Experimental and Numerical Study on Optimizing the Dry Low NOx Micromix Hydrogen Combustion Principle for Industrial Gas Turbine Applications

Abstract

Combined with the use of renewable energy sources for its production, hydrogen represents a possible alternative gas turbine fuel for future low-emission power generation. Due to the 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 to dry low NOx (DLN) hydrogen combustion. The DLN micromix combustion of hydrogen has been under development for many years, since it has the promise to significantly reduce NOx emissions. This combustion principle for air-breathing engines is based on crossflow mixing of air and gaseous hydrogen. Air and hydrogen react in multiple miniaturized diffusion-type flames with an inherent safety against flashback and with low NOx emissions due to a very short residence time of the reactants in the flame region. The paper presents an advanced DLN micromix hydrogen application. The experimental and numerical study shows a combustor configuration with a significantly reduced number of enlarged fuel injectors with high-thermal power output at constant energy density. Larger fuel injectors reduce manufacturing costs, are more robust and less sensitive to fuel contamination and blockage in industrial environments. The experimental and numerical results confirm the successful application of high-energy injectors, while the DLN micromix characteristics of the design point, under part-load conditions, and under off-design operation are maintained. Atmospheric test rig data on NOx emissions, optical flame-structure, and combustor material temperatures are compared to numerical simulations and show good agreement. The impact of the applied scaling and design laws on the miniaturized micromix flamelets is particularly investigated numerically for the resulting flow field, the flame-structure, and NOx formation.

Injector Design Space Exploration for an Ultra-Low NOx Hydrogen Micromix Combustion System

“…The mixing process in a micromix system is significantly improved by the principle of the jet-in-cross-flow. The flame anchoring, which is essential for the reduction in NOx formation, is achieved primarily by the recirculation vortices and by the momentum flux ratio of the crossflow streams [63] [17]. The conceptual micromix combustor design conceived at Cranfield University is shown as in Figure 2.…”

“…It is critical to the design of the micromix system that these micro flames are separated from each other since the merging of adjacent flames could potentially result the formation of larger-scale flames. This would further increase fuel-air mixtures residence time in the hot recirculation regions and subsequently increase the NOx emissions, while potentially overheating the combustor liner [61].…”

“…The other main parameter that influences the micromix combustion system design is the momentum flux ratio between the fuel jet and the air crossflow. This momentum flux ratio is defined as [63]:…”

“…Increasing the momentum flux ratio between the two streams increases the penetration depth of the hydrogen stream into the jet-in-cross-flow air stream thus enhancing the mixing characteristics [61] [63]. The air stream jet air mixes with the penetrating hydrogen stream, the inner recirculation zones are then divided by shear layers between the two streams.…”

“…If the penetration depth is below a critical penetration height, the H2air mixture can be discharged freely, and the residence time is short. If, however, the penetration depth increases beyond this critical value, the hydrogen penetrates the shear layer between the two streams and hence enters into a hot inner recirculation zone [63], as shown in Figure 6. This further increases the residence time of the mixture, subsequently leading to an increase in NOx emissions [17].…”