Abstract
Hydrothermal liquefaction (HTL) of biomass is emerging as an effective technology to efficiently valorize different types of (wet) biomass feedstocks, ranging from lignocellulosics to algae and organic wastes. Significant research into HTL has been conducted in batch systems, which has provided a fundamental understanding of the different process conditions and the behavior of different biomass. The next step towards continuous plants, which are prerequisites for an industrial implementation of the process, has been significantly less explored. In order to facilitate a more focused future development, this review—based on the sources available in the open literature—intends to present the state of the art in the field of continuous HTL as well as to suggest means of interpretation of data from such plants. This contributes to a more holistic understanding of causes and effects, aiding next generation designs as well as pinpointing research focus. Additionally, the documented experiences in upgrading by catalytic hydrotreating are reported. The study reveals some interesting features in terms of energy densification versus the yield of different classes of feedstocks, indicating that some global limitations exist irrespective of processing implementations. Finally, techno-economic considerations, observations and remarks for future studies are presented.
Highlights
It is widely accepted that the need for renewable sources of energy is of utmost importance for the sustainable development of our society
Biomass can be considered the natural substitute for fossil crude oil in the production of liquid fuels and petrochemicals [5]
In this paper, documented reports of continuous Hydrothermal liquefaction (HTL) are reviewed, limited to those applications in which the aim is the production of a liquid fuel from biomass or organic material
Summary
It is widely accepted that the need for renewable sources of energy is of utmost importance for the sustainable development of our society. The liquefaction of solid biomass is a way to increase the energy density, since the energy density of raw biomass is in general quite low This fact directly affects the economic sustainability of a biomass-based process, as it plays a role in the logistics costs. There are essentially two main classes of thermochemical processes able to produce a liquid fuel directly from solid biomass: fast pyrolysis and hydrothermal liquefaction. The former involves a very fast heating of biomass in the absence of oxygen and it is mostly addressed to biomass feedstock with a reduced moisture content (usually lower than 10%), e.g., wood or straw [5,7]. Hydrothermal liquefaction is potentially able to yield a liquid with a considerably lower oxygen content and to work with wet biomass, showing great flexibility in terms of feedstock [9,10]
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