Properties of Chars and Activated Carbons Derived from the Pyrolysis of Used Tyres

Abstract

A nitrogen purged static-bed batch reactor was used to pyrolyse 3 kg batches of shredded scrap tyres at temperatures between 450 and 600 °C. The yield of char remained fairly constant at about 38 wt% irrespective of pyrolysis temperature; however, the yield of oil decreased from 58 wt% to 53 wt% and the gas yield increased from 4.5 wt% to 8.9 wt%. The derived tyre char was characterised for a range of properties, including: surface area, pore size distribution, ultimate and proximate analysis, calorific value and sulphur content and trace metal content. The tyre char was subsequently activated in a steam/nitrogen or carbon dioxide/nitrogen mixture at 835 to 935 °C. The activated carbons were then acid demineralised and the influence of activation process conditions on the surface area, micropore volume, mesopore volume, mesopore surface area and the mesopore size distribution was investigated. The maximum BET surface area generated was 640 m2 g−1 at 65 wt% burnoff. All the activated carbons had a considerably greater mesopore volume than micropore volume, which was due to the predominantly mesoporous structure of the initial tyre char. Examination of the Kelvin mesopore size distribution profiles indicated that activation of tyre char proceeded via the standard mechanism, i.e. micropore formation followed by pore widening and finally pore destruction. Carbon dioxide activation produced carbons with a lower BET surface area, absolute micropore volume and total pore volume, but narrower size distribution than when steam was the activating agent. The activation temperature had little apparent effect on the properties of the activated carbons. Overall, the activation process employed proved a viable means of producing carbons of comparable quality to low grade commercial activated carbons.