Journal of Analytical and Applied Pyrolysis 152 (2020) 104804
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Journal of Analytical and Applied Pyrolysis
journal homepage: www.elsevier.com/locate/jaap
Pyrolysis of plastic waste: Opportunities and challenges
a,
a
a
b
a
Muhammad Saad Qureshi *, Anja Oasmaa , Hanna Pihkola , Ivan Deviatkin , Anna Tenhunen ,
Juha Mannilaa, Hannu Minkkinena, Maija Pohjakallioc, Jutta Laine-Ylijokid
T
a
VTT- Technical Research Centre of Finland, Tietotie 4C, Espoo, Finland
Department of Sustainability Science, School of Energy Systems, Lappeenranta-Lahti University of Technology LUT, 53850, Lappeenranta, Finland
Sulapac Ltd, Iso Roobertinkatu 21, 00120, Helsinki, Finland
d
FCG Finnish Consulting Group Ltd., Osmontie 34, PL 950, 00601, Helsinki, Finland
b
c
ARTICLE INFO
ABSTRACT
Keywords:
Pyrolysis
Plastic waste
Chemical recycling
Circular economy
Environmental impact
With current low recycling rates and exponentially increasing production of plastics there is an increase in
plastic material wastage, and thus new technologies are needed for waste refining. Presently in Europe, only
about 10% of plastic waste is recycled, most of which is achieved through mechanical recycling. Chemical
recycling methods like pyrolysis could significantly increase these recycling rates, as it can utilize mixtures of
waste plastics unlike mechanical recycling. It can also be used to treat waste of many novel materials, such as
composites, especially in the emerging phase when the volumes of the new materials in markets are low making
separate collection of waste not a cost-efficient option. Pyrolysis offers an environmentally sound alternative to
incineration and inefficient landfilling. Currently, main challenges for pyrolysis of plastic waste are unavailability and inconsistent quality of feedstock, inefficient and hence costly sorting, non-existent markets citing lack
for standardized products, and unclear regulations around plastic waste management. Possible solutions could
include tight cooperation between feedstock providers and converters for securing steady quantity and quality of
feedstock. Advanced pre-treatment would provide the basis for cost-effective recycling. The classification of
pyrolysis liquid as a product instead of waste is needed, and the REACH (Registration, Evaluation, Authorisation
and Restriction of Chemicals) registration should be carried out to standardise the liquid oil as a product. In
addition, sustainability impacts need to be clearly positive.
1. Introduction
The recycling industry together with the waste management operators are facing two common challenges related to development of
new business: most of easily recyclable waste is already being recycled
and secondly, the secondary raw materials markets are challenging as
prices of secondary raw materials compared to primary raw materials
are not typically price competitive and overall the markets for secondary raw materials are still not properly established. This increases
the demand for the development of new technological solutions that
techno-economically compete with new materials - technological developments in waste collection, sorting and recycling processes can
result in efficiency gains and increased usage of recycled plastics in new
products. Mechanical recycling of plastics alone, given the limitations
of the technology, is insufficient to meet the demands of the of the
projected recycled plastics markets; therefore, feedstock recycling
technologies where a material is recycled into its basic monomers are
being sought for. Going back to and starting further processing from
⁎
monomers also ensures that legacy substances are not transferred to
new products which enables safe and circular economy.
In 2017, 348 million tonnes of plastic were produced globally and it
is estimated that this demand will quadruple by 2050 [1,2]. Since
1950s plastics mass production, global plastic waste is estimated to be
6300 million tons, of which 79% has accumulated in landfills and environment [3]. It has been almost 40 years since the recycling symbol
for plastics was introduced and still just 14% of plastic packaging is
collected today and only 2% is closed-loop recycled into the same
quality material [4]. The demand for efficient and cost-effective recycling technologies is acute – only about 10% of plastics produced in
Europe is presently recycled (Fig. 1), and almost all commercial plastic
recycling plants use mechanical recycling routes. In mechanical recycling plastic is sorted, washed, melted into granulates and finally
formed into new products. Currently in Europe, approximately over 5
million tonnes of plastic waste is mechanically recycled and only
around 50,000 tons of plastic waste is chemically recycled. [5] Typical
plastic waste recycling systems are logistically more complex with
Corresponding author.
E-mail address: muhammad.qureshi@vtt.fi (M.S. Qureshi).
https://doi.org/10.1016/j.jaap.2020.104804
Received 31 October 2019; Received in revised form 15 February 2020; Accepted 2 March 2020
Available online 06 March 2020
0165-2370/ © 2020 Elsevier B.V. All rights reserved.
Journal of Analytical and Applied Pyrolysis 152 (2020) 104804
M.S. Qureshi, et al.
Fig. 1. Recycled volumes of plastics in Europe in 2015. [7].
separate collections and several different flows than other traditional
waste processing systems, like metals or paper which results in higher
plastic waste management costs [5]. Thus, mechanical recycling works
best with separately collected plastics, and fails to efficiently recycle
mixed plastic waste. However, for example, 40–60 % of separately
collected plastic waste in Finland ends up being incinerated [6]. Situation is similar at the European level, where a large share of plastics
collected for recycling ends-up as rejects during recycling (see Fig. 1).
There are multiple bottlenecks and challenges that hinder the increase of recycling of plastic waste. Apart from economic infeasibility,
there are also multiple technical challenges. Complexity of plastic recycling increases when dealing with more challenging plastic products
like multi-layer materials or plastics that contain harmful substances as
additives like brominated flame retardants, phthalates, etc. [8].
Besides mechanical recycling technologies, there are also chemical
routes to recycle the plastic waste. Chemical recycling technologies
[2,58,60] are likely to play a crucial role in the transition towards
circular economy and closed-loop recycling of materials and compounds like hydrocarbons. These technologies enable the removal of
hazardous substances and thus the handling of challenging waste
streams. Commitment from materials and chemicals producers to use
raw materials from secondary sources is a prerequisite for the development of sustainable, feasible and cost-efficient chemical recycling
value chains.
One of the most important chemical recycling technologies is pyrolysis, also referred to as thermolysis. Pyrolysis is degradation of organic materials under the effect of heat and in the absence of oxygen
[9]. Depending on the process conditions, pyrolysis yields typically a
mixture of molecules in the form of liquid or wax as main products
[10]. Produced liquid or wax can be refined into chemicals or fuels
[11,12,88]. Table 1 presents examples of pyrolysis products from
different polymers. It is of special interest that some resins, like polystyrene (PS), polymethyl methacrylate (PMMA), and polyamides (PA)
produce high yields of their monomers upon thermal degradation.
However, collected plastic waste of municipal origin is often a mixture
of various plastics and, therefore, yields a mixture of products including
different hydrocarbons of various chain lengths.
In order to improve the recycling of mixed plastics, several steps
should be undertaken. This demands the optimization of the whole
value chain from waste collection to recycled products, the efficiency of
collection, monitoring and sorting to produce suitable feeds to target
products. According to the EU Plastics Strategy report published in
2019 and circular economy principals, it is essential to apply eco-design
approach to plastic products and whole value chain. Furthermore, it is
even more fundamental to introduce the approach especially in the
early stages of plastic value chain, e.g. in material and product design,
to ensure circularity at the end-of-life stage. [13]. A holistic approach is
required and optimal solutions can be formed, e.g. by integrating mechanical and chemical recycling. Pyrolysis is an interesting possibility
for safe circular economy as it can handle legacy additives and harmful
substances and stop the transfer of these substances into new products.
Besides techno-economic feasibility, the environmental sustainability of
the value chain is a key issue in further development and commercialization. A life cycle approach is necessary for an overall evaluation
related to environmental impacts and benefits [14]. Important sustainability aspects relate to plastics recycling include possibilities for
reducing greenhouse gas emissions, reducing the need for primary raw
materials and removing harmful substances. In case the products from
pyrolysis would be used as a fuel, the requirements of the European
Renewable Energy Directive (REDII) would need to be considered.
This paper discusses the underlying opportunities and challenges
related to pyrolysis of plastic waste. It includes among others, data
Table 1
Examples of pyrolysis products from different polymers. Modified from Ref. [12].
Resin
Major origin of waste
Low temperature products*
High temperature products*
PE
PP
PS
PA-6
PMMA
PET
PU
PVC
Household, industrial plastic packaging, agricultural plastics
Household and industrial packaging, automotive
Household, industrial plastic packaging, construction, demolition, WEEE
Automotive waste
Automotive, construction waste
Household plastic packaging
Construction, demolition, automotive
Construction plastic waste
Waxes, paraffins,
Waxes, oils
Styrene, its oligomers
ԑ - Caprolactam
Gases and light oils
Gases and light oils
Styrene, its oligomers, PAH
Terephthalic acid,
Di-isocyanate
HCL, benzene
MMA (methyl methacrylate),
benzene, benzoic acid, formaldehyde, acetaldehyde, CO2, CO
Methane, CO, aromatics
Toluene
PE = Polyethylene, PP = Polypropylene, PS = Polystyrene, PA-6=Polyamide 6, PMMA = Polymethyl methacrylate, PET = Polyethylene terephthalate,
PU = Polyurethane, PVC = Polyvinyl Chloride, HCl=Hydrochloride, WEEE = Waste Electrical and Electronic Equipment, PAH = Poly Aromatic Hydrocarbons.
*
Low temperature refers to < 400 °C, High temperature refers to >700 °C.
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M.S. Qureshi, et al.
created in a Finnish Business Finland granted national research project
“Sustainable high value products from low-grade residues and wastes”,
[15] where the overall goal was to generate novel sustainable solutions
for bio and circular economy meeting the research needs of the Finnish
industry. During the project, the whole value chain from pretreatment
of industrial plastic waste feedstocks via pyrolysis to products was
studied.
The paper is organized as follows: First, the main technical opportunities and challenges related to pyrolysis of plastic waste are discussed. Secondly, two potential solutions are presented. Thirdly, main
opportunities and challenges related to the operational environment,
namely sustainability requirements and complex regulatory framework
are discussed. The final part of the paper consists of a concluding
summary that presents recommendations for future development of
pyrolysis of plastic waste.
2. Technical opportunities
simple process, it has the capability to treat various kinds of plastic
waste ranging from packaging waste to more complex materials, like
rubber and plastics from WEEE (waste electrical and electronic equipment), hospital waste, and ELV (end of life vehicle) which are contaminated with toxic and hazardous substances [12,21–27]. Pyrolysis of
plastics besides liquid products generates also hydrocarbon rich gas,
which has, depending of feed and conditions, a heating value of
25–45 MJ/kg [15] making it ideal for energy recovery. Therefore, in
many cases [9] the pyrolysis gas is circulated back into the process to
extract the energy for the heating purpose. This use of pyrolysis gas for
heating substantially reduces the reliance on external heating sources.
Depending on the scale of operation pyrolysis technology allows
efficient downscaling to small plants making them essentially als mobile [15,28]. These portable systems could be installed on the waste
concentration sites where the feedstock is abundant but the transport
costs are high. For example, in Finland scalable pyrolysis solutions are
possible to locate in inhabited regions per environmental permits [29].
2.1. Quality of the feedstock
2.3. Product selection
Plastic waste is generally classified into two broad categories: postindustrial and post-consumer. The characteristics of these waste
streams are quite different. While the post-industrial plastic waste is of
clean, consistent quality and has usually defined composition, the
plastic waste collected from consumers is rather dirty and is contaminated with different sorts of foreign materials including organic
waste, wood pieces, glass, metals etc. This makes the post-consumer
waste non-ideal for mechanical recycling unless otherwise thoroughly
cleaned and separated. This dirty and heterogeneous plastic waste
stream is usually incinerated or landfilled.
Plastic materials deteriorate gradually over their lifetime. This deterioration results from several factors including sunlight, air, water,
thermal stresses from cold and hot conditions, chemical and biological
degradation from different contact materials, etc.. Oxygen contributes
to the photo-oxidative degradation of plastics, which cause material
embrittlement, colour change, cracks, and other changes [16]. In most
cases, these degradations in the properties of the plastic material renders the materials unfit for recycling into high-end applications and in
some cases even unfit for mechanical recycling at all. Pyrolysis of
plastic on the other hand allows such plastic waste to be valorized into
valuable fuel oil/monomers. Although, there is a limit to what can be
fed to pyrolysis process, in comparison to mechanical recycling it can
tolerate considerably higher levels of contaminants in the feed which
makes it attractive in terms of economics, considering reduced number
of pretreatment steps needed upstream [17].
Integrated chemical recycling of wood waste and plastic waste via
pyrolysis is also an interesting option. The possibilities and process
optimisation of pyrolysis based on co-feeding of wood-based waste and
plastics waste have been studied for example by Sajdak et al. [18,19]. It
should also be noted that many novel material innovations are either
composites consisting of several types of polymeric materials or completely novel polymer resins. For them integrated pyrolysis could provide a feasible means for recycling especially in the emerging phase
when the volumes of the new materials on market are low, and thus
separate collection of waste is not a cost-efficient option.
The pyrolysis process gives the freedom to tune the product by
varying the reactor type, operating conditions and use of catalysts. This
is highly advantageous in terms of the economics of the whole process.
While in most cases, the pyrolysis liquid yield is the main goal of the
process, the process can be adapted to optimize the production of wax
[30], monomers [12], aromatics [31–34] or selective chemicals [35,36]
with the use of a suitable catalyst [37–40].
2.4. Energy storage
The main distinction between pyrolysis and other thermochemical
conversion techniques is that it allows the waste to be converted primarily into liquid oil and wax. These products can be efficiently/feasibly stored and utilized flexibly as an energy source. [41]
2.5. Industrial integration
The oil and wax obtained from the waste plastic pyrolysis are rich in
hydrocarbons, which makes them an ideal raw material for a refinery.
Several studies have been carried out to simulate the integration of
pyrolysis of plastics to conventional refinery. Also, the integration has
been tested for example at the ReOil pilot plant at OMV oil refinery in
Austria [42], where the plastic waste is pyrolyzed and the liquid is fed
to the refinery unit. Though compared to the crude oil, the scale is very
insignificant at the moment, but the proof of concept is validated.
A recent report by McKinsey states that waste plastics derived
feedstocks will play a crucial role in the renewal of the whole petrochemical industry. McKinsey estimates that by 2030 one third of the
plastics entering the global market could be manufactured from recycled plastics. [43] This would necessitate considerable increase in
mechanical recycling and in the use of thermochemical technologies
which convert waste plastics to cracker feedstock for the petrochemical
industry. The estimate is that it would also be economically beneficial
for the petrochemical industry and for the plastics industry, assuming a
$75-a-barrel (approximately 159 litres) oil selling price and an effective
regulatory framework reinforced by supportive behaviour from other
industry stakeholders and consumers.
2.2. Simple and advantageous process
Pyrolysis technology has been developed over the years to valorise
organic materials into valuable liquid fuels. Extensive research has been
dedicated to the process to cater the variable needs of the products and
challenges arising from the different feedstock types. It also entails
considerable efforts in designing new reactor types over the years to
optimize the organic yields while minimizing the energy needs.
Pyrolysis is a mature technology and commercial plants are operating
for biomass [20] and for plastics (Table 2). While pyrolysis is rather
3. Technical challenges
3.1. Feedstock availability
The non-availability of plastic waste has been one of the main factors in the past why many plastic recycling initiatives failed despite
being technically sufficient [9,44]. In order to accomplish economically
feasible pyrolysis of plastic waste, a steady flow of rather consistent
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Table 2
Reactor comparison for plastic thermolysis. Modified from [49].
BFB = Bubbling fluidized bed, CFB = Circulated fluidized bed.
quality feedstock is required. Also, pointed out by Ragaert et al [63],
the economics is dependent on the large volumes of feedstock. In the
past, nearly all plastic waste was exported to Asian countries and particularly to China from the EU countries. However, in 2018 China
banned and stopped the import of plastic waste citing environmental
and health consideration. The ban triggered some of the export of
plastic waste to other developing countries. In 2019, the United Nations’ Basel Convention that manages flow of toxic waste from richer to
poorer countries was amended to also include plastic waste and signed
by nearly all, 187, countries. Consequently, large volumes of the waste
plastics now remains in the EU countries to be treated locally. In order
to deal with this change, many initiatives, regulations and programs in
the EU countries are launched to increase the recycling rates of plastics
by bringing innovations to plastic recycling methods. For example, by
2025, 55% of plastic packaging needs to be recycled, which is more
than twice the current volume [13]. Considering these drastic changes
to waste recycling value chains, while it is important to have the waste
supplier in the recycling value chain, the feedstock non-availability will
most likely not be a major bottleneck in future.
designed to tolerate high concentrations of PVC in the feed during
pyrolysis [44–48,59,76,77] or before pyrolysis using hydrothermal
treatment [76], mechano-chemical treatment [77] and subcritical
water extraction [78], it is generally avoided in the feedstock. PVC is
mostly used in the construction sector and is not among the highest
contributor in the packaging industry [61,62]. However, PET is found
in abundance in the packing industry. While mechanical recycling of
PET is well established, chemical recycling of PET such as by glycolysis,
methanolysis, hydrolysis etc. enables PET to be completely depolymerized into its monomer constituents terephthalic acid, dimethyl
terephthalate, bis(hydroxylethylene) terephthalate (BHET), and ethylene glycol [63]. In most cases the recycled PET from mechanical recycling is used in cascaded application owing to inferior material
properties. Furthermore, these techniques are still however limited to
clean PET feedstock which motivates the collection of clean PET plastics. For the PET found in the packaging waste, there has been few
methods to catalytically prevent the formation of high boiling terephthalic acid while treating it with other polymers in pyrolysis
[79–81].
3.2. Feedstock selection
3.3. Pretreatment and material feeding
Plastics collected from different waste streams are very heterogeneous. The types of polymer mixtures vary significantly from
packaging to WEEE or from ELV to agricultural waste streams.
Polyolefins dominate as major shares of plastic items, PET and PVC are
also found in various streams. While polyolefins make an ideal feed for
pyrolysis, degrading into valuable hydrocarbon products, PET and PVC
type plastics are problematic in the feed and are limited in the feedstock
mixtures to less than 5% and typically around 1 to 2% [9]. The thermal
degradation products of these polymers are detrimental to the process
and product. PVC upon thermal degradation produces chlorinated hydrocarbons and HCl, which causes corrosion in the reactor and renders
the oil halogenated. PET tends to decompose into phthalic acids deteriorating the oil quality in addition to clogging the pipes. The pyrolysis
of PET with PVC enhances the formation of organic chlorine [75].
While in many cases in the past, several pyrolysis processes were
Pretreatment of plastic waste is an essential step before pyrolysis. It
must be ensured that plastic waste is not contaminated with foreign
materials such as metals, wood etc. In order to maintain the economic
feasibility of the Plastic to Fuel (PTF) plant, the plastic waste must be
presorted. The origins of the plastic waste also plays an important role
because plastic items from different sources have different shapes and
sizes. In many cases, it is challenging to process such waste without
pretreatment and therefore it is necessary to uniformly size them by
crushing and sieving before feeding into the pyrolysis process. This is an
additional step which incurs an extra cost to the whole process.
However, in some cases, feeding big items and with various shapes and
densities is not a limiting factor. For example, rotary kilns can accommodate plastic waste with varied shapes and sizes. For some reactors,
especially fluidized bed reactors, the feedstock should be more or less
evenly sized to have the uniform thermodynamics in the reactor. In
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M.S. Qureshi, et al.
order to cope up with this challenge, several feeding devices have been
tested. Some plants use screw feeders while the use of hydraulic feeding
system is not uncommon. Plastic films are difficult to feed through the
screw feeders; they tend to entangle around the screw. They also form a
melt on the heated screw making it difficult for the system to continue
with feeding. [9]
waste [12,24–27]. One good example is the Haloclean process developed by Hornung et al [65] to treat the thermosetting composites like
electronic scrap. The system is based on an airtight rotary kiln equipped
with metal spheres to promote heat transfer. Screw reactors are also
used for feedstock recycling, for example, a German project ResolVe
[87] aims at recycling Polystyrene into monomer styrene through depolymerization. Kiln reactors have simple design and are in general
good for treating complex waste of different feedstock shape and size,
however if not optimally designed they suffer from poor heat transfer
and temperature control resulting in fussed polymer material to the
interiors of the reactor.
Vacuum pyrolysis has been mainly developed commercially through
Pyrovac process by Christian Roy [50,53]. In its recent form the pilot
reactor, capacity of 50 kg/h uses a vertical formation which is heated by
the non-condensable gases. The hollow heating plates are disposed one
above the other to allow liquid plastics to travel from top to bottom.
These plates allow the plastic thermal decomposition through internal
circulation of hot eutectic salts. The eutectic salts are acting as a heat
carrier and provide an uniform and elevated temperature in the system.
Another feature of the process is the advanced design of a condensing
system directly enabling a fraction of a diesel fuel (75% v/v), and separately, a fraction of gasoline fuel (25% v/v). Vacuum pyrolysis protects monomers generated from the pyrolysis from repolymerization.
These reactors in general are heated from the outside which means that
they do not have good heat transfer also these reactors could be costly
given the pressure requirements.
Melting vessels or stirred-tank reactors (STR) are the most common
types of reactors used in various chemical processes. They have also
been applied to plastic pyrolysis in the past such as in Smuda process
from Poland and Hitachi Zosen process [51]. The company Plastic
Energy Limited, is a very good example of the STRs employed for plastic
pyrolysis. Various reactor configurations exist employing in situ heating
by an oil or vapor upgrading by a catalyst such as in the Thermofuel
process. These reactors are quite flexible with residence time but are
characterized by insufficient heat transfer though in some case assisted
by a stirrer, which results in heat gradients in the reactor and induce
secondary reactions [15,49,60]. As pointed out by Butler et al [51],
they may require big infrastructure and frequent maintenance.
Microwaves reactors have recently been used to pyrolyse plastic
material. Besides many advantages this technology suffer from poor
thermal conductivity (very low di-electric constants) of plastics especially in the microwave frequency range creates problems. Hence, some
conductor materials need to be included to the shredded plastic waste
for e.g. graphitic carbon or inorganic oxides [51]. Carbonaceous material exposed to microwaves can reach temperatures up to 1000 °C in a
few minutes depending on the microwave strength and the amount of
carbon. Heat is absorbed by carbon and it is conducted to the plastic
waste. Moreover, the reducing nature of carbon prohibits the formation
of undesired oxygenated compounds if the plastic waste contains
oxygen bearing contaminants such as those from paper, bio-waste or
plasticizers. A significant amount of domestic plastic waste contains
plastic/aluminium laminates such as in the packaging of different food
and liquid items. This type of waste is not generally easy to process in
the conventional thermolytic processes, however, owing to the gentle
heating rendered by microwaves, these can be safely processed and
aluminium can be easily recovered and recycled [89]. A recent demonstration unit of microwave pyrolysis is Pyrowave [90], aimed at
recovering styrene from polystyrene waste.
3.4. Choice of reactor
The choice of reactor plays an essential role in defining the pyrolysis
product spectrum. The value of the products obtained from the pyrolysis mainly depends on the feedstock composition but also reactor
technologies play an important role in maximizing the desired product.
There are pros and cons to each pyrolysis reactor type, but the choice
must be based on the required product and the flexibility to handle the
variations in the feedstock. When choosing an appropriate reactor type,
two main factors have to be considered—heat and mass transfer efficiency [60]. A broad comparison of different reactor types is presented
in Table 2. The reactors are assessed based on the different operational
and economical flexibility parameters with findings from several
sources. However, a substantial work is deduced from the work of
Arena and Mastellone [49], Butler et al [49], Sheirs and Kaminsky [12],
Lopez et al [64], and Panda et al [82]. It is important to note that this
assessment is solely based on the application of these reactors for plastic
waste pyrolysis and is subject to change for other feedstock type. Also,
these assessments reflect general characteristics of the reactors therefore special modifications to the reactors are not considered here.
Fluidized bed reactors (Bubbling Fluidised Bed, BFB, Circulated
Fluidised Bed, CFB) have the advantage that operating parameters such
as the temperature can be quite flexibly controlled, which gives direct
freedom to achieve suitable product distribution [11,30–33,36,49].
Fluidized bed reactors are best known for their excellent heat and mass
transfer achieved by a heated fluidizing medium in the reactor. These
reactors can be designed in various configurations to maximize the
intended product yield. Fast pyrolysis of polyolefins in thermal mode of
operation yield waxes as the primary product which can be further
cracked into suitable compounds. Fluidized bed reactors have also been
successfully used with catalysts to achieve lower reaction temperature
and selective product distribution. The catalyst contact mode can be
both in-situ by using the catalyst as the bed material in the reactor or
ex-situ by adding a second reactor with catalyst to allow pyrolysis vapors to interact with the catalyst. In ex-situ configuration, the catalyst
life is better because some inorganic impurities are already removed
through cyclones. In a circulated fluidized bed, the catalyst is regenerated by circulating it to the regenerator. While fluidized bed reactors offer many advantages, they can require high investment costs
especially in small scale. The feeding can also be challenging in terms of
the particle size restriction [51,64]. Superior advantage of fluidized bed
reactors is their scalability. They have been scaled up to demonstration
and commercial scale plant for biomass valorization [20,28].
Rotary kilns are robust and affordable. They have been extensively
used to treat the complex waste streams such as auto shredder residue,
tires, and industrial waste. The heat is usually supplied to the kiln by
externally heated walls. In the past, rotary kiln reactors have been
applied to big scale, for example, the Conrad recycling process treated
plastics and tires in a horizontal auger kiln reactor to produce liquid
petroleum and solid carbonaceous material [12] and Toshiba Sapporo
process treated plastic waste in a rotary kiln having ceramic balls to
avoid coke build up [51]. The pyrolysis of tires in a kiln reactor dates
back to 70 s [12]. The Faulkner process was developed to treat tires; the
unique design allowed varying the temperature of the kiln by externally
heated distinct heaters.
Screw kiln reactors are another variant of the rotary kilns. The
screw or an auger placed coaxially in a fixed kiln transports the feed
through the heated reactor. This allows a better control over the residence time in the reactor and the easier handling of complex plastic
3.5. Wax formation
Pyrolysis of plastics, especially polyolefins, yield wax as one of the
main products. In order to conveniently recover the waxes the recovery
systems must be designed to handle waxes. Inefficient condensing
systems scale wax on the inner surfaces of the condensing systems,
which makes the recovery difficult. One approach used by Kaminsky
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M.S. Qureshi, et al.
et al. [12] is to use heated impact precipitators in the condensing train
to keep the waxes in liquid form and makes the recovery of products
efficient
increased mixing/compounding efficiency and thus shorter length of
needed mixing zone. Furthermore reduced required investments and
reduced floor space compared to traditional pretreatment systems
makes MODIX an attractive pretreatment solution.
3.6. Toxicity of the feedstock and products
4.2. Conversion
As discussed earlier, the plastic waste originating from different
sources for example WEEE, ELV, CDW (Construction and Demolition
Waste), packaging, and agriculture is very heterogeneous. These waste
streams can potentially contain various toxic substances, such as halogens, biohazardous materials, and other harmful additives; [83–86]
therefore, it is necessary to characterize the feedstock carefully before
pyrolysis. This ensures not only good quality product, but also safety in
operations, because these toxic materials could potentially escape from
the recycling system, posing serious environmental and health risks.
Furthermore, it is also important to understand that pyrolysis of plastic
waste eventually turns the polymers back into hydrocarbons; some of
them such as aromatic compounds are dangerous substances and are
classified as possible carcinogens. Before pyrolysis, it is strongly recommended to ensure that a thorough risk assessment is carried out and
all possible risks related to pyrolysis are identified and mitigation
strategies are well devised.
Pyrolysis is thermal or catalytic decomposition of a material in an
oxygen-free or limited oxygen environment. In pyrolysis, the polymer
feedstock is introduced into a reactor at 450–600 ℃ to produce a vapor.
Vapors consists of condensable (liquid and wax) and non-condensable
(gas) fractions. Depending on the technology, liquid products may be
fractionated onsite, usually by distillation, into a range of light, middle
and heavy distillate fuel oils. If fractionation does not occur onsite, the
liquid petroleum product, typically classified as a light sweet synthetic
crude oil, can be sold to a refinery for further processing. Output quality
and quantity from the pyrolysis processes depends on feedstock and the
technology. [9] Monomers (Table 1) can also be produced from certain
resins. Several technologies encompassing diverse reactors with their
pros and cons (Table 2) have been under research for the said purpose.
Normally the choice of the reactor is based on the feedstock and the
product required. Table 3 shows pilots and commercial scale plastic to
liquids plants with planned capacity up to 60 TPD (tons per day)
[9,52,53].
In order to provide constant volumes of adequate quality feedstock
the waste supplier and pyrolysis operator should have long-term
binding agreements. Another alternative would be that the feedstock
collector would also be the pyrolysis oil producer. The latter option is
the case by Pohjanmaan Hyötyjätekuljetus (PHJK [54]), a Finnish SME
which is waste managing company and has also a 12–14 TPD pyrolysis
(cracking unit). The unit is equipped with fractional distillation and
additional refining unit. The present production capacity is 3 million
litres diesel fuel per year, which qualifies i.e. to slow-speed diesel engines (cetane number of upgraded product over 50). A continuous
system is under construction.
3.7. Non-existent analytical standards
Pyrolysis oil derived from plastics is not a standardized product
therefore, standard testing methods are almost non-existent. The absence of validated testing methods originates also from the complexity
and heterogeneity of waste. The composition of the pyrolysis liquid
varies significantly with the feedstock composition making it difficult to
formulate standards. However, despite these challenges, efforts are
being made to formalize the methods to examine the product so it can
reach a standard product status.
3.8. Stability and storage
The stability and ageing properties of pyrolysis liquids are key
properties that determines the quality of the liquid. Pyrolysis liquid is
thermodynamically unstable and tends to repolymerize. Therefore, post
treatment of liquid is necessary to maintain its quality over extended
period. These post treatments could include blending, dewaxing, etc.
[51].
5. Opportunities related to the operational environment:
Sustainability
Compared to the high number of life cycle assessment (LCA) studies
focusing on waste incineration, pyrolysis of waste has not been given
much attention [55]. In the context of the WasteBusters project, the
climate impacts related to pyrolysis of plastic waste were studied using
the LCA methodology. LCA is a quantitative environmental impact assessment method, which can be used for evaluating the environmental
impacts of products covering their whole life cycle. Applied LCA approach was partly framed in the study by Deviatkin and Pihkola [14]
and in the report by Oasmaa et al. [15].
The system boundaries of the studied business-as-usual scenario, as
well as of the alternative scenarios are shown in Fig. 4. The aim of the
studied system was to use plastic waste as a raw material for diesel and
polymer production. The functional unit used in the study was treatment of 1 kg of plastic waste, and production of diesel and polyethylene
in the amounts equal to those produced during pyrolysis process. Pyrolysis of plastic waste was compared to a current business-as-usual situation in Finland, in which plastic waste was either incinerated for
electricity and heat production or recycled mechanically, and the reject
from recycling was sent to incineration. Collection of plastic waste from
consumers was not included in the study, because the collection system
was expected to be the same in all scenarios. Thus, the system boundaries begun with the transportation of plastic waste to either incineration (baseline) or their separation from impurities and transportation to
pre-treatment before pyrolysis (alternative scenarios). Environmental
impacts from the preceding life cycles of collected plastic were not
included in the study, due to lack of proper data. In the LCA methodology, this so called zero-burden approach (or cut off allocation) may
4. Potential solutions
4.1. Pretreatment
Cost-efficient pretreatment is needed however, the number of unit
operations in pretreatment should be minimized to support economical
feasibility. One example utilized and further developed in the
WasteBusters project [15] at VTT is a new type of modular extruder
mixer (MODIX) (Fig. 2), which is capable of processing organic waste
and recycling material otherwise not suitable to be fed to pyrolysis, like
fluffy plastic films, plastic bottles, canisters and mixed plastic waste
including paper, cartoon and pieces of wood (Fig. 3). Due to wide
diameter of hollow rotor and stator, MODIX has a tenfolded size of
feeding zone compared to traditional extruders with the same capacity.
This enables the feed of large heterogeneous particles and thus excludes
the shredding/crushing steps needed with conventional extruders.
Compared to traditional extruder, MODIX has high surface area of
rotor/stator surfaces compared to the volume of the material flow enabling efficient heat transfer to material to be melted and/or thermally
decomposed. In addition, the hollow rotor structure enables easy machining of the components. The inner surface of the stator is nested in
the MODIX, whereas the surface of the stator of traditional extruders is
limited to be smooth. The nested structure of the stator enables highly
6
Journal of Analytical and Applied Pyrolysis 152 (2020) 104804
M.S. Qureshi, et al.
Fig. 2. Traditional extruder (on left) and scalable compact modular extruder (on right).
be applied if the raw material is considered as waste with no other
value or use. The study was geographically representative of Finland in
terms of the plastic waste composition, the technologies studied, and
the energy and raw materials supply chains [15].
The study consisted of a screening carbon footprint study in which
various data sources were applied. Table 4 lists the sources of data and
the names of unit processes retrieved from Ecoinvent 3.4 database [66].
Primary data from the pilot trials conducted in the Wastebusters project
for both pyrolysis and pre-treatment phases was used in the study.
Available database data, literature and expert estimations were used as
additional data sources.
Under studied assumptions and conditions, the results from the assessments showed that pyrolysis of plastic waste has a significantly
lower carbon footprint compared to direct incineration of plastics and
production of diesel and polymers from virgin materials. In the studied
scenarios, the carbon footprints of the alternative pyrolysis scenarios
were 15–60 % lower compared to the business as usual scenario. Yet,
the results are sensitive to the applied assumptions and data. When
climate impacts of sending the separately collected post-consumer
plastics waste to incineration or to pyrolysis were compared, the results
from the two scenarios were quite close to each other. However,
sending the reject from mechanical recycling to incineration was a
significant contributor to the carbon footprint in the business as usual
scenario. Thus, it is assumed that a more favorable result could be received in case the reject from mechanical recycling would be directed to
pyrolysis instead of incineration. Similar findings are reported in the
study of Perugini et al. [56] who showed that low temperature pyrolysis
combined with mechanical recycling of plastic packaging waste result
in a reduction of the carbon footprint of plastic landfilling by 67% and
by 76% compared to plastic incineration in the baseline scenario.
The sensitivity analysis conducted during the case study showed
that a large variation in the results is possible depending on the modelling approach used, e.g. the type of electricity and heat replaced, or
depending on the accuracy of the data. Applied energy production
profiles, assumed yields related to mechanical recycling and assumptions related to incineration of plastic waste all affect the results. Thus,
it is not possible to draw generic conclusions based on one case study,
even if the results seem promising. Future studies should focus on
considering the alternative best case and worst-case scenarios and their
combinations. In addition, future studies should aim for collecting and
applying more accurate data for the processes studied. In addition to
climate impacts, the scope of the studies should be extended to cover
other emissions and impacts to the environment, starting from air
emissions and resource depletion categories. In case the raw material is
expected to include harmful substances, potential impacts related to
toxicity should be considered. If the product from pyrolysis would be
used for transport fuel production, it should comply with the GHG
(Greenhouse Gas) reduction requirements set out in the recast of the
European Union’s Renewable energy directive (REDII) for recycled
carbon fuels. According to the REDII, at least 14% of the final energy
consumed in the transport sector (minimum share) should become from
renewable sources by 2030. EU member states may include the use of
recycled carbon fuels when calculating the required minimum share.
Within the REDII, recycled carbon fuels are defined as “liquid and
gaseous fuels that are produced from liquid or solid waste streams of
non-renewable origin which are not suitable for material recovery”. The
threshold for the required minimum GHG emission savings for the recycled carbon fuels was not yet defined by the time of this study.
6. Challenges related to the operational environment: Legislation
The legislative framework relevant for pyrolysis of plastic waste can
be considered as complex, as it includes policies related to circular
economy, waste management, product safety and fuels. For example,
due to safety reasons, there are several requirements imposed by the
legislation that prevent the use of recycled plastics in food contact
materials and toys, which are among the most common uses for plastic
[57]. In case the end-product is used as a fuel, the GHG savings criteria
Fig. 3. Separately collected plastic packages including fluffy plastic films, plastic bottles, canisters and mixed plastic waste including paper, cartoon and pieces of
wood (left, middle) were treated to homogenous melt (right).
7
Journal of Analytical and Applied Pyrolysis 152 (2020) 104804
M.S. Qureshi, et al.
Table 3
Selected plastic to liquids plants in Europe and in North-America [9,52,53].
Technology provider
Capacity (TPD)
Products
Technology
Location
Status
VadXX
Nexus
Agilyx
Recycling Technologies
Plastic Energy
Susteen Technologies
60
50
10-50
20
20
12
Rotary kiln
Melting Vessel
Dual screw reactor
Fluidized Bed
STR
Screw with recirculation
TCR® Fuel
USA
USA
USA
UK
Spain
Germany
Online
Online
Online
Online
Online
Online
PHJK
12-14
Rotary kiln
Finland
Online
Renewlogy
0.24
10
1.2
12
Syncrude, diesel
Light crude, diesel, gasoline, Kerosene blendstock, wax
Light synthetic crude oil
Low sulphur hydrocarbon Plaxx – wax
raw diesel, light oil, synthetic gas components
Green Crude, Diesel, Gasoline and Jetfuel
Green Hydrogen
Anthracite Coal
Light crude oil
Diesel
Crude oil
Rotary kiln
USA
Online
Diesel and gasoline
Diesel and gasoline
Canada
Online
In construction
2.4
1
0.1-0.2
Synthetic crude oil
Light and heavy wax
Styrene monomers from PS
Multiple Hearth
Vacuum Reactor
Idem
Melting vessel
Fluidized bed
microwave catalytic depolymerization
Pyrovac
Re-oil (OMV)
Thermal cracking (BP process)
Pyrowave
Canada
Austria
Germany
Canada
Online
Dismantled
Online
TPD = Tons Per Day, STR = Stirred-Tank Reactor, TCR = Thermo-Catalytic Reforming (TCR®).
set out in the revised Renewable Energy Directive (2018/2001/EU)
would be important to fulfill.
According to the Waste Framework Directive WFD 2008/98/EC
‘recycling’ means any recovery operation by which waste materials are
reprocessed into products, materials or substances whether for the
original or other purposes. It includes the reprocessing of organic material but does not include energy recovery and the reprocessing into
materials that are to be used as fuels or for backfilling operations. New
addition to WFD is “material recovery” which means any recovery
operation, other than energy recovery and the reprocessing into materials that are to be used as fuels or other means to generate energy. It
includes, inter alia, preparing for re-use, recycling and backfilling.
Thus, one of the major challenges related to the regulatory status of
pyrolysis, is that currently, pyrolysis is not considered as a recycling
technology under EU legislation, if the end product is used in any form
of energy production. No distinction is made between advanced traffic
fuels and production of heat and electricity. This status should be
changed before pyrolysis of plastic waste could really start developing
as a feasible business. However, in the international standard
ISO15270:2008 (Plastics — Guidelines for the recovery and recycling of
plastics waste. International Standards Organization 2008.) pyrolysis is
recognized as forms of feedstock recycling technologies when the
products are used for the production of fuels or raw materials, rather
than for combustion and subsequent energy recovery which would be
considered as a waste-to-energy process [52].
Another important step would be the establishment of the so-called
End-of-Waste (EoW) -criteria to pyrolysis oil. The Waste Framework
Directive (2008/98/EC) includes the option to set EoW criteria, under
which specified waste fractions shall cease to be waste. Fulfilling the
End-of-Waste criteria would mean that the pyrolysis oil would no
longer be classified as waste, but as a product subject to product legislation instead of waste legislation during trade and use. However, this
would require that in addition to fulfilling relevant technical, environmental and safety requirements, a market or demand for the
product or substance would exist [57,71].
Commercial use of pyrolysis as a means of chemical recycling would
be promoted, if the liquid end-product from pyrolysis would be classified as a product with a clear End-of-Waste status and REACH registration.
7. Conclusions
About 10% of plastics is presently recycled in Europe, and almost all
commercial plastics recycling plants use mechanical recycling routes.
Fig. 4. System boundaries used in the LCA study for the business-as-usual scenario and the alternative scenarios for managing three types of plastic waste. Studied
system was considered representative of Finnish conditions in terms of waste treatment and applied assumptions related to energy and heat production.
8
Journal of Analytical and Applied Pyrolysis 152 (2020) 104804
M.S. Qureshi, et al.
Table 4
Sources of inventory data used in the LCA study.
Process
Source of data, comments
Transportation of plastic waste
Incineration in a MSWI plant
LIPASTO [67]
Ecoinvent 3.4
Mechanical recycling
Avoided electricity
Avoided heat
Based on data from an operating plant in Finland. Confidential data.
Ecoinvent 3.4
Mix based on [68] using processes from Ecoinvent 3.4
Production of diesel
Production of polyethylene
Sorting
Ecoinvent 3.4
Ecoinvent 3.4
Modelled using only electricity consumption based on private VTT Ecodata
database
Modelled using only electricity consumption based on test runs
Modelled using only electricity consumption based estimates
Yields as in laboratory test runs. Heat demand [69]. Electricity demand based on
TEA calculations. All demands are covered by a CHP, so no impact.
Formation of carbon dioxide from incineration of char and noncondensible gases
based on carbon content
Expert estimates from industry
Plastics Europe Eco-profiles [70]
MODIX
Size reduction
Pyrolysis
CHP
Refinery
Cracking and polymerization
Name in database, if applicable
Chemical recycling could be applied for the high share of plastics that
cannot be mechanically recycled. Many novel material innovations are
either composites consisting of several types of polymeric materials or
completely novel polymer resins. For them integrated pyrolysis with
feedstocks from several polymeric waste streams, e.g. wood and plastics, could provide a feasible means for recycling especially in the
emerging phase when the volumes of the new materials on market are
low, and thus separate collection of waste is not yet a cost-efficient
option. In order to optimize the whole value chain from waste to products, the efficiency of collection, monitoring and sorting to produce
more suitable feeds to target products needs to be improved. At VTT a
modular extruder was concluded to decrease the process steps in
feedstock recycling.
For circular economy, eco-design - including material and product
design for recycling as well as stopping the transfer of harmful
treatment of waste plastic, mixture, municipal
incineration
market for electricity, medium voltage, FI
41% heat and power co-generation, wood chips,
6667 kW, state-of-the-art 2014
39% heat and power co-generation, hard coal
18% heat and power co-generation, natural gas,
combined cycle power plant, 400 MW electrical
3% heat and power co-generation, oil
market for diesel, low-sulphur
polyethylene production, low density, granulate
market for electricity, medium voltage, FI
market for electricity, medium voltage, FI
market for electricity, medium voltage, FI
substances into new products - is important. A holistic approach and
optimal solutions e.g. by integrating mechanical and chemical recycling
(Fig. 5) should be applied. Besides economics the environmental sustainability of the value chain is a key issue for further development and
commercialization. Life cycle assessment should be used for analyzing
potential impacts and benefits related to alternative routes of plastics
recycling and identifying environmentally sound options. Depending on
the local energy and waste management infrastructure, different solutions may end-up being sustainable, but care must be taken in conducting the evaluations and drawing conclusions. Development of
technologies, value chains, and supporting legislation is still required
for plastic waste-based pyrolysis products to become a reality in industrial scale.
It needs to be emphasized that chemical recycling is not a new
technology but application of chemical processes and methods in waste
Fig. 5. Integration of mechanical and chemical recycling.
9
Journal of Analytical and Applied Pyrolysis 152 (2020) 104804
M.S. Qureshi, et al.
handling and valorization where changes in bond structure of materials
take place. Its comparison to incineration is not by any means justified.
Pyrolysis can treat materials not applicable to mechanical recycling
depending on resin types either to new products or sustainable storable
liquid fuel to replace fossil fuels are produced. Instead, it complements
mechanical recycling and hence it should be given the status of recycling in the waste hierarchy. While pyrolysis is a robust method for
waste valorization, it must be noted that there are still vast number of
challenges related to the feedstock quality, segregation, reactor operations, and stability and standardization of the product. Therefore, efforts along the whole plastic recycling value chain are needed to truly
close the plastic loop and establish circular economy for plastics.
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Acknowledgements
The authors want to acknowledge Mikko Lammi from Pohjanmaan
Hyötyjätekuljetus and from VTT Kirsi Korpijärvi, Jani Lehto, Lisa
Wikström, Margareta Wahlström, Taina Ohra-aho, Christian Lindfors,
Joona Lahtinen, Elmeri Pienhäkkinen, Ismo Ruohomäki, Elina YliRantala, Satu Pasanen, Malin zu Castell-Rüdenhausen and Holger
Pöhler.
Funding
The research received financial support through the Finnish national Business Finland WasteBuster project (BF 5401/31/2016).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jaap.2020.104804.
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