Flammability conditions for ultra-lean hydrogen premixed combustion based on flame-ball analyses

► A one-step H2O2 reduced mechanism is used for analyzing near-limit flame balls. ► Critical combustion conditions in ultra-lean hydrogen-air mixtures are derived. ► A crossover temperature in the kinetics gives the first-order flammability limit. ► Activation-energy asymptotics provides an improved prediction including radiation. ► The flammability limit at p = 1 atm and T = 300 K is predicted to be 3% volume of H2.