A REVIEW OF WOOD-BARK ADHESION: METHODS AND MECHANICS OF DEBARKING FOR WOODY BIOMASS

Abstract

Debarking systems and strategies are used to increase the value of woody biomass by separating wood and bark into two value-added product streams. Several debarking methods have been used for the removal of bark from wood. The selection of a debarking method is often based on the wood type and its end use. Debarking methods discussed briefly in this review include drum debarking, ring debarking, cradle debarking, chain flail debarking, high-pressure water jet debarking, compression debarking, and biodebarking. The performance of these debarking methods is highly dependent on operational parameters of machines, properties and type of woody biomass, and pretreatments. A universal applicable mechanistic model of the debarking process would be especially valuable for the development and optimization of debarking systems. In this respect, the competing objectives of high bark removal, low wood damage, high throughput, and low energy must be balanced against one another to arrive at a truly optimized approach. Wood-bark bond strength plays a vital role in impacting the effectiveness and efficiency of debarking technology. Thus, it is important to understand how mechanical properties of the wood-bark interface are influenced by different factors. Key factors that affect the wood-bark bond strength include MC, harvest season, wood species, temperature, and direction of applied load. The likely reason why MC affects the wood-bark adhesion strength is that the constituent elements of primary plant cell walls (cellulose, hemicellulose, and pectin) behave differently when they are exposed to water molecules. For example, there is negligible change in the length if cellulose microfibrils when exposed to water molecules. However, characteristics of hemicellulose and pectin behave differently than cellulose when they contact with water molecules. Also, the difference in the adhesion strength of wood-bark bond among varieties of woody plants is possibly due to the difference in density of cross linkages of homogalacturonan pectin by Ca2., and arabinan and galactan side chains of pectin. The relevance of this information to the debarking process is discussed.