Numerical Study on Increased Energy Density for the DLN Micromix Hydrogen Combustion Principle

Abstract

Combined with the use of renewable energy sources for its production, hydrogen represents a possible alternative gas turbine fuel within future low emission power generation. Due to the large difference in the physical properties of hydrogen compared to other fuels such as natural gas, well established gas turbine combustion systems cannot be directly applied for Dry Low NOx (DLN) hydrogen combustion. Thus, the development of DLN hydrogen combustion technologies is an essential and challenging task for the future of hydrogen fuelled gas turbines. The DLN Micromix combustion principle for hydrogen fuel is being developed since years to significantly reduce NOx-emissions. This combustion principle is based on cross-flow mixing of air and gaseous hydrogen which reacts in multiple miniaturized diffusion-type flames. The major advantages of this combustion principle are the inherent safety against flashback and the low NOx-emissions due to a very short residence time of reactants in the flame region of the micro-flames. For the low NOx Micromix hydrogen application the paper presents a numerical study showing the further potential to reduce the number of hydrogen injectors by increasing the hydrogen injector diameter significantly by more than 350% resulting in an enlarged diffusion-type flame size. Experimental data is compared to numerical results for one configuration with increased hydrogen injector size and two different aerodynamic flame stabilization design laws. The applied design law for aerodynamic stabilization of the miniaturized flamelets is scaled according to the hydrogen injector size while maintaining equal thermal energy output and significantly low NOx emissions. Based on this parameter variation study the impact of different geometric parameters on flow field, flame structure and NOx formation is investigated by the numerical study. The numerical results show that the low NOx emission characteristics and the Micromix flame structure are maintained at larger hydrogen injector size and reveal even further potential for energy density increase and a reduction of combustor complexity and production costs.