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Molecules 2020, 25, 5367 2 of 13 passive natural air flow at ambient temperature while the wood stack is kept protected from the weather. Practiced for many centuries, this is often referred to as wood “seasoning”, which is a process that can be enhanced by use of industrial dehumidifiers. While gentle on the wood material, seasoning is considered too slow for high-volume industrial wood production to meet demand. Higher throughput can be achieved by the application of heat to stacked green wood, which is a process conducted on large industrial scales in drying kilns [1]. In its simplest form, kiln drying is carried out at atmospheric pressure with fan-circulated hot air [2] at temperatures ranging from 70 up to 100–120 ◦C or higher, with the exhaust from the kiln, comprising steam and volatile compounds derived from thermally modified wood material [3] discharged into the local airspace. Advanced methods of heating the wood cell water to generate water vapour internally include using microwave energy [4] or radiofrequency [5]. All these methods require the phase change of cell water from liquid to vapour, which is an energy-demanding process requiring the breaking of hydrogen bonds between water molecules or between water molecules and wood cell wall polymers (cellulose and hemicelluloses) [2]. Within the structure of wood undergoing drying at elevated temperature, water moves between the tracheid cells via interconnecting pits [2]. Using magnetic resonance imaging (MRI), it also has been shown that during kiln drying, cell water moves anisotropically within the earlywood (the spring-wood located between the tree annual rings, anatomically known as “latewood”) of sapwood towards the drying surface [6]. A consequence of kiln-drying industrial softwood, such as from radiata pine, is the irreversible closure of the cell wall pits [2], the apertures between cell walls that allow movement of water within the stem of the living tree [7]. The pressure differential generated between adjacent cells during active drying snaps the pits shut, which is an irreversible process known as “pit aspiration” [2]. The ensuing internal moisture gradients generated can also result in distortion of the wood bulk material [8,9], which in commercial practice is minimised by placing massive weights on the wood stack. High-temperature drying may also result in a degree of physical collapse of the wood cell walls, resulting in lowered wood strength. Drying may also promote molecular changes leading to formation of “kiln brown stain” [10], which is exhibited as dark-brown patches revealed during the planing and finishing of the dried wood, thereby compromising wood product quality and value. An issue for communities where wood drying kilns are located close to habitation is the often the “nuisance odour” of cooked wood. Kiln emissions from kiln drying of radiata pine wood have been shown to contain the monoterpenes α- and β-pinene as well as toxic molecules such as methanol, formaldehyde, acetaldehyde, furfural, and guaiacol [3]. These compounds arise from the thermal breakdown of the wood cell wall polymers, cellulose, hemicellulose, and lignin. 1.2. New Approaches to Industrial Wood Manufacture Using Dewatering Processes In order to devise a process to rival convention, in which the problems described above might be circumvented, conceptual models whereby the known biophysics and chemistry of cell water movement and hydrogen bonding have been developed to address the fundamental issue of the energy-demanding phase change of cell water from liquid to vapour during wood drying. Novel approaches to removing water from green wood have focussed on dewatering using mechanical rather than thermal processes. Compression-rolling of green wood squeezed water from the cells and increased the density and surface hardness of the wood material once dried, but it also reduced the wood material volume (the measure by which wood material is usually sold, e.g., $/m3) and destroyed the cell wall structure [11]. Another method applied centrifugal force to high-moisture-containing wood, accelerating the cell-wall water sufficiently to overcome the internal forces between cell water and cell lumens and capillaries, enabling dewatering [12]. One promising green wood dewatering process used cycles of compressed air, in which an “incubation” stage, where air molecules were forced into wood cell water under high pressure where they dissolved, before a second, decompression stage, caused dissolution of the air. The subsequent airPDF Image | Dewatering Green Sapwood Using Carbon Dioxide
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