Comprehensive Experimental Assessment of NOx Emissions in Swirling Diffusion Flames of Natural Gas-hydrogen Blends

In the transformation process from fossil-fuel based to carbon-neutral combustion, full or partial replacement of natural gas with hydrogen is considered in numerous industrial applications. As hydrogen flames yield significantly higher NOX emissions than natural gas flames, understanding what factors influence these emissions in flames of natural gas/hydrogen blends is crucial for the retrofitting process. Our work is concerned with the simplest form of industrial retrofitting where hydrogen is injected into the natural gas line without any modifications to the burner construction while keeping the burner power constant. We provide quantifications of NOX emissions with respect to changes in hydrogen content (pure natural gas to 100% hydrogen), swirl number (S=0.6 to S=1.4), excess air ratio ( = 1 to =4.5) and air preheat (ambient air to 300 ◦C). The changes were determined in small steps and over a large range. The emission data is to be used in industrial CFD for both validation and tuning, therefore Laser Doppler Velocimetry was used for precise determination of the burner inlet conditions. Key findings of the investigation include that for hydrogen flames, the NOX emission index [mg/kWh] is 1.2 to 3 times larger than for pure natural gas flames at similar firing conditions. The steepest increase in NOX emissions occurs above 75% volume fraction of hydrogen in the fuel. For natural gas flames, NOX emissions peak at 1.3 to 1.4 excess air, while the maximum for hydrogen and natural gas/hydrogen blends lays at =1.6. NOX emissions decrease slightly as the swirl number increases but this effect is minor in comparison to the effects of hydrogen content, excess air ratio and air temperature.