IADC 2005 - 3rd Inter-American Drying Conference, August 21-23, 2005
Paper III-4
MICROWAVE WOOD STRAND DRYING: ENERGY CONSUMPTION,
VOC EMISSION AND DRYING QUALITY
Siqun Wang, Assistant Professor
Guanben Du, Visiting Scientist
Yang Zhang, Post-Doctoral Research Associate
Tennessee Forest Product Center, Department of Forestry, Wildlife and Fisheries, University
of Tennessee, Knoxville, TN37996-4563, USA
Fax 865 946 1109; Phone 865 946 1120; E-mail: [email protected]
ABSTRACT
Conventional drying system for wood strands operates now at high drying temperatures.
The high temperatures result in high volatile organic compounds (VOCs) emission. By
reducing drying temperatures and transferring energy directly into the moist wood without
wasting heat of the surrounding environments, microwave drying reduces energy required
and decreases thermal degradation of the wood material, thereby reducing VOCs emission. In
this work, the temperature and moisture content changes under different microwave drying
conditions were investigated. The effects of microwave drying on emission of VOCs were
evaluated. Extractives were analyzed using Gas Chromatography / Mass Spectrometry
(GC/MS). The result showed that increasing the microwave power input or decreasing
sample weight results in high drying temperature, high drying rate, and short drying time.
Different strand geometry and initial moisture content resulted in different warm-up curves,
but did not affect final moisture content. Comparing with conventional drying, microwave
drying resulted in low VOC emissions. Effect of microwave drying on the strand surface
energy was investigated as well.
Keywords: Microwave, drying, wood, strand, moisture content, VOC emission, energy,
surface energy.
INTRODUCTION
New environmental regulations in the United States impose stricter controls on wood
composite board manufacturing facilities. Volatile organics released from drying furnish for
particleboard account for about 75 percent of the total volatile organic compounds (VOCs)
emission during particleboard manufacturing (Boswell and Hunt 1991). All wood used in
composite board production must be dried from an average of 60% to 90% moisture content
(on a dry weight basis) to approximately 3% moisture content before entering the production
line. Conventional drying systems for wood strands currently employ a rotary drum dryer that
shoots a raw flame through a 20’-30’ rotating drum while tumbling the wood product around.
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Product scorching and air emission problems, particularly with nitrogen oxides (NOx) and
VOCs, are prevalent because the rotary drum operates at up to 1000 oF as it reduces moisture
from 50-60% to 2-4%. Such high temperatures produce VOCs both from extractives
contained in the wood and from thermal degradation of the wood material itself. It is common
practice to dry wood furnish in panel mills to very low moisture levels to ensure that press
“blows” caused by a build up of steam pressure in the panel do not occur in the subsequent
gluing step. VOCs emission increases when wood is almost dry (<10% moisture content)
because there is little evaporative cooling to limit the wood temperature rise. Regenerative
Thermal Oxidation units (Yeo 1993; Grzanka 1996; Matros et al. 1996) are most often
employed as a VOCs control method, but such units are capital-intensive, and are associated
with substantial operating and maintenance costs. Increased CO2 and NOx emissions from the
VOCs control units are an additional negative. Fundamental process changes that reduce the
VOCs produced can reduce the reliance on expensive control device.
The main objective of this research was to develop microwave drying technology for
wood strand drying for oriented strand board (OSB) manufacturing. OSB currently holds
about 57% of sheathing market in US housing industry and its use is on the increase. There
could be two advantages for wood strand to be dried with microwave. At first of all, the dry
time of wood strand is to be reduced. In microwave processing energy is supplied by an
electromagnetic field directly to the material. This results in rapid heating throughout the
material thickness with reduced thermal gradients and volumetric heating can also reduce
processing times and save energy. Secondly, the VOC release of wood strand is decreased
during its drying with microwave because this kind of drying method applied a littler energy
and lower temperature compared to rotary drum dryer in wood composites mills. However,
besides the two advantages as above it must be also noticed that the change of drying quality
for strands with microwave method. It is value to research the influence of microwave drying
method on the quality of strand specially its surface wettability which has a big influence on
the bonding strength of wood strand composites.
Wetting is a term to describe what happens when a liquid comes into contact with a
solid surface. Good adhesive wetting, proper solidification (curing) of the adhesive, and
sufficient deformability of the cured adhesive are important (Baier et al. 1968). Adhesive
wettability is the ability of the adhesive to make intimate contact with a surface. Wettability is
measured by the contact angle of a drop of liquid on the surface. The contact angle and
wettability are related to the surface free energy. Wettability of the liquid-solid interfaces is
paramount to adhesion strength.
It has been postulated that aged wafers, strands, or particles are not suitable as raw
materials in wood composites manufacturing because of their relatively low surface quality.
This degrade of surface quality is attributed to several possible mechanisms. The migration of
extractives to the wood surface is one mechanism. Other mechanisms influencing wood
gluing properties of aged wood furnish are: chemical modifications/changes at the wood
surface caused by oxidation, absorption of airborne contaminants, and a decrease in fiber
strength caused either by oxidation or an increase in acidity (Stumbo 1964). Sernek et al.
(2001) conducted experiments to find the influence of heat treatment on surface chemical
composition and surface energy properties using electron spectroscopy for chemical analysis
(ESCA) and contact angle measurements. The results show that the oxygen/carbon (O/C)
IADC 2005 - 3rd Inter-American Drying Conference, August 21-23, 2005
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ratio decreases with temperature. A low O/C ratio reflects high concentration of lignin and
extractives on the wood surface. The contact angle decreases with the time at the elevated
temperature, and is dependent on the wood species and drying temperature.
The purpose of this paper was to investigate the temperature and moisture content
changes under different microwave drying conditions, effects of microwave drying on
emission of VOCs, and effect of microwave drying on the and strand surface wettability.
MATERIAL AND METHODS
Southern pine strands were provided by a local OSB mill. The average geometry of
strands was about 111 x 35 x 1mm with an initial average moisture content of 80% (oven-dry
basis). A commercial small microwave oven (Panasonic NN-S761) with oven cavity
dimensions of 278 x 469 x 470 mm and operation frequency of 2450 MHz was used. The
nominal microwave power output was 687 W as calibrated based on the method of the
International Microwave Power Institute (Anonymous 1989). The turntable inside the oven
was not used except for being mentioned specifically. For the purpose of comparison, a
conventional electric oven with internal dimensions 550 x 610 x 730 mm was used (Fisher
ISOTEM oven model 501). An electric balance (Denver Instrument XL-6100) with
accuracy of 0.1g and fiber-optic temperature transducers with an accuracy of 1°C (Fiso
Signal Conditioner UMI-4) were used to monitor the weight and temperature during drying,
respectively. The test set-up is illustrated in Figure 1. The wires of the balance and
temperature sensor were inserted through small diameter holes drilled from the top side of the
microwave oven. Changes of specimens’ weight were monitored using the electric balance
from which the shelf of specimens was suspended. The temperature sensor attached to the
surface of strands was located inside the microwave oven to indicate the mean temperature of
strands. After the microwave oven test, the samples were then placed in a conventional oven
to obtain their over-dry weights. The moisture content was determined by oven drying at
103oC until the mass change over three hours was less than 0.1%.
The two-cold-scrubber collection system was used for collecting VOC emission.
Scrubbers were chilled in a cooler bath to approximately 5°C. The first scrubber contained
100-ml of fresh distilled water and the second contained 150-ml of methylene chloride. Using
a vacuum oil pump, VOC emissions were pulled out from the microwave oven and absorbed
in the two scrubbers. Gas Chromatography/ Mass Spectrometry (GC/MS) was used to
analyze emissions. After drying was completed, a 20-ml solution was taken from the first
scrubber for determination of low molecular weight (LMw) compounds in the VOC
emissions. The remaining solution was combined with methlene chloride solution from the
second scrubber and then was extracted in a separator funnel. The combined solution was
extracted twice with 30-ml methylene chloride. The methylene chloride extracted solution
was used for the determination of chemical compounds released during drying. Gas
Chromatography/ Mass Spectrometry (GC/MS) was used to analyze chemical compounds.
For the control, VOC emission was collected and analyzed using air drying method under
room temperature.
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In order to examine effect of microwave drying on the and strand surface energy,
strands of southern yellow pine, which included fresh and dried with industrial rotary drum
drier, were collected directly from an OSB production line. The fresh strands were to be dried
with microwave in laboratory. The microwave drying condition was 687 W microwave
power input, 4 minutes drying time with a turntable. In the case of industrial rotary drum
drier, the drying temperature at the strand feeding zone was 740 degree Celsius and drying
time was less than one minute. Thirty industrial dried strands and thirty laboratory dried
strands were randomly collected for testing surface contact angle and surface energy. The
contact angle and surface energy on strands were measured using a dynamic contact angle
analyzer (DCA 322), which is based on Wilhelmy plate technique (Zhang and Wang 2004).
Electric Ballance
Temperature Sensors
Flow Meter
Vacuum Pump
Sample Shelf
Microwave Oven
Fiber Optical Conditioner
Figure1. Schematic diagram of the test equipment.
RESULTS AND DISCUSSIONS
Drying Temperature, Moisture Content and Unit Energy Consumption
Figures 2 and 3 summarize some results of experiments comparing strand temperature
and moisture content at various power inputs and drying weights. Typical curves describing
strand temperature and MC change during microwave drying are shown in Figure 2. The
temperature curve during the microwave drying process usually exhibits three distinct periods:
1) a warm-up period which can be characterized by quick rising temperatures and slow water
loss; 2) an evaporation period in which most water evaporates and the drying temperature
reaches a plateau; 3) a heating-up period in which moisture evaporation slows down and the
surface temperature increases rapidly. A similar pattern of temperature and MC changes is
observed for the convective drying process. However, during microwave drying, the
warm-up period was shorter and the plateau temperature was relatively higher (boiling point
of water), which could increase the drying rate. Furthermore, the surface temperature of
specimens during microwave drying begins to rapidly increase at relatively high MC (20% in
Figure 2).
Comparisons of moisture changes between the microwave drying (input power 687 W)
and the convective drying (190 oC) for 50 g strands are shown in Figure 2. The results
indicate that microwave drying was faster in drying wood strands than the convective drying
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method. Good combination of microwave power input with drying time could dry strands
down to 2% final moisture content without damaging strands (Table 1).
Microwave power input and the mass of drying materials in the microwave oven were
found to have a dominant effect on the drying quality. A term of (Power Input)/Mass (PIM)
ratio is used in this study to characterize the microwave drying process. During microwave
drying, the warm up period depends upon the energy input and the drying weight or the
combining parameter: PIM ratio as shown in Figure 3. Higher PIM ratio results in not only
fast temperature increases during the warm-up period, but in high plateau temperatures as
well. In the warm up period, before the temperature reached the boiling point the energy is
mainly used to build up heat for temperature rising. Moisture changes are relatively small.
The notable characteristics during microwave drying is when the drying temperature is close
to the boiling point, the fast evaporation of water results in the decrease of the strand’s
surface temperature, especially in the case of higher PIM ratio (Figure 3). This phenomenon
has never been observed during conventional oven drying processes.
Table 1. Final moisture content variation at different power input (100g).
Power Input
(W)
538
538
538
538
538
538
538
538
538
687
687
687
687
687
687
687
687
687
Drying Time (min.)
Initial MC (%)
Final MC (%)
14
14
14
15
15
15
16
16
16
9
9
9
10
10
10
11
11
11
82.5
81.2
77.3
77.3
80.5
80.5
75.7
78.6
77.0
79.9
78.3
77.9
77.0
77.3
84.2
76.1
81.5
77.6
2.92
2.36
2.48
2.30
1.44
2.35
1.76
1.79
1.59
3.06
3.21
3.56
2.56
2.48
2.58
1.41
1.45
1.42
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200
80
Temp. (583w)
180
70
Temp. (635w)
Temp. (687w)
160
60
MC (583w)
MC (635w)
50
MC (687w)
120
oven (190°C)
100
40
80
30
Moisture (%)
Temperature (°C)
oven (190°C)
140
60
20
40
10
20
0
0
100
200
300
400
500
600
700
800
0
900
Time (sec.)
Figure 2. Temperature and moisture content changes during microwave drying and oven
drying of 50g of wood strands at different power inputs.
200
100
Temp. (300g)
180
90
Temp. (120g)
Temp. (50g)
160
80
MC (300g)
Temperature (°C)
70
MC (50g)
120
60
100
50
80
40
60
30
40
20
20
10
0
0
50
100
150
200
250
300
350
400
450
Moisture Content (%)
MC (120g)
140
0
500
Time (sec.)
Figure 3. Temperature and moisture content changes during microwave drying under 687
W power input for different drying weights.
IADC 2005 - 3rd Inter-American Drying Conference, August 21-23, 2005
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The energy performance was quantified in terms of the energy required to heat and
evaporate a unit mass of water (MJ/kg), and defined as
UEC =
t on × P × (1 + mi )
M i × (mi − m f )
Where: ton = total drying time (s); P = microwave power output (W); Mi = initial mass (kg);
mi = initial moisture content (fraction, dry basis); and mf = final moisture content (fraction,
dry basis). Unit energy consumption, defined previously as unit energy required for
evaporating water from the wood (MJ/kg), was used to describe the drying energy
consumption. It is found that microwave drying is more efficient in drying wood strands in
the 17- to 30-percent moisture range (Table 2). When compared with traditional three-pass
drying processes which require about 3.5~4.4 MJ energy to evaporate one kilogram of water
at high moisture contents (100- to 185-percent range) and about 4.4~5.4 MJ/kg at lower
moisture contents (18- to 33-percent range) (Koch 1985), microwave drying techniques could
save energy consumption up to 50% in drying thin wood strands (Table 2).
Table 2. Unit energy consumption during microwave drying of 500g of wood strands under
687 W power input.
Initial
MC
Final MC Drying Time
UEC*
(%)
(%)
(s)
(MJ/kg)
73.9
30.2
705
3.85
30.2
17.1
210
2.87
17.1
6.8
180
2.81
73.9
6.8
1095
3.90
* Unit energy consumption
VOC Emission
The alpha-pinene was the major chemical components that were released when
softwoods were dried (Danielsson and Rasmuson 2002). By comparing alpha-pinene
concentration collected during microwave drying and air dying (room temperature), the VOC
emissions during microwave drying was quantified. Table 3 shows that the alpha-pinene
concentrations collected during microwave drying were slightly higher than one collected
during air drying. This may be caused by the lower final MC for the case of microwave
drying. This finding was also approved by extractive method (Du et al. 2005). Since VOC
emission came mainly from extractives, the changing of extractives in wood furnishes could
be used to estimate VOC emission.
Surface Energy
The total surface energy γ s of the wood strands is composed of the dispersive component γ sd
and the poplar component γ sp . Figure 4 shows surface energy of wood strands dried by
microwave and industrial rotary drum drier, respectively. The surface polar energy of strand
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dried by microwave was larger than those of strand dried by mill rotary drum. The surface
dispersive energy of strand dried by microwave was a little smaller than that of strand dried
by mill rotary drum. The total surface energy of strand dried by microwave was slightly
higher than that of strand dried by mill rotary drum. The surface contact angle of strands were
also measured in order to compare the surface wettability of wood strands dried with
industrial rotary drum drier and microwave method (see Figure 5 ). From this Figure it was
proved that the surface wettability of wood strands dried with microwave was better than that
with industrial rotary drum drier because both advancing contact angle and receding contact
angle of wood strands dried with microwave were smaller than those with industrial rotary
drum drier.
Table 3.
Concentration of alpha-pinene collected during microwave drying and air drying.
Drying
method
Final MC
(%)
Microwave
drying
5.2
Air drying
8.3
Alpha-pinene
ppm
81.0
82.4
73.5
74.2
Sample
Sample 1
Sample 2
Sample 3
Sample 4
50
45
Rotate drum drier
Surface Energy (mN/m)
40
Microwave
35
30
25
20
15
10
5
0
Dispersive component
Poplar component
Total surface energy
Figure 4. Surface energy of wood strands dried by rotate drum and microwave.
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90
Advancing contact angle
Receding contact angle
80
Contact angle(degree)
70
60
50
40
30
20
10
0
Rotate drum drier
Microwave
Figure 5. Surface contact angle of wood strands dried by rotate drum and microwave.
CONCLUSIONS
Characteristics of temperature, moisture content, unit energy consumption, and surface
wettability during microwave drying of wood strands were examined in this study and
compared with the conventional drying process. The typical curve of temperature change
during the microwave drying of strands indicates that there were normally three distinct
periods: warm-up, water evaporation, and heating-up. Due to its unique heating mechanism,
the microwave drying method could provide faster strand drying, low VOC emission, and
more energy efficiency when compared with the convective drying method. The microwave
drying technique could save energy consumption up to 50% in drying thin wood strands. The
surface wettability of wood strands dried with microwave was better than that with industrial
rotary drum drier.
ACKNOWLEDGEMENT
This work was supported by the USDA Wood Utilization Research Grant and the
Tennessee Agricultural Experiment Station, Project #83. The authors would also like to thank
Huber Engineered Woods LLC for providing experimental materials.
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IADC 2005 - 3rd Inter-American Drying Conference, August 21-23, 2005
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