Insight into the fast pyrolysis of lignin: Unraveling the role of volatile evolving and char structural evolution

Unraveling the role of volatile evolving and char structural evolution, as well as the pyrolysis reaction pathway, is crucial for lignin effective valorization by a pyrolysis-based biorefinery approach. The fast pyrolysis of lignin at 200–800 °C was conducted to explore volatile release characteristics and their link with char structure evolution. The release of simple substituted aryl compounds below 350 °C was aided by the cleavage of β-O-4 and α-O-4. The C-O, C–C, and carboxyl groups of the depolymerized free radical fragments were cleaved further, providing stable mono-phenol (350 °C → 500 °C). Secondary reactions and pyrolytic macromolecular compounds resulted in the synthesis of polycyclic aromatic hydrocarbons (>700 °C). A cubic polynomial was developed to correlate the aromaticity with the atom ratio of H/C. The ring condensation degree and H/C demonstrated an excellent positive linear correlation. Additionally, the aromatization started at 350 °C, and small aromatic rings were changed into more ordered rings (≥6 rings). The fused-ring structures of char had progressed in the following order: 1 × 2, 2 × 3, 4 × 4, and 4 × 5, corresponding to 350 °C, 500 °C, 600 °C, and 800 °C, respectively. High temperatures contributed to the formation of aromatic monomers. Lignin pyrolysis pathway was established based on the correlation between volatile release characteristics and char structural evolution.