A laser absorption sensor for fuel slip monitoring in high-humidity flue gases from ammonia combustion

Abstract

Ammonia has been recently recognized as a promising carbon-free fuel to help decarbone both the power and industrial heating sectors. However, fuel slip during ammonia combustion is of high concern because of the high toxicity of the chemical, motivating the development of sensitive sensors for real-time monitoring of ammonia emission in combustion flue gases. In this work, a near-infrared absorption spectroscopic sensor was developed for trace ammonia (NH3) measurements in high-humidity flue gas environments with water vapor (H2O) mole fractions up to 40%. The focus on high humidity is motivated by the facct that ammonia combustion generally leads to flues gases with notably higher water vapor concentations as compared to conventional hydrocarbons. The sensor used a distributed feedback laser to target the NH3 absorption line near 6612.73 cm−1. Proof-of-concept tests were conducted in a high-temperature multi-pass cell with a controlled temperature of 500 K and pressure of 1 atm. A custom-designed variable humidity generator was used to provide a reference NH3/N2 mixture with a controlled H2O mole fraction (10%–40%) to simulate ammonia combustion flue gases. Direct absorption spectroscopy and wavelength modulation spectroscopy were both used to ensure a relatively large dynamic range. H2O-induced broadening effects on the measured absorption profile were quantified. The developed sensor was firstly validated against NH3/N2 mixtures with different H2O concentrations, with a lower detection limit of 76 ppb achieved. Real-time measurements of NH3 slip in the flue gases from CH4/NH3/air flames and CH4/NH3 co-firing industrial furnace were performed as a demonstration of the sensor for time-resolved ammonia monitoring with sufficient accuracy, sensitivity, and time response.