Okot DK, Bilsborrow PE, Phan AN. Effects of operating parameters on maize
COB briquette quality. Biomass and Bioenergy 2018, 112, 61-72.
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© 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license
DOI link to article:
https://doi.org/10.1016/j.biombioe.2018.02.015
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Embargo release date:
02 March 2019
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Newcastle University ePrints - eprint.ncl.ac.uk
1
EFFECTS OF OPERATING PARAMETERS ON MAIZE COB BRIQUETTE
2
QUALITY
3
4
5
David K. Okot 1, Paul E. Bilsborrow 2, Anh N. Phan 1*
6
1
School of Engineering, Newcastle University, UK
7
2
School of Agriculture Food and Rural Development, Newcastle University, UK
8
9
Corresponding author*:
[email protected]
10
11
Abstract
12
Briquetting is considered as one of the pre-treatment methods available for producing uniform
13
sized and moisture content feedstock which is easy to handle, transport and store. The quality
14
of briquettes in terms of density and durability depends on the physical and chemical properties
15
of the feedstock and briquetting conditions. In this study, the effect of compacting pressure,
16
temperature, moisture content, and particle size on the properties of briquettes for
17
thermochemical applications were investigated. It was found that density, impact resistance,
18
and compressive strength significantly increased with increasing compacting temperature (20-
19
80 oC) and compacting pressure (150-250 MPa). However, increasing moisture content and
20
particle size had a negative impact on briquette quality. The results showed that there was a
21
strong interaction between briquetting parameters with the interaction between moisture and
22
temperature significantly affecting both briquette density and mechanical strength. Briquettes
23
with high density and durability/mechanical strength required to meet quality certification
24
standards could be obtained with course ground material ( < 4mm) from relatively low moisture
25
content feedstock (7-8%) with pressure of 200-250MPa and a compacting temperature of 80oC.
26
27
Keywords: Briquette quality, density, impact resistance, compressive strength, maize cob,
28
agricultural residues
29
1
1
1
Introduction
2
Biomass for energy generation has attracted much attention because it is an abundant resource
3
[1] and CO2 neutral [2, 3]. According to the World Energy Council [4], biomass contributes
4
14% out of the 18% of global energy supply from renewables and contributes 10% of total
5
global energy consumption. It is the predominant source of energy in developing countries e.g.
6
over 80% in sub-Saharan Africa, which is mainly used for cooking [5]. Biomass is
7
heterogeneous in terms of size, shape and composition and has low bulk density (e.g. about 4
8
times lower than the bulk density of diesel) [6], leading to difficulties in handling, storage and
9
transport. Densification of biomass into briquettes/pellets increases bulk density from 40-200
10
kgm-3 to 450-800 kgm-3 [5, 7] and produces a high energy feedstock with uniform moisture,
11
shape and size which makes it suitable for storage and transportation with potential uses in
12
combustion, pyrolysis and gasification [8]. Densification minimises particulate emissions per
13
unit solid fuel transported and improves biomass combustion efficiency as well as conveyance
14
efficiencies (less dust and wastage and lower labour cost) in commercial energy generation
15
facilities [9, 10]. The classification of briquettes and pellets is commonly based on their sizes
16
e.g. 4.0-10.0 mm diameter and 20-50mm length according to the respective Austrian (ONORM
17
M 7135) and German (DIN 51731) quality standards for pellets [11, 12] with 10 - 200 mm
18
diameter and 16 - 400 mm length commonly used for briquettes [13-16].
19
Due to the increase in the share of renewable energy required to achieve national government
20
targets, the demand for densified products increased from 7 to 19 million tonnes for the period
21
2006–2012 [17]. However, shortage of feedstock and sustainability of supply for wood pellet
22
production provides a major challenge especially in the rapidly growing EU pellet market with
23
an urgent need to broaden the feedstock range by using agricultural residues and other sources
24
of biomass e.g. municipal solid waste. Briquetting can be preferred over pelleting for
25
agricultural residues because it can accommodate feedstock with large particle sizes and high
26
moisture content [18], which in turn reduces the energy input in pre-processing of feedstock
27
(grinding and drying). It was reported [19] that the energy required for grinding corn stover
28
decreased 3 fold when increasing particle size from 0.8 mm to 3.2 mm at a moisture content of
29
6-12%.
30
In transport, handling and storage briquettes with high density and mechanical strength are
31
desirable [20]. High density is desired to reduce transport and storage costs [21-23], with high
32
compressive strength, i.e. ≥ 2.56 MPa [24] preferred to prevent breakages [25]. Durability of
2
1
over 80 % [26] is reported to ensure briquettes/pellets remain intact during transport/storage
2
and reduce the amount of fine particles/dust produced [7]. Ensuring moisture content of
3
feedstocks between 5-22 % has been reported to facilitate stable compaction of several
4
feedstocks such as wood, alfalfa, lignite, wheat straw and waste paper [20, 22, 27, 28].
5
Particle size for producing briquettes can be varied from 0.1 to 6 mm depending on type of
6
feedstock [29-34]. However, Ahmed et al [35] also reported that particle size of 6-8 mm
7
together with 13-15% in powder form was recommended to enhance briquette durability by
8
increasing interlockages and minimising spaces between particles [7]. Although pellets have
9
been studied intensively with certified quality standards available (e.g. Austrian ONORM M
10
7135, Swedish SS 187120, German DIN 51731 and DIN EN 15270 and European Standard
11
Committee CEN/TE 335), very little work has been done on briquetting of agricultural residues
12
and the only standards available for briquettes are for wood. Pellet standards therefore have
13
often been used to determine agricultural residue briquette quality. Previous studies
14
[21,24,30,36] showed that briquette properties were strongly dependent upon moisture content,
15
particle size, temperature, compacting pressure and type of feedstock. However, the findings
16
are case-specific and the results are variable. Increasing compacting pressure for mango and
17
eucalyptus leaf [21] from 30 to 100 MPa increased the density from 600 to 1100kgm-3.
18
Similarly increasing pressure from 3 to 11MPa increased the density of palm oil mill residues
19
from 950 to 1010 kgm-3 [24]. Density of tropical hard wood briquettes decreased when particle
20
size was increased from < 1mm to 2-3.35mm, however, there was a weak positive correlation
21
between compressive strength and particle size [36]. The effect of moisture content varies
22
depending on feedstock such that impact resistance (as measured by shatter index) of paper
23
mill briquette increased from 36227 to 168875 when moisture content was increased from 5 %
24
to 15 % and maximum compressive strength of 1299 kgcm-2 was reported at a moisture content
25
of 9 % [30].
26
To date, interactions between different briquetting parameters (compacting pressure, moisture
27
content, particle size and compacting temperature) on properties of briquettes have not been
28
studied. Therefore, fully understanding how chemical composition and physical properties
29
impact upon briquette product quality is essential. The literature shows that low pressures (5-
30
31 MPa) [9, 34], used in the compaction of maize residues resulted in the production of low
31
density ( < 1000 kgm-3) briquettes which did not meet the German Standard DIN 51731 (1-1.4
32
gcm-3). Kaliyan and Morey [33] produced maize cob briquettes at a pressure of 150 MPa and
3
1
reported that density and durability were significantly affected by, moisture content (10 and 20
2
%), pre-heating temperature (25 and 85 oC) and particle size (mean particle diameter 0.85 and
3
2.81 mm), however, the impact of pressure and the interactions between briquetting parameters
4
were not analysed. In this study, the effect of briquetting conditions (pressure, moisture,
5
particle size and temperature) and their interactions on the properties of maize cob briquettes
6
was investigated. The findings from this study have clear potential globally as maize is one of
7
the major crops grown globally but particularly in sub-Saharan Africa regions where a large
8
amount (~7 million tonnes) produced annually [37] are either burnt in open air (without heat
9
recovery) or are dumped to decompose in uncontrollable ways. Converting residue cobs into
10
energy would not only contribute to reducing greenhouse gas emissions but also to a more
11
sustainable waste management strategy.
12
2
Materials and Methods
13
2.1
Material
14
Maize cobs were kindly provided by Barfoots of Botley Ltd, UK. Maize (supersweet varieties)
15
was harvested at Stage R3 (milk stage) of maturity in Senegal, Morocco, United State of
16
America, South Africa, Greece, Germany, United Kingdom, France and Spain and stored at 0-
17
5oC for 1-25 days. Waste cobs were sent to Newcastle University and stored in a cold room at
18
6oC prior to briquetting. The waste maize cobs are representative of material that would be
19
used in processing rather than the production of corn cobs grown to maturity as would be the
20
case for many developing countries including Africa. Residue maize cobs were cut into pieces
21
< 5 mm and oven dried at 105oC for 2-8 hours to obtain a range of moisture contents. All
22
moisture contents presented in this paper are on a % wet basis. Dried maize cobs were crushed
23
using a HGBTWTS3 laboratory blender 8010ES and separated using 2.36 and 4.00 mm sieves
24
to study the effects of particle size.
25
2.2
26
A machine fabricated with a hollow cylindrical mould, internal diameter of 2 cm and length
27
12.5 cm was adapted from the work of Zafari and Kianmehr [28]. The mould was fitted inside
28
two 150W band heaters connected to a temperature controller and was insulated with Fortaglas
29
for operator safety and to reduce heat loss.
30
About 7g of ground maize cob was fed inside the mould and then manually compressed using
31
a 10 tonne Hydraulic Bench Press (Clarke CSA10BB). A dwell time (i.e. duration for which
Briquette preparation
4
1
particles under compression remain under maximum compacting pressure during briquetting)
2
of 20s was chosen for all experiments to minimise briquette relaxation [21, 29] which could
3
have negative impacts on briquette properties (density, impact resistance and compressive
4
strength). The effects of temperature (20-80oC), moisture content, (7-17%) particle size ( <
5
2.36 mm and < 4.00 mm) and pressure (150, 200, 250MPa i.e. within the range of pressures
6
used for briquetting several biomass materials [23, 38, 39]) and their interactions were studied
7
using a 2-level factorial design of experiment. Briquettes were stored in an air tight container
8
at room temperature (approximately 20oC) for 7 days to allow stabilisation [40] prior to
9
analysis of their properties (density, impact resistance and compressive strength).
10
2.3
Briquette characterisation
11
Moisture, ash, volatile matter and fixed carbon content of maize cobs and briquettes were
12
determined according to ASTM D3173, ASTM D3174, ASTM D3175 and ASTM D3172
13
standards respectively. Ultimate analysis was carried out using a Carlo Erba 1108 Elemental
14
Analyser to determine percentage of carbon, hydrogen and nitrogen. High heating value (HHV)
15
was determined using a CAL2K ECO bomb calorimeter. Scanning electron microscopy (SEM)
16
analysis was carried out using a TM3030Hitachi Microscope. Differential Scanning
17
Calorimetry (DSC) analysis was carried out using a DSC Q20 model to identify the range of
18
compacting temperatures to be used in the briquetting experiments. Analysis of neutral
19
detergent fibre (NDF) was carried out by enzymatic gravimetry, while acid detergent lignin
20
(ADL) and acid detergent fibre (ADF) were analysed using an Ankom 220 analyser. The
21
composition of cellulose, hemicellulose and lignin were subsequently determined [41]:
22
Cellulose = Neutral detergent fibre (NDF) – Acid detergent lignin (ADL)
(1)
23
Hemicellulose = Neutral detergent fibre (NDF) – Acid detergent fibre (ADF)
(2)
24
Lignin = Acid detergent lignin (ADL)
(3)
25
Density was determined using the stereometric method which allows briquettes to be used for
26
thermo-chemical applications to remain dry [42]. Height and diameter of a briquette was
27
measured using a digital vernier calliper (error: ± 0.005 mm) to determine volume. For impact
28
resistance, a briquette was released 4 times from a height of 1.85 m to fall freely under gravity
29
onto a metallic plate to determine impact resistance [43]. Percentage residual weight of
30
briquettes was determined after each drop. The remaining piece with the highest weight was
31
taken as the residue and used for the next drop. Impact resistance was defined as the percentage
32
residual weight after the 4th drop. Compressive strength was determined via both the cleft and
5
1
simple pressure tests using a Tinius Olsen H50KS compressing machine. Briquettes were
2
placed between two flat parallel surfaces with surface area greater than the briquette. Briquettes
3
were placed horizontally for the cleft test and vertically for the simple pressure test. An
4
increasing load was then applied to compress briquettes at a rate of 1 mm/min until the briquette
5
failed/cracked. The ultimate load at the point where the briquette cracks, F was used to calculate
6
the compressive strength using Equations (4) and (5). An average of 3 measurements for each
7
test were carried out.
8
Compressive strength σ = F A
(4)
9
Compressive strength, σ = F / l
(5)
10
Where A and l are the cross-sectional area (m2) and length (m) of briquettes.
11
The physical and mechanical properties of briquettes such as density, impact resistance and
12
compressive strength are presented as mean values of at least 6 samples/briquettes. Minitab 17
13
statistical software was used to analyse the impact of the variables and their interactions on
14
density, impact resistance and compressive strength of briquettes. Statistical analysis was
15
carried out at a significance level of α = 0.05 .
16
17
3
Results and Discussion
18
3.1
Characteristics of maize cobs
19
Fresh maize cobs used in this study had high moisture content (73.9 ± 0.74%), which is much
20
higher than in other work e.g. 30.3 % [44]. The high moisture content is likely due to the use
21
of fresh maize cobs which were harvested at early stage of maturity (R3 i.e. milk stage) and
22
also stored at 0-5oC prior to analysis. They cannot be used directly for briquetting according to
23
European Standard Committee CEN/TC 335 for solid fuels as the moisture content in
24
briquettes is required to be 5-15%. In addition, high moisture feedstock/products are prone to
25
fungal decomposition during transportation and storage [27] and poor combustion properties
26
such as low heat output, low combustion temperature, and long fuel residence time in the
27
combustion chamber [17]. Therefore, these fresh maize cobs must be dried/partly dried prior
28
to being briquetted. Maize cob (Table 1) had high volatiles (~76%) and low ash content (3.2%),
29
which agreed well with other work [45, 46]. Fresh maize cobs had a similar high heating value
30
to that of woody materials and anthracite.
6
1
Differential scanning calorimetry analysis was carried out to identify the range of compacting
2
temperatures to be used in the briquetting experiments. An endothermic peak was observed at
3
100.9 oC associated with a loss of moisture, but no transition steps were observed (Fig 1). The
4
non-visibility of the glass transition temperature could be due to interference from the moisture
5
endothermic peak [47], as the glass transition step is likely to overlap with the moisture
6
endothermic peak area.
7
“Table 1 here”
8
A maximum compacting temperature of 80oC was therefore chosen for this study based on the
9
glass transition temperature of 79.2oC identified for corn stover [48]. Furthermore, compacting
10
at high temperatures i.e. ≥100oC is undesirable because it not only requires high energy input
11
which in turn reduces energy efficiency but also reduces compressive strength of briquettes
12
due to the evaporation of water which makes them brittle [49]. A certain amount of moisture
13
is required to reduce friction between particles and the mould during compaction and to
14
enhance the force of attraction between particles [27].
15
Two exothermic peaks at 283.78oC and 337.73oC observed in the DSC thermo-gram (Fig.1)
16
could be due to the decomposition of hemi-cellulose, cellulose, and lignin [50]. The lignin,
17
cellulose and hemicellulose composition identified in this study were 1.5%, 47.1 % and 29.4%
18
respectively with the remaining 22.0% likely to be extractives (e.g. protein, starch, oil and
19
sugar). A low lignin content in this study compared to much higher levels (3-15 %) observed
20
by other researchers, [33, 41, 51, 52] could be due to the analysis method used in this study of
21
which the acid detergent lignin (ADL) only gives a partial value of total lignin content [33].
22
“Fig 1 here”
23
3.2
24
Briquette density ranged between 516 kgm-3 and 1058.2 kgm-3 from variations in briquetting
25
parameters used in this study. The lowest density of 516 kgm-3 was obtained with a low
26
temperature (20 oC), a high moisture content (16.94 %) and a particle size < 4.0 mm, the density
27
of all other treatment combinations being > 700 kgm-3. With the exception of where a high
28
compacting pressure (250 MPa), small particle size ( < 2.36 mm) and a high temperature (80
29
o
30
less than 1000 kgm-3 (Fig 5) which falls below the range of 1-1.4 gcm-3 required to meet the
Density
C) were used, all briquettes produced from the high moisture content of 16.94 % had a density
7
1
German Standard DIN 51731. Highest density briquettes (1054.4-1058.2 kgm-3) were
2
produced from particle size of < 2.36 mm, moisture content of 7.14 % and pressure of 200-250
3
MPa Under these conditions density remained relatively constant likely due to a reduction in
4
original void spaces between particles and an increase in inter-particle bonding at high
5
pressures i.e. > 200 MPa. This trend is consistent with results reported for briquettes from palm
6
oil mill residues [24] and pine [32]. Density increased with increasing compacting pressure and
7
temperature but decreased with increasing particle size and moisture content (Fig 2a). Moisture
8
content and pressure were the predominant factors affecting briquette density. However, Zhang
9
and Guo [38] found that particle size (0.16-5 mm) and moisture content of 5-17 % were the
10
predominant factors that affected density of caragana korshinskii kom briquettes within a range
11
of compacting temperatures of 70-150 oC and compacting pressure of 10-170 MPa. Rhén et al
12
[53] reported that density of spruce pellet was predominantly affected by moisture content (6.3-
13
14.7 %) and compacting tempearture (26-144 oC) for particle size of
14
compacting pressure of 46-114 MPa. A similar observation was reported [54] on density of
15
olive tree pruning residue pellets produced from various particle size ranges < 1 mm to < 4
16
mm, moisture content of 5-20 %, compacting temperature of 60-150 oC and pressure of 71-176
17
MPa. Variable results for factors affecting briquette density are likely due to variation in
18
feedstock properties in addition to which many of the comparative studies have mainly focused
19
on the effects of single factors rather than looking at the interaction among them.
20
“Fig 2 here”
21
All interactions (Table 2) had significant impact on density (P < 0.05) except the interaction
22
between moisture and particle size. Briquettes produced at around 17 % moisture content and
23
pressure <250 MPa (Fig 5) had a density below the German Standard (DIN 51731) for pellets
24
(1-1.4 gcm-3) regardless of particle size and compacting temperature. This is likely due to the
25
incompressibility of water that prevents particles from being completely flattened at high
26
moisture content. Furthermore, the low briquette density could have been attributed to a
27
reduction in briquette weight or an increase in briquette volume upon drying and stabilising. It
28
was also observed that a high proportion of large cracks (Fig. 4) were formed in briquettes
29
produced at high moisture content i.e. 16.94 %. Matúš et al. [27] also reported appearance of
30
cracks on spruce briquettes produced at a moisture content above 16.5 % with 2.56, 12.69,
31
35.92, 26.06 and 27.77 % of particles < 0.50, 0.5- < 1.00, 1.00- < 2.00, 2.00- < 4.00 and > 4.00
32
mm in sizes. Increasing compacting pressure to 250 MPa and reducing particle size ( < 2.36
8
< 3.15 mm and
1
mm) could increase the density into the standard range ~ 1,000 kgm-3 but this will increase the
2
energy requirement for producing briquettes.
3
“Table 2 here”
4
“Fig 3 here”
5
At low moisture content (7.14 %), for small particle size < 2.36 mm, compacting pressure and
6
temperature had little effect on density. Density only increased by less than 4 % when pressure
7
was increased from 150 MPa to 200 MPa and remained almost constant with a further increase
8
to 250 MPa. However, at a moisture content of 7.14 % for a particle size < 4 mm, a significant
9
increase in density (~20 %) was observed when increasing pressure from 150 MPa to 200 MPa;
10
but with only a slight further increase of ~5 % as pressure was increased to 250 MPa. In
11
addition, compacting temperature had a great effect at 150 MPa (~14 % increase). In contrast,
12
at high moisture content (17 %), increasing pressure and temperature significantly increased
13
density for both particle sizes which was probably due to the combined effect of high pressure
14
and heat softening the particles and evaporating moisture. Therefore, with maize cob feedstock
15
at moisture content 7.14-10%, high density briquettes could be produced at either 150 MPa/80
16
o
17
MPa was required. At high moisture content (16.94 %), only a particle size < 2.36 mm could
18
provide briquettes with a density ≥ 1000 kgm-3 and this was under conditions of high pressure
19
and temperature i.e. 250 MPa and 80 oC.
20
“Fig 4 here”
21
“Fig 5 here”
22
3.4
23
Impact resistance is a measure of durability of briquettes which defines their tendency to
24
produce dust or break when subjected to a destructive force. It is an indicator of the mechanical
25
strength of briquettes [55], therefore its value should be as high as possible. In this study,
26
impact resistances ranged from 17.7 % to 99.8 % with variations in the briquetting parameters
27
used. Within all ranges of briquetting parameters studied, impact resistance was increased in
28
response to increased pressure and temperature, but was reduced with an increase in moisture
29
content and particle size (Fig 2b). The optimal moisture content and pressure identified in this
30
study compares well with the optimal moisture content (7.5 %) and pressure (200 MPa)
C or 200 MPa/20 oC for particle size < 2.36 mm but for particle size < 4 mm a pressure > 200
Impact resistance
9
1
required to produce olive waste briquettes with high impact resistance [30]. At high
2
temperature and pressure, moisture evaporates and increases the rate of heat transfer within
3
biomass particles. However, very high moisture prohibits complete flattening of particles
4
which lowers inter-particle bonds [7], causing less stable and weak briquettes. Application of
5
temperature and pressure causes diffusion of molecules thus reducing void space and forming
6
solid bridges which increases bonding between particles and hence the strength of briquettes.
7
The results agreed well with previous studies for paper mill waste briquettes (prepared in a
8
pressure range of 150-250 MPa and moisture content of 9 % [30] and mango and eucalyptus
9
leaf briquettes (pressure of 30-100 MPa and moisture content of 8.6 % and 7.9 % respectively
10
[21]). However, they disagreed with the findings for pulping residue and spruce sawdust
11
briquettes [23] where impact resistance increased as moisture content was increased from 7 to
12
15 %. The variations are likely due to variation in the range of optimal moisture contents used
13
for the different feedstocks.
14
At a fixed compacting temperature of 20 oC, impact resistances of briquettes prepared at high
15
moisture content (16.94 %) and particle size < 4.0 mm were not influenced by compacting
16
pressure (likely due to the incompressibility of water) and remained around 20 %. Decreasing
17
particle size to < 2.36mm had little effect on impact resistance at low compacting pressures but
18
led to a significant increase at 250 MPa. This could be due to the heat generated at high
19
compacting pressure enhancing the release of water within small particles, helping the binding
20
process. Impact resistance was almost 3 fold higher at 150 MPa when temperature was
21
increased to 80 oC most likely due to solid bridge formation, however, particle size had no
22
impact. There were significant interactions (p<0.05) between briquetting parameters on impact
23
resistance (Table 3; Fig 3b) except for the: pressure x particle size, moisture content x particle
24
size x temperature and pressure x moisture content x particle size x temperature interactions.
25
Under high pressure and temperature, low molecular weight components become binding
26
elements of particles whereas at high temperature and pressure, moisture evaporates and
27
increases the rate of heat transfer within biomass particles [56].
28
“Table 3 here”
29
At low moisture content (7.14 %) and particle size ( < 2.36 mm) increasing compacting
30
temperature from 20 oC to 80 oC significantly increased impact resistance i.e. from 50 % to 80
31
% at 150 MPa. However, there was no effect of temperature on impact resistance at higher
32
compacting pressures > 200 MPa (Fig 6). For larger particle size ( < 4 mm), compacting
10
1
temperature had a significant effect resulting in high impact resistance ( > 80 %) but only at
2
high pressure (200 MPa-250 MPa) when a compacting temperature of 20 oC was used. Impact
3
resistance increased significantly (P < 0.05) with an increase in pressure from 150 to 200MPa,
4
but was unchanged above 200 MPa.
5
Briquettes with high impact resistance/durability are desirable to minimise breakage and dust
6
formation during transporting and conveying. Up to now, there are no certified standards for
7
biomass briquettes, however, other researchers [55, 57] have reported that impact resistance of
8
80 - 90 % or over 90 % is required for better handling and transportation. However, very high-
9
quality briquettes (with impact resistance above 95%) were obtained at (i) small particle size (
10
< 2.36 mm), low moisture content (7.14 %) and high pressure (>200 MPa) and (ii) high particle
11
size ( < 4.00 mm), low moisture content (7.14 %), high temperature (80 oC) and high pressure
12
> 200 MPa. These briquettes lost only < 3.5 % of their weight after shattering and are therefore
13
durable thus satisfying the European Standard Committee CEN/TC 335 (durability > 95 %)
14
and are also suitable for transportation, storage and handling with minimal breakage and dust
15
generation.
16
“Fig 6 here”
17
3.5
18
Compressive strength is the maximum load that a briquette can withstand before it breaks [58].
19
It is used to estimate the compressive stress resulting from the weight of the top briquettes on
20
the lower briquettes during storage, transport and handling. Compressive strength (CS) tests
21
were performed via both cleft and simple pressure tests. These two tests have been used
22
independently [9, 13, 53, 59] to determine compressive strength of briquettes and it was found
23
from this study (data not presented) that there was a strong positive correlation between the
24
two tests.
25
Moisture content and compacting temperature were the dominant factors affecting compressive
26
strength in cleft whilst simple pressure was mainly affected by moisture content and particle
27
size i.e. simple pressure decreased with increasing moisture content and particle size (Fig 2c
28
and 2d). The compressive strength (between 75 and 120 MPa) of pine briquettes increased with
29
an increase in compacting pressure in the range of 31 - 318 MPa but was reduced with an
30
increase in particle size i.e. 0.5 - 4.0 mm [32]. Compressive strength of hazelnut shell briquettes
31
produced from particle size of 2-4 mm, moisture content of 8.7 % with pyrolysis oil from
Compressive strength (CS)
11
1
hazelnut shell and some wood as binder (6.5-18.0 %) increased (from around 11 to 38 MPa)
2
when compacting pressure was increased from 300 to 800 MPa [13]. However, the effect of
3
moisture content found in this study contradicts with others. For example, for lupin seed with
4
an average particle size of 0.5 mm, compressive strength of briquettes increased with moisture
5
content from 9.5 % to 15.0 % [60]. A 30% increase in compressive strength of olive refuse
6
briquette was observed when moisture content was increased from 5 % to 15 % [30] using a
7
compacting pressure of 200 MPa and particle size of < 0.250mm. An increase in compressive
8
strength of pulping reject briquettes from 13.0 to 37.2 MPa was reported when moisture and
9
compacting pressure were increased from 7 % to 18 % and 300 MPa to 800 MPa respectively
10
[23].
11
Both compressive strength in cleft and simple pressure increased significantly (P < 0.05) when
12
pressure was increased from 150 MPa to 200 MPa but with no further increase at higher
13
pressures. One can argue that an increase in compacting pressure is associated with an increase
14
in interparticle bonds resulting from an increase in cohesion force [36]. However, above the
15
optimal compacting pressure, in this case 200 MPa, the phenomenon of dilation occurs,
16
producing cracks in briquettes and consequently weakens them [61]. Interaction plots (Fig 3c
17
and d) shows that there were significant interactions (Table 4 and 5) on compressive strength
18
in cleft (Table 4) for all variables with the exception of: pressure x moisture, pressure x particle
19
size, pressure x particle size x temperature, moisture x particle size x temperature and, pressure
20
x moisture x particle size x temperature. For compressive strength in simple pressure all
21
variables showed significant interactions with the exception of particle size x temperature and,
22
pressure x moisture x particle size x temperature (Table 5).
23
“Table 4 here”
24
“Table 5 here”
25
It is recommended [24] that the minimum compressive strength in simple pressure for
26
briquettes is 2.56 MPa to enable storage, transportation and handling with minimum breakage.
27
Compressive strength in simple pressure of all briquettes in this study was above the
28
recommended value (Fig. 7b). The smallest value of 10 MPa was obtained at large particle size
29
( < 4 mm), with low compacting pressure and temperature (150 MPa and 20 oC) and high
30
moisture content (16.94 %).
12
1
At a compacting temperature of 20 oC, compressive strength in cleft was below 10 kNm-1 for
2
all moisture content and particles size variations studied (Fig.7a). Increasing compacting
3
pressure from 150 to 200 MPa resulted in more than 100% increase in compressive strength in
4
cleft for particle size < 4 mm and high moisture content but had little impact where small
5
particle size < 2.36 mm and low moisture content were used. Increasing pressure increased
6
compressive strength because particles undergo plastic and elastic deformation, thereby
7
increasing contact areas of particles which in turn filling void spaces and increasing inter-
8
particle bonds [38, 54]. High compacting pressure could also crush large size particles, leading
9
to increased densification [62]. During briquetting, pressure causes particles to rearrange to
10
form closely packed mass and then to elastically and plastically deform when pressure
11
increases. During the plastic and elastic deformation, particles move and fill void spaces which
12
increases contact area, consequently increasing both density and strength [18, 54]. According
13
to Kers [31] and Antwi-Boasiako and Acheampong [57], too much moisture in the feedstock
14
leaves cracks/void space in briquettes due to the escape of moisture within the briquette. The
15
formation of cracks/void spaces makes briquettes more porous thereby reducing their strength
16
and density. Therefore, a minimum amount of moisture in a feedstock is required to act as a
17
binding/catalyst to release low molecular mass products which binds particles together thereby
18
improving briquette strength. However, low moisture content is associated with low rate of
19
heat transfer between particles and therefore the requirement for high compacting pressure
20
[56]. In addition, moisture is responsible for bringing interfacial forces and capillary pressure
21
into play to increase forces of attraction between particles [27].
22
“Fig 7 here”
23
At a compacting temperature of 80 oC, the effect of compacting pressure was highly significant
24
both with high and low moisture content feedstocks. An increasing temperature releases natural
25
binders such as lignin, cellulose and hemicellulose which form solid bridges upon cooling [49,
26
62, 63] thereby increasing strength and density. A scanning electron microscopy (SEM) image
27
(Fig 8) of a briquette which was broken from the middle in a direction perpendicular to the axis
28
of the cylindrical briquettes showed a relatively smooth surface and particles which were
29
flattened to form a layer. The layer observed in the SEM image could have resulted from solid
30
bridge formation as no evidence of mechanical interlock was observed. Application of high
31
pressure and/or temperature during densification results in diffusion of molecules at the point
32
of contact from one particle to another, thus forming solid bridges [7]. Particles of corn stover
13
1
and switchgrass briquettes/pellets are bonded mainly by solid brigdes resulting from natural
2
binders i.e. mainly lignin and protein [14]. Natural binders can be squeezed out of particles at
3
temperatures near the glass transistion temperature (80 oC for maize cob) and improve particle
4
bonding through formation of solid bridges on cooling [7]. An increase in temperature also
5
results in evaporation of water from the particles of biomass under compression and since
6
water is uncompressible, the density of the briquette is increased.
7
“Fig 8 here”
8
At a fixed pressure, small particles are more densely packed than large particles [43]. In
9
addition, they have large surface area of contact which helps to create strong inter-particle
10
bonding, while large particles cause cracks which reduces density and strength [28]. The larger
11
surface area of small particles also facilitates better heat transfer (necessary for strong bond
12
formation) between particles thereby improving density and strength [54]. High porosity would
13
lower both density and strength. Valence and Van der Waals’ forces can contribute to bonding
14
when seperation between particles are about 10 Å and 0.1µm respectively [14]. Therfore, the
15
forces contributing to bonding become less effective for large pore sizes, thereby weakening
16
the briquettes.
17
4
18
Briquettes properties are an important character to meet the increasing demand for biomass
19
feedstocks, enabling long-term handling, storage and transport. In this study, an increase in
20
compacting pressure and temperature and a decrease in moisture content and particle size
21
increased density, impact resistance and compressive strength of corn cob briquettes. The
22
results showed that compacting pressure of 150MPa led to low quality and is not suitable for
23
briquette production regardless of the other parameters used in briquetting process. Pressure ≥
24
200MPa and temperature had no effect on properties of briquettes made from low moisture
25
content ( < 10 %), or small particle size ( < 2.36mm) maize cob. However, by increasing
26
compacting temperature up to 80oC, the particle size could be increased without trading off any
27
durability properties. This is because temperature releases components such as lignin, cellulose
28
and hemicellulose which act as binders. Compressive strength in simple pressure was in the
29
recommended range (≥ 2.56MPa) for all tested conditions. There was a strong interaction
30
between briquetting parameters and the interaction between moisture and temperature
31
significantly affected all the briquette properties studied most likely because moisture
Conclusions
14
1
accelerates heat transfer between maize cob particles which ease elastic and plastic deformation
2
during compression and also facilitates the release of natural binders.
3
4
Acknowledgements
5
The authors would like to express their sincere gratitude to the Commonwealth Scholarship
6
Commission for financial support; to technicians at the School of Chemical Engineering and
7
Advanced Materials for their help in the fabrication of the briquetting machine and to Barfoots
8
of Botley Ltd for providing maize cobs for this study.
9
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