Reactivities of some biomass chars in air

The reactivities in air of biomass chars, obtained using conventional pyrolysis, are investigated for applications in fixed-bed gasification. Biomasses considered are wheat straw, olive husks and grape residues. Char particles are spread to form a 150 mu m thick layer and are radiatively heated, to achieve a kinetically controlled conversion. Time-weight loss curves, determined under non-isothermal conditions (heating rates of 10 K/min and a final temperature of 873 K), indicate that the reactivity continuously increases with conversion. The olive husk chars present the highest value, whereas that of grape residue chars is the least. All biomass chars are also combusted at different heating rates (20-80 K/min) and final temperatures of 713 K (grape residues) and 673 K (olive husks and straw), so that conversion consists of a dynamic stage, followed by an isothermal period. Again, grape residue chars are the less reactive. Furthermore, the reactivity first attains a maximum, decreases or remains almost constant and then increases again as a function of conversion. This behaviour can be explained by the different roles played by the reaction temperature, the development of surface area as combustion proceeds and the increase in the ratio of ashes (catalytically active) to carbon. Finally, the weight loss curves are well interpreted by a one-step global reaction, whose rate presents a power law dependence on the solid conversion and activation energies in the range 75-94 kJ/mol. (C) 1999 Elsevier Science Ltd. All rights reserved.