## 2 Materials and Methods
### System Definition  
This study aims to analyze the environmental and economic impacts of medium-haul flight operations by using different SAFs compared with fossil kerosene. The analysis was conducted from a well-to-wake perspective (i.e., the underlying foreground system comprises the fuel supply chain as well as the flight operation (Figure 3)). The fuel supply chain consists of different unit processes that are related to feedstock production, electricity generation, and fuel production. The flight operation includes the unit processes related to fuel distribution and storage as well as fuel combustion in the aircraft. The foreground system is linked to version 3.7.1 of the ecoinvent database in the background system [62]. Figure 2: Methodology of life cycle assessment and life cycle costing. Five different types of fuel were considered in this study. These included conventional fossil kerosene as well as four types of SAFs (see Table 1). The supply chain of fossil kerosene begins with the extraction of crude oil [44]. The crude oil is further processed in a refinery by distillation processes and blending in various additives [42; 43]. The crude oil is assumed to be extracted in Tyumen, Russia and further processed in a refinery in Hamburg, Germany [63]. The fossil kerosene was compared to four promising SAFs. They were produced via X-to-liquid pathways, more precisely via BtL and PtL, as their production on an industrial scale is the most researched [25; 27; 43]. The synthesis processes were based on FTS (Section 2.1), as this is also the most widely used method in the industry and the best researched. The BtL pathway focuses on biokerosene of the second generation. The feedstock is miscanthus, being cultivated in Saxony, Germany [64] and further processed in Hamburg, Germany [63]. The individual production processes include biogas production, which is \begin{table}
\begin{tabular}{l l} \hline \hline
**Abbreviation** & **Description** \\ \hline \multirow{2}{*}{Kerosene (fossil)} & Fossil kerosene based on crude oil produced by \\  & refinery processes \\  & Biokerosene based on miscanthus produced by \\  & steam methane reforming and Fisher–Tropsch \\  & synthesis \\  & Synthetic kerosene based on hydrogen \\  & produced by steam methane reforming and \\  & Fisher–Tropsch synthesis \\  & Synthetic kerosene based on hydrogen \\  & produced by polymer electrolyte membrane \\  & electrolysis and Fisher–Tropsch synthesis \\  & Synthetic kerosene based on hydrogen \\  & produced by solid oxide electrolysis and \\  & Fisher–Tropsch synthesis \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Considered aviation fuels. Figure 3: Foreground and background system within the system boundaries, including exchanges between the technosphere, ecosphere, and marketsphere. refined to biomethane by adding various additives, and then processing to hydrogen via steam methane reforming (SMR). In the subsequent production step, the biokerosene is produced using FTS [36]. Concerning the PtL pathway, three types of synthetic kerosene based on hydrogen were investigated [65]. They differ in terms of hydrogen production. Regarding the first synthetic kerosene, hydrogen is produced via the SMR process but by using natural gas, electricity, and water instead of biomethane [66]. The other two production pathways are based on hydrogen production via electrolysis. For this purpose, polymer electrolyte membrane electrolysis (PEM) and solid oxide electrolysis (SOEC) were used. In the PEM process, hydrogen is produced in an energy-intensive production step exclusively from water (55 kWh electricity input per 1 kg of hydrogen) [67]. The SOEC process works similarly, but in addition to water and electricity (36.8 kWh electricity input per 1 kg of hydrogen), a small amount of natural gas is required [68]. The hydrogen is processed to kerosene using FTS. The whole synthetic kerosene production process takes place in Hamburg, Germany [63]. The transports were included for all feedstock materials and further processed fuels.