Waste tire conversion: Understanding the mechanism of decomposition

Currently, in the U.S., nearly 58 million tires per year (∼ 640,000 tons) are discarded typically in landfills which pose serious environmental issues/concerns because of the long life. Markets valuing discarded tires between $50 - $100 per ton have generated interest in recycling applications such as cement kilns, asphalt fills, and waste-to-energy. If alternative routes for waste tire use can be developed there is a potential to mitigate the amount that is discarded. A novel process intensification design to convert waste tires to useful raw materials, such as syngas (CO and H2), has been investigated. This process combines an integrated combustion-gasification reactor consisting of two stages: a modified fixed bed combustor located centrally within a counter-current gasifier. The combustion products directly feed the gasifier, where the heat and CO2 endothermically reform additional tires and water to efficiently produce CO and H2. This presentation will show the results of gasification and combustion experiments using real tire shavings and styrene-butadiene (SBR) polymer, the major constituent of tires, to understand the mechanisms of decomposition. The identities and absolute concentrations of nearly 10 major and minor species have been established and provide insight into potential pollutant emissions during waste tire conversion processes. Light hydrocarbon species such as H2, C2H2, CH4, C2H6, and C4H10 have been identified. In addition, the largest PAH detected was in the family of C24H14 (molecular weight 302), benzo[ghi]perylene with peak concentrations reaching 0.05 ppmv. The measured data was combined with thermodynamic values from the literature and programmed into Aspen™ to simulate the overall process. The results show that 4 million tires per year can produce 18 MWeq of CO and H2 at a rate of return of 14%.