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Experimental investigation on in-situ hydrogenation induced gasification characteristics of acrylonitrile butadiene styrene (ABS) microplastics in supercritical water
Fuel Processing Technology 192 (2019) 170–178
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Experimental investigation on in-situ hydrogenation induced gasification characteristics of acrylonitrile butadiene styrene (ABS) microplastics in supercritical water
T
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Bin Bai, Hui Jin , Shixing Zhu, Pengfei Wu, Chao Fan, Jingli Sun State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi'an Jiaotong University, 28 Xianning West Road, Xi'an 710049, Shaanxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Supercritical water ABS microplastics Gasification In-situ hydrogenation
In order to solve the ecological and environmental problems caused by microplastics remaining in the ocean and improve the gasification efficiency of plastics, the gasification experiments of acrylonitrile butadiene styrene (ABS) microplastics were carried out in supercritical water and the novel in-situ hydrogenation induction gasification was used to achieve optimal gasification. In this paper, the effects of different operating conditions (temperature, time, feedstock concentration, pressure) on gasification performance were investigated. The insitu hydrogenation induction gasification of organic acid (formic acid) and inorganic acid (hydrochloric acid) were introduced to compare the effects of in-situ hydrogenation on gasification with traditional pyrolysis and supercritical water. The experimental results showed that increasing the gasification temperature, prolonging the reaction time and reducing the feedstock concentration can effectively improve the gasification efficiency of the microplastics, and the change in reaction pressure has no effect on the gasification efficiency. It is found that the valuable results that in-situ hydrogenation induction gasification significantly improved the gasification performance of ABS microplastics in supercritical water, in which the catalytic performance of organic acid (formic acid) was the most significant, followed by inorganic acid (hydrochloric acid). In-situ hydrogenation promoted the cracking of unsaturated chain hydrocarbons and polycyclic aromatic hydrocarbons. Finally, it was considered that the optimum gasification condition was at a temperature of 800 °C, a time of 60 min, a feedstock concentration of 3 wt%, a pressure of 23 MPa, and a solution of 1 wt% formic acid, the carbon conversion rate of the microplastics reached 97.0 wt%.
1. Introduction The benefits of plastic, including versatility, resistance and durability to degradation, are well known and it is everywhere in every aspect of life. Since the 1960s, plastic products have grown at an annual rate of about 8.7% [1], which has attracted human attention, in terms of its potential threat to the ecological environment. About 60–80% of the world's wastes are in the form of plastic, and about 10% of these are discarded in the ocean [2]. Due to the presence of natural forces, such as sunlight, wave action, the plastics in the ocean gradually form a large amount of microplastics, which are a general term for a class of plastics having the size of less than 5 mm [3,4]. The accumulation of microplastics in the body threatens wild marine animals, and eventually transfers to the human body along the food chain [2]. They will affect human health due to the chemical composition and pathogens attached to surface of the microplastics, while the entanglement and digestion of
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macroplastics also threaten wild animals. According to the survey, in 21 species of sea fish and 6 species of freshwater fish from the Yangtze estuary, East China Sea and South China Sea, microplastics are abundant in 26 species, accounting for 55.9–92.3% of the total number of plastics items in each species [5,6]. Microplastics are everywhere, such as air, water, soil, food, and so on [6], threatening human health. Scholars all over the world are devoting themselves to the development and utilization of new energy [7–10], and microplastics waste is a potential large scale recyclable resource. How to treat microplastics and plastic waste efficiently and cleanly is a question worth pondering, which is closely related to the living environment of human beings. Microplastics can be defined as primary or secondary, depending on their origin source [11]. The microplastics present in the ocean are mainly secondary products derived from the degradation of large plastic fragments. Other microplastics present in practical applications are micro-sized primary plastics, which are used in cosmetic facial
Corresponding author. E-mail address: [email protected] (H. Jin).
https://doi.org/10.1016/j.fuproc.2019.04.020 Received 23 December 2018; Received in revised form 1 April 2019; Accepted 14 April 2019 Available online 29 April 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.
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used as feedstock to simulate the presence of microplastics in the ocean, and conducted gasification characteristics in supercritical water, in which organic acid (formic acid, FA) and inorganic acid (hydrochloric acid, HCl) were added for improving gasification efficiency.
cleansers, industrial abrasives, sand blasting media, textile applications and so on [1,2,4]. Many scholars have conducted extensive research on the recycling of plastic waste [12–14]. However, traditional plastic waste disposal technologies, such as landfill, incineration, mechanical recycling, have caused some problems, including the release of toxic gases during the disposal process, the high disposal cost, and poor product quality [15,16]. Thermochemical conversions of wet plastics in the ocean, such as pyrolysis and gasification, are also not a suitable choice because the high water content of the plastics require drying pretreatment, which increase the cost of recycling [17,18], and the plastics melted by heat have higher viscosity and poor heat transfer, thus reducing the efficiency of these technologies, like microwave-assisted pyrolysis [19] and liquefaction [20]. The superiority of supercritical water technology, including high diffusivity, high solubility, low viscosity, is well known [21–23] and is a first choice for recycling microplastics in the ocean or production. In the face of the potential threat of plastics to the environment, research scholars have carried out a large number of research on recycling plastics in supercritical water [24,25]. Bei et al. [26] studied the decomposition process of polyethylene naphthalate (PEN) under subcritical water conditions, and observed the change of PEN through fused silica capillary reactor at 240–280 °C. Su et al. [27] studied the effect of different temperature (380 and 400 °C) and pressure increasing process on polypropylene (PP) in supercritical water and found that the critical region of the liquid phase was more beneficial to the decomposition of PP. Wang et al. [28] studied the decomposition process of the brominated flame retardant of ABS plastics in different supercritical fluids (water, methanol, isopropanol and acetone) at 300–420 °C. Tan et al. [29] studied the co-pyrolysis characteristics of heavy oil and low density polyethylene (LDPE) in supercritical water at 693 K and found that LDPE as a H-donor effectively inhibited the condensation of aromatic components and the formation of coke. Pedersen et al. [30] studied the supercritical hydrothermal reaction of nine plastic wastes at a temperature of 400 °C and a pressure of 25 MPa, which found that the polystyrene-butene (SB) liquefaction rate reached 80.8%. Shen et al. [31] investigated the co-liquefaction of coal and polystyrene (PS) in supercritical water at 633.5–703.5 K. The results showed that polystyrene stimulates coal liquefaction as a hydrogen donor. Liu et al. [32] studied the gasification characteristics of ABS plastic in supercritical water at 450–700 °C. It was found that the reaction time temperature and material concentration promoted the gasification reaction. In the early stage, we also investigated the gasification characteristics of high impact polystyrene (HIPS) in supercritical water [33]. It was found that a novel carbon microsphere structure appeared in the solid phase product, and good gasification efficiency was obtained. The supercritical water technology for the recycling of plastics mainly includes two ways of liquefaction and gasification. It can be seen from the above review that most of the existing researches focus on low-temperature liquefaction or degradation of plastics in supercritical water for the purpose that the high-value plastic resources, such as plastic monomer and plastic oil, can be produced in limited energy consumption. However, in actual plastic collection, it is very difficult to classify and recycle different types of plastics, which will affect the quality of plastic oil or plastic monomers because of differences in the composition of the different plastics and the presence of toxic chemicals [34,35]. Therefore, it is more suitable for the promotion of gasification in supercritical water. The challenge to achieve economically and environmentally efficient energy recovery from waste gasification is mainly poor gasification efficiency, resulting in coking damage to equipment and releasing of toxic substances [36,37]. Based on the presence of free radicals and ionic reactions in supercritical water, if the plastics can be rapidly cracked and gasified by in-situ hydrogenation induction with a small amount of H-donor [29,38], the polycondensation between the aromatic rings is hindered, which greatly enhances gasification effectiveness [39]. Therefore, in this paper, the ultrafine ABS microplastics were
2. Materials and methods 2.1. Materials The acrylonitrile butadiene styrene (ABS) was used as experimental feedstock, which was purchased from Sabic Innovative Plastics (Guangdong, China). It is a terpolymer made by polymerizing styrene and acrylonitrile in the presence of polybutadiene [40], as showed in (Supplementary Fig. 1). Analytical grade formic acid (content: 88%) and hydrochloric acid (content: 36–38%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The gasification agent was ultrapure water with a conductivity greater than 18.25 MΩ·cm. The formic acid was mixed with ultrapure water to prepare a solvent of 1 wt% formic acid for in-situ hydrogenation induced gasification. The same H+ mole number of hydrochloric acid solution was prepared based on the H+ mole number in a 1 wt% formic acid solution. The ABS plastic was first cleaned and dried at 60 °C, then pulverized and selected microplastics having a particle size of 50–80 mesh as experimental samples. The elemental analysis of the sample was analyzed by Vari Macro elemental analyzer manufactured by Elementar Corporation, and the proximate analysis of the sample was analyzed by SDTGA5000 produced by Sande Corporation. The results were shown in Supplementary Table 1. In addition, the steam gasification characteristics of sample were also performed on a STA449F3 thermogravimetric analyzer manufactured by NETZSCH company. The crucible was made of Al2O3. The amount of sample added was 10 mg, the rate of steam generated was 2.22 g/h, the carrier gas (N2) flow rate was 4 m3/h, the partial pressure of steam and carrier gas was 50%, the temperature range was from 150 °C to 1000 °C, and the heating rate was 10 °C/min. Supplementary Fig. 2 shows the TG/DTG profiles of the samples under the steam gasification. The weight loss of ABS plastic starts at 370.1 °C and ends at 429.2 °C. The maximum weight loss rate is 25.38 wt%/min at 403.2 °C. It can be seen that the ABS microplastics reacted well with steam, and rapid gasification occured at low temperatures and narrow reaction temperature intervals. 2.2. Methods The supercritical water gasification experiment was carried out using a quartz tube reactor, which was a cylinder (3 mm i.d. × 5 mm o.d. × 200 mm length) with one-end sealed and the maximum designed temperature and pressure of 1000 °C and 35 MPa, respectively. In every experiment, the design pressure was provided by the thermal expansion of the water in the closed quartz tube reactor, so the amount of ultrapure water added was calculated from the design temperature and pressure. First, the plastic sample and ultrapure water measured by a precise electric balance were loaded into the reactor. And the air in the reactor was evacuated with argon to ensure an oxygen-free atmosphere. Then the open end of the quartz tube was sealed by hydrogen flame. Finally, when the temperature reached the designed condition, the prepared quartz tube was placed into the furnace and held for designed time. After the reaction, the quartz tube was moved out of the furnace quickly for cooling. The experimental apparatus is shown in Supplementary Fig. 3. 2.3. Product analysis The volume ratio of gas phase product was obtained by using an Agilent 7890 gas chromatograph produced by Agilent, USA, with 171
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thermal conductivity detectors (TCDS). The column was a Plot-2000 capillary column, and the carrier gas was argon. In order to evaluate the gasification efficiency of the plastic, the gas component yield (YG), the carbon conversion rate (CE), and the hydrogen conversion rate (HE) were calculated by the following formula.
CE =
the mass of carbon in gaseous products × 100% the mass of carbon in feedstock
(1)
HE =
the mass of hydrogen in gaseous products × 100% the mass of hydrogen in feedstock
(2)
YG =
the molar amount of a certain component in the gaseous products total moles of gaseous products × 100%
(3)
The surface morphology of the solid phase product and the element contained in the sample surface was measured and analyzed by Superscan SSX-550 SEM-EDX produced by Shimadzu Corporation. The liquid phase product remaining in the quartz tube was extracted by CH2Cl2, The extracting agent and the liquid phase product were mixed at 1:1, then centrifuged, and the lower layer liquid was extracted for analysis by gas chromatography–mass selective detector (Agilent HP 6980GC/5973MSD, USA).
Fig. 2. Plastic residual liquid component at different reaction temperature: GC/ MS; (time, 10 min; pressure, 23 MPa; feedstock concentration, 5 wt%).
liquid phase products gradually decreases, which means there is a severe cracking reaction. At 800 °C, almost no organic components are detected. In this stage, although CE increases significantly, the CE only reaches 51.3 wt% at 800 °C, indicating that the high temperature promote the graphitization process of the polycyclic aromatic hydrocarbons to form solid phase products. It is considered that 800 °C is an acceptable temperature. The equipment materials used at 800 °C are basically the same as those used at 700 °C. The increase of temperature does not significantly improve the construction cost. If the gasification efficiency can be further increased by changing some means at 800 °C, this would increase economic viability of the gasification technology, though increasing the temperature could increase energy consumption.
3. Results and discussion 3.1. Effect of reaction temperature As can be seen from Fig. 1a, as the temperature increases, the carbon conversion rate (CE) and hydrogen conversion rate (HE) of the ABS microplastics gradually increase. Before 700 °C, the gasification efficiency of the microplastics increase slowly, and after 700 °C, the microplastics gasification increase significantly. At 800 °C, the CE of the microplastics is 51.3 wt%, and the HE is 196.4 wt%. According to the change of gas composition in Fig. 1b, ABS microplastics mainly undergo cracking and volatile matter precipitation at 500–600 °C. At this stage, the yield of CH4 increases obviously, which indicates that the aliphatic hydrocarbons are creaked, such as acrylonitrile and butadiene in the microplastics, which are also confirmed in Fig. 2. At 500 °C, a large amount of toluene, ethylbenzene, styrene products are present in the liquid phase product, which are significantly reduced or even disappeared at 600 °C. Secondly, at 600–700 °C, CO2 and H2 obviously increase in gas phase product, which was related to the water-gas shift (WGS) reaction, and it is shown in Fig. 2 that toluene is cracked at this stage, and polycyclic aromatic hydrocarbons, such as naphthalene, are also slightly reduced. At 700–800 °C, the CO2 in the gas phase product increases slightly, while the CO decreases. The high temperature makes the gasification reaction in the system more intense and thorough. Fig. 2 also shows that at this stage, the residual polycyclic aromatic hydrocarbons in the
3.2. Effect of reaction time It can be seen from Fig. 3a that as the reaction time increases, the CE and HE of the ABS microplastics all gradually increase. According to the gasification efficiency of the microplastic, the entire reaction interval can be divided into two parts: 2–10 min, 10–60 min. At the first 10 min, the ABS microplastics are sharply cracked, and the CE increases from 18.5 wt% at 2 min to 27.3 wt% at 10 min. As shown in Fig. 3b, more H2 are formed at 2–5 min, which is due to the initial dehydrogenation of the microplastic and subsequent methane cracking reaction and the WGS reaction of CO. The microplastics undergo obvious pyrolysis reaction. The aliphatic chain components present in the ABS plastic structure are first decomposed and reformed to form more CH4, H2 and CO2. As shown in Fig. 4, at 5 min, more polycyclic aromatic hydrocarbons, such as naphthalene and phenanthrene, are present in the liquid phase products but no aliphatic chain components. When the
Fig. 1. Effect of reaction temperature on microplastics gasification: (a) CE/HE; (b) gas yield. (C2Hx, x = 4, 6) (time, 10 min; pressure, 23 MPa; feedstock concentration, 5 wt%). 172
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Fig. 3. Effect of reaction time on microplastics gasification: (a) CE/HE; (b) gas yield. (C2Hx, x = 4, 6) (temperature, 700 °C; pressure, 23 MPa; feedstock concentration, 5 wt%).
reduced from 27.6 wt% to 20.8 wt%. In the preliminary work, it was found that the carbon conversion of HIPS plastics reached 33.3 wt% at a feed concentration of 5 wt% [33]. Therefore, it is easy to find that plastic is difficult to achieve gasification. If it is discarded arbitrarily, there will be a long-term impact on the environment. It can be seen from Fig. 5b that as the feedstock concentration increases, the proportion of CH4 in the gas phase component increases gradually, which confirmed again that in a limited reaction time (10 min), the aliphatic chain hydrocarbons in the microplastic structure are first cracked to form more CH4. At the same time, it is also noted that as the feedstock concentration increases, the proportion of CO2 and H2 in the gas phase component gradually decreases, while the proportion of CO remains basically unchanged, which seems to indicate that water is only used for the primary gasification and reforming reaction, without deep reforming of subsequent coking, reducing gasification efficiency. The effect of feedstock concentration on gasification efficiency can be explained in two ways. On the one hand, when the feedstock concentration is high, the water as a gasifying agent only participate in the initial gasification stage of the microplastics, and undergo the reforming reaction with the easily cracked branched hydrocarbons. With the consumption of water, the remaining microplastics have no opportunity to participate in the further gasification reaction, which result in a decrease in the carbon conversion of microplastics. On the other hand, when the feedstock concentration is low, water are seen as a reaction medium, a part of which participates in the gasification reaction, and another part of which dissolves the cracked aromatic component to avoid polycondensation of the polycyclic aromatic hydrocarbon, thereby improving the gasification efficiency. From the economical point of view, the increase of feedstock concentration increases the disposal capacity of waste, but the treatment cost of gasification residue and the operating cost also increases. Meanwhile, there is the risk of clogging the pipeline and damaging the equipment, which reduces the stable operation of the equipment. Therefore, it is important to choose a suitable feedstock concentration.
Fig. 4. Plastic residual liquid component at different reaction time: GC/MS; (temperature, 700 °C; pressure, 23 MPa; feedstock concentration, 5 wt%).
reaction time increases, the WGS reaction and reforming reaction are remarkable at 10 min, and more CO is converted into CO2 in Fig. 3b. At this stage, the ABS microplastics seem to have a continuous depolymerization, cracking, and reforming, which does not result in a decrease in organic component of the liquid phase due to the formation of a large amount of gas phase products, as shown in Fig. 4, the liquid phase components maintain similar content at both 5 min and 10 min. After 10 min, the CE of ABS microplastics increases slowly, there are the further cracking gasification of some polycyclic aromatic hydrocarbons, as shown in Fig. 4. The polycyclic aromatic hydrocarbons in the liquid phase component, such as naphthalene, biphenyl and phenanthrene, are significantly reduced, which promote the improvement of carbon conversion rate. Comparing with the way in which the CE increases by increasing the reaction temperature, it can be seen that the increase in temperature makes the gasification reaction of ABS microplastics more thorough, and there are fewer organic components in the residual liquid. However, it is undeniable that increasing reaction time is an effective way to improve gasification efficiency, but it may reduce economic efficiency, because of the increasing operating cost and energy consumption. According to the research results, it is considered that 60 min, not a long reaction time, may be a good gasification condition which could improve the gasification efficiency, raise waste disposal capacity and reduce maintains costs.
3.4. Effect of reaction pressure As shown in Fig. 6a, it is the gasification change of microplastics under different pressure conditions. It can be seen that the pressure increases from 21 MPa to 29 MPa, which has little effect on the gasification efficiency of microplastics. Fig. 6b also shows the gas phase product components at different pressures. It can be seen that the gas components remain unchanged. The change in pressure causes the ion product of the supercritical water to shift higher temperature. At a certain temperature (400 °C), the increase of pressure increases the concentration of H+ and OH− in the supercritical water. Saqib et al. [41] mentioned that at 400 °C, the pressure increased from 25 MPa to 50 MPa, the ion product of supercritical water increased from −19.4
3.3. Effect of feedstock concentration As shown in Fig. 5a, as the feedstock concentration increases, the CE and HE of the ABS microplastics gradually decreases. When the feedstock concentration is increased from 5 wt% to 20 wt%, the CE is 173
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Fig. 5. Effect of feedstock concentration on microplastics gasification: (a) CE/HE; (b) gas yield. (C2Hx, x = 4, 6) (temperature, 700 °C; time, 10 min; pressure, 23 MPa).
(mol/kg)2 to −11.9 (mol/kg)2. The existence of ionic product promotes the rapid hydrolysis reaction of microplastics and generates a large amount of plastic fragments. However, according to the experimental results, the reaction temperature used in the paper is high temperature gasification. The residence time in the low temperature is so short, and the reaction in the system is mainly based on free radical reaction. Therefore, the change of pressure has little effect on gasification. It would be wise to use low reaction pressure in practical applications, which could reduce the construction cost and operating cost of the equipment and improve the economics of the gasification process.
and 16.0% for PY. These show that two in-situ hydrogenation induction modes can effectively improve the gasification efficiency of plastics and acids promote the cracking gasification reaction of microplastics. In the study of lignin degradation, Lucia et al. [43] found that formic acid as a hydrogen source can successfully achieve the decomposition of lignin. In the early stage of microplastics gasification, formic acid is first creaked to release active species such as hydrogen radicals or protons, which attacked the microplastics to crack [44]. Some studies [45,46] have also shown that formic acid is an intermediate product in the WGS reaction and the active hydrogen formed in the WGS reaction promotes the cracking of organic matter and inhibits the production of low reactive species like coke and tarry compounds [47]. Therefore, the CE is the highest one in in-situ hydrogenation induced gasification with the addition of formic acid, followed by hydrochloric acid which only provides the role of hydrogen protons in the early gasification stage. With the increase of reaction time, the gasification efficiency of traditional pyrolysis always has been maintained at a low level, and the carbon conversion rate of microplastics is 16.0–22.0 wt%, which is far lower than the gasification efficiency of the other three modes. Moreover, under pyrolysis conditions, the proportion of each gas component is also significantly different from that of the other three modes. The proportion of CH4 and CO in the pyrolysis are relatively high, while the proportions of the gas components in the other three modes are close to each other, CO is kept at a low level, and CO2 and H2 maintain a gradual increase with reaction time, which means that the presence of water promotes the reforming reaction of CH4 and WGS reaction of CO, causing changes in the gas phase components to produce more hydrogen. This trend is most prominent in supercritical water, while the change of CO2 in two in-situ hydrogenation induced gasifications is not obvious in Fig. 7b, which means that the acids have a catalytic cracking effect on microplastics, so that the gasification reaction proceeds sufficiently in a short time. In order to further reveal the difference between the four modes, the
3.5. Effect of in-situ hydrogenation induced gasification In order to improve the gasification efficiency of ABS microplastics, based on the idea that H-donors contribute to the decomposition of organic matter, the carbon conversion rate can be improved by increasing the ion and radical reactions in the system. Therefore, in this paper, it was attempted to carry out in-situ hydrogenation induced gasification experiments by using 1 wt% formic acid (FA) solution and the same H+ content of hydrochloric acid (HCl) solution, respectively. Of course, when we evaluated the effect of formic acid on the gasification efficiency, we have already deducted the carbon conversion of the formic acid. We performed a gasification experiment on the addition of 1 wt% formic acid solution as a blank experiment, and found that formic acid was completely gasified at 400 °C, which is similar to the report in the literature [42]. It can be seen from Fig. 7a that the addition of FA increases the gasification reaction of ABS microplastics most significantly at different reaction times, followed by HCl, SCW, and finally PY. The shorter the reaction time, the more obvious difference in carbon conversion rate of microplastics caused by different gasification modes. It can be seen from Fig. 7a that at 10 min, the addition of FA increases the CE of microplastics to 75.9 wt%, while 63.3 wt% for HCl, 52.1 wt% for SCW
Fig. 6. Effect of reaction pressure on microplastics gasification: (a) CE/HE; (b) gas yield. (C2Hx, x = 4, 6) (temperature, 700 °C; time, 10 min; feedstock concentration, 5 wt%). 174
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Fig. 7. Effect of different conversion mode on microplastics gasification: (a) CE/HE; (b) gas yield. (Pyrolysis, PY; supercritical water, SCW; hydrochloric acid, HCl; formic acid, FA) (temperature, 800 °C; feedstock concentration, 5 wt%).
and phenanthrene, but at this time monocyclic aromatic hydrocarbons almost disappear, which means that temperature is the key factor for gasification of microplastics. Comparing the four modes, it can be clearly seen that the presence of acids and supercritical water at low temperature (400 °C) reduce the monocyclic aromatic hydrocarbons containing carbon double bonds (styrene and α-methylstyrene), which is most significant in FA and HCl modes, followed by SCW mode, as shown in Fig. 8a. This indicates that the acids promote the creaking of unsaturated chain hydrocarbons to produces more light products. At the same time, in the retention interval of 35–55 min, except for the PY mode, the other three modes reduce the polycyclic aromatic hydrocarbon content, which is more significant at high temperature (700 °C), as shown in Fig. 8b. The presence of FA and HCl significantly reduce the content of polycyclic aromatic hydrocarbons such as naphthalene, biphenyl and phenanthrene. The effect of HCl mode on the reduction of the residual liquid seems to be more significant than the FA mode. It may be the reason that HCl promotes coking to form a solid phase product. All of these indicate that both acids and supercritical water promote the gasification of ABS microplastics. The addition of acids result in an abundant Hdonor in the supercritical water for creaking, ring opening and debranching reactions of the aromatic ring, which was beneficial to the depolymerization of microplastics at low temperature and the gasification of microplastics at high temperature. The solid phase products are also observed in Fig. 9. The solid phase products are all carbon microspheres. The structure of the microspheres is obvious and uniform, and the surface of the microspheres is smooth in PY and SCW mode. In the HCl and FA modes, the carbon microspheres are bonded to each other, and the surface of the microspheres is relatively rough, attached to tiny debris. The acids molecules penetrate into the crystalline lattice of carbon microspheres, causing the microspheres to swell, thereby causing the microspheres to bond to each other in the process of gasification [48]. In addition, HCl promotes the formation of coke which affects the structure of carbon microspheres. The shape of the microspheres provides some information for the carbon conversion of the microplastics. The formation of the microspheres is related to the residual organic components in the liquid phase components. When the concentration of polycyclic aromatic hydrocarbons in the system reaches the critical point of nucleation, microspheres are formed, and gradually grow up [49,50]. It can be seen from Fig. 8 that in the SCW mode, water participates in the reaction as a gasifying agent, the conversion rate of the microplastic carbon increases, and the organic component in the liquid phase decreases, so that the size of the microspheres is smaller than the size of the microspheres in the PY mode. In the HCl mode, the presence of strong acid has a corrosive effect on the structure of the carbon microspheres, so the structure of carbon microspheres is the worst and bonded to each other. In the FA mode, large-sized microspheres and small-sized microspheres exist simultaneously, and the small-sized microspheres are
liquid phase components at 400 °C and 700 °C were separately analyzed, since no components were detected in the liquid phase product at 800 °C except for the pyrolysis mode. The two reaction temperatures represented the ionization reaction zone and the free radical reaction zone, respectively, to understand the effect of different modes on each reaction zone. As can be seen from Fig. 8, the increase in temperature significantly reduces the liquid component product. At low temperatures (Fig. 8a), the liquid product components are relatively abundant, with a large amount of monocyclic rings such as toluene, ethylbenzene, and styrene as well as a small amount of polycyclic aromatic hydrocarbons such as naphthalene. At high temperatures (Fig. 8b), the liquid phase components are mainly polycyclic aromatic hydrocarbons such as naphthalene
Fig. 8. Plastic residual liquid component at different conversion mode: GC/MS; (a) 400 °C; (b) 700 °C. (Pyrolysis, PY; supercritical water, SCW; hydrochloric acid, HCl; formic acid, FA) (pressure, 23 MPa; time, 60 min; feedstock concentration, 5 wt%). 175
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Fig. 9. Plastic residual solid products at different conversion mode: (a) pyrolysis; (b) supercritical water; (c) hydrochloric acid (d) formic acid (temperature, 800 °C, pressure, 23 MPa; time, 10 min; feedstock concentration, 5 wt%).
while the proportion of CO2 decreases. As explained above, there is not enough water in the reaction to achieve complete gasification and reforming reaction of the microplastics. Therefore, it was considered that the optimum gasification condition was at the temperature of 800 °C, the time of 60 min, the material concentration of 3 wt%, the pressure of 23 MPa, and the solution of 1 wt% formic acid, the carbon conversion rate of the microplastics reached 97.0 wt%, which means the complete conversion of plastics can generate more clean gas and reduce the harm to the environment. However, the economic viability of the technology is particularly important and further research is needed. Just as the reaction temperature and time increase, the feedstock concentration decreases, the investment cost of the gasification process, energy consumption and operating cost also increase, but the gasification process almost requires no subsequent treatment of plastic residues, reduces feedstock storage cost, and products a lot of high-quality hydrogen energy, which are the advantages of the technology. It is an urgent problem to carry out scientific economic evaluation to maximize the technical advantages, but it is undeniable that it provides an effective way for the treatment of plastic waste.
smaller than the microspheres in the SCW mode. There is a large amount of debris in the HCl and FA modes but not in PY and SCW modes. The formation of carbon microspheres is mainly caused by emulsion polymerization. Combined with the liquid phase and gas phase results, the presence of debris indicates that a nucleation reaction occurs in the system. However, as mentioned earlier, the HCl and FA modes have good carbon conversion and the low content of liquid component content, the limited residual organic components do not promote the further polymerization in surface of debris to form carbon microspheres. So there are more debris in the HCl and FA modes. 3.6. Optimal condition for plastic gasification In order to achieve complete conversion of microplastics, the gasification experiment with a solution of 1 wt% formic acid was carried out by increasing the reaction time, increasing the reaction temperature and reducing the feedstock concentration. As shown in Fig. 10, it can be seen that as the feedstock concentration increases, the gasification efficiency gradually decreases. The proportion of CH4 and CO in the gas phase component increase,
Fig. 10. Optimal conditions for microplastics gasification: (a) CE/HE; (b) gas yield. (Temperature, 800 °C; pressure, 23 MPa; time, 60 min). 176
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4. Conclusion
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Now, ABS microplastics are subjected to gasification experiments in supercritical water. The effects of different operating conditions on microplastics gasification are studied. The specific details are as follows: [1] The supercritical water gasification experiment results showed that increasing the gasification temperature, prolonging the reaction time and reducing the feedstock concentration all promoted the gasification reaction of ABS microplastics, but the change of reaction pressure had little effect on the gasification efficiency of plastics. [2] The results of in-situ hydrogenation induced gasification showed that the addition of acid acts as H-donor to promote the cracking of unsaturated hydrocarbons and polycyclic aromatic hydrocarbons, thereby improving the gasification of microplastics. [3] In the four gasification modes, the gasification ability was the best for formic acid, followed by hydrochloric acid, then supercritical water, the lowest pyrolysis, in turn. [4] The optimum conditions for ABS microplastics gasification in supercritical water were a reaction temperature of 800 °C, a reaction time of 60 min, a reaction pressure of 23 MPa, a feedstock concentration of 3 wt% and a solution of 1 wt% formic acid. And under this condition, the plastic carbon conversion rate reached 97.0 wt %. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuproc.2019.04.020. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51776169), China National Key Research and Development Plan Project (2016YFB0600100) and the Fundamental Research Funds for the Central Universities. Declarations of interest None. References [1] M. Smith, D.C. Love, C.M. Rochman, R.A. Neff, Microplastics in seafood and the implications for human health, Curr. Environ. Health Rep. 5 (2018) 375–386. [2] C.G. Avio, S. Gorbi, F. Regoli, Plastics and microplastics in the oceans: from emerging pollutants to emerged threat, Mar. Environ. Res. 128 (2017) 2–11. [3] W.C. Li, H.F. Tse, L. Fok, Plastic waste in the marine environment: a review of sources, occurrence and effects, Sci. Total Environ. 566–567 (2016) 333–349. [4] C.M. Lanctot, M. Al-Sid-Cheikh, A.I. Catarino, T. Cresswell, B. Danis, H.K. Karapanagioti, T. Mincer, F. Oberhansli, P. Swarzenski, I. Tolosa, M. Metian, Application of nuclear techniques to environmental plastics research, J. Environ. Radioact. 192 (2018) 368–375. [5] K. Jabeen, L. Su, J. Li, D. Yang, C. Tong, J. Mu, H. Shi, Microplastics and mesoplastics in fish from coastal and fresh waters of China, Environ. Pollut. 221 (2017) 141–149. [6] A.C.a.C.M. Messika Revel, Micro(nano)plastics: a threat to human health? Curr. Opin. Environ. Sci. Health 1 (2018) 17–23. [7] Y. Li, Z. Ruan, Y. He, J. Li, K. Li, Y. Jiang, X. Xu, Y. Yuan, K. Lin, In situ fabrication of hierarchically porous g-C3N4 and understanding on its enhanced photocatalytic activity based on energy absorption, Appl. Catal. B Environ. 236 (2018) 64–75. [8] K. Chu, J. Chen, A.B. Yu, R.A. Williams, Numerical studies of multiphase flow and separation performance of natural medium cyclones for recovering waste coal, Powder Technol. 314 (2017) 532–541. [9] B. Wang, S. Shen, S.S. Mao, Black TiO2 for solar hydrogen conversion, J. Mater. 3 (2017) 96–111. [10] S. Shen, Z. Wang, Y. Liu, Q. Zhang, K. Zheng, A new experimental method to estimate the leakage current in the solid oxide fuel cell with a mixed ionic and electronic conducting electrolyte, J. Power Sources 406 (2018) 88–95. [11] D.K. Barnes, F. Galgani, R.C. Thompson, M. Barlaz, Accumulation and fragmentation of plastic debris in global environments, Philos. Trans. R. Soc. Lond. Ser. B Biol. Sci. 364 (2009) 1985–1998.
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Experimental investigation on in-situ hydrogenation induced gasification characteristics of acrylonitrile butadiene styrene (ABS) microplastics in supercritical water
T
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Bin Bai, Hui Jin , Shixing Zhu, Pengfei Wu, Chao Fan, Jingli Sun State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi'an Jiaotong University, 28 Xianning West Road, Xi'an 710049, Shaanxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Supercritical water ABS microplastics Gasification In-situ hydrogenation
In order to solve the ecological and environmental problems caused by microplastics remaining in the ocean and improve the gasification efficiency of plastics, the gasification experiments of acrylonitrile butadiene styrene (ABS) microplastics were carried out in supercritical water and the novel in-situ hydrogenation induction gasification was used to achieve optimal gasification. In this paper, the effects of different operating conditions (temperature, time, feedstock concentration, pressure) on gasification performance were investigated. The insitu hydrogenation induction gasification of organic acid (formic acid) and inorganic acid (hydrochloric acid) were introduced to compare the effects of in-situ hydrogenation on gasification with traditional pyrolysis and supercritical water. The experimental results showed that increasing the gasification temperature, prolonging the reaction time and reducing the feedstock concentration can effectively improve the gasification efficiency of the microplastics, and the change in reaction pressure has no effect on the gasification efficiency. It is found that the valuable results that in-situ hydrogenation induction gasification significantly improved the gasification performance of ABS microplastics in supercritical water, in which the catalytic performance of organic acid (formic acid) was the most significant, followed by inorganic acid (hydrochloric acid). In-situ hydrogenation promoted the cracking of unsaturated chain hydrocarbons and polycyclic aromatic hydrocarbons. Finally, it was considered that the optimum gasification condition was at a temperature of 800 °C, a time of 60 min, a feedstock concentration of 3 wt%, a pressure of 23 MPa, and a solution of 1 wt% formic acid, the carbon conversion rate of the microplastics reached 97.0 wt%.
1. Introduction The benefits of plastic, including versatility, resistance and durability to degradation, are well known and it is everywhere in every aspect of life. Since the 1960s, plastic products have grown at an annual rate of about 8.7% [1], which has attracted human attention, in terms of its potential threat to the ecological environment. About 60–80% of the world's wastes are in the form of plastic, and about 10% of these are discarded in the ocean [2]. Due to the presence of natural forces, such as sunlight, wave action, the plastics in the ocean gradually form a large amount of microplastics, which are a general term for a class of plastics having the size of less than 5 mm [3,4]. The accumulation of microplastics in the body threatens wild marine animals, and eventually transfers to the human body along the food chain [2]. They will affect human health due to the chemical composition and pathogens attached to surface of the microplastics, while the entanglement and digestion of
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macroplastics also threaten wild animals. According to the survey, in 21 species of sea fish and 6 species of freshwater fish from the Yangtze estuary, East China Sea and South China Sea, microplastics are abundant in 26 species, accounting for 55.9–92.3% of the total number of plastics items in each species [5,6]. Microplastics are everywhere, such as air, water, soil, food, and so on [6], threatening human health. Scholars all over the world are devoting themselves to the development and utilization of new energy [7–10], and microplastics waste is a potential large scale recyclable resource. How to treat microplastics and plastic waste efficiently and cleanly is a question worth pondering, which is closely related to the living environment of human beings. Microplastics can be defined as primary or secondary, depending on their origin source [11]. The microplastics present in the ocean are mainly secondary products derived from the degradation of large plastic fragments. Other microplastics present in practical applications are micro-sized primary plastics, which are used in cosmetic facial
Corresponding author. E-mail address: [email protected] (H. Jin).
https://doi.org/10.1016/j.fuproc.2019.04.020 Received 23 December 2018; Received in revised form 1 April 2019; Accepted 14 April 2019 Available online 29 April 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.
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used as feedstock to simulate the presence of microplastics in the ocean, and conducted gasification characteristics in supercritical water, in which organic acid (formic acid, FA) and inorganic acid (hydrochloric acid, HCl) were added for improving gasification efficiency.
cleansers, industrial abrasives, sand blasting media, textile applications and so on [1,2,4]. Many scholars have conducted extensive research on the recycling of plastic waste [12–14]. However, traditional plastic waste disposal technologies, such as landfill, incineration, mechanical recycling, have caused some problems, including the release of toxic gases during the disposal process, the high disposal cost, and poor product quality [15,16]. Thermochemical conversions of wet plastics in the ocean, such as pyrolysis and gasification, are also not a suitable choice because the high water content of the plastics require drying pretreatment, which increase the cost of recycling [17,18], and the plastics melted by heat have higher viscosity and poor heat transfer, thus reducing the efficiency of these technologies, like microwave-assisted pyrolysis [19] and liquefaction [20]. The superiority of supercritical water technology, including high diffusivity, high solubility, low viscosity, is well known [21–23] and is a first choice for recycling microplastics in the ocean or production. In the face of the potential threat of plastics to the environment, research scholars have carried out a large number of research on recycling plastics in supercritical water [24,25]. Bei et al. [26] studied the decomposition process of polyethylene naphthalate (PEN) under subcritical water conditions, and observed the change of PEN through fused silica capillary reactor at 240–280 °C. Su et al. [27] studied the effect of different temperature (380 and 400 °C) and pressure increasing process on polypropylene (PP) in supercritical water and found that the critical region of the liquid phase was more beneficial to the decomposition of PP. Wang et al. [28] studied the decomposition process of the brominated flame retardant of ABS plastics in different supercritical fluids (water, methanol, isopropanol and acetone) at 300–420 °C. Tan et al. [29] studied the co-pyrolysis characteristics of heavy oil and low density polyethylene (LDPE) in supercritical water at 693 K and found that LDPE as a H-donor effectively inhibited the condensation of aromatic components and the formation of coke. Pedersen et al. [30] studied the supercritical hydrothermal reaction of nine plastic wastes at a temperature of 400 °C and a pressure of 25 MPa, which found that the polystyrene-butene (SB) liquefaction rate reached 80.8%. Shen et al. [31] investigated the co-liquefaction of coal and polystyrene (PS) in supercritical water at 633.5–703.5 K. The results showed that polystyrene stimulates coal liquefaction as a hydrogen donor. Liu et al. [32] studied the gasification characteristics of ABS plastic in supercritical water at 450–700 °C. It was found that the reaction time temperature and material concentration promoted the gasification reaction. In the early stage, we also investigated the gasification characteristics of high impact polystyrene (HIPS) in supercritical water [33]. It was found that a novel carbon microsphere structure appeared in the solid phase product, and good gasification efficiency was obtained. The supercritical water technology for the recycling of plastics mainly includes two ways of liquefaction and gasification. It can be seen from the above review that most of the existing researches focus on low-temperature liquefaction or degradation of plastics in supercritical water for the purpose that the high-value plastic resources, such as plastic monomer and plastic oil, can be produced in limited energy consumption. However, in actual plastic collection, it is very difficult to classify and recycle different types of plastics, which will affect the quality of plastic oil or plastic monomers because of differences in the composition of the different plastics and the presence of toxic chemicals [34,35]. Therefore, it is more suitable for the promotion of gasification in supercritical water. The challenge to achieve economically and environmentally efficient energy recovery from waste gasification is mainly poor gasification efficiency, resulting in coking damage to equipment and releasing of toxic substances [36,37]. Based on the presence of free radicals and ionic reactions in supercritical water, if the plastics can be rapidly cracked and gasified by in-situ hydrogenation induction with a small amount of H-donor [29,38], the polycondensation between the aromatic rings is hindered, which greatly enhances gasification effectiveness [39]. Therefore, in this paper, the ultrafine ABS microplastics were
2. Materials and methods 2.1. Materials The acrylonitrile butadiene styrene (ABS) was used as experimental feedstock, which was purchased from Sabic Innovative Plastics (Guangdong, China). It is a terpolymer made by polymerizing styrene and acrylonitrile in the presence of polybutadiene [40], as showed in (Supplementary Fig. 1). Analytical grade formic acid (content: 88%) and hydrochloric acid (content: 36–38%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The gasification agent was ultrapure water with a conductivity greater than 18.25 MΩ·cm. The formic acid was mixed with ultrapure water to prepare a solvent of 1 wt% formic acid for in-situ hydrogenation induced gasification. The same H+ mole number of hydrochloric acid solution was prepared based on the H+ mole number in a 1 wt% formic acid solution. The ABS plastic was first cleaned and dried at 60 °C, then pulverized and selected microplastics having a particle size of 50–80 mesh as experimental samples. The elemental analysis of the sample was analyzed by Vari Macro elemental analyzer manufactured by Elementar Corporation, and the proximate analysis of the sample was analyzed by SDTGA5000 produced by Sande Corporation. The results were shown in Supplementary Table 1. In addition, the steam gasification characteristics of sample were also performed on a STA449F3 thermogravimetric analyzer manufactured by NETZSCH company. The crucible was made of Al2O3. The amount of sample added was 10 mg, the rate of steam generated was 2.22 g/h, the carrier gas (N2) flow rate was 4 m3/h, the partial pressure of steam and carrier gas was 50%, the temperature range was from 150 °C to 1000 °C, and the heating rate was 10 °C/min. Supplementary Fig. 2 shows the TG/DTG profiles of the samples under the steam gasification. The weight loss of ABS plastic starts at 370.1 °C and ends at 429.2 °C. The maximum weight loss rate is 25.38 wt%/min at 403.2 °C. It can be seen that the ABS microplastics reacted well with steam, and rapid gasification occured at low temperatures and narrow reaction temperature intervals. 2.2. Methods The supercritical water gasification experiment was carried out using a quartz tube reactor, which was a cylinder (3 mm i.d. × 5 mm o.d. × 200 mm length) with one-end sealed and the maximum designed temperature and pressure of 1000 °C and 35 MPa, respectively. In every experiment, the design pressure was provided by the thermal expansion of the water in the closed quartz tube reactor, so the amount of ultrapure water added was calculated from the design temperature and pressure. First, the plastic sample and ultrapure water measured by a precise electric balance were loaded into the reactor. And the air in the reactor was evacuated with argon to ensure an oxygen-free atmosphere. Then the open end of the quartz tube was sealed by hydrogen flame. Finally, when the temperature reached the designed condition, the prepared quartz tube was placed into the furnace and held for designed time. After the reaction, the quartz tube was moved out of the furnace quickly for cooling. The experimental apparatus is shown in Supplementary Fig. 3. 2.3. Product analysis The volume ratio of gas phase product was obtained by using an Agilent 7890 gas chromatograph produced by Agilent, USA, with 171
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thermal conductivity detectors (TCDS). The column was a Plot-2000 capillary column, and the carrier gas was argon. In order to evaluate the gasification efficiency of the plastic, the gas component yield (YG), the carbon conversion rate (CE), and the hydrogen conversion rate (HE) were calculated by the following formula.
CE =
the mass of carbon in gaseous products × 100% the mass of carbon in feedstock
(1)
HE =
the mass of hydrogen in gaseous products × 100% the mass of hydrogen in feedstock
(2)
YG =
the molar amount of a certain component in the gaseous products total moles of gaseous products × 100%
(3)
The surface morphology of the solid phase product and the element contained in the sample surface was measured and analyzed by Superscan SSX-550 SEM-EDX produced by Shimadzu Corporation. The liquid phase product remaining in the quartz tube was extracted by CH2Cl2, The extracting agent and the liquid phase product were mixed at 1:1, then centrifuged, and the lower layer liquid was extracted for analysis by gas chromatography–mass selective detector (Agilent HP 6980GC/5973MSD, USA).
Fig. 2. Plastic residual liquid component at different reaction temperature: GC/ MS; (time, 10 min; pressure, 23 MPa; feedstock concentration, 5 wt%).
liquid phase products gradually decreases, which means there is a severe cracking reaction. At 800 °C, almost no organic components are detected. In this stage, although CE increases significantly, the CE only reaches 51.3 wt% at 800 °C, indicating that the high temperature promote the graphitization process of the polycyclic aromatic hydrocarbons to form solid phase products. It is considered that 800 °C is an acceptable temperature. The equipment materials used at 800 °C are basically the same as those used at 700 °C. The increase of temperature does not significantly improve the construction cost. If the gasification efficiency can be further increased by changing some means at 800 °C, this would increase economic viability of the gasification technology, though increasing the temperature could increase energy consumption.
3. Results and discussion 3.1. Effect of reaction temperature As can be seen from Fig. 1a, as the temperature increases, the carbon conversion rate (CE) and hydrogen conversion rate (HE) of the ABS microplastics gradually increase. Before 700 °C, the gasification efficiency of the microplastics increase slowly, and after 700 °C, the microplastics gasification increase significantly. At 800 °C, the CE of the microplastics is 51.3 wt%, and the HE is 196.4 wt%. According to the change of gas composition in Fig. 1b, ABS microplastics mainly undergo cracking and volatile matter precipitation at 500–600 °C. At this stage, the yield of CH4 increases obviously, which indicates that the aliphatic hydrocarbons are creaked, such as acrylonitrile and butadiene in the microplastics, which are also confirmed in Fig. 2. At 500 °C, a large amount of toluene, ethylbenzene, styrene products are present in the liquid phase product, which are significantly reduced or even disappeared at 600 °C. Secondly, at 600–700 °C, CO2 and H2 obviously increase in gas phase product, which was related to the water-gas shift (WGS) reaction, and it is shown in Fig. 2 that toluene is cracked at this stage, and polycyclic aromatic hydrocarbons, such as naphthalene, are also slightly reduced. At 700–800 °C, the CO2 in the gas phase product increases slightly, while the CO decreases. The high temperature makes the gasification reaction in the system more intense and thorough. Fig. 2 also shows that at this stage, the residual polycyclic aromatic hydrocarbons in the
3.2. Effect of reaction time It can be seen from Fig. 3a that as the reaction time increases, the CE and HE of the ABS microplastics all gradually increase. According to the gasification efficiency of the microplastic, the entire reaction interval can be divided into two parts: 2–10 min, 10–60 min. At the first 10 min, the ABS microplastics are sharply cracked, and the CE increases from 18.5 wt% at 2 min to 27.3 wt% at 10 min. As shown in Fig. 3b, more H2 are formed at 2–5 min, which is due to the initial dehydrogenation of the microplastic and subsequent methane cracking reaction and the WGS reaction of CO. The microplastics undergo obvious pyrolysis reaction. The aliphatic chain components present in the ABS plastic structure are first decomposed and reformed to form more CH4, H2 and CO2. As shown in Fig. 4, at 5 min, more polycyclic aromatic hydrocarbons, such as naphthalene and phenanthrene, are present in the liquid phase products but no aliphatic chain components. When the
Fig. 1. Effect of reaction temperature on microplastics gasification: (a) CE/HE; (b) gas yield. (C2Hx, x = 4, 6) (time, 10 min; pressure, 23 MPa; feedstock concentration, 5 wt%). 172
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Fig. 3. Effect of reaction time on microplastics gasification: (a) CE/HE; (b) gas yield. (C2Hx, x = 4, 6) (temperature, 700 °C; pressure, 23 MPa; feedstock concentration, 5 wt%).
reduced from 27.6 wt% to 20.8 wt%. In the preliminary work, it was found that the carbon conversion of HIPS plastics reached 33.3 wt% at a feed concentration of 5 wt% [33]. Therefore, it is easy to find that plastic is difficult to achieve gasification. If it is discarded arbitrarily, there will be a long-term impact on the environment. It can be seen from Fig. 5b that as the feedstock concentration increases, the proportion of CH4 in the gas phase component increases gradually, which confirmed again that in a limited reaction time (10 min), the aliphatic chain hydrocarbons in the microplastic structure are first cracked to form more CH4. At the same time, it is also noted that as the feedstock concentration increases, the proportion of CO2 and H2 in the gas phase component gradually decreases, while the proportion of CO remains basically unchanged, which seems to indicate that water is only used for the primary gasification and reforming reaction, without deep reforming of subsequent coking, reducing gasification efficiency. The effect of feedstock concentration on gasification efficiency can be explained in two ways. On the one hand, when the feedstock concentration is high, the water as a gasifying agent only participate in the initial gasification stage of the microplastics, and undergo the reforming reaction with the easily cracked branched hydrocarbons. With the consumption of water, the remaining microplastics have no opportunity to participate in the further gasification reaction, which result in a decrease in the carbon conversion of microplastics. On the other hand, when the feedstock concentration is low, water are seen as a reaction medium, a part of which participates in the gasification reaction, and another part of which dissolves the cracked aromatic component to avoid polycondensation of the polycyclic aromatic hydrocarbon, thereby improving the gasification efficiency. From the economical point of view, the increase of feedstock concentration increases the disposal capacity of waste, but the treatment cost of gasification residue and the operating cost also increases. Meanwhile, there is the risk of clogging the pipeline and damaging the equipment, which reduces the stable operation of the equipment. Therefore, it is important to choose a suitable feedstock concentration.
Fig. 4. Plastic residual liquid component at different reaction time: GC/MS; (temperature, 700 °C; pressure, 23 MPa; feedstock concentration, 5 wt%).
reaction time increases, the WGS reaction and reforming reaction are remarkable at 10 min, and more CO is converted into CO2 in Fig. 3b. At this stage, the ABS microplastics seem to have a continuous depolymerization, cracking, and reforming, which does not result in a decrease in organic component of the liquid phase due to the formation of a large amount of gas phase products, as shown in Fig. 4, the liquid phase components maintain similar content at both 5 min and 10 min. After 10 min, the CE of ABS microplastics increases slowly, there are the further cracking gasification of some polycyclic aromatic hydrocarbons, as shown in Fig. 4. The polycyclic aromatic hydrocarbons in the liquid phase component, such as naphthalene, biphenyl and phenanthrene, are significantly reduced, which promote the improvement of carbon conversion rate. Comparing with the way in which the CE increases by increasing the reaction temperature, it can be seen that the increase in temperature makes the gasification reaction of ABS microplastics more thorough, and there are fewer organic components in the residual liquid. However, it is undeniable that increasing reaction time is an effective way to improve gasification efficiency, but it may reduce economic efficiency, because of the increasing operating cost and energy consumption. According to the research results, it is considered that 60 min, not a long reaction time, may be a good gasification condition which could improve the gasification efficiency, raise waste disposal capacity and reduce maintains costs.
3.4. Effect of reaction pressure As shown in Fig. 6a, it is the gasification change of microplastics under different pressure conditions. It can be seen that the pressure increases from 21 MPa to 29 MPa, which has little effect on the gasification efficiency of microplastics. Fig. 6b also shows the gas phase product components at different pressures. It can be seen that the gas components remain unchanged. The change in pressure causes the ion product of the supercritical water to shift higher temperature. At a certain temperature (400 °C), the increase of pressure increases the concentration of H+ and OH− in the supercritical water. Saqib et al. [41] mentioned that at 400 °C, the pressure increased from 25 MPa to 50 MPa, the ion product of supercritical water increased from −19.4
3.3. Effect of feedstock concentration As shown in Fig. 5a, as the feedstock concentration increases, the CE and HE of the ABS microplastics gradually decreases. When the feedstock concentration is increased from 5 wt% to 20 wt%, the CE is 173
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Fig. 5. Effect of feedstock concentration on microplastics gasification: (a) CE/HE; (b) gas yield. (C2Hx, x = 4, 6) (temperature, 700 °C; time, 10 min; pressure, 23 MPa).
(mol/kg)2 to −11.9 (mol/kg)2. The existence of ionic product promotes the rapid hydrolysis reaction of microplastics and generates a large amount of plastic fragments. However, according to the experimental results, the reaction temperature used in the paper is high temperature gasification. The residence time in the low temperature is so short, and the reaction in the system is mainly based on free radical reaction. Therefore, the change of pressure has little effect on gasification. It would be wise to use low reaction pressure in practical applications, which could reduce the construction cost and operating cost of the equipment and improve the economics of the gasification process.
and 16.0% for PY. These show that two in-situ hydrogenation induction modes can effectively improve the gasification efficiency of plastics and acids promote the cracking gasification reaction of microplastics. In the study of lignin degradation, Lucia et al. [43] found that formic acid as a hydrogen source can successfully achieve the decomposition of lignin. In the early stage of microplastics gasification, formic acid is first creaked to release active species such as hydrogen radicals or protons, which attacked the microplastics to crack [44]. Some studies [45,46] have also shown that formic acid is an intermediate product in the WGS reaction and the active hydrogen formed in the WGS reaction promotes the cracking of organic matter and inhibits the production of low reactive species like coke and tarry compounds [47]. Therefore, the CE is the highest one in in-situ hydrogenation induced gasification with the addition of formic acid, followed by hydrochloric acid which only provides the role of hydrogen protons in the early gasification stage. With the increase of reaction time, the gasification efficiency of traditional pyrolysis always has been maintained at a low level, and the carbon conversion rate of microplastics is 16.0–22.0 wt%, which is far lower than the gasification efficiency of the other three modes. Moreover, under pyrolysis conditions, the proportion of each gas component is also significantly different from that of the other three modes. The proportion of CH4 and CO in the pyrolysis are relatively high, while the proportions of the gas components in the other three modes are close to each other, CO is kept at a low level, and CO2 and H2 maintain a gradual increase with reaction time, which means that the presence of water promotes the reforming reaction of CH4 and WGS reaction of CO, causing changes in the gas phase components to produce more hydrogen. This trend is most prominent in supercritical water, while the change of CO2 in two in-situ hydrogenation induced gasifications is not obvious in Fig. 7b, which means that the acids have a catalytic cracking effect on microplastics, so that the gasification reaction proceeds sufficiently in a short time. In order to further reveal the difference between the four modes, the
3.5. Effect of in-situ hydrogenation induced gasification In order to improve the gasification efficiency of ABS microplastics, based on the idea that H-donors contribute to the decomposition of organic matter, the carbon conversion rate can be improved by increasing the ion and radical reactions in the system. Therefore, in this paper, it was attempted to carry out in-situ hydrogenation induced gasification experiments by using 1 wt% formic acid (FA) solution and the same H+ content of hydrochloric acid (HCl) solution, respectively. Of course, when we evaluated the effect of formic acid on the gasification efficiency, we have already deducted the carbon conversion of the formic acid. We performed a gasification experiment on the addition of 1 wt% formic acid solution as a blank experiment, and found that formic acid was completely gasified at 400 °C, which is similar to the report in the literature [42]. It can be seen from Fig. 7a that the addition of FA increases the gasification reaction of ABS microplastics most significantly at different reaction times, followed by HCl, SCW, and finally PY. The shorter the reaction time, the more obvious difference in carbon conversion rate of microplastics caused by different gasification modes. It can be seen from Fig. 7a that at 10 min, the addition of FA increases the CE of microplastics to 75.9 wt%, while 63.3 wt% for HCl, 52.1 wt% for SCW
Fig. 6. Effect of reaction pressure on microplastics gasification: (a) CE/HE; (b) gas yield. (C2Hx, x = 4, 6) (temperature, 700 °C; time, 10 min; feedstock concentration, 5 wt%). 174
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Fig. 7. Effect of different conversion mode on microplastics gasification: (a) CE/HE; (b) gas yield. (Pyrolysis, PY; supercritical water, SCW; hydrochloric acid, HCl; formic acid, FA) (temperature, 800 °C; feedstock concentration, 5 wt%).
and phenanthrene, but at this time monocyclic aromatic hydrocarbons almost disappear, which means that temperature is the key factor for gasification of microplastics. Comparing the four modes, it can be clearly seen that the presence of acids and supercritical water at low temperature (400 °C) reduce the monocyclic aromatic hydrocarbons containing carbon double bonds (styrene and α-methylstyrene), which is most significant in FA and HCl modes, followed by SCW mode, as shown in Fig. 8a. This indicates that the acids promote the creaking of unsaturated chain hydrocarbons to produces more light products. At the same time, in the retention interval of 35–55 min, except for the PY mode, the other three modes reduce the polycyclic aromatic hydrocarbon content, which is more significant at high temperature (700 °C), as shown in Fig. 8b. The presence of FA and HCl significantly reduce the content of polycyclic aromatic hydrocarbons such as naphthalene, biphenyl and phenanthrene. The effect of HCl mode on the reduction of the residual liquid seems to be more significant than the FA mode. It may be the reason that HCl promotes coking to form a solid phase product. All of these indicate that both acids and supercritical water promote the gasification of ABS microplastics. The addition of acids result in an abundant Hdonor in the supercritical water for creaking, ring opening and debranching reactions of the aromatic ring, which was beneficial to the depolymerization of microplastics at low temperature and the gasification of microplastics at high temperature. The solid phase products are also observed in Fig. 9. The solid phase products are all carbon microspheres. The structure of the microspheres is obvious and uniform, and the surface of the microspheres is smooth in PY and SCW mode. In the HCl and FA modes, the carbon microspheres are bonded to each other, and the surface of the microspheres is relatively rough, attached to tiny debris. The acids molecules penetrate into the crystalline lattice of carbon microspheres, causing the microspheres to swell, thereby causing the microspheres to bond to each other in the process of gasification [48]. In addition, HCl promotes the formation of coke which affects the structure of carbon microspheres. The shape of the microspheres provides some information for the carbon conversion of the microplastics. The formation of the microspheres is related to the residual organic components in the liquid phase components. When the concentration of polycyclic aromatic hydrocarbons in the system reaches the critical point of nucleation, microspheres are formed, and gradually grow up [49,50]. It can be seen from Fig. 8 that in the SCW mode, water participates in the reaction as a gasifying agent, the conversion rate of the microplastic carbon increases, and the organic component in the liquid phase decreases, so that the size of the microspheres is smaller than the size of the microspheres in the PY mode. In the HCl mode, the presence of strong acid has a corrosive effect on the structure of the carbon microspheres, so the structure of carbon microspheres is the worst and bonded to each other. In the FA mode, large-sized microspheres and small-sized microspheres exist simultaneously, and the small-sized microspheres are
liquid phase components at 400 °C and 700 °C were separately analyzed, since no components were detected in the liquid phase product at 800 °C except for the pyrolysis mode. The two reaction temperatures represented the ionization reaction zone and the free radical reaction zone, respectively, to understand the effect of different modes on each reaction zone. As can be seen from Fig. 8, the increase in temperature significantly reduces the liquid component product. At low temperatures (Fig. 8a), the liquid product components are relatively abundant, with a large amount of monocyclic rings such as toluene, ethylbenzene, and styrene as well as a small amount of polycyclic aromatic hydrocarbons such as naphthalene. At high temperatures (Fig. 8b), the liquid phase components are mainly polycyclic aromatic hydrocarbons such as naphthalene
Fig. 8. Plastic residual liquid component at different conversion mode: GC/MS; (a) 400 °C; (b) 700 °C. (Pyrolysis, PY; supercritical water, SCW; hydrochloric acid, HCl; formic acid, FA) (pressure, 23 MPa; time, 60 min; feedstock concentration, 5 wt%). 175
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Fig. 9. Plastic residual solid products at different conversion mode: (a) pyrolysis; (b) supercritical water; (c) hydrochloric acid (d) formic acid (temperature, 800 °C, pressure, 23 MPa; time, 10 min; feedstock concentration, 5 wt%).
while the proportion of CO2 decreases. As explained above, there is not enough water in the reaction to achieve complete gasification and reforming reaction of the microplastics. Therefore, it was considered that the optimum gasification condition was at the temperature of 800 °C, the time of 60 min, the material concentration of 3 wt%, the pressure of 23 MPa, and the solution of 1 wt% formic acid, the carbon conversion rate of the microplastics reached 97.0 wt%, which means the complete conversion of plastics can generate more clean gas and reduce the harm to the environment. However, the economic viability of the technology is particularly important and further research is needed. Just as the reaction temperature and time increase, the feedstock concentration decreases, the investment cost of the gasification process, energy consumption and operating cost also increase, but the gasification process almost requires no subsequent treatment of plastic residues, reduces feedstock storage cost, and products a lot of high-quality hydrogen energy, which are the advantages of the technology. It is an urgent problem to carry out scientific economic evaluation to maximize the technical advantages, but it is undeniable that it provides an effective way for the treatment of plastic waste.
smaller than the microspheres in the SCW mode. There is a large amount of debris in the HCl and FA modes but not in PY and SCW modes. The formation of carbon microspheres is mainly caused by emulsion polymerization. Combined with the liquid phase and gas phase results, the presence of debris indicates that a nucleation reaction occurs in the system. However, as mentioned earlier, the HCl and FA modes have good carbon conversion and the low content of liquid component content, the limited residual organic components do not promote the further polymerization in surface of debris to form carbon microspheres. So there are more debris in the HCl and FA modes. 3.6. Optimal condition for plastic gasification In order to achieve complete conversion of microplastics, the gasification experiment with a solution of 1 wt% formic acid was carried out by increasing the reaction time, increasing the reaction temperature and reducing the feedstock concentration. As shown in Fig. 10, it can be seen that as the feedstock concentration increases, the gasification efficiency gradually decreases. The proportion of CH4 and CO in the gas phase component increase,
Fig. 10. Optimal conditions for microplastics gasification: (a) CE/HE; (b) gas yield. (Temperature, 800 °C; pressure, 23 MPa; time, 60 min). 176
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4. Conclusion
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Now, ABS microplastics are subjected to gasification experiments in supercritical water. The effects of different operating conditions on microplastics gasification are studied. The specific details are as follows: [1] The supercritical water gasification experiment results showed that increasing the gasification temperature, prolonging the reaction time and reducing the feedstock concentration all promoted the gasification reaction of ABS microplastics, but the change of reaction pressure had little effect on the gasification efficiency of plastics. [2] The results of in-situ hydrogenation induced gasification showed that the addition of acid acts as H-donor to promote the cracking of unsaturated hydrocarbons and polycyclic aromatic hydrocarbons, thereby improving the gasification of microplastics. [3] In the four gasification modes, the gasification ability was the best for formic acid, followed by hydrochloric acid, then supercritical water, the lowest pyrolysis, in turn. [4] The optimum conditions for ABS microplastics gasification in supercritical water were a reaction temperature of 800 °C, a reaction time of 60 min, a reaction pressure of 23 MPa, a feedstock concentration of 3 wt% and a solution of 1 wt% formic acid. And under this condition, the plastic carbon conversion rate reached 97.0 wt %. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuproc.2019.04.020. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51776169), China National Key Research and Development Plan Project (2016YFB0600100) and the Fundamental Research Funds for the Central Universities. Declarations of interest None. References [1] M. Smith, D.C. Love, C.M. Rochman, R.A. Neff, Microplastics in seafood and the implications for human health, Curr. Environ. Health Rep. 5 (2018) 375–386. [2] C.G. Avio, S. Gorbi, F. Regoli, Plastics and microplastics in the oceans: from emerging pollutants to emerged threat, Mar. Environ. Res. 128 (2017) 2–11. [3] W.C. Li, H.F. Tse, L. Fok, Plastic waste in the marine environment: a review of sources, occurrence and effects, Sci. Total Environ. 566–567 (2016) 333–349. [4] C.M. Lanctot, M. Al-Sid-Cheikh, A.I. Catarino, T. Cresswell, B. Danis, H.K. Karapanagioti, T. Mincer, F. Oberhansli, P. Swarzenski, I. Tolosa, M. Metian, Application of nuclear techniques to environmental plastics research, J. Environ. Radioact. 192 (2018) 368–375. [5] K. Jabeen, L. Su, J. Li, D. Yang, C. Tong, J. Mu, H. Shi, Microplastics and mesoplastics in fish from coastal and fresh waters of China, Environ. Pollut. 221 (2017) 141–149. [6] A.C.a.C.M. Messika Revel, Micro(nano)plastics: a threat to human health? Curr. Opin. Environ. Sci. Health 1 (2018) 17–23. [7] Y. Li, Z. Ruan, Y. He, J. Li, K. Li, Y. Jiang, X. Xu, Y. Yuan, K. Lin, In situ fabrication of hierarchically porous g-C3N4 and understanding on its enhanced photocatalytic activity based on energy absorption, Appl. Catal. B Environ. 236 (2018) 64–75. [8] K. Chu, J. Chen, A.B. Yu, R.A. Williams, Numerical studies of multiphase flow and separation performance of natural medium cyclones for recovering waste coal, Powder Technol. 314 (2017) 532–541. [9] B. Wang, S. Shen, S.S. Mao, Black TiO2 for solar hydrogen conversion, J. Mater. 3 (2017) 96–111. [10] S. Shen, Z. Wang, Y. Liu, Q. Zhang, K. Zheng, A new experimental method to estimate the leakage current in the solid oxide fuel cell with a mixed ionic and electronic conducting electrolyte, J. Power Sources 406 (2018) 88–95. [11] D.K. Barnes, F. Galgani, R.C. Thompson, M. Barlaz, Accumulation and fragmentation of plastic debris in global environments, Philos. Trans. R. Soc. Lond. Ser. B Biol. Sci. 364 (2009) 1985–1998.
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