Dimensionally reduced modeling of nitric oxide formation for premixed methane-air flames with ammonia content

The paper presented an experimental and numerical study on a combustion process of ammonia doped methane flames with the NH3 content in the fuel from 1% up to 5%. Tests were performed with an atmospheric pressure axisymmetric burner for stoichiometric mixtures and fuel-lean conditions (phi = 0.63, 0.71, 0.83, 1.0) with the substrates preheating up to 573 K. Numerical modelling involved three reduced dimensionally combustion models (0D IdealGasReactor, 1D FreeFlame, 1D BurnerFlame) with four detailed reaction mechanisms for the hydrocarbons and nitrogen chemistry (GRI 3.0, SanDiego, Konnov 0.6 and Tian). Comparison of the 0D/1D calculations results with a complete burner geometry modelling (3D Ansys EDC) was performed for GRI 3.0. Experimental study shown that nitric oxide emission increased with the increase of NH3 content in the introduced fuel, however transition from NH3 to NO was incomplete and trend was not linear for the rising ammonia share. Doubling of the ammonia content in the fuel from 2.5% to 5% resulted in a rise of NO emission by only 55.5% (from 946 to 1471 ppmv) for the lean mixtures and by 48% (from 1451 to 2148 ppmv) for the stoichiometric conditions. The numerical analysis results were in a good agreement with the experimental results for the lean mixtures and ammonia content up to 1% for all investigated combustion models and the kinetic reaction mechanisms. Trends of the nitric oxide emission obtained with the stoichiometric flames were valid for 1D models. However overall values were over predicted for an adiabatic 1D FreeFlame calculations and under predicted for 1D BurnerFlame, which showed a sensitivity of the nitrogen chemistry to the process temperature. Analysis with the 3D Ansys EDC model and GRI 3.0 provided the most accurate results up to 2.5% of the NH3 share in the fuel with a maximum relative differences values to the experimental results from 10% up to 24% for the lean and the stoichiometric conditions respectively.