**3.4 Hydrothermal process**

HTL is emerged to be the most promising method to convert wet algal biomass to liquid fuel and various value-added products. The process is carried out at a low temperature, usually 300–350°C, and high pressure (5-20 MPa) condition with the help of a catalyst and in the presence of hydrogen and yields bio-oil [35, 36]. The process effectively converts the biomass with water activity into smaller molecular components with high energy densities. The drawback of the conventional HTL method paves the way for the two-stage sequential hydrothermal liquefaction (SEQHTL) method, which overcome the limitation of the conventional method in recovering bioactive compounds [37].

In an experiment given as an example, nine species of algae were selected in order to perform HTL at temperatures of 280°C and 320°C to find out the effect of the biochemical composition of the species on bio-oil yields and properties at two different temperatures. They got maximum bio-oil yield at a temperature of 320°C in the algae *Nannochloropsis,* which contains high lipid content [38]. It has been found through a microchip known to control high temperature and pressure that allows the HTL process in situ using fluorescence microscopy [39]. It requires a thermochemical process to convert the algal biomass into biochar products. The process involves heating algal biomass in water at the temperature of 200°C under pressure less than 2Mpa within 60 min of residence time. The process is exothermic and spontaneous [40]. In an experiment, lipid was extracted from Picochlorum oculatum. It was used as an algal biomass for the conversion of algal hydrochar via hydrothermal carbonization and the resultant hydrochar were found to be a promising adsorbent for metal remediation of wastewater [41].

#### **3.5 Torrefaction**

The torrefaction process is designed to offset the drawback of microalgae's poor calorific value. These are the pretreatment process to improve the physicochemical properties of algal biomass and thereby improve the fuel characteristics of algal biomass. The process involves the thermal degradation of algal biomass in an inert or nitrogen environment at one atmospheric pressure and 200–300°C temperature at a residence time of 10 to 60 min [42]. The torrefaction process gives rise to solid biochar. The efficiency of the process can be influenced by certain factors such as temperature, residence time, and composition of the biomass [43]. The torrefaction process shows high similarity with pyrolysis, but the process needs low operating temperatures, so it is called mild pyrolysis [44]. During the process, carbohydrates, proteins, and lipids are all degraded at varying rates resulting in partial carbonization. Few algae such as *Chlorella sp., Nanochloropsis sp.* are analyzed and their thermal degradation of carbohydrates, proteins, and lipids are demonstrated where the activation energies of carbohydrates, lipids, and proteins are in the range of 53.28–53.30, 142.61–188.35 and 40.21–59.23 KJ/mol and the thermal degradation of carbohydrates, proteins, and lipids, are in temperature ranges of 164–497, 209–309, and 200–635°C, respectively. Torrefaction is classified into conventional, microwave, wet, and oxidative torrefaction. These are again categorized as light (200–235°C), mild (235–275°C), and severe (275–300°C) torrefaction depending on the torrefaction temperatures [36–38].
