Measurements of NO in the post-flame region of laminar premixed ammonia/methane-air flames using laser-induced fluorescence

Cofiring of ammonia (NH₃) with methane (CH₄) offers a promising route to enhance the combustion characteristics of pure ammonia while partially decarbonizing methane-based energy systems. However, the presence of fuel-bound nitrogen in ammonia leads to elevated NOx emissions. Although NO measurements in NH₃-containing flames have been reported, there is a notable lack of validation datasets for NH₃/CH₄ cofiring based on non-intrusive diagnostics. In this communication, a laser-induced fluorescence (LIF) approach recently developed for post-flame NO measurements is applied to NH₃/CH₄-air flames with CH₄ contents ranging from 50 vol.-% to 80 vol.-% in the fuel mixture. The LIF signal processing includes corrections for laser absorption, signal trapping, and pulse energy fluctuations. Thermochemical differences between calibration and target flames are addressed through corrections for number density, Boltzmann fraction, spectral line overlap, and electronic quenching. The results show good agreement with recent chemical kinetic models over a broad range of conditions. Within the tested range, increasing the NH₃ content results in a notable reduction of NO emissions under fuel-rich conditions, while the opposite trend is observed under fuel-lean conditions. The observed NO trends are interpreted using normalized radical pool indicators for

and O/H/OH species.

Novelty and Significance

Quantitative data on NO emissions from ammonia-based fuels remain limited, especially those obtained using non-intrusive diagnostics. In this paper, we extend a previously presented laser-induced fluorescence (LIF) method to measure post-flame NO concentrations in NH₃/CH₄-air flames across a broad range of equivalence ratios and fuel compositions. The dataset covers CH₄ contents from 50 to 80 vol.%. By introducing normalized radical pool indicators for

and O/H/OH species, the work helps to interpret observed NO trends. The results provide a basis for refining chemical kinetic models with respect to NO formation.