Maija Kymäläinen

Sorption behaviour of torrefied wood and charcoal determined by dynamic vapour sorption

AbstractThe most important variable of biomass-based fuels is moisture, because it affects the entire logistic chain by creating problems related to transport, handling, storage, and combustion. Recently, there has been a growing interest in thermal pre-treatment of biomass-based fuels by torrefaction. Torrefaction is intended to overcome the moisture-related problems by significantly reducing hygroscopicity and thus the logistics of solid biofuels could be improved. In order to gain a deeper insight on the changes induced by torrefaction, the sorption properties of Finnish birch and spruce wood were investigated. The sorption isotherms, hysteresis, accessibility, and surface area were investigated with dynamic vapour sorption. Also the particle size distributions and the clustering behaviour of water molecules were examined. As a result of the thermal pre-treatment, accessibility, adsorption of water vapour, and hysteresis were reduced. Particle size distribution was shifted towards smaller particles and the surface area measured with the BET method was reduced. Hysteresis was also reduced, which may be linked to the overall reduction in material’s hygroscopicity, as well as changes in porosity. The particle size affects sorption by increasing the surface area, but has a dual effect on accessibility, as it also blocks access to pores through agglomeration. In practice, the most important result is the increased hydrophobicity, but also the increased porosity and reduced particle size are to be considered as they can affect the handling and storage properties of torrefied and charred material negatively.

Sorption properties of torrefied wood and charcoal

Abstract Pre-treating biomass by torrefaction is assumed to improve the materials storage properties because of reducing hydrophilicity. In order to assess the effect of sorption on storage properties, the adsorption of water vapour and capillary absorption of liquid water in torrefied and charred spruce and birch were studied. In addition, the chemical changes were evaluated through Fourier Transform Infrared (FT-IR) spectroscopy. Adsorption decreased notably as severity of treatment increased, as was expected due to degradation of the wood constituents, namely hemicelluloses and amorphous cellulose. Capillary absorption increased with increasing severity in spruce samples while birch showed less change, but the maximum volume for absorption increased with both species. FT-IR results showed an increase in aromatic structures that have a role in forming crystalline structures, possibly leading to increased porosity. Torrefied and charred material should not be stored outside, as liquid water absorbs readily into the material, turning it into a suitable substrate for fungi.

Experimental techniques for characterising water in wood covering the range from dry to fully water-saturated

Water plays a central role in wood research, since it affects all material properties relevant to the performance of wood materials. Therefore, experimental techniques for characterising water within wood are an essential part of nearly all scientific investigations of wood materials. This review focuses on selected experimental techniques that can give deeper insights into various aspects of water in wood in the entire moisture domain from dry to fully water-saturated. These techniques fall into three broad categories: (1) gravimetric techniques that determine how much water is absorbed, (2) fibre saturation techniques that determine the amount of water within cell walls, and (3) spectroscopic techniques that provide insights into chemical wood–water interactions as well as yield information on water distribution in the macro-void wood structure. For all techniques, the general measurement concept is explained, its history in wood science as well as advantages and limitations.

The effect of compression and incision on wood veneer and plywood physical and mechanical properties

ABSTRACT Drying takes the largest share of energy in plywood production, and varying moisture content of veneers necessitates re-drying that often leads to over-dry veneers with deactivated surfaces, which may promote imperfect bonding. In order to decrease the drying time, reduce the need for re-drying of veneers, and improve the quality of plywood, birch and spruce veneers were subjected to pre-treatment by cold compression, incision, or a combination of the two. The effects of pre-treatment on the veneer and plywood quality were assessed by standard tests. Compression had a beneficial effect on water removal of the wettest veneers (spruce sapwood (SW) and birch), but some thickness reduction was observed in the veneers as well as the finished birch plywood. Compression led to thickness reduction of spruce veneers, but had no effect on SW plywood thickness likely due to higher viscoelasticity. Both compression and the combination of incising and compression levelled the moisture variation within the compressed stacks. Incision improved the modulus of elasticity of birch plywood, shear strength of SW plywood, and both bending and shear strengths of heartwood plywood. Higher surface pressure decreased the drying time of spruce SW in both plain compression and combined incision and compression pre-treatment.

Sorption-Related Characteristics of Surface Charred Spruce Wood

Surface charring of wood is a one-sided thermal modification process that can be used to create a hydrophobic, durable surface to exterior claddings. Spruce (Picea abies L.) wood samples were charred with a hot plate and several time-temperature combinations while using simultaneous surface compression. Temperature profile, water sorption, cupping after water exposure and density profile were measured. Furthermore, changes in the microstructure and surface functional groups were investigated by scanning electron microscopy and photoacoustic FT-IR spectroscopy. Results show that surface charring notably improves the hydrophobicity measured by contact angle, water floating and dynamic vapour sorption. Increased holding time during charring reduced the sorption but at the same time increased the dimensional instability measured by cupping. The density profile showed a shifting density peak with more severe modification regimes, indicating a more porous surface. The PAS-FTIR showed increased aromaticity of the surface that was also present in the pyrolysis zone beneath the surface in samples modified with longer holding time. Higher modification temperature affected the sorption as well as cupping positively but it is possible similar results can be obtained with lower temperature and longer holding time.

Erratum to: Sorption behaviour of torrefied wood and charcoal determined by dynamic vapour sorption

1 Department of Forest Sciences, University of Helsinki, P.O. Box 27, 00014 Helsinki, Finland 2 Department of Forest Products Technology, School of Chemical Technology, Aalto University, P.O. Box 16300, 00076 Aalto, Finland 3 Norwegian Forest and Landscape Institute, PO Box 115, 1431 As, Norway 4 Department of Architecture and Civil Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK