Thanks for your comment. In hydrogen engines, the generation of NOx (which does not occur in Fuel Cells) depends on the temperature of combustion which can be mitigated with variations in air/fuel ratio and exhaust gas recirculation, as well as some catalytic converters. I am not sufficiently familier with hydrogen / natural gas mixes sent to conventional natural gas equipment to give you an answer to ways to mitigate NOx in that application.