Is e-fuel production a responsible way to use renewable energy?

The e-fuel route to SAF production, whereby captured CO2 is combined with hydrogen to form syngas, which is subsequently converted to e-kerosene, can produce a highly CO2 efficient drop-in fuel. When compared to the conventional production of aviation fuel from crude oil, it has been suggested that e-kerosene produces 8.4 x less CO2 on a well-to-wake (WTWa) basis. As technology maturity is low these estimates are not based on operational plants, but on scientific modelling. The CO2 business case for moving from conventional aviation fuel produced from crude oil to CO2 derived fuels could therefore be compelling.

Comparison of WTWa CO2 intensity values across different production pathways [1]

The figure above shows the CO2e intensity of different production routes for aviation fuel – with using Direct Air Capture (DAC) derived CO2 as one of the lowest net CO2 producing routes.

Unfortunately, despite being carbon efficient, the electrical energy demands of e-fuel plants are very large when compared to the energy demands of a crude oil refinery. Sourcing sufficient renewable electricity at a commercially attractive price is a key blocker to current e-fuel project development, as the process converts renewable electricity to chemical energy, rather than the refining of existing chemical energy in crude oil. This makes e-fuel production a thermodynamically challenged process.

The attractiveness of refining crude oil from a renewable energy grid management perspective

Crude oil is a highly energy dense substance containing approximately 10,000kWh per cubic metre (a similar energy density to aviation fuel), and releasing this energy into usable fuel liquids through fractional distillation and other processes in a refinery is relatively energy efficient. A well-run refinery can expect to be 82% source efficient, which is to say that 12,195kWh of crude oil produces 10,000kWh of usable kerosene.

In refinery processes, the 18% losses are due mainly to either heating or cooling the different refinery steps. Usually, the bulk of the energy needed for these steps comes from crude oil or the gases produced during the refinery processes. As such, crude oil refineries do not place significant demands on electricity grids, with an EU 2010 average of 7.3% of all refinery energy needs coming from purchased (grid) electricity [2]. In total, refineries purchase 24.5% of the energy needed for the refinery process, with the balance, in addition to grid electricity, being made up from purchased natural gas (12.9%) and steam (4.3%). Optionally, refineries can run a CHP (co-gen) plant to make process steam at desired temperatures, eliminating fired heaters and increasing the proportion of the input crude that is sold as product.

US refineries consume 0.2kWh of electricity per gallon (0.05kWh/litre) of refined product produced. A legacy Boeing 747-400 flying between London and New York consumes 82,000 litres of aviation fuel which contains 820,000kWh of energy – requiring only (approximately) 4,100kWh of grid electricity to produce. In context 4,100kWh is less than the annual electricity consumption of 2 average UK houses.

Electricity demands for E-Fuel production

Mass balance for E-Fuels production via the DAC-RWGS-FT route [3]

E-fuel plants are complex and there are many different potential routes to production, depending on the CO2 and H2 source and the intermediate steps used. A common approach for 1st generation plants under development is to use electrolysis to produce hydrogen and to use DAC to extract CO2 from the atmosphere. This is then converted to Carbon Monoxide (CO) in the Reverse Water Gas Shift (RWGS) reaction. This CO is then synthesised into wax and hydrocarbon condensate using the Fischer-Tropsch (FT) process, and these products are then Hydrocracked to produce liquid fuels. This process is currently 38% percent efficient and is projected to increase to 42% efficiency by 2050.

Producing 0.0232kg of e-fuel containing 1MJ (0.28kWh) of energy requires 2.6385MJ (0.73kWh) of renewable energy (mostly electricity) to produce, composed of:

• Electrolysis: 2.1092MJ

• DAC: 0.1267MJ

• Process Heat: 0.3255MJ

• RWGS: 0.0280MJ

• F-T Synthesis: 0.0161MJ

• CO2 compression: 0.0330MJ

This equates to 25.2kWh of renewable energy to produce 1 litre of e-fuel in a mixed blend of diesel, kerosene and gasoline, of which the e-Kerosene contains approximately 10kWh/litre of energy.

When considered at a plant level, a mid-point production capacity plant with a daily production of 30,000 litres of e-fuel, enough to fly a modern Airbus A320neo some 7,000km or around 8.4 hours at cruising speed,  would require 756MWh of grid electrical energy – the equivalent of over 100,000 average UK households - for a journey only 25% longer than the 2 UK household London to New York example above.

Implications for E-Fuel plant development

Although no trends are yet emerging as to the likely future optimal size for commercial e-SAF plants, with plant sizes under development ranging from sub-1 million litres to over 100 million litres per year in planned capacity, it is clear that e-SAF production will place significantly greater demands on renewable energy infrastructure than conventional hydrocarbon refining. Producing a litre of e-SAF requires in the order of 500 times more electrical energy than refining a litre of hydrocarbon-derived fuel.

E-fuel plants are not alone in demanding renewable energy, with the electrification of existing road transport and wider industry also requiring significant renewables development. In addition, there are growth industries whose demand for renewable energy is increasing markedly. EU data centres currently demand 4% of electricity production, and this is expected to double by 2030 due to the increased power demands of digitisation and AI. On a strictly economic basis, industries which can turn electrons into shareholder returns without requiring government subsidy or incentives, unlike e-fuel facilities, will be better able to compete for future renewable capacity.

The successful development of e-fuels projects therefore sits at a difficult crossroads. Hydrocarbon fuels are carbon intensive, comprising 4% of EU GHG emissions, however their production places little strain on future renewable energy capacity. This capacity is much needed for the less thermodynamically and technically challenging decarbonisation of existing and growth industries, along with heating and surface transport.

The table below shows that SAF is an extremely poor use of renewable energy in comparison to heat pumps, EVs and grid decarbonisation, along with replacement of our existing Steam Methane Reformers for hydrogen production.

CO2 reduction impact from different uses of renewable energy

Regulatory requirements for e-SAF are not onerous, with only 1.2% (552,000 tons) being required at EU airports by 2030. This capacity can be met from current plants under development, whose access to subsidy at national and EU-level will be critical for realisation. Following this, the prospect for future plants is less certain, and in the absence of strong and enduring subsidy support will be almost wholly dependent on access to very cheap (sub EUR 30/MWh) and prevalent renewable energy.

Given that our currently limited renewable electrical energy can be more efficiently used to decarbonise other industries, could the partial decarbonisation of aviation (a 1.2% EU target for e-Fuels by 20230) be better achieved by replacing legacy aircraft with the latest fuel-efficient models - which use up to 30% less fuel per passenger km than legacy aircraft?

References:

[1] https://pubs.rsc.org/en/content/articlehtml/2022/ee/d2ee02439j#imgfig4

[2] https://www.concawe.eu/wp-content/uploads/rpt_12-03-2012-01520-01-e.pdf

[3] https://www.efuel-alliance.eu/fileadmin/Downloads/Rpt_24-4-1.pdf