The pyrolysis of automotive tyre waste

Abstract

The disposal of scrap automotive tyres is an increasing environmental and economic problem. Traditionality the route to disposal has been via landfill sites or open dumping, but pyrolysis of tyres is currently receiving renewed attention. The derived oils may be used directly as fuels, or added to petroleum refinery feedstocks, or as a source of refined chemicals; gases derived from the pyrolysis of tyres are of high calorific value and sufficient to provide the energy requirements for the pyrolysis process plant; and the char may also be useful as a solid fuel, as substitute carbon black or activated carbon. In this paper the pyrolysis of scrap automotive tyres is described in a bench-scale static batch reactor and a commercial 2 t/day batch reactor; the results from the two units are compared and contrasted in relation to the differences in process conditions. Fuel properties of the derived pyrolysis oils are compared with those of petroleum-derived fuel products by means of standard ASTM or IP methods, and the properties measured include calorific value, flash point, viscosity, density, hydrogen content, carbon residue and distillation range. The derived pyrolytic oil has a high calorific value, of the order of 42 MJ kg −1 , with a sulphur content between 0.5 and 1.5% depending on process conditions. Fuel properties of the tyre-pyrolysis oils were similar to those of diesel/light fuel oil petroleum products. The chemical compositions of the oils are described in terms of: chemical class fractionation, functional group composition and molecular weight range. The compositions were related to the process conditions of the pyrolysis: higher temperatures of pyrolysis produced higher concentrations of aromatic compounds. Detailed analysis of the oils showed that they contained significant concentrations of the high-value chemicals benzene, xylene, toluene, styrene and limonene. Analysis of the gas composition and char properties was also carried out: the gases comprised mainly H 2 , CH 4 , CO 2 , C 4 H 6 and C 2 H 6 , with lower concentrations of other hydrocarbon gases. The char was analysed for calorific value and was found to be 29 MJ kg −1 , and had properties that give the char the potential to be used as a low-grade activated carbon or carbon black. The differences in results of the two reactors could be related mainly to the differences in operating temperature and the presence of a purge gas in the bench-scale reactor. An economic review of the tyre pyrolysis process concludes that tyre pyrolysis has the potential to contribute significantly to the disposal of scrap automotive tyres combined with recovery of energy, chemicals and products