Pyrolysis of scrap tyres has been investigated by means of non-isothermal thermogravimetric analysis, at heating rates ranging from 5 to 900 degrees C/min and particle sizes of 1 and 3 mm. The occurrence of primary and secondary pyrolysis stages has been recognized at low heating rates. Based on experimental findings and on the relevant literature on pyrolysis of rubber components, a series-parallel reaction network and a kinetic model are proposed. Accordingly, the first stage of weight loss is related to depolymerization of the rubber components. The residue, upon cyclization and crosslinking, degrades in the secondary stage at larger temperatures. Kinetic parameters of primary and secondary pyrolysis and of the cyclization/crosslinking stages are determined on the basis of experimental results obtained under chemical kinetic controlled pyrolysis conditions. At large heating rates distinction between primary and secondary pyrolysis is no longer observed and a single peak of the pyrolysis rate is observed. This phenomenology can be partly related to decreasing extent of cyclization/crosslinking that take place as the heating rate is raised. Moreover, radial non-uniformities of either temperature, overall volatile pressure or concentration can be established at larger heating rates. These can be further responsible for the modification of the pyrolysis pattern. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.
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