Challenges of Hydrogen Combustion for Sustainable Energy Concepts

At the Institute for Combustion Technology at RWTH Aachen University, headed by JARA-ENERGY Member Prof. Heinz Pitsch, one major research objective deals with the challenges of hydrogen combustion in the context of sustainable energy concepts. Hydrogen represents a promising fuel, which combines a feasible and well understood production process with clean combustion. However, before hydrogen combustion becomes fully applicable in actual engines, a deeper understanding of its combustion behavior is required.

The recent increase of renewable energy resources requires the development of efficient energy storage techniques in order to balance the increasing temporal fluctuations of energy production. According to the vision ‘Energiekonzept 2050’ of the German federal government, energy storage may be realized by transforming excessive electric energy into chemical energy. Therefore, particular attention is paid to the electrolysis splitting water into molecular hydrogen and oxygen by using electric energy. The generated hydrogen is used as chemical storage of the energy, which can be recovered when needed by the electrification of hydrogen via combustion in stationary gas turbines. Hydrogen is of particular interest since its chemical structure ensures that greenhouse gases such as carbon dioxide as well as harmful pollutants such as carbon monoxide and soot are not formed during its combustion.

However, the combustion process of hydrogen is fundamentally different from conventional fuels, so that existing gas turbines require modification to handle hydrogen as a fuel. In particular, hydrogen combustion leads to instabilities known as thermodiffusive combustion instabilities. These instabilities do not occur for conventional fuels, but significantly affect the combustion process of hydrogen at lean conditions under which gas turbines are typically operated. Thermodiffusive combustion instabilities lead to a strong wrinkling of planar flames such that a significant flame speed acceleration and enhanced heat release of such flames is observed. Thus, to enable safe operation and optimize hydrogen combustion in actual engines, a deeper understanding of its combustion behavior is required.

At the Institute for Combustion Technology of RWTH Aachen University, thermodiffusive combustion instabilities of hydrogen are investigated experimentally in a high-pressure combustion chamber. Figure 1 (top row) shows the expansion of a spherical lean hydrogen/air flame. The onset of thermodiffusive instabilities is marked by the formation of cellular structures on the flame front. In contrast, Figure 1 (bottom row) shows the spherical expansion of a stable lean methane/air flame, which possesses a smooth flame front without cellular structures and represents the combustion mode of conventional fuels. The cell formation during hydrogen combustion enhances the overall flame speed and needs to be considered for the design of gas turbines. These fundamental experiments are designed to measure and quantify the flame speed of hydrogen flames under the impact of thermodiffusive instabilities.

To further understand the formation of cellular structures and complex interactions of molecular transport, chemical reactions, and turbulence during hydrogen combustion, numerical simulations have been conducted at the Institute for Combustion Technology. Figure 2 shows a Direct Numerical Simulation of a lean hydrogen/air flame with strongly corrugated flame surface due to thermodiffusive combustion instabilities. The key findings of the current study represent the identification of two distinct length scales that are characteristic of the flame front corrugations. First, small flame front corrugations are observed that can be associated with the thermodiffusive instability mechanism. Second, the flame front develops large-scale corrugations, which arise from a complex interaction of the flame with the flow field.

The insights gained by the experimental and numerical analyses can now guide the development of new combustion models for Large-Eddy Simulations, which are typically used in the design of gas turbine combustors. In Large-Eddy Simulations, the flame is not fully resolved, but modeled instead, in order to reduce the computational cost of such simulations. For the development of such models and validation, the current studies provide an excellent data basis. In the next project phase, the application of the newly developed models to combustion under real-engine conditions will be pursued.