Int.

Agrophysics, 2007, 21, 311-316

INTERNATIONAL
Agrophysics
www.international-agrophysics.org

Temperature variation in a sawdust oven using different wood species

O.A. Ajayi and O.K. Owolarafe*
Department of Agricultural Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria

Received August 12, 2007; accepted November 11, 2007

A b s t r a c t. An investigation was carried out on the use of namely cellulose (C6 H10 O5 ), lignin (C6 H10 O3 ) (OCH3 )
sawdust as fuel for firing oven. An oven was fabricated and fired (0.19-1.7) and hemicelluloses such as xylem (C5 H8 O4 )
with sawdust from four selected wood species (Triplochitus (Tillman, 1978). Hard woods are deciduous trees or wide
scleroxylon, Milicia excelsis, Khaya sinegalensis and Celtis
leafed trees, while soft woods are coniferous trees which are
zenkeri). The temperature of the oven was measured at four
different levels. The results indicate that the temperature of the cone-bearing evergreens. Hard wood generally contains
oven increases with the degree of hardness of the wood species and more energy than softwood on a dry weight basis due to
nearness to the heat source. The temperature was the highest for higher lignin content plus the presence of more resins in the
khaya sinegalensis followed by milicia excelsis, celtis zenkeri and extraction (Brown et al., 1952).
triplochitus scleroxylon, respectively. The average temperatures Specific heat capacity is the amount of heat energy re-
recorded for the species in that order were 209.62, 173.50, 166.37 quired to raise the temperature of a unit mass of a substance
and 104.19°C. Statistical analysis of the data obtained indicates by one degree centigrade. Specific heat capacity differs from
that the separate and interactive effects of all the factors (duration
one species of wood to another. Thermal conductivity is the
of heating, location and wood species) were significant at 99%.
K e y w o r d s : temperature variation, sawdust oven using,
measure of the rate at which heat is conducted through the
wood species material; conductivity increases with the wood density.
Wood thermal conductivity is found to increase with higher
INTRODUCTION moisture content (Panskin et al., 1962).
Sawdust is the powdery wood waste produced by
Wood is a ready source of fuel ever known to man. Its cutting wood with a saw. The size of the sawdust particles
uses in both industry and domestic purposes cannot be over depends on the kind of wood from which the sawdust is
emphasized. In most developing countries wood and obtained and also on the size of the teeth of the saw
charcoal are the predominant fuels for preparation of food as (Afuwape, 1983). Between 10 and 13% of the total content
well as serving as fuel for small and medium scale industries of the log is reduced to sawdust in milling operations; this
(Zerbe, 2004; Bhattacharya et al., 1999). Wood is said to be depends largely on the average width of the saw kert and the
the largest renewable source of energy at present and the thickness of the timber sawed.
fourth largest source of energy ie rated after petroleum, coal Transfer of heat plays an important role in oven, furnace
and natural gas in that order, all of which are non-renewable and dryer designs. Transfer of heat energy could be through
energy sources. Wood was man’s main source of heat until conduction, convection or radiation. The rate of heat transfer
coal, oil, gas and electricity replaced it. In recent years, by conduction through a substance is directly proportional to
however, the high cost of oil and gas has again induced the temperature gradient, dT/dx, and to the cross-sectional
people to burn more wood. area of the path (Bird et al., 2002):
The chemical composition of wood is an important dT
parameter in determining the energy value of wood q = KA ( ) , (1)
dx
materials. Wood is a composite of three basic polymers,

*Corresponding author’s e-mail: owolarafe@yahoo.com © 2007 Institute of Agrophysics, Polish Academy of Sciences
312 O.A. AJAYI and O.K. OWOLARAFE

where: q – heat rate (kW), A – cross-sectional area of flow

Mild steel
path (m 2 ), T – temperature (°C), x – distance through
conducting medium (m), K– thermal conductivity of the Fibre glass
material (W m-1°C).
Mild steel
Transfer of heat by transport of heated fluid material
may be through free or forced convection. The heat rate is
proportional to the difference in temperature between the Fan
surface and the main bulk of fluid and to the surface area,
thus:

q = hc A ( t s - t f ), (2)
where: t s - t f – difference in temperature between bulk of Aluminium
fluid and surface area ( o C), hc – convection heat transfer Exhaust

coefficient. Stove
Radiation is the emission of energy, without need of a con-
ducting or convecting medium, from the surfaces of opaque
bodies and from within semi-transparent objects. Radiation
rate depends upon the area, the nature and absolute tempe- Fig. 1. Saw dust oven.
rature of the surface.
The relationship of those factors is defined as:
Aluminium sheet
q = AEsT 4 , (3)
Exhaust
where: E – emissivity, s – Stefan-Boltzman constant, T –
temperature. Mild steel

If a heating element is located in a thermally insulated Fibre glass

chamber, most of the heat generated is conserved and can be Air duct

applied to a wide variety of heating processes. Such insula-

ted chambers are called ovens or furnaces which depend on Fig. 2. A cross-section of the heat exchanger.
the temperature. Ovens can be classified as electric ovens,
gas ovens, coke ovens, microwaves ovens, wood fired ovens,
coal fired ovens and kerosene fired ovens. They are used for
baking and roasting foods, drying paints and organic
enamels, baking foundry cores, thermal treatment of metals
and drying of agricultural materials (FSTC, 2003). This Oven cabinet
study undertakes the development of a sawdust fired oven as
a way of utilising the wastes from the saw mills to generate 4th layer
energy for various purposes. 300 mm

3rd layer
MATERIALS AND METHODS 300 mm
The oven was designed to have essentially two compo- 2nd layer
nents: a heat exchanger and an oven chamber. 300 mm
Figures 1-3 show the components of the oven (con-
1st layer
structed for the study) and the arrangement for temperature
measurement. The heating unit consists of a heat ex- 300 mm
changing unit and a fire compartment (Tin can stove) as Saw dust stove
shown in Fig. 2.
The heat exchanger consists of four concentric rectan- 300 mm
gular parts (Fig. 2). A little opening is made on one side of 600 mm 100 mm

the rectangular part to allow air intake, for the burning of

sawdust. The other sides of the rectangular box are closed so Fig. 3. Arrangement for temperature measurement in the oven.
as to reduce loss of heat to the surroundings. This rectan-
gular box is constructed from a mild steel sheet – both inner
 TEMPERATURE VARIATION IN A SAWDUST OVEN USING DIFFERENT WOOD SPECIES 313

and the outer parts, with fibre glass in between as lagging rature of the oven was observed to be the highest with saw-
material to prevent heat loss. The upper part of the rectan- dust from Mahogany, while those from Iroko, Ita and Arere
gular box is constructed from aluminium sheet. The base of followed in that order. This result may be explained that
the rectangular box is also constructed from mild steel and Mahogany and Iroko are both hardwood which retains more
lagged with fibre glass. This is done to reduce heat dissipa- energy than softwood (Ita and Arere) (Brown et al., 1952).
tion to the surroundings. Similar results were also reported by Klasnja et al. (2002)
The stove which uses sawdust as fuel is positioned who observed that the calorific value of poplar and willow
inside the rectangular box. It is similar in design and con- wood species vary. This result provides an insight into the
struction to the Thai bucket stove (Bhattacharya et al., 2002, estimation of the preheat time required for the oven when
Dixit et al., 2006). It involves cutting a hole in the bottom on different types of sawdust are used.
one side of a three gallon can. A short length of rod of about The temperature of the oven was also observed to
25 mm diameter is placed horizontally in the hole so that it decrease with increase in the distance from the heat source.
reaches the centre of the stove. The can is filled with saw- The results also agree with the findings of Ekundayo et al.
dust, stamped down with a wooden block during filling. The (1998) on a tunnel oven. The temperature profiles of the
rod is removed. The stove is lit through the air hole at the tunnel oven were observed not only to vary according to the
bottom. The performance of the oven was determined by the heater profile but also to be dependent on the proximity to
preheat time measurement method as specified by ASTM the heating element. The temperature of the oven was ob-
(1999). Preheat time is time required to raise the temperature served to reduce as the distance form the heating source in-
of the cavity form room temperature to cooking temperature creased. For example, for a heating element of 503 W, the
(about175°C). The temperature of the oven was measured at temperature reduced form 95 to about 74°C as the distance
four different levels, A, B, C and D, along the height of the increased form 0 to 80 mm. Related findings were also repor-
oven chamber, with A the closest point to the heat source. ted by Abraham and Sparrow (2004) on another electrically-
The oven temperature was measured both on heating and heated oven. The temperature profiles of the oven were
after removal of heat source. The temperature was moni- observed to differ in the different sections of the oven. The
tored at intervals of 5 min for a period of 1 h. Four different result also corroborates the report of Zareifard et al. (2006)
types of wood, namely Triplochiton scleroxylon (arere who indicate that the time temperature profiles vary and
*wood local names in Nigeria), Milicia excelsis (iroko*), depend on the location of the h-monitor in a baking chamber.
Khaya senegalensis (mahogany*) and Celtis zenkeri (ita*) However, the pattern of temperature variation due to
were used in the form of saw dust for firing the oven. Data heat removal changed as the temperature reduced in the
collected were statistically analysed using SAS statistical mahogany, ita, iroko and arere, respectively (Fig. 5). The
package (SAS, 1987). Analysis of variance (Anova) was variation of the temperature with location follows a similar
carried out; the single and interactive effect of the factors pattern as that of heating. Statistical analysis (SAS, 1987)
(time, wood species and location) on the oven temperature shows that the mean temperatures for arere, iroko, maho-
were also determined using the statistical package. gany and ita were 104.19, 173.50, 209.62 and 166.37°C,
respectively.
RESULTS AND DISCUSSION Table 1 shows the results of the statistical analysis of the
Figure 4 shows the effect of type of wood and location effect of the factors considered (time, location and wood) on
(of temperature measurement) on the temperature of the the temperature of the oven. It could be observed that the
oven. It could be observed that the temperature of the oven separate and the interactive effects of the all the factors on
varies with the type of sawdust (wood) used. The tempe- the oven temperature were significant at 99%.

T a b l e 1. Statistical analysis of the effect of factors in oven temperature

Source Numbers Anova SS Means square F value Pr > F

Time 12 952729.6 79394.1 2564.0 0.0001
Location 3 76675.0 25558.3 825.4 0.0001
Wood 3 299115.2 99705.1 3220.0 0.0001
Time x Location 36 8633.7 239.8 7.8 0.0001
Time x Wood 36 73482.0 2041.2 65.9 0.0001
Location x Wood 9 7042.9 782.5 25.3 0.0001

*Wood local names in Nigeria.

 314 O.A. AJAYI and O.K. OWOLARAFE

A 350
B

350

300

300

250

250
Temperature

200

Temperature
Temperature (°C)

200

150

150

Arere Arere
100 Iroko
Iroko
100
Mahogany
Mahogany
Ita
Ita
50
50

0 0
0 5 10 15 20 25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35 40 45 50 55 60

300 300
C D

250 250

200 200
Temperature (°C)

Temperature
Temperature

150
150

100
100
Arere
Iroko Arere
Mahogany Iroko
Ita Mahogany
50 Ita
50

0
0
0 5 10 15 20 25 30 35 40 45 50 55 60
0 5 10 15 20 25 30 35 40 45 50 55 60
Time (min)
Time Time Time
(min)

Fig. 4. Effect of wood type on oven temperature changes (heat supplied) at location: A, B, C, and D.
 TEMPERATURE VARIATION IN A SAWDUST OVEN USING DIFFERENT WOOD SPECIES 315

A B
350

350

300

300 Arere
Arere
Ita
Ita
250 Iroko
Iroko
Mahogany
250
Mahogany
Temperature

200
(°C)

Temperature
200
Temperature

150
150

100
100

50 50

0
0
0 5 10 15 20 25 30 35 40 45 50 55 60
0 5 10 15 20 25 30 35 40 45 50 55 60
Time
Ti

300
300
C D
Arere
Arere Ita
250
250 Ita Iroko
Iroko Mahogany
Mahogany

200 200
Temperature
Temperature
Temperature (°C)

150 150

100 100

50 50

0
0
0 5 10 15 20 25 30 35 40 45 50 55 60
0 5 10 15 20 25 30 35 40 45 50 55 60
T ime
Time (min) TimeTime
(min)

Fig. 5. Effect of wood type on oven temperature changes (heat removed) at location: A, B, C, and D.
316 O.A. AJAYI and O.K. OWOLARAFE

CONCLUSIONS Bhattacharya S.C., Attalage R.A., Augustus L.M., Amur G.Q.,

Saqlam P.A., and Thanawat C., 1999. Potential of biomass
1. There is an indication that high temperature can be
fuel conservation in selected Asian countries. Energy
obtained in the oven from the use of sawdust as fuel.
Conserv. Manag., 40, 114-1162.
2. The temperature attained in the cavity had a cor- Bird R.B., Stewart W.E., and Lightfoot E.N., 2002. Transport
relation with the type of wood. Phenomena. J. Willey Press, New York.
3. The data not only provide an insight into the choice of Brown H.P., Pansakin A.J., and Forsath C.C., 1952. A Text-
sawdust in the operation of the oven but also on the time book of Wood Technology. McGraw-Hill Press, NewYork.
required to attain a particular temperature within the Dixit Bhaskar C.S., Paul P.J., Mukunda H.S., 2006. Part I:
cooking range. This will be useful in cooking as well as Experimental studies on a pulverised fuel stove. Biomass
drying of agricultural products. and Biomass and Energy, 7, 605-692.
4. Going by the high temperatures recorded, it is evident Ekundayo C.O., Probert S.D., and Newborough M.N., 1998.
that the oven will serve a useful purpose in drying and Improved heating-element configurations for a tunnel oven.
preservation of agricultural products. Applied Energy, 61, 111-124.
Food Science and Technology Centre, 2003. Ovens, 7, 7-31.
REFERENCES Klasnja B., Kopitovic S., and Lovi S., 2002. Wood and bark of
some poplar and willow clones as fuel wood. Biomass and
Abraham J.P. and Sparrow E.M., 2004. A simple model and vali-
Bioenergy, 23, 427-432.
dating experiments for predicting the heat transfer to a load situa-
ted in an lectrically heated oven. J. Food Eng., 62, 409-415. Panskin R.A.J., Hawar E.S, Bethel J.S., and Baker W.J., 1962.
Afuwape F.K., 1983. Design and testing of a sawdust compactor. Forest Products. McGraw-Hill Press, New York.
B.Sc. Thesis, Depart. Agric. Eng., Obafemi Awolowo Uni- SAS, 1987. Guide for Personal Computers. Version Ed. SAS/STA,
versity, Ile-Ife, Nigeria. SAS Inst. Inc.
American Society for Testing and Materials, 1999. Standard Test Tillman D.A., 1978. Wood as an Energy Source. Academic Press,
Method for Performance of Deck Oven. ASTM Designatio, New York.
F1965-99. In: Annual Book of ASTM Standard. West Zerbe J.I., 2004. Energy from Wood. Encyclopedia of Forest
Conshohocken, PA. Sciences, Elsevier Press, New York.
Bhattacharya S.C., Albina D.O., Salam A.P., 2002. Emission Zareifard M.R., Marcotte M., and Dostle M., 2006. A method
factors of wood and charcoal fired cookstoves. Biomass and for balacing heat fluxes validated for newly designed pilot
Bioenergy, 23, 453-69. plant oven. J. Food Eng., 76, 303-312.