# Review of Studies on Joint Recovery of Macroalgae and Marine Debris by Hydrothermal Liquefaction
## 1 Introduction  
Nowadays bursts of macroalgae development have become a frequent occurrence and are associated with an increase in anthropogenic load on the hydrosphere objects and climate change. Objectively, there has been a three-fold increase in the number of recorded cases over the past 30 years [1]. Rotting algae becomes a source of greenhouse gases, creating a secondary anthropogenic load on the biosphere. The potential for the formation of greenhouse gases for algae is relatively high and amounts to 498.75 mL per 1 g of dry matter [2]. At the same time, the biomass of macroalgae has significant resource potential [3, 4]. There are some articles on macroalgae processing with the production of bioethanol [5], biogas [6, 7], biofuels [2, 8, 9] nutrients [10], and biologically active substances [11]. It should be noted that the processing of drifting macroalgae into fuel will allow for solving two problems of reducing the climatic load at once: namely, it will ensure the prevention of methane formation in the decay process and will make it possible to advance in solving the key problem of modern society--the reduction of CO\({}_{2}\) emissions from the use of fossil fuels [12] and their replacement with carbon-neutral sources. This article is focused on recycling drifting algae that washed ashore and has become, in fact, waste. Such algae, together with marine garbage, are collected from the beachesby special sieving machines and stored in heaps until they are taken to a landfill. The accumulation of such biomass varies significantly depending on the season and geography of the coast. The problem of processing algae that has both drifted and washed ashore in order to produce fuel is due to their high moisture content, composition variability, seasonal fluctuations in volumes, and high degree of contamination with foreign impurities. Technologies for direct conversion of biomass into liquid fuel can be conditionally divided into two groups: biochemical and thermochemical. The most suitable processing method depends on the amount and type of biomass, the desired form of energy produced, end-use requirements, environmental standards, and economic feasibility [13; 14]. The major problems associated with the biochemical production of biofuels from algae are low fuel yield (no more than 50%) [15; 16; 17], the need for preliminary preparation of biomass, and the high cost of extracting target products [18; 19]. Direct thermochemical conversion of algae biomass into liquid fuel involves pyrolysis and hydrothermal liquefaction. The main problem of producing biofuel from biomass by pyrolysis is associated with the need to dry raw materials to a moisture content of no more than 10%, which significantly worsens the technical, economic, and energy parameters of the process [20]. Due to the heterogeneity of raw materials, there is a risk of poisoning catalytic systems [21], and the fuel yield does not exceed 30%. Hydrothermal liquefaction (HTL) refers to a thermochemical process in which biomass and organic waste are decomposed by the action of supercritical (i.e., the water pressure and temperature are above the critical point of 374 \({}^{\circ}\)C and 22.1 MPa) or subcritical water (i.e., when the water temperature is significantly below the critical point). Under such conditions, water possesses certain unique properties. For example, near the critical point, the dissociation constant of water increases sharply, forcing water to act as an acidic or basic catalyst for numerous reactions which makes it possible to chemically transform almost all types of organic compounds through hydrolysis, ionic, or radical reactions [12]. Reactions proceed in sub- and supercritical water in fundamentally different ways. In general, ionic reactions are peculiar to the hydrothermal liquefaction in the subcritical water conditions, whereas reactions occurring in supercritical water are the free-radical reactions [12].