Interaction and characteristics of furfural residues and polyvinyl chloride in fast co-pyrolysis

Abstract

This study investigated the interaction between the furfural residue and polyvinyl chloride co-pyrolysis using an infrared heating method. Various analytical techniques including production distribution analysis, thermal behavior, pyrolysis kinetic, simulated distillation and gas chromatography-mass spectrography (GCMS), and X-ray photoelectron spectroscopy were utilized to elucidate the pyrolysis characterization and reaction mechanism during the co-pyrolysis. Initially, the yield of co-pyrolysis oil increased from 35.12% at 5 °C·s^–1 to 37.70% at 10 °C·s^–1, but then decreased to 32.07% at 20 °C·s^–1. Kinetic and thermodynamic parameters suggested non-spontaneous and endothermic behaviors. GCMS analysis revealed that aromatic hydrocarbons, especially mono- and bi-cyclic ones, are the predominant compounds in the oil due to the presence of H radicals in polyvinyl chloride, suggesting an enhancement in oil quality. Meanwhile, the fixed chlorine content increased to 65.11% after co-pyrolysis due to the interaction between inorganic salts in furfural residues and chlorine from polyvinyl chloride.

Keywords

infrared heating / pyrolysis oil / polyvinyl chloride / chlorine / co-pyrolysis

1 Introduction

Polyvinyl chloride (PVC) is widely used because of its durability and low cost. It was estimated that the global demand for PVC increased at an annual rate of about 3.2% until 2021, which will result in an accumulation of over 600 million tons of PVC waste by the end of 2050 [1]. The disposal of PVC (40%) goes to landfill, 32% leaks into the environment, 14% is incinerated, and only 14% is recycled, which poses a huge threat to human health and the nature environment [2]. Furthermore, furfural residue (FR), a byproduct of biomass hydrolysis, is characterized by high salt content and acidity, which have detrimental effect on land, the atmosphere, and waterways when it accumulates in significant quantities. Currently, FR is primarily utilized as a raw material for combustion, with limited high-value applications. Therefore, it is crucial to manage FR and transform it to high-value-added products.

Co-pyrolysis of PVC and FR has been considered a useful method for producing more valuable fuels [3]. Wu et al. [4] investigated how various plastics influence the distribution of products and their cooperative interactions in co-pyrolysis processes with biomass materials, such as rapeseed stalks, alongside PET, PP, and PVC. Their result demonstrated that primarily monocyclic aromatic hydrocarbons were yielded from the co-pyrolysis of rapeseed stalk and PET. The co-pyrolysis of rapeseed stalk and PP provided aliphatic hydrocarbons (mainly olefins) and promoted the generation of aliphatic hydrocarbons and alcohols due to the synergistic effects. The co-pyrolysis of rapeseed stalk and PVC can accelerate the removal of HCl with the Cl⁻ compounds settled in solid residue due to the presence of hydroxyl groups in biomass, which is beneficial for conversion from waste to energy. Yu et al. [5] studied the distribution of chlorine under microwave irradiation during the co-pyrolysis of sophora wood and PVC. The result indicated that microwave pyrolysis of PVC effectively reduced the generation of dioxin and other harmful substances in products, as well as significantly increased oil production with the addition of sophora wood. However, the co-pyrolysis of FR and PVC has rarely been studied, and there have been few efforts to understand the fundamentals behind PVC co-pyrolysis with FR under infrared heating, especially, due to the presence of inorganic salt and metals including K, Zn and Fe in FR. It plays a catalytic role on PVC decomposition and the chlorine capture. However, the specific catalytic effect is unclear.

Therefore, this paper aims to bridge this knowledge gap by investigating the co-pyrolysis behaviors of FR and PVC. The migration and chemical transformation of chlorine and the catalytic interactions of FR and PVC-derived compounds are probed via X-ray photoelectron spectroscopy (XPS). This work is expected to provide a new insight into the co-pyrolysis of FR and PVC and deal with PVC waste while also efficiently utilizing FR.

2 Experimental

2.1 Materials and experiments

PVC (120 mesh with 56% chlorine content) was obtained from Guangzhou, China. The FR powders were dried in an oven at 105 °C for more than 2 h. Proximate analysis and ultimate analysis results are shown in Tab.1. Fig.1 shows the schematic diagram of a fast infrared heating co-pyrolysis, consisting of an infrared heated fixed bed, gas supply, and product collection. Furthermore, the mass flowmeter systematically adjusted the flow of nitrogen into the reactor to ensure an inert environment. Prior to initiating each experiment, a mixture of 4 g of PVC and FR was introduced into the reactor, followed by a 15-min infusion of nitrogen at a rate of 100 mL·min^–1. Pyrolysis experiments were conducted at heating rates of 5, 10, 15, and 20 °C·s^–1, reaching 700 °C. The process was halted after maintaining the target temperature for 30 min. Condensable vapors were gathered in an immersed U-tube maintained at a temperature of –25 °C. The pyrolysis gas underwent purification using acetone within two cleaning breakers, followed by a sodium bicarbonate solution and color-changing silica gel in a washing cylinder. The pyrolysis gas was quantified by a wet gas flowmeter and then stored in a gas bag for further analysis by gas chromatography.

Tab.1 Proximate analysis and ultimate analysis of FR and PVC

Name	Proximate analysis/(wt %, ad)					Ultimate analysis/(wt %, daf)
	Mad	Aad	Vad	FCa)		C	H	Othera)	N	S
FR	5.63	7.4	67.42	19.55		49.4	5.2	44	0.4	0.9
PVC	0.11	0.08	94.06	5.75		39.3	5.04	55.66	–	–

a) Calculated by subtraction of differences. ad: air-dried basis, daf: dry ash-free basis.

Fig.1 Schematic diagram of fixed-bed co-pyrolysis system equipped with fast infrared heating.

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2.2 Analytic methods

Thermal gravimetric (TG) analysis was conducted using an STA449F3 simultaneous (integrated) thermal analyzer (Germany). Gas composition including H₂, CH₄, CO, CO₂, C₂–C₃ was determined using a micro-GC analyzer (Inficon Micro Fusion, Switzerland). Additionally, oil composition analysis was performed using an SHIMADZU GC-MS-TQ80040. An Agilent 78900AGG was used for simulated distillation to determine the fraction and carbon number in oil. The presence of Cl species on the char surface was investigated using X-ray photoelectron spectroscopy (ESCALAB 250XI). Chlorine content in the liquid phase was measured using an ion meter (PXSJ-216F, Shanghai Lei-ci) equipped with chlorine detection capability.

3 Results and discussion

3.1 Thermogravimetric analysis

Fig.2 shows the derivative thermogravimetry (DTG) and TG of mixture of FR and PVC at various heating rates. As can be seen, the co-pyrolysis process was divided into two stages: the first stage was observed in the range of 240–380 °C, and the second stage was observed around 425 °C. Notably, the first stage was the primary pyrolysis stage, characterized by maximum weight losses of 25.6%, 28.2%, and 31.1% at heating rates of 20, 30, and 40 K·min^–1, respectively. In the second stage, weight losses were 62.44%, 60.8%, and 61.61% at heating rates of 20, 30, and 40 K·min^–1, respectively. Due to the PVC’s composition of long polymer chains and chlorine, its decomposition predominantly occurred in the first stage, generating significant quantities of HCl [6]. Concurrently, cellulose, a crystalline material composed of large-molecular-weight polymers from glucose monomer, was transformed into anhydrocellulose and levoglucosan as primary products. Lignin, rich in aromatic rings and diverse branches, exhibits a broad spectrum of chemical bond activities, facilitating its degradation over a wide temperature range [7,8]. The higher decomposition rate in the first stage compared to the second stage can be attributed to the organic volatiles being encapsulated within the softened PVC, leading to a subsequent decline in the decomposition rate. Both TG curves were higher than the theoretical value, indicating the existence of the synergistic effect between FR and PVC.

Fig.2 (a) TG and (b) DTG curves of FR/PVC at different heating rates (20, 30, 40 K·min–1).

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3.2 Kinetic and thermodynamic analysis

Fig.3(a, b) present the calculated results of the KAS and FWO methods at 20 K·min^–1 with a conversion from 0.2 to 0.7 with step of 0.1. The correlation coefficients (R²) for both the KAS and FWO methods exceeded 0.95, indicating the accurate reflection of changes in activation energy (E_a) and good adaptability [9]. As seen, the parallel alignment of FWO and KAS lines across each degradation phase implies a potential singular dominant mechanism or a composite mechanism integrating diverse reactions within the pyrolysis process. The E_a varying with different methods (KAS, FWO) are shown in Fig.2(c). From the data presented in the graph, there was a slight difference in the E_a values, ranging from 116.83 kJ·mol^–1 to 773.82 kJ·mol^–1, indicating the complexity of the interaction effect. The values of E_a could explain the reason behind the interaction effect. Furthermore, the magnitude of E_a reflected the ease with the occurrence of reaction. Notably, the maximum value of 773.82 kJ·mol^–1 was observed at 0.3 conversion, suggesting the presence of hemicellulose cellulose, and PVC [10,11]. The melt of PVC constantly coated on the FR, attributed to the difficulty of the devolatilization of organics. This explained the reason for the decrease in E_a with the conversion increasing. The decomposition of lignin mainly formed aromatic compounds, with the second peak occurring with a continued increase in conversion.

Fig.3 The fitting results of (a) FWO and (b) KAS methods and (c) the variation of activation energy with conversion rates.

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The thermodynamic parameters of FR and PVC are shown in Fig.4. Thermodynamic parameters, including A, ΔH, ΔG, and ΔS were derived from TG data collected at a heating rate of 20 °C·min^–1. The parameter A measures the collision intensity among reactions. It can be seen that the A-values, exceeding 10⁹ s^–1 in the conversion range of 0.2–0.7, suggested the highly reactive nature of co-pyrolysis process and the formation of complex compounds [12−14]. Furthermore, the ΔH represents other essential parameter that delineates the energy disparity between the solid reactant and the activated complex. During the co-pyrolysis of FR and PVC, all ΔH-values were positive, suggesting the necessary acquirement of external heat energy to generate bioenergy within an inert environment and to facilitate the formation of the desired product in the activated complex [14,15]. The ΔG-values are crucial for understanding the overall energy change of the system upon formation of the activated complex. The ΔG-values were used to determine and validate the probability of pyrolysis reactions. As shown in Fig.4(c), the ΔG-values fluctuated greatly with increasing conversion, illustrating the instability of the co-pyrolysis system. The range of ΔG-values were 99.58 to 474 kJ·mol^–1. The positive ΔG-values indicated that the co-pyrolysis required heat absorption and were a non-spontaneous process [15]. The ΔS-values were employed to measure the disorder of the reaction system and the thermodynamic system. The ΔS-values of co-pyrolysis ranged from 0.063 to 0.865 kJ·mol^–1 as the conversion increased from 0.2 to 0.7. The positive ΔS-values suggested that the co-pyrolysis process deviated from thermodynamic equilibrium and exhibited high reactivity [16]. Meanwhile, this illustrated a decrease in the time required for forming complex compounds and an increase in reaction rate.

Fig.4 The change trend of thermodynamic parameters with conversion rate (a) A; (b) ΔH; (c) ΔG; (d) ΔS.

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3.3 Pyrolysis products distribution

The distribution of co-pyrolysis products and gas composition of FR and PVC at varing heating rates is displayed in Fig.5. With increasing heating rates, the oil yield initially increased, followed by a subsequent decrease. The gas yield constantly increased with increasing heating rate. Furthermore, the char yield exhibited minor fluctuations compared to the variability in water yield. As the heating rate increased, the pyrolysis oil yield rose from 35.12% to a peak of 37.7% at a rate of 10 °C·s^–1, before decreasing to 32.07% at 20 °C·s^–1. The yield of water and char exhibited fluctuations around 7% and 35%, respectively. Moreover, the gas yield demonstrated a steady increase from 21.01% to 25.55% with increasing heating rate. This phenomenon can be attributed to the rapid heating rate causing lignin degradation, breaking side chains and opening aromatic rings [7,8]. This process, in conjunction with H radicals from PVC, contributed to an increase in pyrolysis oil yield. Moreover, the elevated heating rate promoted the release of volatiles by generating high pressure in the surrounding environment, thereby contributing to the consistent increase in gas yield [17].

Fig.5 Production distribution at varying heating rates (5, 10, 15, 20 °C·s–1).

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As depicted in Fig.6, H₂ showed a gradual upward trend with increasing heating rate. Additionally, the C₂–C₃ initially increased and subsequently declined. The yield of CO, CO₂, and CH₄ changed slightly. It is noteworthy that CH₄ was the predominant gas component. The C₂–C₃ yield increased from a minimum of 0.76% at a heating rate of 5 °C·s^–1 to a maximum of 1.042% at 15 °C·s^–1, before declining to 0.97% at 20 °C·s^–1. Simultaneously the yield of H₂ gradually rose from 3.17% at 5 °C·s^–1 to 3.55% at 20 °C·s^–1. Furthermore, CO, CO₂, CH₄ fluctuated around 5%, 3.7%, and 14.5%, respectively. The release of CH₄ can be attributed to the cracking of methoxyl groups (–O–CH₃) and methyl groups. Furthermore, certain hydrocarbons could further decompose and recombine to form methane through the cleavage of carbon chains and redistribution of hydrogen. This explained why CH₄ was the predominant gas component. Due to the cracking and reforming of significant amounts of carboxyl (C=O) and carboxylic acid (COOH), CO₂ and CO emerged as the primary gas components after CH₄.

Fig.6 Gas composition variation at different heating rates (5, 10, 15, 20 °C·s–1).

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3.4 Simulated distillation and GC-MS analysis

Fig.7(a) displays the chemical compounds in pyrolysis oil during the co-pyrolysis of PVC and FR. As shown, the furan and ketones constituted the primary chemical components in the pyrolysis oil. Importantly, the concentrations of furans, ketones, and aromatics initially decreased and then increased with increasing heating rate. Concurrently, the concentration of aldehydes consistently increased, reaching a maximum with increasing heating rates. Significantly, the concentration of aldehyde surged notably as the heating rate increased from 10 to 15 °C·s^–1. The content of furans, ketones, and aromatics decreased from 18.49%, 20.14%, and 19.74% at heating rate of 5 °C·s^–1 to a minimum 16.6%, 18.48%, and 15.05% at a heating rate of 15 °C·s^–1, before rebounding to 20.72%, 21.13%, and 16.25%, respectively. Furthermore, the content of aldehydes steadily rose from 10.39% at heating rate of 5 °C·s^–1 to 16.56% at heating rate of 20 °C·s^–1. The content of phenols, and chlorinated compounds both fluctuated around at 11.15% and 4.5%. The concentration of acids fluctuated around 17% below the heating rate of 15 °C·s^–1, experiencing a sharp decrease to 13.34% at a heating rate of 20 °C·s^–1. The result demonstrated that the increasing heating rate accelerated the decomposition of PVC, resulting in an increasing number of H radicals which acted as H-donor species for volatile FR. This enhanced the efficiency of carbon conversion and facilitated the cracking of the aromatic structure of FR. Furthermore, previous studies have reported carbon black not only promoted the chain scission but also the hydrogenation process [18−21], explaining the observed decline in aromatic compounds. As shown in Fig.7(b), the production of HCl initially increased and then decreased with the increasing heating rate. This phenomenon can be attributed to the rate of PVC decomposition which initially accelerates, leading to an increased generation of HCl with an increase in the heating rates. However, the release of HCl from PVC may slow down due to reaching a high-energy stage of the pyrolysis, as the rapid heating rate shortened the time interval to reach specific temperatures. The trend in HCl production coincided with that of phenols production because HCl acted as an acidic catalyst promoting phenols [22−25]. Additionally, hydroxyl groups in FR accelerated the removal of HCl gas. The GCMS results revealed only the presence of methylene chloride and 3-chloro-2-butanone in the pyrolysis oil. This can be explained by the fact that PVC, consisting of hydrocarbons, underwent cracking into short-chain aliphatics during co-pyrolysis which then combined with Cl radicals to form the methylene chloride [26,27]. Furthermore, the short-chain aliphatics reacted with peroxy radicals to form peroxy polyolefin chains, and the chloride radicals substituted a hydrogen atom in the chain via substitution reactions to form 3-chloro-2-butanone.

Fig.7 Co-pyrolysis of (a) oil components and (b) contents of chlorinated at different heating rates (5, 10, 15, 20 °C·s–1).

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Fig.8(a) illustrates the simulated distillation results of oil with varying heating rates. It revealed that the contents of gasoline, diesel oil, and kerosene showed similar trend. There initially was an increase, followed by a decrease. Conversely, the content of heavy oil exhibited an opposite trend. Diesel oil was the primary component of pyrolysis oil throughout the entire process. The contents of gasoline, diesel oil, and heavy oil increased from 10.4%, 38.23%, and 27.37% at 5 °C·s^–1 to 12.66%, 41.67%, and 31.09% at 15 °C·s^–1, respectively, before decreasing to 11.67%, 38.99%, and 28.39% at 20 °C·s^–1. The content of heavy oil steadily decreased from 51.38% at 5 °C·s^–1 to 45.67% at 15 °C·s^–1, then increased to 49.37% at 20 °C·s^–1. The observed trend can be attributed to the gasoline components produced in the initial stage undergoing secondary cracking at higher temperature, breaking down into even smaller molecules like gases. Furthermore, the chemical system reached a new thermal equilibrium, potentially favoring the formation of gases over liquid due to the entropy considerations. The rapid heating rate resulted in hydrocarbon molecules entering a transient high-energy state, making them more susceptible to fragmentation into smaller molecules [28−31]. The cracking reactions reduced the possibility of these intermediates transforming into more complex structures, thus favoring the production of gasoline.

Fig.8 Simulated distillation results of (a) oil and (b) main components of aromatics.

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3.5 Dechlorination and chlorine evolution

Fig.9 illustrates the distribution of chlorine’s chemical forms on the surface of PVC. As demonstrated, a portion of the chlorine from PVC was fixed in the solid after co-pyrolysis. Specifically, raw PVC primarily consists of 99% organic chlorine and 1% inorganic chlorine. After co-pyrolysis at a heating rate of 10 °C·s^–1 and the temperature of 700 °C, the amount of inorganic chlorine fixed in char significantly increased to 65.11%, while that of the organic chlorine decreased to 34.89%. The results suggested that chlorine mainly existed in the form of inorganic salts during the co-pyrolysis of PVC and FR. This can be attributed to Cl partially binding to the inorganic salts in FR and then being fixed in char. Additionally, it was beneficial for inorganic Cl to be absorbed on the microporous and fractured surfaces due to the production of more and larger pore spaces during the co-pyrolysis process [22]. On the other hand, the elimination and substitution reactions were the main pathways that cannot be ignored in causing the decrease of organic Cl. During the co-pyrolysis, the Cl and H on chloroallylic structures were eliminated, generating HCl and forming a conjugated double bond [31−35]. Plenty of –OH radicals produced by phenolic substances substituted the Cl atom on –CHCl–, improving the efficiency of dechlorination.

Fig.9 Cl2p XPS spectra of char from 700 °C, 10 °C·s–1, for (a) 30 min and (b) raw PVC.

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4 Conclusions

Co-pyrolysis of PVC and FR was conducted in an infrared fixed-bed reactor at heating rates of 5 to 20 °C·s^–1 to investigate the synergistic effects from a comprehensive perspective. The kinetic and thermodynamic parameters indicated that the co-pyrolysis process was complex and non-spontaneous. Production distribution results showed that the pyrolysis oil yields increased with the increasing heating rate, from 35.12% to a peak of 37.7% at a rate of 10 °C·s^–1, before decreasing to 32.07% at 20 °C·s^–1. Additionally, H radicals in PVC reacted with oxygen-containing compounds released from FR, facilitating the formation of mono- and bi-cyclic aromatic hydrocarbons. This led to a high content of aromatic hydrocarbons in the pyrolysis oil. Due to the presence of inorganic salts in FR and the formation of pores after pyrolysis, the fixation of inorganic Cl in char significantly increased to 65.11%. These analyses revealed the potential of co-pyrolysis strategies involving biomass and waste synthetic polymers in improving oil quality and enhancing product diversity.

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Competing interests

The authors declare that they have no competing interests.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (Grant Nos. 52104245 and 22178123); the National Natural Science Foundation of Chongqing (Grant Nos. cstc2021jcyj-msxmX0099, 2021XM3073, and 2019LY41). The deepest gratitude goes to the Analytical and Testing Center of Chongqing University.

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