Characterization of emissions during the heating of tyre contaminated scrap

Article abstract of DOI:10.1016/S0045-6535(00)00260-5

In order to characterize the compounds (type and quantities) emitted during melting of organic contaminated scrap and to investigate the mechanism of their formation, an experimental set-up has been designed and built to study precisely the influence of temperature and gas atmosphere in the conditions of an electric arc furnace. These experiments lead to the determination of mass balances (C, H, O, S) and to the quantification of unburnt compounds (tars, carbon monoxide, volatile organic compounds (VOCs), benzene, toluene, ethylbenzene and xylenes (BTEX), polyaromatic compounds (PAHs)). Degradation conditions (gas atmosphere and temperature) corresponding to different areas in the electric furnace have also been investigated. Such experiments lead to a better understanding of degradation mechanisms; this interpretation is not possible from investigations performed in an industrial furnace since there are many uncontrolled parameters (large dispersion of the results).

Full text of DOI:10.1016/S0045-6535(00)00260-5

Chemosphere 42 (2001) 853±859
Characterization of emissions during the heating of tyre
contaminated scrap
a
A. Arion , F. Baronnet , S. Lartiges , J.P. Birat
b,
a
a
*
a
IRSID (Centre Proc eꢀ d eꢀ s, USINOR), Voie Romaine, BP 30320, F-57283 Maizi eꢁ res-l eꢁ s-Metz, France
D eꢀ partement de Chimie Physique des R eꢀ actions, UMR no 7630 CNRS-INPL, 1 rue Grandville, BP 451, F-54001 Nancy Cedex, France
b
Abstract
In order to characterize the compounds (type and quantities) emitted during melting of organic contaminated scrap
and to investigate the mechanism of their formation, an experimental set-up has been designed and built to study
precisely the in¯uence of temperature and gas atmosphere in the conditions of an electric arc furnace. These experi-
ments lead to the determination of mass balances (C, H, O, S) and to the quanti®cation of unburnt compounds (tars,
carbon monoxide, volatile organic compounds (VOCs), benzene, toluene, ethylbenzene and xylenes (BTEX), polya-
romatic compounds (PAHs)). Degradation conditions (gas atmosphere and temperature) corresponding to di€erent
areas in the electric furnace have also been investigated. Such experiments lead to a better understanding of degradation
mechanisms; this interpretation is not possible from investigations performed in an industrial furnace since there are
many uncontrolled parameters (large dispersion of the results). Ó 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Oxidation; Pyrolysis; Recycling
1
. Introduction
ganic and inorganic compounds. As steel is more and
more largely produced in electric arc furnaces using
scrap as a raw material and as regulations on stack
emissions are being drastically enforced, monitoring and
reduction of the e‚uents emanating from the furnace
are essential.
Scrap contains mainly iron, but also non-ferrous
metals and small amounts of organic comaterials (less
than 1%w/w). Their preparation is a very important step
because it aims to separate the non-ferrous elements
from the ferrous ones and to cut to the required size for
the loading into the furnace. For example, shredders
dismantle old household objects (washing machines, etc)
and cars then a fan sucks ¯u€ (foams in a car, etc) and a
magnetic drum separates the cleaned scrap from the
remaining non-ferrous components (plastics and other
metals).
The aim of this study is to characterize precisely these
compounds (type and quantities) and to investigate the
mechanisms of their formation and destruction in order
to de®ne how to obtain non-pollutant smokestack
fumes.
It is rather dicult to give a comprehensive biblio-
graphic review since it is a rather applied subject and our
investigation is speci®cally focused on the electric arc
furnace.
When melting organic contaminated scrap, i.e., when
this scrap preparation has not been properly carried
out, the thermal degradation of organic materials as-
sociated with steel leads to the emission of various or-
However papers from Conesa et al. (1996, 1997)
concern the validation of a kinetic model for the thermal
decomposition of tyre wastes and the nature of products
emitted under pyrolytic conditions. Papers such as that
of Rausa and Pollesel (1997) are focused on the e€ect of
temperature on the pyrolysis of automotive shredder
*
Corresponding author.
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 ( 0 0 ) 0 0 2 6 0 - 5

8
54
A. Arion et al. / Chemosphere 42 (2001) 853±859
residues. More general papers deal with the thermal
recovery from the e‚uents of tyre pyrolysis: Williams
et al., 1995; Avenell et al., 1996; Napoli et al., 1997.
In this paper, we have tried to perform a systematic
investigation by selecting experimental conditions which
are found in the di€erent zones of an electric arc oven
been designed in order to perform small-scale experi-
ments under well-controlled temperatures and gas at-
mospheres, with
interpretability.
a
good reproducibility and
(
Fig. 1). The conditions can be considered as limiting
2. Materials and methods
cases and there are of course a large number of zones in
which the experimental conditions are intermediary.
In an electric furnace, which is a batch reactor, a
large range of atmospheres and temperatures varying
according to time and space is observed (Arion, 1998)
which can be classi®ed as follows:
Old cars are an important source of scrap iron. In
France, two million cars have to be disposed of by
shredding them every year; this generates 1.2 million
tons of scrap metal. Three representative types of or-
ganic compounds contained in this scrap have been se-
lected: used engine oil for the hydrocarbons, PVC putty
for the plastomers, and a new car tyre for the elastomers.
Pyrolysis of used engine oil has been carried out; two
·
·
·
near the injection nozzles and the furnace openings,
at the beginning of the melting process, the gas phase
can be represented by air and the temperatures range
from 300°C to 600°C;
2 2
kinds of atmosphere (air and a mixture N /CO ) and
between the electrodes and the furnace walls, the gas
temperatures ranging from 300°C to 1500°C were tested
and described in detail elsewhere (Arion et al., 1998).
Only emissions emanating from the tyre degradation will
be presented in detail here.
phase can be represented either by air or by N
(
2 2
/CO
60/40 v/v) and the temperatures range from 900°C to
1100°C;
near the steel bath, i.e., at the end of the melting pro-
The tyre rubber was cored through the tyre tread; the
sample therefore contains vulcanized elastomers (47%w/
w), black carbon (25%w/w), steel cord (16%w/w) and
other materials (textile, anti-oxidizing agent, etc). Table 1
shows the elementary analysis of the tyre rubber
studied.
cess, the temperature is about 1500°C and the atmo-
sphere is either air or a mixture of CO /CO.
2
It is not possible to carry out a detailed investigation in a
large scale electric arc furnace (even in a pilot plant)
because this work aims at studying the in¯uence of
temperature and composition of the furnace atmosphere
and also to determine the mechanism of formation of
volatile organic compounds (VOCs), especially benzene,
toluene, ethylbenzene and xylenes (BTEX) and polya-
romatic compounds (PAHs) which are in very small
amounts. Such an investigation requires well-de®ned
conditions, which cannot be obtained in an electric arc
furnace. For this reason, a laboratory equipment has
Fig. 2 shows the experimental set-up used. The small
piece of tyre rubber on its steel sample holder is placed
in a Lenton LTF 75/14/450 furnace at the temperature
2 2
and under the atmosphere (air or N /CO ) studied. The
gaseous e‚uents originating from the oil degradation
are cooled at the furnace exit: heavy products such as
tars and aerosols are then condensed and ®ltered
through Raschig rings and a quartz ®lter. The eight
Fig. 1. Electric arc furnace and the di€erent areas inside the furnace (batch): A ± just after the loading of scrap (temperature: 300±
00°C under air); B ± at an intermediary step of steel melting (temperature: 500±900°C under air or N /CO coming from the oxidation
of injected coal or scrap iron); C ± at the end of the process when most scrap has been melted (temperature 1100±1500°C under N
CO ).
6
2
2
2
/
2

A. Arion et al. / Chemosphere 42 (2001) 853±859
855
Table 1
Elementary analysis of the tyre rubber studied
Element
%w/w)
C
H
O
S
N
Cl
Fe
Ashes
1.6
(
73.8
5.3
0.11
1.71
0.44
0.04
16.2
Fig. 2. Experimental set-up.
PAH emitted in fumes and subject to government reg-
ulations in France are analyzed by high-pressure liquid
chromatography (HPLC) coupled with a ¯uorescence
detector. Elementary analyses are made for tars and
aerosols. The fumes are simultaneously sampled in dif-
ferent ways:
tative areas in an electric furnace and to di€erent kinds
of degradation were chosen (Fig. 1):
·
·
·
·
at 500°C under air (low temperature oxidation);
at 900°C under air (combustion);
at 900°C under N
2
/CO
2
(pyrolysis);
(pyrolysis with part of CO
at 1100°C under N
conversion).
2
/CO
2
·
with on-line speci®c analyzers for CO/CO
Ultramat 22), O (Siemens Oxymat) and total VOC
Siemens Fidamat 5E-E);
2
(Siemens
2
(
·
with a sampling bag in Tedlar and o€-line analysis by
gas chromatography (GC) coupled with a ¯ame ion-
ization detector (FID) for light hydrocarbons or a
GC/TCD (catharometer) for hydrogen and other in-
organic gases;
3. Results
Main products of the thermal degradation were ash
residues remaining on the sample holder, tars trapped in
the cooler (high ratio of C/H), carbon oxides, water,
·
by trapping BTEX with activated charcoal and sub-
sequent analysis by GC/FID.
1 4
VOCs (mainly alkanes and alkenes C ±C , benzene and
toluene, Figs. 3 and 4) and sulphur dioxide (Table 2).
Satisfactory mass balances were obtained (Figs. 5±8)
and Conesa et al. (1996) and Erdogan et al. (1991) ob-
served the same distribution of the products.
In order to characterize the emissions of organic com-
pounds during the recycling of organic contaminated
scrap, four conditions corresponding to both represen-
Fig. 3. Low molecular weight VOCs emitted during the degradation of tyre rubber.

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56
A. Arion et al. / Chemosphere 42 (2001) 853±859
Fig. 4. Benzene and its derivatives emitted during the degra-
dation of tyre rubber.
Fig. 5. Carbon balances obtained during the thermal degra-
dation of tyre rubber.
The e€ect of temperature and gas atmosphere on
oxygen consumption is shown in Table 3. The amount of
oxygen required for the tyre degradation is more im-
portant under air than under N
pounds resulting from primary cracking ignite in air
above 400°C, whereas in the mixture N /CO the dom-
inant reaction is pyrolysis. The sample holder is oxidized
above about 750°C (under air, Fe + 1/2 O ® FeO and
under N /CO , Fe + CO
® FeO + CO).
2 2
/CO , because com-
2
2
2
2
2
2
The formation of small quantities (or traces) of other
monoaromatic compounds and polyaromatic hydro-
carbons (PAHs) was observed. Benzene and toluene
were the main monoaromatic compounds emitted with
increased emission of benzene with temperature and
higher pyrolytic conditions (Fig. 3), whereas ¯uoranth-
ene was the major PAH emitted (Fig. 9).
Fig. 6. Hydrogen balances obtained during the thermal
degradation of tyre rubber.
Under air at low temperature (500°C), one noticed
the pyrolysis of the elastomers and a partial oxidation
of the emitted hydrocarbons leading to large emissions
of unburnt compounds such as initial carbon black of
the tyre, emitted VOCs and CO but few PAHs. Under
air at 900°C, no aromatics (PAHs and BTEX) were
formed because the oxidation conditions were satisfac-
tory and the other products (CO, VOCs) were formed
only in trace amounts. Water and carbon dioxide were
the major products and made up most of the mass
balance.
Under N /CO , pyrolysis was the dominant reaction
2
2
leading to large amounts of VOCs, especially aromatic
compounds. At high temperature, the carbon dioxide
present in the oven atmosphere reacts with the iron
scrap and with the pyrolytic carbon from the tyre; this
reaction leads to the formation of large amounts of
carbon monoxide.
Sulphur was mainly emitted through gaseous sulphur
dioxide but is also present in condensed matter in ash
residues and tars.
Table 2
Major products emitted during the thermal degradation of tyre rubber (in mg/g of tyre)
Conditions
500°C under air
900°C under air
900°C under N
2
/CO
2
2 2
1100°C under N /CO
Non-ferrous ashes
30.2 ‹ 6.2
260 ‹ 17
283 ‹ 79
1101 ‹ 152
178 ‹ 24
21.6 ‹ 5.4
119 ‹ 32
13.5 ‹ 0.7
119 ‹ 25
291 ‹ 3
204 ‹ 19
283 ‹ 39
Tars
CO
211 ‹ 45
136 ‹ 20
Consumption
369 ‹ 28
14.0 ‹ 0.4
53 ‹ 7
20.1 ‹ 3.3
2740 ‹ 767
0.26 ‹ 0.15
26.5 ‹ 6.3
448 ‹ 90
1691 ‹ 32
Consumption
133 ‹ 8
CO
Total VOCs
SO
2
2
11.1 ‹ 0.2
254 ‹ 31
2
H O

A. Arion et al. / Chemosphere 42 (2001) 853±859
857
Fig. 7. Oxygen balances obtained during the thermal degra-
dation of tyre rubber.
Fig. 9. PAHs emitted during the degradation of tyre rubber.
The degradation of the tyre components is due both
to pyrolysis and oxidation reactions; their relative im-
portance depends on the temperature and on the con-
centration of oxygen.
Under pyrolytic conditions (Chauvel et al., 1980),
hydrocarbons are thermodynamically unstable above
5
00°C. Generally their stability decreases when the
molecular weight increases. Methane is also the most
stable compound below 1000°C. A double bond is more
stable than a single one. Because of the resonance en-
ergy, aromatic compounds are very stable at high tem-
perature. Compared to other hydrocarbons excepting
aromatic compounds, alkynes are stable between
1
000°C and 1200°C. Fig. 10 shows the in¯uence of
temperature on the di€erent steps of hydrocarbon py-
Fig. 8. Sulphur balances obtained during the thermal degra-
dation of tyre rubber.
rolysis.
Therefore under oxidizing conditions as seen before,
both pyrolysis and oxidation take place, leading to the
emission of numerous molecules more or less oxidized:
®nal products like carbon dioxide and water, inert gases
4
. Discussion
The composition of the rubber of a car tyre is quite
complex: natural rubber (polyisoprene), synthetic rub-
ber (especially butadiene styrene copolymer), black
carbon and various additives especially those containing
sulphur for the vulcanization process.
but also unburnt compounds such as H
VOCs, PAHs, etc.
2
, CO, tars,
Concerning the thermal degradation of the tyre
rubber, taking into account the chemical composition
(various types of rubbers, additives, etc), the tempera-
ture and the oxygen concentration linked to the resi-
dence time, the decomposition paths can be summarized
as follows:
As a consequence, it is practically impossible to write
a reaction scheme; we can only discuss our results in a
qualitative way, taking into account the basic knowledge
of the thermal reactions of hydrocarbons.
It is also worth noting that in the thermal reactions of
a single reactant (for instance an alkane) there are
common steps for the pyrolysis and oxidation reactions.
The situation should be the same for thermal reactions
of complex mixtures.
·
volatilization of low molecular compounds (plasti®-
ers, etc) (Kim et al., 1995);
·
breaking down of the S±S bonds (reaction 1, Fig. 11),
so that vulcanized rubbers behave like elastomers
(Janowska and Slusarki, 1995);
Table 3
Oxygen mass balance related to the thermal degradation of tyre rubber and to the oxidation of the iron sample holder (in mg/g of tyre)
Conditions
500°C under air
900°C under air
900°C under N
2
/CO
2
2 2
1100°C under N /CO
Fe sample holder
Tyre rubber
Total
9.24 ‹ 1.37
889 ‹ 6
898 ‹ 4
368 ‹ 55
1559 ‹ 421
1927 ‹ 366
81 ‹ 45
80 ‹ 70
181 ‹ 25
289 ‹ 35
1425 ‹ 275
1714 ‹ 240

8
58
A. Arion et al. / Chemosphere 42 (2001) 853±859
Fig. 10. In¯uence of temperature on the di€erent steps of
pyrolysis (Chauvel et al., 1980).
Fig. 12. Degradation of aromatics in an oxidizing medium.
tors have an impact on their formation (Brezinsky et al.,
1
·
·
·
986):
aromatic radicals present in the oven atmosphere;
degradation occurring at high temperature;
pyrolytic conditions.
Aromatic compounds with a side chain behave like ali-
phatic compounds, i.e., only the side chain is decom-
posed producing benzene and toluene, and then phenyl
and benzyl radicals. PAHs are formed by condensation
of aromatic radicals under pyrolytic conditions. Aro-
matic compounds (with or without side chain) can be
formed by a Diels±Alder reaction from diole®ns and
ole®ns. The aromatic compounds are stable at high
temperature under pyrolytic conditions (ratio between
oxygen and hydrocarbon greater than 2), whereas under
oxidizing conditions PAHs are not produced because the
monoaromatic molecules (their precursors) are quickly
oxidized (Fig. 12) into aliphatic hydrocarbons and then
into water and carbon dioxide (Venkat et al., 1982;
Brezinsky et al., 1986).
Fig. 11. Some possible decomposition paths of vulcanized
elastomers.
·
decomposition of the elastomers (primary cracking
by unzip depolymerization and also intramolecular
cyclization followed by scission) leading to oligomers
but also compounds from reactions of these primary
products like alkenes, benzene, etc and hydrocarbons
5. Conclusion
up to C16
Lehrle, 1991).
H26 (Chien and Kiang, 1979; Groves and
The laboratory equipment used allows a reproducible
and detailed study of organic compounds emitted during
pyrolysis of tyre rubber, temperature and atmosphere
being two well-controlled parameters. Such experiments
lead to a better understanding of degradation mecha-
nisms, which is not possible in an industrial furnace, as
there are too many varying factors (large dispersion of
results).
Most of these compounds are aromatic or lead very
easily to aromatics (by Diels±Alder reactions for exam-
ple), which explain why during our experiments large
amounts of benzene and its derivatives were produced in
pyrolytic conditions, even at relatively low temperature.
Due to their carcinogenic properties, aromatic mol-
ecules need to be closely monitored; three di€erent fac-

A. Arion et al. / Chemosphere 42 (2001) 853±859
859
The previous discussion on the possible reaction steps
and our experimental results, especially these shown in
Figs. 4 and 9 show that an ecient oxidation (close to
the combustion conditions) is necessary to avoid the
emission of aromatic species.
Conesa, J.A., Font, R., Marcilla, A., 1996. Gas from the
pyrolysis of scrap tyres in a ¯uidized bed reactor. Energy
Fuels 10, 134±140.
Conesa, J.A., Font, R., Marcilla, A., 1997. Mass spectrometry
validation of a kinetic model for the thermal degradation of
tyre wastes. J. Analy. Pyrol. 43 (1), 83±96.
Erdogan, M., Yalcin, T., Tincer, T., Suzer, S., 1991. Evolved
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Groves, S.A., Lehrle, R.S., 1991. Natural rubber pyrolysis: study
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formation mechanism. J. Anal. Appl. Pyrol. 19, 301±309.
Janowska, G., Slusarski, L., 1995. Thermal analysis of poly-
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