Effect of thermal input, excess air coefficient and combustion mode on natural gas MILD combustion in industrial-scale furnace

•The flow , flame and emission are presented under furnace operation condition.•Smooth thermal field and reduced emissions were observed for MILD mode.•Higer thermal input and excess air ratio anticipate the realization of MILD mode.•From nonpremixed to premixed MILD mode, the NOx formation is suppressed.•Heat balance under premixed MILD combustion was presented in the present work.