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Co-pyrolysis of oil palm empty fruit bunch and oil palm frond with low-density polyethylene and polypropylene for bio-oil production
⁎Corresponding author. chazmier@usm.my (Mohd Azmier Ahmad)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Co-pyrolysis of palm wastes of empty fruit bunch (EFB) and palm frond (PF) with low-density polyethylene (LDPE) and polypropylene (PP) were investigated to find the synergistic effect of biomass-plastic pair on the yield and composition of bio-oil produced. The pyrolysis process of individual materials and their corresponding blends were employed in a fixed bed reactor at heating rate of 20 °C/min with a nitrogen flow rate of 250 mL/min. The co-pyrolysis results showed that EFB:LDPE with weight ratio of 1:1 has the largest synergy on bio-oil yield, while a negative synergy was revealed for PF:PP co-pyrolysis. In regard of bio-oil chemical composition, the synergistic effect was positive for the formation of aliphatic hydrocarbons and all feedstock decreased the overall oxygenated compounds of fuel substitute. The hydrogen generated from plastic promoted the decarboxylation of acids and decarbonylation reactions of carbonyls and sugars.
Keywords
Co-pyrolysis
Bio-oil
Biomass-plastic blend
Empty fruit bunch
Palm frond
Low-density polyethylene
Polypropylene
1 Introduction
The total capacity of fossil fuel-based power plants is enlarging dramatically as a consequence of rapid population growth and faster industrialization, which has resulted in an immense demand for limited fossil fuel resources such as oil, natural gas and coal in the majority of power plants around the world (Alsobaai, 2013; Karmaker et al., 2020). Moreover, fossil fuels are the main contributor to the increase in greenhouse gases that is the main cause for increasing average global temperatures and climate change (Leonard et al., 2020). Due to the depletion of fossil fuel reserves and gas emissions, clean renewable energy sources are increasingly required to meet the energy demand and mitigate the global warming and environmental pollution (Ryu et al., 2020; Balachandar et al., 2013).
Biomass is one of the major future sustainable energy resources which is abundant in all continents, where the global biomass production hits about 100 billion tons per year (Systems, 2017; Wang et al., 2017). Biomass covers a wide range of plentiful agricultural wastes, animal wastes and algae (Papari and Hawboldt, 2015; Ahmad et al., 2021). Moreover, biomass is considered as a carbon–neutral energy source, since the emitted CO2 is originally absorbed from atmosphere during photosynthesis process. Using biofuels as an alternative fuels can prevent the emission of approximately 2.1 giga- tons (Gt) of CO2 into the atmosphere per a year by 2050 (Ojha and Vinu, 2018). Also, emissions of CO, CO2 and SO2 from biofuels is insignificant compared to the conventional fossil fuels (Jiménez-Cruz et al., 2021). These features of biomass have encouraged researchers to develop technologies capable of converting biomass into high valuable forms of biofuels such as thermochemical technologies, including pyrolysis, liquefaction, gasification and combustion (Zhao et al., 2020).
In pyrolysis process, the bio-oil derived from biomass is not suitable for direct use as a fuel due to high acidity and high levels of oxygen and water, which cause low heating value, and thermal instability (Ryu et al., 2020). On the other hand, co-pyrolysis of biomass with hydrogen-rich materials, such as plastic, is a promising way to upgrade the production and quality of pyrolytic bio-oil thru synergistic reactions between formed intermediates (Hassan et al., 2020). The synergetic effect is defined as an effect resulting from co-processing of two of more components that generate an effect greater than the sum of their separate effects (Onay and Koca, 2015). Co-pyrolysis of biomass and plastics increases the yield of bio-oil with more homogeneous composition compared to that collected from blending of the individual oils which tend to separate after a period of time (Gunasee et al., 2017). Co-pyrolysis is a technology that is simple to design and operate without any catalysts or solvents for producing high quality bio-oil from two or more materials as feedstock (Abnisa and Wan-Daud, 2015). Co-pyrolysis improves the quality of the produced bio-oils, since the synergistic effects enhance the oil stability, promote formation of hydrocarbons and reduce reactive oxygenated compounds (Navarro et al., 2018; Gin et al., 2021).
Malaysia is the world's second largest producer of palm oil, contributing about 25.8% of world production and 34.3% of world exports (Zubaidah, 2021; Council, 2020). Among the agricultural biomass, oil palm wastes are the attractive pyrolysis feedstock that can be utilized as liquid fuel. The critical problem facing the oil palm sector is the proper disposal of solid waste such as empty fruit bunches (EFB) and palm frond (PF) (Ong et al., 2020; Uemura et al., 2013). In addition, the disposal of end-life plastics has become a major annoying environmental issue due to the massive increase in plastic consumption (Xue et al., 2017). Significantly, Malaysia annually generates more than 0.94 million tons of mismanaged plastic wastes (Chen et al., 2021). The utilization of plastics and oil palm wastes as biofuels reduces both amount of solid wastes dumped in the landfill (Gu et al., 2019).
Co-pyrolysis is supposed to upgrade pyrolytic oil as a result of the synergistic impact between plastics and biomass intermediates (Hassan et al., 2020; Mu et al., 2020). Previous studies (Ephraim et al., 2018; Deng, 2017; Sfakiotakis and Vamvuka, 2018) focused mainly on the effect of co-pyrolysis temperature and heating rate on the synergistic effect. However, Özsin and Pütün (2018) proved the synergistic effects depended strongly on biomass-polymer pair. They found that the co-pyrolysis of polystyrene (PS) with walnut shells and with peach stones caused a positive synergistic effect on bio-oil yields. Moreover, Aboulkas et al. (2012) reported that co-pyrolysis of lignite-LDPE mixtures enhanced significantly the oil yields. However, there still remains a need to study the effect of biomass-plastic pair on the quantity and quality of co-pyrolytic bio- oil, as well as co-processing PF:LDPE and PF:PP pairs which have not been studied before.
The aim of this study was to investigate the synergistic effect of biomass-plastic pair on the yield and composition of bio-oil. Each of EFB and PF was co-pyrolyzed mutually with LDPE and PP in a fixed bed reactor. The chemical compositions of bio-oil were analyzed using gas chromatography mass spectroscopy (GCMS) technique to gain more insight into the effect of biomass-plastic pair. The thermogravimetric analysis (TGA) has been carried out to investigate the thermal behavior of biomass-plastic blends during co-pyrolysis process.
2 Methodology
2.1 Materials
In this study, biomasses of empty fruit bunch (EFB) and palm fond (PF) were obtained from United Oil Palm Mill, Nibong Tebal, Penang, Malaysia. Low-density polyethylene (LDPE) and polypropylene (PP) obtained from Lotte Chemical Titan (M) Sdn Bhd, Pasir Gudang, Johor, Malaysia. EFB and PF were dried, grinded and screened to achieve particle size of 1–2 mm. Nitrogen and helium gases (purity of 99.9%) supplied by Araztech Engineering, Penang, Malaysia were used as a carrier gas.
2.2 Biomass characterization
The ultimate analysis was conducted using CHNS/O analyzer (Model: Perkin-Elmer 2400, USA). The oxidation furnace temperature was set to 975 °C. The reduction furnace temperature was set to 500 °C. The proximate analysis was conducted using Perkin-Elmer TGA 7 connected to Thermal Analysis Controller TAC7/DX. The thermal gravimetric analysis (TGA) was done according to American Society for Testing and Materials method (ASTM D 7582–10). The sample was heated from 30 °C to 110 °C at 10 °C/min with nitrogen gas flow of 20 mL/min and held at 110 °C for 10 min. Then the sample was heated from 110 °C to 850 °C at 10 °C/min and held at 850 °C for 10 min followed by switching from nitrogen to oxygen atmosphere at similar gas flowrate (Dewayanto et al., 2016). Thermal behavior of pure materials and their corresponding blends was evaluated in order to find the maximum degradation temperature. The thermal behavior was estimated by measuring the weight loss of the sample as a function of temperature with time. In each experimental run, the samples were heated from 30 °C to 800 °C at 10 °C/min and held at that temperature for 10 min with nitrogen (N2) purge of 20 mL/min.
2.3 Pyrolysis experiments
The aim of these experiments was to study the effect of biomass-plastic pair on the yield and composition of bio-oil as a main product. Co-pyrolysis was conducted in a vertical stainless steel fixed-bed reactor with an internal diameter of 25 mm and height of 700 mm. The reactor was installed in an electric furnace which by necessary heat was provided. For each batch, 3 g of biomass and 3 g of plastic (weight ratio of 1:1) were mixed and inserted into the reactor through glass funnel. Nitrogen gas was fed into the reactor at flowrate of 250 mL/min for 10 min to create inert condition as well as to push the vapor products toward the condenser during the co-pyrolysis step.
The co-pyrolysis process was performed at optimum temperature, Topt with heating rate of 10 °C/min, nitrogen flow rate of 250 mL/min and holding time of 45 min (Hassan et al., 2019) in order to achieve maximum degradation of the materials and maximize the bio-oil yields. Initially, pure LDPE, PP, EFB and PF were pyrolyzed separately as basis for the calculation of potential co-pyrolysis yield (theoretical yield). Then biomass-plastic blend (EFB:PP, PF:PP, EFB:LDPE or PF:LDPE) with weight ratio of 1:1 were pyrolyzed at the corresponding Topt obtained from TGA analysis at the same heating conditions. The liquid products from the condenser and wax trap were collected and weighted. Non-condensed gases were collected by gas bags through the vent valve after the condenser. When the reactor was cooled to room temperature, the solid char recovered and weighted. The yield of the oil (Yo), solid char (Yc) and gas (Yg) were calculated using the Eq. (1a-1c):
In order to evaluate the synergy effects during co-pyrolysis process, the experimental co-pyrolytic yields were compared with the predicted co-pyrolytic yields. Predicted yields were calculated based on the individual pyrolysis yields of PP, EFB, LDPE and PP according to Eq. (2) (Cao, 2019; Zhao et al., 2020):
2.4 Bio-oil characterization
The chemical compound distribution in bio-oil product was characterized by gas chromatography mass spectroscopy (GCMS) (Model: Perkin Elmer Clarus 600/600 T, USA) (Hassan et al., 2019). The analysis was performed using a capillary column of Elite-5MS with a length of 30 m, inner diameter of 0.25 mm and a film thickness of 0.25 μm. Highly pure helium was used as the carrier gas with a constant flow rate of 1.0 mL/min. The analysis was conducted by heating the column at 50 °C and was kept for 2 min, the temperature was then ramped to 280 °C at a rate of 5 °C/min and was held at this condition for 20 min. The injected sample was 1 μL volume.
3 Results and discussion
3.1 Characterization of materials
Table 1 displays the proximate and ultimate compositions of the plastics and biomasses. It is obvious that LDPE, PP, EFB and PF were rich in volatile matter thus can be source for production of bio-oil (Boubacar Laougé and Merdun, 2020; Onay, 2007). From elemental analysis, EFB and PF have significant oxygen content which was the prominent factor for the release of volatile oxygenates, leading to the acidity of bio-oil (Ojha and Vinu, 2018). On the other hand, LDPE and PP with high hydrogen content and lower oxygen content could serve as a hydrogen donor to the biomass intermediates, hence promoting the transformation of oxygenates to hydrocarbon compounds (Hassan et al., 2020) (see Table 2).
PF
EFB
LDPE
PP
Proximate analysis (%p/p)
Moisture
7.51
4.52
0.0
0.0
Volatile matter
73.32
71.35
100
100
Fixed carbon
14.11
20.43
0.0
0.0
Ash
5.06
3.71
0.0
0.0
Elemental/Ultimate analysis (%p/p)a
Carbon
39.1
45.3
79.4
77.0
Hydrogen
6.74
7.86
14.1
12.2
Nitrogen
2.30
2.30
1.90
1.90
Oxygen
50.76
43.45
2.10
6.70
Sulfur
1.10
1.09
2.50
2.20
Compounds
GSCM area %
PF-LDPE
EFB-LDPE
EFB-PP
PF-PP
Pred
Exp
Pred
Exp
Pred
Exp
Pred
Exp
Aromatic Hydrocarbons
3.94
0.23
1.61
0.74
1.60
0.0
3.94
0.0
Aliphatic Hydrocarbons
36.64
70.45
40.92
61.24
13.7
40.57
9.42
39.86
Alcohols
10.74
13.52
12.71
13.71
24.51
12.38
22.53
4.96
Phenols
11.30
0.0
2.91
2.06
2.60
0.0
10.90
0.0
Carbonyls
10.90
4.73
4.17
4.61
2.70
2.22
9.41
3.50
Esters
15.83
11.10
25.30
15.90
42.38
39.04
32.96
44.36
Nitrogenates
0.76
0.0
0.57
0.0
0.57
0.0
0.76
0.0
Sugars
0.87
0.0
0.0
0.0
0.0
0.0
0.87
0.0
Acids
8.90
0.0
11.80
2.70
12.14
5.80
9.30
7.30
3.2 Thermal degradation of individual components and blends
Figs. 1 and 2 present the thermogravimetric (TG) and differential thermogravimetry (DTG) curves, respectively for individual and blended samples. In case of individual materials, TG curves illustrate that EFB and PF have low thermal stability, and begin to break down at a lower temperature compared to plastics. At 150 °C EFB and PF lose their moisture about 4.5% and 7.5%, respectively. The next degradation stage of EFB and PF occurred from 150 °C to 470 °C with a maximum degradation rate happen at 390 °C. The maximum mass loss of 62% and 58% for EFB and PF, respectively were due to the thermal overlapped degradation of hemicelluloses and cellulose (Shafaghat, 2019). The next degradation occurs in the range of 470–800 °C with a mass loss of 8% and 15% for EFB and PF, respectively which due to the lignin decomposition (Parthasarathy and Narayanan, 2014). On the other hand, LDPE and PP decompose in a single step. This can be related to the homogeneous structure of plastic unlike biomass that contains different compounds including hemicellulose, cellulose, and lignin (Gunasee et al., 2017). LDPE and PP lose the major parts of their mass approximately 97% and 96%, respectively in a range (440–545 °C). DTG curves illustrated that the maximum degradation rate for both plastics occurred around Topt of 540 °C. Based on the maximum volatiles loss range, LDPE and biomass of DTG curves were overlapped at range of 440–470 °C. Therefore, interactions between them are to be expected when they are utilized as a co-feed during the co-pyrolysis process.
Thermogravimetric (TG) graphs for (a) individual PF, EFB, LDPE and PP and (b) their biomass-plastic blends.
Differential thermogravimetry (DTG) graphs for (a) individual PF, EFB, LDPE and PP and (b) their biomass-plastic blends.
The degradations of the blends are more complex and complicated. The samples lose their moisture up to approximately 150 °C. The first degradation stage started from approximately 150–390 °C for EFB:LDPE, PF:LDPE and EFB:PP and from 150 °C to 410 °C for PF:PP. The second stage occurred in the range of 440–530 °C for PF:LDPE, 450–535 °C for EFB:LDPE, 480–550 °C for EFB:PP and 490–560 °C for PF:PP. According to Fig. 2(b), each of blends revealed two decomposition peaks between 200 °C and 550 °C. The first peak is attributed the major part of biomass decomposition, while the second peak indicated to the decomposition of plastic and remaining biomass (Tsamba et al., 2006). The interactions between plastics and biomass shows slightly decrease in Topt for blends than that for individual plastics. This biomass-derived char was act as a catalyst that promotes the degradation of plastics (Uzoejinwa, 2020). On the other hand, shapes of TG curves for plastic-biomass blends indicate that there is a clear wide area separated the degradation of biomass and plastic which in, the loss of volatiles particles was very less. Accordingly, the optimum temperatures were determined in order to use them in the next pyrolysis process. It can be concluded from Fig. 2(a) that the optimum reaction temperature for EFB and PF was 390 °C, while for LDPE and PP was 540 °C. On the other hand, Fig. 2(b) shows that the optimum reaction temperature was 510 °C, 520 °C, 540 °C and 540 °C for PF:LDPE EFB:LDPE, EFB:PP and PF:PP, respectively.
3.3 Product yields and synergistic effect
3.3.1 Pyrolysis of pure materials
In order to optimize bio-oil yields, the pyrolysis process was performed at optimum temperature of 390 °C for EFB and PF, and 540 °C for LDPE and PP. In terms of plastic pyrolysis, LDPE and PP have completely converted into liquid and gas products with negligible portion of char (Almeida and Marque, 2015; Anuar Sharuddin et al., 2016). In addition, the plastic chemical structure consists mainly of volatile materials as shown in Table 1. Fig. 3(a) shows that the plastic generated the highest bio-oil yields compared to those from biomass. The bio-oil with yield of 82.6% was the key fraction of LDPE pyrolysis which was significantly higher than that produced from pure PP (75.3%). Low bio-oil yield of PP and the formation of more gaseous products were due to the high oxygen content of PP compared to LDPE which led to produce more non-condensable gases such as CO and CO2 (Anene et al., 2018; Miandad et al., 2017). On the other hand, the bio-oil yields from the EFB and PF pyrolysis was about 44% and 40%, respectively. Biomass generated large amounts of char compared with plastics due to the fact that biomass has lower volatile matters and higher fixed carbon content than the plastic as shown in Table 1 which favors char yields (Stančin, 2021).
Products yield of (a) individual substances pyrolysis and (b) corresponding blends co-pyrolysis.
3.3.2 Co-pyrolysis synergistic effect on bio-oil yield
The co-pyrolysis experiments were performed at the corresponding optimum temperature for PF:LDPE (510 °C), EFB:LDPE (520 °C), EFB:PP (540 °C) and PF:PP (540 °C). The biomass-plastic blending ratio and holding time were fixed at 1:1 and 45 min. Fig. 3(b) shows that the yields of bio-oil from co-pyrolysis of LDPE and biomass blends was dramatically higher than those for blends of PP and biomass, meanwhile bio-oil yields generated from EFB and plastic blends were higher than those generated from PF and plastic blends. Co-pyrolysis of EFB:LDPE yielded higher bio-oil (67.1%), than PF:LDPE (65%). On the other hand, PF:PP gave lower bio-oil yield (54.7%) than EFB:PP (59.8%). For byproducts, the char residues were not significantly affected among all feedstock since the plastic decomposition shared negligible amount of solid residue. Regarding gas yields, PF:PP and EFB:PP generated large amounts of non-condensable gases such as CO, CO2, and C2–C3 hydrocarbons (Ojha and Vinu, 2018), since the optimum temperatures of them higher than LDPE:biomass pairs.
In order to evaluate the synergistic effect for each plastic-biomass blend on bio-oil yield, the predicted yields were compared with experimental co-pyrolysis yields as shown in Fig. 4. It can be seen that the obtained results demonstrate different synergy effects on bio-oil production among biomass-plastic pairs. EFB:LDPE blend shows a significant positive synergy with 4% increase in the oil yield, compared to the predicted values. Similarly, significant positive synergistic effect was also observed in co-pyrolysis of LDPE and PF blend with 3.7% increase in the experimental yield. These synergetic effect was attributed to the radical secondary reactions, which cause condensation reactions of non-condensable fragments (Özsin and Pütün, 2018). In addition to the role of LDPE as hydrogenation medium for biomass (Aboulkas et al., 2012), which could prohibit polymerization and cross-linking reactions of biomass, resulting in higher biomass weight loss (Yuan et al., 2018).
Evaluation of synergistic effect for biomass/plastic pair on bio-oil yield.
On the other hand, Fig. 4 also revealed that there was no additive synergistic interaction between the PP and biomass blends in the case of the PP and EFB combination. Meanwhile, PP and PF blend gave negative synergy of −2.9%. This was due to the higher PF:PP optimum temperature than other biomass-plastic blends which promoted the formation of non-condensable gases. The ash content in PF higher than EFB which reduces the production of the bio-oil. Moreover, the biomass-PP curves are sharper compared with those of biomass-LDPE. This means that biomass-PP radical mechanism is very rapid which favored breaking of bond of PP rather than breaking bond, resulting in a lower hydrogen in the atmosphere (Burra and Gupta, 2018).
3.4 Characterization of bio-oil
Chemical composition analysis of the co-pyrolytic oil is the most helpful tool to evaluate the synergistic interaction between biomass intermediates and plastics. The components detected in pyrolysis oil can be classified into aromatic hydrocarbon, aliphatic hydrocarbon (alkane and alkene), alcohol, phenol, carbonyl (aldehyde and ketone), ester, sugar, acid, and nitrogenated compounds. Hydrocarbons and alcohols were classified as a high-value organic chemical in the bio-oil. On the other hand, oxygenated compounds such as acids, ketones, aldehydes, esters and ethers and nitrogenated compounds were undesirable products, because they mainly contribute to corrosiveness, instability and lower heating value of bio-oil (Hassan et al., 2020). The abundances of compounds are compared by the means of their percentage area obtained by GCMS.
3.4.1 Chemical composition of individual pyrolysis bio-oil
Initially, the compositions of pyrolytic oil, derived from pure materials, were specified in order to calculate the predicted bio-oil compositions from the co-pyrolysis of biomass-plastic blends. Fig. 5 reveals the bio-oil product composition from pyrolysis of PF, EFB, LDPE and PP. The components in bio-oil are very different among these feedstocks. In general, the biomass-derived bio-oil has large amount of undesirable oxygenated products compared to the plastic-derived bio-oil which is rich in hydrocarbons. The abundance of oxygenated compounds could be ascribed to the lignin and cellulose in the structure of the biomass (Kabir et al., 2017; Özsin and Pütün, 2018). With regard to the bio-oil derived from biomass, EFB showed slightly less oxygenated content compared to PF. Fig. 5 showed that the major compounds from PF pyrolysis were ester (25.92%), phenols (21.74%), carbonyls (18.66%), acids (17.96%), and (11.96%) of hydrocarbons (aromatic and aliphatic), while the pyrolysis of EFB exhibited (44.37%) of esters, phenols (5.09%) carbonyls (5%) and acids (23.7%), and (15.85%) hydrocarbons. However, the largest involved classes in both biomass-derived oil are oxygenated compounds. This is due to the elemental composition of the EFB and PF, which appears to have a higher oxygen content and less carbon. In contrast, plastic-derived bio-oil contains a large amount of hydrocarbons and alcohol with little amounts of oxygenated compounds except significant amount of esters in PP-derived bio-oil. LDPE-derived bio-oil is rich of aliphatic hydrocarbons compounds (69.18%) and has significant portion of alcohols (20.96%) and share small amount (5.74%) of esters with traces of phenols and carbonyls. On the other hand, the PP derived bio-oil exhibited high portions of alcohol and esters 44.55% and 40% respectively, in addition to significant amount of hydrocarbons (14.74%).
Chemical compositions of bio-oil produced by pyrolysis of individual materials.
3.4.2 Co-pyrolysis synergistic effect on bio-oil composition
The predicted compositions of co-pyrolytic oil were compared with those results obtained from co-pyrolysis experiments. Fig. 6 and Table 1 show the theoretical and experimental bio-oil chemical compositions for the co-pyrolysis of biomass and plastic blends (with the weight ratio 1:1). As expected from the research literature, synergies between plastics and biomass are obvious in the results of bio-oil composition analyses (Özsin and Pütün, 2018; Stančin, 2021; Dewangan et al., 2016). However, it can be seen that degree of synergy depends on biomass-plastic pairs.
Effect of biomass-plastic pair on the relative experimental and theoretical yields in the bio-oil, (a) high-value components and (b) undesired components.
3.4.2.1 High-value chemical compounds
Fig. 6(a) presents the effect of biomass-plastic pair on the distribution of high-value compounds in bio-oil. By comparing the predicted composition distribution calculated based on pyrolysis of individual pyrolysis with those obtained from experimental co-pyrolysis, we can first conclude that co-pyrolysis of biomass-plastic blends enhance the total high-value chemical compounds in oils, depending on biomass-plastic pairs. PF:LDPE bio-oil promote greatly the formation of aliphatic hydrocarbons and alcohols where the experimental value was 70.45% for aliphatic hydrocarbons and 13.52% for alcohols, meanwhile predicted values were 36.64% and 10.74% for aliphatic hydrocarbons and alcohols, respectively. For EFB:LDPE, aliphatic hydrocarbons also increased from 40.91% value to 61.24%, and alcohol increased slightly from 12.71% to 13.71%. However, both biomass-LDPE pairs promoted aliphatic and reduced aromatic hydrocarbons. This was due to the availability of more hydrogen radicals, which converted the aromatics to aliphatic products (Ahmed et al., 2020). This is in agreement with Dewangan et al. (Dewangan et al., 2016) who reported that the liquid produced from co-pyrolysis LDPE with sugarcane bagasse (SCB) consists mainly of significant amount of aliphatic hydrocarbons with smaller amounts of aromatics. On the other hand, the average of desirable (hydrocarbons and alcohols) compounds obtained by PP with biomass co-pyrolysis is considerably lower compared with those from LDPE with biomass. However, in case of PF:PP, aliphatic hydrocarbon increased from 9.43% to 39.86 while alcohols decreased sharply from 22.53% to 4.96%. Similarly, EFB:PP shows increase in aliphatic hydrocarbons from 13.69% to 40.57% and sharp reduction in alcohols from 24.51% to 12.38%. TGA biomass-PP curves are sharper than biomass-LDPE, which means that biomass-PP reaction is very rapid, favoring breaking of bonds rather than breaking bonds (Burra and Gupta, 2018) thus prohibit the synergistic effect of hydrocarbons production. In addition, Fig. 5 suggests that the abundant alcohols derived from PP were interacted with the acids abundant in the biomass, resulting in formation of more esters and reduction of alcohols (Molinero et al., 2013).
3.4.2.2 Undesired chemical compounds
Concerning undesired components, Fig. 6(b) displays the experimental oxygenated and nitrogen-contain composing bio-oil resulted from co-pyrolysis process and the predicted composition. In general, co-pyrolysis of biomass-plastic blends reduce the content of oxygenated compounds, compared with bio-oil obtained from biomass alone which reveals large quantities of oxygen-containing compounds. For PF:LDPE, positive synergic effects were observed regarding diminishing phenols and acids, as well as reduction of esters and carbonyls respectively from 15.83% and 10.9% to 11.1% and 4.73%. EFB:LDPE blend reduces the esters and acids respectively from 25.3% and 11.8% to 15.9% and 2.7%. The reduction in oxygenated compounds in bio-oil derived from biomass:LDPE blends was ascribed to the efficiency of LDPE as a hydrogen donor compared to PP which has lower hydrogen and higher oxygen content (Hassan et al., 2020). For EFB:PP, it was seen that synergistic effects led to insignificant reduction in esters and carbonyls. Acids and phenols dropped respectively from 12.14% and 2.6 to 5.8% and zero. During co-pyrolysis of PF:PP, negative synergistic effects elevated esters content greatly from 32.96% to 44.36% while other major oxygenates such as acids, carbonyls and phenols respectively reduced from 9.3%, 9.41% and 10.9% to 7.3%, 3.5% and zero. This increase in esters in bio-oil derived from biomass:PP blends was attributed to esterification of the acids derived from biomass and abundant alcohols generated from PP (Molinero et al., 2013).
4 Conclusion
The EFB:LDPE has a significant synergy effect on bio-oil yield, as well as PF:LDPE. Concerning bio-oil chemical composition, although they differ in the degree of effect, all feedstocks shown a positive synergistic effect for the formation of aliphatic hydrocarbons. Furthermore, all feedstocks decreased the overall oxygenated compounds of fuel substitute. The hydrogen generated from plastic promoted the decarboxylation of acids and decarbonylation reactions of carbonyls and sugars, meanwhile, oxygen was removed as CO2 and CO. It can be concluded that the quantity and quality of bi-oil highly depended on biomass-polymer pairs. Consequently, selection of feedstock can be an efficient way to maximize and improve bio-oil product.
Acknowledgement
The authors thankfully acknowledge the support obtained from the Lotte Chemical Titan (M) Sdn Bhd, Pasir Gudang, Johor, Malaysia and Universiti Sains Malaysia (Grant Number: 304/PJKIMIA/6050422/L128) in the form of research grant and facilities.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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