Hybrid power plant - hydrogen as an alternative fuel for gas turbines

Author: Dr.-Ing. Nurettin Tekin, Project Manager, KAWASAKI Gas Turbine Europe GmbH

When using renewable energy sources for low-emission power generation, hydrogen can be an alternative to conventional fuels for gas turbines. Kawasaki Heavy Industries, Ltd, in cooperation with FH Aachen University of Applied Sciences and B&B-AGEMA, has developed and successfully tested an innovative DLE (micro-mix combustion) hydrogen combustion system for burning 100% hydrogen. By using this technology you already have the possibility to produce your entire electricity and heat demand CO2-free. Learn more about how the technology works in this technical article!

How does the principle of MMX-Combustion work?

Scientific studies show that NO_x emissions can be significantly reduced, on the one hand, through greater mixing of the reactants in the fuel-air mixture and, on the other hand, through shortened residence times of the reactants in the hot flame or combustion area. The innovative MMX combustion is based on these principles. Here, the fuel is injected into the combustion air stream through micro-bores positioned perpendicular to the air flow. The perpendicular injection (cross-flow) enhances the interaction of both flows, intensifying the mixing of the fuel-air mixture. This so-called micro-mix combustion principle is shown schematically in Figure 1a. The resulting characteristic micro-flames on the test burner can be seen in Figure 1b.

This reduces NO_x formation

The recirculation areas above and below each flame stabilize the micro-flames, which anchor downstream at the trailing edge of the burner segment. The length of the flames is between 5-10 mm. Compared to conventional technologies, these miniaturized flames lead to significantly reduced residence times of the reactants in the hot combustion zone, as the hot combustion zone is much smaller here. In conventional technology, combustion typically occurs within a large flame with an extended combustion zone with increased residence times. This means that many individual small flames -instead of one large flame- significantly reduce NO_x formation during the combustion process. The first prototype design of an MMX test burner has about 1600 miniaturized flames with a fuel injector diameter of d=0.3 mm. For reasons of economy and to simplify the manufacturing process, the number of flames will be successively reduced for subsequent designs. At the same time, the energy density per injector is increased by increasing the bore diameter from initially d=0.3 mm, successively to 0.55 mm up to 1.0 mm. This is done while maintaining the MMX combustion principles. Within the development and optimization process, the number of flames was thus reduced from approx. 1600 to only 410 miniaturized flames.

Figure 2 shows the fabricated test burner in detail: the burner head consists of three ring segments containing the 410 H₂ injectors. The burner head is implemented in a conventional can-type combustion chamber. The individual ring segments are supplied with hydrogen from the center via piping, and each ring segment can be individually controlled depending on the required power.

Figure 3 shows the distribution of NO_x emissions (15vol%O₂) as a function of thermal load. The combustion chamber pressure is 2bar. The three ring segments can be controlled and fired separately depending on the required thermal power. At up to 30% load, the two inner rings are used. From 30% load up to full load of 100%, the third ring is additionally fired. It can be seen that low NO_x values are achieved even in the partial load range of below 70% load. The NO_x values are below 20ppm for the entire load range from 0% to 100%. The Micro-Mix combustion system achieves significantly lower NO_x emissions than conventional combustion systems over the entire load range. In addition, there is inherent safety against flash-back.

The Future of MMX Technology

Provided that suitable and safe hydrogen combustion systems are developed, hydrogen can provide an alternative gas turbine fuel in future low-emission and CO2-free power generation. Figure 4 schematically illustrates such an innovative power plant of the future . The system configuration includes a natural gas-hydrogen gas turbine with integrated H2 generation and storage. With the surplus energy from renewable energy sources such as wind and PV, hydrogen is produced via an electrolyzer. This is temporarily stored and can then be used again for electricity and heat production by combustion via the gas turbine.

The current MMX technology has been developed and optimized for the combustion of pure hydrogen only. In the future, combustion of other gases such as natural gas, biogas, syngases or gas mixtures will also be possible. The increased fuel flexibility will make MMX technology even more attractive and competitive in the future.

Author's References:

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