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Introduction, history and nomenclature, classification of pyrolysis process, pyrolysis mechanism, applications of pyrolysis, conclusion and way forward, acknowledgments.

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A comprehensive review of the pyrolysis process: from carbon nanomaterial synthesis to waste treatment

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Mamta Devi, Sachin Rawat, Swati Sharma, A comprehensive review of the pyrolysis process: from carbon nanomaterial synthesis to waste treatment, Oxford Open Materials Science , Volume 1, Issue 1, 2021, itab014, https://doi.org/10.1093/oxfmat/itab014

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Thermally induced chemical decomposition of organic materials in the absence of oxygen is defined as pyrolysis. This process has four major application areas: (i) production of carbon materials, (ii) fabrication of pre-patterned micro and nano carbon-based structures, (iii) fragmentation of complex organic molecules for analytical purposes and (iv) waste treatment. While the underlying process principles remain the same in all cases, the target products differ owing to the phase and composition of the organic precursor, heat-treatment temperature, influence of catalysts and the presence of post-pyrolysis steps during heat-treatment. Due to its fundamental nature, pyrolysis is often studied in the context of one particular application rather than as an independent operation. In this review article, an effort is made to understand each aspect of pyrolysis in a comprehensive fashion, ensuring that all state-of-the-art applications are approached from the core process parameters that influence the ensuing product. Representative publications from recent years for each application are reviewed and analyzed. Some classical scientific findings that laid the foundation of the modern-day carbon material production methods are also revisited. In addition, classification of pyrolysis, its history and nomenclature and the plausible integration of different application areas are discussed.

Pyrolysis is the key process in carbon nanomaterial synthesis [ 1–4 ], bulk carbon production [ 5 , 6 ], fabrication of carbon-based devices [ 7–10 ], fuel generation from organic waste [ 11–13 ] and molecule fragmentation for their analysis via gas chromatography–mass spectroscopy (GC–MS) [ 14–16 ]. Primary examples of the technologically significant carbon materials prepared via pyrolysis include graphene [ 1 , 17 , 18 ], carbon nanotubes (CNTs) [ 1 , 19 , 20 ], carbon fibers (CFs) [ 1 , 21–23 ], diamond-like carbon (DLC) coatings [ 2 , 24 ] and other industrial carbons such as glass-like carbon (GC) [ 7 ] and graphite [ 5 ].

Unlike other manufacturing materials such as metals, the production of carbon relies heavily on synthetic routes. In fact, certain carbon allotropes (e.g. GC) are exclusively synthetic. Majority of carbon manufacturing pathways are based on pyrolysis, where a suitable organic precursor is heated to elevated temperatures in an inert environment and in some cases, a catalyst. This leads to a thermal decomposition of the precursor and the release of non-carbon atoms in various forms. Owing to carbon’s high thermal stability (in the absence of oxidants), some fraction of solid carbon is always obtained as a residue or in the form of smoke. This reactive solid carbon can potentially adopt numerous microstructural configurations, depending upon the type of precursor, its decomposition pattern, applied pressure (if any), the formation pathways of the larger carbon moieties and the thermodynamic stability of pyrolysis products [ 12 , 25 ]. This principle has been used for manufacturing various crystalline as well as disordered carbons for several decades [ 26 , 27 ]. With the advent of nanotechnology, the pyrolysis process and precursors have been optimized to yield the nano-scale one- and two-dimensional carbon structures. Modern-day chemical vapor deposition (CVD) technique designed for carbon nanomaterial synthesis is indeed based on the principle of pyrolysis [ 1 , 28 ]. Interestingly, the primary reason behind the popularity of carbon in nanotechnology is the ease with which different carbon nanoforms can be deposited onto a range of substrates [ 6 ]. Needless to say, optimization of pyrolysis process is of utmost importance to anyone working in the field of carbon materials and associated technology.

Some large-scale industrial carbon materials such as graphite can be procured via mining. However, their synthetic production yields a high purity material and is therefore preferred. Pyrolysis additionally offers the possibility of tuning the microstructure of the ensuing carbon (e.g., enhancement of graphitic content [ 29 , 30 ]), rendering the pyrolysis-assisted production more popular. Depending upon the type of carbon as well as the scale of reaction, these materials can be produced as films on a substrate or part [ 31 ], or as micro- and nano-scale structures that are pre-patterned employing lithographic techniques [ 7 , 9 ]. The organic precursors may range from very simple molecules such as methane to a complex mixture of high molecular weight polymers and other hydrocarbons [ 12 , 32 ]. Pyrolytic decomposition is also an extensively used process in the petrochemical refineries [ 33 ]. The fact that a variety of heavy hydrocarbons can be broken into smaller molecules that can be further fractionalized is utilized for decomposing the solid organic waste. Here, a mixture of organic (natural and/or synthetic) waste is often heat-treated in large-scale reactors or plants in order to produce useful chemicals [ 34 ].

A reaction is designated pyrolysis if (i) the precursor material is in the decomposition phase rather than bond formation and (ii) the cleavage of bonds is solely thermal. However, in many instances, the heat-treatment may also lead to partial bond formation along with dissociation, which cannot be clearly differentiated from pyrolysis. In certain reactions, the temperature-induced chemical modifications may not be solely thermal. For example, the presence of oxygen within the pyrolyzing precursor can lead to partial combustion of the material. Other reagents such as hydrogen may be present to dilute the precursor for prevention of excessive formation of a particular product [ 35 ]. The use of the term pyrolysis can be often found in the literature of such processes as well, which is acceptable as long as the primary decomposition mechanism is thermal. Notably, pyrolysis is different from both combustion and natural decomposition owing to its very definition.

It is evident that pyrolysis is a very versatile process, which is used in wide range of applications directly or indirectly. Unfortunately, this versatility is also responsible for the fact that this process is often studied only in the context of one specific research field [ 18 , 36–41 ]. Different scientific communities may even use different nomenclature for essentially the same process. Often the connection is missing between various research fields that utilize the same principles and processes, with differences only in terms of process parameters. In this contribution, our goal is to compile a comprehensive review of the pyrolysis process that encompasses (i) its fundamental principles and mechanism, (ii) classification, (iii) process parameters and their tuning, (iv) all application areas with respective examples and finally, (iv) a comparison and possible integration of different application areas. This is particularly of interest for the development of energy materials and systems, which rely on pyrolysis in many ways.

The use of pyrolysis for technological applications dates back to almost two centuries when carbon filaments for incandescent lamps were reportedly derived from cellulose fibers extracted from cotton and bamboo [ 26 ]. Hollow carbon filaments similar to CNTs were observed as early as in 1952, which were formed by the thermal decomposition of gaseous hydrocarbons in a closed retort [ 27 , 42 ]. Single crystal graphite (now known as graphene) was produced by thermal decomposition of acetylene around the same time [ 43 ]. Some early primary batteries utilized during the WW-II contained pyrolytic carbon materials (e.g. charcoal) in the electrodes of the Leclanché cell [ 44 ].

Pyrolysis is also responsible for the formation of carbonaceous materials below the Earth’s crust, which is integrated with the carbon cycle. In fact, an entire branch of carbon science, Deep Carbon study, is dedicated to understanding the fate of different types of organic materials under harsh geophysical and environmental conditions [ 45 ]. These so-called deep carbons constitute approximately 90% of the Earth’s total carbon [ 45 ]. Such investigations reveal the possible pathways of formation of different carbon allotropes due to tectonic movements, sudden changes in the temperatures, meteorite impacts, high-pressure as well as other extreme conditions. Formation of graphite in the sedimentary rocks is believed to have originated from the organic matter trapped within rocks, where each pore of the rock may have served as a ‘reaction chamber’, thus facilitating pyrolysis over millions of years [ 46 , 47 ]. Various carbon allotropes are present at varying depths under the Earth’s crust, depending upon the different temperature and pressure conditions experienced by the initial organic matter.

Nomenclature

The term pyrolysis should only be used to allude to chemical reactions taking place at temperature significantly higher than the ambient temperature in order to differentiate between pyrolysis and natural chemical decomposition. A chemical reaction taking place between 100°C and 300°C, for example, may simply be called thermal degradation rather than pyrolytic decomposition, which typically takes place between 300°C and 800°C. Pyrolysis is also often associated with burning. Burning is a complex combination of combustion, pyrolysis due to heat generated by combustion, evolution of volatile compounds, steam distillation, aerosol formation, etc. [ 48 ]. A clear distinction between the processes of pyrolysis and combustion during burning is extremely difficult because the formation of free radicals during the reaction with oxygen can be involved in the pyrolytic decomposition of molecules [ 48 ]. In addition, the free radicals formed from molecules due to heat (pyrolysis) can be the initiators of a combustion process [ 49 ]. Any organic material undergoing combustion at some stage undergoes pyrolysis to produce gaseous fuels to further initiate the combustion process and generally yields very small molecules like H 2 O, CO 2 , CO and N 2 [ 50 ]. Therefore, utmost care must be taken while differentiating between these high-temperature processes as well as their nomenclature.

In the context of polymeric carbon research, the terms pyrolysis and carbonization are often used interchangeably. Notably, pyrolysis of a polymer produces tars, gases as well as solid carbon (also see the ‘Waste treatment via pyrolysis’ section). If the intended final product is carbon, pyrolysis can be considered as the pathway to carbonization. However, carbonization is the process that entails C–C bond formation that generally takes place between 800°C and 2000°C. If the material is heated further, this region (2000–3000°C) is referred to as graphitization [ 51 ]. There are examples of pyrolysis of coal to extract tars [ 52 ], volatile organic compounds (VOCs) [ 53 ] and char (carbon with impurities) [ 54 ] where each product has its own industrial relevance. This type of pyrolysis is more common for petroleum products. As such, both pyrolysis and carbonization are thermolysis processes but with different target products. Similarly, in the case of pyrolysis of light/gaseous hydrocarbons, the overall process is known as CVD. Importantly, the first step of the CVD process is pyrolysis, which is followed by the collection, migration and deposition/growth of the desired carbon nanomaterial. Discrepancies pertaining to nomenclature of these carbon nanomaterials are also relevant for carbon scientists. However, this vast topic is beyond the scope of this article. Interested readers may find the recommendations by Bianco et al. [ 55 ] helpful.

Pyrolysis can be classified based on (i) the phase of precursor, (ii) scale of reaction (which determines the type of reactor) and (iii) target product(s), as illustrated in Fig. 1 . Based on the precursor, pyrolysis can be classified into solid, liquid and gas phase. Solid phase pyrolysis primarily utilizes synthetic and natural polymers [ 13 , 56 ], solid petrochemicals such as coals and cokes [ 54 ] and hydrocarbons of mixed compositions such as biomass [ 40 , 57 ] or municipal solid waste (MSW) [ 58 , 59 ]. Production of mesophase carbons [a precursor for meso-carbon micro beads (MCMB), carbon foams, etc.] and the production of CF by pyrolysis of petroleum pitches [ 60 ] and naphthenic residues [ 61 ] fall under the category of liquid state pyrolysis. Notably, polymers are often in their liquid state when they are patterned or spun. But before their heat-treatment, they are typically cross-linked, dried and stabilized. Some precursors such as pitches may however be in the semi-solid state also during their heat treatment. Examples of further pyrolytic cracking of pyrolysis oil (the tarry product generated during waste pyrolysis) are also carried out with a liquid precursor [ 62 ]. Gas phase pyrolysis relies on the principle of cracking a hydrocarbon gas such as methane or acetylene at sufficiently high temperatures followed by the collection of solid carbon deposits onto a substrate. As the precursor is present in gas (vapor) phase, this entire process (pyrolysis followed by material deposition and film growth) is known as the CVD. CVD is a more general term that is also applicable to various other chemicals that yield non-carbon element or compound deposits. In the case of carbon materials, however, the precursor gas is essentially a hydrocarbon, and hence, the fundamental process responsible for the CVD is pyrolysis.

Different classification pathways of pyrolysis process.

The second type of classification is based on the reaction scale and reactor type/size. Laboratory scale heat-treatment can be performed in a tube furnace, small reactors or chambers that can facilitate a controlled environment (e.g. inert gas or vacuum) [ 9 , 11 , 18 , 63 ]. In some cases, the size of the precursor sample may be extremely small (micro or even nano-gram scale), for example, in the case of analytical pyrolysis [ 64 ] used for fossils, and in situ pyrolysis investigations performed on a transmission electron microscope (TEM) [ 65 ]. Here, the pyrolysis chamber is associated with another instrument, that may entail specially designed chips [ 66 ], wires [ 67 ] or customized sample holders [ 68 ]. Industrial pyrolysis is either used for large-scale carbon material production or for the purpose of waste treatment. In waste pyrolysis, the availability of waste determines if the process should be batch or continuous. The feed waste is often pelletized prior to pyrolysis [ 69 ]. The common reactors used for waste pyrolysis are rotary kilns [ 70 ], fixed bed [ 71 ], fluidized bed [ 72 ], tubular and certain batch and semi-batch reactors [ 73 ]. Plasma is also used for waste pyrolysis, which requires a specialized plasma reactor [ 73 , 74 ]. Based on the target product, pyrolysis can be divided into three main classes: (i) carbon production, (ii) pyrolysis oil and synthetic gas production and (iii) hydrocarbon fragmentation for analytical purposes. Carbon production can be further divided into synthesis of nanomaterials, preparation of large-scale industrial carbons and carbon-conversion of polymer structures intended for device application. Details on each type of pyrolysis process will be discussed in subsequent sections.

Pyrolysis typically involves covalent bond dissociation and rearrangement, which takes place between 300°C and 800°C for most hydrocarbons. The mechanism may range from simple to very complex, depending upon the nature of the precursor. For example, methane can yield some carbon species along with hydrogen slightly above the temperature where its energy of formation becomes positive [ 75 ]. A polymer, on the other hand, may exhibit complicated fragmentation patterns with parallel secondary and tertiary reactions and release volatile byproducts. Salient features of light and heavy hydrocarbon pyrolysis are described below.

Pyrolysis of light hydrocarbon

Pyrolysis of hydrocarbon gases such as methane, ethane, acetylene and low boiling point liquids such as alcohols is carried out for the purpose of carbon nanomaterial production during their CVD [ 35 , 76 , 77 ]. A hydrocarbon molecule disintegrates at a temperature where its free energy of formation (Δ G f ) becomes positive [ 75 ]. Since, at all temperatures, finite partial pressure of various hydrocarbons is in equilibrium with hydrogen and solid carbon, its pyrolytic disintegration can never quantitatively lead to the formation of only carbon and hydrogen [ 78 ]. The equilibrium compositions are attainable only above the disintegration temperature for a particular hydrocarbon. The carbon solubility (total amount of gaseous hydrocarbons in equilibrium with carbon and hydrogen) reaches a minimum at a certain temperature for a given total pressure of the reaction chamber [ 78 ], which plays an important role in determining the optimum process pressure as well as the type of catalyst for carbon collection. At the temperatures corresponding to this carbon solubility minima, a spontaneous decomposition of the hydrocarbon takes place. Below this, the attainment of equilibrium is very slow. Consequently, other thermodynamically unstable hydrocarbons may exist in the reaction chamber [ 32 ].

For example, at pressure ≤ 10 − 2 bar and temperature > 500°C, the cracking of methane becomes thermodynamically feasible. This leads to the formation of ‘carbon smoke’ in the chamber, which contains various carbon species including thermodynamically unstable ones (i.e. radicals, carbon moieties having two to eight atoms and some cyclic structures). Around 900°C, methane gas approaches equilibrium with these solid carbon species and hydrogen, that is carbon solubility in gas phase exhibits a minimum. Hence, even though thermodynamics suggest that methane disintegrates at temperatures > 500°C), solid carbon deposits are only obtained around 900°C [ 32 ]. These carbon deposits are collected on to a catalytic substrate in the form of carbon films, tubes or other nano structures [ 31 ]. The catalyst plays an important role in determining the film growth rate, film thickness as well as the termination of reaction [ 79 ]. Further details on various catalysts are provided in the ‘Carbon nanomaterial synthesis’ section. Overall, the formation of carbon from light hydrocarbons follows three main reaction stages: (i) cracking of aliphatic hydrocarbons into smaller molecules or reactive species, (ii) cyclization of hydrocarbon chains to form aromatics and (iii) condensation of these aromatics to form polycyclic aromatics on a suitable substrate [ 32 ].

Pyrolysis of high molecular weight hydrocarbon

High molecular weight hydrocarbons include polymers, pitches, cokes and their mixtures. Their pyrolysis can be understood in terms of both chemical and physical changes, as discussed below.

Chemical aspects

During heavy hydrocarbon pyrolysis, a series of primary, secondary and tertiary reactions take place in parallel in a highly dynamic system [ 25 , 80 , 81 ]. The primary chemical changes that occur (generally in sequence) typically include (i) cleavage of C-heteroatom bonds to generate free radicals, (ii) molecular re-arrangement, (iii) thermal polymerization (iv) aromatic condensation and (v) elimination of H 2 from the side chains [ 81 ]. The bond cleavage is based on the bond dissociation energies (BDEs) of the specific carbon-heteroatom bond. Although these reactions take place in parallel, only one of them is dominant at a particular pyrolysis temperature [ 82 , 83 ]. For example, when we pyrolyze coal, at around 300–400°C, condensable coal-tar is released along with other volatiles due to reaction type (i), but at the same time, steps (ii) and (iii) occur in the remaining solid. With increasing temperature, step (iii) becomes dominant over other two steps and char or coke is obtained around 800°C [ 82 ]. One can terminate the heat-treatment process at any temperature, allowing only few of the aforementioned steps to complete. For treatment of waste, for example, the process is terminated at step (iii); hence, the maximum pyrolysis temperature does not exceed 800°C and the final solid residue contains pores and impurities [ 13 ].

In the case of carbon material production, the process is terminated after step (v). Here, the entire heat-treatment can be divided into three stages: pre-carbonation (pyrolysis), carbonization and graphitization (optional) [ 84 ]. Pre-carbonization stage encompasses breaking of C-heteroatoms bond and re-arrangement of the C–C bonds followed by dehydration and elimination of halogens below 500°C due to their lower BDEs (mostly < 450 KJ/Mol −1 ). At this stage, a rapid weight loss is observed due to the elimination of volatiles [ 85 ] and cyclization (formation of aromatic network) [ 86 ]. Above 500°C, bonds with higher BDEs ( > 600 KJ/Mol −1 ) are broken, and oxygen and nitrogen are eliminated. However, at this stage, the thermal polymerization is dominant [ 81 ] and the aromatic networks gets interconnected, resulting in primary volume shrinkage and rapid weight loss in the solid. This phase is called ‘carbonization’ stage, which takes place at temperatures > 800°C [ 51 ] and may extend up to 2000°C for some polymers. It is intuitive that an organic material of high molecular weight will decompose to form carbonaceous hydrocarbons of lower molecular weights. However, it is not always the case, as some organic molecules on pyrolysis may result in molecules larger than the starting ones. For example, during the thermal cracking of n -Hexadecane ( n -C 16 ) [ 87 ], along with the low molecular weight hydrocarbons, (alkanes (C 1 –C 14 ) and olefins (C 2 –C 15 )), higher molecular weight alkyl hexadecanes and alkanes (C 18 –C 31 ) are also obtained [ 87 ], which could be attributed to thermal polymerization.

Further heat-treatment above the temperature of 2000°C leads to gradual elimination of any structural defects due to aromatic condensation and the elimination of the last fragment of volatiles [ 81 ]. This stage is called the ‘graphitization’ stage, which takes place at temperatures above 2000°C [ 51 ]. Here, the crystallite diameter of residual pyrolytic carbon ( L a ) is increased and the stack thickness ( L c ) is decreased. An example of a heavy hydrocarbon precursor is poly-vinyl chloride (PVC), that undergoes all the three stages during its conversion into synthetic graphite [ 86 ].

Physical aspects

In terms of physical changes (e.g. phase, density and morphology), heavy hydrocarbons adopt one of the two possible mechanisms, known as coking and charring, during their pyrolysis. These principles are described in detail elsewhere [ 7 ]. Briefly, if the material experiences softening such that there is a liquid or semi-solid phase during its pyrolysis, it is said to undergo coking. Charring, on the other hand, refers to a relatively high rigidity and the protection of the carbon backbone in its nearly original morphology during and after its pyrolysis. These morphological aspects are of paramount importance when the target product is carbon. Precursors that undergo coking yield the carbon with an extremely flat surface and exhibit mostly microporosity. Charring leads to meso, macro as well as microporosity in the residual carbon.

Both physical and chemical aspects of pyrolysis are strongly influenced by (i) the highest process pyrolysis temperature, (ii) temperature ramp rate and (iii) residence (dwell) time at the highest temperature. The effect of these parameters on the composition and microstructure of the pyrolysis products is detailed in sections ‘Carbon nanomaterial synthesis’ and ‘Waste treatment via pyrolysis.’

Characterization of polymer pyrolysis

Physicochemical changes occurring during heat-treatment of a polymer can be studied by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and by characterization of the material produced at different temperature points. One can also chemically analyze the volatile byproducts generated during the process via GC–MS [ 64 ]. Oils or tars can be separately collected using a condenser unit and then be further analyzed. Other characterization techniques such as elemental analysis, Raman spectroscopy, X-ray diffraction and neutron diffraction can be used for understanding the residual carbon [ 65 , 88–92 ].

In the recent past, some methods for observing the microstructural changes during the heat-treatment ( in situ ) have also been developed. Figure 2 is a collection of TEM, TGA, XRD and wide-angle neutron scattering (WANS)/wide angle X-ray scattering (WAXS) data that indicate the microstructural changes taking place in the solid residue during pyrolysis, which ultimately converts into different types of carbon. It can be clearly observed from the TEM images ( Fig. 2A and B ) that between 600°C and 800°C the material undergoes major microstructural changes and its fragments display a high mobility [ 65 , 88 ]. This is also supported by electrical and mechanical property tests of these intermediate materials [ 93 ]. TGA analysis ( Fig. 2C ) of cellulose indicates that there is a significant mass loss between 300°C and 400°C [ 89 ]. XRD data show an increased peak intensity from the (002) and (100) planes, suggesting a better order and crystallite growth in the resulting carbon with an increase in pyrolysis temperature in the range 500–900°C. It also reveals the shifting of the (002) peaks to higher angles with increasing temperatures ( Fig. 2D ) [ 90 ]. The ex situ WANS and WAXS data for carbon obtained from (poly)-furfuryl alcohol also confirm an increased order due to the annealing of some of the defects ( Fig. 2E ) [ 91 ]. Some other techniques used for in situ observations of pyrolysis include a study of planetary materials by Raman Spectroscopy integrated with Laser-heating [ 92 ].

In situ TEM images of a pyrolyzing SU-8 thin-film up to 1200°C, scale bars—1 nm ( A ), reproduced with permission from Sharma et al. [ 65 ]; TEM images of in-situ heating of photoresist, S1805 showing migration and merging of a small graphitic domains at various temperatures ( B ), reproduced with permission from Shyam Kumar et al. [ 88 ]; TG–DTG curves of in-situ pyrolysis of cellulose at a heating rate of 5°C/min ( C ), reproduced with permission from Zhu et al. [ 89 ]; in-situ XRD studies of pyrolysis of coals at various temperatures (XRD diffractograms) ( D ), reproduced with permission from Li et al. [ 90 ]; ( ex-situ ) WANS AND WAXS data of pyrolysis of poly-furfuryl alcohol at various temperatures ( E ), reproduced with permission from Jurkiewicz et al. [ 91 ]. TG-DTG, thermogravimetry-differential thermogravimetry.

A summary of applications of pyrolysis along with the associated manufacturing pathways is presented in Fig. 3 . Table 1 contains the typical temperature range and pyrolysis environment used in these different applications. As most of the application areas are rapidly progressing, one can find some variations in pyrolysis conditions for specific cases. We have summarized the typical values here. In the subsequent sections, we review the representative examples from each application area.

schematic representation of classification of applications of pyrolysis into four major areas: ( A ) carbon material production, ( B ) fabrication of carbon-micro nano devices, ( C ) chemical analysis of unknown samples by Py-GC–MS, ( D ) treatment of waste. CNF, carbon nanofibers; HOPG, highly oriented pyrolytic graphite.

Typical temperature range and other parameters pertaining to different applications of pyrolysis

Carbon nanomaterial synthesis

CVD of carbon nanomaterials such as graphene, CNTs, vapour grown CF (VGCFs), VD diamonds and DLC films is based on the principle of pyrolysis [ 28 ], where a gaseous hydrocarbon is pyrolyzed. Historically, CVD and similar processes were used for carbon production as early as the 19th century [ 26 , 43 , 100 ]. However, various pyrolytic carbon materials were only considered as byproducts, as the ultimate goal was to synthesize graphite. Only in the last few decades the potential of carbon nanomaterials was recognized and they were studied as independent materials. Experimental work on single (2D) crystals of graphite was reported in the 1960s [ 43 , 101 ]. Prior to its synthesis, the electronic properties of this so-called 2D-graphite were theoretically studied in 1947 by Wallace [ 102 ]. Graphene-oxide, another derivative of single crystal graphite, was reported as early as 1859 [ 103 ] in a different context. The term ‘graphene’ was added to the IUPAC database in 1994, based on its experimental preparation reported in 1962 [ 101 ]. In 2004, Novoselov et al. [ 104 ] developed a novel method for obtaining graphene from HOPG by mechanical exfoliation, for which they were awarded the Nobel prize in 2010. With advances in nano-scale characterization techniques and extensive ongoing research across the globe, graphene has become one of the most technologically important materials of the 21st century. Apart from graphene, other carbon nanomaterials CNTs [ 105 ], VGCFs [ 21 , 106 ] and DLC [ 2 ] are also of immense technological significance. They are also prepared via pyrolysis of gaseous or light liquid hydrocarbons. The pyrolysis conditions as well as the morphology and type of catalytic substrates may differ in these cases. Table 2 contains the standard CVD parameters for synthesis of various carbon nanomaterials. More specific details are discussed below. As there are multiple detailed review articles and books available for each individual nanomaterial, we have only provided the details of their synthesis that fit in the scope of this review. For further reading, relevant reference material is suggested.

Carbon nanomaterials synthesized by gas phase pyrolysis (CVD) and their process parameters

SWCNT, single-walled CNTs, MWCNTs, multi-walled CNTs, VGCF, vapour grown CF.

Graphene is defined as a defect-free single layer of graphite. This material can also be prepared via mechanical or electrochemical exfoliation of HOPG (also see the section ‘Highly oriented pyrolytic graphite’) [ 104 ]. However, CVD is a bottom-up fabrication technique that is preferred for making relatively defect-free, large area graphene films [ 6 ]. For this purpose, a gaseous precursor such as methane, ethylene and benzene along with an inert gas (e.g. He and N 2 ) is fed into a reactor. The precursor gas disintegrates at approximately (600–1000°C) close to the surface of a heated catalyst (transition metals in most cases). The carbon species produced by this decomposition diffuse into the metal and precipitate out onto the metal surface, leading to nucleation and subsequent growth of graphene films. The quality of graphene can be controlled by optimizing the precursor gas flow rate, inert gas flow rate, catalyst, reaction time and the pressure inside the CVD chamber which affects the activation energy required for formation of graphene nuclei on the catalyst surface [ 110 , 111 ]. The formation of single-layer or multi-layer graphene depends upon the solubility of carbon and the catalyst used in the CVD process. Transition metals with unfilled d -orbitals (e.g. Co/Ni) exhibit higher affinity for carbon atoms and hence produce multi-layer graphene by dissolution and precipitation of carbon species, whereas the ones with filled d -orbitals (e.g. Cu/Zn) feature a low affinity to carbon, hence carbon diffuses onto the surface and forms mono-layer graphene [ 110 ]. Further, details on CVD reactor variations, various catalysts used, optimum temperature for graphene growth based on precursor–catalyst combination, can be found in some recent review articles on this topic [ 17 , 112–115 ]. CVD graphene is primarily used for its electrical properties after transferring it from the metal substrate to other suitable substrates [ 116 ]. For a detailed information on the applications of CVD graphene, a recent article by Saeed et al. [ 117 ] may be referred.

Carbon nanotubes

CNTs have the appearance of rolled-up single or multiple layers of graphene, which are designated as single-walled or multi-walled CNTs (SWCNTs and MWCNTs), respectively. As CNTs feature a curvature, they are occasionally also considered a part of the fullerene family of carbon. Fullerenes feature a new hybridization between sp 2 and sp 3 [ 118 ] as the unhybridized p -orbital lies at an angle between 90° (as in an ideal sp 2 carbon material) and 109.5° (as in an ideal sp 3 carbon material). CNTs are produced by CVD, which involved pyrolytic decomposition of gaseous hydrocarbons and carbon deposition (referred to as ‘growth’ in the case of tubes and fibers) on catalytic (nano)particles rather than films. The catalyst particles may be attached to a substrate (seeding catalyst method) or float in the CVD chamber (floating catalyst method) [ 119 ] which is heated to 550–1200°C. The temperature ranges for obtaining SWCNTs and MWCNTs are listed in Table 2 . The carbon precursors are mostly similar to those used for graphene (e.g. methane, acetylene, ethylene and toluene) that are introduced into the chamber at a specific rate in presence of an inert gas (Ar/N 2 ). Elemental carbon moieties diffuse into the catalyst and precipitate carbon either from the top or bottom of the catalyst particle. Typical catalysts used for CNTs are transition metals such as Fe, Co and Ni [ 120 ]. Their size determines whether the CNTs will be SW [ 121 ]. For SWCNTs growth, catalyst particles should be less than 10 nm [ 108 ]. Other process optimization parameters are synthesis temperature and pressure, reaction time and inert gas-flow rate [ 19 , 122 ]. Sometimes CVD is carried out in presence of plasma to enhance the rates of the reactions taking place inside the chamber. Such a CVD process is termed as plasma-enhanced CVD (PECVD). It has been reported that with PECVD, CNTs can be produced at temperatures as low as 120°C [ 123 ]. Further information on CNT synthesis and applications can be accessed in some recent literature on this topic [ 20 , 124–127 ].

Vapor grown CFs

VGCFs are nano-scale solid carbon filaments with an aspect ratio of around 100 [21, 106]. They are different from conventional bulk CF (having diameters of a few micrometers) in their preparation process and hence their properties. Their synthesis involves a hydrocarbon gas (such as natural gas, propane, acetylene, benzene, ethylene and methane) as the precursor, undergoing thermal decomposition in an inert atmosphere at around 950–1100°C on the surface of a catalyst, which are normally metal nanoparticles (Fe/Ni/Co), > 20 nm in size [ 119 , 128–130 ]. Similar to CNTs, the catalyst can be present onto the heated substrate or sometimes can be fed along with the precursor gas as floating catalyst [ 129 , 131 ]. The catalyst particle takes up carbon from the supersaturated hydrocarbon gas and leaves out tubular filaments of mainly sp 2 hybridized carbon. The formation mechanism of VGCFs is similar to the formation of CNTs, with the difference in the size of the catalyst particles used for the decomposition of the hydrocarbons [ 132 , 133 ]. It is because of the catalyst size, instead of tubular cross-section in case of CNTs, fibers with the cross-section consisting of flakes of graphite layers in various orientation precipitate out of the catalyst [ 134 , 135 ]. There are reports that VGCFs having high degree of graphitization can also be prepared by CVD without the use of a catalyst on the surface of ceramic substrates [ 106 ]. VGCFs are excellent candidates as filler materials for polymer matrix composites [ 136–138 ] and carbon–carbon composites [ 139–143 ]. They are also used in energy storage devices as filler in electrodes of lead-acid batteries and Li-ion batteries, and in supercapacitor applications [ 144 , 145 ].

Vapor-deposited diamonds

Carbon thin films (hydrogenated or dehydrogenated) prepared via CVD and having significant portion of sp 3 carbon atoms with negligible sp 2 content are referred to as vapor-deposited diamonds (VDDs) [ 2 ]. The use of CVD for diamond growth started in the late 1960s ([ 2 , 146 , 147 ]). A breakthrough was achieved when atomic hydrogen was used for etching away the graphite deposits. This left a high content of diamond deposits on the substrate. VDDs are used as industrial coatings because of their excellent mechanical properties, especially on various cutting tools [ 148 ].

Diamond films are deposited using PECVD including filament-assisted and microwave PECVD methods [ 149 , 150 ]. Plasma is required to dissociate the hydrogen molecule into reactive atomic hydrogen, which is essential for the formation of diamond instead of its thermodynamically more stable counterpart, graphite. The H atom temporarily bonds with the fourth carbon atom (in the unhybridized p -orbital) to form a tetrahedral geometry (as in the case of sp 3 hybridization). This prevents the structure from forming flat sheets of trigonal planar graphite-like geometry ( sp 2 hybridization). The temperature of the plasma can be as high as 2000°C, but substrate is maintained at lower temperatures ( < 1000°C). At higher temperatures > 1200°C, graphite deposits is more stable. There is no specific requirement when it comes to the substrate. Often industrial machine parts are directly coated with the VDD films. For growth of single crystal diamond, however, a diamond substrate is essential, which renders the process relatively expensive [ 151 ]. For bulk poly-crystalline diamonds, silicon is the widely used substrate [ 6 , 151 ]. The growth of diamond on a non-diamond substrate requires an extra nucleation step that provides the substrate with necessary diamond seeds for diamond growth. These seeds grow three dimensionally until the grain coalesces to form a poly-crystalline film [ 6 ]. The properties of CVD diamonds films have been studied and reviewed in various old and new publications [ 2 , 152–154 ]. Sometimes, along with the precursor hydrocarbon, precursors of boron(B) or phosphorus(P) is also introduced into the CVD chamber to obtain B/P doped diamonds, which are used in the semiconductor industry/power electronics [ 155–157 ].

DLC is a metastable form of carbon, which is physically amorphous in bulk but consists of small diamond-like crystallites (composed of sp 3 hybridized carbon) dispersed randomly in the matrix of sp 2 carbon at the microscale. Hence, it is a disordered type of carbon. It features a higher fraction of sp 2 -content as well as hydrogen impurity ( > 50%) compared with VDDs [ 6 ] which differentiates the two. Both DLC and VDD are used in applications where their optical properties, high hardness and wear resistance can be harnessed [ 148 , 158–160 ]. Some common examples include their coatings on the automotive parts [ 161 ], biomedical tools [ 159 , 162 ], optical devices [ 158 ] and cutting tools [ 148 , 163 ].

DLC film deposition requires a substantially lower (300°C) substrate temperature compared with VDD. Here, the plasma generation for dissociation of hydrogen molecule is induced by a high-frequency discharge [ 164 , 165 ], which does not produce very high temperatures. Consequently, graphite deposits are not etched away by atomic hydrogen and significant sp 2 carbon is retained in the material. Films of up to 0.5-µm thickness can be obtained on any substrates (including polymers) [ 6 ], which is an advantage of the DLC coatings over VDD. A disadvantage of DLC films is their low temperature resistance [ 166 ] that impedes their use in high-performance thermal coatings (operating temperatures > 300°C). DLC coatings also feature a high residual stress and lower toughness, that limits many mechanical applications. These limitations can be overcome by doping DLC with foreign materials such as chromium [ 167 ], nitrogen [ 168 ] and silicon [ 169 ] to form DLC nanocomposites. The source of these dopants in gaseous form can be mixed with the precursor hydrocarbon gas used for DLC deposition [ 168 ], or in solid form can be deposited on the substrate by sputtering (to form an interlayer) and DLC grown on the interlayer [ 167 ]. DLC nanocomposite with Cr doping enhances the mechanical properties by improving the fracture toughness of the material [ 167 ], N and Si doping improves thermal stability of DLC coatings and reduces the friction coefficient [ 168 , 169 ]. For more details on DLC nanocomposites, readers can refer to the article by Abdul et al . [ 163 ].

Manufacture of spun CFs

Another well-known application of the pyrolysis process is the fabrication of CF from various solid/semi-solid precursors (heavy hydrocarbons). CF and CF-based composites are extensively used in the aerospace [ 170 ] and automobile industries [ 171 ]. CF-based composites are also an important candidate for construction of turbine blades due to their high strength and low weight [ 172 ]. For manufacturing CFs, first a viscoelastic polymer or pitch is spun via melt-spinning or electrospinning techniques [ 173 ]. Afterward, they are converted into carbon via pyrolysis, as discussed in the ‘Pyrolysis of high molecular weight hydrocarbon’ section. This selection of polymers for fiber fabrication is restricted to those with a good viscoelasticity. PAN, pitches and rayon are a few examples of polymers that have good viscoelasticity and hence good spinnability; therefore, they are utilized in the commercial production of CFs. The microstructure of carbon obtained from the spun polymer fibers is different from the carbon obtained from bulk polymers because of a high surface-to-volume ratio of the fibers. This facilitates an easy annealing of pyrolysis by-products such as tars and gases, as well as other structural defects during the heat-treatment. CFs (even those derived from PAN having a turbostratic structure) can typically be made more graphitic at high temperatures [ 174 ] which is not possible in the case of bulk carbons. Polymers are typically spun (using melt-spinning or electrospinning processes) prior to their carbonization/pyrolysis. Details of the spinning processes as well as polymer selection of obtaining CF can be found in many reviews [ 175–177 ]. Commercial CFs are produced mainly by carbonization of PAN-based fibers and pitch fibers. Although carbonization of many other polymeric fibers of rayon, polyvinyl alcohol and poly-esters has been attempted, they are yet to hit the market expectations [ 178 ]. Figure 4 shows the electrospinning and melt spinning process for production of spun fibers followed by stabilization and carbonization to obtain CFs. Some polymers that have been employed for CF fabrication are polyacrylonitrile (PAN) [ 179 ], phenolic resins [ 180 ] and cellulose (lignin-based fibers) [ 181 , 182 ] and its derivative (Rayon) [ 183 ].

Production of spun CF by different routes using different precursors. PVA, polyvinyl alcohol.

CF from polymers

Disordered carbons are hard and brittle, which makes it difficult to pull fibers out of them. The production of CF is therefore carried out by first preparing fibers using a suitable polymer and subsequently converting it into carbon via at ≥900°C. In the 1950s, rayon fibers were carbonized and used for high temperature missile applications [ 183 , 184 ], but the technical breakthrough for high strength CF started in the 1960s when PAN precursor was introduced for commercial production of CFs because of its high carbon yield (approximately 50%) [ 185 ], compared with the carbon yield of rayon (approximately 30%) [ 184 ]. Nowadays PAN is the most common precursor for production of CF on a large scale due to its high carbon yield compared with other polymers and also due to that fact that the viscoelasticity of PAN can be altered/modified to produce CF of various diameters. The diameter in turn influences the graphitizability.

Electrospun PAN fibers can be converted to CF by the following steps: (i) stabilizing heat treatment at around 300°C, to prevent the precursor fibers from melting and fusion, (ii) carbonizing heat treatment at ≥ 900°C in an inert environment to drive off the majority of non-carbon elements, (iii) optional high-temperature treatment ( ≥ 2500°C) to improve mechanical properties of the fibers and increase the graphitic content of the fibers. Fibers undergoing steps (i) and (ii) are generally called CF and fibers undergoing all the three steps are also called graphite fibers [ 22 ]. Commercial CFs are either obtained in the form of a tow or a yarn, with each tow/yarn containing thousands of single fibers of diameter ranging from 5 to 10 µm. These fibers are either braided or woven into a mat and are commercially available as ‘preforms’. These preforms are mainly used as filler material/laminates for fabrication of polymer matrix composites [ 186 , 187 ] and carbon–carbon composites [ 188–190 ].

CF from petroleum pitches

Although PAN-based CFs account for approximately 90% of the world’s CF consumption [ 191 ], the carbon yield of PAN is relatively low [ 185 ]. The search for other inexpensive raw materials as precursors for CF started in 1970s, which led to use of petroleum pitches for making precursor fibers having > 70% carbon yield [ 192 ]. Their mechanical properties of pitch fibers are comparable to PAN-derived fibers and they are relatively cost effective [ 193 ].

Pitches are a byproduct of petroleum and coal processing, but can also be synthetically produced, for example, from PVC [ 194 ]. The chemical composition of pitch is very complex and is mainly a mixture of polycyclic aromatic hydrocarbons and tars. However, the composition of pitches also depends on its source [ 195 ]. Pitch-based fibers (isotropic and mesophase) [ 95 , 196 , 197 ] are generally processed via melt spinning to obtain pitch fibers. Pitch fibers are then stabilized/oxidized followed by carbonization to obtain CFs [ 198 ]. However, electrospinning of pitches has also been reported [ 199 , 200 ]. Pitches can also be mixed with PAN to yield a composite of hard and soft CF [ 201 ]. The CF obtained from isotropic pitch and mesophase pitch is different in terms of structure, properties and nanotexture [ 23 ]. Mesophase pitch already contains small graphitic crystallites and the resulting fibers are high-performance fibers, hence and are produced commercially [ 194 , 202 , 203 ]. CFs from isotropic pitch are of general-purpose grade and have low modulus [ 203 ]. Pitch-based fibers are used as an alternative to PAN-based CF in various applications due to its higher stiffness. Apart from that, their electrical properties are utilized in energy storage devices. More information on pitch-based fibers and their applications can be found in the recent reviews by Liu et al. [ 197 ] and Daulbayev et al. [ 204 ].

Bulk industrial carbon production

Highly oriented pyrolytic graphite.

HOPG is a synthetic graphite which is prepared by thermal and/or stress annealing of pyrolytic graphite [ 5 ]. Pyrolytic graphite is nothing but multiple layers of graphene deposited by CVD of hydrocarbons. These graphene layers are initially defect-containing and turbostratic (randomly oriented), but they organize themselves in an ABABA fashion with an interlayer spacing of < 3.36 nm when heated at very high (typically 2500–3000°C) temperatures as shown in Fig. 5 (pathway A–B). When pyrolytic graphite is subjected to high temperatures and uni-axial compressive stress, the mosaic spread (angle between the tiles of graphite) is reduced. HOPG, however, is not a unique material. It is graded based on the mosaic spread. If the mosaic spread is less than 1°, it is called HOPG. Other methods to obtain HOPG include heat-treatment of polymers such as PVC, anthracene that yield graphitizing carbons [ 205 ]. One common application of HOPG is also production of graphene via exfoliation as shown in Fig. 5 (pathway B–C) from HOPG prepared by pathway A–B ( Fig. 5 ). The exfoliation process can be physically, chemically or electrochemically assisted. Physical exfoliation methods use mechanical/ultrasonic forces (sonication) to break the weak van der waals bonds between the individual layers of HOPG and obtain graphene layers [ 104 , 206–208 ]. Chemical exfoliation of HOPG generates reduced graphene oxide (r-GO) as the final product, by treating HOPG with strong acids (sulfuric/nitric acids) at a temperature slightly higher than the ambient temperature [ 209 ]. Electrochemical exfoliation methods involve intercalation of some ions electro-chemically driven in-between the layers of HOPG, leading to mesoscale mechanical exfoliation [ 210–212 ]. HOPG is used for a variety of applications including X-ray optics and spectroscopy [ 213 , 214 ], anode material for Li-ion batteries [ 215–217 ] and as a substrate for thin-film deposition [ 218 ].

schematic of HOPG formation from CVD graphene and graphene formation from HOPG.

Glass-like carbon

GC is a type of non-graphitizing carbon [ 219 ], that is formed by coking during its carbonization from organic precursors. Most common precursors of this type of carbon are phenolic resins [ 220 ] or (poly)-furfuryl alcohols [ 221 ]. The precursor resin is first cured/cross-linked and then heated to elevated temperatures at a very slow rate. The resins are heat treated to temperatures as high as 3000°C, to anneal out structural defects [ 51 ]. During carbonization, inter-twinning of randomly oriented graphene sheets takes place, giving rise to closed inaccessible pores. GC contains fullerene-like structures that also contribute to its low density [ 65 ]. These curved structural units make it difficult for the graphitic planes organize during further heat treatment, hence the value of L c is always > 3.36. The microstructure of this type of carbon has been studied in the past and various models proposed [ 42 , 51 , 65 , 219 ], which reveal the short-range ordering among graphitic crystallites and randomly oriented basal planes.

GC is hard and brittle, resistant to chemical attacks and features higher tensile and compressive strength [ 51 ]. Many large-scale applications of GC-like chemical reactor linings and laboratory crucibles/substrates utilize its chemical inertness, which makes it impermeable to gases and liquids [ 222 ]. Other applications include reference electrodes for electrochemical studies [ 223 ], medical implants [ 224 , 225 ] and molds for glass lenses [ 226 ]. However, production of bulk GC still has scope for optimization due to the following reasons: (i) the precursor resins used for production of GC are expensive and the high carbonization temperatures increase the overall production cost, (ii) inevitable weight loss during carbonization, (iii) difficulty in machining GC to close tolerances and (iv) difficulty in obtaining thicker (>5 mm) GC parts without porosity [ 51 ]. However, this material is studied extensively in the micro and nano-scale by photo patterning the precursor resins and carbonizing them, to obtain GC micro-nano structures, utilized for various applications, which is discussed in the section ‘Fabrication of carbon-based micro and nano devices.’

Activated carbon

Activated carbons exhibit a surface that can easily adsorb foreign molecules (liquids and gases) owing to the presence of porosity and active chemical functional groups. The gas/liquid molecules are held by weak forces (van der waals and london dispersion forces) [ 227 , 228 ] that can often be released at higher temperatures or use of a chemical effluent [ 229 ]. They are prepared by physical or chemical activation of porous carbons, which are in turn obtained by pyrolysis of natural polymers. Activation process generally increases the fraction of micropores ( < 2 nm) and the overall surface area of the material as well create some active functional groups on its surface [ 230 ]. Porous carbons are non-graphitizing. They experience direct charring during their pyrolysis and contain fractal pore geometries (i.e. the pore sizes repeatedly decrease [ 231 ]). During its carbonization, the original skeleton of the precursor material is preserved and these types of carbons exhibit very high porosity (micro/meso/macro pores) and thus, a high surface area [ 229 ]. To produce porous carbons, the heat-treatment temperature should not be very high (typically limited to < 1000°C), as higher temperatures may lead to closing or annealing of some pores [ 51 ]. Common precursors used for obtaining porous carbons include coal, petroleum residues and cellulose-based precursors (coconut shells, rice husk, wood and various biodegradable materials) [ 51 ]. Lately, a large number of agricultural and forestry residues have been utilized for the preparation of porous carbons that can be further activated. Some of these are covered in the section ‘Waste treatment via pyrolysis.’

Physical activation is done on porous carbons prepared at low temperatures, which involves heating these carbons at a higher temperature to get rid of pyrolysis by-products (tars), trapped inside the pores, thereby increasing the porosity. Another method of physical activation is to heat these porous carbona in an oxidizing environment [ 230 ]. Chemical activation is done on bio-polymers before the carbonization process by treating the precursor with some chemicals (acids/metal carbonates/metal chlorides, etc.) to partially degrade the cellulose. The polymer is then carbonized and the carbon is activated [ 230 ]. There are also many other methods of activation of porous carbons that involve combination of both physical and chemical activation processes. Interested readers can refer to the review article by Sevilla et al. [ 96 ]. Applications of activated carbons include water purification [ 232–234 ]; environmental remediation [ 235–237 ]; supercapacitor electrode material [ 238 , 239 ] and as an adsorbent in food, agriculture and pharmaceutical industries [ 240–242 ].

Fabrication of carbon-based micro and nano devices

Carbon-based micro and nano devices can be fabricated using carbon nanomaterials (bottom-up manufacturing) or by directly converting a polymer structure into carbon via pyrolysis (top-down manufacturing). In this section, we will discuss representative examples of micro/nano-scale carbon structures and devices that are fabricated via pyrolysis of pre-patterned polymer structures. Such structures are often first patterned employing lithographic processes such as photolithography [ 11 ], X-ray lithography [ 243 ] and two-photon lithography [ 244 ] on to a silicon substrate, and are subsequently carbonized at temperatures ≥ 900° [ 9 ]. Another top-down approach that has recently gained popularity is the laser-assisted carbonization of a polymer film [ 10 , 245 , 246 ], which will be subsequently discussed.

Carbonization of lithographically patterned polymers

Lithography is a term used for top-down processes where a polymer film is patterned employing an electromagnetic radiation, or a high energy beam of electrons or ions. The energy of the radiation either degrades or crosslinks the exposed part of the polymer, thus modifying its chemical properties and changing its solubility. The polymers used in lithographic techniques are specifically designed for this purpose. For example, polymers that can be pattered using UV/deep-UV (photolithography) or two back-to-back photons (two-photon lithography) are able to crosslink when exposed to a pre-defined dose of the respective light due to the presence of photo-initiators moieties in their chemical structure. Interestingly, many polymers that are used in photolithography are resins that have a high carbon content and an aromatic backbone. Such polymers can yield a high fraction of solid carbon when they are pyrolyzed. This property has been widely explored for the fabrication of carbon-based devices and has been reported in various articles [ 7–11 , 247 , 248 ].

While converting lithographically patterned resins into carbon, the following points must be taken into consideration: (i) structures shrink due to loss of non-carbon atoms, (ii) resulting carbon is of non-graphitizing type [ 65 , 219 ] which shows properties similar to commercial GC and (iii) the pyrolysis temperatures are typically limited to 1200°C, due to the fact that silicon substrates cannot withstand temperatures ≥ 1400°C (process temperature is kept lower for avoiding thermal stresses and fatigue). The pyrolysis temperature should also not be below 900°C, as that would yield carbon with impurities and poor electrical conductivity. Evidently, flexible polymers cannot be used as the substrate. Figure 6 is a compilation of various carbon-based micro/nano devices produced by carbonization of photo-patterned polymers.

SEM images of inter-digitated carbon electrodes ( A , B ), the entire device ( C ), (A–C) reproduced with permission from Mantis et al. [ 248 ]; SEM images of sideview of optimized CNG at the edge of an electrode area ( D ), CNG electrodes with uniform residual bulk carbon layer connecting the CNG, (red arrows) ( E ), the entire device ( F ), (D–F), reproduced with permission from Asif et al. [ 249 ]; SEM images of suspended GCWs before and after the LCVD process ( G ), overview of multiple fibers suspended from scaffolds, illustrating how they can be locally coated (bright fibers) or left uncoated (darker fibers) without contaminating the carbon scaffold ( H ), the entire device ( I ), (G–I) reproduced with permission from Cisquella-Serra et al. [ 247 ]; SEM images of CMN array ( J ), magnified view of a CMN (K), the entire device (L), (J, K), reproduced with permission and ( K ) modified from Mishra et al. [ 250 ]. CNG, carbon nanograss; GCWs, glassy carbon wires; LCVD, localized CVD; CMN, carbon micro-needle.

Some representative applications of carbon structures fabricated using this process include neural sensing electrodes [ 11 , 244 , 249 , 251 , 252 ], cell culture substrates compatible with magnetic resonance imaging [ 8 ], fabrication of atomic force microscopy (AFM) tips [ 9 , 253 ], biosensors [ 254 , 255 ] and various other applications, which are summarized in Table 3 .

Summary of carbon electrodes by pyrolysis of patterned polymeric structures (recent research articles from 2018 to 2021)

Laser-assisted pyrolysis of polymers

A top-down fabrication technique for obtaining polymeric carbon structures is based on conversion of a high carbon containing polymer directly using a laser beam. Laser has been used in the past for production of carbon nanomaterials from thermal decomposition of hydrocarbons [ 261 , 262 ], where reactants are heated by laser in a closed chamber causing the reactants to de-compose and the aggregates undergo homogeneous nucleation and growth to form hydrogen-rich carbon powders. Carbon-rich polymer films, when irradiated by a laser, undergo thermo-chemical decompositions to yield carbon structures, which can be used in micro/nano device applications. This pyrolysis is a combination of photochemical and photothermal mechanisms [ 263 , 264 ]. Laser intensity is insufficient for direct bond dissociation of the polymer, but the radiation induces phonons in the material. The vibrational energy of the phonons is released by bond dissociation of the weaker components of the polymer [ 265 ]. This leads to material ablation from the surface in the form of bubbles and is expressed as ‘bleaching’ at a fluence is below carbonization threshold. Further increase in fluence leads to an immediate burst of the bubbles, resulting in rapid release of volatile products due to fragmentation of the polymer. Under constant radiation, these fragments become ionized and form a plume (plasma-like discharge). The plume-shield prevents further penetration of the beam into the material, resulting in heat generation at the beam front and adjacent areas resulting in carbonization of the material [ 266 ]. Thus, laser-induced carbonization is complete only when the plume has formed (visible as a bright spot by the naked eye).

Laser-induced carbonization has been applied successfully to polyimide [ 10 , 245 , 267 , 268 ], parylene-C [ 269 , 270 ] and polyaramid [ 246 ] to yield carbon structures, different from both glassy and activated carbons owing to the fact that this process happens within a short time and the cleavage of chemical bonds is rapid. The heat generated by the laser and the resulting carbon produced depends on the laser parameters (type of laser, laser power, speed and wavelength) along with the pyrolysis environment [ 10 , 267 ]. The minimum feature size of carbon structures that can be produced by this method depends on the spot radius of the laser [ 270 ]. The microstructure of the laser-induced carbon is thoroughly investigated and applied to various applications such as supercapacitors [ 74 , 267 ], sensors [ 10 , 270 , 271 ], antibacterial coatings [ 246 , 272 ] and carbon-based composites [ 152 ]. Discrepancies in nomenclature of the same material obtained by laser-induced pyrolysis/carbonization of the same polymer are observed in the subsequent literature. For further details on laser-induced carbonization of polymers, interested readers can refer to the review article on laser-induced graphene by Ruquan Ye et al. [ 273 ]. Table 4 summarizes the recent examples of fabrication of carbon-based micro-nano devices by laser-assisted pyrolysis of various polymers and their applications.

Carbon patterns by laser-assisted pyrolysis of polymeric substrates (Research from 2012 to 2021)

Analytical pyrolysis

Pyrolysis induces fragmentation in large hydrocarbon molecules without any foreign chemical reactions such as oxidation. This characteristic turns out to be extremely useful for the analysis of trace amounts of invaluable samples, such as the organic matter found in the fossils [ 14 ]. The analysis of fossil samples is essential for understanding their origin, age and formation mechanism. MS is one of the primary techniques used for the analysis of fossils, which is based on the principle of analyzing the mass of the various fragments of the molecule.

By evaluating this fragmentation mechanism one can detect the original structure of the initial molecule(s) [ 98 ]. Importantly, the sample quantity cannot be increased and needless to say, no amount of sample can be wasted for analytical purposes, hence pyrolysis occurs directly at the ion source to avoid loss of by-products. Pyrolysis MS (Py-MS), however, has one disadvantage that the pyrolytic fragmentation of the molecule is performed in the same chamber of the ion source. This results in contamination of the ion source, affecting long-term reproducibility of mass spectra lines [ 67 ]. Py-MS is therefore often combined with GC to form a set of techniques known as Py-GC–MS [ 283 ]. Py-GC–MS process entails the integration of pyrolyzing unit (Py), GC system and MS together by connecting the pyrolysis unit to the injector port of a gas chromatograph such that pyrolysis by-products (pyrolysates) are chromatographically separated through fused silica capillary columns by inert gas flow, followed by ionization of the products to obtain a mass spectra which is then analyzed with the help of mass spectra libraries [ 98 , 284 ]. Depending upon the sample availability and its possible chemical nature, pyrolysis may be performed using ovens, lasers or by utilizing a filament that can be inductively heated to provide the desired temperatures [ 14 ]. Thus, pyrolysis may be used as a form of sample pre-treatment for analysis of complex organic materials with unknown structures [ 285 ], for example, forensic samples [ 14 , 286 , 287 ], humic materials [ 16 ], geopolymers [ 286 ], environmental samples [ 288–290 ], biological molecules (proteins, peptides and nucleotides) [ 291 ] and various other biochemically important polymers as well as some polymers of non-biological origin [ 64 , 67 ]. A few applications of analytical pyrolysis for analysis of polymers, fossils, archaeological remains and other complex materials are listed in Table 5 .

Applications of analytical pyrolysis methods for analysis of various complex materials

Waste treatment via pyrolysis

Human, animal and plant waste contains a significant fraction of organic matter. While direct burning or combustion of waste polymers is hazardous to the environment, pyrolysis can lead to their safe disposal. The tars, gases and solid carbon residues (often called chars or biochars due to their low purity) produced during the pyrolysis of waste can also be utilized in various applications. As a result, one of the most widely studied applications of the pyrolysis process in the industry and academia at present is the treatment of waste. Here, the process is stopped at the end of the pyrolysis stage itself (generally before 700°C). The desired products are oils (tarry hydrocarbons produced by fragmentation of waste polymers) and synthetic gas (mixture of light hydrocarbons). Similar to other pyrolytic decomposition processes, waste materials which contain organic materials (biodegradable and non-biodegradable) are heated to produce the desired products. Notably, solid carbon fractions obtained at low temperatures (below 700°C) contain a significant amount of impurities and they can only be used for low-cost applications such as soil quality enhancement, oil spillage adsorbents and other industrial cleaning agents [ 298–300 ]. The quality improvement of such carbons is being extensively investigated. In the recent, past several waste-derived carbon materials have been used for advanced applications such as electrode fabrication. It is important to understand that increase in the solid carbon fraction may reduce the oil/gas production. Moreover, higher pyrolysis temperatures increase the cost of the overall process, which may not always be feasible when it comes to large-scale waste treatment. As a result, one needs to evaluate the final products prior to designing the process parameters. Various products of waste pyrolysis along with their calorific value are listed in Table 6 .

Preparation and calorific values of the common pyrolysis products of the current waste pyrolysis facilities

Pyrolytic synthetic (syn) gas

The composition of the pyrolytic gas is strongly dependent on the pyrolysis temperature and feed-stock. Slow pyrolysis of biomass waste such as wood, garden waste and food residue at low temperatures (below 400°C) produces small amounts of gas, which is rich in CO 2 , CO and light hydrocarbons. The yields of gas at these conditions usually do not exceed 30 wt.% of pyrolysis products. On increasing the temperature there is an increase in gas yields, because of the secondary reactions and partial char decomposition. The calorific value of gas from slow pyrolysis is around 10–15 MJ/Nm 3 and varies with temperature and heating rate [ 303 ]. Fast pyrolysis of biomass produces gas with a calorific value of around 14 MJ/Nm 3 . On the other hand, higher temperatures (above 700°C), especially when pyrolysis is combined with gasification, produces syngas, which contains more hydrogen and carbon monoxide. In this case, syngas is the main product of the process. The pyrolysis of plastics produces pyrolytic gas, of which the major components are hydrogen and light hydrocarbons: methane, ethane, ethene, propane, propene and butane. This gas has a significant calorific value, for example, a heating value of gas from PP and PE varied between 42 and 50 MJ/kg [ 304 ]. Similar properties characterized the gas from the pyrolysis of tyres or other artificial products like textiles. In turn, co-pyrolysis of polymers and biomass leads to a higher production of CO and CO 2 especially at lower temperatures. Finally, the pyrogas from MSW consists of CO 2 , CO, hydrogen, methane and other light hydrocarbons with an average heating value of around 15 MJ/Nm 3 , which increases with increasing temperature [ 305 ]. The most suitable demand on pyrogas is its use as a source of the energy required for the pyrolysis process itself. However, the exhaust gas has to be controlled. Pyrogas from tyres contains a relatively high concentration of H 2 S, which can be oxidized to SO 2 [ 306 ]. PVC pyrolysis produces huge amounts of HCl [ 307 ] whereas waste food processing could be a source of dangerous nitrogen compounds [ 308 ]. Usually the precise composition of waste is unknown, thus some unwanted compounds can appear in pyrogas. Therefore, emission control units and gas cleaning devices should be used and it does not matter whether the gas will be combusted or not.

Pyrolytic oil

Pyrolytic oil offers more opportunities for use than syngas, but the composition of the liquid product from pyrolysis may differ radically depending on the composition of the feedstock and the process parameters. Pyrolytic oils derived from biomass consist mainly of the following compounds: acids, ketones, aldehydes, sugars, alcohols, phenols and their derivatives, furans and other mixed oxygenates. Phenolic compounds are mostly present in high concentrations (up to 50 wt.%), consisting of relatively small amounts of cresols, xylenols, phenol, eugenol and much larger quantities of alkylated (poly-) phenols [ 309 ]. It can be used for the production of heat, electricity, synthetic gas or chemicals. The highest yields of oil are gained between 500°C and 600°C. Pyrolytic oil from biomass has calorific values of around 15–20 MJ/kg, on the other hand, pyrolytic oil from plastics has a higher calorific value, about 30–45 MJ/kg, depending on the precursor polymer. Ahmad et al. [ 13 ] compared the oils from the pyrolysis of PP and HDPE with gasoline and diesel via physical properties such as viscosity, the research octane number and the motor octane number, as given in Table 7 . Pour point, flash point or diesel index could be a good indication of pyrolytic oil quality as a fuel [ 13 , 71 ]. The calorific value of oils from mixed plastic waste could be estimated at 40 MJ/kg [ 310 ].

Comparison of pyrolytic oil from some polymers with standard liquid fuels, reproduced from Ahmad et al. [ 13 ]

Pyrolytic char

Currently, pyrolysis conditions are generally optimized in order to maximize the liquid and gas products. Besides these two, a solid fraction named as pyrolytic char is also produced. Char mainly is carbon-rich matrix containing almost all the inorganic compounds present in the wastes with a significant amount of condensed by-products of the pyrolysis process [ 311 ]. Chars are generally porous and its porosity depends upon precursor waste [ 7 ]. The calorific value of char obtained from pyrolysis of waste (mixture of biodegradable and non-biodegradable) is approximately 34 MJ/kg [ 302 ], which is comparable with coal. However, despite all the separation techniques before pyrolysis, some heavy metals and other hazardous elements, like S, Cl and Ni, get retained in the solid products. Therefore, it becomes equally important to characterize chars so as to assess their impact on the environment and humans. In general, this product can be combusted to provide energy for the pyrolysis process or other applications as listed in Table 8 .

Applications of waste-derived pyrolytic carbon

Catalytic pyrolysis

Various catalysts are used during waste pyrolysis that can potentially increase the oil, gas or char fractions, as desirable in the process. Catalytic pyrolysis of plastic waste is typically carried out in presence of natural/modified zeolites to produce pyrolysis oils, which can be used as transportation fuel by mixing or blending with conventional fuels [ 346 , 347 ]. Other catalysis that are being extensively studied include metal oxides and bimetallics [ 152 ]. More information about catalytic pyrolysis of MSW and plastic wastes can be found in these recent articles [ 347–349 ].

Semi-cokes/mesophase carbon from pyrolysis of pitch

It is not possible to commercially exploit all of the crude petroleum of the barrel for commercial purposes and the various distillation and cracking processes produce a huge amount of residues within the refinery, the disposal of the same is of major concern [ 350 ]. These residues are rich in aromatics and has high C/H ratio, hence can be a good feedstock for mesophase carbon. When these residues are heat treat at around 450°C, they convert into a pitch-like isotropic material having a consistency similar to liquid crystals. With increasing temperature, small spheres appear in the pitch-like mass, which grow with time. At some stage in the heating process, the spheres will replace a large part of the pitch-like material and interfere with one another’s enlargement and a ‘mosaic’ begins to form by coalescence when all of the isotropic pitch-like material is replaced by the anisotropic material or mesophase and the mosaic is complete, the mesophase solidifies into ‘semi-coke’, which is readily graphitizable [ 351 ]. With further heat treatment (1400°C), this carbonaceous mesophase coalesces to a state of bulk mesophase before solidification to ‘green coke’ with further loss of volatile compounds [ 350 ]. However, apart from this regular trend, many different behaviors have been observed for varying compositions of the feedstock. The petroleum residues are a mixture of more than 1000 molecular compounds (numbers differ in literature) and contain mixtures of aliphatic and aromatic compounds. To obtain high-quality cokes acceptable for industrial usage (as electrodes for steel industry), high aromaticity in the precursor is essential. Before getting converted to green coke at higher temperatures, this pitch material, known as mesophase pitch can be a good precursor for preparation of high-performance carbon materials [ 352 ] like pitch-derived coke [ 353 ], mesocarbon microbeads (MCMB), CF [ 3 , 23 , 194 ], carbon foams and carbon composites [ 354 ]. A summary of various carbon forms obtained from mesophase pitches and their respective applications is listed in Table 9 .

Different types of carbon materials obtained by pyrolysis of mesophase pitch and their applications

Special cases

Although the term pyrolysis is predominantly used in the context of organic materials, there are certain examples where pyrolysis is performed on inorganic solids/liquids as well. Synthesis of 2D nanomaterials like graphitic carbon nitride from pyrolysis of urea [ 368 ], molybdenum sulfide by CVD [ 369 ] and thin films by CVD of inorganic precursors [ 370 ] are a few examples. Another variation of pyrolysis known as spray pyrolysis [ 371–373 ], in which precursors in liquid phase are sprayed through an atomizer onto a heated substrate (250–500°C) [ 374 ], mainly aimed at deposition of thin and thick semiconductor films for solar cells applications [ 375–377 ]. However, at present, this process has been extended to deposition of thin films for sensors [ 378 ], solid oxide fuel cells [ 379 ] applications as well. Spray pyrolysis technique has also been utilized in the synthesis of various nanomaterials apart from thin films [ 380–382 ] which is beyond the scope of this paper due to vastness of the topic. Another term known as ‘hydropyrolysis’, that is, pyrolysis in presence of hydrogen at high pressure, is predominantly performed on biomass to obtain biofuels/chemicals for industrial use in presence of a catalyst. Hydrogen is used as a reducing agent to form hydrogenated radicals by reacting with the volatiles and to remove oxygen in the form of water, CO and CO 2 , resulting in hydrocarbon generation [ 383 ]. However, hydropyrolysis in itself is a vast topic and is beyond the scope of this paper.

Pyrolysis is extensively used in different applications that are covered in this review in terms of their fundamental principles, history, industrial relevance and process parameters. Evidently, these applications not only belong to entirely different scientific communities, their target products and production scales also widely vary. One important conclusion is that pretty much all synthetic carbon materials, bulk or nano-scale, are derived from organic precursors via the pyrolysis process. Given the significance of advanced carbon allotropes in the cutting-edge technology, there is a compelling need for (i) reducing the cost of pyrolysis, (ii) improving the efficiency of the process and (iii) development of integrated pyrolysis systems. Lowering the energy consumption during pyrolysis is not straightforward, but is possible with the use of sophisticated nano-scale catalysts that can potentially lead to an overall cost reduction. One challenge is to get rid of the catalyst particles through post-processing with a high yield, which demands more focused research. Generally, catalysts can also facilitate an increase in the overall process efficiency. While the idea of efficiency may differ based on the application area, tuning of the underlying process parameters can always be of help. For this purpose, a comprehensive understanding of the pyrolysis mechanism for a given precursor is essential. Key concepts pertaining to this are covered in detail in this review.

The development of the integrated pyrolysis systems may serve multiple purposes. Plenty of work has lately commenced in this direction, where the goal is to further pyrolyze the byproduct(s) of one pyrolytic process. A good example is carbon nanomaterial production via secondary pyrolysis of the synthetic gas obtained during waste pyrolysis. Such innovative ideas need technological support from both academia and industry, for example, an optimized reactor design suitable for the quantity of the feed. Based on the information available in the literature, such multi-stage pyrolysis equipments are already proving to be extremely helpful in improving the commercial viability of the waste treatment plants. Some other integrated processes such as the microbial bioprocessing of pyrolytic oils have also lately gained attention for the generation of fuel with a higher calorific value. An additional future prospect is the quality enhancement of low-grade biochars by increasing the pyrolysis temperature and ensuring a strictly inert environment during the process. The know-how is already available with the activated carbon industry and researchers are rapidly coming up with very promising results. For large-scale pyrolysis, plasma-assisted processes and/or solar energy supported plants are also recommended. The age-old process of pyrolysis is expected to play a major role in the near future in the carbon materials science as well as the expansion of the sustainable energy solutions.

M.D. would like to thank the Ministry of Education, Government of India, for her doctoral fellowship. S.S. acknowledges the financial support from the Seed Research Grant No. IITM/SG/SWS/69, Indian Institute of Technology, Mandi.

AUTHORS’ CONTRIBUTIONS

M.D. prepared the initial draft including figures and tables as well as contributed to editing and finalizing the manuscript. S.R. contributed in drafting the waste pyrolysis section. S.S. conceptualized, edited and finalized the manuscript.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

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• Published: 24 July 2023

Catalytic pyrolysis as a platform technology for supporting the circular carbon economy

• Cody J. Wrasman 1 ,
• A. Nolan Wilson 1 ,
• Ofei D. Mante 2 ,
• Kristiina Iisa   ORCID: orcid.org/0000-0003-1326-901X 1 ,
• Abhijit Dutta   ORCID: orcid.org/0000-0003-4258-7287 1 ,
• Michael S. Talmadge 1 ,
• David C. Dayton 2 ,
• Sundararajan Uppili 3 ,
• Michael J. Watson   ORCID: orcid.org/0000-0003-2244-019X 4 ,
• Xiaochun Xu 3 ,
• Michael B. Griffin   ORCID: orcid.org/0000-0003-0231-1876 1 ,
• Calvin Mukarakate 1 ,
• Joshua A. Schaidle   ORCID: orcid.org/0000-0003-2189-5678 1 &
• Mark R. Nimlos 1  

Nature Catalysis volume  6 ,  pages 563–573 ( 2023 ) Cite this article

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• Chemical engineering
• Heterogeneous catalysis

Catalytic pyrolysis, a process that combines pyrolysis and vapour-phase catalytic upgrading, is a versatile technology platform capable of direct liquefaction of biomass and waste plastic into intermediates that can enable the decarbonized production of chemicals and/or transportation fuels. Recently, catalytic pyrolysis has attracted substantial research and commercialization attention, with over 15,000 journal articles and patents published in the past decade alone. In this Perspective, we chart a path towards commercial-scale catalytic pyrolysis of waste plastic and biomass by identifying key short-term and long-term technological barriers. Within the proposed development roadmap addressing these barriers, catalytic pyrolysis can move from the demonstration scale to integrated biorefinery networks producing fuels and plastics precursors at a scale of between 0.1 and 1 billion tonnes of carbon per year.

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Acknowledgements

This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, for the US Department of Energy (DOE) under contract no. DE-AC36-08GO28308. Funding provided by the US DOE Office of Energy Efficiency and Renewable Bioenergy Technologies Office, in collaboration with the Chemical Catalysis for Bioenergy Consortium (ChemCatBio), a member of the Energy Materials Network (EMN). The views expressed in the article do not necessarily represent the views of the DOE or the US government. The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide licence to publish or reproduce the published form of this work, or allow others to do so, for US government purposes.

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Cody J. Wrasman, A. Nolan Wilson, Kristiina Iisa, Abhijit Dutta, Michael S. Talmadge, Michael B. Griffin, Calvin Mukarakate, Joshua A. Schaidle & Mark R. Nimlos

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Wrasman, C.J., Wilson, A.N., Mante, O.D. et al. Catalytic pyrolysis as a platform technology for supporting the circular carbon economy. Nat Catal 6 , 563–573 (2023). https://doi.org/10.1038/s41929-023-00985-6

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DOI : https://doi.org/10.1038/s41929-023-00985-6

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ORIGINAL RESEARCH article

Catalytic pyrolysis of plastic waste: moving toward pyrolysis based biorefineries.

• 1 Department of Environmental Sciences, University of Peshawar, Peshawar, Pakistan
• 2 Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia
• 3 Department of Environmental Sciences, Faculty of Meteorology, Environment and Arid Land Agriculture, King Abdulaziz University, Jeddah, Saudi Arabia
• 4 Central Metallurgical R&D Institute, Helwan, Egypt
• 5 School of Chemical and Process Engineering, University of Leeds, Leeds, United Kingdom

Pyrolysis based biorefineries have great potential to convert waste such as plastic and biomass waste into energy and other valuable products, to achieve maximum economic and environmental benefits. In this study, the catalytic pyrolysis of different types of plastics wastes (PS, PE, PP, and PET) as single or mixed in different ratios, in the presence of modified natural zeolite (NZ) catalysts, in a small pilot scale pyrolysis reactor was carried out. The NZ was modified by thermal activation (TA-NZ) at 550°C and acid activation (AA-NZ) with HNO 3 , to enhance its catalytic properties. The catalytic pyrolysis of PS produced a higher liquid oil (70 and 60%) than PP (40 and 54%) and PE (40 and 42%), using TA-NZ and AA-NZ catalysts, respectively. The gas chromatography-mass spectrometry (GC-MS) analysis of oil showed a mixture of aromatics, aliphatic and other hydrocarbon compounds. The TA-NZ and AA-NZ catalysts showed a different effect on the wt% of catalytic pyrolysis products and liquid oil chemical compositions, with AA-NZ showing higher catalytic activity than TA-NZ. FT-IR results showed clear peaks of aromatic compounds in all liquid oil samples with some peaks of alkanes that further confirmed the GC-MS results. The liquid oil has a high heating value (HHV) range of 41.7–44.2 MJ/kg, close to conventional diesel. Therefore, it has the potential to be used as an alternative source of energy and as transportation fuel after refining/blending with conventional fuels.

Introduction

Plastic waste production and consumption is increasing at an alarming rate, with the increase of the human population, rapid economic growth, continuous urbanization, and changes in life style. In addition, the short life span of plastic accelerates the production of plastic waste on a daily basis. The global plastic production was estimated at around 300 million tons per year and is continuously increasing every year ( Miandad et al., 2016a ; Ratnasari et al., 2017 ). Plastics are made of petrochemical hydrocarbons with additives such as flame-retardants, stabilizer, and oxidants that make it difficult to bio-degrade ( Ma et al., 2017 ). Plastic waste recycling is carried out in different ways, but in most developing countries, open or landfill disposal is a common practice for plastic waste management ( Gandidi et al., 2018 ). The disposal of plastic waste in landfills provide habitat for insects and rodents, that may cause different types of diseases ( Alexandra, 2012 ). Furthermore the cost of transportation, labor and maintenance may increase the cost of recycling projects ( Gandidi et al., 2018 ). In addition, due to rapid urbanization, the land available for landfills, especially in cities, is reducing. Pyrolysis is a common technique used to convert plastic waste into energy, in the form of solid, liquid and gaseous fuels.

Pyrolysis is the thermal degradation of plastic waste at different temperatures (300–900°C), in the absence of oxygen, to produced liquid oil ( Rehan et al., 2017 ). Different kinds of catalysts are used to improve the pyrolysis process of plastic waste overall and to enhance process efficiency. Catalysts have a very critical role in promoting process efficiency, targeting the specific reaction and reducing the process temperature and time ( Serrano et al., 2012 ; Ratnasari et al., 2017 ). A wide range of catalysts have been employed in plastic pyrolysis processes, but the most extensively used catalysts are ZSM-5, zeolite, Y-zeolite, FCC, and MCM-41 ( Ratnasari et al., 2017 ). The catalytic reaction during the pyrolysis of plastic waste on solid acid catalysts may include cracking, oligomerization, cyclization, aromatization and isomerization reactions ( Serrano et al., 2012 ).

Several studies reported the use of microporous and mesoporous catalysts for the conversion of plastic waste into liquid oil and char. Uemichi et al. (1998) carried out catalytic pyrolysis of polyethylene (PE) with HZSM-5 catalysts. The use of HZSM-5 increased liquid oil production with the composition of aromatics and isoalkanes compounds. Gaca et al. (2008) carried out pyrolysis of plastic waste with modified MCM-41 and HZSM-5 and reported that use of HZSM-5 produced lighter hydrocarbons (C 3 –C 4 ) with maximum aromatic compounds. Lin et al. (2004) used different kinds of catalysts and reported that even mixing of HZSM-5 with mesoporous SiO 2 -Al 2 O 3 or MCM-41 led to the maximum production of liquid oil with minimal gas production. Aguado et al. (1997) reported the production of aromatics and aliphatic compounds from the catalytic pyrolysis of PE with HZSM-5, while the use of mesoporous MCM-41 decreased the aromatic compounds produced due to its low acid catalytic activity. The use of synthetic catalysts enhanced the overall pyrolysis process and improved the quality of produced liquid oil. However, the use of synthetic catalysts increased the cost of the pyrolysis process.

The NZ catalysts can be used to overcome the economic challenges of catalytic pyrolysis which comes with the use of expensive catalysts. In recent years, NZ has gained significant attention for its potential environmental applications. Naturally, NZ is found in Japan, USA, Cuba, Indonesia, Hungary, Italy, and the Kingdom of Saudi Arabia (KSA) ( Sriningsih et al., 2014 ; Nizami et al., 2016 ). The deposit of NZ in KSA mostly lies in Harrat Shama and Jabbal Shama and mainly contain minerals of mordenite with high thermal stability, making it suitable as a catalyst in plastic waste pyrolysis. Sriningsih et al. (2014) modified NZ from Sukabumi, Indonesia by depositing transitional metals such as Ni, Co, and Mo and carried out pyrolysis of low-density polyethylene (LDPE). Gandidi et al. (2018) used NZ from Lampung, Indonesia for the catalytic pyrolysis of municipal solid waste.

This is the first study to investigate the effect of modified Saudi natural zeolite, on product quality and yield from catalytic pyrolysis of plastic waste. Saudi natural zeolite catalyst was modified via novel thermal activation (TA-NZ) at 550°C and acid activation (AA-NZ) with HNO 3 to enhance its catalytic properties. The catalytic pyrolysis of different types of plastics waste (PS, PE, PP, and PET) as single or mixed in different ratios, in the presence of modified natural zeolite (NZ) catalysts in a small pilot scale pyrolysis reactor, was carried out for the first time. The quality and yield of pyrolysis products such as liquid oil, gas, and char were studied. The chemical composition of the liquid oil was analyzed by GC-MS. Furthermore, the potential and challenges of pyrolysis-based biorefineries have been discussed.

Materials and Methods

Feedstock preparation and reactor start-up.

The plastic waste used as the feedstock in the catalytic pyrolysis process was collected from Jeddah and included grocery bags, disposable juice cups and plates, and drinking water bottles, which consist of polyethylene (PE), polypropylene (PP) polystyrene (PS), and polyethylene terephthalate (PET) plastics, respectively. The selection of these plastic materials was made based on the fact that they are the primary source of plastic waste produced in KSA. To obtain a homogenous mixture, all the waste samples were crushed into smaller pieces of around 2 cm 2 . The catalytic pyrolysis was carried out using an individual or mixture of these plastic wastes in different ratios ( Table 1 ). 1000 g of feedstock was used, with 100 g of catalyst in each experiment. Saudi natural zeolite (NZ), collected from Harrat-Shama located in the northwest of Jeddah city, KSA ( Nizami et al., 2016 ), was modified by thermal and acid treatment and used in these catalytic pyrolysis experiments. NZ was crushed into powder (<100 nm) in a ball-milling machine (Retsch MM 480) for 3 h using 20 Hz/sec, before modification and its usage in pyrolysis. For thermal activation (TA), NZ was heated in a muffle furnace at 550°C for 5 h, while for acidic activation (AA) NZ was soaked in a 0.1 M solution of nitric acid (HNO 3 ) for 48 h and continuously shaken using an IKA HS 501 digital shaker with 50 rpm. Afterward, the sample was washed with deionized water until a normal pH was obtained.

Table 1 . Experimental scheme.

The experiments were carried out in a small pilot-scale pyrolysis reactor at 450°C, using a heating rate of 10°C/min and reaction time of 75 min ( Figure 1 ). The obtained yield of each pyrolysis product was calculated based on weight, after the completion of each experiment. The produced liquid oil characterization was carried out to investigate the effect of feedstock composition on the quality of liquid oil produced in the presence of modified NZ. TGA was carried out on feedstock to obtain the optimal process conditions such as temperature and reaction time (75 min) under controlled conditions. In TGA, a 10 μg of each type of plastic waste was taken and heated with a rate of 10°C from 25 to 900°C under a continuous flow of nitrogen (50 ml/min). The authors of this study have recently published work on the effect of feedstock composition and natural and synthetic zeolite catalysts without catalyst modification on different types of plastic waste ( Miandad et al., 2017b ; Rehan et al., 2017 ).

Figure 1 . Small pilot scale pyrolysis reactor ( Miandad et al., 2016b ).

Experimental Setup

The small pilot scale reactor has the capability to be used as both a thermal and catalytic pyrolysis, using different feedstocks such as plastic and biomass materials ( Figure 1 ). In this study, modified NZ catalysts were added in the reactor with the feedstock. The pyrolysis reactor can hold up to 20 L of feedstock and the maximum working safe temperature of up to 600°C can be achieved with the desired heating rates. Detailed parameters of the pyrolysis reactor were published earlier ( Miandad et al., 2016b , 2017b ). As the temperature increases above certain values, the plastic waste (organic polymers) converts into monomers that are transferred to the condenser, where these vapors are condensed into liquid oil. A continuous condensation system using a water bath and ACDelco Classic coolant was used to ensure the condensation temperature was kept below 10°C, and to ensure the maximum condensation of vapor to liquid oil. The produced liquid oil was collected from the oil collection tank, and further characterization was carried out to uncover its chemical composition and characteristics for other potential applications.

Analytical Methods

The pyrolysis oil was characterized using different techniques such as gas chromatography coupled with mass spectrophotometry (GC-MS), Fourier transform infrared spectroscopy (FT-IR),

Bomb Calorimeter and TGA (Mettler Toledo TGA/SDTA851) by adopting the standard ASTM methods. The functional groups in pyrolysis oil was analyzed by a FT-IR, Perkin Elmer's, UK instrument. The FT-IR analysis was conducted using a minimum of 32 scans with an average of 4 cm −1 IR signals within the frequency range of 500–4,000 cm −1 .

The chemical composition of oil was studied using a GC-MS (Shimadzu QP-Plus 2010) with FI detector. A capillary GC 30 m long and 0.25 mm wide column coated with a 0.25 μm thick film of 5% phenyl-methylpolysiloxane (HP-5) was used. The oven was set at 50°C for 2 min and then increased up to 290°C using a 5°C/min heating rate. The temperature of the ion source and transfer line were kept at 230, and 300°C and splitless injection was applied at 290°C. The NIST08s mass spectral data library was used to identify the chromatographic peaks, and the peak percentages were assessed for their total ion chromatogram (TIC) peak area. The high heating values (HHV) of produced liquid oil obtained from different types of plastic waste were measured following the standard ASTM D 240 method with a Bomb Calorimeter (Parr 6200 Calorimeter) instrument, while production of gas was estimated using the standard mass balance formula, considering the difference of weights of liquid oil and char.

Results and Discussion

Tga analysis of feedstock.

TGA was carried out for each type of plastic waste on an individual basis to determine the optimum temperature for thermal degradation. All types of plastic waste show similar degradation behavior with the rapid loss of weight of hydrocarbons within the narrow range of temperature (150–250°C) ( Figure 2 ). The maximum degradation for each type of plastic waste was achieved within 420–490°C. PS and PP showed single step decomposition, while PE and PET showed a two-stage decomposition under controlled conditions. The single step decomposition corresponds to the presence of a carbon-carbon bond that promotes the random scission mechanism with the increase in temperature ( Kim et al., 2006 ). PP degradation started at a very low temperature (240°C) compared to other feedstocks. Half of the carbon present in the chain of PP consists of tertiary carbon, which promotes the formation of carbocation during its thermal degradation process ( Jung et al., 2010 ). This is probably the reason for achieving maximum PP degradation at a lower temperature. The PS initial degradation started at 330°C and maximum degradation was achieved at 470°C. PS has a cyclic structure, and its degradation under the thermal condition involves both random chain and end-chain scission, which enhances its degradation process ( Demirbas, 2004 ; Lee, 2012 ).

Figure 2 . Thermogravimetric analysis (TGA) of PS, PE, PP, and PET plastic waste.

PE and PET showed a two-stage decomposition process; the initial degradation started at lower temperatures followed by the other degradation stage at a higher temperature. PEs initial degradation started at 270°C and propagated slowly but gradually until the temperature reached 385°C. After that temperature, a sharp degradation was observed, and 95% degradation was achieved with a further increase of around 100°C. A similar two-stage degradation pattern was observed for PET plastic and the initial degradation started at 400°C with a sharp decrease in weight loss. However, the second degradation started at a slightly higher temperature (550°C). The initial degradation of PE and PET may be due to the presence of some volatile impurities such as the additive filler used during plastic synthesis ( Dimitrov et al., 2013 ).

Various researchers have reported that PE and PET degradation requires higher temperatures compared to other plastics ( Dimitrov et al., 2013 ; Rizzarelli et al., 2016 ). Lee (2012) reported that PE has a long chain branched structure and that its degradation occurs via random chain scission, thus requiring a higher temperature, while PET degradation follows the ester link random scission which results in the formation oligomers ( Dziecioł and Trzeszczynski, 2000 ; Lecomte and Liggat, 2006 ). The initial degradation of PET was perhaps due to the presence of some volatile impurities such as diethylene glycol ( Dimitrov et al., 2013 ). Literature reports that the presence of these volatile impurities further promotes the degradation process of polymers ( McNeill and Bounekhel, 1991 ; Dziecioł and Trzeszczynski, 2000 ). The difference in TGA curves of various types of plastics could be due to their mesoporous structure ( Chandrasekaran et al., 2015 ). In addition, Lopez et al. (2011) reported that the use of catalysts decreases the process temperature. Therefore, 450°C could be taken as the optimum temperature, in the presence of activated NZ, for catalytic pyrolysis of the aforementioned plastic waste.

Effect of Feedstock and Catalysts on Pyrolysis Products Yield

The effect of thermal and acid activation of NZ on the product yield of the pyrolysis process was examined ( Figure 3 ). The catalytic pyrolysis of individual PS plastic using TA-NZ and AA-NZ catalysts showed the highest liquid oil yields of 70 and 60%, respectively, compared to all other types of individual and combined plastic waste studied. The high yield of liquid oil from catalytic pyrolysis of PS was also reported in several other studies ( Siddiqui and Redhwi, 2009 ; Lee, 2012 ; Rehan et al., 2017 ). Siddiqui and Redhwi (2009) reported that PS has a cyclic structure, which leads to the high yield of liquid oil from catalytic pyrolysis. Lee (2012) reported that PS degradation occurred via both random-chain and end chain scissions, thus leading to the production of the stable benzene ring structure, which enhances further cracking and may increase liquid oil production. Furthermore, in the presence of acid catalysts, PS degradation followed a carbenium mechanism, which further underwent hydrogenation (inter/intramolecular hydrogen transfer) and β-scission ( Serrano et al., 2000 ). In addition, PS degradation occurred at a lower temperature, compared to other plastics such as PE, due to its cyclic structure ( Wu et al., 2014 ). On the other hand, the catalytic pyrolysis of PS produced a higher amount of char (24.6%) with AA-NZ catalyst than with TA-NZ (15.8%) catalyst. Ma et al. (2017) also reported the high production of char from the catalytic pyrolysis of PS with an acidic zeolite (Hβ) catalyst. The high char production numbers were due to the high acidity of the catalyst, which favors char production via intense secondary cross-linking reactions ( Serrano et al., 2000 ).

Figure 3 . Effect of TA-NZ and AA-NZ on the pyrolysis product yield.

The catalytic pyrolysis of PP produced higher liquid oil (54%) with the AA-NZ catalyst than the TA-NZ catalyst (40%) ( Figure 3 ). On the other hand, the TA-NZ catalyst produced large amounts of gas (41.1%), which may be due to the lower catalytic activity of the TA-NZ catalyst. According to Kim et al. (2002) catalyst with low acidity and BET surface areas with microporous structures, favor the initial degradation of PP which may lead to the maximum production of gases. Obali et al. (2012) carried out pyrolysis of PP with an alumina-loaded catalyst and reported the maximum production of gas. Moreover, formation of carbocation during PP degradation, due to the presence of tertiary carbon in its carbon chain, may also favor gas production ( Jung et al., 2010 ). Syamsiro et al. (2014) also reported that catalytic pyrolysis of PP and PS with an acid (HCL) activated natural zeolite catalyst produced more gases than the process with a thermally activated natural zeolite catalyst, due to its high acidity and BET surface area.

The catalytic pyrolysis of PE with TA-NZ and AA-NZ catalysts produced similar amounts of liquid oil (40 and 42%). However, the highest amounts of gases (50.8 and 47.0%) were produced from PE, using AA-NZ and TA-NZ respectively, compared to all other types of plastic studied. The char production was lowest in this case, 7.2 and 13.0% with AA-NZ and TA-NZ, respectively. Various studies also reported the lower production of char from the catalytic pyrolysis of PE ( Xue et al., 2017 ). Lopez et al. (2011) reported that catalysts with high acidity enhanced the cracking of polymers during the catalytic pyrolysis. The increase in cracking, in the presence of a high acidic catalyst, promotes the production of gases ( Miandad et al., 2016b , 2017a ). Zeaiter (2014) carried out catalytic pyrolysis of PE with HBeta zeolite and reported 95.7% gas production due to the high acidity of the catalyst. Batool et al. (2016) also reported the maximum production of gas from catalytic pyrolysis of PE, with highly acidic ZSM-5 catalyst. According to Lee (2012) and Williams (2006) , PE has a long chain carbon structure, and its degradation occurs randomly into smaller chain molecules via random chain scission, which may promote gas production. During the pyrolysis of PE, which holds the C-H and C-C bonds only, initially, macromolecule backbone breaking occurred and produced stable free-radicals. Further, the hydrogenation steps occurred, leading to the synthesis of secondary free-radicals (new stable C-H bond), which resulted into β-scission and produced an unsaturated group ( Rizzarelli et al., 2016 ).

The catalytic pyrolysis of PP/PE (50/50% ratio) did not show any significant difference in the overall product yields when using both AA-NZ and TA-NZ. The liquid oil produced from the catalytic pyrolysis of PP/PE was 44 and 40% from TA-NZ and AA-NZ catalysts, respectively. A slight decrease in the liquid oil yield from AA-NZ could be due to its high acidity. Syamsiro et al. (2014) reported that AA-NZ with HCl has high acidity compared to TA-NZ, produced less liquid oil yield and had high production of gases. Overall catalytic pyrolysis of PP/PE produced the maximum amount of gas with low amounts of char. The high production of gas may be due to the presence of PP. The degradation of PP enhances the carbocation process due to the presence of tertiary carbon in its carbon chain ( Jung et al., 2010 ). Furthermore, the degradation of PE in the presence of catalyst also favors the production of gas with a low yield of liquid oil. However, when PP and PE catalytic pyrolysis was carried out separately with PS, a significant difference was observed in the product yield.

There was a significant difference in the liquid oil yield of 54 and 34% for catalytic pyrolysis of PS/PP (50/50% ratio) with TA-NZ and AA-NZ catalysts, respectively. Similarly, a significant difference in the char yield of 20.3 and 35.2% was observed, whereas the high yield of gases were 25.7 and 30.8% using TA-NZ and AA-NZ catalysts, respectively. Lopez et al. (2011) and Seo et al. (2003) reported that a catalyst with high acidity promotes the cracking process and produces maximum gas production. Furthermore, the presence of PP also enhances gas production due to the carbocation process during degradation ( Jung et al., 2010 ). Kim et al. (2002) reported that PP degradation produces maximum gas in the presence of acid catalysts.

The catalytic pyrolysis of PS with PE (50/50% ratio) in the presence of TA-NZ catalyst produced 44% liquid oil, however 52% liquid oil was obtained using the AA-NZ catalyst. Kiran et al. (2000) carried out pyrolysis of PS with PE at different ratios and reported that an increase in the concentration of PE decreased the liquid oil concentration with the increase in gas. The presence of PS with PE promotes the degradation process due to the production of an active stable benzene ring from PS ( Miandad et al., 2016b ). Wu et al. (2014) carried out TGA of PS with PE and observed two peaks, the first one for PS at a low temperature, followed by PE degradation at a high temperature. Moreover, PE degradation follows a free radical chain process and hydrogenation process, while PS follows a radical chain process including various steps ( Kiran et al., 2000 ). Thus, even when considering the degradation phenomena, PS resulted in higher degradation compared to PE and produced stable benzene rings ( McNeill et al., 1990 ).

Catalytic pyrolysis of PS/PE/PP (50/25/25% ratio) showed slightly lower liquid oil yields as compared to catalytic pyrolysis of all individual plastic types. The oil yield from both catalysts, TA-NZ and AA-NZ, in this case, is similar, 44 and 40%, respectively. The char production was higher (29.7%) with the AA-NZ catalyst than (19.0%) with the TA-NZ catalyst, which may be due to polymerization reactions ( Wu and Williams, 2010 ). Furthermore, the addition of PET with PS, PE and PP (20/40/20/20% ratio) reduced the liquid oil yields down to 28 and 30% overall, using TA-NZ and AA-NZ catalysts, respectively, with higher fractions of char and gas. Demirbas (2004) carried out pyrolysis of PS/PE/PP and reported similar results for the product yield. Adnan et al. (2014) carried out catalytic pyrolysis of PS and PET using he Al-Al 2 O 3 catalyst with ratios of 80/20% and reported only 37% liquid oil. Moreover, Yoshioka et al. (2004) reported the maximum production of gas and char with negligible liquid oil production from catalytic pyrolysis of PET. In addition, maximum char production was also reported when PET catalytic pyrolysis was carried out with other plastics ( Bhaskar et al., 2004 ). The higher production of char from PET pyrolysis was due to the carbonization and condensation reactions during its pyrolysis at a high temperature ( Yoshioka et al., 2004 ). In addition, the presence of the oxygen atom also favors the high production of char from catalytic pyrolysis of PET ( Xue et al., 2017 ). Thilakaratne et al. (2016) reported that production of benzene-free radicals, with two activated carbons, is the precursor of catalytic coke from PET degradation.

Effect of Catalysts on the Composition of Liquid Oil

The chemical composition of liquid oil produced by the catalytic pyrolysis of different plastic waste using TA-NZ and AA-NZ catalysts were characterized by GC-MS ( Figures 4 , 5 ). The produced liquid oil composition is affected by different types of feedstock and catalysts used in the pyrolysis process ( Miandad et al., 2016a , b , c ). The liquid oil produced from the individual plastic types such as PS, PP and PE contained a mixture of aromatics, aliphatic and other hydrocarbon compounds. The aromatic compounds found in oil, from PS and PE, were higher than PP using the TA-NZ catalyst. The aromatic compounds increased in oil from PS and PP but reduced in PE when using the AA-NZ catalyst. The mesoporous and acidic catalyst leads to the production of shorter chain hydrocarbon due to its high cracking ability ( Lopez et al., 2011 ). However, microporous and less acidic catalysts favor the production of long chain hydrocarbons as the cracking process occurred only on the outer surface of the catalysts. Overall, in the presence of catalysts, PE and PP follow the Random-chain scission mechanism, while PS follows the unzipping or end chain scission mechanism ( Cullis and Hirschler, 1981 ; Peterson et al., 2001 ). The end-chain scission results in monomer production while random chain scission produces oligomers and monomers ( Peterson et al., 2001 ).

Figure 4. (A,B) GC-MS of liquid oil produced from different types of plastic waste with TA-NZ.

Figure 5. (A,B) GC-MS of liquid oil produced from different types of plastic waste with AA-NZ.

The liquid oil produced from the catalytic pyrolysis of PE, when using both catalysts, produced mainly Naphthalene, Phenanthrene, Naphthalene, 2-ethenyl-, 1-Pentadecene, Anthracene, 2-methyl-, Hexadecane and so on ( Figures 4A , 5A ). These results agree with several other studies ( Lee, 2012 ; Xue et al., 2017 ). The production of a benzene derivate reveals that TA-NZ enhances the process of aromatization compared to AA-NZ. Xue et al. (2017) reported that intermediate olefins produced from catalytic pyrolysis of PE, further aromatized inside the pores of catalysts. Nevertheless, the aromatization reaction further leads to the production of hydrogen atoms that may enhance the aromatization process. Lee (2012) reported that ZSM-5 produced more aromatic compounds compare to the mordenite catalyst, due to its crystalline structure.

There are two possible mechanisms which may involve the degradation of PE in the presence of a catalyst; hybrid ion abstraction due to the presence of Lewis sites or, due to the carbenium ion mechanism via the addition of a proton ( Rizzarelli et al., 2016 ). Initially, degradation starts on the external surface of the catalysts and later proceeds with further degradation in the inner pores of the catalysts ( Lee, 2012 ). However, microporous catalysts hinder the entrance of larger molecules and thus higher carbon chain compounds are produced from catalytic pyrolysis of PE with microporous catalysts. In addition, in the presence of acidic catalysts, due to carbenium mechanism, the formation of aromatic and olefin compound production may increase ( Lee, 2012 ). Lin et al. (2004) reported highly reactive olefin production, as intermediate products during the catalytic pyrolysis of PE, that may favor the production of paraffin and aromatic compounds in produced liquid oil. Moreover, the presence of an acidic catalyst and free hydrogen atom may lead to alkylation of toluene and benzene, converting intermediate alkylated benzene to the production of naphthalene due to aromatization ( Xue et al., 2017 ).

The liquid oil produced from catalytic pyrolysis of PS with TA-NZ and AA-NZ, contains different kinds of compounds. Alpha-Methylstyrene, Benzene, 1,1′-(2-butene-1,4-diyl)bis-, Bibenzyl, Benzene, (1,3-propanediyl), Phenanthrene, 2-Phenylnaphthalene and so on were the major compounds found in the produced liquid oil ( Figures 4A , 5A ). The liquid oil produced from catalytic pyrolysis of PS, with both activated catalysts, mainly contains aromatic hydrocarbons with some paraffins, naphthalene and olefin compounds ( Rehan et al., 2017 ). However, in the presence of a catalyst, the maximum production of aromatic compounds was achieved ( Xue et al., 2017 ). Ramli et al. (2011) also reported the production of olefins, naphthalene with aromatic compounds from catalytic pyrolysis of PS with Al 2 O 3 , supported with Cd and Sn catalysts. PS degradation starts with cracking on the outer surface of the catalyst and is then followed by reforming inside the pores of the catalyst ( Uemichi et al., 1999 ). Initially, the cracking of polymer is carried out by the Lewis acid site on the surface of catalysts to produce carbocationic intermediates, which further evaporates or undergoes reforming inside the pores of the catalyst ( Xue et al., 2017 ).

The catalytic pyrolysis of PS mainly produces styrene and its derivate as the major compounds in the produced liquid oil ( Siddiqui and Redhwi, 2009 ; Rehan et al., 2017 ). Conversion of styrene into its derivate was increased in the presence of protonated catalysts due to hydrogenation ( Kim et al., 2002 ). Shah and Jan (2015) and Ukei et al. (2000) reported that hydrogenation of styrene increased with the increase of the reaction temperature. Ogawa et al. (1982) carried out pyrolysis of PS with the alumina-silica catalyst at 300°C and found the hydrogenation of styrene to its derivate. Ramli et al. (2011) reported the possible degradation mechanism of PS on acid catalysts that may occur due to the attack of a proton associated with Bronsted acidic sites, resulting in the carbenium ion mechanism, which further undergoes β-scission and is later followed by hydrogen transfer. Moreover, cross-linking reaction was favored by strong Bronsted acidic sites and when this reaction occurred the completing cracking may decrease to some extent and enhance the production of char ( Serrano et al., 2000 ). Furthermore, silica-alumina catalysts do not have strong Bronsted acidic sites, though it may not improve the cross-linking reaction but favor the hydrogenation process. Thus, it may be the reason that styrene was not found in the liquid oil, however, its derivate was detected at high quantities ( Lee et al., 2001 ). Xue et al. (2017) also reported the dealkylation of styrene, due to the delay in evaporation inside the reactor, which may lead to an enhanced reforming process and result in the production of a styrene derivate. TA-NZ and AA-NZ contain a high amount of alumina and silica that leads to the hydrogenation of styrene to its derivate, resulting in the production of styrene monomers instead of styrene.

The catalytic pyrolysis of PP produced a complex mixture of liquid oil containing aromatics, olefins and naphthalene compounds. Benzene, 1,1′-(2-butene-1,4-diyl)bis-, benzene, 1,1′-(1,3-propanediyl)bis-, anthracene, 9-methyl-, naphthalene, 2-phenyl-, 1,2,3,4-tetrahydro-1-phenyl-, naphthalene, phenanthrene etc. were the major compounds found in the liquid oil ( Figures 4A , 5A ). These findings are in line with other studies that carried out catalytic pyrolysis of PP with various catalysts ( Marcilla et al., 2004 ). Furthermore, degradation of PP with AA-NZ resulted in the maximum production of phenol compounds. The higher production was perhaps due to the presence of high acidic sites, as it favors phenol compound production. Moreover, the presence of a high acidic site on catalysts enhanced the oligomerization, aromatization and deoxygenation mechanism that led to the production of poly-aromatic and naphthalene compounds. Dawood and Miura (2002) also reported the high production of these compounds from the catalytic pyrolysis of PP with a high acidic modified HY-zeolite.

The composition of oil from the catalytic pyrolysis of PP with PE contains compounds found in the oil from both individual plastic type feedstocks. Miandad et al. (2016b) reported that feedstock composition also affects the quality and chemical composition of the oil. The produced liquid oil from catalytic pyrolysis of PE/PP contains aromatic, olefin, and naphthalene compounds. The major compounds found were; benzene, 1,1′-(1,3-propanediyl)bis-, mono(2-ethylhexyl) ester, 1,2-benzenedicarboxylic acid, anthracene, pentadecane, phenanthrene, 2-phenylnaphthalene and so on ( Figures 4B , 5B ). Jung et al. (2010) reported that the aromatic production from PP/PE catalytic pyrolysis might follow the Diels–Alder reaction mechanism and is then followed by dehydrogenation. Furthermore, catalytic pyrolysis of PP and PE carried out individually with PS, mainly produced aromatic compounds due to the presence of PS. The produced liquid oil from PS/PP contains benzene, 1,1′-(1,3-propanediyl)bis, 1,2-benzenedicarboxylic acid, disooctyl ester, bibenzyl, phenanthrene, 2-phenylnaphthalene, benzene, (4-methyl-1-decenyl)- and so on ( Figures 4A , 5A ). PS catalytic pyrolysis with PE mainly produced liquid oil with major compounds of azulene, naphthalene, 1-methyl-, naphthalene, 2-ethenyl, benzene, 1,1′-(1,3-propanediyl)bis-, phenanthrene, 2-phenylnaphthalene, benzene, 1,1′-(1-methyl-1,2-ethanediyl)bis- and some other compounds as well ( Figures 4B , 5B ). Miskolczi et al. (2006) carried out pyrolysis of PS with PE with a ratio of 10 and 90%, respectively, and reported the maximum production of aromatics even at a very low ratio of PS. Miandad et al. (2016b) reported that thermal pyrolysis of PE with PS without a catalyst, resulted in the conversion of PE into liquid oil with a high composition of aromatics. However thermal pyrolysis of the only PE without a catalyst converted it into wax instead of liquid oil due to its strong long chain branched structure ( Lee, 2012 ; Miandad et al., 2016b ). Wu et al. (2014) carried out TGA of PS with PE and reported that the presence of PS favors the degradation of PE, due to the production of stable benzene rings.

The chemical composition of pyrolysis oil, by different functional groups, was studied using FT-IR. The obtained data revealed the presence of aromatics and aliphatic functional groups in the oil ( Figures 6 , 7 ). A very strong peak at 696 cm −1 was observed in most of the liquid oils obtained using both catalysts, which corresponds to the high concentration of aromatic compounds. Two more peaks, that are obvious, were visible at around 1,456 and 1,495 cm −1 for C-C with single and double bonds, corresponding to aromatic compounds. Furthermore, at the end of the spectrum, strong peaks at 2,850, 2,923, and 2,958 cm −1 were observed in all types of liquid oils except the PS, corresponding to the C-H stretch of alkanes compounds. Overall, the liquid oil obtained from catalytic pyrolysis of different plastic waste using the AA-NZ catalyst, showed more peaks than the samples from the TA-NZ catalysts. These extra peaks corresponded to aromatics, alkanes and alkene compounds. This indicates that, as expected, the AA-NZ had better catalytic properties than the TA-NZ. Various researchers have reported similar results, that liquid oil produced from PS was dominant with aromatics. Tekin et al. (2012) and Panda and Singh (2013) also reported the presence of aromatics with some alkanes and alkenes from catalytic pyrolysis of PP. Kunwar et al. (2016) carried out the thermal and catalytic pyrolysis of PE and reported that produced liquid oil contained alkanes and alkenes as a major functional group. Overall, the FT-IR analysis provided more insight into the chemical composition of liquid oil produced, from catalytic pyrolysis of different plastic waste, using modified NZ catalysts and further confirmed our GC-MS results.

Figure 6 . FT-IR analysis of liquid oil produced from catalytic pyrolysis with TA-NZ.

Figure 7 . FT-IR analysis of liquid oil produced from catalytic pyrolysis with AA-NZ.

Potential Applications of Pyrolysis Products

The liquid oil produced from the catalytic pyrolysis of different types of plastic feedstock has a high number of aromatic, olefin, and naphthalene compounds that are found in petroleum products. Moreover, the HHV of the produced liquid oil has been found in the range of 41.7–44.2 MJ/kg ( Table 2 ) which is very close to the energy value of conventional diesel. The lowest HHV of 41.7 MJ/ kg was found in liquid oil obtained from PS using the TA-NZ catalyst, whereas the highest HHV of 44.2 MJ/kg was from PS/PE/PP using the AA-NZ catalyst. Thus, the pyrolysis liquid oil produced from various plastic wastes has the potential to be used as an alternative source of energy. According to Lee et al. (2015) and Rehan et al. (2016) , the production of electricity is achievable using pyrolysis liquid oil in a diesel engine. Saptoadi and Pratama (2015) successfully used pyrolytic liquid oil as an alternative in a kerosene stove. Moreover, the produced aromatic compounds can be used as raw material for polymerization in various chemical industries ( Sarker and Rashid, 2013 ; Shah and Jan, 2015 ). Furthermore, various researchers utilized the produced liquid oil as transportation fuel after blending with conventional diesel at different ratios. The studies were carried out to explore the potential of produced liquid oil in the context of engine performance and vehicle exhaust emission. Nileshkumar et al. (2015) and Lee et al. (2015) reported that 20:80% blend ratio of pyrolytic liquid oil and conventional diesel, respectively, gave similar engine performance results than conventional diesel. Moreover, at the same blended ratio the exhaust emissions were also similar, however the exhaust emissions increased with the increase in the blended amount of pyrolysis oil ( Frigo et al., 2014 ; Mukherjee and Thamotharan, 2014 ).

Table 2 . High Heating Values (HHV) of pyrolysis oil from various feedstocks using TA-NZ and AA-NZ catalysts.

The residue (char) left after the pyrolysis process can be utilized for several environmental applications. Several researchers activated the char via steam and thermal activation ( Lopez et al., 2009 ; Heras et al., 2014 ). The activation process increased the BET surface area and reduced the pore size of the char ( Lopez et al., 2009 ). Furthermore, Bernando (2011) upgraded the plastic char with biomaterial and carried out the adsorption (3.6–22.2 mg/g) of methylene blue dye from wastewater. Miandad et al. (2018) used the char obtained from pyrolysis of PS plastic waste to synthesize a novel carbon-metal double-layered oxides (C/MnCuAl-LDOs) nano-adsorbent for the adsorption of Congo red (CR) in wastewater. Furthermore, the char can be used as a raw material for the production of activated carbon as well.

Limitations of GC-MS Analysis of Pyrolysis Oil

There are some limitations in conducting the accurate quantitative analysis of chemical components in pyrolysis oil using GC-MS. In this study, we used the mass percentage of different chemicals found in oil samples, calculated based on the peak areas identified by a normal phase DP5-MS column and FID. The identified peaks were matched with the NIST and mass bank spectra library. The compounds were chosen based on the similarity index (SI > 90%). Further comparison with known (CRM) standards enabled confirmation of the identified compounds. The used column and detectors were limited only with hydrocarbons. In reality however, oil from most plastic waste has a complex chemical structure and may contain other groups of unidentified chemicals such as sulfur, nitrogen, and oxygen-containing hydrocarbons. This is why a more in-depth and accurate qualitative chemical analysis is needed to fully understand the chemistry of pyrolysis oil, using advanced calibration and standardization and using different MS detectors like SCD and NCD as well as different GC columns.

The Potential and Challenges of Pyrolysis Based Biorefineries

Waste biorefineries are attracting tremendous attention as a solution to convert MSW and other biomass waste into a range of products such as fuels, power, heat and other valuable chemicals and materials. Different types of biorefineries, such as an agriculture-based biorefinery, animal waste biorefinery, wastewater biorefinery, algae-based biorefinery, plastic waste refinery, forestry-based biorefinery, industrial waste biorefinery, and Food waste biorefinery etc., can be developed depending on the type and source of waste ( Gebreslassie et al., 2013 ; De Wild et al., 2014 ; Nizami et al., 2017a , b ; Waqas et al., 2018 ). These biorefineries can play a significant role to reduce waste-related environmental pollution and GHG emissions. Furthermore, they generate substantial economic benefits and can help achieve a circular economy in any country.

A pyrolysis based biorefinery can be developed to treat a range of biomass waste and plastic waste to produce liquid and gas fuels, energy, biochar, and other higher value chemicals using an integrated approach. The integrated approach helps to achieve maximum economic and environmental benefits with minimal waste production. There are many challenges and room for improvement in pyrolysis-based biorefineries, that need to be addressed and optimized to ensure maximum benefits. Although pyrolysis oil holds more energy than coal and some other fuels, pyrolysis itself is an energy-intensive process, and the oil product requires more energy to be refined ( Inman, 2012 ). This means that pyrolysis oil may not be much better than conventional diesel or other fossil-based fuels in terms of GHG emissions, though much detailed research studies on mass and energy balance across the whole process's boundaries are needed to confirm this. To overcome these process energy requirements, more advanced technologies can be developed using the integration of renewable energies such as solar or hydro with pyrolysis-based biorefineries, to achieve maximum economic and environmental benefits.

The availability of plastic and biomass waste streams as feedstocks for pyrolysis based biorefineries, is another major challenge, since recycling is not currently very efficient, especially in the developing countries. The gases produced from pyrolysis of some plastic waste such as PVC are toxic, and therefore pyrolysis emission treatment technology has to be further refined to achieve maximum environmental benefits. The pyrolysis oil obtained from various plastic types need to be cleaned significantly before it is used in any application, to ensure minimal environmental impact. The high aromatic contents of the pyrolysis oil is good and some aromatic compounds such as benzene, toluene, and styrene can be refined and sold in an already established market. However, some of the aromatic hydrocarbons are known carcinogens and can cause serious human health and environmental damage. Serious consideration is therefore needed in this regard.

Other aspects for optimization of pyrolysis based biorefineries, such as new emerging advanced catalysts including nano-catalysts, have to be developed and applied in pyrolysis processes in order to increase the quality and yield of products, and to optimize the overall process. The market for pyrolysis based biorefinery products should be created/ expanded to attract further interest and funding, in order to make this concept more practical and successful. Similarly, more focus is needed to conduct further research and development work on enriching the biorefinery concept and to tap into its true potential. Furthermore, it is vital to conduct a detailed economic and environmental impact assessment of biorefineries during a design stage, using specialized tools such as the life-cycle assessment (LCA). The LCA can analyze the environmental impact of the biorefinery and all products by conducting detailed energy and material balances of all life stages including raw material extraction and processing, manufacturing, product distribution, use, maintenance, and disposal/recycling. The outcomes of LCA will help to determine the sustainability of biorefineries, which is crucial in making the right decision.

Conclusions

Catalytic pyrolysis is a promising technique to convert plastic waste into liquid oil and other value-added products, using a modified natural zeolite (NZ) catalyst. The modification of NZ catalysts was carried out by novel thermal (TA) and acidic (AA) activation that enhanced their catalytic properties. The catalytic pyrolysis of PS produced the highest liquid oil (70 and 60%) compared to PP (40 and 54%) and PE (40 and 42%), using the TA-NZ and AA-NZ catalysts, respectively. The chemical composition of the pyrolysis oil was analyzed using GC-MS, and it was found that most of the liquid oil produced a high aromatic content with some aliphatic and other hydrocarbon compounds. These results were further confirmed by the FT-IR analysis showing clear peaks corresponding to aromatic and other hydrocarbon functional groups. Furthermore, liquid oil produced from different types of plastic waste had higher heating values (HHV) in the range of 41.7–44.2 MJ/kg similar to that of conventional diesel. Therefore, it has the potential to be used in various energy and transportation applications after further treatment and refining. This study is a step toward developing pyrolysis-based biorefineries. Biorefineries have a great potential to convert waste into energy and other valuable products and could help to achieve circular economies. However, there are many technical, operational, and socio-economic challenges, as discussed above, that need to be overcome in order to achieve the maximum economic and environmental benefits of biorefineries.

Data Availability

All datasets generated for this study are included in the manuscript and/or the supplementary files.

Author Contributions

RM performed the pyrolysis experiments and helped in manuscript write up. HK, JD, JG, and AH have carried out the detailed characterization of the process products. MR and ASA have analyzed the data and written parts of the manuscript. MAB, MR, and A-SN have corrected and edited the manuscript. ASA and IMII supported the project financially and technically.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

MR and A-SN acknowledge the Center of Excellence in Environmental Studies (CEES), King Abdulaziz University (KAU), Jeddah, KSA and the Ministry of Education, KSA for financial support under Grant No. 2/S/1438. Authors are also thankful to Deanship of Scientific Research (DSR) at KAU for their financial and technical support to CEES.

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Keywords: catalytic pyrolysis, pyrolysis based biorefineries, natural zeolite, plastic waste, aromatic compounds, modified natural zeolite, catalyst

Citation: Miandad R, Rehan M, Barakat MA, Aburiazaiza AS, Khan H, Ismail IMI, Dhavamani J, Gardy J, Hassanpour A and Nizami A-S (2019) Catalytic Pyrolysis of Plastic Waste: Moving Toward Pyrolysis Based Biorefineries. Front. Energy Res . 7:27. doi: 10.3389/fenrg.2019.00027

Received: 15 November 2018; Accepted: 22 February 2019; Published: 19 March 2019.

Reviewed by:

Copyright © 2019 Miandad, Rehan, Barakat, Aburiazaiza, Khan, Ismail, Dhavamani, Gardy, Hassanpour and Nizami. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Mohammad Rehan, [email protected]

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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Recent advances on waste plastic thermal pyrolysis: a critical overview.

1. Introduction

1.1. thermal pyrolysis process, 1.2. co-pyrolysis and mixed waste plastics, 2. common influential factors of waste plastic thermal pyrolysis, 2.1. chemical composition of the feedstock, 2.2. reactor type.

• Fusion of the waste plastics.
• Pyrolysis of the fused waste plastics.

2.3. Decomposition Temperature and Heating Rate

2.4. residence time, 2.5. pressure, 3. quality guarantee of waste plastic pyrolysis and petroleum products with emphasis on diesel: a brief comparative analysis.

4. Active Commercial Plastic Pyrolysis Processes and Technologies

4.1. the impacts of feedstock, 4.2. plastic pyrolysis technology, 4.2.1. feeding system, 4.2.2. pyrolysis reactor, 4.2.3. separation and collection system, 4.3. practical implications of this literature, 5. conclusions, author contributions, institutional review board statement, informed consent statement, data availability statement, acknowledgments, conflicts of interest.

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Yansaneh, O.Y.; Zein, S.H. Recent Advances on Waste Plastic Thermal Pyrolysis: A Critical Overview. Processes 2022 , 10 , 332. https://doi.org/10.3390/pr10020332

Yansaneh OY, Zein SH. Recent Advances on Waste Plastic Thermal Pyrolysis: A Critical Overview. Processes . 2022; 10(2):332. https://doi.org/10.3390/pr10020332

Yansaneh, Osman Y., and Sharif H. Zein. 2022. "Recent Advances on Waste Plastic Thermal Pyrolysis: A Critical Overview" Processes 10, no. 2: 332. https://doi.org/10.3390/pr10020332

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Pyrolytic Conversion of Plastic Waste to Value-Added Products and Fuels: A Review

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Plastic production has been rapidly growing across the world and, at the end of their use, many of the plastic products become waste disposed of in landfills or dispersed, causing serious environmental and health issues. From a sustainability point of view, the conversion of plastic waste to fuels or, better yet, to individual monomers, leads to a much greener waste management compared to landfill disposal. In this paper, we systematically review the potential of pyrolysis as an effective thermochemical conversion method for the valorization of plastic waste. Different pyrolysis types, along with the influence of operating conditions, e.g., catalyst types, temperature, vapor residence time, and plastic waste types, on yields, quality, and applications of the cracking plastic products are discussed. The quality of pyrolysis plastic oil, before and after upgrading, is compared to conventional diesel fuel. Plastic oil yields as high as 95 wt.% can be achieved through slow pyrolysis. Plastic oil has a heating value approximately equivalent to that of diesel fuel, i.e., 45 MJ/kg, no sulfur, a very low water and ash content, and an almost neutral pH, making it a promising alternative to conventional petroleum-based fuels. This oil, as-is or after minor modifications, can be readily used in conventional diesel engines. Fast pyrolysis mainly produces wax rather than oil. However, in the presence of a suitable catalyst, waxy products further crack into oil. Wax is an intermediate feedstock and can be used in fluid catalytic cracking (FCC) units to produce fuel or other valuable petrochemical products. Flash pyrolysis of plastic waste, performed at high temperatures, i.e., near 1000 °C, and with very short vapor residence times, i.e., less than 250 ms, can recover up to 50 wt.% ethylene monomers from polyethylene waste. Alternatively, pyrolytic conversion of plastic waste to olefins can be performed in two stages, with the conversion of plastic waste to plastic oil, followed by thermal cracking of oil to monomers in a second stage. The conversion of plastic waste to carbon nanotubes, representing a higher-value product than fuel, is also discussed in detail. The results indicate that up to 25 wt.% of waste plastic can be converted into carbon nanotubes.

1. Introduction

Plastic products play a critical role in our lives and are being used in large quantities due to their durability, versatility, light weight, and low cost [ 1 , 2 ]. Plastic waste materials, generated in different sectors of the economy, such as agriculture, residential and commercial, automobiles, construction and demolition, packing materials, toys, and electrical equipment are growing rapidly and are either recycled, combusted (waste incineration), or disposed of [ 3 ]. Plastic waste consists mainly of low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polyvinylchloride (PVC), polystyrene (PS), and polyethylene terephthalate (PET) [ 4 ]. Polyethylene and polypropylene constitute the greatest portion of plastic waste [ 4 ].

The increase in the world population and subsequent living standards have caused a rapid increase in municipal solid waste generation of to up to 1.3 billion tons per annum [ 3 ]. Reportedly, plastic waste is the third largest contributor of municipal solid waste [ 4 ]. The global production of plastic has increased, from 1.5 million tons in 1950 to approximately 359 million tons in 2018 and is attributed to a rapid rise in the packaging/wrapping sector [ 5 ]. Today, over 250 million tons per year are either landfilled or dispersed in the environment and an estimated 10 million tons per year end up in the oceans. Considering an increase of 9–13% of plastic waste per year [ 4 ], it is predicted that billions of tons of plastic could be produced by 2050, of which the greatest portion could go to landfills or be dispersed, both in the land environment and in the oceans.

An increase in daily demand of plastic materials, which are petroleum-based substances, can result in the depletion of non-renewable fossil resources. Approximately 4% of crude oil production is directly utilized in plastic production [ 6 , 7 ]. In addition to contributing to a global energy crisis, plastic waste can affect the environment and, therefore, either disposing or reusing/recycling is crucial. It is well known that plastics can persist in the environment for a prolonged period. The continuous disposal of plastic wastes is destructive to both terrestrial and marine ecosystem, as they are not readily biodegraded and can take several years to vanish [ 8 ]. Photo-degradation, auto-oxidation, thermo-oxidation, thermal degradation and biodegradation are plastic nature-based degradation mechanisms, however with a very slow rate [ 4 ]. On the other hand, the pollutants, such as toluenes, xylenes, benzenes, and phenols, released into the air, water, and soil as a result of plastic degradation cause undeniable issues, such as impacts on human and animal health and deterioration of soil fertility [ 4 ]. Photo-degradation converts plastic waste into fine pieces (micro-plastics), which float on the surfaces of rivers, ponds, lakes, end up in seas and oceans and can penetrate into the food chain and subsequently pass to humans [ 9 ].

In addition to landfilling, there are four distinctive approaches for plastic waste management. Primary mechanical recycling is a technique in which single-type uncontaminated and clean plastic wastes are reprocessed, resulting in a product without changing the basic structure and equivalent quality. Although this method is cost-effective, washing the waste materials generates a new waste stream and, more importantly, plastic wastes usually consist of mixtures of different plastics, often arranged as composites with other materials and are either difficult or impossible to recycle. Secondary recycling is another mechanical recycling approach that follows a decontamination process, remelting, remolding and re-extruding. Size reduction, contaminants removal, separation from other waste materials, make this approach less favorable in terms of operating cost. Tertiary recycling is a chemical or/and thermochemical recycling, which includes chemolysis/solvolysis (i.e., glycolysis, hydrolysis, methanolysis, and alcoholysis), gasification, partial oxidation, and pyrolysis. In this approach, large polymer molecules of plastic wastes are converted into shorter molecules through the use of heat and/or chemical reactions. This technique produces fuels or value-added chemicals that are useful for the synthesis of new plastics and other products. Quaternary recycling is waste combustion (incineration) of the waste material for energy recovery. It seems to be the simplest method; however, it generates pollution and may not meet the circular economy milestones [ 10 , 11 ].

According to the waste management hierarchy (from the most to the least preferred), we identify prevention, minimization, reuse, recycle, energy recovery, and disposal. Energy recovery through incineration is in a lower rank compared to reuse and recycling [ 12 ]. As mentioned, the primary and secondary recycling methods suffer some drawbacks, such as labor-intensive operation for the separation process prior to recycling, a high material loss, possible water contamination, and, overall, a high cost. More importantly, the recycled products are often more expensive than the virgin plastics and may not maintain the original properties. As such, the thermochemical conversion (i.e., tertiary recycling) can be an economically and environmentally friendly solution, leading to a high value fuel/chemical production from plastic waste.

The interest in thermochemical conversion of plastic waste, particularly pyrolysis, has increased considerably over the last few years, primarily since China stopped accepting post-consumer plastic waste in 2018, after having taken up to 45% of the world’s plastic waste for recycling, landfilling and incineration [ 13 , 14 , 15 , 16 ]. Zhang et al. [ 17 ] conducted a comprehensive review on various advanced non-biodegradable plastic waste treatment technologies. They concluded that physical recycling methods are the most sustainable technologies with consideration of a decrease in the performance of plastic after several recycling cycles. It has been stated that pyrolysis is the most widely used thermal remediation, and gasoline/diesel yield is an indicator parameter which can reflect the actual valuable yield of the process and its industrial applicability. Degradation is another promising technology, but most studies focus on using selected kinds of microorganism to degrade specific polymers. As such, research on microorganisms able to degrade mixtures of various plastics is needed. Fojt et al. [ 18 ] critically reviewed the overlooked challenges associated with the accumulation of micro-plastics in the soil. Products made from biodegradable plastics are beginning to replace conventional plastics. Composting is highly suggested for bioplastic disposal; however, the compost formed could contain micro-bioplastic particles resulting from incomplete biodegradation, causing soil contamination. These authors addressed this problem by summarizing sample pre-treatments and analytical techniques. The analytical techniques include both thermo-analytical (i.e., pyrolysis) and non-thermo-analytical (i.e. pre-sorting and respective detection limits) approaches. They concluded that, due to the poor knowledge of the production rate of micro-plastics, fate, sorption properties and toxicity, a rapid and suitable approach is required for their determination. Yet, thermo-analytical approach is the most promising strategy. Murthy et al. [ 19 ] carried out an in-depth review study of the plastic pyrolysis process and discussed the influence of various operating parameters as well as the characterization of the liquid oil obtained from the process. The results revealed that most of the plastics produce oil with reasonable calorific values (i.e., approximately similar to conventional fuels). The plastic pyrolysis product distribution depends on the type of reactor used. Significant studies have been conducted on batch-style reactors due to the easy design, fabrication, operation, and control. On the other hand, continuous fluidized bed reactors can provide a uniform mixing of feedstock and heat carriers or catalysts during operation and, therefore, generate more stable products. Nanda and Berruti [ 20 ] systematically reviewed solid waste technologies, such as pyrolysis, liquefaction and gasification for converting waste plastic into fuels/chemicals. They stated that pyrolysis and hydrothermal liquefaction technologies are able to reduce the volume of plastics to landfills/oceans, reduce the overall carbon footprints, and, more importantly, have high conversion efficiencies and relatively lower costs when compared to higher temperature processes, such as gasification. Selectively, plastics can be converted either to bio-oil, bio-crude oil, synthesis gas, hydrogen and aromatic char. As such, the influence of process parameters, such as temperature, heating rate, feedstock concentration, reaction time, reactor type, and catalysts, have been discussed thoroughly. Damodharan et al. [ 21 ] conducted a review on the utilization of waste plastic oil in diesel engines. They used waste plastic oil obtained from the pyrolysis of mixed waste plastics in the presence of catalyst (e.g. silica, alumina, ZSM-5 and kaolin), with up to 80 wt.% yield. The pyrolysis oil had a lower cetane number than fossil diesel and, therefore, longer ignition delays and higher heat releases. NOx emissions are higher with plastic pyrolysis oil. Smoke emissions were chiefly low with plastic oil and could be further decreased to Euro levels by the use of oxygenated additives. They finally concluded that, plastic pyrolysis oil is a good candidate for fossil diesel replacement and found it to run smoothly in diesel engines. Williams [ 22 ] carried out a review on converting waste plastic to hydrogen and carbon nanotubes via pyrolysis coupled with catalytic steam reforming. This author investigated the influence of reactor designs, catalyst type, and operating conditions on the yield and quality of the carbon nanotubes. He concluded that the process temperature along with the type of catalyst are the prominent factors in plastic to hydrogen and carbon nanotubes pyrolysis. There is a balance between introduction of steam, which enhances hydrogen yield, and carbon nanotubes quality, since higher steam flowrates tend to oxidize the carbon nanotubes.

This review aims to thoroughly discuss the different types of plastic waste pyrolysis processes (i.e., slow, fast, and flash) with respect to the quality and quantity of the products. Furthermore, the application of pyrolysis plastic oil, as a fuel and/or material, is reviewed. Upgrading plastic oil through different methods (e.g., thermal cracking for monomer recovery, hydrogenation, and blending) along with the conversion of plastic waste to carbon nanotubes is reviewed and discussed in detail. This paper contributes to the science of waste management and waste valorization, providing the most updated information and insight through a comprehensive study of the most advanced literature on the pyrolytic conversion of waste plastics.

2. Plastic Waste Properties

To achieve a very good heat/mass transfer during the pyrolysis process, plastic wastes are typically crushed, shredded and sieved to obtain small size flakes, i.e., less than 2 mm. Proximate and ultimate analysis of different plastic wastes are presented in Table 1 and Table 2 . A high volatile matter content (above 90 wt.%) along with a high carbon and hydrogen content make plastic waste an excellent candidate for the pyrolysis process, leading to a high conversion to the liquid/gas products.

Ultimate analysis of different plastic wastes [ 23 ].

Proximate analysis of various plastic types [ 2 ].

3. Pyrolysis Process

Pyrolysis is a versatile thermal cracking process that occurs in the absence of oxygen at temperatures above 400 °C. Typically, pyrolysis processes are classified as slow, fast, and flash [ 24 , 25 ]. This thermochemical process breaks down the long chain polymer molecules into smaller and less complex molecules through heat and chemical reactions. Slow pyrolysis is typically performed at temperatures between 350 and 550 °C, with 1 to 10 °C/min heating rates, and a prolonged vapor residence time. The major product of slow pyrolysis is a solid residue, called char, as a slow heating rate favors solid formation among various parallel-competitive reactions [ 25 ]. Fast pyrolysis often takes place at temperatures between 500 and 700 °C. The heating rate experienced by the feedstock is above 1000 °C/min, and vapor residence times are normally in the range of a few seconds [ 26 ]. Fast pyrolysis favors liquid production and, depending on the feedstock type, the liquid yield can surprisingly reach up to 90 wt.% for the pyrolysis of polyolefin materials [ 27 ]. In flash pyrolysis, the temperature is usually above 700 °C, the heating rate experienced by the feed is extremely fast, and vapor residence times are in the range of milliseconds [ 25 ]. Flash pyrolysis can produce higher yields of oil than fast pyrolysis for biomass feedstocks, while it differs for plastic waste, as the latter produces more gas compared to other products [ 28 ]. The products obtained from the pyrolysis of plastic wastes (all types, alone, or as mixtures) are categorized into liquid/wax, solid residues, and gas [ 29 ].

Pyrolysis is a robust technique and can be used for either fuel or monomer recovery, particularly while addressing plastic waste management challenges. For example, liquid pyrolysis oil obtained from fast pyrolysis is an excellent source of gasoline and diesel [ 29 ]. Unlike water-rich pyrolysis bio-based oils derived from biomass feedstocks, the plastic oils have a high heating value, almost three times more than bio-oils, and similar to diesel fuels, due to absence of highly oxygenated compounds and water [ 30 ]. The acid content of plastic oils also is dramatically lower than that of bio-oils and, therefore, no further upgrading may be required for fuel applications. The pyrolysis coproducts include solid (char), which can be used as an adsorbent [ 31 , 32 , 33 ], and gas, a valuable resource that can be used as an energy supplier for the pyrolysis process.

The produced plastic oil can be a liquid or a wax. The wax is yellowish with a high viscosity at room temperature and is predominantly composed of alkanes and alkenes hydrocarbons with a high boiling point (C20+) [ 34 ]. Wax is typically an intermediate product, and a further process, such as fluid catalytic cracking (FCC), is required to convert it into liquid fuels. Liquid plastic oil is comprised of mainly aliphatic compounds as well as mono- and polyaromatics [ 29 ]. In addition to being a promising precursor for fuel applications, the plastic oil can be used as an intermediate and converted into ethylene and propylene through further cracking at higher temperatures and extremely low contact times. The major gaseous species forming the “gas product” are methane, ethylene, ethane, propylene, butadiene, and butane [ 35 ]. The gas product can be used as an energy source to provide the required pyrolysis energy, making the process self-sustaining and independent from external energy sources. In addition, the valuable olefin components present in the gas stream can be separated and recovered for chemical recycling. The solid residue is the remaining pyrolysis product, mostly made of coke and ash [ 36 ].

3.1. Slow Pyrolysis

Table 3 summarizes slow pyrolysis trials conducted for the conversion of plastic wastes with/without catalyst utilization using various reactors and under a wide range of operating conditions. The results reveal that the liquid produced during slow pyrolysis is typically oily rather than waxy. The oil yield can reach up to 93 wt.% when LDPE is pyrolyzed at 550 °C, which implies a remarkable yield with a broad range of applications [ 21 ]. The plastic oil is versatile and can be used either directly in steam boilers for electricity generation or as a platform chemical for other applications, such as transportation fuels, monomer recovery, and carbon nanotubes (CNTS) production. The solid residue yield is significantly less than bio-based char, as a consequence of the lower fixed carbon and a higher volatile matter associated with plastic wastes compared to biomass ( Table 1 ). The gasoline fraction, C6–C12, can constitute up to 90 wt.% of the liquid product, making it valuable for conventional gasoline replacement.

Slow pyrolysis of different plastic wastes.

3.1.1. Influence of Plastic Types

Table 3 indicates that the pyrolysis of polyolefins, including LDPE, HDPE, and PP, typically produces a liquid oil with a significant fraction of aliphatic (alkanes and alkenes), specifically in the absence of catalyst. The impact of catalyst on the composition of plastic oil is discussed later, in Section 3.1.2 . The desired pyrolysis temperature to achieve a high conversion of polyolefins is above 450 °C, since, below this temperature, the solid residue drastically increases. Polystyrene (PS), which is composed of styrene monomers, can generate a liquid with a remarkable amount of aromatic compounds, such as benzene, toluene, and ethyl benzene [ 2 ]. Although the pyrolysis of polyolefins and polystyrene leads to the formation of a liquid oil which can be an excellent precursor for fuels/chemicals, the pyrolysis of PET and PVC generates a significant amount of benzoic acid and hydrochloric acid, respectively, which are toxic and corrosive to the reactors [ 2 , 37 ]. As such, these two polymers are typically excluded from pyrolysis.

3.1.2. Influence of Catalyst

Among the typical plastic pyrolysis process catalysts (e.g., FCC, HZSM-5, MCM-41, HY, Hβ, HUSY, mordenite and amorphous silica-alumina), acidic zeolites have been widely investigated [ 38 , 39 , 40 , 41 , 42 , 43 ]. Zeolite catalysts have shown an excellent catalytic efficiency on cracking, isomerization and oligomerization/aromatization, attributed to their specific physicochemical properties, including a strong acidity and a micropore crystalline structure [ 44 ]. As illustrated in Table 3 , the plastic pyrolysis in the presence of catalysts, particularly HZSM5, tends to produce remarkably more aromatic and polycyclic aromatic hydrocarbons, compared to the uncatalyzed pyrolysis process, therefore contributing to the gasoline fraction. Further, a significantly higher production of gases is typically observed in the presence of zeolite catalysts, due to the enhanced cracking reactions [ 45 ]. Amorphous silica-alumina catalysts significantly contribute to the production of light olefins, with no noticeable changes in the aromatics formation [ 46 ]. ZSM-5 and zeolite-Y promote the formation of both aromatics and branched hydrocarbons, along with a significant increase in the proportion of gaseous hydrocarbons. These results are consistent with other published reports in the literature [ 47 , 48 ]. Catalytic reforming over Al-MCM-41 proactively contributes to the gasoline production with a lower impact on gas generation, likely due to the weaker acid properties and larger pore dimensions of the catalyst [ 1 ]. In the presence of both Y-zeolite and ZSM-5 catalysts, the oil yield dramatically decreases in favor of gas production [ 49 ]. The superiority of Y-zeolite compared to the ZSM-5, in terms of aromatic compounds production, is rationalized by the differences in physical and chemical catalyst properties, i.e., pore size, surface area, and surface acidity [ 50 ].

3.2. Fast Pyrolysis

Unlike slow pyrolysis, which is typically performed in the batch reactors, fast pyrolysis takes place in continuous systems. The faster char removal from the reactor space associated with continuous processes prevents undesirable catalytic effects leading to the excessive cracking of vapors, which, coupled with short vapor residence times, minimizing secondary cracking reactions, result in a higher liquid production. Table 4 indicates that fast pyrolysis can convert up to 95 wt.% of plastic waste into the liquid/wax product (e.g., pyrolysis of HDPE at 600 °C). In addition, PE yields a higher liquid/wax product compared to the PP. The waxy fraction (C20+) of fast pyrolysis is greater than that of slow pyrolysis, attributed to a shorter vapor residence time and, consequently, reduced cracking reactions. As previously mentioned, the waxy product, which is an excellent source of paraffins and olefins, can be used as a feedstock in FCC units for the production of transportation fuels and other valuable petrochemical compounds.

Fast pyrolysis of different plastic wastes.

Influence of Temperature

Temperature plays a key role in all pyrolysis processes, regardless of the feedstock type. In the pyrolysis of plastic wastes, as in any other pyrolysis process, the increase in temperature results in a rapid increase in gas yields from the enhanced cracking reactions and, correspondingly, in a decrease of the oil/wax yield ( Table 4 ). In addition to the alteration of yields, temperature expectedly affects the products quality, due to its impacts on the pyrolysis kinetic mechanisms. Generally speaking, high temperature favors the production of less waxy and more oily compounds production, attributed to the conversion of long-chain paraffins/olefins to shorter molecules. Conversely, the solid residue yield decreases at elevated temperatures. A qualitative assessment of plastic oil shows high temperature favors an increase in gasoline production corresponding to a higher concentration of aromatics [ 52 ]. The yield of ethylene and propylene are found increase as the temperature rises.

3.3. Flash Pyrolysis

In order to avoid over-cracking reactions during pyrolysis, especially at high temperatures (above 700 °C), which converts a significant amount of liquid to gaseous products, flash pyrolysis taking place within milliseconds is a suitable option. Unlike the fast pyrolysis of biomass, which generates the highest yields of bio-oil, flash pyrolysis of plastic waste produces more gas rather than liquids ( Table 5 ). As illustrated in Table 5 , up to 75 wt.% of monomers, i.e., ethylene and propylene, can be recovered through flash pyrolysis. The by-product, which is oil, can be used to provide the required energy for the process. Kannan et al. [ 56 ] performed a flash pyrolysis of LDPE in a microreactor with a minimal heat/mass transfer resistance at temperatures of 600–1000 °C, and vapor residence time of 250 ms to investigate the effect of temperature on monomer recovery (yield of olefins). They found that the 950–1000 °C temperature range is optimal to recover up to 68 wt.% of monomers.

Flash pyrolysis of LDPE with different experimental parameters.

Influence of Vapor Residence Time

Together with temperature, the vapor residence time in the hot zone (i.e. reactor) is the key parameter that significantly affects the pyrolysis products yields and compositions. Previous work [ 28 , 57 , 58 ] on the flash pyrolysis of different waste plastics in micro and large scale reactors indicated that short vapor residence time resulted in a high olefin content gas. Although the Internally Circulating Fluidized Bed (ICFB) reactor illustrated in Figure 1 has been shown to be suitable for thermochemical processes requiring short residence times, this capability is not achieved without cost [ 58 ]. The control of the residence time, particularly for large scale reactors, presents a significant challenge to the reactor designer. Nielsen et al. [ 60 ] invented a fenestrated centrifugal riser terminator for use in an ICFB and/or in conventional fluid bed reactor risers. This terminator can separate solids from gas in less than 20 ms with 99.5% separation efficiency. Such a fast separation is critically important when the objective is to minimize the vapor residence time at high temperature and rapidly quench the reaction ( Table 5 ).

Internally circulating fluidized bed (ICFB) and riser terminator (adapted from [ 58 , 60 ]).

4. Plastic Oil Cracking

Tsuji et al. [ 35 ] utilized a two-stage unit, including a liquid–liquid extraction followed by a pyrolysis reactor, to investigate the thermal cracking of pyrolysis plastic oils containing considerable amounts of aromatic compounds, such as styrene. In the first stage, sulfolane solvent was used to remove the aromatic compounds prior to the pyrolytic cracking, in order to mitigate the coking effects, since stable aromatics (e.g., styrene) tend to be coked rather than cracked during pyrolysis. The results were promising, and the gas yield reached 85 wt.% at 750 °C, corresponding to a 20 wt.% increase compared to non-extracted oil. A schematic diagram of the pyrolysis oil cracking set-up is shown in Figure 2 .

Schematic diagram of the experimental apparatus for cracking of raw plastic pyrolysis oil and extracted oil after separation of aromatics (adapted from Reference [ 35 ]).

The results of cracking of different plastic oils are summarized in Table 6 . SL and SM are the light and medium fractions of pyrolysis plastic oil obtained from Sapporo Plastic Recycle Co. DH and DL stand for heavy and light fractions of plastic oil obtained from Dohoh Recycle Center Co. in Japan. Model plastics waste oil was obtained in the lab [ 35 ] from the pyrolysis of mixed plastics at 450 °C. The analysis of data presented in Table 6 suggests that a higher cracking temperature (above 850 °C) and a lower vapor residence time (less than 730 ms) are required to potentially achieve the best monomer recoveries.

Pyrolysis of different plastic wastes [ 35 ].

5. Upgrading of Pyrolysis Plastic Oils

The comparison between diesel fuel and a sample plastic oil obtained from the pyrolysis of mixed plastics, including 58 wt.% of HDPE and LDPE, 27 wt.% of PP, 9 wt.% of PS, 5 wt.% of PET [ 61 ] is shown in Table 7 . The GC-MS results reveal that the carbon number distributions of the produced plastic oil is as follows: 35.41 wt.% of C6–C9, 48.40 wt.% of C10–C14, 13.21 wt.% of C15–C20, and 1.83 wt.% of C20+ [ 61 ]. The comparison between fuel properties of pyrolysis plastic oil and diesel indicates similar heating values and kinematic viscosity, while the plastic oil has a higher ash content and a lower cetane number compared to diesel fuel.

A comparison between plastic oil and diesel physicochemical properties [ 61 ].

* Composition: 35.41 wt.% of C6-C9, 48.40 wt.% of C10-C14, 13.21 wt.% of C15-C20, and 1.83 wt.% of C20+.

5.1. Blending

As discussed earlier, raw pyrolysis plastic oil has a high heating value (40–55 MJ/kg), a low water content (<1 wt.%), and approximately neutral pH. Therefore, boilers can readily burn it as-is for the electricity generation. Damodharan et al. [ 21 ] stated that diesel engines can smoothly run plastic oil and no modification is required to the existing engine infrastructure. In contrast, there are several researchers [ 8 , 62 , 63 , 64 ] who believe improvements in plastic oil quality are needed to meet EN590 standards. In terms of drawbacks associated with the utilization of pyrolysis plastic oil in internal combustion engines, a high heat release and delayed ignition have been reported [ 61 ]. As such, a blend of conventional fuel and plastic oil can be a potential solution. The fuel trials using blends in conventional engines reveal a stable performance with less emission of NOx and SOx compared to diesel and gasoline fuels [ 65 ]. A reduced specific fuel consumption has also been reported [ 61 ]. Awasthi and Gaikwad [ 66 ] stated that the overall performance of the blend of diesel and plastic oil in a single cylinder four stroke VCR diesel engine was very satisfactory, particularly with 20 wt.% of pyrolysis plastic oil. Singh et al. [ 61 ] experimentally showed that the blend of plastic oil with diesel up to 50% can be easily utilized in conventional diesel engines.

5.2. Hydrogenation

Hydrogenation process takes place in the presence of three components: hydrogen, a catalyst, and an unsaturated compound. The transfer of hydrogen pairs to the unsaturated compound is facilitated via a heterogeneous catalyst which enables the reaction to occur at a lower temperature and pressure. For instance, hydrogenation converts alkenes to alkanes in plastic oil [ 67 ]. Due to the significant presence of unsaturated compounds in the plastic oils, some storage instability challenges may be experienced over time. Hydrogenation of pyrolysis oil occurring at temperatures above 700 °C, pressures around 70 bar, and in the presence of catalyst (such as ZSM-5) can alter unsaturated compounds into saturated and makes the oil more stable. A combination of hydrogenating and blending have been suggested to upgrade the plastic oil quality in order to meet the EN590 standard [ 67 ]. The fuel properties of plastic oil, diesel, and hydrogenated plastic oil along with the EN590 standard are compared in Table 8 .

Physicochemical properties of diesel, plastic oil and hydrogenated plastic oil [ 67 ].

5.3. Liquid-Liquid Extraction

The high aromatic content of plastic oils, particularly those obtained from pyrolysis of mixed plastics containing polystyrene, leads to a decrease in the engine performance and an increase in emissions due to a long ignition delay [ 68 ]. Generally, the low cetane number fuels, caused by high aromatic content, are not suitable for conventional diesel engines as they can cause unstable combustion. As such, the aromatic compounds can be separated and removed via solvent extraction prior to the utilization of plastic oil in diesel engines. Sulfolane as solvent was proven effective by Tsuji et al. [ 35 ] for the separation of aromatic compounds.

6. Carbon Nanotubes

Carbon nanotubes (CNTs) have gained recognition as very attractive materials due to their unique properties, including great electrical conductivity (100 times greater than copper), excellent mechanical strength (100 times greater than steel), high thermal conductivity, stable chemical properties, extremely high thermal stability, and an ideal one-dimensional (1D) structure with anisotropy [ 69 , 70 , 71 , 72 ]. Conventionally, methane, natural gas, acetylene, and benzene from nonrenewable resources have been utilized as a feedstock for CNTs production. Recently, the potential fabrication of CNTs from the pyrolysis of plastic waste has drawn researchers’ attentions, adding a significant value to the plastic wastes. The process of converting plastic waste into CNTs is composed of two successive stages ( Figure 3 ). In the first stage, the plastic waste is converted to the volatile vapor in the absence of oxygen and at a moderate temperature (approximately 550 °C). Then, the produced vapor is introduced into the second stage at a high pressure of 1 MPa and a temperature of 750 °C in the presence of Ni-based catalyst where it is converted into CNTs on the surface of the catalyst through the chemical vapor deposition mechanism. In this advanced process, CNT yields can reach up to 25 wt.% [ 73 ]. In the second stage, during pyrolysis at 750 °C, plastic waste vapors further decompose to the mixtures of their monomers (e.g., ethylene, propylene, and styrene). These light gases serve as carbon donors for CNTs formation. Moreover, the produced vapors from the first stage contain a significant amount of hydrogen, which plays an undeniable role in the formation of CNTs. Hydrogen moderates the rate of carbon deposition and prevents catalyst deactivation and poisoning by continuous surface cleansing of the catalyst surfaces [ 69 , 70 , 71 , 72 ]. A SEM image of CNT growth on Ni-based catalyst is shown in Figure 4 .

Schematic diagram of two-stage pyrolysis reactor system (adapted from [ 73 ]).

CNT growth on Ni-based catalyst (adapted from [ 73 ]).

7. Conclusions

The pyrolytic conversion of plastic waste into value added products and/or fuels is extensively reviewed in this paper. Plastic waste, which can be a source of detrimental problems to terrestrial and marine ecosystems, can be thermochemically converted into valuable products, such as gasoline, diesel, and wax. Fast pyrolysis leads to the production of waxy hydrocarbon mixtures, whereas slow pyrolysis typically produces more oil than wax. This is attributed to a difference in the vapor residence times, since longer residence times in slow pyrolysis allow for more cracking reactions breaking down the larger molecules into smaller and lighter fragments. The utilization of catalyst in plastic pyrolysis favors aromatic compounds in the liquid phase and gas production. Higher pyrolysis temperatures result in enhanced secondary cracking reactions, and, therefore, in a greater conversion of waxy compounds to oily and gaseous products. PE and PP produce pyrolysis oils with more aliphatic compounds, while PS generates higher aromatic hydrocarbons. In flash pyrolysis, conducted at 1000 °C and 250 ms of vapor residence time, up to approximately 70 wt.% of olefin monomers including 50 wt.% of ethylene can be recovered, making it a promising process for monomer recovery. The plastic oil can be blended with diesel and utilized as a fuel in conventional diesel engines. A two-stage process, including a pyrolysis unit followed by a fixed bed reactor with a nickel-based catalyst can be utilized to convert up to 25 wt.% of plastic waste into very valuable carbon nanotubes.

Acknowledgments

The authors are grateful to the Natural Sciences and Engineering Council of Canada and to the industry sponsors of the NSERC Industrial Research Chair in “Thermochemical Conversion of Biomass and Waste to Bioindustrial Resources” for the financial support for this project.

Author Contributions

S.P. contributed to the writing, data analysis, and organization of the literature review on slow, and fast pyrolysis. H.B. contributed to the literature review and data analysis/organization of CNTs and application of the section on pyrolysis plastic oil. F.B. supervised the overall project and contributed to developing and expanding the whole sections of this manuscript, particularly to the flash pyrolysis of plastic waste and plastic oil cracking. All authors have read and agreed to the published version of the manuscript.

This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the industry partners involved in the NSERC Industrial Research Chair program entitled “Thermochemical Conversion of Biomass and Waste to Bioindustrial Resources”.

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The authors declare no conflict of interest.

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Characterization of Biomass and Studies of Pyrolysis on Rice Husk in a Lab-Scale Pyrolyzer: A Step Toward Environmental and Energy Sustainability

• First Online: 13 August 2024

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• Pushpa Jha   ORCID: orcid.org/0000-0002-9263-7206 3  

Part of the book series: Sustainable Development Goals Series ((SDGS))

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“Biomass characterization” describes biomass in terms of the characteristics relevant to its intended use. Biomass appropriateness may depend on physical and chemical factors, availability, acquisition cost, and disposal. Carbon is the main constituent of any biomass. The biomass materials selected for the characterization were bagasse, bamboo dust, coconut coir, cotton stalk, Acacia nilotica branches, Lantana , pine needles, groundnut shell, rice husk, and sal seed husk. Standard methods were used for the analysis. The vital characteristics that must be established for determining suitable biomass for various thermochemical operations (pyrolysis, gasification, and combustion) include proximate analysis, ultimate analysis, calorific values, ash fusion temperatures, and devolatilization rate. The assessment concluded that rice husk was the most sustainable raw material for the investigation of pyrolysis, a thermochemical process. The effects of the size of rice husk particles, heating rate, and the maximum temperature attained by the residues on the pyrolysis process were studied. Redfern and Coats equation was used to estimate the activation energy of pyrolysis. Various applications of rice husk char are suggested, which align with the sustainable development goals (SDGs) 3, 6, 7, 8, 9, 11, 12, 14, and 15. The chapter also elucidates the life cycle assessment (LCA) technique used to evaluate the environmental impact of converting rice husk into pelletized char, used as a sustainable solid fuel.

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Acknowledgments

The author expresses her deep gratitude toward the technical staff of “The Biomass Conversion Laboratory” of IIT Delhi for carrying out various tests and experiments mentioned in this chapter. Heartfelt thanks are also expressed toward the administrators of SLIET, Longowal, for their support in preparing the chapter.

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Jha, P. (2024). Characterization of Biomass and Studies of Pyrolysis on Rice Husk in a Lab-Scale Pyrolyzer: A Step Toward Environmental and Energy Sustainability. In: Nasr, M., Negm, A. (eds) Solid Waste Management. Sustainable Development Goals Series. Springer, Cham. https://doi.org/10.1007/978-3-031-60684-7_5

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Green Chemistry

Opening pathways for the conversion of woody biomass into sustainable aviation fuel via catalytic fast pyrolysis and hydrotreating.

Meeting aggressive decarbonization targets set by the International Civil Aviation Organization (ICAO) will require the rapid development of technologies to produce sustainable aviation fuel (SAF). Catalytic fast pyrolysis (CFP) can support these efforts by opening pathways for the conversion of woody biomass into an upgraded biogenic oil that can be further processed to SAF and other fuels. However, the absence of end-to-end experimental data for the process leads to uncertainty in the yield, product quality, costs, and sustainability of the pathway. The research presented here serves to address these needs through a series of integrated experimental campaigns in which real biomass feedstocks are converted to a final SAF product using large bench-scale continuous reactor systems. For these campaigns, the degree of catalytic upgrading during CFP was varied to produce CFP-oils with oxygen contents of 17 and 20 wt% on a dry basis. The CFP-oils were then hydrotreated and distilled into gasoline, diesel, and SAF fractions. Detailed yield and compositional data were obtained for each step of the process to inform technoeconomic and lifecycle analyses, and the fuel properties of the SAF fraction were evaluated to provide first-of-its-kind insight into the quality of the final product. This research reveals opportunities to optimize process carbon efficiency by tuning the degree of catalytic upgrading during the CFP step and highlights routes to produce a high-quality cycloalkane-rich SAF with 85-92% reduction in greenhouse gas emissions compared to fossil-based pathways.

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Journal Article Presents a Novel Approach to Measuring Thermal Conductivity of Materials for Pyrolysis Modeling

Fire safety engineers leverage fire models to predict the burning behavior of materials, which helps them design safer buildings and improve firefighting strategies. Accurate model predictions require an understanding of the thermal decomposition mechanism and a corresponding set of material properties, which may be measured using bench-scale tests. Of the required properties, the thermal conductivity of thermally reactive materials (e.g., wood and plastics) is notoriously difficult to measure at elevated temperatures using existing methodologies.  

A new peer-reviewed journal article from the Fire Safety Research Institute (FSRI), part of UL Research Institutes, was recently published in Fire and Materials to address this issue. This article presents and evaluates a method for measuring the thermal conductivity of thermally reactive materials in a manner that is compatible with contemporary pyrolysis models. The paper was authored by FSRI research engineers Matthew DiDomizio, Mark McKinnon, and Grayson Bellamy as part of the Thermal Decomposition of Materials research project. 

Measuring the thermal conductivity of thermally reactive materials

As a material is heated in an inert atmosphere, such as the conditions generated between a diffusion flame and a burning material in an open burning configuration, it will thermally decompose from its original species to a residual species (e.g., char). Intermediate species may also be produced during this process. In a contemporary pyrolysis model, the thermal conductivity must be defined for each distinct species. Thus, characterizing the thermal conductivity is essential for understanding the physics of decomposition. 

In light of this modeling paradigm, FSRI researchers developed a methodology for isolating and measuring the thermal conductivity of each distinct species. This involved a series of bench-scale tests, including:  

• thermogravimetric analysis to derive a reaction mechanism, identify distinct species of the decomposed material, and assess the thermal stability of those species;
• differential scanning calorimetry to derive heats of reaction and specific heat capacities;
• the production of gram-scale samples of each species using a controlled-atmosphere pyrolysis chamber; and
• laser flash analysis on gram-scale samples to measure the thermal conductivity of each species. 

The researchers selected eucalyptus hardboard, colloquially known as “masonite board” in some locales, to evaluate the effectiveness of this methodology. Using this method, the researchers successfully gathered all the necessary information about the material to accurately model how it burns. 

Evaluation of the measured material properties with a pyrolysis experiment and simulation

Next, researchers conducted experiments in which the mass and temperature of hardboard samples were measured as they were exposed to heat representative of a fire. They also developed a pyrolysis model of that experiment using the previously measured material properties. It was shown that the mass loss rate and temperature rise predicted with the model matched well with measurements in the experiments. These results demonstrated the suitability of the material properties, the pyrolysis model, and the overall utility of this approach. 

“We have demonstrated the value of a novel approach for measuring the thermal conductivity of materials in a way that aligns with an accompanying thermal decomposition process, ensuring that the measured data are compatible with contemporary pyrolysis models.” —Matt DiDomizio, research engineer, FSRI

By demonstrating the utility of this methodology, this study sets the foundation for measuring thermal conductivity of materials in future pyrolysis research. 

About Fire and Materials : 

Fire and Materials is the leading journal at the interface of fire safety and materials science. The publication covers all aspects of the fire properties of materials and their applications, including polymers, metals, ceramics, and natural products such as wood and cellulosics. Papers on all areas of fire safety science and engineering are welcomed, including those on passive and active fire prevention, modeling, fire retardant chemicals, human behaviour and wildland and large fires.

Material and Products Database

Access FSRI's repository of materials and products properties and fire test data.

Conducting an Uncertainty Analysis of Pyrolysis Models

Discover how to understand uncertainty in data and how that uncertainty impacts model predictions. 

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