A fuel system for a gas turbine engine comprises an injector disposed to inject fuel and air into a combustor of the gas turbine engine. In a first embodiment, the fuel system further comprises an air separation module configured to supply oxygen-enriched air into the combustor via the injector for combustion. In a second embodiment, the fuel system further comprises a barbotage system and a heating element. The barbotage system is configured to feed hydrogen to the injector, and the heating element is configured to pre-heat the fuel.