Catalytic pyrolysis of Abandoned Fishing Nets using Activated Carbon Derived from Wasted Clamshell with in-situ and ex-situ Catalyst Configuration
Article information
Abstract
Fishing net is considered as one of the biggest problem in the world owing to the release of micro-plastics from abandoned fishing nets, which contributes to marine pollution. Although disposal and recycling strategies are considered as effective methods for overcoming these problems, the pyrolysis of abandoned waste has emerged as a strategy to recover massive quantities of waste materials. In this study, to develop an effective method to valorize abandoned fishing net, the effect of the use of a CaCO3 catalyst after the loading method (i.e., in-situ and ex-situ) on pyrolytic products of abandoned fishing net was investigated using micro-gas chromatography and gas chromatography/mass spectrometry. Compared to non-catalytic pyrolysis, catalytic pyrolysis increased the yield of non-condensable and condensable gas. Particularly, the ex-situ method significantly increased the yield of non-condensable gas to up to 37.2 wt.% at 900. Compared to the ex-situ method, in-situ loading method increased the yield of condensable products to up to 82 wt.%. The understanding of the difference between different catalyst loading configurations will provide useful insight on thermocatalytic waste conversion processes.
Abstract
Graphical Abstract
1. Introduction
Fish is one of the most consumed organic food in the world [1]. According to the Food and Agriculture Organization of the United Nations (FAO), the global per capita consumption of fish was approximately 20.5 kg in 2019 [2]. With the growth of the marine industry, the amount of plastic wastes on the sea has increased as people abandon numerous used plastic materials (fishing nets, buoys) in the sea [3]. Hence, untreated plastic wastes have remained in the ocean [4]. Particularly, fishing net is considered as the most serious problem in marine biology, and it makes up 10% of the total marine waste [5]. For example, abandoned fishing net interrupts the swimming pattern of fishes as the net binds them [6]. In addition, plastic nets are sometimes regarded as food, and are consumed by fishes (or they emit feed smell, which lures fishes into the net, where they cannot escape from) [7]. Consequently, these have resulted in the death of numerous marine creatures and the destruction of the marine ecology [8]. Moreover, the destruction of wasted fishing nets by waves results in the production of micro-plastic, which are very harmful to marine organism [9]. However, these plastic nets cannot be degraded naturally [10]; thus, it is essential to develop an appropriate treatment approach for micro-plastics.
Generally, wasted plastic fishing nets are treated using recycling and landfill strategies. The recycling of plastic nets is a good method for treating wasted plastic fishing nets owing to its ability to remove waste without polluting the environment [11]. However, this method requires a high labor intensity [12] and costs [13]. Landfill is one of the easiest and common method for disposing plastic waste. Although this method is convenient [14] and simple [15], it can pollute underground water [16], occupy large area [17], and emit harmful materials [18].
To overcome the disadvantages of the consisting methods, thermochemical processes such as pyrolysis [19], gasification [20] and liquefaction [21] were emerged. Recently, these methods have been developed to increase efficiency by adding catalysts to obtain high yield of production as well as to suppress coke formation [22]. Thermochemical processes have upgraded the quality of products using two different substances (e.g., biomass and plastic) simultaneously [23–25].
In this study, we performed pyrolysis of plastic to treat plastic efficiently as an effective chemical recycling strategy [26]. Pyrolysis has been used as a promising method to valorize various plastic materials, such as polyethylene terephthalate (PET) [27, 28], low-density polyethylene (LDPE) [29], and polypropylene (PP) [30]. The pyrolysis of plastic materials produces pyrolytic oil and pyrolytic gas in the absence of oxygen. Pyrolytic oil can be used as an alternative fuel because it is composed of various kinds of condensable compounds [31]. In addition, pyrolytic gas produces H2 (Hydrogen) CO (Carbon monoxide), CH4 (methane), and CO2 (Carbon dioxide), as well as hydrocarbons from C1 to C3, which can be used as energy sources [32–34].
Although pyrolysis is a relatively simple method and offers value-added chemicals, it exhibits a high energy consumption owing to the requirement of an external energy supply to sustain the required temperature during the process. To solve this problem, catalysts are employed during pyrolysis. For example, a recent study demonstrated the use of calcinated scallop shells as catalyst during pyrolysis, and they found that the catalyst reduced the peak temperature [35]. In addition, the use of CaCO3 as a catalyst for increasing the quantity of oil production has been demonstrated [36]. Gulab et al. reported that the use of CaCO3 as a catalyst in the co-pyrolysis of biomass and polyethylene increased the yield of oil compared to the yield under non-catalyst condition. Moreover, CaCO3 was observed to favor the formation of aromatic hydrocarbons [37].
Although the degradation of nylon-6 has been widely investigated for a long time, it has rarely been used to recover polymer monomers from waste material [38, 39]. In this study, we performed the valorization of wasted fishing nets through catalytic pyrolysis over two different catalyst configurations (i.e., in-situ and ex-situ) using clamshell waste. Two types of pyrolytic products, including gaseous and liquid products, were identified and analyzed.
2. Materials and Methods
2.1. Materials and Chemicals
Abandoned fishing net and clamshell waste (CSW) were obtained from a harbor located in the city of Buan, Republic of Korea. The obtained plastic net was washed to remove soil, debris, and salt, after which it was soaked in water for 24 h and then dried in a drying oven at 60 ºC for 48 h. Dichloromethane (DCM; 99.9% purity) supplied by Samchun chemical (Seoul, Republic of Korea) was used as the solvent. 5-methlyfurfural (5MF; 99% purity) was purchased from Sigma–Aldrich (St. Louis, MO, USA), and N2 gas was purchased from DK gas (Hwaseong, Republic of Korea).
2.2. Preparation of Activated Carbon Material
CSW catalyst was prepared in this study using the following procedure. First, the purchased CSW was cleaned using water to remove surface impurities, after which it was dried at 60 ºC for 24 h. Subsequently, the dried shells were broken using a mortar and pestle, and then ground into a fine powder using a pulverizer (New Korea Metal Company, Republic of Korea). Thereafter, the powders were separated based on their particle size using a sieve shaker (SS-D-S, Woo Ju Scientific, Republic of Korea) to ensure that the small particles (>600 mm) of CSW powders were obtained. Subsequently, the powders were placed in a tube furnace for carbonization at 400 ºC for 2 h under flowing N2 gas at 100 mL min−1. Thereafter, the powders were mixed with KOH solution (6 M) at a ratio of 1:3, and then dried at 60 ºC for 48 h. The dried powders were heated again using a tube furnace from 400 to 700 ºC with a holding time of 30 min for 1 h at a heating rate of 3 ºC min−1 under N2 gas. Lastly, the obtained activated materials were mixed with HCl solution (2 M) in a water bath at 90 ºC to purify and remove the metal-oxide produced during the carbonization process. The solution containing the powder was washed with deionized water several times until the pH of the solution was neutral, after which they were dried at 60 ºC.
2.3. Feedstock Characterization
Proximate analysis of the wasted fishing net was performed using a batch furnace. First, the moisture content was measured by placing the fishing net on an uncovered crucible in the furnace at 105 ºC for 24 h. Subsequently, the volatile matter content of the feedstock was determined by heating the covered crucible at 450 ºC for 1 h. Thereafter, the ash content was estimated when the open crucible was heated at 750 ºC for 1 h. Lastly, the fixed matter content was calculated by subtracting the moisture, volatile matter, and ash contents from the original mass.
The ultimate analysis of the elemental compositions of the fishing net was performed using a Thermo Scientific FlashSmart 2000 elemental analyzer. The C, H, N, and S contents were calculated using a copper wire and tungstic anhydride catalysts at 1000 ºC. The content of O was estimated by calculating the difference between the initial mass and the sum of ash, C, N, H, and S contents. Table 1 shows the result of proximate and ultimate analysis.
Proximate and Ultimate Analysis of Fishing Net
2.4. Pyrolysis Experiment
Three types of pyrolysis of fishing net (i.e., without catalyst, ex-situ, and in-situ) were conducted using a tube furnace (Tube furnace-60, Hantech, Republic of Korea). Figure S1 shows the scheme of the pyrolysis process. The feedstock was placed in a quartz tube located at the center of the heating zone with a gas mask. For the ex-situ experiment, the catalyst was loaded next to the feedstock between quartz wools. In contrast, for the in-situ process, the catalyst was mixed with the fishing net, after which the mixture was placed between quartz wools. Mass flow controller (KOFLOC, Japan) was connected to a gas mask, through which N2 flows into the quartz tube at 100 mL min−1 to achieve an oxygen-free atmosphere. The pyrolysis temperature was controlled using the temperature controller attached to the tube furnace.
The pyrolytic oil produced during the pyrolysis process was collected using four inpingers. Because the oil was composed of volatile compounds, the inpingers were located in the cold region. The first inpinger contained 40 mL of DCM and it was placed in an ice bath where the temperature was maintained at −1 ºC. Next to the first inpinger, three inpingers were consecutive soaked in cold traps where the temperature was sustained at −55 ºC using a mixture of acetone and dry ice. To collect all the pyrolytic oil, the inpingers, quartz tube, and line were washed with DCM. The collected oil was dried at 60 ºC for 24 h to remove the DCM.
2.5. Pyrolytic Product Analysis
The pyrolytic oil produced during the pyrolysis process was analyzed using gas chromatography/mass spectrometry (GC/MS; Agilent 5975C, USA). The components of the pyrolytic oil were qualified and quantified using an Agilent HP-5ms column (30 m × 0.25μm × 0.25 mm). The detailed conditions, including the temperature, heating rate, and flow rate, are listed in Table S1. The quantitative analysis was conducted via internal standard method using Methylfurfural (5 μg mL−1).
The pyrolytic gas was analyzed using micro GC (Inficon, Switzerland). In addition, H2 and carbon monoxide (CO) gases were quantified using Rt-Molsieve 5A (0.25 mm × 10m); carbon dioxide (CO2), methane (CH4,), and hydrocarbons (from C1 to C4) were identified using the Rt-Q-Bond (0.25 mm × 8 m). The specific conditions are listed in Table S2.
3. Result and Discussion
3.1. Characterization of Fishing Net
Fig. 1 shows the change in the weight of the fishing net samples with a change in temperature under N2 flow. Fig. 1 shows the TGA (thermogravimetric analysis) and DTG (Derivative Thermogravimetry) curves of the fishing net. The first weight loss of the fishing net was observed at 400 ºC, after which the weight rapidly decreased until 500 ºC. With an increase in the temperature to 900 ºC, more than 97 wt.% of the fishing net was thermally degraded, which could be attributed to the thermal decomposition of volatile matter (i.e., devolatilization occurred). However, approximately 3.0% of the fishing net was not thermally decomposed owing to the presence of fixed carbon. This result is very consistent with the proximate analysis result presented in Table 1: the fishing net sample consisted of fixed carbon (0.01 wt.%), volatile matter (97 wt.%), and moisture (2 wt.%) without ash. This indicates the high consistency of the TGA and proximate analysis results. Ultimate analysis also revealed that fishing net were composed of 63.06 wt.% carbon, 12.27 wt.% nitrogen, 10.28 wt.% hydrogen, and 15.35 wt.% oxygen
TGA result of the fishing net.
3.2. Analysis of Non-condensable Gas
Fig. 2a shows the yield of non-condensable gas obtained from the pyrolysis of fishing net with and without catalyst with a change in temperature. The total yield of non-condensable gas increased with an increase in the pyrolysis temperature. For example, there was no significant difference in the total yield of gas at 500 and 700 ºC; however, with a further increase in temperature to 800 °C, the total yield from the ex-situ process dramatically increased from 7.7 to 29.3%, and increased to 37.2% with a further increase in temperature to 900 ºC. This could be attributed to the enhancement of the pyrolysis of volatile substances through gas and gas–solids reactions at high temperatures [40]. Further, the gas with the highest concentration obtained during the pyrolysis processes was CO2 (Fig. 2b). Although there was no significant difference in the evolution of the gas species in the absence of a catalyst, the difference was enhanced when the catalyst was loaded. Particularly, the ex-situ loading method produced the highest quantity of CO2 compared to the non-catalytic and in-situ process. This may be attributed to the generation of CO2 by the decomposition of the CSW catalyst. The catalysts were prepared using CSW, which consists of calcium carbonate (CaCO3). With an increase in temperature, CaCO3 was decomposed into CO2, which was enhanced in the presence of a catalyst. Accordingly, as more CSW was used during the ex-situ process compared to the in-situ reaction, more CO2 was produced during the ex-situ reaction, which was evident at temperatures of 700 ºC or higher.
(a) Yield of non-condensable gas at different pyrolysis temperatures; and (b) product distribution of non-condensable gas at 900 ºC for the non-catalytic, in-situ catalytic, and ex-situ catalytic pyrolysis of fishing net.
As shown in Fig. 3, the major gaseous products from the pyrolysis of fishing net were H2, CO, CH4, and C2H4. In addition, C2H6, C3H6, and C3H8 were observed in the product, but their concentration was not comparable to those of the four major gas (Fig. 2). Further, the concentration of H2, CO, and CH4 increased with an increase in temperature under all conditions; however, that of C2H4 decreased between 500 and 600 ºC, and then increased until 900 ºC. The H2 produced from catalytic pyrolysis was higher than that produced during the non-catalytic pyrolysis at all temperatures, and the difference between the results was not negligible (Fig. 3a). However, at temperatures above 700 ºC, the difference in the H2 yield increased significantly. This could be attributed to the fact that an increase in the pyrolysis temperature promoted the decomposition of vaporized species released from the feedstock during the pyrolysis process [41]. In addition, compared to the non-catalytic pyrolysis, the ex-situ pyrolysis method generated more H2, but this was lower than that generated from the in-situ pyrolysis method. This indicates that the use of CSW catalyst for the pyrolysis of fishing net can increase the production of H2 gas, and the in-situ method generated a higher quantity than the ex-situ method. The quantity of generated CO from the pyrolysis of fishing net is shown in Fig. 3b. Further, the use of CSW enhanced the generation of CO. It was expected that CO formation via reverse water-gas-shift reaction (rWGS, H2 + CO2 −> CO + H2O) could be realized using the CO2 produced from the CSW catalysts. Thus, the CO2 produced by the calcination reaction of CSW catalyst (CaCO3 −> CaO + CO2 +183kJ/mol) was used as a source of the WGS reaction [42]. Compared to the aforementioned gases (Fig. 3c and 3d), there was no significant in the amount of CO2 generated from non-catalytic and ex-situ catalytic pyrolysis with an increase in the pyrolysis temperature. In contrast, the quantity of gas generated by the in-situ method was higher than those produced by other methods. These results offer two messages. First, the effect of catalysts is evidence at temperatures above 700 ºC. Second, the in-situ method is more effective than the ex-situ method for producing combustible gas except CO.
Volumetric portion of (a) H2, (b) CO, (c) CH4, and (d) C2H4 gas produced from the in-situ, ex-situ, and non-catalytic pyrolysis of fishing net under N2 flow.
3.3. Analysis of Condensable Products
The weight of condensable products produced by the fishing net was 48.5 to 88.8 wt.%, and the yield of liquid was highest at 500 and 600 ºC and was lowest at 900 ºC at all conditions. The condensable gas produced from the pyrolysis of fishing net was composed of 3-pyrrolidinopropionitrile, four kinds of amine compounds (azepan-2-one, 7-butyl-3,4,5,6(2H)-tetrahydroazepine, 2,3,4,5,6,7-hexahydro-2-octylimino-1H-azepine, and oleylamine), 4-hexyl-2,5-dihydro-2,5-dioxo-3-furanacetic acid, 13-heptadecyn-1-ol, and 1,8-diazacyclotetradecane-2,9-dione.
Fig. 4 shows the total yield of condensable products (Fig. 4a) and the distribution (Fig. 4b) of the condensable gas obtained from the fishing net with and without catalysts at various pyrolysis temperatures. The in-situ and non-catalytic pyrolysis exhibited similar trend, in which the concentration of condensable products decreased with increasing the pyrolysis temperature. However, there was a slight difference in the trend of the ex-situ pyrolysis compared to the other methods. With an increase in temperature from 500 to 600 ºC, the concentration of condensable products increased, and then decreased until 900 ºC. For example, the amount of condensable products generated during the non-catalytic pyrolysis and in-situ pyrolysis decreased from 45.1 to 33.7 wt.% and from 63.6% to 53.3 wt.%, respectively. In contrast, the concentration of condensable products produced during the ex-situ pyrolysis increased from 46.2 to 50.2 wt.%, and then decreased to 39.3 wt.%. Particularly, the quantity of condensable products generated by the in-situ pyrolysis was higher than those generated by other methods at all temperatures. This result implies that the mixture of solid particles and feed would exhibit an effect on heat and mass transfer [43], as well as increase the intimate contact of the feed with the catalyst and the change in the vapor residence time in the reactor [44]. This is consistent with the result of a recent study on the comparison of the in-situ and ex-situ co-catalytic pyrolysis of high-density polyethylene and torrefied yellow poplar, which revealed that in-situ catalytic co-pyrolysis exhibited higher performance than ex-situ catalytic co-pyrolysis [45]. The distribution of the products obtained from the Non, in-situ and ex-situ catalytic pyrolysis of fishing net at 500 ºC is shown at Figure 4b. Condensable products were composed of amide, amine, acid, alcohol, and ketone compounds. Particularly, amine compounds exhibited the highest proportion (amine compounds consisted of 95.5 wt.%% of the total products) at all conditions. However, the remained products were not produced as high as amine compound. Particularly, in in situ loading method, the proportion of amide, acid, alcohol, and ketone compounds in the condensable gas product was 0.6, 1.1, 0.2, and 3.7 wt.%, respectively. This could be attributed to the composition of fishing net: fishing nets are composed of polyamide, so the high-temperature depolymerization reaction enabled the extraction of the major compounds from the complete product.
(a) Yield of condensable products at different pyrolysis temperatures for the non-catalytic, in-situ catalytic, and ex-situ catalytic pyrolysis of fishing net; (b) Distribution of the condensable gas obtained from the non-catalytic, in-situ catalytic, and ex-situ catalytic pyrolysis at 500 ºC.
4. Conclusions
This study performed the catalytic pyrolysis of fishing net consisting of nylon-6 using CSW catalyst (under in-situ and ex-situ loading conditions) under N2 condition to retrieve value-added chemical material. The study was conducted within the temperature range from 500 to 900 ºC. The total yield of non-condensable gas increased with an increase in temperature. Particularly, at temperatures above 700 ºC, the difference between the yield of non-condensable gas generated during the non-catalytic pyrolysis and ex-situ catalytic pyrolysis increased significantly owing to the occurrence of calcination reaction. However, a higher amount of the major gases, except CO, was produced during the in-situ method compared to the ex-situ method. The HHV of pyrolytic gas was 5.4 MJ/kg, indicating that it can be used as an alternative energy source for the pyrolysis reaction. The generated condensable products were mainly composed of amine compounds at all condition. Particularly, in-situ catalytic pyrolysis produced the highest yield of condensable products at 500 ºC. This work revealed that not only value-added chemical can be retrieved via pyrolysis of marine wastes, such as fishing net, but alternative energy sources could be generated.
Supplementary Information
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1A4A1031357). This work was also supported by C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2015M3D3A1A01064899).
Notes
Declaration of competing interest
The authors declare that they have no conflict of interest.
Author contributions
S.K. (Researcher) conducted all the experiments and wrote the manuscript. S.L. (MS student), H.S.L. (MS student), and W.Y. (MS student) supported to conduct experiments and prepared the manuscript. J.L. (Associate Professor) supervised the experiments and wrote and revised the manuscript.