in the annular zone and co-current but highly turbulent in the
spout, which allows for a very efficient contact.
The surface area of the tyre char obtained in the process of
pyrolysis in the conical spouted bed increases in a very pronounced
way with temperature, Fig. 2, from approximately 40 m2/g at
425 8C to 120 m2/g at 600 8C. The latter is even higher than that
obtained by Roy et al. [33] by operating in a moving bed reactor
under vacuum.
The char sample is heated in an inert atmosphere (nitrogen)
flow until the activation temperature is reached. Once this
temperature has been reached, nitrogen flow is maintained for
1 h in order to complete sample carbonization. The activation gas
mixture, which is made up of steam and nitrogen at a ratio of
75:25, is then continuously fed into the reactor. The gas flowrate
used under both inert and activation conditions is 400 cm3/min
measured at normal conditions. When the reaction has finished,
the activation mixture is replaced by nitrogen and the reactor is
cooled. Finally, the sample is removed from the reactor and
weighed to determine the burn-off undergone in the activation.
Surface area, and pore volume and size distribution, have been
determined from nitrogen adsorption–desorption isotherms carried
out in a Micromeritics ASAP 2000. The technique based on Hg
porosimetry (Micromeritics Autopore II 9220) has been used to
characterize macropores.
The composition of the active carbon samples obtained in the
activation process has been determined in an LECO CHNS-932
elemental analyzer. Sulphur content is a parameter of great
relevance, given that its application as active carbon or reuse as
carbon black requires this content to be lower than 1%.
3. Results and discussion
Activation runs have been carried out at 850 and 900 8C for
several times. Fig. 3a shows the evolution of burn-off with
activation time for both temperatures. As observed, the pyrolytic
tyre char shows a high reactivity using steam as activation agent at
both studied temperatures, this high reactivity can be mainly
attributed to the catalytic effect of inorganic components [34] such
as zinc oxide. There is a clear increase in reaction rate with
temperature, given that it approximately doubles from 850 to
900 8C. For both temperatures studied, the evolutions of burn-off
with time are almost linear, Fig. 3a. The burn-off curves do not start
at zero, because there is a mass loss during the carbonization step.
These initial mass losses are 10.6% at 850 8C and 12.0% at 900 8C.
Moreover, the BET surface areas of the chars grow during
carbonization. Thus, the char obtained in tyre pyrolysis has a
BET surface area of 65.2 m2/g, but the samples carbonized at 850
and 900 8C have 89.5 and 93.2 m2/g, respectively.
Fig. 3b shows the BET surface area values obtained for the
sample for different burn-off levels. As observed, the evolutions are
very similar, so temperature affects mainly the activation kinetics,
but not porous structure development. The maximum BET areas
for both temperatures studied are higher than 500 m2/g. Concerning
the BET areas published in the literature, they vary in a
relatively wide range from 300 to 1000 m2/g. These differences are
a consequence of several factors, such as the experimental device
used for the activation process, the original tyre characteristics,
pyrolysis conditions (heating rate, secondary reactions) and other
factors that may affect the char porous structure and reactivity.
The BET area increases steadily with burn-off until levels of
around 60%, when the maximum BET surface area values are
obtained. For longer treatments, a reduction in surface area is
observed. This trend is observed at the two temperatures studied
in this paper.
The shape of the adsorption isotherms gives useful information
about the porous structureof the tyre-derived-carbons. Fig.4ashows
a comparison between the isotherms of the original pyrolytic char
and the carbon obtained after 1-h activation at 900 8C. The active
carbon predominantly exhibits a type IV isotherm, which is characteristic
of mesoporous materials. The initial adsorption capacity at
low relative pressures records limited micropore development.
Moreover, an important mesoporous structure is created, as is
evidenced by nitrogen adsorption at high relative pressures and by
the typical hysteresis loop of mesoporous materials [35].
Fig. 4b shows the evolution of the total pore volume and
micropore volume of the carbons obtained for different activation
times. The data presented correspond to the activation reactions
carried out at 850 8C, and both the trend and the values obtained
Fig. 3. (a) Evolution of burn-off in the activation process at the two temperatures
studied. (b) BET surface area values of the active carbons obtained at different burnoff
levels.
Fig. 2. Effect of pyrolysis temperature over the char samples BET surface area.