Fuels from Algae or Waste

Bio-crude is a black, highly viscous liquid. It contains various compounds depending on the applied feedstock. The HTL of protein rich feedstocks result in a nitrogen rich oil. Lignocellulosic based bio-crude from HTL process has higher viscosity. High oxygen and nitrogen contents in the bio-crude make it unsuitable for direct use. But compared to bio-oils from pyrolysis bio-crude from HTL has lower oxygen and water contents.

In the upgrading step oxygen content has to be decreased and the hydrogen content has to be increased to receive high quality hydrocarbon fuels. Suitable processes and catalysts for upgrading bio-crude from HTL are still under development, especially if it comes to integration of all process steps in one facility.

Hydroprocessing, already discussed in Lesson 2.2.2., can be used to upgrade bio-crude. By applying hydrogen, 300-400°C, and 150-300 bar, undesirable compounds such as oxygen-containing compounds (e.g., phenols, ketones, and fatty acids) can be removed. This process is also known as hydrodeoxygenation. Suitable catalysts include ruthenium on carbon, cobalt-molybdenum, nickel-molybdenum, or sulfide catalysts.

Although several pilot plants using HTL technology are operating continuously, further research is needed to scale up to commercial levels. Specifically, heat recovery, reactor scale-up, the pumpability of slurries with high solids content, and upgrading systems need further development (Biller & Roth 2018).