Global warming is a major threat demanding revolutionary reductions in greenhouse gas (GHG) emissions in all sectors, including transportation. GHG emission reductions in the transport sector are achievable by energy savings with system optimisation and new technologies, e.g., modal shifts, route planning, vehicle sizes, driving patterns and high efficiency engines. Further, the energy still needed in form of electricity or advanced fuels should be sustainable and low-carbon.

Actual potential to mitigate GHG emissions from transport sector by using low-carbon energy depends on the lifecycle, or well to wheels (WTW),  emissions of solutions. In this respect, the upstream Well-to-Tank (WtT) emissions of raw materials and conversion processes are determining. The calculation models and sustainability criteria used for evaluation of the GHG emissions for fuels are defined in regional regulations to ensure meeting required reductions and sustainability criteria with the classified fuels. For example in Europe, Renewable Energy Directive (RED) defines calculation principles and required GHG emission savings for fuels accounted as biofuels, bioliquids and biomass fuels being at least 65% starting from 1.1.2021. In the U.S., both the low-carbon fuel standard in California and the renewable fuel standard in the entire U.S. include WTW emissions of a variety of transportation fuels.

Despite of importance of upstream GHG emissions for energy, vehicle regulations today account only for tailpipe Tank-to-Wheels (TtW) emissions. Tailpipe CO₂ emissions may reduce up to 30% with the best combination of carbonaceous fuel and internal combustion engine (such as methane fuel in efficient dual fuel engine) compared with diesel technology. Considerable are also tailpipe emissions of further greenhouse gases such as methane and nitrous oxide, as well as black carbon emissions, having high global warming potentials, which emphasizes the need for appropriate emission control technologies. Overall, compromising the tailpipe emissions is not an option, since respecting “no-harm” principles in health and environmental perspectives is essential.

Lifecycle GHG emissions including the upstream and tailpipe emissions can be low for renewable fuel alternatives, and WtW basis would offer holistic basis and technology neutrality in meeting the targets in transport sectors that are challenging for electrification, such as long haul transport, shipping and aviation. Synthesis of desired fuel chemistries is possible regardless of raw materials (Table 1), which is an important aspect, since fuels compatible with the present infrastructure and vehicles (“drop-in” fuels) would enable immediate impact. For example, paraffinic fuels, methane, or methanol can be produced from biogas, vegetable oils, cellulosic feedstock, waste streams (animal fats, plastics), or as electro-fuels from renewable hydrogen and circular carbon dioxide. Lignocellulosic biomass represents the greatest part of the bio-origin raw material potential. Carbon-neutral fuels could be tailored regionally depending on the available raw materials, infrastructure, and vehicle fleet. For sophisticated engines and exhaust aftertreatment technologies, high-quality fuels are crucial, since impurities of fuels may lead to premature deterioration of exhaust aftertreatment devices jeopardizing their long-term emission performance. Renewable fuels, which are clean, enable tailpipe emissions at “zero-level” with optimally performing exhaust aftertreatment devices.

Development of markets for carbon-neutral fuels depends on regulations, subsidies, raw materials, production, by-products and competitive technologies. There are already regional policies promoting clean technologies and restricting use of fossil fuels. However, efforts are needed to recognize low-carbon renewable fuels in the vehicle CO₂ emission regulations and to create credit systems and mandates to support this development. Integration of fuel and vehicle GHG emissions would convey the way of low-carbon fuels to complement other zero-emission options to mitigate global warming.

Bioenergy TCP has conducted studies on GHG emissions of different raw materials and processes. For example, IEA Bioenergy Task 39 publications are available at link.

• Biofuels LCA models comparison- Summary Draft - July 2019
• CTBE biofuels LCA comparison Final Report (Phase 2 Part 1) - February 2019
• Comparison of biofuel life-cycle GHG emissions assessment tools: The case studies of ethanol produced from sugarcane, corn, and wheat
• Comparison of Biofuel Life Cycle Assessment Tools - Conventional Ethanol

Table 1. Different pathways from raw materials to fuels.

To summarise

• Sustainable fuels have low GHG emissions over the whole life cycle.
• Fast reduction of GHG emissions of transport sector is achievable with carbon-neutral “drop-in” fuels compatible with present engines and aftertreatment devices.
• Fuel quality needs to be maintained to secure long-term durability of engines and after-treatment devices.
• Tailpipe exhaust emissions cannot be compromised by low-quality fuels to avoid harmful effects on health and environment.
• Carbon-neutral fuels complement other clean technologies, e.g. full battery vehicles for transport decarbonization.
• Deployment of carbon-neutral fuels requires integration in vehicle CO₂ regulations.