36 Drying of Wood

set at a constant value at constant wet-bulb temperatureafter an initialization period, followed by aperiod at constant dry-bulb temperature.The measurement of the boards’ moisture content,at the end of drying, has frequently been done withhand-held resistivity meters, often complementedwith more time-consuming gravimetric measurementswith sample boards (EN 13183-1). The Standard EN13183-2, Round and Sawn Lumber—Procedure toMeasure the Moisture Content, employs a hammerprobe with insulated measuring pins, which areinserted to a depth of approximately one third ofthe board’s thickness, and is valid over the moisturecontent range from about 7 to 30%. This techniquehas been adapted by inserting probes in the kilnstacks to give a continuous reading of the moisturecontent in the later stages of drying. Nonpenetrativesystems are available in which the resistivity probesare aluminum fillets placed along the length of thelumber charge to measure the resistance of all theboards that the probes are in contact with. All resistivemethods, however, are limited to measuring moisturecontents from just above fiber saturation andbelow, with one system having a claimed repeatabilityof +2% when the average moisture content is lessthan 16% (Furniss, Priv. Comm., 2002).The alternative use of microwave-sensing technologiesis attractive in enabling a wider range ofmoisture contents to be determined from green todry (Holmes and Riley, 1996; Riley and Holmes,2001). The first prototype tested used a waveguide,40 20 1.2 mm, with angled slots in the long face,connected by a high-temperature coaxial cable to theoscillator and placed in a stack so that the slots werein contact with the lower face of a board. A laterdevelopment enabled the possibility of the stack’smoisture distribution to be determined. The experimentshave demonstrated the potential for the manufactureof a rugged industrial system. However, unlessthe timber is very uniform in properties, this techniqueis uncertain as the relationship between themicrowave signal and the moisture content becomesfuzzy because of factors such as variations in wooddensity among the boards.An online contact-free method of measuring themoisture content of wood, which involves traversingthe boards on a band between sensor heads, could bedeveloped based on techniques used in papermaking.The method would be suitable for sorting boardsafter drying. At least one such method is commerciallyavailable, but the measurement principle hasnot been divulged (Smith, Priv. Comm., 2002).The benefits of even a relatively simple kilncontrolsystem are illustrated in an example quotedby Culpepper (1990). The incorporation of aprogrammable logic controller at one site increasedthe overall grade recovery from 70.7 to 81.9%, with areduction in energy costs of 43% for steam and 10%for electrical power. The total saving represented a1.2-y payback on the investment.36.3.4.3 Volatile EmissionsAcidic corrosion is a well-recognized feature of kilndrying, particularly with certain species such as oaks.Packman (1960) notes that, of some 150 species studiesof both hardwoods and softwoods, the majority(80%) has a pH between 4 and 6 and only one had apH consistently above 7. In an extreme case, with ahardwood of high extractives content, sufficiently severerusting of steel trolley wheels and other ferrousfittings in a pilot-plant kiln required their replacementafter a 2-week period. Extensive use of aluminumlinings with plastic piping in modern kilns has largelyavoided the problems of corrosion.The corrosive nature of the volatile substances releasedduring kiln drying is derived from the presence offree acetic acid in the wood and from the hydrolysis ofthe various acetyl groups attached to the hemicelluloses.Softwoods generally yield greater emissions thanhardwoods, because the latter are normally dried atlower temperatures and have lower resin content.Some volatile substances, such as formaldehyde, comefrom the thermal degradation of the hemicelluloses andlignin, whereas green pines release terpenes and theirderivatives. Table 36.11 lists the concentrations of volatileemissions from two high-temperature schedules.Milota (2001) compared the emissions from variousNorth American lumber species in a small-scale kilnTABLE 36.11Concentrations of Volatile Emissions Arisingfrom Two High-Temperature Kiln Schedulesfor Pinus radiataCompoundConcentration, g m 3 at dry/wet-bulb temperatures (8C)120/70 140/90Formaldehyde 19.5 31.0Acetic acid 21.7 38.2Monoterpenes 34.8 66.4Hydroxylated monoterpenes 12.6 10.7Condenser residues(resins and fatty acids)16.4 16.4Source: McDonald, K.A. and Wastney, S., Analysis of volatileemission from kiln drying of radiata pine, Proceedings of theEighth International Symposium on Wood Pulp Chemistry, Vol. 3,431–436, 1995.ß 2006 by Taylor & Francis Group, LLC.