Kinetics of scrap tyre pyrolysis under vacuum conditions

Gartzen Lopez, Roberto Aguado, Martín Olazar

*

, Miriam Arabiourrutia, Javier Bilbao

Departamento de Ingeniería Química, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain

a r t i c l e

i n f o

Article history:
Accepted 3 June 2009
Available online 8 July 2009

a b s t r a c t

Scrap tyre pyrolysis under vacuum is attractive because it allows easier product condensation and control
of composition (gas, liquid and solid). With the aim of determining the effect of vacuum on the pyrolysis
kinetics, a study has been carried out in thermobalance. Two data analysis methods have been used in the
kinetic study: (i) the treatment of experimental data of weight loss and (ii) the deconvolution of DTG (dif-
ferential thermogravimetry) curve. The former allows for distinguishing the pyrolysis of the three main
components (volatile components, natural rubber and styrene–butadiene rubber) according to three suc-
cessive steps. The latter method identifies the kinetics for the pyrolysis of individual components by
means of DTG curve deconvolution. The effect of vacuum in the process is significant. The values of acti-
vation energy for the pyrolysis of individual components of easier devolatilization (volatiles and NR) are
lower for pyrolysis under vacuum with a reduction of 12 K in the reaction starting temperature. The
kinetic constant at 503 K for devolatilization of volatile additives at 0.25 atm is 1.7 times higher than that
at 1 atm, and that corresponding to styrene–butadiene rubber at 723 K is 2.8 times higher. Vacuum
enhances the volatilization and internal diffusion of products in the pyrolysis process, which contributes
to attenuating the secondary reactions of the repolymerization and carbonization of these products on
the surface of the char (carbon black). The higher quality of carbon black is interesting for process viabil-
ity.

The large-scale implementation of this process in continuous mode requires a comparison to be made

between the economic advantages of using a vacuum and the energy costs, which will be lower when the
technologies used for pyrolysis require a lower ratio between reactor volume and scrap tyre flow rate.

Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The devolatilization of wastes generated by human activity

(such as plastics and tyres) is one of the 21st century’s greatest
challenges and its technological progress is crucial for sustainable
development. This valorisation must be carried out following
routes that contribute to a better exploitation of raw materials that
attenuate climate change and decrease fossil fuel consumption.

The pyrolysis of scrap tyres produces flammable gases, high en-

ergy density liquids and an adulterated carbon black. The viability
of its industrial implementation requires maximizing jointly the
efficiency of pyrolysis process and the valorisation of each product
stream (

Zhang et al., 2008

). The gas has a higher heating value of

around 38 MJ/N m

3

, which is sufficient to sustain the process of

pyrolysis and offset heat losses (

Aylon et al., 2007

). The pyrolysis

liquid has a higher heating value of around 42 MJ/kg (

Laresgoiti

et al., 2004; Olazar et al., 2008a

) and a high content of BTX and lim-

onene (

Islam et al., 2008; Olazar et al., 2008a

), which has promoted

studies on its use for the replacement of conventional liquid fuels
and as a source of chemicals and raw materials for the petrochem-
ical industry. The yields of BTX and light olefins may be increased

by using in situ acid catalysts (HY or HZSM-5 zeolites) or by
reforming the volatile stream at the outlet of the pyrolysis reactor
by using these catalysts (

Arabiourrutia et al., 2008

). The carbon

black obtained in thermal flash pyrolysis has a high heat value of
29 MJ/kg and a sulphur content of 2.0–2.8% (

Olazar et al., 2008a

),

which should be removed for most of the applications.

The pyrolysis of scrap tyres has already been studied and re-

ported in the literature and encouraging results have been ob-
tained at different scales and using different technologies,
amongst which the following are worth mentioning: fixed bed
(

Berrueco et al., 2005; Islam et al., 2008

), fluidized bed (

Kaminsky

and Mennerich, 2001; Dai et al., 2001

), rotary oven (

Li et al., 2004

),

vacuum moving bed (

Pantea et al., 2003; Gupta et al., 2004

) and

conical spouted bed reactor (

Arabiourrutia et al., 2007

).

The interest of vacuum pyrolysis lies in the advantages associ-

ated with the decrease in the inert gas flowrate and residence time
of volatiles in the reactor: (a) lower energy requirements for the
process (although this advantage depends largely on the technol-
ogy used, given that it will condition the energy requirement for
vacuum); (b) simpler devices for volatile product condensation;
(c) higher liquid yield and better control of its composition, either
for increasing the yield of high value added components, such as
dl-limonene (

Pakdel et al., 2001

) or for improving its fuel quality

(

Zhang et al., 2008

) and (d) better quality of the carbon black, given

0956-053X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:

10.1016/j.wasman.2009.06.005

*

Corresponding author. Tel.: [phone]; fax: [phone].
E-mail address:

[e-mail]

(M. Olazar).

Waste Management 29 (2009) 2649–2655

Contents lists available at

ScienceDirect

Waste Management

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / w a s m a n

that the undesired processes of volatile carbonization by secondary
reactions are minimized and the surface properties of the carbon
black are similar to the commercial ones (

Benallal et al., 1995;

Zhang et al., 2008

). Furthermore, as determined in a previous paper

(

Olazar et al., 2008b

), in which the same material has been pyrol-

ysed in a conical spouted bed reactor, the controlling step in the
kinetics of scrap tyre pyrolysis is an intermediate active com-
pound. Presumably, the formation of this intermediate is enhanced
by operating under vacuum, which will contribute to increasing
the production rate. This effect has already been qualitatively ob-
served in the pyrolysis of tobacco, in which vacuum enhances
the volatilization of non-polymeric semivolatile compounds of
high molecular weight, such as nicotine, solanesols, polyphenols,
sterols, fatty acids and carboxylic acids (

Oja et al., 2006

).

Vacuum tyre pyrolysis has been developed at the Universite LA-

VAL using a moving bed reactor (

Benallal et al., 1995; Yang et al.,

1995a,b; Roy et al., 1997, 1999; Darmstadt et al., 1997

) and has

been subsequently studied by

Zhang et al. (2008)

in a fixed bed

reactor. Nevertheless, and although it is an interesting technology,
no results have been published on the kinetics under vacuum nor
has any comparison been made with the extensively studied kinet-
ics under atmospheric pressure.

Studies on the kinetics of scrap tyre pyrolysis under atmo-

spheric pressure have been carried out by means of thermal ana-
lytical techniques (

Brazier and Schwartz, 1978; Bouvier et al.,

1987; Bhowmick et al., 1987; Kleps et al., 1990; Williams and Bes-
ler, 1995; Teng et al., 1995; Yang et al., 1995b

) and fast heating

pyrolysis reactors (

Aguado et al., 2005

), but studies in reactors

operating under conditions similar to those of industrial ovens
(fast heating and short residence times for the products) are scarce
(

Aylon et al., 2005; Olazar et al., 2005

).

Given that tyres are made up of different pyrolysable materials,

kinetic models based on the volatilization of these components
have been established (

Conesa and Marcilla, 1996; Lin et al.,

1998; Aylon et al., 2005; Seidelt et al., 2006

). The results obtained

have confirmed the kinetics of the overall weight loss published in
the literature under atmospheric pressure (

Seidelt et al., 2006

).

2. Experimental

The composition and properties of the material studied are set

out in

Table 1

.

The kinetic study has been carried out by thermogravimetry in a

Setaram TG-DSC 111 thermobalance. The operating method con-
sists in subjecting the sample (approximately 25 mg) to a heating
ramp of 10 K min

1

from room temperature to 800 K, in an inert

atmosphere (helium) under two different absolute pressures:
1 atm and 0.25 atm. The heating rate has a great influence on pyro-
lysis kinetics and a ramp of 10 K min

1

is suitable for avoiding the

thermal lag and non-uniform evolution of products. In fact, when
the heating ramp is higher than 30 K min

1

, heat and mass transfer

equations must be coupled to the kinetic model in order to fit the
experimental results (

Quek and Balasubramanian, 2009

).

The particle size of the sample is smaller than 0.2 mm, which

has been obtained by grinding the material (following to immer-
sion in liquid nitrogen) in a Retsch ZM100 mill. This size is smaller
than that commonly used in the literature in order to avoid heat
and mass restrictions within the particle (

Koufopanos et al.,

1989; Ahuja et al., 1996

).

3. Results

The kinetic parameters have been obtained by following two

variants of the differential method for data analysis: (a) by using
the evolution of the overall conversion (overall weight loss) with
time of a tyre sample; (b) by deconvolution of the DTG curve, in or-
der to identify the weight loss corresponding to each one of the
three tyre components (

Mui et al., 2008

).

3.1. Kinetics based on the weight loss of tyre material

Assuming a first order kinetics, the kinetic constant has been

calculated as:

k ¼

dX=dt

ð1 XÞ

ð1Þ

where conversion is:

X ¼

W

o

W

W

o

W

1

ð2Þ

Table 1
Composition and properties of the material studied.

Components

wt%

Natural rubber

29.59

Styrene–butadiene rubber

29.59

Carbon black

29.59

Zinc oxide

2.96

Sulphur

0.89

Aromatic oil

2.37

Phenolic resin

2.37

Estearic acid

0.59

IPPD (antioxidizing agent)

0.89

CBS (vulcanization accelerator)

0.89

Other additives

0.27

Tyre material properties
Density

1140 kg m

3

Particle diameter

<0.2 mm

Higher heating value

38,847 kJ kg

1

Nomenclature

BTX

benzene, toluene and xylenes

DSC

differential scanning calorimetry

DTG, TG differential thermogravimetry and thermogravimetry
k, k

i

kinetic constant for the overall weight loss and for the
weight loss of each i component, s

1

k*, k

i

kinetic constants at a reference temperature, s

1

k

0

, (k

0

)

i

frequency factors for the pyrolysis of the sample and of
each i component, s

1

L

number of experimental data available

NR, SBR, V natural rubber, styrene–butadiene rubber and volatile

components in the tyre, respectively

R

gas constant, 8.314 kJ mol

1

K

1

T, T

o

temperature and reaction starting temperature, K

W, W

0

, W

1

weight of tyre sample at t time, at the beginning of

pyrolysis and at the end of pyrolysis, respectively, mg

W

i

, W

i,0

, W

i,1

weight of i component in the sample at t time, at

the beginning and at the end, mg

X

conversion of the tyre sample by mass unit of pyrolysa-
ble mass

X

i

, X

i,1

conversion of i component at t time and at the end

2650

G. Lopez et al. / Waste Management 29 (2009) 2649–2655

In Eq.

(2)

, W is the weight of the sample at a given time t, W

o

is

the initial weight and W

1

the amount of sample that has not re-

acted at the end of the run.

Fig. 1

shows the evolution of tyre material conversion with time

under the two pressures studied. Given that weight loss is more ra-
pid under vacuum than under atmospheric pressure, conversion at
a given temperature is also higher under vacuum. This difference is
observed from the beginning of the process and was also observed
when determining the reaction starting temperature, T

o

. As is qual-

itatively observed, the kinetics is faster at vacuum even at low
temperatures and the difference in conversion increases up to
625 K. It is then almost constant as temperature is increased to to-
tal devolatilization of the material. Total conversion of the material
is reached 20 K below that of atmospheric pressure. This result
does not allow making quantitative comparisons.

Fig. 2

shows the Arrhenius plot corresponding to the runs under

both pressures. Certain scattering is observed in the fitting to the
Arrhenius equation for the experimental points corresponding to
low temperatures. As temperature is increased, three well-defined
steps are observed, which are due to the three main components
that make up the material; natural rubber, styrene–butadiene rub-
ber, and a variety of volatile additives. Given that each component
shows a different behavior during pyrolysis, these steps have been
studied by thermal degradation in thermobalance of tyres made up
of mixtures of natural rubber, styrene–butadiene and polybutadi-
ene (

Yang et al., 1993; Williams and Besler, 1995; Kim et al.,

1995; Conesa and Marcilla, 1996; Leung and Wang, 1998; Galvag-
no et al., 2007; Mui et al., 2008

). Other authors (

Gonzalez et al.,

2001; Aylon et al., 2005

) have considered three steps in the devol-

atilization of tyres; the first one associated with the thermal
decomposition of the aromatic oil and other additives, the second
one with the natural rubber and the third one with the styrene–
butadiene and polybutadiene rubber.

Table 2

shows the values of activation energy and frequency

factor calculated for the three steps proposed, which have been ob-
tained by fitting the experimental results of each step in

Fig. 2

to

the Arrhenius equation. The overall regression coefficient of the fit-
ting is r

2

= 0.97 and the average relative error is 4%. Furthermore,

the value of the kinetic constant has been calculated for an inter-
mediate temperature in each range, 503, 623 and 723 K, which al-
lows quantifying the effect of vacuum on the kinetics of pyrolysis.
As observed in

Table 2

, the values of activation energy correspond-

ing to the three steps decrease when the reaction is carried out un-
der vacuum. Thus, 50.6, 130.8 and 245.9 kJ mol

1

activation energy

values are obtained for pyrolysis under atmospheric pressure, and
43.5, 104.7 and 243.0 kJ mol

1

under vacuum. In view of the com-

position of the material studied, the first of these activation ener-
gies corresponds to the devolatilization of the aromatic oil and
other volatile additives, the second one to the natural rubber and
the third one to styrene–butadiene.

Pressure has a significant effect on the value of the kinetic con-

stant in the three steps at a reference temperature. Thus, the ki-
netic constant of the third step under vacuum (0.025 s

1

) is

almost double that corresponding to atmospheric pressure
(0.014 s

1

).

Fig. 1. Evolution of conversion with time under atmospheric pressure and vacuum,
by following a heating ramp of 10 °C min

1

.

b

a

Fig. 2. Comparison of experimental results (points) and those estimated using the Arrhenius equation (lines) for the overall sample pyrolysis. Graph a, 1 atm. Graph b,
0.25 atm.

G. Lopez et al. / Waste Management 29 (2009) 2649–2655

2651

Table 2

shows the values of activation energy corresponding to

the three steps published in the literature for the atmospheric
pyrolysis of similar materials, in which three components with dif-
ferent devolatilization kinetics (volatile additives, natural rubber
and synthetic rubbers) are identified. These values are very similar
to the results in this paper. It should be noted that the materials
compared have certain differences (particularly concerning the
nature and composition of the synthetic rubber). Other authors
have distinguished several steps in the thermal degradation of
tyres, but they have not associated each step with a given compo-
nent or they have studied mixtures of undefined composition
(

Senneca et al., 1999; Galvagno et al., 2007; Mui et al., 2008

), which

makes it difficult to compare data.

The aforementioned results show that vacuum enhances the

process of pyrolysis from low temperatures and from the initial
steps of devolatilization. It should be noted that operation under
vacuum generates a pressure gradient that enhances the diffusion
of the volatile components formed within the particle towards its
outside. Given that the residence time of the volatile components
within the porous structure is shorter, secondary reactions are
minimized, reducing the deposition of heavy hydrocarbons, and
consequently pore blockage is reduced (

Zhang et al., 2008

).

López

et al. (2009)

have pyrolysed the same material studied in this paper

in a conical spouted bed reactor at 425 °C and they have obtained a
carbon black with a BET area of 106 m

2

g

1

in vaccum operation,

whereas the area was 45 m

2

g

1

in atmospheric operation.

3.2. Kinetics based on DTG deconvolution

Fig. 2

shows three kinetic regimes corresponding to the three

pyrolysable components that make up the tyre material used in
the runs under both atmospheric pressure and vacuum. The model
for DTG curve deconvolution considers the kinetic scheme made up
of three parallel and independent reactions, which correspond to
the degradation of volatile additives (V), natural rubber (NR) and
styrene–butadiene rubber (SBR). The overall conversion of the sam-
ple is the sum of the conversions of these three components:

X ¼

X

3

i¼1

X

i

ð3Þ

where X

i

is i component conversion:

X

i

¼

W

i;0

W

i

W

i;0

W

i;1

ð4Þ

In Eq.

(4)

, W

i

, W

i,0

and W

i,1

are the weights of i component at t time,

at the beginning and at the end.

The change in the overall conversion of the sample, X, with time

is calculated as:

dX

dt

¼

X

3

i¼1

dX

i

dt

ð5Þ

in which the conversion evolution of each component is:

dX

i

dt

¼ k

i

ðX

i;1

X

i

Þ

ð6Þ

where k

i

is the kinetic constant corresponding to the weight loss of i

component and X

i,1

is its final conversion.

Given that the runs are carried out following a temperature

ramp (10 K min

1

in this study), the Arrhenius equation is consid-

ered in Eq.

(6)

:

dX

i

dt

¼ ðk

o

Þ

i

expð

E

i

RT

ÞðX

i;1

X

i

Þ

ð7Þ

The calculation of the kinetic parameters (k

0

)

i

and E

i

has been

carried out by fitting the experimental results obtained by decon-
volution to Eq.

(7)

integrated for each component. From the exper-

imental TG curve and following a second order approximation with
central derivatives, the theoretical DTG curve is obtained by mini-
mizing the error objective function:

EOF ¼

P

L
i¼1

ðDTG

theo

DTG

exp

Þ

2

L

ð8Þ

where L is the number of experimental data.

The fitting is carried out by means of a program written in SCI-

LAB using the subroutine optim with the option NDcost (finite dif-
ferences) in order to minimize the error objective function, Eq.

(8)

, and the subroutine ode with the option rkf (based on Fehlberg’s

Runge–Kutta pair of order 4 and 5 method) for solving the differen-
tial equations for each component in the sample, Eq.

(7)

.

Fig. 3

compares the experimental results (points) and those cal-

culated (line) for the evolution of the overall conversion of the
sample with temperature under atmospheric pressure (

Fig. 3

a)

and under vacuum (

Fig. 3

b). The results calculated for the evolu-

tion of the conversion of the three components with time are also
shown. The fitting between the experimental and calculated re-
sults is satisfactory, given that there is only a slight deviation at
the initial stretch of the curve. The number of experimental points
used in the overall fit is 390 and the regression coefficients are
0.994 for vacuum and 0.995 for atmospheric pressure.

Fig. 4

compares the DTG curves calculated for the components

by deconvolution. They correspond to the best fitting of the DTG
curve for the whole sample when pyrolysis is carried out under
atmospheric pressure (

Fig. 4

a) and under vacuum (

Fig. 4

b). As

mentioned above, there is a slight difference at the initial stage
of the process, in which the model predicts a lower weight loss
than that experimentally obtained. This deviation is clearly ob-
served in

Fig. 3

. There is also a slight deviation in the valley be-

tween the two peaks corresponding to natural and synthetic
rubber. In view of the shape of these peaks, the effect of vacuum
may be qualitatively understood; that is, it accelerates the pyroly-
sis process, given that the second peak is sharper and the reaction
finishes in a shorter time.

Table 2
Values of frequency factor, k

o

, activation energy, E, and kinetic constant (at a reference temperature, 503, 623 and 723 K), k*, for the three steps defined in the model based on the

overall weight loss of the sample and activation energies for the same steps obtained by other authors.

Step I

Step II

Step III

1 atm

k

o

(s

1

)

2.46(±0.21) 10

1

5.63(±0.53) 10

7

8.10(±0.71) 10

15

E (kJ mol

1

)

50.6 ± 4.9

130.8±12.7

245.9 ± 18.9

k* (s

1

)

0.00014

0.00060

0.014

0.25 atm

k

o

(s

1

)

7.62(±0.61) 10

1

6.03(±0.45) 10

5

9.09(±0.77) 10

14

E (kJ mol

1

)

43.5 ± 3.9

104.7 ± 9.2

243.0 ± 20.9

k* (s

1

)

0.00022

0.0010

0.025

Literature data
1 atm

Kim et al. (1995)

, E (kJ mol

1

)

42

195

204

Conesa and Marcilla (1996)

, E (kJ mol

1

)

70

212

249

Aylon et al. (2005)

, E (kJ mol

1

)

70

212

265

2652

G. Lopez et al. / Waste Management 29 (2009) 2649–2655

Table 3

shows the values calculated for the frequency factors

and activation energies for the three components. Furthermore, it
also shows the kinetic constants calculated at the same tempera-
tures as in the overall kinetic study of the sample pyrolysis (503,
623 and 723 K). The values of activation energy under vacuum
for the volatile components and natural rubber are lower than
those corresponding to atmospheric pressure, whereas the reverse
is true for styrene–butadiene rubber. This result is explained by the
evolution of material composition during devolatilization. Thus,
when the devolatilization of styrene–butadiene rubber starts un-
der vacuum operation, the volatile additives associated with this
elastomer have already been volatilised, whereas some of these re-
main in the material in the volatilization at atmospheric pressure.
This fact agrees with the kinetic study by

Leung and Wang (1999)

,

who consider the devolatilisation of each component, made up of

the corresponding rubber and their volatile additives. Furthermore,
the importance of considering the evolution of the reaction med-

a

b

Fig. 3. Comparison of experimental results (points) and those estimated using the kinetic model (dashed line) for the evolution of sample conversion with time, and results
calculated for each component (V, NR and SBR). Graph a, 1 atm. Graph b, 0.25 atm.

a

b

Fig. 4. Comparison of experimental results (points) and those estimated using the kinetic model (dashed line) for DTG, and the DTG curves calculated by deconvolution for
each component (V, NR and SBR). Graph a, 1 atm. Graph b, 0.25 atm.

Table 3
Values of frequency factor, k

o

, activation energy, E, and kinetic constant (at a

reference temperature, 503, 623 and 723 K), k*, for the pyrolysis of the three
components in the tyre.

Additives

NR

SBR

1 atm
(k

o

)

i

(s

1

)

8.06(±0.73) 10

5

5.45(±0.41) 10

7

2.19(±0.18) 10

12

E

i

(kJ mol

1

)

84.1 ± 6.9

126.7 ± 11.7

201.1 ± 13.9

k* (s

1

)

0.0015

0.0013

0.0064

0.25 atm
(k

o

)

i

(s

1

)

2.17(±0.18) 10

5

2.67(±0.21) 10

7

1.31(±0.10) 10

14

E

i

(kJ mol

1

)

76.2 ± 6.5

107.9 ± 8.1

219.6 ± 18.7

k* (s

1

)

0.0026

0.0023

0.018

G. Lopez et al. / Waste Management 29 (2009) 2649–2655

2653

ium is analysed by

Mazloom et al. (2009)

by comparing discrete

and continuous kinetic models for describing the kinetics of scrap
tyre pyrolysis.

The kinetic constants at the reference temperature are signifi-

cantly higher under vacuum than those obtained under atmo-
spheric

pressure.

Thus,

the

kinetic

constants

for

the

devolatilisation of volatile additives and natural rubber are 1.7
times higher at 0.25 atm than at 1 atm and the constant for
the devolatilisation of styrene–butadiene rubber is 2.8 times
higher.

4. Conclusions

Pressure considerably affects the process of tyre pyrolysis.

Vacuum enhances the process even at low temperatures and
from the initial stages of the reaction. The reaction starting tem-
perature is 485 K under vacuum and 497 K under atmospheric
pressure.

Based on an overall kinetic model, three steps are identified,

which are associated with the degradation of three components
(volatile components, NR and SBR), whose activation energies are
lower under vacuum: 43.5, 104.7 and 243.0 kJ mol

1

under

0.25 atm vs. 50.6, 130.8 and 245.9 kJ mol

1

under atmospheric

pressure.

The same trend has been observed based on a kinetic model of

parallel reactions for each one of the pyrolysable components,
although the value of activation energy for SBR degradation under
vacuum is slightly higher than that corresponding to atmospheric
pressure (219.6 kJ mol

1

vs. 201.1 kJ mol

1

).

A comparison of the results obtained with the two kinetic

models shows that pyrolysis reaction rate increases with vac-
uum. This effect of vacuum is attributed to the enhancement
of the volatilization of primary products and their diffusion
within the particle, which reduces the residence time of these
products in the particle. Consequently, secondary reactions of
repolymerization and carbonization and char pore blockage are
minimized.

The increase in the kinetic constants of devolatilisation, which

are 1.7 times higher at vacuum (0.25 atm) than at atmospheric
pressure for the volatile additives and natural rubber and 2.8 times
higher for the styrene–butadiene rubber contained in the tyre, is a
significant result for improving process viability. Furthermore, the
production of a carbon black that is less adulterated by deposition
of organic components on the surface may also contribute to via-
bility. Nevertheless, the study carried out in this paper has the lim-
itations inherent to thermobalance studies. It has been carried out
in discontinuous mode and under conditions (particle size and
heating rate) in which there are no heat and mass transfer limita-
tions. Scaling up and continuous operation are required to confirm
the results obtained in this paper when operating in thermobal-
ance. Furthermore, a more concise assessment of the interest of
vacuum pyrolysis requires an economic study whose result will
depend on the reactor technology, given that energy costs under
vacuum will depend on the ratio between reactor volume and
material flow rate in the feed.

Acknowledgements

This work was carried out with the financial support of the Uni-

versity of the Basque Country and Department of Education of the
Basque Government (Project IT-220-07), the Ministry of Science
and Education of the Spanish Government (Project CTQ2004-
01562/PPQ) and the Ministry of Environment of the Spanish Gov-
ernment (Project 242/2006/2-5.3).

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