To achieve the goal of ecologically sustainable aviation, a significant amount of alternative fuels from renewable sources is required. However, a safe use of these alternative fuels must be always ensured. As a consequence, new fuels have to undergo a complex approval process. In this approval process, specifications for over 70 different properties have to be met, starting from material properties (e.g. density, viscosity, ...) to more complex performance properties such as re-ignition capability at high altitudes.
Research and development projects therefore focus on reducing the costs, the time required as well as the uncertainties in fuel tests and in the evaluation process in order to enable an efficient and timely certification, qualification and acceptance of potential alternative aviation fuels. The MAT department is therefore building-up, further developing and applying a platform for the simulation and assessment of new, alternative and future fuels.
In addition to reducing costs, the improvement of performance characteristics is also an important research and development goal. The synthetic production of aviation fuels offers new possibilities for fuel composition. In case pollutant emissions can be significantly improved or the fuel consumption reduced, airlines could be willing to accept higher prices for a premium designer fuel as long as the saving effects outweigh the costs.
Besides the numerical assessment of the fuels, the MAT department coordinates a variety of projects, in which the emissions of the designer fuels are characterized by means of laboratory investigations, ground measurements, and flight tests. In collaboration with the DLR Institute of Atmospheric Physics, the influence of the emissions on the formation, microphysics and life time of contrails is studied. This is intended to demonstrate the positive influence of designer fuels on local air quality, cruise flight emissions and the climate. It has already been shown that an addition of bio-kerosene to conventional kerosene can reduce particle and soot emissions.
NASA and DLR investigate the impact of aviation on the climate - joint flight tests on alternative fuel emissions
A journey through an exhaust plume – DLR flight tests for alternative fuels
Jetscreen – Low Carbon transportation fuels in aviation
Solar reactors are a promising technology for an environment-friendly thermochemical fuel production. Solar radiation is concentrated by a heliostat field and efficiently absorbed in a solar reactor that thermochemically converts H2O and CO2 to syngas which is subsequently processed to Fischer-Tropsch hydro-carbon fuels. The solar synthesis gas is produced based on a two-stage solar redox cycle. In this process, a metal oxide is exposed to concentrated solar radiation and therefore very high temperatures (around 1500 °C). This reduces the metal oxide, which means that oxygen is released. If, in a next step, carbon dioxide and water are introduced into the solar reactor at a lower temperature (about 1000 °C), the reduced metal oxide can separate the oxygen from these molecules and thus generate carbon monoxide and hydrogen. A mixture of carbon monoxide and hydrogen is called synthesis gas. In subsequent process steps the synthesis gas can be further processed into liquid fuels. The MAT department contributes to the development of this technology by analyzing the heat and mass transfer in the porous media of the cavity receiver. The cavity receiver is located in a cavity at the top of the solar tower. Furthermore, the MAT department is active in the assessment of the resulting solar fuel and its application as drop-in fuel. A so-called drop-in fuel does not require adaptation of the engine, fuel system or the fuel distribution network.
S. Kyrimis, P. Le Clercq, and S. Brendelberger, 3D Modelling of a Solar Thermochemical Reactor for MW Scaling-up Studies, AIP Conference Proceedings 2019
Sun to Liquid – Fuels from concentrated sunlight
Solar Fuels (only in German)
Solar-Jet – Zero-carbon jet fuel from sunlight
Entrained flow gasification is a promising process for the conversion of low-grade feedstock, e.g. highly viscous slurries from biomass and suspensions with a significant content of solid particles, to synthetic gas (syngas). Syngas, a mixture mainly of hydrogen and carbon monoxide, can be used directly as a fuel in gas engines or gas turbines. However, as its energy density is about half the one of natural gas, it is mostly suited for the production of transportation fuels and other chemicals. Actually, syngas is a building block for the production (synthesis) of various fuels in particular synthetic natural gas, methanol, or synthetic liquid fuels: dimethyl ether (DME), gasoline, jet fuel (e.g. Synthetic Kerosene with Aromatics (SKA)) or diesel. The latter liquid fuels are often produced by means of a heterogeneous catalytic process like Fischer-Tropsch. A major scientific challenge is the prediction of the physical and chemical phenomena occurring in high-temperature and high-pressure multi-phase flow systems like entrained low gasifiers. Furthermore, biomass based slurries are heterogeneous mixtures of immiscible liquids (emulsions) and solid, non-uniform particles (suspensions). The research focus of the department MAT is on the development of new models and methods for the prediction of these complex multi-phase flow systems.
G. Eckel, P. Le Clercq, T. Kathrotia, A. Saenger, S.Fleck, M. Mancini, T. Kolb and M. Aigner, Entrained flow gasification. Part 3: Insight into the injector near-field by Large
Helmholtz Virtual Institute for Gasification Technology
Most Bio-kerosene projects exclusively investigate the production of an alkane-based basic kerosene formulation, while functional additives such as oxidation inhibitors continue to come from petrochemical sources. However, these high performance additives can account for up to 25% of the total kerosene costs. The development of biogenic kerosene additives is therefore reasonable from both an economical and ecological point of view. Microalgae are active consumers of the greenhouse gas CO2 and form under metabolic stress conditions, e.g. with increased salinity of the water, intracellular lipids, which can be converted into usable products by thermo-catalytic conversion. Algae grow up to 10 times faster than land plants and can be cultivated on fallow land using sea water. Therefore, the algae-based fuel production does not compete with food production. To certify a biogenic antioxidant additive, however, it must be ensured that it does not influence any other physical or chemical process besides fuel oxidation (for which it was developed).
As part of the Bio@Jet project, phenol-producing algal strains were selected and characterized on a laboratory scale at the Werner Siemens Chair for Synthetic Biotechnology (TUM). The TUM Chair for Bioprocess Engineering explored the technical optimization of algae-based phenol production under specific climate scenarios. DLR investigated the effects of anti-oxidative additives from algae biomass on the fuel properties and the combustion behavior. The change in the combustion reaction kinetics and the fuel preparation (atomization and evaporation) caused by a phenol addition was analyzed in detail on the basis of experimental data and modeling results.
D.V. Woortman, S. Jürgens, M. Untergehrer, J. Rechenberger, M. Fuchs, F. Qoura, G. Eckel, M. Stöhr, P. Oßwald, P. LeClercq, L. Hintermann, D. Weuster-Botz, F. Bracher, T. Brück and N. Mehlmer, Greener aromatic antioxidants for aviation and beyond, Sustainable Energy & Fuels, 2020