Ind. Eng. Chem. Res. 1995,34,3102-3111
3102
Reprocessing of Used Tires into Activated Carbon and Other Products Hsisheng Teng: Michael A. Serio," Marek A. Whjtowicz, Rosemary Bassilakis, and Peter R. Solomon Advanced Fuel Research, Inc., 87 Church Street, East Hartford, Connecticut 06108
Landfilling used tires which are generated each year in the United States is increasingly becoming an unacceptable solution. A better approach, from an environmental and economic standpoint, is to thermally reprocess the tires into valuable products such as activated carbon, other solid carbon forms (carbon black, graphite, and carbon fibers), and liquid fuels. In this study, high surface area activated carbons ('800 m2/g solid product) were produced in relatively high yields by pyrolysis of tires at up to 900 "C, followed by activation in CO2 at the same temperature. The surface areas of these materials are comparable with those of commercial activated carbons. The efficiency of the activation process (gain in specific surface a r e d o s s in mass) was greatest (up t o 138 m2/g original tire) when large pieces of tire material were used (-170 mg). Oxygen pretreatment of tires was found to enhance both the yield and the surface 0 is area of the carbon product. High-pressure treatment of tires at low temperatures ( ~ 4 0 "C) a n alternative approach if the recovery of carbon black or fuel oils is the primary objective.
Introduction
COAL
Landfilling the 280 million tires generated each year in the United States is increasingly becoming an unacceptable solution (New York Times, 1990). The tires take up large amounts of valuable landfill space and also represent a fire hazard. Recently, a large mountain of tires caught on fire in Canada with widespread environmental consequences due to the oils and gases generated from the decomposingtires. A better solution from an environmental and economic standpoint is to thermally reprocess the tires into valuable products (Schulman and White, 1978). The largest-scale efforts employ tires either as a fuel or as a filler for asphalt. These two technologies consume annually about 28 million tires (U.S. Environmental Protection Agency, 1991). However, tire burning has had repeated problems with tire feeding and slagging, while the rubber asphalt costs 40% more than conventional material. An alternative is to reprocess the used tires into activated carbon, other solid carbon forms (e.g., carbon black, graphite, and carbon fibers), and liquid fuels. The key to the manufacture of such products is controlling the chemistry of low-temperature carbonization, which is the subject of this paper. Pyrolysis has been widely used for converting solid fossil fuels into liquid and gaseous hydrocarbons, a process which results in a solid char residue. Coal pyrolysis has been extensively studied (Howard, 1981; Gavalas, 1982; Solomon et al., 1992),but investigations of tire pyrolysis are rarely reported in the open literature. Used automotive tires contain polymeric aromatic structures which are similar to those of coal in some respects. Thus, the well-developed techniques used in coal pyrolysis should also be applicable to the pyrolysis of waste tires. Tires contain vulcanized rubber and various reinforcing materials. The most commonly used vulcanized tire rubber is a Gyrene-butadiene copolymer (SBR) or a mixture of natural rubber and SBR (Merchant and Present address: Department of Chemical Engineering, Chung Yuan Christian University, Chung-Li, 32023, Taiwan. * To whom correspondence should be addressed.
versus
TIRES
1 Volatile Matter
Y
- CH2 - CH
-
-
SBR Rubber
- -
-
-
-
CH CH2 CH2 CH I CH2 CH = CH CH2
C H
82.4 5.5
0 N
8 1.7
S
2.4
c
H 0 N
S
-
88 8 2
0.5 1.5
Figure 1. Comparison of the structure and composition of coal and tires.
.
Table 1 Composition of Tire Rubber" wt % wt % component (as received) component (as received) sulfur 1-2 60-65 SBR (and natural rubber) -2 29-31 extender oil carbon black additives -2 2-3 zinc oxide a After
Williams et al., 1990, and Ogasawara et al., 1987.
Petrich, 1993; Williams et al., 1990; Petrich, 1991; Torikai et al., 1979; Ogasawara et al., 1987). A typical composition of tire rubber is shown in Table 1. Also, a comparison of the structure and composition of tire rubber with that of a bituminous coal is shown in Figure 1. In most cases, tire pyrolysis studies were performed under inert conditions (Merchant and Petrich, 1993; Williams et al., 1990; Petrich, 1991;Torikai et al., 1979).
0888-5885/95/2634-3102$09.00/0 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 3103 However, pyrolysis may be carried out in mildly oxidizing atmospheres, such as steam or carbon dioxide, t o improve the quality of pyrolytic products (Ogasawara et al., 1987; Funazukuri et al., 1987; Merchant and Torkelson, 1990). Tire pyrolysis experiments have usually been conducted in the 500-900 "C range (Williams et al., 1990; Petrich, 1991; Ogasawara et al., 1987). Similar to coal pyrolysis, the principal products of tire pyrolysis are gases, liquid oils, and solid carbon residues. The following yields are typical for tire pyrolysis (as-received basis): 33-38 wt % char, 38-55 wt % oil, and 10-30 wt % gas. Product yields are affected by process conditions, such as temperature and heating rate. The literature on the analysis of tire pyrolysis products is summarized below. Gas Analysis. Gases produced from tire pyrolysis are mainly hydrogen, carbon dioxide, carbon monoxide, methane, ethane, and butadiene, with lower concentrations of propane, propene, butane, and other hydrocarbon gases (Williams et al., 1990). The temperature for the maximum evolution rate of each gas shifts to higher temperature levels as the heating rate is increased. There is an increase in total gas emission observed with increasing heating rate, and this is accompanied by a corresponding decrease in oil yield (Williams et al., 1990). Pyrolysis carried out in the presence of water increases the production of hydrogen and carbon monoxide (Ogasawara et al., 1987). This is thought to result from the occurrence of carbon gasification by steam, i.e., C HzO CO H2. Oil Analysis. The yield of oil from tire pyrolysis is high, reflecting the potential of tire rubber to act as a substitute for fossil fuel and chemical feedstocks. The oils have low sulfur content and are considered relatively good fuels (Merchant and Torkelson, 1990). In one study, the molecular weight of the oils from pyrolysis in nitrogen was found t o be up to 1600, with an average molecular weight in the 300-400 range (Williams et al., 1990). The average molecular weight increases with increasing pyrolysis temperature and with decreasing heating rate. Infrared analysis of the oils indicated the presence of alkanes, alkenes, ketones, or aldehydes as well as aromatic, polyaromatic, and substituted aromatic groups (Williams et al., 1990).An increase in pyrolysis temperature produced a decrease in the aliphatic fraction and an increase in the aromatic fraction (Williams et al., 1990). Aliphatic hydrocarbons and alkylbenzenes were found t o be the major oil components in another study (Ogasawara et al., 19871, in which the pyrolysis was performed in the presence of water. The average molecular weights of the aliphatic hydrocarbons and alkylbenzenes were 164 and 180, respectively. These molecular weights are lower than those characteristic of tire pyrolysis in an inert environment. The difference in molecular weight may be due t o the different tire materials used in different studies or due to the fact that the cracking of oil is promoted by water. The oil product from tire pyrolysis is a potential source of energy and chemicals. The oils may be used directly as fuel or added to petroleum refinery feedstocks. The composition of the gasoline boiling fraction was reported to be comparable t o that of petroleum gasoline (Merchant and Torkelson, 1990). The derived oil product may also be an important source of refined chemicals as it contains high concentrations of such chemical feedstocks as benzene, toluene, and xylene (Williams et al., 1990).
+
+
Carbon Residue Analysis. The carbon residue will become a marketable product if its properties are similar to those of commercial carbons. The production of valuable solid products and gaseous and/or liquid fuels from what is currently a waste material would make tire pyrolysis economical if a large supply of an inexpensive raw material is readily available. This situation exists in many regions of the United States. It should be realized that activated carbons derived from used tires would fall into the category of commoditytype products rather than specialty carbons. Such carbons could be utilized in applications in which the presence of appreciable amounts of mineral matter and sulfur can be tolerated. Examples of such applications, for which large-volume markets can be identified, include wastewater treatment, adsorption of organic vapors, and possibly the use of activated carbons as landfill-liner components. As mentioned previously, tire pyrolysis performed in an inert environment can produce 33-38 wt % of carbon residue. It has been reported that the char yield increases with decreasing pyrolysis temperature and heating rate, whereas surface area shows the opposite trend (Williams et al., 1990). According to Petrich (19911, chars prepared at low temperatures retain a large fraction of volatile material, which results in low surface areas, and those prepared a t excessively high temperatures may sinter and thus also lose surface area. The surface area of a tire char produced by pyrolysis in an inert gas usually ranges from 30 to 90 m2/g(Merchant and Petrich, 1993;Williams et al., 1990; Petrich, 1991; Ogasawara et al., 1987). In general, there are two uses of tire chars: as a reinforcing filler and as an adsorbent. Commercial carbon black is typically used for filling polymers and vulcanizates. The use of tire char as carbon black for the tire and printing-ink industries has been reported to be unsatisfactory (Williams et al., 1990), mainly due to the high mineral-matter content. Chars from tire pyrolysis contain as much as 15 wt % of minerals, mostly in the form of zinc oxide (Petrich, 1991). A means of removing mineral matter is an important issue in the process of producing carbon black from waste tires. Carbon adsorbents are usually evaluated by their surface area, which can be determined, for example, using the nitrogen BET technique. As mentioned above, the surface area of tire char is in the range of 30-90 m2/g. This is too low for an activated carbon since commercial activated carbons have surface areas well above 400 m2/g. Therefore, an activation process is required to produce activated carbon from tire char. Carbons can be activated at high temperatures by mild oxidation with steam or carbon dioxide. The slow gasification kinetics allow gas molecules to diffuse into the carbon micropores and develop large surface area. The activation process usually follows hydrocarbon pyrolysis performed in an inert environment, but it is possible t o accomplish pyrolysis and activation in one stage by pyrolyzing under mildly oxidizing conditions (Ogasawara et al., 1987). Torikai et al. (1979) pyrolyzed tires at 550 "C for 30 min and activated the granulated char using C02 at 900 "C. They found linear relationships between activation time and burn-off, burn-off and surface area, and burnoff and Methylene Blue index (an adsorption test). This activated carbon had a surface area up to 400 m2/g. Ogasawara et al. (1987) carried out the pyrolysis and activation of tires in one stage. In their study, water was continuously introduced to the sample with helium.
3104 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995
The carbon residue from 1h steam activation at 900 "C had a surface area of 1260 m2/g, while pyrolysis in helium gave a char with a surface area of 87 m2/g. The carbon residue produced from this "wet method" is as good as the commercial activated carbon in terms of surface area, but the carbon yield was only 9 wt % of the original tire material. In studies on coal liquefaction and pyrolysis (Serio et al., 1990,1991),it has been found that oxygen functional groups, such as carboxyl and hydroxyl, appear to play a major role in promoting the cross-linking reactions between coal molecules. These cross-linking reactions are usually encountered at low temperatures (
