LEL and UEL

Hello Aida, it depends...

There is most certainly a theoretical stoichiometric ratio for methane/air mixtures, but that provides little more than a starting point.

Also, the above-noted may not necessarily be the ideal ignition ratio...

Your reference to a "burner" suggests continuous combustion rather than cyclic, but is this at near-atmospheric pressure [such as in a boiler furnace] or at an elevated pressure [such as in the combustion chamber of a gas turbine]?

Properly graduated and matching adjustment of fuel admission valves and air admission dampers or equivalent is commonly achieved using flue gas analysis, and the optimum excess air ratio is determined by the application, burner style, fuel composition, firing rate, primary and secondary air temperatures, etc., etc.

More information leads to better answers.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]