Hydrothermal conversion of biomass has been known since the 1970’s, and the first relatively pilot plants were built in the following years. During that time, the term Hydrothermal Liquefaction (HTL) was used to describe the process, in simple terms a conversion of solid (waste) biomass to a liquid product – bio-crude- employing high temperature and pressure in the presence of liquid water. Essentially, the process resembles the process that has taken place over millions of years in our planet converting sedimented biomass to crude oil, with the distinct difference that the process takes place in minutes rather than millennia.
Hydrothermal Liquefaction should not be confused with various pyrolysis processes taking place at even higher temperatures without water. The pyrolytic oils produced will have a significantly higher oxygen content and thus be less stable. The pyrolytic processes are fast and the often the pyrolysis equipment has lower CAPEX. Nevertheless, in most cases the process will not be competitive in terms for overall economy
Another alternative is the Bio-Diesel process, in which oleanous feedstocks are converted via esterification and/or trans-esterification of vegetable oils – so called first generation feed stocks.
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