Marina Braun-Unkhoff

This paper summarizes the findings of a detailed assessment of synthetic, electricity-based fuels for use in aviation, shipping, and road transport. The fuels considered correspond to the most promising alternatives that were analyzed as part of the German research project BEniVer (Begleitforschung Energiewende im Verkehr—Accompanying Research for the Energy Transition in Transport) initiated by the German Federal Ministry for Economic Affairs and Climate Action (BMWK). Focusing on usage, infrastructure, and ecological analyses, several e-fuels were evaluated and compared to fossil fuels according to the specific sector. It turns out that for all sectors evaluated, the existing sustainable synthetic fuels are already compatible with current technology and regulations. In shipping and road transport, the use of advanced, sustainable fuels will allow for a more distinct reduction in emissions once technology and regulations are adopted. However, standard-compliant synthetic gasoline and diesel are considered the most promising fuels for use in road transport if the transition to electricity is not realized as quickly as planned. For the aviation sector, the number of sustainable aviation fuels (SAFs) is limited. Here, the current aim is the introduction of a 100% SAF as soon as possible to also tackle non-CO₂ emissions.

Iso-octane is widely recognized as a prominent candidate to represent the oxidation of iso-alkanes within jet fuel and gasoline surrogates. This work evaluated a chemical kinetic mechanism for iso-octane focusing on the model's capability to predict the formation of polycyclic aromatic hydrocarbons (PAHs). As the model is intended to be further coupled with soot models, the chemical kinetic mechanism must supply good predictability of the formation and consumption of PAHs considered as major soot precursors. A first validation of the iso-octane sub-model as incorporated within ESTiMatE-Mech, using experimental data from literature, reveals the need to improve the sub-model. Considerable deviations were observed in the prediction of the PAHs, although concentration profiles of major species and fundamental combustion properties such as ignition delay time and laminar flame speed were accurately predicted. Through rate of production and sensitivity analyses of the mechanism, nine reactions were identified to have a strong influence in the (over)prediction of the PAHs. These reactions have been modified based on information gathered from literature resulting in an updated version of the mechanism called ESTiMatE-Mech_mod. Simulation results with this modified mechanism showed that this updated mechanism is now capable of predicting well the targeted PAHs, while retaining the good initial prediction of the major species concentration profiles as well as of laminar flame speeds and ignition delay times.

Trimethyl orthoformate (TMOF: HC(OCH3)3) has recently been examined as a viable biofuel. TMOF is a branched isomer of oxymethylene ether-2 (OME2) that, due to its high oxygen content and lack of direct carbon-carbon bonds, considerably reduces the formation of soot particles. To meet the challenges of a more flexible and sustainable power generation, a detailed understanding of its combustion properties is essential for its safe and efficient utilization, neat or in blends. In this work, two fundamental combustion properties of TMOF were studied: (i) Auto-ignition of TMOF/synthetic air mixtures (φ = 1.0; diluted 1:5 with N2) using the shock tube method at pressures of 1, 4, and 16 bar, and (ii) Laminar burning velocities of TMOF/air mixtures using the cone angle method at ambient and elevated pressures of 3 and 6 bar. Furthermore, the impact of TMOF addition to a gasoline surrogate (PRF90) on ignition delay times was studied using the shock tube method at φ = 1.0, 1:5 dilution with N2, T = 900–2000 K, and at 4 bar. The experimental data sets have been compared with predictions of the in-house chemical kinetic reaction mechanism (DLR concise mechanism) developed for interpreting the high-temperature combustion of a broad spectrum of different hydrocarbon fuels as well as oxygenated fuels, including TMOF. The results demonstrate that the ignition delay times of TMOF and OME2 are nearly identical for all pressures studied in the moderate-to high-temperature region. The results obtained for the blend indicate that ignition delay times of the TMOF/PRF90 blend are shorter than those of the primary reference fuel 90 (PRF90) at 4 bar. In the lean-to stoichiometric region, the results obtained for laminar burning velocities of TMOF and OME2 are similar. However, in the fuel-rich domain (φ > 1.0), laminar burning velocities for TMOF are noticeably lower, indicating a decreased reactivity. The model predictions based on the in-house model reveal a good agreement compared to the measured data within the experimental uncertainty ranges. In addition, sensitivity analyses regarding ignition delay times and laminar flame speeds were performed to better understand TMOF oxidation.