**2.1 Combustion**

Combustion is defined as high-temperature exothermic oxidation of biomass in the presence of oxygen and the presence of consecutive heterogeneous and homogeneous reactions which resulted in the production of heat as the main product. Combustion is divided into four stages: drying, pyrolysis (de-volatilization), volatiles combustion, and char combustion. As soon as biomass particles enter the burning environment, the particles moisture evaporate, on further heating, volatile gases and tars are released which follow by their combustion. The remaining char will essentially retain its original shape. The process outcomes mostly depend on the properties of the feedstock, particle size, temperature, and combustion atmosphere that can have char and ash (typically includes inorganic oxides and carbonates) as the solid byproducts of combustion [21, 22]. Carbon dioxide (CO2) and water vapor (H2O) are also produced during the complete combustion of biomass, however, it is not achieved under any conditions which cause the production of carbon monoxide (CO), methane (CH4), non-methane hydrocarbons (NMHCs), particulate matter (PM) and nitrogen and sulfur species mainly NOx and SOx during the incomplete combustion the biomass material [23]. The drawbacks are mainly controlled through modification of combustion processes via flue gas recirculation, boiler modification, and re-burning technology which often mitigate such emissions economically [24].

#### **2.2 Hydrothermal conversion**

The hydrothermal conversion process is a suitable technology especially for wet biomass into bio-fuel which is defined as a thermochemical transformation of biomass in high temperatures (100–700°C) and high pressures (5–40 MPa) in a liquid media or hot supercritical water [25]. In hydrothermal liquefaction (HTL) as an important hydrothermal process, raised temperatures (200–350°C) and high pressures (5–20 MPa) in the presence of solvent (sub−/super-critical water) applied to boost biomass decomposition and reformation to produce bio-crude (as the main output) bio-char, water-soluble organic polar fractions and gaseous [26–28]. During the HTL process, several complex mechanisms such as hydrolysis activate which degrade biomass macromolecules and then decompose them into smaller components to reactive fragments by bond cleavage and several reactions such as dehydration, dehydrogenation, deoxygenation, and decarboxylation while some complex chemicals such as bio-crude produce through depolymerization [29–31]. Derived bio-crude oil through HTL shows a higher heating value between 36 to 40 MJ/kg which is close to petroleum-derived oil characteristics [32–34]. HTL technology which is currently at the pilot/demonstration scale has several positive points in comparison to another thermochemical process including the ability to use high moisture content biomass inputs, lower operating temperature, higher throughput, and removal of oxygen from the bio-crude [35, 36]. In addition to biomass feedstock elemental composition, various operational parameters such as temperature, reaction time, pressure, presence of a catalyst (catalyst type and amount), solvent/biomass ratio, and reaction medium influence the process in terms of quantity and quality of produced bio-crude [37–40]. HTL in comparison with other processes reveals various advantages including application feedstock with high moisture content without drying requirement, exploitation of the properties of superheated fluids to reduce mass transfer resistances, and penetration of the solvent to biomass structure to enable the fragmentation of biomass molecules due to high pressure which result to obtain high-quality bio-crude oil [41].

Hydrothermal Carbonization (HTC) is the second hydrothermal conversion which is performed in a temperature range of 180 to 350°C during which the biomass is submerged in water and heated under pressure (2 to 6 MPa) for 5 to 240 min while the main product of HTC is hydro-char [42]. Hydrothermal gasification (Supercritical water gasification) is a thermochemical conversion process in which, wet biomass was directly converted into combustible gases under 400 to 500°C (processed till 700°C) and 24 to 36 MPa pressure with/without catalyst aid. Further, supercritical water is (<374°C, 22.1 MPa pressure) is acting as a reactant and solvent that splits organic compounds. During gasification, decomposition of biomass causes dissolution of reactive species that promote the yield of gaseous products by impeding the biochar production at supercritical [39, 43]. The hydrothermal gasification technologies) have considerable economic, environmental, and technical advantages over other high-demand energy conversion technologies. These processes are compatible with wet feedstock (not suitable candidates for another thermochemical process). Also due to the reactions taking place at lower temperatures (less energy consumption) and the use of a wide range of feedstock processing [44]. Due to the unique dissolution properties of water during hydrothermal gasification, less coke and tars are produced while pressurized produced syngas is typically free from gaseous that do not usually require further processing and can lower compression costs [45].
