Green wood, moisture content

The hardness of wood varies markedly from soft balsa to hard ironwood with pine, oak, and maple in between. It is measured either by determining the force needed to push a hard ball (diameter = 0.444 in) into the wood to a depth equal to half the ball s diameter (Janka hardness) or by the initial slope of the force vs. penetration-depth curve (Hardness modulus). Average values of Janka hardnesses for typical woods are listed in Table 13.1. The data are from Green et al., (2006), and are for penetration transverse to the tree axis. The values are for moisture contents of about ten percent. The first set of five items are hardwoods, while the second set are softwoods. To roughly convert Janka hardnesses to VHN multiply by 0.0045. 

Biomass differs from conventional fossil fuels in the variability of fuel characteristics, higher moisture contents, and low nitrogen and sulfur contents of biomass fuels. The moisture content of biomass has a large influence on the combustion process and on the resulting efficiencies due to the lower combustion temperatures. It has been estimated that the adiabatic flame temperature of green wood is approximately 1000°C, while it is 1350°C for dry wood [41]. The chemical exergies for wood depend heavily on the type of wood used, but certain estimates can be obtained in the literature [42]. The thermodynamic efficiency of wood combustors can then be computed using the methods described in Chapter 9. 

Dry Form Process, The advent of the pressurized refiner led eventually to dry-formed wood fiberboards because high-consistency pulps can be produced. Green wood chips contain equal parts water and dry wood matter and can be pressure refined in saturated steam with little change in moisture content. Fiber from the refiner at fifty percent consistency can be made fluffy and readily suspendable in air. This enables vapor-phase dewatering in hot-air driers to produce a fine, dry fiber that can be formed in air and dry pressed. Without liquid-phase dewatering, solubles formed by the steam cooking of chips remain in the fiber furnish going to the board machine. 

Moisture Content of Green Wood. The moisture content of wood in a freshly felled tree is designated as the green moisture content. The green moisture content may vary considerably among different kinds of trees and between heartwood and sapwood within a tree. It may also vary with height in the tree and with the season of the year in which the tree is felled. 

Water in green wood is found in three basic forms bound water in the cell walls, free or capillary water in the cell cavities, and water vapor, also in the cell cavities. The total amount of water in vapor form is normally only a small fraction of the total and is negligible at normal temperatures and moisture contents. When green wood dries... 

Moisture Sorption Isotherms. Green wood loses moisture to the atmosphere and approaches a moisture content designated as the equilibrium moisture content (EMC) for the particular atmospheric conditions. The EMC is a function of relative humidity, temperature, previous exposure history (hysteresis), species, and other miscellaneous factors. 

Values given are based on results of tests of approximately 50 species of green wood. Values for wood in the air-dried condition (12% moisture content) may be assumed to be approximately of the same magnitude. 

Free Radical Characteristics and Reactions in Weathered Wood. Wood, wood fiber components, and isolated lignin contain certain amounts of free radicals that are detectable by ESR spectroscopy (SS, S9). Unexposed green wood with 69% moisture content (in dark and in vacuo) was found (77a) to contain no free radicals. A trace amount of free radicals may be produced in the presence of oxygen, and most of these free radicals are generated in wood during mechanical preparation (90) as well as in wood exposed to electromagnetic irradiation. ESR studies revealed that wood interacts readily... 

Thus, if the green density of a piece of wood at 50% moisture content is 600 kg m its nominal density will be 400 kg m . The wood will contain 400 kg m of oven-dry material per cubic metre of green wood. 

The concept of the fibre saturation point cannot be over emphasized. Its importance lies in the fact that the manner in which water is held is different in the adsorbed and absorbed states. The fact that the absorbed water can be removed before stripping off the adsorbed water indicates that a distinction can be made between these two states of sorption. For example, when green wood is dried there is no appreciable change in its mechanical properties until the fibre saturation point is reached. Below this moisture content they increase almost linearly with any further decrease in moisture content. Furthermore, wood only shrinks when the adsorbed water is removed from the cell walls. 

Some typieal moisture content values for green wood are noted in Table 3.1. These values are considerably greater than the fibre saturation point. Absorbed water at the surface will evaporate and the lumber will dry provided the surrounding atmosphere is not totally humid. Indeed the absorbed water in the lumens cannot remain there in equilibrium with the atmosphere unless the relative humidity of the air is in excess of 99% (Table 3.3). If the wood is left under cover - keeping the rain off - it will eventually dry to a moisture content that will vary according to the temperature and humidity of the air (Figure 3.2). This moisture content will be below the fibre saturation point so all the absorbed water and some of the adsorbed water will have evaporated. If an even lower moisture content is required it is necessary to use a kiln to lower the relative humidity and raise the temperature (Figure 3.2). 

Even with a permeable wood diffusion assumes increasing importance as the average moisture content approaches the irreducible moisture content indeed, in every part of the board where the moisture eontent approaehes this value drying is diffusion controlled. Permeable and impermeable timbers of similar densities should dry from fibre saturation at about the same rate. The behaviour of mixed heart/sapwood boards is eomplieated sinee, at first, there is both an evaporative interface near the sapwood surfaee and one in the interior at the zonal boundary between heart and sapwood. For a board with only a slither of heartwood along one face, mass flow can only move to the sapwood faee so in effeet the board appears to be twice the width than it aetually is. Pang et al. (1994) predieted that such a 50 mm thick board would dry from green to 6% moisture eontent using a 140°C/90°C schedule in 14 hours, compared to 10 hours for sapwood and 11 hours for heartwood. 

When wood is green the diffusion eoeffieient of water vapour in the lumens is about 10 times greater than that of adsorbed water in the eell wall, but when the wood is dry, c. 5% moisture content, the diffusion eoeffieient of water vapour in the lumen is about 1000 times greater than that of water in the eell wall the difference is least when the moisture content of the eell wall is elose to the fibre saturation point and greatest when the moisture content approaehes oven-dry. 

Koshijima T and Watanabe T (2003) Association between lignin and carbohydrates in wood and other plant tissues. Springer-Verlag, Berlin Kretchmann DE (2003) Velcro mechanics in wood. Nature Materials, 2 775-6 Kretschmann DE and Green DW (1996) Modeling moisture content-mechanical properly relationships for clear Southern Pine. Wood and Fiber Science, 25(3) 320-37 Kubler H (1987) Growth stresses in trees and related wood properties. Forestry Abstracts, 45(3) 131-89... 

Green wood contains a lot of water. In the outer parts of the stem, in the sapwood, spruce and pine have average moisture content of about 130%, and in the inner parts, in the... 

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