In situ NO sensing in methane/ammonia premixed flame by calibration-free Faraday Rotation Spectroscopy

With the rapid development of ammonia combustion, the detection of NOx content in flame has become an increasingly important task. While infrared tunable diode laser absorption spectroscopy (TDLAS) has presented a viable solution to combustion diagnostics for temperature and species detection, difficulties such as transmission baseline fitting and spectral interference still remain to be major hurdles for its deployment in complex practical environment. A novel in situ NO sensing system based on Faraday Rotation Spectroscopy (FRS) is developed and deployed for calibration-free ammonia flame measurement in this work. Laser absorption from diamagnetic species including H2O and CO2 in flame bears no influence from the exerted magnetic field, such that NO as a paramagnetic species can be detected with high sensitivity. A group of R-branch transition lines from the midinfrared fundamental band of NO near 1884.3 cm-1 are selected for NO concentration detection, and a new strategy is developed for simultaneous temperature inference so that no on-site calibration will be needed. Flame measurement results are validated against thermocouple and conventional direct absorption measurements, and temperature and concentration uncertainty of 3.0 % and 4.1 % has been evaluated from experimental data analysis. NO detection limit of 5.1 ppm-m is achieved over 1000-2000 K in ammonia-seeded methane flames. The study shows great potential of tunable diode laser FRS sensors for in situ, calibration-free, and quantitative species diagnostics in combustion flames, where accuracy and sensitivity are typically hindered by the harsh, dynamically changing practical conditions.