# Review of Studies on Joint Recovery of Macroalgae and Marine Debris by Hydrothermal Liquefaction
## 2 Experience of Hydrothermal Liquefaction of Macroalgae  
### Using Macroalgae for Biorfied Production in a One-Step Process  
**Macroalgae composition analysis. ** The total lipid content of macroalgae ranges from 1% to 6% (dry weight), which is much lower than that of microalgae (7-23% d.m. ), and the ash content of macroalgae ranges from 21% to 90% (dry weight) depending on the species [31; 52; 53; 54]. Analysis of literature data and preliminary field studies carried out in the summer and autumn of 2021 in the Kaliningrad region showed that the dominant species are macroalgae of four genera: _Ulva_ sp., _Cladophora_ sp., _Furellaria_ sp., and _Polisyphonia_[55]. Table 1 presents the main parameters of their physical and chemical composition [55; 56; 57; 58; 59; 60]. According to some studies, the main disadvantages of marine multicellular algae, from the point of view of obtaining fuel, are low lipid content, large size, and a rigid cell wall which does not allow the use of algae directly as raw material in a one-stage mode; a preliminary processing stage is required [56; 57]. A recent study of the composition of 10 macroalgae species found in the Red Sea off the coast of Jeddah showed a slightly different picture. A fairly high lipid content was \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Species** & _Derbesia_ & _Ulva_ & _Cladophora_ & _Polisyphonia_ \\ \cline{2-5}  & & **Value in Mass \% d.m.** & \\ \hline Ash & 34.7 & 30.7–31.4 & 17.8–25.5 & 15.1–32.32 \\ Moisture & 6.4 & 7.2 & 5.7–6.7 & - \\ Lipid & 10.4 & 8.3–12.5 & 4.6–5.3 & 0.33–2.78 \\ Protein & 21.6 & 14.27–16.35 & 17.8–26.8 & 11.7–34.45 \\ Carbohydrates & 26.9 & 28.65–43.6 & 44.4–45.4 & - \\ C & 29.2 & 27.7 & 30.9–37.5 & - \\ H & 4.8 & 5.5 & 5.0–5.9 & - \\ O & 27.4 & 41.1 & 32.9–34.9 & - \\ N & 4.5 & 3.5 & 5.2–6.5 & - \\ S & 2.8 & 5.0 & 1.8–2.3 & - \\ HHV (MJ/kg) & 12.4 & 11.7 & 12.7–16.4 & 15.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The composition of the Baltic Sea macroalgae. found in the biomass of _Ulva lactuca_ (12.5%), _Padina boryana_ (10.9%), and _Ulva intestinalis_ (11.6%) [58]. [PERSON] et al. [59] also studied several types of macroalgae for biofuel production and determined that the algae _Derbesia tenuissima_, _Ulva olnoi_, and _Oedogonium_ sp. are promising candidates in terms of high bio-oil yields and quality. The same research group conducted an economic analysis of the process and concluded that protein extraction before HTL will increase the value of the feedstock compared to one-stage hydrothermal liquefaction [60] and will provide additional economic benefits from protein sales. Careful consideration of the data on the macroalgae composition shows their rather low calorific value in comparison with woody biomass (for example, for conifers HHV is 18.66 MJ/kg), which is associated with the high moisture and ash content in algae (for example, the ash content of coniferous firewood species is lower by an order of magnitude and is no more than 3.2% of dry weight) and a low carbon concentration in the composition of macroalgae (1.7 times lower than in coniferous wood). Simultaneously, there is a significant content of nitrogenous compounds (primarily proteins), posing an obvious task for scientists to search for alternative methods of transforming macroalgae that ensure maximum utilization of algae resource potential. **Main results of hydrothermal liquefaction of macroalgae. ** Analysis of the presented means of processing macroalgae show that one-stage hydrolysis without catalytic systems provides a low degree of conversion. [PERSON] et al. used the macroalgae _Sargassum tenerrinum_ for the production of bio-oil by hydrothermal liquefaction. The process was carried out at 260-300 \({}^{\circ}\)C; the reaction time was 15 min. The maximum oil yield (16.3 wt%) was obtained at 280 \({}^{\circ}\)C. The analysis showed that the content of organic acids in bio-oils decreased with an increase in temperature from 260 to 300 \({}^{\circ}\)C [61]. Similar results were obtained by [PERSON] et al.