Combustion of high calorific value waste material: Organic atmospheric pollution

Article abstract of DOI:10.1021/es990289o

Waste tire combustion in an atmospheric fluidized-bed (AFB) reactor (7 cm i.d., 76 cm height) has been performed in a laboratory plant with the aim of studying the polycyclic aromatic hydrocarbon (PAH) emissions as a function of combustion temperature. The main aim has been to compare these organic emissions with the ones obtained when coal is burned at the same combustion conditions. PAH emissions have been analyzed in solids collected in two cyclons at the exit of the reactor and in a trap system formed by a condenser, a filter (20 μm), and an adsorbent. After PAH extraction with dimethylformamide (DMF) by sonication, fluorescence spectroscopy in the synchronous mode (FS) has been used as an analytical technique to quantify the PAHs emitted. It could be concluded that higher PAH emissions are generated when this waste material is burnt at the same conditions used for coal atmospheric fluidized-bed combustion (AFBC).

Full text of DOI:10.1021/es990289o

Environ. Sci. Technol. 1999, 33, 4155-4158
Combustion of High Calorific Value
Waste Material: Organic
Atmospheric Pollution
TABLE 1. Efficiencies (%) at the Different Combustion
Temperatures Used in an AFBC Pilot Plant (860 L/h, 20%
Excess O )
2
tem perature (°C)
efficiencies (%)
650
90
750
94
850
95
950
92
A N A M . M A S T R A L , * M A R IÄ A S . C A L L EÄ N ,
R A M OÄ N M U R I L L O , A N D T O M AÄ S G A R C IÄ A
Instituto de Carboqu ´ı mica, Mar ´ı a de Luna, 12,
elemental analysis shows that their contents in C, N, H, O,
and S are very similar with similar aromatic structures in
some aspects (16). Anyway, tires show a remarkable difference
in the moisture (17) content and ashes, which are lower than
in coal.
5
0015, Zaragoza, Spain
In general, in fluidized-bed combustion, it will be neces-
sary to use higher combustion temperatures than for coal
due to the lower reactivity of CB (18). This has been observed
in coal-tire blend combustion (19). However, it is also
necessary to study the different behavior of tires with regard
to organic emissions. During combustion and pyrolysis
processes of fossil fuels, polycyclic aromatic hydrocarbons
are released (20). Although, studies about PAH emissions in
coal combustion are receiving interest, very little is known
about PAH emissions in tire combustion. First, due to the
different nature of the fuels, different combustion conditions
could be required. In this paper, PAH emissions from the
fluidized combustion of waste tires have been studied in a
pilot plant working at the same conditions used in coal
combustion (21) varying the combustion temperature and
keeping constant the air total flow and the percentage of
excess oxygen.
Waste tire combustion in an atmospheric fluidized-bed
AFB) reactor (7 cm i.d., 76 cm height) has been performed
(
in a laboratory plant with the aim of studying the polycyclic
aromatic hydrocarbon (PAH) emissions as a function of
combustion temperature. The main aim has been to compare
these organic emissions with the ones obtained when
coal is burned at the same combustion conditions. PAH
emissions have been analyzed in solids collected in two
cyclons at the exit of the reactor and in a trap system formed
by a condenser, a filter (20 µm), and an adsorbent. After
PAH extraction with dimethylformamide (DMF) by sonication,
fluorescence spectroscopy in the synchronous mode
(FS) has been used as an analytical technique to quantify
the PAHs emitted. It could be concluded that higher
PAH emissions are generated when this waste material is
burnt at the same conditions used for coal atmospheric
fluidized-bed combustion (AFBC).
Experimental Section
A fluidized-bed combustion laboratory plant described in
previous work (22) was used to perform the combustion of
waste tires, a nonspecific sample supplied by AMSA enterprise
after shredding and separating the metal components (0.6-
mm particle size).
The elemental analysis is as follows: % C ) 86.35 (ar); %
H ) 7.29 (ar); % S ) 1.60 (ar); % N ) 0.18 (ar). The proximate
analysis is as follows: % moisture ) 0.94 (ar); % ashes ) 3.28
Introduction
In the last part of this century, the increase in the automotive
and trucking industry has generated a huge amount of waste
tires (1). Due to the economic and environmental problems
that these wastes create, the non-biodegradability and the
adverse environmental effects (2) have led to looking for new
ways of elimination. On the other hand, the high calorific
value (28-37 MJ/ kg) (3) of tires as well as the low mineral
matter are their most important advantages, which makes
tires a potential nonfossil fuel.
Tires are formed primarily by a copolymer mixture (4, 5):
about two-thirds natural rubber and synthetic rubber,
composed mainly of styrene-butadiene (SBR) and poly-
butadiene (PB) and approximately one-third carbon black
(
(
ar); % volatiles ) 56.09 (ar); % Zn ) 1.49. Calorific value
kcal/ kg) ) 9220.
Sand was utilized as the fluidizing agent. Due to problems
in the feeder because of the friction between particles, a
blend 1:1 of tire-sand was used to feed the tire into the
reactor.
The combustion experiments were carried out keeping
constant the air total flow (860 L/ h) and the percentage of
excess oxygen (20%) and varying the combustion temperature
(
CB). CB (6) is one of the constituents used to strengthen
(
650, 750, 850, and 950 °C).
rubber to avoid abrasion. It is an aggregate of esferic coal
particles (7) characterized by a high ratio of surface/ weight.
Moreover, there are other additional components such as
aromatic hydrocarbons, trace metals (8), and stearic acid
that appear in different proportions for several kinds of tires
for variation in some of their properties.
The main ways of eliminating waste tires (9, 10) have
been related to landfilling (11), incineration (2), and com-
bustion in power generation (12). One of the most used ways
has been pyrolysis (13, 14) with the aim of obtaining oils that
can be used as fuels or solvents in coal liquefaction (14, 15).
Comparing the nature of tires and coal, used worldwide
as the main fuel in power generation, it can be deduced that,
although both of them are very different materials, their
The combustion experiences were performed during 2 h
once the plant worked at the regime conditions. Every
experiment was carried out at least in duplicate.
The five samples from first cyclone, second cyclone,
condenser, nylon filter, and XAD-2 resin obtained in each
experiment were extracted with an ultrasonic bath for 15
min with DMF, filtered and concentrated in rotary vacuum.
Afterward, they were analyzed by fluorescence spectroscopy
at the conditions for each specific PAH (22) previously
determined with model compounds.
The combustion efficiencies at each temperature were
calculated, and the results are shown in Table 1.
Results and Discussion
*
Corresponding author e-mail: AMastral@carbon.icb.csic.es;
All combustion processes are influenced by three factors that
must be considered when PAHs emissions are studied: (a)
phone: 34-976-733977; fax: 34-976-733318.
1
0.1021/es990289o CCC: $18.00

1999 Am erican Chem ical Society
VOL. 33, NO. 23, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4 1 5 5
Published on Web 10/21/1999

three different trends. The first trend is between 650 and 750
C, in which a decrease of the total PAHs emitted is produced
with an increase of the temperature. A second trend is from
50 to 850 °C, in which a huge increase in the PAHs amount
°
7
is emitted. Finally, a third trend is from 850 to 950 °C in
which PAH emissions are maintained practically constant.
In general, the increase of the temperature seems to favor
the PAHs formation.
The temperature variation is the determining factor taking
into account that the total air flow is kept constant (860 L/ h)
along with the percentage of excess oxygen (20%). With
variable temperatures, the exit velocity of the flue gas will
vary; therefore, the contact time between the radicals released
will vary as a function of the combustion temperature. At
low combustion temperatures, the exit velocity of the gas is
lower and the residence time in the reactor is higher. This
indicates that, for the reactions in the reactor, both oxidation
and condensation reactions will be favored because the
residence time is higher. Anyway, at these conditions,
elimination by the radical oxidation appears to be the
predominant factor since the fluidization and the effective
oxygen are under control. When the combustion temperature
is increased, the exit velocity of the gases in the interior of
the reactor is higher and the oxidation time is lower. As a
consequence, PAH emissions increase. Moreover, it is
necessary to take into account the influence of the tire
combustion efficiency since incomplete combustion is
important in these emissions. CB seems to be the responsible
component due to its high ratio of surface/ weight and
inertness. It is also important to consider the physical aspects
of the bed.
The composition of the tire, formed mainly by a mixture
of styrene, polybutadiene, and carbon black (approximately
one-third) will also influence the amount of total PAHs
emitted. Tire pyrolysis leads to ethene, propene, and 1,3-
butadiene formations that react to form cyclic alkenes.
Because all combustion process experiments involve a
pyrolytic process, which implies secondary reactions of the
pyrolysis, the formation of aromatic and polycyclic com-
pounds has been attributed to Diels-Alder ciclyzation
reactions with alkenes (23, 24). These are produced especially
at high temperatures with long residence times. The lowest
combustion efficiency values favor these kinds of processes,
but also the inertness of CB must be considered. This favors
soot formation and greater associations of aromatics with
coronene (Co) in elemental structures such as nanotubes,
semifullerenes, etc. In this way, Sahouli et al. (6) have stated
that, during tire pyrolysis, the long elastomer chains break
down and are adsorbed on the CB surface. As a result, carbon
deposits and small aromatic compounds are formed, de-
pending on the pyrolysis conditions. The deposits increase
with increasing temperatures. We have also observed this
phenomenon (20, 25). According to Figure 1, combustion
temperatures between 700 and 750 °C for tires are more
desirable due to lower PAH formation and emissions.
The PAHs distribution in different parts of the trap system
(Table 2) shows that most of the PAHs emitted are trapped
on the particulate matter of the cyclons and on the nylon
filter. That is to say, most of the PAHs emitted are supported
on the particulate matter. The high PAH emissions supported
on particulate matter of the cyclons are more pronounced
at high combustion temperatures.
FIGURE 1. Total PAHs emitted (µg/kg) during tire combustion from
the reactor (AFBC, 860 L/h, 20% excess oxygen) as a function of the
combustion temperature.
bad combustion or incomplete combustion in which frag-
ments, mainly aromatic, from the fuel structure are emitted
as unburned material, (b) the pyrolytic process, a conse-
quence of the thermal changes that consist of the rupture of
the fuel structure into radicals, and (c) oxidation reactions
with the elimination of radicals as oxides.
Influence of Incom plete Com bustion. Concerning in-
complete combustion, the combustion efficiencies reached
give an idea about the influence of this factor. From Table
1
it can be deduced that, in general, the higher the combustion
temperature, the higher the efficiency. The efficiency values
reached are high, but not so high as to discard the influence
of the entrainment of unburned material that could increase
the organic emissions. Therefore, to study PAHs emitted by
waste tires as a function of the combustion temperature, not
only will the pyrolytic and oxidation processes be taken into
account but also the influence of unburned emissions. In
comparison, when a sub-bituminous coal is burnt at the
same conditions (21), the efficiency values are very close to
1
00%.
Influence of the Pyrolytic Process. The influence of the
previous volatilization step to the combustion process on
PAH emission as a function of the combustion temperature
has been analyzed. As consequence of the thermal breaking
or devolatilization, rubber structures release organic radicals.
These radicals have a very brief average life due to reduction
both by reaction between themselves (condensation) or by
reaction with oxygen (combustion). These two possibilities
will compete depending on the oxygen availability, which at
the same time will depend on the proximity to the flame.
The higher radical concentration leads to a higher rate of
their association.
When radicals interact in order to stabilize, and as a
consequence of their association, aromatic structures are
promoted or preferentially formed. These reactions are the
origin of PAHs formation during the combustion process.
These reactions will be more prominent close to the burning
particle.
Oxidation Reaction. When conforming radicals or mol-
ecules resulting from the pyrolytic process are entrained by
the flow, their ability to react with oxygen will increase and
their gradual oxidation will be carried out. The further the
fuel particle is from the flame, the easier radical elimination
is by oxidation. The final reaction being their conversion
2 2
into CO and H O. That means elimination of PAHs formed
in a previous step and a good combustion.
The consequences of the influence of the three analyzed
steps, the total PAH amounts emitted, are shown in Figure
When the individual PAHs emissions are studied as a
function of the combustion temperature (Table 3), it is
observed that the highest contribution to the total PAHs is
due to fluorene (Fu), pyrene (Py), chrysene (Cry), anthracene
(An), acenaphthene (Ac), benz[a]anthracene (BaA), and Co.
It is more interesting to examine the PAHs variation as
a function of the combustion temperature of the compounds
with higher carcinogenic propertiessbenzo[a]pyrene (BaP),
1
. The distribution of the total PAHs emitted in tire combus-
tion as a function of the temperature shows the existence of
4
1 5 6
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 23, 1999

TABLE 2. PAHs Distribution (µg/kg) between Five Traps in Tire
Combustion (AFBC, 860 L/h, 20% Excess O ) as a Function of
Combustion Temperature
2
temperature (°C)
750 850
19 18107
6
50
950
cyclone 1
cyclone 2
nylon
condenser
XAD-2
681
1316
2172
267
19745
5581
6655
401
314
33
20
4
7925
5617
279
41
270
244
total (µg/kg)
4477
390
32198
32626
TABLE 3. Totals of Individual PAHs (µg/kg) Collected in Five
Traps from Tire Combustion (AFBC, 860 L/h, 20% Excess O
as a Function of Combustion Temperature
2
)
FIGURE 3. Total distribution of Co and BaA emitted during tire
combustion in the plant (AFBC, 860 L/h, 20% excess O ) as a function
of the combustion temperature.
2
temperature (°C)
650
750
850
950
fluorene
benzo[a]pyrene
pyrene
chrysene
antraceno
acenaphthene
benz[a]anthracene
dibenz[a,h]anthracene
coronene
perylene
benzo[k]fluoranthene
792
nd
90
1
5531
611
9893
375
a
309
697
9
145
351
338
1836
nd
18
16
7
238
7
4
9
nd
nd
8136
3067
2093
8060
3926
65
590
119
nd
8704
2656
1976
2605
3951
42
2317
107
nd
nd
total (µg/kg)
4477
3990
32198
32626
a
nd, not detected.
FIGURE 4. Distribution of the total PAHs (µg/kg) emitted between
solid and gas phases during tire combustion (AFBC, 860 L/h, 20%
excess O ) as a function of the combustion temperature.
2
function of the temperature although it is emitted in higher
amounts. As happened with the other two compounds, the
lowest emissions of BaA and Co are produced at 750 °C.
Concerning environmental pollution control, it is im-
portant to study how PAHs are distributed between solid
and gas phases. The PAHs supported on particulate matter
of the first and second cyclone are considered as a solid
phase. The PAHs trapped in the system composed of the
condenser, the nylon filter, and the XAD-2 resin are con-
sidered as gas phase. The representation of the PAH
distribution as a function of both phases (Figure 4) shows
that both phases have the same profile in PAHs emissions
at different combustion temperatures. Between 650 and 750
FIGURE 2. Total distribution of BaP and D(a,h)A emitted during tire
combustion (AFBC, 860 L/h, 20% excess O ) as a function of the
combustion temperature.
2
°
C, emissions are practically constant. Upon increasing the
temperature to 850 °C, there is a remarkable increase in solid
phase emissions. At this temperature, PAH emissions seem
to be associated or supported on the particulate matter of
cyclones as a consequence of the high flow and the
entrainment of particles partially burned corresponding to
CB. At the lowest combustion temperature, gas phase
contributes the majority to the total PAHs emitted. An
important difference between coal and tire combustion
emissions is that, with coal, emissions in the gas phase are
more prominent. In tire combustion, the solid phase emis-
sions show a higher contribution, especially at 850 °C, a
temperature that power stations commonly use. This seems
to be due to the nature of the tire and the CB that serves as
a support to PAHs formation.
dibenz[a,h]anthracene (D(a,h)A), and the most stable, Co.
BaA is also represented due to the abundance of this
compound in tire combustion and its carcinogenic properties,
although it has a lower incidence than BaP and D(a,h)A.
These variations are shown in Figures 2 and 3. As shown in
Figure 2, it is deduced that BaP has the highest emission at
6
50 °C. D(a,h)A shows a maximum at a temperature of 850
°
C.
Both compounds have high carcinogenic properties and
are emitted at the extreme combustion temperatures. At 750
C, their emissions are minimum, which seems to be the
°
most favorable combustion temperature to diminish these
emissions.
Figure 3 illustrates results similar to those with D(a,h)A
in that BaA, in general, shows the same distribution as a
At 850 °C, most of the PAHs emitted are deposited on the
particulate matter of the cyclones. In this way, the contami-
VOL. 33, NO. 23, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4 1 5 7

Literature Cited
(
(
(
(
1) Kujioka, M. The Japan Automobile Tire Manufacturers As-
sociation, Inc., Tokyo, Japan, June 1995, personal communica-
tion.
2) Proceedings: 1991 Conference on Waste Tires as a Utility Fuel;
EPRI GS-7538; Electric Power Research Institute: Palo Alto, CA,
1
991.
3) Ekmann, J. M.; Smouse, S. M.; Winslow, J. C.; Ramezan, M.;
Harding, N. S. Cofiring of coal and waste; IEACR/ 90; IEA Coal
Research: London, 1996; p 18.
4) Dodds, J.; Domenico, W. F.; Evans, D. R.; Fish, L. W.; Lassahn,
P. L.; Toth, W. J. Scrap Tires: a Resource and Technology
Evaluation of Tire Pyrolysis and Other Selected Alternative
Technologies; Report EGG-2241; U.S. Department of Energy:
Washington, DC, 1983.
(
(
5) Mastral, A. M.; Murillo, R. Caucho 1995, 436, 23-26.
6) Sahouli, B.; Blacher, S.; Brouers, F.; Darmstadt, H.; Roy, C.;
Kaliaguine, S.; Fuel 1996, 75 (10), 1244-1250.
FIGURE 5. Distribution of total PAHs (µg/kg) by ring size during tire
combustion (AFBC, 860 L/h, 20% excess O ) as a function of the
combustion temperature.
(7) Rivin, D.; Medalia, A. I. In Soot in Combustion Systems and its
Toxic Properties; Lahaye, J., Prado, G., Eds.; Plenum Press: New
York, 1983; p 25.
2
(
(
8) Jost, K. Automot. Eng. 1992, 10, 23.
9) Martin, A. Garbage 1991, May/June, 28.
nation focus can be controlled more easily than if it were
emitted as gas phase.
(10) Coal Synfuels Technol. 1993, 21, 3.
(
11) Farcasiu, M.; Smith, C. M. ACS Fuel Chem. Div. Prepr. 1992, 37,
72.
12) Tesla, M. R. Power Eng. 1994, 43.
4
It is also worthy to comment that from the five traps used
as the sampling system, biphenyl (Bf) has been detected.
This has not been quantified because it is not present in the
U.S. EPA list of priority PAH pollutants.
(
(13) Williams, P. T.; Besler, S. Fuel 1995, 74 (9), 1277-1283.
(14) Mastral, A. M.; Murillo, R.; P e´ rez-Surio, M. J.; Call e´ n, M. S. Energy
Fuels 1996, 10, 941-947.
(
15) Murillo, R. Reutilization of waste tires: coprocessing with coal.
Ph.D. Dissertation, Universidad de Zaragoza, 1998, in Spanish.
16) Teng, H.; Serio, M. A.; Bassilakis, R. Preprints of Papers Presented
at the 203rd ACS National Meeting, San Francisco, CA; American
Chemical Society: Washington, DC, 1993; Vol. 37, p 533.
17) Mastral, A. M.; Call e´ n, M. S.; Murillo, R.; Mayoral, C. In
Proceedings of the International Conference of Coal Science;
Ziegler, A., Van Heek, K., Eds.; DGMK: Alemania, 1997; Vol. II,
pp 1155-1158.
The PAHs distribution by ring size as a function of the
combustion temperature is shown in Figure 5. PAHs with
three and four rings are the main compounds emitted at all
combustion temperatures, especially at the highest tem-
peratures in which the increase of three- and four-ringed
polyaromatics is drastic. So, at high temperatures the radicals
formed are released and entrained quickly in such way that
the recombination and interaction between radicals is less
favored, leading to the formation of the most volatile
compounds.
(
(
(
18) Howe, W. C. Proceedings: 1991 Conference on Waste Tires as a
Utility Fuel; EPRI GS-7538; Electric Power Research Institute:
Palo Alto, CA, 1991; p 10.
(
(
19) Atal, A.; Levendis, Y. A. Fuel 1995, 74 (11), 1570-1581.
20) Call e´ n, M. S. PAHs formation and emission in power generation.
AFBC. Ph.D. Dissertation, Universidad de Zaragoza, 1999, in
Spanish, pp 137-145.
In summary, PAH emissions obtained during tire com-
bustion in a fluidized-bed combustion system have been
analyzed, and the results are reported and discussed. The
nature of the tires seems to promote PAHs formation as
compared to coal combustion at the same combustion
conditions, showing the importance of the pyrolytic process
in PAHs formation and emission. Concerning PAH emissions,
(21) Mastral, A. M.; Call e´ n, M. S.; Murillo, R. Fuel 1996, 75 (13),
1
533-1536.
22) Mastral, A. M.; Call e´ n, M. S.; Mayoral, M. C.; Galb a´ n, J. Fuel
995, 74, 1762-1766.
23) Williams, P. T.; Taylor, D. T. Fuel 1993, 72 (11), 1469-1474.
24) Cypr e` s, R.; Bettens, B. In Pyrolysis and Gasification; Ferrero, G.
L., Maniatis, K., Buekens, A., Bridwater, A. V., Eds.; Elsevier
Applied Science: London, 1989.
(
1
(
(
7
50 °C seems to be the optimum temperature to burn tires
in a fluidized-bed reactor.
(
25) Report CSIC to EU DG XVII, ECSC; Ref 7220/ ED/ 089; Report 1,
2
, 3.
Acknowledgments
The authors thank the European Community for Steel and
Coal (ECSC), Project Ref. 7220/ EC/ 089, and the Intermin-
isterial Commision for Science and Technology (CICYT),
Project Ref. Amb 98-1583, for financial support.
Received for review March 15, 1999. Revised manuscript
received August 10, 1999. Accepted September 7, 1999.
ES990289O
4
1 5 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 23, 1999