Interpreting isothermal thermogravimetric data of complex reactions: Application to cellulose pyrolysis at low temperatures

Abstract

Isothermal thermogravimetric data are most informative when plotted in an unusual way, namely, as -d M′/dt vs. M′, where M′ is dimensionless mass and t is time. The resulting ‘DMM plots’ are particularly useful in the case of complex reaction schemes, as in the pyrolysis of lignocellulosic materials. DMM curves have been used previously, together with multistage isothermal thermogravimetry, to show that the pyrolysis of woods and barks can be interpreted as the independent volatilization of several fractions. The volatilization of most of these fractions follows first-order kinetics. The fraction identified as cellulose, however, shows a complex DMM pattern which recurs in various materials, including pure cellulose (filter paper, cotton wool). In this work, various reaction schemes were tried to model the DMM curves generated by cellulose pyrolysis. The simplest one consists of three consecutive first-order reactions. The first reaction produces about 30% of the total volatiles. The second reaction produces little or no volatiles: it is probably a rearrangement or activation of the solid, before further reaction. The third reaction produces most of the total volatiles.