Interest is increasing in the potential use of lignin as a feedstock for biofuels and aromatic chemicals. Lignin is a highly cross-linked amorphous polymer of phenylpropane units, accounting for up to 40 wt% of dry biomass. Empirical and global models of lignin pyrolysis using overall apparent kinetics derived from TGA studies are well established and often adequately predict yields of lumped products within a small range of experimental conditions. The main limitations of these models are their inability to predict the composition and distribution of product tars and gases, or to cover a wide range of heating rates and temperatures with one set of kinetic parameters. A fully mechanistic model of lignin pyrolysis would be of great interest for these reasons, but developing such a model is challenging due to the random structure of lignin polymers.
As a step toward a more mechanistic model, we explore the use of a semi-detailed kinetic model for lignin pyrolysis to predict the compositions of gases and tar components from the fast pyrolysis of lignin over a wide range of conditions. The model combines elementary and lumped reaction steps to track approximately 100 real molecules, radicals, lumped heavy species, and functional groups linked to the degrading polymer. The sensitivity of permanent gases and tar precursor molecules to experimental conditions is examined, and using this model we are able to explore new mechanistic details about the evolution of functional groups.