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.
