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Lumber drying is a mandatory technological operation that gives wood the properties necessary for the production of items, during which the wood is given shape and size stability during operation and/or increases biostability during transportation. Chamber drying is carried out at elevated temperatures, which predetermines the transfer of wood to a chemically active state. As a result, the first stage of extraction is realized – the release of a complex of substances in a solid, liquid and gaseous states onto the surface of the board. With all the variety of substances that accumulate during the growth process in each tree species, phenol and formaldehyde, which are common to all, are released during drying. They belong to class II hazard to humans. In the specialized literature, this issue is given unjustifiably little attention. The problem is complicated by the fact that a significant part of the wood drying production capacity is currently located within the urban area. Therefore, the entire spectrum of substances that are released from the dried wood is concentrated in the residential area, having a negative impact on humans. The aim of the research is to determine the degree of environmental impact of substances released from wood during drying. The list of substances that are released during convective drying from larch and pine wood, as well as data on the amount of these substances, which are given in the article, indicate the presence of an environmental pollution problem. In Russia, the entire volume of lumber for the production of products is dried in drying plants that do not have devices that neutralize pollutants. Consequently, harmful substances released from dried wood accumulate both on the territory of wood processing enterprises and in residential areas. From the above, it follows that drying lumber using low-temperature conditions cannot be considered an environmentally friendly type of production. This problem requires a more detailed study, which will allow us to develop a set of measures to reduce the negative impact of convective lumber drying on the environment.

Investigating the moisture migration during the wood drying process can help improve wood utilization. Single-sided nuclear magnetic resonance (NMR) technology facilitates such investigation with its advantage in conducting one-dimensional measurements along different directions of wood, allowing for the detection of moisture transfer at different depths along the axial and transverse directions during the wood drying process. This research focused on Pinus sylvestris var.mongolica wood, on which a glue sealing technique was employed to ensure that the moisture only transfers along the axial or transverse axis, and the apparent transverse relaxation time (T2app) was utilized to delve into the changes of moisture content at various depths during the drying process. The results showed that at the first 2 hours of the drying process, there was a little free water near the evaporation surface of Pinus sylvestris var.mongolica wood, followed by a scarcity of free water during the drying process, and a noticeable moisture content gradient was observed near the evaporation surface. When the moisture transferred along the axial direction, the farther the moisture was away from the evaporation surface, the more uniform the moisture distribution was. When the moisture transferred along the tangential direction, the farther the moisture was away from the evaporation surface, the more obvious the moisture difference in each layer was. By single-sided NMR technology, it is possible to ascertain the moisture content of wood at various depths, thereby offering a theoretical framework for revealing the migration mechanism of water within wood.

In recent decades, wood drying has emerged as an important method to ensure wood quality and improve wood utilization. However, the addition of modifiers to wood can hinder the transfer of moisture, making the drying process difficult. Therefore, the drying of modified wood is challenging. This study focuses on poplar wood and reports the preparation of modified wood capable of releasing health-beneficial negative oxygen ions. In particular, this study examined the effect of hexacyclite on the transfer of internal moisture and heat in wood. Hexacyclite was dispersed using sodium hexametaphosphate as a dispersing agent, and the modified poplar wood was prepared via vacuum pressure impregnation. The process involved six cycles of the following treatments: a vacuum treatment (−0.08 MPa, 20 min), followed by a pressure treatment (1 MPa, 120 min), and a final atmospheric treatment (5 min). The modified wood was dried via kiln drying, during which the overall wood temperature rapidly increased, with the total drying rate also increasing by 1.05%/h. The introduction of hexacyclite potentially altered the way modified wood bound with moisture, leading to permanent changes in the porosity of cell walls and the structure of polymers and thus altering the movement of water molecules. The modified wood continuously emitted far-infrared rays with a wavelength of 8–15 μm. In some large water clusters, after the far-infrared energy was absorbed, the movement of water molecules accelerated, thereby increasing the drying rate in the wood below the fiber saturation point. Ultimately, this resulted in the modified wood drying faster and thus exhibiting a better quality. [ABSTRACT FROM AUTHOR]

In this paper, an Improved Pelican Optimization Algorithm (IPOA) was proposed to optimize a BP neural network model to predict the dielectric loss factor of wood in the RF heating and drying process. The neural network model was trained and optimized using MATLAB 2022b software, and the prediction results of the BP neural network with POA-BP and IPOA-BP models were compared. The results show that the IPOA-optimized BP neural network model is more accurate than the traditional BP neural network model. After the BP neural network model with IPOA optimization was used to predict the dielectric loss factor of wood, the value increased by 4.3%, the MAE decreased by 68%, and the RMSE decreased by 67%. The results provided by the study using the IPOA-BP model show that the prediction of the dielectric loss factor of wood under different macroscopic conditions in radio frequency heating and drying of wood can be realized without the need for highly costly and prolonged experimental studies. [ABSTRACT FROM AUTHOR]

Deformation and surface-checking defects in wood are closely related to the drying stress caused by shrinkage and moisture content (MC) gradients throughout the drying process. In this study, 50-mm-thick Masson pine (Pinus massoniana Lamb.) lumber was used as the research object. The drying stress caused by shrinkage behavior and MC changes was monitored, and the development of surface checks was observed during the drying process. Results showed that when the average MC was 60%, the free water in the surface layer of lumber evaporated completely, causing the MC to decrease below the fiber saturation point (FSP). The surface layer was subjected to tension stresses, resulting in surface checks which primarily distributed in the latewood area and propagated longitudinally. When the average MC was between 30% and 40%, the combined effects of the high MC gradient and anisotropic shrinkage of the wood resulted in further expansion of the checks. As the average MC decreased to less than 20%, the MC of the core layer lumber decreased below the FSP, resulting in the closure or even disappearance of most surface checks. The critical point (average MC of 60%) of drying stress was obtained, which can benefit the fine-tuning of kiln-drying schedules and improve the drying quality of Masson pine wood. [ABSTRACT FROM AUTHOR]

Drying is the most energy consuming process in the industrial transformation of wood. The current energy and climate crisis makes it imperative to adapt the process to energy availability, in terms of quantity, cost and temperature level. Wood drying schedules are historically based on practice. In the present work, a mechanistic drying model is used as a predictive tool to adapt conditions to specific situations. A multiscale computational model, Multi_Wood_DryS, has been combined with a probabilistic optimization code to propose tailor-made drying schedules that meet operators' expectations in terms of energy consumption, quality, drying time and cost. Optimizations of drying schedules of a stack of boards are proposed. The cost-optimized schedule is advantageous for all criteria with a 48% reduction in drying time. The resulting metamodel is a first step toward an intelligent controller for wood drying. [ABSTRACT FROM AUTHOR]

There are no systematic approaches available in current specialist literature when it comes to the optimization of the process of drying out hardwood species of wood, such as birch, aspen, or black alder, all of which find industrial uses in Estonia. In order to investigate the drying of these three hardwood species, this paper has made use of an experimental and theoretical drying optimization methodology which was developed previously for pine sapwood within the framework of the EIC Grant No. 16200 project. The same methodology was used to determine the critical diffusion coefficient and corresponding critical relative humidity levels (RH) in relation to the surrounding air, with those levels being important from the point of view of drying optimization, and calculating those levels separately for each species of hardwood. For alder it was found that the critical diffusion coefficient is Dcr = 36.57 * (10−4 mm2 s−1), while for aspen the figure was Dcr = 30.71 * (10−4 mm2 s−1), and for birch it was Dcr = 16.35 * (10−4 mm2 s−1). It was found that the time dependencies for the total deformation of different tree species tend to differ considerably, although the same drying regime was used for all of those tree species which were incorporated into the experiments.

This article explores the research and development undertaken as part of a Master's degree in Computer Engineering, with a primary focus on enhancing control mechanisms for natural wood drying. While this method is known for its cost-effectiveness in terms of labor and energy, it suffers from slower and unstable drying cycles. The project's objective is to implement an intelligent control system that significantly improves monitoring and recording of humidity levels in each wooden stack. Additionally, the system incorporates the capability to predict humidity based on data sourced from a weather forecasting API. The proposed solution entails a three-layer system: data collection, relay, and analysis. In the data collection layer, low-computing devices, utilizing a Raspberry Pi, measure humidity levels in individual wood stacks. These devices then transmit the data via Low Power Bluetooth to the subsequent layer. The data relay layer incorporates an Android application designed to aggregate, normalize, and transmit collected data. Furthermore, it provides users with visualization tools for comprehensive data understanding. The data storage and analysis layer, developed with Django, serves as the back-end, offering management functionalities for stacks, sensors, overall data, and analysis capabilities. This layer can generate humidity forecasts based on real-time weather information. The implementation of this intelligent control system enables accurate insights into humidity levels, triggering alerts for any anomalies during the drying process. This reduces the necessity for constant on-site supervision, optimizes work efficiency, lowers costs, and eliminates repetitive tasks. [ABSTRACT FROM AUTHOR]

The article presents the impact of drying process and selected parameters on the color changes on the surface of pine wood (Pinus sylvestris L.). Three predetermined process conditions (modes), mild, normal, and intensive, were investigated. The experiments were conducted using a semi-industrial scale dryer designed at the Gdańsk University of Technology with a loading capacity of 0.55 m3. The drying process was applied to pine wood grown in the northern part of Pomeranian region in Poland. The specimens were selected taking into consideration the radial angle of the growth rings. During each drying mode, 25 pieces of wood with dimensions (thickness, width, length) of 0.03 m, 0.2 m, and 1.5 m, respectively, were dried. The total color change (ΔE) after drying process and color saturation (h*) before and after drying processes of the wood surface was determined using the normalized CIELAB methods. The obtained results indicated that the color change of the pine wood surface increased simultaneously with the intensification of the drying process. However, the normalized wood quality after drying under intensive drying process conditions remained within the standard limits. The application of intensive drying process conditions remarkably changes the surface color of the obtained material, while remarkably reducing the drying process duration.

The article presents the impact of drying process and selected parameters on the color changes on the surface of pine wood (Pinus sylvestris L.). Three predetermined process conditions (modes), mild, normal, and intensive, were investigated. The experiments were conducted using a semi-industrial scale dryer designed at the Gdańsk University of Technology with a loading capacity of 0.55 m3. The drying process was applied to pine wood grown in the northern part of Pomeranian region in Poland. The specimens were selected taking into consideration the radial angle of the growth rings. During each drying mode, 25 pieces of wood with dimensions (thickness, width, length) of 0.03 m, 0.2 m, and 1.5 m, respectively, were dried. The total color change (E) after drying process and color saturation (h*) before and after drying processes of the wood surface was determined using the normalized CIELAB methods. The obtained results indicated that the color change of the pine wood surface increased simultaneously with the intensification of the drying process. However, the normalized wood quality after drying under intensive drying process conditions remained within the standard limits. The application of intensive drying process conditions remarkably changes the surface color of the obtained material, while remarkably reducing the drying process duration. [ABSTRACT FROM AUTHOR]

There are no systematic approaches available in current specialist literature when it comes to the optimization of the process of drying out hardwood species of wood, such as birch, aspen, or black alder, all of which find industrial uses in Estonia. In order to investigate the drying of these three hardwood species, this paper has made use of an experimental and theoretical drying optimization methodology which was developed previously for pine sapwood within the framework of the EIC Grant No. 16200 project. The same methodology was used to determine the critical diffusion coefficient and corresponding critical relative humidity levels (RH) in relation to the surrounding air, with those levels being important from the point of view of drying optimization, and calculating those levels separately for each species of hardwood. For alder it was found that the critical diffusion coefficient is Dcr = 36.57 * (10−4 mm2 s−1), while for aspen the figure was Dcr = 30.71 * (10−4 mm2 s−1), and for birch it was Dcr = 16.35 * (10−4 mm2 s−1). It was found that the time dependencies for the total deformation of different tree species tend to differ considerably, although the same drying regime was used for all of those tree species which were incorporated into the experiments. [ABSTRACT FROM AUTHOR]

Drying green sawn timber to a specific moisture content is needed for further processing. Large batch kilns of approx. 100 cubic meters of timber loads are commonly used with heated air blowing across the packages. When the air reaches the end of the blow depth, it has considerably increased its relative humidity by the evaporation from the wet timber. To even out the climate at the beginning and end of the blow depth, the direction of air circulation is reversed periodically. This exposes timber at those locations to fluctuations in climate while the middle packages are exposed to a more even climate. This study aimed to find the influence of the location of timber along the blow depth in liquid-water-flow behavior during the initial stages of drying. A research drying kiln combined with a CT-scanner was used to mimic this environment across the load and obtain CT images simultaneously, which were used to study the moisture content evolution. The results found that the transition between the capillary and the diffusion regimes can be identified with this method. Additionally, the transition regime was entered earlier for specimens drying in the more even climate which modeled the middle of a drying kiln, but with larger water pockets nearer to the surface of the specimens. [ABSTRACT FROM AUTHOR]

In the present work, it was demonstrated that mass transfer and mass transfer coefficients related to the wood drying process can be satisfactorily quantified using thermography. The method was based on continuous measurements of the wood's surface temperature, which were converted to a vapor pressure at the wood surface. The results showed that the values of the experimentally obtained transfer coefficients were in the same order of magnitude as values obtained with classical empirical correlations that apply in boundary layer theory. The measurements also showed that an average value of the mass transfer coefficient obtained during drying satisfactorily describes the complete process. The measurement set-up makes it possible to determine a surface potential accurately and continuously, which is useful in the assessment of wood drying processes. [ABSTRACT FROM AUTHOR]

Taking the conventional drying process of Pinus sylvestris square wood with pith as the research material, based on the Back Propagation (BP) neural network algorithm, a model was constructed using the real-time online-measurement data. Softening treatment time and temperature, variable treatment time and temperature, initial moisture content of wood, and position of wood core and sapwood were used as model inputs. Wood drying rate and longitudinal cracking degree were used as outputs to indicate wood drying quality. The results showed that with a suitable model structure of 6-9-2 (input layer-hidden layer-output layer), the coefficient of determination R2 and mean square error of the test samples were 0.96, 0.99, and 0.00605, respectively, indicating that the neural network model has good generalization ability. Compared with the experimental value, the predicted value basically conforms to the change law and size of the experimental value, and the error distribution is approximately 2%. This shows that the BP neural network model can simulate the drying rate and longitudinal cracking degree in the drying process and realize the prediction of the drying process.

In this paper, an Improved Pelican Optimization Algorithm (IPOA) was proposed to optimize a BP neural network model to predict the dielectric loss factor of wood in the RF heating and drying process. The neural network model was trained and optimized using MATLAB 2022b software, and the prediction results of the BP neural network with POA-BP and IPOA-BP models were compared. The results show that the IPOA-optimized BP neural network model is more accurate than the traditional BP neural network model. After the BP neural network model with IPOA optimization was used to predict the dielectric loss factor of wood, the value increased by 4.3%, the MAE decreased by 68%, and the RMSE decreased by 67%. The results provided by the study using the IPOA-BP model show that the prediction of the dielectric loss factor of wood under different macroscopic conditions in radio frequency heating and drying of wood can be realized without the need for highly costly and prolonged experimental studies.

Wood drying, such as boxed-heart square timber, often involves surface checks, which spoils the appearance of wood products and reduces their market value. Pretreatments are performed to avoid the surface checks, but detailed information about surface strain during timber drying remains unclear. In this study, surface strain detections were performed using two types of strain sensors (strain gauge and optical fiber strain sensor) to understand the surface dimensional changes during a series of pretreatments, consisting of steaming and high temperature and low humidity treatment. Simultaneous strain measurements based on the optical fiber sensor grasped the surface strain distributions in each lateral surface during timber drying; contraction behaviors were observed in the middle part of most surfaces in the early steaming stage, while one surface showed expansion. A remarkable expansion was detected in one surface during the high temperature and low humidity treatment, although most of the other surfaces showed gradual contraction behaviors. It was also discovered that the above detected behaviors were gradually reduced with the progress of each pretreatment.

At present, green low-carbon economy has become the need of the times, and the wood drying industry should also follow the trend. At present, there is no evaluation model to comprehensively evaluate the greenness of the whole drying process in the wood drying process, therefore, a comprehensive evaluation model was established for wood green drying. The evaluation indexes were selected, the indexes affecting the green drying process of wood were classified, and the hierarchical analysis method was used to analyze the weight of the indexes affecting the green drying of wood, and finally, the fuzzy comprehensive evaluation was used to make a comprehensive evaluation of each index in the wood drying process. When establishing the judgment matrix, the quantitative indicators were selected according to the actual measurement values and the intermediate trapezoidal distribution function was used to establish the affiliation function; the fuzzy comprehensive judgment matrix of the qualitative indicators was constructed by the affiliation degree determined by the fuzzification process of the expert judgment, and finally the comprehensive evaluation level of green wood drying was obtained. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) was used to analyze the impact of air temperature, relative humidity, wind speed, and their interactions on drying time and energy consumption in conventional steam drying of birch wood. The aim was to obtain optimal drying parameters for birch wood with shorter drying time and lower energy consumption. Optimal Latin Hypercube Design (Opt LHD) sampling was applied to determine sample points in the design space and conduct CFD numerical simulation. The third-order response surface method was used to construct an approximate proxy model and analyze the effects of drying conditions on drying time and energy consumption. Based on the Pareto solution set of multi-objective optimization using the Fast Non-dominated Sorting Genetic Algorithm II (NSGA-II), the best wood drying parameters were obtained. The results showed that this experimental method can effectively reduce drying time and energy consumption and provide reference for wood drying process optimization. [ABSTRACT FROM AUTHOR]

Drying is an indispensable process for the industrial use of wood because it results in greater stability, durability, quality, and value of the final product. However, pre-established programs for tropical species have been poorly evaluated. The objective of this study was to formulate and evaluate drying programs for tropical species. The woods of Astronium lecointei (muiracatiara), Dinizia excelsa (faveira-ferro) and Hymenolobium heterocarpum (angelim-pedra) were used. The anatomical characteristics and physical properties of these species were evaluated. The drying programs for each wood were elaborated based on the drying potential to estimate the initial and final temperatures. The programs were executed in a laboratory oven with forced air circulation and afterward, the drying rates and defects were evaluated. The highest values of basic density were observed for faveira-ferro (0.92 g cm-³) followed by muiracatiara (0.70 g cm-³), and angelim-pedra (0.59 g cm-³). As for the volumetric shrinkage, the values were increased in the following order: angelim-pedra (9.87%), faveira-ferro (12.70%), and muiracatiara (14.23%). The anisotropy coefficient of the three wood evaluated was less than 1.5. The density and vessel diameter influenced the drying rate of the woods. The application of the drying programs in this work resulted in a high rate of top and surface cracking, which can compromise the use of wood for more demanding purposes. The woods were classified as first-class because the warping did not exceed the allowed limits. [ABSTRACT FROM AUTHOR]

Forestry companies have invested in genetic improvement to increase wood production in a shorter amount of time. Thus, studies are needed to compare the properties of clonal and seminal wood materials. The objective of this study was to analyze physical and mechanical properties of Tectona grandis from clonal (C1 and C2) and seminal (S) origin and evaluate the yield and quality of sawn wood subjected to outdoor and oven drying. Genetic material was collected from six, 15-year-old trees. Clone C2 presented the lowest amount of bark, and 51 % heartwood up to half the commercial height, while the heartwood of C1 and S went up to 25 % of the height. The three materials did not differ statistically for maximum angular deviation, pith eccentricity, basic density, Janka hardness, anisotropy, commercial income of sawn wood and the presence of knots. After the drying processes, the bowing and crooking indexes were less than 5 mm.m-1, however, the seminal material showed a higher cracking incidence after outdoor and oven drying. In conclusion, the wood properties of the three materials are similar. In addition, the oven drying process is recommended.

The surface checks or internal cracks from drying could significantly affect the processing and utilization of the wood. Therefore, it is of great significance to investigate the formation mechanism of cracks and reduce its occurrence. In this work, the development of cracking was quantified and its relationship with moisture content (MC) gradient and anisotropic shrinkage was analyzed. Results showed that the slight shrinkage occurred at the MC of 40%, and the shrinkage ratio at the end of drying was about 7.38% in the tangential (T) direction and 4.35% in the radial (R) direction. The shrinkage ratio of T/R in the MC range of 30%–15% was about 1.8, which was higher than that (1.65) in the MC range of 15%–5%. Cracks initiation was observed at average MC of 45%. The area and number of cracks reached a maximum at 15% MC. During the late stage of drying, the drying stress of the surface layer was reversed from tensile to compressive, which leads to the narrowing or closure of the cracks. Moreover, the fracture morphology of wood cells was observed at the microscopic level, indicating that the failure firstly occurred in the intercellular layer between the ray tissue and the tracheid in the latewood. The ray cells presented intact, and the lamellar structure between ray tissues and between the ray cells and the tracheids were the weak points of wood during drying. The results would contribute to keeping in minimum drying defects and improving the drying quality of wood. [ABSTRACT FROM AUTHOR]

Wood drying is a key operation of wood processing to ensure that the wood retains its physical integrity and stability in remanufacturing and in use. However, in industrial drying various drying defects still occur, which include uneven moisture content between and within boards, board distortion, residual drying stresses that may induce internal and external checking, and wood collapse. This paper presents a review on advances in wood drying research and development, which lay foundation for better understanding of the drying process, mechanisms of drying defect occurrence and affecting factors. In particular, recent progresses in understanding drying stress development and wood collapse formation during drying are presented in more details. Based on these advances, inter-linkage and interactions are analyzed among wood physical and microstructural properties, moisture movement in wood during drying, stress development and reversal, and wood collapse formation. Following this, drying schedule improvements and novel drying technologies are presented with targets to mitigate the drying defects, reduce energy consumption and increase the productivity. Application of artificial intelligence for modeling and control of the timber drying is also briefly described. Finally, recommendations are made for future studies and practical implementation. [ABSTRACT FROM AUTHOR]

In order to investigate the transverse grain deformation of elm square timber during drying, Digital Image Correlation technology(DIC) was used to monitor the full-field strain distribution of the transverse grain at different temperatures in real time and with 50 mm thick elm square timer as the object. The law and difference of radial strain at different temperatures were analyzed, and the relationship between water content distribution, strain law and cracking was explored. The results showed that the transverse strain distribution of elm square timber was small in the middle and large in the edge at three different temperatures. The maximum chord and radial strains at 40 ℃ were 0. 0366 and 0. 0336. At 60 ℃, 0. 0452 and 0. 0423; at 80 ℃,those were 0.059 8 and 0. 0511. With the decrease of water content, the chord strain and radial strain gradually increased, and crossover occured when the water content was about 30% (60 ℃ ) and 40% (80 ℃ ), respectively, and the strain direction changed. The uneven distribution of water content led to the difference of strain distribution. The cracking occurred after the moment of maximum strain difference, and the cracking occurred when the water content was about 15% and 30%, respectively. Temperature had significant effects on moisture and strain distribution of elm square timber (P<0. 05). [ABSTRACT FROM AUTHOR]

In this work, an updated review of solar wood drying is presented. Experimental investigations, numerical studies and recent patents on the solar wood drying process were reviewed. The review outlines the different techniques for solar drying of wood with or without the use of thermal energy storage materials. From the review study, about 63% of published works are research papers and only a few review papers were available. A review of the numerical study aspect showed that most models were based on the simplified lumped modeling approach where several physical phenomena were ignored. However, the complex aspect of wood drying should be studied and could be modeled successfully using a new approach such as a multi-physical modeling method instead of a simplified lumped modeling approach. Therefore, more experimental and numerical studies are needed and should also consider the environmental impact and the economic viability of these solar systems. [ABSTRACT FROM AUTHOR]

The strain variations in Elm earlywood and latewood during drying were studied with digital image correlation method. Also, the moisture contents of different water states were quantified based on time-domain nuclear magnetic resonance technique to reveal the strain-water relationships. Although the shrinkage strains gradually increased with increasing drying time, there was a significant difference in the strain distribution of the earlywood and latewood. Owing to latewood large stress, the earlywood was pulled by it to shrink together, and the earlywood maximum shrinkage appeared in the early/latewood junction, while the maximum shrinkage of latewood happened in its middle part. Wood shrinkage strain increased fast in the early-drying stage, mainly influenced by moisture content variation; in the mid-drying stage, the shrinkage increased slowly due to the synergistic effect of free and bound water migration; in the late-drying stage, the bound water variation led to a mild shrinkage. [ABSTRACT FROM AUTHOR]

The surface temperature of a drying wooden board is strongly related to the drying process. However, it is relatively difficult to determine the surface temperature accurately during drying. In this paper, an experimental setup for analyzing the wood surface during drying by thermal imaging as well as dry and wet-bulb temperature reference surfaces was tested. Spruce sapwood samples were dried in various climates and evaluated with respect to both mass loss and surface temperature. The experimental setup enabled both qualitative and quantitative analysis of the wood drying process. The results showed that thermal imaging enabled a detailed view of the drying progression. The distinct correlation between surface temperature and mass change showed that an accurate determination of a basic, often considered, and difficult-to-determine drying potential is possible. [ABSTRACT FROM AUTHOR]

The moisture content (MC) of wood affects its industrial performance, but it is difficult to monitor spatial variations in MC. Here, a multivariate regression was developed to estimate the MC from near infrared (NIR) spectra and was used to monitor the spatial variation in the MC of wood during air- and oven-drying. The spectra and mass of wood pieces were measured at five stages during drying (at each 20% loss of initial water mass). Wood pieces were dried naturally and oven-dried at 60 °C. Initially, 25 spectra were recorded at equidistant points covering the entire longitudinal × radial surface of the sample. Then, a planing machine was used to access the inside of the wood, and NIR spectra were measured for each new surface, at a total of 100 points spatially distributed within the wood pieces. The wood pieces were analyzed in their original state, and when they had lost 20, 40, 60, and 80% of their initial water mass. An NIR-based regression (R2p = 0.90 and RMSEP = 10.51%) was applied to estimate the MC, and its spatial gradient during drying was then mapped. These analyses revealed the spatial variation in MC within wood pieces during drying. [ABSTRACT FROM AUTHOR]

This article demonstrates that it is possible to optimize the drying process for pine wood in two independent ways. Those ways involve either the use of the critical diffusion coefficient (DC) which is determined by the experiment shown below, and/or the drying air moistening impulse in the second drying phase. When processing data which has been gained from the pine wood drying experiment, both coincidences and differences were found when compared to the results from a simulation of the drying process which was carried out using the same drying schedule. There is a relatively good level of agreement between the drying experiment and the simulation results of TORKSIM v5.11 simulation program in the case of the simulated and experimentally determined drying curves. The magnitude of numerical values for the DC agrees with this finding, as do the wood's moisture profile in the final phase of the drying experiment at 142 hours, the simulated and measured wood surface temperatures from the beginning of drying to a point at 60 hours into the process, and the simulated wood stresses when compared with the maximum values for the electrical surface-core ratio (ESCR) graph, as determined by the experiment. It was found that the DC's numerical value decreases sharply by about 1.5 times after transitioning from the first drying phase to the second drying phase.

Drying crack is a common phenomenon occurring during moisture discharge from wood, reducing efficient wood utilization. Drying crack is primarily caused by drying stress, and the reasonable methods for determining drying stress are sparse. In this study, the initiation and propagation of cracks during wood discs drying were simulated using the extended finite element method (XFEM). The distribution of drying stress and displacement was analyzed at different crack conditions based on the simulation results. This study aimed to solve the problem of the limitation of drying stress testing methods and provide a new idea for the study of wood drying stress. The numerical simulation results are found in good agreement with the experimental results, thus corroborating the feasibility of XFEM in modeling drying crack of wood discs. The stress concentration was observed at the crack tip region, while a minor stress was presented in the region of crack passing through, indicating that the crack formation process was also a process of releasing drying stress. Further, more energy was required to form double cracks in comparison with the single crack mode. [ABSTRACT FROM AUTHOR]

Wood is widely considered to be a green and renewable building material. However, many of the challenges to using wood as an engineering material arise from the shrinkage and swelling behaviors during moisture content (MC) loss or gain. In this study, the shrinkage behavior of Queensland peppermint (Eucalyptus exserta F. V. Muell) wood was investigated at the macroscopic and cellular levels, and the interdependence between the two levels was explored. Results revealed that the tangential shrinkage of 2% and radial shrinkage of 1% was observed as MC decreased from about 60% to 32% in the macroscopic level. The most obvious anisotropic shrinkage was presented at the MC region of 32–22%, and the ratio of T/R was ranged from 1.6 to 1.75 below the fiber saturation point. At the cellular level, a considerable shrinkage was observed at MC of 42%. The cell wall was shrunk, but the lumen in radial (TR plane) was expanded with MC loss. The shrinkage of the total wood cell showed a linear relationship with MC. The shrinkage ratio of T/R was maintained around 1.5 below the fiber saturation point, indicating that the MC states have little effect on the anisotropic shrinkage. Besides, the shrinkage value at the macroscopic level was slightly smaller than the cellular level, but the anisotropic shrinkage showed an opposite trend. [ABSTRACT FROM AUTHOR]

Due to wood moisture sensitivity, shrinkage cracks tend to present wooden structures. These failures are caused by moisture-related shrinkage behavior. In order to avoid it, it is necessary to have a better understanding of shrinkage behavior. In this respect, studying the dimension changes in wood at different scales is of utmost significance for a better understanding of the shrinkage properties. Herein, the shrinkage behavior of Masson pines (Pinus massoniana) wood was investigated at macroscopic and cellular levels during moisture loss via digital image correlation using VIC-3D and digital microscopic systems, respectively. According to the full-field strain maps, shrinkage strain near the external face was higher than that at the internal face, which increased susceptibility to cracking at the external face of lumber. Additionally, the anisotropic shrinkage of wood was explored. The shrinkage ratio at the end of drying was about 5.5% in the tangential (T) direction and 3.5% in the radial (R) direction. However, at a cellular level, the shrinkage ratios in the T and R directions of earlywood tracheids were 7.13% and 2.46%, whereas the corresponding values for latewood tracheid were 9.27% and 5.52%, respectively. Furthermore, the maximum T/R shrinkage ratio at the macroscopic level (1.7) was found to be similar to the value of latewood tracheid (1.72). The earlywood showed high anisotropic, its T/R shrinkage ratio was 2.75. The macroscopic shrinkage was the result of the interaction of the tracheids of earlywood and latewood and was mainly dominated by latewood tracheids. [ABSTRACT FROM AUTHOR]

A CT-scanner was used to scan 25 mm thick radiata pine boards during drying at 90 °C dry-bulb and 60 °C wet-bulb temperatures. Twenty transverse sections spaced by 10 mm along the length of the boards were CT-scanned six times during drying, and an elastic image registration technique was used to transform the CT-scanned images into moisture content profiles. The resulting images suggested that moisture content distributions did not always resemble diffusion-like moisture transfer mechanism, so common assumptions based on moisture content gradients as the main driving force for wood drying may not always be enough to explain the development of wet and dry zones coexisting below and above fiber saturation point. This paper discusses these findings based on both experimental CT-scanner data and the theory of water potential, which it is argued provides a more complete description of driving forces in wood drying. [ABSTRACT FROM AUTHOR]

Selles artiklis uuriti kahte erinevat vahelduvvoolu meetodit puidu niiskussisalduse määramiseks. Dielektrilise mahtuvusmeetodi (lühidalt mahtuvusmeetod) korral asetatakse puit, mille niiskussisaldust määratakse, omavahel, ja ühtlasi uuritavast puidust elektriliselt isoleeritud metallplaatide vahele. Moodustub nn. mõõtekondensaator, milles puit täidab dielektriku rolli. Plaatide vahelise elektrimahtuvuse mõõtmiseks rakendatakse plaatidele vahelduvpinge. Puidu niiskussisaldus muudab dielektriku (antud juhul puidu) elektrilisi omadusi, ning seetõttu muutub ka mõõtekondensaatori elektrimahtuvus, mis mõõdetakse ja seostatakse mõõtemudelis ehk kalibreerimismudelis puidu niiskussisaldusega. Teise vahelduvvoolu meetodi, elektrilise impedantsi spektromeetria meetodi korral antakse vahelduvpinge otseselt puitu sisestatud roostevabast terasest või süsinikkiust nõelelektroodidele, ja mõõdetakse puidu elektrilist kogutakistust (impedantsi), seejärel koostatakse vastav mõõtemudel puidu niiskussisalduse määramiseks.

A non-linear elastic constitutive model for a swelling/shrinking orthotropic wood matrix is proposed. The model is thermodynamically consistent, derived based on the principles of continuum mechanics and the hybrid mixture theory. Moisture induced strains are introduced considering finite deformations by assuming a multiplicative split of the deformation gradient tensor into a swelling and an elastic part. Novel definitions of the Cauchy stress tensor and the moisture-dependent elastic material tangent matrix are obtained. The model is coupled to a multi-phase transient mass and heat transport model developed in Part I of this work. In this part of the work 2-D and 3-D test examples are used to describe the ability of the model to simulate moisture-induced distortions when drying wood within the hygroscopic and also from the over-hygroscopic moisture ranges. Despite deriving the model considering wood, the obtained constitutive relations can be suitably adopted to other orthotropic porous materials displaying properties similar to that of wood.

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The aim of this study was to determine how varying the inputted surface area value of wood samples would affect the determination of kiln-drying schedules using the drastic drying test. For this purpose, eight individuals of two Eucalyptus clones were selected. Specimens were obtained for drastic drying tests at 100 °C, to measure the basic density and to determine the initial moisture content. The initial and final temperatures and the drying potential were calculated in 100 mm × 50 mm × 10 mm samples, considering the surface area to be 130 cm² (Updated Method), in contrast to the surface area of 100 cm² that is commonly used in the method known as the Standard Method. Based on these findings, kiln-drying schedules were set for the lumber from each clone. Although the significant differences aforementioned, it was observed that the drying schedules developed by Standard Method and Updated Method are similar.

Using the molecular dynamics software Materials Studio, a micro-level carbon dioxide - cellulose model was established to study the supercritical carbon dioxide drying of eucalyptus wood. The change of the primary components of the eucalyptus wood cellulose were also studied, by simulating various pressures, i.e., 10, 15, 20, 25, and 30 Pa, and simulating a temperature of 323 K. Results showed that the diffusion coefficient of carbon dioxide decreases as the pressure increases, and it reaches the maximum at 20 Pa, which was confirmed by the number of hydrogen bonds in the carbon dioxide cellulose model. Combined with the comprehensive analysis of the mechanical parameters, the Poisson's ratio and the ratio of bulk modulus to shear modulus (K/G) values of cellulose first increased and then decreased as the pressure increased, and the Young's modulus increased as the pressure increased. From a microscopic point of view, the study shows that eucalyptus cellulose has good mechanical properties when dried by supercritical carbon dioxide under a pressure of 20 Pa. The simulation results of the dynamic model agreed well with the measured results, and the simulation results support the previous experimental data and the practical application results in production. [ABSTRACT FROM AUTHOR]

In spite of being both a one-step solution to several problems associated with woodworking and also energy efficient, the application of microwave (MW) modification in wood research has remained very limited and this promising method has practically no use in wood industries across the globe. Research done so far in this field primarily sheds light on its potential in enhancing wood permeability, treatability and uniform wood drying. While MW treatments are mostly used on wet or green wood, another modification technique, plasma, has potential benefits to synergistically enhance the effects of MW treatment, but so far has not been applied on wet or green wood specimens. This study takes a first step to investigate the effects of plasma treatments (PT) on green wood specimens, as well as combinations of MW and plasma treatments. As a preliminary study, the methodology focuses on water contact angle measurements, since these are most commonly used as indicators for surface modifications in industrial applications. An exponential time dependence was found for the contact angle on the investigated samples of Norway spruce (Picea abies Karst.). Initial contact angles after droplet deposition increased due to drying and migration of organic molecules during treatments. In comparison with the literature, the effect of plasma was significantly less pronounced on wet wood specimens. The initial contact angles showed the lowest statistical variations after MW treatment, whereas plasma increased inhomogeneities. The final contact angles on treated specimens was lowest for PT-only specimens as well as specimens treated with plasma after MW. In contrast to the initial contact angles, the final contact angles showed the lowest variations after PT. Wetting rates were insignificantly improved by plasma, with reduced statistical variations after all treatments.

A research program was undertaken with the goal of developing dehumidifier solar wood dryers and wood thermal phytosanitary treatment cells. The first step of the program, was dedicated to the development of the numerical tools that would assist in the design and optimization of the dryers. These were based on an approach that combines micro (at the level of the wood board) and macro (at the level of the entire dryer and its solar heating system) modellings, and that uses typical year weather data, to perform the simulations. A finite difference numerical code was employed to predict the time evolution of the wood and air properties during the thermal phytosanitary treatment of wood. These predictions are employed as input to the TRNSYS© software to model the retained solar phytosanitary treatment installation configuration. In this system, the air leaving the treatment cell is recycled. But prior to injecting it back, it is mixed, in the right proportions, with outside air in order to adjust its humidity. The recycled air temperature is then risen to the operating value in an air-water heat exchanger fed from the storage tank of the solar water loop. The latter includes solar evacuated collectors, two pumps, a storage tank and a heat exchanger. The concept technical feasibility was prooved. The sytem optimization showed that, for a cell to be installed in Tunis, a 22 m2 evacuated tube collector and a 1.5 m3 water reservoir are needed for the treatment of 1 m3 wood stack i.e. 0.7 m3 of wood (reduced to 0.42 m3 in the winter season). A prototype of the optimized system was constructed and preliminary experimental tests are presented. [ABSTRACT FROM AUTHOR]

Key message: X-ray diffraction allows indirect measurement of the microfibril angle (MFA) in green and dry wood, revealing that there are statistical differences between the MFAs of these woods. Wood drying is essential for most wood uses; however, the removal of water from wood can lead to defects. Although it is consolidated that water removal to below the fibers saturation point causes a decrease in the distance between cellulose microfibrils, little is known about how the microfibril angle (MFA) behaves in dry wood in terms of the cell wall ultrastructure. The hypothesis investigated in this study is that with the reduced distance between cellulose microfibrils after drying, there is a simultaneous decrease in MFA. Therefore, the objective of the work was to estimate the microfibril angle of dry and green wood using X-ray diffraction (XRD) and then to compare the MFA of the wood in both conditions mentioned. The basal discs from stems of Eucalyptus grandis with age of 22 years were used to produce the samples used in the measurement of MFA by polarized light microscopy and of the T value by XRD. The mean MFA measured by polarized light microscopy was 7.0°. MFA could be estimated by T value obtained from XRD using the cubic model. The mean MFA estimated by this model was 9.0° for green wood and 7.5° for dry wood, concluding that the MFA in dry wood is slightly lower than in green wood. [ABSTRACT FROM AUTHOR]

A novel Mg(II) metal-organic framework (Mg-MOF) was synthesized based on the ligand of 2,2'-bipyridine-4,4'-dicarboxylic acid. Single-crystal X-ray structural analysis confirmed that three-dimensional-nanostructure Mg-MOFs formed a monoclinic system with a channel size of 15.733 Å × 23.736 Å. N2 adsorption isotherm, Fourier transform infrared spectroscopy, thermogravimetric analysis and high-resolution transmission electron microscopy were performed to characterize the thermal stability and purity of the Mg-MOFs. The adsorption studies on four typical volatile organic compounds (VOCs) emitted during wood drying showed that Mg-MOFs have noteworthy adsorption capacities, especially for benzene and β-pinene with adsorptions of 182.26 mg g−1 and 144.42 mg g−1, respectively. In addition, the adsorption of Mg-MOFs mainly occurred via natural adsorption, specifically, multi-layer physical adsorption, accompanied by chemical forces, which occurred in the pores where the VOCs molecules combined with active sites. As an adsorbent, Mg-MOFs exhibit versatile behaviour for toxic gas accumulation.

Summary: A conventional stereo light microscope was used to image polished wood surfaces at cellular resolution over size scales of the growth ring or larger. Bandpass filtering and local area contrast enhancement were used to aid automatic image thresholding and binarisation. An estimate for the location and proportion of cell collapse was introduced based on the distance between uncollapsed cell lumens. Additionally, spatial associations between vessels were determined using a Euclidean distance transform. The analysis of pith to bark cores provided sufficient detail to show significant intra and inter-annual trends in Pinus radiata tracheid dimensions (wall thickness, wall area, and radial widths). These trends were consistent with expectations and in agreement with the literature. Measured cell dimensions may be influenced by cell collapse and deformation as a result of drying. The analysis of air, kiln and oven-dried Eucalyptus nitens showed that cell collapse was highly variable but generally more prominent in the outer third of growth rings. There were significant changes in vessel shape across the growth rings and vessel area was significantly reduced by drying. The technique provides an intermediate step between detailed microscopy and macroscopic imaging that allows spatial analysis at the wood cell level. [ABSTRACT FROM AUTHOR]

• A detailed modeling of a hybrid solar kiln is addressed. • A Combined system reducing drying time by 18% compared to the heat pump drying alone is achieved. • Electricity saving improved by 250% by increasing the concentration ratio from 5 to 25% • An effective combined system competing with solar models described in the literature is presented. This work proposes a hybrid solar dryer model combining a heat pump and a concentrated photovoltaic thermal using air as a cooling fluid for the solar cells. The proposed system will be able to simultaneously produce electricity and heat necessary to conduct the wood drying operations. Mathematical models are developed, verified and validated for the various components constituting the combined system. The discrepancy between measurement and theoretical results did not exceed 8%. The results showed that the combined system can reduce the drying time up to 18%, compared to the drying method using the heat pump alone. Effect of drying parameters on system performance was analyzed. The results demonstrated that the overall efficiency of the concentrated photovoltaic thermal system ranged from 34 and 52%. Compressor operating hours increased as mass flow rate increased. The electricity saving and the moisture extraction rate improved by 57 and 39% by increasing the regenerator efficiency from 25 to 75%, respectively. The proposed system could significantly reduce the power consumption of hybrid solar dryers and can compete effectively with solar models described in the literature. [ABSTRACT FROM AUTHOR]

The issue of energy consumption in the field of wood drying has attracted widespread concern worldwide. In this paper, combining with the self-heat recuperation technology, a novel self-heat recuperation wood drying system (SHRWDS) with a fixed bed dryer is proposed to recover the sensible heat and latent heat of humid air, which is not fully utilized in the general drying process of wood. Mathematical models of the SHRWDS based on the conservation of mass and energy are established, and the effects of drying temperature, airspeed, and compression ratio on the energy and exergy input as well as the economy of the whole system are also carried out. The simulation results show that the drying temperature and the airspeed have a positive influence on the system performance, with optimal values, while the effect of the compression ratio is closely related to the relative humidity of the humid air at the outlet of the dryer. Furthermore, the minimum energy input of 77.23 kJ/s and exergy input of 28.27 kJ/s for the system are obtained at the condition of t= 60°C, v= 1.5 m/s, and φ= 90%, while the minimum annual consumption of standard coal volume and the lowest cost of wood drying for the SHRWDS are 68.34 t and 16.54 $/t. All of this indicates the remarkable superiority of the presented system compared with the traditional device. [ABSTRACT FROM AUTHOR]

This article demonstrates that it is possible to optimize the drying process for pine wood in two independent ways. Those ways involve either the use of the critical diffusion coefficient (DC) which is determined by the experiment shown below, and/or the drying air moistening impulse in the second drying phase. When processing data which has been gained from the pine wood drying experiment, both coincidences and differences were found when compared to the results from a simulation of the drying process which was carried out using the same drying schedule. There is a relatively good level of agreement between the drying experiment and the simulation results of TORKSIM v5.11 simulation program in the case of the simulated and experimentally determined drying curves. The magnitude of numerical values for the DC agrees with this finding, as do the wood's moisture profile in the final phase of the drying experiment at 142 hours, the simulated and measured wood surface temperatures from the beginning of drying to a point at 60 hours into the process, and the simulated wood stresses when compared with the maximum values for the electrical surface-core ratio (ESCR) graph, as determined by the experiment. It was found that the DC's numerical value decreases sharply by about 1.5 times after transitioning from the first drying phase to the second drying phase. [ABSTRACT FROM AUTHOR]

Research into the possibility of applying the electric impedance spectrometry (EIS) method and the dielectric capacitance method (DECM) simultaneously above fibre saturation point (FSP) and in harsh kiln conditions has been relatively scarce. In the framework of this research, tests were carried out on the operational reliability of the measuring capacitor (MEC) prototype used for calibrating the DECM in the harsh internal climate (50°C and 98% RH) of the kiln. Condensation of water vapor on MEC plates, leakage of MEC insulators and the emergence of static electric charges on MEC plates were studied. Quantitative ranges were found for MEC performance-disrupting effects on the parasitic capacities induced by each effect. The DECM was found to be less reliable than the EIS method for application in harsh kiln conditions. Secondly, under the same test conditions and for the same wood species (birch), the possibilities of the DECM method and the EIS method were comparatively modeled with the predetermined Rozema quality criterion of ±1.75% MC for predicting the moisture content (MC) of birch wood above FSP. It was found that, under the same test conditions, the DECM method proved more accurate than the EIS method for predicting birch wood MC above FSP. Based on the tests, it was concluded that DECM can be used in practice by applying a non-destructive method to reliably determine the average moisture content of a wood batch immediately prior to commencing the wood-drying process. Selles artiklis uuriti kahte erinevat vahelduvvoolu meetodit puidu niiskussisalduse määramiseks. Dielektrilise mahtuvusmeetodi (lühidalt mahtuvusmeetod) korral asetatakse puit, mille niiskussisaldust määratakse, omavahel, ja ühtlasi uuritavast puidust elektriliselt isoleeritud metallplaatide vahele. Moodustub nn. mõõtekondensaator, milles puit täidab dielektriku rolli. Plaatide vahelise elektrimahtuvuse mõõtmiseks rakendatakse plaatidele vahelduvpinge. Puidu niiskussisaldus muudab dielektriku (antud juhul puidu) elektrilisi omadusi, ning seetõttu muutub ka mõõtekondensaatori elektrimahtuvus, mis mõõdetakse ja seostatakse mõõtemudelis ehk kalibreerimismudelis puidu niiskussisaldusega. Teise vahelduvvoolu meetodi, elektrilise impedantsi spektromeetria meetodi korral antakse vahelduvpinge otseselt puitu sisestatud roostevabast terasest või süsinikkiust nõelelektroodidele, ja mõõdetakse puidu elektrilist kogutakistust (impedantsi), seejärel koostatakse vastav mõõtemudel puidu niiskussisalduse määramiseks. Elektrilise impedantsi spektromeetria ja mahtuvusmeetodi kasutamise võimalusi on üheaegselt üle kiu küllastuspunkti puidu niiskussisaldustel ja puidukuivati karmides kliimatingimustes suhteliselt vähe uuritud. Selle uurimuse raames tehti mahtuvusmeetodi kalibreerimiseks kasutatud mõõtekondensaatori prototüübi töökindluse katsetused puidukuivati karmi sisekliima (50°C ja 98% RH) tingimustes. Uuriti veeauru kondenseerumist mõõtekondensaatori plaatidel, plaatide isolaatorite lekkimist ja staatiliste elektrilaengute tekkimist plaatidel. Mõõtekondensaatori töökindlust häirivate efektide jaoks leiti iga efekti poolt esile kutsutud parasiitmahtuvustele kvantitatiivsed vahemikud. Leiti, et mahtuvusmeetod on puidukuivati karmides kliimatingimustes kasutamiseks vähem töökindel kui elektrilise impedantsi meetod. Samuti modelleeriti võrdlevalt samades katsetingimustes ja sama puuliigi (kask) korral mahtuvusmeetodi ja elektrilise impedantsimeetodi võimalusi etteantud Rozema kvaliteedikriteeriumiga ±1.75% kasepuidu niiskussisalduse ennustamiseks üle puidu kiu küllastuspunkti niiskussisaldustel. Samades katsetingimustes osutus kasepuidu niiskussisalduse ennustamisel üle kiu küllastuspunkti puidu niiskussisalduste piirkonnas mahtuvusmeetod täpsemaks kui elektrilise impedantsi meetod. [ABSTRACT FROM AUTHOR]

This study aimed to evaluate the spatial distribution of the air circulation speed and the final moisture content of pine lumber after kiln drying by using geostatistics. Two kilns acting on boards of 27 mm and 42 mm were evaluated. Air circulation speed and final moisture content were collected in different regions of the kilns. There was no significant difference for air circulation speed and final moisture content between the front and rear regions of the equipment. In the horizontal axis (Y-axis), air circulation speed averages did not differ statistically in both situations, but higher values ​​for this variable were obtained from the spaces between the piles. Final moisture content tended to increase when closer to the door opening. In the vertical axis (Z-axis), the lower part tended to reach higher air circulation speed values. Thus, the spatial behavior influences the air circulation speed and final moisture content during lumber drying. Besides, the variograms presented the same tendency in comparison to the data obtained in a conventional manner, which indicates that geostatistics can be used to represent variables in kilns during the drying process.