Hydrogen is widely regarded as a beacon of hope in the energy sector. When produced using electricity from renewable sources such as solar or wind power, it can be used, stored and, when required, converted back into electricity or heat with virtually no direct carbon emissions. This makes hydrogen particularly valuable for an energy system that is increasingly characterised by fluctuating renewable energy output. For this reason, hydrogen is also attracting interest in the field of gas turbines. They play a key role in power plants, electricity grids and industrial production facilities because they can respond very rapidly to fluctuations in energy demand.
Today, however, gas turbines are still predominantly fuelled by natural gas and therefore produce CO₂ emissions. Replacing natural gas with hydrogen appears to be the obvious solution. In practice, however, the challenge is greater than it might initially seem. This is because hydrogen behaves differently to natural gas during combustion. “It is significantly more reactive and much more difficult to control,” says Mirko Bothien from the Institute of Energy Systems and Fluid Engineering (IEFE) at the ZHAW. Together with his team, he is working to ensure that hydrogen can take over the role currently played by natural gas in gas turbines in the future.
High temperatures, nitrogen oxides, flashbacks
The key difference lies in hydrogen’s physical properties. “Hydrogen is the smallest molecule and therefore diffuses extremely quickly,” explains Bothien. While methane, the principal component of natural gas, diffuses at approximately the same rate as heat and is therefore distributed evenly throughout the reaction zone, hydrogen spreads much faster and less evenly. This results in local variations within the flame: in some places more fuel accumulates, in others less. Bothien describes this as a “self-reinforcing mechanism,” known in technical terms as thermodiffusive instability.
The consequences are evident during combustion. Localised hotspots – areas of high temperature – form. This gives rise to a further problem: nitrogen oxides. “Although hydrogen itself does not release CO₂, its combustion can cause harmful NOx emissions if it is not precisely controlled,” he says. In addition, hydrogen burns significantly faster than natural gas – in some cases up to ten times faster. This increases the risk of a flashback. “In such situations, the flame can travel back into the burner.” This so-called flashback is one of the greatest risks and can result in damage to components.
Mirko Bothien
Dr.-Ing, Head of the Renewable Energy focus area, ZHAW
“Although hydrogen itself does not release any CO₂, its combustion can generate harmful NOx emissions if the process is not controlled precisely.”
The interaction between flames and sound waves
Combustion is also influenced by a phenomenon that is little known outside specialist circles: thermoacoustics. It describes the interaction between flames and sound waves and is one of the core research areas of Bothien’s team. “A turbulent airflow is needed to ensure that the fuel and air mix properly,” explains the expert. This keeps the flame constantly moving and generates small pressure waves. “It’s not like a candle flame, which burns calmly and produces virtually no sound.” These sound waves propagate through the combustion chamber, strike and reflect off the walls and return to the flame. If these oscillations reinforce one another, a great deal of acoustic energy can build up within a short space of time. In extreme cases, vibrations may become strong enough to damage turbine components. “Unlike natural gas, this mechanism is highly pressure-dependent when hydrogen is involved and is therefore much more difficult to predict.”
To gain a better understanding of these interactions, the team is studying hydrogen flames under atmospheric conditions in the laboratory. Based on these measurements, the researchers are developing models capable of predicting behaviour under real operating conditions. These resulting insights are incorporated directly into technological development. This work is based on a unique experimental infrastructure for various burner systems, complemented by numerical simulations and analytical models. The objective is to enable gas turbines to operate with hydrogen contents ranging from 0 to 100% without nitrogen oxide emissions rising to critical levels. Combustion stability should nevertheless be maintained.
Mirko Bothien, Head of the Renewable Energy focus area at the School of Engineering
Research with a practical focus
Another key consideration is retrofitability: the technologies are not only intended for new facilities, but are also designed to be used in existing gas turbines. In addition, several projects are running in parallel with various industry partners. Many of these form part of Horizon Europe and are linked to the EU Green Deal. In the HyPowerGT project, for example, a burner technology for a 16-megawatt gas turbine is being developed and tested under real operating conditions. The InsigH2t project brings together leading European research institutions and aims to develop a fundamental understanding of the pressure-dependent behaviour of hydrogen. In the H2FHYFLEXGT project, which is funded by the Hightech Zentrum Aargau, laboratory findings are being applied to a modular burner system that can be adapted to different classes of turbine using 3D printing.
The research is deliberately kept close to practical application. “Our task is not simply to develop a technology in isolation,” Bothien emphasises. “We are working together with industry partners from Switzerland and abroad to find practical solutions.” The objective is clear: to make hydrogen sufficiently controllable so that it can be used safely, efficiently and with the lowest possible emissions in gas turbines in the future, thereby becoming a key building block of a carbon-free energy system.
