A mid-infrared laser absorption sensor for calibration-free measurement of nitric oxide in laminar premixed methane/ammonia cofired flames

Abstract

An interband cascade laser (ICL)-based absorption sensor was developed for calibration-free and high-fidelity measurement of nitric oxide (NO) in laminar premixed methane/ammonia cofired flames using a combination of a micro-probe and a low-pressure, high-temperature gas cell. The developed sensor, which has been shown to be particularly suitable for kinetic studies of ammonia combustion, used a tunable, continuous, narrow-linewidth, free-space ICL operating at ~5.2 μm to target the optimal NO transition centered at 1900.07 cm−1. A direct absorption spectroscopy technique with a reduced line model was implemented for calibration-free measurements. Comprehensive experiments were first conducted to evaluate the sensor performance against the standard gas mixture at varied pressure from 20 to 101 kPa at 393 K, showing a relative difference of ≤2.5% and a measurement uncertainty of ≤2.0%. Such a sensor shows a minimum detection limit of ~2 ppm at the integration time of 1 s. The sensor was then applied for investigating the NO formation in ammonia-methane cofiring flames with various ammonia blending ratios; the acquired experimental data was further used as benchmark data for evaluation of literature chemical mechanisms for ammonia combustion. The developed sensor system was demonstrated to be capable of providing quantitative and spatially resolved measurement of NO in ammonia-methane cofired flames.