Scrap Tire Recycling Through Pyrolysis

Scrap tire (rubber) recycling is not a new idea it has just gained a little more notoriety as we have become more aware of the environmental consequences. In the early 1900's the average recycled content of rubber products was 50% and by the 1960's the tire and rubber manufacturing industries used only 20% recycled rubber (Reschner, 2000). Cheap oil imports, widespread use of synthetic rubber, and the development of the steel belted radial tire have led to the continued decline in recycled content.

Although tire scrap only represents 2% of the solid waste stream in most industrialized countries; it is the illegal or improper stockpiling and dumping of the estimated 500 million to 3 billion scrap tires in the US that poses serious health and environmental concerns. Tire stockpiles are breeding ground for disease carrying insects and rodents and are susceptible to fire, which is extremely polluting and difficult to extinguish (Department of Energy, 2000). Because of these problems, most states have comprehensive scrap tire management and recycling programs. Consequently, of the estimated 270 million tires taken out of service every year in the US, about 66% are recycled.
There has been a great deal of research in the area of tire recycling and many useful strategies have been developed as a result. One potentially useful technology is Pyrolysis. Pyrolysis is the thermal distillation or degradation of organic materials under the exclusion of ambient oxygen (Rich, Burrell, & Hanson, 1993) (Department of Energy, 2000). The decomposition process of tire pyrolysis produces hydrocarbon gases and oils, carbon black and steel, which can be processed for reuse. Pyrolysis is seen (by some) as an environmentally friendly way of dealing with large stockpiles of tires. However, pyrolysis has its drawbacks, chief among them being that it is cost prohibitive as crude oil prices remain reasonable (averaging between $20-$25 / barrel). The process itself is expensive and the products yielded are of comparatively low quality adding the cost of further refining to bring these products to market specifications (Department of Energy, 2000).
To counter some of the problems associated with tire pyrolysis, researchers in the Department of Chemistry at the University of Northern Iowa have looked into the use of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-IR) to analyze the formation of secondary hydrocarbons in the gas phase and determine if they can control soot formation or recycle these hydrocarbons back into the process as a fuel for the pyrolysis oven (Rich, et. al., 1993). It was hoped that by modeling aromatic and aliphatic production, more information can be gained regarding the formation of larger hydrocarbons from smaller, and the role the C3H3+ cation plays in this process. The research found that the C3H3+ cation is the major precursor to the formation of larger hydrocarbons in both aromatic and aliphatic production. In aliphatic production, the C3H3+ cation participates indirectly to form the C6H10+ cation through charge exchange, as opposed to aromatic production where the C3H3+ cation produces the aromatic phenyl cation through reactions with the target gas furan (Rich, et. al., 1993).

The dynamics of producing larger molecular weight hydrocarbons from smaller ones is abstract. Nonetheless, the research advances the technology of tire pyrolysis and contributes to the body of knowledge that may make tire pyrolysis a much more economically viable means of dealing with the scrap tire problem as both economics and technology change.

U.S. Department of Energy. (2000). Scrap Tire Recycling. Consumer Energy Information EREC Reference Briefs.
Reschner, Kurt. (2000). Scrap Tire Recycling. Webpage:
Rich, M., Hammer, M. & Hanson, C. (1993). Gas Phase Reactions of Conjugated Dienes as a Model for Pyrolysis Product Formation. Recycling Reuse Technology Transfer Center.