Thermochemical Recycling of Plastic Wastes to Alternative Fuels: History
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Due to its resistance to natural degradation and decomposition, plastic debris perseveres in the environment for centuries. As a lucrative material for packing industries and consumer products, plastics have become one of the major components of municipal solid waste today. The recycling of plastics is becoming difficult due to a lack of resource recovery facilities and a lack of efficient technologies to separate plastics from mixed solid waste streams. This has made oceans the hotspot for the dispersion and accumulation of plastic residues beyond landfills. 

  • catalysts
  • clean fuels
  • co-processing
  • gasification

1. Introduction

With rapid population growth and resource consumption, the accumulation of municipal solid waste necessitates appropriate reduction, reuse and recycling methods [1]. Among the major sources of municipal solid waste, end-of-life plastic waste is a major component because of its occurrence in large quantities around the world. Plastic products have been every day, ubiquitous and practical materials with massive versatility in molding, durability, affordability and easy tuning of their physical properties. Thus, plastics have long been used in various industries, including packaging, manufacturing, electronics, automobiles, toys, tools, home appliances, construction, etc. Since plastic products are not degraded naturally by microorganisms, they tend to accumulate in the environment, especially in landfills and oceans. The problem of plastic waste accumulation occurs when plastic production surpasses recycling and/or effective valorization, thus leading to negative environmental and health impacts on aquatic and terrestrial ecosystems [2].
Plastic waste has long been a major issue on a global scale, as the consumption of single-use plastic products increases globally. Examples of polymers used to manufacture single-use plastic products are polyethylene terephthalate, high-density polyethylene, low-density polyethylene, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polycarbonate and polyurethane (Figure 1). Table 1 lists the common types of synthetic plastics and their main characteristics and consumer applications.
Figure 1. Common examples of single-use plastic products.
Table 1. Main characteristics and applications of plastic products.

2. Pyrolysis

Pyrolysis is one of the most widely used thermochemical conversion techniques of various materials such as lignocellulosic biomass (e.g., agricultural and woody residues) or polymeric wastes (e.g., plastics, rubber and tires) into liquid and solid fuels under oxygen-deficient conditions [3]. Co-pyrolysis is an iteration of pyrolysis where mixed feedstocks such as biomass with plastics or rubber are thermochemically converted to liquid hydrocarbon oil and char with superior physicochemical and fuel properties. Pyrolysis can efficiently liquefy polymers including plastics into polyolefins and hydrocarbons [4][5]. Additionally, it provides a way to reprocess the polyolefins, which are too expensive to handle through traditional mechanical recycling, preventing incineration and the production of dangerous chemicals like dioxins and furans.
Pyrolysis is classified into three types such as fast, flash, slow and intermediate pyrolysis based on the process parameters such as reaction temperature, vapor residence time and heating rate of the reactor [6]. Both fast and flash pyrolysis are mostly carried out at high temperatures with rapid heating rates for a short vapor residence time. These characteristics lead to higher bio-oil yields in comparison with biochar and gases. The temperature range for fast pyrolysis typically varies from 400 °C to 600 °C at a high heating rate of 10–200 °C/s and a short vapor residence time of 30–1500 s [7]. High reaction temperatures of 700–1000 °C, fast heating rates (>1000 °C/s) and vapor residence times ranging in the milliseconds are characteristic of flash pyrolysis, leading to greater quality and yields of bio-oil. On the contrary, slow pyrolysis is typically characterized by temperatures of 300–500 °C, low heating rates (0.1–1 °C/s) and longer vapor residence times (10–100 min), leading to higher char yields compared to bio-oil [7]. Intermediate pyrolysis is usually conducted at temperatures of 500–600 °C with heating rates varying from 2–10 °C/s for a vapor reaction time of 10–20 s, resulting in moderate yields of bio-oil and biochar.
The physicochemical properties of the feedstock, including its particle size, moisture content, ash, volatile matter, impurities and elemental content, can affect the reaction rate of pyrolysis and the overall product distribution [8][9]. The type of pyrolytic reactor also plays a role in the pyrolysis process as well as in product yield. Reactors for pyrolysis can be selected based on the process control, liquid and gas phase flow, mass and heat transfer, reactant mixing, retention time, fluidization media and catalyst used. Driven by the growing interest in pyrolysis, significant research has resulted in the development of different pyrolysis reactors such as fixed beds, moving beds, fluidized beds (bubbling, circulating and sprouted), ablative, auger, rotary kiln, drum, vortex and entrained flow [10]. In addition, depending on the flow of materials, pyrolysis reactors can be operated as batch, semi-batch or continuous systems.
A temperature rise coupled with a controlled heating rate enhances several reactions during pyrolysis including decomposition, dehydration, depolymerization and fragmentation, which leads to an increase in condensable and non-condensable vapors, thereby improving bio-oil yield and quality. Condensation is a quenching process where vapor residence time plays a vital role in determining the bio-oil quality and composition. For example, rapid quenching generates various volatile chemicals which may condense, cleave or interact with other intermediate components at extended vapor residence times. Certain amounts of non-condensable gases and other lighter hydrocarbons are also emitted after the quenching of hot vapors from biomass pyrolysis. The gaseous fraction of pyrolysis mainly contains H2, CO2, CO and CH4 along with trace amounts of other lighter hydrocarbons, including ethane (C2H6), ethylene (C2H4), propane (C3H8), propene (C3H6), butane (C4H10) and butene (C4H8).
Another product of pyrolysis, biochar, is a result of the secondary polymerization and aromatization of decomposed organics at longer vapor residence times. Various reactions such as dehydration, decarboxylation, deamination, dehydrogenation and aromatization lead to the formation of biochar containing a significant amount of fixed carbon [6]. The physicochemical properties of char, such as carbon content, hydrogen content, sulfur content, elemental composition, porosity, surface area, crystallinity, pH, aromaticity, salinity and electrical conductivity, depend on the pyrolysis process parameters and feedstock properties, which also determine their post-treatment and applications [11]. Biochar has various applications such as in energy recovery for heat and power generation, as a solid fuel, as a soil amendment agent, as fertilizer, catalyst support, absorbent in water purification, wastewater treatment for producing chemicals and in pharmaceuticals and cosmetics industries [12].
Bio-oil produced from pyrolysis can be utilized as a drop-in liquid fuel, a precursor for jet fuels or as a raw material for biochemicals [3]. The direct utilization of bio-oil is not convenient because of its high aqueous content, low heating value, high viscosity, acidity, corrosiveness, inferior thermal stability and presence of heteroatoms such as nitrogen, sulfur and oxygen [13]. Before its applications, crude bio-oil requires some upgrading through catalytic (e.g., hydrotreating, hydrocracking, esterification and transesterification) and non-catalytic (e.g., emulsification, solvent extraction, supercritical fluid extraction and electrochemical stabilization) processes to enhance its thermal stability, physicochemical and fuel properties with the exclusion of heteroatoms and oxygenated compounds [14].
Pyrolysis of plastics respects four mechanisms, namely depolymerization or end-chain scission, cross-linking, chain stripping and random-chain scission [15]. Thermal cracking of plastics can produce oil, gases and char along with chemicals including paraffin, olefins, benzene, xylene, ethylene glycol, terephthalic acid, acetophenone, acetaldehyde, alcohols, amines and phosphorous-containing oligourethanes. The oil produced from the pyrolysis of plastics generally contains hydrocarbons in the range of light and heavy crude oil, mid-distillates and naphtha. The light oil, with a boiling point of 250–350 °C, is made up of olefins and paraffin. In contrast, heavy oil, containing olefins, paraffin, aromatics and high molecular weight components, has a boiling point of more than 350 °C [16]. The composition of mid-distillates is C12–C28 hydrocarbons, while naphtha comprises C5–C15 hydrocarbons containing paraffin, olefins and aromatics. Unlike the pyrolysis of lignocellulosic biomass, which typically results in about 35–55 wt% of bio-oil, catalytic pyrolysis of certain plastics can produce more than 80% of liquid product [17]. This is because plastics have lesser impurities (elements and ash) and primarily contain long-chain polymers of carbon and hydrogen compared to lignocellulosic biomass. This attribute of plastics makes them suitable for use as a co-feed in co-pyrolysis, co-liquefaction and co-gasification with lignocellulosic biomass for enhancing the overall yield of the oil.
The components of the plastic can have an impact on the yields and properties of the final product. Moreover, the type of plastic used for conversion has a great impact on liquid fuel. Pyrolysis of polyethylene can increase the alkane content, while polystyrene can enhance the aromatic content in the liquid fuel [18]. Alkene production in the pyrolysis oil resulting from polypropylene can increase the octane number of liquid fuel [19]. Furthermore, pyrolysis of polypropylene and polyethylene can produce more aliphatic hydrocarbons, thereby improving the concentration of paraffin, olefins and waxes in the oil. Waxes are intermediary products consisting of long-chain hydrocarbons (C20+) with a high boiling point. Therefore, after pyrolysis, they must be separated and further cracked into combustion products [4]. Due to the presence of unsaturated hydrocarbons, the pyrolysis oil also needs additional processing such as distillation, refining and hydrogenation to enhance its physicochemical and fuel characteristics [20].
Free radical reactions such as β-scission, hydrogen transfer, hydrogen abstraction, radical recombination and disproportionation can occur during the pyrolysis of plastics, primarily producing aromatic monomers, dimers and trimers [20]. Most aromatic monomers are created through an “unzipping of carbon chains” process in which the terminal aromatic ring separates from the other aromatic ring because of the C–C bond cleavage. The liquid product from the pyrolysis of plastics (e.g., polyvinyl chloride) could also contain polycyclic aromatic hydrocarbons [21]. Pyrolysis of polyvinyl chloride generally involves three stages, i.e.: (i) dichlorination with interior cyclization, (ii) aromatic chain scission, and (iii) release of aromatics with a two-four rings structure [22]. Similarly, the primary thermal degradation route for polyethylene terephthalate during pyrolysis involves β-scission and retro-hydroalkoxylation, which produce benzoic acid and vinylic products and allow the breakdown of bridging glycol O–C bonds and transformation of β–H atoms to carbonyl groups [23]. Additionally, a significant quantity of CO2, CO and ethylene are also released in the gas products from the pyrolysis of plastics [24].
The oil obtained from the pyrolysis of plastics can be grouped as: (i) hydrocarbons, e.g., n-paraffins, iso-paraffins, olefins, naphthene, monoaromatics, di-aromatics, tri-aromatics, tetra-aromatics, naphthenoaromatics, naphthenodiaromatics and naphthenotriaromatics; (ii) oxygenated compounds, e.g., aldehydes, ketones, phenols, esters and ethers; (iii) nitrogenated compounds, e.g., indole, nitriles, caprolactam, pyridines and quinolines; (iv) sulfur-containing group, e.g., sulfides/thiols, benzothiophenes, disulfides/thiophenes and dibenzothiophenes [25].
Pyrolysis is one of the preferred thermochemical conversion technologies considering the diversity of product distribution of added value. However, the flexibility relating to the preference of the product yield and quality can be achieved by adjusting the process parameters such as temperature, vapor residence time, reaction time, feedstock loading and reaction type (i.e., batch, fed-batch or continuous). As discussed earlier, pyrolysis oil, char and gases can have their dedicated applications upon upgrading, activation and refining, respectively.

3. Catalytic Pyrolysis

Using catalysts in pyrolysis can accelerate the reaction rate, process performance, reactant conversion and product yield. Catalysts can also lower the activation energy and reaction time to enhance the conversion rate and selectivity of products, thus lowering energy consumption. A benefit of catalytic pyrolysis over conventional methods is the ability to generate liquid products with desired properties such as high heating value and hydrocarbons like jet fuels, diesel and gasoline along with low tar or wax formation [26]. Catalysts are broadly classified as homogeneous (i.e., catalyst and reactants are in the same phase) and heterogeneous (i.e., catalyst and reactants are in separate phases).
Protonic Zeolite Socony Mobil-5 (HZSM-5), H-style ultrastable Y (HUSY), hydrogen bonding (H-β) and hydrogen-type mordenite (HMOR) are some examples of nanocrystalline zeolites widely used in the pyrolysis of plastics [27]. Additionally, non-zeolite catalysts like silicalite, silica-alumina (SiO2/Al2O3) and Mobil Composition of Matter No. 41 (MCM-41) have also attracted a lot of interest in recent studies [28]. Zeolites, fluid catalytic cracking (FCC) catalysts and silica-alumina catalysts are also frequently used in the pyrolysis of plastic [29]. ZSM-5 zeolite is one of the most widely used catalysts for plastic conversion. It has a three-dimensional structure in which the tetrahedral sides are linked via oxygen atoms. Various ratios of SiO2/Al2O3 are used for the formation of this type of catalyst and the ratio has a great influence on the final products of pyrolysis. Although amorphous ZSM-5, Y-type zeolites, SiO2/Al2O3 and other diverse acidic catalysts have promising catalytic effects, their high cost of manufacturing and regeneration increases the overall expenditures of the pyrolysis process.
An amorphous acid catalyst, SiO2/Al2O3, is also used for the catalytic pyrolysis of plastics. It includes Lewis acid sites, which take electrons and Brønsted acid sites with an ionizable hydrogen atom. In contrast to zeolites, SiO2/Al2O3 catalysts have an acid strength decided by the high molar ratios of SiO2/Al2O3. Various acidity strengths in the catalyst have a greater impact on the final products from the pyrolysis of plastics. Light olefin production is greatly increased by amorphous SiO2/Al2O3 catalysts, with no appreciable changes in aromatics formation [4].
Zeolite catalysts have demonstrated outstanding catalytic effectiveness on cracking, isomerization, aromatization and oligomerization owing to their unique physicochemical characteristics, which include a strong acidity along with a crystalline microporous structure. Dai et al. [30] found Zn/SBA-15 as an effective heterogeneous catalyst to produce short-chain olefins from high-density polyethylene pyrolytic wax via catalytic cracking. Marino et al. [31] used three types of zeolites such as ZSM-5 (11), ZSM-5 (25) and ZSM-5 (25-des) for pyrolysis of electric and electronic equipment plastic waste in a stainless-steel downdraft fixed-bed reactor at temperatures of 450 and 600 °C with a catalyst/feedstock ratio of 0.2. In these catalysts’ nomenclature, (11) and (25) indicate the molar ratio of Si/Al, whereas (25-des) means that the ZSM-5 catalyst was prepared from desilication treatment. When the pyrolysis vapors were cracked using ZSM-5 zeolites, the oil and gas yields increased significantly compared to non-catalytic pyrolysis, which created waxes. During non-catalytic pyrolysis, more wax and a trace amount of oil and gas were produced because of random thermal cracking of the polymer chain. However, the usages of zeolite introduce additional cracking of long-chain hydrocarbons into lighter components due to its high reactivity. The highest oil yield (60 wt%) was obtained using ZSM-5 (25-des). The formation of light hydrocarbons was enhanced by using catalysts, leading to a sharp rise of gas products, paraffin and olefins owing to the expansion of end-chain cracking reactions.

4. Microwave-Assisted Pyrolysis

In microwave-assisted pyrolysis, the thermal decomposition of plastics takes place via microwave irradiation. Microwave-assisted pyrolysis has become popular over conventional heating due to advantages such as fast heating, uniform distribution of heat, low chances of localized temperature zones, short reaction time and less infrastructure requirement [32]. In contrast to traditional electrical heating which distributes temperature and transfers heat via conduction, convection or radiation, microwave irradiation (i.e., 1000–300,000 MHz) passes through the heated material and transforms thermal energy within it within seconds [33]. The long hydrocarbon chains in plastics can easily break down into lighter hydrocarbons through microwave irradiation via chain-end scission mechanisms and generate high-quality syngas and oil [34].
Although microwave heating has several benefits, a major barrier that impedes its widespread commercial application is the lack of data needed to estimate the dielectric properties of materials. The efficacy of microwave heating directly depends on the dielectric properties of the raw material as dielectric properties absorb the microwave radiation and lead to its heating [35]. The use of dielectric materials (adsorbents) such as activated carbon, silicon dioxide and graphene is necessary to enhance the pyrolysis process [5]. Absorbents significantly increase the heating and process efficiency and reduce the reaction time. They also increase the heating rate by distributing the temperature evenly throughout the reactor with the minimal energy intake provided by microwave irradiation. Hence, high temperatures can be reached in a matter of seconds or minutes as opposed to the hours needed for traditional heating. Efficiency can also be improved by adding various metals such as iron, copper and aluminum. The use of carbon black material as a susceptor can be another promising method of enhancing process efficiency as they absorb electromagnetic energy and directly transform it into heat energy. It must be noted that this technique typically involves the introduction of polymers without cleaning to accelerate the absorption of microwaves, as moisture, dust and waste aid in the absorption of microwaves [36].

5. Plasma-Assisted Pyrolysis

In plasma-assisted pyrolysis, electromagnetic radiation and electricity are the main energy providers. Plasma is the fourth state of matter and consists of positively and negatively charged particles in the form of a conducting gas containing ions and electrons [37]. The two types of plasma are thermal and non-thermal. High-temperature equilibrium plasma (≥10 keV) and low-temperature quasi-equilibrium plasma (≥10 eV) are the two classes of thermal plasma [38]. In contrast, non-thermal non-equilibrium plasma has a low-temperature range of 25–125 °C [37]. Plasma carrier gases such as argon, nitrogen, hydrogen and steam, when excited with a high-power supply, can generate plasma in the form of ions and electrons. Plasma can be applied to nuclear reactors, combustion, gasification and pyrolysis to generate alternative energy.
Plasma-assisted pyrolysis has several advantages over conventional and microwave-assisted pyrolysis such as higher energy efficiency, greater energy density, better reaction kinetics and lower carbon emissions [37]. However, the major drawbacks of this process are high energy requirements and a low technology readiness level. This technique is usually used to neutralize large-scale hazardous and toxic wastes because of the high energy requirements. Plasma-assisted pyrolysis can process toxic substances and refractory compounds while generating limited inhibitors compared to conventional pyrolysis technologies. The major reactions that usually occur in the presence of plasma are oxidation, substitution, elimination, rearrangement and reduction. These reactions occur rapidly and often simultaneously.
Thermal cracking is the primary mechanism of plasma-assisted pyrolysis. Charged particles have high kinetic energy. Initiation, propagation and termination make up the main radical chain for the thermal degradation of waste materials, wherein initiation entails the generation of free radicals, propagation entails intermolecular abstractions and termination follows second-order reactions [39]. In the plasma reaction zone, homogeneous and heterogeneous processes happen at the same time. The reaction mechanism is highly affected by temperature, feedstock type, residence time and intensity and type of plasma. 

6. Liquefaction

Liquefaction is an emerging technique for direct conversion of feedstocks (biomass and polymers) into liquid fuels at moderate temperatures (150–450 °C) under high pressures in the range of 0.1–25 MPa [40]. Liquefaction involves using various solvents to dissolve the organic matter and polymers through solvolysis, hydrolysis and cracking to produce bio-crude oil. The speed and selectivity of these reactions can be altered via temperature, feedstock/water ratio, reaction time and catalyst, enabling the control of functional group conversion reactions. The physicochemical and fuel properties of bio-crude oil derived from liquefaction are superior to the bio-oil produced from pyrolysis due to greater heating value, low oxygen content, more carbon content, enhanced thermal stability, low acidity, better flowability and low polymerization potential. Hence, the bio-crude oil produced from the liquefaction of organic matter and polymers requires less intense upgrading conditions to improve its fuel grade.
Solvents and catalysts can be used to enhance the liquefaction of feedstocks, make conditions milder and enhance mass and heat transfer. Alcohols (e.g., methanol, ethanol and propanol), toluene, acetone and water are widely used as organic solvents because they are convenient and easy to separate due to their low boiling point [41]. Supercritical ethanol is a popular solvent used in liquefaction to enhance the solubility and cracking of organic components. The scission of polymer chains via reaction with solvent takes place during liquefaction. Several base catalysts (e.g., NaOH, Na2CO3, KOH and KCO3), acid catalysts (e.g., H2SO4, H3PO4 and p-toluenesulfonic acid), heterogeneous catalysts (e.g., Ni, Pd, Pt and Ru) and metal oxide catalysts (e.g., CeO2, Y2O3, ZrO2, Raney-Ni and HZSM-5) have been used for liquefaction to enhance bond cleavage, dehydration, decarboxylation and decarbonylation reactions [42].

7. Gasification

Gasification is a thermochemical biomass-to-gas technology that transforms organics into a gas phase mostly consisting of syngas (a mixture of H2 and CO) along with a small amount of CH4, CO2, C2H2, C2H4 and C2H6 [43]. Although the main product of gasification is syngas, char and a trace amount of tar are also produced, depending on process conditions such as temperature, pressure, reaction time, equivalence ratio, feedstock concentration, catalysts and gasifier type [44]. Gasification is an appealing process over other thermochemical technologies because it produces H2, which can decrease energy loss during combustion in power plants due to its superior calorific value of 120–142 MJ/kg. The occurrence of CH4 in the gas products from gasification also enhances the combustion properties of the gases due to its reasonable energy density of 50–55 MJ/kg.
Hydrogen is considered one of the cleanest fuels because its combustion releases heat energy and water. As opposed to steam reforming of fossil fuels to produce hydrogen, which generates significant levels of greenhouse gas emissions, clean hydrogen production technologies such as gasification of organic wastes have the potential to attain decarbonization and demonstrate economic viability and performance [45]. Apart from energy applications, hydrogen can be applied in multiple sectors such as fuel cells, metallurgy, cogeneration, aviation, chemical refineries and pharmaceutical industries [46]. Hydrogen is also used to upgrade heavy and light gas oils, pyrolysis-derived bio-oil and liquefaction-derived bio-crude oil through a variety of hydrotreating technologies such as hydrodeoxygenation, hydrodenitrogenation, hydrodesulfurization and hydrodemetallization [47][48][49]. Hydrogen is also used as a raw material to produce clean fuels in the range of gasoline, diesel and jet-fuels, chemicals and lubricants through the catalytic Fischer–Tropsch process [50].
Gasification can be classified based on the medium, such as air, steam, subcritical and supercritical water gasification. Subcritical and supercritical water gasification are categorized as hydrothermal gasification because of the aqueous reaction media. As mentioned previously, subcritical water occurs at temperatures and pressures below the critical point of water, i.e., 374 °C and 22.1 MPa. On the contrary, the water turns into supercritical water when the reaction temperature and pressure exceed the critical points [43]. Supercritical water possesses superior solvation properties owing to its ionic products and free radicals that lead to hydrothermal denaturation of complex organic substances including woody and agricultural biomass, plastics, tires, municipal solid waste and sewage sludge [51]. Supercritical water gasification is advantageous over conventional air or steam gasification because of the comparatively lower reaction temperatures, use of water as a green solvent, utilization of wet biomass and recovery of hydrogen at high pressures, thus lowering the cost of biomass drying, gas compression and overall energy requirement [52]. The gaseous product obtained from air gasification generally has low energy content because of the diluting effect of nitrogen (carrier gas).

This entry is adapted from the peer-reviewed paper 10.3390/ma16134563


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