Resource Recovery from Used Rubber Tires

 

 

 

Published in Resources Policy, 25 (1999), 170-188

 

 

 

November 8, 2000

 

 

 

 

 

Takeshi Amari (1), Nickolas J. Themelis (2), and Iddo K. Wernick* (3)

 

 

 

 

1.      Mitsubishi Heavy Industries (Visiting Scholar, Columbia University, 9/97-9/98)

Technical Headquarters, Yokohama R&D Center

1-8-1, Sachiura, Kanazawa-ku, Yokohama, 236-8515, Japan

Tel: 81-45-775-0829; Fax: 81-45-770-1122

E-mail: Takeshi_Amari@d.ydmw.mhi.co.jp

 

2.      Columbia University, Earth Engineering Center

500 West 120th St., New York, NY 10027

Tel: (212) 854-2138; Fax: (212) 854-5213

Email: njt1@columbia.edu

 

3.      Columbia Earth Institute

405 Low Library, New York, NY 10027

Tel: (212) 854-9462; Fax:      (212) 854-6309

Email: iw4@columbia.edu

 

 

* Corresponding author

 

Keywords: Scrap Tires, Tire Derived Fuel, Recycling, Life Cycle Energy Accounting
Abstract

 

            Including car, truck, bus, and airplane tires, 266 million tires were scrapped in the U.S. in 1996 (Scrap Tire Management Council (STMC) 1997). More than three-quarters of these tires were used as fuel, recycled for material applications, or exported. The remainder accumulates in junkyards or landfills where they pose a fire hazard and provide a breeding ground for disease carrying rodents and insects. Using information on scrap tire composition and the current markets using them, we examine the technologies used to recover their value either for energy or as rubber. As the majority of scrap tires are used as fuel, we calculate their life cycle energy budget considering both the energy consumed for tire production and the energy recovered from their use as fuel. Based on our findings, we draw some preliminary conclusions on how to maximize value recovery from this ubiquitous artifact of industrial societies.

 

 

1.            Composition of Rubber Tires

 

            Globally, motor vehicles are the manufactured product of the highest value. They roll on rubber tires that wear out several times over the average vehicle’s lifetime. Tires are made of vulcanized (i.e. cross-linked polymer chains) rubber and various reinforcing materials. The most commonly used rubber matrix is the co-polymer styrene-butadiene (SBR) or a blend of natural rubber and SBR. In addition to the rubber compound, tires contain:

 

 

Reinforcing fillers: Carbon black, used to strengthen the rubber and aid abrasion resistance.

Reinforcing fibers: Textile or steel fibers, usually in the form of a cord, used to provide the reinforcing strength or tensile component in tires. (The materials used for this purpose have progressed steadily from natural cotton through man-made rayon to totally synthetic suite of nylons and polyesters. By the mid 1990’s the use of steel tire cord has increased substantially, occupying about 50% of the reinforcing fiber market (Corallo 1995; Shemenski 1994)).

Extenders: Petroleum oils, used to control viscosity, reduce internal friction during processing, and improve low temperature flexibility in the vulcanized product. (By the mid 1990’s, naphthenic oil captured more market share at the expense of aromatic oils, because the latter contain hazardous materials that require special handling.)

Vulcanizing agents: Organo-sulfur compounds, used to act as the catalyst to accelerate the vulcanization process; and Zinc oxide and stearic acid, used to activate the curing (cross-linking) system and to preserve cured properties.

 

A typical composition of synthetic rubber compound is shown in Table 1.

 

Table 1. Rubber compounding composition (Dodds et al 1983)

 

 

The amount of reinforcing steel or synthetic fibers used in rubber tires varies among manufacturers. For Western Europe, Guelorget et al (1993) reports the following average composition of reinforcing fibers as a percentage of all material inputs.

 

Rayon     :        2.8%

Nylon      :       1.3%

Polyester :       0.1%

Steel        :     13.1%

 

We use an average tire composition of 8% fabric (synthetic or steel), 3% steel wire and 89% rubber compound as reported by Brown et al (1996), for the life cycle energy analysis shown later.

 

2.            Combustion/Pyrolysis of Used Rubber Tires

 

2.1       Tires as Fuel

 

Tires can be used as fuel either in shredded form (Tire Derived Fuel, or TDF) or whole, depending on the type of combustion furnace. The Scrap Tire Management Council reports that over 57% of scrap tires in the U.S. were used as fuel in 1996, Figure 1 (STMC 1997).

 

Figure 1 here

 

In considering the value of tires as fuel it is interesting to compare the typical composition of tires with that of coal, Table 2. The Babcock & Wilcox Company conducted tests with three types of shredded tires (Granger and Clark 1991): 1.25 cm rubber ‘fuzz’ and 5 cm rubber tire pieces with and without steel reinforcement. The results of the analysis show that, compared to coal, the tire samples had less moisture, significantly more combustible matter, and less fixed carbon. The heat content of the shredded tire samples tested was 10 to 16% higher than that of coal. The TDF ash sample contained 1.2 to 1.3% sulfur. This corresponds to approximately one-half of the sulfur content of U.S. eastern coal and is about the same as low sulfur-western coals.

 

Table 2. Analysis of Various Tires, TDF, and Coal

 

 

            The ash residue from the TDF samples was 16%, 23%, and 9%, which compares to coal combustion, which yielded 11% ash.  Charred steel, that can be recovered, accounts for the higher values for the TDF ash. The ash chemistry varies significantly between coal and TDF. Table 3 shows the principal constituents and their concentrations expressed as oxides, in the coal and TDF ash samples. Zinc oxide (ZnO), added during the rubber compounding process to control the rate of vulcanization, results in the high concentration of zinc which can be recovered from the ash. Generally, the ash residues from TDF contain a lower heavy metals content, making them less of a solid waste disposal burden than ash from standard coal combustion. Finally, using rubber tires as fuel results in lower NOx emissions when compared to many US coals (Ohio Air Quality Development Authority 1991).

 

Table 3. Principal Chemical Elements in Ash of Coal and Three Rubber Samples

(Granger and Clark 1991)