This project aims to characterize the influence of hydrogen addition on the local flame structures and NOx emission formation in a premixed laminar counterflow configuration for carbon-free ammonia combustion using combined Raman/Rayleigh spectroscopy and

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The use of the carbon-free fuels ammonia and hydrogen as chemical energy storage for renewable energies is of great importance for the necessary decarbonization in some sectors (e.g. maritime propulsion). Co-combustion of ammonia and hydrogen opens up options to overcome the difficulties of using the two fuels individually. Hydrogen can be produced energy-efficiently from green ammonia. By adding a few mole fractions of hydrogen to ammonia, a significant increase in flame stability is observed compared to, for example, natural gas combustion, with similar laminar burning rates. At the same time, the addition of hydrogen changes the flame structure and the local formation of nitrogen oxides. So far, these phenomena have been investigated mainly numerically. Detailed experimental investigations are not available in the literature, but are urgently needed for further understanding and validation of mathematical-chemical models. The aim of this project is to fundamentally understand the influence of hydrogen addition on flame structure, flame stability and nitric oxide formation for premixed ammonia/hydrogen flames. Using advanced laser diagnostic methods, spatially resolved thermochemical states and the very different nitric oxide formation in the reaction zone compared to hydrocarbon-based combustion processes will be investigated. The investigations will be carried out up to the aerodynamic extinction limit in order to analyze the impact of hydrogen addition on the flame structure and the interacting mechanisms of flame stability and nitric oxide formation. In a parametric variation, different equivalence ratios and different ammonia/hydrogen/nitrogen/air mixtures are systematically explored in a counterflow configuration. Initially, the mechanisms caused by hydrogen addition will be investigated for laminar flow conditions, and in a second project phase for turbulent flow conditions. Thermochemical states (temperature, species concentrations) and flame structures will be measured by one-dimensional spatially resolved combined Raman/Rayleigh spectroscopy and subsequently analyzed. In preparation for this, the previously unknown, temperature-dependent Raman scattering cross sections of the ammonia molecule are determined experimentally. Nitric oxide concentration will also be quantitatively determined using laser-induced fluorescence. Local stretching rates near the reaction zone will be identified by particle image velocimetry in combination with the simultaneously determined hydroxyl radical distribution via laser-induced fluorescence. These experimental data will be used to evaluate reaction kinetic models that show significant deviations, particularly in predicting nitric oxide formation and extinction limits. These comprehensive investigations are intended to fill a knowledge gap in order to close the potentials of the thermal utilization of green ammonia-hydrogen mixtures.