Hydrogen is a clean alternative to conventional hydrocarbon fuels, but it is very important to reduce the nitrogen oxides (NOx) emissions generated by hydrogen combustion. The rich-lean combustion or staged combustion is known to reduce NOx emissions from continuous combustion burners such as gas turbines and boilers, and NOx reduction effects have been demonstrated for hydrocarbon fuels. The authors applied rich-lean combustion to a hydrogen gas turbine and showed its NOx reduction effect in previous research. The present study focused on experimental measurements of NO and NO 2 emissions from a co-axial rich-lean burner fueled with hydrogen. The results were compared with diffusion combustion and methane rich-lean combustion. Significant reductions in NO and NO 2 were achieved with rich-lean combustion. The NO and NO 2 reduction effects by rich-lean combustion relative to conventional diffusion combustion were higher with hydrogen than with methane.Keywords: Hydrogen, Rich-Lean Combustion, Staged Combustion, Nitrogen Oxides, Zel'dovich NO, Fenimore NO, prompt NO, NO 2 INTRODUCTIONHydrogen is considered an ideal alternative to conventional hydrocarbon fuels, because hydrogen can be produced from any kind of energy source and as it burns without emitting soot or carbon dioxide. Nitrogen oxides (NOx) are the only potential pollutants from hydrogen combustion, and it is crucial to reduce NOx emissions from hydrogen combustion.Rich-lean combustion or staged combustion is known as a technique to reduce NOx formation in continuous combustion burners such as gas turbines and boilers. The effect has been demonstrated with conventional hydrocarbon fuels by many researchers [1][2]. The authors have applied rich-lean combustion to a hydrogen gas turbine and shown its effect of NOx reduction in previous research [3][4]. The rich-lean combustion reduces the thermal NO, the Zel'dovich NO [5][6], which is the main part of combustion-generated NOx. At the same time, the prompt NO, Fenimore NO [7], is formed in hydrocarbon combustion especially under fuel-rich conditions. Though the amount of prompt NO is quite small for conventional premixed or diffusion combustion, it could become a significant portion of the total NO emissions from rich-lean combustion due to the largely decreased thermal NO and the existence of fuel-rich mixture. Also, the NOx emissions from combustion include NO 2 which is more harmful to human health than NO and also has a strong green house effect. The current paper analyzes the emission characteristics of NO and NO 2 from a coaxial rich-lean burner fueled with hydrogen. The results are compared with rich-lean combustion with methane.

Hydrogen is expected as a clean and renewable alternative to the conventional hydrocarbon fuels. Because the only possible pollutants from the hydrogen combustion are nitrogen oxides (NOx), it is crucial to reduce the NOx emission in the hydrogen utilization. The rich-lean combustion is well known as a technique to reduce the emission of the Zel’dovich NO from the continuous combustion burners for such as gas turbines and boilers. Because the Zel’dovich NO occupies a large part of the total NOx emission, the rich-lean combustion is quite effective to reduce the NOx emission. However, the NOx reduction effect of the rich-lean combustion has not yet been proven for the hydrogen fuel, while the effect has been demonstrated for the hydrocarbon fuels. On the other hand, the prompt NO is emitted from the hydrocarbon combustion especially under the fuel-rich conditions. Though the amount of the prompt NO is quite small for premixed or diffusion combustion, it could be a relatively significant part in the total NO emission from the rich-lean combustion due to the decreased Zel’dovich NO. The authors estimate that hydrogen is more suitable for the rich-lean combustion compared with hydrocarbons, because hydrogen does not emit the prompt NO even under the fuel-rich conditions which necessarily exist in the rich-lean combustion. This research proposes the rich-lean combustion as a method to reduce the NOx emission from hydrogen combustion and experimentally analyzes the characteristics using a coaxial rich-lean burner with varying the mixture conditions.