large microwave wood drying kiln

effects of microwave pretreatment on drying of 50mm-thickness chinese fir lumber | journal of wood science | full text

Low permeability of wood causes problems during drying of timber. This study evaluated the effects of microwave (MW) pretreatment on the conventional drying behavior and mechanical damages of Chinese fir lumber. MW pretreatment of lumber was performed at applied MW energy of 43 kWh/m3, and then, the samples were dried in a laboratory drying kiln. The results showed that the drying rate was effectively increased after MW pretreatment. The moisture content (MC) deviation in thickness and residual stress indexes of MW-pretreated samples were significantly decreased in comparison with the control samples, and the appearance quality of wood samples was not clearly affected by the MW pretreatment. Scanning electron microscope (SEM) micrographs demonstrated that pit membranes were damaged after MW pretreatment, and the micro-cracks in radial section as well as detachments between ray parenchyma cells and tracheids were also observed. Consequently, new pathways for moisture migration during drying process were formed after MW pretreatment, which contributed to the improved permeability of Chinese fir lumber and decreased drying time.

Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is one of the most important plantation species in China. It has been widely used in timber construction and manufacturing of bridge, boat, and furniture [1]. However, the large proportion of heartwood and low permeability of Chinese fir causes problems during drying process, such as long drying times, high energy consumption, and a number of drying defects, which seriously restrict its high efficient utilization.

Wood permeability reflects the easiness with which a fluid moves inside the wood under a given pressure gradient. The wood permeability is influenced by the conditions of cell tissues, chemical components, and physical properties of wood, and conditions of cell tissues are the most important factors [2]. In softwood, bordered pit is the primary structure that governs the permeability [3, 4]. Pit aspiration, pit occlusion with extractives, and pit incrustation are effective in reducing the capillary size of the pit pairs [5]. Increasing the radius and the number of pit membrane openings can improve the permeability of wood; thus, making the wood easier to dry and impregnate [6]. Furthermore, the fluid flow inside softwood includes three primary pathways. In longitudinal direction, fluid moves through the tracheid lumen, pit aperture, and pit membrane pores [4, 7, 8], while in the tangential direction, fluid flow is primarily through tracheids and intertracheid bordered pits [3, 8], and the ray tracheids are regarded as the main radial pathways [9, 10]. Therefore, it could be an effective way to facilitate fluid flow in different pathways to achieve good permeability improvement.

Microwave (MW) treatment is an effective method to increase wood permeability [11, 12]. The MW energy vaporizes the water in wood rapidly during the treatment and the increasing steam pressure causes varying degrees of damage to each cell tissue of wood. Pit membranes, ray cells, tyloses, and even main cell walls are ruptured to form new pathways for moisture migration, which lead to increased permeability and create favorable conditions for wood drying, wood functional improvement, and new material preparation [13, 14]. Different degrees of MW modification depend on the process conditions and MW process parameters. With the increase in MW energy, the wood permeability increases and mechanical properties decrease [15]. Low degree of MW modification accelerates the drying rate and does not significantly affect the wood mechanical properties [15, 16]. Meanwhile, drying defects (surface and internal checks; collapse) are reduced with the decrease in drying stress [15, 17, 18]. The increased permeability makes moisture move faster during the drying process, ensuring that the evaporative front is maintained at the wood surface for a long time and moisture gradient is reduced in the thickness direction, thereby decreasing the drying stress. Therefore, MW pretreatment is a proven way to reduce the drying time and drying degradation of refractory wood. However, previous studies have usually concentrated on the MW pretreatment for rapid drying of hardwood; the MW pretreatment of softwood was seldom reported in literature.

In this study, Chinese fir lumber was treated with MW radiation before conventional drying. The effects of MW pretreatment on drying time, drying rate, drying quality, and moisture diffusion were investigated. Scanning electron microscope (SEM) observations were conducted to investigate the occurrence of mechanical damages of cell tissues of wood via MW pretreatment. The results can provide a theoretical and technical support for rapid drying and high-value utilization of Chinese fir.

Chinese fir logs with 24cm diameter at breast height were procured from Yaan of Sichuan province, South-west part of China. The basic density and oven-dry density of this wood species were 0.33g/cm3 and 0.37g/cm3, respectively. Lumbers with dimensions of 900mm (Length)90mm (Width)50mm (Thickness) were sawed from the heartwood region of these logs. Prior to MW pretreatment, all lumbers were cut off the ends and then divided into two groups with dimensions of 300mm (Length)90mm (Width)50mm (Thickness). One group was used for MW pretreatment, while the other was used as the control group. During this process, 20mm length specimens were cut as the moisture content (MC) test pieces from 900mm lumber length direction in order to determine its initial MC. Both groups were also divided into quarter-sawn samples and flat-sawn samples. All the samples were stored in a freezer with a temperature of 5C to prevent the moisture loss. The average initial MC of the samples before MW treatment ranged from 50 to 60%.

Continuous feeding MW equipment (Frequency: 915MHz) (Manufacturer: Nanjing Sanle MW Co, WX20L-19 type) was used for wood MW pretreatment experiments (Fig.1). The rated power of this equipment was 20kW, and max cross-sectional dimensions of timber were 100mm (Width)100mm (Thickness). According to preliminary treatments, internal checks could occur in Chinese fir lumber when the applied MW energy per unit volume of wood was higher than 43 kWh/m3, and the mechanical properties of lumber could be significantly decreased. Therefore, the MW energy of 43 kWh/m3 was chosen for MW pretreatment of Chinese fir lumber in this study, with the aim to reduce drying time without decreasing the quality of dried lumbers. The corresponding parameters of MW power, processing time, and conveyor speed were set as 15kW, 92s and 1.3m/min. In order to prevent the steam pressure inside the wood from leaking quickly along the longitudinal direction, the ends of samples were sealed with epoxy resin before the MW pretreatment. 20 replicas were considered for MW pretreatment, of which the half were quarter-sawn samples, while the other half were flat-sawn samples. The weights of the samples were measured to determine the moisture loss before and after MW pretreatment. The MC decrease after MW pretreatment ranged from 1.2 to 2.5%.

Following MW pretreatment, 12 wood samples (6 quarter-sawn samples; 6 flat-sawn samples) with their respective control pairs, totaling 24 samples were dried in a laboratory drying kiln (Type: HD74/TAII. Manufacturer: HILDEBRAND Co, LTD. Japan). The drying schedule is listed in Table 1. The preheating stage and the final treatment last for 10 and 6h, respectively. 6 random samples (3 quarter-sawn samples; 3 flat-sawn samples) with their respective control pairs were selected to record the MC change during drying process.

The appearance quality of wood samples after drying was analyzed as per China National Standard (Drying quality of sawn timber, GB/T 6491-2012), which is similar to the quality testing method for kiln samples (Chapter 6) [19]. Then, the test pieces were cut for measuring the final MC, MC deviation in thickness, and residual stress index. The cutting schematic of the samples is shown in Fig.2.

Final MC was determined by oven-dry method using the specimens cut from each sample. As shown in Fig.3a, the layered MC specimens were divided into thin slices in the thickness direction. The No. 1 and No. 5 slices were surface layers and No. 3 was the center layer. MC deviation in thickness MCd (%) could be calculated using Eq.1:

The residual stress index of lumber was determined using the prong test method. The cutting lines were drawn on the stress specimens, as shown in Fig.3b. The stress specimens were cut into fork tooth shape after measuring S and L. All the prong specimens were equilibrated in room conditions for 24h to obtain evenly distribution of MC throughout the specimens [20]. Then, S1 value was measured and residual stress index Y (%) was calculated using Eq.2:

The moisture diffusion coefficient represents the rate of moisture migration through wood. The following equation was used to describe the falling rate period (MC

where MR is the fraction of moisture content (%), M and M0 are the moisture content at time t and time t=0 (%), Me is equilibrium moisture content (%), A is a constant, k is the drying rate constant (s1), and t is the drying time (s).

As shown in Eq.3, a plot of lnMR versus t was obtained. The Fick's second law of diffusion was used to perform moisture diffusion analysis according to the linear relationship between lnMR and t. The moisture diffusion coefficient De (m2 s1) was calculated using Eq.4 [22]:

In order to investigate the occurrence of mechanical damages of cell tissues before and after MW pretreatment, several 6-mm cubes were cut from the MW-pretreated and control samples. Then, specimens with standard radial surface and tangential surface were prepared for image analysis under Scanning electron microscope (SEM, Hitachi S-4800N, Tokyo, Japan) instrument. Pits, tracheids, and ray parenchyma cells were selected for primary analysis.

Figure4 depicts the conventional drying curves of Chinese fir lumber. The drying time of MW-pretreated samples was shorter than that of the control samples. When the MC reached 10%, the drying time of quarter-sawn and flat-sawn samples with MW pretreatment were decreased by 9.51% and 12.61%, respectively, compared to the control group. In addition, during the preheating stage, the wood samples absorbed moisture under high humidity conditions. The MC of MW-pretreated quarter-sawn and control samples were increased by 0.68% and 0.59%, while those of MW-pretreated flat-sawn and control samples were increased by 1.27 and 0.37%, respectively. The decreased drying time and increased moisture uptake indicate that the moisture migration during the drying process was accelerated after MW pretreatment.

Drying process can be divided into two stages, depending on whether the MC is above or below the fiber saturation point (FSP). The drying rate of wood samples in different drying stages is listed in Table 2. The results showed that MW pretreatment effectively increased the drying rate. In comparison with the control group, the drying rate (whole process) of quarter-sawn and flat-sawn samples with MW pretreatment were increased by 8.66 and 16.56%, respectively. Furthermore, the drying rate were increased in both stages, i.e., early stage (MC>FSP) and later stage (MC

The MC and residual stress indexes of wood samples after conventional drying are listed in Table 3. The drying uniformity, MC deviation in thickness, and residual stress indexes of the MW-pretreated and control wood samples met the requirements of the 1st drying grade in accordance with China National Standard. During the drying process, the stress develops due to moisture content gradients and non-uniform shrinkage properties of the wood [23]. The moisture gradient occurs because the moisture evaporating rate from wood surface is higher than that moving from inside wood to its surface [24]. According to Table 3, MW pretreatment improved the moisture distribution in the thickness direction and decreased the drying stress. The MC deviation in the thickness and residual stress index of MW-pretreated quarter-sawn samples were decreased by 48.06 and 42.92%, respectively, in comparison with the control samples. Those indexes of MW-pretreated flat-sawn samples were decreased by 31.79 and 48.59%. Statistical analysis revealed that these differences were significant between the MW-pretreated and control samples. A greater moisture gradient was developed in the control groups in comparison to the MW-pretreated samples. The reasons could be that the MW pretreatment accelerated the moisture movement across the grain, which contribute to reduced moisture gradient, thereby decreasing the residual stress indexes after drying.

Table 4 shows the appearance quality of Chinese fir lumber after drying. The bow, crook, cup, twist, and internal check indexes of the MW-pretreated and control samples met the requirements of the 2nd grade in accordance with China National Standard. Statistical analysis revealed that there is no significant difference in the appearance quality between the MW-pretreated and control samples, except for twist between the MW-pretreated quarter-sawn and its control samples. The results indicate that the appearance quality of wood samples was not clearly impacted by the MW pretreatment.

During the later stage (MC

The increase in drying rate may be due to high pressure generated from fast evaporation because of high-intensity MW radiation on wet wood. The high pressure resulted in the rupture of some elements in the wood [25], such as pit membranes, ray cells, and even main cells. Thus, new pathways for moisture migration were formed, leading to an increase in wood permeability, thereby improving drying efficiency of Chinese fir lumber. Several researchers have obtained similar conclusions. The permeability and drying rate of plantation eucalyptus were effectively increased because of damage of cell tissues caused by MW radiation [26]. Zhang et al. [27] found that the MW treatment could significantly reduce the number of aspirated pits in larch wood. Pit membranes and the radial parenchyma were also ruptured which lead to increased moisture absorption and drying rate.

The SEM images of MW-pretreated and control wood samples are displayed in Figs.6 and 7. In general, the main pathways for tangential fluid flow in softwood are tracheids and intertracheid bordered pits, while the radial fluid flow occurs primarily through ray cells. Nevertheless, the pits of Chinese fir heartwood are usually covered with the amorphous material and most of the pit membranes are encrusted. Figure6a, b shows that the bordered pits and cross-field pits of control samples were intact and mostly occluded, which severely hindered the migration of moisture. In MW-pretreated samples (Fig.6c, d), fine fractures were visible on the pit membranes. Hence, the radius and number of pit membrane openings were increased, which led to a faster movement of moisture through the pits; thus, the permeability in tangential direction was improved.

SEM images of longitudinal tracheids and ray cells: a radial section of control sample; b tangential section of control sample; c radial section of MW-pretreated sample; and d tangential section of MW-pretreated sample

The transient cell wall capillary network has a great influence on the fluid flow from cell to cell. Several micro-cracks were generated in the radial cell walls of longitudinal tracheids after MW pretreatment, and the cracks mostly originated from the both ends of cross-field pit apertures (Fig.7c). According to Muzamal, the presence of cross-field pits in the cell walls resulted in stress concentration under steam pressure, and the resultant stress concentration promoted the formation of cracks in these regions [28]. These micro-cracks would provide new capillaries for moisture diffusion and make moisture easier to move through cell wall to adjacent cell lumen. In addition, detachments between ray parenchyma cells and longitudinal tracheids also occurred (Fig.7d), new pathways for moisture migration were created in the radial direction and the radial permeability was improved as well [29].

Overall, MW pretreatment improved the permeability of wood in tangential and radial directions by creating new pathways for moisture migration. Therefore, the drying time was shortened effectively. These damages are in agreement with reports from He et al. [30]. Furthermore, the cited authors observed that this degree of MW treatment could significantly increase liquid permeability without much decrease in the mechanical properties.

MW pretreatment (applied MW energy: 43kWh/m3) can be used for improving drying characteristics of Chinese fir lumber. Drying rate was effectively increased in conventional drying process. MW pretreatment is more effective during the early stage of drying; this stage involves the migration of free water in wood. The MC deviation in thickness and residual stress indexes of MW-pretreated samples were decreased significantly in comparison with the control samples, which can improve dimensional stability during the remanufacturing of the dried timber. The appearance quality of wood samples was not clearly impacted by the MW pretreatment. MW pretreatment damaged a part of pit membranes, radial cell walls of longitudinal tracheids, as well as separated the ray parenchyma cells and longitudinal tracheids, which makes moisture easier to move during wood drying. The new moisture pathways due to MW pretreatment were likely the primary causes of the improved permeability and decreased drying time.

Guo J, Guo XX, Xiao FM, Xiong CY, Yin YF (2018) Influences of provenance and rotation age on heartwood ratio, stem diameter and radial variation in tracheid dimension of cunninghamia lanceolata. Eur J Wood Wood Prod 76(2):669677

Balboni BM, Ozarska B, Garcia JN, Torgovnikov G (2018) Microwave treatment of eucalyptus macrorhyncha timber for reducing drying defects and its impact on physical and mechanical wood properties. Eur J Wood Wood Prod 76(3):861870

XW and YZ conceived and designed the experiments. XW, ZF, XG, FZ, and JJ performed the experiments and analyzed the data. XW wrote the draft of this manuscript. YZ reviewed and edited the manuscript. All the authors read and approved the final manuscript.

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit

Weng, X., Zhou, Y., Fu, Z. et al. Effects of microwave pretreatment on drying of 50mm-thickness Chinese fir lumber. J Wood Sci 67, 13 (2021).

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kilns - an overview | sciencedirect topics

Kiln drying is a process that results in green or (semi-) air-dried wood being stacked into large rooms or containers, which are then raised to high temperature (>212F) to drive off the moisture in the wood.

Wet kilns are those that are usually fed with slurry materials. Wet kilns are usually long with kiln lengths on the order of 150180m (about 500600ft). The feed end is usually equipped with chains that serve as a heat flywheel by recuperating the heat in the exhaust gas for use in preheating the feed to assist the drying. Chains are also used to break up any lumps that the material might form during the transition phase of changing from slurry to solids upon drying. In the cement industry these kilns are often not efficient and are becoming a thing of the past replaced by long dry kilns. Nevertheless, there are certain applications that are not amenable to the alternative use of long dry kilns, for example, lime mud kilns found in the pulp and paper industry and some food applications.

Kiln seasoning is an accelerated method of drying wood that was developed in the 1920s. In this method, rapid and controlled drying is achieved by enclosing the wood in a building called a kiln and circulating heated air through the piles of timber. In order to avoid splitting the wood by drying it too fast (removing water too quickly), steam is often injected into the kiln to re-dampen the air. The circulation of heated air is controlled by baffles and fans to give uniform distribution in the kiln which is critical. The kiln drying schedule differs depending upon the type of wood and the required dryness. Kiln drying of softwoods, such as the southern yellow pines, will normally take 14 days to reach 15% moisture content. It takes longer to dry dense hardwoods if serious splitting, warping and other drying defects are to be avoided. The acceleration factors i.e. temperature of heated air, air flow and humidity are varied to achieve the optimum dryness. Unlike air seasoning, kiln seasoning is an energy-intensive process. Its main advantages include: precise control of the moisture content best suited for the application of wood; quicker drying, giving a rapid turnover of stock and reduction in capital investment; and avoidance of degradation due to fungal staining or insect attacks during storage for seasoning.

The kiln is the most important component of a cement manufacturing plant as most of the chemical reactions take place in it at high temperature. Rotary kilns are used which consist of a huge rotating steel furnace with a length and diameter ratio between 10 and 40 [37]. The range of its length varies from 60 to 200m while the diameter ranging from 3 to 9m [4]. The slight inclination (2.54.5%) together with the slow rotation (0.54.5 revolutions per minute) allow for a material transport sufficiently long to achieve the thermal conversion processes required [37]. Kilns are operated in a counter-current configuration where gases and solids flow in opposite directions through the kiln, providing more efficient heat transfer. The raw meal is fed in at the upper end of the rotary kiln, and the slope and rotation cause the meal to move toward the lower and hot end. The kiln is fired at the lower end, usually with coal or petcoke as the primary fuel. As the meal moves through the kiln and is heated, it undergoes drying and pyroprocessing reactions which cause chemical and physical changes to form the clinker. The red-hot clinker is discharged from the end of the kiln and passed through coolers to cool down. Depending on the manufacturing process, rotary kilns can be classified into the following types [38]:

The main energy-intensive phases of the cement production process take place inside the precalciner, kiln and during the production of clinker. A large amount of thermal energy is required to create enough heat for the cement kiln and precalciner. Typical thermal energy consumption of clinker manufacturing with different kiln processes is illustrated in Table 9.1 [5].

A schematic diagram of the temperature profile along with the qualitative phase of the main reaction in the kiln is given in Figure 9.2 [39]. The exact temperature of the kiln flame is not typically measured as adequate temperature sensors are not generally built into the kiln. However, a predicted temperature profile can be constructed based on the temperature of the kiln gas and the state of the solid inside the kiln [39]. About 40% of total thermal energy is required in the kiln to complete the clinkerization process. Depending on the process type one or more alternative fuels can be utilized in the kiln along with the primary fuel, coal. Typically, alternative fuels are injected with the primary fuel by using a multi-channel burner, which is capable of introducing solid and liquid fuel at the same time in the burning zone.

Large kilns for brick and refractory firing, up to 3 m in internal height and width and up to 100 m long, require firing systems which give even heating in various temperature zones with particular attention to uniformity across the load. This is normally obtained by using a large number of small input burners firing between the cars and/or through spaces arranged within the charge at low level. The burners are usually partial premix, the remainder of the air to complete combustion being obtained from the flow along the kiln. Full premix is also used in some instances. These kilns can also be roof-fired in a manner similar to continuous kilns, and also by burners firing along the kiln between the walls and the ware.

The manufacture of ceramic products such as bricks, pottery, refractories and industrial ceramics involves heating the ceramic to temperatures in excess of 800C. The firing or sintering typically involves the use of either direct or indirect gas flames, or electrical resistance elements to radiate the heat. In the UK the main method of radiant heating, accounting for over 80% of the energy market is through the use of direct gas firing.

Kilns and furnaces are becoming more sophisticated in order to meet the more stringent demands, required by the end manufacturer. Another driver for improvement is from the tightening of environmental regulations. Examples of the areas in which major advances have been made include;

However, energy efficiency surveys carried out by the ceramic industry over the past five years have identified that the specific energy consumption is often more than ten times the theoretical minimum energy required to fire the product. The main reasons for this are due to heat loss from the kilns, and the need to ensure uniformity of temperature within the components. Much work has been carried out to minimise the heat loss from kilns, and include the measures listed above.

The firing or sintering a ceramic component or powder is one of the most critical steps in the manufacturing process. One must achieve the required properties, whether maximum density, correct colour or particle size, at minimal cost, energy consumption, time, and defect rate. In order to achieve these properties it is necessary to take all possible actions to ensure the uniformity of temperature within the components or powders.

Heat in a conventional furnace is first transferred to the surface of the product, primarily by radiative thermal transfer, with minor contributions form thermal conduction and thermal convective transfer. This heat is then passed to the centre of the ceramic product by thermal conduction.

Ceramics are poor thermal conductors, are usually only 60% dense after the processing stages prior to firing and in essence can be regarded as thermal insulators. For an unsintered homogeneous material the thermal conductivity is extremely low, normally less than 1W/mK.

Under the required high radiant loading, which is then necessary in order to fire the product to a given temperature, in a specific time period, the temperature gradient across the specimen, which is determined primarily by the thermal conductivity, becomes steep, with the surface being at a much higher temperature than the centre.

In larger components this temperature gradient leads to a large overall thermal mismatch between centre and surface. As a result of the thermal mismatch, there develops a stress at the surface proportional to Eeff, the effective Young's Modulus and the thermal expansion co-efficient, , of ceramics (typically 8 106K1). During the first stages of heating, if the component is heated uniformly on all sides, this mismatch causes compressive thermal stresses in the surface, and as a result crack propagation at the surface is inhibited and failures are comparatively rare. However if the presence of the thermal gradient is sufficiently large, cracking will occur, and the maximum temperature gradient will vary according to material and temperature, thereby making it difficult to predict, and involving extensive trials to determine the optimum firing schedule for each component and material composition.

However if there is non-uniform radiant heating, for example, if the components are stacked or multiple fired, one side will be in tension and prone to crack propagation. In extreme cases catastrophic fracture will occur, in less severe cases the stresses will cause distortion of the components. Once the sintering regime is reached, the linear shrinkage associated with the densification process tends to overshadow the thermal mismatch and the stress distribution becomes more complicated to predict.

The problems of non-uniform heating are the same for powders as components. In the case of powders, the properties, whether required colour or particle size, will vary throughout the powder bed. Furthermore, the thermal conductivity of a powder bed is generally worse than that of a component, and at high temperatures, the radiative transfer across the pores, which in a component improves as the densification process begins, does not improve to the same extent in a powder bed.

The problem of poor heat transfer is accentuated with conventional fast firing. In many cases it is preferable to fast fire a ceramic in order to promote better energy efficiency and allow greater throughput of product. Grain growth is also inhibited as the time is decreased and it should then be possible to achieve an optimum fine-grained microstructure. Slower heating rates tend to give a coarser grain size and a deterioration in mechanical strength, as large grains act as flaws.

However, to rapidly fire a component it is necessary to use a higher radiant surface loading (W/m2). In the case of electrical heating this gives the heating elements a difficult role to perform and may shorten their life expectancy. When gas is used, in addition to the higher radiant loading, there is an increased turbulence from the gas flames, which is detrimental to some products.

The higher radiant loading also enhances the temperature gradients within the products, with the temperature gradient becoming steeper. The presence of a severe temperature gradient can, apart from causing cracking, result in uneven sintering, with the surface sintering before, and at a faster rate, due to the sintering rate being also dependant on temperature. This can result in non-uniform properties within the material, which can make the quality specification difficult to meet and can, in addition, result in a lot of waste material if the final component is to be machined from a block of sintered material. Furthermore, in the firing of ceramics, it is not uncommon to have major crystallographic phase transformations, which can be accompanied by volume changes. In the firing of quartz-containing clay bodies, for example, tableware, the inversion of / quartz requires a uniform temperature profile throughout the component. This requires specific firing schedules and restricts the design of the components. In some areas of the ceramic industry, firing schedules lasting over 2 weeks are used for large components and, as a result, extremely large tunnel kiln systems have to be used to achieve throughput. This makes the firing process within the industry not only energy intensive, but also highly capital and labour intensive.

Kilns are used for heating or firing products made by the pottery and refractories industries. In the firing of clayware, three stages of heating are involved drying, oxidation and finishing (or soaking). In the first stage the moisture is removed by hot air or gas until the ware is dry, in the second the carbonaceous matter in the product is oxidized, and in the final stage a sufficiently uniform temperature is achieved to develop the required degree of vitrification.

The net heat required by many of the materials fired in these industries is very small, since the material gives up a lot of heat during cooling. Older kilns were of batch type in which heat given up during cooling was not utilized; consequently they had very large fuel requirements. One such batch-type equipment is the conventional beehive kiln, which is of hemispherical shape fired by a number of fuel beds around the circumference, with waste gases going down through the ware into bottom flues. Such kilns are still used for firing pottery and silica bricks. These were later developed into battery kilns in which the air for combustion is preheated by using it to cool the previous charges, while the waste gases go to preheat the following charges.

The kilns that make use of heat given up by the ware in cooling are made in two forms semi-continuous and continuous types. The semi-continuous kiln consists of a number of chambers connected through suitable openings. In such kilns goods are set and stay stationary, and firing is applied to each chamber in turn thereby moving the preheating, firing and cooling zones. Combustion takes place in the hottest chamber, the combustion air is preheated in the preceding chamber and the gases leaving the hottest chamber preheat the ware in successive chambers, the last of these being connected to the stack. Such kilns give a compact layout.

Tunnel kilns are the continuous types in which preheating, firing and cooling zones are fixed and the ware is moved on bogies operated by an external pusher mechanism. The tunnel kilns are also made circular with an annular moving hearth used instead of cars.

In continuous and semi-continuous kilns, although fuel consumption is very small, control of the firing stages of the batch is very difficult, since the same gases are used to heat different batches at different stages of firing. Where high-grade heating is a requirement, and a variety of batches is to be treated, fuel economy may well have to be sacrificed by the use of batch-type equipment.

In pottery firing the ware is often protected from combustion gases by enclosing it in muffles or refractory containers (saggers). The use of these results in increasing fuel consumption, but in this case the fuel cost is small compared to the value of the ware fired, and fuel economy takes second place.

Most kilns in the heavy clay and pottery industries are coal-fired, but continuous bogie-type tunnel kilns are very often fired by gas, pulverized coal or even oil. The efficiency of kilns, besides being affected by control of air to meet combustion requirement, depends on setting of the ware in the kiln and the firing schedules.

Kilns for firing pottery are simple structures, but they are large and heavy, and that means a lot of material goes into making them. Is this a case in which the embodied energy of the materials dominates the life energy? An energy audit will tell us.

At its most basic, a kiln is a steel frame lined with refractory brick that encloses the chamber in which the ceramics are fired. Nichrome heating elements are embedded in the inner face of the brick and connected to a power source with insulated copper cable. The chamber is closed during firing by a brick-lined door (Figure 8.13). Table 8.9 gives a bill of the principal materials in a typical small pottery kiln capable of firing a chamber of 0.28m3 (10.5 cubic feet) at up to 1,200C. It is rated at 12kW. We will suppose that the kiln is installed in an art department of a school, where it is fired once per week for 40 weeks per year. To get there, it had to be transported by a large truck over a distance of 750km. The kilns life is 10 years.

The data in Table 8.9 allow the embodied energies of the materials of the kilns to be summed: they amount to 3.6GW. The transport energy data of Table 6.8 gives the energy consumption of a 32-metric-ton truck as 0.46MJ/metric tonkm, so transporting this half-metric-ton kiln over 750km consumes a trifling 0.2GJ.

In a typical firing cycle the kiln operates at full power (12kW) for 4.5 hours to heat the interior to 1,200C, and then it is cut to 4.9kW to maintain this temperature for another 2.5 hours, after which point power is cut, using a total of 66kWh. Multiplying this by the duty cycle of 40 firings per year for 10 years gives the electrical energy used over life as 26.4GWh. When you multiply this by 3.6 to convert the units to MJ, and then divide it by 0.33, the approximate conversion efficiency of fossil fuel to electric power gives the oil-equivalent use energy of 288GJ.

Figure 8.14 displays the outcome of the audit. In 1 year the kiln consumes nearly 10 times more energy in electrical heating power than in material energy; in the 10-year life (short, for a kiln) it consumes nearly 100 times more. This is the most extreme example so far of the great energy commitment of the use-phase of devices that consume energy during use.

It has profound implications for material selection. The embodied energy of the material of the kiln hardly mattersit is conserving heat that is the central objective. Thats where material selection comes in, but well keep that for Chapter 10.

Kiln cooling gases usually have a high flow rate and temperature of about 100250C. These gases contain no pollutants, as they come from the kiln cooling zone. They can therefore be used directly in other process facilities, such as dryers.

Kiln combustion gases are exhausted through the stack. The temperature of this gas stream is about 200C. The gas stream has a variable composition, as it contains combustion products from the fuel used and products from the chemical reactions that occur in the ceramics being made. As a result, the recovery of the stream in other process facilities makes it necessary previously to clean these gases or to use a heat exchanger in which part of the energy contained in the stream is transferred to a fluid (either air or a thermal fluid), in order to be able to use the transferred heat in other process facilities.

At present, airair heat exchangers of thermal fluid heat exchangers may be used. Figure 5 schematically illustrates combustion gas heat recovery from two kilns to several dryers, using a heat exchanger.

If heat is to be exchanged via a thermal fluid, heat exchangers need to be installed at each facility in which combustion gas heat is to be recovered, as the thermal fluid cannot be directly used in other process facilities. Once the thermal fluid has transferred its heat, this returns to the kiln heat exchanger through a closed circuit.

The energy saving attained by this measure depends on the replaced amount of gas, dryer working temperature, and so forth. Reference (7) reports that savings of up to 70% have been attained recently in dryer energy consumption.

Kiln drying is a process that results in green or (semi-) air-dried wood being stacked into large rooms or containers, which are then raised to high temperature (>212F) to drive off the moisture in the wood. During the drying process, VOCs are also expelled. Thus the process results in the emission of VOCs from the kiln and emissions from the fueling operation, which are often gas or distillate oil. Estimates should be made, therefore, for both of these pollutants.

There are few reliable emission factors reported in the literature. Emission factors generally must be developed by actual testing of kiln units. Emissions are also dependent on the wood species. A draft memo published by the North Carolina Department of Air Quality (NC DENR, 1998) reports emission factors of 2.11 pounds of VOC per 1000 board feet for steam-heated kilns for pine and 0.211 for hardwood; however, it states that these are extrapolations and in lieu of more precise information recommends a general emission factor of 3.4 pounds of VOC per 1000 board feet for steam-heated kilns, based on limited tests of a plant it cited. Another study published in the Forest Products Journal (Milota, 2000) reported 14 lb of total organic emissions per 1000 board feet of softwood lumber.

Forintek Canada Corp. prepared a Power Point presentation summarizing the state of knowledge of kiln emission factors ( The company reported that hazardous air pollutants emitted during kiln drying include acetaldehyde, acrolein, benzene, ethanol, formaldehyde, methanol, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), toluene, and other chemicals. While some emission factors are reported in their presentation, they are mill specific and are not recommended for calculation purposes.

Emissions from kilns and dryers include wood dust and other solid particulate matter (PM), VOCs, and condensable PM. If direct-fired units are used, products of combustion such as carbon monoxide (CO), carbon dioxide (CO2), and nitrogen oxides (NOx) are also emitted. The condensable PM and a portion of the VOCs leave the dryer stack as vapor but condense at normal atmospheric temperatures to form liquid particles or mist that creates a visible blue haze. Both the VOCs and condensable PM are primarily compounds evaporated from the wood, with a smaller constituent being combustion products. Quantities emitted are dependent on wood species, dryer temperature, fuel used, and other factors including season of the year.

The PM and PM10 emissions from strand dryers can be controlled with an electrified filter bed (EFB) and a wet electrostatic precipitator (WESP). These electrostatic control devices provide efficient control of PM and PM10, but lesser control of condensable organic pollutants in the exhaust streams from dryers.

Regenerative thermal oxidizers (RTOs) can be used to control emissions of VOCs and condensable organics from strand dryers or veneer dryers. An RTO can also control emissions of CO from direct-fired dryers. Thermal oxidizers destroy these pollutants by burning them at high temperatures. The RTOs are designed to preheat the inlet emission stream with heat recovered from the incineration exhaust gases. Up to 98% heat recovery is possible, although 95% is typically specified. Gases entering an RTO are heated by passing through preheated beds packed with a ceramic medium. A gas burner brings the preheated emissions up to an incineration temperature between 788 and 871C (1450 and 1600F) in a combustion chamber with sufficient gas residence time to complete the combustion. Combustion gases then pass through a cooled ceramic bed, where heat is extracted. By reversing the flow through the beds, the heat transferred from the combustion exhaust air preheats the incoming gases to be treated, thereby reducing auxiliary fuel requirements.

The above information may be applied to developing approximate fugitive emissions from the kiln itself. Separate computations for the fueling operations should also be made using methodology and emission factors provided in AP-42's section on combustion and fueling operations.

A solar kiln is a piece of equipment where solar energy is used in the form of thermal energy for drying materials. The lightweight and simple design, together with the simple operational procedure, of solar kilns means fewer resources and less supervision are required in manufacturing and maintaining them than conventional kilns. Unlike some solar energy capturing systems that require resource-intensive photovoltaic solar panels, solar kiln technology uses solar energy through a simple combination of layered plastic, a ventilation chamber, and an air circulation system. This natural system of heat generation means that this equipment produces few by-products other than heat. Also, this technology does not need boilers and wood-waste burners to provide heat, so there are less pollutants and environmental hazards. The low infrastructure requirements for greenhouse-type solar kilns, as described in this chapter, mean that the kilns do not have to be installed around a high-energy supply center and/or static boiler system, resulting in flexibility with choosing the location of the equipment.

Also, there has been some evidence, including [6,15], and [4], indicating that kiln drying of materials (wood) improves the productivity and quality of the end-use timber, when compared with the open-air drying systems. This better productivity and quality of kiln-dried timber, together with the rising price of conventional fuel (fossil fuel), have driven the use of solar energy for drying processes to ensure that the final kiln-dried products are sustainably processed and are economically competitive in the market.

a decade of improved lumber drying technology | springerlink

In this paper, we comprehensively review the relevant literature published from 2005 to 2016, focused on lumber drying and provide a summary of where we feel future research will focus. Drying is a critical part of most wood products manufacturing process, and the methods used and proper control are key to achieving the appropriate production level, quality, and costs. While a combination of drying methods may be used, most lumber is dried in a kiln at some point in the process. The most common commercial kilns can be classified as conventional, high temperature, and vacuum; however, there continues to be some interest in solar and compression drying. While no new drying technologies have been proposed, work has continued on improving the existing methods. Control of the drying process varies with the type of kiln used, the species being dried and the temperatures used in the process; however, it usually involves some type of measurement of the moisture content of the wood being dried. The development of new methods for controlling the drying process focuses on new ways to measure moisture content or moisture content variation, temperature drop across the load, and drying stresses. While wood quality can be defined differently by its various users, for example, industrial or end users, certain aspects of quality remain constant across these groups, such as minimizing warp, checks, and splits, and discoloration, and maintaining or enhancing mechanical properties. New schedules have been proposed to increase drying rate and improve drying quality. Methods to reduce drying defects and improve its quality have focused mainly on mechanical restraint to prevent warp, better understanding of defect formation, and pre-treatments to speed up the drying process or reduce final moisture content variation. Finally, concerns regarding the environmental impacts of wood drying, most importantly the high energy demands and emissions, have increased in importance as concerns about sustainability and health issues become more mainstream.

Most lumber must be dried prior to use since drying reduces shrinkage, increases strength, reduces weight, allows wood to be treated and adhesives to be applied, and improves overall manufacturing quality [1]. Lumber is typically dried using some combination of air-drying, accelerated air-drying or pre-drying, and kiln drying, where proper control of the drying process allows the highest quality to be attained economically. Control usually consists of the timely application of the appropriate temperature, relative humidity, and air circulation. Poor control of the drying process leads to defects that can adversely affect the value and quality of the product and higher drying costs. For example, improper moisture content in dried pine lumber can lead to losses between $1.2 and $2.8/m3 and poor warp control losses between 50 and 150 dollars per thousand board feet (MBF). Several methods or combination of methods exist for the drying of solid wood products, and there are a variety of ways to control the drying process. Much of the recent work on drying has focused on improving the methods used to dry lumber, on improving or developing new methods of control, and on the environmental aspects of the drying process. The objective of this paper is to comprehensively review literature published during the last decade and discuss the body of work related to the methods of drying solid wood, including control of the drying process, avoidance of defects, energy consumption, and environmental issues. By disseminating recent relevant research findings, this review should help promote improved drying processes that will result in faster drying times, raw material with better quality, and improved environmental performance, outcomes that should lead to increased competitiveness for those who adopt these technologies.

The methods used to dry wood and to control the process vary based on species, desired moisture content (MC), size of the material, quality aspects, and economics. While air-drying is commonly used for large timbers or in combination with kiln drying at some point in the process, most lumber is dried in a kiln. Kiln drying allows the best control over the environmental conditions that promote drying. The most common kiln types are convectional steam, dehumidification, vacuum, and solar.

The majority of softwood and hardwood lumber is dried in conventional steam-heated kilns [2]. Conventional steam kilns vary between those that are a batch process or progressive or continual process. While there has been an increase in the use of progressive kilns in the softwood industry, little research has been published over the last decade on the use of progressive kilns, other than the determination that for drying Scots pine with similar quality, a two-zone progressive kiln had lower energy consumption and drying costs than single-zone batch kilns [3]. Most hardwoods and many softwood species are dried using maximum temperatures below 100C, whereas some softwood species such as southern yellow pine are currently dried using temperatures higher than 100C. Recent work on high-temperature drying has focused on its application to sub-alpine fir [4], spruce, and pine lumber [5] to reduce drying times (up to 3.5%) and warp. Superheated steam used in combination with hot air has been suggested as a drying method to reduce the drying time for rubberwood (by 62%) and pitch pine compared to convectional drying and improve modulus of elasticity (MOE) [6], compression parallel to grain, and hardness [7].

Vacuum drying differs from conventional drying in that it allows for rapid drying to occur at lower temperatures, and one of the driving forces of moving water vapor is the pressure gradient. Current research on vacuum drying has focused on comparisons of the technology to conventional drying technology, its use for different species or MCs, and the potential improvements of its abilities.

While most previous work comparing vacuum drying to conventional drying has focused on the reduction of drying times, Brenes et al. [8] compared the ability of both to support lean manufacturing operations. Simulation of drying and processing operations was performed to determine that the use of vacuum drying in flooring manufacturing plants could reduce total cycle time between 78 and 90%, work-in-process inventory by 5257%, leading to a potential cost savings of USD7.313.6 million a year [8].

While vacuum drying is used to dry many hardwoods and softwoods, it does have some limitations when drying certain species with high green MC. Hansmann et al. [9] developed a technique to rapidly dry green Eucalyptus globulus with shorter drying times than conventional drying and with better quality (less cracks) [9]. Others have attempted to combine vacuum drying with different technologies to improve drying time and quality. He et al. [10] used ultrasonic energy in combination with vacuum drying to accelerate the drying process for Chinese catalpa (Catalpa ovata); they determined that samples treated with ultrasound had diffusion coefficients 12 to 41% higher than untreated samples, which resulted in a 27% faster drying rate [10]. Zhangjing and Lamb [11] explored the use of a hot water-heated vacuum-drying system to dry green red oak dimension parts at different drying temperatures. They were able to dry red oak parts from green to approximately 6% MC within 30h at a temperature of 50C and pressure of 12mmHg with good quality [11]. A combination of low radio-frequency and vacuum contact drying was tested for birch by Lopatin et al. [12], who were able to achieve a drying rate 25% higher than vacuum alone [12]. Elustondo et al. [13] proposed the use of radio frequency vacuum (RFV) drying as a cost-effective way to re-dry wets or boards that remain under-dried after conventional drying. They concluded that this strategy reduces drying time by 4.6%, final MC variability, area shrinkage (0.51% reduction), and degrade (increase in lumber value of USD2.8 per m3) [13].

The use of the suns energy to dry lumber continues to be of interest, especially as the focus on energy in manufacturing increases. However, the majority of interest and continued research on solar drying focuses on smaller volume, lower cost, or low technology solutions for drying lumber. There are many examples of new variations on old designs such as a low-cost design, easy-to-construct and operate, suited for tropical countries, especially for remote communities where electricity is scarce and expensive [14, 15]. Others have compared the indicative life-cycle embodied energy and embodied carbon values for the construction and maintenance of two different wood-drying solar kiln designs with the same timber load capacity over an assumed service life of 20years and determined that one kiln had 37% lower life-cycle embodied energy and 43% lower embodied carbon values than the other [16].

Other works on solar kilns have focused on the modeling of the drying process. Solar drying in a greenhouse-type chamber with two glazed walls was modeled by Bekkiou et al. [17, 18] with input values of temperature, MC, and relative humidity. Hasan and Langrish [17] developed a numerical simulation for the modeling of solar kilns for hardwood timber drying with different boundary conditions. The simulation was used to predict the key behavior of the wood and the kiln itself under different geographical and weather conditions [17].

A very useful review and analysis of the development of solar-heated dry kilns was completed [18], where the authors analyzed each main component of the solar kiln. They then compared developments of each component group using eight laws of evolution. They determined that most modifications have focused on optimizing their thermal and drying efficiency. Based on the analysis, they suggest future developments should focus on the arrangement of components, use of storage with independent heating, integration of an air heater in the storage and not in the drying chamber, and management of different drying cycles according to quality control of the product [18].

Press-drying, or compression drying, is a technique where wood is subject to compression forces from heated platens, with the purpose of increasing moisture loss and reducing warp. Recent work has focused on its use for plantation wood, which contains a high percentage of juvenile wood. The effect of press drying on the mechanical properties of plantation-grown loblolly pine (Pinus taeda) has shown that it does not significantly change the specific gravity or bending properties, but can decrease work to maximum load under some conditions [19]. Mikkola and Korhonen [20] looked at the mechanical and structural changes caused by compression drying of Finnish pine (Pinus sylvestris L.) and determined that compression drying enhanced the tangential mechanical properties and surface hardness as a result of the increased latewood-to-earlywood ratio (due to earlywood deformation) [20]. Combining press-drying with other drying techniques such as drilled holes as a way to improve drying and mechanical properties has been shown to significantly increase moisture loss and reduced compressive stress for Japanese cedar (Cryptomeria japonica) specimens [21].

Control of the drying process is critical to achieve the highest quality with the lowest cost. Control of the drying process is done using programmable logic controllers (PLC), where the temperature, relative humidity, and air-flow are controlled by the PLC, often with a PC interface. The PC interface allows the kiln operator to manually set the environmental conditions or to have the computer automatically control the process based on time or some other monitoring input. The methods used to monitor the drying process vary greatly between the type of kiln used, the species being dried and the temperatures used in the process (low temperature versus high temperature). Common monitoring methods currently used in commercial lumber drying include the use of sample boards that are manually or automatically weighed to determine MC, the use of probes to measure electrical resistance related to MC, dielectric measurement of MC, temperature drop across the load, and time schedules. Improvements on these methods and the development of new methods focus on measuring MC or MC variation, temperature drop across the load, and drying stresses.

One of the most common methods of process control in the conventional drying of hardwoods is to set the temperature and relative humidity based on the MC of the lumber. Specific methods for this technique include the sampling of fast and slow drying material and controlling the drying process such that the slowest drying samples control the majority of the process and the fastest drying samples are used to prevent over-drying. This method can be implemented by manually or remotely weighing the samples, for which many commercial systems are currently in use; however, no systems are currently available that measure the distribution of moisture in the wood. Current research focuses on new methods to automatically measure or estimate the MC of drying lumber, which include the use of microwaves, X-rays, NIR, and vibration.

Microwave systems for MC measurement have been developed and tested in laboratory settings. Schajer and Orhan [22] developed a microwave system that measured grain angle, moisture, and density. The prototype microwave system successfully measured MCs with standard errors of 1.2 and 1.9%, with a range between 7 and 28% MC for hemlock (Tsuga spp.) and Douglas fir (Pseudotsuga menziesii) [22]. Moschler et al. [23, 24] developed and tested an in-kiln dielectric meter which could measure MCs from 6 to 100% with a standard deviation of less than 1.5% MC. While this system showed great promise, it utilized frequencies within the range of 4.56GHz, which are currently restricted for commercial use [23, 24].

Several investigators have continued to look at the use of X-rays to measure the moisture gradient in convectively dried wood. One system used a newly developed soft X-ray digital microscope to demonstrate that that X-ray imaging could be used to determine moisture gradients during drying. The technique involved comparing the oven-dry MC of samples taken at various stages in the drying process and relating it to average gray-scale values from X-ray images [25]. Similar work on moisture gradients but for vacuum drying was conducted on Norway spruce (Picea abies) boards at different drying times, where the theoretical uncertainty was less than 1% for MCs below 100% [26].

Computer tomography (CT) has been used to determine moisture gradients in wood and to further investigate how wood behaves during the drying process. For example, Hansson and Cherepanova [27] demonstrated an image-processing algorithm for investigating density, MC, and moisture loss during drying and suggested that such a system could be further developed to control the drying process [27]. Sobue and Woodhead [28] worked on a CT method for estimating moisture distribution in squared timbers and stated that the reconstituted moisture distribution matched well with that determined by the oven-dry method. However, for both research projects, no data in regard to accuracy was presented [28].

Near-infrared (NIR) radiation has been used to measure many different chemical and physical properties in wood [29], and work continues on its use to measure the MC in the drying process. NIR was successfully used to measure MC of Scots pine (Pinus sylvestris L.) logs using multi-step sample preparation and NIR scanning procedure, where the root mean square error of prediction was 0.8 and 10% for heartwood and sapwood, respectively [30]. NIR has also been used to sort hem-fir timbers more successfully than with a capacitance-type moisture meter [31]. Vibration as a method for measuring MC in wood has been demonstrated such that as resonance frequencies decreased as the MC increased; however, no data or discussion about the accuracy of the method was presented [32].

Special techniques have been proposed for measuring MC during vacuum drying. One such technique uses measurement of temperature and pressure in wood to monitor MC during radio frequency vacuum (RFV) below fiber saturation point where the methods developed had an absolute error within 0.8 to 1.8% [33]. Lui et al. [34] used the relationship between equilibrium moisture content (EMC) and temperature, relative humidity, and ambient pressure as a basis for monitoring MC under various pressures during RFV drying. They determined that as ambient pressure decreased, EMC increased more than what is indicated by Kollmanns chart. The MC estimated from temperature and pressure for Hinoki wood (Chamaecyparis obtusa) was smaller than the MC determined by the oven drying method. The absolute errors of their method ranged from 1.0 to 1.5%; however, when the estimate included the modified EMC, absolute errors dropped to within 0.6% [34].

While the use of temperature drop across a load (TDAL) for drying wood is currently used for several softwood species, efforts to develop this technique with new sensors continue. Elustondo et al. [35] suggested that when drying occurs with low evaporation, the sensors typically used in commercial kilns are not accurate enough to measure TDAL; therefore, they developed and tested a new and more accurate sensor to measure this parameter and demonstrated that TDAL could be used to estimate drying curves in conventional lumber drying [35]. They further developed and tested the system to demonstrate that the TDAL sensor can be satisfactorily used for detecting the transition point between wet and dry wood regardless of the drying process, determining the drying end-point after sensor calibration, and monitoring drying rate on the basis of airflow volume rate, lumber volume, and lumber basic density [36].

The ability of directly or indirectly measure drying stresses has been proposed as a method to control the drying process of wood species prone to checks and splits. Research regarding new methods for measuring of drying stresses can be divided into the development of new methods to control the drying processes and for understanding the development of drying stresses. New methods include the use of acoustic emissions, direct sensor measurement, MC gradient combined with shrinkage data, and the use of NIR.

While acoustic emissions (AE) do not directly measure the stresses developed during drying, they have been suggested as a method to control the drying process as they are related to stress formation. Beall et al. [37] demonstrated that acoustic emission (AE) monitoring can lead to drying times up to 40% shorter when drying above the fiber saturation point, compared with conventionally controlled loads [37].

Several different types of sensors that directly measure the stress in drying wood have been proposed. Allegretti and Ferrari [38] developed and tested a sensor to measure internal compressive drying stresses based on a silicon micro-machined pressure gage inserted in a cylindrical Teflon shell in the wood [38]. Fe et al. [39] used Lurethane (a thermoset elastomeric polyurethane with high toughness), as a medium to transfer the pressure from wood to a pressure transducer [39]. Diawanich et al. [40] used a restrained half-sawn specimen, a restrained free shrinkage specimen, and a load cell as a real-time technique for measuring internal stress perpendicular to grain [40]. Diawanich et al. [41] used the restoring force measured from a half-sawn specimen to measure the magnitude of internal stress within the kiln-dried lumber during cooling and conditioning. They determined that cooling under relatively high humidity after drying improves the internal stress relief within kiln-dried rubberwood lumber (Hevea brasiliensis) during conditioning [41]. Watanabe et al. [42] demonstrated that NIR could be used to predict drying stress levels to detect critical periods in drying [42]. While all the methods described demonstrated being able to measure drying stresses to some degree, no commercial system using these techniques is yet available at the time of writing.

Other researchers have focused on measuring drying stresses to increase the understanding of their formation, knowledge that can be used to improve drying. For example, Clair [43] determined the contribution of maturation stresses to drying shrinkage. He analyzed the strains in the longitudinal and tangential planes of reference in both tension wood and normal wood and determined that part of the shrinkage is caused by the release of internal stresses during the desorption process [43]. Other attempts to use drying stress information include the use of restoring force measurements on half-sawn specimens under various patterns of wet-bulb temperature and using the information to optimize a drying schedule [44] and to estimate the drying stress by measuring the MC gradient of the surface and core layers, and the shrinkage of the board [45]. Tarmian et al. [46] used physical measurements for both longitudinal and transverse drying stress in poplar with mixed tension/normal wood to develop a schedule that results in the least amount and length of checks and warp [46].

A drying schedule is the outline of environmental conditions to control the removal of moisture until the desired MC is reached. Commonly, schedules are listed to set air temperature and humidity levels in the kiln based on time or on the MC of lumber in the kiln. New schedules are constantly being developed and tested on new or known species to dry with optimal quality, in the shortest time possible, or both. This review will not focus on schedule development for individual species but will focus on the application of new ideas in schedule development and the impact of schedules on wood quality.

While most conventional and high temperature schedules use a constant temperature and relative humidity for each step, some studies suggest that oscillating or cyclic drying conditions may reduce drying stress and possibly reduce drying times. For example, Mili et al. [47] compared conventional drying to oscillating the EMC or temperature for drying beech (Fagus sylvatica L.) and found less case-hardening for schedules that oscillated EMC or temperature; however, oscillations of both temperature and EMC did not reduce case-hardening [47]. De la Cruz-Lefvre et al. [48] determined that the mechano-sorptive effect, activated by MC oscillations, leads to a significant stress relaxation [48]. Rmond and Perr [49] focused their work on the oscillation frequency and stress development using simulation models and determined that 30-min oscillations reduced the average absolute stress beneath the surface by about 30% and that longer oscillation times were less effective [49]. While oscillating drying shows some promise for reducing drying stresses, the potential for drying time reduction remains unproven.

Wood quality is often defined differently depending on the material being dried, those drying the material and those using the material. However, certain aspects of quality remain constant across these groups, such as minimum warp, checks and splits, and discoloration in wood. Work to improve wood quality through drying includes the modification of schedules and techniques specific to the defect type; therefore, discussion of wood quality will be done by defect type.

Warp can be divided into four categories: cup, twist, bow, and crook. Cup is caused by differences in shrinkage in two faces of the board. Xaing et al. [50] proposed the use of a surface coating as a way to minimize warp during drying of southern red oak (Quercus falcata) and demonstrated that this method can be effective in reducing cup if it is applied to the pith side of the tangential face of the specimens [50]. Bow, crook, and twist are caused by longitudinal shrinkage in the wood, which is usually a result of reaction wood, spiral grain, diagonal grain, or growth stresses.

Interest in understanding, modeling, and preventing warp in softwoods has increased recently. Twist in lumber results from the presence of juvenile or reaction wood and slope of grain [51]; however, some have suggested that annual growth ring curvature has the greatest impact relative to grain angle and tangential shrinkage [52]. While some have investigated the extent of the twisting force during drying with the goal of being able to predict the force required to keep lumber from twist [53], others have successfully used mechanical restraint to reduce crook, bow, and twist [54]. These methods of warp control can increase the value of lumber between 50 and 150 US dollars per MBF. Fruhwald [55] further demonstrated that the reduction in twist by restraint was permanent during subsequent moisture variations. The lateral restraining of drying loads was also applied to reduce warp (not only twist but bow and crook) by use of a pressure bar and pneumatic cylinders [55]. Research in the reduction of warp in hardwoods suggests that lower temperature or more mild schedules lead to minimal warp [5659].

Stacking practices have great influence on the occurrence of warp. Bond and Wiedenbeck [60] looked at how differences in stacking practices affected drying degrade, kiln capacities, and rough mill yields of red oak (Quercus rubra) lumber. They determined that drying degrade was not significantly different between the two trimming and stacking practices, that kiln capacity can be increased by an average of 4 to 12% for precision end-trimmed lumber; and that using lumber with over-length leads to an increase in rough mill yield [60].

Checks and honeycomb in wood are a direct result of excessive drying stresses during the process and discussion of the control and understanding of drying stresses has already been covered. However, Song and Shida [61] attempted to monitor surface checking during the drying process using NIR, and they determined that the coefficient of variation of the surface temperature increased in the checked areas of cross-section, whereas it decreased in the unchecked areas [61]. Others determined that growth site and the location of the wood within the tree greatly influenced the percentage of honeycomb or internal checking occurring during drying for radiata pine (Pinus radiata) [62].

Collapse is a defect that often occurs with woods of high MC, where removal of the free water from the lumen is too rapid, which results in collapse of the cell due to capillary forces. Recent work on collapse has focused on rapidly grown plantation species and methods on how to recondition collapsed wood. When drying Eucalyptus urophylla, the use of an intermittent drying process (drying followed by lower temperature and higher humidity periods) decreased total shrinkage and collapse by one third, compared to a continuous drying process; however, the schedule leads to longer drying times [63]. For collapse recovery, the authors suggested using a temperature difference between the drying period and the intermittent period. A comprehensive summary of collapse and collapse prevention was provided by Goo [64]. While he focused more on plantation-grown eucalypts, the summary includes information pertinent to many hardwood species, where he concludes that collapse is largely related to the properties of the wood being dried. Only freeze-drying was demonstrated to completely avoid collapse in the species discussed. While the authors reviewed many different strategies to reduce collapse, their use by the industry is limited to those that provide the greatest benefit versus the cost. Blakemore and Langrish [65] suggested that the application of heat, rather than moisture pick-up was the most important component of the steaming reconditioning process [65].

Wet pockets are common in many commercial species, and their presence leads to increased drying times, degrade, and high variation in MC after drying; all factors reduce lumbers value and usefulness. Several methods have been developed to detect wet pockets and determine their severity. Alkan et al. [66] demonstrated that the computerized tomography (CT) scanning technology could be used to detect wet pockets in lumber [66], and Watanabe et al. [67] developed an NIR system that could work at a line speed of 0100mm per second and detect surface wet-pockets in kiln-dried lumber [67]. They further developed the system to include both visible and near-infrared spectroscopy to distinguish wet-pockets in normal subalpine fir (Abies lasiocarpa) [68].

While the impact of air velocity on the drying rate above and below fiber saturation point in convention kiln drying is well known, several investigators looked at its effects in more detail. Steiner et al. [69] studied the effects of air velocity to determine when and how much it can be reduced without affecting the drying rate of the Norway spruce timber dried at 70C. They determined that too early or too sharp a reduction in air velocity results in a reduced drying rate and a large variation in MC; however, a reduction in air velocity can occur at 40 and 20% MC, without considerable changes in the drying schedule but did result in an increase in final MC variation [69]. Vikeberg et al. [70] looked at airflow distribution in an industrial kiln and how it is affected as the fan speed is reduced. They determined that airflow distribution did not significantly change as the fan speed was reduced, and no locations where the air movement stopped were found. They also found that relatively more air ran in the bolster spaces in comparison to the adjacent packages. The application of these results is limited to the kiln types used in the experiments [70].

Stain or discoloration in wood usually results from either a fungal or a chemical reaction of components already present. Enzymatic or chemical stain in hardwood continues to be of interest to researchers. Several have confirmed that elevated temperature and liquid flow transport of solutes on the surface chemistry are shown to influence the formation of stain in maple (Acer spp.) [71]. A new method to prevent enzymatic stain includes the Elder process, a patented treatment, where lumber is heated to 120F and wet-bulb depression near zero, followed by cooling, which resulted in significant stain reduction [72]. They estimated that the process would on average increase dry lumber value between $32/MBF and $58/MBF in red oak. Using inert gasses or oxygen-free environments during the high-temperature drying of wood to prevent stain has been successfully demonstrated in radiata pine [73] and Norway spruce (Picea abies) [74]. McCurdy et al. [75] used a spectrophotometer and the CIELab scale to measure surface color change during drying of radiata pine, while studying the formation of kiln brown stain. The authors concluded that color changes above and below fiber saturation, and that drying temperature is significantly correlated with color change, with color change accelerating at temperatures higher than 60C [75].

Wood color is a critical attribute in some applications, particularly in high value-added products. As customers increasingly favor lighter colors, researchers have focused on factors that may have an impact on color and color uniformity during manufacturing. A considerable amount of work has been conducted on the impact of drying variables, such as drying temperatures and residence time, and how they influence the final color of wood. For example, Ratnasingam and Grohmann [76] studied color changes in rubberwood (Hevea brasiliensis) under different drying schedules and determined that discoloration increased with higher temperatures and drying time, while lower relative humidity tended to minimize discoloration. They recommended the use of lower drying temperature and relative humidity schedules to minimize discoloration of rubberwood [76].

Mottonen and Karki [77] studied the effects of wet-bulb depression, timber thickness, and initial MC on the color of high-temperature-dried birch. The increased drying force increased the lightness and decreased the redness and the yellowness of wood; however, the difference in color between the surface layer and the interior of boards increased. They also found that an increase in thickness and initial MC accentuated the difference in color between the surface and the interior of boards and that pretreatment with water soaking decreased the difference in color between the surface layer and the interior of boards when low drying force was used, but this difference was increased when higher drying force was used [77]. Luostarinen [78] further determined that that the color of birch was correlated with microscopic characteristics of wood, such as cell types and their dimensions, and by drying processes. They found that in conventional drying, the most important factor causing darkened wood was wide latewood, while for vacuum drying it was thickness of the vessel walls, broad rays, and large amounts of axial parenchyma. Phenolics were abundant in ray parenchyma and tended to darken at elevated temperatures, less in conventional drying than in vacuum drying [78]. In two other studies, the discoloration of the surfaces of European white birch during vacuum drying was investigated, and it was determined that the yellowness of the surface layer was associated with the accumulation of low-molecular-weight phenolic extractives, and the redness with Brauns lignin and possibly proanthocyanidins [79, 80]. For vacuum drying of oak (Quercus spp.), the lack of oxygen during drying was stated to improve lightness in color; however, temperature could affect the antioxidant potency [81].

Asghar et al. [82] investigated the surface color change of compression wood in spruce (Picea abies L.) and tension wood in poplar (Populus nigra L.) The color change of compression wood was found to be more remarkable than that of tension wood. Overall, the difference in the colorimetric parameters between the reaction woods and their corresponding normal woods was less significant after drying [82]. Nemeth et al. [83] investigated the color change of Robinia (Robinia pseudoacacia) and hybrid poplars during drying when temperatures from 20 to 80C and relative humidity from 95 to 20% were used. The color of Robinia was shown to be more sensitive to heat than the poplars; poplars actually became brighter with drying at all temperatures [83].

A number of studies focused on color variation of tropical species due to drying. One experiment showed that lightness and yellowness are prevalent in the heartwood and sapwood of plantation Vochysia guatemalensis, and that visually perceptible changes occur in color during drying and under different drying conditions. For example, only lightness increased significantly after drying in heartwood, while all other parameters decreased and color differences between sapwood and heartwood decreased after drying. Different drying conditions accentuate differences in color between green and dried wood [84]. In another study, the color of marup (Simarouba amara) was characterized, with focus on the effects of drying (kiln- and air-dried) and sawing direction (tangential and radial). Marup is a grayish-white species, due mostly to its position on the yellow-blue axis (b*). Regardless of the drying method considered, the tangential direction presents a lighter color than the radially-sawn wood and air-drying produced wood with a lighter color [85].

The influence of storage conditions before kiln-drying on color was the subject of two research efforts. Stenudd [86] determined that log storage for 13weeks under low-temperature conditions had no visible effect on the color of non-steamed sawn beech. He concluded that the reddish discoloration was mainly temperature-related, while the grayish discoloration was mainly controlled by the equilibrium MC (EMC) during the initial drying. Within the investigated climate interval, it was determined that EMC was twice as important as temperature for the final color [86]. In a similar effort involving conventional and vacuum drying, Katri and Mottonen [87] determined that different periods of log storage affected the synthesis of soluble proanthocyanidins during conventional drying and that the concentration of proanthocyanidins also correlated with changes in the color of birch wood. Discoloration appeared differently in conventionally dried and vacuum-dried wood, which indicates that the discoloration mechanism in these drying methods may differ chemically, and/or the compounds that take part in discoloration may be different at different drying temperatures [87].

Several approaches to sorting lumber before drying have been suggested over the years to avoid problems such as under- or over-drying, dimensional stability, and low-grade recovery. More recent work in this area includes pre-sorting based on MC and drying schedule modification as means to improve drying times and recovery [88]. Elustondo et al. [89] sorted hem-fir lumber prior to drying into three groups by electric capacitance and weight. They were able to reduce the drying time by approximately 10% and over-dried lumber to practically zero [89]. Elustondo et al. [90] further demonstrated improvements in drying by sorting using the scaledry/sort/re-dry (Q-Sift) strategy for drying of 2 by 4 Pacific Coast hemlock (Tsuga heterophylla) lumber. When using combined conventional and radio frequency vacuum drying technologies, the Q-Sift strategy reduced drying time by 4.6%, increased lumber grade quality (4.7% reduction in degrade), and reduced the planed lumber area [90]. It was estimated that by using a pre-sorting system that $1.2 and $2.8/m3 could be gained in sales value. Elustondo et al. [91] went on to develop a method of sorting the entire lumber population into three sorts and then combining the sorted lumber into six subgroups. Laboratory and industrial tests on hem-fir lumber demonstrated that sorting into three lumber groups can potentially reduce drying time by 1day approximately and increase lumber value in approximately $8 USD per thousand board feet [91]. Another multi-variable pre-sorting approach was used by Sugimori et al. [92], who used cluster analysis in an attempt to pre-sort sugi (Cryptomeria japonica) lumber to minimize final MC variation. Their results indicated that while green lumber should be sorted by MC, it should first be sorted based on red and non-red heartwood colors; then non-red heartwood should be sorted by heartwood ration [92]. Berberovi and Milota [93] studied how drying rate of lumber within Western hemlock (Tsuga heterophylla) logs are impacted as basic density, initial MC, heartwood percentage, and growth ring angle change based on location from the parent log and determined that sorting based on these variables should reduce drying time and greatly reduce final MC variability [93].

Recent work on pre-treatment of lumber to accelerate drying include the use of green kerfing, steaming, ultrasound pretreatment, microwave pre-treatment, and compression. Green kerfing is a technique where thin slots (3mm or narrower) are cut on the wide faces of dimension lumber, perpendicular to the grain [94]. It is claimed that kerfing accelerates drying by increasing the moisture loss through the end-grain (much faster than moisture loss in the tangential or radial direction of wood), and reducing warp during drying, all this with minimum loss of strength. The steaming of wood to reduce permeability has been shown to increase the drying rate and reduce defects in rubberwood (Hevea brasiliensis) [95]. Ultrasound pretreatment prior to vacuum drying has been shown to enhance the effective water diffusivity; where the higher the ultrasound power level, the longer the pretreatment time, and the lower the absolute pressure, the shorter is the drying time [96]. The increased mass transference rate and effective water diffusivity are attributed to changes in wood microstructure and removal of extractives [97]. Pre-treatment by compression to reduce the MC in wood in a short time period was shown to be effective for Poplar (Populus tomentosa) and Chinese fir (Cunninghamia lanceolata) [98].

Microwave pretreatments have been used to reduce the number and depth checks and honeycomb in the conventional kiln drying of backsawn/flatsawn messmate stringybark (Eucalyptus obliqua) [99] and to increase the diffusion coefficient of hinoki (Chamaecyparis obtusa) timber up to 3% [100]. A pre-treatment with dielectric heating at radio frequency (RF) on sub-alpine fir results in higher permeability and increased drying rates both above and below the fiber saturation point, 1830% and 2155%, respectively [101]. The use of high temperature and low humidity as a pre-treatment to radio frequency vacuum (RFV) drying of boxed heart Japanese cedar (Cryptomeria japonica) timber resulted in a reduction of MC up to 25% and a reduction in drying times from green to 15% MC [102].

The use of high temperatures in the drying process is known to degrade wood, leading to weight and strength losses. These losses depend on factors that include MC, heating medium, temperature, exposure period, and to some extent, species and size of the pieces involved [1]. Borrega and Karenlampi [103] studied the mechanisms that affect the mechanical properties of spruce dried at high temperatures and determined that thermal degradation of cell wall components and formation of irreversible hydrogen bonds influenced both the hygroscopicity and the mechanical properties of dried wood. Significant mass loss, caused by thermal degradation of cell wall components, occurs in slow high-temperature drying processes. Hornification influences strength and stiffness more than mass loss. Ductility was negatively affected by the mass loss and the hornification, the inelastic ductility being more sensitive than the elastic one. Microscopic cell wall damage caused by incompatibility of drying shrinkage did not affect the mechanical properties of macroscopic wood specimens [103]. When examining the effects of high-temperature drying on the cell wall porosity, it was suggested that high-temperature drying seemed to close large-diameter cavities in early wood cells, which was explained by irreversible hydrogen bonding in Norway spruce [104].

Oltean et al. [105] offered an excellent review of work done on the effects of drying temperature on cracks and mechanical properties of wood. Among other conclusions, the authors stated that higher temperatures were associated with strength loss and increased brittleness, potentially due to depolymerization of hemicellulose and reduced hygroscopicity. Also, drying temperature had a greater effect on modulus of rupture than on modulus of elasticity. The authors noted that little research existed on the simultaneous effect of temperature on mechanical properties and occurrence of cracks, and that a disproportionate amount of literature deals with high temperature drying and very little with conventional temperature drying [105].

Mold growth on lumber has become an increasing concern over the last decade due to concerns over mold spores and their impact on human health. While drying reduces the MC of lumber to support mold growth, there are some concerns about how drying may affect mold growth after drying. Also, drying methods can influence mold growth. For example, during the drying process for Norway spruce and Scots pine sapwood boards, it is possible to direct the migration of nutrients in sapwood toward one chosen side of each board by double stacking; the opposite side leaches out, which has a great impact on surface mold growth. Chemical analyses of monosaccharide sugar gradients beneath the boards surfaces confirmed these results [106]. Others have noted a clear difference in discoloring mold fungus between wood dried at room temperature and kiln-dried wood [107].

Sehlstedt-Persson and Wamming looked the effect of the duration of high temperatures used in drying when the wood is at high MC and determined that these factors have a critical impact on the decay resistance of the heartwood of Scots pine. They also determined that steam conditioning after drying decreased durability in sapwood of Scots pine [108].

Concerns for environmental degradation and resource depletion have motivated industries to implement initiatives to reduce their impact on the environment, from waste reduction to the implementation of energy-saving equipment. Science has demonstrated abundantly that wood products require less energy to manufacture than alternative materials (e.g., steel, concrete), and that wood products have a negative balance of carbon (i.e., they store carbon). However, manufacturing of wood products has some environmental impacts, and much of this impact occurs during drying, mostly due to the high energy needs, the emission of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). These issues have been the subject of numerous research efforts, which are discussed in this section.

Life cycle assessment (LCA) has become the standard science-based method to evaluate the environmental impacts of products, from extraction of raw materials to disposal. LCA is based on the accounting of energy and materials associated with transformation processes. A number of studies on the environmental impacts of wood-based products using LCA were performed under the Consortium for Research on Renewable Industrial Materials project [109], including LCA on hardwood and softwood lumber manufacturing. The results from these studies confirm that drying represents a large part of the environmental costs associated with lumber manufacturing, including energy inputs, and volatile organic compounds (VOCs). In hardwood lumber manufacturing, for example, drying generates 1.2kg/m3 VOCs and consumes 70 to 80% of the total energy [110]. For softwood lumber, CORRIM researchers found that drying consumes most of the fuel and VOCs emission amounted to 0.652kg/m3 [111].

The release of volatile organic compounds (VOCs) and total organic compounds (TCOs) during the dehumidification-drying of air-dried hardwood lumber was investigated by Beakler et al. (2005), by analyzing the effluent from a dehumidification kiln. Of the 13 hardwood species dried in 6 kiln charges, the largest amounts of TOCs were released by a mixed charge of white and red oak (Quercus alba and Quercus rubra, respectively) [112]. A similar experiment focused on conventional kiln-drying of green red and white oak, where red oak released the largest amount of VOCs, 0.154 to 0.358lb per thousand board feet (MBF), while white oak released 0.058 to 0.227lb per MBF; the authors suggested this difference may be related to the higher concentrations of acetic acid and acetaldehyde in red oak and presence of tyloses in white oak, which may limit the release of VOCs. In both species, most of the VOCs release occurred before fiber saturation point [113]. Lumber samples from a single loblolly pine log (Pinus taeda) were dried to measure emissions of terpenes, major chemical components in the VOC emissions of this species. VOCs from heartwood were 90% larger than those from sapwood (on a pounds per dry ton as carbon), and 50 to 60% of terpenes remained in the lumber after the drying process [114].

Dahlen et al. (2011) determined that when drying southern pine from green to 19 and 8% MC, higher temperature schedules resulted in the largest emissions of hazardous air pollutants (HAPs, e.g., methanol, phenol, formaldehyde, acetaidehyde, propionaldehyde, and acrolein) and VOCs. The authors concluded that a southern pine mill would become a major source of HAPs (10t of a single HAP or 25t of total HAPs per year) when drying more than 53 to 67 million board feet of lumber (MMBF) (or 106 to 124m3) to 8% MC. As for VOCs, relatively larger emissions were measured for material with larger presence of knots and higher temperatures [115]. HAPs emissions during the drying of five softwood species and one hardwood (red alder, Alder rubra) were measured under the National Council for Air and Stream Improvement (NCASI) Method 105. Species included ponderosa pine (Pinus ponderosa), white wood (mix of western pines, fir, and spruce), Douglas-fir (Pseudotsuga menzeisii), western hemlock (Tsuga heterophylla), white spruce (Picea glauca), and red alder (Alnus rubra). Two drying schedules were used, one conventional and another with high temperature; and a target MC was 15% was used. Results revealed a strong association between temperature and emissions of methanol and formaldehyde. Red alder exhibited the largest amounts of HAPs, which was attributed to the large number of methoxyl groups in hardwood lignin and the higher hemicellulose content and number of acetyl groups [116].

The removal of moisture from wood is one of the most energy-intensive processes in the manufacturing of wood products. Investigations into the energy used in drying and its reduction have focused on comparing the energy used between drying methods and modeling energy use. TBi-Guang et al. [117] compared the energy consumption between conventional, dehumidification, and combined conventional-dehumidification drying methods. They found that dehumidification drying used the least energy but had the longest drying time. Energy consumption for the combined drying method was 18% more than that in the dehumidification drying but 21.5% less than that in the conventional drying, and the drying time is half of that in the dehumidification drying [117]. When comparing the energy requirements between conventional, all electrical and hybrid kiln drying, reductions in total (electrical and fossil) energy consumption for the all-electrical and hybrid drying cycles ranged between 42 and 48% compared with the total energy consumption for conventional drying [118].

The energy consumption used in kiln-drying was modeled by Elustondo and Oliveira [119] where their model used an empirical equation that is calibrated with experimental data consisting of lumber initial and final MC and total drying time. The model also assumed that diffusion controls the drying process for the total moisture range, not only below fiber saturation point, as the theory indicates and was demonstrated to accurately reflect energy used in three trial runs [119].

While the methods to dry wood have not significantly changed over the last decade, there have been many advances in improving the technology and understanding the process. The modification or addition of technology to current methods has allowed for the more rapid drying and attainment of better quality for specific species and thicknesses of materials. Efforts to develop new methods to control the drying process to further reduce drying times and improve quality continue. The increasing use of lesser-known species has also motivated development of drying schedules to achieve specific quality requirements. We also have a much better understanding of the energy use and environmental impact of wood drying.

Based on the trajectory of the research published over the last decade and the current needs of commercial drying operations, we predict that future research on wood drying will focus on two thematic areas: (1) a focus on improving the quality of wood dried and reducing the time required to dry the material and (2) a focus on reducing the energy requirements and environmental impact of wood drying. These two thematic areas are related, as the first will lead to improvements in the second. Also, we believe that the first thematic area can be further subdivided into (a) research on vacuum drying applied to higher moisture content material, its application to different species, and increasing its efficiency with current species and thicknesses of material; (b) research on the control of the drying process of conventional, high temperature, and vacuum drying technologies; (c) research on the development of new schedules for each of the technologies mentioned previously for tropical and hybrid species. Each of these thematic areas is related, as each will ultimately improve the quality, reduce drying time, and reduce the energy used for wood drying.

Brenes-Angulo OM et al. The impact of vacuum-drying on efficiency of hardwood products manufacturing. BioResources. 2015;10(3):458898. The majority of vacuum drying research has been focused on modifying a technique that has been available and used for many decades. This work introduces a new research direction regarding the adoption of vacuum drying for what would typically be considered a prime use of conventional dry kiln technology.

Luna D, Nadeau JP, Jannot Y. Solar timber kilns: state of the art and foreseeable developments. Renew Sust Energ Rev. 2009;13(67):144655. Paper provides a comprehensive review of what has been done with solar drying technology and they analyze each main component of solar kilns. Based on their analysis, they suggest the most likely direction for future development.

Baettig R, Remond R, Perre P. Measuring moisture content profiles in a board during drying: a polychromatic X-ray system interfaced with a vacuum/pressure laboratory kiln. Wood Sci Technol. 2006;40(4):26174.

Liu H, Yang L, Cai Y, Sugimori M, Hayashi K. Effect of EMC and air in wood on the new in-process moisture content monitoring concept under radiofrequency/vacuum (RF/V) drying. J Wood Sci. 2010;56(2):959.

Tomad S, Matan N, Diawanich P, Kyokong B. Internal stress measurement during drying of rubberwood lumber: effects of wet-bulb temperature in various drying strategies. Holzforschung. 2012;66(5):64554.

Schroll MS, Ray CD, Wiedenbeck JK, Stover LR, Blankenhorn PR, Beakler BW. A comparison of kiln-drying schedules and quality outcomes for 4/4-thickness black cherry lumber sawn from small-diameter logs. For Prod J. 2008;58(12):418.

Blakemore P, Northway R. Review of, and recommendations for, research into preventing or ameliorating drying related internal and surface checking in commercially important hardwood species in south-eastern Australia. In: Manufacturing and products. Melbourne, Victoria: Forest and Wood Products Research and Development Corporation; 2003. p. 84.

Jocak T, Mamonova M, Babiak M, Teischinger A, Muller U. Effects of high temperature drying in nitrogen atmosphere on mechanical and colour properties of Norway spruce. Holz Roh Werkst. 2007;65(4):28591.

Hiltunen E, Mononen K, Alvila L, Pakkanen TT. Discolouration of birch wood: analysis of extractives from discoloured surface of vacuum-dried European white birch (Betula pubescens) board. Wood Sci Technol. 2008;42(2):10315.

Tarmian A, Foroozan Z, Gholamiyan H, Grard J. The quantitative effect of drying on the surface color change of reaction woods: spruce compression wood (Picea abies L.) and poplar tension wood (Populus nigra L.). Dry Technol. 2011;29(15):18149.

Zhao Y, Wang Z, Iida I, Huang R, Lu J, Jiang J. Studies on pre-treatment by compression for wood drying I: effects of compression ratio, compression direction and compression speed on the reduction of moisture content in wood. J Wood Sci. 2015;61(2):1139.

Lee N-H et al. Effect of pretreatment with high temperature and low humidity on drying time and prevention of checking during radio-frequency/vacuum drying of Japanese cedar pillar. J Wood Sci. 2010;56(1):1924.

Oltean L, Teischinger A, Hansmann C. Influence of temperature on cracking and mechanical properties of wood during wood dryinga review. Bioresources. 2007;2(4):789811. The authors provide an excellent review of work done on the effects of drying temperature mechanical properties of wood, a very important aspect in regards to the use of high temperature drying of structural lumber. The work has implications for both wood quality and the shortening of drying times.

Dahlen J, Prewitt L, Shmulsky R, Jones D. Hazardous air pollutants and volatile organic compounds emitted during kiln drying of southern pine lumber to interior and export moisture specifications. For Prod J. 2011;61(3):22934.

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At Nova Dry Kiln, we strive to provide a complete drying solution for our primary and secondary woodworking industry clients. Novas large kilns range from 18,000 to 110,000 board feet chambers. Our kinder, gentler method of drying lumber dramatically reduces the lumbers degrade and maintains the time parameters of conventional kiln schedules. We have designed our large kilns to surpass new regulations, as well as consumer requirements, for kilns that decrease degrade and reduce waste. This design translates to maximum efficiency and improved financial results for the plant. All Nova kilns are of the highest quality, longevity, and operational economy, plus they are the easiest to use. We can provide turnkey construction services along with startup assistance as part of our overall drying solution. Contact our technical staff today to discuss the best drying solution for your facility!

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the effect of microwave drying on norway spruce woods strength: a comparison with conventional drying - sciencedirect

The purpose of the present work was to investigate whether the drying method itself affects strength of wood apart from fibre direction, density, temperature in the wood, moisture content and with which angle the microfibril is placed in the middle layer at the secondary cell wall S2. The drying methods compared were microwave drying and conventional air-circulation drying, and the species tested was Norway spruce. The result shows that it is not possible to demonstrate any difference between the two drying methods with respect to the strength of the wood. What affects wood strength are such variables as moisture content, number of annual rings and the density properties weight, width and thickness.