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Fostering Sustainable Chemical Energy Conversion Technologies through a Novel Interdisciplinary Approach which combines Combustion Science and Additive Manufacturing

 

High-temperature thermochemical processes combined with carbon-free chemical energy sources, such as hydrogen (H2) and ammonia (NH3), have key roles in the future energy system and the preceding transition phase.  Compared to conventional fuels,  H2 and NH3  have fundamentally different combustion properties, which are reflected, for example, in extraordinarily high burning rates and different flammability limits and ignition energies. In this Priority Program, SPP 2419 HyCAM, a new interdisciplinary approach is being pursued that combines combustion research and additive manufacturing competencies. The hypothesis of SPP2419 HyCAM is that only a comprehensive understanding of combustion fundamentals, as well as the integration of modern 3D manufacturing processes through simulation-based design, can enable the simultaneous improvement of flexibility, efficiency and emissions in thermochemical energy conversion processes.  As a coordinated and network-building measure, this Priority Program is intended to bring together the relevant disciplines of chemistry, combustion research, materials science and manufacturing technologies. This approach facilitates the systematic, rapid, and interdisciplinary development of technology-relevant fundamentals for for advancing technology in the field of thermochemical utilization of carbon-free fuels, thereby making a significant contribution to climate protection in a timely manner.
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Thermochemical utilization of hydrogen-based fuels is essential for transforming the energy system towards carbon-neutral energy conversion, as it offers significant advantages in this context. Thermochemical energy conversion processes are widely available and reliable and can be made completely carbon-free using fuels such as hydrogen and ammonia. They are “drop-in” capable and allow gradual substitution of fossil fuels, avoiding complex, disruptive technology changes.
Therefore, high-temperature thermochemical processes combined with carbon-free chemical energy sources have key roles in the future energy system and the preceding transition phase. These technologies require fuel flexibility, which will be achieved here together with the optimization of efficiency, emissions and safety by combining combustion science with innovative manufacturing processes, e.g. additive manufacturing, and the associated degrees of freedom in materials and design. Compared to conventional fuels, hydrogen and ammonia have fundamentally different combustion properties, which are reflected, for example, in extraordinarily high burning rates and different flammability limits and ignition energies. 
Due to the subordinate technological relevance of these fuels to date, many of these aspects have been insufficiently investigated and are not sufficiently well understood for further technology development. However, these fuels hold great potential for significantly improving thermal efficiencies and pollutant emissions. In this context, simulation-based design and modern manufacturing processes enable new burner geometries that can meet the high requirements when using these fuels.
In this Priority Program, SPP 2419 HyCAM, a new interdisciplinary approach is being pursued that combines the competencies of combustion research and additive manufacturing. The hypothesis of the SPP is that only a comprehensive understanding of combustion fundamentals, as well as the integration of modern 3D manufacturing processes through simulation-based design, can enable the simultaneous improvement of flexibility, efficiency and emissions in thermochemical energy conversion processes.
As a coordinated and network-building measure, the SPP is intended to bring together the relevant disciplines of chemistry, combustion research, materials science and manufacturing technologies. In this way, the technology-relevant fundamentals are to be developed systematically, quickly and interdisciplinarily so that the basis for further technology development in the field of thermochemical use of carbon-free fuels can be made possible in a timely manner, thus making an important contribution to climate protection.
Relevance of Thermochemical Energy Conversion Processes of Hydrogen-based Chemical Energy Carriers.
Hydrogen (H2) plays a special role among renewable synthetic fuels (RSFs). It can be used in industrial processes, for heating in buildings, and as fuel in transportation. Its use for time-shifted power generation is also possible. In addition to hydrogen, ammonia (NH3) can also be mentioned as a carbon-free energy carrier, the production of which is well understood and entirely possible through renewable energy sources. A significant advantage of ammonia compared to hydrogen arises from its significantly higher volumetric energy density in the liquid state, which favors long-term storage and transport over long distances.
Thermochemical Energy Conversion Processes
The energy stored in hydrogen or ammonia can be exploited through thermochemical energy conversion processes. Such processes are currently widespread in the energy sector, particularly reliable, and offer significant further optimization potentials through operation with new fuels. Relevant areas of application include back-electrification in gas turbines and, in particular, the provision of process heat using industrial gas burners. Ammonia/hydrogen mixtures are also particularly promising, as they partially compensate for the challenges in the combustion of pure substances. Potentials of thermochemical energy conversion of hydrogen also arise when mixed with natural gas, as hydrogen can be gradually added to the existing natural gas infrastructure, enabling a low-risk transition to a carbon-free energy economy. In this context, its use for space heating is also interesting. The term “hydrogen-containing fuels” refers to fuel mixtures of hydrogen, ammonia, and hydrocarbons with a high hydrogen or ammonia content.
Challenges in the use of Hydrogen-containing Fuels and the relevance of Additive Manufacturing (AM)
Molecular hydrogen is the carbon-free energy carrier closest to energy technology and mobile use. Nevertheless, there is a great need for research, mainly due to the specific molecular dynamic and flame-specific properties of molecular hydrogen, with questions regarding flame stability, flame dynamics, nitrogen oxides (NOx) formation, and safety. Potentials arise from the high burning speeds of hydrogen/air mixtures, allowing for ultra-lean combustion concepts and efficiency improvements. For ammonia, the challenges lie in the tendency to form nitrogen oxides, toxicity, which limits its use to well-monitored applications such as central industrial processes, large engines, and power generation, and combustion-specific properties such as high minimum ignition energy and low flame speeds. However, in mixtures with hydrogen, these properties can be advantageously influenced, opening up new possibilities for process control.
To optimally exploit the potential of these fuels, additive manufacturing (AM) can make an important contribution. As opposed to conventional manufacturing methods, it allows for almost complete design freedom, material combinations, and thus extensive possibilities for functional integration (e.g., sensors). This enables entirely new burner geometries to be realized, tailored to the flame dynamics of hydrogen-containing flames, which are not feasible in conventional manufacturing processes. Flame investigation and simulation results can be used for inverse design optimization. The resulting flow-optimized geometries are hardly or not at all feasible with conventional manufacturing methods. In contrast, components can be produced without the need for traditional tools in a coordinated digital and physical process chain through the use of AM and based on coupled models with the claim “form follows function”. Both entire burners or only critical components in terms of cost-effectiveness can be manufactured by AM. The challenge here lies, among other things, in processing suitable materials, which are needed due to the higher flame temperature, higher water content in the exhaust gas, and material/hydrogen interaction. New concepts for process control, such as new preheating concepts, must also be developed for realization. Furthermore, safety risks can already be addressed through component design. Further challenges lie in achievable detail resolution, resulting in surface roughness and the feedback of manufacturing-specific features (e.g., build direction, support) in the functional design process.
Structuring relevant Research Fields
 It is important to understand the connection between additive manufacturing and combustion and the interrelationships in the individual disciplines.
Burner design (e.g., topology optimization), sensor integration, and materials are important for AM. For thermochemical energy conversion, the relevant processes take place on length and time scales spanning several orders of magnitude. This requires the integration of numerous disciplines on laboratory and system scales. Fuel flexibility is an aspect that plays a central role during the transition phase from today’s carbon-based energy system to a CO2-neutral future. Therefore, all thermochemical energy conversion processes require separate investigations regarding pure hydrogen, relevant mixtures of hydrogen and ammonia, and mixtures with conventional hydrocarbons.
AM can make an important contribution to all areas of combustion that need to be investigated. On the laboratory scale, specifically designed burners can be manufactured for experimental investigation, such as flame dynamics, which enable deeper insights through sensor integration or built-in gas sampling channels. Furthermore, insights from the laboratory scale can be transferred to the system scale using AM to enable the development of fuel-flexible, scalable industrial burners and gas turbines. For this purpose, fundamental questions in the areas of inverse design optimization must be addressed. Examples include digital materials with locally manipulable properties (e.g., shape memory effects), thin-walled structures (e.g., channel geometries with locally variable cross-sections), achievable surface roughness (e.g., in the area of overhangs), multiphysical topology optimization, component-integrated and/or printed sensors, as well as the processing of high-temperature-resistant materials in AM.

Goals

For a reliable and gradual transition of thermochemical energy conversion processes to hydrogen-based fuels, the necessary fundamentals and new innovative solutions through the integration of AM in burner development are to be researched interdisciplinary and cross-domain within this DFG Priority Programme. The goal is to investigate the fundamental questions of hydrogen and ammonia combustion and to establish a foundation for overcoming application-specific challenges, among others, in industrial burners and gas turbines. AM can already support this today. However, comprehensive and integrative further development of fluid mechanics and combustion fundamentals is necessary to overcome these challenges.
The SPP2419 HyCAM aims to provide insights and technologies that can be used in the short term while laying the groundwork for the long-term development of future technologies. In the short term, the knowledge gained from the SPP2419 HyCAM is intended to enable a gradual substitution of fossil fuels with hydrogen-containing energy carriers in already existing carbon-based thermochemical conversion processes, thereby paving the way for a fully ‘drop-in’ capable continuous path of energy transition. In the medium term, increased fuel flexibility and process optimization under the requirements of stability, safety, and pollutant formation are to be implemented, allowing for an expansion of the selective use of hydrogen-containing fuels. In the long term, technologies operating with carbon-free hydrogen-containing fuels are to be applied in a tailored carbon-neutral energy system of the future. For all developments, it is essential that the unique properties of these fuels, together with the opportunities opened up by the use of AM in burner design and process control, are strategically utilized to achieve targeted improvements in efficiency, emissions, and fuel and operational flexibility.

Methodic Goals

The methodological challenges in the analysis, modelling, and practical implementation of thermochemical energy conversion processes lie in the significant scale separation of physically relevant phenomena (from the molecular scale to system scale – “muti-scale“), the number of physically relevant phenomena (“multi-physics“) in combustion, and the resulting highly non-linear behavior. The conception of highly complex AM-manufactured burner concepts and coordinated operating strategies requires an integrated process using predictive simulation, AM, and experimental analysis. Methodical goals, therefore, consist of the further development of individual aspects as well as the integrative design process.
Predictive and reliable simulation models are required to optimize thermochemical energy conversion processes using inverse design simulation. However, current combustion simulation approaches often use an ad-hoc selection of models, methods, and data and do not adequately address the multiscale-multiphysics problem considered here. Model developments based on both mechanistic and data-driven approaches are therefore needed. A focus lies on quantifying uncertainties of the developed and used models.
To resolve the processes relevant to many macroscopic phenomena inside the flame structure of hydrogen-containing fuels, advanced experimental measurement methods are required, such as temporally and spatially high-resolution laser-optical techniques. Furthermore, the reduction of uncertainties in reaction kinetic parameters still represents a significant hurdle in the development of reaction mechanisms. A comprehensive reduction of model uncertainties is intended to be achieved in this Priority Program through systematic coupling of quantum mechanical calculations and experimental results. Here, the freedoms of AM are particularly advantageous in designing new fundamental experiments to reduce measurement uncertainties through sensor integration, e.g., by using printed sensors in flow reactors for measuring pressure and temperature. Furthermore, AM could provide new, cost-effective, and robust flame sensing methods, as hydrogen flames are hardly visible and ion current sensors used in many assemblies are not usable for hydrogen due to the absence of the CH radical. Additionally, high-temperature-resistant materials are integrated into the production of alloy components through rapid alloy development. The design freedom, the use of tailored alloys, and the possibilities of sensor integration through AM are intended to enable the realization of innovative concepts for the development of future highly efficient, emission-free, stable, and safe combustion systems.

Short-term Goals of the SPP (First Project Phase)

Experimental Database for Kinetic Modeling
  • Acquisition of physical insights and generation of a database from fundamental laboratory experiments and direct numerical simulations on the internal structure of reaction zones, flame stabilization, flame flashback, intrinsic instabilities, and pollutant formation; initial modeling approaches
  • Establishment of extensive, well-documented, and publicly accessible datasets for system-scale standard configurations with initial AM-manufactured burners (gas turbine, industrial burners)
  • Derivation and initial implementation of necessary development steps (design, material, process) in the field of AM (based on requirements from combustion technology) for future realization of low-emission hydrogen and ammonia combustion
  • Development of specialized, fuel-flexible, and scalable burners/turbines for experimental investigations through sensor integration and/or gas sampling channels by AM

Medium-term Goals of the SPP (Second Project Phase)

  • Predictive kinetic models for complex mixtures of hydrogen-based fuels (hydrogen and ammonia), including pollutant formation
  • Model validation for laboratory- and system-scale configurations
  • Development of hierarchical simulation methods for turbulent combustion of hydrogen-containing fuels
  • Simulative and experimental material development for AM manufacturing of combustion-relevant components (mixture, combustion, analysis)
  • Hierarchical design optimization of fuel-flexible burners manufactured by AM based on simulated and experimental investigations of flame behavior
  • Demonstration of new burner concepts on a laboratory scale through AM

Long-term Goals (Vision)

  • Automatic optimization for industrial implementation for different units with fuel flexibility up to 100% hydrogen or hydrogen/ammonia mixtures
  • Computer-aided upscaling of energy conversion plants for the energy transition (Automated) design and manufacturing of high-temperature-resistant burners using AM using multi-material processes and new concepts for temperature control of high-temperature-resistant materials (e.g., nickel-based superalloys, refractory metals)
  • Establishment of 3D-printed burner concepts on a laboratory scale, as well as automation and further development of sensor-integrated measurement technology and production of multi-material components

Limitation of scientific questions considering the duration of a focus program

The central research needs for thermochemical energy conversion of hydrogen-containing fuels arise in the areas of stability/safety and emissions/efficiency. The scientific questions encompass phenomena that occur equally on the laboratory scale and system scale of combustion and must be taken into account. Additive manufacturing (AM) supports at every level through innovative manufacturing possibilities.
Below are some relevant scientific questions and the associated research fields (RF), with the areas of AM, system scale, and laboratory scale of combustion highlighted in color. If not explicitly mentioned, all scientific questions include the topics of process optimization and fuel flexibility and require investigations regarding mixtures of hydrogen, ammonia, and/or hydrocarbons. An intrinsic coupling of all combustion laboratory and system levels with AM is desired in SPP2419 HyCAM. Numerous scientific questions relevant to practical applications can be abstracted from the combustion system level to the underlying laboratory level to investigate them in isolation and in greater detail. The special requirements for the production of customized components, which are limited by conventional manufacturing processes, can be passed on to AM, which can then offer optimized solutions on both the laboratory and system levels. Insights gained at the laboratory level can be used to understand the interaction of various phenomena and subsequently implement technological innovations at the system level.
RF1: Ignition and extinction processes, laminar burning velocities, and flame data
Hydrogen-containing mixtures are easily ignitable and have high burning velocities, promoting pre-ignition but significantly influencing flame stability and blow-off. The exact measurement of these fundamental flame quantities is of great importance for the design of numerous systems. Measurements of ignition delay times, minimum ignition energy, laminar burning velocity, and the distribution of chemical components in flames (internal flame structure) serve the validation of chemical reaction mechanisms in RF2 and are also used as parameters in modeling more complex systems in RF5 and RF6 as well as for topologically optimized designs and materials in RF7. Furthermore, the development of new diagnostic techniques or sensors should be advanced here. For example, the measurement of slow-burning velocities and critical stretch rates of ammonia flames to assess the risk of flame blow-off and local extinction is challenging because established methods cannot be applied due to radiation and buoyancy effects. Flame detection also requires new developments since hydrogen flames have very low flame luminosity and produce hardly any ions. To further develop diagnostics, AM contributes individually tailored, customized components, allowing, for example, the investigation of flames in situ through burners with integrated sensors or gas sampling channels from RF9 or the determination of chemical compositions at different points of the burner.
RF2: Reaction mechanisms considering pollutant formation
Suitable reaction mechanisms for hydrogen/ammonia/hydrocarbon mixtures are needed for the analysis and modeling of any combustion processes. This also includes investigations on the transport data of hydrogen-containing fuels, which play a significant role in many macroscopic phenomena and have not yet been adequately quantified. A particular focus lies on the reaction kinetics of ammonia combustion and pollutant formation, especially nitrogen oxide formation. Reaction rates of individual important reactions are determined using quantum chemical simulations or targeted experiments. By integrating RF7, RF8, and RF9, the experimental setups are improved using AM, thereby reducing uncertainties. The reaction mechanisms are validated with the data from RF1 and also serve as input for modeling the measured quantities. Reduced reaction mechanisms with sufficient accuracy should be provided for use in the other RFs. A particular focus is placed on the development of reaction mechanisms for conditions at elevated pressure.
RF3: Flame instabilities, instability/turbulence interaction
RF3 aims to investigate the mechanisms of intrinsic instabilities strongly pronounced in hydrogen-containing fuels depending on the mixture composition (fuel flexibility) under elevated pressure and in interaction with turbulent flows. The latter aspects describe typical conditions found in gas turbines or industrial burners, whose influence on instabilities is not yet understood. Part of RF3 is developing and validating predictive simulation models, dimensionless stability maps, and design rules for RF4, RF5, and RF6. Here, the sensors from RF9 can be integrated into experiments on the laboratory scale to obtain relevant data for investigations and validation of simulation models. The fundamental physical insights gained here can be used for inverse burner design in RF7.
RF4: Instability/turbulence interaction regarding pollutant formation in turbulent flames
RF4 aims to develop reliable predictive models for large eddy simulations based on validated experimental data for predicting pollutants and methods for pollutant reduction. The formation of pollutants such as NOx and CO is significantly determined by the interaction of relevant chemical reactions with the local flow field, especially by the formation of hot spots due to flame intrinsic instabilities, leading to increased NOx formation. Since the coupling of pollutant formation reactions with intrinsic flame instabilities and turbulent flows is highly complex and not yet sufficiently researched, these connections shall be investigated in close collaboration with RF3. The predictive models developed here will be used in the design and topological optimization in RF7.
RF5: Process control considering safety-relevant aspects
RF5 aims to develop new methods and knowledge to prevent or detect phenomena such as flame flashback, flame blow-off, and pre-ignition early on to ensure the safe use of hydrogen-containing fuels with varying hydrogen content (fuel flexibility). Insights from RF1 and RF2 are required for the knowledge-based development of such methods. Furthermore, improvements in diagnostics from RF1 and integrated sensors from RF9 should be addressed. Within RF5, concepts of AM-compatible burner design to suppress the mentioned undesired phenomena using the manufacturing-specific design freedoms in collaboration with RF7 should also be developed. For example, targeted modification of surface structure through the introduction of microstructures can prevent flame flashback and influence the flow behavior of the fuel mixture. The research field of thermo-acoustic instabilities, which also has high safety relevance, for example, for gas turbines, shall not be addressed due to the numerous other questions in this SPP.
RF6: Process control considering pollutant formation and efficiency increase
Stoichiometric hydrogen flames have a higher adiabatic flame temperature than stoichiometric natural gas flames, significantly increasing nitrogen oxide emissions. Additionally, the formation of CO emissions for mixtures with hydrocarbons is not yet sufficiently understood. Therefore, in RF6, the fundamentals for suitable combustion concepts for pollutant reduction should be established. This includes various combustion processes such as premixed, partially premixed, non-premixed, or staged combustion, lean-burn concepts, developing and adapting burner and combustion chamber geometries, and introducing cooling channels. In particular, pollutant formation at elevated pressures, as typical in gas turbines, shall also be investigated. Furthermore, the combustion efficiency of hydrogen-containing fuels (burnout) shall be analyzed to avoid molecular hydrogen and ammonia in the exhaust gas. Due to the high complexity and large number of influencing parameters and to ensure simultaneously stable, safe, and low-emission combustion processes, the analysis and modeling at the system level require contributions from all other RFs.
RF7: Topological optimization and inverse burner design
The aim of RF7 is to explore topologically optimized burner configurations and designs for an optimized operating state considering combined reactive fluid flow and heat transfer problems enabling the development of new hydrogen- and ammonia-operated flexible combustion systems. The modeling here should be done in inverse, i.e., automated based on validated numerical models from RF4, RF5, and RF6 to subsequently generate algorithm-based inverse simulation methods for creating burner designs. In particular, the complex interactions of turbulent flow and chemical reaction kinetics, as well as the instability effects from RF3, must be considered in simplified models suitable for inverse modelling. A particular importance is attached to the rough surface of AM components, which has an influence on the flow dynamics. Furthermore, data generated in RF1 can also be directly used to design new combustion concepts. RF7 also interacts with RF8 for high-temperature resistant materials and coating requirements and with RF9 for sensor integration.
RF8: High-temperature resistant materials and coating requirements
The aim of RF8 is to develop new concepts enabling the processing of high-temperature resistant materials. Ideas in the testing phase, such as preheating using flat infrared radiators (e.g., VCSEL) or with the help of an (additional) laser beam to reduce the temperature gradient and thus prevent crack formation, shall be investigated. Additionally, simulations of solidification conditions and thermal balances should enable the estimation of stresses and component warping to compensate for them or avoid them through the appropriate use of preheating concepts. Thermal barrier coatings (TBC) can be directly applied during the manufacturing of components using AM via graded structures to protect the burners from the high temperatures of the combustion chamber. TBC’s processing and coherent connection in additive multi-material processes requires further developments, especially in the desired complex geometries in RF6 and RF7. In this RF, there is also the opportunity to build on SPP2122 “Materials for AM” to advance material development for high-temperature applications.
RF9: Measurement technology with sensor integration
RF9 aims to advance various developments in sensor integration to ensure successful integration (in RF7) and later functionality of the sensors. These serve not only for better experimental investigation of burners and flame dynamics in RF1, RF3, and RF5 but also enable real-time monitoring of the burner in operation and adaptive process control, e.g., in RF6. In addition to the integration of existing sensor technology, especially multimaterial systems should be used, which combine various manufacturing techniques, such as LPBF for component build-up in connection with aerosol jet printing or a dispenser for applying sensors, to directly print sensors during the printing process, e.g., strain gauges. For these systems, the optimized process control for direct printing of sensors must first be developed to avoid damaging the sensors during the LPBF process. To achieve this goal, coatings developed in collaboration with RF8, which protect the sensors from high temperatures, can also be developed. The possibility of printing a channel into which the sensor is subsequently inserted shall also be investigated. Regardless of sensor integration, channels for gas extraction from the burner can also be incorporated to investigate the chemical composition of the flame in the stabilization area (RF1, RF3, and RF6) and to determine pressures. These measurement data shall be validated against non-invasive measurement methods.
 
The overarching vision of the SPP2419 HyCAM is to accelerate the transition towards a carbon-neutral energy economy based on hydrogen-containing fuels by transforming thermochemical energy conversion processes using innovative solutions from additive manufacturing (AM). The central challenges for this endeavor lie in realizing fuel-flexible burners and process controls while considering flame stability, pollutant reduction, and efficiency enhancement. To ensure a comprehensive investigation of the central research needs, the structure of the focus program has been aligned accordingly. Furthermore, the structure of the focus program addresses the intrinsic multiscale, multiphysics, and multidisciplinary nature of thermochemical energy conversion processes, as a holistic exploration of the central problem areas can only be realized through the collaboration of various disciplines. Moreover, AM is utilized here as a connecting link, as depicted in Figure 3, without which the planned structure will not function. Defining relevant engineering questions at the system level is intended to generate questions at the laboratory level. With the help of AM, innovative, problem-specific solutions for process optimization and fuel flexibility can then be achieved.
SPP2419 HyCAM aims to bring together various disciplines, so there is a clear preference for primarily cooperative proposals to be included in the Priority Program,  where complementary expertise is directly connected. To support the necessary interdisciplinary approach, a project is expected to typically consist of one experimental and one theoretical/simulation/modeling subproject from the combustion domain, in addition to an AM subproject.