Laser Absorption Diagnostic for Chemical Kinetics Studies of Ammonia

Abstract

Avoiding global warming necessitates using alternative fuels on a large scale to decarbonize the power generation sector. Ammonia (NH₃), the second-most produced chemical globally, is a carbon-free fuel that has the potential to be the primary driver of this sector. However, the combustion chemistry of NH₃ is not well understood, limiting its large-scale application. In particular, the species-specific kinetics interactions during NH₃ combustion have been seldom studied, and new measurements of this kind provide optimization targets for the enhanced understanding of NH₃ combustion chemistry.

In this study, a new NH₃ laser absorption diagnostic, intended for chemical kinetics measurements, was developed to access twelve NH₃ transitions in the v₂ fundamental band near 10.4 μm. Spectroscopic characterization of these transitions, located between 957.5 and 963.0 cm⁻¹, was conducted via scanning-wavelength absorption experiments to measure the line strength and broadening coefficients due to collisions of Ar, He, O₂, N₂, and NH₃.

Using this new and other diagnostics, laser absorption experiments were performed to study NH₃ chemical kinetics at high temperatures and near-atmospheric pressures in a shock tube. First, experiments were performed to monitor NH₃ time histories during the thermal decomposition of ~ 0.5% NH₃/Ar and ~ 0.42% NH₃/2% H₂/Ar over a temperature range of 2096–3007 K. Using these data, along with literature data, a detailed NH₃ thermal decomposition kinetics mechanism was proposed and validated. Second, experiments were conducted to monitor N₂O time histories during NH₃/O₂/Ar oxidation between 1829 and 2198 K for equivalence ratios of 0.54, 1.03, and 1.84. These equivalence ratios were determined accurately by measuring the NH3 concentration in the mixtures using the new diagnostic. Third, simultaneous measurements of NH₃ and H₂O time histories were obtained during the oxidation of several NH₃/O₂/Ar and NH₃/H₂/O₂/Ar over a temperature range of 1474–2307 K, equivalence ratios varying from 0.56 to 2.07, and NH₃:H₂ ratios of 100:0, 80:20, and 50:50. These large time-history datasets were used to investigate the literature NH₃ kinetics mechanisms and illustrate several improvements. The reported time-history data offer stringent constraints for the accurate assessment and validation of future NH₃ kinetics mechanisms.