**2. Hydrothermal liquefaction (or upgrading) of biomass**

Researches related to hydrothermal liquefaction of biomass have gained significant momen‐ tum during the last decade. Search for published literatures under the keywords "hydrother‐ mal liquefaction" using SCOPUS database resulted into heavy turn outs of about 257 published articles in the recent 5 years as compared to only 40 articles during the previous 5-year term. Several review articles of the method have also been published recently including the most

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comprehensive review of the technology summarized by Peterson et al. [3]. Reviews on its application to various biomass components such as protein, carbohydrates, lignin and fats have also been reported [4]. The most recent applications to the liquefaction of microalgae and marine biomass have also been summarized [5–8]. This hot-compressed water has also been compared with other hydrolysis methods [9] and shows it to be very attractive economically and environmentally because of higher sugar recovery even in the absence of acid and chemical catalysts.

The increasing popularity of this method is due to the deemed tremendous opportunities that biomass of any sources can offer as energy carrier, combined with the promising properties of water at high temperature and high pressure, especially under sub- and supercritical condi‐ tions.
