Journal of Sustainable Bioenergy Systems, 2013, 3, 243-249
Published Online December 2013 (http://www.scirp.org/journal/jsbs)
Open Access JSBS
Wood Pyrolysis in Pre-Vacuum Chamber
Hiroki Homma1, Hiroomi Homma2*, Yusrizal2, Muhammad Idris2
1Matsue College of Technology, Matsue, Japan
2University of North Sumatera, Jl. Almamater, Medan, Indonesia
Received October 16, 2013; revised November 12, 2013; accepted December 1, 2013
Copyright © 2013 Hiroki Homma et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Climate change, global warming, and energy crisis are critical issues to be solved urgently in a global framework. Al-
ternative energy and renewable energy technologies must be quickly developed to be substituted for fossil fuels like oil,
gases, and coal. USA, UE, and Japan invested huge budgets to develop biomass renewable energy technology. Their
target is to develop a commercial base large-scaled plant. On the other hand, in developing countries, especially in rural
areas, people who can access electricity is still less than 70%, To decelerate or prevent global warming and improve
electrification in rural areas, a new technology for wood pyrolysis, which requires low manufacturing cost and less
maintenance, and of which gases are directly applicable to the gas engine generator, is developed in a laboratory scale.
This paper reports the performance of this new plant and effects of several parameters on the performance. It is con-
cluded that the new technology is quite feasible in rural areas, and upgrading of the plant is easily possible.
Keywords: Pyrolysis; Rubber Wood; Pre-Vacuum Chamber; Pyrolysis Yields
Global issues of energy crisis and climate change are
forcing to develop renewable energy technology in a
global framework. In such situation, pyrolysis of biomass
has been receiving great attention and researches on py-
rolysis are increasingly carried out recently. United
States and European countries have established a task
group for pyrolysis technology development as Task 34
of Bioenergy, International Energy Agency (IEA) since
1995 and the task work is still continued until now .
United States has also conducted a biomass multi-year
program since 2007 . In this program, pyrolysis of
woody biomass is positioned as one key technology for
bio-fuel production. In IEA and US, Department of En-
ergy focus on the development of fast pyrolysis technol-
ogy, because high yield of bio-oil can be produced by
this pyrolysis method. Therefore, many efforts for tech-
nical development and development of the sustainable
feedstock supply system are made to down the produc-
tion cost of bio-fuels for fossil fuel prices. Thus, the py-
rolysis plant must be necessary in large scaled.
The pyrolysis technology is rather old, but pyrolysis
proc ess is not fully clarified yet until now. Pyrolysis is a
thermal decomposition process from a solid phase to gas
and liquid phases. Many experimental works have been
done to investigate gas, char and tar yields by various
wood pyrolysis methods in this decade [3-7] and the re-
view articles are also published . On the other hand,
for basic understanding of a pyrolysis process, chemical
kinetic parameters of thermal decomposition processes
are obtained by several researchers, and kinetic equations
are derived from simplified forms [9-12]. It is well
known that woods consist of main three constitutions,
cellulose, hemicellulose, and lignin, but there are many
other minor compositions contained in woods, such as
organic extractives and minerals . Kinetic equations
are identified for the main constitutions, respectively ,
but the composition ratio varies in one wood to another,
and kinetic equations of minor compositions are not
available. As a result, there is no unique kinetic equa-
tion that can express the overall pyrolysis process, uni-
Developing countries are also going to take part in de-
celerating or preventing climate change, global warming
and in solve energy crisis together with developed coun-
tries. Developing countries have plans to develop rene-
wal energy using their natural resources. However, en-
ergy crisis increasingly becomes serious, because devel-
oping countries are surely going to use more energy in
future than in past years. Developing countries have an-
H. HOMMA ET AL.
other energy issue that is a low electrification rate. Ac-
cording to the estimation by IEA in 2011 , average of
electrification rate in the world is 80.5%. However, the
electrification in developing countries is 75%. Especially,
in rural areas, 32% people cannot access electricity.
Looking at ASEAN countries, electrification in Indonesia
is 76% in 2012, which is remarkably lower than that of
other Asian counties, Vietnam (89.3%), Philippines
(89.7%), Thailand (99.3%), Malaysia (99.4%), and Sin-
gapore (100%). Electrification in whole Indonesia is low,
nevertheless, in urban areas like Jakarta and North Suma-
tra, more than 90% of people can access electricity, while
only 50% of rural area people can access electricity.
Electricity is one form of energy, but it is quite a versa-
tile energy source. In urban areas, electricity is usually
supplied through the power grid from large scale power
stations. Many rural areas in developing countries, how-
ever, are not covered by those power grids. Constructing
power grid for rural areas, newly, is not efficient and too
expensive. Therefore, off-grid power generation systems
are receiving strong attention to improve electrification
in rural areas, because of efficiency and easiness.
Biomass potential in rural areas is rather high in gen-
eral. However, as mentioned above, large-scale and so-
phisticated biomass technology developed by United
States and European countries is not always necessary
for rural areas, because frequent maintenance service is
not available. A simple and low-maintenance system is
appropriate there. There are several methods to create an
oxygen-absent or oxygen-rare condition for wood pyro-
lysis reactor. Oxygen-absent conditions by filling a py-
rolysis reactor with an inert gas like nitrogen, oxy-
gen-rare conditions by piling feedstock upper solid fuel,
and vacuum condition by evacuating a reactor are widely
used for biomass pyrolysis. The former two methods are
suitable for large scale plants for the sake of high effi-
ciency. However, post processes to clean pyrolysis
products, such as fine particle removal, or liquid fuel
cleaning and stabilizing [2,15,16] are required. On the
other hand, a vacuum pyrolysis process has been devel-
oped for converting biomass to liquid fuels. Several
drawbacks are pointed out for industrial use, but despite
those drawbacks, vacuum pyrolysis does have attractive
advantages: clean oil production and usage of larger
This work aims at the enhancement of electrification
in rural areas using a pyrolysis gas engine generation. A
new technology of wood pyrolysis using a pre-vacuum
reactor chamber taking advantages of the vacuum
pyrolysis is developed so that the pyrolysis gases could
be directly used for a gas engine generator. In this new
pyrolysis technology, no pre-treatment of feedstock and
post-treatment of pyrolysis gases are necessary, and the
system is asimple and requires less maintenance. In this
paper, performance of the pyrolysis system is evaluated
by a series of experiments.
2.1. Wood Pyrolysis System with a Pre-Vacuum
A pilot plant of wood pyrolysis system was constructed
in the laboratory. The constructed system is shown in
Figure 1. The system consists of five components. Those
are a furnace, a pre-vacuum reactor chamber, a tar trap, a
carbon dioxide absorber, and a gas storage tank as shown
from the left hand side to the right in Figure 1(a). The
pre-vacuum reactor chamber is placed inside the furnace.
The reactor chamber is made of a stainless steel pipe of
which inner diameter is 254.0 mm, wall thickness is 13.0
mm, and inner height is 500.0 mm. A blind flange is bol-
ted on the top of the chamber for opening of the reactor
chamber, charging of new wood and taking out chars be-
fore and after experiments.
All the components of the system are connected with
stainless steel pipes of half inch diameter. In the tar trap,
gases containing tar vapor are cooled with circulated wa-
ter and tar is liquefied to remain inside the trap. The car-
bon dioxide absorber utilizes the property that carbon di-
oxide can be dissolved in water.
Before starting an experiment, the pre-vacuum reactor
chamber, the tar trap, the carbon dioxide absorber and the
gas storage tank are evacuated by a vacuum pump up to
0.02 MPa (0.2 atm). Then, a valve between the reactor
chamber and the tar trap is close. T1, T2, and T3 shown
in Figure 1(b) indicate positions where K-type thermo-
couples are installed.
Figure 1. Constructed pyroly sis system.
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H. HOMMA ET AL. 245
2.2. Experimental Procedures
Rubber wood pieces undergoing pyrolysis is supplied
from a local furniture company. Those are all the waste
wood pieces from trunk part after manufacturing furni-
ture. Pieces sizes are 100.0 mm to 200.0 mm long, 50.0
mm to 100.0 mm wide and 20.0 mm to 25.0 mm thick.
In the furniture company, the rubber wood timbers are
dried in kiln. Moisture content of the rubber wood tim-
bers has been reduced into 8 to 10% in kiln drier, but it
may rise to around 12% while the timbers are stored in
storage after taken out from the kiln drier. Therefore, the
sample used for the experiment may contain moisture of
around 12%. No additional water is added in the pre-va-
cuum reactor chamber as a pyrolysis agent. Experiments
were carried out according to the following procedures.
1) 5 kg of rubber wood pieces are charged in the reac-
2) Two K-types of thermocouples are installed in the
reactor chamber. One is placed at the center of bottom
and the other is placed at the center of top. Then, the
chamber is closed with a flange. The additional thermo-
couple is installed on the outer surface of the flange as
shown in Figure 1(b).
3) The chamber, the tar trap, the carbon dioxide absor-
ber, the gas storage tank, and the piping lines are evacu-
ated by a rotary vacuum pump. The level of vacuum is
around −0.08 MPa in gage pressure, 0.02 MPa in abso-
4) The weight of the pre-vacuum reactor chamber
containing the 5.0 kg rubber wood pieces is measured by
a digital scale with 50.0 g resolution during an experi-
5) The pre-vacuum reactor chamber is heated by firing
up the furnace.
6) Temperatures at three positions, the pressure in the
pre-vacuum reactor chamber, and the weight of the reac-
tor chamber are monitored.
7) Heating rate of the reactor chamber is kept constant.
8) After the pressure in the reactor chamber reaches a
specified value, the chamber pressure is kept constant by
adjustment of valve opening.
9) The termination of the pyrolysis in the reactor
chamber is judged by the asymptotic change in the reac-
tor chamber weight.
10) After a pyrolysis process has been terminated,
mass of gas, tar and char products is measured to evalu-
ate pyrolysis yields.
This system is characterized by the reactor chamber,
which is evacuated initially and then filled with pyrolysis
gases, called a pre-vacuum chamber in this paper. This
system is different from vacuum pyrolysis in which
whole thermal decomposition undergoes in vacuum .
In this experiment, wood charge in the reactor chamber is
varied to 1.0, 3.0, and 5.0 kg. In addition, 0.1 kg wood
charge is also examined. The specified chamber pressure
is 0.1 MPa and 0.2 MPa in gage pressure.
3. Experimental Results
The temperature and the pressure were monitored during
the experiment. After the experiment, mass of the pyro-
lysis yields, gases, char, and tar was measured. These
experimental results are shown in next subsections.
Temperature is measured at three points as shown in
Figure 1. Measured temperature results at three points
are shown in Figure 2, for case of 5.0 kg wood charge
and chamber pressure of 0.1 MPa. Under the same con-
dition, three experiments were carried out. Three expe-
rimental results are shown in the figures. In Figure 2(a),
the temperature at the bottom of the chamber inside is
plotted as a function of time, minutes after firing up the
furnace. Outside surface of the chamber bottom is di-
rectly exposed to flames of burning woods. The tem-
perature increases almost linearly with time up to 400˚C.
Then, suddenly the temperature drops in three experi-
ments. After these drops, the temperature gradually in-
creases to return to the initial curves’ extrapolation.
However, beyond 500˚C, the temperature increment be-
comes moderate and reaches the maximum of around
600˚C.The pyrolysis process has terminated after 100 mi-
Temperature at the top of the chamber inside is rather
lower than the temperature at the bottom as shown in
Figure 2(b). It reaches the maximum, 350˚C at near 60
minutes, unlike the temperature at the bottom. Then, the
temperature gradually decreases toward the process ter-
mination. Temperature at the top surface of the flange is
shown in Figure 2(c). Its evolution with the time is
similar to that of the chamber top temperature, although
the maximum value is much lower. All the data of three
experiments fall near one curve shown in the figure,
which is drawn using the average values of three experi-
In Figure 3, the time evolution results of temperature
measured at three points are shown for the chamber
pressure levels of 0.1 and 0.2 MPa. The chamber pres-
sure has no influence on the time evolution of tempera-
The gage pressure in the reactor chamber is plotted as a
function of time in Figure 4. The initial pressure of the
chamber is set to −0.08 MPa. For seven to eight minutes
after beginning of the experiment, pressure does not
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H. HOMMA ET AL.
Figure 2. Temperature measured at three positions T1, Ts,
and T3 shown in Figure 1(b) for 5 kg wood charge, and
chamber pressure of 0.1 MPa.
change. Then, the pressure steeply increases up to 0.1
MPa and 0.2 MPa. Before the valve is open at 0.1 MPa,
two pressure evolution with time should be the same in
Lift of the chamber pressure may be caused by thermal
decomposition of wood pieces into pyrolysis gases and
thermal expansion of gasses inside the reactor chamber.
The pressure of the chamber is kept constant 0.1 or 0.2
MPa until more than 100 minutes, termination of the
pyrolysis in the chamber.
3.3. Pyrolysis Yields
After termination of pyrolysis process in the chamber is
judged by asymptotic change in the chamber weight with
time, char in the chamber, tar in the tar trap, and gases in
the storage tank are weighted, respectively. It should be
noted that gases stored in the tank is not all the gases
generated by pyrolysis process, because a certain amount
of carbon dioxide has been removed by the carbon diox-
ide absorber. Measured yields are summarized in Table
In this table, a value in a parenthesis is a percentage of
Figure 3. Averaged temperature of three experiments
for 0.1 MPa for 0.2 MPa.
Figure 4. Chamber pressure vs. time for 5 kg wood charge,
and specified pressure of 0.1 and 0.2 MPa.
each product to the wood charge 5.0 kg, and it can be
seen that yields of tar, char, and gases are around 40%,
30%, and 30% of the wood charge, respectively. This
pilot plant yields slightly higher tar than the other
Aiming at improvement of electrification in rural areas,
one pilot plant of a wood pyrolysis system has been de-
veloped based on a new technology in a laboratory scale.
In order to ensure sustainable usage, low-maintenance
equipment is inevitable. Based on this concept, the plant
was constructed and its performance was evaluated. In
contrast to commercial wood pyrolysis plants using small
feedstock like wood pellets and fast pyrolysis process,
this plant uses rather big wood pieces and the pyrolysis
process is slow. Therefore, there are several factors
dominating pyrolysis performance of this plant. In the
next subsections, those factors are discussed.
4.1. Effect of Pressure
The chamber pressure is monitored during the experi-
ment. After the reactor chamber pressure reaches 0.1
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H. HOMMA ET AL.
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Table 1. Pyrolysis yields for 5.0 kg wood charge, and chamber pressure of 0.1 and 0.2 MPa.
Wood Charge kg Pressure MPa Tar kg (wt%) Char kg (wt%) Gases kg (wt%) CO2, & others kg (wt%)
0.1 1.96 (39.2) 1.43 (28.6) 1.13 (22.6) (32.2)* 0.47 (9.6)
0.2 2.00 (40.0) 1.53 (30.6) 1.18 (23.6) (29.4)* 0.29 (5.8)
*CO2 and other gases removed by absorber are added to gases.
Table 2. Gas composition (vol%).
MPa, or 0.2 MPa in gage pressure, the chamber pressure
is kept constant at that level until the pyrolysis process
terminates. This intends that wood pieces may undergo
thermal decomposition under the constant pressure. As
seen in Figure 3, though the reactor chamber pressure is
changed between 0.1 MPa and 0.2 MPa, temperature at
three points of the reactor chamber similarly varies with
time. In addition, pyrolysis yields shown in Table 2 in-
dicate that there is no significant difference between two
chamber pressure levels. It can be drawn from these re-
sults that the chamber pressure up to 0.2 MPa does not
affect thermal decomposition process of wood in the
Wood Charge (kg)
1.0 3.0 5.0
H2 17.1 19.5 19.8
CH4 32.2 37.7 3.6.6
CO 28.9 22.1 18.6
CO2 21.8 20.7 14.9
4.2. Effect of Wood Charge
Wood mass charged in the reactor chamber must be one
of the most dominant factors to determine the plant per-
formance. The 5.0 kg of wood fills the reactor chamber
up to almost a half of its total depth. Then, the reactor
chamber is placed in the furnace so that the reactor
chamber sinks in the furnace up to 300.0 mm from the
bottom. This means that all the wood pieces are im-
mersed in the furnace. Even if all the woods in the reac-
tor chamber is immersed in the furnace, large tempera-
ture distribution in the wood pieces exists after the fur-
nace is fired up. Simple thermal conduction analysis was
done neglecting thermal convection in the chamber, be-
cause the chamber is evacuated into vacuum. One exam-
ple of temperature distributions inside the reactor cham-
ber is shown in Figure 5. In the analysis, wood pieces
are modeled as a rectangular shape, and they are shown
with dotted lines in the figure. The most left wood piece
is located in the center of the reactor chamber and the
most right piece contacts with the chamber wall. The
bottom of each wood piece contacts with the chamber
bottom. Therefore, the bottom of wood piece is directly
heated by the chamber bottom. In the figure, temperature
at the bottom of the wood piece is shown with a red line,
temperature at the top of wood piece, with a blue line,
and temperature at the middle of wood piece, with a yel-
low line, and temperature at the chamber tope, with a
black line. From this numerical calculation result, it can
be seen that there is a large temperature variation in
wood pieces. This analysis does not take into account
convection of volatile gasses emitted from wood pieces.
If the volatile gas convection is considered, the tempera-
Figure 5. Temperature distribution in wood at 800 seconds
after furnac e firing.
ture distribution may go to uniform. However, it is easily
understandable that for small wood charge like 1.0 kg
and 3.0 kg, wood pieces may undergo less temperature
For wood charge of 0.1 kg, 1.0 kg, and 3.0 kg, ex-
periments were also carried out to examine the wood
charge effect on the pyrolysis yields ... The controlled
chamber pressure was 0.2 MPa. The heating rate was
almost the same in four cases including 5.0 kg wood
Three main pyrolysis yields are plotted as a function of
wood charge mass in Figure 6. For wood charge mass of
more than 1.0 kg, three yields are insensitive against the
wood charge. However, it should be noted that for 0.1 kg
wood charge, tar yield is significantly low while gas
yield is remarkably high
The following reason can be considered for this fact.
The 0.1 kg wood charge is very small volume as com-
pared with the reactor chamber volume. When primary
H. HOMMA ET AL.
0.0 1.0 2.0 3.0 4.0 5.0
Figure 6. Pyrolysis yields as a function of wood charge.
volatile gases are emitted from wood pieces, the gas vo-
lume is not much enough to raise the chamber pressure
up to 0.2 MPa. Then, the primary volatile gases are
heated further to more than 500˚C in the furnace and the
secondary decomposition of tar may take place for gen-
eration of new gasses . As a result, the chamber
pressure increases up to 0.2 MPa. This suggests that 1.0,
3.0, and 5.0 kg wood charges mainly produce the pri-
mary volatile gases as the pyrolysis yield.
In addition, this hints that upgrading of this plant can
be easily realized to obtain high gas yield if the secon-
dary decomposition of volatile gasses is induced and ex-
pedited by attaching another vacuum chamber heated up
to more than 500˚C.
4.3. Gas Compositions
Main compositions of wood pyrolysis gas product are
usually hydrogen, methane, carbon monoxide, and car-
bon dioxide. Gas products from 1.0, 3.0, and 5.0 kg
wood charge were analyzed by gas chromatography. Ob-
tained results are summarized in Ta ble 2 . As seen in this
table, composition ratios of four gases are not susceptive
against the wood charge mass. The major gas composi-
tion is methane. Other gases are similar in volume con-
centration. Concentration of methane is very high as
compared with other researcher’s results , in which the
methane concentration is given in wt%, and is con-
verted to 15.5 vol% by calculation.
Pyrolysis experiments were carried out by the other
group using the same rubber wood as those used in this
experiment, but the small pieces less than 10.0 mm cubic,
and the different equipment that can be heated up to
900˚C. The experimental results indicated that the pyro-
lysis process produced gases with methane content of
17.6 %, 19.8%, 18.0% and 10.1% at the pyrolysis tem-
perature of 600˚C, 700˚C, 800˚C, and 900˚C, respect-
tively [19,20]. These data are calculated as methane con-
tents of the combustible gases volume totaled from car-
bon monoxide, hydrogen, and methane. This indicates
that methane gas content decreases with increase of py-
rolysis temperature. On the other hand, other result 
indicates that methane gas content increases with pyroly-
sis temperature in range of 350˚C to 450˚C.
It is not clear why this plant can produce methane-rich
gas. Two factors can be considered for this reason. One
is wood species and the second is the pyrolysis system.
Further experiments are necessary to clarify this problem.
In addition, heating value of the gases produced by this
plant is calculated as 9.7 MJ/kg from the results in Ta-
bles 1 and 2. This gas heating value is also higher than
that of other wood pyrolysis gas . If carbon dioxide is
completely removed by the absorber, the heating value
rises to 19.0 MJ/kg. This is almost same as heating value
of dry wood .
To prevent global warming and improve electrification in
rural areas, a pilot plant of wood pyrolysis was con-
structed in a laboratory, based on a new technology and
the concept that for sustainable usage, the plant should be
easy-operating, low maintenance, and low cost.
Experiments were carried out to evaluate performance
of the plant and characterize the plant. The following
conclusions are obtained.
1) This plant is characterized by a pre-vacuum cham-
ber, in which wood pieces undergo thermal decomposi-
tion in the absence of oxygen.
2) The chamber pressures levels of 0.1 MPa and 0.2
MPa under which pyrolysis process proceeds do not af-
fect pyrolysis yields of tar, char, and gases.
3) The wood mass of 1.0 kg to 5.0 kg charged in the
pre-vacuum chamber does not affect the pyrolysis yields,
4) However, 0.1 kg wood charge yielded high gas
content as compared with tar and char. This can be inter-
preted as a result of secondary decomposition of tar. In
addition, this hints how to upgrade of this plant for high
5) This plant yields ethane-rich pyrolysis gas. As a re-
sult, heating value of the pyrolysis gas is around 9.7
MJ/kg for gases in which carbon dioxide is partially re-
moved by a water resolution method and 19.0 MJ/kg if
carbon dioxide is completely removed.
Further investigation is necessary to understand pyro-
lysis process in the pre-vacuum chamber. Numerical
analysis is also useful for this. Experimental and nu-
merical research results will be reported late.
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