Characterization of gaseous and solid product from thermal plasma pyrolysis of waste rubber

Article abstract of DOI:10.1021/es034193c

Pyrolysis of waste rubber in thermal plasma is studied for the purpose of producing gaseous fuel and recovering carbon black filler. The plasma reactor has a dc arc nitrogen plasma generator with a maximum electric power input of 62.5 kVA and a reaction chamber of 50 mm inner diameter and 1000 mm height. The results of a series of experiments have shown that the main components of the gaseous product are H₂, CO, C₂H₂, CH ₄, and C₂H₄; the heat value of the gas is about 5-9 MJ/Nm³. The solid product contains more than 80 wt % elemental carbon, has a surface area of about 65 m²/g, and is referred to as pyrolytic carbon black (CB_p). X-ray photoelectron spectroscopy (XPS) analysis has revealed that the CB_p has mainly graphitic carbon structure similar to those of commercial carbon black. The CB_p may be used as semireinforcing carbon black in nontire rubber applications, or, after upgrading, as carbon black filler for tire. Thus thermal plasma pyrolysis is potentially a useful way of treating waste rubber for resource recovery.

Full text of DOI:10.1021/es034193c

Environ. Sci. Technol. 2003, 37, 4463-4467
Pyrolytic carbon black (CB
waste tires is another potentially marketable product (10-
4). CB is a mixture of the recovered carbon black filler used
p
) from thermal treatment of
Characterization of Gaseous and
Solid Product from Thermal Plasma
Pyrolysis of Waste Rubber
1
p
in the tire manufacturing process and other inorganic tire
components as well as carbonaceous deposits that may have
been formed during the pyrolysis process. Carbon black is
an important industrial material widely used in automotive-
related and other rubber product applications. In view of
this, we study the pyrolysis of waste rubber in a dc arc plasma
reactor with an emphasis on the properties of the solid
product and its potential uses in the present paper.
H . H U A N G , * L A N T A N G , A N D C . Z . W U
Guangzhou Institute of Energy Conversion, Chinese Academy
of Sciences, 81 Xian Lie Zhong Road,
Guangzhou 510070, China
2. Experimental Section
Pyrolysis of waste rubber in thermal plasma is studied for
the purpose of producing gaseous fuel and recovering
carbon black filler. The plasma reactor has a dc arc nitrogen
plasma generator with a maximum electric power input
of 62.5 kVA and a reaction chamber of 50 mm inner diameter
and 1000 mm height. The results of a series of experiments
have shown that the main components of the gaseous
product are H2, CO, C2H2, CH4, and C2H4; the heat value of
2.1. Material. The tire particle sample was provided by
Guangzhou Resource Recycling Company. The particle size
of the sample was from 50 mesh (308 µm) to 80 mesh (180
µm). The proximate, ultimate analyses and heat value of the
sample are summarized in Table 1.
2.2. Plasm a Pyrolysis Experim ent. The experimental
setup shown schematically in Figure 1 consists of two main
parts: dc arc plasma generator and the reaction chamber as
well as some accessories. The details have been described
in ref 8, so only a brief description is given here. The plasma
generator was designed and built by the Department of
Mechanics, Tsinghua University; it has a tungsten cathode
and a water-cooled copper anode. Typical operating condi-
tions are listed in Table 2. The reaction chamber has a 50
mm inner diameter and a 1000 mm height; it is made of
3
the gas is about 5-9 MJ/Nm . The solid product contains
more than 80 wt % elemental carbon, has a surface area of
2
about 65 m /g, and is referred to as pyrolytic carbon
black (CBp). X-ray photoelectron spectroscopy (XPS) analysis
has revealed that the CBp has mainly graphitic carbon
structure similar to those of commercial carbon black. The
CBp may be used as semireinforcing carbon black in
nontire rubber applications, or, after upgrading, as carbon
black filler for tire. Thus thermal plasma pyrolysis is
potentially a useful way of treating waste rubber for resource
recovery.
1
Cr18Ni9Ti steel with internal graphite lining. Running water
was used to cool the system. Sample particles were put in
the reaction chamber by a screw feeder; the particles were
heated by the plasma, and the pyrolysis reaction occurred.
Water steam can be injected into the reaction chamber when
needed. Gaseous product was withdrawn through the
sampling line at the exit of the reactor and collected by rubber
bags and analyzed in a GC-20B-1 gas chromatography system
1. Introduction
(
(
Shimadzu, Japan) equipped with a GC-Carboplot column
30 m × 0.53 mm × 3.0 µm). Solid residues were collected
A large amount of used tires is produced every year. It is
6
estimated that 2.5 × 10 t/ year are generated in the European
in the ash tank.
6
6
Union, 2.5 × 10 t/ year in North America, 1 × 10 t/ year in
2
.3. Analysis of Solid Sam ple. Volatile matter was
6
Japan, and 1 × 10 t/ year in China (1). Disposal means such
estimated from the weight loss of sample at 800 °C in nitrogen
atmosphere; fixed carbon was from the weight loss of sample
at 800 °C in air. Analysis of C, H, O, N, and S elements was
done using a Vario EL element analyzer manufactured by
Elementar Analysis System, Germany. The surface area was
measured by the nitrogen adsorption BET method using an
ASAP 2010 type device manufactured by Micromeritics, U.S.A.
SEM pictures were taken by XL-30ESEM scanning electron
microscopy (Philips, The Netherlands). X-ray photoelectron
spectroscopy (XPS) characterization was done with a PHI-
as combustion, pyrolysis, and gasification have been studied
actively. Among these processes, plasma pyrolysis is a new
technology. It has a number of unique advantages. For
example, it provides conditions of high temperature and high
energy for the reaction; the sample is heated to a high-
temperature rapidly, and reaction velocity is fast. Some
reactions can take place which would not appear in con-
ventional pyrolysis. Laboratory studies of plasma pyrolysis
of coal (2-5), wood (6, 7) ,and plastics (8) have been reported.
However, plasma pyrolysis of waste rubber has not yet been
studied in detail. Chang et al. (9) tested thermal plasma
pyrolysis of old tires and found that the components of the
5
300/ ESCA equipped with an aluminum anode; the analyzer
energy was 100 eV for survey scans (0-1000 eV) and 50 eV
for the detailed analysis of each peak.
2 2 4 2 4 2
produced gas were C H , CH , C H , H , CO, etc., and the
combustion heat value of the produced gas was 4-7 MJ/
Nm , so the gaseous products were considered as useful fuels.
3. Results and Discussion
3
Nevertheless, the gas product value did not appear sufficiently
high to give an economically viable operation; other high-
value products were required if the conversion process is
not to be heavily subsidized.
3.1. Gaseous and Solid Yields. Experimental conditions and
product yields are given in Table 3. It can be seen that as the
input power/ feed rate ratio is increased, the gas yield
increases, and the solid yield decreases. The main gas
2 2 2 4 2 4
components are H , CO, C H , CH , C H , etc.; the heat value
3
of the gas ranges from 5.3 to 7.9 MJ/ Nm , in agreement with
*
Corresponding author phone: 86-20-87696675; fax: 86-20-
8
7608586; e-mail: huanght@ms.giec.ac.cn.
the report by Chang et al. (9).
1
0.1021/es034193c CCC: $25.00

2003 Am erican Chem ical Society
VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4 4 6 3
Published on Web 08/30/2003

TABLE 1. Analysis of Waste Tire Sample
proximate analysis (wt %)
ultimate analysis (wt % dried)
calorific
H/C
material
moisture
volatile
fixed carbon
ash
C
H
O
N
S
value (kJ/kg)
atomic ratio
tire particle
0.93
63.67
26.43
8.97
80.5
7.33
10.27
0.33
1.57
37328
1.09
FIGURE 1. Schematic of the dc arc plasma reactor: 1. plasma generator, 2. control system of plasma generator, 3. plasma reaction chamber,
. control system of screw feeder, 5. feed hopper, 6. gas sampling system, 7. ash tank, 8. cyclone separator, 9. filter, 10. water tank, 11.
cooling system, 12. observational port, 13. water steam generator, 14. nitrogen gas cylinder.
4
indicate that there are some chemical reactions occurring
during plasma pyrolysis when injecting water steam as
discussed later.
TABLE 2. Typical Operating Conditions of the dc Arc Plasma
Reactor
plasma generator parameter
range
optimum
In Table 3 the solid yield from run No. F1 to P4 is in the
range 39.4-69 wt % of feed, which is much higher than the
values expected from concentration of the virgin carbon black
in tires (20-30% typically). Besides the inorganic tire
component and some amount of carbonaceous deposits on
the carbon black surface that were formed during the
pyrolysis process, there might be a certain amount of tire
particles unreacted when the power input/ feed rate ratio is
low (in particular, run No. F4). When injecting water steam
voltage (V)
current (A)
power (kVA)
220-250
220
160
35.2
5
0.4
2
120-250
26.4-62.5
3
working gas (N2) flow rate (Nm /h)
working gas pressure (MPa)
5
0.4
2
3
carrier gas (N2) flow rate (Nm /h)
sam ple powder feed rate (g/m in)
water steam pressure (MPa)
20-100
0.4
0.4
(
run No. S1), the solid yield is 23 wt %, which is less than the
concentration value of the virgin carbon black and inorganic
components in tires; a portion of the carbon black might
have been oxidized in this case.
In run No. S1, water steam was injected into the plasma
reaction chamber together with the tire particles in order to
improve product quality and get syngas so that the range of
application can be extended. With water steam injection,
Similar to plasma pyrolysis of coal (5), a reaction scheme
for rubber pyrolysis in dc arc plasma may be described. Under
conditions of the high temperature in the plasma, primary
pyrolysis reactions take place; the volatile matter is released
including heavy hydrocarbons (tar), light hydrocarbons, and
the amounts of H
2
and CO were increased significantly; the
sum of H and CO concentrations reached up to about 38%.
2
Similar trends have also been found when testing polypro-
pylene pyrolysis with steam injection (8). These changes
TABLE 3. Products from dc Arc Plasma Pyrolysis of Waste Rubbera
gas composition (balance N2)
power power input/
solid
yield
(wt %) (wt %)
gas
yield
(vol %, dry basis)
gas calorific
value
(MJ/Nm )
feed rate
(g/min)
input
feed rate
ratio
3
no.
(kVA)
H2
CH4
CO
C2H4
C2H2
CnHm + unknown
F1
F2
F3
F4
P1
P2
P3
P4
S1
44.04
89.1
96.4
122.5
78.06
75.36
86.6
80.04
75
35.2
35.2
35.2
35.2
30.8
39.6
44
0.8
0.4
39.4
57.8
59.02
69.02
61.32
55.7
56.9
55
60.6
42.2
40.98
30.98
39.68
44.3
43.1
45
8.75
14.2
0.71
1.02
0.85
1.01
0.71
0.6
0.67
0.69
0.98
3.07
3.21
3.92
3.27
2.75
4.2
4.13
4.25
14.17
0.28
0.54
0.38
0.54
0.25
0.2
0.25
0.27
0.41
2.04
3.92
2.76
3.6
2.57
1.57
1.54
1.42
1.75
4.57
5.55
5.7
5.1
3.6
5.3
7.56
7.34
7.94
6.76
6.01
6.48
6.12
8.96
0.37
0.29
0.4
0.53
0.51
0.61
0.47
18.38
18.54
12.07
15.23
15.77
16.15
24.12
6.2
6.34
5.83
6.2
48.4
35.2
23
77
a
Note: F1-F4 ) test with different feed rate, P1-P4 ) test with different input power, S1 ) test with addition of steam (80 m L/m in).
4
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 19, 2003

FIGURE 2. SEM pictures of solid product sample (left: CBp1, right: CBp2).
TABLE 4. Characteristics of Commercial Carbon Black
TABLE 6. Elemental Composition and Ash Content of Solid
Samplea
ASTM
designation
mean particle
diameter (nm)
mean aggregate
size (nm)
surface
area (m /g)
2
ultimate analysis (wt % dried)
calorific H/C
value atomic
ash
N110
N330
N351
N550
N774
27
46
50
93
124
93
146
159
240
265
143
80
75
43
28
material
C
H
O
N
S
(wt %) (kJ/kg) ratio
CBp1
CBp2
82.69 0.42 13.9 0.42 2.57 15.14 28703 0.061
85.06 0.24 12.35 0.38 1.97 16.25 28565 0.034
com m ercial tire 97.1 0.2
carbon black
1.1 0.2 1.0
0.4
0.025
a
CBp1 is sam ple from run No. F2; CBp2 is sam ple from run No. S1.
TABLE 5. Surface Area and Sulfur Content of Solid Product
Sample
TABLE 7. XPS Surface Composition (Atom %) of Solid Product
Sample
surface area
sulfur
(wt % dried) no.
surface area
sulfur
(wt % dried)
2
2
no.
(m /g)
(m /g)
C
O
S
Zn
F2
P1
64.8
61
2.57
2.55
P4
S1
66
70
2.7
1.97
CBp1
CBp2
93.82
91.13
4.79
7.18
0.44
0.47
0.95
1.22
other gaseous components, leaving behind solid char:
lower carbon content and a larger proportion of ash and
impurities than commercial carbon black. Pyrolytic carbon
black with such a high ash content may be used as a
semireinforcing agent for nontire rubber applications. Only
after further treatment and upgrading can it be used as tire
carbon black. For example, about half of the inorganics could
be removed from pyrolytic carbon black by a simple acid
wash according to Piskorz et al. (10).
3.2.2. Particle Morphology and Surface Chemistry. SEM
pictures of the pyrolytic carbon black are presented as Figure
2. Significant aggregate formation can be seen as in com-
mercial carbon black (10). XPS analysis is used to identify
the surface chemistry and carry out a quantitative analysis.
Wide-scan spectra in the binding energy (BE) range 0-1000
eV are shown in Figure 3. The spectra are dominated by the
rubber f char + heavy hydrocarbons +
light hydrocarbons +
gas (H , CO, CH , C H , C H , etc.)
2
4
2
2
2
4
Further cracking of the tar then occurs:
heavy hydrocarbons f light hydrocarbons +
gas (H , CH , C H , C H , etc.)
2
4
2
2
2
4
Light hydrocarbons may also decompose:
light hydrocarbons f H + CH + C H + C H +
2
4
2
2
2
4
C H
n
m
With addition of water steam, the following overall reaction
is likely important
C1s photoelectron peak, representing the major constituent
of the pyrolytic carbon black. Other smaller signals confirm
that oxygen and sulfur are present in the surface region, and
weak peaks of other elements such as zinc can be found.
Table 7 gives the surface elemental composition of the
samples from XPS analysis.
Detailed scans of C1s, O1s, and S2p are then performed.
The C1s spectra in Figure 4 show an intense peak of graphitic
carbon. Graphitization degree is indicated by the full width
at half-maximum (fwhm) of the C1s peak; the more complete
the graphitic structure is, the smaller the fwhm of the C1s
peak will be (14). The fwhm of the C1s peak of sample CBp1
is about 2.1 eV, and that of CBp2 is about 1.9 eV, indicating
that pyrolytic carbon black obtained from plasma pyrolysis
with steam injection has a more complete graphitic structure.
Probably, aliphatic or adsorbed hydrocarbon compounds
char + H O f CO + H + solid residue
2
2
which could be one of the reasons for the observation that
and CO concentrations were increased, and solid yield
H
2
decreased with steam injection (Table 3). The solid residue
includes the inorganic tire component, carbon black filler,
and carbonaceous deposit. Plasma pyrolysis is a complex
process involving a large number of physical and chemical
factors, so detailed analysis of the chemical reaction kinetics
is needed to give a more accurate description.
3
.2. Properties of Pyrolytic Carbon Black. 3.2.1. Elemental
Composition and Surface Area. Commercial carbon blacks
used as reinforcing agents for rubber are commonly prepared
in a number of standard ASTM grades shown in Table 4. Our
analysis results of solid residue sample from plasma pyrolysis
of tire particles are presented in Tables 5 and 6. It can be
seen that the sample has surface area comparable with those
of medium-grade commercial carbon black, though it has a
p
on the CB surface can react with steam and thus are oxidized/
removed, resulting in a higher graphitization degree of CB
p
.
In the O1s narrow-scan spectra of the samples (O1s and S2p
narrow-scan spectra are not shown in this paper), an O1s
peak was detected at BE ) 532.5 eV, suggesting that most
VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. XPS wide scan spectra of solid product sample.
FIGURE 4. XPS narrow scan of C1s peak.
oxygen is present in the carbonyl group, in agreement with
the C1s results. In the narrow scan of the S2p region, the peak
processing rubber waste 300 kg/ h is $1 500 000; (2) specific
energy consumption is 1 kWh/ kg rubber feed (for industrial-
scale plasma systems, 0.8-1 kWh/ kg has been reported (16));
(3) the electricity price for industrial sector is $0.05/ kWh; (4)
carbon black recovery is 23 wt % of rubber feed; market price
for semireinforcing carbon black is $500/ ton; (5) the gate fee
for receiving rubber waste is $30/ ton; and (6) gas yield is 3
Nm / kg rubber feed, with a calorific value 9 MJ/ Nm ; the gas
is combusted in a boiler or gas engine for power generation
at efficiency 26%.
position is about 162.9 eV for CBp1 and 162.7 eV for CBp2
.
Darmstadt et al. (13) have estimated that for sulfide, BE )
1
)
)
62.1 ( 0.2 eV; for organic sulfur not bound to oxygen, BE
164 ( 0.1 eV; for organic sulfur bound to one oxygen, BE
166.1 eV. In our sample, most sulfur is likely present as
3
3
sulfide such as ZnS.
.3. Process Application Potential. The above results
3
show that plasma pyrolysis of waste rubber gives two product
streams: combustible gas and pyrolytic carbon black; both
are valuable, easy-to-handle products. In comparison, con-
ventional pyrolysis of waste rubber usually leads to gas, liquid,
and solid products (11, 12, 15). The liquid product is a tarry
oil consisting of a variety of heavy hydrocarbon compounds,
and separation and collection of the oil from other gas and
solid products are difficult.
This paper reports only a lab-scale investigation. Industrial
applications of plasma process are dependent on economic
factors. Our preliminary analysis indicates that plasma
pyrolysis of rubber waste has economic potential, given the
following assumptions: (1) capital investment for a plant
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