Improving fast pyrolysis of lignin using three additives with different modes of action

Lignin holds the potential to obtain key monoaromatic compounds upon its depolymerization. Depolymerization of woody biomass by pyrolysis is well established but often unsuccessful for lignin due to a combination of its melting, agglomeration, and modest yields towards aromatics. Therefore, several lignin additives have been put forth to overcome one or more of these hurdles. Although some seem promising, a direct comparison is obscured by differences in applied technical lignin types, reactor configurations/scales, and product analyses. Moreover, the effects of additives have either been evaluated mostly on an analytical scale or their mode of action is not entirely understood. This work involves the addition of clays, calcium hydroxide and sodium formate to lignin, each having a different (putative) mode of action, in a well-defined and comparable manner. Organosolv lignin and lignin with additives were analysed by TGA/DSC and py-GC/MS. Pyrolysis was performed in a lab-scale reactor (350 g feeding). The pyrolysis liquids were characterised through elemental analysis, GCxGC-FID, GCxGC-HR-ToF-MS, GPC, and HSQC NMR analyses. All additives overcame melting issues and led to increased liquid yields but the most promising were attapulgite and calcium hydroxide. Lignin with attapulgite resulted in a heavy phase with the highest carbon yield (25.7%) and a substantial monomer yield (18.9%, mostly alkylphenols). Lignin with calcium hydroxide resulted in a heavy phase with the highest monomer yield (23.8%, mostly alkylphenols) at a substantial carbon yield (15.1%). The pyrolysis mechanisms for lignins with additives are elaborated and updated in this work.