Pyrolysis and gasification of 5–20 mm tyre rubber cubes under carbon dioxide flow

Abstract

The thermal behaviour of waste tractor tyre tread was investigated using 5-, 10-, 15- and 20 mm cubes and video recording of the process – an experimental approach for which no precedent could be found in the literature. Pyrolysis and gasification under CO₂ flow in the range of 400 °C to 1000 °C were studied using a pre-heated tube furnace under near-isothermal reaction conditions. The video-graphic timeline and thermal history observations were used to correlate the results with first-order heat-transfer calculations and TGA-derived kinetics published previously. For pyrolysis, heat-transfer becomes the rate limiting step in the region 800–900 °C and above. Experimental evidence shows that the full pyrolysis time may be estimated from the algebraic sum of the local kinetic component and a heat-transfer component. The pressure build-up due to the release of gaseous products results in shattering of the solid into sub-millimetre char fragments. The kinetics of the reverse-Boudouard reaction can be described by a standard gas–solid shrinking particle model; however the character of the charred remains complicates this. Mass transfer limits are predicted only to become significant above 1200 °C, for a well-characterised char surface.