Tackling climate change
Although we have made progress in decarbonising many aspects of the world’s energy matrix, with solar, wind and hydro all adding to “green” electricity generation, longdistance transport has proven much more difficult to decarbonise. Increasing electrification of urban transport is anticipated, with a steady increase in hybrid and fully electrified cars. In contrast, long-distance transport, particularly aviation, marine and road freight, will be much harder to electrify. Similarly, other possible “green”
options such as using renewable natural gas and green hydrogen, are proving challenging to commercialise and are likely to be at least a decade away. The aviation sector contributes about 2% of the world’s CO₂ emissions and about 12% of all transport emissions, amounting to 915 million tonnes (Mt) of CO₂ in 2019 (ATAG, 2020). Non-CO₂ emissions from aviation also have a significant climate impact, contributing almost two-thirds of net radiative forcing (Lee et al., 2021). Until the impact of COVID-19 on the global economy, aviation was one of the fastest-growing transport sectors, with demand increasing at about 4% per year (Wheeler, 2018) and pre-COVID projections suggesting
that the sector’s emissions could grow to 2.1 gigatonnes (Gt) per year (IRENA, 2018).

Figure 1 illustrates the potential contribution of different options, although considerable uncertainties remain. Various factors such as decarbonising airport/land operations, increasing the efficiency of aircraft and new infrastructure investment will help reduce the sector’s carbon emissions. Sustainable aviation fuels will provide the lion’s share of the sector’s decarbonisation potential (the orange section).

Sustainable aviation fuels
Biojet (i.e. aviation fuel produced from biomass) is currently the most certified type of sustainable aviation fuel. Over time synthetic aviation fuels produced from green hydrogen could also play a role as drop-in fuels, but production is currently very limited and costs are very high, exacerbated by a lack of demand for the fuels at the current price point (IRENA, 2020a). Biojet therefore holds the most promise for cost-effective scale-up and use in the 2020s and 2030s.


The aviation sector contributes about 2% of the world’s CO₂ emissions and about 12% of all transport emissions in 2019 (ATAG, 2020). Although not widely recognised, the vast majority of global transport energy needs are met by oil and petroleum products (96.7%), with biofuels (3.0%) and renewable electricity (0.3%) contributing only small amounts (REN21, 2020). Before COVID affected global transport, various groups had projected global demand for all liquid transport fuels (primarily fossil-derived), including biofuels (IRENA, 2016a). It was anticipated that total biofuels would increase significantly from over 100 billion litres today to about 600 billion litres by 2050. IRENA’s 1.5°C
Scenario (1.5-S) , which has a goal of holding the global temperature rise to no more than 1.5°C, estimates that about 740 billion litres per year of liquid biofuels will be required, of which around 200 billion litres is biojet fuel (IRENA, 2021).

As summarised in Table 1, although the number of renewable diesel facilities continues to grow globally, only two facilities produced biojet fuel in 2019, Neste (Rotterdam) and World Energy (California). The Neste Singapore facility is currently undergoing infrastructure modifications to produce biojet on a routine basis by 2022. It should be noted that only about 15% by volume of the total capacity of these refineries can be fractionated to produce biojet. Although this fraction can be increased (up to a maximum of 50%), it comes at a higher cost4 and a loss of yield. At this point in time it is not economically attractive for companies to produce a higher biojet fraction (UOP, 2020).


In the future, electric and hydrogen-powered aircraft will help reduce the sector’s CO₂ emissions as well as providing additional environmental benefits such as decreasing contrail emissions, improving local air quality and reducing noise pollution (ICAO, 2019a). However, emission reduction calculations must be based on a full life-cycle assessment, not just tailpipe emissions. For example, overall emission reductions will only occur if the source of electricity or hydrogen is from a renewable resource such as using solar/wind/ hydroelectricity to recharge batteries or using “green” hydrogen to power a fuel cell. Although there is enormous potential in these alternative propulsion systems, as mentioned earlier, most carbon emissions occur during long-distance flights while transporting large numbers of passengers. As aircraft have an average operating lifespan of about 30 years, and as it typically takes years to design and build these new propulsion systems, fleet replacement will not happen soon. It is likely that challenges such as the weight and size of batteries and sourcing green hydrogen will result in these technologies first being pioneered in short-distance flights involving a small number of passengers. Thus, any alternative propulsion systems used in long-distance flights are unlikely to have a significant impact on emission reductions before 2050 (Baraniuk, 2020).


The two main feedstock categories that are primarily being pursued are the oleochemical/lipid-based processes (using fats, oils and greases [FOGs], vegetable oils and animal/rendered fats) and the socalled biocrude-based processes derived from various lignocellulosic/biomass feedstocks (such as agricultural residues and woody biomass). The vast majority of biojet fuel that is produced today is derived via the oleochemical/lipid “conventional” route, and this conventional route to biojet fuels is likely to dominate the biojet market in the coming decade. As demonstrated by companies such as Neste, lipids are a relatively high-density feedstock, which allows them to be more readily transported over long distances, achieving benefits such as larger economies of scale. For example, the Neste facilities in Rotterdam and Singapore produce over one billion litres of renewable diesel per year, partly as a result of the company successfully establishing a global supply chain, sourcing fats and oils from New Zealand, Australia, China, Canada and other countries. Currently, alternative crops such as camelina, carinata and salicornia are also being assessed as lipid feedstocks for potential biojet fuel production. However, further work is required to fully develop and optimise these supply chains, including the establishment of more routine processing infrastructure.

As the technology used to make conventional biojet is relatively mature, the amount, cost and overall sustainability of the feedstock will be very important. For example, structuring policies to link with the carbon intensity of fuels in California has encouraged the increased use of waste lipid feedstocks such as used cooking oil and tallow (Figure 3), and this is reflected in the current feedstock mix used by companies such as Neste (Figure 4). One of the critical aspects of sustainability is the need for overall emission reductions, with CORSIA requiring at least a 10% reduction across the entire supply chain (ICAO, 2019b).

In contrast, advanced biojet fuel based on biomass (biocrudes), waste carbon, alcohols and sugars is still very much under development. As described earlier, lignocellulosic materials, such as agricultural residues and woody biomass, will be the primary feedstocks for thermochemical technologies such as gasification and thermochemical liquefaction (pyrolysis, etc.) (Karatzos et al., 2017). In addition, biomass feedstocks can be used to produce sugars that may be fermented to alcohols for ATJ production, or directly fermented to hydrocarbons such as farnesene (Karatzos, Mcmillan and Saddler, 2014).


As summarised in Figure 9, multiple technology pathways can be used to make biojet fuels.

this means that biojet produced via the HEFA pathway is the only technology that is at this level. As described earlier, CAAFI has developed Fuel Readiness Level (FRL) and Feedstock Readiness Level (FSRL) tools to assess potential feedstock supply chains, and progression between FRL levels can take three to five years (Mawhood et al., 2016). Figure 10 provides an estimate of the commercialisation status of various technologies.


ICAO members have agreed that global market-based measures will be one of the strategies used to address the environmental impact of the aviation sector. The ICAO CORSIA is designed to work in conjunction with efficiency improvements, innovative technologies and biojet fuels to achieve the indicated carbon reduction targets. However, to achieve carbon-neutral growth, offsets will be used by the airlines as an interim measure by financing emission reductions in other sectors. Until the COVID pandemic, the airline sector’s baseline emissions were due to be defined as an average of the 2019 and 2020 international flight emissions, with the offset requirements calculated against this baseline to ensure carbon-neutral growth. However, as a result of the pandemic, only the 2019 emissions will be used as the baseline. targets will be mandatory for all members unless they have obtained an exemption. Exceptions will include flights to and from least-developed countries, small island developing states, landlocked developing countries and states/countries that represent less than 0.5% of international revenue tonne kilometres. However, some exempt states may still volunteer to participate. The countries/regions that will participate or who are exempt from the various phases are summarised in Figure 11.


Biojet fuel is significantly more expensive than conventional jet fuel and is likely to remain so for some time to come. Estimates of the price difference vary from 2-7 times (IATA, 2015a) to 3-4 times higher (Hollinger, 2020). Although airlines and airports have signed several offtake agreements, the price of the biojet fuel in these contracts is not publicly disclosed. Thus, it has proven difficult to project accurate prices for biofuels, with most cost estimates derived from sources such as reports, online media, presentations and academic papers (Bann et al., 2017; IATA, 2015a, 2015b; de Jong et al., 2015; de Jong, Hoefnagels, et al., 2017; Pavlenko et al., 2019; Staples et al., 2014; Yao et al., 2017; Zhao, Yao and Tyner, 2016).

Figure 12 shows the estimated MFSPs for different biojet technologies according to a number of publications and the production costs, broken down by capital expenditure (CAPEX), operating expenditure (OPEX) and feedstock cost (see Annex B). MFSP represents the break-even price at which fuel products have to be sold to attain a zero NPV. It may include additional costs or benefits compared to the production cost, such as tax credits, additional infrastructure cost, environmental benefits and by-product revenue. HEFA is currently the most commercialised technology option, with low CAPEX and OPEX, but with high feedstock cost (Figure 12). Feedstock costs vary depending on the source, and waste-based feedstock such as UCO or tallow can lead to a significant reduction in the overall fuel production costs. HEFA is expected to play a pivotal role as a primary, short-term accelerator for biojet, but availability of sustainable oil-based raw material will become limiting. These feedstocks are already used extensively for biofuels in the road transport sector and biojet is likely to be in competition with renewable diesel for them.


As discussed earlier, the primary motivation for developing biojet fuels is to reduce the carbon footprint of the aviation sector. Thus, ensuring the overall sustainability of biojet production and use will be a critical component of its development. Overall sustainability is typically assessed by certification bodies who consider the complete supply chain against a number of principles and criteria. Two prominent certification bodies that have been involved in the assessment of the sustainability of biojet fuels are the Roundtable on Sustainable Biomaterials (RSB)14 and International Sustainability and Carbon Certification (ISCC).15 Other organisations such as the Programme for the Endorsement of Forest Certification (PEFC), the Sustainable Forest Initiative (SFI),16 the Forest Stewardship Council (FSC)17 and the Sustainable Biomass Program (SBP)18 are arms-length certification systems specifically designed to assess the sustainability of forestry-based feedstocks and forestry practices and to ensure that woody biomass is sourced


Potential feedstock supply and challenges
Multiple feedstocks can be used to produce biojet fuels, including lipids (vegetable oils, UCO and other waste lipids), starches, sugars and lignocellulosics (forest residues, agricultural residues and energy crops). Theoretical feedstock availability is not expected to a barrier to biojet production, although high prices may be a barrier. The challenge relating to feedstock is rather the competing uses for other applications, such as bioenergy for heat and electricity, and availability once these have been satisfied. The cost of feedstock can be a significant obstacle and biojet fuel production is very sensitive to the cost and sustainability of feedstock. The low energy density of lignocellulosic feedstocks such as forest residues makes transport cost an important factor, and feedstock can only be transported economically over limited distances.


Ambitious, stable and internationally relevant policies will be needed if there is to be a significant increase in biojet fuel production and use. Several recent reports have assessed the types of policies that will be required to increase biojet fuel production and use (World Economic Forum, 2020). Such policies will need to address factors such as unfavourable economics, limited feedstock availability, competition from other low-carbon fuel uses and immature technologies, which currently limit the use of sustainable aviation fuel, including biojet. If the aspirational 2050 carbon reduction targets of the sector are to be met, the construction and repurposing of hundreds of facilities will be needed, requiring hundreds of billions of dollars of investment. These facilities will then produce the low-carbon jet and other low-carbon fuels via many of the production pathways discussed in this report (Staples et al., 2018). Effective policies will also be required to catalyse many of the components of viable supply chains, from the production of low-cost and sustainably derived feedstocks through to the preferential production and use of low-carbon-intensive jet fuels. Aviation may compete with other modes of transport (e.g. cars, trucks, ships and trains) for similar fuels as they all seek to decarbonise. However, it is likely that biojet fuel production would still benefit from these types of incentive, as loan guarantees and grants could play an important role in encouraging initial investment in drop-in biofuel/ biojet fuel facilities. Other policies, such as producer incentives, would mitigate the longer-term risk to investors, with the information in Figure 14 illustrating the impact of incentives on five different transport fuels under the California LCFS (Lane, 2020). The current situation in California with respect to biofuel incentives and their impact on the value of the biofuel shows that conventional ethanol, cellulosic ethanol and renewable diesel can earn incentives under the federal Renewable Fuel Standard (orange), as well as LCFS credits under California’s policies (light green), while the two advanced biofuels (cellulosic ethanol and renewable diesel) can also earn further federal tax credits (yellow).


Although oleochemical/lipid feedstocks are currently the primary feedstocks used to make biojet fuels, in the longer term thermochemical technologies will use feedstocks such as biomass and residues to make lowcarbon-intensity fuels. While some parts of the process pathway have established regional/local supply chains (e.g. wood pellets and waste lipids), future biojet fuel pathways are likely to be based on current infrastructure (such as repurposed oil refineries), available feedstocks, and regional conditions and policies.


The deep decarbonisation of the aviation sector by 2050 will require concerted action on multiple fronts
and the use of a range of different solutions, including new propulsion systems such as electric and hybrid aircraft and the use of hydrogen. However, to achieve early reductions in emissions in the 2020s and 2030s, and deep reductions by 2050, the use of sustainable aviation fuels will be essential. Biojet is currently the most certified type of sustainable aviation fuel and whilst over time synthetic fuels will become available, biojet holds the most promise for cost-effective scaleup and use in the 2020s and 2030s, and is likely still to be playing a major role in the 2040s. However, to date very little biojet fuel has been used, with limited availability, high costs and a lack of policy drivers impeding its development. Significant progress has been made recently in several areas, such as ASTM certifying new biojet production routes, various airports successfully using airport fuel hydrant systems to deliver biojet/jet fuel blends and over 315 000 flights using some percentage of biojet fuels. But the production and use of these lower-carbon-intensive fuels remains small, contributing less than 1% of all of the jet fuel that is currently used.


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