Raynard Christianson Sanito1,2, Ya-Wen Chen2, Sheng-Jie You2,3,4, Hsi-Hsien Yang5, Yen-Kung Hsieh6, Ya-Fen Wang This email address is being protected from spambots. You need JavaScript enabled to view it.2,3,4

1 Department of Civil Engineering, Chung Yuan Christian University, Taoyuan 32023, Taiwan
2 Department of Environmental Engineering, Chung Yuan Christian University, Taoyuan 32023, Taiwan
3 R & D Centre for Membrane Technology, Chung Yuan Christian University, Taoyuan 32023, Taiwan
4 Center for Environmental Risk Management, Chung Yuan Christian University, Taoyuan 32023, Taiwan
5 Department of Environmental Engineering and Management, Chaoyang University of Technology, Taichung 413310, Taiwan
6 Marine ecology and conservation research center, National Academy of Marine Research, Kaohsiung 80661, Taiwan


 

Received: May 22, 2020
Revised: July 14, 2020
Accepted: July 14, 2020

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.


Download Citation: ||https://doi.org/10.4209/aaqr.2020.05.0252  


Cite this article:

Sanito, R.C., Chen, Y.W., You, S.J., Yang, H.H., Hsieh, Y.K. and Wang, Y.F. (2020). Hydrogen and Methane Production from Styrofoam Waste Using an Atmospheric-pressure Microwave Plasma Reactor. Aerosol Air Qual. Res. 20: 2226–2238. https://doi.org/10.4209/aaqr.2020.05.0252


HIGHLIGHTS

  • Hydrogen and methane production from PSF waste are discussed in this manuscript.
  • PSF waste can be recycled as gases in an atmospheric-pressure microwave plasma reactor.
  • Nitrogen gas produces better concentration of hydrogen and methane compared to argon gas.
  • Microwave power plays an important role to generate an optimum hydrogen and methane.
 

ABSTRACT


Polystyrene foam (PSF), which is widely used in oyster farming in Taiwan, generates approximately 120,000–200,000 pieces of floating waste annually. The issues related to processing this waste, however, include the financial cost, incinerator clogs, human exposure to carcinogenic and non-biodegradable components, and potential debris, which threatens the seashore. In this study, we obtained methane (CH4) and hydrogen (H2), two crucial gases in power generation, by treating PSF waste with an atmospheric-pressure microwave plasma reactor. Substituting argon with nitrogen as the carrier gas and increasing the microwave power (1200 W) produced a higher concentration of H2 (4739 ppm) but a lower one of CH4 (less than 300 ppm). Treating a larger quantity of waste (0.2 g) resulted in CH4 and H2 levels of 19,657 ppm and 440 ppm, respectively. SEM-EDX and XRD testing confirmed the transformation of the PSF structure and a reduction in carbon (C) content in the final residue. This research demonstrates how solid waste can be recycled into valuable gases by applying plasma technology.


Keywords: CH4; H2; Microwave power; Nitrogen; PSF.


INTRODUCTION


Plastic debris has a significant impact in various sectors of society, including beaches, biodiversity, tourism, ecosystems, and human health (Pomeroy and Guieb, 2006; Liang and Zhang, 2010; Allen et al., 2013; Tanaka, 2013; UNEP, 2013; Hong et al., 2014; Lee et al., 2015; Lee et al., 2017; Vince and Hardesty, 2018). Styrofoam, which is made from a polystyrene foam (PSF), is a type of plastic debris, and that is quite difficult to degrade naturally (Chen et al., 2018). According to Tanaka et al. (2013), the transferring of chemical pollutants from marine environments to marine organisms occurs through the food chain, and is harmful to the ecosystem, and also to organism itself. Accumulation of plastic debris in animals, for examples, birds, occurs because of accidental ingestion, and was reported in 171 birds from 9 species by Codina-Garcia et al. (2013). Provencher et al. (2010) and Holland et al. (2016) confirmed that plastic debris is ingested by diving seabirds in eastern Canadian Arctic and affects their health. Thereby, plastic waste is harmful to organisms and environments.

In several countries, PSF waste has become a serious environmental issue that should be managed properly because of the large amount that has contaminated the environment. Laglbauer et al. (2016) found that macro-debris in the ocean in Slovenia is comprised of as much as 64% plastics. Fujieda and Sasaki (2005) discovered that 99.5% of foamed plastic fragments can be found along approximately 48.6 km of Japan’s coastline. A finding of Lee et al. (2015) stated that 1,800,000 buoys contaminate ocean annually in the South Korean sea. In Taiwan, between 3.7 million and 7.9 million approximately of plastic debris in the form of shopping bags, plastic bottle caps, tableware, fishing equipment, and plastic drinking straws, contaminated the coastline of Taiwan from 2004 to 2016 (Walther et al., 2016). Chen et al. (2018) reported that in Taiwan, approximately 120,000–200,000 large floating pieces of Styrofoam used for oyster farming every year. Polystyrene foam has been used widely in oyster farming and has become a main issue because of their shorter lifetime of approximately three years, and it is difficult to recycle (Liu et al., 2013; Chen et al., 2018). Thus, contamination with plastic waste is a serious issue, and should be handled correctly to prevent environmental pollution.

Several technologies have been developed to deal with plastic waste, namely, incineration (Ganeshprased, 2002; Choi et al., 2008; Wang et al., 2018, Yang et al., 2019) and recycling (Xu et al., 2013). Unfortunately, there are some drawbacks to both incineration and recycling. Unstable combustion, lack of air pollution control, high concentrations of dioxin, halogenated polycyclic hydrocarbons, and remaining pathogens are disadvantages of incineration (Nema and Ganeshprasad, 2002; Choi et al., 2008; Wang et al., 2018; Yang et al., 2019), as well as limited energy recovery and heavy water consumption (European Commission, 2006). Xu et al. (2013) reported that the recycling areas are contaminated by tetrachlorodibenzo-p-dioxins (TCDDs) and octachlorodibenzo-p-dioxin (OCDD) from melting plastic of e-waste. The total concentrations of TCDDs to OCDD were 2816–17,738 pg g–1 in soil. Thus, proper technology is required to tackle these issues.

Investigations of plasma technology in environmental application to treat waste have been performed by several authors (Heberlein and Murphy, 2008; Gomez et al., 2009; Deng et al., 2019; Sanito et al., 2020). Transformation of electrical energy to thermal energy occurs because of the invention of an electrical arc between two electrodes; subsequently, ionization occurs, which leads to the conversion of gas to electrically conductive compounds for elimination of elements (Taylor and Pirzada, 1994; Chang, 2009). Heberlein and Murphy (2008) and Wang et al. (2010) stated that the benefits of plasma technology include start-up of the reactor and plants, variety of chemical processes, low gas flow rate, high electron density, straightforward operation procedure, and easier application of recycling products in manufacturing processes. Thereby, plasma technology is a prospective technology to treat waste.

Previous studies have stated that in term of plasma technology, hydrogen (H2) and methane (CH4) can be generated from biomass and plastic waste (Huang et al., 2003; Chu et al., 2006; Mazzoni and Janajreh, 2007; Dave and Joshi, 2010; Mackzka, 2013; Tang et al., 2013; Shie et al., 2014; Materazzi et al., 2015; Huang et al., 2016; Sanlisoy and Carpinlioglu, 2017). Huang et al. (2013) found that the main components of the gaseous products from waste rubber were CO, C2H2, CH4, C2H4 and H2. Materazzi et al. (2015) reported that the final products, CO, H2 and H2S, can be obtained from tar under plasma gasification, which reduced complex organics by more than 96% v/v. In the treatment of paper mill waste, approximately 90% of CO and 99% of H2 can be obtained with a 2 min reaction time in a Plasmatron reactor, and 1.2 tons of paper mill waste can be recovered per day (Byun et al., 2011; Shie et al., 2014). Therefore, plasma technology is a promising procedure for treatment of solid waste and gasification of materials.

Previous researchers have reported on how to handle the plastic waste and produces synthetic gas (“syngas”) through plasma pyrolysis (Ruj and Chang, 2003; Dave and Joshi, 2010; Maczka et al., 2013; Zhou et al., 2020). Tang et al. (2003) reported that H2 gas can be produced from polypropylene in a nitrogen plasma reactor in a DC arc nitrogen plasma generator, with a maximum electric power input of 62.5 kVA in a 50 mm reaction chamber. Dave and Joshi (2010) found that the integration of thermo-chemical properties of plasma generates pyrolysis gas from polyethylene (PE), and polypropylene (PP) at temperatures ranging from 800–1000°C and can be considered on energy recovery with 78% propylene, and CO, H2, and hydrocarbons can be successfully recovered. NOx and H2S, which are toxic gases, can be eliminated during plasma pyrolysis (Ruj and Chang, 2002). Maczka et al. (2013) discovered that plasma pyrolysis of waste plastics produces H2, CH4, and CO with values of 1.1%, 2%, and 0.3%, respectively. Zhou et al. (2020) found that industrial atmospheric dielectric-barrier-discharge plasma can be used to treat plastic bags using a mechanism that breaks the chemical bonds of the plastic bag. Therefore, plasma technology can be used to produce a syngas from plastic waste. However, the treatment of PSF waste in plasma technology for such applications has not been thoroughly investigated or successfully performed.

In this study, potential hydrogen (H2) and methane (CH4) production as a side product in an atmospheric-pressure microwave plasma reactor is investigated. Furthermore, the composition of organic compounds is analyzed. To the author’s knowledge, PSF waste has not yet been discussed in any scientific literature, especially in terms of treatment using an atmospheric-pressure microwave plasma reactor. This paper is a study of the use of atmospheric-pressure microwave plasma to produce gas as a side product derived from PSF waste. In this study, the effects of carrier gases and microwave power on the total concentration of H2 and CH4 are then presented to understand the gas production mechanism.


METHODS



Collection and Preparation of the Samples

PSF waste was collected from an oyster farm in Tainan, Taiwan. The PSF waste product measured 30 × 40 × 95 cm. The sample was cut into smaller pieces (1 cm) and was measured with a standard analytical balance (AUY-220; Shimadzu, Japan) before being used in the experiment.


Pyrolysis of PSF Waste

Fig. 1 describes the atmospheric-pressure microwave plasma reactor. In this experiment, an atmospheric-pressure microwave plasma reactor was used with a frequency ranging between 0.3 GHz and 10 GHz, and the slurry frequency was set at 2.45 GHz (wavelength: 12.2 cm). An MCw signal was generated by a magnetron through a waveguide. Pyrolysis of the PSF waste was performed in a 20 cm3 crucible, for which height and diameter were 4 cm and 2.5 cm, respectively. A 0.1-cm-thick quartz tube with a diameter of 3 cm and a length of 32 cm was positioned near the path of magnetron propagate where plasma jet discharged.

Fig. 1. The system of an atmospheric-pressure microwave plasma reactor in this study.Fig. 1. The system of an atmospheric-pressure microwave plasma reactor in this study.

Fig. 2 illustrates the preparation of the sample and treatment in an atmospheric-pressure microwave plasma reactor. Fig. 2(a) shows a schematic of the plasma treatment set-up. Approximately 0.1 g of PSF waste was used as the sample. Fig. 2(b) shows how the waste prepared and treated in atmospheric-pressure microwave plasma reactor. To generate a plasma jet, the microwave power was kept constant at 1000 W. Argon gas and nitrogen gas were used as the carrier gases to generate the plasma jet for 4 min with 0.1 g of PSF waste. The parameters of the experiment are explained in Table 1

Fig. 2. Setup of plasma treatment and preparation. (a). Schematic set up of plasma treatment. (b). PSF treatment in an atmospheric-pressure microwave plasma reactor.Fig. 2. Setup of plasma treatment and preparation. (a). Schematic set up of plasma treatment. (b). PSF treatment in an atmospheric-pressure microwave plasma reactor.

Table 1. Effect of sample weight on gas production rate of CH4 and H2.

To investigate the effects of microwave power on the treatment, different amounts of power were utilized. The plasma power was maintained at 800 W and 1200 W, respectively. The plasma jet was discharged through a magnetron, and gases were collected via a 1 L gas bag (Tedlar gas bags) each 30 seconds. Plasma pyrolysis was performed at 4 min using nitrogen as the carrier gas (fast reaction and high metastable energy), while argon gas is used with 5 min of pyrolysis. The details of this process are shown in Table 2

Table 2. Effect of sample weight on gas production rate of CH4 and H2.

To assess the effects of the amount of PSF waste on production of H2 and CH4, 0.1 g, 0.2 g, and 0.3 g of PSF waste were used as the samples treated in the atmospheric-pressure microwave plasma reactor. The microwave power was maintained at 1000 W; nitrogen was used as the carrier gas, and pyrolysis was performed at 4 min. The experiment is displayed in . In this study, the liquid product and mass balance were not determined because of their small quantities in the final residue after treatment. 

Table 3. Effect of sample weight on gas production rate of CH4 and H2.


Gas Analysis

CH4 and H2 gases in the samples were analyzed via gas chromatography (GC-14A/B and GC-2014; Shimadzu, Japan). The lock syringe size of 1 mL (Taiwan) was used to inject the gas. A thermal conductivity detector (TCD) and flame ionization detector (FID) were used to detect the H2 and CH4, respectively. The columns were Porapak Q 80/100 mesh and a molecular sieve (5A Column). Standard carrier gases (CH4 and H2) were purchased from the Ming Yang Company (Taiwan). The flow rate was set up at 30 mL min–1 and maintained isothermally at 120°C. Concentrations of CH4 gas standard were 100 ppm, 200 ppm, 400 ppm, and 800 ppm, and concentrations of H2 gas were 100 ppm, 200 ppm, and 400 ppm. In this study, only analyses of H2 and CH4 were considered


Analysis of Sample Properties

Scanning electron microscopy (SEM-EDX; S-4800; Hitachi, Japan) was used to analyze the surface morphology of the PSF. The results of this analysis provided the surface and morphological structure of the residue.

To analyze the functional groups available in the PSF before and after treatment, Fourier transform infrared spectroscopy (FTIR; FT/IR-6000; Jasco, USA) was used with wavenumbers ranging from 400–4000 cm–1. The PSF was pelleted with KBr, and samples were pressed using a manual hand press until samples were 1 mm in size. After that, the samples were placed in the FTIR instrument and scanned with infrared rays for 5 min. FTIR curves were determined using Origin software (version 9.1).

To analyze the characteristics and crystalline structures of the resin, X-ray diffraction (XRD; D8 Advance Eco; Bruker, Germany) was used. The scanning process was performed for 5 min using X-rays with 2θ scanning at a range 10–80°. The X-rays were performed using asymmetric diffraction from the modular components. The voltage was controlled at 40 kV, and the current was set up at 25 mA. The power was maintained at 1000 W, and 2θ peaks represented the PSF waste.


RESULTS AND DISCUSSION



Characterization Analysis of PSF

The surface morphology and elemental composition of the samples were determined using a scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDX; Hitachi S-4800, Japan). Fig. 3 provides information about the surface morphology of the PSF samples before treatment and after treatment. Fig. 3(a) explains the regular morphological structure of Styrofoam, which is comprised of a chain of hydrogen and carbon. Fig. 3(b) shows the breakage surface of the materials after treatment with an argon plasma, which was found to assume a typical shape after treatment. Fig. 3(c) displays the irregular, inhomogeneous surface of the materials after treatment using nitrogen plasma, indicating destruction of the materials. Farrelly and Shaw (2017) confirmed that Styrofoam consists of vinyl benzene. In this study, carbon in the form of vinyl benzene was detected in the PSF waste before treatment. Material transformation refers to the degradation of linkages between materials and elements inpyrolysis process (Benedikt, 2010) because of free electrons at high concentrations that establish electrical conductivity from the carrier gas (Safa and Soucy, 2014). Specifically, ionization from the gas creates the collision of atoms, and enthalpy from the plasma produce the gases (Mountouris et al., 2006). 

Fig. 3. SEM images of materials surface of Styrofoam. (a) Materials of Styrofoam before treatment. (b) Surface material of Styrofoam after treatment with argon gas. (c) Surface material of Styrofoam after treatment with nitrogen gas.Fig. 3. SEM images of materials surface of Styrofoam. (a) Materials of Styrofoam before treatment. (b) Surface material of Styrofoam after treatment with argon gas. (c) Surface material of Styrofoam after treatment with nitrogen gas.

Fig. 4 explains the carbon composition before and after treatment. Fig. 4(a) gives an information about carbon in the PSF waste before treatment. Fig. 4(b) provides the carbon distribution after treatment in nitrogen plasma, and Fig. 4(c) depicts information about the PSF waste after treatment. All of this information shows that, based on the final residue after the plasma treatment, carbon was degraded better using the treatment. Lower concentrations of carbon could be obtained after treatment with nitrogen plasma because the high thermal degradation of the material plasma pyrolysis broke carbon bonds (Yang and Shibasaki, 1998). 

Fig. 4. Composition of carbon in the Styrofoam before treatment and its distribution of sample after treatment based on the EDX data. (a) Composition of carbon before treatment of PSF waste. (b) Composition of carbon after treatment of PSF waste using nitrogen as the carrier gas. (c) Distribution of carbon after treatment of PSF waste with argon gas.Fig. 4. Composition of carbon in the Styrofoam before treatment and its distribution of sample after treatment based on the EDX data. (a) Composition of carbon before treatment of PSF waste. (b) Composition of carbon after treatment of PSF waste using nitrogen as the carrier gas. (c) Distribution of carbon after treatment of PSF waste with argon gas.

Fig. 5 illustrates the FTIR patterns of the PSF waste after treatment in an atmospheric-pressure microwave plasma reactor. A qualitative analysis of functional groups was carried out to determine charactristics in the residue ranging from 400–4000 cm–1 of wavenumbers (Table 4). The analysis shows that C–H and C–C stretching vibration could be detected at 1493 cm–1 and 1601 cm–1, respectively. This finding was confirmed by Song et al. (2015), who stated that 721 cm–1, 1378 cm–1, 2825 cm–1 and 2923 cm–1 wavenumbers represent C–H groups. C–H aliphatic and C–H aromatic compounds, respectively, can be detected at wavenumbers ranging from 2910–2928 cm–1 and 3001–3082 cm–1 because of the presence of carbon. In the present, conversion of PSF waste to smaller components was due to the degradation of hydrocarbon and chemical bonds. There was a significant conversion of residue from the broadband peaks between 500 cm–1 and 1600 cm–1

Fig. 5. Characterization of final residue (a) FTIR test of PSF waste before and after treatment. (b) XRD test of PSF waste before and after treatment.Fig. 5. Characterization of final residue (a) FTIR test of PSF waste before and after treatment. (b) XRD test of PSF waste before and after treatment.

Table 4. Spectrum analysis of PSF on the FTIR analysis.

PSF waste materials after treatment using an argon plasma jet were found at wavenumbers of 1601 cm–1, 1493 cm–1, 2910–2928 cm–1 and 3001–3082 cm–1. However, the peaks became smaller at 3200 cm–1. This indicates the transformation of these materials during the plasma treatment. A wavenumber at 3500 cm–1 was attributed to hydroxyl groups (–OH). This functional group relates to C=C double bonds, a peak at 1633 cm–1. Interestingly, 1633 cm–1 and 3440 cm–1 wavenumbers were not detected during the analysis because of the elimination of chemical compounds after treatment using nitrogen plasma. Wavenumbers of 500–1600 cm–1 confirm C–H and C–C bonds (Singh et al., 2015; Ismail et al., 2017). In this study, peaks in the range 500–1600 cm–1 were detected in the final residue, which were C–H bonds and C–C bonds, confirming the transformation of carbon to H2 and CH4.

Fig. 6 compares the XRD analysis of the PSF waste before treatment and after treatment with the different carrier gases. Before treatment, the analysis showed a peak in the sample of 2θ at 10° and 20°. The highest intensity peaks appeared in the final residue. Interestingly, the 2θ peak was not detected in the final residue after treatment using nitrogen plasma because of low intensity at 10° and 25°, thus confirming the transformation of PSF waste due to carbon degradation. Liu et al. (2010) confirmed that diffraction peaks of 2θ, ranging from 15–30°, explain amorphous carbon structures. In this study, the detection of peaks from 10° and 20° prior to treatment confirmed the amorphous carbon structure of the PSF waste. Thus, gases can be possibly produced from the PSF waste because of the transformation of carbon. 

Fig. 6. Total concentration of CH4 and H2 (a) treatment with nitrogen gas. (b) Treatment with argon gas.Fig. 6. Total concentration of CH4 and H2 (a) treatment with nitrogen gas. (b) Treatment with argon gas.


Effects of Microwave Power and Carrier Gases on the Generation of CH4 and H2

Table 5 compares the results of H2 and CH4 after treatment with different microwave power settings and carrier gases in an atmospheric-pressure microwave plasma reactor. The highest concentrations were CH4 and H2 at 188 ppm and 4739 ppm, respectively, in 5 min, with 1200 W of microwave power and using nitrogen as the carrier gas. In this study, 1200 W of microwave power had a significant impact on greater generation of the gases. At 2 min, 2659 ppm of H2 and 85 ppm of CH4 were obtained with nitrogen plasma. This was a significant difference from plasma treatment with argon gas, for which the concentrations of CH4 and H2 were 7 ppm and 1114 ppm, respectively. The highest concentrations using argon plasma can be obtained at 188 ppm of CH4 and 4739 ppm of H2

Table 5. Analysis of hydrogen and methane on different microwave power.

Figs. 5(a) and 5(b) provide concentrations of H2 and CH4 in the final residue with different carrier gases and microwave power settings. In the plasma treatment, the final concentrations of H2 were 3500–5000 ppm. Furthermore, the final concentration of CH4 was less than 500 ppm with treatment using the selected parameters (nitrogen gas, 800 W and 1200 W). With the use of argon as the carrier gas, the final concentrations of H2 ranged from approximately 700 ppm to almost 1200 ppm, while the result for CH4 were less than 100 ppm using the selected parameters (800 W and 1200 W). From this analysis, it can be inferred that higher microwave power and nitrogen gas play an important role in generating gases due to high thermal plasma enthalpy (Mountouris et al., 2006; An’shakov et al., 2007). The conversion of carbon to gases during plasma pyrolysis occurs because of the reaction of molecules (Maczka et al., 2013). During plasma treatment, a higher temperature plays an important role in degrading the components of the residue to simple molecules (Dave and Joshi, 2010). Specifically, the collision of molecules and reactive species occurs in the plasma, causing the degradation of complex molecules to simpler molecules. Furthermore, high ionization of atoms from energetic electrons colliding with the material rapidly decomposed the material into gaseous products that release radicals such as H, O, and N (Mackza et al., 2013). Thus, CH4 and H2 gases can be formed directly in the plasma pyrolysis process, for which the mechanism described in Fig. 7, where it is shown that electrons collide with atoms from the PSF waste, and then carbon is converted into gases. The final residues after treatment of PSF waste are both solid and liquid*9. Approximately 0.002 g of solids were obtained. However, the amount of liquid was difficult to measure due to its limited amount. Fig. S1 shows the details for final PSF waste after treatment. The information of the mass balance was shown in Fig. S2. In this study, 0.1 g, 0.2 g and 0.3 g of PSF waste were the input amounts, respectively. Outputs were hydrogen and methane gases, and black carbon residue. Dave and Joshi (2010) confirmed that plasma gasification generates 65% hydrogen. Maczka et al. (2013) stated that a common gasification reaction can be associated with the pattern:

Polyethylene (CH2CH2) + Plasma + H2O → X (CH4) + z (CO) + Radicals n (CO) + m (H2)         (1)

According to Mountouris et al. (2005) the common gasification reaction known as an equilibrium reaction, that can be described by an ultimate analysis (CHxOy), as follows:

CHxO + wH2O + mO2 + 3.76 mN2 → aH2 + bCO + cCO2 + dH2O + eCH4 + fN2 + gC          (2)

Mattox (2010) stated that electrons accelerate rapidly through an electric field from the plasma source, where ionizing collisions are responsible for the loss of atoms from the target, and the electrons from plasma bombarded the surface area of the sample, thus transforming the final residue. Furthermore, H and CH4 gases are obtained as side products. 

Fig. 7. Mechanism of gas production during treatment in an atmospheric-pressure microwave plasma reactor.Fig. 7. Mechanism of gas production during treatment in an atmospheric-pressure microwave plasma reactor.


Effect of Different PSF Amounts to Generate CH4 and H2

Fig. 8 shows the different sample weights used for the CH4 and H2 production using nitrogen as the carrier gas. The highest H2 gas concentration was 19,657 ppm obtained from 0.2 g sample (Table 6). More striking was the highest CH4 concentration of 440 ppm with same amount of PSF waste (Table 6). The lowest concentrations of CH4 and H2 were 170 ppm and 5642 ppm, respectively, which were obtained with 0.1 g of sample material. When using higher amounts of PSF waste (0.3 g), a longer duration of pyrolysis should be used to obtain a better result. The higher microwave power improves the chemical reaction occurring during the plasma treatment, which results in higher H2 concentrations due to the higher temperature (Dominguez et al., 2007; Dave and Joshi, 2010). In this study, the rapid high-temperature plasma pyrolysis can be obtained on the higher set-up of microwave power. Dave and Joshi (2010) stated that a higher plasma temperature plays an auxiliary role in the transformation of a material into simpler molecules. In this study, lower concentrations of H2 and CH4 were achieved because of a slower reaction of the argon plasma jet at lower temperature corresponds to the microwave power. H2 formation can be improved by increasing the pyrolysis temperature, causes dehydrogenation reaction of aromatic hydrocarbons and condensates associated with the pyrolysis temperature, dehydrogenation, and aromatic hydrocarbon (Zhao et al., 2012; Dominguez et al., 2017). A higher microwave power for plasma increases the temperature reaction, which was linked with the better results for H2 and CH4 found in this study. Thereby, higher concentrations of H2 can be obtained when the microwave power is kept constant at a higher power. In this study, a 5 min plasma reaction increased the temperature from 600°C to 700°C. 

Fig. 8. Effect of sample weight on gas production rate of CH4 and H2.Fig. 8. Effect of sample weight on gas production rate of CH4 and H2

Table 6. CH4 and H2 gases concentration after treatment in an atmospheric-pressure microwave plasma reactor using nitrogen as the carrier gas.

Carbon and other volatile materials in PSF waste are converted to CO, H2 and CxHy through chemical reactions (Ruj and Chang, 2012). Tang et al. (2003) stated that tar can be degraded effectively into CO and H2 as the final products, thus confirming the significant production of H2 because of the higher feed rate in a plasma system. This finding confirmed that PSF waste leads to good gas production results after 4 min of pyrolysis. Carbon decompose because of an oxidation process that generates CH4 and H2 (Fabry et al., 2013; Maczka et al., 2013). H2 and CH4 gases are the byproduct bond dissociation and endothermic reactions where the exothermic reaction releases energy from the heat at plasma jet discharge (Nema and Ganeshprasad, 2002). Thus, these gases can be generated from plasma pyrolysis.

In this investigation, H2, the main product of plasma pyrolysis, was described as the main primary gas obtained during the plasma treatment. The pyrolysis of gas plays an important role in converting solid waste to syngas, namely, H2 and CH4, which can potentially be substituted for fossil fuels and as alternative sources of energy (Gomez et al., 2009) and may be potentially used into the electricity application (Maczka et al., 2013). H2 and CH4 can be obtained from the plasma pyrolysis of PSF waste, owing to carbon conversion in an atmospheric-pressure microwave plasma reactor. The pyrolysis of syngas depends on the parameters used in plasma treatment and amount of PSF waste. These results may be considered as preliminary information to give the information on the conversion of PSF waste to CH4 and H2.


CONCLUSION


Employing an atmospheric-pressure microwave plasma reactor, and argon and nitrogen as carrier gases, we produced syngas on a pilot scale by converting carbon into CH4 and H2 during the treatment of PSF waste. Using nitrogen instead of argon as the carrier gas and increasing the microwave power proved to be the optimal method, achieving CH4 and H2 concentrations of 19,657 ppm and 440 ppm, respectively, for 0.2 g of PSF. The SEM-EDX and XRD results for the final residue confirmed the structural transformation of the PSF and a decrease in carbon content. Future research should focus on maximizing the concentration of the generated syngas, which depends partially on the quantity of the waste. Mixing the carrier gas with H2O, increasing the duration of the pyrolysis, and performing a cost analysis are also potential steps. Furthermore, investigating the particulate matter (PM) and aerosols released by the atmospheric-pressure microwave plasma reactor will enable us to control pollutive emissions associated with this technology.


ACKNOWLEDGEMENT


This research was financially supported by Ministry of Science and Technology (MOST), Taiwan, with a grant number of 107-2221-E-033-005. We thank the Department of Environmental Engineering, Chung Yuan Christian University, where the study was undertaken.


DISCLAIMER


Authors declare no competing of financial interest in this research. Facts and opinions in articles are solely the primary results of authors.


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Aerosol Air Qual. Res. 20 :2226 -2238 . https://doi.org/10.4209/aaqr.2020.05.0252  


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