Experimental investigation of fundamental flame characteristics, N2O and NF3 decomposition, and NOX formation in hydrogen/methane diffusion flames

In this study, the thermal decomposition characteristics of high-global-warming-potential (GWP) greenhouse gases (N 2 O and NF 3 ) and the associated NO x emission behaviors were experimentally investigated under methane/hydrogen diffusion flame conditions. The experiments were conducted by varying fuel composition (based on lower heating value, LHV), combustion conditions (Fixed LHV vs. Fixed fuel velocity), and gas injection locations (fuel line, oxidizer line, flame base, and flame tip). Under fixed LHV conditions, allowing the high-temperature characteristics of hydrogen to be fully reflected; as the hydrogen mole fraction increased, both flame temperature and OH* chemiluminescence intensity increased accordingly. In contrast, under constant fuel velocity conditions, the lower volumetric heating value of hydrogen led to a reduction in total heat release and a corresponding decrease in flame temperature. These observations indicate that controlling the LHV provides a more appropriate basis for evaluating hydrogen combustion characteristics. Complete decomposition of N 2 O and NF 3 was achieved when the gases were injected through the fuel line; however, flame instability was observed due to the oxidizing properties of NF 3 . Injection at the flame tip demonstrated superior decomposition performance for both gases, particularly under high hydrogen content conditions, due to elevated flame temperatures and enhanced radical activity. Nevertheless, NO x formation accompanied the decomposition process, with a sharp increase in NO x emissions observed when injection occurred near the flame reaction zone. The findings suggest that, for hydrogen-fueled POU scrubber systems, optimization of fuel composition, flame structure, and injection location is critical to simultaneously maximize decomposition efficiency and minimize NO x formation.