Fractioning and chemical characterization of barks of Betula pendula and Eucalyptus globulus

Abstract

The composition of birch (Betula pendula Roth.) and eucalypt (Eucalyptus globulus Labill.) barks was studied after grinding and fractioning into different particles sizes.

There was a significant difference in the fractionation of both barks in relation to the yield of fines (5.9% and 28.3% of particles under 0.450 for birch and eucalypt, respectively) and of coarser particles over 2 mm (70.7% and 41.4%).

The chemical composition of birch and eucalypt barks, as a mass weighed average of all granulometric fractions was, respectively: ash 2.9% and 12.1%; total extractives 17.6% and 6.5% (hydrophilic extractives were dominant), lignin 27.9% and 28.8% and holocellulose 49.8% and 62.6%. Birch bark contained a considerable amount of suberin (5.9%) whereas eucalypt bark contained a very small amount (<1%). The carbohydrate composition differed between birch and eucalypt barks, i.e., respectively, glucose 47.0% and 68.4%, and xylose 33.8% and 23.2% of total neutral monosaccharides.

Ash elemental composition was different in both species. Birch bark contained in relation to eucalypt bark, in the 0.250–0.450 mm fraction, more N (0.69% vs. 0.26%) and P (0.075% vs. 0.001%), and less Ca (0.39% vs. 0.62%), K (0.24% vs. 0.31%) and Mg (0.07% vs. 0.15%). High concentration of Zn was found in birch bark (217 mg/kg vs. 11 mg/kg in eucalypt bark).

After grinding and granulometric separation, extractives were present preferentially in the finest fraction with an enrichment in dichloromethane and ethanol solubles especially in the case of birch bark. Eucalypt bark had a high content of cellulose and hemicelluloses especially in the coarser fraction. The fibrous character of this fraction shows its potential as a fiber source.

Introduction

Bark represents a substantial proportion of the aboveground total biomass of trees. During industrial processing for timber or for pulping, bark is removed from the logs and constitutes an important mill residual material that is usually burnt for energy production. In addition to being considered a valuable solid biofuel, bark is also scrutinized for more added-value products that will consider potential specific chemical composition or properties (Demirbas, 2010).

Bark valorization, namely if envisaged within a biorefinery platform, therefore requires a careful examination of composition and processing characteristics. Barks are usually rich in extractives, including organic solvent and water solubles, and in polyphenolics, and they also contain a high amount of inorganic material (Fengel and Wegener, 1984, Pereira et al., 2003). Structurally barks are complex tissues and their sampling, characterization and processing have difficulties that are not found, e.g. in wood processing.

Hardwood species are presently the most important source of wood for pulp production (Patt et al., 2006). White birch (Betula pendula) is the dominant pulpwood species in Northern European countries (especially in Finland and Russia) and eucalypts (mainly Eucalyptus globulus) are dominant in Portugal and Spain, in Southern Europe. The pulpwood consumption in Europe in 2010 of birch and eucalypt was, respectively, 18 425 million m³ and 13 708 millions m³, corresponding to 12.5% and 9.3% of the total pulpwood consumption (CEPI, 2010).

In both cases considerable amounts of bark are made available from log debarking, and are separated in the mill as a residual product and used as fuel.

In E. globulus trees at the age used for pulping (9–13 years in temperate climates) bark has a thickness in the range of 3–16 mm and corresponds to 7–20% of o.d. mass of the stem depending on site and genetics (as compiled in Pereira et al., 2010).

There are few studies on B. pendula bark content. Jensen (1948) reported 3.4% outer bark in birch logs. Bhat (1982) referred that bark thickness is strongly associated with stem diameter and reported a mean value of 16.6 mm for the double bark thickness of trees aged 65–95 years. Repola (2008) reported a double bark thickness at breast height between 2.5 and 38.7 cm in trees within an age range of 7 and 132 years. For young trees with 1–16 years of age and 0.3–24.0 cm stem diameter, Trockenbrodt (1991) reported bark thickness values between 7.1 and 26.1 mm.

Some studies have characterized eucalypt bark anatomically (Quilhó et al., 1999, Quilhó et al., 2000) and chemically (Sakai, 2001, Bargatto, 2010) as well as birch bark (Bhat, 1982, Trockenbrodt, 1991, Harkin and Rowe, 1971). However little is known on their fractioning behavior and on the chemical characteristics of different fractions. The use of fractioning is usually involved in biomass processing and may be used for selective enrichment of specific components by taking advantage of the biomass chemical and structural heterogeneity (Miranda et al., 2012, Silva et al., 2011). However, this is species specific and depends on the specific bark characteristics, as recently shown for Picea abies, Pinus sylvestris and Quercus cerris (Miranda et al., 2012, Sen et al., 2010).

This paper studies the chemical composition of the barks of these two important European hardwoods, silver birch (B. pendula) and Tasmanian blue gum (E. globulus), obtained as residual by-products of commercial debarking in pulp mills, after fractionation into different particle sizes. Summative chemical analysis and inorganic composition were determined for each granulometric fraction, as well as bulk density. The objective is to analyze the potential of triturating and particle fractioning as a biomass pre-treatment step for a selective component enrichment within a bark valorization chain for energy, composite materials and chemicals.