Gas turbines
Task 5 pertains to the use of combustion in gas turbine engines for power generation, and represents the required enabling step that completes the CCS value chain on the Norwegian Continental Shelf, throughout Europe and worldwide.
The overall objective is to assess and improve the stability and operability of gas turbine combustion systems facing issues related to novel and unconventional fuel mixtures. Specific focus and significant efforts are aimed at the characterisation of “reheat” hydrogen combustion, which is a firing layout that relies on the longitudinal staging of the combustion system that is divided into two combustion zones arranged in sequence: a first “conventional” dry low-emission burner and a second “reheat” dry low-emission burner.
Ultimately, Task 5 aims to assess the overall impact on power generation, thermodynamic efficiency, and pollutants emissions in order to reduce costs related to clean and efficient energy conversion in gas turbines, and improve their safety and robustness.
Results 2021
In 2021, the research work built upon the significant findings and fundamental insights about hydrogen combustion acquired in 2017-2020 to investigate more applied topics of increasing relevance to the development of fuel-flexible gas turbines.
The characteristics of hydrogen flames at reheat combustion conditions that are present in Ansaldo’s GT36 gas turbine have been investigated by SINTEF using a computationally intensive numerical modelling approach named Large Eddy Simulation (LES), in order to accurately represent the reactive flow, combined with a detailed representation of the chemical reaction kinetics.
Results from the LES calculations have provided important information for combustion conditions corresponding to part- and full-load operation of the GT56 gas turbine about the response of hydrogen reheat flames to prescribed variations of the reactants’ temperature. The calculations revealed a robust, stable and predictable behaviour of hydrogen reheat flames in high-pressure conditions, which is relevant to gas turbine applications.
Interestingly, a very different flame behaviour, characterised by a strong tendency of flame instability in response to temperature fluctuations, was observed for atmospheric pressure conditions. However, these are irrelevant in the context of reheat combustion. This knowledge is key to assessing and improving the robustness of gas turbine combustors that aim for stable and clean operation based on pure hydrogen firing, and will speed up combustor development by OEMs.
In parallel to the computational activity at SINTEF, the experimental activity performed at NTNU has focused on the ignition dynamics and its effect on the flashback tendency of hydrogen-enriched flames (hydrogen-methane blends). The experimental work, conducted by the PhD student Tarik Yahou under the supervision of Professor James Dawson, and in collaboration with Thierry Schuller of IMFT Toulouse, revealed that the ignition dynamics cannot be fully described by classical kinematics arguments and proposed a new mechanism to explain the empirical observations.
The NCCS-sponsored KSP “Reheat2H2” complements Task 5’s research activities by investigating combustion dynamics (thermo-acoustic instabilities) of hydrogen reheat flames and, based on numerical simulation and experimental measurement, is building a low-order model that can represent the flames’ response to unsteady effects emerging from the surroundings (acoustic waves, reactants temperature fluctuations, heat loss etc).
Combustion of hydrogen (from the pre-combustion capture route) in state-of-the-art gas turbine combustion systems has been the main focus of the research activities in Task 5. Enabling the near-future clean and efficient hydrogen-firing of gas turbines creates the attractive possibility of a very convenient value-chain synergy. This combines and optimizes power generation based on hydrogen feedstocks obtained via natural gas reforming with CCS and via water electrolysis, powered by Renewable Energy Sources (RES) in periods of excess production.
In 2020, through advanced numerical modelling (DNS), we have made considerable progress in our fundamental understanding of the characteristics of hydrogen combustion at “reheat” conditions. The “reheat” combustion scheme is implemented in Ansaldo’s state-of-the-art commercial heavy-duty gas turbines and represents the most promising technology to achieve stable, clean and efficient hydrogen-firing of H-class gas turbines (the largest and most efficient engines). In parallel with the numerical modelling activity, experimental investigations have focused on the effects of increasing hydrogen content in the fuel mixture on the stability of a model, lab-scale combustor. In the framework of the NCCS-sponsored KPN “Reheat2H2”, the necessary preparations and preliminary activities including analytical modelling, numerical simulation tool and lab setup have been completed during 2020. The research work itself is set to start in 2021.
There were two main achievements in 2020:
Firstly, the unsteady spontaneous ignition process that takes place in hydrogen flames at reheat conditions has been fully characterized with the help of DNS calculations. The conditions (reactants’ temperature, pressure and composition) that lead to a self-excited instability of the flame are charted for a range of relevant flame temperatures and pressure levels to derive a simple relation that is able to predict the time scale of the instability.
Secondly, based on experimental measurements, features of flame transfer function (FTF) have been characterized for turbulent, non-swirled, bluff body stabilized “M” flames for different hydrogen and methane blends, including pure hydrogen flames. A developed model separately considers the impulse response of acoustic velocity fluctuations and vortex shedding and is interpreted as a distribution of time lags between velocity fluctuations and the unsteady heat release rate. This novel distributed time lag (DTL) model is shown to capture all the features of the FTFs, as shown in the figure.
These new fundamental insights are key to assessing and improving the robustness of gas turbine combustors operating on increasing fractions of hydrogen in the fuel mixture and will speed up combustor development by OEMs.
Main results 2019
In 2019, we have investigated the spontaneous ignition process in hydrogen flames at reheat conditions, with and without inlet forcing utilizing state-of-the-art direct numerical simulations (DNS). Results indicate the occurrence of unsteady ignition and combustion patterns peculiar to hydrogen reheat flames that have not been observed before. Furthermore, results from full-fledged, three-dimensional DNS have provided turbulent flame speed estimates for hydrogen reheat flames (see left figure) spanning a range of turbulence levels and pressure conditions.
Finally, we have performed measurements of hydrogen/methane premixed flames. Flame Transfer Functions (FTFs) extracted from these measurements revealed a characteristic response pattern not observed earlier.
All of this fundamental knowledge is key to assess and improve the robustness of gas turbine combustors operating on pure hydrogen as fuel and will speed up combustor development by OEMs.
NCCS-sponsored KPN “Reheat2H2” was awarded in January, contract work is completed in late spring and project kick-off is arranged on October 23rd (in connection with NCCS CD 2019). The research planned in the KPN Reheat2H2 will optimally complement the ongoing activities of Task 5 and allow special focus to the important issue of combustion dynamics (thermo- acoustic instabilities) in hydrogen reheat flames.
Main results
- “Strategy” to start & stabilize reheat flame of 100% hydrogen is established.
- Comprehensive validation of skeletal chemical kinetics scheme for H2/CH4 fuels.
- First laboratory experiments on combustion dynamics of H2/CH4 flames.
The research activities started in mid-2017 at SINTEF and focused on two modelling topics:
- High-definition numerical modelling of the reactive flow in Ansaldo’s reheat combustion chamber
- Tuning of the chemical kinetics model to efficiently represent combustion at reheat conditions.
The technical work on both topics was planned and performed in close collaboration with Ansaldo’s corporate combustor R&D group with frequent mutual visits between Trondheim and Baden (Switzerland) and with the University of California San Diego (developers of the chemical kinetics model).
Results obtained from the Direct Numerical Simulation (DNS) of a scaled, and geometrically simplified, version of the reheat combustor operating on the target hydrogen-air reactive mixture have provided the first detailed quantification of the combustion characteristics (flame propagation vs auto-ignition) in the device.
On the academic side, the work at NTNU has been mainly related to the preparation/commissioning of the experimental rigs and to the selection and set up of the academic positions (Postdoc/PhD).
Among NCCS industrial partners, Equinor is actively following the research with particular interest in the development of hydrogen-fired gas turbines, providing input and feedback.