Renewable fuels for aviation

Introduction

Renewable and drop-in capable jet fuels are needed to meet the industry’s self-imposed targets for greenhouse gas (GHG) emission reductions.This contribution addresses the potentials of renewable jet fuels as an important pillar of a future sustainable aviation sector. First, aviation-specific targets in the context of internationally agreed goals to limit global warming are described. Then, the issue of drop-in capability and technical approval of synthetic jet fuel is addressed, and a brief overview of selected production pathways and the current use of renewable jet fuel is presented. Several production pathways, based on biomass feedstock as well as non-biogenic options, are under development but have not yet been industrially demonstrated.The specific GHG balance of renewable jet fuels and the scalability of production are discussed as key factors determining the potential of renewable jet fuel alternatives to facilitate the energy transition in the aviation sector. Many pathways show very favourable GHG performances and production potentials that are in principle sufficient to supply the worldwide fleet with renewable fuel. Finally, the economic competitiveness of renewable jet fuel is discussed as a key challenge for large-scale market uptake, and a concluding outlook on further developments is presented.

The need for renewable jet fuel

The world’s economy has to be “decarbonized” in the long term. While this expression in its literal sense is misleading, as carbonaceous materials will always be eaten, burnt, consumed and used in many ways, it transports a serious and urgent meaning: The carbon loop of our economy and our daily life has to be closed, thus stopping the accumulation of carbon in Earth’s atmosphere, biosphere and the oceans through combusting fossil fuels.

At the 2015 United Nations Climate Change Conference (COP 21) held in Paris, the parties agreed on the long-term target of “holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above preindustrial levels” (UN, 2015). In order to achieve this target, enormous efforts are required worldwide and by all sectors. It should nevertheless be noted that international aviation is not explicitly included in the Paris Agreement — the mandate for emissions abatement from international aviation has been assigned to ICAO as responsible UN body. ICAO has defined a “basket of measures to reduce aviation CO, emissions” (described in ICAO, 2016a: Chapter 4), and, as part of that, decided in 2016 “to implement a GMBM¹ scheme in the form of the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) to address any annual increase in total CO, emissions from international civil aviation” (ICAO, 2016b). However, the Paris Agreement of course implicitly includes the entire (national and international) aviation sector through the specification of the overarching goal of limiting global warming to 2 °C or even 1.5 °C.

The fifth assessment report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) assesses various mitigation scenarios and their compatibility with targets to limit global warming. According to AR5, the 2°C target is met with a probability of greater than 66%, if the CO, concentration in the atmosphere does not exceed the range of 430 to 480 ppm at the end of this century (IPCC, 2014: Chapter 6). This compares to a global average level of already 410 ppm today (NOAA, 2018a) and an annual increase of well above 2 ppm in the past five years (NOAA, 2018b). In terms of emission trajectories, this means that by 2050 the global annual GHG emissions have to be reduced by 41% to 72% relative to 2010 and even by 78 to 118% by the end of this century (IPCC, 2014).To meet the 1.5°C goal, the reductions have to be substantially more severe, with annual emissions peaking already in 2020, reaching values below todays level by 2030 and even more strongly declining beyond 2030 (Millar et al., 2017).

The IPCC emission trajectories are global and include emissions from all sectors. However, it is obvious that any sector or any nation failing to meet the required average emission reduction would translate into even higher reductions required in the other sectors or nations. While there are currently no binding emission reduction targets in place for commercial aviation, the aviation industry itself, recognizing its responsibility to contribute to the global efforts to limit global warming, has announced a set of ambitious, albeit non-binding targets to reduce its carbon footprint (illustrated in Figure 3.1, top). Specifically, the aviation industry committed to increase the fleets fuel efficiency by 1.5% per year until 2020, to reach carbon-neutral growth² from 2020 onwards and to halve the net CO, emissions of the aviation sector by 2050 relative to the level of 2005 (ATAG, 2012). This target for 2050 lies well within the required emission reduction corridor associated with a greater than 66% probability of keeping global warming below 2 °C, according to the IPCC (2014) assessment. Please note that the reference year for quoted IPCC scenarios is 2010, while the aviation industry’s reduction targets are defined relative to 2005; as emissions in 2005 were lower than in 2010, this means that a reduction of 50% relative to 2005, as targeted by the aviation industry, would correspond to an even higher reduction relative to 2010. However, it must also be emphasized that

C0₂ index (2005= 100)

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Figure 3.1 Jet fuel consumption and CO, emission reduction targets.

Top: Schematic illustration of the CO, emission reduction targets of the aviation industry. Bottom: Historic global jet fuel consumption.

Sources: Top: Adapted from ATAG (2012). Carbon-neutral growth from 2020 and halving of fleetwide emissions relative to 2005 levels by 2050; emission reductions envisioned through improvements in technology and operations as well as through market-based measures (carbon trading or offsetting); an “emissions gap” remains to be filled through use of sustainable fuels and “additional technologies”. Bottom: EIA (2018). The 2005 level of consumption is indicated as reference for the ATAG target of halving emissions relative to 2005 level by 2050.

Renewable fuels for aviation 19 according to the IPCC assessment, even higher reductions are required for the second half of the century, essentially resulting in a complete decarbonization of all sectors by 2100. In this sense, the goal of halving emissions by 2050 can be only an intermediate step on the pathway of the aviation sector towards the 2 °C target (or even the 1.5 °C target).

To understand the challenges associated with the aviation industry’s emissions reduction goals, it is useful to look at the recent development of the sector’s fuel consumption (Figure 3.1, bottom). From the plotted data it is apparent that fuel consumption of global aviation is rapidly increasing, albeit with temporary declines following the Gulf War in 1991, the September 11 attacks in New York in 2001 and the global economic crisis in 2008. But apart from these temporary declines, global jet fuel consumption has grown annually by 1% to 8% since 1992. As the jet fuel consumed today is essentially 100% conventional (i.e., fossil), this means that CO, emissions of aviation have increased approximately with the same rate.

The extent of the problem associated with the future development of the aviation sector’s emissions vs. the targeted emission reductions becomes clearly visible when studying the market development forecasts of the industry. For example, aircraft manufacturers Boeing (2017) and Airbus (2017) forecast a global annual air traffic growth (in revenue kilometres) of 4.7% and 4.4%, respectively, for the next 20 years (2017-2036). Such growth rates would substantially surpass the sector’s ambitious targets in terms of increased fuel efficiency, for example 1.5% annual efficiency gains until 2020, as expressed by the global aviation industry (ATAG, 2012), or even 2% annual gains until 2050, as set forth by ICAO (2011).

Such a development would, if it becomes reality, result in a future situation where the growth in air traffic continues to outpace gains in efficiency, translating into a net growth in fuel consumption and, consequently, in rising GHG emissions, if the consumed jet fuel remains largely conventional. For international aviation, ICAO (2016a) projects a more than fourfold increase in CO, emissions from fuel combustion by 2050 when compared to 2005 levels, even under optimistic assumptions with respect to development and implementation of fuel-efficient technologies. It can therefore be concluded that an energy transition is needed in the aviation sector, in the course of which essentially the entire volume of currently fossil kerosene is replaced in the long-term future by renewable energy carriers with very low specific GHG emissions. In this context, however, aviation’s contribution to global warming cannot be measured only by the emitted quantities of CO, and other GHGs; another relevant contribution comes from the fact that aircraft cause emissions of soot, water, nitrogen oxides, sulphur oxides and other species in high altitudes, thereby affecting atmospheric physico-chemical processes and impacting the global radiation budget (Lee et al., 2010; IPCC, 2013: Chapter 7; Grewe in this volume).

Several technology options enabling an energy transition in aviation are currently discussed and researched, such as electric propulsion using batteries and/or hydrogen fuel cells, hybrid electric solutions and liquefied gaseousor synthetic liquid fuels. However, given the long development and product cycles of aircraft, the high cost associated with the implementation of disruptive technologies and the to-date limited performance of required key technologies, e.g. batteries with sufficient storage capacities, it is generally acknowledged that the only feasible renewable energy option for aviation in the near- to mediumterm future are synthetic liquid fuels. Such fuels should be designed in a way that allows direct use without requiring any changes of aircraft and fuel systems and are therefore commonly referred to as drop-in fuels.