Sectors that are difficult to electrify, such as aviation, shipping, and the chemical industry, remain dependent on liquid fuels with high energy density. To make these more sustainable, three routes are typically considered: advanced biofuels from waste streams, synthetic e-fuels from hydrogen and captured CO2, and continued use of fossil fuels combined with negative emissions elsewhere in the system. A fourth route remains largely overlooked in research and policy. This is stated by researchers in a recent preprint that calculates the implications of the European energy system. This route, the addition of hydrogen to biomass conversion to better utilize the available carbon, is referred to as e-biofuel.
The impetus is a computational problem in existing biofuel routes. In thermochemical processes such as Fischer-Tropsch, only about 30 percent of the carbon from biomass ends up in the final liquid fuel. The remainder is separated from the syngas as CO2. By adding extra renewable hydrogen, this excess carbon can be utilized directly, without the need for capture and transport to a separate conversion plant. According to the study, this principle can be applied to multiple chains, including biomass-to-liquid, biomethanol, synthetic natural gas, and methanol from biogas.
Cost advantage with scarce biomass
For the calculations, the authors used the open-source model PyPSA-Eur, which optimizes the European electricity, transport, heat, and industrial sectors together for the lowest system costs. The key finding: e-biofuel becomes cost-effective as soon as biomass is scarce and fossil fuels are phased out, for example due to limited carbon storage capacity or strict blending obligations. In those situations, adding e-biofuel reduces total system costs by up to 2,7 percent, or about 23 billion euros per year. The costs of the liquid fuel system itself decrease by more than 10 percent.
Behind these savings lies a shift in the required infrastructure. The model shows that the investment in fuel conversion can decrease by up to 27 percent and the required CO2 capture capacity by 28 percent. The CO2 price in the system also decreases by up to 16 percent. In addition, the demand for biomass decreases by up to 221 terawatt-hours, equivalent to nearly 19 percent of the assumed European potential for domestic residual streams. The gain lies primarily in carbon efficiency: for the biomass-to-liquid route, the share of utilized carbon in the calculations increases from 0,38 to 0,91, and the steps are similar for the other chains.
The authors present e-biofuel primarily as a way to spread risks. Because the technology can build modularly on existing biofuel plants, investments can be made incrementally as more clarity emerges regarding the availability of green hydrogen and carbon storage. If hydrogen remains scarce and storage capacity increases, excess carbon can still be stored. Conversely, if hydrogen scales up more rapidly, the same plant can make the switch to e-biofuel. The researchers call this flexibility a hedge against uncertainty regarding biomass, hydrogen, carbon storage, and policy itself.
EU rules do not yet specify the route
It is precisely on this last point that the study identifies a bottleneck. Current EU regulations do not explicitly mention e-biofuels and appear to favor e-fuels. According to the authors, the Renewable Energy Directive provides insufficient guidance regarding the category of renewable fuels of non-biological origin, while the ReFuelEU aviation regulation gives the impression that synthetic fuels are based exclusively on captured CO2. The consequence is that less energy-efficient e-fuels count in full, whereas for e-biofuels, only the hydrogen component, approximately two-thirds, would be taken into account. As a result, there is a risk of lock-in in a suboptimal system, according to the researchers, who advocate for a more technology-neutral classification of fuels.
The results come from a preprint that has not yet been peer-reviewed and are based on model calculations with associated assumptions regarding costs, demand, and available resources. The authors themselves point out limitations, including the rough spatial resolution of the model and the uncertainty surrounding future fuel demand in aviation.
Source: Cornell University
Photo: Ross Parmly, Unsplash
