There are severe issues on increasing amount of carbon dioxide (CO2) emission in the world. Many studies are devoted to alternative fuels. One of promising candidates is the utilization of ammonia which is zero emission of CO2, a hydrogen energy carrier, and also can be burned directly as a fuel.
For direct combustion of ammonia in industrial furnaces, there were two issues which were weaker radiative heat flux and a huge amount of NOx emission compared with the combustion of methane. We already have reported  the solution of the former issue by using the oxygen enriched combustion.
The objective of this research is to study the reduction mechanism of NOx emissions in the ammonia / methane co-combustion in an industrial furnace both experimentally and numerically. Experimentally we measured the radiation spectra and the total radiative thermal flux under the condition of the ammonia fuel burned in a 10 kW furnace with a coaxial jet flame and additional two oxidizer inlets for the staging combustion. The spectrum measurement results show that the amount of NOx emission was in reverse proportion with the intensity of N2O spectrum in the downstream of the reaction zone in the furnace. This indicates that N2O, which is one of main intermediate species of NH3, reacts with NOx as a reduction reactant to nitrogen molecule.
Numerical simulations were performed to clarify these hypotheses. Firstly we carried out the simulation to overview the combustion in the whole area of a furnace, with the condition of three dimensions, eddy dissipation concept model in the turbulent combustion and GRI-Mech3.0 in the ammonia and methane reaction. In addition, the two dimensional simulation with Okafor mechanism  was conducted to research the combustion process in detail.
When the amount of NOx emission was smaller at the furnace exit, more N2O existed in the middle region of the furnace. This simulation could explain the experimental data with good agreement.
 R. Murai et al, The radiative characteristics of NH3/N2/O2 non-premixed flame on a 10 kW test furnace, Energy Procedia, vol. 120, August 2017, P325-332
 E. Okafor et al, Experimental and numerical study of the laminar burning velocity of CH4 –NH 3–air premixed ﬂames, Combustion and Flame, vol. 187(2018), P185-198