Semivolatile and volatile compounds in combustion of polyethylene

Article abstract of DOI:10.1016/j.chemosphere.2004.06.020

The evolution of semivolatile and volatile compounds in the combustion of polyethylene (PE) was studied at different operating conditions in a horizontal quartz reactor. Four combustion runs at 500 and 850°C with two different sample mass/air flow ratios and two pyrolytic runs at the same temperatures were carried out. Thermal behavior of different compounds was analyzed and the data obtained were compared with those of literature. It was observed that α,ω-olefins, α-olefins and n-paraffins were formed from the pyrolytic decomposition at low temperatures. On the other hand, oxygenated compounds such as aldehydes were also formed in the presence of oxygen. High yields were obtained of carbon oxides and light hydrocarbons, too. At high temperatures, the formation of polycyclic aromatic hydrocarbons (PAHs) took place. These compounds are harmful and their presence in the combustion processes is related with the evolution of pyrolytic puffs inside the combustion chamber with a poor mixture of semivolatile compounds evolved with oxygen. Altogether, the yields of more than 200 compounds were determined. The collection of the semivolatile compounds was carried out with XAD-2 adsorbent and were analyzed by GC-MS, whereas volatile compounds and gases were collected in a Tedlar bag and analyzed by GC with thermal conductivity and flame ionization detectors.

Full text of DOI:10.1016/j.chemosphere.2004.06.020

Chemosphere 57 (2004) 615–627
www.elsevier.com/locate/chemosphere
Semivolatile and volatile compounds in combustion
of polyethylene
*
´
Rafael Font, Ignacio Aracil , Andres Fullana, Juan A. Conesa
Chemical Engineering Department, Universidad de Alicante, E-03080, Alicante, P.O. Box 99, Spain
Received 28 October 2003; received in revised form 4 June 2004; accepted 10 June 2004
Abstract
The evolution of semivolatile and volatile compounds in the combustion of polyethylene (PE) was studied at differ-
ent operating conditions in a horizontal quartz reactor. Four combustion runs at 500 and 850 °C with two different
sample mass/air flow ratios and two pyrolytic runs at the same temperatures were carried out. Thermal behavior of dif-
ferent compounds was analyzed and the data obtained were compared with those of literature.
It was observed that a,x-olefins, a-olefins and n-paraffins were formed from the pyrolytic decomposition at low tem-
peratures. On the other hand, oxygenated compounds such as aldehydes were also formed in the presence of oxygen.
High yields were obtained of carbon oxides and light hydrocarbons, too. At high temperatures, the formation of poly-
cyclic aromatic hydrocarbons (PAHs) took place. These compounds are harmful and their presence in the combustion
processes is related with the evolution of pyrolytic puffs inside the combustion chamber with a poor mixture of semi-
volatile compounds evolved with oxygen. Altogether, the yields of more than 200 compounds were determined.
The collection of the semivolatile compounds was carried out with XAD-2 adsorbent and were analyzed by GC–MS,
whereas volatile compounds and gases were collected in a Tedlar bag and analyzed by GC with thermal conductivity
and flame ionization detectors.
Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: PAHs; PE; Combustion; Furnace; Emissions
1. Introduction
2002 the consumption of polymers for plastic applica-
tions in Western Europe was 38123000 tonnes, an in-
crease of 6% from 2000. Each individual in this region
consumed on average 94.8 kg of virgin plastics. The
plastics with the greatest demand were polyethylene,
both low (LDPE) and high density (HDPE), polypropyl-
ene (PP), polyvinyl chloride (PVC) and polystyrene (PS).
The total production of plastic waste in Europe in 2002
was 20391000 tonnes, and 38% of the total collectable
plastic waste was recovered, up from 36% in 2000. In
tonnage terms, this represented an increase of 11%.
Energy was recovered from 4688000 tonnes of plastic
waste in 2002, an increase of 6.3% on 1999 figures and
There is a steady increase in the consumption of plas-
tics worldwide due to some of their properties such as
flexibility, affordability, endurance, durability, etc.,
which allow them to be suitable in almost every field.
According to a report published by the Association of
Plastic Manufacturers in Europe (2003), in the year
*
Corresponding author. Tel.: +34 965 903 546; fax: +34 965
903 826.
E-mail address: nacho.aracil@ua.es (I. Aracil).
0045-6535/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2004.06.020

616
R. Font et al. / Chemosphere 57 (2004) 615–627
representing 23% of the total collectable plastic waste.
Thermal treatments, both pyrolysis and combustion,
are important alternatives to the disposal of plastic
wastes in landfills. However, thermal processes in gen-
eral have been shown to release toxic compounds. For
this reason, it is important to know more about the
behavior of plastics when they are burnt.
consisting of identifying more than 100 PAH com-
pounds with 2–7 fused rings with a high-performance
GC–MS. Li et al. (2001) studied the presence of PAHs
in the raw waste, stack flue gas and bottom ashes in con-
trolled-air incineration of PVC, HDPE and PP plastic
waste, and found that PAHs present in the raw waste
were almost entirely destroyed and proposed CO as a
surrogate indicator for total PAH emissions, since both
could be correlated.
Some recent papers concerning the analysis of
organic by-products in great or small concentrations in
combustion and pyrolysis of different kind of wastes
such as plastics or wood can be found in literature
(Esperanza et al., 2000; Fullana et al., 2000; Launhardt
and Thoma, 2000; Ciccioli et al., 2001; Friedli et al.,
2001). There is an increasing interest in the determina-
tion, analysis and proposal of solutions for the semivol-
atile compounds evolved in many thermal processes of
coal, industrial oils, biofuels and wastes, since toxic
by-products must be processed or destroyed correctly.
On the other hand, pyrolytic puffs can be evolved in a
furnace, so many of the toxic by-products generated in
a pyrolysis process can also be present in the combustion
of the waste.
PAHs are always present in any thermal process of
an organic compound. Both their formation and
destruction have been considered by different research-
´
ers. Mastral and Callen (2000) and Richter and Howard
(2000) have reviewed the emission and generation of
PAHs from energy generation processes. In literature,
there are some references concerning the mechanisms
of formation and destruction of organic compounds
in different types of flames, and the catalytic combus-
tion, but this does not form part of the aims of this
paper.
The main objective of this work has been the quali-
tative and quantitative determination of the toxic
by-products that can be evolved in the combustion of
polyethylene considering both the effect of the tempera-
ture and sample mass/air flow ratio in their formation
and thermal destruction. Combustion experiments were
carried out in substoichiometric conditions. The estima-
tion of the yields corresponding to the different com-
pounds at these conditions can be useful, since the
formation of puffs of the semivolatile ones is likely to
occur in combustors, where they can circulate maintain-
ing their identity.
Pyrolysis of polyethylene has been widely studied in
several papers (Wampler, 1989; Conesa et al., 1997; Wil-
liams and Williams, 1999; Predel and Kaminsky, 2000;
Mastral et al., 2002). However, there are fewer works
concerning combustion of polyethylene.
Hodgkin et al. (1982) determined some low molecular
weight products from burning polyethylene, including
some oxidative degradation compounds such as acetone,
acetaldehyde, acetic acid or acrolein. Mitera and Michal
(1985) characterized some compounds detected in the
combustion of polyethylene, comparing them with those
produced using polypropylene, polystyrene and poly-
amide (PA).
2. Experimental
Hawley-Fedder et al. (1984, 1987) studied the prod-
ucts evolved in the combustion of polyethylene in a
polymer mixture (PE, PS and PVC) at 800–950 °C,
observing alkyl benzenes, hydrocarbons, alkenes, biphe-
nyls and PAHs. Wheatley et al. (1993) made an explor-
atory study on the combustion and PAH emissions of
selected municipal waste plastics. The gas temperature
and residence time were varied and the effects of both
the flame and the postflame conditions on the PAH
emission levels were investigated. Minimization of toxic
hydrocarbons species was found only at the highest
postflame gas temperature and longest residence time ex-
plored in this study, 1150 °C and 2 s. Panagiotou et al.
(1996a,b) determined the aromatic hydrocarbon emis-
sions from combustion of PS, PE and PVC, concluding
that, unless the resulting particles of PE were very small
and burnt in extremely dilute clouds, low PAH emis-
sions were not ensured. Piao et al. (1999) characterized
combustion products of PE, observing the presence of
olefins and n-hydrocarbons at low temperature and
PAHs at higher temperatures, their major contribution
2.1. Raw material
High-density polyethylene (HDPE 10062E) from
Dow Plastics was used in this work. Table 1 shows some
characteristics of the material studied in this paper. In a
previous work by Font et al. (2003), an elementary ana-
lysis of the major components was carried out in a
Table 1
Elementary analysis and net calorific value of polyethylene
Element
% weight
C
85.3
14.7
0.0
H
N
S
O (by difference)
0.0
0.0
Net calorific value:
10273 kcalkgꢀ1

R. Font et al. / Chemosphere 57 (2004) 615–627
617
Perkin–Elmer 2400. The calorific value was determined
in a Leco AC-350 calorimetric bomb. From a TG run
carried out in a Setaram TG-DSC at 15 °Cminꢀ1 both
in He and He:O2 4:1, it was tested that decomposition
takes place in the range 440–500 °C, and the amount
of solid residue is negligible.
that the packing of quartz rings produces a local good
mixing of the gas, avoiding bypass of the gas. The resi-
dence time of the gas inside the central part of the reac-
tor was calculated at about 4 s.
Table 2 presents the operating conditions of the six
runs performed in this work. The mass flow of the sam-
ple was calculated assuming that the sample is uniformly
distributed in the holder and burnt in a fixed front (as
previously done by Fullana et al. (2000)), so the rate
of advance of the boat can be related with the burning
rate of the sample.
2.2. Laboratory scale furnace
The equipment used was a horizontal quartz reactor
(Fig. 1), where the sample was introduced inside a cruci-
ble or holder. A small engine allowed the sample holder
to be moved through the reactor at different velocities
between 0.05 and 20 mmsꢀ1. An electric furnace with
two separate parts was placed around the tube. The vol-
atile compounds evolved underwent cracking reactions
mainly in the second part of the reactor, which was filled
with quartz rings.
A parameter named bulk air ratio (k) was defined as
the ratio between the actual air flow and the stoichiom-
etric air flow necessary for complete combustion, assum-
ing that the combustion of the solid occurs at the same
rate as that is being introduced. Therefore, it can be cal-
culated as
ðmair
Þ
stoichiometric
actual
k ¼
The procedure to carry out an experiment was as fol-
lows: first, the furnace was switched on and the gas flow
adjusted; then, when the temperature was stabilized at
its nominal value, the sample was introduced inside
the holder; afterwards, the motor pushing the holder
was connected and the sample was introduced into the
hot part of the reactor, where it remained for a short
time. Finally, the holder came back out of the furnace.
After the runs, no solid residue was observed inside
the holder.
ðmair
Þ
mair ꢁ 23
¼
ꢀ
ꢁ
msample ꢁ v %C %H %S %O
þ
þ
ꢀ
32
ꢁ 32
L
12
4
32
In the previous expression, mair is the air flow intro-
duced, msample is the total sample mass, v is the rate of
sample inlet, L is the length of the crucible and %C,
%H, %O and %S are, respectively, the weight percent-
ages of carbon, hydrogen, oxygen and sulfur in the sam-
ple. It is clear that k = 0 in pyrolytic conditions, k = 1 in
stoichiometric conditions and k > 1 in excess of oxygen.
In all the runs, the velocity of the holder with the
sample going into the furnace was 1 mmsꢀ1 for 120 s.
Then, the sample was maintained for 100 s in the hot
part of the reactor to ensure total decomposition.
The volatile compounds were collected for 220 s
(100 + 120 s) in a Tedlar bag to perform the analysis
of light hydrocarbons and gases, and a small tube con-
taining the resin XAD-2 was used to collect the semivol-
atile compounds.
Altogether six runs were performed, since two differ-
ent nominal temperatures (500 and 850 °C) and three
sample mass/air flow ratios (one of them in pyrolytic
conditions with nitrogen) were employed. Gas flow (air
or nitrogen) was 280 mlminꢀ1 (measured at 1 atm and
20 °C). The flow inside the hollow reactor was laminar
with Reynolds numbers around 10–20, but it was tested
2.3. Analytical method
The compounds collected in the Tedlar bag were ana-
lyzed by gas chromatography in a Shimadzu GC-14A.
Fig. 1. Scheme of the reactor.
Table 2
Experimental conditions
Run
Nominal temperature (°C)
Sample mass (mg)
Sample mass flow (mgsꢀ1
)
Bulk air ratio (k)
P1
C2
C3
P4
C5
C6
500
500
500
850
850
850
51
49
20
53
49
20
2.04
1.96
0.80
2.12
1.96
0.80
0
0.187
0.458
0
0.187
0.458

618
R. Font et al. / Chemosphere 57 (2004) 615–627
A capillary alumina-KCl plot column from Supelco with
a flame ionization detector was used to analyze the light
hydrocarbons, such as methane, ethane, ethylene, pro-
pylene, butane, acetylene, benzene, toluene, etc. For
the analysis of O2, N2, CO2 and CO, a packed CTR I
column from Alltech with a thermal conductivity detec-
tor was used. This column consists of a column within
a column. This permits two different packings to be
used for the analysis of the sample. Identification and
quantification was performed using standards of each
compound.
The results were discussed from two viewpoints, since
both the temperature and the bulk air ratio are two vari-
ables that influenced the yields of the compounds.
Therefore, the compounds obtained were also classified
according to their behavior related to these parameters.
Some symbols were used to characterize the behavior:
(a) When bulk air ratio increases, IO, iO, DO, dO, SO,
MO, mO and ?O mean that yields increase with oxy-
gen, slightly increase, decrease, slightly decrease, are
similar, have an intermediate maximum, a minimum
or do not follow a similar behavior at the two tem-
peratures, respectively.
The semivolatile compounds adsorbed in the resin
were extracted with dichloromethane in accordance with
the EPA 3540C method. Before the extraction, 10 ll of
an internal deuterated standard (1,4-dichlorobenzene-d4,
naphthalene-d8, acenaphthene-d10, phenanthrene-d10,
chrysene-d12 and perylene-d12), in a concentration of
2000 mglꢀ1, were spiked to the resin.
(b) When temperature increases, IT, iT, DT, dT, ST and
?T mean, in this case, that yields increase with tem-
perature, slightly increase, decrease, slightly decrease,
are similar or do not follow a similar behavior at the
three bulk air ratios, respectively. Due to the fact
that the experiments were only carried out at two
temperatures, it was not possible to consider inter-
mediate maximum or minimum values.
A DB-5 30 m column in a chromatograph gas (Fi-
sons GC 8000) coupled to a mass spectrometer (Fisons
MD 8000) was used to analyze semivolatile com-
pounds. A qualitative identification of compounds
was performed comparing their mass spectrums with
those of the NIST database. However, other indicators
such as Lee and Kovats retention indices (Lee et al.,
1979; Rostad and Pereira, 1986), boiling points and
logical correlation in chromatograms of different runs
were used. The semivolatile compounds were quantified
using the deuterated standards. The response factors of
the different compounds were calculated by interpola-
tion between the two nearest standards. For an exact
identification and quantification of the compounds, a
standard for each compound would have been re-
quired, but this was considered out of the scope of this
work.
Some different kinds of compounds were identified,
including gases, volatile and semivolatile compounds.
In this last group compounds such as substituted
benzenes, aliphatic hydrocarbons, polycyclic aromatic
hydrocarbons, alcohols, aldehydes or ketones could be
distinguished.
3.1. Effect of temperature
When increasing temperature, carbon oxides de-
creased their yields. In the same way, volatile com-
pounds such as light hydrocarbons from C3 to C6, also
behaved like this. They were classified as DT or dT com-
pounds. Nevertheless, methane, ethane, ethylene, ben-
zene or toluene increased their yields in a considerable
amount at high temperatures (IT and iT). One possible
explanation for this last fact could be as follows: meth-
ane, ethane and ethylene are final products in cracking
reactions, which take place extensively at high tempera-
tures. On the other hand, toluene and benzene are both
products in pyrosynthesis reactions with a high thermal
stability (Taylor et al., 1990). Consequently, their yields
are expected to increase and therefore, these results seem
to be logical according to this explanation.
3. Results and discussion
The variation of the emission factors or yields (mg
compound/kg sample) with both temperature and sam-
ple mass/air flow ratio was studied in the six runs carried
out. Table 3 shows the results for the different com-
pounds detected. Each compound has a code related
to the confidence of its determination:
(a) an authentic quantitative standard was used,
(b) the confidence parameter of the NIST database
was greater than 90%, referred to the coincidence
between the experimental mass spectrum and the
proposed compound mass spectrum,
Heavy aliphatic hydrocarbons were found at low
temperatures, whereas they practically disappeared at
850 °C (DT compounds). It was tested that the charac-
teristic triplets of peaks corresponding to a,x-olefins,
a-olefins and n-paraffins were formed from the primary
decomposition, as indicated in previous works (Hodgkin
et al., 1982; Hawley-Fedder et al., 1984; Wampler, 1989;
Piao et al., 1999; Williams and Williams, 1999; Font
et al., 2003). These compounds showed a behavior
(c) the confidence parameter was between 80% and 90%,
(d) the differences in Kovats and Lee retention indices
were less than 1%,
(e) the boiling point of the proposed substance was log-
ical in relation to the other substances.

Table 3
Emission factors or yields in ppm (mg compound/kg sample)
Compound
Pyrolysis (P)/combustion (C)
P1 C2
Temperature (°C)
500 500
Bulk air ratio (k)
Authors(1)
Type(2)
C3
P4
C5
C6
500
850
850
850
0
0.187
0.458
0
0.187
0.458
Carbon dioxidea
Carbon monoxidea
Methanea
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
398
947297
198402
16043
4612
1229416
317775
12045
2669
59879
1554
25744
6852
633
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
nm
695567
84082
71564
8066
144817
279
977852
98127
34298
2290
Mi, Li
Mi, Li
IO-DT
IO-DT
DO-IT
DO-iT
DO-IT
DO-DT
DO-DT
SO-ST
dO-dT
DO-DT
dO-dT
dO-DT
DO-DT
dO-DT
?O-?T
Wi, Co, Ma
Ho, Wi, Co, Ma
Ho, Wi, Co, Ma
Ho, Wi, Co
Ho, Mi, Wi, Co
Ethanea
Ethylenea
80823
2252
63522
155
4527
Propanea
Propylenea
33383
9910
964
9154
8534
615
Isobutanea
9445
427
Acetylenea
Co
n-Butanea
16889
482
511
11995
348
357
369
143
Mi, Co
Co
Ho, Mi, Co
trans-2-Butenea
1-Butenea
111
1,3-Butadienea
n-Pentanea
21884
1321
6152
12835
956
8811
87
5121
Co
Mi
Propynea
3660
816
7361
422
1-Pentenea
1359
3665
536
4888
8995
284
dO-DT
DO-iT
dO-dT
DO-DT
DO-DT
DO-IT
dO-iT
2-Butynea
2339
9855
1456
5350
7875
946
2246
8519
1-Butynea
15823
2295
9509
n-Hexanea
1-Hexenea
68
63910
4210
Ho, Mi
Benzenea
12160
3039
26108
3013
Ho, Mi, Wi, Co
Ho, Mi, Wi, Co
Ho, Mi
Toluenea
1-Octeneb,c
DO-DT
iO-DT
DO-DT
IO-DT
IO-DT
IO-DT
SO-iT
n-Hexanalb
125
567
n-Octaneb,d,c
2-Cyclopenten-1-oneb
2-Methylcyclopentanonee
2-Butanoneb
Ethylbenzeneb,c
p-Xyleneb,c
249
Mi
207
107
144
1075
694
654
76
139
32
100
32
110
Mi
Mi, Co
40
51
125
180
SO-ST
iO-DT
SO-iT
3-Hepten-2-ol
m-Xyleneb,c
Ethylidenecyclohexane
Phenylethynec,e
100
174
47
65
Mi, Co
13
dO-dT
iO-IT
(continued on next page)
95
317
564

Table 3 (continued)
Compound
Pyrolysis (P)/combustion (C)
P1 C2
Temperature (°C)
Authors(1)
Type(2)
C3
P4
C5
C6
500
500
500
850
850
850
Bulk air ratio (k)
0
0.187
0.458
432
0
0.187
4491
0.458
5088
5-Hepten-2-oneb
7-Octen-2-onee
2-Propenylcyclohexaneb
1,8-Nonadieneb,c
Styreneb,c
55
MO-dT
IO-DT
dO-dT
iO-DT
?O-IT
36
135
108
505
474
86
410
172
5994
Mi, Pi, Co
Ho, Mi
1-Noneneb,c
1450
676
1549
926
mO-DT
IO-DT
DO-DT
IO-DT
IO-DT
iO-DT
IO-DT
iO-DT
SO-iT
Oxygenated
n-Nonaneb,d,c
3-Hepten-2-ole + others
Oxygenated
2,5-Hexanedionee
2-Cyclohexen-1-oneb
2,4-Hexadien-1-ol
Allylbenzeneb,c
Benzaldehydeb
1,3,5-Trimethylbenzeneb,c
4-Octanonee
57
Mi
170
90
1164
389
251
687
293
38
157
58
32
43
37
177
737
IO-DT
mO-iT
IO-DT
iO-dT
149
15
Wi
368
279
Oxygenated
Oxygenated
815
2402
IO-DT
DO-DT
dO-iT
1,9-Decadieneb,c
m-Methylstyreneb,c
1-Deceneb,c
745
Ha, Wi
Ha
Ho, Ha, Mi, Wi
100
10
5914
1010
1257
DO-DT
dO-IT
dO-iT
p-Methylstyreneb,c
o-Methylstyreneb,c
Oxygenated
n-Decaneb,d,c
n-Octanalb
trans-1-Propenylbenzeneb,c
Indanee
Indeneb,d,c
Benzylic alcohole
2-Propylheptanol
trans-Decahydronaphthaleneb,d
4-Nonanone
631
255
526
194
309
173
Ha
Ha
335
313
725
IO-DT
mO-DT
IO-DT
dO-iT
1638
155
113
Ha, Mi, Wi
235
124
153
98
103
67
Wi
Ha, Pi, Wi
dO-iT
iO-IT
467
73
413
305
311
2873
3146
4330
IO-DT
IO-DT
dO-iT
276
17
328
420
IO-DT
IO-DT
Oxygenated

1,10-Undecadieneb
o-Allyltolueneb,c
1-Undeceneb,c
2748
5022
1619
833
510
749
1422
358
Ha, Wi
DO-DT
?O-IT
1716
588
19
49
5338
1970
1136
490
Ho, Ha, Mi, Wi
Ha, Mi, Wi
DO-DT
?O-IT
p-Ethylstyreneb,c
n-Undecaneb,d,c
214
DO-DT
DO-IT
IO-DT
SO-IT
DO-IT
?O-iT
cis-Decahydronaphthaleneb,c
n-Nonanalb
314
149
92
664
m-Divinylbenzeneb
Divinylbenzeneb
1-Methyl-1H-indeneb
1-Methylene-1H-indeneb
1,2-Dihydronaphthaleneb,d
Oxygenated
334
353
352
612
229
204
80
252
118
223
417
131
252
301
78
Pi
Pi
Pi
dO-IT
dO-iT
iO-dT
198
682
1105
195
428
395
157
218
243
292
679
969
430
1,11-Dodecadieneb
1-Dodeceneb,c
Naphthaleneb,d,c
n-Dodecaneb,d,c
n-Decanal
659
3054
429
1149
287
Ha, Wi
mO-DT
DO-DT
SO-IT
mO-DT
IO-DT
iO-dT
Ho, Ha, Mi, Wi
Ha, Wh, Pa, Li, Pi, Wi
Ha, Mi, Wi
9011
7911
7995
1733
263
112
5-Decanone
5-Undecanone
iO-dT
iO-dT
iO-dT
Oxygenated
Oxygenated
1,12-Tridecadieneb
1-Trideceneb,c
10-Undecenal
n-Tridecaneb,d,c
2-Methylnaphthaleneb,d,c
605
3443
539
1029
Ha, Wi
Ho, Ha, Mi, Wi
SO-DT
DO-DT
IO-DT
DO-dT
DO-IT
IO-DT
DO-IT
dO-iT
1390
232
106
59
Ha, Mi, Wi
Ha, Wh, Pa, Pi, Wi
1892
1098
622
n-Undecanal
305
1-Methylnaphthaleneb,d,c
1-Ethylidene-1H-indeneb
Byphenylb,d,c
61
1548
109
875
71
457
33
Ha, Wh, Pi, Wi
Pi
Ha, Wh, Pi
2289
1227
853
DO-IT
iO-dT
Oxygenated
1,13-Tetradecadieneb
1-Tetradeceneb
224
632
665
92
731
4169
1442
518
932
197
Ha, Wi
Ho, Ha, Mi, Wi
Ha, Mi, Wi
SO-DT
DO-DT
DO-DT
IO-DT
iO-dT
n-Tetradecanee,d,c
11-Dodecenal
n-Dodecanal
365
293
40
1-Allylnaphthaleneb,c
176
23
69
50
47
73
dO-dT
SO-iT
Biphenylene
Ha, Wh, Pa, Li, Pi
Ha, Pi
1-Vinylnaphthaleneb
2,3-Dimethylnaphthaleneb,c
2,6-Dimethylnaphthaleneb,d
2-Vinylnaphthalene
Acenaphthyleneb,d,c
822
151
54
345
60
179
36
DO-IT
dO-iT
dO-iT
Pi, Wi
Wi
20
64
182
2939
4491
1441
2367
823
2796
Ha, Pi
Ha, Wh, Pa, Li, Pi, Wi
DO-IT
?O-IT
(continued on next page)

Table 3 (continued)
Compound
Pyrolysis (P)/combustion (C)
P1 C2
Temperature (°C)
500 500
Bulk air ratio (k)
0.187
Authors(1)
Type(2)
C3
P4
C5
C6
500
850
850
850
0
0.458
0
0.187
0.458
1,14-Pentadecadieneb
1-Pentadeceneb,c
933
2412
427
1070
333
495
Ha, Wi
DO-DT
DO-DT
DO-IT
DO-DT
dO-iT
Ho, Ha, Mi, Wi
Pa, Li, Pi, Wi
Ha, Mi, Wi
Ha, Pi
Acenaphtheneb,c
937
380
109
243
90
n-Pentadecaneb,d,c
4-Methyl-1,1-biphenylb,d,c
12-Tridecenal
1563
209
148
242
86
30
316
195
IO-DT
iO-dT
dO-iT
n-Tridecanal
2,30-Dimethyl-1,1-biphenyle
3-Methyl-1,1-biphenylb,c
1H-Phenalene/fluorene or similarb
1H-Phenalene/fluorene or similarb
1,15-Hexadecadieneb
1H-Phenalene/fluorene or similarb,d
Methylbiphenyl or similare
1-Hexadeceneb,c
154
296
416
368
79
96
58
55
81
94
Ha, Pi
dO-iT
150
136
Ha, Wh, Pa, Li, Pi, Wi
Ha, Wh, Pa, Li, Pi, Wi
Ha, Wi
dO-IT
dO-IT
DO-DT
DO-IT
dO-iT
867
401
286
2094
129
976
39
920
17
Ha, Wh, Pa, Li, Pi, Wi
3518
1602
793
291
458
116
Ho, Ha, Mi, Wi
Ha, Wi
DO-DT
DO-DT
DO-IT
iO-dT
n-Hexadecaneb,d,c
Methylfluorene or similare
13-Tetradecenalb
7-Tetradecen-1-ol
830
364
329
176
178
167
132
276
41
23
MO-dT
DO-IT
iO-dT
1H-Phenalene/fluorene or similarb,c
n-Tetradecanal
Ha, Wh, Pa, Li, Pi, Wi
1H-Phenalene/fluorene or similare
1H-Phenalene/fluorene or similare
1,16-Heptadecadieneb
4-Vinylbiphenyle
650
24
282
56
82
157
Ha, Wh, Pa, Li, Pi, Wi
Ha, Wh, Pa, Li, Pi, Wi
Ha, Wi
DO-IT
iO-iT
1046
478
DO-DT
dO-iT
66
28
Pi
Ho, Ha, Wi
Ha, Mi, Wi
1-Heptadeceneb,c
2735
1261
825
240
344
99
DO-DT
DO-DT
iO-dT
n-Heptadecaneb,d,c
14-Pentadecenalb
9-Pentadecen-1-ol
82
36
42
SO-dT
iO-dT
dO-iT
n-Pentadecanal
75
9,10-Dihydrophenanthreneb
2-Methyl-9H-fluoreneb,d
Diphenylethyne
108
497
467
190
88
41
252
145
78
19
135
76
Ha, Pi
Ha, Wh, Pi
Ha, Pi
Ha, Wh, Pi
dO-IT
dO-IT
dO-iT
1-Methyl-9H-fluorene
9-Methylene-9H-fluorenee
1-Phenyl-1H-indenec,e
38
27
48
47
dO-iT
dO-iT
119
26
Pi

1,17-Octadecadieneb
1-Octadeceneb
990
2590
1234
512
768
479
111
145
272
Ha, Wi
DO-DT
DO-DT
DO-DT
DO-IT
iO-dT
Ho, Ha, Mi, Wi
Ha, Wi
Ha, Wh, Pa, Li, Pi, Wi
n-Octadecaneb,d,c
Phenanthreneb,d,c
15-Hexadecenalb
Anthraceneb,d,c
58
71
3956
1605
1567
663
1714
504
21
31
64
Ha, Wh, Pa, Li, Pi
DO-IT
SO-dT
iO-dT
11-Hexadecen-1-ol
n-Hexadecanal
81
1-Phenylnaphthaleneb,d,c
Methylanthracene/phenanthreneb
1,18-Nonadecadieneb
1-Nonadeceneb
375
110
139
48
67
19
Ha, Pi
Ha, Wh, Pi
Ha, Wi
dO-iT
dO-iT
1208
2877
1379
496
938
304
95
138
28
DO-DT
DO-DT
DO-DT
iO-dT
Ho, Ha, Mi, Wi
Ha, Mi, Wi
n-Nonadecaneb,d
16-Heptadecenalb
n-Heptadecanal
47
52
22
iO-dT
dO-iT
Methylanthr./phenant.b
Methylanthr./phenant.b
Methylanthr./phenant.b,d
Benzo(def)fluoreneb
Methylanthr./phenant.b,d
1,19-Eicosadieneb,d
2-Phenylnaphthaleneb,d,c
1-Eicoseneb
236
189
150
750
200
72
67
36
35
Ha, Wh, Pi
Ha, Wh, Pi
Ha, Wh, Pi
Pi
dO-iT
dO-iT
50
320
72
233
56
DO-IT
dO-iT
Ha, Wh, Pi
Ha, Wi
Ha, Pi
1164
620
79
DO-DT
DO-IT
DO-DT
DO-DT
iO-dT
661
324
156
2949
1578
1036
288
31
117
38
Ho, Ha, Mi, Wi
Ha, Wi
n-Eicosaneb,d
17-Octadecenalb
25
26
n-Octadecanale
24
iO-dT
dO-iT
dO-iT
Phenylnaphthalenee
Vinylanthracenee
159
102
726
85
67
22
20
Pi
Fluorantheneb,d,c
609
370
Ha, Wh, Pa, Li, Pi
Ha, Wi
dO-IT
DO-DT
DO-DT
DO-DT
dO-IT
iO-dT
1,20-Heneicosadienee
1-Heneicoseneb
1305
3216
1795
675
1056
464
48
44
71
36
Ho, Ha, Mi, Wi
Ha, Wi
Wh, Pi
n-Heneicosaneb,d
Acephenantrylene/aceanthryleneb
18-Nonedecenalb
Nonadecanol
367
307
143
20
17
35
27
SO-dT
SO-dT
dO-iT
n-Nonadecanalb
Acephenant./aceanthr.b
Pyreneb,d,c
b,d
C22 (diene+ene+ane)
72
1164
47
1045
26
513
109
Wh, Pi
Ha, Pa, Li, Pi
Ho, Ha, Mi, Wi
64
2462
62
DO-IT
DO-DT
MO-dT
SO-dT
dO-iT
6868
145
Eicosanol
n-Eicosanal
23
7H-Benzo(c)fluoreneb
11H-Benzo(b)fluoreneb
Methylpyreneb,d
238
136
55
233
140
60
95
88
14
Ha, Wh, Pi
Ha, Wh, Pi
Ha, Wh, Pi
dO-iT
dO-iT
(continued on next page)

Table 3 (continued)
Compound
Pyrolysis (P)/combustion (C)
P1 C2
Temperature (°C)
500 500
Bulk air ratio (k)
0.187
Authors(1)
Type(2)
C3
P4
C5
C6
500
850
850
850
0
0.458
0
0.187
0.458
Methylpyreneb,d
211
111
137
86
52
66
Ha, Wh, Pi
Ha, Wh, Pi
Ho, Mi, Wi
dO-iT
dO-iT
Methylpyreneb,d
C23 (diene+ene+ane)b,d
Heneicosenol
n-Heneicosanal
C24 (diene+ene+ane)b,d
21-Docosenal
7161
8060
2782
87
122
135
104
DO-DT
MO-dT
MO-dT
DO-DT
MO-dT
MO-dT
SO-iT
32
3330
67
70
Ho, Mi, Wi
n-Docosanal
96
Benzo(c)phenanthreneb
Benzo(a)anthracene + Cyclopenta(cd)pyreneb,d
Triphenylene/chryseneb,d,c
1,20-Binaphthalenee
C25 (diene+ene+ane)b,d
Tricosenol
75
182
108
26
124
242
298
58
78
152
232
32
Ha, Wh, Pi
Ha, Wh, Pa, Li, Pi
Ha, Wh, Pi
Pi
MO-iT
MO-iT
SO-iT
8298
9592
4462
77
156
Ho, Mi, Wi
DO-DT
MO-dT
MO-dT
?O-?T
n-Tricosanal
70
29
60
Bis(2-ethylhexyl)phthalateb
C26 (diene+ene+ane)b,d
23-Tetracosenal
100
4238
128
58
50
61
176
56
183
Mi, Wi
Mi, Wi
DO-DT
MO-dT
MO-dT
DO-DT
MO-dT
MO-dT
DO-DT
MO-iT
MO-iT
MO-iT
MO-iT
DO-DT
MO-iT
dO-iT
n-Tetracosanal
C27 (diene+ene+ane)b,d
Pentacosenol
11502
13452
4209
61
257
453
133
71
Pentacosanal
42
4081
C28 (diene+ene+ane)b,d
Benzo(j)fluorantheneb
Benzo(b)fluorantheneb,d
Benzo(k)fluorantheneb,d
Benzo(e)pyreneb
287
120
80
195
Mi, Wi
Ha, Wh
43
12
13
26
Ha, Wh, Pa, Li, Pi
Ha, Wh, Pa, Li, Pi
84
118
623
260
93
290
212
Li, Pi
Wi
C29 (diene+ene+ane)b,d
Benzo(a)pyreneb
Perylenee
C30 (diene+ene+ane)b,d
C31 (diene+ene+ane)b,d
1,12-Benzperylenee
C32 (diene+ene+ane)b,d
Indeno(1,2,3-cd)pyrenee
C33 (diene+ene+ane)b,d
17520
4372
778
51
10
Li, Pi
Li, Pi
Wi
16628
14591
3602
2713
753
571
600
393
183
257
DO-DT
DO-DT
dO-iT
Wi
10
15
Pi
Wi
11799
10439
2073
1531
348
214
239
45
89
DO-DT
SO-iT
DO-DT
Li, Pi
Wi
50

R. Font et al. / Chemosphere 57 (2004) 615–627
625
similar to the light aliphatic hydrocarbons previously
commented. They are primary products in polyethylene
decomposition evolved in cracking reactions, so at high
temperatures they turn into gases and polycyclic aro-
matic hydrocarbons. This last group of compounds is
produced as a result of pyrosynthesis reactions. There-
fore, their yields rise when increasing temperature (IT
and iT). Compounds such as naphthalene, styrene, ace-
naphthylene, phenanthrene, 2-vinylnaphthalene, indene,
biphenyl, fluorene, 2-methylnaphthalene or anthracene
were obtained with the highest yields.
Some oxygenated compounds appeared in combus-
tion experiments at low temperatures. They are interme-
diate products that are not very stable, so high
temperatures allow their destruction. Saturated and
unsaturated aldehydes were the main compounds of this
group. Two peaks of the a-enal and n-anal with n-2
carbon atoms were mainly observed to appear after
the corresponding peaks of the three hydrocarbons with
n carbon atoms previously commented. On the other
hand, some linear alcohols, linear and cyclic ketones
were also identified. These oxygenated compounds were
included in the DT and dT groups.
3.2. Effect of oxygen (k)
Regardless of temperature, both carbon dioxide and
carbon monoxide yields rose when bulk air ratio did
(IO compounds). This is logical, since there is more oxy-
gen available to react. However, the ratio between the
former and the latter would be expected to follow the
same trend, but they did not comply with this at 500
°C, as can be observed in Fig. 2. Anyway, the great yield
of these two compounds is remarkable, compared with
the rest. Therefore, as carbon oxides cannot be evolved
in pyrolysis experiments, it can be concluded that
the yields of light hydrocarbons must have been much
greater in pyrolysis, since carbon total balance must be
attained.
8.00
4.00
T (ºC)
850
0.00
500
C2, C5
C3, C6
Fig. 2. CO2/CO ratios.

626
R. Font et al. / Chemosphere 57 (2004) 615–627
The volatile compounds, benzene and toluene identi-
450000
volatile
fied decreased their yields when bulk air ratio increased
(DO and dO). There was a marked drop in compounds
such as methane or ethylene at 850 °C. It is important to
emphasize the great yields obtained for ethylene and
propylene, which are expected taking into account the
chemical structure of polyethylene. Moreover, it must
be pointed out that there is a carcinogenic compound
such as benzene that appeared in a remarkable quantity.
On the other hand, propyne had a different behavior,
since it did not appear at intermediate bulk air ratios
but it suddenly raised at higher values at 850 °C.
With respect to semivolatile compounds, the results
showed that, in general, both the aliphatic and the aro-
matic hydrocarbons decreased their yields when there
was an increase in the bulk air ratio (DO and dO).
However, the decrease was not observed in some cases:
aliphatic compounds such as 1-nonene, n-decane, n-
dodecane, 1,13-tetradecadiene showed a minimum at
intermediate air bulk ratios (mO), while aromatic com-
pounds such as benzofluoranthenes or benzo(a)pyrene
had a maximum (MO), and even 1,3,5-trimethylbenzene,
indene or p-ethylstyrene rose in a significant way at the
highest bulk air ratio. This behavior is likely to indicate
that either the amount of oxygen available was not en-
ough to attack every compound evolved in pyrolysis
or mixing with oxygen was inadequate and, thus, pyro-
lysis ÔpuffsÕ appeared. Having used higher bulk air ratios,
a great decrease in the yields of these compounds should
have been observed.
compounds
300000
150000
0
T (ºC)
850
C2, C5
500
C3, C6
Fig. 3. Variation of total yields of volatile compounds vs.
temperature and air bulk ratio (k).
250000
200000
150000
100000
50000
semivolatile
compounds
T (ºC)
850
0
500
P1, P4
C2, C5
C3, C6
Fig. 4. Variation of total yields of semivolatile compounds vs.
temperature and air bulk ratio (k).
It was observed that the yields of partially oxygen-
ated compounds rose with bulk air ratio (IO and iO),
except the heaviest ones, which showed an intermediate
maximum (MO). The rise for the lighter oxygenated
compounds is explained because of the higher availabil-
ity of oxygen. These are, however, only intermediate
products in the total oxidation of hydrocarbons, so a de-
crease should be expected at higher bulk air ratios. On
the other hand, it is reasonable that the yields of the
heaviest compounds decreased at lower air bulk ratios
more than the lightest ones, because the former can be
considered as precursors of the latter.
temperatures and air bulk ratios leads to a global de-
crease in yields for those compounds, which involves
an increase in carbon oxides yields, as been pointed
out previously.
Table 3 also presents a review of the authors who
also detected the different compounds present in the
combustion or pyrolysis of polyethylene, observing
many coincidences. For example, the results presented
by Piao et al. (1999) in the combustion of polyethylene
show yields of PAHs that fall within the same range of
order as in this paper. On the other hand, it must be
noted that there are some compounds that appear in
the paper of these authors that have not been detected
in this work.
It can be noted that the presence of oxygen very
probably enhances the formation of radicals, which
contributes to a more reactive situation. This could
explain why some aromatics at 500 °C (p-xylene, styrene,
benzaldehyde, indene, o-allytoluene, p-ethylstyrene, m-
4. Conclusions
divinylbenzene,
1-methyl-1H-indene,
naphthalene,
methylnaphthalene, 2-vinylnaphthalene, phenanthrene,
acenaphthylene, pyrene, . . .) and some aliphatic hydro-
carbons at 850 °C (alkanes, alkenes and/or alkadienes
from C22 to C32) appeared in combustion experiments
whereas they were not identified in those of pyrolysis.
Figs. 3 and 4 show the total yields obtained for vol-
atile and semivolatile compounds, respectively, in the
experiments. It appears that the combination of high
It was tested that there is a wide range of products
evolved in the combustion of polyethylene that can be
obtained in a horizontal laboratory furnace, where both
the influence of bulk air ratio and temperature were
analyzed.
More than 230 products were detected, observing that
higher values of the available oxygen led to a decrease in

R. Font et al. / Chemosphere 57 (2004) 615–627
627
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atile compounds and carbon oxides increased. On the
other hand, when increasing temperature both cracking
and pyrosynthesis reactions were enhanced, so the results
showing a rise in the yields of methane, ethane, ethylene,
benzene or PAHs were logical, whereas the production of
other linear hydrocarbons, oxygenated compounds and
carbon oxides fell.
Lee, M.L., Vassilaros, D.L., White, C.M., Novotny, M., 1979.
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Acknowledgments
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Mastral, F.J., Esperanza, E., Garcıa, P., Juste, M., 2002.
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