Environ. Sci. Technol. 2003, 37, 4463-4467
Characterization of Gaseous and Solid Product from Thermal Plasma Pyrolysis of Waste Rubber H. HUANG,* LAN TANG, AND C. Z. WU Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, 81 Xian Lie Zhong Road, Guangzhou 510070, China
Pyrolytic carbon black (CBp) from thermal treatment of waste tires is another potentially marketable product (1014). CBp is a mixture of the recovered carbon black filler used 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 automotiverelated 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.
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 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 (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. Introduction A large amount of used tires is produced every year. It is estimated that 2.5 × 106 t/year are generated in the European Union, 2.5 × 106 t/year in North America, 1 × 106 t/year in Japan, and 1 × 106 t/year in China (1). Disposal means such 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 hightemperature rapidly, and reaction velocity is fast. Some reactions can take place which would not appear in conventional 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 produced gas were C2H2, CH4, C2H4, H2, CO, etc., and the combustion heat value of the produced gas was 4-7 MJ/ Nm3, so the gaseous products were considered as useful fuels. Nevertheless, the gas product value did not appear sufficiently high to give an economically viable operation; other highvalue products were required if the conversion process is not to be heavily subsidized. * Corresponding author phone: 86-20-87696675; fax: 86-2087608586; e-mail:
[email protected]. 10.1021/es034193c CCC: $25.00 Published on Web 08/30/2003
2003 American Chemical Society
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. Plasma Pyrolysis Experiment. 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 conditions are listed in Table 2. The reaction chamber has a 50 mm inner diameter and a 1000 mm height; it is made of 1Cr18Ni9Ti 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 (Shimadzu, Japan) equipped with a GC-Carboplot column (30 m × 0.53 mm × 3.0 µm). Solid residues were collected in the ash tank. 2.3. Analysis of Solid Sample. Volatile matter was 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 PHI5300/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.
3. Results and Discussion 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 components are H2, CO, C2H2, CH4, C2H4, etc.; the heat value of the gas ranges from 5.3 to 7.9 MJ/Nm3, in agreement with the report by Chang et al. (9). VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Analysis of Waste Tire Sample material
moisture
tire particle
0.93
proximate analysis (wt %) volatile fixed carbon 63.67
26.43
ash
C
8.97
80.5
ultimate analysis (wt % dried) H O N 7.33
10.27
0.33
S
calorific value (kJ/kg)
H/C atomic ratio
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, 4. 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.
TABLE 2. Typical Operating Conditions of the dc Arc Plasma Reactor plasma generator parameter
range
optimum
voltage (V) current (A) power (kVA) working gas (N2) flow rate (Nm3/h) working gas pressure (MPa) carrier gas (N2) flow rate (Nm3/h) sample powder feed rate (g/min) water steam pressure (MPa)
220-250 120-250 26.4-62.5 5 0.4 2 20-100 0.4
220 160 35.2 5 0.4 2 0.4
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, the amounts of H2 and CO were increased significantly; the sum of H2 and CO concentrations reached up to about 38%. Similar trends have also been found when testing polypropylene pyrolysis with steam injection (8). These changes
indicate that there are some chemical reactions occurring during plasma pyrolysis when injecting water steam as discussed later. 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 (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. 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
TABLE 3. Products from dc Arc Plasma Pyrolysis of Waste Rubbera gas composition (balance N2) (vol %, dry basis)
no.
feed rate (g/min)
power input (kVA)
power input/ feed rate ratio
solid yield (wt %)
gas yield (wt %)
H2
CH4
CO
C2H4
C2H2
CnHm + unknown
gas calorific value (MJ/Nm3)
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 48.4 35.2
0.8 0.4 0.37 0.29 0.4 0.53 0.51 0.61 0.47
39.4 57.8 59.02 69.02 61.32 55.7 56.9 55 23
60.6 42.2 40.98 30.98 39.68 44.3 43.1 45 77
8.75 14.2 18.38 18.54 12.07 15.23 15.77 16.15 24.12
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 6.2 6.34 5.83 6.2
5.3 7.56 7.34 7.94 6.76 6.01 6.48 6.12 8.96
a
Note: F1-F4 ) test with different feed rate, P1-P4 ) test with different input power, S1 ) test with addition of steam (80 mL/min).
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FIGURE 2. SEM pictures of solid product sample (left: CBp1, right: CBp2).
TABLE 4. Characteristics of Commercial Carbon Black ASTM designation
mean particle diameter (nm)
mean aggregate size (nm)
surface area (m2/g)
N110 N330 N351 N550 N774
27 46 50 93 124
93 146 159 240 265
143 80 75 43 28
TABLE 6. Elemental Composition and Ash Content of Solid Samplea ultimate analysis (wt % dried) material
no.
surface area (m2/g)
sulfur (wt % dried)
F2 P1
64.8 61
2.57 2.55
no.
surface area (m2/g)
sulfur (wt % dried)
P4 S1
66 70
2.7 1.97
other gaseous components, leaving behind solid char:
rubber f char + heavy hydrocarbons + light hydrocarbons + gas (H2, CO, CH4, C2H2, C2H4, etc.) Further cracking of the tar then occurs:
heavy hydrocarbons f light hydrocarbons + gas (H2, CH4, C2H2, C2H4, etc.) Light hydrocarbons may also decompose:
light hydrocarbons f H2 + CH4 + C2H2 + C2H4 + CnHm With addition of water steam, the following overall reaction is likely important
char + H2O f CO + H2 + solid residue which could be one of the reasons for the observation that H2 and CO concentrations were increased, and solid yield 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
H
O
N
S
CBp1 82.69 0.42 13.9 0.42 2.57 15.14 28703 0.061 CBp2 85.06 0.24 12.35 0.38 1.97 16.25 28565 0.034 commercial tire 97.1 0.2 1.1 0.2 1.0 0.4 0.025 carbon black a
TABLE 5. Surface Area and Sulfur Content of Solid Product Sample
C
calorific H/C ash value atomic (wt %) (kJ/kg) ratio
CBp1 is sample from run No. F2; CBp2 is sample from run No. S1.
TABLE 7. XPS Surface Composition (Atom %) of Solid Product Sample CBp1 CBp2
C
O
S
Zn
93.82 91.13
4.79 7.18
0.44 0.47
0.95 1.22
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 commercial 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 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 on the CBp surface can react with steam and thus are oxidized/ removed, resulting in a higher graphitization degree of CBp. 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 position is about 162.9 eV for CBp1 and 162.7 eV for CBp2. Darmstadt et al. (13) have estimated that for sulfide, BE ) 162.1 ( 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 sulfide such as ZnS. 3.3. Process Application Potential. The above results 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, conventional 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 4466
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processing rubber waste 300 kg/h is $1 500 000; (2) specific energy consumption is 1 kWh/kg rubber feed (for industrialscale 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 Nm3/kg rubber feed, with a calorific value 9 MJ/Nm3; the gas is combusted in a boiler or gas engine for power generation at efficiency 26%.
Literature Cited (1) Shulman, V. L. The Recycling in the EU Member States; Spanish Waste Management Association: Madrid, 2000. (2) Dlugogorski, B. Z.; Berk, D.; Munz, R. J. Treatment of pitch in argon/hydrogen plasmas. Ind. Eng. Chem. Res. 1992, 31, 818827. (3) Georgiev, I. B.; Mihailov, B. I. Some general conclusions from the results of studies of solid fuel steam plasma gasification. Fuel 1992, 71, 895-901. (4) Dai, B.; Fan, Y.; Yang, J.; Xiao, J. Effect of radicals recombination on acetylene yield in process of coal pyrolysis by hydrogen plasma. Chem. Eng. Sci. 1999, 54, 957-959.
(5) Baumann, H.; Bittner, D.; Beiers, H. G.; Klein, J.; Juntgen, H. Pyrolysis of coal in hydrogen and helium plasmas. Fuel 1988, 67, 1120-1123. (6) Brown, D. S. Plasma arc reduction of biomass for production of synthetic fuel gas. Preprint Paper Div. Petrol. Chem., Am. Chem. Soc. 1979, 24, 429-431. (7) Zhao, Z.; Huang, H.; Wu, C.; Li, H.; Chen, Y. Biomass pyrolysis in an argon/hydrogen plasma reactor. Chem. Eng. Technol. 2001, 24, 197-199. (8) Tang, L.; Huang, H.; Zhao, Z.; Wu, C.; Chen, Y. Pyrolysis of polypropylene in a nitrogen plasma reactor. Ind. Eng. Chem. Res. 2003, 42, 1145-1150. (9) Chang, J. S.; Gu, B. W.; Looy, P. C.; Chu, F. Y.; Simpson, C. J. Thermal plasma pyrolysis of used old tires for production of syngas. J. Environ. Sci. Health A 1996, 31, 1781-1799. (10) Piskorz, J.; Majerski, P.; Radlein, D.; Wik, T.; Scott, D. S. Recovery of carbon black from scrap rubber. Energy Fuels 1999, 13, 544551. (11) Roy, C.; Chaala, A.; Darmstadt, H. The vacuum pyrolysis of used tires: end-uses for oil and carbon black products, J. Anal. Appl. Pyrolysis 1999, 51, 201-221.
(12) de Marco Rodriguez, I.; Laresgoiti, M. F.; Cabrero, M. A.; Torres, A.; Chomo´n, M. J.; Caballero, B. Pyrolysis of scrap tires. Fuel Process. Technol. 2001, 72, 9-22. (13) Darmstadt, H.; Roy, C.; Kaliaguine, S. Characterization of pyrolytic carbon blacks from commercial tire pyrolysis plants. Carbon 1995, 33, 1449-1455. (14) Darmstadt, H.; Chaala, A.; Roy, C.; Kaliaguine, S. SIMS and ESCA characterization of bitumen reinforced with pyrolytic carbon black. Fuel 1996, 75, 125-132. (15) Yin, X. L.; Zhao, Z. L.; Xu, B. Y.; Wu, C. Z.; Chen, Y. Effect of dolomite and limestone on waste tire pyrolysis. J. Fuel. Chem. Technol. 2001, 29, 283-285 (in Chinese). (16) Rutberg, P. G. Plasma pyrolysis of toxic waste. Plasma Sources Sci. Technol. 2002, 11, A159-A165.
Received for review March 5, 2003. Revised manuscript received July 16, 2003. Accepted July 30, 2003. ES034193C
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