivity in situ measurements of ammonia ($\hbox {NH}_3$) in high-temperature environments, using scanned wavelength modulation spectroscopy (WMS) with first-harmonic-normalized second-harmonic detection (scanned WMS-2f/1f) 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 $\hbox {NH}_3$ at $10.39\,{\upmu }\hbox {m}$ 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 $\hbox {H}_2\hbox {O}$ and $\hbox {CO}_2$ without significantly sacrificing signal-to-noise ratio. The sensor performance was validated by measuring known concentrations of $\hbox {NH}_3$ in a flowing gas cell. The sensor was then demonstrated in a laboratory-scale methane-air burner seeded with $\hbox {NH}_3$, achieving a demonstrated detection limit of 2.8 ± 0.26 ppm $\hbox {NH}_3$ 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.