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A review on gasification and
pyrolysis of waste plastics
Hamad Hussain Shah
1
, Muhammad Amin
2
, Amjad Iqbal
3
,
4
*,
Irfan Nadeem
5
, Mitjan Kalin
5
, Arsalan Muhammad Soomar
6
and
Ahmed M. Galal
7
,
8
1
Department of Engineering, University of Sannio, Benevento, Italy,
2
Department of Energy Systems
Engineering, Seoul National University, Seoul, Republic of Korea,
3
Department of Materials Technologies,
Faculty of Materials Engineering, Silesian University of Technology, Gliwice, Poland,
4
CEMMPRE - Centre for
Mechanical Engineering Materials and Processes, Department of Mechanical Engineering, Rua Luís Reis
Santos, Coimbra, Portugal,
5
Laboratory for Tribology and Interface Nanotechnology, Faculty of Mechanical
Engineering, University of Ljubljana, Ljubljana, Slovenia,
6
Faculty of Electrical and Control Engineering,
Gdańsk University of Technology, Gdańsk, Poland,
7
Mechanical Engineering Department, College of
Engineering, Prince Sattam Bin Abdulaziz University, Wadi ad-Dawasir, Saudi Arabia,
8
Production Engineering
and Mechanical Design Department, Faculty of Engineering, Mansoura University, Mansoura, Egypt
Gasification and pyrolysis are thermal processes for converting carbonaceous
substances into tar, ash, coke, char, and gas. Pyrolysis produces products such as
char, tar, and gas, while gasification transforms carbon-containing products (e.g., the
products from pyrolysis) into a primarily gaseous product. The composition of the
products and their relative quantities are highly dependent on the configuration of
the overall process and on the input fuel. Although in gasification, pyrolysis processes
also occur in many cases (yet prior to the gasification processes), gasification is a
common description for the overall technology. Pyrolysis, on the other hand, can be
used without going through the gasification process. The current study evaluates the
most common waste plastics valorization routes for producing gaseous and liquid
products, as well as the key process specifications that affected the end final
products. The reactor type, temperatures, residence time, pressure, the fluidizing
gas type, the flow rate, and catalysts were all investigated in this study. Pyrolysis and
waste gasification, on the other hand, are expected to become more common in the
future. One explanation for this is that public opinion on the incineration of waste in
some countries is a main impediment to the development of new incineration
capacity. However, an exceptional capability of gasification and pyrolysis over
incineration to conserve waste chemical energy is also essential.
KEYWORDS
gasification, pyrolysis, plastic waste, valorization, chemistry
OPEN ACCESS
EDITED BY
Mina Mazzeo,
University of Salerno, Italy
REVIEWED BY
Chiara Costabile,
University of Salerno, Italy
Sajjad Hussain,
Ghulam Ishaq Khan Institute of
Engineering Sciences and Technology,
Pakistan
*CORRESPONDENCE
Amjad Iqbal,
amjad.iqbal@polsl.pl
SPECIALTY SECTION
This article was submitted
to Polymer Chemistry,
a section of the journal
Frontiers in Chemistry
RECEIVED 03 June 2022
ACCEPTED 16 December 2022
PUBLISHED 03 February 2023
CITATION
Shah HH, Amin M, Iqbal A, Nadeem I,
Kalin M, Soomar AM and Galal AM (2023), A
review on gasification and pyrolysis of
waste plastics.
Front. Chem. 10:960894.
doi: 10.3389/fchem.2022.960894
COPYRIGHT
© 2023 Shah, Amin, Iqbal, Nadeem, Kalin,
Soomar and Galal. 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.
Abbreviations: MSW, municipal solid waste; PE, polyethylene; HDPE, high-density polyethylene; FBR,
fluidized bed reactor; CBF, circulating fluidized bed; BFB, bubbling fluidized bed; DFB, dual fluidized bed;
CSBR, conical spouted bed reactor; PP, polypropylene; LDPE, low-density polyethylene; MJ, mega joules;
EDAX, energy-dispersive X-ray spectroscopy; WGS, water–gas shift; ER, equivalence ratio; PET, polyethylene
terephthalate; PVC, polyvinyl chloride; PS, polystyrene; PETRA, the PET Resin Association; EPA,
Environmental Protection Agency; GC-MS, gas chromatography–mass spectroscopy; HIP, high-impact
polystyrene; RPM, revolutions per minute; TG, thermogravimetry analysis; DTG, derivative thermogravimetry
analysis; SEM, scanning electron microscope; FFC, fluid catalytic cracking; SDG, sustainable
development goal.
Frontiers in Chemistry frontiersin.org01
TYPE Review
PUBLISHED 03 February 2023
DOI 10.3389/fchem.2022.960894
1 Introduction
Plastics are adaptable, flexible, and lightweight, allowing them to be
used in a wide variety of applications. In recent years, the political
agenda has focused on the economic, environmental, and social
influences of plastics, with an emphasis on sustainable
manufacturing and the decoupling of negative ecological outcomes
from waste generation. Waste plastics disposal has become a
significant global environmental issue. Around 55 million tons of
postconsumer plastic waste are produced annually in the
United States, Japan, and Europe (Sun et al., 2021). Previously, these
waste products were discarded in landfills, which was an unsustainable
and environmentally unsound practice. Furthermore, the number of
landfill sites and their capabilities are steadily declining, and landfill
regulation is becoming more stringent in most countries. Recycling is
being considered as another option for managing plastic waste in order
to reduce its disposal in landfills. Because of the restrictions on water
pollution and insufficient separation prior to recycling, which is labor
intensive, recycling plastic has proved difficult and expensive (Jaafar
et al., 2022). Since plastics come in a variety of colors, resin compounds,
and transparencies, separation is required. Plastics that are pigmented or
dyed typically have a lower market value. Manufacturers choose clear
transparent plastics because they can be colored and turned into new
goods, giving them more flexibility (Thompson, 2022). Recycling plastic
has become difficult in recent years due to the strict requirements for
obtaining high-value products.
The disposal of plastic waste presents a significant problem that
must be tackled immediately. As a result, plastics’low degradability
poses significant ecological issues, particularly in marine
environments (de Sousa, 2021). Furthermore, insufficient waste
plastics management contributes to environmental concerns due to
the depletion of essential and limited resources obtained from
petroleum. As a result, in recent years, public policies aimed at
strengthening waste plastics management have been promoted. In
fact, in Europe over the last decade, the quantity of plastic waste
disposed of in landfills has decreased by 38% while the fraction of
waste plastics used for energy valorization and recycling has
increased by 46% and 64%, respectively (Plastics, 2016).
Although the situation with waste plastics management in
developed countries is slowly improving, it is still far from
satisfactory, and in developed countries, plastics management is
obviously less promising. Different methods, such as reuse,
recycling, energy recovery, and waste minimization are being
considered with the goal of minimizingthevolumeofwastethat
is disposed of in landfills. However, neither minimization nor reuse
has been extensively utilized in the case of waste plastics (Aguado
et al., 2008). Combustion is a viable valorization route due to the
high calorific value of plastics, but it is hampered by the emissions
generated (Thimoteo et al., 2022). Chemical recycling routes have
been the best chance of being implemented on a wide scale because
these permit the formation of syngas/hydrogen, chemicals, and
fuels from plastic waste. Figure 1 depictsthemajorchemical
valorization pathways for waste plastics. Pyrolysis of waste
plastics is widely recognized as the most efficient method for
producing chemicals and fuels from plastic waste (Aguado et al.,
2008), (Al-Salem et al., 2009;Al-Salem et al., 2010;Butler et al.,
2011;Wong et al., 2015;Anuar Sharuddin et al., 2016;Kunwar
et al., 2016;Ma et al., 2016;Yu et al., 2016;Lopez et al., 2017).
FIGURE 1
Primary chemical routes for plastic waste valorization.
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Shah et al. 10.3389/fchem.2022.960894
The solid waste incineration is an attractive technology for thermal
energy generation and reducing the volume of landfill waste. However,
municipal waste incineration involves climate-relevant emissions
(CO
2
, SOx, NOx, and N
2
O). One tonne incineration of municipal
waste generates about 0.7–1.7 tonnes of CO
2
, thus making significant
greenhouse effect contribution. When compared to other
conventional plastic recycling techniques (such as gasification and
pyrolysis), the energy produced by incineration has significantly high
emissions of greenhouse gases (340 g CO
2
eq per kWh). Therefore,
waste incineration is not an environmentally friendly alterative due to
subsequent greenhouse gas emissions.
Various plastic pyrolysis processes have been developed for the
selective processing of waxes (Berrueco et al., 2002;Arabiourrutia
et al., 2012a;Yansaneh and Zein, 2022), light olefins (Milne et al., 1999;
Mastral et al., 2006a;Hernandez et al., 2007;Elordi et al., 2011;Artetxe
et al., 2013a), and monomers (Achilias et al., 2007;Mo et al., 2014).
Furthermore, in recent years, plastic waste and biomass co-pyrolysis
have gained a lot of attention (Xue et al., 2015;Zhang et al., 2016a).
Despite the growing interest in plastic waste pyrolysis, it is still in the
developmental stages of implementation (Butler et al., 2011). Waste
plastics and their derivatives, such as pyrolysis wax oil products, can
also be fed into traditional refinery units to produce fuels (Arandes
et al., 1997;Lopez et al., 2017;Lovás et al., 2017;Palos et al., 2022a).
Numerous studies have been conducted on the potential of
different plastic types for gasification and pyrolysis procedures to
produce gas and liquid products. It is important to note that the setup
parameters have a significant impact on product quality and yield.
Therefore, this review concentrates on the various plastic gasification
and pyrolysis processes that have been investigated along with the key
factors that affect these processes and those that require attention in
order to maximize the production of gas and liquid oil and improve
the quality of the final product. The primary parameters include
pressure, residence time, the reactor type, temperature, the use of
various catalysts, and the type and flow rate of the fluidizing gas. The
obtained results from various valorization methodologies have been
compared, and their potential values have been discussed critically.
Furthermore, this study also presents some important discussion
concerning product yield optimization.
2 Gasification
By partial oxidation with a gasification agent, gasification refers
to the chemical and thermal conversion of carbon-based materials
into a primarily gaseous output (usually air, oxygen, or steam). If
gasification is preceded by pyrolysis, the pyrolysis outputs (gas, tar,
and char) can be improved further by partial oxidation of the more
complex hydrocarbons, particularly those found in the char
and tar.
Temperature range from 800 to 1,100°C when using air as an
oxidant, and up to 1,500°C when using oxygen. While most
gasification processes are exothermal, that is, they generate heat,
some of the associated reactions are endothermal and require heat,
which could be provided by steam as the gasification agent. In general,
the products of gasification are
Solid: non-volatile metals and other inorganic elements are found
in ashes. Solids may account for 30–50% of the input weight.
Liquid: smaller amounts of oil and tar, about 10–20% by weight of
the input, are used in some conditions.
Gas: same as pyrolysis gas but with higher CO
2
fractions. The
heating value varies depending on the gasification agent, but it is
usually 3–12 MJ/Nm
3
with oxygen as the gasification agent. By weight
of the supply, the gas yield can range from 30 to 60% (Belgiorno et al.,
2003;Hu et al., 2021;Tezer et al., 2022).
Like pyrolysis products, gasification products are strongly
influenced by the temperature, waste input, and overall process
framework. The waste input, in particular, is often
underrepresented in the literature, and the waste is frequently
composed of distinct industrial segments instead of mixed MSW.
The heating value for the gas output can therefore be considered as the
upper limit for MSW. Char and tar formed by pyrolysis reactions are
further converted to CO
2
, CO, CH
4
, and H
2
by heating to higher
temperatures than pyrolysis and adding a gasification agent. The
gasification agent used has a considerable impact on the processed
gas composition, and “dilution”from the gasification agent has a
substantial impact on the gas heating value, again contingent on the
agent (medium) used. For example, air gasification is less expensive
than using pure oxygen as a gasification agent but produces a gas that
contains up to 60% nitrogen (Tezer et al., 2022).
3 Chemical reactors for gasification of
plastic waste
Plastic waste gasification processes are exactly the same as those
used to gasify other feedstocks such as coal and biomass. However, the
unique properties of plastic wastes, particularly their high volatility
and high thermal resistivity; sticky, viscous, and adhesive nature; and
exceptional tar production, obstruct their processing in traditional
gasification technologies and pose a significant challenge for process
realization. As a result, an adequate gasifier design for plastic handling
must incorporate the following characteristics: it should 1) be capable
of providing high rates of heat transfer aiming to facilitate rapid
depolymerization of plastic waste, 2) evade operative issues caused by
the sticky and adhesive behavior of plastics by maintaining a tight
control over the operating parameters and conditions, 3) have
adequate residence time dispensation to favor the cracking of tar
and enable the use of primary (fundamental) catalyst in situ while
maintaining virtuous contact with the catalyst.
Traditional waste gasification systems are fixed bed, entrained
flow, downdraft, updraft, fluidized bed, plasma reactor, and rotary kiln
(Heidenreich and Foscolo, 2015;Ahmad et al., 2016;Mahinpey and
Gomez, 2016;Molino et al., 2016;Ud Din and Zainal, 2016;
Sansaniwal et al., 2017). However, because of the complexities of
waste plastics, some of these technologies have been limited in their
application. Each gasification system is available in a number of basic
configurations, each with benefits for a specific product or feedstock
applications. Each system type’s basic design revolves around the
reaction chamber with feedstock insertion, but each has a unique air
entry, heating mechanism, and syngas removal area.
3.1 Spouted conical bed gasifier
Conical spouted reactors are a substitute for heterogeneous
fluidized beds (FBRs) for waste valorization processes due to their
unique characteristics. As a result, these reactors have high mass and
heat transfer rates, appropriate fluidization, and excellent solid mixing
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(Makibar et al., 2011). Furthermore, their dynamic solid cyclic
circulation eliminates agglomeration and de-fluidization issues and
facilitates the manipulation of irregular and discrete particulates,
particles with a wide distribution size, and adhesive substances. In
gasification processes, the primary drawbacks are the volatiles’short
residence (stay) time, which impedes the cracking tar reactions
(Erkiaga et al., 2014). In bench-scale units, this technology is
extensively applied in the pyrolysis of various solid wastes (Lopez
et al., 2009;Lopez et al., 2010;Amutio et al., 2012;Artetxe et al., 2013a;
Alvarez et al., 2015). Furthermore, the biomass pyrolysis process has
been effectively generalized up to 25 kg/h (Fernandez-Akarregi et al.,
2013;Makibar et al., 2015). The first time the spouted beds were used
in the gasification processes, coal was used as the feedstock (Foong
et al., 1981;Teo and Watkinson, 1986;Sueaquan et al., 1995;
Fernandez et al., 2022). Gasification of different feedstocks (raw
materials) has recently been added to this technology, such as
waste plastics and biomass (Erkiaga et al., 2013a;Erkiaga et al.,
2013b;Bernocco et al., 2013;Erkiaga et al., 2014;Lopez et al.,
2015a;McCullough et al., 2015). To decrease the content of tar
and improve the efficiency of the process in the gaseous product,
various primary (fundamental) catalysts have been investigated in situ
(Erkiaga et al., 2013a;Erkiaga et al., 2013b), or in a second reactor,
secondary catalysts have been utilized (Lopez et al., 2015b). Figure 2
depicts a spouted conical bed gasifier design.
3.2 Fixed (packed) bed reactor
Packed bed reactors are used in the gasification of plastic because of
the ease in their operation and design, and their low investment cost, with
the key problem being scaling up, limited gas–solid contact, continuous
operation, and a low heat transfer rate. There are many different designs
of fixed-bed reactors, but they all have one thing in common: they are
used in small-scale units (Ahmed and Gupta, 2009;Wu and Williams,
2010a;Wu and Williams, 2010b;Wu and Williams, 2010c;Friengfung
et al., 2014;Parparita et al., 2015;Baloch et al., 2016). Usually, plastic waste
gasification (Ponzio et al., 2006;HeMXiao et al., 2009a;Wang et al., 2012;
Lee et al., 2014;Ongen, 2016) or their coprocessing with biomass and coal
(Straka and Bicáková, 2014a;Akkache et al., 2016;Singh et al., 2022)in
fixed-bed reactors has received little attention. Ahmed and Gupta (2009)
used a laboratory-scale fixed-bed (packed) reactor operating in the batch
mode for steam co-gasification of polystyrene and plastic–wood samples
(Singh et al., 2022). Moreover, experiments were performed in a bench-
scale fixed-bed reactor designed by HeMXiao et al. (2009b) at a plastic
continuous feed rate of 0.3 kg/h, and the impact of utilizing reforming in
situ Ni/Al
2
O
3
catalyst was investigated. Li et al. (2012) also developed a
similar continuous-mode experimental setup for MSW steam gasification.
Lee et al.( 2014) conducted their research in a semi-batch laboratory-scale
reactor with a steam (condensation) atmosphere. Guo et al. (2015,2016)
investigated polyurethane air gasification by utilizing various in situ
catalysts in a laboratory-scale fixed-bed (packed) reactor.
3.3 Fluidized bed reactors
In gasification processes, two classes of fluidized bed reactors have
traditionally been utilized: circulating fluidized beds (CFBs) and bubbling
fluidized beds (BFBs) (Mahinpey and Gomez, 2016;Molino et al., 2016).
Despite the intriguing characteristics of CFBs for the gasification of plastic
waste operations, particularly the ability to achieve low tar and high
conversion yields (McKendry, 2002), plastic gasification research has
been limited to BFBs. The primary benefits of BFBs are their excellent
gas–solid contact, high mass and heat transfer rates, good temperature
control and flexibility, and good solid-mixing regime. Their primary
drawbacks are their limitations in particle size both in feed and bed,
high investment cost, unreacted material entrainment, and defluidization
issues (Molino et al., 2016). These reactors run in a continuous mode and
have a high scale and development degree, with various research being
conducted in pilot plant scale units (Arena et al., 2010;Arena et al., 2011;
Ruoppolo et al., 2012;Martínez-Lera et al., 2013a;Wilk and Hofbauer,
2013;Arena and Di Gregorio, 2014;Brachi et al., 2014;Narobe et al., 2014).
In the co-gasification with coal and biomass or in plastic waste gasification,
these are generally used with air as the gasifying medium (Sancho et al.,
2008;Kim et al., 2011;Toledo et al., 2011;Ruoppolo et al., 2012;Cho et al.,
2013a;Martínez-Lera et al., 2013b;Martínez-Lera et al., 2013c;Arena and
Di Gregorio, 2014;Brachi et al., 2014). Despite the low gas heating value
obtained, this approach offers functional benefits like lesser content of tar in
the product gas and autothermal process (Gil et al., 1999;Devi et al., 2003).
Mastellone and Arena (2008),Arena et al. (2009),andArena et al. (2010)
conducted research with continuous feed rates in a pilot plant up to 100 kg/
h, while gasifiers have been employed by other researchers with feed rates of
plastic ranging from 1 to 4 kg/h, running in a continuous mode (Xiao et al.,
2007;Sancho et al., 2008;Toledo et al., 2011). Because steam gasification is
considerably endothermic, it has high requirements of energy that are
resolved in biomass gasification by utilizing dual fluidized bed (DFB)
reactors, which combine a fast fluidized bed puffed with air with a steam-
blown fluidized bed, where the residual char is burned (Goransson et al.,
2011;Schneider et al., 2022). The research group led by Prof. Hofbauer used
this operating methodology to gasify waste plastics in a pilot plant with a
capacity of 15 kg/h (Martínez-Lera et al., 2013a;Narobe et al., 2014).
However, as a result of the low yield of char and the problem in maintaining
the heat balance between the combustion and gasification operations, issues
may arise. Figure 3 depicts various types of gasifier schemes.
3.4 Plasma gasification reactors
The primarily use of plasma gasification is in industries where
hazardous waste is disposed of at relatively high temperatures. The
FIGURE 2
Spouted conical bed gasifier.
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plasma torch in the gasifier (Figure 4) generates high temperatures (up
to 10,000°F). There are two plasma gasification arrangements available
depending on where the plasma torch is used in the gasification
process. The first is plasma-assisted gasification, and the second is
plasma-assisted gasification combined with traditional thermal
gasification. This methodology has been utilized sparingly for the
gasification of plastic waste, and studies have usually been conducted
on a small scale (Tang and Huang, 2007;Rutberg et al., 2013a;
Gibadullina et al., 2015;VishwajeetPawlak-Kruczek et al., 2022).
However, the level of development achieved by Hlina et al. (2014)
in their gasification unit, which works in a continuous mode with
11 kg/h plastic feed rate, is remarkable. Park et al. (2016) proposed
combining continuous pyrolysis processes with 1.3 kg/h feed rate in a
plasma reactor with gasification–pyrolysis (in-line) of volatiles.
4 Temperature and heating rate
The temperature reached in the reactor is critical because
temperature changes affect the majority of the chemical
reactions for waste conversion. Higher temperatures, in general,
alleviate lower tar content and higher carbon conversion in the
waste in the gas phase, but in the case of gasification, a lower
heating value of the gas may also result. In pyrolysis, higher
temperatures produce more gas, while lower temperatures
produce more liquid. Figure 5 depicts a relationship between
temperature and output products, demonstrating that
temperature is a very important factor and that uniform
distribution of temperature across the reactor is crucial.
Another factor that affects the outputs significantly is the heating
rate (Hu et al., 2021). Char generation is increased by slow heating
rates combined with relatively low final temperatures (e.g., slow
heating at relatively low temperatures is required for the
production of charcoal from wood). Mild heating rates up to mild
temperatures give a more even weight distribution of pyrolysis
outputs. High heating rates to high temperatures, possibly
accompanied by rapid quenching, are commonly referred to as
FIGURE 3
Gasification system types.
FIGURE 4
Plasma gasification (Oliveira et al. 2022).
FIGURE 5
Reactor temperature (°C; RT) and mass yields (wt%, MY) correlation
(Tezer et al., 2022).
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flash pyrolysis and can result primarily in a liquid product; however,
the oils can be further broken down to enhance the gas output, if
quenching is evading. Slow heating rates to high final temperatures
typically result in a primarily gaseous product.
Some gasification processes use steam as the gasification agent and
operate at high pressures (up to about 20 bar). High pressure favors
the gas yield, though these processes may be circumscribed in their use
with fuel as waste.
5 Gasification mechanism
The plastics gasification aims for the highest possible conversion to a
syngas or gas product, with char and tar being the most unwanted
derivatives. Gasification is a complex process and consists of many
chemical reactions. Figure 6 depicts these steps. The importance of these
steps in terms of kinetics and process performance is determined by the
gasification conditions and feedstock properties. The main steps of
gasification are
•Drying: around temperatures between 20 and 100°C, moisture is
converted into steam. The feedstock is not decomposed, and no
chemical reaction occurs at these temperatures. The
predominant part of the gasification system is feedstock with
a moisture content ranging from 10 to 20% for a high calorific
value of produced gas.
•Pyrolysis: is devolatilization (thermal degradation), at
temperatures between 150 and 700°C in the absence of
oxygen, of the dry feedstock, liberating the volatile elements
and a residue consisting of ash and char. The produced volatiles
are a mixture of hydrogen, CO
2
, tar, CO, water vapor, and light
hydrocarbons.
•Oxidation: in a gasification scenario, various oxidation
chemical reactions occur, liberating the heat required for
endothermic reactions. Carbon dioxide is produced due to
the reaction between oxygen and char. Water is produced
by oxidizing the hydrogen in the feedstock.
Substoichiometric amounts of oxygen are present; partial
oxidation of carbon may transpire, ensuing carbon
monoxide production.
•Reduction: due to the consumption of oxygen in oxidation
reactions, several chemical reactions, primarily endothermic
ones occur in the absence of O
2
.CH
4
, CO, and H
2
are the
reduction reactions’main products.
The following is a list of the most important chemical reactions
that take place during the gasification process:
Carbon reactions C +CO2→2CO +172 MJ kmol (1)
Water −gas or steam C +H2O→CO +H2+131 MJ kmol (2)
Hydrogasif ication C +2H2→CH4–74.8MJkmol (3)
C+0.5O2→CH4–111 MJ kmol (4)
Oxidation reactions C +O2→CO2–394 MJ kmol (5)
CO +0.5O2→CO2–284 MJ kmol (6)
CH4+2O2→CO2–803 ML kmol (7)
H2+0.5O2→H2O–242 MJ kmol (8)
Shift reaction
CO +H2O→CO2+H2O–41 MJ kmol (9)
Methanation reaction
2CO +2H2→CH4+CO2–247 MJ kmol (10)
CO +3H2→CH4+H2O–206 MJ kmol (11)
CO2+4H2→CH4+2H
2O–165 MJ kmol (12)
Steam reforming reaction
CH4+0.5O2→CO +2H2–36 MJ kmol (13)
CH4+H2O→CO +3H2+206 MJ kmol (14)
The gasification process, as far as can be determined, is
globally endothermic, with the required heat obtained in one
of the two ways: direct (autothermal) gasification occurs when
heat is generated inside the reactor as a result of exothermic
reactions, while indirect (allothermal) gasification occurs when
the required heat is generated outside of the reactor (Milhé et al.,
2013).
FIGURE 6
Potential gasification routes (Aguado et al., 2008).
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6 Processes involved in gasification of
plastic waste
Valorization of waste plastics through gasification processes has
been considered using a variety of schemes, with the goal of producing
syngas of various compositions and potential applications. Research
on the gasification of waste plastics is still in its early stages, and the
number of studies is restricted. On the other hand, investigations on
biomass and coal co-gasification have been conducted.
Due to process simplification, air gasification is the most widely used
process as there are no external energy prerequisites. Moreover, as
compared with steam gasification, tar content is typically lower in the
gasproducts(Gil et al., 1999). As a result, this gas is primarily used in the
production of energy (Arena et al., 2010;Arena, 2012). Steam gasification
produces an H
2
-rich syngas with high ratios of H
2
/CO, which is more
suitable for chemical synthesis applications than direct air gasification
syngas (Erkiaga et al., 2013a). The main difficulty with this alternative is
the amount of heat that must be introduced into the reactor in order to
perpetrate the endothermic steam reforming reactions.
Direct air gasification is the utmost investigated of these,
compassing a gas product with a comparatively low heating value
because of the diluting result of nitrogen.
Gasification with pure O
2
is an alternative to air and steam that
combines the benefits of both gasifying agents. Although, due to the
operating costs and high capital assets for air separation, this choice is
more expensive and complex for medium-size utilizations in particular
(Xiao et al., 2007). Recently, pyrolysis–reforming (in-line) of pyrolysis
volatiles has been intended as a favorable waste plastics H
2
production
TABLE 1 Different gas compositions obtained by authors in steam plastics waste gasification.
Plastic
type
Reactor Reaction
conditions
Bed
material
Composition of
gas (% vol)
Gas
produced
(m
3
/kg)
LHV
(MJ/m
3
)
Tar
content
(g/m
3
)
References
Waste plastics Plasma reactor Gasifying agent:
steam/O
2
T: 1,200
—CO: 34, H
2
: 62, CH
4
:–,
CO
2
:–
3.5 10.1 —Rutberg et al.
(2013b)
PE Spouted bed reactor
(0.1 kg h
−1
)
T: 900, S/P: 1 Olivine CO: 27, H
2
: 58, CH
4
:7,
CO
2
:3
3.2 16.2 15 Erkiaga et al.
(2013a)
PE Spouted bed reactor T: 900, S/P: 1 γ-Alumina CO: 26, H
2
: 59, CH
4
:8,
CO
2
:2
3.3 16.2 16.1 Erkiaga et al.
(2013a)
PE Two steps: Spouted
bed plus packed bed
reactor (0.1 kg h
−1
)
T: 900/600–700,
S/P: 1
Olivine/
NiCa-Al
2
O
4
CO: 8–12, H
2
:71–73,
CH
4
:3–0.3, CO
2
:17–15
4.4–5.6 —0Lopez et al.
(2015b)
PET Semi-batch and fixed
(packed) bed reactor
T: 1,000 —CO: 6, H
2
: 61, CH
4
:2,
CO
2
:12
—7.8 —Lee et al. (2014)
PS + PE Fluidized bed (dual)
(15 kg h
−1
)
T: 850, S/P: 1.8 Olivine CO: 24, H
2
: 52, CH
4
: 12,
CO
2
:7
1.4 17 110 Martínez-Lera
et al. (2013a)
PET + PE Fluidized bed (dual)
(15 kg h
−1
)
T: 850, S/P: 1.2 Olivine CO: 20, H
2
: 27, CH
4
: 15,
CO
2
:29
1 16.4 160 Martínez-Lera
et al. (2013a)
PE + PP Fluidized bed (dual)
(15 kg h
−1
)
T: 850, S/P: 2.0 Olivine CO: 22, H
2
: 46, CH
4
: 16,
CO
2
:5
2.1 19.4 30 Martínez-Lera
et al. (2013a)
PP Fluidized bed (dual)
(15 kg h
−1
)
T: 850, S/P: 2.0 Olivine CO: 4, H
2
: 34, CH
4
: 40,
CO
2
:8
1 27.2 180 Martínez-Lera
et al. (2013a)
PE Fluidized bed (dual)
(15 kg h
−1
)
T: 850, S/P: 2.0 Olivine CO: 7, H
2
: 38, CH
4
: 30,
CO
2
:8
1.2 25.8 190 Martínez-Lera
et al. (2013a)
HDPE Fixed (packed) batch
bed (0.1 g)
Gasifying agent:
steam/O
2
1:1,
T: 850
Ni-dolomite CO: 43, H
2
: 35, CH
4
: 11,
CO
2
:10
2.4 —17 Friengfung et al.
(2014)
PS Fixed (packed) batch
bed (0.1 g)
Gasifying agent:
steam/O
2
1:1,
T: 850
Ni-dolomite CO: 43, H
2
: 29, CH
4
: 1.7,
CO
2
:26
1.3 —290 Friengfung et al.
(2014)
PP Fixed (packed) batch
bed (0.1 g)
Gasifying agent:
steam/O
2
1:1,
T: 850
Ni-dolomite CO: 45, H
2
: 38, CH
4
:9,
CO
2
:8
1.9 —140 Friengfung et al.
(2014)
Plastic waste Fixed (packed) batch
bed (0.1 g)
T: 850 (15 °C/min) —CO: 19, H
2
: 44, CH
4
: 20,
CO
2
:13
—20.4 —Akkache et al.
(2016)
Plastic waste
and refuse
paper
Fixed (packed) batch
bed (0.1 g)
T: 900 —CO: 22, H
2
: 38, CH
4
: 12,
CO
2
:17
0.9 17.9 —Hwang et al.
(2014)
PW waste Fixed (packed) bed
(0.3 kg h
−1
)
T: 700–900,
S/P: 1.33
Ni/γ-Al
2
O
3
CO: 20–27, H
2
:17–37,
CH
4
:21–10, CO
2
:35–21
1.22–2.04 12.4–11.3 106–13 HeMXiao et al.
(2009a)
Frontiers in Chemistry frontiersin.org07
Shah et al. 10.3389/fchem.2022.960894
valorization route (Czernik and French, 2006;Wu and Williams, 2010a;
Park et al., 2010;Namioka et al., 2011;Barbarias et al., 2016a;Barbarias
et al., 2016b). Furthermore, this alternative makes use of highly active
reforming catalysts, which enable the production of tar-free syngas,
overcoming the key problem in standard gasification of plastics.
6.1 Steam gasification
Plastic steam gasification has received little attention in the
literature. In contrast to air gasification studies, which have almost
entirely been conducted in fluidized bed reactors, plastics waste
steam gasificationhasbeeninvestigatedinvariousreactortypes
(Table 1), such as fluidized beds (FBRs) (Martínez-Lera et al.,
2013a), fixed (packed) bed (HeMXiao et al., 2009a;Wang et al.,
2012;Friengfung et al., 2014;Lee et al., 2014), and conical spouted
beds reactors (CSBRs) (Erkiaga et al., 2013a;Lopez et al., 2015b).
Heat requirement and the content of tar in the product gas are the
challenges that steam gasification faces. To overcome this
limitation, Wilk and Hofbauer (2013) investigated steam
gasificationofvariousplasticsinadualfluidized bed reactor,
with a 100-kW pilot plant. At 850°C, the gasification reactor
runs an in situ primary catalyst of olivine with an S/P ratio of 2.
Erkiaga et al. (2013a) investigated the HDPE steam gasification in
a spouted bed conical continuous bench scale reactor (0.1 kg/h)
operating at temperatures ranging from 800 to 900°C. Operating at
temperatures above 850°C and with an S/P of 1, the product stream
H
2
content was slightly higher than 60%, accounting for an 18 wt%
production. Because of the decrease in hydrocarbon content, the gas
heating value decreased from 19.3 to 15.4 MJ/m
3
as the gasification
temperature was raised. At the highest temperature investigated for
an inert sand bed, a minimal tar content of 16.8 g/m
3
was obtained,
and this tar was interestingly composed primarily of single-ring
aromatics. In the syngas, the tar content was slightly reduced and had
little effect on the gas composition, by utilizing γ-alumina and
olivine as the primary catalysts. The same authors used Ni
reforming commercial catalyst with a fixed-bed (packed) reactor
connected in-line with the spouted conical bed gasifier in a
subsequent study (Lopez et al., 2015b). The operating
temperature of the fixed bed is between 600 and 700°Cwith
gasification experimental parameters being the same as those used
in a previous study. The production of H
2
increased up to 36.5 wt%
by the addition of a catalytic reforming step and also enabled the full
reforming of tar and hydrocarbons.
The gasification of PP and PE generates syngas with up to 40% H
2
concentration, accordingly with 4–3 wt% of H
2
production rates (gH
2
100 g/plastic). However, the most notable aspect of the composition
gas product were the high concentrations of CH
4
(40% and 30%,
respectively) and C
2
H
4
(11% and 15%) in the PP and PE gasification.
The heating value of the produced gas up to 25 MJ/m
3
due to high
hydrocarbon content. However, the high concentration of light
hydrocarbons and methane as previously investigated by other
authors is a clear indication for the presence of tar (Pohorelyl
et al., 2006;Mastellone and Arena, 2008;Pinto et al., 2009a;
Mastellone et al., 2010a), and for both plastics, the values of tar
content were higher than 120 g/m
3
, with naphthalene as the
prevailing compound. In utilizing the same experimental
parameters in biomass gasification, lower tar values have been
reported by the same authors (Schneider et al., 2022).
Using an Ni-Al
2
O
3
catalyst, HeMXiao et al. (2009a) investigated
the PE gasification (0.3 kg/h) with 1.33 S/P ratio between 700 and
900°Cinafixed (packed) bed reactor. The production and
concentration of H
2
improved significantly to 3.7 and 6.6 wt%;
conversion of plastic improved with temperature increase; and at
900 C, gases’yield reached 2.04 m
3
/kg. On steaming after 3 h time, no
deactivation was evident by reforming the (Ni-based) catalyst. The gas
product heating value ranged from 12.3 to 11.4 MJ/m
3
, at the lowest
temperature, with the highest value being obtained.
Dou et al. (2016) recently conceived a laboratory-scale
continuous reaction system consisting of a fluidized bed (FBR)
gasifier followed by CO
2
/steam reforming adsorption in a moving
bed reactor. The combination of steam reforming on CO
2
retention
on CaO and a Ni-Al
2
O
3
catalyst resulted in the high production of
H
2
; however, they discovered that below 700°C, adsorption of CO
2
was only effective.
In the literature, the values of H
2
production with high
concentrations of H
2
vary between 3 and 18 wt% (g 100 g/plastic)
of polyolefins steam gasification (HeMXiao et al., 2009a;Erkiaga et al.,
2013a;Martínez-Lera et al., 2013a). Furthermore, the syngas obtained
is suitable for the synthesis of various fuels (methanol, DME, and
hydrocarbons) (Zhang, 2010). Temperature is the most critical and
important parameter in the steam gasification of plastics. Its increase
facilitates the cracking and reforming of endothermic reactions that
include tar and light hydrocarbons, which facilitates the yield of both
gas (Figure 7A) and H
2
(Figure 7B). However, for synthesis
applications in the gaseous stream, the tar content must be
considerably decreased to achieve stringent tar content constraints
(Devi et al., 2003). Steam gasification of plastic waste, as previously
reported, results in high concentrations of tar in the gas product, even
exceeding 100 g/m
3
(HeMXiao et al., 2009a;Martínez-Lera et al.,
2013a). In fact, it is widely acknowledged that air gasification
results in less tar than that obtained through steam gasification
(Gil et al., 1999;Devi et al., 2003), and as compared with the
gasification of biomass and coal, the gasification of plastic waste
yields more tar (Pinto et al., 2009b;Mastellone et al., 2010b;
Martínez-Lera et al., 2013a).
By using fixed-bed batch reactor, Friengfung et al. (2014)
studied the laboratory-scale gasification of steam/O
2
(0.1 g of
sample) of various plastics. Byutilizing(Ni-impregnated)
dolomite and dolomite at 850°C, the experiments were carried
out without a catalyst. In all cases, the tar production was higher
(more than 80 wt%) and the results obtained without a catalyst with
various polyolefins, PP, LDPE, and HDPE were poor. In the HDPE
case, promising results were achieved by utilizing Ni-impregnated
dolomite catalyst for which a tar production of 10 wt% or below
was achieved. The gasification efficiency is enhanced by utilizing a
dolomite catalyst, but the tar production was on the higher side
(more than 50 wt%). For full-scale development, steam gasification
faces considerable challenges due to its high process heat
requirement, however N
2
absence improves the gas heating
value over 15 MJ/m
3
(Erkiaga et al., 2013a;Martínez-Lera et al.,
2013a;Hwang et al., 2014). In fact, the well-designed dual fluidized
beds scheme is also scarce by the low fixed carbon of waste plastics,
which impedes the heat balance closure process (Wilk and
Hofbauer, 2013;Schneider et al., 2022). Generally, steam
gasification of waste plastics has received little attention and
development and is not as advanced and promising when
compared to air gasification.
Frontiers in Chemistry frontiersin.org08
Shah et al. 10.3389/fchem.2022.960894
6.2 Air gasification
The main challenge of gasification of plastic processes, regardless
of the gasifying agent utilized, is the yield of the gas product tar,
though when O
2
or air is utilized in the place of steam, the tar yield is
lower (Gil et al., 1999;Devi et al., 2003). Thus, the content of tar must
be less than 10 mg N/m
3
for the utilization of syngas for the
production of energy in turbines and engines but much lower for
synthesis applications (Devi et al., 2003). Deposition in the process
equipments, especially in heat exchangers, and the characteristics of
tar, mainly its dew point, play a vital role in the problems that it causes
(Guan et al., 2016). The dew point is determined by the amount of tar
present, and its composition, since single-ring aromatic hydrocarbons
are non-condensable even at concentrations of 10 g N/m
3
. At the
concentration of just 1 mg N/m
3
, polyaromatics with more than four
rings condense, resulting in serious operational problems (Anis and
Zainal, 2011).
Air gasification studies on plastic waste have primarily been
conducted in fluidized bed reactors (FBRs), with substantial
advancement in experimental units, especially the bench scale or
pilot plants functioning in a continuous mode. Table 2 summarizes
the important outcomes in air gasification of plastic waste. Air
gasification has been broadly examined by the research group of
Prof. Arena. They used plastic mixtures and different plastics in a
pilot plant fluidized bubbling bed with a surmised capacity between
30 and 100 kg/h (Mastellone and Arena, 2008;Arena et al., 2009;
Arena et al., 2010;Arena et al., 2011;Arena and Di Gregorio, 2014).
Their early research focused on PE waste gasification with equivalence
ratios (ERs) in between 0.21 and 0.33 at 850°C to investigate the role of
olivine as the main catalyst for tar diminution (Mastellone and Arena,
2008;Arena et al., 2009). The gasification process efficiency improved
significantly with the use of olivine, resulting in significant tar content
reduction in the product gas. This result is linked not only to direct tar
cracking but also to the removal of its promoters, i.e., light olefins. By
improving the reforming reactions, the composition of the gas was also
improved, resulting in a significant increase in H
2
content. Thus, in
experiments using inert silica powder, the content of tar in the product
gas was about 100 g N/m
3
, while when calcined olivine was utilized in
situ as the catalyst, the tar was almost completely removed. The
efficiency of carbon conversion, or the fraction of carbon in the
feed that is altered into products in the stream outlet, has been
shown to increase the overall process output when olivine is used.
At low ERs, this parameter increased by 60%–66%, while at high ERs,
it increased by 70%–82%. In the gas product, the increase in
equivalence ratio had a positive impact on the content of tar yield.
However, the dilution effect due to the increased gas output for high
ER values may also be a factor. The same authors have equated the
gasification efficiency of various plastic waste mixtures retrieved from
MSW and postconsumer packaging in a subsequent study (Arena
et al., 2010). The in situ waste gasification of a mixture of polyolefin
with olivine yields a gas fraction composition, process efficiency, and
tar yield that are close to those which have result with pure PE,
demonstrating the versatility of this valorization path. Poor results
however have been obtained in the case of complex plastic mixture
gasification with low process efficiencies and high tar yields. This is
due to the reduction in the performance of the primary olivine catalyst.
In a bench scale two-step unit, Kim et al. (2011) investigated air
gasification with a continuous feed rate of 0.50 kg/h of plastic waste
mixture composed of polyolefins and other waste plastics (PET, PVC,
and PS). Both phases were conducted at about 800°Cinfluidized bed
reactors, with the first containing sand, followed by the second, i.e., tar
cracking catalysts. Dolomite and activated carbon were among the
catalysts investigated, with activated carbon proving to be a better
option for tar removal. Apart from reducing the tar content, the
utilization of activated carbon as a primary catalyst significantly
improved the content of H
2
in gas products. Based on
experimental parameters, the tar yields ranged from 3 to 7 wt%
with the impact of catalytic bed mass being particularly noticeable.
The same authors suggested a similar approach in a subsequent study
conducted under similar conditions, but they substituted sand with
olivine in the first bed and dolomite as the primary cracking catalyst
(Cho et al., 2013a;Cho et al., 2013b). The fraction of gas composition
improved significantly with the utilization of dolomite. Furthermore,
combining both these catalysts in the first bed with active carbon in the
second bed provided a tar yield of less than 2 wt%. In bubbling
fluidized bed gasifier (4 kg/h bench scale), Xiao et al. (2007)
FIGURE 7
(A) Temperature effect on yield of gas in steam plastic waste gasification. (B) Temperature effect on H
2
production.
Frontiers in Chemistry frontiersin.org09
Shah et al. 10.3389/fchem.2022.960894
TABLE 2 Gas compositions achieved by various researchers in air gasification of plastics waste.
Plastic type Reactor Reaction
Conditions
(°C)
Bed
material
Composition of
gas (% vol)
Gas
produced
(m
3
/ kg)
LHV
(MJ/
m
3
)
Tar
Yield
(g/m
3
)
References
Plastic waste Fixed (packed) bed
(0.06 kg/h)
T:700–900,
ER: 0.4
- CO: 0.2–4, H
2
:0–2, CH
4
:
21–20, CO
2
:5–7
- 7.8–818–12 Kaewpengkrow et al.
(2012)
Mixture of waste
plastic
Moving grate *
fueled with pure O
2
(80 kg/h)
T:700–900, ER:
0.15–0.6
- CO: 22–33, H
2
:41–29,
CH
4
: 4.3–10, CO
2
: 8.2–22
1.2-1.5 9.0–11.8 - Lee et al. (2013)
PE Bubbling fluidized
(aggregative) bed
(100 kg/h)
T: 845–897, ER:
0.20–0.31
Sand CO: 2.8–2.2, H
2
: 9.1–9.5,
CH
4
: 10.4–7.1, CO
2
:
9.1–10.4
3–4.3 7.9–6.3 160–81 Arena et al., (2010)
PE Bubbling fluidized
(aggregative) bed
(100 kg/h)
T: 807–850, ER:
0.2–0.29
olivine CO: 18.4–20.9, H
2
:
30.1–29.1, CH
4
: 3.4–1.5,
CO
2
:1.6–1.2,
4.2-6.2 7.6–6.3 0 Arena et al., (2010)
Mixture of waste
plastic
Bubbling fluidized
(aggregative) bed
(100 kg/h)
T: 869–914, ER:
0.22–0.31
Olivine CO: 3.7–4.8, H
2
: 6.8–6.6,
CH
4
: 7.3–6.3, CO
2
:
11.1–11.6
2.5-3.2 6.8–5.2 99-56 Arena et al., (2010)
Mixed waste
(polyolefins)
Bubbling fluidized
(aggregative) bed
(100 kg/h)
T: 887, ER: 0.25 Olivine CO: 4.5, H
2
: 5.9, CH
4
: 6.6,
CO
2
: 10.3
3.3 6.6 59 Arena and Di
Gregorio, 2014
Mixed cellulosic
and plastic waste
Bubbling fluidized
(aggregative) bed
(100 kg/h)
T: 869, ER: 0.24 Olivine CO: 6.6, H
2
: 6.0, CH
4
: 6.5,
CO
2
: 12.7
2.73 7.4 34 Arena and Di
Gregorio, 2014
Recycled plastic
waste from
packaging
Bubbling fluidized
(aggregative) bed (5
kg/h)
T: 887, ER: 0.25 Silica sand CO: 6.6, H
2
: 6.0, CH
4
: 6.5,
CO
2
: 12.7
3.5 7.9 46 Zaccariello and
Mastellone, 2015
PP Fluidized bed
(FBR) (1/kg h)
T: 850, ER:
0.32–0.36
Sand CO: 5, H
2
:5,CH
4
:3,
CO
2
:12
4.5 2.9 17 Sancho et al. (2008)
PP Fluidized bed
(FBR) (1/kg h)
T: 850, ER:
0.32–0.36
70%
sand−30%
dolomite
CO: 7, H
2
:6,CH
4
:8,
CO
2
:16
5.3 7.4 1.5 Sancho et al. (2008)
PP Fluidized bed
(FBR) (1/kg h)
T: 850, ER:
0.32–0.36
70%
sand−30%
olivine
CO: 4, H
2
:5,CH
4
:7,
CO
2
:14
2.9 5.8 10 Sancho et al. (2008)
PP Fluidized bed
(FBR) (1/kg h)
T: 850, ER:
0.32–0.36
olivine CO: 8, H
2
: 10, CH
4
:7,
CO
2
:11
662Sancho et al. (2008)
PP Fluidized bed
(FBR) (4/kg h)
T: 690–950, ER:
0.2–0.45
bottom ash CO: 20–15, H
2
:4–5, CH
4
:
6–4, CO
2
:9–15
2-3.8 11.3–5.2 40-1.3 Xiao et al. (2007)
Mixture of
plastic waste
Fluidized bed plus
fixed bed (0.5 kg/h)
T: 800/830, ER: 0.2 olivine/active
carbon
CO: 6.7, H
2
: 27.1, CH
4
:
6.4, CO
2
: 8.5
- 5.8 - Cho et al. (2013b)
Mixture of
plastic waste
Fluidized bed plus
fixed bed (0.5 kg/h)
T: 800/800, ER: 0.2 silica sand/
dolomite
CO: 6.6, H
2
: 14.2, CH
4
:
15.7, CO
2
: 4.0
- 13.4 - Kim et al. (2011)
Mixture of
plastic waste
Fluidized bed plus
fixed bed (0.5 kg/h)
T: 800/800, ER: 0.2 silica sand/
active carbon
CO: 6.7, H
2
: 15.2, CH
4
:
14.8, CO
2
: 4.5
- 13.2 - Kim et al. (2011)
PE Bubbling fluidized
(aggregative) bed
(1 kg/h)
T: 750, ER: 0.3 silica sand CO: 6.1, H
2
: 2.7, CH
4
: 7.0,
CO
2
: 8.8
3.6 3.9 128 Martínez-Lera
et al. (2013b)
Polyolefins waste Bubbling fluidized
(aggregative) bed
(1 kg/h)
T: 750, ER:
0.25–0.35
silica sand CO: 8.5–10, H
2
:3,CH
4
:
8.5-10, CO
2
: 7.8–6.5
3.2–4.4 4.9–5.7 150-55 Martínez-Lera
et al. (2013b)
PE waste Bubbling fluidized
(aggregative) bed
(1 kg/h)
T: 750, ER: 0.3 silica sand CO: 8.7, H
2
:3,CH
4
: 8.7,
CO
2
: 7.4
3.7 4.9 102 Martínez-Lera
et al. (2013b)
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Shah et al. 10.3389/fchem.2022.960894
investigated the impact of various operating variables like equivalence
ratio, gas velocity, and residence time on PP air gasification. The
presence of Fe, Al, Mg, and Ca caused tar cracking activity in the
bottom ash from a boiler. The most important variable analyzed was
ER, which induced a substantial increase in the temperature of the
gasifier from 705 to 917 °C when ER was increased from 0.23 to 0.47.
Furthermore, in the ER range investigated, the gas product tar content
decreased from 40.3 to 0.25 g N/m
3
. A higher yield of gas and the high
temperature were both responsible for this reduction. For high ER
values, the authors found that the equivalence ratio should be
thoroughly calibrated to prevent a decrease in the heating value of
the gas product.
At 850°C, Sancho et al. (2008) studied PP air gasification in a
continuous fluidized bed reactor (bench scale) with an equivalence
ratio of about 0.35 at 1 kg/h. This study evaluated the catalytic
efficiency of dolomite and olivine as the primary catalysts and
compared the findings to those procured with inert sand. They
found that the use of dolomite is restricted by its low physical
ability, which drives it to be ejected from the gasifier. Moreover,
olivine has material characteristics that make it ideal for use in
fluidized beds, with a catalytic activity that is just marginally lower
than dolomite. As a result of the use of olivine, the content of tar in the
product gas was decreased from 17 g N/m
3
achieved with sand to
2 g N/m
3
. Furthermore, olivine facilitates reforming hydrocarbon
reactions, which increases the amount of hydrogen in the syngas.
The same authors went on to investigate the use of olivine in PP air
gasification, demonstrating olivine permanence over long gasification
runs (Toledo et al., 2011). Furthermore, the values of the equivalence
ratio were dropped from 0.37 to 0.24 to increase the heating value of
the gas product while maintaining the tar content at a low. This was
accomplished by raising the gasifier freeboard region temperature up
to 915°C by using an external heat source.
In a moving grate pilot plant gasifier, Lee et al. (2013) studied the
gasification of waste plastics with an output of 80 kg/h. The gasifying
agent used was pure oxygen. Under these parameters, the ideal
equivalence ratio was between 0.30 and 0.45, and the gas yield was
from 1.35 to 1.48 m
3
/kg with the heating value above 10 MJ/m
3
. Plastic
waste air gasification is an intriguing option for producing a gas stream
adequate for a variety of energy applications, the most viable one being
electricity generation in engines and turbines (Heikkinen et al., 2004).
As shown in Figure 8A (ER 0.2 and 0.45), the heating value is 3–12 MJ/
m
3
of produced gas. This heating value is primarily influenced by two
factors: 1) equivalence ratio and 2) waste plastics composition. In the
gasification of plastic waste, the heating value (average) is
approximately 6–8 MJ/m
3
(Table 2).
The air gasification of pure PP, PE, and PE waste has been studied
by Martínez-Lera et al. (2013c) in a bubbling fluidized bed bench-scale
gasifier with a capacity of 1 kg/h. The bed was composed of inert silica
sand with an equivalence ratio of 0.25–0.35, and the experiments were
carried out at 750°C. Pure PP and PE gasification produced similar gas
compositions and yields. However, waste PE gasification produced
better results than pure polyolefins gasification. As a result, the gas
yield achieved with PE waste was 92.7%, while that of pure PE was
90.6%, with the tar content difference being more substantial. The tar
content obtained from pure PE and waste plastics was 127 g N/m
3
and
103 g N/m
3
. Despite the fact that the ER was only changed to a small
degree (0.25–0.35), it had a significant impact on the process
efficiency, especially tar yield. As a result, it was lower from
around 150 g N/m
3
to below 60 g N/m
3
in the case of PE waste. A
semiempirical model was developed by Martínez-Lera and Pallarés
Ranz (2017) for polyolefin in FBR gasification, with the model
predictions confirmed by previously described findings and others
from the literature.
The equivalence ratio is undoubtedly the most significant
parameter in terms of impact on air gasification operating
conditions since it specifies the composition and yield of the gas
(Xiao et al., 2007;Martínez-Lera et al., 2013c). Increased ER
contributes to higher gas production, but it also reduces the gas
heating value (Figure 8B). In the gas product, the presence of N
2
increases with an increase in ER value, and the combustion of CH
4
,
CO, and H
2
and the resulting increase in CO
2
. An increase in the ER
usually reduces the gas product tar content, which not only increases
the gasifier temperature but also the volumetric gas yield.
The gasifier’s design is also essential for improving tar removal
quality. To favor the cracking of tar in FBRs, an increase in
temperature and residence time in the freeboard area is typically
sought (Toledo et al., 2011;Martínez-Lera et al., 2013b). In fluidized
bed gasifiers, the feed location also affects the tar yield (Wilk et al.,
FIGURE 8
(A) ER effect on gas LHV product in air plastic waste gasification. (B) ER effect on the yield of gas by Lee et al. (2013).
Frontiers in Chemistry frontiersin.org11
Shah et al. 10.3389/fchem.2022.960894
TABLE 3 Gas compositions obtained by authors in the plastics waste co-gasification.
Plastic type Reactor Bed
material
Gasifying
agent
Reaction
Conditions
(°C, -)
Composition of
gas (% vol)
Gas
yield
(m
3
/kg)
LHV
(MJ/
m
3
)
Tar
yield
(g/m
3
)
Reference
PE(0.3)/wood
pellets (0.7)
Fluidized bed (dual)
(15 kg/h)
olivine steam S/F: 1.6, T: 850 CO: 23, H
2
: 41, CH
4
:
14, CO
2
:16
1.9 16 47 Wilk and
Hofbauer,
(2013)
MSW
plastic(0.5)/
wood
pellets (0.5)
Fluidized bed (dual)
(15 kg/h)
olivine steam S/F: 0.94, T: 850 CO: 24, H
2
: 35, CH
4
:
6, CO
2
:19
1.1 16 39 Wilk and
Hofbauer,
(2013)
PE(0.33)/
lignite (0.66)
Fluidized bed (dual)
(15 kg/h)
olivine steam S/F: 0.90, T: 850 CO: 24, H
2
: 45, CH
4
:
8, CO
2
:10
-139Kern et al.
(2013)
Wood (0.2)/
recycled
plastic (0.8)
Bubbling Fluidized
(aggregative) bed
(5 kg/h)
SiO
2
air T: 872, ER: 0.25 CO: 7, H
2
: 10, CH
4
:8,
CO
2
:11
3.4 7 34 Zaccariell o and
Mastellone,
(2015)
wood (0.2)/
Recycled plastic
(0.3)/ coal (0.5)
Bubbling Fluidized
(aggregative) bed
(5 kg/h)
SiO
2
air T: 868, ER: 0.25 CO: 13, H
2
: 14, CH
4
:
2, CO
2
:14
2.7 6 41 Zaccariell o and
Mastellone,
(2015)
Wood(0.5)/
HDPE(0.5)/
PE (0.5)
Spouted (conical) bed
reactor (0.1 kg/h)
olivine steam S//F: 1.00, T: 900, CO: 27, H
2
: 57, CH
4
:
6, CO
2
:7
2.64 - 9.7 Lopez et al.
(2015a)
PE(0.5)/coconut
shell (0.5)
Fluidized bed/fixed
(packed) bed (2/kg)
Commercial
Ni catalyst/
dolomite
steam S/F: 2, T: 800/600 CO: 9, H
2
: 82, CH
4
:7,
CO
2
:2
2.7 12.4 0 Alipour
Moghadam
Esfahani et al.
(2017)
PE (0.5)/rice
straw (0.5)
Fixed (packed) bed - steam T: 900 CO: 30, H
2
: 46, CH
4
:12 CO
2
: 12,
1.1 13.9 - Baloch et al.
(2016)
Wood and
biomass paper
fiber(0.45)/
waste
polyolefins
(0.55)
Updraft (60kg/h) - air ER: 0.19-0.24, T:
800–930
CO: 15–14, H
2
:
10–15, CH
4
: 6-5,
CO
2
:8
2.6-3.4 9.5-79 22-11.2 Ponzio et al.
(2006)
Biomass (0.5)/
PP (0.5)
Dual Fixed bed
(0.04g)
Fe-CeO
2
steam T: 850/700 CO: 5, H
2
: 40, CH
4
:6,
CO
2
:16
2.55 35.5 - Parparita et al.
(2015)
PET (0.5)/
wood (0.5)
Fluidized
(heterogeneous) bed
reactor
olivine air ER: 0.19-0.31, T:
725-875
CO: 13-9, H
2
: 4.3-5.4,
CH
4
: 3-2.7, CO
2
:17
- 4.5-3.5 145-63 Robinson et al.
2016
PE(0.2)/ rice
husk (0.8)
Fluidized
(heterogeneous) bed
reactor (0.3 kg/h)
- oxygen ER:0.20, T:850 CO: 12, H
2
: 38, CH
4
:
12, CO
2
:37
11312Pinto et al.
(2016)
PE(0.2)/ rice
husk (0.8)
Fluidized
(heterogeneous) bed
reactor (0.3 kg/h)
- air ER:0.20, T:850 CO: 24, H
2
: 19, CH
4
:
13, CO
2
:33
1.3 8 12 Pinto et al.
(2016)
PE(0.2)/ rice
husk (0.8)
Fluidized
(heterogeneous) bed
reactor (0.3 kg/h)
- steam S/F: 1, T:850 CO: 15, H
2
: 41, CH
4
:
11, CO
2
:24
0.35 13 15 Pinto et al.
(2016)
PE (0.1)/ pine
wood (0.9)
Fluidized
(heterogeneous) bed
reactor (0.75 kg/h)
- steam S/F: 0.8, T:
740–880
CO: 34–31, H
2
:
25–44, CH
4
:15–10,
CO
2
:14–9,
0.63–1.28 21–15 - Pinto et al.
(2002)
PE (0.1)/
coal (0.9)
Fluidized
(heterogeneous) bed
reactor (6 kg/h)
- steam/air S/F: 0.85, ER: 0.2,
T: 850
CO: 17, H
2
: 40, CH
4
:
17, CO
2
:16
1.3 - 19 Pinto et al.
(2009b)
PE (0.2)/pine
wood (0.2)/
coal (0.6)
Fluidized
(heterogeneous) bed
reactor (5.5 kg/h)
- steam/air Air/ F: 1.14, S/F:
1, T: 740–880
CO: 18–17, H
2
:
25–40, CH
4
:18–15,
CO
2
:24–20,
0.6–1.35 24–18 - Pinto et al.
(2003)
(Continued on following page)
Frontiers in Chemistry frontiersin.org12
Shah et al. 10.3389/fchem.2022.960894
2013;Brachi et al., 2014). Secondary air injections in the gasifier’s free
board are another popular technique for improving tar cracking and
increasing the temperature in this region (Narváez et al., 1996;Pan
et al., 1999). In a plastic waste air gasification, the amount of tar in the
gas produced by different researchers varies greatly and depends on
various factors, i.e., catalyst utilization, design of the reactor, the
composition of plastics, and experimental parameters, in particular
residence time, temperature, and equivalence ratio (Table 2). In
general, the contents of tar are higher than biomass gasification
(Pinto et al., 2009b;Mastellone et al., 2010b;Pinto et al., 2016),
whose average value in the FBR reactors is 10 g/m
3
(Anis and
Zainal, 2011).
Since the content of tar has a significant impact on the direct
use of the gas generated, various strategies for eliminating or
reducing it have been suggested. As a result, using a primary
catalyst in situ reduces the tar content of the gas component
substantially (Sancho et al., 2008;Arena et al., 2009;Toledo
et al., 2011). While in tar cracking, dolomite is more effective
than olivine (Rapagna et al., 2000;Corella et al., 2004;Sancho et al.,
2008;de Andres et al., 2011). In FBR reactors, olivine is the more
commonly utilized catalyst because of its refined mechanical
characteristics (Sancho et al., 2008;Arena et al., 2010;Toledo
et al., 2011;Arena and Di Gregorio, 2014). The olivine catalytic
function is generally linked to the content of iron (II) oxide (Kumar
and Singh, 2011), with interest stemming from not only the
enhanced removal of tar promoters but also the ability of the
catalyst in the direct cracking of tar, preventing further formation
oftarinthegasification system (Arena et al., 2010;Schneider et al.,
2022). Different catalysts, like active carbon (Kim et al., 2011;Cho
et al., 2013a;Cho et al., 2013b), zeolite (Cho et al., 2014), dolomite
(Kim et al., 2011;Cho et al., 2013b), and active carbon filled with Ni
(Cho et al., 2015), have been proposed for catalytic cracking of tar
in secondary beds. Furthermore, for the removal of tar from the gas
product, electrostatic precipitators and filters have been
recommended (Kim et al., 2011;Cho et al., 2013a).
6.3 Co-gasification
The degree to which the product distribution is dependent on the
composition of the feed is a notable differentiation between
gasification and pyrolysis processes. As a result, the yield and
composition of the products derived from pyrolysis of various solid
wastes are extremely different. Moreover, the variations in gasification
of various feed materials are limited to the composition of gas and
small byproduct yields such as char and tar. The analysis of waste
plastics co-gasification has been aided by the flexibility of the
gasification process, and the higher advancement level of the
gasification of biomass and coal.
Pinto et al. (2003) used a fluidized bed gasifier (5.5 kg/h) to
investigate coal air/steam co-gasification with lower concentrations
of PE and biomass (20% each). Plastic co-feeding increased the
hydrocarbon content in the product gas; however, this result could
be prevented by working at higher ERs or temperatures. A similar
pattern was observed in the formation of tar. In order to achieve an
appropriate performance for each mixture of feedstock, they found
that the gasifier operating parameters had to be thoroughly calibrated.
The same authors were able to fully eliminate tar by using two
secondary fixed-bed (packed) tar cracking reactors, the first of
which used dolomite and the second of which used Ni-Al
2
O
3
(Pinto et al., 2009b). Surprisingly, holding unwanted halogen and
sulfur mixtures in the dolomite bed bettered the durability and
TABLE 3 (Continued) Gas compositions obtained by authors in the plastics waste co-gasification.
Plastic type Reactor Bed
material
Gasifying
agent
Reaction
Conditions
(°C, -)
Composition of
gas (% vol)
Gas
yield
(m
3
/kg)
LHV
(MJ/
m
3
)
Tar
yield
(g/m
3
)
Reference
PE (0.2)/ pine
wood(0.8)
Fluidized
(heterogeneous) bed
reactor (5 kg/h)
quartz sand air ER: 0.23, T: 780 CO: 16, H
2
: 17, CH
4
:
12, CO
2
:15
- 7.3 60 Ruoppolo et al.
(2012)
PE (0.2)/ pine
wood(0.8)
Fluidized
(heterogeneous) bed
reactor (5 kg/h)
Ni-γAl
2
O
3
air ER: 0.23, T: 780 CO: 14, H
2
: 30, CH
4
:
3CO
2
:10
- 6.5 27 Ruoppolo et al.
(2012)
Polyolefins waste
(0.4)/ coal (0.6)
Fluidized
(heterogeneous) bed
reactor (4 kg/h)
sand-dolomite air ER: 0.36, T: 850 CO: 22, H
2
: 40, CH
4
:
5.5, CO
2
:16
2.9 8.3 1.3 Aznar et al.
(2006)
biomass (0.2)/
polyolefins waste
(0.2)/coal (0.6)
Fluidized
(heterogeneous) bed
reactor (4 kg/h)
sand-dolomite air ER: 0.36, T: 850 CO: 12, H
2
: 11, CH
4
:
2, CO
2
:14
3 5.5 1 Aznar et al.
(2006)
PET (0.25)/ olive
husk (0.75)
Fluidized
(heterogeneous) bed
reactor (5 kg/h)
γ-Al
2
O
3
steam/air S/F: 0.76, ER: 0.1,
T: 752
CO: 13, H
2
: 33, CH
4:
9, CO
2
:19
1.3 10.2 90 Brachi et al.
(2014)
PET (0.25)/ olive
husk (0.75)
Fluidized
(heterogeneous) bed
reactor (5 kg/h)
Ni-γAl
2
O
3
steam/air S/F: 0.62, ER: 0.1,
T: 845
CO: 22, H
2
: 40, CH
4
:
5.5 CO
2
:16
1.4 9 29 Brachi et al.
(2014)
Frontiers in Chemistry frontiersin.org13
Shah et al. 10.3389/fchem.2022.960894
performance of the Ni-based catalyst. Pinto et al. recently investigated
rice husk co-gasification (80%)/PE (20%) in a fluidized bed gasifier
utilizing various gasifying agents, such as air, pure oxygen, steam, and
mixtures of these agents (Kaewpengkrow et al., 2012;Pinto et al.,
2016). The findings show that working with steam and pure oxygen
produces the best gas, but that the usage of pure oxygen is restricted
due to high oxygen production cost, thereby considering enriched air
as a viable alternative.
Pinto et al. (2002) investigated the PE/biomass mixture steam
gasification (continuous) in an FBR. The PE maximum content
studied (60%) resulted in effective conversion, as demonstrated by
a particular gas yield and a heating value, i.e., 1.96 kg/m
3
and 18.3 MJ/
m
3
. Furthermore, an increase in PE feed resulted in an increase in
methane concentrations and H
2
(to 52%) on the one side, but a
decrease in CO
2
and CO concentrations on the other.
Despite the fact that plastic waste has mostly been co-gasified with
biomass (Pinto et al., 2002;Wilk and Hofbauer, 2013;Alvarez et al., 2014;
Narobe et al., 2014;Lopez et al., 2015a;Zaccariello and Mastellone, 2015;
Arena and Di Gregorio, 2016;Singh et al., 2022), it has also been co-
processed with ternary mixtures (Ahmed and Gupta, 2011;Jung et al.,
2013;Lopez et al., 2016)andcoal(Mastellone et al., 2010b;Kriz and
Bicakova, 2011;Straka and Bicáková, 2014b). Steam, air, or their mixtures
were used as the gasifying agent in these experiments. Table 3 summarizes
the key findings in the co-gasification of plastic waste.
At 900°C in a laboratory fixed-bed batch reactor, Ahmed and
Gupta (2011) studied PE steam co-gasification and wood chips. In the
co-processing of biomass and plastics, they also discussed the
synergistic impact on gas yields, hydrocarbons, and hydrogen, as
well as on thermal performance. Furthermore, in the feed, the
optimized content of plastic was found to be within 65 and 80%.
Lopez et al. (2015a) confirmed the previously recorded synergistic
effects in a spouted bed conical gasifier (0.1 kg/h) using biomass and
PE co-gasification. This effect is particularly noticeable at a 1/
1 blending ratio.
The gas product tar content of a 1/1 mixture of biomass and PE
gasification was decreased to 9.5 N/m
3
with an S/F ratio of 1, by
utilizing a primary olivine catalyst operating at 900 °C. Furthermore,
while the gas yield (2.67 kg/m
3
) was close to the theoretical value
predicted in accordance with the results achieved for biomass and PE
particular feeds, a synergistic impact on the char yield reduction and
H
2
content in the syngas was observed.
By utilizing olivine as the bed material in the dual fluidized bed
gasifier (15 kg/h), Wilk and Hofbauer (2013) investigated biomass
pellets steam co-gasification with various waste plastics types and their
mixtures (such as PE). Thus, a 16 MJ/m
3
LHV value of 1.6 m
3
/kg gas
yield was reported for 1/1 ratio of blended HDPE/biomass, which is
significantly less than that obtained with pure plastic. Moreover, when
plastics and biomass were co-gasified, a synergistic effect on the
formation of tar was observed, with the tar contents being less
than that predicted based on their particular gasification.
Furthermore, tar composition was also affected by an increase in
the content of plastic in the feed thus lowering furan and phenol while
enhancing naphthalene content. Similarly, by utilizing different
blending ratios, non-linear patterns were perceived, and the
composition of the gas product cannot be directly evaluated from
the outcomes achieved with particular feedstocks. The impact of
lignite co-feeding in the PE steam gasification was investigated by
the same authors (Kern et al., 2013). Furthermore, lignite co-feeding
was found to have a synergistic impact on cold gas efficacy, and lignite
co-feeding also enabled a reduction in the content of tar when
contrasting with those results from pure plastic.
In a fluidized bed pilot scale gasifier, Ruoppolo et al. (2012)
explored pellets gasification containing 20% PE and 80% wood,
and correlated the results to those from pure biomass. Ni-Al
2
O
3
and inert quartzite catalyst were utilized as bed materials. Mixtures
of air and air/steam were utilized as gasifying agents, and they
discovered that by improving the reforming reactions, air/steam
mixtures resulted in a higher hydrogen concentration and a lower
content of tar. The high concentration of H
2
obtained during PE
pellets gasification was the most promising result (30% vol.). Despite
their utilization of comparatively low Ni-Al
2
O
3
catalyst and the
content of plastic in the pellets, the tar content as compared with
biomass (below 30 g N/m
3
) was significantly higher (around 46 g N/
m
3
). Therefore, the above synergies in steam gasification were
apparently less pronounced when air was used as the gasifying
agent. The same authors investigated gasification of pellets
composed of olive PET (25%) and husk (75%) with mixtures of
steam/air, but with low ERs to increase syngas efficiency (Brachi
et al., 2014). When a nickel-based catalyst (Ni-Al
2
O
3
) was compared
with an Al
2
O
3
catalyst, the former produced better gas composition
and tar content. Furthermore, when the effects of feeding from a bed
middle point were compared to those from the top bed feeding, a
substantial increase in gasifier efficiency was observed.
A two-step gasification framework was developed by Park et al.
(2016) that included oxidative pyrolysis at 526°C and a plasma thermal
reactor operating at 626°C. Different mixing ratios and equivalence
ratios were used to investigate the biomass and HDPE co-gasification.
With an ER of 0.46 and 70% biomass in the feed, the best results were
achieved.
In a fluidized bed pre-pilot gasifier, Mastellone et al. (2010b),
Mastellone et al. (2012), and Zaccariello and Mastellone (2015)
investigated the air gasification of ternary mixtures composed of
biomass, coal along with plastic mixtures, and coal. Because of the
higher light hydrocarbon content, the key result of plastics co-feeding
was an improvement in heating value and gas yield. When plastics
were used in the feed, they found a rise in tar formation and a decrease
in H
2
concentration, i.e., for various ERs (0.21–0.31), the co-
gasification of coal/plastics tar contents ranged from 26 to 48 gm
−3
.
Surprisingly, biomass had the opposite effect than that predicted,
which is tar formation reduction. As a result, the authors assessed that
by promoting synergistic effects in the feed by using appropriate
component proportions, the process’viability can be increased.
Moghadam et al. (2014) and Alipour Moghadam Esfahani et al.
(2017) proposed a two-stage method for HDPE steam gasification and
a palm kernel shell/coconut shell mixture between 660 and 880°Cin
FBR using in situ Ni catalyst (powder), followed by cracking of tar in
an FBR dolomite reactor at 600 °C. This method produces syngas with
high H
2
content and allows for effective tar removal. Hence, at the
maximum gasification (880 °C) temperature, a hydrogen yield of
29.4 wt% was recorded, with an 87% concentration (by vol).
Furthermore, plastics in the feed had a positive effect on the
content of tar and gas heating value; but on tar formation, this effect
was found to be the opposite of that stated by other authors (Ruoppolo
et al., 2012;Wilk and Hofbauer, 2013;Zaccariello and Mastellone,
2015). Thus, for binary and ternary mixtures, very low contents of tar
(1.35 gm
−3
) were achieved by operating at 850°C, utilizing dolomite as
the primary catalyst and with an ER of 0.36, with heating values in the
range of 5–8 MJ/m
3
due to the high equivalence ratio used.
Frontiers in Chemistry frontiersin.org14
Shah et al. 10.3389/fchem.2022.960894
Aznar et al. (2006) studied air co-gasification of binary and ternary
mixtures in a fluidized bed reactor. The mixtures were made up of
plastic waste, i.e., PP and PE, biomass, and coal. In binary mixtures,
the content of plastics was comparatively high (40%), while in ternary
mixtures, the content was low (10–20%). The concentration of
hydrocarbon in the gas production increased due to the presence
of plastics in the feed while lowering H
2
,CO
2,
and CO.
According to these results, plastic waste co-gasification with various
feedstocks produces fascinating synergies, highlighting the strategy’s
utility (Wilk and Hofbauer, 2013;Lopez et al., 2015a;Singh et al.,
2022). The reciprocations between product polymer degradation and
biomass chars are usually due to these synergies (Antelava et al., 2021),
with a positive correlation in their thermal joint degradation being well
established (Zhang et al., 2016b;Lopez et al., 2017). As shown in Figures
9A,B, increasing the content of plastics in its co-gasification with coal and
biomass increases both H
2
concentrationandgasyield.Theseoutputsare
explicated by the higher content of carbon and H
2
in waste plastics when
compared to coal and biomass, as well as the lower or non-existent char
yield. An increase in the formation of tar is the key plastics co-feeding
disadvantage as shown in Figure 10A. The higher gas heating value
generated when compared to that in the gasification of biomass, as shown
in Figure 10B, also facilitates the plastic co-feeding benefit(Pinto et al.,
2016). Another benefitofco-gasification of plastic and biomass is that this
reduces plastics gasification operational issues, such as formation of fine
char particulates and reactor feeding (Pinto et al., 2002).
7 Pyrolysis
Pyrolysis is organic matter thermal decomposition without
oxidizing agents like CO
2
, oxygen, or steam. The temperature for
pyrolysis processes is generally inbetween 300 and 850°C, depending
on various process parameters. Usually, pyrolysis processes are
endothermic, which means that energy is required to proceed with
the process. The energy content and composition of pyrolysis products
are dependent largely on the input of waste and can differ significantly
(Hu et al., 2021;Tezer et al., 2022):
•Solid: a char-like substance that contains residual solid products,
such as sand, glass, and metals. The heating values and char content
(by weight) are around 10–35 MJ/kg and 20–50%, respectively,
which may have substantial content of ash (10–50%).
•Liquid: a complex mixture of hydrocarbons, such as organic
acids, phenols, PAHs, and alcohols, made up of water, tar, and
oil. The heating values and liquid amount (by weight) are
around 5–15 MJ/kg and 30–50%.
•Gas: a mixture of CO, CH
4
,CO
2
,H
2
, and other volatile waste
constituents. The heating value and gas yield may be around
3–12 MJ/Nm
3
and 20–50%, respectively.
Moisture is released and waste is dried during the pyrolysis
process, which involves heating the waste to about 100–120°C.
Following this process, a series of complex reactions take place,
resulting in the release of volatile compounds and the breakdown
of more complex carbon-containing compounds into simpler ones.
Gaseous outputs are formed by breaking nitrogen, hydrogen, and
oxygen bonds at temperatures ranging from about 200°C to 800 C (see
Table 4). The primary reactions are those that result in the production
of gas and tar/oil, while the secondary reactions are those that result in
the conversion of gas and tar/oil. During gasification, these secondary
reactions can also occur. Secondary reactions convert further tar to
gases and char, along with the enhancement in the concentrations of
CH
4
and CO
2
in the gas product.
Pyrolysis product heating values and mass yields differ greatly
from one process to another and also depend highly on the
composition of the waste input. With well-sorted solid recovered
fuel (SRF), automotive shredder residue (ASR), or biomass waste as a
process input, the above values can only be considered suggestive and
typically representing an upper limit. Mixed plastics generally produce
high amounts of inorganic residues and char, whereas high quality
plastic waste and rubber promote higher oils and gases ratios.
The amount of water in the waste input has an impact on both the
process conditions and outputs, especially on the liquid and gas
outputs. Heat is mostly supplied indirectly via the reactor walls,
but waste compaction and friction can also lead to waste heating.
Pyrolysis takes place in an inert atmosphere, but in practice, it occurs
in the pyrolysis gaseous atmosphere that go through various secondary
conversion reactions.
8 Pyrolysis of waste plastics
Plastics come in a variety of compositions which are usually stated
based on their proximate analysis. The proximate analysis includes the
determination of volatile matter, moisture content, fixed carbon, and
waste sample ash content. If samples of solid waste are to be utilized as
a fuel, all of these characteristics are very significant (Kreith, 1998).
The main factors that affect the yield of liquid oil in the pyrolysis
process are ash content and volatile matter. A high content of ash
decreases the liquid oil yield, while high volatile matter enhances the
production of liquid oil (Abnisa and Wan Daud, 2014). The proximate
study of various plastics is summarized in Table 5, which shows that all
plastics have high volatile matter and low ash content. These
properties show that plastics have a high capacity for pyrolysis to
produce significant amounts of liquid oil.
8.1 High-density polyethylene
Polyethylene is the most popular plastic in the world. It is the most
basic of all commercial thermoplastics in terms of structure. Its
molecules are made up of long-chain carbon atoms joined by two
atoms of hydrogen. The straight chain (no branching) is called high-
density polyethylene (HDPE) or linear PE, short for high-density
polyethylene. Although linear PE is far more durable than branched
PE, branched PE is easier to manufacture and less expensive. Its
different uses account for 17.6% of the plastic waste group, which is the
third most common plastic form of MSW (Michael, 2010). HDPE is
therefore suitable for applications like weaving, Raschel knitting,
reinforcement applications, and braiding. Many studies on the
pyrolysis of HDPE at various operating conditions have been
performed to determine the yield of the product.
Using a batch reactor, Marcilla et al. (2009a) explored the pyrolysis
of HDPE at 550°C. The gaseous product produced was 16.4 wt% and
the yield of liquid oil was 84.8 wt%. The findings showed that at higher
temperatures, more liquid oil yield could be produced, but there was
also a drawback that should be observed; since the process had reached
the utmost thermal decomposition stage, too high temperatures would
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increase the gaseous product while decreasing the yield of liquid oil.
Mastral et al. (2001) studied the pyrolysis of HDPE at 650 °Cinan
FBR. During experimentation, they noted that the production of the
gaseous product was 31.7 wt% and liquid oil yield was 68.3 wt%. They
found that when the temperature exceeds 550°C, the liquid further
cracks into the gaseous products.
Kumar and Singh (2011) investigated the thermal pyrolysis of
HDPE at 400–550°C utilizing a semi-batch reactor. At 550°C, gaseous
product (24.73 wt%) and the maximum liquid yield (79.06 wt%) were
obtained, while at temperatures of 500–550°C, wax began to dominate
the fraction of the product. The pyrolysis produced a dark brownish
oil with no clear residue and a boiling point ranging from 83 to 351°C.
This indicated that the oil contained a mixture of components of
various oils, like diesel, kerosene, and gasoline, which coordinated the
characteristics of conventional fuel (see Table 6). In addition, the
pyrolytic oil of HDPE had a very low sulfur content (0.018%), making
it environmentally friendly.
In a micro steel reactor, Ahmad et al. (2014) explored the pyrolysis
of HDPE by utilizing nitrogen as a fluidizing medium at 5–10°C/min
heating rate at 300–400°C. They discovered that the maximum total
conversion occurred at 350°C, with liquid yield as the primary product
(80.83 wt%). At 300°C, the solid residue was fairly significant (33.07 wt
%), but it decreased to 0.53 wt% at the maximum temperature of
400 °C.
8.2 Low-density polyethylene
Low-density polyethylene (LDPE) is a semi-rigid, translucent
plastic polymer. It has a large proportion of long and short side-
chain branching than HDPE. Tubular and stirred autoclave processes
are the two most used methods for producing LDPE. Because it has
greater rates of ethylene conversion, the tubular method is becoming more
popular than the autoclave method. Squeeze bottles, containers, carrier
FIGURE 9
(A) The effect of the feed’s plastic content on the yields of gas in plastics co-gasification with coal and biomass. (B) Plastic content effect in the feed on
the production of H
2
in plastics co-gasification with coal and biomass.
FIGURE 10
(A) Plastic content effect in the feed on the content of tar in the produced gas in plastics co-gasification with coal and biomass. (B) Plastic content effect
in the feed on the produced gas heating value in plastics co-gasification with coal and biomass.
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bags, wash bottles, laboratory molded apparatus, and high-frequency
insulation are among the most common uses for LDPE. Plastic bags
are the most common use for LDPE, therefore day by day, LDPE waste has
been accrued and is now the second most used plastic after PP in MSW
(Michael, 2010). Apart from that, LDPE also has the potential for energy
recovery, i.e., converting it into liquid and gaseous products.
Uddin et al. (1996) investigated the pyrolysis of LDPE at 430 °Cin
a batch reactor. The yield of liquid product was about 75.7 wt%. By
utilizing a similar reactor type as Uddin et al. (1996),Aguado et al.
(2007) obtained a yield of 74.6 wt% at 450 °C which is closer to the
yield obtained by Uddin et al. However, even at lower temperatures in
the reactor, when pressure was applied during the operation, the yield
of liquid oil could be increased. Onwudili et al. (2009) demonstrated
this at 425 °C in LDPE pyrolysis using a pressurized batch reactor
(0.7–4.2 MPa). They obtained 0.4 wt% char, 10 wt% gaseous products,
and 89.6% liquid oil from the experiment. This suggests that pressure
can have an effect on the pyrolysis product’s composition.
With a 10 °C/min heating rate, Bagri and Williams (2001) at
500 °Cinafixed-bed (packed) reactor studied the pyrolysis of
LDPE by utilizing nitrogen as the fluidizing gas. During the
experimentation, it was discovered that a 95% liquid yield was
achieved with a low gas and char yield. Marcilla et al. (2009a) at
550 °C also investigate the LDPE pyrolysis in a batch reactor with a
5°C/min heating rate. During experimentation, a high yield of liquid
oil was obtained (93.2 wt%), while the gas yield was notably low.
8.3 Polyvinyl chloride
Polyvinyl chloride (PVC) is a thermoplastic resin that is widely
utilized in the manufacturing of a wide range of products. PVC is a
cost-effective and versatile polymer that is used in a variety of
industries, such as the packaging, construction, automotive, and
medical industries. PVC is different from other thermoplastics in
terms that it is made up of a combination of carbon (43%) and chlorine
(57%) (British Plastics Federation, 2015). Due to the content of
chlorine in PVC, recycling it is more complex and challenging
than recycling other polymers such as PET. To recycle PVC
plastics, dechlorination is required.
In batch reactors under vacuum, Miranda et al. (1998) studied
PVC pyrolysis at a 10°C/min heating rate, with applied pressure of
2 kPa, and at a temperature between 220 and 520°C. The accumulation
of tar increased dramatically as the temperature increased and reached
19.5%, which was even higher than the liquid oil yield (12.78%). From
the experiment, the primary product yield was hydrogen chloride
(HCl) (58.32 wt%). When heated mildly, HCl is toxic and corrosive,
resulting in equipment damage. This was one of the key reasons for the
pyrolysis pilot plant in Germany (Ebenhausen), being shut down
(Miranda et al., 1998). Therefore, PVC is not an ideal material for the
pyrolysis process. There are two major reasons for this: firstly, PVC
waste accumulation in MSW is very less (less than 3%) (Michael,
2010), and secondly, the presence of HCl in the liquid product is very
harmful to the process equipment due to its corrosive properties. PVC
dechlorination is required to overcome the problem and to make the
pyrolysis process effective. This is possible through various techniques
like catalytic pyrolysis, adding adsorbents to PVC, and stepwise
pyrolysis (López et al., 2011). As a result, when an extra
dechlorination phase is necessary, the PVC pyrolysis requires an
additional cost, which has been one of the industry’s drawbacks.
8.4 Polyethylene terephthalate
Polyethylene terephthalate (PET) polymer is utilized in several
applications, such as sheets, packaging, and industrial parts. PET has
outstanding mechanical strength, transparency, and gas barrier
characteristics. Printing pads, electrical insulations, photographic
films, and X-ray and magnetic tapes and films are some of the
other uses of PET (Çepeliogullar Ö Pütün, 2013). PET is the most
extensively used and highly recycled plastic in the world. As reported
by the PET Resin Association (PETRA), the PET recycling rate in the
EU is about 52%, whereas in the United States the rate is 31%.
The recycling rate in the United States dropped below 29% in
2016. Over 1.8 billion pounds of PET had been recycled in 2015 and
was utilized to produce a range of products. PET containers are
estimated to account for 1% of MSW in the United States,
according to the EPA. As a result, other options for the
recovery of PET, like the pyrolysis process, have been
investigated, and the yield of products has been studied by a
number of researchers. Cepeliogullar et al. (2013) studied the
pyrolysis of PET in a fixed-bed reactor by using nitrogen as the
sweeping gas at a 10 °C/min heating rate and at a temperature of
500°C. The authors found that the yield of liquid oil (23.2 wt%) was
significantly lower than the gaseous product (76.90 wt%). There
was no solid residue left after the process. As shown in Table 5,the
volatile content of PET is 86.83%,whichisrelatively low when
TABLE 4 Temperature-dependent pyrolysis reactions (Bilitewski et al., 1997).
Chemical reaction Temperature range
Dehydration, thermal drying 100–120
Desulfurization, deoxidation, CO
2
and H
2
O molecular splitting, H
2
S splitting 250
Aliphatic hydrocarbon bonds breakage, methane and other aliphatic hydrocarbons splitting 340
Carbonization 380
C-O and C-N bonds breakage 400
Bituminous (asphalt) compounds disintegration into low temperature tars and oils 400–600
Bituminous (asphalt) compounds cracking into thermal resistant elements, aromatic organic compounds formation 600
Thermal aromatization of ethene to hexanaphthene to C
6
H
6
formation and other volatile aromatic hydrocarbons >600
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compared to that of other plastics, which is the main reason for the
low liquid oil yield. Unfavorably, GC-MS (gas chromatography
mass spectroscopy) studies have revealed that benzoic acid is the
primary product in the oil composition, which is about 49.93%. The
acidic characteristic of the pyrolysis oil is unfavorable because of its
corrosiveness, which degrades the fuel efficiency (Cepeliogullar
et al., 2013). Moreover, the benzoic acid content in pyrolysis oil is
generally sublime and can clog the piping of heat exchangers,
necessitating close monitoring if used on an industrial scale
(Shioya et al., 2005;Wan Ho, 2015).
TABLE 5 Proximate analysis of plastics (Abnisa and Wan Daud, 2014).
Plastics types Marks on plastics Volatile (wt%) Fixed carbon (wt%) Ash (wt%) Moisture (wt%) References
Polyethylene terephthalate (PET) 91.75 7.77 0.02 0.46 Zannikos et al. (2013)
86.83 13.17 0 0.61 Heikkinen et al.
(2004)
High-density polyethylene
(HDPE)
99.81 0.01 0.18 0 Ahmad et al. (2013)
98.57 0.03 1.40 0 Heikkinen et al.
(2004)
Polyvinyl chloride (PVC) 93.70 6.30 0 0.80 Hong et al. (1999)
94.82 5.19 0 0.74 Heikkinen et al.
(2004)
Low-density polyethylene (LDPE) 99.70 0 0 0.30 Park et al. (2012)
99.60 - 0.40 - Aboulkas et al. (2010)
Polypropylene (PP) 95.08 1.22 3.55 0.15 Jung et al. (2010)
97.85 0.16 1.99 0.18 Heikkinen et al.
(2004)
Polystyrene (PS) 99.63 0.12 0 0.25 Abnisa et al. (2014)
99.50 0.20 0 0.30 Park et al. (2012)
Polyethylene (PE) Acrylonitrile
butadiene styrene (ABS)
Polyamide (PA) or Nylons
Polybutylene terephthalate (PBT)
98.87 0.04 0.99 0.10 Jung et al. (2010)
97.88 1.12 1.01 0 Othman et al. (2008)
99.78 0.69 0 0 Othman et al. (2008)
97.12 2.88 0 0.16 Heikkinen et al.
(2004)
TABLE 6 Properties comparability of conventional fuel and pyrolytic HDPE oil.
Oil type Properties of conventional fuel (Boundy et al., 2011) Characteristics of HDPE pyrolysis oil (Kumar and
Singh, 2011)
Boiling point (°C) Cv (MJ/kg) Boiling point (°C) Cv (MJ/kg)
Gasoline 40–200 42.9 82–352 43.4–46.5
Diesel 150–390 42.8–45.8
Kerosene 150–300 43.0–46.2
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8.5 Polypropylene
Polypropylene (PP) is a crystalline, rigid, and tough polymer made
from the monomer of propylene (or propene). It is a hydrocarbon
resin with a linear structure. PP is a polymer that belongs to the family
of polyolefin and is one of the top three most extensively utilized
plastics in the world. PP is a material that can also be used as a fiber
and plastic in the furniture market, the automobile industry, consumer
goods, and industrial applications. Polypropylene accounts for around
24.3% of the total amount of plastics contained in MSW (Michael,
2010). The pyrolysis of polypropylene has been investigated by many
researchers, which are given below.
Ahmad et al. (2014) investigated PP pyrolysis in a micro steel reactor
with temperatures between 250 and 400°C.Theyfoundthatat300
°C, the
yield of liquid oil obtained was 69.82 wt%, which was the highest at this
temperature with 98.66% of total conversion. They also noted that an
increase in temperature (400°C) increased the solid residue (1.33–5.70 wt%)
and decreased the conversion of the product (94.30%). This means that at
higher temperatures, coke formation increases. Sakata et al. (1999) explored
the pyrolysis of polypropylene at 380°C. They obtained an 80.10 wt% yield
of liquid oil, along with a 6.6 wt% gaseous yield and 13.30 wt% solid residue.
Fakhrhoseini and Dastanian (2013) also explored the pyrolysis PP at 500°C.
They obtained a higher yield of liquid product (82.12 wt%), but an increase
in temperature above 500°C decreased the production of liquid oil.
Demirbas (2004) proved this by investigating PP pyrolysis in a batch
reactor at a very high temperature of 740°C. The yield of liquid produced
was 48.8 wt%, with 49.6 wt% of gaseous product and 1.6 wt% of solid
residue.
8.6 Polystyrene
Polystyrene (PS) is a versatile material that can be utilized in a
wide range of customer goods. Its common applications are in
products that demand limpidity, such as in laboratory ware and
food packaging. PP is used to produce electronics, appliances, toys,
automobile parts, and gardening pots when mixed with different
additives, colorants, or polymers. Recycling of PS can be achieved
in thermal, chemical, and mechanical ways. For mechanical
recycling, high-impact polystyrene (HIP) is a propitious
material because, despite several processing cycles, its properties
remain the same. The liquid and gaseous products production
depend highly on the reaction conditions. For the production of
both gaseous and liquid products, high-selectivity catalysts
are used.
In an autoclave pressurized batch reactor, Onwudili et al. (2009)
investigated the pyrolysis of PS for a duration of 1 hour at 300–500°C.
The experimental pressure was 0.32 MPa–1.6 MPa, and the rate of
heating was 10°C/min. They noted that at 452°C, the gas yield
production was only 2.6%, while the production of liquid oil was
very high and around 97.0 wt%. Liu et al. (1999) investigated PS
pyrolysis at 450–700°Cinafluidized bed reactor. At 600°C, the highest
amount of liquid oil (98.7 wt%) was obtained. But at 450°C, the
production of liquid oil was also considerably high which was
97.6 wt%. Demirbas (2004) also studied the pyrolysis of PS in
batch reactors at 581 °C. From the experiments, the highest yield of
liquid product was 89.5 wt% which is less when compared to those
obtained by Onwudili et al. (2009) and Liu et al. (1999). Therefore, PS
is not a favorable material for the pyrolysis process at a high
temperature because of its effect on the end products.
8.7 Mixed plastics
The pyrolysis process has a benefit over recycling in that it does
not require a thorough sorting process. Many plastics are incompatible
with one another in their cycling processes and cannot be recycled
together. For instance, a small PVC contamination quantity in the
stream of PET recycling can degrade the whole resin of PET, turning it
brittle and yellow, necessitating reprocessing (Hopewell et al., 2009).
This demonstrates that the recycling process is so vulnerable to
pollutants that all plastics must be sorted by transparency, color,
and resin type. The pyrolysis process, on the other hand, appears to be
more viable because liquid oil can still be obtained from any sort of
plastic present in the feedstock. Donaj et al. (2012) explored the
pyrolysis of mixed plastics in a bubbling fluidized bed reactor at
temperatures between 650°C and 730°C. The plastics mixture
comprised of 24 wt% PP, 30 wt% HDPE, and 75 wt% LDPE. They
noted that at 650°C, the yield of liquid oil was 48 wt%. This oil fraction,
on the one hand, was composed of 52% heavy fractions that included
carbon black, wax, and heavy oil. The yield of liquid oil at 730°C (44 wt
%), on the other hand, contained a liquid light fraction of up to 70%.
Therefore, higher temperatures facilitate gaseous or light hydrocarbon
liquids. Therefore, the distribution of the product changes
dramatically when the temperature is increased to a high extent.
Kaminsky et al. (1996) studied mixed plastic pyrolysis,
approximately composed of 25% PS and 75% polyolefins (PP, PE).
The product yield contained a small amount of chlorine which
demonstrated the presence of PVC content in the mixture (1 wt%).
The yield of liquid oil obtained was 48.4wt% at 730°CinanFBR.
Demirbas (2004) also investigated mixed plastic pyrolysis that involved
PS and polyolefins (PE, PP). The solid and gaseous yields were about
2.2 and 35 wt%, respectively. The yield of liquid oil was around 46.6 wt
%, which was very similar to the yield obtained by Kaminsky et al.
(1996). The composition of liquid oil also contained small amounts of
chlorine (4 ppm) which was due to the presence of PVC in the feedstock.
However, the presence of chlorine did not affect the quality of the liquid
oil because its content was below 10 ppm. Moreover, the majority of the
chlorine content was found in the solid residue. Therefore, in order to
get a quality liquid oil yield, the feedstock’s chlorine content could not
exceed 1 wt%. From the above results, it may be observed that when
compared to the pyrolysis of single plastics, mixed plastics pyrolysis
produces lower than 50 wt% liquid oil. Nonetheless, the produced oil
had a composition similar to that of pyrolysis of the single plastic,
making it suitable for petrochemical refineries for further processing.
9 Chemical reactors for pyrolysis of
plastic waste
In the pyrolysis process, the reactor type used has a profound
influence on the catalysts and plastics mixing, heat transfer,
reaction efficiency, and residence time in order to achieve the
desired final end product. At a lab scale, most of the
experimentationisdoneinafixed bed, fluidized bed,
continuous flow, CSBR, and batch reactors.
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9.1 Fixed-bed (packed) reactor and fluidized
bed reactor
The catalyst is normally packed and palletized in a static bed in a
fixed-bed reactor, as shown in Figure 11A. The key advantage of these
is their design simplicity, but on the other hand, there are some
limitations, like the irregular shape and size of the plastic particles used
as feedstock, which during the feeding process cause difficulties.
Another disadvantage is that the reaction’s access to the catalyst’s
usable surface area is limited. For the pyrolysis of plastic waste, many
researchers have utilized the fixed-bed reactor (Bagri and Williams,
2001;Ballice, 2001;Choi et al., 2010;Renzini et al., 2011;Cepeliogullar
et al., 2013;Saad et al., 2015a). Because it is easy to feed the primary
pyrolysis product into the fixed-bed reactor, which is usually
composed of gaseous and liquid phases, these reactors in some
cases are only used as secondary pyrolysis reactors (Fogler, 2010).
Onu et al. (1998) and Vasile et al. (2000) studied different plastic
pyrolysis using a two-step procedure. The two-step process for plastic
pyrolysis does not get much attention because it is not cost-efficient,
and the product composition procured is equivalent to that obtained
using the single-step process.
In plastics catalytic cracking, several studies have favored fluidized
bed reactors to fixed-bed reactors (Sharratt et al., 1997;Garfoth et al.,
1998;Williams and Williams, 1998;Liu et al., 1999;Mastral et al.,
2001;Lin et al., 2004;Lin and Yen, 2005;Yan et al., 2005;Mastral et al.,
2006b;Marcilla et al., 2007). Jung et al. (2010) studied the PE and PP
pyrolysis processes in an FBR at temperatures between 290 and 850°C.
The yield of liquid product was dramatically high because the reactor
provides constant temperature with high heat and mass transfer,
reliable mean time distribution, and uniform products spectrum.
Luo et al. (2000) also studied the pyrolysis processes of PP and
HDPE in an FBR at 500°C by utilizing a silica–alumina catalyst.
The yield of liquid oil by HDPE was 85 wt%, while PP produced
had a high liquid composition, which was 87 wt%.
In an FBR, unlike in a fixed-bed (packed) reactor, the catalyst sits
on a distributor plate, as shown in Figure 11B, through which the
fluidizing gas moves and the particulates are held in a fluid state. Since
the catalyst is mixed thoroughly with the solvent, there is greater
accessibility to the catalyst, resulting in a wider surface area for the
reactions to take place (Kaminsky and Kim, 1999). With effective and
viable heat transfer, this decreases process volatility. Furthermore,
when compared to batch reactors, the FBR reactor is more flexible as it
does not require regular feedstock charging, which makes the process
steady. Therefore, because of the lower operating cost, the FBR will be
the better reactor to use in the pilot plant on a traditional design scale.
Therefore, the FBR is more feasible to perform plastic catalytic
degradation as it provides uniform catalyst mixing with the fluid,
resulting in a high surface area for the reaction to take place.
Furthermore, it does not require regular feedstock charging which
makes the process steady. As a result, in terms of economics, the FBR
will be the utmost appropriate reactor for large- and extensive-scale
applications.
9.2 Batch and semi-batch reactors
Batch reactors are the most basic reactors used in chemical
reactions. They are closed systems that work in an unsteady state,
which means that no reactants or products inflow or outflow are
possible during the reaction. In batch reactors, high residence time
means higher conversion rate, which is one of their main
advantages. The downsides of batch reactors are high labor cost
and the difficulty in maintaining extensive production (Fogler,
2010). A semi-batch reactor, on the other hand, allows product
removal and reactant addition at the same time. Concerning
reaction selectivity, the semi-batch reactor has the advantage of
being able to incorporate reactantsovertime.Highlaborcostand
small-scale production are the main downsides of a semi-batch
reactor.
Because of the easy configuration and ability to monitor the
operating conditions readily, many researchers utilize batch and
semi-batch reactors in the pyrolysis of plastic waste in laboratory-
scale experiments (Cardona and Corma, 2000;Uemura et al., 2001;
Kim and Kim, 2004;Miskolczi et al., 2004;García et al., 2005;Lee and
Shin, 2007;Jan et al., 2010;Shah et al., 2010;Adrados et al., 2012;
AdnanShah and Jan, 2014). The ideal temperature for catalytic and
thermal pyrolysis in these reactors is in the range of 300–800°C. To
increase the yield of hydrocarbons, many researchers have added
catalysts to plastics. The main drawback of catalytic pyrolysis is the
formation of coke on the catalyst surface which reduces the efficiency
of the catalyst due to the blockage of its active sites, thus causing high
residues during the reaction.
Abbas-Abadi et al. (2014) studied the pyrolysis of PP in semi-
batch reactors and found a very high liquid yield of 92.3 wt%. The
experiment was conducted at 450°C using an FCC catalyst. As shown
in Figure 12, some batch reactors and semi-batch reactors were also
fitted with stirrers that ran at various speed depending on the
necessary setting. Seo et al. (2003) explored the pyrolysis of HDPE
at 450 °C by utilizing the stirrer batch reactor. The speed of the stirrer
was 200 RPM. They found a high yield of liquid oil of 84.0 wt% in
thermal pyrolysis than did Sakata et al. (1999). Furthermore, by using
a silica–alumina catalyst, the liquid oil obtained by Sakata et al. (1999)
was 74.3 wt%, while Seo et al. (2003) obtained a high liquid oil yield
which was 78 wt%. As a result, it has become clear that in the batch
reactor, the stirrer improved the mixing of plastics and catalysts within
the reactor, thus increasing the yield of liquid oil. Kyong et al. (2002),
Lee (20080, and Abbas-Abadi et al. (2013) conducted additional
research on semi-batch reactors with stirrers. Sakata et al. (1999)
studied the HDPE and PP processes with and without catalysts at
430°C and 380 °C, respectively, in batch reactors. For certain catalysts,
the liquid oil yield through catalytic pyrolysis was even less than that
obtained through thermal pyrolysis. The yield of liquid in thermal
pyrolysis from HDPE was 69.4 wt%, and 80.2 wt% from PP. In
catalytic pyrolysis, the yield of liquid for both plastics was
decreased to 49.9–67.7 wt% (HDPE) and 47–78 wt% (PP). This
might be due to the formation of coke on the catalyst surface
which degraded the catalyst efficiency. The catalysts used in the
experimentation were HZSM-5 and silica–alumina (SA-1).
However, for both plastics, the liquid yield increased very slightly
about 1.0–7.0 wt% than the thermal pyrolysis, by utilizing mesoporous
silica and silica–alumina (SA-2) catalysts. Thus, the reactivity of
various catalysts to various plastic types might be different. Based
on the above results, it was found that the batch reactors and semi-
batch reactors are favorable and feasible to be utilized in waste plastics
pyrolysis process because it is easy to monitor the parameters of these
reactors which promote the high yield of liquid. These reactors were
however not appropriate for catalytic plastic pyrolysis due to the
formation of coke on the outer surface of the catalyst which would
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affect the overall product composition. These reactors are only suitable
for laboratory experiments because, on a large scale, it is difficult to
maintain per unit of production.
9.3 Spouted bed reactors
The spouted bed reactor (CSBR) offers fine amalgamation and can
accommodate a broad particle size distribution, different particle
densities, and larger particles (Fogler, 2010). The CSBRs have been
used by several researchers in the catalytic pyrolysis of plastics
(Aguado et al., 2002;Elordi et al., 2009;Olazar et al., 2009;
Arabiourrutia et al., 2012b;Elordi et al., 2012;Artetxe et al.,
2013b). The CSBR, according to Olazar et al. (2009), have lower
bed segregation and attrition when compared to the bubbling fluidized
bed. The CSBR offers inconsiderable defluidization issues while
processing sticky materials and also provides excellent heat transfer
between the phases. However, the main downsides of this reactor are
product collection, entrainment and feeding of the catalyst, and high
operating cost (Lopez et al., 2009).
The CSBRs are particularly well suited for preventing problems of
agglomeration in the polyolefins pyrolysis, even when the process is
performed under maximum stickiness conditions. Aguado et al.
(2005) investigated the LDPE, HDPE, and PP pyrolysis processes
using the 1:30 g of plastic/sand ratio. The experiments were performed
at 400, 500, 550, and 600°C. The authors found that, for a certain sand
amount, the amount of plastic fed into the reactor increases almost
linearly as the gas velocity rises, which results in increasing particle
velocity. Moreover, the particles’rapid velocity causes collisions that
have enough energy to prevent agglomeration.
Elordi et al. (2007) studied the pyrolysis of HDPE illustrated in
Figure 13 at 500 °C in the CSBR by utilizing HY zeolite catalyst. The
gasoline fraction yield was 68.6 wt% (C5–C10). The octane number of
the gasoline was RON 96.6, which is similar to the conventional
gasoline quality. Arabiourrutia et al. (2012b) utilized the CSBR to
investigate the depiction and wax yield from the pyrolysis processes of
PP, LDPE, and HDPE at 450–600 C. They claimed that the CSBR has
the ability to handle sticky solids that are difficult to handle in the FBR.
The spouted bed scheme was specifically well suited to low-
temperature wax pyrolysis. They found that with the temperature,
the yield of waxes decreased. More wax is cracked into gaseous and
liquid products at higher temperatures. The yield of waxes from the PP
pyrolysis was 92 wt%, while that from LDPE and HDPE was very
similar at 80 wt% waxes.
Artetxe et al. (2015) studied the flash pyrolysis of PS in the CSBR
for styrene recovery at 450–600°C. The results showed that gas velocity
and temperature have a significant impact on the yield of styrene,
having maximum recovery of the monomer (70.6 wt%) at 500°C.
Regarding light olefins recovery, the same authors performed a
two-step pyrolysis process in CSBR. The light olefins yield was
77 wt% in the second step at 900°C. Moreover, the yield of butene,
propylene, and ethylene was 17.5%, 19.5%, and 40.4% respectively. On
the other hand, the yield of aromatics was only 6.2 wt% (Artetxe et al.,
FIGURE 11
Representations of (A) fixed-bed (packed) reactor and (B) fluidized bed reactor (FBR) (The University of York, 2013).
FIGURE 12
Batch reactor (sequential) with stirrer (The University of York, 2013).
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Shah et al. 10.3389/fchem.2022.960894
2012;Barbarias et al., 2018). These results show that the CSBR enables
optimizing wax yield and preventing problems of defluidization.
9.4 Microwave-assisted technology
Microwave heating is used in microwave pyrolysis, and it now
provides a novel method for the recovery of waste through the process
of pyrolysis. In this method, waste materials are mixed with a
microwave-absorbent substance like particulate carbon. The
microwave energy is absorbed by the microwave absorbent to
generate enough thermal energy to reach the temperatures required
for comprehensive pyrolysis (Lam and Chase, 2012). Microwave
pyrolysis has a number of benefits over the traditional pyrolysis
process, such as lower particle levels in bio-oils, ease of control,
and standardized large biomass internal heating. In this process,
the material is directly heated with microwave energy that is
directly delivered by using molecular interactions with the electron
beam (Fernandez et al., 2011). This would heat up the environment
without wasting any time. Regardless of the benefits of microwave
heating, there is a significant drawback that prevents this technique
from being extensively investigated on the commercial scale, such as
the lack of adequate evidence to measure the treated waste stream’s
dielectric properties. Microwave heating efficiency is highly dependent
on the material’s dielectric properties. Plastics, for example, have a low
relative permittivity, so during pyrolysis, combining them with a
microwave absorber (carbon) can allow more energy to be
absorbed and altered into heat in less time (Lam and Chase, 2012).
As a result, the heating efficacy of each material can vary, posing a
significant problem to the industries.
Ludlow-Palafox and Chase (2001) studied microwave-assisted
pyrolysis by using two different substances: i) HDPE small pallets
and ii) toothpaste packaging combined with polyethylene laminates
and aluminum. This experiment is unique in that it includes a 180-cm
diameter quartz vessel reactor with a 6-RPM impeller that mounted
within the microwave. With a microwave power of 5 kW, carbon is
used as the absorber. The liquid oil yield from the pyrolysis of HDPE
was 79–81 wt%, with a gaseous yield of 19–21 wt% and no solid
residue at 500–600°C. On the other hand, no product was produced
from polyethylene laminates and aluminum pyrolysis. Moreover, the
authors have noted that at the same operating temperatures, there was
no discernible variance in product yield between the HDPE pellets and
laminates. The average molar mass was somewhat higher in both
cases, but the molar mass distribution was corresponding. Since
aluminum was easily removed by sieving, it had no effect on the
product yield. They noticed a substance called titanium dioxide
(TiO
2)
adhered to the side walls of the reactor as a white powder
during the experiment. TiO
2
can be seen on the toothpaste tube’s
painted surface. Since it had segregated from the laminate’sorganic
content during pyrolysis, this material had no effect on the yield of
the pyrolysis product. Conclusively, using the microwave-assisted
pyrolysis process, real waste like toothpaste packaging was
pyrolyzed successfully.
Undri et al. (2014) also studied the microwave heating technology
in the pyrolysis process by utilizing two sorts of absorbers (carbon and
tires). The waste plastics used were HDPE and PP (polyolefin).
Microwave power ranging from 1.2 to 6.0 kW was used. They
obtained 74.8 wt% liquid yield from PP, whereas they found the
highest yield of 90 wt% from HDPE. Carbon was used as the
microwave absorber in both experiments, with microwave powers
varying from 3 to 6 kW because the polymers’residence time in the
oven was shortened by using high power. As a result, instead of non-
condensable gases, more polymers were transfigured to liquid. The
solid residue level increased to 33 wt% when tires were used as the
microwave absorber, which was due to other non-pyrolyzable
compounds in the tires. Due to the cocking phase, the accumulated
solid residue was at its lowest at 0.4 wt% when compared to when
using carbon as the microwave absorber. Carbon was found to be a
strong microwave absorbent, with a high capability for converting and
absorbing microwave energy into heat. In order to optimize the yield
of liquid in microwave pyrolysis, special attention must be given to the
absorber type and microwave power.
Khaghanikavkani (2013) also studied microwave technology and
elaborated on multiple variables that effect the performance of
microwave heating in the pyrolysis of plastic such as the design of
microwave rotation, absorber type, and nitrogen volume velocity. Lam
and Chase (2012),Fernandez et al. (2011), and Undri et al. (2011) have
also published comprehensive analyses of the microwave heating
technology in the plastic pyrolysis.
10 Process parameters that influence
pyrolysis process
In any process, the parameters play an important role in the
optimization of the product composition and yield. The yield of the
final end products, for instance, char, liquid oil, and gas, can be
influenced by the main process parameters in plastic pyrolysis.
Monitoring the parameters at various settings will result in the
required product. These parameters are elaborated on in the
following section.
10.1 Residence time and pressure
Residence time is one of the key parameters that influences the end
product yield and is defined as the amount of time (average) that a
particle or substance takes in the reactor (Mastral et al., 2001).
Prolonged residence time improves the primary products’
conversion, resulting in more thermally persistent products like
non-condensable gas and lower-molecular-weight compounds
(Ludlow-Palafox and Chase, 2001). In the fluidized bed reactor,
Mastral et al. (2003) explored the temperature effect and residence
time on the HDPE pyrolysis product distribution. They discovered
that high residence time yields a higher liquid product when the
temperature does not exceed 685°C. At temperatures above 685°C,
however, the influence of residence time is less on the yield of the
gaseous and liquid products.
Murata et al. (2004) investigated the effect of pressure in a
continuous stirred tank reactor on the HDPE pyrolysis at
0.1–0.8 MPa at an elevated temperature. They found that as the
pressure increased from 0.1 to 0.8 MPa, the gaseous product yield
increased dramatically from about 6 wt% to 13 wt% at 410°C, but only
slightly at 440°C from 4 wt% to 6 wt%. This demonstrates that at
elevated temperatures, pressure had a significant effect on the gaseous
product distribution. At a high pressure, the liquid product carbon
number distribution shifted to the smaller molecular weight side.
Murata et al. (2004) discovered that when the pressure was increased, a
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Shah et al. 10.3389/fchem.2022.960894
decrease in the double bond formation occurs. This indicated that the
C–C links scission rate in polymers was directly influenced by
pressure, suggesting that pressure had a major impact on the rate
of formation of double bonds. They also found out that at lower
temperatures, pressure had a significant effect on residence time. As
the temperature exceeded 430°C, however, the pressure effect on the
residence time became less noticeable.
As a result, it was concluded that both residence time and pressure are
temperature-dependent variables that at lower temperatures, may affect the
product distribution of plastic pyrolysis. The yield of gaseous products
improved at higher pressures and influenced the gaseous and liquid
products’molecular weight distribution, but only at very high
temperatures. The residence time effect at higher temperatures becomes
less evident, which is why most of the researchers focus more on the
temperature parameter rather than on residence time while conducting
plastic waste pyrolysis studies. Furthermore, if the pressure factor is deemed,
additional units like pressure transmitter and compressor must be
augmented to the entire system, thus increasing the operating cost.
10.2 Temperature
In the pyrolysis process, temperature is one of the most important and
key variables because it controls the polymer chain’s cracking process. The
molecules are prevented from collapsing by the Van der Waals force
which attracts them together. In the system, the molecules’vibration
increases when the system temperature rises, thus causing the molecules
to evaporate from the system surface. When the energy impelled by the
intermolecular force along the polymer chains exceeds the C–C bond
enthalpy in the chain, the carbon chain breaks (Sobko, 2008). The
thermogravimetry analyzer is used to measure the plastics’thermal
cracking behavior. The thermogravimetry analysis (TG) curve and
derivative thermogravimetry analysis (DTG) curve are two types of
graphs produced by the analyzer. The TG curve calculates a
substance’sweightchangeasafunctionof temperature and time,
while the DTG curve provides data on the degrading phase that
occurs through the process, as shown by the number of peaks (Kumar
and Singh, 2011). Cepeliogullar et al. (2013) studied the pyrolysis of PET
in which they observed that at a temperature of 427.8°C, the material’s
maximum weight loss occurred. At 400°C, actual degradation of PET
started and when the temperature was between 200 and 400°C, small
changes in weight loss occurred. They also noted that above 470°C, there
were no considerable alterations. Hence, the temperature range of the PET
thermal degradation is 350–520°C.
Cepeliogullar et al. (2013) studied the thermal behavior of PVC
andreportedtwosignificant weight losses at two distinct
temperature variations. The first temperature variation was
260–385°C, which resulted in a peak weight reduction of 62.26%
when compared to the starting weight. The second temperature
variation was between 385 and 520°C, which resulted in a decrease in
weight of 21.76% when compared to the initial weight. The material
weight loss became minimal as the temperature was increased to
800°C (1.63%). Thus, the PVC degradation temperature was between
200 and 520°C. Chin et al. (2014) studied the HDPE thermal
degradation at the heating rates of 10–50°C/min. Based on the
TG analysis, they observed that the thermal degradation of HDPE
was completed almost at 517–538°C that began at 377–404°C. The
weight loss was accelerated with higher heating rates, which
increased the reaction rate. In a subsequent study conducted by
Marcilla et al. (2005), the authors observed that at 468°C, the HDPE
degradation rate was at its maximum.
Jung et al. (2010) investigated the temperature effect on the
pyrolysis processes of HDPE and PP in a fluidized bed reactor and
observed that the main HDPE and PP degradation started at
FIGURE 13
CSBR illustration in pyrolysis of HDPE in the presence of zeolite catalyst (Elordi et al., 2007).
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Shah et al. 10.3389/fchem.2022.960894
400–500°C according to the DTG curves. However, while comparing
the PP and HDPE fractions, it was discovered that the PP fraction had
begun losing weight at temperatures lower than 400°C. Marcilla et al.
(2005), on the other hand, had found out that for HDPE, the greatest
degradation temperature was at 467°C, while PP degraded at 447°C. In
principle, the degradation rate of HDPE was slower than that of PP
because of its linear structure which contains very little branching that
has stronger intermolecular forces (Jung et al., 2010).
Marcilla et al. (2009b) found that at 360–385°C, a small volume of
liquid oil was formed during the LDPE pyrolysis. At 469–494°C, the
highest liquid yield was obtained. Onwudili et al. (2009) found that
below 410°C, a brown waxy substance was produced, and at 410°C, the
actual LDPE oil conversion had started. They also observed that at
425°C, the highest liquid yield was obtained. Marcilla et al. (2009a) had
also observed the highest liquid oil yield at 550°C from LDPE
degradation. Increasing the temperature to 600°C did not improve
the liquid oil yield (Williams and Williams, 1998). Therefore, the
optimum temperature for liquid oil production from LDPE is
360–550°C. Onwudili et al. (2009) explored the pyrolysis of PS in a
batch reactor. At 350°C, they obtained highly viscous dark-colored oil,
and no PS degradation occurred at 300 C. At 425°C, the highest yield
of liquid oil was achieved. Higher temperatures up to 580°C increased
the production of gaseous products while lowering the liquid oil yield
(Demirbas, 2004). Therefore, PS thermal cracking temperature is
estimated to be between 350 and 500°C.
As a result of the previous discussion, it has been established that
the reaction rate depends mainly on temperature which greatly affects
the output composition for all plastics. The product selectivity
depends strongly on the operating temperature. Lower
temperatures of 300–500°C yield liquid products while a higher
temperature of 500°C or more yields char or gaseous products.
10.3 Catalysts
Catalysisreferstoasubstance’s ability to accelerate the rate at
which a chemical reaction occurs. Catalysts help to make chemical
production faster, easier, and safer. They do so by controlling the
activation energy necessary to initiate chemical reactions. Because
heat is the most costly factor in industries, using a catalyst for it could
help save energy. As a result, catalytic degradation is especially
appealing for obtaining commercially valuable products such as
C2–C4 olefins and automobile fuel (gasoline and diesel) which
are in high demand in the petrochemical industry (Elordi et al.,
2009). Furthermore, many studies have employed catalysts for
product enhancement to increase hydrocarbon distribution to
generate pyrolysis liquid with qualities comparable to traditional
fuels such as diesel and gasoline.
10.3.1 Zeolite-based catalysts
Zeolites are basically crystalline aluminosilicate (Al
2
SiO
5
) sieves
with open pores that have ion exchange properties (Degnan, 2000;
International Zeolite Association, 2005). The framework is made up of
a three-dimensional structure that connects the tetrahedral sides with
oxygen atoms. Zeolite catalyst is build by various SiO
2
/Al
2
O
3
ratio
which depends largely on its type. The reactivity of zeolite is
determined by the SiO
2
/Al
2
O
3
ratio, which influences the pyrolysis
end product. The aromatics and light alkanes yield is also decreased
when zeolite’s SiO
2
/Al
2
O
3
ratio is increased. Table 7 equates the
gasoline fraction fuel qualities achieved with three types of HZSM-
5 zeolite with varying SiO
2
/Al
2
O
3
ratios. As shown, the lowest catalyst
acidity with the highest SiO
2
/Al
2
O
3
ratio resulted in higher olefin
content but lower benzene and aromatics content and lower octane
number. In the pyrolysis of HDPE, HZSM-5 zeolite’s SiO
2
/Al
2
O
3
ratio
had a considerable impact on the yield of product fraction (Artetxe
et al., 2013b). The zeolite’s low acidity was signified by a high SiO
2
/
Al
2
O
3
ratio. When compared to the highly acidic catalyst (SiO
2
/
Al
2
O
3
= 30), the low acidic catalyst (SiO
2
/Al
2
O
3
= 280) was less
dynamic in breaking waxes, resulting in higher C12–C20 fractions and
lower light olefins. The yield of light olefins decreased from 58.0 to
35.5 wt% when the SiO
2
/Al
2
O
3
ratio was increased from 30 to 280, but
the yield of C12–C20 fraction had increased from 5.3 wt% to 28.0 wt%.
Marcilla et al. (2008) investigated the performance of HZSM-5 and
HUSY in a batch reactor at 550 °C with a 10 wt% polymer-to-catalyst
ratio on HDPE and LDPE. The HZSM-5 catalyst produced more
gaseous product (LDPE = 70.7 wt%, HDPE = 72.6 wt%). Conversely,
when compared to the HZSM-5 catalyst (LDPE = 18.3 wt%, HDPE =
17.3 wt%), more liquid oil was obtained with the HUSY catalyst
(LDPE = 61.6 wt%, HDPE = 41.0 wt%). Lin and Yen (2005)
showed a similar tendency of product selectivity utilizing HUSY
and HZSM-5 zeolites on the pyrolysis of PP. This demonstrates
that product selectivity varies depending on the catalyst and
different zeolite catalysts may have distinct product predilection in
terms of selectivity. In the catalytic pyrolysis of plastics, HMOR and
HUSY are extensively utilized zeolite catalysts. Garfoth et al. (1998)
examined the efficacy of three zeolite catalysts for HDPE pyrolysis:
HUSY, HMOR, and HZSM-5 with a 40 wt% polymer-to-catalyst (P/
C) ratio. In their experimentation, they observed that the catalytic
activity of HUSY and HMOR was less when compared to that of
HZSM-5. HUSY and HMOR left 7.08 wt% and 8.94 wt% residues,
while HZSM-5 left 4.53 wt% residue which indicates higher catalytic
capabilities of HZSM-5 over HUSY and HMOR.
Miskolczi et al. (2009) studied the effect of zeolite catalyst in real
municipal plastic waste pyrolysis. PP and HDPE waste sources were
collected from packaging agriculture sectors, and before pyrolysis, they
were chopped and washed. Both polymers (PP = 35 mg kg
−1
and
HDPE = 238 mg kg
−1
) contain sulfur according to the properties
TABLE 7 Comparison of gasoline fraction fuel properties achieved with various SiO
2
/Al
2
O
3
ratios using three HZSM-5 types (Artetxe et al., 2013b).
SiO
2
/Al
2
O
3
Aromatics (vol%) Olefins (vol%) Benzene (vol%) Octane number
280 6.9 68.9 0.46 85.9
80 13.5 62.2 1.3 86.7
30 43.3 33.1 4.2 94.1
Required <35 <18 <195
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analysis, although more contaminants were found in the HDPE waste
generated from agricultural sectors such as calcium (103 mg kg
−1
),
phosphorus (47 mg kg
−1
), and nitrogen (963 mg kg
−1
). The
contaminants were most likely caused by fertilizer containing
superphosphate and ammonium nitrate, which could have collected
in the HDPE waste after the washing method failed to remove them.
With 40 wt% HZSM-5 catalyst, the catalytic pyrolysis was performed at
520°C. The utilized structure of the catalyst was analyzed by EDAX and
SEM after the pyrolysis. Aside from the silica–alumina HZSM-5 zeolite
structure, traces of phosphorus, nitrogen, and sulfur were found on the
catalyst surface (sodium, oxygen, magnesium, aluminum, calcium,
silica, and potassium). This shows that the contaminants were
derived from plastic waste. However, the product characteristics were
not affected by the catalyst surfacecontaminants, which were influenced
more by the catalytic pore structure and grain diameter. In fact, the
plastic waste pyrolysis catalyst may be reused because the pore diameter
is determined to be the same as that of the new catalyst (Miskolczi et al.,
2009). In the pyrolysis of HDPE waste, sulfur content was reduced
dramatically from 75 mg kg
−1
–37 mg kg
−1
when the HZSM-5 catalyst
was used which shows that the catalyst usage facilitates minimizing
contaminants in the oil. The phosphorus and nitrogen content showed a
similar reduction pattern. Calcium content was exclusively found in
heavy fuel oil fraction, while no calcium was found in the light oil or
gasoline fractions.
Seo et al. (2003) studied the HZSM-5 effect at 450°C in the
pyrolysis of HDPE. They found that HZSM-5 produced a higher
gaseous yield (63.6 wt%) but very low liquid product (35 wt%) with a
20 wt% catalyst-to-polymer ratio. Hernández et al. (2007) acquired a
high gaseous yield (86.2 wt%) but lower liquid product (4.4 wt%) than
did Seo et al. (2003) at 500 C. Lin and Yen (2005) obtained very low
2.31 wt% and 3.75 wt% liquid yield at 360 C in PP pyrolysis by
utilizing HZSM-5 and HUSY zeolites with a 40 wt% catalyst-to-
polymer ratio. However, the coking resistance of HZSM-5 was
higher than that for HUSY when the product stream such as
pentene and butane increased during the process while iso-
pentanes and iso-butane persisted unaffected (Uemichi et al., 1998;
Lin and Yen, 2005;Obeid et al., 2014).
Apart from direct plastic cracking, some authors have also studied
the effectiveness of zeolite catalysts in two-step reaction processes
incorporating catalytic and thermal reactors (Vasile et al., 2000;
Syamsiro et al., 2014). In the two-step reaction process, Aguado
et al. (2007) investigated the LDPE catalytic conversion in a batch
reactor and fixed-bed reactor. In the batch reactor, the plastic would be
thermally cracked, and in the fixed-bed reactor, the generated vapors
were carried out where the catalyst HZSM-5 (10 wt%) was placed.
Pyrolysis was carried out at a temperature of 425–475°C. Catalytic
reforming with a zeolite catalyst increased the gas fraction significantly
which was around 74.4 wt%, however, the liquid oil yield was only
22.0 wt% at the maximum temperature. As a result, the observed trend
was extremely corresponding to catalytic direct degradation, which
yielded a high gaseous product when the HZSM-5 catalyst was used.
10.3.2 Fluid catalytic cracking
Fluid catalytic cracking (FFC) catalysts are commonly employed
in oil refinery processes to break the chains of high-molecular-weight
hydrocarbons, which is required to maximize the amount of gasoline
produced. Nowadays, FCC catalysts are made up of zeolitic materials
and different promoters and binders (Humphries and Wilcox, 1989;
Rajagopalan and Habib, 1992;Magee and Mitchell, 1993;Degnan,
2000). Due to its strong thermal stability and product selectivity,
Zeolite-Y has been the major FCC catalyst component for over
40 years (Marcilly, 2000). Kyong et al. (2002) studied the spent
FCC catalyst effect at 400°C on LDPE, HDPE, PS, and PP pyrolysis
in a stirred semi-batch reactor at 7°C/min heating rate. They found
that PS produced 90 wt% liquid yield which was the highest among all
other plastics (others produced over 80 wt%). In terms of the gaseous
product yield, PE produced the highest gaseous yield followed by PP
and PS. The liquid product yields which had an opposing order with
PS produced the highest followed by PP and PE (LDPE, HDPE). PS
produced less gaseous product because it had a benzene ring that
formed a more firm structure. Overall, spent FCC catalyst exhibits
good catalytic efficacy, with liquid yields over 80% for all plastic
specimens. Furthermore, because it is a “reused”catalyst, it is less
expensive.
Using the same experimental conditions, Kyong et al. (2003)
studied the spent FCC catalyst’sefficiency in comparison to HDPE
thermal pyrolysis without a catalyst, but with a temperature of
430°C. They observed that with the catalyst, the gaseous yield
slightly reduced from 20.0 to 19.5 wt% while increasing the yield
of liquid oil from 75.6 to 79.8 wt%. The presence of the catalyst also
reduced the solid residue from 4.5 to 0.8 wt%. Furthermore, the
formation of liquid oil from HDPE was observed at 350°C, which
means that the FCC catalyst lowered the HDPE reaction’s
temperature. In the case of thermal pyrolysis, the initial liquid
formed at 430 °C after 30 min. This means that in thermal pyrolysis,
using the spent FCC catalyst improved the overall product
conversion while also increasing the reaction rate.
Apart from this, the plastic pyrolysis product distribution may be
affected by different FCC conditions. FCC steaming catalyst, for
example, would change the composition and structure of the
catalyst. Olazar et al. (2009) proved this by conducting a study on
severe, mild, and fresh FCC catalyst steaming. Severe steaming was
performed for 8 h at 816 °C, while mild steaming was performed for
5 h at 760°C. The results exhibited that steaming increased the FCC
catalytic performance. As shown in Table 8, the fresh FCC catalyst
yields a high gaseous fraction and low diesel fraction, while severe FCC
steaming produced less gaseous products (C1–C4 hydrocarbon) and a
high diesel fraction (C10 + hydrocarbon).
TABLE 8 FCC fresh and steaming product distribution (Olazar et al., 2009).
FFC catalyst type Medium gasoline (C5–C9) (wt%) Diesel (C10+) (wt%) Gaseous (C1–C4) (wt%)
Severe steaming 20 70 5
Mild steaming 38 40 25
Fresh FCC 35 15 52
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Besides this, the polymer to catalyst ratio also greatly affects the
product composition and yield. Abbas-Abadi et al. (2014) investigated
the various HDPE to FCC catalyst ratios ranging from 10 to 60 wt% at
450°C in a stirred semi-batch reactor. They observed that the catalyst
to polymer ratio of 20 wt% was the prime ratio for higher liquid yield
conversion. The coke obtained was around 4.1 wt% with a very high
liquid yield of 91.2 wt% and gaseous product of 4.7 wt%. A decrease in
liquid production was observed by increasing more than 20 wt% of the
catalyst to polymer ratio, thus producing high gaseous product and
coke. This indicates that the catalyst/polymer ratio had to be
constrained in order to improve the conversion of the product,
particularly the yield of liquid oil and catalyst coke formation.
Kyong et al. (2002) explored the effectiveness of the FCC catalyst
with a catalyst to polymer ratio of 10 wt% to the various plastics types.
The results showed that a high yield of liquid was produced (80–90 wt
%) for PS, PP, HDPE, and LDPE, which shows the productivity of the
FCC catalyst in the pyrolysis of different plastic waste. Similarly,
Rodríguez et al. (2019) studied the pyrolysis of HDPE waxes using the
FCC catalyst at 3, 5, and 7 g
cat
g
oil
−1
ratio at 500–600°C and obtained
HDPE waxes product distribution of 36.7–65.1 wt%. Furthermore,
Palos et al. (2022b) under similar conditions used three FCC catalysts
and obtained 82.0 wt% heavy cycle oil and 12.5 wt% light cycle oil.
Moreover, Abbas-Abadi et al. (2014) also achieved a very high yield of
liquid oil (92.3 wt%) at 450 °C in PP pyrolysis with a 10 wt% catalyst/
polymer ratio. Conclusively, the utilization of the FCC catalyst was
recommended in the pyrolysis of plastic to optimize the production of
liquid oil. But in order to obviate the gaseous product and coke
dominance, the catalyst/polymer ratio must not exceed 20 wt%.
10.3.3 Silica–alumina catalyst
The amorphous acidic catalyst silica–alumina has Bronsted acid
sites with ionizable hydrogen atoms and Lewis acid sites which accept
electrons. The SiO
2
/Al
2
O
3
molar ratio determines the acid content of
silica–alumina catalyst. Opposite to zeolite, a high SiO
2
/Al
2
O
3
ratio
means high silica–alumina catalyst acidic strength. For example, SiO
2
/
Al
2
O
3
= 0.27 (SA-2) has a low acidic strength than SiO
2
/Al
2
O
3
= 4.99
(SA-1), and both are commercial silica–alumina (Sakata et al., 1997).
The catalyst mode also plays an important role in product distribution
and the product yield. This was proven by Sakata et al. (1997) on the
pyrolysis of PP at 380°C by utilizing the silica–alumina catalyst with
different contact modes: vapor phase and liquid phase. The catalyst
was assorted with PP pallets and in the liquid phase was placed into the
batch reactor. In contrast to the vapor phase, the catalyst was
suspended 10 cm from the reactor’s bottom on a stainless steel net.
From the experiment, they observed that a higher gaseous product
(35 wt%) was produced when the catalyst was in the vapor phase and a
low liquid product was produced because over the silica–alumina
catalyst, the polymer decomposed further into the gaseous product.
Alternatively, the catalyst in the liquid phase produced little gaseous
product but a higher yield of liquid (68.8 wt%) because the wax residue
over the silica–alumina catalyst disintegrated into a lighter
hydrocarbon.
In plastic pyrolysis, the final end product is greatly influenced by the
catalyst’s acidic strength. At 430°C, Sakata et al. (1997) investigated catalysts’
acidity effect on HDPE pyrolysis product distribution in semi-batch reactor
where 10 g of HDPE was mixed with 1 g of the catalyst (SA-1, SA-2, ZSM-
5). The catalysts’acidic strength was determined by using TPD (NH
3
temperature programmed desorption). From the results, it was shown that
SA-1 had high acidity followed by ZSM-5 and SA-2. From the
experimentation, they found the following liquid oil yield order: ZSM-5
(49.8 wt%) <SA-1 (67.8 wt%) <SA-2 (74.3 wt%). The catalyst with lower
acidity (SA-2) produced high liquid oil, while ZSM-5 having strong acidic
sites produced a low liquid yield when compared to the other catalysts but
produced high gaseous product. Uddin et al. (1996),byusingthesame
experimental conditions as Sakata et al. (1997), also investigated the SA-2
effect on LDPE and HDPE pyrolysis processes and obtained high liquid oil
by using LDPE (80.2 wt%) than using HDPE (77.4 wt%). The LDPE
structure was weaker than that of HDPE because of its branched chain,
thus LDPE produced high amounts of liquid yield. Moreover, the catalyst
reactivity can also be augmented under specific temperature ranges. Luo
et al. (2000) studied PP and HDPE pyrolysis processes in a fluidized bed
reactor at 500 °Cbyutilizingasilica–alumina catalyst, and they obtained
higher liquid oil than did Sakata et al. (1999) and Uddin et al. (1996).The
liquid product obtained was 90 wt% for PP pyrolysis and around 85.0 wt%
for HDPE pyrolysis. This demonstrates that temperature is also crucial in
maximizing catalyst effectiveness in the process of plastic pyrolysis to
optimize the yield of the liquid oil product. In conclusion, the FCC catalyst
is the ideal catalyst in plastic pyrolysis for optimizing liquid oil production.
The FCC catalyst in the pyrolysis processes of PP and HDPE produced
90 wt% liquid oil while the highest yield of liquid for HDPE and PP by
using silica–alumina was around 85–87 wt% (Luo et al., 2000;Abbas-Abadi
et al., 2013;Abbas-Abadi et al., 2014). This shows the effectiveness of the
FCC catalyst in plastic pyrolysis for product optimization and is also more
economically attractive than zeolite-based catalysts.
TABLE 9 The effect of carrier gas on the product yield and the condensed product composition (Abbas-Abadi et al., 2014).
Fluidizing
gas
Molar
mass
Yield of non-
condensable
product (%)
Yield of
condensed
product (%)
Olefins
(%)
Coke
yield
(%)
Naphthenes
(%)
Paraffins
(%)
Olefins/
paraffin
ratio
Aromatics
Ar 37 9.8 84.8 45.21 5.4 21.93 25.27 0.66 7.59
Propylene 42 9.7 87.8 42.36 2.5 20.92 31.85 1.33 4.87
Ethylene 28 5.1 93.8 41.76 1.1 19.75 34.76 1.2 3.73
N
2
28 4.1 92.3 44.63 3.6 17.23 32.87 1.36 5.27
He 4 3.2 94.7 43.32 2.1 19.29 33.41 1.3 3.98
H
2
2 3 96.7 30.86 0.3 20.54 46.53 0.66 2.07
Frontiers in Chemistry frontiersin.org26
Shah et al. 10.3389/fchem.2022.960894
TABLE 10 Results obtained by different authors by using pyrolysis and steam reforming technique.
Reactor Approach Plastic
type
Reaction
Conditions
(°C, -)
Bed
material
Composition
of gas (% vol)
Gas
yield
(m
3
/
kg)
Production
of H
2
(100g/
plastic)
Tar
yield
(g
/m
3
)
References
Plasma reactor (11
kg h
−1
)
Plasma
gasification
with CO
2
PE, PET
and PP
mixture
T: 1200–1400 - CO: 50, H
2
: 42, CH
4
:
0, CO
2
:7
-- <0.001 Hlina et al.
(2014)
Dual fixed bed (1 g) Pyrolysis–dry
reforming of
plastic
PE T: 500/800 -/Ni-Co-Al - - 15 0 Saad and
Williams,
(2016)
Dual fixed bed (1 g) Pyrolysis–dry
reforming of
plastic
PP T: 500/800 -/Ni-Co-Al - - 13.6 0 Saad and
Williams,
(2016)
Dual fixed bed (1 g) Pyrolysis–dry
reforming of
plastic
PS T: 500/800 -/Ni-Co-Al - - 7.6 0 Saad and
Williams,
(2016)
Dual fixed bed (1 g) Pyrolysis–dry
reforming of
plastic
PET T: 500/800 -/Ni-Co-Al - - 2.5 0 Saad and
Williams,
(2016)
Fluidized bed
(heterogeneous)
(0.08 kg h
−1
)
Plastic pyrolytic
oil Steam
reforming
Pyrolysis
oil of PE
S/C: 3.5(molar),
T: 570–800
Ni-Al
2
O
3
CO: 7–18, H
2
:70,
CH
4
:<1, CO
2
:19–12
4.7–5.8 37 - Tsuji and
Hatayama,
(2009)
Fluidized bed
(heterogeneous)
(0.08 kg h
−1
)
Plastic pyrolytic
oil Steam
reforming
Pyrolysis
oil of PE
S/C: 3.5(molar),
T: 600–800
Ni-Al
2
O
3
CO: 8–16, H
2
: 68,
CH
4
:<1, CO
2
:20–18
4.6–5.2 31.5 - Tsuji and
Hatayama,
(2009)
Two Fixed (packed)
bed (0.06 kg/h)
Pyrolysis–in
line steam
reforming
PP S/C: 3.6(molar),
T:400/580–680
-/Ru-Al
2
O
3
CO: 9–11, H
2
:71–70,
CH
4
: 1.5–1.4, CO
2
:
19–16
5.4–8.8 36.5 0 Park et al.
(2010)
Two Fixed (packed)
bed (0.06 kg/h)
Pyrolysis–in
line steam
reforming
PP S/C: 3.6 (molar),
T:400-600/630
-/Ru-Al
2
O
3
CO: 9–8, H
2
:71–72,
CH
4
: 1.5–0.9,
CO
2
:19
5.4–5.6 36 0 Park et al.
(2010)
Two Fixed (packed)
bed (0.06 kg/h)
Pyrolysis–in
line steam
reforming
PS S/C: 3.7(molar),
T:400/580–680
-/Ru-Al
2
O
3
CO: 5–10, H
2
:69–68,
CH
4
:0,CO
2
:25–21
4.2–5.2 33 0 Namioka et al.
(2011)
Spouted (conical)
bed/packed bed
(fixed) (0.05 kg/h)
Pyrolysis–in
line steam
reforming
PE S/C: 3.1, T:
500/700
sand/Ni-
CaAl
2
O
4
CO: 11, H
2
:71, CH
4
:
<1, CO
2
:17
5.4 34.5 0.11 Erkiaga et al.
(2015)
Spouted (conical)
bed/FBR
(heterogeneous)
(0.05 kg/h)
Pyrolysis–in
line steam
reforming
PE S/C: 3.1, T:500/
600–700
sand/Ni-
CaAl
2
O
4
CO: 11, H
2
:71, CH
4
:
<1, CO
2
:17
5.4 37.3 0 Barbarias et al.
(2016a)
Spouted (conical)
bed/FBR
(heterogeneous)
(0.05 kg/h)
Pyrolysis–in
line steam
reforming
PS S/C: 2.89, T:
500/700
sand/Ni-
CaAl
2
O
4
CO: 14, H
2
: 65, CH
4
:
<0.1, CO
2
:21
5 29.1 0 Barbarias et al.
(2016b)
FBR/FBR (0.06
kg/h)
Pyrolysis–in
line steam
reforming
PP S/C: 4.6 (molar),
T: 650/850
sand/
commercial
Ni catalyst
CO: 12, H
2
: 71, CH
4
:
1.2, CO
2
:16
5.4 34 0 Czernik and
French (2006)
FBR/FBR (0.06
kg/h)
Pyrolysis–in
line steam
reforming
PP S/C: 4.6 (molar),
T: 650/850,
ER: 0.25
sand/
commercial
Ni catalyst
CO: 12, H
2
: 65, CH
4
:
1.6, CO
2
:21
4.1 24 0 Czernik and
French (2006)
Dual fixed bed (1 g) Pyrolysis–in
line steam
reforming
PP T: 500/600–900 -/Ni-CeO
2
ZSM-5
CO: 8–26, H
2
:62–67,
CH
4
:7–4, CO
2
:16–4
-27–61 0 Wu and
Williams,
2009a
Dual fixed bed (1 g) Pyrolysis–in
line steam
reforming
PP T: 500/600–900 -/Ni-CeO
2
Al
2
O
3
CO: 9–27, H
2
:62–65,
CH
4
:4–1,
CO
2
:18–4,
-13–52 0 Wu and
Williams,
2008
(Continued on following page)
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Shah et al. 10.3389/fchem.2022.960894
10.4 Fluidizing (medium) gas effect in the
pyrolysis process
Fluidizing gas (also known as inert gas) is a carrier gas that is only
used to carry vaporized products and does not participate in the
pyrolysis process. The reactivity of the fluidizing gas (each type)
depends on its molar mass. Propylene, ethylene, hydrogen, argon,
nitrogen, and helium are some of the fluidizing gases that can be
utilized in the pyrolysis of plastics. According to Abbas-Abadi et al.
(2014), the carrier gas’s molecular size aids in defining the product
composition, which is also affected by temperature. The PP catalytic
pyrolysis product distribution was affected by the carrier gas’s
molecular weight, as shown in Table 9. High amounts of liquid oil
(condensed product) were produced by the lighter gas; 33.8 wt% liquid
was produced without using any carrier gas, while 96.7 wt% of the
liquid oil was produced by using H
2
as shown in Table 9. This
demonstrates the importance of carrier gas in improving pyrolysis
product yield. Apart from this, it has been discovered that the carrier
FIGURE 14
(A) Temperature effect on the production of H
2
in pyrolysis–reforming of plastic. (B) Steam/carbon ratio effect on the production of hydrogen in the
reforming process of plastic waste.
TABLE 10 (Continued) Results obtained by different authors by using pyrolysis and steam reforming technique.
Reactor Approach Plastic
type
Reaction
Conditions
(°C, -)
Bed
material
Composition
of gas (% vol)
Gas
yield
(m
3
/
kg)
Production
of H
2
(100g/
plastic)
Tar
yield
(g
/m
3
)
References
Dual fixed bed (1 g) Pyrolysis–in
line steam
reforming
PP T: 500/800 -/Ni-Al
2
O
3
CO: 20, H
2
:56, CH
4
:
6, CO
2
:9
-27 0Wu and
Williams,
2009e
Dual fixed bed (1 g) Pyrolysis–in
line steam
reforming
PP T: 500/800 -/Ni-CeO
2
CO: 6, H
2
:75, CH
4
:5,
CO
2
:7
-27 0Wu and
Williams,
2009e
Dual fixed bed (1 g) Pyrolysis–in
line steam
reforming
PP T: 500/800 -/Ni-Mg-Al CO: 24, H
2
: 64, CH
4
:
1, CO
2
: 10,
4.65 26.6 0 Wu and
Williams,
2010c
Dual fixed bed (1 g) Pyrolysis–in
line steam
reforming
PS T: 500/800 -/Ni-Mg-Al CO: 25, H
2
: 58, CH
4
:
1, CO
2
:10
3.57 18.5 0 Wu and
Williams,
2010c
Dual fixed bed (1 g) Pyrolysis–in
line steam
reforming
PE T: 500/800 -/Ni-Mg-Al CO: 20, H
2
: 67, CH
4
:
1, CO
2
:12
3.94 26.0 0 Wu and
Williams,
2010c
Dual fixed bed (1 g) Pyrolysis–in
line steam
reforming
MSW
plastics
waste
T: 500/800 -/Ni-Mg-Al CO: 20, H
2
: 67, CH
4
:
1, CO
2
:12
3.94 23.6 0 Wu and
Williams,
2010c
Conical spouted
bed/fluidized bed
(0.75g/min)
Pyrolysis–in
line steam
reforming
MSW
plastics
waste
T:500/700 -/Ni-Al
2
O
3
CaAl
2
O
4
CO: 9.9, H
2
:71 CH
4
:
0, CO
2
: 29.3
- 30.3 0 Barbarias et al.
(2018)
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Shah et al. 10.3389/fchem.2022.960894
TABLE 11 Experimental results of pyrolysis of plastic waste by different authors.
Reactor
configuration
Feedstock Temperature Heating
rate
(°C/min)
Pressure Residence
time (min)
Solid
(wt%)
Oil
(wt
%)
Gas
(wt%)
References
Batch (sequential) HDPE 450 ——60 19.7 74.5 5.8 Miskolczi et al.
(2004)
Batch (sequential) HDPE 550 5 —— 0 84.7 16.3 Marcilla et al.
(2009a)
Batch (sequential) LDPE 430 3 —— 7.5 75.6 8.2 Uddin et al. (1996)
Batch (sequential) LDPE 550 5 —— 0 93.1 14.6 Marcilla et al.
(2009a)
Batch (sequential) PP 380 3 1 atm —13.3 80.1 6.6 Sakata et al. (1999)
Batch (sequential) PP 740 ——— 1.6 48.8 49.6 Demirbas, (2004)
Batch (sequential) PS 500 ——150 0 96.73 3.27 AdnanShah and
Jan, (2014)
Batch (sequential) PS 581 ——— 0.6 89.5 9.9 Demirbas, (2004)
Semi-batch
(semi-flow)
HDPE 400 7 1 atm —28216Kyong et al. (2002)
Semi-batch
(semi-flow)
HDPE 450 25 1 atm —4.7 91.2 4.4 Abbas-Abadi et al.
(2013)
Semi-batch
(semi-flow)
PP 400 7 1 atm —28513Kyong et al. (2002)
Semi-batch
(semi-flow)
PP 450 25 1 atm —3.6 92.3 4.1 Abbas-Abadi et al.
(2014)
Semi-batch
(semi-flow)
PS 400 7 1 atm —4906Kyong et al. (2002)
Fluidized bed
(multiphase)
HDPE 500 ——60 5 85 10 Luo et al. (2000)
Fluidized bed
(multiphase)
HDPE 650 ——23 0 68.5 31.5 Mastral et al.
(2001)
Fluidized bed
(multiphase)
LDPE 600 —1 atm —0 51.0 24.2 Williams and
Williams, (1998)
Pressurized batch PS 425 10 0.31–1.6 MPa 60 0.5 97 2.50 Onwudili et al.
(2009)
Horizontal steel PP 300 20 —30 1.34 69.82 28.84 Ahmad et al.
(2014)
LDPE 500 6 1 atm —0.16 80.40 19.43 Fakhrhoseini and
Dastanian, (2013)
Pressurized batch LDPE 425 10 0.8–4.3 MPa 60 0.5 89.53 10 Onwudili et al.
(2009)
Vacuum batch PVC 520 10 2 kPa —28.13 12.79 0.34 Miranda et al.
(1998)
Horizontal steel HDPE 350 20 —30 1.88 80.88 17.24 Ahmad et al.
(2014)
PET 500 6 1 atm —8.98 38.89 52.13 Fakhrhoseini and
Dastanian, (2013)
Fixed bed (packed) PET 500 10 —— —23.1 76.9 Cepeliogullar et al.
(2013)
Fixed bed (packed) PVC 500 10 —— 0 12.3 87.7 Mastral et al.
(2001)
Fixed bed (packed) LDPE 500 10 —20 0 95 5 Bagri and
Williams, (2001)
Frontiers in Chemistry frontiersin.org29
Shah et al. 10.3389/fchem.2022.960894
gas’s reactivity influenced the formation of coke. Ar coke formation
was very high followed by N
2
, propylene, and helium, while H
2
coke
formation was very low. The molecular weights of nitrogen and
ethylene were the same. However, the reactivity of ethylene was
higher and produced lower coke and high liquid oil yield than
nitrogen because it could cause the equilibrium to shift, resulting
in a higher liquid yield (Abbas-Abadi et al., 2014). However, in plastic
pyrolysis, propylene and hydrogen were used the least by many
researchers because of the flammability risk, while nitrogen was the
more commonly utilized fluidizing gas since it is safer and easier to
handle.
Besides this, the flow rate of the fluidizing gas may also affect the
final end product. This was proven by Lin and Yen (2005) by using the
HUSY catalyst over PP pyrolysis at 360 °C. They observed that at
300 ml/min (the lowest fluidizing flow rate), the degradation rate
decreased instantly. At a lower flow rate, the primary product contact
time was high, leading the coke precursor formation to enhance with
the by-product achieved despite the rate of degradation being slower
(Lin and Yang, 2007). At 900 ml/min (highest fluidizing flowrate), the
hydrocarbon gases and gasoline fractions were increased. As a result,
in plastic pyrolysis, the rate and type of fluidizing gas are particularly
important, as they certainly affect the composition of the end product.
11 Pyrolysis and in-line steam reforming
Because of the high production of H
2
and operational dominance,
the two-step pyrolysis and in-line catalytic reforming of plastic waste
(Table 10) is likely the most propitious (Wu and Williams, 2009a;Wu
and Williams, 2010a;Namioka et al., 2011;Barbarias et al., 2016b;
Arregi et al., 2017). Moreover, waste plastic contaminants remain in
the reactor, avoiding contact which resulting catalyst deactivation (Wu
and Williams, 2010c). The steam catalytic reforming and thermal
degradation steps may potentially benefit from independent
temperature maximization (Park et al., 2010). In addition, when
compared to direct gasification, the process temperature is
substantially lower, reducing reforming catalyst sintering issues and
material costs (Barbarias et al., 2016a;Barbarias et al., 2016b). As a
result, this process removes tars completely from the gaseous product
because of the usage of the very active reforming catalyst, which is the
main advantage of this process. This approach has also been shown to
be useful in H
2
production from biomass (Xiao et al., 2013;Ma et al.,
2014;Arregi et al., 2016). Furthermore, a new option for the plastic
waste pyrolysis–reforming method has recently been presented, which
entails producing carbon nanotubes and H
2
, simultaneously utilizing
various Fe- and Ni-based catalysts (Yang et al., 2015;YangRChuang
and Wey, 2016;Bajad et al., 2017;Liu et al., 2017;Yao et al., 2017).
Prof. Williams conducted a detail study on the
pyrolysis–reforming (in-line) method by using various catalysts on
waste plastics (Wu and Williams, 2009b;Wu and Williams, 2009c;Wu
and Williams, 2009d;Wu and Williams, 2009e;Wu and Williams,
2010a;Wu and Williams, 2010c;Acomb et al., 2014;Saad et al.,
2015b). The experimental setup comprised of dual fixed-bed (packed)
reactors operating in batches for the pyrolysis and reforming steps. At
40°C–500°C/min, the volatiles formed in the pyrolysis reactor were
subsequently processed in the reforming packed bed reactor (800°C).
The production of H
2
was 26.6 wt% when PP was fed, while PS
produced only 18.5 wt% H
2
.Inbothcases,theNi-Mg-Alcatalyst
was used (Cho et al., 2013a). Furthermore, in the reforming of
derived PP volatiles, the production of H
2
was approximately 65%.
But this time, the authors used Ni-based commercial catalyst (Wu
and Williams, 2008;Wu and Williams, 2009c). The same authors
recently used CO
2
instead of steam to investigate the dry reforming
of pyrolysis volatiles from plastics (Saad and Williams, 2016). This
innovative approach is an intriguing CO
2
valorization technique
since it achieves nearly complete conversion, with the produced
syngas primarily consisting of CO and H
2
. By using PET, PS, PP,
and PE at 500 and 800°C, the values of H
2
production are the
following: 2.5, 7.6, 13.6, and 15.0 wt%, respectively. The catalyst
utilized was Ni-Mg-Al in the pyrolysis and reforming steps. These
results are significantly inferior to those found in pyrolysis and in
line with steam reforming.
Czernik and French (2006) studied pyrolysis and in-line steam
reforming of PP by using commercial Ni-based catalyst at 650 and
800°C in two FBR reactors. The derived plastic volatiles were
completely altered into gaseous stream free of tar, with 34 wt%
H
2
production (34 g 100 g PP
−1
). This yield is 80% of the highest
stoichiometrically allowable. By co-feeding air with an equivalence
ratio of 0.25 into the reforming step while operating under
reforming autothermal parameters, the production of H
2
was
lowered to 24 wt%.
Surprisingly, the process has been run successfully at a steady
(equilibrium) state for 10 h without detecting any deactivation of
the catalyst. Moreover, plastic pyrolysis and in-line reforming were
also investigated by Erkiaga et al. (2015), who developed an
experimental unit consisting of a CSBR reactor and fixed-bed
(packed) reactor for pyrolysis and the steam catalytic reforming
step. HDPE was pyrolyzed at 500 °C and then the reforming step
was performed at 700°C using a commercial Ni catalyst. The
reforming catalysts showed remarkable efficiency and completely
converted the waste plastics into gaseous products, with 34.5 wt%
of H
2
yield, which is 81.6% of the stoichiometric maximum
allowable. The formation of coke (4.4 wt% of the feed) is the
biggest issue in this process, as it obstructs the flow of the
reactant in the reforming fixed-bed (packed) reactor. To avoid
these operational and functional challenges, Barbarias et al. (2016a)
replaced the fixed-bed reactor with the FBR reactor for the
reforming step. During experimentation, they obtained a higher
H
2
yield (38.1 wt%) than did Erkiaga et al. (2015), which accounts
for 92.6% allowable stoichiometry. This demonstrates the benefits
of employing an FBR reactor for the reforming process. Further
research using PS validated the high efficiency of this setup (CSBR
and FBR) for the pyrolysis and reforming process (Barbarias et al.,
2016b). Namioka et al. (2011) also conducted studies on pyrolysis
and in-line reforming, but they used PS instead of HDPE and
obtained 29.1 wt% H
2
yield which was lower than that obtained by
Barbarias et al. (2016a).ThevaryingH
2
concentration of these
polymers is related to this result. The deactivation kinetics, as well
as the type of the deposited coke, are thus dependent on the
hydrocarbons produced during the degradation of the polymer
(Barbarias et al., 2016b;Barbarias et al., 2016c).
Park et al. (2010) and Namioka et al. (2011) developed a two-step
PP pyrolysis and reforming method based on two fixed-bed
(packed) reactors (1 g min
−1
) operating in a continuous
framework. The pyrolysis and reforming steps were carried out
between 400–600°C and 580–680°C. The reforming step was
Frontiers in Chemistry frontiersin.org30
Shah et al. 10.3389/fchem.2022.960894
performed on a Ru-Al
2
O
3
commercial catalyst. Because of a
considerable increase in the yield of coke at high temperatures,
the optimal outcomes were achieved at 630°C, which is the average
temperature studied. As a result, the hydrocarbon liquids were
altered completely into coke and gaseous products at 630 °Cand
the production of H
2
reached 34.2 wt%. Using similar
experimental conditions and units (Namioka et al., 2011), the
same authors investigated the PS two-step pyrolysis and
reforming process and obtained a lower H
2
yield (33.0 wt%)
than obtained when using PP.
In comparison to conventional gasification, the pyrolysis two-step
and in-line volatiles reforming process allow for 100% conversion,
resulting in a gaseous stream with high H
2
concentration and no tar or
liquid hydrocarbons. As a result, different authors have reported the
values of H
2
production above 30 wt% (Tsuji and Hatayama, 2009;
Park et al., 2010;Namioka et al., 2011;Erkiaga et al., 2015;Barbarias
et al., 2016a;Barbarias et al., 2016b;Arregi et al., 2017). In the
pyrolysis–reforming process, the most important parameters which
affect the end products are the steam/carbon ratio and reforming
phase temperature. Figure 14A depicts the effects of both factors on H
2
production, respectively. As shown in Figure 14A, the H
2
production
improves by enhancing the reforming temperature, thus
increasing the endothermic steam reforming reactions (ESRRs)
comprising hydrocarbons, despite the water–gas shift (WGS)
reaction equilibrium limiting this improvement. In the reaction
environment, the partial steam pressure increases as the steam/
carbon ratio rise, enhancing both the water–gas shift reaction and
reforming processes, thus favoring the production of H
2
,butat
high steam/carbon ratios, this effect is reduced as seen in
Figure 14B. However, the indirect approach for the production
of H
2
via biomass oil (pyrolysis oil) reforming has been
extensively investigated (Trane et al., 2012;Chen et al., 2017;
Nabgan et al., 2017), and this route has been studied infrequently
in the plastic waste case. Tsuji and Hatayama (2009) only studied
the H
2
production indirect route from plastic waste. The oil
produced by the pyrolysis of LDPE was evaporated at 600 and
800 °C and subjected to catalytic steam reforming in a fluidized
bed reactor on a Ni-Al
2
O
3
catalyst. The gas generated has an H
2
composition of roughly 70% volume, which is near the equilibrium
value and contributes to 37.0 wt% of the total production. The oil
reforming derived from PS pyrolysis has also been investigated, with
the production of H
2
being 31.6 wt% in this case. Even under ideal
conditions, the values of H
2
production produced in the pyrolysis and in-
line reforming approach are substantially greater than those normally
achieved in the steam plastics gasification, which are often below 20 wt%
(HeMXiao et al., 2009a;Erkiaga et al., 2013a;Martínez-Lera et al., 2013a).
Similarly, due to the high H
2
and carbon content in plastics, the
production of H
2
achieved through biomass pyrolysis–reforming and
steam gasification is significantly lower, ranging from 2–8wt%(Rapagna
et al., 2000;Luo et al., 2009;Umeki et al., 2010;Erkiaga et al., 2014)to
4–11 wt% (Xiao et al., 2013;Ma et al., 2014;Arregi et al., 2016). As a result,
the pyrolysis–reforming technique for plastic waste valorization is a
promising approach.
12 Summary of pyrolysis of plastic waste
Table 11 outlines the various parameters which affect the
composition of the final end products (gas, liquid, and solid) at
different conditions in catalytic and thermal pyrolysis processes.
The fluidizing media used in all of the studies was nitrogen gas.
Compared with other plastics, PVC and PET generate very low
liquid oil yield (based on Table 11), making them less commonly
investigated by authors. In pyrolysis, PVC was not recommended
since it produces toxic HCL acid and has a low liquid oil yield.
Furthermore, the oil produced by PVC includes chlorinated
compounds, which potentially decrease the quality of the oil
and are also harmful to the environment.
In thermal degradation, the ideal temperature in plastic
pyrolysis for maximizing liquid oil production is between
500 and 550°CasshowninTable 11. Nevertheless, the
utilization of the catalyst in plastic waste pyrolysis allowed the
optimal temperature to be decreased to 450°C, resulting in a
significant increase in liquid yield production. Among the
plastics, polystyrene (PS) is the best plastic for the pyrolysis
process and produced 97 wt% of liquid oil without any catalyst
compulsions (Onwudili et al., 2009). In terms of polyolefinplastic
types in thermal pyrolysis, PP provided the lowest yield of liquid oil
(82.12 wt%) and LDPE provided the highest (93.1 wt%). However,
product optimization of 90 wt% or above is possible by using the
appropriate catalysts and performing experiments at the right
operating parameters.
The preparation of useful materials in tribology is also an
interesting application of plastic waste recycling (Iqbal et al., 2020;
Iqbal et al., 2022). This is another alternative to disposing of plastic
waste and recycles it to develop lubricating oil for tribological
applications. Recently, Hackler et al. (2021) compared the
tribological performance of synthetic lubricants derived from
HDPE, LLDPE, and bubble wrap with industrial-grade oils. Their
findings suggest that the lubricants derived from the waste plastics
outperformed the traditional mineral oil with a 43% improvement in
wear volume when compared to Group III minerals. Furthermore,
Sikdar et al. (2020) studied the frictional behavior of pyrolyzed oils
derived from waste plastics, and their results indicate that these
pyrolyzed plastic waste oils exhibit similar frictional behavior when
compared to bio-based lubricants. Moreover, the waxes obtained
during polyolefin plastics (PP and PE) fast pyrolysis and oil
produced during tire pyrolysis together can be co-fed with the
industrial current stream units. It is an opportunity for
conventional refineries to operate as a waste refineries by co-
feeding these feeds alternatively and adjusting the fuel
characteristics and raw materials produced, to be tailored to
commercial objectives within the oil economy framework (Palos
et al., 2021).
Considering the Sustainable Development Goals (SDGs),
lubricants derived from pyrolysis and gasification of plastic
waste not only have the potential to reduce plastic pollution but
also the potential to replace industrial-grade oils for tribological
applications.
13 Conclusion
This study gives a comprehensive overview of gasification
and plastic pyrolysis for each classification, as well as a
discussion of the most important influencing aspects for
optimizing H
2
production and liquid oil yield. In contrast to
conventional combustion (incineration), one of the key
Frontiers in Chemistry frontiersin.org31
Shah et al. 10.3389/fchem.2022.960894
contentions for gasification and pyrolysis is to enhance
ecological performance and the possibility for ameliorating
emission control. In the literature studies, most researchers
have preferred pyrolysis process over gasification because it
has the greatest potential for converting most of the waste
plastics energy into useful char,gas,andliquidoil.The
fundamental obstacle of gasification of plastic waste is the
formation of tar, which leads to major operational challenges,
thus reducing the gas yield and influencing the total process
productivity. The pyrolysis process also has drawbacks, such as a
more complex product stream and the inability to directly vent
product gases due to high concentrations of CO. The
composition and variable quality of the feed is a considerable
challenge for all plastic conversion processes. The long-term
viability of these processes are indisputable because by using
these valorization routes, the management of waste becomes
highly systematic, with less landfill space required, lower cost,
and less pollution. As a final conclusion, the ideal way to
encounter plastic pollution is to recycle plastic waste either by
gasification or pyrolysis.
Author contributions
HS, MA, and AI suggested the idea of this work, wrote the
manuscript and made the final improvements. HS, MA, and AI
provided help with the alignment of this article. HS, MA, AI,
IN, MK, AMS and AMG proofread, edited, and made
improvements to this article. IN and MK provided the
financial assistance.
Acknowledgments
Publication supported under the Excellence Initiative - Research
University program implemented at the Silesian University of
Technology, one year 2022 under the project no 32/014/SDU/10-
22-25, Also Irfan Nadeem and Mitjan Kalin would like to acknowledge
the partial financial support from the Slovenian Research Agency
(ARRS), Slovenia (research core funding No. P2-0231).
Conflict of interest
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.
Publisher’s note
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, editors, and 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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