e-diesel

The main feedstocks for producing e-diesel are low-emissions electricity, water, and renewably sourced CO₂, i.e. biogenic CO₂ or CO₂ removal solutions such as direct air capture (DAC) or direct ocean capture (DOC). In the main pathway for e-diesel production, carbon monoxide (derived from renewably sourced CO₂) is combined with hydrogen (produced through electrolysis powered by low-emissions electricity) to create a synthetic hydrocarbon, which can then be refined to e-diesel.

Mature low-emissions electricity generation technologies like solar, wind, and hydro are commercially available, but large-scale build-out is needed to support e-diesel production. The potential use of nuclear power for e-diesel production remains uncertain and an area for further exploration. Scaling up low-emissions electricity generation is one of the main feedstock challenges for e-diesel production.

While water is globally plentiful, accessible fresh water makes up just under 1% of the planet’s water. Of the remaining water, almost 99% is seawater, which is accessible and can be purified by desalination – but the briny discharge requires sustainable disposal.

Renewable CO₂ can be waste CO₂ released by combustion or decomposition of biomass and its derivatives. Point sources of renewable CO₂ are too limited to provide for full maritime decarbonization, but global volumes should be sufficient in the near term to supply e-fuels for a small fraction of the world fleet. However, access to this CO₂ is challenged by competition from other uses, such as permanent sequestration. Carbon removal solutions such as DAC and DOC are very costly due to low efficiencies of selecting ppm-level concentrations of CO₂ in air and dilute concentrations in the ocean, respectively.

Key challenges to producing e-diesel are therefore the availability of renewably sourced CO₂ and of low-emissions electricity. Competition with the aviation sector for the feedstock is also a concern for this pathway.

An alternative synthesis pathway for e-diesel uses e-methanol as the feedstock. However, the set of reactions needed to produce e-diesel from e-methanol has a limited yield due to lack of selectivity and potential catalyst deactivation. This alternative e-diesel pathway therefore reduces the overall fuel yield from the already constrained feedstock of e-methanol. For this reason, although the chemical pathway is a mature process, the commercialization of the e-methanol to e-diesel fuel pathway is currently constrained.

In the established Fischer-Tropsch (FT) process, carbon monoxide (CO) is reacted with hydrogen to produce hydrocarbons, which can be further refined and upgraded to e-diesel. To achieve low carbon intensity in e-diesel production, this synthesis route requires both a supply of e-hydrogen produced using low-emissions electricity and a renewable source of CO₂ (to be converted to CO).

As the selectivity of the FT reaction can vary, it must be engineered to ensure that e-diesel, rather than e.g. kerosene or gasoline, is produced. Conversely, e-diesel could be obtained as a minor co-product for shipping if the FT synthesis is used to produce other hydrocarbons, such as e-kerosene, for other industries.

The FT synthesis pathway for e-diesel is commercially certified, and e-diesel is being produced using this method at a pilot plant in Norway. Production of e-diesel using e-hydrogen is a new process, enabled by the recent adoption of electrolysis.

The electrolyzer stack is the core technology needed to split water into oxygen and hydrogen. Global electrolyzer production today is challenged by a scarcity of raw materials, the low stack manufacturing technology maturity and capacity, and the need to produce replacement stacks (current stacks have an average lifespan of 3-12 years).

Additionally, substantial infrastructure for dedicated low-emissions electricity is required to generate hydrogen via electrolysis at an industrial scale. However, the build-out of low-emissions electricity is constrained by issues like the expansion of electricity grids, availability of raw materials such as copper, and competition from other sectors.

An e-diesel synthesis plant requires large quantities of renewable CO₂ (see also feedstock availability tile for e-diesel) at a centralized location. Such CO₂ sources typically come from industrial waste streams like flue gas. The industrial plants that produce these waste streams are often located where it is not possible to build adjacent CO₂ capture and e-diesel production infrastructure. As a result, transportation of CO₂ feedstock from its source to a production facility presents logistical challenges, which can increase the well-to-wake environmental impact of the resulting fuel.

Therefore, the primary challenges to the advancement of e-diesel synthesis lie in expanding low-emissions electricity infrastructure, centralized access to renewably sourced CO₂, and increasing the manufacturing capacity of electrolyzer stacks.

As an alternative to the FT synthesis route, e-methanol can be used as an intermediate for conversion to olefins, which are then oligomerized to e-diesel. However, this production route is not yet commercially mature, and less mature than the FT route. The keys to industrializing this production route will be obtaining significant volumes of e-methanol, achieving the complete conversion of e-methanol while maintaining catalytic performance when converting methanol to olefins, and establishing commercial certification.

Fuel supply logistics and bunkering are well-established for conventional fossil-based diesel (and very low-sulfur fuel oil, VLSFO). Given that the molecular composition of e-diesel is similar to that of VLSFO, its use does not represent any fundamentally new challenges in fuel storage, logistics, and bunkering.

Existing port infrastructure is technically adequate for use with e-diesel, but e-diesel is currently not widely used due to a lack of regulatory clarity. Standardization and certification are required to enable legal bunkering and fuel logistics as for conventional fuels.

The properties of e-diesel are similar to those of conventional fuels and hydrotreated vegetable oil (HVO). As a result, we do not consider onboard storage and fuel conversion to be a challenge for this fuel.

E-diesel has properties that resemble those of conventional maritime fuels in many ways. Technology and safe operations are mature and not considered a challenge.

When e-diesel is produced with low-emissions electricity and carbon capture, the well-to-wake emissions associated with this fuel can reach close to net-zero. This is because the CO₂ emitted during onboard fuel combustion is offset by the well-to-tank captured CO₂ used for fuel production (see also the tiles for feedstock availability and fuel production for e-diesel).

Emissions of local air pollutants such as sulfur oxides (SO_x) and particulate matter (PM) are significantly reduced when using e-diesel compared to conventional diesel. This reduction is due to e-diesel’s low sulfur content and more uniform composition. Notably, e-diesel combustion does not result in increased nitrogen oxides (NO_x) emissions, making it compatible with existing NO_x emission standards.

E-diesel has the same composition as the conventional diesel used today. There are therefore few regulatory barriers to its use as a maritime fuel, although more clarity in the International Maritime Organization (IMO) safety regulations and International Organization for Standardization (ISO) standards could be beneficial. The main regulatory barrier for this fuel pathway is to certify and regulate the feedstock so that e-diesel is more sustainable than fossil diesel.

The IMO is advancing its development of well-to-wake-based regulations to promote the use of sustainable fuels, including e-diesel. Regulating the climate impact of fuel use from a life-cycle (well-to-wake) perspective offers the industry the opportunity to establish sustainable fuel production and consumption patterns. Such regulation can help mitigate the risk of shifting climate impact from the downstream (tank-to-wake) segment of the value chain to the upstream (well-to-tank). This is a crucial consideration for alternative marine fuels, as a significant portion of their climate impact is associated with upstream activities (see also tiles for feedstock availability and fuel production). However, many elements of these regulations remain to be discussed and finalized, including certification, sustainability criteria, rules for electricity production, and implementation in the IMO mid-term measures.

The European Union (EU) has made progress with the introduction of the EU Emissions Trading Scheme and the FuelEU Maritime regulation, which may promote the uptake of e-diesel. However, certification procedures are not yet in place for these fuels in the EU. The certification of renewably sourced CO2 should be a particular area of attention.