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GREENEST Report Summary

Project ID: 247322
Funded under: FP7-IDEAS-ERC
Country: Germany

Final Report Summary - GREENEST (Gas turbine combustion with Reduced EmissioNs Employing extreme STeam injection)

The increasing use of renewable energy sources also increases the demand for highly flexible and efficient fossil fuel power plants. Gas turbines will play a major role in the future energy supply, as they provide excellent abilities to complement the renewables. Ultra-wet combustion gas turbines offer a significant increase in efficiency and operational flexibility compared to the dry gas turbine cycle, while at the same time reduces emissions. This combustion concept is the topic of the ERC funded project GREENEST at the Chair of Fluid Dynamics at the TU Berlin.

In the scope of the project several aspects of ultra-wet combustion in swirl-stabilized burners have been investigated. A specialized combustor has been designed and tested for combustion stability - thermoacoustic and flashback instabilities – and emissions with a very broad variety of fuels. Both atmospheric and high pressure experiments have been carried out, which demonstrated that steam injection can make combustion of highly reactive fuels, like pure hydrogen, in swirl stabilized combustors feasible with very low emissions and no flashback. The blowout limits for very wet flames have been explored and it was shown that generally the operation with fuel-rich mixtures and very low emissions is possible, when ultra-wet combustion is applied. These results demonstrate the technical feasibility of wet combustion and also open new possibilities for the design of gas turbines, which traditionally operate with very lean mixtures, in order to reduce NOx emissions.

Moreover several experimental and numerical (CFD) studies have been carried out on the chemical kinetics of wet combustion. In particular, the influence of steam injection on the reaction mechanism of various fuels and fuel blends has been studied. These studies resulted in adaptations of known reaction mechanisms for hydrogen and natural gas flames, so that they can take into account the role of steam in the combustion reactions. The main takeaway of these studies is that steam dilution reduces the burning velocity at constant preheat temperature and equivalence through the combination of thermal, dilution and chemical effects. The importance of each of these effects depends on the particular conditions of each flame.

Very detailed experimental and theoretical analysis has been carried out on the combustor flow field and the observed flame shapes. Three basic flame shapes and adjacent flow fields were identified. The detailed mechanisms behind the observed flame shapes were clarified by applying linear stability analysis of the reactive and the isothermal flow. The role of the so-called wave maker in the occurrence of a precessing vortex core and relevant flow instabilities has been highlighted. This analysis lead to the development of several methods to avoid these instabilities in swirl-stabilized combustors and extended considerably the state-of-the-art in this field.

In conclusion a new combustor concept for the wet combustion of stoichiometric H2/O2 mixtures has been developed, based on the experience gained in wet combustion of hydrogen. For a broad range of adiabatic flame temperatures, a stable combustion process was observed without any evidence that the life time of the combustor might be limited for example due to flashback or chemical reaction attached to walls or injection holes. The combustor was proven a viable option for steam generation and the idea was further developed in the scope of the ERC proof of concept project Blue Step.

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