Elucidating radical-mediated pyrolysis behaviors of preoxidized lignins

Abstract

Effect of lignin preoxidation on subsequent radical-mediated pyrolysis was discussed in this study. Technical hot-water-extracted lignin was preoxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in diverse degrees and pyrolyzed under different temperatures. Characterizations indicated that preoxidation increased lignin oxygen contents and converted α-hydroxyls to α-carbonyls. These structural modifications caused by preoxidation reduced the thermal stability and pyrolysis reactivity of lignin, shifting lignin thermal decomposition to the low temperature region and inhibiting lignin pyrolysis into bio-oil fractions. However, recognition of species and yields of specific compounds via analytical pyrolysis declared that although preoxidation reduced product yields, it did not alter the reaction pathways. The fixed bed experiments proved the above findings and gave the gas compositions, mainly CO₂ derived through decarbonylation. Both radicals in chars and bio-oils were monitored, and char radical concentrations were proportional to the preoxidation degrees. This work sorted out the performances of lignin pyrolysis after preoxidation and determined their negative effects.

Keywords: Lignin; Mechanism; Preoxidation; Pyrolysis; Radical.