5-7 OCTOBERISFR2011

5TOLEDO (SPAIN)

THERMAL AND PYROLYTIC PROCESSES FOR THE VALORIZATION OF WASTE
PLASTICSPaul T. Williams
Energy & Resources Research Institute, The University of Leeds, Leeds LS2 9JT, UKe-mail: p.t.williams@leeds.ac.uk

Abstract
Waste plastics are produced in various municipal and industrial waste streams. Each waste stream presents particular
problems for feedstock recycling, however, each stream also offers potentially great benefits in terms of not only plastic
waste feedstock recycling, but for the recovery of other high value fractions from the waste. This paper describes theapplication of thermal and pyrolytic processes for the recycling of various plastic wastes including mixed plastic waste frommunicipal solid waste, waste electronic & electrical equipment, and composite plastic wastes. Examples of the recycling of
the plastic waste streams are presented in relation to the recovery of oils for feedstock recycling, synthesis gas/hydrogen forfuels and solid materials recovered from the waste feedstock.

Keywords: MSW; WEEE; Printed Circuit Boards; Composites; Hydrogen

1. Introduction
Approximately 230 million tonnes of plastics areproduced worldwide each year with European production
representing more than 50 million tonnes [1]. It isestimated that only about 50% of the plastics produced inEurope each year are available for collection andrecycling, the remainder accumulating in the environment
as long term uses of plastics such as building works,including, plastic window frames, pipes, electrical wiring,etc. Of this collectable waste plastic, only about 5.5
million tonnes, is recycled with the remainder incinerated
with other wastes, mainly as municipal solid waste, or isdisposed of, mostly to landfill. Recycling is dominated bymechanical routes to produce low grade products. The
low grade uses for plastic recycled materials has led toresearch into alternative processing methods to produce
higher value products. One example is via feedstock
recycling where the plastic waste materials areprocessed back to produce basic petrochemicals thatcan be used as feedstock to make virgin plastic.
Pyrolysis of plastics, thermally degrades the plastic,
breaking the bonds of the polymer to produce lower
molecular weight oligomers and monomers.
Plastic waste occurs in a variety of waste streams,
including, mixed plastic waste from municipal solidwaste, waste electronic and electrical equipment, andcomposite plastic wastes. The plastic component of thewaste can be recycled by pyrolysis to produce an oil for
use as a petrochemical feedstock and also a gas toprovide the energy requirements of the pyrolysis process.
In addition, the plastic can be gasified using catalytic
steam gasification to enhance the production of synthesisgas. Some of the plastic waste types contain additional
material that can be recovered, for example, waste
electrical and electronic waste contains printed circuit

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boards where valuable metals can be recovered,composite plastic materials contain valuable glass orcarbon fibres which can be recovered. This paper
presents thermal and pyrolysis processes for a variety of
waste plastics for the recovery of oils for feedstock
recycling, synthesis gas/hydrogen for gaseous fuels andhigher value solid materials.

2. Waste Plastic Materials and Processes
2.1. Municipal solid waste
The six main plastics in municipal solid waste arethermoplastics consisting of, high density polyethylene(HDPE), low density polyethylene (LDPE), polypropylene(PP), polystyrene (PS), polyvinyl chloride (PVC) andpolyethylene terephthalate (PET). The product yield frompyrolysis is related directly to the type of plastic, thereactor type and the process conditions, particularly
pyrolysis temperature. Pyrolysis of plastics produces anoil/wax product. The distinction between the yield ofcondensed hydrocarbon wax and oil is a consequence ofthe pyrolysis reaction system, temperature, condensationcollection system and condensation temperature. For
example, fluidised bed pyrolysis of various plastics at 550°C in a fluidised bed reactor with rapid cooling of thecondensate using water cooled condensers produced anoil/wax ratio of 1.23 for HDPE, 0.50 for LDPE, 0.82 for
PP, 4.75 for PS and 1.48 for PET [2]. In addition, it was
shown that PVC produced 100% oil and no wax [2].
Polyethylene has been pyrolysed in a fluidised bed
reactor at 530 °C and an oil yield of 50.3wt% and a wax
yield of 42wt% was reported [3]. Vacuum pyrolysis ofvarious plastics produced 88wt% conversion of LDPE towax and 8wt% to an oil. For HDPE, the conversion was92.3wt% to wax and 5.4wt% to oil, for PP the conversion
to wax was 70wt% and 25wt% oil [4]. It was also shown

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66th International Symposium on Feedstock Recycling of Polymeric Materials

that when polystyrene was pyrolysed in the vacuum
reactor, 58.6wt% light oil was produced and 40.7wt%
heavy oil [4].
Further developments in the process of feedstock
recycling of plastics have included the use of catalysts.
The use of a catalyst has been proposed as a way of
producing a high aromatic product for the production ofgasoline or as a chemical feedstock or for the productionof speciality chemicals. Catalysts can reduce therequired reaction temperature, improve the yield ofvolatile products and provide selectivity in the productdistributions. Different catalyst types can alter thetemperature of thermal degradation of the plastic andcould also be used to define the composition. For
example, pyrolysis of a number of plastic types over azeolite ZSM-5 catalyst has shown that the use of acatalyst increased the aromatic content of the derived
product oil [5].
The gasification of plastics produces a syngas which hasa high content of hydrogen. Hydrogen has been identified
as a fuel which might play an important role in futureenergy supplies. Czernik and French [6] havedemonstrated the potential of the gasification process forplastics to produce hydrogen. Different process
conditions such as gasification temperature, introductionof a catalyst and introduction of steam etc. have aninfluence on the production of hydrogen from thegasification of plastics [7,8]. A key factor in maximising
the production of hydrogen from waste plastics using
pyrolysis-gasification is the use of catalysts. Nickel
catalysts are commonly used in the gasification process
due to their lower price and effective catalytic activity.
Furthermore, in order to obtain high concentrations ofhydrogen in the product gases, multiple stage reaction
systems have been applied to the gasification process[7,8].
2.2. Waste electrical & electronic equipment (WEEE).
Total electrical and electronic equipment waste
generated in the European Union have been estimated at
approximately 10 million tonnes per year. Pyrolysis ofthree plastic fractions from a commercial waste electricaland electronic equipment (WEEE) processing plant havebeen investigated [9]. The plastic waste included
material from equipment containing cathode ray tubes,
from refrigeration equipment, and from mixed WEEE.
The major pyrolysis product of each plastic was pyrolysis
oil with only small amounts of gas being produced. Eachof the pyrolysis oils was characterised by GC-MS, whichshowed that all the oils contained mainly aromaticcompounds and both nitrogenated and oxygenatedcompounds. The pyrolysis oils contained very few
organic halogens and it was also reported that the realworld waste samples of WEEE plastic had low halogen
content. It has been reported that only 2.5% of WEEE
plastic contains halogens, usually in the form ofbrominated flame retardants [9].
Virtually all electrical and electronic equipment contain
printed circuit boards. Printed circuit boards are
particularly problematic to recycle because of theinteriores libro.indd 6

heterogeneous mix of organic material, metals, and glass
fibre and low recycling rates are reported of about 15%[10]. It has been estimated that a typical printed circuitboard is composed of 40 wt% metals, 30 wt% plastics
and 30 wt% of ceramic material. The metals present
include large amounts of copper, aluminium and iron, butalso precious metals such as gold, silver and palladium
which increase the interest in recycling technologies torecover such metals. In addition, the plastic fraction of
the printed circuit board represents an important fraction
to meet recycling and recovery targets.
Pyrolysis of printed circuit boards has been an area ofextensive research. De Marco et al [11] pyrolysed
printed circuit board waste produced from a WEEE
recycling plant. The product yield from pyrolysis,
produced 76.5 wt% solid residue, 16.2 wt% oil and 7.3
wt% gas. The solid residue consisted of 5.8 wt% charderived from the pyrolysis of the epoxy resin and themetals and ceramic materials. The pyrolysis liquid oil wascomposed of products related to the original polymer ofthe plastic fraction, being mainly epoxy resin which
produced a pyrolysis oil of mainly aromatic oil of low
viscosity containing high concentrations of phenol andphenol derivatives. Pyrolysis gases were composed ofmainly hydrocarbons from C1 – C5 consisting of mainlyethane and ethane and also CO and CO2. The pyrolysis
of printed circuit boards derived from waste computers,televisions, and mobile telephones has been reported[12]. The pyrolysis gases consisted mainly of CO2 andCO and C1 – C4 gases and some inorganic bromine wasalso present, inorganic chlorine was also present in themobile telephone printed circuit board pyrolysis gas. The
pyrolysis oils contained high concentrations of phenol, 4(1-methylethylphenol), and p-hydroxyphenol, which are
all regarded as decomposition products of epoxy resins.
After pyrolysis, the metals and pyrolysis char can beeasily separted, either by mechanical separation or mildoxidation. The metals consist of copper in highconcentration with high concentrations of calcium, iron,nickel, zinc, aluminium, lead, and silver also found in
most of the residues.
2.3. Composite plastic waste.
Advanced composite materials offer a combination ofhigh strength, lightness and corrosion resistance thathave made them increasingly popular in the
manufacturing industries Fibre reinforced composite
plastics, comprising thermoset or thermoplastic polymer
matrices with glass, carbon and/or polymer fibre
reinforcement, offer a combination of strength, durability,low weight and corrosion resistance that has made themincreasingly important products, especially in theautomotive vehicle sector. Thermoset unsaturated
polyester resins are employed in the manufacture ofglass-reinforced polyester (GRP), sheet moulding
compound (SMC) and dough moulding compound(DMC), extensively used in the production of automotive
vehicles. When the vehicle reaches the end of its life it
has to be scrapped and it has been estimated that the26/9/11 13:12:37

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ISFR

2011

number of end-of-life vehicles will reach almost 19 million
cars in Western, Central and Eastern Europe by 2015.
However, the complex nature of composite materials,
coupled with the thermoset resins used, presents major
problems to recycling and much of the composite waste
currently produced must ultimately be disposed of via
landfill
As a process for recycling fibre-reinforced composite
plastic waste, pyrolysis has the advantage that all of theproducts can potentially be recovered and reutilised inone form or another. The polymeric components
decompose to produce potentially useful oil and gas
products, and controlled pyrolysis enables the recoveryof the fibre reinforcement with minimum degradation.
The recovered fibres could then be recycled back into thecomposite production industries.
Pyrolysis of a thermoset polyester/styrene copolymer
reinforced with glass fibre has been pyrolysed at 450°C[13]. The main gases evolved were CO and CO2, which
accounted for more than 75vol % of the total gas
composition. Other gases identified were H2, CH4 andother hydrocarbons from C2-C4. The properties of thepyrolytic oil, including calorific value, viscosity andelemental analysis, suggested that it could be a viableliquid fuel. The condensable products could also be avaluable source of chemical feedstocks including styrene
and phthalic anhydride, which could potentially berecycled into polyester/styrene resins. The pyrolysis
process produced a solid char, filler and glass fibres.
The glass fibres could be easily separated from thepyrolysis char and filler and could be recycled to produce
new composite plastic material. The glass fibre
recovered from the solid residue had mechanicalproperties which showed that they could replace up to
20wt% of virgin glass fibre in Dough Moulding Compound(DMC).
Carbon fibre reinforced plastics comprising thermoset
and thermoplastic polymer matrices, are considered as
one of the most attractive products due to their strength,durability, low weight and corrosion resistance and arecommonly used in the aerospace and automotive
sectors. Carbon fibres are regarded as high value
products and their recovery from the composite waste via
pyrolysis is economically attractive. Carbon fibre
composite waste composed of carbon fibres andpolybenzoxazines resin has been pyrolysed in a fixed
bed reactor at temperatures between 350 and 700 ºC[14]. Solid residues of between 70-83.6 wt%, liquid yields
14-24.6 wt% and gas yields 0.7-3.8 wt% were reported
depending on pyrolysis temperature. The derived
pyrolysis liquids contained aniline in high concentration
together with oxygenated and nitrogenated aromaticcompounds. The pyrolysis gases consisted mainly of
CO2, CO, CH4, H2 and other hydrocarbons. The carbon
fibres used in the composite waste were separated fromthe char of the solid residue via oxidation andinvestigated for their mechanical strength properties. The
recovered carbon fibres exhibited mechanical properties

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which were 90% of that of the original virgin carbonfibres.

3. Conclusions
Many types of plastic waste can be recycled using
thermal and pyrolyitic processes to produce high value
fuels, chemical feedstocks, gases, hydrogen, or for therecovery of solid materials such as high value metals or
glass/carbon fibres from the waste. The influence ofprocess conditions can be used to determine the yieldand composition of the product stream or the optimised
recover process for the targetted end-product.

References[1] Anon. Plastics-The Facts 2010. Plastics Europe,Brussels, Belgium (2010).
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[3] W. Kaminsky, M. Predel, A. Sadiki, Feedstock
recycling of polymers by pyrolysis in a fluidised bed.
Polym. Degrad. Stabil. 85 (2004) 1045-1050.
[4] R. Miranda, H. Pakdel, C. Roy, C. Vasile, Vacuum
pyrolysis of commingled plastics containing PVC II
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[5] R. Bagri, P.T. Williams, Catalytic pyrolysis ofpolyethylene. J. Anal. Appl. Pyrol. 63, (2002) 29-41.
[6] S. Cernik, R.J. French, Production of hydrogen fromplastics by pyrolysis and catalytic steam reform. Energ.
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[8] C. Wu, P.T.Williams, Pyrolysis-gasification of postconsumer municipal solid plastic waste for hydrogenproduction. Int. J. Hydrogen Energ. 35 (2010) 949-957.
[9] W.J. Hall. P.T. Williams, Analysis of products from thepyrolysis of plastics recovered from the commercial scale
recycling of waste electrical and electronic equipment.
Journal of Anal. Appl. Pyrol. 79 (2007) 375-386.
[10] P.T. Williams, Valorization of printed circuit boards
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[11] I. de Marco, B.M. Caballero, M.J. Chomon, M.F.
Laresgoiti, A. Torres, G. Fernandez, S. Arnaiz, Pyrolysis
of electrical and electronic wastes, J. Anal. Appl. Pyrol.
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[12] W.J. Hall, P.T. Williams, Separation and recovery ofmaterials from scrap printed circuit boards. Resour.
Conserv. Recy. 51 (2007) 691-709.
[13] A.M. Cunliffe, P.T. Williams, Characterisation ofproducts from the recycling of glass fibre reinforced
polyester waste by pyrolysis. Fuel 82 (2003) 2223-2230.
[14] M.A. Nahil, P.T. Williams, Recycling of carbon fibre
reinforced polymeric waste for the production of activatedcarbon fibres. J. Anal. Appl. Pyroly. 91 (2011) 67-75.

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