Influence of Process Conditions on the Rate of Activation of Chars

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Energy & Fuels 1999, 13, 166-175

Influence of Process Conditions on the Rate of Activation of Chars Derived from Pyrolysis of Used Tires Adrian M. Cunliffe and Paul T. Williams* Department of Fuel and Energy, The University of Leeds, Leeds, LS2 9JT, U.K. Received July 1, 1998. Revised Manuscript Received October 8, 1998

Used tires were pyrolyzed in a nitrogen-purged 3 kg static bed, batch reactor. The char was subsequently activated in an equimolar mixture of either steam and nitrogen or carbon dioxide and nitrogen. Activation was a two-stage process, with an initial more rapid gasification of carbonized rubber deposits followed by less rapid gasification of carbon black. The activation energy in steam was 201 kJ mol-1. The burnoff achieved by carbon dioxide under otherwise identical conditions to that of steam was on average 72% of that found when steam was used. The reactivity of acid-demineralized char in steam was around 22% less than that of raw char. It was suggested that calcium ions in the raw tire char catalyzed the gasification reactions. The tire pyrolysis temperature had an unclear influence on the rate of activation of the derived chars. The influence of particle size on the rate of activation was also investigated, and the results are discussed in terms of ash content and distribution. The outflow gas composition and gross calorific value of the gas were also determined in relation to activation conditions. BET surface areas of the derived activated carbons reached a maximum of 640 m2 g-1.

Introduction Approximately 180 million used tires are produced in the European Union each year and 150 million in North America, representing an enormous treatment and disposal problem.1 The main disposal route in both the European Union and North America is a landfill, however, this route is being discouraged because it is seen as a waste of a resource. In addition, the minimal degradation, high buoyancy, and high bulk of tires make landfilling a less than satisfactory option.1 Illegal open dumping or surface storage may result in accidental fires, which not only release high amounts of environmentally harmful pollutants but which are also extremely difficult to extinguish. Technical and economic concerns mean that reuse options such as retreading tire carcasses and the production of reclaimed rubber are likely to have reached market saturation.2 Other treatment and disposal routes include the production of rubber crumb for use in sports surfaces and road asphalt and rubber reclaim via devulcanization under high temperature and pressure to produce a rubber which can be used in low-grade applications such as bicycle tires, conveyor belts, and footwear. Incineration with energy recovery is gaining favor as a disposal option, although the combustion of tires presents a variety of obstacles, such as snagging of the grate by the steel wires and the expense of meeting pollution legislation. The application of pyrolysis as a means of reusing scrap tires has recently been the subject of renewed interest. Pyrolysis of tires produces an oil, char, and gas (1) Williams P. T. Waste Treatment and Disposal; John Wiley & Sons: Chichester, 1998. (2) Dufton, P. Scrap Tires- A Wasting Asset. RAPRA Technology Report; RAPRA Technology Ltd.: Shrewsbury, U.K., 1991.

product, in addition to the steel cord, all of which have the potential to be recycled. Tire-derived pyrolysis oils may be combusted directly or added to petroleumderived fuels.3-6 Valuable compounds used in the chemical industry, such as benzene, toluene, and limonene, are also present in high concentrations in the oils.3,4,7-9 The pyrolysis gases can be used to provide the energy requirements of the pyrolysis process. The solid char has potential as a solid fuel or as a low-grade carbon black.7,10,11 However, activation of the char to produce a high surface area activated carbon may represent a more economically attractive option.10,12 Commercially available activated carbons are highly porous carbonaceous (3) Williams, P. T.; Besler, S.; Taylor, D. T. Proc. Inst. Mech. Eng. 1993, 207, 55-63. (4) Cunliffe, A.; Williams P. T. J. Anal. Appl. Pyrolysis 1998, 44, 131-152. (5) Roy, C.; Unsworth, J. Pilot plant demonstration of used tires vacuum pyrolysis. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Bridgwater, A. V., Eds.; Elsevier Applied Science: London and New York, 1989. (6) Benallal, B.; Pakdel, H.; Chabot, S.; Roy, C. Fuel 1995, 74, 15891596. (7) Wolfson, D. E.; Beckman, J. A.; Walters, J. G. Destructive Distillation of Scrap Tires. U.S. Department of the Interior Bureau of Mines Report of Investigations 7302, 1969. (8) Kaminsky, W.; Sinn, H. Pyrolysis of plastic waste and scrap tires using a fluidised bed process. In Thermal Conversion of Solid Wastes and Biomass; Jones, J. L., Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980. (9) Pakdel, H.; Roy, C.; Aubin, H.; Jean, G.; Coulombe, S. Environ. Sci. Technol. 1992, 25, 1646-1649. (10) Cypres, R.; Bettens, B. Production of benzoles and active carbon from waste rubber and plastic materials by means of pyrolysis with simultaneous post-cracking. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Bridgwater, A. V., Eds.; Elsevier Applied Science: London and New York, 1989. (11) Kawakami, S.; Inoue, K.; Tanaka, H.; Sakai, T. Pyrolysis process for scrap tires. In Thermal conversion of solid wastes and biomass; Jones, J. L., Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980.

10.1021/ef9801524 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/20/1998

Chars Derived from Pyrolysis of Used Tires

Energy & Fuels, Vol. 13, No. 1, 1999 167

Table 1. Typical Composition of Scrap Tire Feedstock Rubber elemental composition proximate analysis gross calorific value (%) (%) (mJ kg-1) C 86.4 H 8.0 N 0.5 S 1.7 O 3.4 (Ash 2.4)

volatiles 62.2 fixed carbon 29.4 ash 7.1 moisture 1.3

40.0

materials with a high BET (Brunauer, Emmett, and Teller13) surface area, typically in the range from 400 to 1500 m2 g-1.14 These properties mean that activated carbon is an excellent adsorbent and is commonly used to remove pollutants from gas or liquid streams. The most commonly used feedstocks in the production of activated carbon are wood, coal, lignite, coconut husks, and peat, respectively.14 Several studies have shown that the activation of tire char can result in a product with a comparable surface area to commercially available activated carbons.12,15-19 Similarly, activated tire chars have exhibited promising wastewater remediation performance12,15 and could also have potential as a storage medium for methane.19 However, most studies of the activation of tire char have concentrated on characterizing the activated products rather than on the factors which affect the rate of activation. In this paper, tire chars were generated from the pyrolysis of used tires and their properties characterized. The influence of time, temperature, activating agent, acid demineralization, particle size, and pyrolysis temperature on the rate of tire char gasification were investigated to allow comparison with the behavior of other chars, most notably those derived from coal. Experimental Section Tire Samples. Shredded used tires in narrow strips approximately 3 cm wide × 1.5 cm thick and of between 50 and 150 cm maximum dimension retaining both steel and fabric cords were kept in dry conditions prior to pyrolysis. The tires used in this work represented a mixture of used passengercar tires. Cypres and Bettens10 pyrolyzed different makes and brands of tires and found small but significant differences (of the order of 10%) in the yield of char, oil, and gaseous products. The proximate and ultimate analyses, on a steel- and fabricfree basis, and the calorific value of the raw tire tread used are given in Table 1. Pyrolysis Unit. The tires were pyrolyzed in a stainless steel, static bed, batch reactor of inner diameter 24 cm and depth 36 cm with a feedstock loading of 3 kg, as shown in (12) Bilitewski, B.; Ha¨rdtle, G.; Marek, K. Usage of carbon black and activated carbon in relation to input and technical aspects of the pyrolysis process. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Bridgwater, A. V., Eds.; Elsevier Applied Science: London and New York, 1989. (13) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-319. (14) Bansal, R. C.; Donnet, J.-B.; Stoeckli, F. Active Carbon; Marcel Dekker Inc.: New York and Basel, 1988. (15) Giavarini, C. Fuel 1985, 64, 1331-1332. (16) Ogasawara, S.; Kuroda, M.; Wakao, N. Ind. Eng. Chem. Res. 1987, 26, 2552-2556. (17) Teng, H.; Serio, M. A.; Wojtowicz, M. A.; Bassilakis, R.; Solomon, P. R. Ind. Eng. Chem. Res. 1995, 34, 3102-3111. (18) Merchant, A. A.; Petrich, M. A. Am. I. Chem. Eng. J. 1993, 39, 1370-1376. (19) Sun, J.; Brady, T. A.; Rood, M. J.; Lehmann, C. M.; RostamAbadi, M.; Lizzio, A. A. Energy Fuels 1997, 11, 316-322.

Figure 1. Schematic diagram of the tire pyrolysis rig. Figure 1. The reactor was externally electrically heated, with the heating regime being electronically controlled. The heating rate was approximately 5 °C min-1. Purge gas distributor rings were positioned in the base and upper part of the reactor. The purge gas was nitrogen and was preheated before entering the reactor to 400 °C. The use of an inert purge gas was to minimize the residence time of the pyrolysis vapors in the hot zone to reduce the extent of any secondary reactions. In addition, the purge gas increased the gas mixing within the reactor. Gases were exited via a downtube to a water-cooled condenser of mesh-screen design and passed to a second watercooled condenser which also condensed oils back to the first condenser. A tertiary water-cooled condenser trapped the remaining oil. The heavier molecular weight pyrolysis oils were collected in the first and second condenser and lighter oils in the third. However, it was found that approximately 95 wt % of the oils were collected in the first condenser. A gas sampling point was positioned just after the third condenser, allowing gas samples to be taken via gas syringes for off-line analysis by packed-column gas chromatography. Batches of 3 kg of tires, including the steel cord, were loaded into the unit in horizontal layers, and the reactor was purged with nitrogen. The maximum residence time of the gases in the reactor was calculated as approximately 120 s. An initial heater ramp temperature of 150 °C was set in each case, followed by final temperatures of 450, 475, 500, 525, 560, and 600 °C measured in the tire stock. Once the temperature had stabilized near to the final set point, the system was held at this temperature for 90 min. Temperatures throughout the reactor were taken at timed intervals. The oils collected from the condenser system were centrifuged at 3000 rpm for 15 min to remove any water and sediment present prior to all analyses, except for those samples used for water-content determinations. The oil samples were stored under refrigerated conditions prior to analysis. Detailed analysis of the oils and gases and the methods used for their determination for these experiments have been reported previously.4 The residual char was removed from the reactor at the end of each experiment, separated into char and steel using magnetic separation, and weighed. The char was characterized using the following standard American Society of Testing Materials (ASTM) methods: proximate analysis ASTM D31735; elemental analysis ASTM D3178; sulfur content ASTM D3177; and light transmission of toluene extract ASTM D1618. The raw chars which were derived directly from the pyrolysis of the tires were activated using a 200 cm3 internal capacity stainless steel reactor shown in Figure 2. The unit was again of static-bed batch design and was heated by an electric ring furnace. Prior to activation, the char was sieved to generate a 1.4-5.6 mm size fraction and dried at 125 °C for 1 h. The sized

168 Energy & Fuels, Vol. 13, No. 1, 1999

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Figure 3. Yield of tire pyrolysis products. Figure 2. Schematic diagram of the char activation rig. char (25 g) was placed in the reactor, and nitrogen was passed through the unit in order to purge air from the system. The sample was heated at 20 °C min-1 to final temperatures of 835, 865, 900, and 935 °C. After the sample temperature had stabilized, either steam or carbon dioxide was introduced to the nitrogen stream, mixing in the distributor tube before entering the reactor. The activating agent flow rates were 8 L h-1 carbon dioxide and 6.43 mL h-1 deionized water, respectively, at ambient conditions, which gave an equimolar activating-agent/nitrogen mixture. The activating-agent molar flow rate per unit weight of char was 0.0143 mol g-1 h-1. Samples of the outflow gas were taken in syringes at regular time intervals and analyzed using off-line gas chromatography. Activation time ranged from 0.5 to 11.5 h, which achieved increasing degrees of burnoff, after which the char was left to cool overnight under nitrogen. The degree of carbon loss (burnoff) achieved was calculated from:

burnoff wt % daf )

(

)

w1 - w2 × 100 w1

where w1 ) initial char mass on a dry ash free (daf) basis, g, and w2 ) mass of char remaining after activation on daf basis, g. Only char generated at a final pyrolysis temperature of 450 °C was activated, since it was found that above 450 °C the temperature has a limited influence on the char characteristics (see later). Nitrogen adsorption experiments were conducted to determine the surface area of the chars and activated carbons using a Quantachrome Corp. Quantasorb instrument. The surface area was determined using the nitrogen adsorption method of Brunauer, Emmett, and Teller (BET).13 Porosity data was also determined on the chars and activated carbons, including mesopore volume, mesopore surface area, mesopore size distribution, and micropore volume, which has been reported previously.20 It was found that because of the high ash contents of the derived activated carbons, there were some problems of reproducibility for surface area determination. Consequently, each activated carbon was demineralized after activation to remove the high ash content. After activation, ash contents were, in some cases, in the region of 20-25 wt %, and in order to reduce this, activated chars were crushed and heated in 5 (20) Cunliffe, A. M.; Williams, P. T. Properties of chars and activated carbons derived from the pyrolysis of used tires, submitted to Environ. Technol. 1998.

M hydrochloric acid to the boiling point for 90 min, washed, dried, and sieved to give a 180-212 µm size fraction for the BET determination. In addition, in a certain set of experiments, the char was demineralized before activation to determine the effects of deashing on the rate of activation. In this case, the demineralization procedure was as before, using 5 M hydrochloric acid at its boiling point for 90 min. The gases from the activation process were sampled periodically throughout the experiment using gas syringes and analyzed off-line by packed-column gas chromatography. The gases were analyzed for CO, H2, CH4, and O2 using a molecular sieve SA 60-80 column with Ar as the carrier gas and a thermal conductivity detector. Nitrogen, which was the carrier gas used in the reactor, was also determined on this column, and the volumetric flow rates of all the derived gases were calculated by comparison with the nitrogen flow rate. CO2 was determined separately using a silica-gel column, argon as the carrier gas, and a thermal conductivity detector, and gaseous hydrocarbons up to C5 were determined on a Porosil C 80100 column with nitrogen as the carrier gas and a flame ionization detector. The total mass of gas evolved was calculated from the measured concentrations and molecular masses of the gases and compared well with the gaseous product yield calculated by difference.

Results and Discussion Product Yields. The product yields of oil, char, and gases in relation to process conditions in the batch reactor are shown in Figure 3. The results corrected for the percentage mass of steel cord show that the char yield remains fairly constant with a mean of 37.8 wt %. As the temperature of pyrolysis was increased from 450 to 600 °C, there was a decrease in oil yield and a corresponding increase in gas yield. The oil yield is high, reflecting the potential of tire pyrolysis as a source of liquid hydrocarbons. Similar high yields of oil have been found by other workers, for example, Roy and Unsworth5 reported 56.6 wt % oil yield, Bennalal et al.6 reported 57 wt % oil, Kawakami et al.11 reported 53 wt %, and Williams et al.21 reported 58.8 wt %. Higher oil yields from tire pyrolysis have usually been reported where there is rapid removal of the pyrolysis vapors from the hot (21) Williams, P. T., Besler, S.; Taylor, Fuel 1990, 69, 1474-1482.



Table 2. Properties of Tire-Derived Pyrolysis Chars temp of pyrolysis (°C) 450

475

500

525

560

Proximate Analysis (wt % as received) moisture content 0.5 0.4 0.4 0.4 0.3 volatiles 3.0 2.7 2.8 2.8 2.6 ash content 11.7 11.6 11.9 12.4 12.3

600 0.4 2.3 12.1

Ultimate Analysis (wt % dry ash free) carbon 93.3 94.8 90.6 95.0 94.6 95.9 hydrogen 1.1 0.9 0.9 1.1 0.8 0.8 nitrogen 0.7 0.7 0.7 0.9 0.9 1.1 sulfur 2.4 2.6 2.3 2.6 2.4 2.3 toluene discoloration 93.9 98.2 99.6 99.9 100.0 100.0 (% transmission) BET surface area (m2 g-1) 61 65 64 68 67 65 Acid Demineralized Char BET surface area (m2 g-1) 63 68 71 69 ash content (wt%) 1.6 2.1 1.5 2.0

75 1.1

74 2.6

pyrolysis zone before the occurrence of significant secondary reactions which are known to reduce the oil yield and tend to increase the gas and char yields.3,21 The decrease in oil yield with increasing temperature and corresponding increase in gas yield have been observed by other workers. For example, Williams et al.,3,21 Kaminsky and Sinn,8 and Lucchesi and Maschio22 all found significant decreases in oil yield and increases in gas yield with increasing temperature of pyrolysis. Similar char yield results from tire pyrolysis to those reported in this work have been reported by, for example, Kawakami et al.11 with char yields of between 38 and 40 wt % over a temperature range of 540-750 °C and Teng et al.17 with char yields of between 36 and 40 wt % over a temperature range of 500-900 °C. Tire Char Properties. Table 2 shows the characteristics of the tire-derived pyrolysis chars in relation to the pyrolysis temperature. In addition, the BET surface area and ash content of the derived char after acid demineralization using hot, 5 M hydrochloric acid for 90 min are also shown. The chars contained over 90 wt % carbon on a dry ash free (daf) basis. There was a slight decrease in the hydrogen content with increasing temperature, which may indicate either increasing removal or lower formation of solid hydrocarbons as the temperature increased.23,24 The decrease in the volatile-matter content with increasing temperature supports this supposition. In addition, the toluene discoloration test represents the solvent (toluene) extractable organic fraction of material, which can be extracted from the carbon under standard conditions. This showed that the percentage transmission through the toluene extract increased with temperature, which indicated that the amount of tolueneextractable organic compounds in the char decreased with increasing temperature. The nitrogen content of the chars ranged from 0.7 wt % at 450 °C to 1.1 wt % at 600 °C, while the sulfur content appeared to not be influenced by pyrolysis (22) Lucchesi, A.; Maschio, G. Semi-active carbon and aromatics produced by pyrolysis of scrap tires. Conserv. Recycling 1983, 6, (3), 85-90. (23) Dodds, J.; Domenico, W. F.; Evans, D. R.; Fish, L. W.; Lassahn, P. L.; Toth, W. J. Scrap Tires: A Resource and Technology Evaluation of Tire Pyrolysis and Other Selected Alternative Technologies. U.S. Department of Energy Report EGG-2241, 1983. (24) Sahouli, B.; Blacher, S.; Brouers, F.; Darmstadt, H.; Roy, C.; Kaliaguine, S. Fuel 1996, 75, 1244-1250.

temperature, being between 2.3 and 2.6 wt %. Darmstadt25 reported similar nitrogen and sulfur contents to those found in this work at 0.7 and 3.6 wt %, respectively. Also, Wolfson et al.7 reported sulfur contents of between 0.7 and 3.2 wt %, and Giavarini15 obtained sulfur contents between 2.7 and 2.8 wt % for chars derived from the pyrolysis of tires. The ash content of the char was high compared to most activated carbons used for effluent gas and liquid cleanup. Since the raw tire feedstock had a typical ash content of 7.1 wt %, the theoretical ash content if all the ash was included in the char would be of the order of 18 wt %. However, the ash content of the raw tire can be variable depending on make; in addition, some losses of inorganic material, such as zinc, can be significant at higher pyrolysis temperatures. High ash contents for chars derived from tire pyrolysis have been reported in the literature. For example, Wolfson et al.7 reported a wide range of ash contents from 7.0 to 16.5 wt %, Darmstadt et al.25 at 14.7 wt %, and Merchant and Petrich18 between 11.1 and 11.9 wt %. Table 2 shows the analysis of the chars for their ash content after acid demineralization. The ash content was 1.12.6 wt %, a reduction of 80-90% of the original char ash content. Previous work has shown that the major components of the ash are zinc and calcium compounds.20 The surface areas of the chars shown in Table 2 were quite similar, ranging from 61 to 68 m2 g-1; there was an indication that increasing the pyrolysis temperature produces an increase in the surface area, as found in earlier work.21 After acid demineralization, the surface area increased slightly, reaching a high value of 75 m2 g-1. Comparable tire char BET surface areas to those found in this study have been reported in the literature, for example, Merchant and Petrich18 pyrolyzed tires in a stream of nitrogen at 416-560 °C. The surface area of the derived char was 48-83 m2 g-1. Cypres and Bettens10 reported that the surface area of tire char was around 60 m2 g-1 and independent of heating rate. Further detailed characterization of the chars, including porosity, trace metal content, chlorine content, calorific values, etc., have been reported previously.20 Influence of Temperature and Time on Steam Activation. Figure 4 shows the influence of activation temperature and time on the degree of burnoff for steam achieved by the 1.4-5.6 mm size fraction of the 450 °C tire char. A linear relationship between activation time and burnoff was exhibited at the same temperature. When the lines were extrapolated back to the y-axis, the intercepts did not pass through the origin. This suggested that the activation of tire char proceeds through two stages: A higher rate of weight loss at low degrees of burnoff. A linear period of burnoff, occurring at a lower rate. The results of carbon dioxide activation in the staticbed reactor support this suggestion (see later). Merchant and Petrich18 reported that tire char consists of around 15-25 wt % carbonized rubber, in addition to the carbon black originally used in tire manufacture. Similarly, Dodds et al.23 showed that tire char contained a solid hydrocarbon residue in addition to the original carbon (25) Darmstadt, H.; Roy, C.; Kaliguine, S. Carbon 1995, 33, 14491455.



Figure 4. Carbon burnoff in steam for the 1.4-5.6 mm size fraction of a 450 °C tire char in relation to activation temperature and time.

Figure 5. Carbon burnoff in carbon dioxide for the 1.4-5.6 mm size fraction of a 450 °C tire char in relation to activation temperature and time.

black used in the manufacturing process. Sahouli et al.24 analyzed the surface morphology of pyrolytic chars by a variety of techniques, including electron spectroscopy and small X-ray scattering. They found that the surface of pyrolytic chars contained considerable amounts of carbonaceous deposits derived from the polymerization/ aromatization of adsorbed hydrocarbon species during pyrolysis. Radovic et al.26 and Marsh and Kuo27 have shown that disorganized carbon reacts more quickly than better organized carbon, exhibiting a more graphitic structure. The disorganized carbon is preferentially burnt off in the initial stages of the activation process.14 It is possible that the bulk of the weight loss in the first stage of tire char activation was due to the burning off of carbonized rubber deposits, with a less organized structure than the carbon black derived from the tire manufacturing process. Hence, this carbon would burn off at a faster rate than the carbon black, which gasifies in the second, linear weight-loss stage. Merchant and Petrich18 have suggested a similar theory. Burnoff had a linear correlation with time, but there was no initial higher rate of conversion. The linear nature of the second stage suggests that the physical and chemical nature of the char is relatively stable, i.e., that the carbon graphitic structure being burnt off is essentially similar and that active sites are being produced at the same rate as they are being used. By assuming that the burnoff Xc over the residence time tr approximates the instantaneous overall rate of reaction dXc/dt, a zero-order Arrhenius-type plot was produced for tire char.18 The activation energy derived from the plot was 201 kJ mol-1, which was high enough to suggest that gasification was chemically rather than mass-transfer controlled. The data showed a fair degree of scatter, reflected by the linear coefficient of correlation being 0.819, so the activation energy found should be treated as an estimate. The burnoff in the more rapid first stage, as postulated above, would represent a greater proportion of the overall burnoff achieved at lower conversions. dXc/dt would, hence, be higher at lower conversions and, thus, cause a spread of data in

the Arrhenius plot. Also, the batch nature of the apparatus did not allow continuous measurement of the weight loss. Figure 5 shows the carbon burnoff achieved in carbon dioxide by the 1.4-5.6 mm size fraction of 450 °C tire char plotted against time in relation to activation temperature. The burnoff achieved by carbon dioxide at a given temperature exhibited a linear increase with increasing activation time. Again, when the lines were extrapolated toward the y-axis, they did not pass through the origin. Similarly, the position of the lowest burnoff data point at 935 °C supports the higher rate of reaction at low degrees of burnoff suggested by the steam activation results. It may be noted that while the trends between burnoff, time, and temperature were similar to those for steam, the burnoff achieved by carbon dioxide was on average 72% of that produced by steam under otherwise identical conditions. Other researchers have commented on the relative reactivities of these two activation agents.28-31 Wigmans reviewed the literature pertaining to carbon activation31 and reported that steam was approximately 30% more reactive than carbon dioxide. Lu and Do30 gasified coal reject char and reported that at an activation temperature of 950 °C and activation time of 100 min, gasification in steam resulted in 90% conversion. Under the same conditions, the conversion achieved in carbon dioxide was 65% or 72% of that achieved by steam. The difference between the relative reactivites was dependent on temperature. For example, at 850 °C, steam gave 70% conversion in 300 min whereas carbon dioxide gasification resulted in 81% at 800 min, a reactivity under one-half that of steam. The activation energy of the char in carbon dioxide was 199 and 170 kJ mol-1 for steam. Similarly, Kuhl et al.28 reported that coke derived from Westerholt coal gasified more rapidly in steam than carbon dioxide. The activation energies for the two agents were 153 and 180 kJ mol-1, respectively. Walker and Almagro29 found that the reactivity of anthracite char in 0.1 MPa steam at 950 °C was 6-7 times greater than in carbon dioxide.

(26) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983, 62, 849-856. (27) Marsh, H.; Kuo, K. In Introduction to Carbon Science; Marsh, H., Ed.; Butterworths: London, 1989.

(28) Ku¨lh, H.; Kashani-Motlagh, M. M.; Mu¨hlen, H.-J.; van Heek, K. H. Fuel 1992, 71, 879-882. (29) Walker, P. L.; Almagro, A. Carbon 1995, 33, 239-241. (30) Lu, G. Q.; Do, D. D. Carbon 1992, 30, 21-29. (31) Wigmans, T. Carbon 1989, 27, 13-22.


Wigmans31 related the lower reactivity of carbon dioxide to diffusion effects brought about by the greater size of the carbon dioxide molecule. Other workers have postulated that carbon dioxide forms more stable oxygen groups on the carbon surface than steam, which remain longer on the surface and result in a slower reaction rate.32-33 For example, Molina-Sabio et al.33 used temperature-programmed desorption to determine the amount of oxygen-containing gases on the surface of activated chars. They found that chars activated in carbon dioxide released greater amounts of these species than steam-activated chars and concluded that carbon dioxide formed more stable oxygen-containing groups on the carbon surface than steam. The major gaseous products of steam activation were hydrogen, carbon monoxide, and carbon dioxide. Methane was the main hydrocarbon gas, comprising 1-2 mol % on a nitrogen-free basis of the gas stream at all activation temperatures. Trace amounts (5.6 mm

temp gross CV (°C) (mJ m-1) 835 865 900 935 835 865 900 935 935 935 935 935 935

3.59 4.84 6.00 6.55 10.47 10.19 11.07 11.85 11.59 11.29 11.31 11.85 11.47

Figure 8 shows the gas yield from the activation of the tire char when carbon dioxide was used as the activating agent. On a nitrogen-free basis, the outflow gas was comprised almost exclusively of carbon monoxide and unreacted carbon dioxide. Again, the molar ratio of carbon dioxide to carbon monoxide fell with increasing temperature, as shown in Figure 7. As a


Figure 8. Molar gas yield for carbon dioxide activation in relation to activation temperature for tire pyrolysis char derived at a 450 °C pyrolysis temperature.

Figure 9. Influence of acid demineralization of the char prior to steam activation of the 450 °C pyrolysis char at a 935 °C activation temperature.

consequence of this, the GCV of the gas increased from 3.59 MJ m-3 at 835 °C to 6.55 MJ m-3 at 935 °C (Table 3). By comparison, coal gas derived from the gasification of raw coal in steam typically comprises 49 vol % hydrogen, 20% methane, 18% carbon monoxide, 4% carbon dioxide, 2% other hydrocarbons, and the remainder nitrogen and oxygen.34 Blue water-gas is derived from the gasification of coal chars or cokes and typically comprises 49 vol. % hydrogen, 41% carbon monoxide, 5% carbon dioxide, and 1% methane, with the remainder being nitrogen.34 The GCV of these two coal-derived gases are 19 and 11 MJ m-3, respectively. The similar composition of the gas outflow streams of coal coke and tire char reflects the low volatile matter and oxygen contents of both materials. Influence of Char Acid Demineralization. Figure 9 shows the effect of demineralization of the char using acid dissolution on the burnoff achieved by steam (34) Technical Data on Fuel; Spiers, H. M., Ed.; British National Committee; World Power Conference, London, 1962.


activation under otherwise identical conditions. The reactivity of the demineralized char was on average 22% less than that of the raw char. Several other workers have noted that demineralization influences the rate of gasification of a char.35,36 For example, Linares-Solano et al.36 demineralized low-rank coals by washing them with mineral acids before pyrolysis. The reactivity of the chars in a steam/nitrogen mixture at 910 °C was significantly reduced. Conversely, some high-rank coals displayed an increase in reactivity after demineralization. The authors concluded that this was because the inert mineral matter blocked the pore structure of the char, making it less accessible to the activating agent and hampering the removal of the products. Otto et al.37 gasified a lignite and a brown coal in steam at 850 °C and found that the reactivity was 2 and 10 times less, respectively, for demineralized samples compared to the original lignite and brown coal. Again, the reactivity of a higher rank coal (Pittsburgh seam) was actually increased by demineralization. Lignite was acid-demineralized prior to carbonization by Samaras et al.35 The resulting char showed a substantially reduced gasification rate in carbon dioxide. As a result of such work, the ash composition and content and distribution of the coal char have been shown to have a marked influence on the rate of gasification. Some metallic ash components, most notably forms of potassium, sodium, calcium, magnesium, and iron, can cause an increase in the rate as they have been found to have a catalytic effect.36,38 Calcium compounds have been found to possess an especially strong catalytic effect.38 The tire chars generated in this work were found to contain 0.56-1.81 wt % calcium ions, as determined by atomic absorption spectrometry. It was shown earlier that hydrochloric acid demineralization effectively removed the calcium ions in the form of calcium compounds from the char.20 It is, therefore, suggested that the lower reactivity of demineralized tire char is due mainly to the removal of catalytic forms of calcium present as calcium compounds in the char. The carbon dioxide:carbon monoxide and hydrogen:carbon monoxide outflow gas molar ratios were both higher for demineralized tire char than for raw char. These results suggest that the calcium ions in the char catalyzes both the steam and carbon dioxide gasification reactions, which as a consequence decreases the importance of the water-gas shift reaction. Influence of Particle Size. Figure 10 shows the effect of char particle size on the burnoff achieved. Sieved size fractions of the 450 °C char were activated in steam at 935 °C for 5 h. The burnoff achieved showed signs of a small increase from 45 to 50 wt % with increasing particle size fraction up to the 1.4-5.6 mm fraction then a decrease to 45 wt %. Repeatability tests showed a standard deviation of 0.5 wt % burnoff for three repeat samples, while this indicates that the results are significant, the effect of char particle size on the burnoff is small. (35) Samaras, P.; Diamadopoulos, E.; Sakellaropoulos, G. P. Fuel 1996, 75, 1108-1114. (36) Linares-Solano, A.; Mahajan, O. P.; Walker, P. L., Jr. Fuel 1979, 58, 327-332. (37) Otto, K.; Bartosiewicz, L.; Shelef, M. Fuel 1979, 58, 85-91 . (38) Cazorla-Amoros, D.; Ribes-Perez, D.; Roman-Ma´rtinez, M. C.; Linares-Solano, A. Carbon 1996, 34, 869-878.


Figure 10. Influence of particle size range on the carbon burnoff of the 450 °C tire pyrolysis char activated in steam at 935 °C for 5 h.

It has been shown that smaller particles can lead to greater rates of reaction because of the greater availability of external active surface to the reagent and the shorter intraparticular diffusion transport paths.39,40 For example, Zanzi et al.,39 gasifying agricultural waste chars in steam, reported an increase in reactivity with decreasing particle size in the range 0.5-1.3 mm. Fung et al.40 found that anthracite and semi-anthracite chars showed an increase in reactivity in air with decreasing particle size in the range 50-140 µm. Linares-Solano et al.36 reported an increase in reactivity with decreasing particle size for a bituminous coal. However, a lower rank coal exhibited no such effect. This was attributed to the more open pore structure of the latter, which gave free access to the carbon structure regardless of particle size, whereas in the former, access was enhanced by the greater external active surface available on the smaller particles. Similarly, the inhibitory products of the gasification reaction can leave the particle. As was seen previously, some of the components of the ash have a catalytic effect on the rate of carbon gasification. The ash contents of the char particle size fractions were 180 µm to 0.5 mm, 12.86 wt %; 0.5-1.4 mm, 13.45 wt %; 1.4-5.6 mm, 11.56 wt %; and >5.6 mm, 9.48 wt % ash. As the 180 µm to 0.5 mm and 0.51.4 mm fractions had higher ash contents than the 1.45.6 mm fraction and the >5.6 mm fraction, one might expect that the rate of gasification would be the greatest for these fractions if gasification catalysis was in operation. However, Figure 10 shows that this was not so. The more open structure of the tire chars and the dominance of mesopores suggests that particle size effects would be minimal. Also, the previous section regarding the influence of ash showed that for a significant reduction in burnoff to be seen, the majority of the ash needed to be removed. Consequently, the changes in ash content observed in the chars in relation to particle size ranging between 9.48 and 13.45 would not have a major effect on burnoff, and again, porosity of the chars would be a dominant phenomenon regarding the amount of burnoff. (39) Zanzi, R.; Sjo¨stro¨m, K.; Bjo¨rnbom, E. Fuel 1996, 75, 545-550. (40) Fung, D. P. C.; Fairbridge, C.; Anderson, R. Fuel 1988, 67, 753757.


Figure 11. Molar ratio of carbon dioxide/carbon monoxide and hydrogen/carbon monoxide for steam activation in relation to char particle size.

Figure 11 shows that the carbon dioxide:carbon monoxide product gas molar ratio exhibited a decrease with particle size fraction up to 1.4-5.6 mm and then increased. The hydrogen:carbon monoxide ratio showed a similar trend. As before, this indicates that the steam gasification reaction and/or the carbon dioxide gasification reaction become more important with particle size up to the 1.4-5.6 mm size fraction than the water-gas shift reaction. However, as the particle size increased further, these trends were reversed. This might be due to the reduced catalysis of the gasification reactions in the largest particle size fraction because of the lower ash content. Alternatively, it could indicate that the intramolecular transport paths of the carbon monoxide products were sufficiently increased to increase the chance of reaction with unreacted steam entering the particle. Buekens and Schoeters41 calculated an effectiveness factor which was a measure of the importance of internal diffusion on the rate of reaction for the gasification of biomass chars. They suggested, based on their effectiveness factor, that internal diffusion became noticeable for particles of greater than a 5 mm particle size. Influence of Pyrolysis Temperature. Figure 12 shows the burnoff achieved by activating the 1.4-5.6 mm fraction of tire chars generated at different pyrolysis temperatures in steam at 935 °C for 3 h. The burnoff achieved showed a rather indistinct effect with increasing pyrolysis temperature. Increasing the pyrolysis temperature has often been found to reduce the rate of gasification, although this appears to be dependent on the initial feedstock employed.32,42,43 For example, Mu¨hlen and van Heek43 studied the effects of increasing the pyrolysis temperature from 700 to 900 °C on the reactivity of two coal chars in steam. A bituminous coal char exhibited a strong decrease in reactivity, while a char derived from anthracite showed no effects. Kumar and Gupta42 found that increasing the temperature at which acacia and eucalyptus woods were carbonized from 800 to 1000 °C resulted in a 15-60% decrease in the reactivity in carbon dioxide. (41) Buekens, A. G.; Schoeters, J. G. In Fundamentals of Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science: London, 1985. (42) Kumar, M.; Gupta, R. C. Fuel 1994, 73, 1922-1925. (43) Mu¨hlen, H.-J.; van Heek, K. H.; Juntgen H. Fuel 1985, 64, 944949.


Figure 12. Influence of tire pyrolysis temperature on the carbon burnoff of the derived char; activation at 935 °C for 3 h in steam for the 1.4-5.6 mm size fraction.

The lack of a clear correlation between tire pyrolysis temperature and char gasification rate is probably due to the nature of the tire feedstock and the narrow temperature band studied compared to the studies mentioned and, hence, to the essentially similar nature of the chars. Also, the chars may have undergone further pyrolytic reactions during the heating period before activation, which could have reduced any differences. Any temperature effects are likely to be due to the slightly higher volatile matter content and greater amount of carbonaceous deposits on the surface of tirederived chars produced at lower temperatures.24 These deposits are less organized and, therefore, more reactive than the carbon black.26,27 Influence of Process Parameters on the Surface Areas of the Derived Activated Carbons. A key property of activated carbons is their surface area. Commercial activated carbons typically have a surface area in the range from 400 to 1500 m2 g-1.14 This paper has outlined the influence of various process parameters on the rate of activation of tire pyrolysis chars. The potential of activated carbons produced from tirederived pyrolysis chars to compete with commercially produced activated carbons from conventional sources such as wood, coal, lignite, coconut, and peat14 lies in achieving an end product activated carbon with a high surface area. Figure 13 shows the influence of burnoff on the BET surface area of tire-derived carbon produced by activation of tire char produced at 450 °C pyrolysis of tires in steam and carbon dioxide at 935 °C. After activation, the carbons were then acid-demineralized and the surface area determined in relation to the process parameters. Postactivation, acid demineralization of the activated carbons was required, as outlined in the Experimental Section, to improve the reproducibility of the BET surface area determination because of the high ash content of the samples after activation. Increasing burnoff was achieved by increasing the time at the activation temperature, which was from 0.5 to 11.5 h. The raw char surface area is also shown. Figure 13 also shows that for the case of steam there was an initial relatively slow increase in BET surface area with increasing burnoff, followed by a linear increase up to a maximum surface area at 65 wt % burnoff, which then decreased. The maximum BET surface area attained was 640 m2 g-1 for the steam


Figure 13. BET surface area of the char and activated carbon in steam and carbon dioxide at a 935 °C activation temperature with post-activation acid demineralization in relation to burnoff.

activation at 935 °C. The surface area of activated carbons in the presence of carbon dioxide also produced a linear increase in the BET surface area with burnoff. However, the surface area was approximately 20% lower than that produced by steam in the linear region. Ku¨hl et al.28 also showed that a higher BET surface area was produced with steam compared to carbon dioxide for activated coal and pitch coke activated at 900-1100 °C. The linear relationship between surface area and carbon burnoff has been reported by other workers for tire-derived pyrolysis chars16-18 and also to the activation of other chars such as pine wood char, agriculturalwaste-derived chars, and coal.14,28,32,44 It has also been reported that the surface area reaches a maximum at a limiting degree of burnoff, which is specific to the initial char.14,32,44 High surface areas for activated tirederived chars similar to those reported in this work have also been reported in the literature. For example, Bilitewski et al.12 obtained a maximum BET surface area of 520 m2 g-1, Merchant and Petrich18 obtained 607 m2 g-1, and Teng et al.17 reported a maximum BET surface area of 813 m2 g-1. Comparison with surface areas produced from more conventional feedstocks show, for example, surface areas of 1300 m2 g-1 at 60-70 wt % burnoff for coconut activated carbon, 700 m2 g-1 at 40 wt % burnoff for brown coal activated carbon, and 900 m2 g-1 at 60 wt % burnoff for walnut shell activated carbon.44 The influence of acid demineralization on the activated carbon was to increase the surface area, for example, after 5 h steam activation at 935 °C, the surface area of the activated carbon was 449 m2 g-1, whereas following acid demineralization it was 526 m2 g-1. Conclusions Tire char had high carbon, sulfur, and ash contents and a low volatile matter content. Pyrolysis temperature had a limited influence on the character of the char. Boiling the char in hydrochloric acid reduced the ash content of tire char by 80-90% by effectively removing zinc and calcium ions. (44) Heschel, W.; Klose, E. Fuel 1995, 74, 1787-1791.


Tire char activation in steam was a two-stage process. A more rapid first stage at low burnoff was followed by a less rapid second stage, which exhibited a linear relationship between activation time and burnoff. This was consistent with the composition of the tire char. The apparent activation energy of gasification in steam was 201 kJ mol-1. Carbon dioxide activation resulted in similar trends between burnoff, time, and temperature to steam. The burnoff achieved by carbon dioxide under otherwise identical conditions was on average 72% of that produced by steam. Acid demineralization of tire char prior to activation resulted in a reactivity in steam 22% less than that of the raw char. The results suggested that the demineralization process removed calcium ions which thereby reduced the catalytic effect of calcium on the gasification reactions. The influence of particle size on the char burnoff was small, which was attributed to the overriding influence


of the open porous structure of the tire chars, minimizing particle size effects. The pyrolysis temperature of the tires had an unclear effect on the rate of gasification of the derived chars. Steam activation of tire char with postactivation acid demineralization produced an activated carbon with a maximum BET surface area of 640 m2 g-1 at 65 wt % burnoff. Carbon dioxide activation produced carbons with a 20% lower BET surface area than steam. Acknowledgment. This research was funded by a grant from the University of Leeds Research Fund and a grant from the U.K. EPSRC (GR/L35331). A.M.C. was supported by an award from the John Henry Garner Scholarship. The authors thank Ed Woodhouse for his work in the construction of the pyrolysis rig. We also acknowledge the help of Peter Thompson, Tony McHugh, and Alan Wheeler in the char analyses. EF9801524

Influence of Process Conditions on the Rate of Activation of Chars

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