Already in 1944, Berl34 reported that the biomass could be converted in hot compressed water into a petroleum-like product. In the 1970s and 1980s, the interest in alternative energy sources, such as biomass, was high due to the oil crises. Liquefaction research was started in 1971 by the US Bureau of Mines,27 with conversion of carbohydrates in hot compressed water in the presence of CO and Na2CO3. This combination of CO and Na2CO3 was reported in early HTC
developments to produce in-situ hydrogen,27 but Molton et al.24 showed that the use of CO in combination with alkali only leads to a limited increase in the oil yield.

Early work by the US Bureau of Mines led to the development of an 18 kg wood per hour process development unit (Albany pilot plant).35 In this installation, Douglas fir was liquefied, first using the product oil itself (the ‘Pittsburgh Energy Research Center/PERC process’) and later using water (the ‘Lawrence Berkeley Laboratories/LBL process’) as a carrier (see Fig. 18.5). For the LBL process, slurries, formed from acid pre-hydrolyzed wood chips and water, were used as

feedstocks. Operating problems led to several process modifications. However, not all issues were completely successfully resolved.36 This, along with a large number of parameters that needed to be studied,37 caused a shift to research in a much smaller scale (continuous 1 l autoclave).37,38

HTL, using biomass/water slurries of high organic/water ratios, was studied at the University of Arizona39-41 and the University of Saskatchewan42-44 by using special feeding systems.

Another important development involved sewage sludge treatment in the so-called sludge-to-oil reactor system (STORS). This process was developed using autoclaves and continuous installation with the capacity of 30 kg of concentrated sewage sludge (20 wt.% solids) per hour in the Battelle Pacific Northwest laboratories of the US Department of Energy.45 Sodium carbonate was employed as a catalyst.

After a period of reduced attention, the interest in conversion of biomass into energy carriers was renewed in the mid-1990s driven by political, environmental and economical incentives. For example, work on the Hydro-Thermal Upgrading (HTU®) process, developed during 1980s in the Shell Laboratories in Amsterdam, was restarted using a bench-scale experimental setup (10 kg water-biomass slurry per hour)5 and a pilot plant (20 kg dry matter per hour).4 To the best of the authors’ knowledge, this plant is now mothballed.

Several demonstration and (semi) commercial activities can be identified as well. A five 5-ton per day STORS process demonstration plant was built in Japan, with the aim of converting sewage sludge into a combustible energy carrier (see Fig. 18.5).46 After a successful municipal wastewater treatment STORS demo project in Colton, California, ThermoEnergy (USA) has patented the improved wastewater treatment process marketed under the name ‘Thermofuel process’. EnerTech Environmental Inc. (USA) is also developing a process for converting sewage sludge into a solid energy carrier, the ‘Slurrycarb process’. The company operates a 1-ton per day process development unit, a 20-ton per day process demonstration unit in cooperation with Mitsubishi Corporation in Ube City (Japan), and is currently commissioning a commercial-scale facility in Rialto, California. When completed, the installation will convert more than 880 wet tons of bio-solids per day from five municipalities in the Los Angeles area into approximately 170 tons per day of the product called E-Fuel.

Changing World Technologies was developing a so-called thermo­depolymerization and chemical reformer process for conversion of turkey waste (carcasses) to fuel products and fertilizer. The company used a 15-ton per day pilot plant and a 200-ton per day processing unit (the Renewable Environmental Solution unit in Carthage, Missouri).

From this overview, it appears that the HTL of specific feedstocks to hydrophobic fuels for combustion (specifically solids) is nearing commercial operation. On the other hand, application of HTL for broader range of feedstocks and for production of transportation fuel precursors is still in the development stage.

18.2 Current research

Next to the pilot plant studies, a significant amount of laboratory-scale research was performed over the last four decades.23,26,47-52 In the past and also currently, this research has been dominated by chemical and kinetic studies. Mostly, these investigations use model components instead of real biomass. Recently, several complete reviews have appeared on these items.13,29-31 There is hardly any process development research ongoing. Also, the link between the insights gained by the chemical research with possibilities for process improvement is not well worked out. It is interesting to note that several research groups have realized that the knowledge obtained from HTL research is very useful for the development of other processes such as high-pressure thermal treatment of bio-liquids (HPTT),53,54 hydrodeoxygenation (HDO)55-58 and solvolysis.59,60 In particular, the knowledge of polymerization reactions of biomass’ decay products in HTL has provided, and can still further offer, many insights in the mechanisms and problems in HPTT, HDO and solvolysis.