Sustainable Synthetic Carbon Based Fuels for Transport

(The Royal Society) The need to achieve net-zero greenhouse gas emissions from all human activities has never been clearer. One area requiring urgent action is the transition from the use of fossil fuels for transport. Whilst the decarbonisation of electricity is progressing and many transport modes can feasibly be electrified, some transport modes, such as heavy-duty vehicles, aircraft and shipping will require different technological options. The cost, volume and energy density of alternative fuels are of critical importance.

This briefing considers the science allowing a pathway to achieving this transition, through the sustainable production and use of carbon based synthetic fuels. Synthetic fuels can be manufactured, via chemical conversion processes from ‘defossilised’ carbon dioxide sources such as point source capture from the exhausts of industrial processes, direct capture from air or from biological sources. Whilst synthetic fuels emit carbon dioxide when burnt, this report demonstrates that synthetic fuels could, in the medium to long term (5 to 10+ years), displace fossil fuels. However, a full life cycle assessment of their manufacture has yet to be evaluated in depth.

Two methods of making carbon based sustainable synthetic fuels are explored in this briefing (Figure 1); i. electro fuels (efuels) made using captured carbon dioxide in a reaction with hydrogen, generated by the electrolysis of water, and
ii. synthetic biofuels made through the chemical or thermal treatment of biomass or biofuels.

The background technology to produce synthetic fuels is well known and used at scale (eg Fischer Tropsch synthesis using carbon monoxide). However, these existing processes use fossil carbon sources and new technologies and further innovation will be required to enable non-fossil carbon dioxide sources to be used.

The advantages of sustainable synthetic fuels are:
• They can be manufactured as ‘drop in’ replacements for fossil jet fuel, diesel and fuel oil
• Both the volume and energy density of synthetic fuels are similar to existing fuels
• They can be designed to burn cleanly, reducing other pollutants associated with fossil fuel use, such as particulates and nitrogen oxides
• Existing infrastructure can be utilised for distribution, storage and delivery to the vehicle

and the disadvantages are:
• Synthetic fuels from both biomass and carbon dioxide are currently more expensive than fossil fuels, for example around €4.50/litre for diesel equivalent efuel and around €1/litre petrol equivalent biofuel. Innovation in each process stage has the potential to reduce these costs in the future to enable production and scale up to defossilise the current and growing future transport demands. Estimated future costs vary greatly but range from 60 cents to €1.50 per litre for diesel equivalent efuel by 2050.
• The energy losses from manufacturing and using synthetic fuels are high due to the many processes involved. However, this might be justified where electrical propulsion is not practical and renewable electricity is cheap and plentiful.

Further scientific research is required in: improving the fundamental understanding of catalysis; the need to produce cheap green hydrogen at scale; and developing sources of competitively priced low-carbon energy are key to the development of synthetic efuels and biofuels.

With further development, synthetic fuels could offer a pathway to achieving net-zero carbon for transport in the long term. The UK has the research skills and capacity to improve many of these process steps such as in catalysis and
biotechnology, and to provide a further area of UK leadership in low-carbon energy.

Scope of this briefing
Synthetic fuels: Carbon based liquid fuels manufactured, via chemical conversion processes, from a carbon source such as coal, carbon dioxide, natural gas, biogas or biomass. This includes established conventional fossil-based processes (see Box 2).
1. Electrofuels (efuels)
These are synthetic fuels manufactured using captured carbon dioxide or carbon monoxide together with low-carbon hydrogen. They are termed electro- or efuels because the hydrogen is obtained from sustainable electricity sources eg wind, solar and nuclear power.

2. Synthetic biofuels
In this report, these are defined as fuels synthesised from biomass or waste or biofuels using chemical or thermal processes. The production of fuels using biomass and only biological processes are outside the scope of this briefing (for example, bioethanol produced through fermentation of sugars)

Sunfire e-crude
A 20 MW facility is to be installed by Sunfire in Herøya, Norway, which will produce 8000 tonnes of ‘e-crude’ liquid fuel/year from renewable energy and carbon dioxide with a target price of below €2/litre. The plant will be built in 2022 and operated by Norsk e-Fuel.

Carbon dioxide is partly extracted on-site through Direct Air Capture (DAC) technology.

From carbon dioxide and water, syngas is produced through electrolysis using renewable electricity. Via Fischer Tropsch, the syngas is converted to ‘e-crude’, a renewable crude oil substitute.

‘E-crude’ can be refined to generate e-diesel or e-jet fuel; this is currently being investigated by several leading manufacturers. READ MORE