As decarbonization of our society becomes more important, gas turbine manufacturers are investigating the utilization of hydrogen as a fuel in their systems. In parallel, a need exists to minimize pollutant emissions associated with power generation. Traditionally, gas turbine manufacturers have adopted lean premixed strategies to minimize pollutant emissions. However, the consideration of hydrogen as a fuel while using traditional premixing strategies for low emissions requires one to consider hydrogen’s unique combustion features compared to natural gas including 1) high flame speed and associated flashback risk, 2) high diffusivity, and 3) high adiabatic flame temperatures for a given fuel/air mixture. “Micromixing” technology is a current promising fuel injection strategy under development for low emission hydrogen fueled ground-based engines to 1) avoid challenges with flashback found in premixed systems by rapidly mixing fuel and air together at a small spatial scale and 2) to produce compact reactions with short residence times which help minimize NOx formation. The general goal of micromixing is to attain premixed conditions and compact reactions using small fuel and air passages immediately upstream of the combustion zone.A relatively new approach is derived from successful low emissions technology developed for aeroengines. The highly transient operation of aeroengines and the need for passenger safety require an even more robust combustion system performance compared to power generation. “Lean direct injection” (LDI) involves larger physical scales (order of 0.1-0.2 in2 effective area) compared to the typical 0.01-0.02 in2 micromixers) and seeks to attain rapid mixing of the fuel and air. In the present effort, an LDI concept developed by Collins Aerospace for liquid fuel applications is adapted for a ground-based turbine system to be operated on hydrogen. Of particular interest in the present effort is the fuel flexibility of these injectors (e.g., Can they operate on a range of natural gas and hydrogen mixtures?) as well as the emissions performance. A set of carefully designed injectors were fabricated to allow air flow splits, air swirl strength, fuel swirl strength to be studied. In addition, operating conditions controlled via the pressure drop, preheat temperature, fuel composition, and adiabatic flame temperature were investigated. With seven factors, a statistical test matrix was designed using a Box-Behnken approach to yield 43 test points per injector for a total of 688 test points for 16 injectors. Analysis of variance was used to develop response surface correlations for the pollutant emissions produced. The analysis identified that the geometric parameters had some influence, on emissions, but that fuel composition, air preheat, and flame temperature were most significant. Optimization identified that a higher air pressure drop, higher air swirl, and more air into the inner part of the injector with low fuel swirl yielded the best emissions performance regardless of fuel composition or calculated adiabatic flame temperature. The results indicate that the Collins injector concept rivals other manufacturers’ micromixing injectors by producing NOx levels below 10 ng/J while having the flexibility to operate on methane and hydrogen. A chemical kinetics study using a reactor network with the GRI Mech 3.0 mechanism was also conducted to understand differences between NOx formed by hydrogen and methane. The network was developed with the ability to account for some unmixedness expected for the LDI configuration. The results identified the NNH (nitrogen-nitrogen-hydrogen) pathway as the main cause for NO emissions when using hydrogen for all conditions studied. Whereas hydroxyl (OH) leads to the direct formation of NO when hydrogen is burned, the bulk of this intermediate species forms carbon dioxide (CO2) rather than NO when methane is combusted. For methane fuel, at reaction temperatures below 1675K, NO is formed mainly through the nitrous oxide (N2O) intermediate mechanism route. At temperatures above 1800K, NO is mainly formed through the Zeldovich (or thermal) mechanism route for both fuels as expected.