**2.4 Lignin hydrothermal carbonization, HTC**

HTC is an eco-friendly, convenient, and low-cost method [22]. Comparably, HTC offers distinctive advantages, which are not limited to no need for pre-drying, low processing temperature (180−350°C), avoiding air-polluting nitrogen oxides, and sulfur oxides dissolution in water [23]. During HTC operation, lignin is heated in a highpressure autoclave in the presence of subcritical water and pressure ranging between 2 and 10 MPa. Reaction mechanisms comprise hydrolysis, dehydration, recondensation, decarboxylation, and aromatization [15], which are collaborative and concurrent. Similar to pyrolysis, key roles are played by the reaction temperature, residence time, and source of raw materials in the HTC process that yields hydrochar [24].
