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Article

Characteristics of Molten Salt Gasification of Waste PVC

by
Chengqian Lin
1,*,
Tianfeng Yang
2,
Siyu Chen
1,
Minjie Li
1,
Shoukang Wang
1 and
Qunxing Huang
1
1
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China
2
Huangzhou Mingsheng New Energy Technology Co., Ltd., Hangzhou 310027, China
*
Author to whom correspondence should be addressed.
Processes 2024, 12(2), 306; https://doi.org/10.3390/pr12020306
Submission received: 8 January 2024 / Revised: 23 January 2024 / Accepted: 27 January 2024 / Published: 1 February 2024
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Abstract

:
Molten salt oxidation is a robust thermal process with the inherent capability to catalytically oxidize the organic compounds while retaining the inorganic ingredients in salt bath. In the present study, molten salt gasification was used for the disposal of waste PVC. The characteristics of molten salt gasification of PVC under different temperatures and air equivalence ratios (ERs) on the gasification characteristics, chlorine retention efficiency, PCDD/F generation, and the distribution of heavy metals such as Cu, Pb, and Zn were investigated. The results showed that increasing the temperature and ER could effectively enhance the yield of gasification gas and carbon conversion efficiency. The highest gasification efficiency of 41.2% was achieved at 750 °C and ER = 0.4, with a gas yield of 0.442 Nm3/kg PVC. Molten carbonates showed an absorption and retention efficiency of more than 99.5% for chlorine under all conditions. Increasing temperature resulted in a significant reduction in the generation of PCDD/F. At 750 °C, the PCDD/F generation was less than 19 pg/g PVC with an I-TEQ of less than 1.4 pg/g PVC, and the ER had a minor effect on PCDD/F. During the molten salt gasification process, most of the heavy metals, such as Cu, Pb, and Zn, were retained in the salt bath.

1. Introduction

Polyvinylchloride (PVC) is widely used in modern society and daily life due to excellent properties such as good machinability, long lifespan, low cost and chemical resistance [1,2,3]. PVC is the second most-produced versatile plastic after polyethylene, with a global production capacity of more than 60 million tons, with China accounting for more than half of this capacity [4]. The huge production of PVC results in a significant amount of waste PVC. Waste PVC is a distinctive solid waste containing chlorine because PVC monomers contain 56.7 wt.% chlorine [5]. The recycling and disposal processes of waste PVC are different from those of other types of plastic wastes due to the incorporation of chlorine, various stabilizers, plasticizers, and flame retardants during the manufacturing of PVC products [6].
Traditional disposal methods of waste PVC include incineration [3], landfilling [7], and mechanical recycling [8]. However, incineration of PVC may release HCl, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) and other hazardous materials [3]. Air pollution control devices are required to control the emissions of these substances, which increases the investment and operation costs. Landfills are becoming full, and places suitable for new landfills are becoming fewer, and the leachates and volatiles may cause severe environmental pollution [7]. Another significant drawback of landfilling is that the chemical energy in waste PVC is linearly lost rather than circularly recycled. Mechanical recycling involves cleaning, crushing, and granulating collected waste PVC plastic to recover the PVC plastic [8]. The recovered PVC plastic serves as raw material for production or as a filler in other plastic products. Mechanical recycling offers simplicity, cost-effectiveness, full energy recovery from waste products, and reduced pollution. However, recycling waste PVC imposes higher requirements on the homogeneity of raw materials. Otherwise, recycling may result in a significant decline in product performance.
On the other hand, the treatment of waste PVC by chemical recovery methods, including pyrolysis, hydrothermal treatment, ionic liquid dissolution, mechanical ball milling, and gasification, has been the focus of research for decades, not only for the waste management of waste PVC, but also for the energy recovery [1,2,9]. Blazsó et al. [10] investigated the composition of pyrolysis products from a blend of small amounts of PVC plastics with low-density polyethylene at different temperatures. It was observed that the presence of PVC resulted in pyrolysis products containing HCl and chlorine-containing organics. Poerschmann et al. [11] conducted a hydrothermal carbonization test on PVC plastics and found that the dechlorination efficiency of PVC increased with higher hydrothermal reaction temperatures. At temperatures above 235 °C, PVC undergoes nearly complete chlorine removal, with coke being the predominant recycling form. Colnik et al. [12] demonstrated the degradation performance of waste PVC with supercritical water. It was observed that the PVC in SCW decomposed into gas, oil, water-soluble, and solid products. The yield of gas and oil phases increased with increases in temperature and reaction time. The main part of chlorine remained in the aqueous phase. Glas et al. [13] investigated the use of different ionic liquids for the removal of chlorine from PVC plastics. It was found that anions in ionic liquids played a significant role in the dechlorination process. Tetrabutylphosphonium chloride ionic liquids achieved more than 98% chlorine removal from PVC within one hour. Yoshinaga et al. [14] investigated the hydrolysis and dechlorination characteristics of PVC plastics in organic solutions. It was demonstrated that NaOH effectively removes chlorine from plastics in an organic solution at 80 °C. After 3 h of hydrolysis, PVC dechlorination efficiency reached 98.8%, with chlorine primarily existing in the solution as NaCl. Tongamp et al. [15] combined PVC plastic with oyster shells for mechanochemical dechlorination. After stirring for 2 h, more than 95% of the chlorine reacted with oyster shells and calcium oxide to produce CaCl2. Gasification of plastics pursues a maximum conversion to a gas product with tar and char being the undesirable byproducts, covering air gasification, steam gasification, co-gasification with other materials, and novel methods like pyrolysis and in-line catalytic steam reforming. Because of the characteristics of waste plastics, such as low thermal conductivity, high volatile content, and remarkable tar formation, they require a suitable gasifier [16]. In the gasification process of waste PVC, the high chlorine content is a big and special challenge that separates it from other plastics. Zabłocka-Malicka et al. [17] conducted gasification tests on PVC wrapping materials for electrical wires. It was reported that 28% of the chlorine in PVC appeared in the gasification residue as CaCl2, while 71% was present in the gasification gas condensate as HCl. Cho et al. [18,19] investigated the air gasification of mixed plastics containing PVC, employing dolomite and activated carbon as catalysts. It was observed that most of the chlorine in the material was retained in the gasification residue, and the gasification condensate contained thousands of ppm of chlorine. Borgianni et al. [20] conducted a comparative study to investigate the impact of CaCO3 and Na2CO3 on the gasification of 10% PVC plastics. Na2CO3 demonstrated higher dechlorination efficiency than CaCO3. When the Na2CO3 to PVC material ratio exceeded 1, it effectively removed chlorine from the gas, with most chlorine absorbed and stored in the ash.
For these chemical disposal methods of waste PVC, the main challenge remains to achieve efficient dichlorination of waste PVC and effectively recycle the chemical energy of waste PVC simultaneously. Molten salt gasification (MSG) offers the best available method to efficiently dechlorinate and utilize the resources from PVC waste. Molten salt gasification is a highly efficient thermal disposal technology that involves the controlled oxidative degradation of organic matters within alkali metal molten carbonates, leveraging the unique properties of molten salt [21,22]. Molten salt plays several crucial roles in the molten salt gasification process. Firstly, it transforms the reaction phase interface between the material and carrier gas from the ‘gas–solid interface’ in the air to the ‘gas–liquid, liquid–solid interface’ in the molten salt [23]. This renewal occurs cyclically, with molten salt serving as an effective diffusion medium for both the material and carrier gas. Simultaneously, the molten salt acts as a catalyst for the decomposition and oxidation of organic components in the material [24]. Additionally, oxygen chemically dissolves in the molten salt, generating peroxide ions (O22−) and superoxide ions (O2−), which intensify the oxidation of organic components in the material [25]. Furthermore, molten carbonate dissolves the ash produced after oxidative degradation of the material, storing insoluble matter and heavy metals in the salt bath [24,26], which reduces the generation of fly ash and heavy metal emissions. Acidic gases such as HX and SO2 are absorbed and neutralized by the molten salt, reducing the need for off-gas desulphurization and deacidification equipment [27]. The absence of chlorine in the exhaust gas inhibits the synthesis reaction of PCDD/F, leading to the decomposition and destruction of dioxin precursors in the molten salt, thereby eliminating the precursor synthesis pathway of PCDD/F [28,29]. Last but not least, molten salt has a high heat capacity, which helps to minimize the impact of temperature and feed fluctuations on the reaction process due to the thermally stable environment provided by the salt bath [30].
Hsu et al. [27] conducted an experiment on disposing of waste organic solvents containing polychlorinated biphenyls (with a polychlorinated biphenyl concentration of 1567 mg/L) through molten salt oxidation. The results showed that at 950 °C, the oxidative degradation efficiency of polychlorinated biphenyls exceeded 99.999%, HCl emission was only 0.0015 g/h, and the concentration of PCDD/F in the exhaust gas was below 10 pg TEQ/Nm3. Pandeti et al. [31] investigated the oxidative degradation of C6H5Cl through molten salt. The results showed that molten carbonate almost completely absorbed the Cl in C6H5Cl. The degradation efficiency of chlorobenzene in molten salt oxidation was influenced by the temperature of molten salt and the residence time of bubbles in the salt bath. Higher temperatures and longer residence times resulted in a decrease in incomplete oxidation products in the exhaust gas. Yang et al. [32] conducted a study on the molten salt oxidation of halogenated sulfurous plastics. The results indicated that molten salt effectively absorbed hydrogen halide and halogen monomer gases released from halogen-containing plastics. The entrapment of heavy metals in plastics during molten salt oxidation was primarily influenced by gas flow rate, temperature, and the nature of the heavy metal elements. Flandinet et al. [23] conducted a study on the thermal cracking of printed circuit boards in molten alkali. At 300 °C, molten alkali effectively absorbed the bromine in the circuit boards, with more than 99.99% of bromine being absorbed and neutralized by the molten salt, and all metals were efficiently recovered. In our previous works, molten salt demonstrated high halogen retention efficiency and low PCDD/F generation for the disposal of waste printed circuit boards and hazardous waste with high salt content [33,34]. It should be noted that during the molten salt oxidation process, an ash content of less than 20 wt.% must be maintained to sustain the fluidity of the melt, and the concentration of alkali metal carbonates should be higher than 15 wt.% to maintain the acid gas absorption capability. Periodic carbonates replacement is required to maintain excellent performance [24]. In conclusion, molten salt exhibits notable efficiency in halogen retention and thermal destruction of organic components during the treatment of halogenated wastes.
In the present work, molten salt gasification was employed to treat waste PVC. Molten salt gasification experiments were conducted to evaluate the gasification characteristics, chlorine retention efficiency by molten salt, PCDD/F generation, and heavy metal distribution during the MSG process. The results showed that the molten salt significantly improved the gasification performance of waste PVC together with low HCl and PCDD/F generation in the gas product.

2. Materials and Methods

2.1. Materials

Waste PVC water pipes obtained from a recycling enterprise in Jinhua, Zhejiang province (China), were selected as the feeding material in this study. The waste PVC pipes were crushed, dried for 24 h at 105 °C, and sieved into particles. The fraction passed through a 20-mesh screen but retained at a 40-mesh screen was chosen as the experimental sample. Its proximate and ultimate analysis, along with the chlorine and heavy metal contents (Cu, Pb, and Zn), are presented in Table 1. The values of moisture, ash, volatiles, and fixed carbon were determined according to the Chinese GB/T 212-2008 method [35]. The content of C and H were determined according to the Chinese GB/T 476-2008 method [36]. The content of N was determined according to the Chinese GB/T 19227-2008 method [37]. The content of S was determined according to the Chinese GB/T 214-2007 method [38]. The chlorine content of the waste PVC scraps was determined using the pyrohydrolytic-IC method with an ion chromatograph (792 Compact IC, Metrohm, Herisau, Switzerland), while the Cu, Pb, and Zn content was analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES, 720ES, Agilent, Santa Clara, CA, USA).
A ternary carbonate salt mixture (Li2CO3-Na2CO3-K2CO3) with a molar ratio of 0.44/0.30/0.26 and a eutectic point of 393 °C [39] was selected for the present study. All the chemical reagents used in the present study were of analytical grade (≥99.5% purity) and were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China).

2.2. Experiment Setup and Procedure

The molten salt gasification experiments were performed using a lab-scale molten salt reaction system with a 30–150 g/h waste PVC treatment capacity, which was modified from the reaction setup used in our previous works [33,34]. Figure 1 shows the diagram of the setup. The setup primarily included a screw feeding, a control section for air and nitrogen injection, a stainless-steel reaction vessel heated up by surrounding ceramic heaters, and an off-gas analysis unit. The reaction vessel had an internal diameter of 68 mm and was made of 310S stainless steel. During the MSG process, waste PVC scraps were introduced into the salt bath via a 12 mm internal diameter vertical stainless-steel tube and the carrier gas. To introduce the PVC and carrier gas into the molten salt bath, the screw feeding unit of the PVC scraps was sealed to generate sufficient pressure. The feeding tube was equipped with two parallel baffles, each containing two dozen small holes (3 mm in diameter) designed to produce small bubbles instead of the giant bubbles produced by a nozzle. This design enhanced the mass transfer between the carrier gas, waste PVC, and the molten carbonates. The immersion depth of the feeding tube was fixed at 120 mm during the MSG process.
The quantity of (LNK)2CO3 salt used was 1500 g. The experimental temperatures for the MSG process were selected as 600 °C, 650 °C, 700 °C, and 750 °C, with air equivalence ratio (ER) varying from 0.2 to 0.8. Air and nitrogen acted as the carrier gases, with nitrogen added in each test. The total volume of carrier gas in each test was maintained constantly at ER = 1.0, ensuring relative stability in the residence time of the gas mixture in the salt bath and the fluctuation of the molten salt. The scraps were fed at a rate of 1.0 g/min. After the system had achieved stability, gas sampling was conducted. All experimental tests were repeated three times, and the average data were selected.

2.3. Methods of Sampling and Analysis

The gas composition was analyzed using a gas chromatographer (micro GC-490 analyzer, Agilent), and mainly comprised N2, CO2, CO, H2, CH4, and C2Hm. Total gas yield was quantified based on the nitrogen balance from the known nitrogen flow rate and the nitrogen content in the gas product [40]. The gasification efficiency, carbon conversion efficiency, and the lower heating value (LHV) of the gas product were calculated based on the gas composition [41]. The chlorine in the gas was absorbed by an alkaline solution (0.05 M Na2CO3 and NaHCO3), and the concentration of chlorine in each solution was later analyzed by an ion chromatograph (792 Compact IC, Metrohm). The chlorine retention efficiency of the molten carbonates was calculated from the chlorine concentration in the alkaline solution. The PCDD/F in the flue gas was collected by XAD-2 resin together with a toluene solution. The clean-up procedure and the analysis of the dioxins were performed according to the US EPA method 1613 (US EPA, 1994) [42]. A high-resolution gas chromatograph (HRGC) in the 6890 series from Agilent Co., Ltd (USA) coupled with a JMS-800D high-resolution mass spectrometer (HRMS) from JEOL Co., Ltd. (Tokyo, Japan) were used for PCDD/F detection and analysis. Two H2O2/HNO3 impingers were used to collect the heavy metals in the off-gas, and the concentrations of heavy metals were analyzed using the ICP-OES method (720ES, Agilent). XRD analysis was performed on the fresh and treated drained salt to determine the crystal structures using an X-ray diffractometer (PANalytical X’Pert’3 Power, Panalytical, Almelo, The Netherlands).

3. Results

3.1. Gasification Characteristics of PVC during MSG

Figure 2 illustrates the gasification characteristics of waste PVC during the MSG process. Figure 2a displays the trends of gas product yield as a function of the temperature and ER. The gas yield noticeably increased with temperature due to a higher temperature providing more favorable conditions for the thermal cracking of PVC and the secondary cracking of the intermediates, and a higher ER also enhanced the yield of gas products. For example, at 750 °C, the gas yield increased rapidly from 0.37 Nm3/kg PVC to 0.52 Nm3/kg PVC as the ER increased from 0.2 to 0.8. It was likely that the partial oxidation reactions of the intermediates (C + O2 → CO2, C + 1/2O2 → CO, CnHm +n/2O2 → nCO + m/2H2) were strengthened at higher ERs, and led to an increase in the gas yield. The highest gas yield achieved for PVC molten salt gasification disposal was 0.52 Nm3/kg PVC at 750 °C and ER = 0.8. The variation of the syngas (CO and H2) yield as a function of the temperature and ER is shown in Figure 2b. The trend in syngas yield had a similar tendency to that of the gasification gas yield. Both the increase of the temperature and ER enhanced the yield of the syngas.
Figure 2c depicts the LHV of the gas product at the different gasification temperatures and ERs. As presented in Figure 2c, the LHV of the gas diminished crucially from 8.3 to 5.8 MJ/Nm3 as the ER increased (750 °C). It was likely that the increase of ER promoted the further oxidation of the intermediates of PVC cracking (CO + 1/2O2 → CO2, H2 +1/2O2 → H2O), produced more CO2, and diminished the contents of CO, H2, and other combustible components in the gas product, thus reducing the LHV of the gasification gas. The impact of increasing temperature on the LHV of the gas was much less significant than that of increasing ER, which could be attributed to the fact that the temperature had a minor effect on the components of gas during the MSG process of waste PVC.
The effect of the temperature and ER on the gasification efficiency of waste PVC during the MSG process is exhibited in Figure 2d. With an increasing temperature, the gasification efficiency increased from 28.2% to 41.2% (ER = 0.4) as the gas yield increased from 0.37 to 0.52 Nm3/kg PVC. With an increasing ER, the gas gasification efficiency initially increased and then decreased under all the temperatures. A higher ER enlarged the gas yield, which promoted the gasification efficiency. However, higher ER also led to a lower LHV of the gas product, leading to a negative impact on the gasification efficiency. The gas gasification efficiency was the result of the combination of the two opposing effects. As a result, the gas gasification efficiency initially increased and then decreased as the ER rose from 0.2 to 0.8.
Figure 2e shows the relationship of carbon conversion efficiency as a function of temperature and ER. The carbon conversion efficiency of PVC increased as the temperature increased due to the promotion of the gasification gas yield. The effect of ER on the carbon conversion efficiency in PVC was more significant than that of the temperature. Increasing ER could promote the oxidation of the intermediates and generate more gasification gas, thereby increasing the carbon conversion efficiency in PVC.
Figure 3 illustrates the composition of gas products under variation temperatures and ERs. Figure 3a shows the impact of different temperatures on the gasification gas composition at ER = 0.4. The gas product of PVC by MSG exhibited a markedly higher CO2 content (70.3 vol.%–78.1 vol.%) compared to the situation of PVC gasification in a fluidized bed [43]. This phenomenon could be attributed to the catalytic effect of the molten carbonates and the superoxide ions, which promoted the oxidation of the intermediates (C + O2 → CO2, CO + 1/2O2 → CO2), thereby increasing the yield of CO2. With the rise of the gasification temperature, the contents of CO and H2 in the gas gradually increased while the fraction of CO2 decreased. This may be attributed to the enhancement of reactions such as C + 1/2O2 → CO and CnHm +n/2O2 → nCO + m/2H2 during the MSG process of waste PVC, as the gasification temperature increased with an unchanged ER. These reactions led to an increase in the contents of CO and H2 and a decrease in the proportion of CO2. It should be noted that HCl was absorbed by the molten carbonates during the MSG process that concurrently generated some CO2, and this part of the CO2 was excluded from the calculation of CO2 yield in gas products in this paper. Figure 3b depicts the influence of increasing ER on the composition of the gas phase at 750 °C. As the ER increased, an increase in the contents of CO and CO2, accompanied by a decline in H2, CH4, and C2Hm contents, was observed. This was primarily attributed to the enhanced oxidation of the intermediates of PVC cracking with the ER increase (CnHm + (n + m/2) O2 → nCO2 + m/2H2O). The formation of H2, CH4, and C2Hm was less than that of CO and CO2, as CH4 and C2Hm were consumed by oxidation, leading to a decrease in H2, CH4, and C2Hm contents.

3.2. Chlorine Retention and PCDD/F Generation

Table 2 shows the retention efficiency of chlorine by molten carbonates under different gasification temperatures and ERs. It was evident that molten carbonates could efficiently absorb chlorine, with a retention efficiency of more than 99.6% under all conditions. Most of the chlorine in PVC was absorbed and neutralized by molten carbonates, and was retained in the salt bath. The proportion of chlorine in the gas phase was less than 0.5% of that in the feed PVC.
The thermal decomposition of PVC took place in two stages [6]. The first step involved the removal of HCl from PVC and the release of the intermediate volatile components, occurring at a temperature range of 250–350 °C. The second stage involved the further oxidation and decomposition of the intermediate products. At the MSG temperature range, chlorine in PVC was completely released. During the MSG process of PVC, PVC began to dechlorinate soon after it descended. Upon the mixture of solid and gas being immersed into the salt bath, HCl was absorbed rapidly by the molten carbonates, with minimal impact from the temperature changes of the molten salt. The release of HCl was weakly affected by the atmosphere, resulting in weak effects from the change in ER on the absorption of chlorine. During the MSG processing of PVC, most of the chlorine was absorbed by the molten salt and retained in the salt bath, leading to a low HCl content in the gas-phase product, which inhibited the generation of PCDD/F. Glas et al. [13] used the different ionic liquids for the removal of chlorine from waste PVC, and found the chlorine removal efficiency was more than 98% within 1 h. It seemed that the chlorine absorption through MSG needed less time and had a higher efficiency.
Chlorine in PVC is the main source of chlorine for PCDD/F generation [44]. The generation of PCDD/F during the thermal disposal of waste PVC has a significant impact on its clean and efficient disposal. This paper presents a preliminary study on the generation characteristics of PCDD/F of waste PVC during MSG processing. Table 3 shows the PCDD/F generation characteristics in the gas phase at different temperatures and ERs during the MSG processing of PVC. It was observed that as the gasification temperature increased from 600 °C to 650 °C, there was a significant decrease in the yield of PCDD/F and the corresponding toxic equivalents, from 632.83 pg/g PVC and 11.98 international toxicity equivalent quantities (I-TEQ) pg/g PVC to only 42.38 pg/g PVC and 1.95 I-TEQ pg/g PVC, respectively. When the temperature was further increased to 750 °C, the generation of PCDD/F and I-TEQ values decreased to 18.41 pg/g PVC and 1.38 I-TEQ pg/g PVC, respectively. The decreasing trend of PCDF was greater than that of PCDD, resulting in a decreasing PCDF/PCDD ratio with increasing temperature. The generation of PCDD/F during the MSG treatment of PVC was much lower than that of normal gasification and incineration. This was due to the low chlorine content in the gas phase as the molten carbonates absorbed and neutralized most of the HCl released during the thermal decomposition of PVC, and the lack of chlorine source had a significant inhibitory effect on the generation of PCDD/F. Katami et al. [45] investigated the generation of PCDD/F during the incineration of mixed plastics containing PVC. The results showed that incineration of the PVC contained plastics led to a PCDD/F generation of 824–8920 ng/g. Yasuhara et al. [46] investigated the effect of PVC addition on PCDD/F generation during the incineration disposal of waste newspapers. It was reported that the PCDD/F generation was only 0.186 ng/g when the waste newspapers were burned alone. However, when PVC was added (mixture chlorine content of 5.1 wt.%), the PCDD/F generation jumped to 146 ng/g. A comparison of the results indicated that the PCDD/F generation in the disposal of pure waste PVC by MSG processing was 2–3 orders of magnitude less than that from other thermal disposal processes of PVC-containing materials.
During the MSG processing of waste PVC, most of the released chlorine reacted with the molten carbonate (M2CO3 + 2HCl → 2MCl + HCl + H2O + CO2) to produce the corresponding salts that were retained in the salt bath. On the other hand, the Deacon reaction (2HCl + 1/2O2 → H2O + Cl2) was inhibited due to the absence of HCl [44], resulting in a significant reduction in the generation of PCDD/F during the MSG process. The low generation of PCDD/F was due to the lack of the chlorine source, which was also confirmed by the low fraction of chlorine in the gas product.
Figure 4 shows the distribution of homologue groups (PCDD + PCDF = 100%) at different temperatures and ERs. The main homologues comprising PCDD/F at different temperatures were OCDD, HpCDD, and OCDF, which coincided with the high chlorination level of PCDD/F shown in Table 3. The proportions of these three homologue groups were more than 70%, and the proportion of the three groups declined with increasing temperature. The proportions of low-chlorinated homologue groups (TCDD, PeCDD, HxCDD, TCDF, and PeCDF) increased, which was mainly due to the increase of temperature promoting the dechlorination reaction of high-chlorinated homologues, leading to a decrease in the chlorinated level of PCDD/F. The homologue distribution of PCDD/F was not significantly affected by different ERs during the MSG process. This was mainly because the absorption of HCl by molten carbonate was not affected by ER. Moreover, the molten salt exhibited high HCl retention efficiency under different ERs, which led to a reduction in the generation of PCDD/F. Therefore, the difference in the homologue distribution of PCDD/F under different ERs was relatively small.

3.3. Heavy Metals Entrainment and Drained Salt

The distribution of heavy metals (Cu, Pb, Zn) in the exhaust gas during the MSG processing of PVC was analyzed to determine their relationship with temperature and ER, as illustrated in Figure 5. Due to the melting effect of molten salt on the inorganic components, the entrainment of heavy metal elements (Cu, Pb, Zn) in PVC in the gas phase was less than 0.2%, while the retention efficiency of the Cu, Pb, and Zn by molten carbonates was higher than 99.8% in all conditions. The influence of the gasification temperature and ER on the entrainment of heavy metals was minimal. During the MSG process, the entrainment of Cu, Pb, and Zn by gas increased slowly with temperature. At 750 °C and ER = 0.4, molten salt demonstrated an absorption and retention efficiency of more than 99.9% for these three heavy metals. As the total amount of carrier gas fed into the molten salt bath was fixed, changing the ER had little effect on the gas flow rate and the disturbance of the gas flow in the salt bath. Therefore, the ER had little impact on the distribution of heavy metals. The retention efficiencies of Cu and Zn in the molten salt were both more than 99.9%, and the retention efficiency of Pb in the molten salt was more than 99.8%. The low entrainment of heavy metals was due to the relatively low temperature of the MSG process compared to incineration, which reduced the volatilization of heavy metals [27]. Furthermore, the gasification process of PVC occurred in the salt bath, where the molten carbonates absorbed most of the HCl released by PVC directly. The generation of the volatile metal chlorides was surpassed, thus reducing the distribution of heavy metals in the gas phase [26].
Figure 6 shows the XRD patterns of the drained salt before and after PVC disposal. It shows that the major crystalline phases of the fresh salt mixture were sodium-lithium carbonate, sodium-potassium carbonate and lithium-potassium carbonate. After the MSG of waste PVC, the major phases remained the same, while NaCl was formed due to the absorption of HCl during the treatment. It should be noted that as the proportion of NaCl increased, the proportion of molten carbonate decreased, which may have adversely affected the retention efficiency of HCl by the molten salts.

3.4. Reaction Mechanism Discussion

The MSG reaction of waste PVC is a complex process involving solid–gas, gas–liquid, and gas–gas reactions. The main reactions of the PVC MSG process in molten carbonates with carrier gas involved are summarized and shown in Figure 7. According to the relevant research [1,24,29] and the experimental results under different conditions, the main reactions of the waste PVC MSG process include PVC’s thermal degradation reaction, oxygen chemical dissolution reaction, gasification reaction, and the absorption reaction between carbonates and HCl. Specifically, they are:
R1: PVC’s thermal degradation reaction. During this process, HCl and intermediates are produced, which are mainly affected by the reaction temperature.
R2: Oxygen was chemically dissolved and catalytically oxidized to form potent oxidizing agents O2− and O22−. It was likely that the potent oxidizing agents, together with the molten carbonates, enhanced the oxidation process of the intermediates.
R3: the intermediates were further oxidized by the oxidizing agent (O2, O2−, and O22−), forming CO, CO2, CH4, H2, and H2O.
R4: The HCl absorption reaction between the released HCl and the carbonates resulted in the formation of NaCl, as indicated by the XRD results.

4. Conclusions

In this study, MSG was used for the disposal of waste PVC. The characteristics of the MSG of waste PVC, including the gasification characteristics, chlorine retention efficiency, PCDD/F formation, and the distribution of heavy metals, were investigated. High temperature and ER enhanced the yield of PVC gasification gas and carbon conversion efficiency. The gasification efficiency of PVC reached 41.2% at 750 °C and ER = 0.4, with a gasification gas yield of 0.442 Nm3/kg PVC. Molten carbonates efficiently absorbed chlorine, with a chlorine retention efficiency of more than 99.6%. Increasing the gasification temperature resulted in a significant reduction in the generation of PCDD/F during MSG processing. The PCDD/F generation was less than 19 pg/g PVC with an I-TEQ of less than 1.4 pg/g PVC at 750 °C. Most of the heavy metals were retained in the salt bath during the MSG process. Further research will be conducted to investigate the optimum MSG conditions, given the factors such as the ratio of feed to carbonates, molten salt composition, and the composition of the waste plastics mixture.

Author Contributions

Conceptualization, C.L. and T.Y.; methodology, C.L.; validation, Q.H.; formal analysis, S.C.; investigation, M.L. and C.L.; data curation, S.W.; writing—original draft preparation, S.C., M.L. and C.L.; writing—review and editing, C.L. and T.Y.; supervision, Q.H.; project administration, C.L.; funding acquisition, Q.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research program was supported by the Key Research and Development Program of Zhejiang province (No. 2022C03082), Department of Science and Technology of Zhejiang Province.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors acknowledge financial support from the Key Research and Development Program of Zhejiang province (No. 2022C03082).

Conflicts of Interest

Author Tianfeng Yang was employed by the company Huangzhou Mingsheng New Energy Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

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Figure 1. Scheme diagram of the lab-scale molten salt gasification reaction system.
Figure 1. Scheme diagram of the lab-scale molten salt gasification reaction system.
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Figure 2. Effect of the ER and temperature on the gasification characteristics of waste PVC using molten salt: (a) gas yield; (b) syngas yield; (c) LHV of gas product; (d) gasification efficiency; (e) carbon conversion efficiency.
Figure 2. Effect of the ER and temperature on the gasification characteristics of waste PVC using molten salt: (a) gas yield; (b) syngas yield; (c) LHV of gas product; (d) gasification efficiency; (e) carbon conversion efficiency.
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Figure 3. Influence of temperature and ER on the gas composition: (a) gas composition under different temperatures (ER = 0.4); (b) gas composition under different ERs (temperature =750 °C).
Figure 3. Influence of temperature and ER on the gas composition: (a) gas composition under different temperatures (ER = 0.4); (b) gas composition under different ERs (temperature =750 °C).
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Figure 4. Distribution of homologue groups under different temperatures and ERs: (a) homologue groups under different temperatures; (b) homologue groups under different ERs.
Figure 4. Distribution of homologue groups under different temperatures and ERs: (a) homologue groups under different temperatures; (b) homologue groups under different ERs.
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Figure 5. Distribution influence of temperature and ER on the entrainment and retention efficiency of (a) Cu; (b) Pb; (c) Zn.
Figure 5. Distribution influence of temperature and ER on the entrainment and retention efficiency of (a) Cu; (b) Pb; (c) Zn.
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Figure 6. XRD patterns of carbonate mixture before and after MSG disposal of waste PVC.
Figure 6. XRD patterns of carbonate mixture before and after MSG disposal of waste PVC.
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Figure 7. The reaction mechanism of waste PVC scraps in molten salt bath.
Figure 7. The reaction mechanism of waste PVC scraps in molten salt bath.
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Table 1. The characteristics of the waste PVC.
Table 1. The characteristics of the waste PVC.
Proximate analysis
(wt.%)		MoistureAshVolatilesFixed carbonTotal
0.4337.8549.9011.82100
Ultimate analysis
(wt.%)		CHNSO
27.642.220.790.3519.11
High heating value
(J/g)		Cl (wt. %)Cu (mg/g)Pb (mg/g)Zn (mg/g)
765111.610.392.4812.69
Table 2. Effect of the ER and temperature on chlorine retention efficiency of the molten salt.
Table 2. Effect of the ER and temperature on chlorine retention efficiency of the molten salt.
Temperature/ER600 °C650 °C700 °C750 °C
ER = 0.299.65 ± 0.0899.78 ± 0.1099.81 ± 0.0599.65 ± 0.11
ER = 0.499.75 ± 0.0699.92 ± 0.0599.71 ± 0.0899.77 ± 0.17
ER = 0.699.84 ± 0.0799.70 ± 0.1499.72 ± 0.1199.78 ± 0.10
ER = 0.899.71 ± 0.1599.79 ± 0.0699.83 ± 0.1299.61 ± 0.13
Table 3. Effect of the temperature and ER on the yield of PCDD/F, I-TEQ, the ratio of PCDF to PCDD, and chlorination level of PCDD/F.
Table 3. Effect of the temperature and ER on the yield of PCDD/F, I-TEQ, the ratio of PCDF to PCDD, and chlorination level of PCDD/F.
ConditionsPCDD/F
(pg/g PVC)		I-TEQ
(pg/g PVC)		PCDF/ PCDDCl-PCDDCl-PCDFCl-PCDD/F
600 °C, ER = 0.4632.83 ± 15.2111.98 ± 0.641.60 ± 0.137.65 ± 0.127.30 ± 0.217.43 ± 0.19
650 °C, ER = 0.442.38 ± 1.821.95 ± 0.081.36 ± 0.117.52 ± 0.197.02 ± 0.107.23 ± 0.16
700 °C, ER = 0.420.94 ± 1.771.35 ± 0.131.04 ± 0.197.47 ± 0.206.85 ± 0.197.15 ± 0.19
750 °C, ER = 0.418.41 ± 0.861.38 ± 0.170.84 ± 0.297.40 ± 0.176.75 ± 0.127.10 ± 0.14
750 °C, ER = 0.218.25 ± 1.031.16 ± 0.090.72 ± 0.107.50 ± 0.206.84 ± 0.117.22 ± 0.15
750 °C, ER = 0.616.75 ± 1.371.20 ± 0.110.85 ± 0.137.46 ± 0.166.83 ± 0.217.17 ± 0.17
750 °C, ER = 0.818.50 ± 1.821.13 ± 0.081.02 ± 0.097.50 ± 0.296.86 ± 0.237.18 ± 0.24
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Lin, C.; Yang, T.; Chen, S.; Li, M.; Wang, S.; Huang, Q. Characteristics of Molten Salt Gasification of Waste PVC. Processes 2024, 12, 306. https://doi.org/10.3390/pr12020306

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Lin C, Yang T, Chen S, Li M, Wang S, Huang Q. Characteristics of Molten Salt Gasification of Waste PVC. Processes. 2024; 12(2):306. https://doi.org/10.3390/pr12020306

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Lin, Chengqian, Tianfeng Yang, Siyu Chen, Minjie Li, Shoukang Wang, and Qunxing Huang. 2024. "Characteristics of Molten Salt Gasification of Waste PVC" Processes 12, no. 2: 306. https://doi.org/10.3390/pr12020306

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