Proposing a New Route to Solve Marine Debris Pollution Issues: Low-Temperature Eco-friendly Pulverization System by Utilizing LNG Cold Energy

Developing an effective and ecient recycling process for marine debris (MD) is one of the most urgent issues to maintain Earth’s sustainability. However, the restricted circumstances for collecting and separating MD in the ocean limit proper MD recycling. Here, we proposed a complete eco-friendly low-temperature MD pulverizing system that utilizes excessive liqueed natural gas (LNG) cold energy (LCE) in an LNG propulsion ship to improve the eciency and effectiveness of MD recycling. The prototype design of the low-temperature pulverization (LTP) system showed that consumable refrigerant (liquid nitrogen) up to 2831 kg per hour could be substituted. Furthermore, we estimated the additional refrigerant needed for desired MD disposal depending on the ship speed to determine the optimal energy requirement. In addition, LTP systems utilizing LCE can signicantly improve the storage capacity by pulverizing bulky MD. To determine the feasibility of LTP for MD recycling, four types of plastics obtained from actual MD from a coastal area in Busan, Korea were classied and tested.


Introduction
Since the 1970s, marine debris (MD) has increased due to rapid industrialization 1,2 . MD can take a severe toll on biological 3-6 , economic 7,8 , and aesthetic (tourism) 9,10 factors. Plastic production, which surged with industrialization in the 1950s, exceeded the cumulative production of 8. 3

billion tons in 2017.
According to Geyer et al., 59% of plastics are left unattended without being recycled or incinerated 11 .
These plastics ow naturally into the ocean from land 12,13 . According to Eriksen et al., there are about 85 to 150 million tons of marine plastic debris (MPD) divided into 5 trillion pieces in the world's oceans, causing severe marine ecological issues 14 . Additionally, due to the COVID-19 pandemic, there are concerns that the increasing use of plastics, including personal protective equipment (PPE), is exacerbating marine pollution 15,16 . The lifetimes of MPD are relatively long and unpredictable. Therefore, they accumulate in the ocean for decades without decomposition 17 . In addition, secondary pollution of MPD owing to the marine environment such as corrosion, adhesion of shell sh, ingestion of marine organisms, leads them to be non-recyclable 1,18 . Currently, many non-pro t environmental organizations are nding ways to resolve the issue of MPD distributed in the oceans, especially the Paci c Ocean [19][20][21][22][23] .
In general, cleaning ships equipped with facilities to dispose of oating and immersed wastes, collect and process MD. Figure 1 shows currently operating or developing MD collection vessels in the coastal and ocean areas. The ship in Fig. 1(a) is equipped with a system to gather plastic and trash at domestic and international locations by targeting local and land-based coastline issues. According to road transportation regulations, the ship in Fig. 1(b) can retrieve, compress, and pack waste. Furthermore, Fig. 1(c) shows a ship that can handle MD on board with an eco-friendly cleaning system that uses renewable energy. This vessel can gasify plastic and use it as fuel. Lastly, Fig. 1(d) shows a ship that collects oating MD in the Great Paci c Garbage Patch (GPGP) and picks up plastic and ghost nets with the support of a U-shaped arm. However, these ships are limited in terms of their operating radius and time because of the limited size of cargo capable of storing MD. Therefore, increasing the size of the MD cargo hold is a critical parameter that should be considered for better MD recycling.
Unfortunately, most collected MD is incinerated and disposed of in land lls, which causes not only severe environmental pollution 24-27 but also additional time and costs. As ecological pollution intensi es, there have been more efforts to increase the e ciency of MPD recycling. According to the Northwest Paci c Action Plan (NOWPAP), current plastic recycling technologies can be classi ed into three categories: material recycling (or mechanical recycling), chemical recycling (or feedstock recycling), and thermal recycling (or energy recovery) 28 . Each methodology depends on the plastic's properties and potential usages of recycled plastics. Notably, all approaches have common issues in terms of pulverizing bulky MPD as a preprocessing step to enhance its portability and readiness for another use 29,30 . However, because of the low melting point of plastic (e.g., thermoplastics (TP)), it is di cult to pulverize plastics into smaller particle sizes. As an alternative, a low-temperature pulverization (LTP) process was proposed to improve pulverizing e ciency [31][32][33][34] . Furthermore, there was an attempt to construct a cooling system by utilizing the cold heat from a liquid gas storage tank such as lique ed natural gas (LNG) [35][36][37][38] .
Additionally, tightened ship emissions legislations have been increasing the demand for LNG propulsion ships. Marine emission legislations (e.g., Tier III requirements of the revised MARPOL Annex VI mandate) have required the reduction of NOx emissions by 20% (by 2020) and 50% (by 2050). Meanwhile, LNG in a cryogenic state uses an eco-friendly fuel in the transportation industry and onshore energy resources.
Accordingly, many nations are making great efforts to demonstrate LNG-fueled propulsion systems 39 since LNG can reduce the energy e ciency design index by 20% [40][41][42] .
According to Tian et al., given the dual-fuel engine ship, about 860 kJ/kg of cold heat is wasted when LNG is vaporized and overheated 43 . Therefore, it is advantageous to improve and/or develop a system to maximize the usage of excessive LCE. The utilization of excessive LCE such as power generation, storage/transportation, desalination can be seen in many studies 35 . For example, in an LNG carrier (LNGC), boil-off gas (BOG) is generated by heat ingress in the LNG cargo containment system (CCS) 44 .
Since BOG can increase the pressure of LNG CCS, it has to go through a relique cation process, thus limiting LCE utilization offshore. Therefore, cryogenic power generation systems through the Organic Rankine cycle (ORC) and Brayton cycle are primarily applied 45,46 . However, the cold heat generated by LNG propulsion ships is less than that of LNGC and has rarely been used.
This study proposes a conceptual design that combines the MD disposal system and the residual cold energy utilization of an LNG-powered ship to build an eco-friendly and cost-effective LTP system. The amount of additional refrigerant used for freezing MD when the LCE system is operational is also quanti ed and evaluated in the prototypical ship for collecting MD. Further, LTP on MPD samples collected from a coastal shoreline of Busan, Korea was tested to show the feasibility of the proposed LTP system. As a result, this study shows that 1) LTP systems can be used to treat MD by processing MD into ner particles to improve the ship storage's capacity, and 2) building LCE-based LTP systems in LNG-fueled propulsion ships can provide an alternative route to improve MD recycling and upcycling to ensure Earth's sustainability.

System Description
Pulverization is an essential process for recycling marine waste. It turns processed plastics into different products in a single form, allowing for consistency in subsequent processes. Furthermore, this pretreatment process can be more economical and e cient if the energy required to collect and preprocess MD is from surplus resources. For example, refrigeration using LCE can reduce initial investment and maintenance costs due to the simpli cation of facilities. In addition, using the existing refrigerant circulation system for the condensation-expansion process when using surplus LCE does not require additional equipment. Figure 2 shows the layout of the main facilities of an MD collection and cleaning ship equipped with an LTP facility. The facility is divided into two parts. The rst is the propulsion part containing the LNG fuel tank. According to the eco-friendly trend in shipbuilding, MD collection and cleaning vessels are using LNG as fuel. LNG in the cryogenic state causes phase changes in the fuel gas supply system (FGSS), resulting in heat exchange. The gas is then combusted to generate the energy needed for power. Propulsion can also be carried out through the direct internal combustion of LNG, but in ships such as ferries, electric propulsion is also applied using an LNG power generator 47,48 . The second is the MD disposal part. In oating MD, collection through a conveyor is effective and can operate at a constant rate to bring the debris from the ocean directly to the storage cargo hold 49 . Furthermore, magnetic separators and dechlorination facilities are included. A detailed description of the pulverization process will be provided later (see Fig. 6). Figure 3 shows a detailed schematic diagram of the system used to freeze MD for LTP. LNG lowers the temperature of ethylene glycol water (EGW) in the heat exchanger of the FGSS 50 . Ethylene glycol is typically used as a heat transfer medium owing to its low freezing point, which suits the low-temperature condition of the LNG stream 35 . Therefore, cold air with the circulating EGW decreases the temperature of MD via contact (i.e., air-blast method). As a result, MD is frozen to a brittle temperature. Furthermore, this LTP system (upper-right side of Fig. 3) supplies continuous cold energy without a heat exchanger.
To evaluate its potential cooling capacity and feasibility, we constructed a prototypical LNG propulsion cleaning ship with proper parameters. The ship has a cargo capacity of 1,300 m 3 for loading MD and is equipped with an LTP facility capable of handling 20 tons of MD per day. The LTP facility operates in two units for cleaning e ciency, considering an eight-hour workload per day. Table 1 lists the speci cations of the prototypical cleaning ship. Based on the ship's speci cation, the heat transfer rate for freezing MD is calculated as follows: where M is the ow rate of MD (kg/h m 2 ), C pM is the speci c heat of MD (J/kg K), T i is the inlet temperature of MD (K), T s is the temperature of MD at the end of the pre-cooling section (K), G is the ow of refrigerant (kg/h m 2 ), C pR is the speci c heat of refrigerant (J/kg K), and T gO is the outlet temperature of refrigerant (K).
If the ow rate of MD is expressed as the ratio of refrigerant ow, the amount of refrigerant needed to pulverize MD (i.e., M/G) can be calculated as follows; 3 .It is worth noting that the available LCE for MD collection and cleaning is limited when the cleaning ship moves at a relatively low speed because of the less excessive LCE. Therefore, it is necessary to determine the MD freezing capacity depending on the speed of a ship, which can be calculated from the fuel consumption. Assuming the prototypical ship is equipped with a Himsen engine (Hyundai Heavy Industry, HHI) and its speci c gas consumption (SGC) based on maximum continuous rating (MCR) is 163.42 g/kWh, the amount of freezing capacity using LCE per hour (W LCE ) according to the output of the ship (P E ) is as follows; 4Furthermore, P E is proportional to v 3 , where v is the ship's speed 58 . Figure 4 shows the calculated W LCE depending on the ship's speed, v. It is worth noting that the estimated W LCE based on MCR, which is less than 10% (around 5 knots in this study), is inaccurate. Therefore, the minimum speed for collecting marine waste is assumed to be 5 knots. In addition, MD collection and LTP are independent processes, suggesting that two processes can be done simultaneously (i.e., independently) when a ship is in operation. However, much fuel is consumed when a ship sails at a high output after MD collection. For example, 1,858 kg of MD can be frozen per hour at the speed of 10 knots/2831 kg at the design speed.
Therefore, more effective LTP can be done at a high speed. In general, MD collection ships need to stay in the ocean for a long time compared to merchant and passenger ships. Therefore, the targeted collection area and LTP throughput should be designed by adjusting the size of the LNG fuel tank. Considering that optimal MD collection is operated at speeds of 5 knots or less, it is possible to freeze up to 250 kg of MD per hour without any additional energy. Therefore, if an additional refrigerant (e.g., LN 2 ) is used, the extra MD can be frozen and pulverized. The additional amount of liquid nitrogen (W LN2 ), needed for over ow MD freezing and pulverizing can be derived as follows from Equations (3) and (4):  Figure 6 shows the detailed LTP process of MD using LCE. The collected waste is classi ed into MD and marine organisms. Since marine organisms such as echinoderms and seaweeds inhabit the seabed, they should be separated. Further, among the classi ed MD, ber-type waste, such as dumped shing nets or rope, are sorted out because entanglement and overload can be induced in the shredding and grinding process 59 . In addition, oating MD may contain metals and/or other high-density materials. In the case of wasted metals, the magnetic separator is used to lter out any pieces. At the same time, high-density materials should be separated through speci c gravity sorting prior to the cutting process. The remaining MD is primarily crushed by a shredding machine. The shredding machine has the advantage of a high grinding capacity. However, the ground particle size is relatively large at ~ 50 mm 60 . Therefore, improving the LTP e ciency requires further processing to a particle size of 20 mm or less. To do this, the particles are stored in a low-temperature freezer (e.g., ~ 233 K) for a while prior to the LTP process. To lower the refrigerant temperature in the freezer, the LCE, which is waste energy, is supplied to the FGSS.
Some collected plastic MD contains chlorine. For example, polyvinyl chloride (PVC) is a TP amorphous with a high molecular compound used in various places due to its low price, rigidity, and high immutability 61 . However, since PVC contains chlorine, many toxic substances such as dioxins and furans may be generated during incineration and thermal decomposition. Therefore, a separate dechlorination process is required 62 . In addition, electrochemical treatment is essential due to the high salinity of MD and wastewater generated from the pulverizing process. IrO 2 electrodes have been widely used for wastewater desalination. However, boron-doped diamond (BDD) electrodes were developed to generate strong oxidizing agents such as OH-. Strong oxidants can react with Cl in plastics (or Cl-of waste seawater) to produce additional oxidants such as hypochlorous acid (HCLO) and perchlorate (CLO 4 − ), which can remove chlorine. Figure 7 shows the schematics of drum-type capacitive dichlorination (CD) equipment with a ball mill reactor and the detailed chemical process related to dichlorination. Drum-type dechlorination facilities are designed to perform plastic dechlorination treatments at a 470 K or higher temperature with BDD electrodes.

Marine Debris Pulverization
LTP, which is a pre-treatment process for waste recycling, is known to improve storage e ciency. Figure 8 shows the typical bulk MD with a small density (106 kg/m 3 ) and a large volume. Therefore, collection bulk MD without processing prevents mass collection. Although collecting oating MD using ocean currents, not loading onto the cargo of a cleaning ship, has been proposed to save on storage space, the usages of this technique are still limited to certain regions and speci c environments (Jambeck and Johnsen, 2015; Sterenborf et al., 2019). However, pulverizing MD into particles smaller than 5 mm increases the density to 420-770 kg/m 3 , increasing the loading e ciency up to seven times. Furthermore, the additional compression process increases the packing density by more than 10 times. Therefore, an energy-e cient pulverizing (e.g., LTP process) and compression process is essential to enhance a ship's cleaning capacity and long-term operation.
A practical test to determine the feasibility of the LTP process of TP-MPD was performed. The MPD used for the pulverization test was collected within the range of 4 km off the coast of Busan, Korea as shown in Fig. 9(a). A cleaning ship operated by the Korean government collected oating MD ( Fig. 9(b)) and seabed MD (Fig. 9(c)). As mentioned in Sect. 2, MD collected by cleaning ships is currently stored in warehouses prior to moving to a land ll or incineration since recycling is ine cient due to contamination and chemical decomposition 64 . In particular, shing nets and rope from shing boats, as shown in Fig.  9(c), are highly corroded and decomposed, so the recycling cost is very high. Furthermore, the processing procedure is very complicated.
Randomly collected oating MD from a conveyor method was primarily classi ed by manual labor into four materials: polyethylene terephthalate (PET), expanded polystyrene (EPS), polyamide (PA), and polypropylene (PP) (Fig. 10(a-d)). PET was acquired through land-based household waste, and EPS was chosen from buoys among the oating waste. PA and PP were obtained from abandoned nets and rope among those dumped from shing boats. Furthermore, classi ed MD was con rmed through Fouriertransform infrared spectroscopy (FT-IR) analysis (Lee et al., 2020) that allowed a comparison with reference materials (Jung et al., 2018). In general, ultrasonic mill, jet mill, and ball mill are used to make ne particles 65-67 . However, it is advantageous to select a cutter mill or an impactor mill for large pulverizing volumes such as waste. Therefore, the impactor mill, which had two opposite blades at 24,000 RPM, was used to pulverize the material while circulating LN 2 to maintain a low temperature. The temperature was set to 220 K followed by an hour of pre-cooling before pulverization. In addition, the temperature was monitored in real time to prevent the increase in temperature during pulverization.
A sieve test was conducted to analyze the pulverized particle size distribution. The sieve sizes were 0.25, 0.5, 1, 2, and 4 mm, respectively. The standard sieve had a squared mesh so that particles could pass through up to times the mesh size for ground particles with irregular shapes as shown in Fig. 11(a). Figure 11(b) shows the sieve test results.
In the case of PET particles pulverized at room temperature (PET-RT), about 76% of the particles failed to pass the largest sieve. In addition, melting and clumping around the edges were observed because of the high temperature of the pulverizing environment (Fig. 11(c)). PET particles pulverized at low temperature √ 2 (PET-LT) around ~ 223 K showed no noticeable edge melting and clumping. In particular, 82% of the particles formed a particle size of less than 4 mm. Since EPS is produced by foaming polystyrene, it comprises a cell structure. The bond was broken between cells when it was ground even at room temperature, while the cell structure of EPS was crushed under low temperature (Fig. 11(c)). It is worth noting that rising temperatures in the pulverizing process cause plastic to melt. Due to the generation of a signi cant amount of endocrine-disrupting chemicals in this process, this should be resolved in the process of recycling 68 . In addition, existing crushing processes produce particles randomly distributed particle sizes, which degrades recycling quality. Uniform ne particles through LTP are eco-friendly and enable high-quality recycling.

Summary
This study demonstrated a prototypical concept for an eco-friendly low-temperature MD pulverizing system that utilizes the cold energy from an LNG-powered cleaning ship. Typical cleaning ships used these days have a limited loading e ciency due to the low bulk density and larger volume of MD. As such, they mainly operate in coastal areas. However, the proposed concept can collect MD in the oceanic region because the MD loading capacity increases by more than 10 times through LTP and compression processes. Furthermore, the energy source for the LTP is mostly from excessive cold energy from LNG propulsion ships, which is essential for the upcoming low-CO 2 emission requirement. It is also expected to dramatically reduce refrigerants used in LTP processes. By utilizing LCE at the ship's designed speed (e.g., 11.5 knots in this study), it is expected that more than 2 tons of MD per hour can be frozen, and 200 kg of MD can be processed per hour even during collection at ~ 5 knots. In addition, the savings according to the ship's output were calculated in the cooling of MD using LN 2 . With an output of 20% MCR, more than 514 kg of MD can be processed per hour without using additional refrigerants. These results suggest that up to 253 kg of LN 2 per hour can be saved during ship operation. To show the feasibility of the conceptual design, we estimated the amount of energy needed for a proper cleaning capacity on various ship speeds. The outcomes are promising even though there is room for improvement. For example, the energy conversion e ciency and optimal system con gurations in designing an LNG-powered cleaning ship can be improved with technological development. To evaluate the low-temperature process for adequate MD pulverization, residual particle size analysis was conducted. The results showed that LTP is advantageous for ne particle production and is eco-friendly by preventing melting. The proposed idea can resolve critical environmental issues in the ocean, but it can be generalized to ensure the utilization of any excess energy in modern industries.  Layout for LNG-fueled propulsion system equipped with an LTP system utilizing LCE.

Figure 3
Schematic diagram of LNG-fuelel propulsion system with LTP system. It contains the LNG propulsion part (blue line), EGW system (green line), and pulverizing chamber with an air-blast freezing system (red line).

Figure 4
Maximum MD freezing capacity according to the ship's speed. Low-temperature MD pulverizing process utilizing LCE.

Figure 7
Schematic of the CD equipment. It uses a BDD electrode instead of a IrO2 electrode.

Figure 8
Page 21/21 Storage e ciency according to the stages of MD treatment. Bulk MD contains seawater and has poor storage e ciency due to its low bulk density. Even if only the pulverizing process is carried out, it is possible to achieve a storage e ciency of more than ve times and is expected to store more than 10 times through compression 69.   (a) Relationship between sieve opening size and particle size. Typically, MPD is not in a cube or spherical shape after grinding, so it is ltered up to particles equal to √2 times the sieve opening size. (b) Sieve test results. More than half of the PET-CT passed through the 1 mm sieve. (c) Photo of pulverized particles. In the case of PET-RT, the melted edges were observed, and the pulverized particles were coarser than PET-LT (Left). In the case of EPS-RT, pulverization was performed in foam cell units (Right).