**2.3 Lignin hydrothermal liquefaction, HTL**

Recently, HTL has been extensively studied as a biomass conversion method with water as a reaction medium. HTL is an efficient process of lignin transformation during which pyrolysis and hydrolysis take place simultaneously [14]. HTL is considered an eco-friendly and sustainable technology with the advantages of short residence time, high conversion rate, and less pollution.

Although lignin is abundant in plethoric quantities naturally, HTL has not reached the industrial scale, not even the pilot level because of troublesome depolymerization and separation. In contrast, lab-scale studies have been successful in lignin transformation in which 60 wt% low-weight aromatic molecules were obtained from oxidized lignin [21].
