## 1 Introduction
### Microalgal Biomass Processing Technologies
Supercritical or superheated water is water of 374 \({}^{\circ}\)C at a pressure of 218 atm, where a liquid homogenized phase is formed that has gaseous and liquid properties. With this method, 99.2% of the carbonate loads of liquefaction water have been recorded, as C-C bonds are broken down, allowing its reuse in microalgae cultures. From the complete gasification achieved, the hydrogen generated can be applied to the liquefaction of microalgae biomass to produce crude bio-oil. After being treated with supercritical water, minerals recovered from liquefaction water can be utilized as fertilizers for plants or other biomass sources. Lastly, from an energy standpoint, compared to HTL alone, the combination of HTL and SCWG can increase energy recovery from microalgae biomass by 5.53 to 18.30% [18]. Pyrolysis achieves decomposition of microalgae biomass under temperature conditions (400-600\({}^{\circ}\) C) and in an inert environment with main products, bio-oil, solid biochar, and gases such as synthetic gas. The produced bio-oil is more stable than that of liquefaction and its quality depends both on the operating parameters of the pyrolytic process and on the type of microalgae. Pyrolysis can be carried out at slow rates of temperature increase (5-10 \({}^{\circ}\)C/min) and last an hour, so it is referred to as slow pyrolysis, while when carried out at 600 \({}^{\circ}\)C for a few seconds, it is defined as rapid pyrolysis, and its application is more widespread, especially in continuous processes [9]. Catalysts contribute to increasing the efficiency of the pyrolysis process and shortening its timing. Catalytic cracking can increase product selectivity or reduce the effect of reaction conditions, and can be applied either on-site or off-site. For on-site catalytic pyrolysis, the raw material and catalyst are mechanically mixed and then fed simultaneously into the pyrolysis reactor. For catalytic pyrolysis outside, the products, immediately after the reactor, pass through beds, which contain the catalyst. Because pyrolysis generally requires the moisture content of the mate rial not to exceed 10% wt., microalgae biomass should undergo dehumidification, which can be achieved either by dissolved air flotation or centrifugation [19]. Gasification is a process applied at temperatures above 700 \({}^{\circ}\)C and under controlled oxygen conditions. It is mainly applied to low-moisture biomasses, but can also be used for microalgae biomass. It can be either conventional or supercritical gasification. Conventional gasification is oxidized with air, oxygen, or steam in a gasifier. The temperature in the gasifier is between 700 and 1000 \({}^{\circ}\)C and the process steps include drying of the remaining moisture, pyrolysis to break down the solid mass into smaller molecule structures, oxidation of biomass, and finally gasification of pyrolysis products. The products produced by the gasification of microalgae are synthetic gas, char, tar, and ashes. As mentioned above, supercritical gasification is more efficient for microalgae biomass than the conventional method due to its increased humidity. It is applied under 400-500 \({}^{\circ}\)C and 24-36 MPa, with products such as those in the conventional method [9]. Hydrothermal carbonization of microalgae biomass is a mild process carried out between 150 and 250 \({}^{\circ}\)C at a pressure below 100 bar with long lead times, usually over an hour. The main product of the process is a solid substance, biochar. This contains essential nutrients and minerals, and can be used as fertilizer in agricultural crops and many other applications. Experimental yields of 60% weight in less than an hour at 200 \({}^{\circ}\)C at a pressure of 20 bar have been recorded. The carbon content derived from carbonization varies between 20 and 60%, and the exact percentage depends on the type of microalgae and operating conditions. In addition, the char produced absorbs the hydrophobic lipids present in the biomass mixture, which are recovered using solvents [9].