Title:
Comparison of Thermal and Hydrothermal Lignin Degradation - Reaction Mechanisms and a Strategy for Lignin Recovery
Author(s):
Hornung, U., Kruse, A., Forchheim, D., Breunig, M.
Document(s):
Paper
Abstract:
Lignin is generated in huge amounts of about 50 Mio t from wood pulping as by-product and enormous efforts in research were made to develop a process for sustainable phenol production from lignin. To understand that problem and to identify the bottlenecks we compare the different reaction mechanisms of thermal and hydrothermal lignin depolymerisation. Whereas the thermal lignin degradation is a radical chain reaction starting with the cleavage of weak ether bonds, consecutive .-scissions, hydrogen abstractions, termination reactions, the hydrothermal lignin depolymerisation is influenced by the competitive hydrolysis reaction in the aqueous medium and thermal degradation reactions. Hydrotreating with organic sovents and catalyst influence the radical chain process of degradation reactions in the way that radicals are captured and hydrogenated and functional groups are eliminated. Due to the special properties of water near the critical point and in situ hydrogenation via the water gas shift reaction, the hydrothermal lignin depolymerisation is more selective and favors the formation of catechols. The hydrothermal degradation of lignin starts with a fast hydrolysis of weak ether bonds and lead to methoxy phenols and reactive oligomers as primary products. Nevertheless the amount of methoxy phenols is very limited, which is the main reason for the insufficient amount of monocycles from lignin degradation. The reactive oligomers further degrade to catechols, stable dimers and perform crosslinking reaction to chars which are comparable to chars from pyrolysis. The oligomers are monitored by GPC and the monocyclic compounds are quantified by GC-FID. On that basis the reaction mechanism for hydrothermal lignin depolymerisation is developed and validated. A strategy for a possible lignin recovery could be to stop the hydrothermal lignin depolymerisation at an early point of reaction and to take out the methoxy phenols and catechols in the water phase and to extract the solvable reactive oligomers from the solid residues and to recycle the unreacted lignin back to the process.
Keywords:
lignin, hydrothermal liquefaction, thermal degradation of lignin, reaction mechanisms
Topic:
Biomass Conversion Technologies for Liquid and Gaseous Fuels, Chemicals and Materials
Subtopic:
Pyrolysis and other biomass liquefaction technologies
Event:
23rd European Biomass Conference and Exhibition
Session:
3AO.6.1
Pages:
923 - 927
ISBN:
978-88-89407-516
Paper DOI:
10.5071/23rdEUBCE2015-3AO.6.1
Price:
FREE
