Dynamic Dielectric Properties of a Wood Liquefaction System Using Polyethylene Glycol and Glycerol

Microwave-assisted liquefaction has shown potential for rapid thermal processing of lignocellulosic biomass. The efficiency of microwave heating depends largely on the dielectric properties of the materials being heated. The objective of this study was to investigate the dynamic interactions between microwave energy and the reaction system during the liquefaction of a woody biomass sample. The dielectric properties of poplar wood particles, model compounds representing the three main chemical components of wood, and individual liquefaction solvent components, along with their mixtures, were measured to evaluate their abilities to convert microwave energy to heat at different frequencies. Dry samples of wood particles, cellulose, xylan, and lignin were all poor microwave energy absorbers as indicated by their low dielectric values relative to the liquefaction solvent components and their mixtures; among the two solvents, polyethylene glycol had lower dielectric values than glycerol, likely due to its larger molecular size. Ionic conduction significantly affected the dielectric loss factor of the liquefaction solvent mixture upon the addition of the sulfuric acid catalyst. During the wood liquefaction reactions, temperature was the main factor that governed the dielectric properties through the preheating stage, and then when the system reached 130 °C, the dielectric properties were governed by changes in chemical composition.