Simultaneous Measurement of NO and NH₃ in Ultrahigh Humidity Flue Gases from Ammonia Combustion Using Mid-Infrared Laser-Absorption Spectroscopy

Accurate and simultaneous monitoring of ammonia (NH3) and nitric oxide (NO) emissions is critical for the practical implementation of NH3-based fuels in industrial combustion systems from an emission control perspective. Unlike conventional hydrocarbon combustion, ammonia combustion generates flue gases with exceptionally high humidity levels, which can reach 75% when pure oxygen combustion strategies are implemented. Existing measurement technologies struggle under these extreme humidity conditions, resulting in unreliable detection, in addition to the requirement of frequent calibration. In this work, we address these issues by developing a novel mid-infrared laser-absorption spectroscopy sensor specifically designed for robust, simultaneous quantification of NH3 and NO in ultrahigh humidity flue gases. The sensor employs a quantum cascade laser (QCL) and an interband cascade laser (ICL), targeting carefully selected absorption lines at 962.17 cm–1 for NH3 and 1900.07 cm–1 for NO. Utilizing a calibration-free wavelength modulation spectroscopy (CF-WMS) technique integrated with frequency-division multiplexing (FDM), the sensor achieves high sensitivity and accurate measurements without extensive calibration or corrections for humidity-induced broadening. Comprehensive experimental validations confirmed the sensor’s capability to reliably detect trace NH3 and NO concentrations in controlled flue gas mixtures containing water vapor concentrations ranging from 50% to 80%. The achieved detection limits are 621 ppb for NH3 and 88.7 ppb for NO, demonstrating robust performance, long-term stability, and rapid response under challenging industrial conditions. This research provides a critical tool for enhancing emission control strategies in ammonia combustion applications, ensuring compliance with environmental standards, and advancing the feasibility of ammonia as a sustainable industrial fuel.