The HyFlexFuel process
The conceptual flow diagram of the HyFlexFuel process is depicted in the following figure
In HyFlexFuel a broad range of biomass feedstocks is processed via hydrothermal liquefaction (HTL) for biocrude production. The yielded biocrude is subsequently upgraded via catalytic hydrotreatment and distillation to final fuel products. The aqueous phase from HTL is processed via catalytic hydrothermal gasification (cHTG) for methane production and energetic valorisation of the organic components. As alternative technology for the energetic valorisation of organic components in the aqueous phase, anaerobic digestion (AD) is studied. Finally, inorganic salts are recovered from HTL solids and the aqueous phase in the form of marketable fertilisers.
In HyFlexFuel, very different types of non-food biomass feedstock are utilised to produce liquid fuels through hydrothermal liquefaction (HTL) and catalytic upgrading. Initially, three model feedstocks are examined:
- the energy grass Miscanthus, representing lignocellulosic materials
- Microalgae as aquatic biomass and representing high-protein feedstock
- Sewage sludge as model waste feedstock with high ash content
More feedstocks will be converted via HTL in the further course of the project.
In hydrothermal liquefaction (HTL), temperatures around 350 °C and pressures of about 200 bar are applied to biomass feedstocks in aqueous slurries, resulting in a liquid raw product generally referred to as biocrude. Furthermore, the process yields an aqueous phase, containing water-soluble organic and inorganic components, solid residues and a gaseous phase. Biocrude can be upgraded and refined into liquid fuels.
Biocrude from HTL can undergo catalytic upgrading with high pressure hydrogen (hydrotreating) in order to be converted into a mixture of hydrocarbons. Through this process, undesired elements, such as oxygen and nitrogen, can be effectively removed. Moreover, conditions can be tuned to increase the yield of the desired fuel fractions (e.g. gasoline, diesel, jet fuel). From the upgraded biocrude, drop-in fuels are obtained by means of distillation, just as with fossil crude oil.
Catalytic hydrothermal gasification
In catalytic hydrothermal gasification (cHTG), the organic content of an aqueous slurry or solution is converted under supercritical conditions into an energy-rich gaseous mixture of methane (CH4), hydrogen (H2) and carbon dioxide (CO2). After separation of CO2, the remaining methane and hydrogen (“synthetic natural gas”, SNG) can be used for electricity and/or heat generation. In HyFlexFuel, cHTG is applied to energetically valorise the organic load dissolved in the aqueous phase from the HTL step.
In anaerobic digestion (AD), organic residues (e.g. manure, sewage sludge, organic household waste, agricultural and industrial by-products) are biologically converted in the absence of oxygen at mesophilic (± 37°C) or thermophilic (± 52°C) temperature and normal pressure. This biological process results in the production of biogas (CH4 + CO2), which can be used as renewable energy source, and nutrient-rich digestate. The digestate can be directly applied as an organic fertiliser or subjected to nutrient recovery procedures. In HyFlexFuel, AD is examined as technology option to convert the organic content of the aqueous phase from the hydrothermal liquefaction (HTL) step.
Biomass feedstocks contain different levels of inorganic components that are worth to be recovered. Nitrogen and phosphorus are of special interest in the HTL process-chain, but they accumulate in different waste streams. The nutrient recovery aims to combine and alter waste streams in our process chain to isolate nutrients in a superior form, such as struvite that represents a marketable fertiliser, by precipitation from mixed aqueous phases.