Chemical Structure and Pyrolysis Characteristics of the Soda-Alkali Lignin Fractions

In the present work, three different kinds of lignin fractions (L1, L2, and L3) were isolated from the alkali black liquor of Cunninghamia lanceolata by selective precipitation at the pH values of 8 (fraction L1), 5 (fraction L2), and 2 (fraction L3). Elemental analysis, gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FT-IR), P-31 nuclear magnetic resonance (P-31-NMR), thermal gravimetric analysis (TG), and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) were used to characterize the chemical structure and thermochemical properties of the three lignin fractions. The results showed that L1 presented higher heating value (HHV) and molecular weight compared with L2 and L3. The structures and functional group types were similar for the three lignin grades. However, there was more hydroxyl group in L1 than that in L2 and L3, and the L3 contained a higher amount of carboxylic hydroxyl. L1 exhibited the lowest weight loss value (47.8%) at 200 to 600 degrees C and the highest amount of charred residue (45.1%), which was exactly the opposite for L3. Phenols, the dominant pyrolysis products, constituted 92.17% of all the pyrolysis products for L1, 93.93% for L2, and 88.09% for L3.