Waste tyre pyrolysis : sustainable recovery and reuse of a valuable resource

Abstract

End-of-life car passenger tyres represent a major waste management problem, with more than 450,000 tonnes of used tyres being generated each year in the UK alone (approximately 55,000,000 tyres). Recent legislation has made 100% material or value recovery from waste tyres an imperative. There are many different waste tyre management options available. The most commonly used processes include: retreading, use as fuel in incinerators and cement kilns and reprocessing to produce rubber crumb which has further applications in asphalt mixtures, carpet underlay and playgrounds. The theme of the research presented is a fundamental study and development of tyre pyrolysis. Pyrolysis is an attractive method to sustainably recover valuable components of the tyre rubber through the generated carbonaceous solid, oil and gas. Both conventional and microwave-induced pyrolysis have been investigated. Microwave pyrolysis offers an exciting alternative to traditional pyrolysis because of the potentially significant energy efficiency advantage offered. In the first instance an investigation of the optimisation of the pyrolysis conditions by statistical design using conventional means to produce a carbonaceous solid char has been undertaken. This was followed by the investigation of microwave-induced pyrolysis using modified commercially available microwave equipment as well as a bespoke and novel 2450 MHz, 2 kW rotary microwave furnace developed for the purposes of this project by Cobham Microwave Ltd. Carbonaceous solids produced by varying different experimental parameters, such as power levels and pyrolysis duration are reported and analysed. In addition, the main constituents of the oils generated have been identified and comparisons made with traditional thermal pyrolysis oil. Reuse of the solid residue is an important activity which has been researched. This char is often described as carbon black, but given that it contains all the solid, inorganic components present in the tyre, as well as carbon black, it is better described as “char filler”. The suitability of this waste tyre-derived char for reuse in the rubber industry as a filler substitute has been evaluated and is presented. The results show that the char has similar properties to a semi-reinforcing filler with potential for improvement.