Plasma-assisted pyrolysis for converting oil palm fronds into reduced graphite oxide

One of the oil palm tree’s solid waste is oil palm fronds. Due to its lignocellulosic composition, it has the potency to convert it into carbon. The common heat treatment method to convert oil palm fronds into carbon-based material is pyrolysis. However, this process has some disadvantages, such as being time-consuming and just producing amorphous carbon. Different from common pyrolysis temperatures, in this study we use a higher temperature (4000 °C) generated by DC current arc plasma. This process is faster than the common pyrolysis process (less than 10 minutes) and produces crystalline material. This product is then characterized by Raman spectroscopy, SAED-TEM, XRD, FT-IR, and SEM/EDX. Based on Raman spectroscopy, this crystalline material shows the characteristics of semiconductor carbon. Based on the SAED-TEM, there is a (002) plane of hexagonal crystal structure detected (graphite phase characteristics). An XRD analysis shows the characteristics of the trigonal crystal structure (P3) constructed by O and H atoms. An FT-IR characterization shows that there are C=C, C-H, and O-H bonds, while the EDX analysis result shows a carbon-to-oxygen ratio of about 4.23. Therefore, based on the whole interpretation, the plasma-assisted pyrolysis treatment is successfully used to convert oil palm fronds into reduced graphite oxide (rGO).


Introduction
Despite a production decline of 42.7 million tons compared to 2022, oil palm production in Indonesia remains one of the largest in the world.The total oil palm production in Indonesia will reach about 8.5 million tons until February 2023 [1,2].This high amount of oil palm production causes some problems in the post-harvest process, including the large volume of solid waste.These solid wastes include oil palm fronds (OPF), palm kernel shells (PKS), empty fruit bunches (EFB), and oil palm trunks (OPT) [3].Among these solid wastes, OPF and PKS have the greatest potential for damaging the environment.The higher organic acid content in both has a major contribution to river pollution and damage to river ecosystems [3].In order to prevent this problem, OPF and PKS need to be reduced.The most widely used and common way to reduce both of them is to use the pyrolysis method [4,5].However, conventional pyrolysis has several disadvantages, such as requiring a long duration and additional expensive catalysts.Therefore, one way that can be done is by accelerating the pyrolysis process without 1309 (2024) 012001 IOP Publishing doi:10.1088/1755-1315/1309/1/012001 2 using a catalyst.To conduct it, plasma technology has the potential to be applied in this pyrolysis process due to its short exposure time and high temperature.
Although there are rarely reports of plasma technology applications for OPF and PKS pyrolysis processes, there is potential for both to be reduced using this method because the lignocellulosic content is relatively the same as that of biomass in general.As an example, OPF consist of 51.5 wt.% cellulose, 25.64 wt.% hemicellulose, and 19.10 wt.% lignin [6] by lignocellulosic content; and of for about 47.11 wt.% C, 44.92 wt.% O, and 5.16 wt.% H by element content [7].Some latest reports show that plasma technology can be applied on some biomass pyrolysis processes [8][9][10][11][12][13].All of these reports show the same plasma source, i.e., N2 gas [8][9][10][11][12][13].This application can be applied for extracting biochar [8] and for pyrolysis gas refinement [9][10][11][12][13].For only pyrolysis biochar extraction, plasma zone is only designed for main pyrolysis reactor so that this plasma zone is only used for biomass reduction [8].Recent latest reports show that plasma zone is applied as a second step process after pyrolysis [9][10][11][12][13].This second step is applied plasma zone for plasma catalysis process to extract H2 from pyrolysis gas [9][10][11][12].Another report shows this plasma catalysis system is used for converting pyrolysis gas into bio-fuel [13].
These reports show some biomasses as feedstock, such as cellulose fibre [9], wood pellet [8,10], pine sawdust [11,12], and biomass pellet [13].It means that there is no OPF or PKS used for biomass feedstock, so the study of plasma application for pyrolyzing them is still an interesting thing to be observed.In this study, we try to use plasma technology for OPF pyrolysis applications.Different from some of the latest reports [9][10][11][12][13], this study applies a different plasma source, i.e., Ar gas.The use of Ar gas as a plasma source in this study is expected to result in a higher plasma arc temperature compared to N2-source plasma arcs.Then, this high temperature is hypothetically applied to convert OPF biomass into a crystalline graphitic product.

Material and Plasma Treatment Method
Oil palm frond (OPF) biomass was used as a sample in this study.This biomass is obtained from oil palm trees planted in the Puspiptek area, South Tangerang, Indonesia.The lifespan of this oil palm tree is about 15 years.This bulk frond sample is first washed with distilled water (Aqua SCiencΣ Aquadest) and then dried in the oven (70 o C) for 24 hours.This sample is then cut into small pieces (20 mm long and about 2-3 mm thick) and placed on a cylindrical graphite pot with an inner diameter of 30 mm and a height of 40 mm.Then, it is placed on the sample holder in the pyrolysis chamber (see no. 3 in FIGURE 1).In order to determine the composition of C, H, and O of OPF biomass, ultimate analysis was conducted using ASTM E870-82 method [14].Plasma-assisted pyrolysis is then conducted using the schematic reactor illustrated in FIGURE 1.The plasma source is Ar gas (flow rate of 15 L.min -1 ) generated by electric current from a DC source (12 V and 75 A parameters).The temperature obtained by this system is about 4000 o C and was conducted for 8 minutes.
For SEM/EDX characterization, the preparation method used is the general powder preparation method.For TEM characterization, powder samples were dissolved in ethanol, ultrasonicated for 15 minutes, and then dripped on a grid-holder sample.
The whole interpretation was then conducted to determine the actual structure and phase of the plasma-assisted pyrolysis solid product (plasma char).

Plasma Char Characterization
The Raman spectroscopy characterization result is shown in FIGURE 2. In this result, there are two peaks with Raman shift values of 1341 cm -1 and 1609 cm -1 .A Raman shift value of 1341 cm -1 indicates the characteristics of D-peak, while a Raman shift value of 1609 cm -1 indicates the characteristics of Gpeak [15,16].The appearance of both peaks shows the characteristics of carbon materials [14,15].The intensity ratio between D and G peaks is 1.0575 which means that this ratio is about 1, so the characteristic of the carbon material detected is semiconductor carbon [15,16].The TEM analysis result is shown in FIGURE 3. Based on selected area electron diffraction (SAED) in FIGURE 3.b, the appearance of the (002) plane of the hexagonal crystal structure is clearly shown.The SAED-TEM analysis result shows that the lattice constant is about 2.55 Å (see FIGURE 4.a).This value tends to be close to the a-lattice constant characteristics of a graphite phase, which has an ideal alattice constant of about 2.5 Å [17].FIGURE 5 shows an x-ray diffraction (XRD) pattern of plasma char.Some diffraction peaks are detected, which are indexed as some hkl planes of (110), (002), ( 201), ( 210), ( 202), (300), (003), ( 220), (113), (310), (303), and (411).The crystal structure determined is trigonal.The calculation result determines two lattice constants, i.e., a = 7.6553 Å and c = 6.3105Å.By using the temperature factor (B-factor) of 0.4, the predicted atomic coordinates of (0,0,0), (½,½,½), (½,0, ½), (0, ½,½), and (0,0,¼) are determined as shown in TABLE 1.Then, based on the determined lattice constants and these atomic coordinates, the crystal structure detected based on XRD can be illustrated as shown in FIGURE 4.b.The illustration in FIGURE 4.b shows that the space group of the trigonal crystal structure tends to be P 3. Therefore, according to SAED-TEM and XRD analysis results, the overlapping structure of the (002) layer can be illustrated as shown in FIGURE 6.a.The determined structure (FIGURE 6.a) is then compared with the analytical result of Fouriertransform infrared (FT-IR) spectroscopy (FIGURE 7).An FT-IR result shows that there are O-H, C=C, and C-H bonds.O-H bonds are detected at the wavenumbers of 3439 cm -1 and 1427 cm -1 , while C=C and C-H are detected at the wavenumbers of 1642 cm -1 and 873 cm -1 .The comparison of FIGURE 6.a with these bonds generates the predictive structure shown in FIGURE 6.b.The structure shown in FIGURE 6.b indicates the characteristics of the graphene oxide (GO) layer.The accumulative lamination of graphene oxide layers will construct graphite oxide, as shown in FIGURE 3.a.The mass ratio of carbon (C) and oxygen (O) will determine the type of GO layer.If the C/O ratio is between 2 and 3, the type is GO layer; on the other hand, if the C/O ratio is greater than 3, the type is reduced graphene oxide (rGO) layer.Thus, by the EDX analysis result shown in FIGURE 8, the C/O ratio is 4.23 (by weight), so the type of the layer is rGO.Therefore, After the whole interpretation of Raman, TEM, XRD, FT-IR, and EDX, it can be determined that the characteristic of plasma char derived from plasma-assisted pyrolysis of oil palm fronds is reduced graphite oxide, which is accumulated in some rGO layers.

Process Yield and Chemical Exergy Efficiency
The ultimate analysis of the pre-pyrolysis OPF biomass sample is shown in TABLE 2. Based on the dry basis mass composition divided by molecular weight, the mole ratio of C : H : O will be 4 : 6 : 3. Therefore, the chemical formula of pre-pyrolysis OPF biomass will be approximately (C4H6O3)n.The plasma-assisted pyrolysis process shows some yields for five samples, as shown in TABLE 3. The average plasma char yield of this process is calculated as 6.5 ± 2.4 %.Based on the predicted plasma char structure in FIGURE 6, the chemical formula of plasma char will be C24H10O7.Then, based on the mass balances in TABLE 3, the predicted chemical reaction during the plasma-assisted pyrolysis process can be predicted as written in the "Predicted Pyrolysis Reaction" column in TABLE 4.
These stoichiometric reactions written in the "Predicted Pyrolysis Reaction" column in TABLE 4 form the basis for calculating the chemical exergy efficiency.It is calculated by dividing the total values of pyrolysis gas product (CO, H2, CH4) chemical exergies by the biomass (C4H6O3) chemical exergy [18].The standard chemical exergies used for the calculation are 1846 kJ.mol -1 , 275 kJ.mol -1 , 236.09 kJ.mol -1 , and 831.7 kJ.mol -1 for C4H6O3, CO, H2, and CH4 [19].The average determined chemical exergy efficiency of the plasma-assisted pyrolysis process is 94.8 ± 3.2 % as shown in the last column of TABLE 4. Note that this study was still conducted on a laboratory scale.Therefore, research and development on a higher scale (e.g., pilot scale and industrial scale) still needs to be done in the future to ensure the feasibility of this process being upscaled to pilot scale and industrial scale, especially in the yield and chemical exergy efficiency parameters.

Conclusion
The plasma-assisted pyrolysis process in this study was successfully used to convert oil palm fronds (OPF) into reduced graphite oxide.The reduced graphite oxide is constructed by the overlapping structure of hexagonal graphite and the P 3-type trigonal structure constructed by O and H atoms.
The average yield of this process is determined to be about 6.5 ± 2.4 %, and the average chemical exergy efficiency is calculated to be about 94.8 ± 3.2 %.

Acknowledgement
This research is fully funded by the BPDP-KS program, Ministry of Finance, Republic of Indonesia, and held in the Plasma Centre, National Research and Innovation Agency (BRIN), KST B. J. Habibie, South Tangerang, Indonesia.

FIGURE 5 .
FIGURE 5. XRD pattern and hkl of plasma char.

FIGURE 7 .
FIGURE 7. FT-IR analysis result of plasma char.

TABLE 1 .
Atomic coordinate analysis results based on XRD characterization.

TABLE 2 .
Ultimate analysis of oil palm fronds.

TABLE 4 .
Predicted reaction and chemical exergy analysis results during plasma-assisted pyrolysis.