On the utility of thermogravimetric analysis for exploring the kinetics of thermal degradation of lignins
Annan Xiang, John R. Ebdon, A. Richard Horrocks, Baljinder K. Kandola
Abstract
The kinetics of pyrolysis of organosolv (TcA) and hydroxypropyl-modified (TcC) lignins have been investigated using thermogravimetric analysis (TGA). Three isothermal models (single first order, Guggenheim and Avrami-Erofeev) and one non-isothermal model (Kissinger) were used to analyse the mass-loss data. Sensible derived kinetic parameters, i.e., activation energy and pre-exponential factor, were obtained only for the initial stages of pyrolysis where the kinetics were approximately first order. Models that analysed TGA data beyond the initial stage gave inconsistent results, indicating the complexity of subsequent decomposition steps occurring at higher temperatures and/or longer times. The kinetics of the initial stage are important for designing routes to lignin's valorisation into useful products, such as carbon fibres, activated carbons, polymer additives, etc. TcC had a higher activation energy (41.5 kJ/mol) for initial decomposition than TcA (39 kJ/mol), consistent with its greater thermal stability observed previously during conversion of lignin-based fibres into carbon fibres.