activacion de vapor de carbon pirolitico a diferentes temperaturas

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Steam activation of pyrolytic tyre char at different temperatures

G. Lopez, M. Olazar *, M. Artetxe, M. Amutio, G. Elordi, J. Bilbao

University of the Basque Country, Department of Chemical Engineering, P.O. Box 644, E48080 Bilbao, Spain

1. Introduction

Although they account for only 2% of the overall amount of waste, tyres are of special concern in developed countries due to

the problems they may generate through inappropriate manage-

ment. The world generation of used tyres in 2005 was over 2.5

million tonnes in North America, 2.5 million tonnes in Europe and

0.5–1.0 million tonnes in Japan, which means 6 kg (the approx-

imate weight of a tyre) per inhabitant and year [1]. According to

estimates, this figure will increase to over 17 million tonnes per

year (approximately 1.4 billion tyres) by 2012, given that the gross

national product in developing countries encourages car demand

and tyre substitutionas a measure forsafe driving. These trends are

not offset by the measures adopted for prolonging tyre life [2]. The

increase in car sales in Asia, especially in China, will rapidly

increase the number of used tyres in this area, given that 85% of

tyres are from cars, andChina will become the first world producer

of used tyres (1 million tonnes in 2005, and according to the

estimates this figure will double by 2010, with a 12% annual

increase) [3].

Over the past 20 years, numerous pyrolysis plants have been

built at pilot and demonstration scale, based on the perspectives

that an industrial plant is economically profitable by selling

recovered products when 81,000 tonnes of used tyres are

processed [4]. Nevertheless, these projects have not been

commercially successful due to the low price of fuel and carbon

black on the market [5]. According to the more recent studies [6],

economic viability is only obtained when pyrolysis processes arenot limited to primary products, but rather they include stages for

obtaining higher value added products, such as high quality carbon

black, active carbon or chemical compounds such as benzene,

xylene, limonene, and so on. The char or solid residue accounts for

30–40% of the original tyre mass, which means that finding a

commercial application for that product is of great interest. In fact,

certain authors [7–9] state that the profitability of the tyre

pyrolysis process at industrial scale depends on char application.

The most straightforward application of tyre char is its reuse as

carbon black for new tyre production [10,11], but the feasibility of

this option depends on char properties such as particle diameter,

surface morphology and sulphur content.

Activation is an alternative for producing high quality active

carbons from wastetyres. Chemical activation is commonly carried

out using KOH as activation agent [12] and this process allows for

integrating pyrolysis and activation in a single step. Physical

activation is commonly carried out by using steam or carbon

dioxide as activation agents, but it may also be suitably carried out

using NO and O2 [13]. Steam is considered to be more active than

carbon dioxide [14–16] and, moreover, the carbons obtained with

steam have higher BET surface areas. This different behaviour may

be related to the smaller molecular size of water, which facilitates

diffusion within the char porous structure [17].

The active carbons obtained in the steam activation process are

mainly mesoporous with limited microporosity. In order to

increase microporosity, an acid pre-treatment has been applied

J. Anal. Appl. Pyrolysis 85 (2009) 539–543

A R T I C L E I N F O

Article history:

Received 27 May 2008

Accepted 4 November 2008Available online 12 November 2008

Keywords:

Active carbons

Steam activation

Tyre char

A B S T R A C T

Activationof tyrechar has beencarried out using steam as activation agent, and the effect of temperature

and activation time has been studied. The char samples used in the activation have been obtained by

continuous flash pyrolysis carried out in a conical spouted bed reactor at 500 8

C. The activation has beencarried out at 850 and 900 8C in a fixed bed reactor. During the process, a mesoporous structure is

developed, with a predominant pore diameter of around 500 A and BET surface areas above 500 m2/g for

both the temperatures studied. Moreover, sulphur content significantly decreases during activation and

activated carbons with lowsulphur contentare obtained. This reduction in contentmay be thekey forthe

industrial application of tyre-derived-carbons, either as active carbons or as carbon blacks for tyre

manufacturing.

2008 Elsevier B.V. All rights reserved.

* Corresponding author.

E-mail address: [email protected] (M. Olazar).

Contents lists available at ScienceDirect

Journal of Analytical and Applied Pyrolysis

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 / j a a p

0165-2370/$ – see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.jaap.2008.11.002

mailto:[email protected]

http://www.sciencedirect.com/science/journal/01652370

http://dx.doi.org/10.1016/j.jaap.2008.11.002

http://dx.doi.org/10.1016/j.jaap.2008.11.002

http://www.sciencedirect.com/science/journal/01652370

mailto:[email protected]



to the char prior to activation [18,19]. Carbons with high surface

areas are obtained in the steam activation of tyre chars, although

important differences are found in the literature depending on the

different experimental devices and pyrolysis conditions usedin the

process. Moreover, different trends have been observed in the

evolution of the surface area with burn-off. Some authors

[14,20,21] have observed a maximum in the BET surface area

with burn-off values of around 60% and an increase above these

values gives way to a decrease in BET surface area. Others

[16,22,23] have observed a continued increase in the surface area

up to burn-off values of 80%. Nevertheless, activation processes

with such a burn-off extension involve a major reduction in the

amount of product obtained.

The mesoporous carbons obtained in the activation process can

be applied to the adsorption of different pollutants in an aqueous

medium, such as phenols [18,19,24,25], mercury [26], pesticides

[27], chromium IV [28]. Moreover, the performance of tyre-

derived-carbons is also promising for their application in gaseous

phase, as is the case of SO2 retention in natural gas [29].

2. Experimental

The activation of tyre char has been addressed using steam as

activation agent. The char samples used in theactivation have been

obtained by continuous flash pyrolysis carried out in a conical

spouted bed reactor described elsewhere [30,31]. Operating at

500 8C, char yield reaches 34%, whichaccounts forthe total amount

of carbon black contained in the tyre. Pyrolysis conditions are an

important factor for porosity development during the activation

process. Some authors [32] report that a reduction in secondary

reactions and tar formation during the pyrolysis process improves

the subsequent activation process.

Fig. 1 shows a schematic representation of the activation unit

used in this study. The unit’s main component is the fixed bed

reactor where activation takes place. The reactor is placed inside a

radiant oven that provides the heat to operate at temperatures up

to 1000 8C. The unit is provided with a pressure meter to ensure

that pressure in the reactor during the activation reaction is not

higher than 1.2 atm. Both the reactor and the oven are located in a

hot box at 270 8C to ensure that water is vaporized before entering

the reactor. The heat in the hot box is supplied by two blowers

provided with four cartridges. Nitrogen is fed into the reactor by

means of a mass flowmeter. The water flowrate is controlled by a

high precision HPLC Gilson 307 pump.

2-g samples of tyre char have been used in each reaction. It is

noteworthy that the char particles obtained in the conical spouted

bed are the same size as the original tyre particles used in the

pyrolysis process, which are smaller than 1 mm. Furthermore, the

excellent gas–solid contact of the conical spouted bed (and theshort gas residence time) avoids additional carbonaceous material

deposition on the original carbon black of the tyre. This is due to

the fact that gas–particle circulation in this bed is countercurrent

Fig. 1. Scheme of the steam activation unit used in this work.

G. Lo pez et al. / J. Anal. Appl. Pyrolysis 85 (2009) 539–543540



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 bedincreases 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 thatobtained 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 beendetermined from nitrogen adsorption–desorption isotherms car-

ried 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 withactivation 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 arealmost linear, Fig. 3a. Theburn-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 8

C have 89.5 and 93.2 m

2

/g, respectively.

Fig. 3b shows the BET surface area values obtained for the

sample for differentburn-off levels. As observed,the evolutions arevery 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. Con-

cerning 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

aboutthe porous structureof thetyre-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 chara-

cteristic 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 8

C, 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 valuesof theactive carbons obtained at differentburn-

off levels.

Fig. 2. Effect of pyrolysis temperature over the char samples BET surface area.

G. Lo pez et al. / J. Anal. Appl. Pyrolysis 85 (2009) 539–543 541



are similar to those obtained at 900 8C. As observed, the microporevolume represents a small fraction of the total pore volume,

around 10% of the total pore volume. Both the total pore volume

and micropore volume reach the maximum for burn-off values in

the 50–60% range and longer activation times give way to lower

values of these parameters. This trend is explained by the growth

and destruction of micropores walls to produce meso- and

macropores [14].

Fig. 5 shows the pore size distribution obtained for samples

activated at 850 8C for 1, 2.5 and 3 h. The results obtained for the

activation carried out at 900 8C are very similar to those at 850 8C.

The Barrett–Joyner–Halenda (BJH) method [35] was used to

deduce the pore size distribution. As observed for all the carbons,

there is an important amount of pores of around 500 A , which is a

result observed by other authors [20]. Moreover, for longactivation

periods, thereis also an importantamount of large pores of 1000 A .

This trend of increasing pore size with activation time has been

observed by other authors [16]. It suggests that the activation

process consists of micropore formation, followed by pore

enlargement.

Table 1 shows the elemental analysis of the original char and

carbons obtained for different activation times. The charobtained by pyrolysis has a similar composition to coal, mainly

made up of carbon, with limited hydrogen and nitrogen contents.

The ashcontent is in allcasesaround 10%, whichis mostly ZnO. In

fact, the starting material we used for pyrolysis is vulcanized

rubber without steel chords or other additives. The pyrolytic char

in the pyrolysis process has a considerable sulphur content due

to the addition of this compound as vulcanization agent. This

content is a problem for pyrolytic char reuse as carbon black,

given that sulphur content for this purpose must be lower than

1% [36]. Apart from the improvement of char surface properties

during the steam activation process, an important reduction in

sulphur content is attained, which has also been observed by

other authors [32,37] but has been scarcely commented in the

literature. It should be noted that sulphur content is aspecification of commercial active carbons, so the reduction in

sulphur content could be the key for the industrial application of

tyre-derived-carbons.

4. Conclusions

The steam activation of pyrolytic tyre char obtained in a conical

spouted bed reactor produces good quality active carbon.

Commercial active carbons are microporous materials, but those

obtained from fast pyrolysisof waste tyres aremainly mesoporous.

The presence of mesopores and macropores makes pyrolytic tyre

char suitable for the adsorption of largemolecular size compounds.

The properties of the carbons obtained depend largely on

activation time, but temperature seems only to have a kineticeffect. Steam activation has another important advantage, namely,

sulphur removal from the char during activation. This removal is

enhanced by the structure of the char obtained in the conical

spouted bed reactor. This reduction in sulphur content may be the

key for the industrial application of tyre-derived-carbons, either as

active carbons or as carbon blacks for tyre manufacturing.

Acknowledgements

This work was carried out with the financial support of the

University of the Basque Country (Project GIU06/21), the Ministry

of Science and Education of the Spanish Government (Project

CTQ2007-61167) and of the Ministry of Industry of the Basque

Government (Project IE05-149).

Fig. 5. Pore size distribution for active carbons obtained at 850 8C for different

activation times.

Fig. 4. (a) Comparison of adsorption–desorption curves of the original tyre char

with thoseof theactive carbonobtainedfor 1-hactivationat 900 8C.(b) Evolutionof

the micropore and total pore volume during the activation process carried out at

850 8C.

Table 1

Elemental analysis of the original pyrolytic char and of active carbon samples

obtained with different burn-off levels and temperatures.

Sampl e Carbon (%) Hydrogen (%) Nitroge n (%) Sul phur (%)

Original char 86.92 1.17 0.51 3.34

850 8C, 1 h 87.23 0.89 0.12 1.26

850 8C, 3 h 77.29 1.10 0.06 0.57

900 8C, 0.5 h 86.49 1.00 0.10 1.25

900 8C, 1.5 h 78.22 1.10 0.08 0.34

G. Lo pez et al. / J. Anal. Appl. Pyrolysis 85 (2009) 539–543542



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