Study on the thermal degradation behaviors and kinetics of alkali lignin for production of phenolic-rich bio-oil using TGA-FTIR and Py-GC/MS

Alkali lignin, an aromatic polymer extracted from soda pulping black liquor, is considered to be a potential source of phenolic-rich bio-oil using pyrolysis technology. This paper investigated the pyrolysis behaviors and kinetics of alkali lignin using thermogravimetric analyzer coupled with Fourier transform infrared spectrometry (TGA-FTIR) and pyrolyzer coupled with gas chromatography/mass spectrometer (Py-GC/MS). Results showed that the pyrolysis process of alkali lignin consisted of the three stages: drying stage, fast degradation stage and slow degradation stage. The weight loss rate reached its maximum value of 0.2448 mass%/°C at the temperature of 327 °C. The aromatics (at 1512 cm−1) and phenols (at 1261 cm−1) were the dominant volatile components according the FTIR spectrum. As the conversion rate increased from 0.05 to 0.9, the activation energy estimated from Flynn-Wall-Ozawa (FWO) method, was raised from 124 to 721 kJ mol−1. The phenolic compounds, namely guaiacol type (G-), the phenol type (P-), the syringol type (S-), and the catechol type (C-) were the main products from Py-GC/MS experiment. The total phenols and the G-type phenols reached their maximum contents of 78% and 63.43% at 500 °C, respectively. The content of S-type phenols decreased from 17.63% to 10.2% as the temperature increasing from 400 °C to 700 °C, while the contents of P-type and C-type phenols increased from 0.81% to 8.16% and 0.12% to 3.11%, respectively.