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Determining The specific energy Consumption
for Grinding Rice straw with a Hammer mill
Rezvani Z1, Chegini GH1, Arabhosseini A 1, Kianmehr, M. H.1,Asadi M. R5 ,*
1
Department of Agrotechnology, College of Abouraihan, University of Tehran, Tehran, Iran
Department of Mechanical Engineering, Buinzahra branch, Islamic Azad University, Buinzahra, Iran
5

ABSTRACT
The objective of this study was determining the relationships
between moisture content (8 and 12 % w.b) and hammer mill
in four screen sizes (2, 4, 7 and 8 mm) with energy required for
grinding rice straw. An amount of rice straw was grinded in
each test and the particle size of grinded rice straw was
determined. A watt metering system was used to measure the
energy consumption of grinding of grinding for each test. The
highest specific energy consumption was 32.46kWh.t-1 for
hammer mill screen size of 2mm at 12 % (w.b) moisture
content. The hammer mill screen size was negatively connected
with specific energy consumption. Therefore, the larger the
hammer mill screen size, use less the specific energy
consumption. The particle size was normally distributed for
the rice straw grinded by at 2 and 4 mm hammer mill screen
sizes. The grinds from screen size of 8 mm had a large size
distribution with a geometric mean particle diameter of 1.04
mm.
KEYWORD
Energy consumption, rice straw, hammer mill, grinding.
INTRODUCTION
B
iomass has become an important renewable energy
resource for producing electric power and other forms
of energy with low greenhouse gas and low acid gas
emissions. Direct-combustion and co-firing technologies
have become commercially available. Co-firing biomass
with coal in existing coal-fired power plants has emerged to
be an attractive way for increased use of biomass as an
energy source (Costello, 1998).
Agricultural biomass residues such as barley, canola, oat
and wheat straw are potential feedstocks for the sustainable
production of bio-fuels and to offset greenhouse gas
emissions [1, 2]. Milling is an important pretreatment of
biomass for energy conversion. It is also crucial to the
densification process. For example, in the production of fuel
pellets and briquettes, the feedstock has to be ground before
being transformed into a denser product. Particle size
reduction increases the total surface area, pore size of the
material and the number of contact points for inter-particle
bonding in the compaction process [3]. The grinding process
is an important unit operation prior to densification.
Tabil (1996) obtained consumption of specific energy for
alfalfa pellet mill at two hammer mill screen sizes of 2.4 and
3.2 mm using a watt-hour meter with a data logger
attachment and sampling time of 15 s [4]. Specific energy
consumption of grinding material depends on moisture
content, bulk and particle densities, feed rate of the material,
particle size distribution (initial/final particle size) and
machine variables [5].
Energy consumption for grinding wheat straw, corn stover,
and rice straw increases when screen opening size are
changed from coarser to finer [6].
Several models such as Kick, Rittinger [7] and Bond [8]
have been used to predict the required specific energy
consumption for grinding agricultural materials. All three
models proposed for dry grinding have given reasonably
good results based on the type of grinding: the Kick model
for coarse grinding; the Bond model for intermediate
grinding and the Rittinger model for fine grinding [9].
MATERIALS AND METHODS
Rice straws in square bales were obtained from
agricultural land at Daniel village of Mazandaran, Iran.The
bales were of standard dimensions of 1.00 × 0.45 × 0.35 m
with the moisture content of 8 % (w.b)
*Corresponding Author: Asadi M. R.
E-mail: [email protected]
Telephone Number: 00989124086314
A. Chopping
Fax. Number :
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The rice straw was chopped manually with cutter to the
size of 40-50 mm (Fig.1). The chop size was determined
using a chopped forage size analyzer specified in ASAE
Standard S424.1 MAR 98 [10].
Fig.1. Chopped rice straw
B. Conditioning of rice straw
Conditioning of the rice straw to the required moisture
content was done by spraying water uniformly into the
chopped material .The wetted material was placed in a
plastic bag and stored in a room at 22 ͦC for 72 h for
moisture equilibration prior to grinding.
C. Moisture content
The moisture content of rice straw was determined
according to ASAE standard S358.2 FEB03 for forage [11] .
A sample of 25 g was oven dried for 24 h at 105± 3 ͦC. The
moisture content of the grind was determined by the
procedure given in ASTM Standard D 3173-87 for coal and
coke [12]. One gram of pulverized sample passing through a
sieve with opening of 0.25mm (sieve no. 60) was taken and
oven dried for 1 h at 104–110◦C .The moisture content of
the grind was determined by weighing and expressed in
percent wet basis [13].
D. Grinding test apparatus /Hammer mill
A hammer mill was used for grinding of rice straw (Fig.2).
The hammer mill used in this study consisted of 25
swinging hammers, attached to a shaft powered by a 2.2 kW
(3 hp) electric motor. The shaft rotated at a speed of 3000 r
min-1 .A tapering hopper (with 500 mm small diameter 500
mm large diameter and 300mm height) was used for feeding
system.
Fig.2. Hammer mill
E. Particle size distribution
The particle size of rice straw was determined according
to ASAE standard S319.3 FEB03 [14]. A sample grind of
100 g was placed in a stack of sieves arranged from the
largest to the smallest opening for 10 min. The sieves
number of 10, 16, 30 and 50 (nominal openings of 2, 1.18,
0.6 and 0.3 mm, respectively) were used for grinding from
2, 4, 7 and 8 hammer mill screen opening. After sieving, the
mass retained on each sieve was weighed. Sieve analysis
was repeated three times for each grind sample
The hammer mill screen size of 4mm and 2mm were
selected. The hammer mill was started and a known quantity
of rice straw was fed into the hammer mill. The hammer
mill was started .A known quantity of rice straw was
manually fed into the hammer mill and the time required to
grind the straw was recorded along with the power drawn by
the hammer mill motor which was recorded every 6 s .The
power required to run the empty hammer mill was measured
before the material was introduced .This allowed
determining the net power required to grind the material.
The specific energy required for grinding was determined by
integrating the area under the power demand curve for the
total time required to grind a sample [15]. Each test was
repeated three times.
The geometric mean diameter (dgw) of the sample and
geometric standard deviation of particle diameter (Sgw) were
calculated according to the ASAE Standard S319.3 [14].
(1)
(2)
(3)
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Where
dgw : geometric mean diameter or median size of particles by
mass (mm);
Slog : geometric standard deviation of log normal distribution
is the specific energy for grinding the rice straw samples. In
other words, fine grinding requires high specific energy.
Similar results have been reported for four biomasses
namely corn stover, switch grass, wheat and barley straw
[13].
by mass in 10 based logarithm (dimensionless);
Sgw : geometric standard deviation of particle diameter by
mass, mm
Wi : mass on ith sieve, g
n: number of sieves plus one pan
i :
(di×di+1)1/2
di: nominal sieve aperture size of the ith sieve , mm
di+1 : nominal sieve aperture size of the ith+1 sieve, mm.
RESULTS AND DISCUSSION
Fig.3. Specific energy requirement for grinding of rice straw at 8 % (w.b)
moisture content
A. Energy requirement for grinding
Table 1 lists the average specific energy consumption for
grinding rice straw using the hammer mill with four
different screen sizes at two moisture content levels.
Table.1. Specific energy requirement for grinding of rice straw using
hammer mill
Moisture
content (%
w.b)
Screen opening size of
hammer mill (mm)
8
8
8
8
12
12
12
12
2
4
7
8
2
4
7
8
Average specific
energy
consumption (kW h
t-1)
19.73
11.58
8.24
8.12
32.46
26.52
15.84
14.69
The specific energy consumption for grinding rice straw
with 8 % (w.b) moisture content by the hammer mill screen
sizes of 2, 4, 7 and 8 mm were 19.73, 11.58, 8.24 and 8.12
kW h t-1, respectively. As the size of screen on the hammer
mill increased from 2 to 8mm, the specific energy
consumption for grinding of 8% (w.b) decreased from 19.73
to 8.12KW ht-1. The specific energy consumption for
grinding rice straw with 8% (w.b) decreased by 59% when
the screen size was increased from 2 to 8 mm. Among the
four hammer mill screen sizes, grinding from screen size 8
and 2 required the highest and lowest the specific energy
consumption for grinding, respectively.
B. Effect of hammer mill screen sizes on energy
consumption for grinding
Figure 3 represents the specific energy requirement for
grinding rice straw as a function of screen size at moisture
content of 8% (wb). The smaller the screen size, the higher
Figure 4 shows the relationship between the specific
energy requirement for grinding rice straw and hammer mill
screen sizes at 12% (wb) moisture content. Among the four
hammer mill screen sizes, grinding from screen size 8 and 2
required the highest and lowest the specific energy
consumption for grinding, respectively at 12% moisture
content. The hammer mill screen size of 8mm at 12%
moisture content used the least specific energy of 32.46 kW
h t−1.
Fig.4.Specific energy requirement for grinding of rice straw at 12% (w.b)
moisture content
C. The effect of moisture content on grinding
Figure 5 represents the effect of moisture content on the
specific energy requirement for grinding rice straw.
At 12% (w.b) moisture content, hammer mill screen size
of 2 mm, consumed the highest amount of energy with a rate
of 32.46 kW h t−1. The fig. 5 shows that the hammer mill
screen size was negatively connected with specific energy
consumption. Therefore, the larger the hammer mill screen
size, use less the specific energy consumption. Similar
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results have been reported for grinding of alfalfa stem [16]
and for four biomasses namely corn stover, switch grass and
wheat and barley straw grind [13]. Sitkei reported a secondorder polynomial relationship between the specific energy
requirement and the mean particle size for alfalfa stems with
a R2 value of 0.99. Similarly, Holtzapple et al. (1989)
reported the relationship for grinding energy with the length
of wood cubes. They also concluded that grinding energy
increased greatly as the particle size is reduced [17].
Moisture content is also an important factor for energy
consumption during grinding. Moisture content had a
positive connected with specific energy consumption. The
higher the moisture content, use more the specific energy
consumption. It can be stated that an increase in moisture
content of rice straw would increase the shear strength of the
material, although shear strength decreases with
decomposition of rice straw [18].
Balk (1964) reported that for alfalfa grinding, moisture
content had a positive correlation with specific energy
consumption [15].
particles rearrange and fill in the void space of larger
(coarse) particles producing denser and durable compacts
[4]. Coarse particles are also suitable feed for boilers and
gasifiers.
Fig.6. Particle size distribution of rice straw grinds (average of three tests)
for different sizes
CONCLUSION
The highest specific energy consumption was 32.46KW.h
t-1 for hammer mill screen size of 2mm at 12 % (w.b)
moisture content. The hammer mill screen size was
negatively connected with specific energy consumption.
Therefore, the larger the hammer mill screen size, use less
the specific energy consumption. The particle size was
normally distributed for the rice straw grinded by at 2 and 4
mm hammer mill screen sizes. The grinds from screen size
of 8 mm had a large size distribution with a geometric mean
particle diameter of 1.04 mm.
Fig.5.Specific energy requirement for grinding of rice straw at 8% (w.b)
and 12% (w.b) moisture content
REFERENCES
D. Particle size distribution
Figure 6 shows a typical particle size distribution of rice
straw grind from four different hammer mill screen sizes.
Figure 6 shows that particle size distribution of rice straw
grind at 2 and 4 mm hammer mill screen sizes were
normally distributed. Similar observations were reported for
wheat and barley straw [14] and for barley, canola, oat and
wheat straw [19].
The grinds from screen size of 4 mm had a large size
distribution with a geometric mean particle diameter of 1.04
mm.
Grinds from the hammer mill screen size of 2, 4, 7 and 8
mm were distributed in wide range (Fig. 4) and produced
particles with geometric mean particle diameter of 0.66,
0.76, 1.03 and 1.04 mm, respectively. Wider particle size
distribution is suitable for compaction (pelleting or
briquetting) process. During compaction, smaller (fine)
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