A review of scientific studies, databases, and regional reports was used to estimate available feedstocks for HTC and microalgae cultivation. These feedstocks include sewage sludge, digestate, agricultural residues, livestock manure, food and agro-industrial waste, and byproducts from sectors such as dairy, slaughterhouses, pulp and paper, and breweries. The analysis shows that Europe generates enough waste and biomass to support several HTC biorefineries with capacities of 30,000 tDM per year, highlighting strong potential for regional development.
Various microalgae strains were also tested for their ability to grow on different effluents—both incoming streams (like wastewater and digestate) and recycled flows (such as the HTC aqueous phase and APR effluent)—while producing biomass suitable for HTC, especially rich in carbohydrates. Some strains achieved high carbohydrate yields when grown in NIAGARA recirculating streams, which is a crucial first step toward validating the NIAGARA concept.
At the core of the NIAGARA project is the development of thermochemical processes that convert these feedstocks into valuable gaseous biofuels. Over 60 HTC experiments have been conducted at the lab scale using different wastes and various microalgae biomass, supported by the creation of an AI-based process model. Ongoing lab- and pilot-scale tests will further refine this model.
The HTC aqueous effluent has been characterized, pretreated, and processed through APR to produce hydrogen (about 15 mmol H2/gC) and methane. This marks the first time APR has been applied to such a complex, nitrogen-rich wastewater. Strategies to reduce catalyst deactivation are already in place, and further work—such as phosphate removal—is planned.
The main HTC product, hydrochar, has also been analyzed. Hydrochars derived from waste need to be blended with those from biomass to enable effective gasification in a fixed-bed reactor. To support pilot-scale testing, 700 kg of hydrochar-like material were produced by torrefying 1.5 tonnes of rapeseed cake pellets. These torrefied pellets show similar characteristics to hydrochars, except for the oxygen content, and meet the criteria required for upcoming gasification trials.
The biosyngas produced in the NIAGARA project combines the gaseous products from the gasification of HTC hydrochar and the APR process. This renewable fuel is then used to power a SOFC, generating both electricity and heat. A carbon-safe gas composition was defined to ensure stable SOFC operation at temperatures above 600 °C, providing a reliable basis for efficient, carbon-free energy production. NIAGARA also established quantitative links between operating conditions, performance, and degradation, creating a predictive tool for optimizing SOFC operation with varying biosyngas compositions. The team also developed a novel diagnostic method—Total Harmonic Distortion Analysis (THDA)— and successful electroplating methods to increase the lifespan of the fuel cells. Finally, the development of regenerable hot-gas sorbents marks a major advance in sulfur control above 500 °C, enabling the direct and efficient use of the NIAGARA biosyngas in SOFC systems.
Enhancing the circularity of the NIAGARA process is a key objective. The CO2 generated during thermochemical processes will be captured using a newly developed solvent with high absorption capacity, fast reaction rates, and strong thermal stability. Its performance surpasses that of the conventional solvent monoethanolamine (MEA), which will be tested in a pilot-scale unit already commissioned on MEA.
NIAGARA aims to minimize liquid effluents by recycling them into microalgae cultivation, with the goal of discharging only clean water—mainly from the algae harvesting stage. In addition, the solid residue from gasification, known as biochar, has shown strong potential as a plant growth medium. When properly conditioned and washed, it performs as well as or better than conventional substrates.
The industrial scale-up of the NIAGARA process is being explored through modeling and simulation of best-case scenarios. Preliminary models have provided initial mass and energy balances, which will be refined as more detailed data become available. A preliminary life cycle assessment (LCA) offered an early environmental evaluation of the NIAGARA system. Three feedstock scenarios were analyzed using a parameterized LCA approach to account for variability in feedstock composition and process conditions. A model plant will also be developed to perform a techno-economic assessment over the system’s lifetime.
Finally, the NIAGARA project is guided by the principles of Responsible Research and Innovation (RRI), ensuring that all outcomes are not only scientifically robust but also socially responsible, transparent, and environmentally sustainable.
