Teng, Lin, andPAPER
Hsu
TECHNICAL
ISSN 1047-3289 J. Air & Waste Manage. Assoc. 50:1940-1946
Copyright 2000 Air & Waste Management Association
Production of Activated Carbons from Pyrolysis of Waste Tires
Impregnated with Potassium Hydroxide
Hsisheng Teng, Yu-Chuan Lin, and Li-Yeh Hsu
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
ABSTRACT
Activated carbons were produced from waste tires using a
chemical activation method. The carbon production process consisted of potassium hydroxide (KOH) impregnation followed by pyrolysis in N2 at 600–900 °C for 0–2 hr.
The activation method can produce carbons with a surface area (SA) and total pore volume as high as 470 m2/g
and 0.57 cm3/g, respectively. The influence of different
parameters during chemical activation, such as pyrolysis
temperature, holding time, and KOH/tire ratio, on the
carbon yield and the surface characteristics was explored,
and the optimum preparation conditions were recommended. The pore volume of the resulting carbons generally increases with the extent of carbon gasified by KOH
and its derivatives, whereas the SA increases with degree
of gasification to reach a maximum value, and then decreases upon further gasification.
INTRODUCTION
The disposal of waste tires has become an increasingly
important issue worldwide in recent years. The tires not
only take up large amounts of valuable landfill space, but
also create fire hazards and provide a refuge for disease-
IMPLICATIONS
Conversion of waste tires into valuable products through
pyrolysis has been explored by numerous researchers.
Reprocessing of the solid residue from pyrolysis into activated carbon has been considered to be a profitable way
to do this. In developing the technology, almost all efforts
have been devoted to producing activated carbon through
physical activation, a process that would result in low
carbon yield and high energy consumption. The present
work has demonstrated that in the presence of KOH, activated carbon can be produced directly from the pyrolysis of waste tires. Without the secondary heating required
in the physical activation, the production of activated carbon through KOH impregnation is very beneficial in terms
of energy saving and emission control. This research can
stimulate more in-depth investigations and other more
practical studies on the pyrolysis of waste tires.
1940 Journal of the Air & Waste Management Association
carrying creatures. Because of their high heating value,
tire incineration presents an option of solving the disposal problem as well as of producing additional energy.1
From an economical standpoint, however, the profit derived from energy recovery in tire incineration would be
offset by the expense of meeting pollution legislation. The
use of waste tires as an auxiliary fuel in cement kilns has
recently been recognized as an acceptable means for disposing of waste tires through incineration. Apart from
incineration, other reuse options, such as retreading and
the production of rubber crumb, have been recommended.2
Recently, much effort has been devoted to the thermal degradation of tires into gaseous and liquid HCs and
a solid char residue, all of which have the potential to be
reprocessed into valuable products.2-12 The resulting HCs
from thermal treatment can be used directly as fuel or
added to petroleum refinery feedstocks. As for the solid
residue, the char can be used either as low-grade reinforcing filler or as activated carbon. The use of the tire char as
carbon black for tires has been reported to be unsatisfactory,6 mainly due to the high mineral matter content,
while activation of the char with steam or CO2 to produce a high-surface-area (SA) carbon has been shown to
be applicable.2,7-12 Because of the low reaction rate between
the activating agent (steam or CO2) and the char, a high
temperature (>800 °C) for activation was necessary. Under this circumstance, the energy consumed in heating
the activating agent would be relatively high.
The production of activated carbon can also be
achieved by chemical activation,13 which usually produces
carbons with very high SA. This method consists of impregnation of the precursor with a dehydrating reagent,
such as ZnCl2, H3PO4, or potassium hydroxide (KOH), followed by carbonization in an inert environment. In comparison with activation with steam or CO2, there are two
important advantages of chemical activation. One is the
lower energy consumption due to the fact that heating
the activating agent is not required, and the other is that
tire pyrolysis and activated carbon production can be
Volume 50 November 2000
Teng, Lin, and Hsu
achieved in one stage. Among the numerous chemical
reagents, KOH was found to be effective in creating porosity in chars derived from tires in our preliminary study,
due to the difference in the activation mechanism. The
recent literature has reported that activated carbons prepared from KOH treatment have their specific uses, such
as the removal of SO2 from flue gas and halogenated HCs
and pesticides in drinking water purification.14
In this paper, the properties of the final carbons prepared from the pyrolysis of tires impregnated with KOH
were the main concerns. The influence of the preparation parameters, such as the solvent during impregnation,
the ratio of KOH to tires, and the pyrolysis temperature
and holding time, on the pore structure of the activated
carbons was extensively explored.
EXPERIMENTAL APPROACH
Materials
Waste tires are the precursors used in the pyrolysis process.
The tires used in the present work represent a mixture of
used truck tires, containing no steel or synthetic cord. The
proximate and ultimate analyses of the waste tires are
shown in Table 1. Before being treated, the tires were ground
and sieved to a size of 0.2–0.3 mm. Pulverization of the tire
rubber provided the rubber a better contact with the chemical reagent during the course of reagent impregnation.
Pyrolysis
Pyrolysis of the waste tires was performed in the presence
of a chemical reagent, KOH. The tires were impregnated
with KOH prior to pyrolysis. In a 250-mL glass-stoppered
flask, the impregnation process was initiated by mixing
1 g of tires with a KOH solution containing 50 g of water
or C2H5OH as the solvent. The flask was immersed in a
constant temperature shaker bath, with a shaker speed
of 100 rpm. The mixing was performed at 85 °C and
lasted 3 hr. The concentration of the KOH solution was
adjusted to give a w/w ratio of chemical reagent to tire
(i.e., a chemical ratio) varying in the range of 0–8.
After mixing, the tire-KOH slurry was subjected to
vacuum drying at 110 °C for 24 hr. The chemical-loaded
samples were then pyrolyzed in a horizontal cylindrical
furnace (60-mm i.d.) in N2 atmosphere, with a flow of 100
mL/min. Pyrolysis was carried out by heating the samples
at 30 °C/min from room temperature to heat-treatment
temperatures in the range of 600–900 °C, followed by holding the samples at the treatment temperature for different
lengths of time (0–2 hr) before cooling under N2.
After cooling, the pyrolyzed products were washed
by stirring with 250 mL of 0.5 N HCl solution at 85 °C for
30 min, followed by filtration. The acid-washed sample
was then leached by mixing with 250 mL of distilled water at 85 °C, followed by filtration of the mixtures. Leaching was carried out several times until the pH of the
water-carbon mixture was above 6. The leached products
were then dried in a vacuum at 110 °C for 24 hr to give
the final carbon products.
Carbon Characterization
Specific SAs and pore volumes of the activated samples
were determined by gas adsorption. An automated adsorption apparatus (Micromeritics, ASAP 2010) was employed for these measurements. Adsorption of N2 was
performed at –196 °C. Before any such analysis, the sample
was degassed at 300 °C in a vacuum at ~10–3 Torr. Nitrogen SAs and micropore volumes of the samples were determined from the application of the BET and
Dubinin-Radushkevich (D-R) equations,15 respectively, to
adsorption isotherms at relative pressures between 0.06
and 0.2. The amount of N2 adsorbed at pressures near unity
corresponds to the total amount adsorbed at both
micropores and mesopores; consequently, the subtraction
of the micropore volume (from the D-R equation) from
the total amount (determined to be p/p0 = 0.98 in this
case) will provide the volume of the mesopores.16 Although
this procedure of determining the mesopore volume is
justifiable only if the isotherm is typical of type IV, which
has a distinct plateau at high p/p0,17 it still provides a rough
estimation of pore-size distribution of different samples.
If the pores are assumed to be parallel and cylindrical, the
average pore diameter would have a value of 4V/BET, where
V is the total pore volume and BET represents the SA.15
A scanning electron microscope (Jeol, JXA-840) was
used to study the structural features of the carbon surface.
Table 1. Analysis of waste tires.
Ultimate
(wt %, Dry-Ash-Free Basis)
Carbon
Hydrogen
Sulfur
Oxygen
Other
86.1
7.1
1.6
3.3
1.9
Volume 50 November 2000
Proximate
(wt %, As-Received)
Moisture
Volatile matter
Fixed carbon
Ash
0.2
66.1
29.4
4.3
RESULTS AND DISCUSSION
Previous studies18-21 have shown that chemical activation
of bituminous coals with ZnCl2 or H3PO4 gives the highest porosity of the resulting activated carbon at a carbonization temperature of about 500–600 °C, which is much
lower than the temperature in physical activation with
steam or CO2.13,22,23 In our preliminary study, chemical
activation of the tires with ZnCl2 and H3PO4 was tested,
and the results are presented in Table 2, showing that high
Journal of the Air & Waste Management Association 1941
Teng, Lin, and Hsu
porosity carbon could not be produced through these activation processes. These preliminary results will not be
further discussed in the present work.
KOH was found to be more effective than ZnCl2 and
H3PO4 in creating porosity in carbons derived from the
tires. Two solvents—water and C2H5OH— were tested in
the impregnation of tires with KOH. Experimental results
showed that carbons prepared from KOH/alcohol solutions exhibit higher SAs. This can be attributed to the fact
that alcohol, which is less polar than water and organic
in nature, can more easily penetrate into the tire particles
than water can. Therefore, carbons obtained from impregnation in KOH/alcohol solutions were employed to discuss the effects of pyrolysis parameters.
The porosity development of the carbons prepared
from KOH activation was investigated by N2 adsorption.
Figure 1 shows the typical isotherms of N2 adsorption on
the carbons prepared from the tires at different pyrolysis
temperatures (600–900 °C) with a zero holding time. The
chemical ratio of KOH to tires was fixed at 4. It can be
seen from Figure 1 that the isotherms are not type I, which
is typical of microporous carbons. In a type I isotherm,
the knee of the isotherm is sharp and the plateau is fairly
horizontal. The shapes of the isotherms in Figure 1 reflect
that these samples are microporous carbons containing a
high percentage of mesoporosity, since the knees of the
isotherms are not obvious and, above the knees, the quantity of N2 adsorbed still increases significantly with the
increasing relative pressure, indicating the adsorption of
second and higher layers.15 The data show that the adsorption capacity at relative pressures near unity increases
with the pyrolysis temperature in the low temperature
regime (600–800 °C) and reaches a maximum at 800 °C.
Above 800 °C, the capacity decreases with increasing
pyrolysis temperature. However, at low relative pressures,
the 700 °C carbon was found to have the highest capacity, reflecting that the proportion of the pore volume in
small pores is relatively high for this carbon.
To explicitly discuss the influence of the variation in
pyrolysis parameters, the isotherms of N2 adsorption on
different carbons were obtained and employed to deduce
the surface characteristics of these carbons. In addition,
the yield of activated carbon was estimated according to
carbon yield (%) = [Wac/Wtire] × 100
(1)
where Wac and Wtire are the weights of the activated carbon product and the tire precursor, respectively. The effects of the pyrolysis temperature, the chemical ratio of
KOH to tire, and the holding time in pyrolysis were explored, and the data are presented in Tables 3, 4, and 5,
respectively.
The yield of the final carbon products was found to
vary with the pyrolysis parameters. Table 3 shows that
the carbon yield is a decreasing function of the pyrolysis
temperature. The decrease in carbon yield with temperature can mainly be attributed to the increased carbon gasification by CO2 or oxygen in the alkali.18,24 It has been
reported that at high temperatures the release of CO2 from
K2CO3 formed during carbonization becomes significant.24
The evolved CO2 can react with carbon atoms to open up
closed pores and enlarge existing micropores, resulting in
the increase in porosity. Apart from the gasification by
CO2, at a temperature higher than 700 °C, the potassiumcontaining compounds (such as K2O and K2CO3) can be
reduced by carbon to form K metal,25-27 thus causing the
carbon gasification as well as the oxidation. A previous
study has pointed out that alkaline surface salt complexes
can be formed through the interactions between KOH and carTable 2. The surface characteristics of the carbons from pyrolysis of waste tires impregnated with ZnCl2 and H3PO4 at different
a
bonaceous materials at low
temperatures.
temperatures.28 These complexes
are the active sites in gasification.
Pore Size Distribution
Because of the gasification
Pyrolysis
Carbon
BET SA
Pore Vol.
Micropore
Mesopore
Average Pore
Temp. (°C)
Yield (%)
(m2/g)
(cm3/g)
(cm3/g)/(%)
(cm3/g)/(%)
Diam. (nm)
mechanisms, the carbon yield was
found to decrease with the inZnCl2
creases in chemical ratio and in
500
48
129
0.24
0.055/23
0.18/77
7.4
holding time, as shown in Tables
600
46
117
0.24
0.048/20
0.19/80
8.4
4 and 5, respectively. In Table 4,
700
36
85
0.33
0.036/11
0.29/89
15.4
the final carbons were obtained by
carrying out the pyrolysis at a fixed
H3PO4
temperature of 700 °C for zero
500
50
74
0.33
0.030/9
0.30/91
18.0
holding time. A pyrolysis experi600
24
55
0.22
0.022/10
0.20/90
16.1
ment with no KOH added (i.e.,
700
14
37
0.18
0.016/9
0.16/91
18.8
zero chemical ratio) was also cona
The pyrolysis was performed by impregnating the tires with a reagent/tire ratio of 4, followed by heating the sample to the ducted at 700 °C and the results
pyrolysis temperature and holding for 1 hr.
are presented in Table 4. It was
1942 Journal of the Air & Waste Management Association
Volume 50 November 2000
Teng, Lin, and Hsu
formation of a rigid matrix, less
prone to volatile loss upon heating to high temperatures.24 On the
other hand, the low carbon yield
obtained at high chemical ratios
suggests that carbon loss resulting
from gasification should be considered as an important issue in
the pyrolysis of tires impregnated
with KOH.
How the chemical ratio would
affect the amount of ash retained
in the resulting carbons should be
a concern in the application of
these carbons. It has been reported
that ash removal by caustic digestion may be enhanced by an increase in KOH.24 The amounts of
ash retained in the carbons were
evaluated. Figure 2 shows that a
significant amount of ash was removed by the treatment with
KOH, and the extent of demineralization increased with the
chemical ratio, especially for ratios
Figure 1. Adsorption isotherms of N2 on the carbons prepared from pyrolysis of KOH-treated tires at
different temperatures (prior to the pyrolysis, which had a zero holding time at the pyrolysis temperature, less than 4. The ability of caustic
the tires were impregnated with KOH to have a chemical ratio of 4).
digestion seems to reach its limit
at a chemical ratio of 4.
found that the carbon yield from pyrolysis of tires having
As for the variation of surface characteristics with the
a zero chemical ratio is lower than those reported by other
activation parameters, it can be seen from Table 3 that
studies for tire pyrolysis.2-12 This can be attributed to the
the pore volume increases with increasing carbonization
mixing of tires with water at 85 °C, which would remove
temperature to a maximum at 800 °C, and then decreases
some functional groups capable of promoting cross-linkwith further increase of temperature. This was expected
ing reactions facilitating char formation during pyrolysis.29
according to the isotherm data in Figure 1. The increase
The acid-washing process might also remove a significant
in pore volume caused by raising the temperature from
portion of ash to reduce the carbon yield. The table also
600 to 800 °C can be attributed to the enhanced carbon
reflects that the carbon yield can be enhanced by the addigasification. In addition, the metallic K formed at high
tion of an appropriate amount of KOH. This can be attribtemperatures may also intercalate to the carbon matrix,
uted to the dehydrating properties of KOH, which may
resulting in the increase in pore volume.27
promote the formation of cross-links, leading to the
At temperatures higher than 800 °C, the pore volume was found to decrease with
a
Table 3. Effects of pyrolysis temperature on the surface characteristics of the carbons from waste tires impregnated with KOH.
the carbonization temperature.
The decrease can be attributed to
Pore Size Distribution
the severe thermal treatment,
Pyrolysis
Carbon
BET SA
Pore Vol.
Micropore
Mesopore
Average Pore
which causes the breakdown of
2
3
3
3
Temp. (°C)
Yield (%)
(m /g)
(cm /g)
(cm /g)/(%)
(cm /g)/(%)
Diam. (nm)
cross-links in carbon matrix,
with a consequent rearrange600
26
116
0.22
0.051/23
0.17/77
7.7
ment of carbonaceous aggregates
700
16
474
0.38
0.23/60
0.15/40
3.2
and the collapse of pores. It is
800
12
411
0.57
0.19/33
0.38/67
5.5
also possible that the extensive
900
11
306
0.45
0.11/24
0.34/76
5.9
gasification at high temperatures
a
The pyrolysis was performed by impregnating the tires with a KOH/tire ratio of 4, followed by heating the sample to the pyrolysis resulted in the destruction of
pore structures.22
temperature and then cooling.
Volume 50 November 2000
Journal of the Air & Waste Management Association 1943
Teng, Lin, and Hsu
Table 4. Effects of chemical ratio (KOH/tire) on the surface characteristics of the carbons from waste tires impregnated with KOH.a
Tables 4 and 5 also show how
the pore size distribution varied
Pore Size Distribution
with the chemical ratio and the
Chemical
Carbon
BET SA
Pore Vol.
Micropore
Mesopore
Average Pore
holding time. It can be seen that
Ratio
Yield (%)
(m2/g)
(cm3/g)
(cm3/g)/(%)
(cm3/g)/(%)
Diam. (nm)
the increase in the values of
these two parameters results in
0
22
86
0.31
0.040/13
0.27/87
14.4
a decrease in the micropore vol2
25
334
0.33
0.16/48
0.17/52
3.9
ume and a corresponding in4
16
474
0.38
0.23/60
0.15/40
3.2
crease in the mesopore fraction
6
15
367
0.43
0.17/40
0.26/60
4.6
and average pore diameter.
8
10
240
0.44
0.11/25
0.33/75
7.3
Again, these observed trends can
a
be explained by the fact that
The pyrolysis was performed by heating the impregnated sample to 700 °C and then cooling.
carbon gasification was enhanced by the increased KOH
a
impregnation and the extended
Table 5. Effects of pyrolysis holding time on the surface characteristics of the carbons from waste tires impregnated with KOH.
holding time, thus causing the
Pore Size Distribution
removal of carbon atoms on
Holding
Carbon
BET SA
Pore Vol.
Micropore
Mesopore
Average Pore
pore walls, which widened the
2
3
3
3
(cm /g)
(cm /g)/(%)
(cm /g)/(%)
Diam. (nm)
Time (hr)
Yield (%)
(m /g)
micropores.
Figure 3 shows scanning elec0
16
474
0.38
0.23/60
0.15/40
3.2
tron micrographs of the external
1
14
397
0.57
0.19/33
0.38/67
5.7
surface structures of the carbons
2
12
285
0.57
0.080/14
0.49/86
8.0
prepared from different extents
of KOH impregnation. It can be
a
The pyrolysis was performed by impregnating the tires with a chemical ratio of 4, followed by heating the sample to 700 °C and
seen from Figure 3a that the carthen holding for different periods of time.
bon obtained from pyrolysis in
the absence of KOH had an inAs for the variation of SA with pyrolysis temperature,
tact external surface. As the chemical reagent was added
unlike the variation of pore volume, the maximum value
to the tires in pyrolysis, the resulting carbons showed a
occured at 700 °C, rather than at 800 °C. Obviously, extenrugged surface, as indicated in Figures 3b and 3c for
sive gasification to break through pore walls occurred by
chemical ratios of 4 and 6, respectively. The degree of
raising the temperature from 700 to 800 °C, thus resulting
erosion appearing on the external surface increased with
in a decrease in SA and an increase in pore volume.30
the extent of KOH impregnation. Because of the void
In addition to the temperature, increasing the chemistructure, the amount of KOH added in pyrolysis should
cal ratio and holding time also results in enhanced carbe well adjusted to assure the mechanical properties of
bon gasification and, thus, higher porosity development,
the carbons.
as reflected in Tables 4 and 5, respectively. However, the
Although the production of activated carbons from
development of SA does not show an increasing trend
tires using steam or CO2 has been the subject of a large
with increasing gasification. For pyrolysis performed at
number of publications,4-12 results from chemical activa700 °C, a chemical ratio of 4 and a zero holding time would
tion have rarely been reported in the literature. A previgive the highest value for the SA of the resulting carbons.
ous study has provided the results of tire activation with
The results indicate again that severe gasification should
KOH,10 but the influence of different parameters during
be avoided if carbons with high SAs, rather than pore
activation was not examined. The KOH-activated tire carvolumes, are preferred in the process.
bon was reported to have a BET SA and micropore volPore size distribution and average pore diameter are
ume of 820 m2/g and 0.274 cm3/g, respectively. The
significantly affected by the pyrolysis parameters. Table 3
micropore volume found in the previous study is compashows that the mesopore volume generally increases with
rable to those obtained here, whereas the SA is higher.
an increase in pyrolysis temperature. This increase at high
The difference in porosity might come from different chartemperatures can be attributed to the increased extent of
acteristics of the material employed, or from less effective
gasification, which enlarges the existing pores. Because of
surface contact with KOH in the present work.
the increase in the mesopore fraction, a corresponding inPrevious studies4-12 have shown that the porosity
crease in average pore diameter with carbonization temof activated carbons derived from physical activation
perature can be observed.
generally increased with the extent of burn-off in steam
1944 Journal of the Air & Waste Management Association
Volume 50 November 2000
Teng, Lin, and Hsu
gasification, which diminished
SA.30
It has been shown that activated carbons with a specific SA
as high as 1000 m2/g can be produced from tires with physical
activation.10 However, the yields
of the high SA products are generally low. Considering the effectiveness of activation, the total SA
in units of m2/(g tire) is the actual measure of activation efficiency for producing activated
carbons from tires. The maximum value of total SA obtained
in the present study is as high as
83 m2/(g tire), which is comparable to the reported results from
physical activation.4-12 It should
be noted that the hydrophobic
nature of the tire surface restricted
the penetration of KOH into the
rubber matrix. Providing better
surface contact of tires with KOH
is expected to promote efficiency
of activation.
Figure 2. Influence of the chemical ratio of KOH to tire on the amount of ash retained in the carbons
prepared from pyrolysis at 700 °C with zero holding time.
or CO2, and high porosity carbons were obtained with
low yields. This trend of porosity variation with yield
is also obeyed in the case of KOH activation. Tables 3-5
show that the total pore volume of the carbons generally increased with a decrease in the yields. The results
indicate that the extent of carbon removal by gasification predominantly controls pore formation, which is
similar to the situation in physical activation. As for
BET SA, it shows a decreasing trend with a decrease in
yield at high extents of gasification, and this has been
attributed to destruction of pore walls upon severe
(a)
(b)
CONCLUSIONS
This study has demonstrated that activated carbons with
medium porosities can be produced from pyrolysis of
waste tires impregnated with KOH. The pyrolysis process
produces carbons having SAs and pore volumes as high
as 470 m2/g and 0.57 cm3/g, respectively.
The effects of different parameters during pyrolysis were
investigated. The carbon yield was found to decrease with
the pyrolysis temperature, holding time, and KOH/tire ratio. At a KOH/tire ratio of 4 and zero holding time, the SA
of the resulting carbons increased with the pyrolysis
(c)
Figure 3. Scanning electron micrographs of carbons prepared from 700 °C pyrolysis of tires impregnated with KOH at a KOH/tire ratio of (a) 0,
(b) 4, and (c) 6.
Volume 50 November 2000
Journal of the Air & Waste Management Association 1945
Teng, Lin, and Hsu
temperature to a maximum at 700 °C and then began to
decrease with temperature, whereas the pore volume
passed through a maximum at 800 °C. At a fixed temperature of 700 °C and zero holding time, the pore volume of the carbons was found to be an increasing function
of the KOH/tire ratio, while the SA reached a maximum
at a ratio of 4. Under the experimental conditions investigated, the mesopore volume and the average pore diameter generally increased with the pyrolysis temperature,
KOH/tire ratio, and holding time. It is recommended,
based on the results of the present work, that a KOH/ratio
of 4, a pyrolysis temperature of 700 °C, and a zero holding time are the optimum values of the parameters for
producing high SA carbons from pyrolysis of waste tires
impregnated with KOH.
ACKNOWLEDGMENTS
The financial support from the National Science Council
of Taiwan, through Project NSC 89-EPA-Z-006-004, is gratefully acknowledged.
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About the Authors
Hsisheng Teng (corresponding author), Ph.D., is a professor of Chemical Engineering at National Cheng Kung University, Tainan 70101, Taiwan. He can be reached by e-mail
at [email protected] or by fax at 886-6-2344496. At
the time of this research, Yu-Chuan Lin and Li-Yeh Hsu were
research assistants at National Cheng Kung University.
Volume 50 November 2000