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A new generation of hydrothermal liquefaction (HTL) process can convert all kinds of biomasses to crude bio-oil, which is close enough to fossil crude oil that a simple thermal upgrade and existing refinery technology can subsequently obtain all the liquid fuels we know today.

Add on top of that the HTL process only consumes 10-15 percent of the energy in the feedstock biomass, yielding an energy efficiency of 85-90 percent.

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This HTL process accepts all biomasses – sewage sludge, manure, wood, compost and plant material along with waste from households, meat factories, dairy production and similar industries.

To date, it is the most feedstock flexible of any liquid fuel producing process, including pyrolysis, bio-ethanol, gasification with Fischer-Tropsch or catalytic upgrading of different vegetable or agro-industrial residual oils, and does not carry higher costs than these.

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HTL is basically pressure cooking, but instead of cooking the biomass in batches, one pot-full at a time, this new generation of HTL works off of flow production, where the biomass injects into a 400°C pre-heated reactor, “cooked” under high pressure for about 15 minutes and then quickly cooled down to 70°C.

At 400°C and high pressure the water is in a supercritical state, neither liquid nor gas, at which it easily decomposes the biomass. The process is environmentally friendly, since no harmful solvents are a part of the process, and the energy efficiency is very high: The HTL process only consumes approximately 10-15 percent of the energy in the feedstock biomass, because the heat energy recycles between the heating and cooling of the process medium.

The wet medium means HTL readily accepts moist or wet biomasses, such as those mentioned above. Wet biomasses are in vast majority on Earth. All other known processes for liquid bio-fuel production either require expensive drying or only make use of a limited proportion of the biomass, e.g. the carbohydrate content.

The water phase emanating from the HTL process has low carbon contents and can either recycle into the process or ultimately end up purified to attain drinking water quality, which is the long-term goal. As such HTL replaces the burden of disposal with the benefit of recycling.

The HTL process has the following benefits:
• Crude HTL oil has high heating values of approximately 35-39 MJ/kg on a dry ash free basis
• The HTL process only consumes approximately 10-15 percent of the energy in the feedstock biomass, yielding an energy efficiency of 85-90 percent
• Crude HTL oil has very low oxygen, sulphur and water content (compared to e.g. pyrolysis oil which typically contains approx. 50 percent water)
• HTL oil recovers more than 70 percent of the feedstock carbon content (single pass)
• HTL oil is storage stable, and has comparatively low upgrading requirements, due in part to a high fraction of middle distillates in the crude oil. It is much less upgrade intensive than e.g. pyrolysis oil, which needs immediate upgrading in order not to deteriorate.
• The bio-oil from HTL can see use as-produced in heavy engines or it can end up hydrogenated or thermally upgraded to obtain diesel-, gasoline- or jet-fuels by existing refinery technology. In this sense, HTL bio-oil is directly comparable to fossil crude oil. This is unique among liquid bio-fuels and means that it can directly enter the existing fuel distribution network for automotive transportation in any concentration, giving it full drop-in properties.

In Denmark, Aarhus University and Aalborg University are in partnership on HTL research at all levels. In Aarhus, Dept. of Chemistry focuses on fundamental understanding of the process and quick surveys of the effects of different feedstocks and catalysts along with subsequent upgrading. Dept. of Agro-Ecology develops energy crops while Dept. of Engineering works on pilot-scale HTL. Energy crop development is also under the microscope at Aalborg University (Dept. of Energy Technology), which focuses on pilot-scale production and process efficiency, as well as upgrading of HTL bio-oil along with end user testing of oils and upgraded distillates in engines and turbines. The Dept. of Biotechnology, Chemistry and Environmental Engineering, AAU Esbjerg, directs its activities toward extracting value not only from the oil, but also from the effluents.

About the HTL process:
• HTL operates in hot, compressed water at >300 degrees C and >200 bar, often assisted by catalysts
• Boiling ends up suppressed, i.e. no energy is expended to overcome the latent heat of evaporation
• Successful HTL is dependent on extremely swift heating and cooling of the biomass, in order to avoid the formation of tar or ash compounds
• Heat energy recycles between the heating and cooling of the process medium
• A typical pilot-scale plant yields 30 liters of bio-oil per day – one such plant is under development at Aalborg University
• An industrial scale plant can be anything from 300 barrels per day up several thousand, comparable to fossil oil wells
• Some HTL has made use of heterogeneous as well as homogeneous catalysts, e.g. ZrO2

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