Abstract: Ammonia is a typical zero-carbon, hydrogen-rich fuel. Its large-scale application can achieve decarbonization at the source of high-temperature industry. However, ammonia fuel has low reactivity and a narrow stable combustion range, making it prone to significant ammonia escape due to misfiring under high-throughput fuel flow conditions. In addition, excessive ammonia injection also constitutes a major source of ammonia escape during the selective catalytic reduction stage. This study employs a quantum cascade laser with a center wavelength of 10.4 μm (962.15 cm−1 ) and a self-developed compact single-pass high-temperature gas cell (T = 120 ℃, P = 30 kPa) to develop a mid-infrared spectroscopic gas detection system for high-temperature trace ammonia escape. Firstly, based on direct absorption spectroscopy (DAS), the absorption characteristics of ammonia (NH3) standard gases with various volume fractions in four typical background gases (N2, air, Ar, and 20%H2O) are measured, and the Voigt line fitting method is applied to extract absorption information correlated with gas concentration, thereby verifying the feasibility of the spectral parameters. Subsequently, wavelength modulation spectroscopy (WMS) is adopted for low-volume-fraction NH3 measurement, with the second-harmonic-normalized-by-first-harmonic (2f /1f) method used to suppress signal fluctuations induced by non-absorption effects and background noise. The linear response characteristics of the system are then determined by establishing the correlation between the harmonic signal peak and the gas volume fraction. The experimental results show that, using DAS to measure NH3 with a volume fraction of 100 cm3 /m3 in different background gases, the measurement error is ≤ 2.1%, and using WMS technology enables NH3 detection with a minimum volume fraction of 1 cm3 /m3 . And Allan deviation analysis indicates that the detection limit of the system for NH3 is 0.02 cm3 /m3 at an integration time of 13 s.

Key words: spectroscopy, trace ammonia detection, mid-infrared absorption spectroscopy, direct absorption spectroscopy technology, wavelength modulation spectroscopy technology