Structural changes of chars produced from fast pyrolysis of lignin at 100-300 °C

Lignin pyrolysis experiences complicated reactions because lignin has a heterogeneous structure and its low-molecular-weight portion melts at low temperatures during pyrolysis to form a liquid intermediate phase. This study presents a systematic study to understand the structural changes of the chars produced from lignin fast pyrolysis at 100-300 degrees C, focusing more on the low-molecular-weight portion (i.e., the THF-soluble portion) of lignin. The results show that low temperature lignin pyrolysis mainly proceeds with the THF-soluble portion via decomposition reactions into volatiles and polymerization reactions into the THF-insoluble portion. The decomposition of the THF-soluble portion starts at similar to 150 degrees C, mainly due to the loss of some thermally-labile structures such as hydroxyl groups and the alkyl aliphatic chains. Decomposition of the THF-soluble portion becomes significant as the pyrolysis temperature increases to similar to 175 degrees C, mainly because of the cleavage of weak ether bonds linked with beta-carbon or gamma-carbon to release phenolic monomers or oligomers. Meanwhile, significant polymerization also takes place at similar to 175 degrees C or above, leading to an increase in the yield of the THF-insoluble portion from similar to 31% in the raw lignin to similar to 67% in the char at 250 degrees C. Further decomposition of the THF-insoluble portion occurs as pyrolysis temperature increases. Because of enhanced polymerization reactions at higher temperatures (> 250 degrees C), the char structure becomes more condensed with decreased H/C atomic ratio and increased aromaticity.