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][3,4]. There are some articles on macroalgae processing with the production of bioethanol [5], biogas ^[6][7][6,7], biofuels ^[2][8][9][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 CO₂ 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 beaches by 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][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][15,16,17], the need for preliminary preparation of biomass, and the high cost of extracting target products ^[18][19][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 °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]. Despite said general dependency of the reaction type on the water conditions, there are studies reporting that ionic reactions can still happen in supercritical water, as was shown by Ciuffi et al. [22].

The process is typically carried out in an aqueous medium at temperatures ranging from 250 °C to 380 °C and pressures ranging from 4 to 25 MPa [23]. The usual dry matter content in the introduced mixture is from 5% to 20% [24], so the biomass can be processed without preliminary drying.

Hydrothermal liquefaction, like conventional pyrolysis, yields three main products: bio-oil, solid residue, and gas phase, as well as an aqueous phase containing soluble products. Liquid organic fraction (bio-oil) is obviously the most valuable product among them. In this regard, the majority of current research is devoted to the search for methods to increase the yield of bio-oil, including the use of catalysts ^[25][26][25,26], the selection of solvents for the extraction of bio-oil from an aqueous solution [27], and the joint processing of various types of raw materials [28].

Using the HTL process typically results in higher quality fuel after drying. As a result, the fuel has a lower oxygen content and a higher calorific value (HHV 33–36 MJ/kg) and thus requires much less effort to condition than pyrolysis oil [29]. According to the literature, hydrothermal liquefaction ^{[30][31][32][33]}[30,31,32,33] can also provide fuel with a lower cost price of $0.57–1.2 per L (for gasification processes this figure is 1.1–1.73 [12] and for pyrolysis is 1.54–1.78 [12]), while the fuel will be of reasonably good quality. The ability to process wet raw materials with a moisture content of up to 80% (for example, sewage sludge, manure, algae, etc.) without the need for drying or other pretreatment is one of the benefits of HTL [34]. Thus, the HTL process should be considered the most promising for the direct transformation of wet algal biomass.

According to the literature data, a mixture of drifting macroalgae is inevitably contaminated with impurities of foreign fractions to one degree or another. The world’s total plastic reserves in the oceans amount to five trillion elements weighing more than 0.25 million tons, while in the surface layer of water, the concentration of plastic reaches half a million elements per square kilometer [35]. Plastic marine debris significantly affects the quality and safety of the aquatic environment, posing a threat to aquatic organisms and human health, and becoming a problem on the path to ensuring food safety ^{[32][33][34][35][36][37][38][39]}[32,33,34,35,36,37,38,39].

Polymers account for approximately 60–90% of the total mass of marine waste, according to an analysis of the component composition of marine debris ^[38][39][38,39]. Typical polymers and polymer-containing composite in marine debris are high- and low-pressure polyethylene, polyethylene terephthalate, Tetra Pak, polypropylene, and nylon ^[39][40][39,40].

There are few studies on the hydrothermal liquefaction of polymers or their mixtures ^{[40][41][42][43][44][45][46][47][48][49]}[40,41,42,43,44,45,46,47,48,49], and only a few studies are devoted to the joint recovery of biomass of macroalgae and polymers to produce liquid fuel ^[50][51][50,51].

2. Prospects for Joint Processing of Macroalgae Biomass and Marine Debris

There are few studies on the HTL of polymers or their mixtures ^{[40][41][42][43][44][45][46][47][48][49]}[40,41,42,43,44,45,46,47,48,49] and only two studies are devoted to the joint recovery of macroalgae and polymers with the production of liquid fuel ^[50][51][50,51]. In this connection, it was decided to consider the results of studies pertaining to the HTL of polymers with/or without adding other types of biomasses and to assess the perspectives of their joint processing with algae. Table 13 presents an overview of current research in the field of HTL of polymers and their mixtures with biomass.