Towards carbon-neutral and clean propulsion in heavy-duty transportation with hydroformylated Fischer-Tropsch fuels

The transport sector’s greenhouse gas emissions have grown from 5.1 GtCO2e in 1990 to nearly 8.0 GtCO2e in 2022, and it remains a major source of urban air pollutants such as particulate matter (PM) and NOx. While direct electrification is a key strategy, hard-to-electrify modes (aviation, shipping, agricultural machinery, long-haul heavy-duty trucks) will continue to require liquid energy carriers. Synthetic fuels can leverage existing infrastructure, but to accelerate a clean transition they must simultaneously: (1) be scalable via mature synthesis processes; (2) comply with fuel standards and engine components; (3) reduce urban air pollutants; and (4) enable net-zero GHG emissions without shifting burdens to other environmental impacts. This study investigates whether hydroformylated Fischer-Tropsch (HyFiT) fuels—optimized alkane–alcohol blends—can meet these requirements through a holistic approach spanning process design, engine/fuel testing, vehicle emissions, and well-to-wheel life cycle assessment (LCA).

Multiple synthetic fuels have been considered as diesel substitutes in heavy-duty applications, including gaseous carriers (H2, methane, dimethyl ether) and liquids such as fatty acid methyl ester (biodiesel), hydrotreated vegetable oil (HVO), Fischer-Tropsch (FT) fuels, polyoxymethylene ethers (OME), and long-chain alcohols. FT diesel, with low aromatics and high cetane number, shows good compatibility and reduced PM (up to 40%) and NOx (up to 30%) engine-out emissions. LCAs report 70–98% GHG reductions for FT fuels when using biomass or CO2 feedstocks. Prior work demonstrated that blending long-chain alcohols (for example C8, such as 1-octanol) with diesel can drastically lower PM emissions, though neat 1-octanol’s low cetane number limits its direct use. Attempts to produce higher alcohols directly in FT processes have yielded moderate alcohol selectivities and mostly C2–C5 products, unsuitable for diesel-range fuels. Industrial hydroformylation/hydrogenation of olefins to alcohols is mature, and previous studies (including REDIFUEL) showed that FT alkane–olefin cuts can be directly hydroformylated, enabling integration of olefin upgrading into FT-derived product streams.

Process and fuel design: The HyFiT production concept integrates Fischer-Tropsch (FT) synthesis with downstream hydroformylation and hydrogenation to convert FT olefins into C1-elongated alcohols, generating designed alkane–alcohol blends. FT operation is tuned to increase olefin content in the naphtha cut (C5–C10) suitable for hydroformylation, while distillate hydrocarbons (C13–C22) are hydrocracked and remixed to deliver a HyFiT product composed of C6–C11 alcohols and C9–C17 alkanes. Alcohol/alkane ratios are adjusted by combining low-temperature FT (LT-FT; cobalt catalysts; higher chain growth) and high-temperature FT (HT-FT; iron catalysts; higher olefin content). Two syngas-loop scenarios are considered: open loop (no tail-gas recycle) and closed loop (recycling unconverted CO). Feedstocks include biomass- and CO2-derived syngas, with H2 supplied by water electrolysis. Process yields and selectivities are evaluated; open-loop results are emphasized in the main text, with closed-loop results in the Supplementary Information. Engine/fuel property testing: HyFiT fuels with 15–65 wt% alcohol were characterized. Properties measured (per EN and DIN/ISO standardized methods) include density (buoyancy/Archimedes), viscosity (Ubbelohde; DIN 51562), lubricity (HFRR; DIN/EN/ISO 12156-1), and derived cetane number (DCN; AFIDA following ASTM D6890/D8183). Lower heating value (LHV) was calculated. Additized fuels used 2,000 ppm Infineum R655 to meet lubricity targets without affecting other properties. Material compatibility: Compatibility with common elastomers (FKM, NBR, HNBR) was tested following DIN/ISO 1817 and 13226, assessing changes in mass, hardness, and volume across 15–65 wt% alcohol. Vehicle emissions testing: A plug-in hybrid light commercial van (3.3 t test mass; 5.5 t max; Euro 6d baseline) was tested on the WLTC (max 120 km/h) for diesel and additized HyFiT at 20 wt% and 40 wt% alcohol (HyFiT-20%, HyFiT-40%). Tests were repeated to ensure reproducibility. Battery state-of-charge deviations were corrected to maintain comparability. Three engine calibration cases were considered: (i) drop-in (manufacturer calibration), and two adjusted calibrations to (ii) achieve the same tailpipe NOx as diesel, or (iii) the same engine-out PM as diesel by modifying EGR rates. Emissions recorded: CO2, NOx, and PM (tailpipe and engine-out where relevant). Life cycle assessment: A well-to-wheel LCA (ISO 14040/14044; EF 3.0) assessed HyFiT-20% in a hybrid heavy-duty van for a 2030 global scenario. Functional unit: 1 km of transportation. System boundary includes feedstock and utility supply; syngas and HyFiT production; battery production; and vehicle operation. Benchmarks: hybrid van on fossil diesel and fully electric BEV. Supply chain optimization minimized carbon footprint for bio-based and CO2-based HyFiT across a range of electricity carbon intensities and resource availability (biomass, renewable electricity). Sensitivity analyses covered battery capacity/lifetime and indirect land use change; broader environmental impacts were analyzed for the carbon-footprint-optimal supply chains.

• Process performance: The integrated FT–hydroformylation concept delivers high overall carbon efficiency and fuel yields up to 83% at CO conversions above 95%, matching or exceeding state-of-the-art routes producing C2–C7 alcohols via modified FT catalysts. Open-loop gas handling already yields strong performance; closed-loop variants are provided in the Supplementary Information. - Fuel properties and standards: Adjusting alcohol content tunes properties. Alcohol contents of 20–40 wt% keep DCN ≥51 (EN 590 minimum), LHVs within diesel-like ranges (diesel ~42.8 MJ kg−1), and viscosities within EN 590; densities fall within common global diesel standards (EN 15940) even if not always within EN 590 bounds. Lubricity can be brought into EN 590 compliance via 2,000 ppm Infineum R655 without altering other properties. - Material compatibility: Across 15–65 wt% alcohol, mass and volume changes in FKM, NBR, and HNBR were small; hardness showed larger sensitivity but remained within tolerated limits for FKM and, with some limitations, HNBR. Overall, 20–40 wt% alcohol blends are most promising for compatibility. - Vehicle emissions (drop-in calibration): HyFiT-20% and HyFiT-40% reduced tailpipe CO2 by ~3–5% vs diesel due to a lower C/H ratio (0.45 vs 0.53). Engine-out NOx rose ~8%, leading to tailpipe NOx of 29 mg km−1 (HyFiT-20%) and 35 mg km−1 (HyFiT-40%) vs diesel 24 mg km−1; all are far below the Euro 7 limit of 75 mg km−1. Engine-out PM decreased substantially: −55% (HyFiT-20%) and −70% (HyFiT-40%) vs diesel, attributed to higher oxygen content (2.5–5.0 wt% vs diesel ~0.8 wt%), no aromatics, and higher volatility; with a particulate filter, tailpipe PM was near zero. - Vehicle emissions (optimized EGR calibrations): Leveraging lower PM, EGR adjustments reduced NOx markedly. Under the calibration achieving the same engine-out PM as diesel, NOx tailpipe emissions fell to 16 mg km−1 (HyFiT-20%) and 15 mg km−1 (HyFiT-40%), a 33–38% reduction vs diesel. HyFiT-20% provided the best overall balance (lower PM and NOx with slightly higher CO2 than HyFiT-40%). - LCA (well-to-wheel): Bio-based HyFiT exhibits a lower carbon footprint than diesel and is competitive with BEV across electricity carbon intensities considered (from today’s EU grid to wind-power levels). CO2-based HyFiT is more electricity-sensitive and requires electricity below ~65 gCO2e kWh−1 to outperform diesel; off-grid renewables or favorable regions could enable this. Resource availability defines scale-up: biomass availability limits bio-based routes; low-carbon electricity availability limits CO2-based routes. Hybrid supply chains (combining biomass and CO2) can reach carbon footprints comparable to BEVs powered by wind or today’s EU grid. - Environmental trade-offs: For long-haul ranges, bio-based HyFiT reduces impacts vs a BEV in 10/16 (ranges >550 km) or 12/16 (ranges >850 km) EF 3.0 categories, with land use as a notable trade-off. Combining biomass and CO2 feedstocks (‘bio-hybrid’) can mitigate land-use impacts while maintaining low carbon footprints.

The study addresses the core challenges for synthetic drop-in fuels. By integrating mature FT and hydroformylation technologies and tuning FT olefin content with LT-FT/HT-FT combinations, HyFiT enables scalable, feedstock-flexible production from biomass and CO2 while achieving high yields and carbon efficiency. Fuel testing confirms that, with appropriate alcohol proportions (20–40 wt%) and a standard lubricity additive, HyFiT blends meet common diesel fuel standards and are compatible with key elastomers, facilitating retrofits. Vehicle testing demonstrates substantially lower PM and the potential for significantly reduced NOx via EGR recalibration, while maintaining CO2 advantages and compliance far below Euro 7 limits even in drop-in operation. The well-to-wheel LCA shows that bio-based HyFiT can match or beat BEVs in carbon footprint under a wide range of electricity mixes and that CO2-based HyFiT can outperform diesel when powered by very low-carbon electricity; broader environmental impacts are generally favorable for HyFiT in long-range use cases, with land use as the main trade-off for biomass. Collectively, these findings position HyFiT as a complementary solution to electrification for heavy-duty transport, leveraging existing infrastructure to accelerate decarbonization and air-quality improvements.

HyFiT fuels—alkane–alcohol blends produced by integrating FT synthesis with hydroformylation—fulfill key requirements for de-fossilized, drop-in heavy-duty fuels: scalable and flexible production from biomass and CO2; compliance with fuel standards and elastomer compatibility; substantial reductions in PM and potential NOx decreases below diesel through calibration; and well-to-wheel carbon footprints enabling net-zero trajectories without major burden shifting in most environmental categories. Bio-based HyFiT is competitive with BEVs on carbon footprint across diverse electricity mixes, while CO2-based HyFiT competes with diesel when supplied by very low-carbon electricity. The concept provides a practical pathway to harness renewable resources for hard-to-electrify segments using existing infrastructure. Future work should quantify techno-economics, cost trajectories, and capacity scale-up timelines, assess broader material compatibility and long-term engine durability, and develop policy frameworks to prioritize deployment in sectors where liquid fuels remain essential (for example, long-haul heavy-duty).

• Economic analysis and capacity development timelines were out of scope; the study explicitly calls for future assessment of costs and scale-up potential. - Vehicle emissions were tested on a single plug-in hybrid light commercial van and specific calibrations (drop-in and EGR-adjusted) under WLTC conditions; generalization to other engine types, duty cycles, and after-treatment systems requires further validation. - Material compatibility was assessed for standard reference elastomers (FKM, NBR, HNBR); broader component/material testing and long-term durability were not covered. - LCA results reflect a 2030 global scenario, specific assumptions on electricity carbon intensity, battery lifetimes/capacities, and supply-chain configurations; regional variations and future technology changes may shift outcomes. - Closed-loop syngas recycling and additional process variants are presented in Supplementary Information; main-text results focus on the open-loop design.