## 1 Introduction
### Microalgal Biomass Processing Technologies  
In general, the conversion of biomass into fuels is performed through thermochemical, biochemical, and physicochemical processes (Figure 1). The conversion of microalgae biomass is mainly performed by thermochemical methods, which are classified into five categories: liquefaction, pyrolysis, gasification, carbonization, and combustion. The method chosen each time depends on a series of parameters, with the required final product and its application playing a dominating role. Figure 1: Biofuel production from microalgae. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & **Criterion** & **Description** \\ \hline
1 & Biomass quantity & When it is required to produce a large amount of biomass; therefore, the application needs a large area. \\ \hline
2 & Biomass quality & When the quality of harvested cells should be good and not release organic matter. \\ \hline
3 & Cost & When low operating costs are needed, it is the first requirement. \\ \hline
4 & Process time & When harvesting should be performed at a fast pace \\ \hline
5 & Special species of microalgae & When a method specific to the needs of a particular species of microalgae needs to be applied. \\ \hline
6 & Toxicity & When the biomass produced requires as a first requirement the absence of toxicity. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Basic criteria for choosing a harvesting method, inspired by [14]. Hydrothermal liquefaction (HTL) achieves the conversion of microalgae biomass, without a pre-treatment stage for drying, and thus avoids operating and energy costs, which due to the increased water content (-80%) of biomass would be particularly increased. In addition, heat transfer is successfully exploited due to the small size of the microalgae, and this supports the efficiency and cost-effectiveness of the process. During the liquefaction process, the microalgae biomass, in the form of a slurry, enters a reactor, where using an aqueous medium, it is converted into liquid fuel under high pressure conditions (5-20 MPa) and at temperatures from 250 to 370 \({}^{\circ}\)C. Usually the biomass of microalgae makes up 5-20% of the total power supply of the reactor, while the rest is the liquid medium. When water is used as a liquid medium, the main product is a mixture of crude bio-oil that is more deoxygenated and degraded than that produced by the pyrolysis method. It can be refined to be used in a variety of ways because it is similar to crude mineral oil. The process residue is bitumen, a mixture of cyclic and polycyclic high-boiling hydrocarbons (such as 2-5-ring aromatics). The conditions applied to liquefaction may change and affect the final products produced, depending on their end use. Below 250 \({}^{\circ}\)C, hydrothermal carbonization takes place, as hydrochar (carbon residues), a material similar to coal (coal), is produced as the main product, while when the temperature rises above 380 \({}^{\circ}\)C, the process is referred to as hydrothermal gasification and its main product is syngas. The advantage of gasification over liquefaction is that the syngas produced is a mixture of hydrogen, carbon monoxide, carbon dioxide, and methane, which does not contain nitrogen and has a high energy content [9]. A major disadvantage of liquefaction is the large amount of organic material remaining in the process fluid, resulting in loss of energy charge and thus hindering the cost of the process. This liquid contains from 8.01 to 21.55% of the carbon of microalgae, in the form of acetic acid and acetamide, and the difference in distribution is due to the type of microalgae. In addition, when it is discharged without treatment into the environment, it creates an environmental burden. Therefore, efficient management or reuse of the liquid medium is an important stage for the economic viability of the process. By gasifying biomass with supercritical water (SCWG), residual organics can be converted into hydrogen and methane and easily separated from water.