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PEER-REVIEWED ARTICLE EFFECT OF MICROWAVE RADIATION AND PRE-STEAMING TREATMENTS ON THE CONVENTIONAL DRYING CHARACTERISTICS OF FIR WOOD (ABIES ALBA L.)

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Lignocellulose Dashti et al. (2012). "Drying characteristics of Fir wood," Lignocellulose 1(3), 166-173. 166 a In this research, the effect of microwave radiation and steaming pretreatments on drying rate and residual stresses of fir wood (Abies alba L.) was investigated. Wood samples with green dimensions of 340 × 100 × 50 mm and initial moisture content of about 50% were exposed to either steam or microwave radiation treatment before being conventionally dried. The pre-steaming was performed at temperatures of 120, 140, and 160°C for 1 hour, and the microwave treatment was applied with 2.45GHz frequency for 7 and 10 minutes at three different conditions. Results revealed that the pre-steaming at 140 and 160°C and the microwave radiation for 10 minutes imposed greater effect on the drying rate. The residual drying stresses were reduced due to the microwave radiation; in contrast, they were increased as a result of steaming at 140 and 160°C.
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PEER-REVIEWED ARTICLE Lignocellulose
Dashti et al. (2012). “Drying characteristics of Fir wood,” Lignocellulose 1(3), 166-173. 166
EFFECT OF MICROWAVE RADIATION AND PRE-STEAMING
TREATMENTS ON THE CONVENTIONAL DRYING
CHARACTERISTICS OF FIR WOOD (ABIES ALBA L.)
Hadi Dashti,a Asghar Tarmian,a,* Mehdi Faezipour,a and Mahdi Shahverdi a
In this research, the effect of microwave radiation and steaming
pretreatments on drying rate and residual stresses of fir wood (Abies
alba L.) was investigated. Wood samples with green dimensions of 340 ×
100 × 50 mm and initial moisture content of about 50% were exposed to
either steam or microwave radiation treatment before being
conventionally dried. The pre-steaming was performed at temperatures
of 120, 140, and 160°C for 1 hour, and the microwave treatment was
applied with 2.45GHz frequency for 7 and 10 minutes at three different
conditions. Results revealed that the pre-steaming at 140 and 160°C and
the microwave radiation for 10 minutes imposed greater effect on the
drying rate. The residual drying stresses were reduced due to the
microwave radiation; in contrast, they were increased as a result of
steaming at 140 and 160°C.
Key words: Drying rate; Residual stress; Steaming; Microwave; Fir wood
Contact information: a: Department of Wood and Paper Science & Technology, Faculty of Natural
Resources, University of Tehran, Karaj, Iran. *Corresponding author: Department of Wood and Paper
Science & Technology, Faculty of Natural Resources, University of Tehran, P. O. Box 31585-4314, Karaj,
Iran, E-mail: tarmian@ut.ac.ir
INTRODUCTION
Wood drying is time consuming and cost intensive. Research is being conducted
to find new and highly efficient drying methods to be adapted industrially. For this
purpose, some new drying methods, such as dielectric (microwave and radiofrequency),
vacuum or combined drying methods have been applied to achieve the mentioned
objectives in this industry. In addition, some pretreatments of wood before drying, such
as steaming and microwave radiation were used to increase the wood drying rate (LV et
al. 1994; Zhao et al. 1998; Zielonka and Dolowy 1998; Zhao et al. 2003; Yu et al. 2002;
Zhang and Cai 2006). Alexiou et al. (1990) reported the increase of wood drying rate was
a result of pre-steaming due to the movement and elimination of some parts of wood
extractives which increases water molecules accessibility to the cell walls. Harris et al.
(Harris et al. 1989) also found that the drying rate of red oak increased by pre-steaming.
Zhang and Cai (2006) observed the rupture development in Abies lasiocarpa wood due to
pre-steaming above 130°C, and the rupture intensity increased by increasing steaming
temperature from 130°C to 160°C. Turner et al. (1998) demonstrated that the drying rate
and quality of pine wood significantly increased by microwave radiation before drying.
Brodie (2009) performed microwave pretreatment on two species of poplar and
eucalyptus, and then dried them in a solar oven. He found out that as a result of
PEER-REVIEWED ARTICLE Lignocellulose
Dashti et al. (2012). “Drying characteristics of Fir wood,” Lignocellulose 1(3), 166-173. 167
microwave pretreatment, drying rate increased due to occurrence of microscopic cracks
in the cell walls, and permeability and diffusion coefficients were consequently
increased. Fei et al. (2003) and Zhao et al. (2003) also showed that microwave
pretreatment improved moisture diffusion coefficient and reduced drying time of
eucalyptus wood. In recent years, fir wood (Abies alba) comprises a great percentage of
wood used in Iran. This research aims to investigate the effects of microwave and
steaming pretreatments on the drying rate and quality of Abies alba.
EXPERIMENTAL
Sampling
Fir wood (Abies alba L.) flat-sawn boards with green dimensions of 340 × 100 ×
50 mm and initial moisture content of about 50% were selected from a wood yard for the
study. Six replications were tested for each set of experiment.
Microwave Radiation and Steaming Procedures
Pre-steaming was applied at three temperatures of 120 (ST120), 140 (ST140) and
160°C (ST160) for 1 hour under a pressure of 2-3 bars inside a laboratory steaming
device. A microwave oven with frequency of 2.45 GHz was used for microwave
radiation under three conditions (Table 1). To prevent the occurrence of severe checking
of wood samples, at every 30 to 60 seconds intervals during microwave application, the
heating was stopped for 60 to 120 seconds to equilibrate for the temperature of the wood
specimens (rest time).
Table1. Three Different Conditions Applied for Microwave Radiation of Abies
alba Wood Specimens
Rest time (s)
Microwave radiation
period (s)
Total time (min) Treatment
- - - Control
120 30 7 MW1
120 60 7 MW2
60 60 10 MW3
Drying Method
After either microwave radiation or steaming, the boards were end coated using
oil-based paint to avoid the moisture flow through the end sections. Subsequently, they
were conventionally dried inside a laboratory kiln at a constant temperature of 60°C and a
relative humidity of 50% to the final moisture content of 10%.
PEER-REVIEWED ARTICLE Lignocellulose
Dashti et al. (2012). “Drying characteristics of Fir wood,” Lignocellulose 1(3), 166-173. 168
Initial moisture content was determined based on the primary dry weight of each
board. In addition, drying rate was also assessed according to the percentage of moisture
content of each sample before and after the drying procedure with taking the overall
drying time into account.
Residual Stresses and Moisture Gradient Measurement
In equation (1) , PR is prong response (or casehardening) of test sample (mm-1),
x
is the distance between outer prong edges before cutting (mm), xis the distance
between outer prong edges after cutting (mm), and l is the length of each test sample’s
prong (Fig. 1). To determine the moisture content gradient along the thickness of dried
boards, slice cutting method using four layers of 10 mm in thickness was applied.
2
l
xx
PR
(1)
Fig. 1. Cutting method of specimens for measurement of casehardening (internal residual
stresses)
Surface and Internal Checking Measurement
After drying, the intensity of surface and the internal checking were determined
for all dried boards. Five specimens with 20 mm in length were used for measuring of
internal checks. Internal crack abundance was assessed with millimeter precision and
then reported in the four ranges from 1 to 40mm in length.
RESULTS AND DISCUSSION
Effect of Microwave Radiation and Steaming on Drying Rate
Figure 2 shows the drying rate of the wood samples exposed to either microwave
radiation or steaming compared to the control samples. Steaming at 140°C and 160°C
increased the drying rate. However, no increasing effect was observed by increasing the
drying temperature from 140°C to 160°C. Microwave radiation for the duration of 10 min
and rest time of 60 seconds (MW3 condition) improved the moisture loss rate. In
PEER-REVIEWED ARTICLE Lignocellulose
Dashti et al. (2012). “Drying characteristics of Fir wood,” Lignocellulose 1(3), 166-173. 169
contrast, microwave radiations under MW1 and MW2 conditions were not effective to
improve the drying rate. A close relationship between wood drying rate and its
permeability and diffusion coefficients was observed. In fact, these two factors play a
pivotal role in the drying behavior of wood within the free water and bound water
domains. The permeability coefficient is affected by the wood porous and anatomical
structure (Tarmian and Perre 2009), and the diffusion coefficient by the structure of cell
walls (Tarmian et al. 2012). In previous study, Dashti et al. (2012) showed that the radial
permeability and diffusion coefficients of fir wood significantly increased as a result of
pre-steaming at 160°C and microwave radiation. Thus, the improved drying rate can be
related to the increasing of both permeability and diffusivity parameters of fir wood. The
increasing effect of microware radiation on the drying rate of Abies alba L may be
attributed to the rupture occurrence in the ray parenchyma cells. Torgovnikov and Vinden
(2009) mentioned that when the microwave energy is applied to wood, steam is generated
within the wood cells, and thus under high internal steam pressure, the pit membranes on
the cell walls and the weak ray cells rupture to form pathways for easy fluid transfer. The
hydrolysis of bordered pit torus material in fir wood due to steaming as previously
reported by Dashti et al. (2012) can be the main reason for the increasing of the drying
rate. The similar findings were also reported by other researchers (Nicholas and Thomas
1968; Jianxiong et al. 1994; Zhang and Cai, 2008).
a
a
a
b
a
bb
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Control MW1 MW2 MW3 ST120 ST140 ST160
Dryingrate(%/h)
Fig. 2. Drying rate of microwave and steam exposed fir wood specimens compared to the
unexposed ones
Effect of Microwave Radiation and Steaming on Residual Stresses and
Moisture Content Gradient
In all specimens, the prongs of casehardened samples showed inwards deviation,
suggesting the residual stresses in the dried boards. Steaming at 140°C and 160°C
resulted in a higher intensity of the residual stress compared to the control specimens
(Fig. 3). In contrast, the microwave exposed specimens showed lower residual stress
intensity than the unexposed ones. As shown in Fig. 4, the moisture gradient was more
uniform in the microwave exposed boards compared to the unexposed ones, and a fairly
flatter moisture profile was attained. In contrast, steaming had a negative impact on the
PEER-REVIEWED ARTICLE Lignocellulose
Dashti et al. (2012). “Drying characteristics of Fir wood,” Lignocellulose 1(3), 166-173. 170
moisture profile uniformity, and the most heterogeneous moisture profile occurred due to
steaming at 160°C (Fig. 5). In contrast to the microwave exposed specimens, a typical
MC profile pattern with a parabolic shape was developed along the thickness of all
steamed and control specimens. This can be due to the different heating and moisture
flow mechanisms occurred in microwave drying method. The more heterogeneous
moisture profile of steamed specimens compared to the control ones may be a reason for
their greater residual stress intensity. While there was no internal and surface checks in
the microwave exposed specimens, the intensity of both surface and internal checking
increased as a result of steaming (Tables 2 and 3). The higher checking in the steamed
specimens can be attributed to their higher drying stresses. When the drying stresses
exceed wood strength, they cause surface and internal checks.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Control MW1 MW2 MW3 ST120 ST140 ST160
Residualstress(mm
1
)
Fig. 3. Intensity of residual drying stresses in microwave and steam exposed fir wood specimens
compared to the unexposed ones
9
9.5
10
10.5
11
11.5
12
012345
Moisturecontent(%)
Numberoflaye rsin thickness
MW1
MW2
MW3
control
Fig. 4. Moisture content gradient in microwave exposed fir wood specimens compared to the
unexposed ones
PEER-REVIEWED ARTICLE Lignocellulose
Dashti et al. (2012). “Drying characteristics of Fir wood,” Lignocellulose 1(3), 166-173. 171
8
9
10
11
12
13
012345
Moisturecontent(%)
Numberoflayersinthickness
ST120
ST140
ST160
control
Fig. 5. Moisture content gradient in steam exposed fir wood specimens compared to the
unexposed ones
Table 2. Intensity of Internal Checking in Microwave and Steam Exposed Fir
Wood Specimens Compared to the Unexposed Ones
ST160 ST140 ST120 MW3 MW2 MW1 Control
Crack length
(mm)
1 0 0 0 0 0 1 1-10
11 3 2 0 0 0 1 11-20
4 6 0 0 0 0 2 21-30
3 1 2 0 0 0 0 31-40
Table 3. Intensity of Surface Checking in Microwave and Steam Exposed Fir
Wood Specimens Compared to the Unexposed Ones
ST160 ST140 ST120 MW3 MW2 MW1 Control
Crack length
(mm)
0 0 2 0 0 0 0 1-30
2 0 0 0 0 0 0 31-60
1 1 1 0 0 0 0 61-90
0 1 0 0 0 0 0 91-120
PEER-REVIEWED ARTICLE Lignocellulose
Dashti et al. (2012). “Drying characteristics of Fir wood,” Lignocellulose 1(3), 166-173. 172
CONCLUSIONS
In the present study, effect of pre-steaming and microwave radiation on the
conventional drying characteristics of fir wood (Abies alba L.), a gymnosperm species
with torus margo pit membrane was investigated. Results revealed that both steaming and
microwave radiation improve the drying rate of fir wood. However, the effectiveness of
pre-steaming method depends on the steaming temperature and that of microwave on the
microwave radiation duration. Based on our previous study regarding the effect of
microwave radiation and pre-steaming on the air permeability and water vapor diffusivity
of fir wood (Dashti et al. 2012), it can be concluded that the improved drying rate is due
to the modification effect of both pretreatments on the wood permeability and diffusivity.
In addition to the drying rate improvement, the microwave radiation resulted in the lower
residual drying stresses and better drying quality (more uniform MC profile and less
drying checking) compared to the unexposed specimens. In contrast, pre-steaming
imposed negative effects on the wood drying quality. Since considerable drying residual
stress occurred in all pre-steamed specimens, stress relief treatment with sufficient
duration time is recommended. The effectiveness of both microwave radiation and pre-
steaming methods to increase the drying rate also depends on the wood anatomical
structures. Therefore, the potential application of such pretreatments to improve the
drying rate of other wood species is recommended for further research.
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Article submitted: May 13, 2012; Peer review completed: June 20, 2012; Revised version
received and accepted: July 23, 2012; Published: September 5, 2012.
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In this research, the effect of pre-steaming on mass transfer properties, including air permeability and water vapor diffusivity of fir wood (Abies alba L.), a gymnosperm species with torus margo pit membrane, was evaluated. The pre-steaming was performed at temperatures of 120, 140, and 160°C for 1 hour under a pressure of 2-3 bars. Then, the pre-steamed specimens were conventionally dried at a constant temperature of 160°C and a relative humidity of 50% to the final moisture content of 10%. Subsequently, the mass transfer properties of the dried specimens were measured in longitudinal and radial directions. Overall, the pre-steaming was found to be an effective modification method to improve the mass transfer properties of Abies alba L. The improvement was more remarkable for the air permeability as well as through the radial direction. The specimens steamed at the temperature of 160°C had higher mass transfer rates than those steamed at the temperatures of 120 and 140°C. Results of chemical analyses, FT-IR spectroscopy, and SEM imaging provide some explanations for the effects of pre-steaming.
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