Pilot scale fast pyrolysis of lignin in a twin-screw reactor

Fast pyrolysis is a promising method for converting lignin into valuable bio-oils, yet it faces significant processing challenges related to melting and agglomeration. In this study, the use of a twin-screw reactor for lignin fast pyrolysis at pilot scale was investigated. The twin-screw reactor design was changed to promote mechanical breakup and better address the issues encountered with melting and agglomeration. Two types of lignin were tested: Indulin AT, a Kraft lignin, and Miscancell, a Miscanthus-derived alkali lignin. The experiments with Indulin AT encountered significant challenges related to melting and agglomeration, which ultimately led to its interruption. Changes in the screw reactor design successfully reduced agglomeration inside the reactor, which led to the feeding system becoming the bottleneck for long term operation in terms of agglomeration. In contrast, pyrolysis of the Miscancell lignin proceeded successfully without any agglomeration issues. The difference in processing behavior between the two lignins can be explained by their distinct chemical nature and properties. Analysis of the bio-oils showed that the Indulin AT oil is rich in phenolics, particularly phenols and guaiacols, while the Miscancell oil displayed a broader variety of components, including phenols, guaiacols and syringols, as well as sugars and furans. These findings highlight the potential of the twin-screw reactor for lignin pyrolysis, particularly with Miscancell lignin or similar types that do not exhibit severe melting behavior. However, further optimization of the feeding system is essential to address the issues encountered with Indulin AT, and enable continuous processing of lignins prone to severe melting behavior, such as Kraft lignin.