This study investigates flame stabilization and flashback in a trapped vortex combustor operating on a lean premixed hydrogen–air mixture at an equivalence ratio of ϕ=0.35. The combustor geometry features a U-bend in conjuction with a liner plate that aerodynamically stabilizes the flame. Particle Image Velocimetry (PIV) was used to study the (reacting) flow in detail at two Reynolds numbers: Re=9.68×10³ (case R-1, marginally stable flame) and Re=13.55×10³ (case R-2, highly stable flame). Within the U-bend, the flame front shows steady laminar-like behaviour where the velocity is primarily tangential to the flame front. Downstream of the U-bend, the shear layer weakens and the flame front becomes more intermittent. This intermittency may cause flame bulges to reach low-velocity zones near the U-bend wall, increasing the possibility of flame flashback through the boundary layer that wall. An analysis of the strain rate tensor shows that within the U-bend, the angle between the flame front normal and the most extensive strain rate direction remains close to 45°, indicating the dominance of shear straining in this region. Further downstream, alignment with the most extensive strain rate increases, indicating that combustion-induced expansion becomes more dominant.

We report on boundary layer flashback of a turbulent premixed, pure hydrogen flame using well-resolved LES. This numerical work is based on flashback experiments of the TU Delft (TUD) jet flame at a jet Reynolds number of Re=11000. Flashback is a highly sensitive process, which is why (i) the turbulent inflow conditions, (ii) chemistry modeling and (iii) the wall temperatures of the mixing tube are crucial parameters to predict accurately this transient process. The presence of thermo-diffusive flame instabilities is the main contributor for flashback in this setup. We identify quasi-coherent turbulent structures in the mixing tube, namely an ejection event, which transports slow, preheated and hydrogen-enriched fluid away from the wall and triggers the flashback event. As a result, the flame forms a convex cusp upstream of the tube exit pointing towards the unburnt gas mixture. During the transition from unconfined (no walls around the flame) to confined (flame surrounded by walls) boundary-layer flashback, this cusp further bends and propagates towards the jet exit center, while, at the same time, its curvature and the reaction rate of hydrogen significantly increase by a factor of two. We repeated the flashback simulations several times and also for various flow conditions: all cases feature the same FB characteristics and, hence, confirms the generality of the conclusions. Moreover, the numerical flashback mechanism confirms the process hypothesized by the experiments. Based on the identified governing key parameters that affect flame flashback, we performed parametric variations of the Lewis number and wall temperature. By varying the Lewis number, we can clearly state that the flashback is driven by thermo-diffusive instabilities, while a hotter wall significantly deteriorates the flashback behavior of this setup. Novelty and significance statement Hydrogen combustion plays a crucial role in various energy applications due to no CO₂ emissions. However, lean premixed hydrogen/air combustion can lead to safety challenges, particularly in the form of flame flashback, potentially causing catastrophic failures in combustion chambers. Understanding and controlling flashback is essential to ensure the safe and efficient use of hydrogen for instance in gas turbines. With this study, we address a number of open questions: (i) root cause of boundary layer flashback in turbulent premixed lean 100% hydrogen jet flames. (ii) transition from unconfined to confined boundary layer flashback. (iii) investigate key parameters that govern flame flashback: Lewis number and wall temperature. This study demonstrates for the first time that flashback in turbulent premixed lean hydrogen combustion is driven by the characteristic behavior of thermo-diffusive instabilities.

This study focuses on flame-induced pressure gradients in turbulent premixed jet flames and its potential role in the occurrence of flame flashback. A new procedure is proposed to determine these pressure gradients experimentally from the Favre-averaged momentum equations. The procedure involves a novel experimental method to determine Favre-averaged quantities from particle image velocimetry data. The resulting pressure distributions are compared for two fuel-air mixtures with identical unstretched laminar flame speed (a stoichiometric natural gas-air mixture and a lean (ϕ=0.49) hydrogen-air mixture) for stable and near-flashback conditions. In all four cases the flame-induced pressure gradients are closely related to the intermittent behavior of the flame. Furthermore, the pressure gradients for the stable and near-flashback flames show only small differences indicating that the mean pressure distribution is not a suitable indicator for the occurrence of flame flashback. Detailed analysis shows a mild, but systematic shift in the orientation of the instantaneous flame fronts, which tend to align more perpendicular to the flow for the flames closer to flashback. This change in orientation results in local deceleration of the flow, thus increasing the probability of flashback. Novelty and significance This work presents original results of experiments in premixed hydrogen-air and natural gas-air turbulent jet flames. A new methodology is introduced to calculate Favre-averaged quantities and the pressure field in a flame from a combination of PIV and Mie scattering measurements. The focus of the experiments and follow up analyses is on the flame characteristics near flashback, since flame flashback is one of the phenomena that hampers the transition from the use of natural gas to hydrogen in, for example, gas turbines.

Hydrogen is presently emerging as a convenient, chemically simple and carbon-free chemical for large-scale energy transport and storage with good balancing potential in future energy systems dominated by unsteady, non-dispatchable renewable power generation from solar and wind resources. Therefore, the capability to operate with hydrogen-enriched fuels reliably, cleanly and efficiently is an increasingly important requirement for gas turbines combustion systems. In this context, the innovative FlameSheet™ combustion system platform, developed by PSM with continued technology refinements by Thomassen Energy, both sister Hanwha companies, represents a competitive Dry Low Emission (DLE) device that has already proven able to handle gaseous fuel blends with high hydrogen fractions at 1350 C gas turbine firing conditions and above. This is mainly due, among a number of crucially important characteristics, to a carefully designed fuel-injection system and to an aerodynamic flame-stabilization strategy characterized by a unique flow pattern (U-bend) of the premixed reactants, ultimately resulting in increased resistance to premixed flame flashback. In the present work, we report a joint research effort consisting of a comprehensive numerical modelling study and of a experimental measurements campaign conducted on a geometrically simplified “FlameSheet^TM-like” burner fired with hydrogen-air mixtures at varying equivalence ratios. A two-dimensional, planar version of FlameSheet^TM (originally a cylindrical burner) is developed at TU Delft in collaboration with Thomassen Energy to enable better optical access and improved diagnostics of the turbulent reactive flow. Massively parallel Large Eddy Simulation (LES) of several geometrically simplified FlameSheet^TM configurations are performed at SINTEF in conjunction with detailed chemical kinetics and a Partially Stirred Reactor (PaSR) model for the turbulence-chemistry interaction. The LES results are validated against the experimental measurements and used, jointly with the latter, to provide new insights about the physical mechanisms that lead to stable flames or, alternatively, to the occurrence of flashback. It is found that, depending on the shape of the tip of the inner combustor-liner wall, flashback takes place along an inner route, around the blunt-shaped tip, or follows an outer route along the outer wall of the U-bend, for a sharp-shaped tip. Furthermore, as the critical equivalence ratio is approached, the amplitude of acoustic pressure fluctuations, excited by the interaction of the flame with the vortex-shedding immediately downstream of the U-bend, significantly increases ultimately leading to abrupt upstream flame displacement and to the occurrence of flashback. Finally, the LES model predictions confirm that the ratio of the channel thickness confining the flow upstream and downstream of the U-bend represents one of the main tuning parameters in flashback control.

Industry is a major contributor to the rise in global CO2 emissions, constituting one fifth of the global energy consumption, of which a significant amount is provided by fossil fuel combustion. Following the Paris agreement, emphasis has been made on the decarbonization of the industrial sector. This study focuses on industrial decarbonization by employing Carbon Capture and Storage for Combined Heat and Power (CHP) gas turbine plants. The scope of this study includes conceptual modelling and thermodynamic analysis of potential decarbonization options for zero carbon CHP plants. The studied options include post-combustion capture, exhaust gas recirculation, precombustion capture and oxyfuel combustion. As conventional air Brayton cycles are not applicable for oxy-fuel combustion in gas turbines, different working fluids and cycle configurations are proposed and thermodynamic performance is evaluated. Selected cycles were then compared based on thermodynamics, economics and off-design performance at a typical constant power to heat ratio of 0.78. It was observed that oxyfuel CHP cycle with CO2 working fluid is a promising solution for zero carbon CHP with relatively low costs and 100% CO2 capture. However, this solution requires new turbomachinery design.

Hydrogen is considered as a promising zero carbon battery fuel to deliver balancing power for the future electricity system with an increasing share of variable renewable power generation. Flame flashback is one of the main challenges for the application of hydrogen in gas turbines. Lean premixed hydrogen combustion is more prone to flashback than natural gas combustion due to higher flame speed and Lewis number effect. The TU Delft developed a boundary layer flashback model based on a previous work by TU Munich. The TU Delft model includes amongst others the effect of the laminar flame speed, boundary layer profile, Lewis number and adverse pressure gradient of the mean flow. The model was successfully validated on academic experiments from TU Munich. In the present paper the turbulent flame speed closure in the TU Delft flashback model is updated for gas turbine like conditions using experimental data from the University of California, Irvine (UCI). This updated model is validated against data from the Paul Scherrer Institute (PSI) and back tested on the original academic experiments from TU Munich. The updated TU Delft boundary layer flashback model and the flashback model from the Paul Scherrer Institute (PSI) have been applied to a scaled version of the Flamesheet^TM combustor. The outcome of the PSI flashback model correlates very well with test results from the TU Delft laboratory, the TU Delft flashback model only with the original flame speed correlation.

New technologies are being developed to produce electricity cleaner and more efficient. Promising technologies among these are the solid oxide fuel cell and the supercritical carbon dioxide Brayton cycle. This study investigates the potential of integrating both technologies. The solid oxide fuel cell is known as a potentially clean and highly efficient technology to convert chemical energy to electricity. The high operating temperatures (600–1000 °C) allow the possibility of a bottoming cycle to utilize the high quality excess heat and also facilitate reforming processes, making it possible to use higher hydrocarbons as fuel. The supercritical carbon dioxide Brayton cycle has received attention as a promising power cycle. It has already been identified as a suitable cycle for relatively low temperature, compared to traditional gas turbines, heat sources for several reasons. Firstly because of the high efficiency, around 40%–45% for the common simple recuperative cycle. Secondly, because the turbine inlet temperature of a supercritical carbon dioxide is around 700 °C is low, compared to well over 1000 °C for a common air Brayton cycle. This is especially of interest because solid oxide fuel cell developers are targeting lower operating temperatures to avoid the use of exotic and expensive materials. And thirdly, the cycle can operate entirely above the critical point. Therefore the temperature increases gradually with the energy added to the cycle. This is more suitable for waste heat because the exergy loss decreases and more low temperature heat can be utilized compared to a steam Rankine cycle where most of the heat is added above the relatively high boiling point of pressurized water. A thermodynamic model of the solid oxide fuel cell- supercritical carbon dioxide Brayton cycle hybrid system is developed to explore and analyze different concepts of integration. Several conclusions are drawn. Firstly it is found that recirculating cathodic air increases the efficiency of the system and decreases the size of the heat exchangers. Secondly, applying a pinch point optimization decreases the size of the heat exchangers but increases the complexity of the system while the efficiency is not much affected. Thirdly, applying the recompression cycle in stead of a simple recuperative supercritical carbon dioxide cycle increases the efficiency of the system but not as significantly when operating the supercritical carbon dioxide as a stand-alone system while the complexity of the system increases even more. And finally, compared to a directly coupled solid oxide fuel cell-gas turbine system the solid oxide fuel cell- supercritical carbon dioxide Brayton cycle hybrid system is more efficient but significantly more complex.

The combustion properties of hydrogen make premixed hydrogen-air flames very prone to boundary layer flashback. This paper describes the improvement and extension of a boundary layer flashback model from Hoferichter et al. (2017, "Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory," ASME J. Eng. Gas Turbines Power, 139(2), p. 021505) for flames confined in burner ducts. The original model did not perform well at higher preheat temperatures and overpredicted the backpressure of the flame at flashback by 4-5×. By simplifying the Lewis number-dependent flame speed computation and by applying a generalized version of Stratford's flow separation criterion (Stratford, 1959, "The Prediction of Separation of the Turbulent Boundary Layer," J. Fluid Mech., 5(1), p. 1), the prediction accuracy is improved significantly. The effect of adverse pressure gradient flow on the flashback limits in 2 deg and 4 deg diffusers is also captured adequately by coupling the model to flow simulations and taking into account the increased flow separation tendency in diffuser flow. Future research will focus on further experimental validation and direct numerical simulations to gain better insight into the role of the quenching distance and turbulence statistics.

The combustion properties of hydrogen make premixed hydrogen-air flames prone to flashback. Several combustor concepts have been proposed and studied in the past few years to tackle the problem of flame flashback in premixed high hydrogen fuel combustors. This study looks at one of the concepts which uses the Aerodynamically Trapped Vortex to stabilize the flame. Burner concepts based on trapped vortex flame stabilization have a higher resistance towards flame blowout than conventional swirl stabilized burners. This work looks at the flow and flame behavior in the proposed Aerodynamically Trapped Vortex Combustor for 100% premixed hydrogen operation. Numerical simulations for the analysis were performed with the commercial CFD simulation package AVL FIRE^TM. The flow field characterization was focused on the investigation of the influence of both the inlet velocity and inlet turbulence intensity on the mean velocity, wall velocity gradient and turbulence intensity in the combustor. To study the flame stabilization mechanism, reactive simulations were performed at two fuel equivalence ratios. The combustion regime of the flame, turbulent flame speed and temperature distribution in the combustor were quantified from the simulation results. Combustion is modelled using a detailed chemistry solver with the k - e turbulence model to resolve turbulence. No additional turbulence-chemistry interaction model is used in the current research. To reduce chemistry computational time, the multi-zone method is employed. To capture the effect of preferential diffusion, two approaches were used to quantify the diffusion coefficient of each species. The diffusion coefficients were calculated using both mixture averaged approach and the multi component diffusion approach. The proposed design for the Aerodynamically Trapped Vortex combustor was able to stabilize a 100% premixed hydrogen flame without flashback for the simulated conditions.

This work is focused on the process system modelling of an indirectly heated gasifier (10 MW_th) using torrefied wood as feedstock and its integration with methanol and power production using Aspen Plus®. The modelling of the gasification process along with the obtained reaction kinetics were validated with experimental data found in literature. Different processing steps such as gasification, gas cleaning and upgrading, methanol synthesis and energy conversion, were modelled and their performance was optimized through a series of sensitivity studies. The results obtained were then used to investigate the effect of different technologies and the variation of operational parameters on the overall process performance. Three cases were examined: “syngas production” (case 1), “methanol production” (case 2), and “power production” (IGCC) (case 3). Case 1 and case 2 were simulated using sand and dolomite as bed materials respectively, in order to study the incorporation of Absorption Enhanced Reforming (AER) on the syngas and methanol production efficiency. For case 3 the simulation was performed for two different configurations: a conventional Integrated Gasification Combined Cycle (IGCC) and an innovative Inverted Brayton Cycle (IBC) turbine system. Dolomite was used as the bed material for both configurations. For case 1, an increase of 5% in hydrogen yield in the product gas when AER is applied was observed. For case 2, higer values of Cold Gas Efficiency and Net Efficiency (34% and 60% instead of 33% and 55%, respectively) and a slightly lower value of Carbon Conversion (96% instead of 100%) were obtained when AER was employed. Gasification temperature was lowered by 110 °C in this scenario. For case 3, a lower value of Net Efficiency was obtained when IBC was considered (43% instead of 47%), while a value of 60% was obtained for methanol production with AE. Moreover, the results of case 3, showed that the latent heat in the hot syngas is best utilised when IBC is considered. The developed model accurately predicted the composition of the produced gas and the operational conditions of all the identified blocks within the methanol synthesis and power production processes. This way the use of this model as a generic tool to compare the utilization of different technologies on the performance of the overall process was validated.

The rapid growth of renewable generation and its intermittent nature has modified the role of combined cycle power stations in the energy industry, and the key feature for the operational excellence is now flexibility. Especially, the capability to start an installation quickly and efficiently after a shutdown period leads to lower operational cost and a higher capacity factor. However, most of existing thermal power stations worldwide are designed for continuous operation, with no special focus on an efficient start-up process. In most current start-up procedures, the gas turbine controls ensure maximum heat flow to the heat recovery steam generator, without feedback from the steam cycle. The steam cycle start-up controls work independently with as main control parameter the limitation of the thermal stresses in the steam turbine rotor. In this paper, a novel start-up procedure of an existing combined cycle power station is presented, and it uses a feedback loop between the steam turbine, the boiler and the gas turbine start-up controls. This feedback loop ensures that the steam turbine can be started up with a significant reduction in stresses. To devise and assess this start-up methodology, a flexible and accurate dynamic model was implemented in the Simulink environment. It contains >100 component blocks (heat exchangers, valves, meters and sensors, turbines, controls, etc.), and the mathematical component submodels are based on physical models and experimental correlations. This makes the model generally applicable to other power plant installations. The model was validated against process data related to the three start-up types (cold start, warm start, hot start). On this basis, the optimization model is implemented with feedback loops that control, for example, the exit temperature of the gas turbine based on the actual steam turbine housing temperature, resulting in a smoother heating up of the steam turbine. The optimization model was used to define the optimal inlet guide vanes position and gas turbine power output curves for the three types of start-up. These curves were used during real power station start-ups, leading to, for cold and warm starts, reductions in the start-up time of, respectively, 32.5% and 31.8%, and reductions in the fuel consumption of, respectively, 47.0% and 32.4%. A reduction of the thermal stress in the steam turbines is also achieved, thanks to the new start-up strategy.