Timber Drying Research Articles

Emissions From Kiln Drying of Pinus radiata Timber: Analysis, Recovery, and Treatment

The effect of drying schedules was investigated on the volatile organic compounds (VOCs) emission from kiln drying of Pinus radiata timber. An enclosed drying system has been invented to recover the VOCs emission and the heat in the drying medium. A new technology, called TOCKDOST, was also developed for treatment of the emission condensate for safe discharge to the environment. The condensate had high chemical oxygen demand (COD) of 256–280 mg l−1, high total organic carbon (TOC) of 56–69 mg l−1, and low pH of about 3.7, and thus required treatment before being discharged to the environment. After numerous trials, it has been found that the most effective treatment was to use Fenton's reagent followed by activated carbon adsorption. With optimum treatment conditions, the average COD was reduced by 93%, TOC was reduced by 87%, and pH was increased to about 6.

Nondestructive evaluation of drying stress level on wood surface using near-infrared spectroscopy

A nondestructive technique for swiftly measuring the stress level of the surface of wood is proposed, which is important for process control in timber drying. Partial least squares (PLS) regression models for predicting surface-released strain (e) were developed using NIR spectra obtained from Sugi (Cryptomeria japonica D. Don) samples during drying. The predictive ability of the models was evaluated by PLS analysis and by comparing NIR-predicted e with laboratory-measured values. The PLS regression model using the NIR spectra pre-processed by MSC and second derivatives with a wavelength range of 2,000–2,220 nm showed good agreement with the measurement (R 2 = 0.72). PLS analysis identified the wavelengths around 2,035 nm as making significant contributions to the prediction of e. Orthogonal signal correction (OSC) was an effective pre-processing technique to reduce the number of factors required for the model using the wavelength range 1,300–2,500 nm. However, the predictive ability of the OSC-corrected model was not improved. Elapsed times to reach the maximum tensile stress (T max) and the stress reversal point (T rev) at the wood surface during drying were detected correctly for 75 % of the samples. The results show that NIR spectroscopy has potential to predict the drying stress level of the timber surface and to detect critical periods in drying, such as T max and T rev.

Correlative light and scanning electron microscopy of the same sections gives new insights into the effects of pectin lyase on bordered pit membranes in Pinus radiata wood

Bordered pits are structures in the cell walls of softwood tracheids which permit the movement of water between adjacent cells. These structures contain a central pit membrane composed of an outer porous ring (margo) and an inner dense and pectin-rich disc (torus). The membrane is overarched on each side by pit borders. Pits may be aspirated, a condition where the torus seals against the pit border, effectively blocking the pathway between cells. In living trees this maintains overall continuity of water conduction in xylem by sealing off tracheids containing air. Drying of timber results in further pit aspiration, which reduces wood permeability to liquid treatment agents such as antifungal chemicals. One possible way to increase permeability is by treating wood with pectin lyase to modify or remove the torus. The effectiveness of this treatment was initially evaluated using light microscopy (LM) of toluidine blue stained wood. Pectic material is coloured pink-magenta with this stain, and loss of this colour after treatment has been interpreted as indicating destruction of the torus. However, correlative light (LM) and scanning electron (SEM) microscopic observations of identical areas of toluidine blue stained sections revealed that many unstained pits had intact but modified tori when viewed with SEM. These observations indicate that LM alone is not sufficient to evaluate the effects of pectin lyase on pit membranes in wood. Combining LM and SEM gives more complete information

Velocity and uniformity of air circulation in conventional kilns for sawn timber

This paper presents the results of a research of velocity of air circulation in the conventional kiln drying of sawn timber and its influence on the uniformity of final moisture content. The data showed that the air velocity (between 0.8 - 1.2 m?s -1) is significantly lower than the one that is optimal for timber drying of coniferous species (3 - 4 m?s-1). This results in a reduction in the capacity of installations for drying, increased energy (thermal and electrical) consumption and thereby increase in the cost of drying. The correlation between the speed of air circulation and the final MC of timber due to prolonged drying was not established. Recommendations that should eliminate the identified deficiencies are also given.

Non‐destructive methodologies for the evaluation of moisture content in sawn timber structures: ground‐penetrating radar and ultrasound techniques

ABSTRACTThe moisture content is a critical parameter for most physical and chemical pathologies of timber and, in the case of structural wood, a can be dangerous for any load‐bearing construction. The complexity of evaluating while timber is in use by means of the current methods (oven‐drying and resistance wood meter) led us to test non‐destructive techniques to evaluate this parameter on site.With this in mind, measurements with two non‐destructive techniques, ground‐penetrating radar (GPR) and ultrasound, were carried out on joists of Pinus pinaster Ait. from their initial green state until the point of hygroscopic equilibrium moisture content. In particular, the analysis presented in this paper focuses on the capacity of each technique to register the velocity variations of their waves during the timber drying process.Prior to the GPR analysis, it was necessary to distinguish between differences in the propagation velocity of electromagnetic waves attributable to the wood anisotropy and those due to variations in . The propagation velocity of the electromagnetic waves was always found to be lower when the electrical field was parallel to the grain of the wood than when it was perpendicular to it. However, when the field was perpendicular, its direction whether radial or tangential, did not significantly affect the . The direct measurements illustrate the ability of the GPR technique to characterize the of timber as a clear decrease in the resulted in an increase in the . A strong correlation was obtained between the two parameters with coefficients of determination, .Longitudinal elastic wave velocities were assessed by means of a ultrasound technique during the timber drying process. Despite the fact that the increased with the decreasing of each joist, the determination coefficient between these two variables was very low.The analysis presented in this paper is a successful application of the GPR technique to the study of wood’s physical properties and has a promising future for the non‐destructive, on‐site analysis of timber .

Density and Moisture Profile Evolution during Timber Drying by CT Scanning Measurements

Timbers of 105 × 105 mm2 in cross section and 1000 mm long were computed tomography (CT) scanned with a commercial unit in equal time steps during a drying cycle. The study aimed to convert the scanning numbers into wood density values, observe the changes that occur during the dehydration process, and, from these, differentiate moisture content as a function of time and space. The reference for calculating the moisture content of each image pixel was the oven-dry wood scan. Analysis of the results allowed visualizing the water distribution, the mechanism of water removal, as well as check formation. Although only a single drying run was analyzed and a limited number of timbers were scanned, the results revealed trends and thus show promise, so further research is urged because it may lead to a better understanding of water movement and check formation in wood during kiln drying.

Intermittent and Continuous Drying of Red Beech Timber From the Green Condition

Intermittent and continuous schedules for drying New Zealand's indigenous red beech timber (Nothofagus fusca) from the green condition have been compared. Both schedules can be used to dry red beech with good timber quality. Continuous schedules are appropriate for use in conventional kilns, where better humidity control can be used to minimize timber degrade and equalize moisture contents. Intermittent schedules can be adapted for use in dehumidifiers, where the lower achievable humidities—often a result of poor insulation and air leakages—can be compensated for by the relaxation periods, when the fans and heaters are switched off intermittently to relieve drying-induced stresses. However, equalization cannot be easily applied in a dehumidifier. In this case, a presort of the green timber into two classes—a heartwood heavy class and a mixed heartwood light and sapwood class—would then prove beneficial to address the issue of significant timber property and green moisture content variations that occur in red beech timber.

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

This study investigated the effect of pretreatment with high temperature and low humidity (HT-LH) on characteristics of radio-frequency/vacuum (RF/V) drying of boxed heart timber of Japanese cedar (150 × 150 mm in cross section, 3600 mm long). From green to approximately 15% moisture content, the drying times including HT-LH treatment were 170 h for the nonkerfed control, 190 h for the kerfed control, and 150 h for both the kerfed and nonkerfed HT-LH specimens. Surface checks were effectively prevented by the HT-LH treatment while the kerfed HT-LH specimens were free from surface checks. In order to prevent the formation of internal checking during RF/V drying, it is suggested that HT-LH treatment should finish around the fiber saturation point.

Drying of timber in progressive kilns: Simulation, quality, energy consumption and drying cost considerations*

Progressive (semi-continuous) kilns for softwood drying are very common in Finland and Sweden and are used in some other countries too. A simulation program has been developed that covers all three types of this kiln. The program calculates the climate in the length direction of the kiln and moisture content, moisture profile, wood temperature, slicing test gap and stress development. Energy consumption and drying costs may also be determined. The temperature level is the most important parameter regarding kiln efficiency. Two-zone progressive kilns are found to be more efficient than single-zone kilns. Progressive kilns have lower energy consumption and drying costs than batch kilns. Experimental full-scale tests show that the timber quality from a progressive kiln is comparable to, or in some cases even better than, timber dried in batch kilns.

Variability in Mechanical and Drying Properties for Blackbutt Timber in New South Wales

AbstractVariability is a key issue in the processing of biological materials, in this case the drying of hardwood timber. This paper reports measurements of variability of mechanical properties and correlations with basic density and diffusion coefficients, which are relevant to the drying of blackbutt, Eucalyptus pilularis Sm, from Northern New South Wales. Specifically, radial, circumferential and height variations are reported for two regrowth blackbutt logs. Both blackbutt logs consisted of more than 80% heartwood, thus diffusion was considered as the main transport mechanism of moisture for modelling purposes.The modulus of elasticity for boards taken from two blackbutt trees increased from their green state to their kiln-dried state, generally by a factor of around two. An analysis of variance of wood samples from different parts of the two blackbutt logs showed that the variability of basic density, diffusion coefficient and most mechanical properties of timber, are significant in the radial direct...

Optimization of Kiln Drying for Softwood through Simulation of Wood Stack Drying, Energy Use, and Wood Color Change

An integrated modeling system was developed to simulate the drying processing, energy use, and wood color change in kiln drying of softwood timber. The model has been applied for a temperature range from 50 to 70°C and an airspeed from 3 to 9 m/s. The model is based on theoretical analysis and contains components such as kiln configuration and practical operations. From the model simulation, optimized drying schedules for minimizing color change and energy use are recommended with dry bulb temperature of 60 to 70°C and wet bulb depression of 15 to 20°C.

Effect of high-temperature drying on properties of Norway spruce and larch

In timber drying, mechanical properties may be changed due to treatment temperature and treatment duration. In general, when increasing the kiln temperature, drying time is decreased and some timber properties are negatively affected. In this study, the effect of different drying temperatures (80, 120 and 170 °C) on equilibrium moisture content and sorption rate, on bending strength and stiffness was investigated for Norway spruce and larch from four proveniences and the results were compared to those obtained for heat-treated wood (Thermowood

Microwave Treatment Accelerates Solar Timber Drying

Drying is essential before wood can be used. The tortuosity of the moisture transport path in wood profoundly affects drying speed and timber quality. Solar drying can reduce drying costs and energy consumption while producing high-quality timber. Unfortunately, solar drying is much slower than many other kiln systems. Intense microwave heating rapidly generates steam inside moist wood. The resulting internal pressure selectively ruptures the wood structure, creating radial pathways through which moisture may readily move. This modification process does not dry the wood, but vastly increases its permeability, allowing the wood to dry faster with minimal degradation of the timber quality. Subsequent experimentation in a solar kiln resulted in a 17% reduction in drying time for microwave-treated Populus alba samples, compared with untreated samples dried in the kiln at the same time. In a second experiment, microwave-treated Eucalyptus regnans samples dried in 33% less time than untreated samples dried in the solar kiln at the same time. There was no difference in timber quality between treated or control samples during either experiments. These reductions in drying time are attributable to a combination of wood modification and the initial moisture loss associated with microwave treatment. The energy requirements for microwave treatment were approximately 104 kW-h m-3.

A Solar Timber-Dryer Equipped with an Air Collector

A generally well-known high energy requirement for technological processes of timber drying and the fact that the world's supply of the conventional energy sources has considerably decreased are the decisive factors forcing us to look for some new, if possible, renewable energy sources for this process emphasising their environmental reliability.One of the possibilities of how to replace, at least partly, the conventional energy sources - heat in a drying process is solar energy.Air-drying of timber usually has a series of disadvantages such as time expenditure, drying defects in the wood and inadequate final moisture content. To avoid these drawbacks, kiln drying is used in many cases to dry the timber, but this involves higher investments for equipment and greater operating costs. The energy used to dry timber is approximately 60 to 70 % of the total energy used to manufacture timber.A method that obviates or reduces the disadvantages of air-drying and, at the same time, reduces the costs of kiln drying, is drying with solar heat. For the most part, fossil fuels are currently used to dry timber products. Solar energy can replace a large part of this depletable energy since solar energy can supply heat at the temperatures most often used to dry wood (i.e. from 40 to 100 °C). Solar timber-dryers offer an attractive solution.

Variability in Transport Properties Regarding Drying Behavior for Blackbutt Timber in New South Wales

ABSTRACT Variability is a key issue in the processing of many biological materials, in this case the drying of hardwood timber. This article reports the measurements of variability of the diffusion coefficient (a transport property), the initial moisture content, and the basic density that are relevant to the drying of blackbutt, Eucalyptus pilularis Sm, from northern New South Wales in Australia. The diffusion coefficient was quantified using a mathematical model solving Fick's second law of diffusion for mass transfer, and Fourier's law for heat transfer. The initial moisture content and the basic density were measured using experimental procedures. Specifically, within-tree and between-tree variations are reported. The coefficients of variation of the initial moisture contents and final moisture contents are 0.24 and 0.19, respectively, for within-tree variability. A similar result was found for the amount of between-tree variability. Compensating differences in the diffusion coefficients of the timber boards were a significant reason for the small dispersion of final moisture contents, despite the large variation in initial moisture contents. An analysis of variance showed that some timber properties were affected by the board positions within trees and between trees. Circumferential and radial effects were significant for the within-tree variability of most transport properties. Moreover, principal components analysis suggested that timber boards with low densities have high initial moisture contents and high diffusion coefficients. A potential reason is that if there is less wood material per unit volume (lower density), then there is more space to be occupied by water (higher initial moisture content), and there is also less resistance to the diffusive transport of moisture (higher diffusion coefficients).

Current advances in superheated-steam drying and processing

Purpose of the review: This article summarises progress in research on drying and processing with superheated (SS) steam over t he last six years, highlighting the current status of mathematical modelling used in SS drying of timber, fluidised drying, impi ngement drying and flash drying. Findings : A new advance in mathematical modelling of SS drying is th e inclusion of the initial condensation on a product surface in m odels. Several studies show that SS is used for simultaneous drying and processing of food products where the food q uality aspect is a primary issue. Directions for future research: Th e quality aspects of products dried or processed with SS need to be investigated and address ed in more systematic studies. There are also challenges asso ciated with complex laboratory e quipment, appropriate instrum entation of equipment, and with results that can be duplicated and confirmed in other centres for the purpose of scaling-up SS pr ocessing units. Keyw ords: mo delling; drying; drying systems; processing; quality; food

THE INFLUENCE OF SOME FACTORS ON THE TIMBER DRYING PROCESS, ANALYZED BY A GLOBAL SIMULATION MODEL

A model for the simulation of timber drying in a batch kiln has been used for the investigation of a set of different questions related to this drying process. The model is global in the sense that it takes into account that the drying air local climate changes as it passes through the kiln load. In Scandinavian, practice kiln stacks are normally built from boards of random lengths so that every second board is located flash at one end of the stack and the other boards flash at the other end. This kiln stack geometry is also included, as well as airflow reversal.This model has been utilized to investigate the following situations:The effect of sticker thickness on airflow, drying time and final MC standard deviation is illustrated.Thicker stickers mean less timber in a given kiln, but results in shorter drying time. The kiln drying capacity is not influenced very much, but the final MC spread is decreased.The influence of kiln parameters on the final MC standard deviation is investigated and simple formulas are presented for the estimation of this variation.

Assessing the Influence of the Drying Schedule on Moisture Variability in the Batch Drying of Timber

Timber drying schedules influence the dispersion in moisture content between boards at the end of drying. Both a single set-point schedule and a double set-point schedule are selected to illustrate the concept. A simple deterministic drying model that can predict moisture content as a function of time for a single and double set-point schedule is presented. Given random variability in both the initial moisture content and drying rate of the product, theoretical expressions for the range in moisture content as a function of time are developed. The predictions of these equations are compared with experimental measurements from a laboratory kiln. A general methodology whereby the basis of commercial schedules can be analyzed as to their performance with respect to minimizing product moisture dispersion is constructed. The approach should prove useful in assisting drying process designers, concerned about moisture variability, in distinguishing between otherwise equal schedules.

High‐Pressure Treatment of Wood – Combination of Mechanical and Thermal Drying in the “I/D Process”

AbstractThermal drying of materials with internal pores is always a time‐consuming and energy‐intensive step within a production process. For chemical and pharmaceutical mass products and, in particular, for wood as an important raw material it is desirable to reduce the water content before thermal treatment by mechanical operations. The wood‐processing industry, facing a rising stress of competition, is forced more than ever to offer high‐quality products at lowest prices. Today, drying of timber is mostly done by air drying or by technical drying in kiln dryers. In any case, drying is necessary to prevent deterioration in quality by shrinkage, formation of cracks, discoloration or infestation. A new process of dewatering wood by combining mechanical and thermal means has been developed at the University of Karlsruhe. Compared to conventional drying processes, short drying times and a low residual moisture content can be achieved and, thus, energy consumption and costs can be reduced. In industrial wood drying only thermal processes (e.g., convective kiln drying, vacuum drying, etc.) have been established because so far no method has been known for removing liquid by mechanical force without significant change in wood structure. With the new I/D process chances for alternatives to conventional thermal drying or for mechanothermal applications are offered.

Air emission from timber drying: high temperature drying and re-drying of CCA treated timber

The nature and mass flows of atmospheric emissions from an industrial kiln, drying radiata pine (Pinus radiata D.Don) timber at 140°C dry bulb and 90°C wet bulb, which was subsequently copper-chrome-arsenic (CCA) treated and redried at 90°C dry bulb and 60°C wet bulb, were assessed by measuring the concentration of chemical components in the kiln atmosphere at regular time intervals and determining air-flows and temperature differentials across the stack. The volatile organic compounds (VOCs) emitted were trapped and analyzed by GC-MS. The two major VOCs found during high temperature pre-drying and CCA re-drying were α-pinene and β-pinene, which made up 87% and 60% of the total discharge (2433 and 145 g/m3 wood), respectively. Most of the VOC fraction was released during the early stages of drying. The polar compounds were trapped in water. Total combined amount of methanol, ethanol, acetic acid and formic acid released over the two runs were 283, 281, 117, and 260 g/m3 wood, respectively. The aldehydes were trapped in 2,4-dinitrophenyl-hydrazine solution and the aldehyde derivatives analysed by HPLC. The total release of formaldehyde and acetaldehyde during the high temperature pre-drying run were 13 and 15 g/m3 wood and the CCA re-drying run were 14 and 4 g/m3 wood, respectively.