Flameless combustion, the thermal NO formation becomes less significant than other routes because the low average and fluctuating temperatures; hence other routes, such as the N2O/NO intermediate path and NNH, the latter in case of presence of H2 in the fuel, should be taken into account. However such scheme is usually not included in commercial CFD codes. Reduced NO mechanisms (including thermal, prompt, N2O and NNH pathways) suited for flameless conditions are derived on the basis of Rate of Production Analysis in onedimensional Perfectly Stirred Reactor (PSR) using detailed kinetic schemes. The reduced models are tested using experimental NO data taken in the lab-scale Jet in Hot Coflow burner fed with CH4/H2, emulating flameless combustion. The same approach is applied to the IFRF NG-fired pilot-scale burner in presence of injection of an ammonia solution.