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[[Sustainable biofuel]]s are an alternative to [[electrofuels]].<ref>{{Cite web|author=Mark Pilling|title=How sustainable fuel will help power aviation's green revolution|url=https://www.flightglobal.com/flight-international/how-sustainable-fuel-will-help-power-aviations-green-revolution/143044.article|date=2021-03-25|website=Flight Global}}</ref> Sustainable aviation fuel is certified as being [[sustainable]] by a third-party organisation.<section end="summary" />
{{TOCLimit}}SAF technology faces significant challenges due to feedstock constraints. The oils and fats known as hydrotreated esters and fatty acids (Hefa), crucial for SAF production, are in limited supply as demand increases. Although advanced [[Electrofuel|e-fuels]] technology, which combines waste {{Chem2|CO2}} with [[Green hydrogen|clean hydrogen]], presents a promising solution, it is still under development and comes with high costs. To overcome these issues, SAF developers are exploring more readily available feedstocks such as [[Lignocellulosic biomass|woody biomass]] and agricultural and municipal waste, aiming to produce lower-carbon jet fuel more sustainably and efficiently.<ref>{{Cite web |date=2024-05-10 |title=New Technology Helps Advance Non-Hefa SAF Projects |url=https://www.energyintel.com/0000018f-5ac5-d00d-a7df-7ff56da40000 |access-date=2024-05-14 |website=Energy Intelligence |language=en}}</ref><ref>{{Cite web |date=2024-08-14 |title=New SAF Process Could Transform Industry |url=https://www.ainonline.com/aviation-news/aerospace/2024-08-14/new-saf-process-could-transform-industry |access-date=2024-08-14 |website=Aviation Industry News |language=en}}</ref>
==Environmental impact==
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[[Jatropha oil]], a non-food oil used as a biofuel, lowers {{chem2|CO2}} emissions by 50–80% compared to Jet-A1, a [[kerosene]]-based fuel.<ref name=AImar2009>{{cite magazine |title= A Greener Future? |magazine= [[Aircraft Illustrated]] |date= March 2009}}</ref> Jatropha, used for [[biodiesel]], can thrive on [[marginal land]] where most plants produce low [[crop yield|yield]]s.<ref>{{cite news |author= Ron Oxburgh |url= https://www.theguardian.com/commentisfree/2008/feb/28/alternativeenergy.biofuels |title= Through biofuels we can reap the fruits of our labours |newspaper= [[The Guardian]] |date= 28 February 2008}}</ref><ref>{{cite news |author= Patrick Barta |title= As Biofuels Catch On, Next Task Is to Deal With Environmental, Economic Impact |url= https://www.wsj.com/articles/SB120631198956758087 |newspaper= [[Wall Street Journal]] |date= 24 March 2008 |url-access= subscription}}</ref> A [[life cycle assessment]] on jatropha estimated that biofuels could reduce greenhouse gas emissions by up to 85% if former agro-pastoral land is used, or increase emissions by up to 60% if natural woodland is converted.<ref>{{Cite journal | last1 = Bailis | first1 = R. E. | last2 = Baka | first2 = J. E. | doi = 10.1021/es1019178 | title = Greenhouse Gas Emissions and Land Use Change from Jatropha Curcas-Based Jet Fuel in Brazil | journal = Environmental Science & Technology | volume = 44 | issue = 22 | pages = 8684–91 | year = 2010 | pmid = 20977266| bibcode = 2010EnST...44.8684B }}</ref>
[[Palm oil]] cultivation is constrained by scarce land resources and its expansion to forestland causes [[biodiversity loss]], along with direct and indirect emissions due to [[land-use change]].<ref name="Doliente2020" /> [[Neste Corporation]]'s renewable products include a refining [[byproduct|residue]] of food-grade palm oil, the oily waste [[Resource recovery|skimmed]] from the palm oil mill's [[wastewater]]. Other Neste sources are [[used cooking oil]] from [[Deep fryer#Oil filtration|deep fryers]] and animal fats.<ref>{{Cite web|url=https://www.neste.com/products/all-products/raw-materials/waste-and-residues|title=Waste and residues as raw materials |date=15 May 2020 |publisher=Neste Corporation website}}</ref> [https://www.neste.com/en-us/products-and-innovation/sustainable-aviation/sustainable-aviation-fuel Neste's sustainable aviation fuel] is used by [[Lufthansa]];<ref>{{Cite press release|url=https://www.neste.com/releases-and-news/aviation/neste-and-lufthansa-collaborate-and-aim-more-sustainable-aviation|title=Neste and Lufthansa collaborate and aim for a more sustainable aviation|date=October 2, 2019|publisher=Neste Corporation website}}</ref> [[Air France]] and [[KLM]] announced 2030 SAF targets in 2022<ref>{{Cite press release|url=https://news.klm.com/klm-groups-co2-emission-reduction-targets-for-2030-approved-by-sbti/|title=KLM Group's CO2 emission reduction targets for 2030 approved by SBTi|date=16 December 2022 |publisher=KLM website |access-date=2023-01-02}}</ref> including multi-year purchase contracts totaling over 2.4 million tonnes of SAF from Neste, [[TotalEnergies]], and [[DG Fuels]].<ref>{{Cite news| url=https://www.reuters.com/business/energy/totalenergies-air-france-klm-agree-sustainable-jet-fuel-deal-2022-12-05/|title=TotalEnergies and Air France KLM agree sustainable jet fuel deal|date=5 December 2022 |publisher=[[Reuters]] |access-date=2023-01-02}}</ref>
Aviation fuel from wet waste-derived feedstock ("VFA-SAF") provides an additional environmental benefit. Wet waste consists of waste from landfills, sludge from wastewater treatment plants, agricultural waste, greases, and fats. Wet waste can be converted to volatile fatty acids (VFA's), which then can be catalytically upgraded to SAF. Wet waste is a low-cost and plentiful feedstock, with the potential to replace 20% of US fossil jet fuel.<ref name="auto">{{Cite journal |last1=Huq |first1=Nabila A. |last2=Hafenstine |first2=Glenn R. |last3=Huo |first3=Xiangchen |last4=Nguyen |first4=Hannah |last5=Tifft |first5=Stephen M. |last6=Conklin |first6=Davis R. |last7=Stück |first7=Daniela |last8=Stunkel |first8=Jim |last9=Yang |first9=Zhibin |last10=Heyne |first10=Joshua S. |last11=Wiatrowski |first11=Matthew R. |last12=Zhang |first12=Yimin |last13=Tao |first13=Ling |last14=Zhu |first14=Junqing |last15=McEnally |first15=Charles S. |date=2021-03-30 |title=Toward net-zero sustainable aviation fuel with wet waste-derived volatile fatty acids |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=118 |issue=13 |pages=e2023008118 |doi=10.1073/pnas.2023008118 |issn=1091-6490 |pmc=8020759 |pmid=33723013 |doi-access=free |bibcode=2021PNAS..11823008H }}</ref> This lessens the need to grow crops specifically for fuel, which in itself is energy intensive and increases {{chem2|CO2}} emissions throughout its life cycle. Wet waste feedstocks for SAF divert waste from landfills. Diversion has the potential to eliminate 17% of US methane emissions across all sectors. VFA-SAF's carbon footprint is 165% lower than fossil aviation fuel.<ref name="auto"/> This technology is in its infancy; although start-ups are working to make this a viable solution. Alder Renewables, BioVeritas, and ChainCraft are a few organizations committed to this.
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: Processing solid [[biomass]] using [[pyrolysis]] can produce [[pyrolysis oil|oil]] or [[gasification]] to produce a [[syngas]] that is processed into FT SPK ([[Fischer–Tropsch process|Fischer–Tropsch]] Synthetic Paraffinic Kerosene).{{Citation needed|date=February 2023}}
; ATJ-SPK
: The alcohol-to-jet (ATJ) pathway takes alcohols such as [[ethanol]] or [[butanol]] and de-oxygenates and processes them into jet fuels.<ref>{{Cite web|url=https://advancedbiofuelsusa.info/tag/atj-spk-alcohol-to-jet-synthetic-paraffinic-kerosene/|title=Advanced BioFuels USA – Truly Sustainable Renewable Future|website=advancedbiofuelsusa.info}}</ref> Companies such as LanzaTech have created ATJ-SPK from {{chem2|CO2}} in [[flue gas]]es.<ref>{{Cite web|url=https://www.lanzatech.com/2018/04/03/jet-fuel-derived-ethanol-now-eligible-commercial-flights/|title=Jet Fuel Derived from Ethanol Now Eligible for Commercial Flights|access-date=2020-12-22|archive-date=2022-01-25|archive-url=https://web.archive.org/web/20220125161620/https://www.lanzatech.com/2018/04/03/jet-fuel-derived-ethanol-now-eligible-commercial-flights/|url-status=dead}}</ref> The ethanol is produced from CO in the flue gases using microbes such as ''[[Clostridium autoethanogenum]]''. In 2016 LanzaTech demonstrated its technology at Pilot scale in NZ – using Industrial waste gases from the steel industry as a feedstock.<ref>Voegele, E. November 2009. "Waste to ethanol projects move forward", Ethanol Producer Magazine</ref><ref>{{Cite web|url=https://www.triplepundit.com/story/2013/interview-lanzatech-ceo-jennifer-holmgren/52196|title=Interview: LanzaTech CEO Jennifer Holmgren|website=www.triplepundit.com}}</ref><ref>{{Cite journal|url= |title=Genome editing of Clostridium autoethanogenum using CRISPR/Cas9|first1=Shilpa|last1=Nagaraju|first2=Naomi Kathleen|last2=Davies|first3=David Jeffrey Fraser|last3=Walker|first4=Michael|last4=Köpke|first5=Séan Dennis|last5=Simpson|date=October 18, 2016|journal=Biotechnology for Biofuels|volume=9|issue=1|pages=219|doi=10.1186/s13068-016-0638-3|pmid=27777621|pmc=5069954 |doi-access=free |bibcode=2016BB......9..219N }}</ref> [[Gevo]] developed technology to retrofit existing [[ethanol]] plants to produce [[isobutanol]].<ref>{{Cite web |url=https://gevo.com/wp-content/uploads/2020/05/Gevo-Whitepaper-Sustainable-Aviation-Fuel.pdf |title=Archived copy |access-date=2021-11-23 |archive-date=2021-06-23 |archive-url=https://web.archive.org/web/20210623212715/https://gevo.com/wp-content/uploads/2020/05/Gevo-Whitepaper-Sustainable-Aviation-Fuel.pdf |url-status=dead }}</ref> Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) is a proven pathway to deliver bio-based, low-carbon fuel.{{Citation needed|date=February 2023}}
=== Future production routes ===
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* The SUN-to-LIQUID project is examining Fischer-Tropsch hydro-carbon fuels (solar kerosine) through the use of a [[Chemical reactor|solar reactor]].<ref>{{Cite web|url=https://solar-jet.aero/page/about-solar-jet/news-events.php|title=SOLAR-JET project terminated and succeeded by SUN-TO-LIQUID project|website=solar-jet.aero}}</ref><ref>{{Cite web|url=https://ec.europa.eu/commission/presscorner/home/en|title=Press corner|website=European Commission - European Commission}}</ref><ref>{{Cite web|url=https://www.sun-to-liquid.eu/|title=SUN to LIQUID project - SUN to LIQUID project|website=www.sun-to-liquid.eu}}</ref>
* Alder Fuels is proposing to convert [[lignocellulosic biomass]] (a common type of waste from forestry and agriculture) into a hydrocarbon-rich "greencrude" via [[pyrolysis]] (see: [[pyrolysis oil]]). Greencrude can be turned into fuel in refineries like crude oil.<ref>{{Cite news |date=August 17, 2022 |title=Ways to make aviation fuel green |newspaper=The Economist |url=https://www.economist.com/science-and-technology/2022/08/17/ways-to-make-aviation-fuel-green |access-date=2023-02-23 |issn=0013-0613}}</ref>
* Universal Fuel Technologies is marketing its Flexiforming technology that can use different feedstocks and even the byproducts from existing renewable fuel manufacturing processes to produce SAF.<ref>{{Cite news |date=August 14, 2024 |title=New SAF Process Could Transform Industry |newspaper=Aviation Industry News |url=https://www.ainonline.com/aviation-news/aerospace/2024-08-14/new-saf-process-could-transform-industry}}</ref>
=== Piston engines ===
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[[Sustainable biofuel]]s do not use [[food crop]]s, prime [[agricultural land]] or fresh water. '''Sustainable aviation fuel''' (SAF) is certified by a third-party such as the [[Sustainable biofuel#Roundtable on Sustainable Biomaterials|Roundtable For Sustainable Biofuels]].<ref>{{cite news |url= http://aviationweek.com/commercial-aviation/glacial-pace-advancements-biofuel-threatens-emissions-targets |title= Glacial Pace Of Advancements In Biofuel Threatens Emissions Targets |date= Oct 10, 2017 |author= Kerry Reals |work=[[Aviation Week & Space Technology]]}}</ref>
As of 2022, some 450,000 flights had used sustainable fuels as part of the fuel mix, although such fuels were ~3x more expensive than the traditional fossil jet fuel or [[kerosene]].<ref name=Economist18aug2022>{{Cite news |title=Ways to make aviation fuel green |newspaper=The Economist |url=https://www.economist.com/science-and-technology/2022/08/17/ways-to-make-aviation-fuel-green |date=2022-08-17 |issn=0013-0613}}</ref>
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