Detecting the inner mechanism of agglomeration behaviors and product properties during fast pyrolysis of lignin via alkaline additives

Abstract

Lignin can be converted into valued-added phenol-rich bio-oil by pyrolysis, but the melting and agglomeration nature causes serious problems for thermal conversion of lignin. To investigate the inner melting and agglomeration mechanism during lignin pyrolysis, three types of alkaline additives (monoacidic base NaOH, diacidic base Ca(OH)2 and triacidic base Al(OH)3) were employed in this study. Results showed that agglomeration behaviors during pyrolysis was determined by both of active functionalities and molecular structure in lignin. It was found that char foaming was partially reduced in L-3%Na, for the NaOH modification only inhibited active functionalities in lignin structure. For Ca(OH)2 modification, the Ca(OH)2 not only reacts with active functionalities in lignin but also links single lignin molecules to cross-linked macromolecule polymers, thus the agglomeration was completely eliminated in L-3%Ca. While, effects of Al(OH)3 modification on agglomeration inhibition was negligible, due to its weak alkalinity. Analysis on bio-oil revealed that three alkali additives also promoted the mono-phenolics content in bio-oil. Specially, NaOH modification preferred to the formation of phenol, Ca(OH)2 modification raised alkylphenols content, while the effect of Al(OH)3 modification was the least. The maximum mono-phenols content (91.14%) and alkylphenols content (60.53%) was both obtained in L-10%Ca which pyrolyzed at 500 °C.