December, 2014
Int J Agric & Biol Eng
Open Access at http://www.ijabe.org
Vol. 7 No. 6
67
Optimization of binder addition and compression load for pelletization of wheat straw using response surface methodology Lu Donghui1, Lope G. Tabil2, Wang Decheng1*, Wang Guanghui1, Wang Zhiqin1 (1. College of Engineering, China Agricultural University, Beijing 100083, China; 2. Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon S7N 5A9 SK, Canada) Abstract: Densification is required for efficiently handling and transporting biomass as feedstock for biofuel production. Binders can enhance straw pellet strength and improve the pellet performance.
The present investigation aimed to optimize
binders and compression load for wheat straw pelletization using a single pelleting unit. Response surface methodology was employed by using a four-factor, five-level central composite design with wood residue (%, w/w), bentonite (%, w/w), crude glycerol (%, w/w), and compression load (N) as process parameters. of pelleting, and pellet density were the response variables.
The pellet tensile strength, specific energy consumption
The higher heating value, ash content of the pellet product and the
cost of the feedstock were also considered in optimizing binder addition. adequate for binder analysis and optimization.
The developed model fitted the data and was
Wheat straw pellet, with the addition of 30% wood residue, 0.80% bentonite,
and 3.42% crude glycerol, in addition to 4 000 N of compressive load, was identified as optimal with good performance of pellet tensile strength (1.14 MPa), specific energy consumption (32.6 kJ/kg), and pellet density (1 094 kg/m3) as well as low ash content (6.13%) and high heating value (18.64 MJ/kg).
Confirmation tests indicated high accuracy of the model.
Keywords: biomass, wheat straw pellet, binder, wood residue, bentonite, crude glycerol, RSM, compression load DOI: 10.3965/j.ijabe.20140706.009 Citation: Lu D H, Tabil L G, Wang D C, Wang G H, Wang Z Q. pelletization of wheat straw using response surface methodology.
1
Introduction
Optimization of binder addition and compression load for Int J Agric & Biol Eng, 2014; 7(6): 67－78.
handle or transport.
Logistics cost is often high due to
the great distance between the origin of biomass and the Agricultural residues, the largest available biomass [1]
location for energy production[2].
Densification of
resource in the world for bioenergy production , are
biomass into pellets can significantly reduce the cost of
environmental-friendly feedstock with huge potential for
handling, transportation, and storage[3].
sustainable energy production.
However, agricultural
calorific value and physical properties can be improved,
straw has low bulk density which does not make it easy to
and the size and shape of biomass can be made uniform
Received date: 2014-02-24 Accepted date: 2014-11-23 Biographies: Lu Donghui, PhD, research interests: postharvest processing, processing and densification of agricultural material to biofuel, life cycle assessment of biomass fuel system. Tel: +86-10-62737147, Fax: +86-10-62737208, Email: lvdonghui.cau@ gmail.com. Lope G. Tabil, PhD, PEng, Professor, research interests: pelleting of feeds and forage and optimization of the process involved in feed and forage processing; physical properties
[email protected]. Wang Guanghui, PhD, Associate Professor, research interests: drying and densification technology of biomass; manure processing; grassland improvement mechanization. Tel: +86-10-62737845, Email:
[email protected]. Wang Zhiqin, PhD, Associate Professor, research interest: techno-economic analysis of agricultural bioenergy system. Tel: +86-10-62737845, Email:
[email protected]. *Corresponding author: Wang Decheng, PhD, Professor,
of agricultural materials and postharvest technology of agricultural crops; bioprocess engineering; value-added engineering and postharvest handling of crops; storage, drying and cooling of biological materials; infrared spectroscopy; biomass processing and utilization. Tel: +1-306-9665317, Fax: +1-306-9665334, Email:
research interests: postharvest processing, practaculture machine, processing and densification of agricultural material. Mailing address: P.O. Box 134, No.17 Qinghuadonglu, Beijing, 100083, China. Tel: +86-10-62737208, Fax: +86-10-62737208, Email:
[email protected].
The volumetric
68
December, 2014
Int J Agric & Biol Eng
through densification[4].
Open Access at http://www.ijabe.org
The lack of natural binder in
the agricultural straw is one of the important reasons for
act as a lubricant during pelleting and its addition could potentially enhance the heating value of biomass pellets.
poorly formed pellets which are often dusty, difficult to [4]
handle, and costly to manufacture .
Vol. 7 No. 6
Response surface methodology (RSM) is a statistical
Some pretreatment
tool based on the multivariate non-linear model, which is
methods have been explored to improve the durability
useful in analyzing various parameters affecting the
[5-8]
By controlling the
process[16]. Process optimization is based on the
processing method and related parameters, high quality
simultaneous combination of all goals for the responses
pellets could be produced. Binder addition is a method
and factors. RSM has been widely used in development
employed in this study.
of new processes and the optimization of their
and strength of biomass pellets
.
Optimization of binder addition
for the pelletization process is one of the most important
performance[17].
steps in the development of an efficient and cost-effective energy conservation strategy using biomass.
In the present study, the optimization of the addition of binders and applied compression load during wheat
Lignin is one kind of natural binder for biomass [9]
straw pellet production was conducted by using RSM.
densification . Wood residue with high lignin content
The objective of this study is to determine the most
can act as a potential binder for wheat straw densification.
promising binder combination and compression load for
The heating value of wheat straw pellets can be
wheat straw pelletization by maximizing pellet strength,
potentially improved with the addition of wood residue
minimizing energy consumption of pelleting, and
because
maximizing pellet density.
of
its
higher
heating
value.
It
is
environmentally friendly to recycle wood residue, which also can solve a disposal problem by using them to make biofuel[10]. Ståhl and Berghel[10] investigated the effect of mixing sawdust and rapeseed cake to make pellets.
Heating value, ash content
and binder cost is also considered.
2 2.1
Materials and methods Materials and pelletization experimental setup
They reported that the durability of the pellets increased
The details of the processes and experimental single
proportionally with an increase in the amount of sawdust.
pellet unit in these experiments are presented in an earlier
[9]
Kaliyan and Morey
suggested that biological binders
paper[18].
A brief description of the process and
such as sawdust (more than 20% by weight) can help
equipment is as below. Wheat straw (Triticum aestivum
improve the quality of the densified product.
L.) was acquired in the form of small square bales with
Colloidal hydrated magnesium aluminum silicate clay
dimensions of 0.45 m×0.35 m×1.00 m from the Central
or bentonite had been used for making durable feed
Butte area (50.83°N, 106.51°W) in Saskatchewan,
[11]
pellets
.
Bentonite added at a rate of 2.6% can reduce
dust generation of feed pellets
[12]
.
Bentonite has been
reportedly used at an inclusion rate of 5.0% to produce [11]
alfalfa pellets which can improve pellet durability
.
Canada, which was harvested in the fall of 2011. Bentonite was obtained from Canadian Clay Products, Inc. (Wilcox, SK, Canada).
Crude glycerol was acquired
from Milligan Bio-Tech, Inc. (Foam Lake, SK, Canada).
Crude glycerol, a byproduct of the transesterification
Wood residue was purchased in the form of soft wood
process in the biodiesel industry, is an additive
shavings with 60% spruce and 40% pine from Western
considered in this study.
Wood Shavings, Inc. (Saskatoon, SK, Canada).
One kilogram of crude
glycerol is co-produced for every 10 kg biodiesel produced
[13]
.
Wheat straw and wood residue samples were first
There is a positive expectation for
ground using a hammer mill (Serial no. 6M13688; Glen
glycerol’s production with the spread of biodiesel
Mills Inc., Maywood, NJ, USA) powered by a 1.5 kW
[14]
production plants
so that crude glycerol supply may
electric motor which was connected to a dust collector
Crude glycerol is an energy
(Model No. DC-202B, House of Tools, Saskatoon, SK,
with a higher heating value compared to
Canada) to control dust during operation, provide
agricultural biomass or wood residue. Glycerol could
flowability of the ground biomass through the hammer
exceed its demand. source
[15]
December, 2014
Lu D H, et al. Optimization of binder addition and compression load for wheat straw pelletization
Vol. 7 No. 6
69
mill, and collect the ground biomass.
A screen size of
USA) universal testing machine compressed the sample
3.2 mm was used in the hammer mill.
The particle size
at a speed of 50 mm/min.
analysis of wheat straw grinds and wood residue grinds was conducted using ASABE Standard S319.3
[19]
.
A
The cylindrical die was slip
fitted into a stainless steel base with a hole matching the outer diameter of die.
This steel base allowed the
Ro-Tap sieve shaker (W. S. Tyler Inc., Mentor, OH, USA)
plunger to move straight down with no lateral movement
and U.S. sieve numbers 16, 20, 30, 50, 70, and 100 (sieve
during pelletization.
opening sizes: 1.190, 0.841, 0.595, 0.297, 0.210, and
loaded into the cylindrical die for each run. Compressive
0.149 mm, respectively) were used for the particle size
force applied can be adjusted by changing the program.
analysis in 3 replicates. The mean geometric particle
After compression and achieving the pre-set load, the
size of wood residue grinds and wheat straw grinds was
plunger was stopped and held in position for 60 s. Then
(1.022±0.017) mm and (0.858±0.001) mm, respectively.
the pellet was ejected from the die at a speed of
As the geometric particle size of wood waste grinds was
50 mm/min using the plunger after the base plate was
larger than wheat straw grinds, the geometric particle size
removed.
of wood residue grinds was adjusted by changing the
recorded using the Bluehill software (Version 2.12,
mass percentage on sieve no.16 by using the formula
Illinois Tool Works, Inc., 2010). The single pelletization
suggested in the ASABE Standard S319.3
[19]
to obtain the
Approximately 0.5 g sample was
The force-displacement curve and data was
process was replicated eight times for each sample.
same geometric mean diameter with wheat straw grinds. The adjusted mass percentage of particles retained on each sieve size was presented in an earlier paper[18].
In
brief, the average percentage of wood particles on sieve number 16 was changed from 61.77% to 41.27%, and the average percentages of wood particles on other sieves were increased slightly and accordingly.
Wood residue
samples were recreated by mixing grinds from each sieve with the adjusted proportion. The moisture content of the ground material and the pretreated wood residue samples were measured using oven drying at 103±2ºC for 24 h
[20]
1. Plunger 2. Cylindrical die 3. Heating element 4. Base plate 5. Pellet sample 6. Base
in three replicates.
Figure 1
The average initial moisture contents of the wheat straw grinds and the softwood grinds were 7.30% (w.b.) and
2.2
Schematic diagram of the single pellet unit
Experimental design
The moisture content of
Based on the results of an earlier study[18], bentonite,
wheat straw grinds and wood residue grinds were both
wood residue, and crude glycerol performed well as
adjusted to a range from 9% to10% by spraying water.
binders or additives.
Then the samples were kept in air-tight plastic bags for
pellet strength and reducing energy consumption.
one week to allow for moisture equilibration.
residue can increase the pellet strength, density, and
6.66% (w.b.), respectively.
Wheat straw samples were pelleted in a single pelleting unit (Figure 1) used in previous research
[21-26]
Bentonite is good at improving
heating value of wheat straw pellets.
Wood
Crude glycerol can
.
significantly increase the heating value of the straw pellet.
In brief, the device is composed of a steel cylindrical die
So, in this work these three binders were combined as a
with an internal diameter of 6.35 mm and a length of
mixed binder to improve pellet quality.
125 mm.
A heating element maintaining the die wall
compression load for pelleting was also considered to be
temperature at 95±1°C was wrapped on the outside of the
a factor and then the interaction between adding binder
die. A plunger connected to the upper moving crosshead
and energy consumption would be observed more clearly.
of an Instron Model 3366 (Instron Corp., Norwood, MA,
Pellet tensile strength, specific energy consumption of
In addition, the
70
December, 2014
Int J Agric & Biol Eng
Open Access at http://www.ijabe.org
Vol. 7 No. 6
pelleting, and pellet density were chosen to be responses
Pellets were firstly cut diametrically into approximately
because strength, energy, and density are three of the
2 mm thick specimens and placed longitudinally at the
most important characteristics for evaluating pellet
centre of the base plate for testing.
quality.
Response surface methodology (RSM), using a
plunger of the Instron were padded with blotting paper.
central composite factorial design (CCD) with four
Each of the specimens was pressed by the plunger of the
factors, six central points, and eight axial points, was used
Instron at a speed of 1 mm/min.
to design the experimental trials.
Design-Expert
displacement data were recorded and the fracture force
software, version 7.0 (Stat-Ease Inc., Minneapolis, MN,
was determined by choosing the point that the specimen
USA) was used to conduct the experimental design and
was split apart into two semi-circular segments.
the optimization.
strength of the pellet was determined and calculated using
performed.
A total of 30 experiments were
The order of the experiments was
randomized. Factors and their respective levels are
Equation (1)[21, 25].
-1
0
1
Wood residue/%
A
10
20
30
Bentonite/%
B
1
2
3
Glycerol%
C
2
4
6
D
2 000
3 000
4 000
thickness, m. 2.5
Specific energy consumption Energy consumption was calculated by integrating the
area under the force-displacement curve recorded by the computer.
Mixing binders with wheat straw Binders were directly mixed into wheat straw grinds
according to the experimental design except crude glycerol.
(1)
fracture, N; d is specimen diameter, m; l is specimen
Symbol
Compression load/N
2.3
2F
dl
where: σx is tensile (horizontal) stress, Pa; F is load at
Code and level Independent variable
Tensile
This test was replicated eight times
x
Experimental codes and actual levels of the independent variables
The force and
for each sample.
summarized in Table 1. Table 1
The base plate and
Then, it was converted to specific energy
values in MJ/t by dividing the pellet mass. 2.6
Crude glycerol was heated to above 160°C
Pellet density Immediately after the ejection of the pellet, its
using a hot plate and stirred with a mini size magnetic
dimensions were measured using a digital caliper.
stirrer at 220 r/min to reduce its viscosity before spraying
pellet mass was also measured.
it into wheat straw grinds. In order to achieve uniform
calculated using mass divided by pellet volume.
distribution of each material, the mixing was carried out
2.7
The
Pellet density was
Model development and diagnostics
for 15 min; after mixing, the samples were stored in
The Design-Expert software, version 7.0 (Stat-Ease
air-tight bags. Subsequent mixing was carried out every
Inc., Minneapolis, MN, USA) was used to create the
12 h for at least 72 h.
model and analyze data. The relationship between factors
2.4
(wood residue addition, bentonite addition, crude glycerol
Tensile strength of pellets The strength and durability of the densified products
addition, and compression load) and the responses (pellet
would help optimize the material, pretreatment, and
tensile strength, specific energy consumption, and pellet
[5]
equipment to produce high quality densified products .
density) was evaluated by the best fit model suggested by
Pellet tensile strength was determined in lieu of pellet
the Design Expert software.
durability because of the limited number of pellets
derived by excluding insignificant terms.
produced in single pelleting experiments.
Diagnostic Plots were conducted by the Design Expert
The tensile
strength of the single pellet was evaluated using the [21, 25]
diametral compression test
The Model
software as well.
A Dremel rotary tool
The main indicators demonstrating the significance
(Robert Bosch Tool Corp., Racine, WI, USA) with a
and adequacy of the model are: the modified models were
[25]
was used to cut the pellet
significant; the lack of fit was insignificant; the adjusted
due to the high strength of the binder-straw pellet.
and predicted R2 values were within 0.2 of each other; the
diamond cutting wheel bit
.
The modified models were
December, 2014
Lu D H, et al. Optimization of binder addition and compression load for wheat straw pelletization
Vol. 7 No. 6
71
adequate precision was over 4[27]; and all the residuals
was measured in three replicates using a Parr 1281
were random.
automatic isoperibol oxygen bomb calorimeter (Parr
good
Then the model is considered to give
predictions
for
average
outcomes.
The
Instrument Co., Moline, IL, USA) based upon ASTM
significance of model terms was estimated based on
D5865-03[28].
P-values at 95% confidence level.
using AOAC standard method 942.05[29].
2.8
on binder costs was gathered and also considered to be a
Optimization method The goals for optimization were set for both responses
and variables.
Other factors were also taken into
consideration during optimization.
These objectives
were input in the Design-Expert software, version 7.0 (Stat-Ease
Inc.,
Minneapolis,
MN,
USA).
The
Design-Expert software was also used to draw the three-dimensional plot of response surfaces.
The total ash content was determined
factor in optimization.
Information
The cost data of the wheat straw
and binders were acquired by consulting companies selling these products.
3
Results and discussion The results of the experimental runs are shown in
Table 2. The mean pellet tensile strength varied from
Other factors (heating value, ash content, and cost)
0.69 to 1.24 MPa. The specific energy consumption for
were also considered to be minor factors that affect the
pelleting ranged from 18.6 to 36.6 MJ/t. Pellet density
final optimization.
varied from 883.0 to 1 105.6 kg/m3.
Table 2
The heating value of the samples
Mean values of pellet tensile strength, specific energy consumption and pellet density during the experimental runs
Run
Wood residue /%
Bentonite /%
Crude glycerol /%
Compression load /N
Tensile strength /MPa
Specific energy consumption /MJ·t-1
Pellet density /kg·m-3
1
30
1
2
2 000
0.89
26.0
1 010
2
20
2
4
3 000
0.97
31.0
1 052
3
30
1
6
4 000
0.88
31.8
1 071
4
20
0
4
3 000
0.91
33.8
1 041
5
30
1
2
4 000
1.24
33.5
1 071
6
20
2
4
3 000
0.92
30.1
1 050
7
10
1
6
4 000
0.80
33.5
1 055
8
20
2
4
3 000
0.97
29.5
1 062
9
20
2
8
3 000
0.81
27.5
1 029
10
40
2
4
3 000
1.04
30.1
1 075
11
30
3
2
2 000
1.01
24.8
1 017
12
0
2
4
3 000
0.93
30.7
1 071
13
20
2
4
1 000
0.77
18.6
883
14
10
3
6
4 000
0.74
31.8
1 055
15
20
2
4
3 000
0.91
29.8
1 057
16
10
1
2
2 000
0.86
26.3
991
17
10
3
2
4 000
1.09
33.9
1 106
18
30
3
2
4 000
1.11
31.7
1 104
19
20
2
0
3 000
0.98
33.6
1 029
20
10
1
6
2 000
0.80
26.4
1 009
21
10
3
6
2 000
0.95
27.0
1 004
22
20
2
4
3 000
0.86
29.6
1 065
23
20
2
4
5 000
1.15
36.1
1 077
24
10
3
2
2 000
1.09
25.2
1 012
25
20
4
4
3 000
1.03
28.9
1 072
26
30
3
6
4 000
0.77
30.3
1 071
27
10
1
2
4 000
1.09
36.6
1 069
28
30
3
6
2 000
0.87
24.1
9 91
29
20
2
4
3 000
0.97
29.9
1 059
30
30
1
6
2 000
0.69
27.0
980
72
December, 2014
3.1
Int J Agric & Biol Eng
Open Access at http://www.ijabe.org
for each model are all within 0.2, which represents good
Modeling and analysis of variance (ANOVA) Table 3 shows the ANOVA of the response surface 2
2
model, coefficient of determination (R ), adjusted R , and predicted R
2
of all the responses models. 2
The 2
differences between adjusted R and predicted R values Table 3
Vol. 7 No. 6
agreement of the models for the three responses. The adequate precision of the models is greater than 4 as shown in this table, indicating that the models were adequate.
Analysis of variance (ANOVA) for the response variables during pelletization of wheat straw
Response variable
Selected model*
R2
Adjusted R2
Predicted R2
Adequate precision
Tensile strength
2FI
0.83
0.78
0.70
16.07
Specific energy consumption
Quadratic
0.95
0.93
0.88
32.01
Pellet density
Quadratic
0.96
0.94
0.86
31.74
Note: * 2FI = 2 factor interaction model; Quadratic = Quadratic model.
Tables 4, 5, and 6 show the analysis of variance
Table 6
affected by binder used and compression load
(ANOVA) of the response variables (pellet tensile strength, specific energy consumption, and pellet density, respectively) as affected by the binders and interactions after stepwise regression. Table 4
Analysis of variance (ANOVA) of pellet tensile
strength as affected by binder used and compression load Sourceⅰ
Sum of squares
Degree of freedom
Mean square
F value
P value
Model
0.41
6
0.07
17.76
