PARTⅠ PYROLYSISINFLUENCE OF TEMPERATURE ON THE PRODUCTSOBTAINED IN PYROLYSIS OF WASTE PACKING AND
PACKAGING PLASTICS
Alexander López*, Isabel de Marco, Blanca Mª Caballero, Mª Felisa Laresgoiti, Amelia
Torres, Aitziber Adrados
Chemical and Environmental Engineering Department, School of Engineering of Bilbao,Alda. Urquijo s/n, Bilbao 48013, Spain.
E-mail: alex.lopez@ehu.es
Abstract: The objective of this work is the study of pyrolysis as a feedstock recycling process,for valorising the rejected streams that come from industrial plants, where packing andpackaging wastes are classified and separated for their subsequent mechanical recycling. A
representative sample of these rejected streams has been pyrolysed under nitrogen in a 3,5
dm3 autoclave at 460, 500 and 600º C for 30 minutes to analyze the influence of temperaturein the products obtained. Waste packing pyrolysis liquids are a complex mixture of organic
compounds containing valuable chemicals as styrene, ethyl-benzene, toluene, etc. Pyrolysis
gases are mainly composed of hydrocarbons together with some CO and CO2, and have very
high gross calorific values (GCV). Almost no solids are obtained in pyrolysis of waste
packing plastics.
Temperature plays a significant role in the pyrolysis process. At 460º C total conversionof the waste plastics is achieved. Raising temperature gives rise ti higher gas yields and lower
liquid yields. The characteristics of pyrolysis products, especially the liquid fraction, varysignificantly with temperature. At 460º C wax-like products are obtained, at 500º C highviscosity liquids and at 600º C more fluid and aromatic liquids are obtained.
Keywords: pyrolysis; thermolysis; feedstock recycling; plastic wastes.
1. Introduction
Nowadays plastic consumption is more than ever increasing. Annual consumption ofplastics in Occidental Europe is about 60 million tons, of which about 40 % are used inpacking and packaging applications, that in a very short period are converted to wastes,yielding about 15 million tons of such wastes per year [1]. At present, in Europe, about 50%of the annually generated plastic wastes are valorized, 60 % by incineration with energy
recovery and about 40 % by recycling. Most of the recycled plastics are mechanicallyrecycled, while less than 1% is chemically recycled.
- 35 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
The EU legislation (Directive 2004/12/CE) obliges to valorise (energy recovery +recycling) 60 % of the packing and packaging wastes, with at least 55-85 % of recycling. For
this to be possible, on the one hand municipal (yellow) containers for such wastes have been
set out in different UE countries, and on the other hand industrial separation and classification
plants, which receive the yellow containers contents as raw materials, are being operated on
an industrial scale. In such plants the wastes are separated in different fractions (steel,tetra-brick, aluminium cans, different plastics, etc.) and then sent to recycling companies.
However, a significant amount of the income materials cannot be properly classified orseparated and are rejected. Such rejects are composed of many different materials (PE, PP, PS,PVC, PET, ABS, aluminium, film, etc.), which are very much intermingled; consequently it is
not technical or economically viable an ulterior separation, and therefore they can only beincinerated or land-filled. In this paper a mixture of plastics with the same components andproportions as a real rejected stream was prepared and pyrolysed at different temperatures, inorder to on the one hand evaluate the potential of pyrolysis as a feedstock recycling techniquefor plastic packaging wastes and on the other hand determine the influence of this parameterin the characteristics of the products obtained.
In the pyrolysis process (heating without oxygen), the organic components of thematerial are decomposed generating liquid and gaseous products, which can be useful as fuels
and/or sources of chemicals. If there were inorganic components in the waste (metals,fillers…) they would remain practically unaltered after pyrolysis, and they might be recycled
or reused. The rejects from plastic packages coming from separation plants have rather low
inorganic contents, so if it was not worth recycling pyrolysis solids they would be a minimum
waste to be land-filled. Pyrolysis is especially appropriate for products or streams of wastes
which are a mixture of different plastics for which mechanical recycling is not feasible; suchis the case of the fraction that has been studied in this work.
2. Experiment
2.1 Characteristics of the sample pyrolysed
The typical composition of the rejects of a waste packing separation and classification
plant was determined characterizing real samples rejected from an industrial plant located inAmorebieta, in the north of Spain. Homogeneous samples with such typical composition wereprepared for the pyrolysis experiments, using pellets (≈3 mm size) of the following materials:
polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET)and polyvinyl chloride (PVC). Additionally a finely ground sample (< 1 mm) was preparedfor characterization purposes. The determined composition, which is presented in tab. 1 , isquite similar to those proposed by other authors as representative of municipal plastic waste
compositions [2,3].

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Tab. 1 Composition of the sample pyrolyzed (weight %)MaterialPE

PP

PS

PET

PVC

Composition

40351843

The moisture and ash contents of the samples were determined by thermogravimetricanalysis, and the elemental composition with automatic CHN and S determinators, and byanalysing the gases generated in the combustion of the sample for Cl determination. The grosscalorific value (GCV) was determined with an automatic calorimetric bomb. The results arepresented in Tab. 2.
Tab. 2 Elemental composition and GCV of the samples pyrolysed (weight % in wet basis)
Parameter

Moisture

AshCHNCl

OthersH/C

GCV (MJ kg-1)

Value

0.1

0.0

84.7

12.5

<0.1

1.1

1.5

1.8

43.9

2.2 Pyrolysis experiments
The pyrolysis experiments were carried out at 460, 500 and 600º C in nitrogen
atmosphere, using an unstirred stainless steel 3,5 dm3 autoclave. In a typical run 100 g of thesample are placed into the reactor, which is then sealed. Nitrogen is passed through at a rate of
1 dm3 min-1 and the system is heated at a rate of 20 ºC min-1 up to the selected temperature,and is maintained there for 30 minutes. It has been proved by the authors that in thementioned installation after 30 minutes no more pyrolysis products evolve from the autoclave[4]. All through the run the vapours leaving the reactor flow to a series of water cooled
gas-liquid separators where the liquids are condensed and collected. The uncondensed
products are passed through an activated carbon column and collected as a whole in Tedlar
plastic bags, to be afterwards tested by gas chromatography. Solid and liquid pyrolysis yields
are determined in each experiment by weighing the amount of each obtained, and calculating
the corresponding percentage, while the gas yield is determined by difference.
3. Results and Discussion
The mean value of the liquid, gas and solid yields (weight %), obtained in three pyrolysis
runs carried out at each temperature, which did not differ more than 3 points in the percentage,
are presented in Tab. 3 .
Tab. 3 Pyrolysis yields (weight %)FRACTION

460º C

500º C

600º C

Liquids

72.0

65.2

42.9

Gas

26.9

34.0

56.2

Solids

1.1

0.8

0.9

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It can be seen in Tab. 3 that while the solid yield has a constant value in all theexperiments, the gas and liquid yields are strongly influenced by temperature, and the higher
the temperature, the higher the gas yield and the lower the liquid yield obtained. This has also
been reported by many other authors, e.g. [2,5], and it is attributed to the stronger cracking ofC-C bonds that is produced at higher temperatures, which gives rise to lighter hydrocarbons
with shorter carbon chains. In all cases, a small quenatity of solid product (≈1% wt) wasobtained in spite of the absence of inorganic matter in the original sample (see Table II); thisis attributed to char formation, due to secondary repolymerisation reactions among thepolymer derived products. The authors [4,6] and also other research groups [7,8] haveobtained a certain amount of char in the pyrolysis of many polymeric materials. A study
carried out by Van Krevelen [9] about the thermal decomposition of polymers, concludes thatchar is formed in the decomposition of certain polymers, and that the char forming tendency
depends on the chemical structure of the polymer; such tendency increases when the polymer
contains groups capable of reacting with hydrogen atoms of the polymeric structure, such as–Cl, =O, etc., which is the case of PVC and PET, both of them contained in the raw sample.
Tab. 4 GC-MS analysis (area %) and GCV of the liquids
Aromatics

460º C
71.7

500º C73.9

600º C
99.3

Non aromatics
Non identified
C5-C922.3
6.0
78.1

22.4
3.7
79.6

0.4
0.3
70.6

C10-C13

7.4

9.4

23.8

>C138.5

7.3

5.3

Main components

Styrene: 45.5
Toluene: 9.9
Ethylbenzene: 7.1

Styrene: 48.4
Toluene: 8.1
Ethylbenzene: 5.0

Styrene: 32.4
Toluene: 17.5
Ethylbenzene: 8.1

GCV (MJ kg-1)

43.5

43.3

40.8

Tab. 4 shows the main characteristics of the liquids obtained in the experiments. Theliquid derived from the 460º C experiment was a wax-like product, which is due to thepresence of long non aromatic chains. The characteristics of the 500º C experiment liquid arequite similar to those of the 460º C run, however it was not a wax but a high viscosity liquidat room temperature, which may be due to its slightly higher aromatics and C5-C9 contents.
On the other hand, the liquid derived from the 600º C experiment was a dark coloured fluid
product which resembled petroleum fractions and which contained large quantities ofaromatics, more than 99 area%. Several authors have reported that the formation of aromatics
in thermal processes at low temperatures (460-500º C) the aromatics are probably derived
from the aliphatic hydrocarbons formed in the primary decomposition, via Diels-Alder- 38 -

PARTⅠ PYROLYSIS
reaction and followed by a dehydrogenation step [10,11]. Since Diels-Alder reactions areexothermic and reversible, they will be produced to a lesser extent at higher temperatures.
Therefore, such mechanism does not justify the increase in aromatics with temperatureobserved in this study, since if Diels-Alder reactions are reversed, polyenes formation will befavoured and less aromatics will be produced. Alternatively, it has been proposed [11] that at
higher temperature highly stable aromatics are directly produced by means of the so calledpyrosynthesis reactions.
In all cases, styrene is the main component of the organic liquids, reaching almost 50
weight% in tha 500ºC experiment. Other aromatic products obtained in significant
proportions are toluene and ethyl-benzene. Therefore valuable chemicals can be recoveredfrom pyrolysis organic liquids. On the other hand, Table IV shows that pyrolysis oils havevery high GCVs, similar to those of conventional liquid fuels, so they may be considered as
an appropriate alternative to fossil fuels.
Concerning gases, Tab. 5 shows that they are composed of hydrocarbons from C1 to C6,hydrogen and some carbon dioxide and monoxide. The gases from the 600º C experiment
have the highest H2 and short chain hydrocarbons (C1-C2) contents, which is due to thestronger cracking of C-C bonds produced at higher temperatures. In any case, the GCV ofevery of the pyrolysis gases is higher than the GCV of natural gas (44 MJ kg-1) and for thisreason they can be used as energetic source for the process, and the surplus may be valorized.
Tab. 5 GC-TCD/FID analysis (weight %) and GVC of the gases
Experiment

H2

CO+CO2

C1

C2C3

C4

C5+C6 GCV (MJ kg-1)

460º C

0.4

3.6

7.9

21.3

29.8

18.1

18.8

48.6

500º C

0.4

3.6

8.3

22.2

29.1

17.6

18.7

48.6

600º C

0.7

2.7

13.0

29.628.2

16.3

7.5

49.8

4. Conclusion
Feedstock recycling by pyrolysis of packing and packaging plastic wastes is a promisingtechnique for valorising this kind of wastes, since conversions to liquid + gas higher than 95%
can be obtained. The temperature of the process plays a very important role in the distribution
and quality of the pyrolysis products. On the one hand, low temperatures leads to thegeneration of a high proportion of wax-like paraffinic/olefinic liquids. On the other hand, hightemperatures give rise to very aromatic liquids and to more H2 and short chain hydrocarbons
in the gases. In every case, some char is formed as a result of secondary repolymerisation
reactions among the polymer derived products.
Concerning the applications of pyrolysis products, liquids may be used as high GCValternative fuels or as a source of valuable chemicals, such as styrene, toluene orethyl-benzene. The gaseous fraction can be used to supply the energetic demand of the- 39 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
process and the surplus may be used for additional power generation. Finally, the remaining
solid may find applications such as solid fuel, pigment, activated carbon, low quality carbon
black, etc.
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[2] J. Aguado, D. P. Serrano, Feedstock Recycling of Plastic Wastes, 1999..
[3] K. H. Lee, D. H. Seen, Waste Management, 2007, 27, 168.
[4] A. Torres, PhD Thesis, University of the Basque Country, 1998.
[5] A. Demirbas, J. Anal. App. Pyrol., 2004, 72, 97.
[6] I. de Marco, B. M. Caballero, M. A. Cabrero, et al., J. Anal. App. Pyrol., 2007, 79, 403.
[7] W. Kaminsky, Makromol. Chem., Macromol. Symp., 1991, 48/49,381.
[8] N. Grittner, et al., J. Anal. Appl. Pyrolysis, 1993, 25, 293[9] D.W. Van Krevelen, K. Te Nijenhuis, Property of Polymers, 1990, 21.
[10] H. Bockhorn, A. Hornung, U. Hornung, J. Anal. App. Pyrol., 1999, 50, 77.
[11] M. Sablier, L. Ji, A. Hervier, Chemosphere, 2006, 65, 1120.

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