A thermogravimetric study of nonfossil solid fuels. 2. Oxidative pyrolysis and char combustion

The study addresses the competition between the course of purely pyrolytic processes and heterogeneous oxidation during oxidative pyrolysis of nonconventional fuels. This feature is one key, together with volatile matter flammability and effectiveness of oxygen transport to the particle, to establish whether flaming or glowing ignition of the fuel takes place. A selection of six nonconventional high volatile solid fuels is considered in the study, namely two plastics (polyethylene and poly(ethylene terephthalate)), two lignocellulosic materials (Robinia Pseudoacacia and waste wood), and two rubber-derived materials (scrap tires and ebonite). The analysis is based on the comparison of the behavior of fuels when heated in a thermogravimetric apparatus under inert and oxidizing atmospheres. Further analysis is directed to assess the heterogeneous combustion kinetics of chars obtained by pre-pyrolyzing in inert conditions samples of the fuels. The study is complementary to a companion paper (Senneca, O. Chirone, R. Masi, S. Salatino, P. A Energy Fuels 2002, 16, 653) where pyrolysis in nitrogen of the same fuels has been addressed. Results indicate that heterogeneous oxidation and pyrolytic processes play different roles depending on the nature of the fuel. A general feature of all fuels tested is that burnoff cannot be simply described as sequential reaction paths corresponding to purely thermal degradation and heterogeneous oxidation. Synergistic effects between these processes are significant and need to be taken into account.