Editing IPCC:AR6/WGIII/Chapter-6 (section)

==== 6.6.2.4 Alternative Fuels in Sectors not Amenable to Electrification ====

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Net-zero energy systems will need to rely on alternative fuels – notably hydrogen or biofuels – in several sectors that are not amenable to electricity and otherwise hard to decarbonise ( ''medium confidence'' ). Useful carbon-based fuels (e.g., methane, petroleum, methanol), hydrogen, ammonia, or alcohols can be produced with net-zero CO 2 emissions and without fossil fuel inputs (Sections 6.4.4 and 6.4.5). For example, liquid hydrocarbons can be synthesised via hydrogenation of non-fossil carbon by processes such as Fischer-Tropsch (MacDowell et al. 2017) or by conversion of biomass ( [[#Tilman--2009|Tilman et al. 2009]] ). The resulting energy-dense fuels can serve applications that are difficult to electrify, but it is not clear if and when the combined costs of obtaining necessary feedstocks and producing these fuels without fossil inputs will be less than continuing to use fossil fuels and managing the related carbon through, for example, CCS or CDR ( [[#Ueckerdt--2021|Ueckerdt et al. 2021]] ).

CO 2 emissions from some energy services are expected to be particularly difficult to cost-effectively avoid, among them: aviation; long-distance freight by ships; process emissions from cement and steel production; high-temperature heat (e.g., &gt;1000°C); and electricity reliability in systems with high penetration of variable renewable energy sources (NAS) ( [[#Davis--2018|Davis et al. 2018]] ; [[#Luderer--2018|Luderer et al. 2018]] ; [[#Sepulveda--2018|Sepulveda et al. 2018]] ; [[#Chiaramonti--2019|Chiaramonti 2019]] ; [[#Bataille--2020|Bataille 2020]] ; [[#Madeddu--2020|Madeddu et al. 2020]] ; [[#Rissman--2020|Rissman et al. 2020]] ; [[#Thiel--2021|Thiel and Stark 2021]] ). The literature focused on these services and sectors is growing, but remains limited, and provides minimal guidance on the most promising or attractive technological options and systems for avoiding these sectors’ emissions. Technological solutions do exist, but those mentioned in the literature are prohibitively expensive, exist only at an early stage, and/or are subject to much broader concerns about sustainability (e.g., biofuels) ( [[#Davis--2018|Davis et al. 2018]] ).

Liquid biofuels today supply about 4% of transportation energy worldwide, mostly as ethanol from grain and sugar cane and biodiesel from oil seeds and waste oils ( [[#Davis--2018|Davis et al. 2018]] ). These biofuels could conceivably be targeted to difficult-to-electrify sectors, but face substantial challenges related to their lifecycle carbon emissions, cost, and further scalability ( [[#Tilman--2009|Tilman et al. 2009]] ; [[#Staples--2018|Staples et al. 2018]] ), ( [[#6.4.2|Section 6.4.2]] ). The extent to which biomass will supply liquid fuels or high temperature heat for industry in a future net-zero energy system will thus depend on advances in conversion technology that enable use of feedstocks such as woody crops, agricultural residues, algae, and wastes, as well as competing demands for bioenergy and land, the feasibility of other sources of carbon-neutral fuels, and integration of bioenergy production with other objectives, including CDR, economic development, food security, ecological conservation, and air quality ( [[#Fargione--2010|Fargione 2010]] ; [[#Williams--2010|Williams and Laurens 2010]] ; [[#Creutzig--2015|Creutzig et al. 2015]] ; [[#Chatziaras--2016|Chatziaras et al. 2016]] ; [[#Laurens--2017|Laurens 2017]] ; [[#Lynd--2017|Lynd 2017]] ; [[#Bauer--2018|Bauer et al. 2018]] a, b; [[#Strefler--2018|Strefler et al. 2018]] ; [[#Muratori--2020b|Muratori et al. 2020b]] ; [[#Fennell--2021|Fennell et al. 2021]] ) ( [[#6.4.2.6|Section 6.4.2.6]] ).

Costs are the main barrier to synthesis of net-zero emissions fuels ( ''high confidence'' ), particularly costs of hydrogen (a constituent of hydrocarbons, ammonia, and alcohols) ( [[#6.4.5|Section 6.4.5]] ). Today, most hydrogen is supplied by steam reformation of fossil methane (CH 4 into CO 2 and H 2 ) at a cost of 1.30– USD1.50 kg –1 ( [[#Sherwin--2021|Sherwin 2021]] ). Non-fossil hydrogen can be obtained by electrolysis of water, at current costs of USD5–7 kgH 2 –1 (assuming relatively low electricity costs and high utilisation rates) ( [[#Graves--2011|Graves et al. 2011]] ; [[#DOE--2020a|DOE 2020a]] ; [[#Newborough--2020|Newborough and Cooley 2020]] ; [[#Peterson--2020|Peterson et al. 2020]] ). At these costs for electrolytic hydrogen, synthesised net-zero emissions fuels would cost at least USD1.6 per litre of diesel equivalent (or USD6 gallon –1 and USD46 GJ –1 , assuming non-fossil carbon feedstock costs of USD100 per tonne of CO 2 and low process costs of USD0.05 litre –1 or USD1.5 GJ –1 ). Similar calculations suggest that synthetic hydrocarbon fuels could currently avoid CO 2 emissions at a cost of USD936–1404 tonne –1 ( [[#Ueckerdt--2021|Ueckerdt et al. 2021]] ). However, economies of scale are expected to bring these costs down substantially in the future ( [[#IRENA--2020c|IRENA 2020c]] ; [[#Ueckerdt--2021|Ueckerdt et al. 2021]] ), and R&amp;D efforts are targeting 60–80% reductions in costs (to less than USD2 kg –1 (H 2 ) –1 ) possibly by use of less mature but promising technologies such as high-temperature electrolysis and thermochemical water splitting ( [[#Kuckshinrichs--2017|Kuckshinrichs et al. 2017]] ; [[#Pes--2017|Pes et al. 2017]] ; [[#Schmidt--2017|Schmidt et al. 2017]] ; [[#Saba--2018|Saba et al. 2018]] ; [[#DOE--2018|DOE, 2018]] , 2020b). Technologies capable of producing hydrogen directly from water and sunlight (photoelectrochemical cells or photocatalysts) are also under development, but are at an early stage ( [[#Nielander--2015|Nielander et al. 2015]] ; [[#DOE--2020a|DOE 2020a]] ). High hydrogen production efficiencies have been demonstrated, but costs, capacity factors, and lifetimes need to be improved in order to make such technologies feasible for net-zero emissions fuel production at scale ( [[#McKone--2014|McKone et al. 2014]] ; [[#DOE--2020a|DOE 2020a]] ; [[#Newborough--2020|Newborough and Cooley 2020]] ).

The carbon contained in net-zero emissions hydrocarbons must have been removed from the atmosphere either through DAC, or, in the case of biofuels, by photosynthesis (which could include CO 2 captured from the exhaust of biomass or biogas combustion) ( [[#Zeman--2008|Zeman and Keith 2008]] ; [[#Graves--2011|Graves et al. 2011]] ). A number of different groups are now developing DAC technologies, targeting costs of USD100 per tonne of CO 2 or less ( [[#Darton--2018|Darton and Yang 2018]] ; [[#Keith--2018|Keith et al. 2018]] ; [[#Fasihi--2019|Fasihi et al. 2019]] ).

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[[File:5d330b84bdcc07db002b9a1b3473e03c IPCC_AR6_WGIII_Figure_6_23.png]]

'''Figure 6.23 | Schematic of net-zero emissions energy system, including methods to address difficult-to-electrify sectors.''' Source: with permission from [[#Davis--2018|Davis et al. (2018)]] .

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