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• The reaction images for different equivalence ratio were shown. • The OH* imges and NO x production in the MILD combustion zone were analyzed. • The NO x reduction mechanism prevails over the NO x formation mechanism under MILD mode. The effect of equivalence ratio and staging ratio on the CH 4 MILD (Moderate or Intense Low-oxygen Dilution) combustion was examined in an axially staged MILD combustor. The results were presented on reactant temperature and dilution as well as the ignition delay time through CRN (chemical reaction network) calculation. Also, the data on the OH* radicals distribution and exhaust emissions were obtained using experiments. Higher staging ratio induces the high temperature low oxygen thermodynamic condition required for MILD scheme, yet reduces the ignition delay time. An expanded and downstream moved reaction region was observed at higher equivalence ratio. Reduced and even negative NO x emissions from the MILD combustion zone are acquired at high staging ratio, implying that the NO x reduction mechanism is superior to the NO x formation mechanism in the MILD combustion zone. The opposite effect of staging ratio on the CO emission for low equivalence ratio and high equivalence ratio is also observed. With primary air swirled in the primary combustion zone, the staged combustor exhibits an excellence performance with distributed reaction region and ultra-low NO x and CO emissions under gas turbine relevant lean operating condition. [ABSTRACT FROM AUTHOR]

• 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 NO x formation is suppressed. • Heat balance under premixed MILD combustion was presented in the present work. To promote the application of MILD scheme in furnace, the present work was performed to evaluate the effect of thermal input, excess air coefficient and combustion mode on the MILD combustion at high thermal level (thermal input varies from 600 to 786 kW); meanwhile, the heat from the utilized furnace was extracted by an annular water jacket and a corundum tube was installed in the furnace tube to create the high temperature atmosphere. The results were presented on flow field using numerical simulations and on global flame signatures, temperature/species distribution and exhaust emissions using experiments. Better combustion performance of MILD mode than conventional swirling diffusion combustion mode was verified in the present work. The positive effect of increased thermal input and excess air coefficient on gas recirculation, thermal uniformity and exhaust emissions reduction was observed. In the MILD combustion regime, the switching from nonpremixed to premixed combustion resulted in lower oxygen level and reduced reaction temperature in the combustion region, also much lower NO x emissions and slightly higher CO emission were observed. At 786 kW thermal input, the reaction zone covers the entire combustion chamber; meanwhile, low NO x emission level of 32 ppm@3%O 2 are achieved for premixed MILD combustion mode. The MILD regime was established for natural gas fueled furnace with colorless distributed combustion and suppressed NO x and CO emissions. [ABSTRACT FROM AUTHOR]