Non‐intrusive, in situ detection of ammonia in hot gas flows with mid‐infrared degenerate four‐wave mixing at 2.3 µm

Abstract

We demonstrate non‐intrusive, in situ detection of ammonia (NH3) in reactive hot gas flows at atmospheric pressure using mid‐infrared degenerate four‐wave mixing (IR‐DFWM). IR‐DFWM excitation scans were performed in the v2 + v3 and v1 + v2 vibrational bands of NH3 around 2.3 µm for gas flow temperatures of 296, 550 and 820 K. Simulations based on spectroscopic parameters from the HITRAN database have been compared with the measurements in order to identify the spectral lines, and an absorption spectrum at 296 K has also been measured to compare with the IR‐DFWM spectra. The signal‐to‐noise ratio of the IR‐DFWM measurement was found to be higher than that of the absorption measurement. Some spectral lines in the measured IR‐DFWM and absorption spectra had no matching lines in the HITRAN simulation. The detection limit of NH3 diluted in N2 with IR‐DFWM in this spectral range was estimated at 296, 550 and 820 K to be 1.36, 4.87 and 7.06 × 1016 molecules/cm3. The dependence of the NH3 IR‐DFWM signal on the quenching properties of the buffer gas flow was investigated by comparing the signals for gas flows of N2, Ar and CO2 with small admixtures of NH3. It was found that the signal dependence on buffer gas was large at room temperature but decreased at elevated temperatures. These results show the potential of IR‐DFWM for detection of NH3 in combustion environments. Copyright © 2016 John Wiley & Sons, Ltd.