Waste Management 24 (2004) 463–469
www.elsevier.com/locate/wasman
Pyrolysis of tyres. Influence of the final temperature of the process
on emissions and the calorific value of the products recovered
C. Dı́ez, O. Martı́nez, L.F. Calvo, J. Cara, A. Morán*
Department of Chemical Engineering, Institute of Natural Resources, University of León, Avenida de Portugal, 41 24071 León, Spain
Accepted 19 November 2003
Abstract
A study was made of the pyrolysis of tyre particles, with the aim of determining the possibilities of using the products resulting
from the process as fuel. Three final temperatures were used, determined from thermogravimetric data. The design of the experiment was a horizontal oven containing a reactor into which particles of the original tyre were placed. After the process, a solid
fraction (char) remained in the reactor, while the gases generated went through a set of scrubbers where most of the condensable
fraction (oils) was retained. Finally, once free of this fraction, the gases were collected in glass ampoules. Solid and liquids fractions
were subjected to thermogravimetric analyses in order to study their combustibility. The gas fraction was analysed by means of
gas chromatography to establish the content of CO, CO2, H2 and hydrocarbons present in the samples (mainly components of gases
produced in the pyrolysis process). A special study was made of the sulphur and chlorine content of all the fractions, as the presence
of these elements could be problematic if the products are used as fuel. Tyre pyrolysis engenders a solid carbon residue that concentrates sulphur and chorine, with a relatively high calorific value, although not so high as that of the original tyre. The liquid
fraction produced by the process has a high calorific value, which rises with the final temperature, up to 40 MJ/kg. The chlorine
content of this fraction is negligible. Over 95% of the gas fraction, regardless of the final temperature, is composed of hydrocarbons
of a low molecular weight and hydrogen, this fraction also appearing to be free of chlorine.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: Pyrolysis; Tyres; Thermogravimetry; Gas chromatography; Combustibility
1. Introduction
One of the many problems recently observed in the
area of waste is that of scrap tyres, a waste with special
characteristics of its own, among them their complex
nature, which makes them extremely difficult to recycle,
and their high calorific value, an important fact for
assessing their value.
In the European Union over 2,500,000 tonnes of tyres
are produced per year, with a very similar figure for the
United States. The management of this type of waste
leaves much to be desired, as in the European Union
almost 40% of tyres are thrown away untreated. There
are alternatives, such as re-treading, shredding, etc.,
although with many limitations (Mastral et al., 2000).
* Corresponding author. Tel.: +34-987-291841. Fax: +34-987291839.
E-mail address: dfqamp@unileon.es (A. Morán).
0956-053X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.wasman.2003.11.006
Yet it is possible to evaluate their energy potential, as
there are great possibilities for this type of waste.
The only way of assessing energy so far put into
practice is incineration, mainly using tyres as a back-up
fuel, especially in cement works, paper factories and
power stations. There are other techniques, such as
gasification and pyrolysis. The pyrolysis process for the
treatment of tyres has hardly been developed at all,
although there have been several studies concerning the
great possibility of this process for assessing energy
potential (Sharma et al., 2000).
The process of pyrolysis consists in a decomposition
of the material by means of temperature, in the total
absence of oxygen, which is why it is sometimes called
thermolysis. It leads to the production of a solid carbon
residue, a condensable fraction and gases. The solid
fraction contains the mineral matter initially present in
the tyre (Napoli et al., 1997). The liquid by-products of
tyre pyrolysis consist of a very complex mixture of
organic compounds of 5–20 carbons with a very high
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C. Dı´ez et al. / Waste Management 24 (2004) 463–469
proportion of aromatics (Laresgoiti et al., 2000). Finally,
the gas fraction is composed mainly of CO, CO2, H2 and
light hydrocarbons (Cunliffe, and Williams, 1998).
The main aim of the present study is to assess the
influence of the final temperature on the process of
pyrolysis, with a view to evaluating the energy (combustion) potential of the products of the process
(Inguanzo et al., 2002). Moreover, a special study was
made of elements in the samples that could pose problems from the point of view of this application, such as
sulphur and chlorine. The final temperatures chosen
were obtained from thermogravimetric data.
2. Experimental work
The design used for the experiment is shown in Fig. 1.
A horizontal oven was used, in which a reactor was
placed, consisting of a quartz tube 40 cm long and 7 cm
in diameter, which was loaded with the sample. The
sample was of tyre material ground mechanically to
pass through a 420 mm screen and was free of metal
reinforcing. About 50 g of the sample was used in each
experiment.
The oven was heated electrically with a heating gradient that was not constant but which increased with
temperature, from 5–60 C/min. The heating conditions
of the oven were reproduced in the thermobalance to
obtain a thermogram characteristic of the process of
pyrolysis of tyres and from there to choose the final
temperatures of pyrolysis. Three pyrolysis assays were
then carried out, one for each of the three final temperatures chosen, with the same residence time (about
15 min) of the tyre in the reactor for all three owing to
the difference in the oven heating time dependent on
temperature.
In the forward part of the reactor there is a gas intake,
the carrying gas used being helium with a flow rate of
200 mL/min. The gas products of the process came out
through the lower part of the reactor. To clean the gas
and separate it from the condensable fraction, three
stages of traps were set up. Firstly, the gases leaving the
reactor made direct contact with ice, then passed
through two columns placed in ice and packed with
raschig rings, to be finally filtered through a tube
packed with cotton and silicagel.
The liquid fraction obtained in each of the trials was
subjected to thermogravimetric analyses by means of an
SDT2960 thermobalance capable of taking a simultaneous reading of TGA and DTA signals. These analyses
consisted in combustion, of approximately 40 mg in a
controlled atmosphere, with a constant gas flow (100
mL helium/min) and a constant heating gradient
throughout the experiment of 15 C/min. The solid
residual fraction was subjected to the same thermal
analyses as the liquid fraction, weighed samples of
about 5 mg being used. Immediate, elemental and
calorific power analyses were carried out on both the
liquid and solid fractions, according to ASTM standard
procedures.
Samples were taken of the gases in ampoules that
were initially full of water, to verify the passage of gases,
samples corresponding to gases given off at 250–350 C
(sample taken at 350 C), 350–450 C (sample taken at
450 C) and 450–550 C (sample taken at 550 C). They
were analysed by gas chromatography, in an HP 5890
using three different columns for separation and two
signal detectors. Two packed columns were used: one
with a molecular sieve for permanent gases (hydrogen,
nitrogen, methane and CO) and a Chromosorb, specifically designed to detect CO2, together with an HP-1
semi-capillary column used to determine hydrocarbons.
The detectors used were a thermal conductivity detector
(TCD) and a flame ionization detector (FID). Three
Supelco reference standards were used to quantify
gases: mixture of olefins in helium (C2–C6), a reference
Fig. 1. Diagram of the experiment.
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C. Dı´ez et al. / Waste Management 24 (2004) 463–469
mixture for natural gas and a mixture of permanent
gases in helium. Once the composition of the gases had
been determined, their calorific value was calculated.
A more detailed study was made of sulphur and
chorine, for, as has been pointed out, their presence
could jeopardize the use of the pyrolysis products as
fuel. These elements were therefore analysed both in the
original tyre and in the different fractions obtained
during the process. In the gas fraction, they were determined by difference.
3. Results
3.1. Yields of the pyrolysis products
Fig. 2 shows a derived thermogravimetry (DTG) profile corresponding to the process of pyrolysis of the tyre
dust, revealing its mass loss rate at temperatures similar
to those used in the reactor. From the figure three stages
are to be observed: 200–350, 350–450 and 450–550 C.
For this reason, 350, 450 and 550 C were chosen as the
final temperatures for reactor tests. It can be said that
after 550 C the pyrolysis process is complete (Sørum et
al., 2001). Some authors (Leug, 1999) attribute the first
peak to the thermal decomposition of the mixture of
oils, moisture, plastifiers and other additives; and the
second and third ones to the thermal decomposition of
natural rubber (NB), polybutadene (BR) and polybutadene-styrene (SBR), the main components of tyres.
Other authors (Senneca, 1999) attribute part of the
second peak to the thermal decomposition of natural
rubber and the rest of it, together with the whole
third peak, to the decomposition of BR and SBR.
Table 1 shows the yields of the solid, liquid and gas
fractions for each final temperature used for pyrolysis.
The solids and liquid fractions were weighed directly
and the gas fraction calculated by difference. As the
temperature increased throughout the process, the production of oils and gases increased to a maximum of
550 C, in agreement with González et al.’s results (2001).
If the temperature continued rising, the production
of char would remain almost constant, according to
thermogravimetric data, while that of gases would
continue to rise as that of oil fell, owing to secondary
cracking reactions occurring in the process once pyrolysis
itself were over (Font et al., 2001).
4. Characterization of the pyrolysis fractions
4.1. Solid fraction (char)
The results of the elemental, proximate and calorific
value analyses of the initial sample and of the solid
fraction are given in Table 2. No major differences are
to be observed between the char yields for each temperature, but the drop in the volatile matter content with
the increase in the final temperature is significant,
because pyrolysis does not finish until 550 C is reached.
There is also a noticeable decrease in the hydrogen
content as temperature increases, probably due to the
great proportion of hydrogen compounds in the volatile
matter.
The high calorific value of char must be stressed:
although it is less than that of the original tyre, it can be
compared with that of good quality coal.
Fig. 3 shows DTGs for the combustion at predetermined temperatures of the solid fraction for the
three final temperatures used for pyrolysis. The final
combustion temperature (Tf) will be observed not to
depend on the final temperature at which pyrolysis occurs,
as the three samples contain the same non-combustible
Table 1
Yields for each fraction (%)
Char
Oil
Gasa
a
350 C
450 C
550 C
50
30
20
40
33
27
33
38
29
Obtained by difference.
Table 2
Proximate, elemental and heating value analyses of the tyre and char
Tyre (%)
Ash
Volatile matter
Fixed carbon
Moisture
Carbona
Hydrogena
Nitrogena
Sulphura
Oxygena (by subtraction)
HCV (KJ/Kg)
LCV (KJ/Kg)
Fig. 2. DTG pyrolysis of tyre dust.
a
Results in dry ash free.
7.1
61.9
29.9
1.1
89.5
7.3
0.3
1.9
0.9
37,352
35,931
350 C
Char (%)
450 C
550 C
16.1
6.7
75.9
1.3
95.3
1.3
0.1
3.2
0.06
28,942
28,696
16.0
3.1
79.5
1.5
95.9
0.7
0.1
3.3
0.02
28,600
28,462
16.5
1.2
81.3
1
95.9
0.5
0.2
3.4
0.01
28,574
28,495
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C. Dı´ez et al. / Waste Management 24 (2004) 463–469
material. On the other hand, the initial temperature (Tv)
rises as the final temperature rises, because the samples
with incomplete pyrolysis begin to give off volatile
matter at low temperatures, which favours the earlier
reaching of the initial temperature. The volatile matter
favours combustion, which means that the less pyrolysed the samples are, the earlier the temperature at
which maximum mass loss (Tmax) is reached.
The differential thermal analysis (DTA) profiles for
combustion of the solid fraction at predetermined temperatures are shown in Fig. 4. The area under the DTA
peak may be related to the heat produced in the combustion process, that is the high calorific value (HCV).
It will be observed that the higher the final temperature
of pyrolysis is, the smaller the area under the combustion curves of de solids fractions will be, that is to say
that an increase in the final temperature of pyrolysis
means a decrease in the heat given off by the combustion of the solid residues from the process, which confirms the data of the calorific value of the samples. This
is the consequence of a greater presence of volatile
matter in the samples pyrolysed at 350 and 450 C (with
values of 6.7 and 3.1%, respectively) as opposed to the
sample completely pyrolysed at 550 C, which had a
value of 1.2%.
4.2. Liquid fraction
Table 3 shows the results for the elemental and
calorific analysis of the liquid products from the
pyrolysis trials. In this case, unlike that of the solid
residues, there are significant differences between the
samples obtained at different temperatures. It will be
observed that an increase in the pyrolysis temperature
will be accompanied by a rise in carbon content and a
fall in oxygen content, which can be attributed to the
fact that the increase in temperature provides the activation energy necessary for the pyrolysis reactions to
continue and for carbon enrichment to occur. Also to be
noted is the increase in calorific power of the oils with
the rise in final pyrolysis temperature, the result of the
carbon concentration in the oils. The high calorific
value of these compounds together with the negligible
presence of chlorine makes this mixture suitable for use
as a fuel.
As for the thermal analysis of the combustion of the
solids products at predetermined temperatures, no significant differences were observed between the oils
obtained at different final temperatures of pyrolysis.
Fig. 5 shows the TG combustion curves of the liquids
samples oils, which are a complex mixture of olefins,
paraffins and aromatics. Apart from their complexity,
another characteristic of these liquid samples is their
great volatility, mass loss being observed from the sample at the beginning of the experiment, at around 50 C.
The combustion of the sample is complete at 610 C.
The same figure shows the DTA curve for one of the
Table 3
Elementary analysis and calorific value of the oils
Carbona
Hydrogena
Nitrogena
Sulphura
Oxygena (by difference)
HCV (KJ/Kg)
LCV (KJ/Kg)
a
350 C
450 C
550 C
76.5
10
0.3
1.3
11.9
37428
35308
78.2
10
0.4
1.4
10
38468
36349
84.9
9.6
0.4
1.6
3.5
40776
38740
Results expressed with regard to dry matter and free of ash.
Fig. 3. DTG of waste pyrolysed at 350, 450 and 550 C.
Fig. 4. DTA of waste pyrolysed at 350, 450 and 550 C.
Fig. 5. TG and DTA combustion curves of the liquid samples
obtained at different final temperatures.
C. Dı´ez et al. / Waste Management 24 (2004) 463–469
samples, revealing the endothermic character of the
boiling process of much of it at low temperatures and
the exothermic nature of the combustion process of the
rest of it at high temperatures.
4.3. Gas fraction
The gas fraction was analysed by gas chromatography, with results being obtained for CO, CO2, H2,
N2, O2, methane, methane+ethylene, propylene, butadiene, remaining C4, C5 and C6. Figs. 6 and 7 show the
chromatograms corresponding to the sample obtained
at the highest final temperature (550 C). Fig. 6 corresponds to the FID signal, when the sample of gases taken
at 550 C is passed through the HP-1 semi-capillary
column. The first two peaks correspond to methane and
ethane+ethylene respectively, the third one probably
corresponds to propylene, and next come peaks
grouped according to the number of carbons, in the
order C4, C5 and C6. Within the C4 group there is a
467
peak that is higher than the rest, corresponding to
butadiene. It should not be forgotten that this compound is a part of SBR rubber, the main constituent of
the tyres used. Fig. 7 shows the chromatogram corresponding to the analysis of the same gas sample when the
molecular strainer column and TCD detector were used.
The oxygen and nitrogen in the sample must be associated with the sample being contaminated by air, as
they are in the same proportion as in the atmosphere.
The hydrogen signal shows a double peak, normal and
inverted, concentration being reckoned from the sum of
the areas of the two peaks, as though they were one.
The results for the composition of the gases are shown
in Table 4, where it is to be observed that the higher the
final pyrolysis temperature is, the more enriched with
methane and hydrogen the gases become, while the
remaining hydrocarbons decrease (Conesa et al., 1996).
This drop in the longer hydrocarbon chains and the rise
in the lighter ones (methane and hydrogen) with temperature is due to the mechanism of the pyrolysis process
Fig. 6. Chromatogram of the sample at 550 C using an HP-1 column.
Fig. 7. Chromatogram of the sample at 550 C using the molecular strainer.
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C. Dı´ez et al. / Waste Management 24 (2004) 463–469
MJ/m3 (Cunliffe and Williams, 1998; de Marco et al.,
2001). Calorific strength diminishes with temperature,
which may be attributable to the decreasing proportion
of hydrocarbons with more than one carbon atom and
with greater calorific value, occurring as temperature
increases (Laresgoiti et al., 2000). Once the energy balance has been calculated, the error in the values
obtained for the low calorific value (LCV) and the percentage of the different fractions is seen to be under 1%.
Table 4
Composition of the pyrolysis gases
Methane
Ethane+Ethylene
Propylene
Butadiene
Remaining C4
C5
C6
CO
CO2
H2
Total
350 C
(% vol.)
450 C
(% vol.)
550 C
(% vol.)
20
29
12
3.6
2.1
3
1
1
2.3
24
98
24
26
9
2.7
1.6
2
0.2
1.1
1.9
30
98.5
26
20
6
1.5
1.3
1.1
0.1
1.6
1.4
40
99
4.4. Sulphur and chlorine
Table 6 gives the concentrations of sulphur and
chlorine for each of the pyrolysis products. The process
leads to a slight change in the sulphur content in the
solid waste, the 100% of the original tyre becoming
51.1–75.0% of the carbon black. In the case of the oils,
the sulphur values are somewhat lower than in the original tyre, rising slightly with the final temperature of
pyrolysis. Finally, the gas fraction shows fairly low
values for sulphur, probably in the form of hydrogen
sulphide (H2S).
If we bear in mind the yields of the three fractions, it
will be observed that amount of sulphur liberated in the
liquid and gas fractions after the pyrolysis process is
greater as the final temperature rises, although the percentages are not very different, for the amount of oil
and gas produced also rises, owing to the fact that
higher temperatures favour the breaking of the C–S
bonds in the original tyre.
The case of chlorine is different, as it is completely
retained in the solid fraction. This fraction has similar
chorine values for the three final temperatures, about
0.1%, a relatively high value compared with the original
tyre, which has a chlorine percentage of 0.04. The presence of chlorine in the liquid and gas fractions is negligible, regardless of the final temperature of pyrolysis.
Table 5
Calorific value of the gas samples
HCV (MJ/m3)
LCV (MJ/m3)
350 C
450 C
550 C
55,01
50,80
47,66
43,83
39,60
36,16
itself. As temperature rises, the heavier hydrocarbons
are cracked, giving rise to lighter ones, methane and
hydrogen (Dai et al., 2001). Consequently, the gas samples become enriched with the lighter hydrocarbons as
the temperature increases (Arion et al., 2001). Furthermore, the higher the final temperature of pyrolysis is,
the less CO2 there is, while CO increases, probably as a
consequence of the secondary reactions taking place
during the pyrolysis process, in which CO is formed
from reactions in the gas phase between CO2 and
hydrocarbons or from other cracking reactions (homogeneous phase reactions). Tests were also run for several
gases not shown in the table, such as H2S.
From the concentrations shown in Table 4, an estimate was made of the calorific value, bearing in mind
the densities of each of the gas species considered,
enthalpies of combustion and specific heats. The results
obtained are shown below (Table 5).
The gases generated in the process of pyrolysis of
tyres have a significant calorific value, of the order 40
5. Conclusion
Tyre pyrolysis leads to the production of a solid
waste, liquids and gases. The amount recovered of
Table 6
Distribution of sulphur and chlorine in the pyrolysis products
Original tyre
Carbon black
S (%)a
Cl (%)b
S (%)b
Cl (%)b
a
b
1.8
0.04
–
–
Oil
Gas
350 C
450 C
550 C
350 C
450 C
550 C
350 C
450 C
550 C
2.7
0.10
75.0
100
2.7
0.11
60.0
100
2.8
0.12
51.1
99.0
1.3
<0.01
21.7
—
1.4
<0.01
25.7
—
1.6
<0.01
33.8
<1.0
0.3
<0.01
3.3
–
1.0
<0.01
14.3
–
1.0
<0.01
15.1
<1.0
Results of analysis of S and Cl, on dry basis.
Percentage of S and Cl retained in each fraction.
C. Dı´ez et al. / Waste Management 24 (2004) 463–469
each of them depends on process conditions, mainly the
final temperature. A previous study to determine the final
temperatures uses was carried out in a simple and reliable way by means of thermogravimetric analysis.
The solid fraction is a carbonic residue, with a carbon
content of around 80%. Its properties as a fuel can be
compared with those of good quality coals, the carbon
content and the calorific value rising as does the final
temperature of the process. Nevertheless, the pyrolysis
process may concentrate such substances as sulphur and
chlorine in this residue, jeopardizing its use as a fuel.
Ash content of the solid fraction also increases in comparison with that of the original tyre, an important factor to consider if combustion is contemplated.
The liquid fraction is a complex mixture of organic
compounds with a high calorific value that rises with the
final temperature of pyrolysis and which has good
combustibility as is borne out by the thermogravimetric
analyses. Furthermore, the presence of chlorine in these
compounds is negligible and the sulphur and nitrogen
contents are similar to those of other liquid fuels. All
this favours their use as fuels.
The gas fraction is basically a mixture of hydrocarbons of low molecular weight (up to C6) and hydrogen, which together make up over 95% of the gas
content in all cases. The composition of this gas fraction
depends very much on the final temperature of pyrolysis, so high temperatures favour the presence of
hydrogen and methane, while low ones mean a higher
proportion of heavier hydrocarbons.
The pyrolysis process creates several by-products with
a number of applications, although the main use in all
cases would be combustion to produce energy. We can
therefore conclude that the pyrolysis process is an
interesting alternative for dealing with used tyres, provided that it is subject to the appropriate controls,
especially regarding chlorine.
Acknowledgements
The authors wish to thank the University of León for
funding the project with grant ULE-2001-05 B. We also
469
thank the MAPFRE Trust for co-funding the project
with a grant to assist research.
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