(82g) What Can We Learn About Fast Pyrolysis of Lignin From Solvent Liquefaction?

Fast pyrolysis of lignocellulosic biomass produces bio-oil consisting of an emulsion of lignin-derived phenolic oligomers in an aqueous phase derived mainly from polysaccharides.  The phenolic oligomers present several problems to the commercial utilization of bio-oil, including high viscosity, low volatility, and poor thermal stability.  Ideally, lignin would depolymerize into more tractable phenolic monomers and dimers.  Achieving such an outcome requires a better understanding of lignin pyrolysis than currently exists.  Many researchers suggest than the oligomers represent fragments of lignin that have been thermally ejected from the pyrolysis zone as liquid droplets.  Others suggest that these oligomers derived from volatile depolymerization products that recombine in the gas phase and nucleate to liquid aerosols.

Solvent liquefaction is an alternative thermal depolymerization process that can be described as “pyrolysis in a solvent.”  Investigating the chemical and physical phenomena that control thermal depolymerization of lignin in some respects is more easily conducted under conditions of solvent liquefaction than fast pyrolysis because reactions occur more slowly and in a single phase.  Solvent liquefaction studies suggests the rapid and complete depolymerization of lignin to monomers and dimers followed by repolymerization to phenolic oligomers unless reaction is quenched by either dispersing or “capping” the depolymerization products.  This suggests a path forward to produce more desirable fast pyrolysis products from lignin.