Recent advancements of thermochemical conversion of plastic waste to

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Recent advancements of thermochemical conversion of plastic waste to biofuel-A review

Mohammed B. Al Rayaan

Department of Environmental Sciences and Engineering, King Abdullah University of Science&Technology, Thuwal, Saudi Arabia

A R T I C L E I N F O Keywords:

Plastic waste Biofuel

Themochemical conversion Thermal behaviour Techno-economic

A B S T R A C T

Advancement and modernization have brought about a signiﬁcant rise in all kind of commodities production.

Plastics have been labelling as one of the materials due to its wide range of applications, light weight, versatility and low cost. Over the decades, the global plastic production has continuously increased to fulﬁl the global market. This phenomenon has resulted in the depletion of non-renewable fossil fuel since plastics are the petroleum-based material. Apart from that, the disposal of plastic wastes in landﬁlls are also imposing risk on human and animal’s health as well as causing groundwater contamination. Thus, a sustainable and an effective plastic waste treatment is essential to overcome the related issue. Recycling and energy recovery methods are of the alternatives that have been discussed and practiced in many countries to reduce the plastic waste generation.

Although recycling methods are highly effective, the high labor cost for the separation process and the emission of greenhouse gases during the process have reduced the process attractiveness. Due to these drawbacks, researchers have diverted their attention to the energy recoveryﬁeld speciﬁcally on the thermochemical conversion method.

Extensive research works have been carried out on the technology development to convert the plastic waste to bio-fuel. Although extensive research works have been carried out on the technology development to convert the plastic waste to bio-fuel, but there is still a lack of comparative discussion between the recent technologies, specifically on the economic feasibility in industrial scale. Thus, this paper aims to provide an overview on the strength and weakness of each thermochemical technologies such as a) Microwave assisted pyrolysis b) Super- critical gasification and C) Plasma gasification.

1. Introduction

According to the municipal solid waste (MSW) generation report from the United Nations (UN),c.a100% of goods originally bought for use by consumers are turned into waste after six months of acquisition (Ayeleru et al., 2020). This has leads to the rise of the annual growth rate of global MSW of 3.2–4.5% in developed nations and 2–3% in developing nations (Tang et al., 2018) For instance, In 2010, the EU-28 member states have generatedc.a875.2 million tons of MSW waste and the amount increased to 890.8 million tons in 2014. Meanwhile, In China, the MSW generation has increased drastically from 178.6 million tons to 204 million tons from 2014 to 2015, and it is predicted that China’s annual amount of MSW will reach up to 282 million tons by 2020 (Ma et al., 2020). Despite close to 70–80% of the waste is being recyclable, about 90–95% of the collected wastes are still disposed of in landﬁlls which lead to severe environmental consequences such as water contamination and high greenhouse gases (GHG) emission (Gaeta-bernardi and parente, 2016).

In most developing countries, the major components of MSW are food

wastes and plastics (Zhou et al., 2015). The increase in demand for plastics is due to the enormous growing dependency in human life as a key material in many applications such as construction, healthcare, engineering and packaging (Al-salem et al., 2017). The plastics in MSW include polypropylene (PP), polyvinyl chloride (PVC), polyethylene (PE), polystyrene (PS) and High-density polyethylene (HDPE) as shown in Table 1. From an engineering perspective, non-degradable plastics are no longer an environmental issue in landfills since the plastics can be recycled. However, run-away plastic wastes are still a critical hazard to the environment such as waterways, oceans, endangering safe life for both animals and humans (Khan et al., 2016). Lately, promising dis- coveries have been made, including the usage of plastic waste as biofuel generation. Due to their intrinsic properties (e.g. calorific value) which are almost similar to petroleum fuel, plastics can be used as a throughput material to produce heat and steam through various thermochemical techniques such as gasification and pyrolysis (Arjanggi and kansedo, 2020). The thermochemical techniques are also considered faster and environmentally safe to convert plastic waste to bio-gas, bio-oil and heat

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Contents lists available atScienceDirect

Cleaner Engineering and Technology

journal homepage:www.journals.elsevier.com/cleaner-engineering-and-technology

https://doi.org/10.1016/j.clet.2021.100062

Received 19 July 2020; Received in revised form 30 January 2021; Accepted 11 February 2021

nc-nd/4.0/).

Cleaner Engineering and Technology 2 (2021) 100062

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as compared to hydrothermal and biological methods (Munir et al., 2019).

Although the advantages of thermochemical techniques in trans- forming plastic wastes to value-added products have been widely reported, the review on the application of advanced thermo-chemical technologies (ATT) such as microwave assisted pyrolysis, supercritical gasiﬁcation and plasma reforming are still limited in the literature. This observation may be due to its being a relatively new technology, a moderate technology readiness level (TRL) and also a highly energy- intensive process. However, ATT are claimed to be the most effective technologies for conversion of the carbon-based plastic landﬁll waste into syngas and fuels, with achieving a circular economy with zero-waste accumulation (Bai et al., 2019). Here, we review the progress that has been made recently in this arena, with an emphasis on the 1) physio-characteristic of plastic waste 2) application of ATT in conversion of plastic waste to bio-fuel and 3) Future prospect of conversion of plastic waste to bio-fuel through ATT.

2. Material recovery through thermochemical conversion Based on the deﬁnitions related to plastic waste management for the European Union (EU), established by Directive (2008/98/EC Waste legislation and policy, the tertiary recycling is acknowledged as the most economically viable and environmentally friendly method for a circular economy (Yin et al., 2019). According to the Ellen Macarthur Founda- tion,“A circular economy is one that is restorative and regenerative by design and aims to keep products, components, and materials at their highest utility and value at all times, distinguishing between technical and biological cycles (Jubinville et al., 2020). Amongst all the recycling methods, thermochemical process is one such routes for plastic waste material recovery. Thermochemical process has been proven to be one of the best technologies to substituting the conventional incineration and refuse derived fuel (RDF) burning due to the releases toxic chemicals and gases such as hydrochloric acid, sulfur dioxide, dioxins, furans and heavy metals as well as particulates which causes signiﬁcant hazard to the environment (Li et al., 2015). In addition, thermochemical processes can convert the unwanted plastic waste to liquid fuel and gaseous products

without the need of expensive chemical reagents (Loy et al., 2018b).

Also, there is still a lack of studies have been deliberated for recycling plastic waste management with energy recovery process through thermo-chemical process (Banu et al., 2020). One of the contributing factors might due to the lack of knowledge and the ignorance of the underutilized physical characteristic of plastic waste. Besides, the ﬁnancial risk which attributed to high initial capital investment and long payback period have been the crucial conundrums that caused the in- vestors to be sceptical in venturing into this industry.

2.1. Physio-characteristic of plastic waste

Table 2shows the characteristics such as proximate, ultimate and heating value of different plastic wastes. Most of the plastic wastes consists of high volatile mater>90 wt% dry-basis except plasticﬁlm, indicating a high ignition biofuel can be generated such as syngas and bio-oil (Ahmad et al., 2017). Besides that, the relatively low content of S and N elements in plastic wastes indicate that a low emission of NOx, HCl, and SO2 associated with a high heating value (HHV) gaseous products will be produced during pyrolysis process due to the high content of C and H elements (Loy et al., 2018a). Meanwhile for individual components, HDPE, LDPE and PP consisted mainly of carbon and hydrogen with a high volatile matter except to PET, suggesting that PET is not a potential candidate for biofuel generation. Overall, the intrinsic features such as high volatile content, high viscosity along with low ash content show that plastic wastes are comparable to its counterparts (e.g biomass and coal) as a feedstock for biofuel industrial applications (see Table 3).

2.2. Thermal degradation behaviour of plastic waste

Understanding the thermal degradation behaviour and degradation mechanism of plastic waste are the critical components to the reactor design, feasibility assessment and scale-up purpose. Generally, the design of pyrolyzer or gasifier require a series of hydrodynamics and transport simulation which involves information about mass, heat transfer as well as the kinetics. Most importantly, the optimal conditions for the process need to meet the requirements of mass and heat transfer efficiency and pyrolysis kinetics parameters (Al-salem et al., 2020). In order to inves- tigate the degradation mechanism thoroughly, researchers often analyse the kinetic parameters of the plastic waste in a non-isothermal condition using Thermogravimetric analyser (TGA) (Gou et al., 2019). For instance, Ozsin and co-authors have analysed the thermal degradation of PVC at heating rates of 5–40C/min using TGA (Ozsin and pütün, 2019). Based€ on the TG/DTG profiles shown inFig. 1, it can be deducted that the PVC degradation occurred within the temperature range between 230 and 580C, while the maximum rate of mass loss (DTG peak) are between 270 and 320C. Generally, there are two degradation phases can be found in the profile which are dehydrochlorination (250–380C) and dehydrogenation (280–580C). During thefirst stage, polyene radicals Table 1

Global consumption of individual plastics (Cleetus et al., 2013).

Type of plastic Consumption

%

Polythene (PE) 33.5

Polypropylene (PP) 19.5

Polyvinylchloride (PVC) 16.5

Polystyrene (PS) 8.5

Polyethylene terephthalate (PET) and polyurethane (PU) 5.5

Styrene copolymers (ABS, SAN, etc.) 3.5

Blends, alloys, high performance and specialty plastics, thermosetting plastics, and so forth

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Table 2

Physiochemical characteristic of plastic waste.

Plastic Source Proximate analysis (wt

% dry basis)

Ultimate analysis (wt% dry basis) Ref

Volatile Matter Ash C H N S O^a

HDPE Train station, China 98.2 1.8 84.5 13.8 0.1 0.1 1.5 Zhang et al. (2020)

LDPE Petro China Daqing Petro-chemical Company 99.8 0.0 86.8 12.9 0.1 – 0.2 Zhang et al. (2020)

PP Train station, China 99.6 0.4 85.0 13.9 0.1 0.0 1.0 Zhang et al. (2020)

PET O’Brien Recycling Centre, UK 85.7 8.3 62.9 4.1 0.0 – 32.9 Diaz silvarrey and phan (2016)

Municipal plastic Waste

Brazil 93.2 3.3 68.9 12.8 14.3 0.5 0.2 Lazzarotto et al. (2020)

Colour plasticﬁlm Spain 74.2 9.8 72.7 12.3 2.4 0.3 0.6 Gala et al. (2020)

Medical plastic waste Wuhan hospital, China 99.13 0.0 81.8 5.6 0.1 0.1 12.3 Qin et al. (2018)

Reclaimed plastic solid waste Kuwait 95.18 4.81 77.9 12.9 0.1 – 6.3 Al-salem et al. (2020)

aBy difference.

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and a small quantity of hydrocarbons especially unsubstituted aromatics are being released; followed by a second step which is the formation of alkyl aromatics through condensation and dehydrogenation reactions to form various gaseous products. After most of the reactions have taken place,c.a.10–12 wt% solid residue ash remained which can be used as addictive.

In recent year, new technologies such as the coupling of TG and FTIR (TG-FTIR) or TG and MS (TG-MS) including simultaneous and continuous real-time analysis to obtain the transient mass loss and the evolution of volatile product during the pyrolysis (Ong et al., 2020). Based on the TG-FTIR analysis, Qin and co-authors managed to obtain the gaseous evolution proﬁles as well as the function groups of the decomposition during degradation of medical plastic waste (Qin et al., 2018). The in-situ FTIR result indicates that the medical plastic waste starts degrading around 300 C and reaches to the maximum near 400 C, produces mainly styrene monomer, benzene, toluene, and small amounts of C1–C4

hydrocarbons as the initial pyrolysis products. Apart from that, Gunasee et al. reported on combustion characteristics of PVC, PP and PS using TGA-MS under dynamic conditions at 20 K/min (Gunasee et al., 2016).

Since there is no oxygen is present in the structure of PVC ([CH2-CHCL]

n-), PP ([CH2–CHCH3]n-) and PS ([CH2–CHC6H6]n-) and thus, H2, CH₄and other hydrocarbons (HCs) should be the main volatile components. This theory is in good agreement with the study reported in which the relative abundance intensity of H2and CH4in the gaseous evolu- tionary proﬁle are much higher than CO and CO₂.

3. Recent thermochemical conversion of plastic waste to biofuel methods

3.1. Microwave irradiation -assisted pyrolysis

Microwave irradiation -assisted pyrolysis is a thermal decomposition process where plastic wastes are pyrolyzed at high temperatures (573 K–1073 K) through microwave radiation to produce bio-gas and bio-oil (Lam et al., 2019c). Microwave irradiation has been reported to have vast advantages compare to conventional heating method such as fast heating rate, higher temperature distribution temperature achieved at low operating temperature, and shorter reaction time (Loy et al., 2019).

Moreover, the microwave irradiation could easily break the heavier hydrocarbon component of plastic wastes into lighter hydrocarbon component via chain-end scission mechanism, producing syngas or other high-quality bio-oil.Table 2shows a series of microwave-assisted pyrolysis of plastic wastes to value-added products found in the literature.

During microwave-assisted pyrolysis of plastics, additional use of dielectric material or absorbent is required such as silicon dioxide, activated carbon or graphene (Borges et al., 2014). This is because the use of absorbent can signiﬁcantly improve the heating rate of the microwave irradiation. With the minimal energy input through microwave irradiation, the temperature of the reactor is distributed uniformly when the plastic waste is dropped directly onto the heated absorbent. Thus, high temperatures can be achieved in second or minute rather than hours as Table 3

Microwave-assisted pyrolysis of plastic wastes to bioenergy.

Type of feedstock Types of catalyst/absorbent Microwave power (W) Reaction temperature (C) Products Ref

Waste plasticþwaste cooking oil Activated carbon 800 400–550 Gasoline, Diesel Lam et al. (2019c)

LDPE NiO and HY 150–200 450–600 Gasoline Ding et al. (2019)

Frying oil and plastic waste – 800 550 Biodiesel, Diesel Wan mahari et al. (2018)

Plastic waste – 5000 – Bio-char Aishwarya and sindhu (2016)

Polystyrene and Polypropylene mixtures Activated carbon 900 – Gasoline, Diesel Rex et al. (2020)

Plastic Waste Carbon-coated AF 900 1000 Syngas Jiang et al. (2020)

Polystyrene Iron Mesh 700 1100–1200 Bio-oil Hussain et al. (2010)

Fig. 1. TG and DTG curves of PVC at different heating rates.

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needed in conventional heating (Bu et al., 2019).

Moreover, a few studies have revealed that microwaves irradiation can effectively upgrading of in-situ pyrolytic vapors into enhanced bio- oils during pyrolysis (Lam et al., 2019b). For instance, Wan adibah et al. (2018) have reported that a complete microwave co-pyrolysis of waste cooking oil and plastic waste can be achieved up to 500C within 10 min (Wan mahari et al., 2018). The prevailing synergistic effect between waste cooking oil and plastic waste during microwave assisted co-pyrolysis reactions produced a high yield of liquid fuel (81 wt%).

Notably, the liquid fuel showed an almost similar properties as transport-grade diesel with high stability, low oxygen content, free of nitrogen and sulfur and higher energy content (42–46 MJ/kg). Besides that, Rex et al. have also investigated the microwave irradiation assisted pyrolysis of two different plastic wastes, PSW and PP (Rex et al., 2020). A high oil yield of 84.30 wt% was obtained through microwave pyrolysis at 900 W using coconut sheath activated carbon as an absorbent. The results have demonstrated that microwave pyrolysis has a great potential for energy recovery from plastic waste to value added products.

A techno-economic study of the application of microwave co- pyrolysis of waste plastic and used cooking oil has been reported lately (Lam et al., 2019c). Lam’s group has revealed that the production cost of liquid oil from microwave approach is estimated to be USD 0.25/L, which is lower than the price of diesel fuel in most countries in this world), suggesting that this pyrolysis approach shows promise to be scaled up for industrial operation. A similar observation is attained by Wang and Lei study, where a small scale microwave assisted ex-situ catalytic pyrolysis of wood pellet into aromatic hydrocarbon fuels can generate a proﬁt of $135,494 yearly, suggesting that the employment on a larger scale for further utilization is highly attractive (Wang et al., 2015).

3.2. Supercritical water gasiﬁcation

Supercritical water gasification (SCWG) is a clean and effective thermal degradation of plastic waste (Bai et al., 2019), biomass (Okolie et al., 2019), and coal (Chen et al., 2018) which provides vast advantages as compared to the conventional method. Firstly, a homogenous reaction environment can be formed through the high solubility of organics and gases of SCWG which speed up the reaction rate (Nurcahyani et al., 2020). Secondly, the gasification process can be obtained at lower temperature compare to conventional gasification (Su et al., 2020). Thirdly, high purity product can be obtained in short reaction time (Wang et al., 2020). Fourthly, no or less emissions of toxic gaseous such as NOx (Killilea et al., 1992), andfine particles (Ferreira-pinto et al., 2019) in SCGW to the environment compared to other conventional gasification.

Table 4confirmed that the feasibility for energy conversion of plastic via SCWG as supported by many previous literatures, reporting that high syngas composition can be obtained in a short reaction time. Bai and co-authors have reported that the optimum conditions for degradation of PS plastic via SCWG were reaction temperature of 1073 K, a reaction time of 60min, a reaction pressure of 23 MPa to obtain high carbon conversion rate reaches 94.48 wt% and hydrogen yield of 12.0 wt% (Bai et al., 2019). Meanwhile, Okajima et al. studied the effects of supercritical fluids on carbon fiber reinforced plastics and they reported that the decomposition rate of plastics is mainly affected by the size of solvent molecules at the initial stage of reaction (Okajima et al., 2017).

In addition, Liu et al. studied the inﬂuence of operating parameters such as temperature and time on acrylonitrile-butadienestyrene (ABS) plastics in supercritical water at 673–973 K and 10–80 min (Liu et al., 2019). Notably, most of the monomers were converted into more stable substances at long residence time and, the optimal reaction condition for monomer recovery was attained at 673 K and 3 min. Onwudili and co-authors had studied the catalytic SCWG of various types of plastics including HDPE, LDPE, PP and PS using RuO2-based catalyst (Onwudili and williams, 2016). A promising yield of CH4 was obtained in the presence of RuO₂-based catalysts. The aliphatic plastics produced

relatively more CH4than CO2, suggesting that the RuO2-based catalyst is more active in C–C bonds than C–O cleavage. The promise of recovering energy value from plastic waste, environmentally benign process con- tinues to attract attention to SCWG. Most of the studies in the literature has reveals that substituting the traditional gasiﬁcation or hydrothermal liquefaction with SCWG is an economic viable option where a high rate of return can be achieved with a reasonable payback period ~5–7 years (Piazzi et al., 2020).

3.3. Plasma gasiﬁcation

Recent year, intensification of research on the use of plasma techniques for utilization of plastic wastes have been revealed, especially from the European Union countries (Mączka et al., 2013). Plasma gasification is an allo-thermal gasification process where the heat required by the endothermic reactions is given by thermal plasma typically generated by direct current (DC) non-transferred arc plasma torches (Mazzoni and janajreh, 2017). As shown inFig. 2, there is a large fraction of electrons, ions and excited molecules together with the high energy radiation within the highly reactive plasma zone. Plasma gasification usually tak- ing place at reaction temperature (approximately 2000–14,000 C), residence time (less than 30 min) with the aid of plasma gas, oxidant, or steam streams (Munir et al., 2019). When the carbonaceous particles are injected into a plasma, they are heated very rapidly by the plasma arc;

and the volatile matter is released and cracked giving rise to syngas ((Paulino et al., 2020) and methane (Kuo et al., 2020).

In 2012, the bench scale plasma gasifier with a 20 kg/h capacity plasma arc pyrolyzer for energy recovery of plastic has been proposed by Puncochar et al. (2012). They revealed that reveals that there is a great potential for development of thermal plasma pyrolysis technologies applicable to plastic waste disposal management with energy and material recovery. Meanwhile, Zhang and co-authors also developed a plasma gasification melting system, consisting an updraft moving-bed gasifier with plasma torches placed next to air nozzles to heat the incoming air to 6000C. They analysed the gasifier using a solid waste consists of 10 wt% of plastic materials and they found out that a high LHV (6–7 MJ/Nm3) syngas can be obtained. Besides experimental studies, there is also literature reporting on the ASPEN PLUS simulation of municipal plastic waste plasma gasification (Tavares et al., 2019). The Table 4

Supercritical water gasiﬁcation of plastic waste to hydrogen production.

Type of Feedstock

Reaction time (Min)

Reaction Temperature (K)

Reaction Pressure (MPa)

Yield of gaseous product (mol%)

Ref

Plastic wastes and soda lignin

30.0 1023.0 26.0 63.3^a Cao et al.

(2020)

High impact PS

60.0 1123.0 23.0 12.0 Bai et al.

(2019)

PET 10.0 1123.0 23.0 20.0 Bai et al.

(2020b)

PP 60.0 1023.0 23.0 22.0 Bai et al.

(2020c)

PC 10.0 1023.0 23.0 29.0 Bai et al.

(2020a)

LDPE 60.0 723 38.0 10.0 Onwudili

and williams (2016)

HDPE 60.0 723 38.0 12.0 Onwudili

and williams (2016) a mol/kg.

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simulation is focused on the behaviour of the equivalence ratio (ER), steam ratio and gasiﬁcation temperature as the function of three different gasifying agent to assess theﬁnal syngas composition. Based on the data, highest hydrogen composition is obtained with steam, followed by air and O2.

In a nutshell, in order to improve the technology readiness level and economic sustainability of plasma gasiﬁcation; the revenue generation beyond tipping fees will be the key factor, depending on the quantity and quality of value added products produced such as hydrogen and long chain hydrocarbon (Shahabuddin et al., 2020). This is because the reduction of the initial capital cost of the process is an unrealistic option since sophisticated and high-end equipment and process plants are ne- cessity for operation.

4. Conclusion

In summary, the advanced thermochemical technologies (ATT) are potential viable methods of converting plastic wastes to value-added chemicals. All the technologies reported have circular economic advantages of reducing the high plastic waste volume that being landfill, save the costs for waste treatment, and alleviate the environmental problems associated with plastic wastes disposal. However, there are various challenges associated with the ATT which require to be solved before being successfully commercialize in near future: 1) The high initial capital costs of ATT operation plant. 2) The high energy consumption of the ATT process needs to be reduced through process integration and intensification. 3) Improve the technology readiness levels through additional safety analysis 4) Economic and environmental studies need to be carried out to further confirm their feasibility.

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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