Editing IPCC:AR6/WGIII/Chapter-10 (section)

=== 10.3.1 Alternative Fuels – An Option for Decarbonising Internal Combustion Engines ===

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The average fuel consumption of new internal combustion engine (ICE) vehicles has improved significantly in recent years due to more stringent emissions regulations. However, improvements are now slowing down. The average fuel consumption of LDVs decreased by only 0.7 % between 2016 and 2017, reaching 7.2 litres of gasoline-equivalent (Lg-eq) per 100 km in 2017, much slower than the 1.8 % improvement per year between 2005 and 2016 ( [[#GFEI--2020|GFEI 2020]] ). Table 10.4 summarises recent and forthcoming improvements to ICE technologies and their effect on emissions from these vehicles. However, these improvements are not sufficient to meet deep decarbonisation levels in the transport sector. While there is significant and growing interest in electric and fuel-cell vehicles, future scenarios indicate that a large number of LDV may continue to be operated by ICE in conventional, hybrid, and plug-in hybrid configurations over the next 30 years ( [[#IEA--2019a|IEA 2019a]] ), unless they are regulated away through ICE vehicle sales bans (as some nations have announced) ( [[#IEA--2021a|IEA 2021a]] ). Moreover, ICE technologies are likely to remain the prevalent options for shipping and aviation. Thus, reducing CO 2 and other emissions from ICEs through the use of low-carbon or zero-carbon fuels is essential to a balanced strategy for limiting atmospheric pollutant levels. Such alternative fuels for ICE vehicles include natural gas-based fuels, biofuels, ammonia, and other synthetic fuels.

'''Table 10.4 | Engine technologies to reduce emissions from light-duty ICE vehicles and their implementation stage.''' Table nomenclature: GDI = Gasoline direct injection, VVT = Variable valve technology, CDA = Cylinder deactivation, CR = compression ratio, GDCI = Gasoline direct injection compression ignition, EGR = exhaust gas recirculation, RCCI = Reactivity controlled compression ignition, GCI = Gasoline compression ignition. Source: [[#Joshi--2020|Joshi (2020)]] .

{| class="wikitable"
|-
| '''Implementation stage'''

| '''Engine technology'''

| '''CO''' 2 '''reduction'''

'''(%)'''

|-
| Implemented

| Baseline: GDI, turbo, stoichiometry

| 0

|-
| rowspan="12"| Development

| Atkinson cycle (+ VVT)

| 3–5

|-
| Dynamic CDA + Mild hybrid or Miller

| 10–15

|-
| Lean-burn GDI

| 10–20

|-
| Variable CR

| 10

|-
| Spark assisted GCI

| 10

|-
| GDCI

| 15–25

|-
| Water injection

| 5–10

|-
| Pre-chamber concepts

| 15–20

|-
| Homogeneous lean

| 15–20

|-
| Dedicated EGR

| 15–20

|-
| 2-stroke opposed-piston diesel

| 25–35

|-
| RCCI

| 20–30

|}

'''Natural Gas.''' Natural gas could be used as an alternative fuel to replace gasoline and diesel. Natural gas in vehicles can be used as compressed natural gas (CNG) and liquefied natural gas (LNG). CNG is gaseous at relatively high pressure (10 to 25 megapascal (MPa)) and temperature (–40 to 30°C). In contrast, LNG is used in liquid form at relatively low pressure (0.1 MPa) and temperature (–160°C). Therefore, CNG is particularly suitable for commercial vehicles and light- to medium-duty vehicles, whereas LNG is better suited to replace diesel in HDVs ( [[#Dubov--2020|Dubov et al. 2020]] ; [[#Dziewiatkowski--2020|Dziewiatkowski et al. 2020]] ; [[#Yaïci--2021|Yaïci and Ribberink 2021]] ). CNG vehicles have been widely deployed in some regions, particularly in Asian-Pacific countries. For example, there are about 6 million CNG vehicles in China, the most of any country ( [[#Qin--2020|Qin et al. 2020]] ). However, only 20% of vehicles that operate using CNG were originally designed as CNG vehicles, with the rest being gasoline-fuelled vehicles that have been converted to operate with CNG ( [[#Chala--2018|Chala et al. 2018]] ).

Natural gas-based vehicles have certain advantages over conventional fuel-powered ICE vehicles, including lower emissions of criteria air pollutants, no soot or particulate, low carbon to Hydrogen ratio, moderate noise, a wide range of flammability limits, and high octane numbers ( [[#Kim--2019|Kim 2019]] ; [[#Bayat--2020|Bayat and Ghazikhani 2020]] ). Furthermore, the technology readiness level (TRL) of natural gas vehicles is very high (TRL 8–9), with direct modification of existing gasoline and diesel vehicles possible ( [[#Transport%20and%20Environment--2018|Transport and Environment 2018]] ; [[#Peters--2021|Peters et al. 2021]] ; [[#Sahoo--2021|Sahoo and Srivastava 2021]] ). On the other hand, methane emissions from the natural gas supply chain and tailpipe CO 2 emissions remain a significant concern ( [[#Trivedi--2020|Trivedi et al. 2020]] ). As a result, natural gas as a transition transportation fuel may be limited due to better alternative options being available and due to regulatory pressure to decarbonise the transport sector rapidly. For example, the International Maritime Office (IMO) has set a target of 40% less carbon intensity in shipping by 2030, which cannot be obtained by simply switching to natural gas.

'''Biofuels.''' Since AR5, the faster than anticipated adoption of electromobility, primarily for LDVs, has partially shifted the debate around the primary use of biofuels from land transport to the shipping and aviation sectors ( [[#IEA--2017a|IEA 2017a]] ; [[#Davis--2018|Davis et al. 2018]] ). At the same time, other studies highlight that biofuels may have to complement electromobility in road transport, particularly in developing countries, offering relevant mitigation opportunities in the short- and mid-term (up to 2050) ( [[#IEA--2021b|IEA 2021b]] ). An important advantage of biofuels is that they can be converted into energy carriers compatible with existing technologies, including current powertrains and fuel infrastructure. Also, biofuels can diversify the supply of transport fuel, raise energy self-sufficiency in many countries, and be used as a strategy to diversify and strengthen the agro-industrial sector ( [[#Puricelli--2021|Puricelli et al. 2021]] ). The use of biofuels as a mitigation strategy is driven by a combination of factors, including not only the costs and technology readiness levels of the different biofuel conversion technologies, but also the availability and costs of both biomass feedstocks and alternative mitigation options, and the relative speed and scale of the energy transition in energy and transport sectors (Box 10.2).

Many studies have addressed the lifecycle emissions of biofuel conversion pathways for land transport, aviation, and marine applications ( [[#Koeble--2017|Koeble et al. 2017]] ; [[#Staples--2018|Staples et al. 2018]] ; [[#Tanzer--2019|Tanzer et al. 2019]] ). Bioenergy technologies generally struggle to compete with existing fossil fuel-based ones because of the higher costs involved. However, the extent of the cost gap depends critically on the availability and costs of biomass feedstock ( [[#IEA--2021b|IEA 2021b]] ). Ethanol from corn and sugarcane is commercially available in countries such as Brazil and the US. Biodiesel from oil crops and hydro-processed esters and fatty acids are available in various countries, notably in Europe and parts of Southeast Asia. On the infrastructure side, biomethane blending is being implemented in some regions of the US and Europe, particularly in Germany, with the help of policy measures ( [[#IEA--2021b|IEA 2021b]] ). While many of these biofuel conversion technologies could also be implemented using seaweed feedstock options, these value chains are not yet mature ( [[#Jiang--2016|Jiang et al. 2016]] ).

Technologies to produce advanced biofuels from lignocellulosic feedstocks have suffered from slow technology development and are still struggling to achieve full commercial scale. Their uptake is likely to require carbon pricing and/or other regulatory measures, such as clean fuel standards in the transport sector or blending mandates. Several commercial-scale advanced biofuels projects are in development in many parts of the world, encompassing a wide selection of technologies and feedstock choices, including carbon capture and sequestration (CCS) that supports carbon dioxide removal. The success of these projects is vital to moving forward the development of advanced biofuels and bringing many of the advanced biofuels value chains closer to the market ( [[#IEA--2021b|IEA 2021b]] ). Finally, biofuel production and distribution supply chains involve notable transport and logistical challenges that need to be overcome ( [[#Mawhood--2016|Mawhood et al. 2016]] ; [[#Skeer--2016|Skeer et al. 2016]] ; [[#IEA--2017a|IEA 2017a]] ; [[#Puricelli--2021|Puricelli et al. 2021]] ).

Table 10.5 summarises performance data for different biofuel technologies, while Figure 10.3 shows the technology readiness levels.

'''Table 10.5 | Ranges of efficiency, GHG emissions, and relative costs of selected biofuel conversion technologies for road, marine, and aviation biofuels.'''

{| class="wikitable"
|-
| '''Main application'''

| '''Conversion technology'''

| '''Energy efficiency of conversion''' a

| '''GHG emissions of conversion process (gCO''' 2 '''-eq per MJ of fuel)''' b

| '''Relative cost of conversion process'''

|-
| Road

| Lignocellulosic ethanol

| 35% c

| 5 d

| Medium

|-
| Road/aviation

| Gasification and Fischer-Tropsch synthesis

| 57% e

| &lt;1 d

| High

|-
| Road

| Ethanol from sugar and starch

| 60–70% f

| 1–31 d

| Low

|-
| Road

| Biodiesel from oil crops

| 95% g

| 12–30 d

| Low

|-
| Marine

| Upgraded pyrolysis oil

| 30–61% h

| 1–4 h

| Medium

|-
| Aviation/marine

| Hydro-processed esters and fatty acids

| 80% i

| 3 i

| Medium

|-
| Aviation

| Alcohol to jet

| 90% j

| &lt;1 k

| High

|-
| Road/marine

| Biomethane from residues

| 60% l

| n/a

| Low

|-
| Marine/aviation

| Hydrothermal liquefaction

| 35–69% h

| &lt;1 h

| High

|-
| Aviation

| Sugars to hydrocarbons

| 65% m

| 15 m

| High

|-
| Road

| Gasification and syngas fermentation

| 40% n

| 30–40 n

| High

|}

Notes: a Calculated as liquid fuels output divided by energy in feedstock entering the conversion plant; b GHG emissions here refers only to the conversion process. Impacts form the different biomass options are not included here as they are addressed in Chapter 7; c [[#Olofsson--2017|Olofsson et al. (2017)]] ; d [[#Koeble--2017|Koeble et al. (2017)]] ; e [[#Simell--2014|Simell et al. (2014)]] ; f [[#de%20Souza%20Dias--2015|de Souza Dias et al. (2015)]] ; g [[#Castanheira--2015|Castanheira et al. (2015)]] ; h [[#Tanzer--2019|Tanzer et al. (2019)]] ; i [[#Klein--2018|Klein et al. (2018)]] ; j Narula et al. (2017); k [[#de%20Jong--2017|de Jong et al. (2017)]] ; l [[#Salman--2017|Salman et al. (2017)]] ; m [[#Moreira--2014|Moreira et al. (2014)]] ; [[#Roy--2015|Roy et al. (2015)]] ; [[#Handler--2016|Handler et al. (2016)]] ; n [[#Salman--2017|Salman et al. (2017)]] ; [[#Moreira--2014|Moreira et al. (2014)]] ; [[#Roy--2015|Roy et al. (2015)]] ; [[#Handler--2016|Handler et al. (2016)]] .

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[[File:3c805cf0be412a504291c2d8f8af6da3 IPCC_AR6_WGIII_Figure_10_3.png]]

'''Figure 10.3 | Commercialisation status of selected biofuels conversion technologies.''' The grey boxes represent the current technology readiness level of each conversion technology. Source: based on [[#Mawhood--2016|Mawhood et al. (2016)]] , [[#Skeer--2016|Skeer et al. (2016)]] , [[#IEA--2017a|IEA (2017a)]] , and [[#Puricelli--2021|Puricelli et al. (2021)]] .

Within the aviation sector, jet fuels produced from biomass resources (so-called sustainable aviation fuels, or SAF) could offer significant climate mitigation opportunities under the right policy circumstances. Despite the growing interest in aviation biofuels, demand and production volumes remain negligible compared to conventional fossil aviation fuels. Nearly all flights powered by biofuels have used fuels derived from vegetable oils and fats, and the blending level of biofuels into conventional aviation fuels for testing is up to 50% today ( [[#Mawhood--2016|Mawhood et al. 2016]] ). To date, only one facility in the US is regularly producing sustainable aviation fuels based on waste oil feedstocks. The potential to scale up bio-based SAF volumes is severely restricted by the lack of low-cost and sustainable feedstock options (Chapter 7). Lignocellulosic feedstocks are considered to have great potential for the production of financially competitive bio-based SAF in many regions. However, production facilities involve significant capital investment and estimated levelised costs are typically more than twice the selling price of conventional jet fuel. In some cases (notably for vegetable oils), the feedstock price is already higher than that of fossil jet fuel ( [[#Mawhood--2016|Mawhood et al. 2016]] ). Some promising technological routes for producing SAF from lignocellulosic feedstocks are below technology readiness level (TRL) 6 (pilot scale), with just a few players involved in the development of these technologies. Although it would be physically possible to address the mid-century projections for substantial use of biofuels in the aviation sector (according to the International Energy Agency (IEA) and other sectoral organisations ( [[#ICAO--2017|ICAO 2017]] )), this fuel deployment scale could only be achieved with very large capital investments in bio-based SAF production infrastructure, and substantial policy support.

In comparison to the aviation sector, the prospects for technology deployment are better in the shipping sector. The advantage of shipping fuels is that marine engines have a much higher operational flexibility on a mix of fuels, and shipping fuels do not need to undergo as extensive refining processes as road and aviation fuels to be considered drop-in. However, biofuels in marine engines have only been tested at an experimental or demonstration stage, leaving open the question about the scalability of the operations, including logistics issues. Similar to the aviation sector, securing a reliable, sustainable biomass feedstock supply and mature processing technologies to produce price-competitive biofuels at a large scale remains a challenge for the shipping sector ( [[#Hsieh--2017|Hsieh and Felby 2017]] ). Other drawbacks include industry concerns about oxidation, storage, and microbial stability for less purified or more crude biofuels. Assuming that biofuels are technically developed and available for the shipping sector in large quantities, a wider initial introduction of biofuels in the sector is likely to depend upon increased environmental regulation of particulate and GHG emissions. Biofuels may also offer a significant advantage in meeting ambitious sulphur emission reduction targets set by the sectoral organisations. More extensive use of marine biofuels will most likely be first implemented in inner-city waterways, inland river freight routes, and coastal green zones. Given the high efficiency of the diesel engine, a large-scale switch to a different standard marine propulsion method in the near to medium-term future seems unlikely . Thus, much of the effort has been placed on developing biofuels compatible with diesel engines. So far, biodiesel blends look promising, as it is used in land transport. Hydrotreated vegetable oil (HVO) is also a technically good alternative and is compatible with current engines and supply chains, while the introduction of multifuel engines may open the market for ethanol fuels ( [[#Hsieh--2017|Hsieh and Felby 2017]] ).

'''Ammonia.''' At room temperature and atmospheric pressure, ammonia is a colourless gas with a distinct odour. Due to relatively mild conditions for liquefaction, ammonia is transferred and stored as a liquefied or compressed gas and has been used as an essential industrial chemical resource for many products. In addition, since ammonia does not contain carbon, it has attracted attention as a carbon-neutral fuel that can also improve combustion efficiency ( [[#Gill--2012|Gill et al. 2012]] ). Furthermore, ammonia could also serve as a hydrogen carrier and be used in fuel cells. These characteristics have driven increased interest in the low-carbon production of ammonia, which would have to be coupled to low-carbon hydrogen production (with low-carbon electricity providing the needed energy or with CCS).

For conventional internal combustion engines, the use of ammonia remains challenging due to the relatively low burning velocity and high ignition temperature. Therefore, [[#Frigo--2014|Frigo and Gentili (2014)]] have suggested a dual-fuelled spark ignition engine operated by liquid ammonia and hydrogen, where hydrogen is generated from ammonia using the thermal energy of exhaust gas. On the other hand, the high-octane number of ammonia means good knocking resistance of spark ignition engines and is promising for improving thermal efficiency. For compression ignition engines, the high-ignition temperature of ammonia requires a high compression ratio, causing an increase in mechanical friction. Since [[#Gray--1966|Gray et al. (1966)]] , many studies have shown that the compression ratio can be reduced by mixing ammonia with secondary fuels such as diesel and hydrogen with low self-ignition temperatures, as summarised by [[#Dimitriou--2020|Dimitriou and Javaid (2020)]] . Using a secondary fuel with a high cetane number and the adoption of a suitable fuel injection timing has enabled highly efficient combustion of compression ignition engines in the dual fuel mode with ammonia ratios up to 95% ( [[#Dimitriou--2020|Dimitriou and Javaid 2020]] ). One major challenge for realising an ammonia-fuelled engine is the reduction of unburned ammonia, as described in [[IPCC:Wg3:Chapter:Chapter-6#6.4.5|Section 6.4.5]] ( [[#Reiter--2011|Reiter and Kong 2011]] ). Processes being examined include the use of exhaust gas recirculation (EGR) (Pochet et al. 2017) and after treatment systems. However, these processes require space, which is a constraint for LDVs and air transport but more practical for ships. Shipbuilders are developing an ammonia engine based on the existing diesel dual-fuel engine to launch a service in 2025 ( [[#Brown--2019|Brown 2019]] ; [[#MAN-ES--2019|MAN-ES 2019]] ). Ammonia could therefore contribute significantly to decarbonisation in the shipping sector ( [[#10.6|Section 10.6]] ), with potential niche applications elsewhere.

'''Synthetic fuels.''' Synthetic fuels can contribute to transport decarbonisation through synthesis from electrolytic hydrogen produced with low-carbon electricity or hydrogen produced with CCS, and captured CO 2 using the Fischer-Tropsch process ( [[#Liu--2020a|Liu et al. 2020a]] ). Due to similar properties of synthetic fuels to those of fossil fuels, synthetic fuels can reduce GHG emissions in both existing and new vehicles without significant changes to the engine design. While the Fischer-Tropsch process is a well-established technology ( [[#Liu--2020a|Liu et al. 2020a]] ), low-carbon synthetic fuel production is still at the demonstration stage. Even though their production costs are expected to decline in the future due to lower renewable electricity prices, increased scale of production, and learning effects, synthetic fuels are still up to three times more expensive than conventional fossil fuels ( [[IPCC:Wg3:Chapter:Chapter-6#6.6.2.4|Section 6.6.2.4]] ). Furthermore, since the production of synthetic fuels involves thermodynamic conversion loss, there is a concern that the total energy efficiency is lower than that of electric vehicles ( [[#Yugo--2019|Yugo and Soler 2019]] ). Given these high costs and limited scales, the adoption of synthetic fuels will likely focus on the aviation, shipping, and long-distance road transport segments, where decarbonisation by electrification is more challenging. In particular, synthetic fuels are considered promising as an aviation fuel ( [[#10.5|Section 10.5]] ).

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