As early as the 1920s, the concept of using hot water and alkali catalysts to produce oil out of biomass was proposed. In 1939, U.S. patent 2,177,557, described a two-stage process in which a mixture of water, wood chips, and calcium hydroxide is heated in the first stage at temperatures in a range of , with the pressure "higher than that of saturated steam at the temperature used." This produces "oils and alcohols" which are collected. The materials are then subjected in a second stage to what is called "dry distillation", which produces "oils and ketones". Temperatures and pressures for this second stage are not disclosed.
These processes were the foundation of later HTL technologies that attracted research interest especially during the 1970s oil embargo. It was around that time that a high-pressure (hydrothermal) liquefaction process was developed at the Pittsburgh Energy Research Center (PERC) and later demonstrated (at the 100 kg/h scale) at the Albany Biomass Liquefaction Experimental Facility at Albany, Oregon, US. In 1982, Shell Oil developed the HTU™ process in the Netherlands. Other organizations that have previously demonstrated HTL of biomass include Hochschule für Angewandte Wissenschaften Hamburg, Germany, SCF Technologies in Copenhagen, Denmark, EPA’s Water Engineering Research Laboratory, Cincinnati, Ohio, USA, and Changing World Technology Inc. (CWT), Philadelphia, Pennsylvania, USA. Today, technology companies such as [http://www.licella.com.au Licella/Ignite Energy Resources] (Australia), [https://arbiosbiotech.com Arbios Biotech], a Licella/Canfor joint venture, [http://altacaenergy.com Altaca Energy] (Turkey), [https://circlianordic.com/ Circlia Nordic] (Denmark), [http://steeperenergy.com/ Steeper Energy] (Denmark, Canada) continue to explore the commercialization of HTL. Construction has begun in Teesside, UK, for a catalytic hydrothermal liquefaction plant that aims to process 80,000 tonnes per year of mixed plastic waste by 2022.