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Hydrothermal liquefaction: Enhanced performance and feedstock flexibility for efficient biofuel production

Periodic Reporting for period 3 - HyFlexFuel (Hydrothermal liquefaction: Enhanced performance and feedstock flexibility for efficient biofuel production)

Reporting period: 2020-04-01 to 2021-09-30

The European Green Deal aims at a 90% reduction in transport emissions to achieve net-climate neutrality by 2050. In this context, liquid biofuels from advanced biomasses or wastes (such as sewage sludge, wheat straw or miscanthus) are expected to play an important role. Hydrothermal liquefaction (HTL) is emerging as feedstock-flexible biofuel technology to produce various renewable transportation fuels by converting wet biomass into crude-like oil under moderate temperature and high pressure.

HyFlexFuel is dedicated to the development of a cost-competitive production pathway for liquid fuels based on HTL. All high-level project objectives of HyFlexFuel were achieved:
1) Demonstrating compatibility of the process with a diverse feedstock base
2) Maturing HTL-based fuel production from technological readiness level TRL 2-4 to TRL 5
3) Increasing heat integration and product recovery
4) Understanding the relation between feedstock and process conditions vs. product yield and quality
5) Efficient valorisation of residual process streams
6) Quantification of techno-economic and environmental performance potentials, risks and benefits
7) Demonstrating drop-in capability of HyFlexFuel products
8) Identification and quantification of technology gaps

HyFlexFuel covered the entire production chain to finished fuels, as well as the utilisation of further process streams. The availability of a broad range of biomass feedstock was studied in detail. The project demonstrated and optimized the liquefaction of lignocellulosic biomass, microalgae and wet wastes into biocrudes. The biocrudes were subsequently upgraded into transportation fuels, such as gasoline, jet fuel and diesel. The substantial content of organic compounds in HTL process water is converted into biogas via catalytic hydrothermal gasification (cHTG) and anaerobic digestion (AD), e.g. for on-site electricity and heat generation. Valuable inorganic nutrients are recovered from the process and can be used as fertilisers.
The four-year HyFlexFuel project produced many significant results, including:
- Quantification and mapping of waste and residue availability in Europe for sustainable HTL biofuel production
- Biocrude production from various biomasses in a unique HTL pilot plant
- Successful upgrading of HTL biocrudes from sewage sludge, wheat straw and algae to hydrocarbon fuels
- Development of technologies for the valorisation of further process streams for energy generation and nutrient recovery
- Quantification of the economic and environmental performance

The achievements of HyFlexFuel are discussed in more detail in the following paragraphs.

The availability of sustainable biomass streams in Europe was investigated by project partner DBFZ (German biomass research centre). DBFZ quantified the potential availability of waste streams such as sewage sludge, biogenic municipal wastes, manure, and various kinds of agricultural residues such as straw and sugar beet leaves. Feedstock density maps were compiled for each biomass to identify suitable locations for HTL plants in Europe.

The unique pilot-scale HTL plant at Aarhus University demonstrated the feedstock flexibility of hydrothermal liquefaction by converting various biomasses under industrially relevant conditions. During these experimental campaigns more than 800 kg of biocrude were produced. A long-duration campaign demonstrated continuous HTL conversion over 48 h. In parallel, HTL models were developed based on screening experiments with smaller reactors to optimize the biocrude yield and quality. This work also showed that the co-processing of different biomasses can improve the performance. To understand the complex HTL reaction pathways, HyFlexFuel partners developed and applied sophisticated analytical techniques to reveal the chemical composition of biocrudes and other product phases.

The chemical composition of biocrude varies depending on feedstock and process conditions. Aalborg University and Haldor Topsoe derived multi-step strategies, including an appropriate use of catalysts, which enabled the successful upgrading of HTL biocrudes for more than 300 h of continuous operation. The kerosene and diesel products from the upgrading of biocrudes from sewage sludge, wheat straw and algae showed a promising composition. A blend of HTL kerosene and conventional jet fuel was successfully tested on a laboratory-scale jet engine test at Aalborg Airport. Project partner ENI investigated the alternative option to co-feed HTL biocrudes in crude-oil refineries. It was found that specific properties of biocrudes limit the co-feeding ratios to few percent. However, mildly upgraded biocrudes can improve the co-feeding ratio significantly.

The biocrudes are not the only product from HTL conversion. Typically 90% of the throughput of a HTL plant is process water, which contains a significant amount of minerals and organics. HyFlexFuel investigated the production of methane from the organic content via catalytic hydrothermal gasification (cHTG) and anaerobic digestion (AD). cHTG experiments were carried out on a pilot facility at the Paul Scherrer Institute. Strategies for the separation of salts and of sulfur-containing species were developed. Anaerobic Digestion experiments at project partner OWS (Organic Waste Systems) showed substantial biogas production for HTL process water from miscanthus, while process waters from sewage sludge and microalgae proved to be more challenging. The University of Hohenheim successfully recovered nitrogen and phosphorus in form of struvite, a fertilizer product, from the process water and from solids that also form during HTL.

Bauhaus Luftfahrt established process models covering all abovementioned process steps, to evaluate the economic and environmental performance of HTL fuel production. In case of sewage sludge conversion, near-competitive production costs and GHG reductions of more than 80% can be achieved. For the conversion of lignocellulosic feedstock, it will be important to provide renewable heat and green hydrogen to achieve a comparable level of GHG reductions.
The HyFlexFuel project results, summarised in the previous section, are a major step beyond the state of the art of HTL conversion at project start. They can become key-enabling solutions for the change from conventional transportation fuels towards renewable alternatives and the transformation of Europe into a net-carbon neutral society by 2050. The HyFlexFuel technology innovates feedstock flexible biofuel production, in particular from wet waste streams such as sewage sludge, manure or food waste, which need to be disposed otherwise. One example of particular interest is the integration of the HyFlexFuel process in wastewater treatment plants, to destroy micro-pollutants in sewage sludge, to recover phosphorus, and to produce fuels. Thereby, HyFlexFuel also contributes to the European target to establish a more circular economy. HTL conversion provides new market opportunities for fossil fuels replacement and can create employment in various regions in Europe, especially when agricultural residues are utilized for fuel production. HyFlexFuel advanced key process steps for HTL conversion and demonstrated the potential to produce drop-in capable fuels in a scalable, sustainable and economically competitive way. Stakeholder dialogues indicate that commercial HTL fuel production may be achieved as early as 2025.
The HyFlexFuel process