Heliyon 7 (2021) e07802

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Heliyon
journal homepage: www.cell.com/heliyon

Research article

Biomass valorization for energy applications: A preliminary study on

millet husk
Aondoyila Kuhe *, Achirgbenda Victor Terhemba, Humphrey Iortyer
Mechanical Engineering Department, Federal University of Agriculture, Makurdi, Nigeria

A R T I C L E I N F O A B S T R A C T

Keywords: This study used millet husk which is a waste and gum Arabic as binder to develop briquettes for domestic cooking
Compaction pressure in Northern Nigeria. The objective was to investigate the effect of particle sizes, compaction pressures and binder
Particle size concentrations on the physical, mechanical and thermal characteristics of the briquettes. Furthermore, the study
Compressive strength
also accessed the economic viability of the usage of millet husk briquettes as fuel. Particle sizes of 0.3, 0.4, 0.6 and
Agro Residue
Millet husk
1.7mm; compaction pressures of 10, 15, 20 and 25 MPa and binder concentrations (gum Arabic) of 25, 30, 35 and
Gum Arabic 40% were used to densify the millet husk mixed with gum Arabic at room temperature with the aid of hydraulic
press. The caloric value (15.27 MJ/kg) was determined using ASTM D2015, other physical and chemical prop-
erties of the millet husk was determined by proximate and ultimate analysis which showed that volatile matter
(76%), ash content (6.5%) and sulphur content (0.3%) are within the recommended range for domestic cooking
fuels. It was found that the density (438 kg/m3 and 669 kg/m3), impact resistance index (70–93%) and
compressive strength of the millet husk briquettes increased with compaction pressures and binder concentrations
and decreases with increase in particle sizes, while for porosity of the briquettes, the above case was a reversal.
The performance of the briquettes for domestic cooking were accessed by ignition time (109 and 140 s); burning
rate (0.09 g/s and 0.18 g/s) and water boiling test which took 5 and 11 min to boil 1 L of water as compared to
fuel wood that takes longer. Economic analysis showed that utilizing the millet husk generated in northern
Nigeria will lead to huge savings in fuel wood consumption, monetary savings of about ₦ 9,257,869,268.62, and
reduction in deforestation and its attendant problems. A structured questionnaire as used to ascertain the
acceptability of the produced briquettes. Most of the respondents (90%) in a survey expressed willingness to use
millet husk briquette as replacement for wood. The study concludes that millet husk is good for briquetting for
energy applications with high potential to reduce energy poverty, minimal waste and reduce indoor pollution for
domestic cooking therefore, making millet cultivation more profitable in Northern Nigeria.

1. Introduction coal and kerosene [4]. The rate of usage of fuel wood as source of energy
has its own disadvantages hardship on women and children, emission of
It is generally acknowledged that the looming depletion of fossil fuels toxic atmospheric pollutants which are responsible for many health is-
is one of the most pressing concerns confronting many countries. The sues [5,6]. As a result, any fuel alternative that is both cost-effective and
ever-increasing price of such fuels on the global market appears to be environmentally friendly would be welcomed.
having a negative impact on economies all over the world and the Agriculture produces significant quantities of residues which could
greenhouse effect of fossil fuels [1,2]. These situations have forced many contribute to energy production and reduce the use of fuel wood [7]. .
persons especially rural dwellers in Northern Nigeria from the use of Northern Nigeria is at risk of desertification, owing to increased tree
fossil fuel as a source of domestic energy to the use of wood and wood felling for domestic heating and cooking. In this part of the country,
charcoal as they are cheap, abundant, accounting for about 51% of household cooking consumes more energy than any other end-use ser-
Nigeria's annual total energy consumption [3]. In Nigeria, the bulk of the vices [9]. Humans, plants, and animals that rely on the forest/woodland
energy usage is mostly for cooking at household level, which is mainly ecosystem for food and shelter are now being affected by deforestation
derived from biomass (67% fuelwood, 6% charcoal) and fossil fuel (18% [10]. Categories of biomass include agricultural residues, biomass

* Corresponding author.
E-mail address: amkuhe@uam.edu.ng (A. Kuhe).

https://doi.org/10.1016/j.heliyon.2021.e07802
Received 28 May 2021; Received in revised form 12 July 2021; Accepted 12 August 2021
2405-8440/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
A. Kuhe et al. Heliyon 7 (2021) e07802

plantation and forest residues, of these agricultural residues which rep- and ratio on the physical and combustion properties of charcoal bri-
resents a large and under-exploited potential energy resource, is quettes produced from wood residues of neem. The authors used starch
accounted to be the most abundant and cheapest of them all [10]. . The and gum Arabic as the binder types, the study showed that gum Arabic
amount of agricultural residue produced at any given time is proportional bonded briquettes had better physical and combustion properties than
to the amount of main crop produced and the amount of residue. As a starch bonded briquettes.
result, it is possible to estimate the amounts of agricultural residues From available literature, there is very little research on pearl millet
produced using the residue to product ratio (RPR). According to [13] and hulls briquettes. Therefore, a knowledge gap on pearl millet hull
the work of [14] the resource base of residue in Nigeria has it that cas- briquette exists, which this research intends to fill. Pearl millet hulls
sava, groundnut, sorghum, oil palm, millet, maize, cowpea, plantain, briquettes using gum arabic as binder will be produced and the effect a
palm kernels have residue product ratio (RPR) as 2.0, 2.3, 1.25, 0.230, feed process variable, particle size distribution and some operating s such
1.75, 4.328, 1.75, 0.50 and 0.25 respectively. Agricultural residues have as compaction pressure and binder ratio on the combustion properties of
been identified as a potential source of biomass for energy application, the briquettes will be investigated via an experimental and economic
but direct usage in their raw form is often difficult as it gives raise to toxic analysis on the feasibility of using pearl millet hulls to produce briquettes
emissions, poor ignition quality, excessive smoking and low combustion as an alternative to fuel wood for cooking in the rural areas of Nigeria,
efficiency due to their uneven and troublesome characteristics as high particularly northern Nigeria, where there is a large abundance of pearl
moisture content, low bulk density, fibrous nature etc. [15]. millet and gum Arabic.
Based on the large amount of millet grown, millet husk a by-product is
gotten in large quantities. There are nine species of millet in the world, 2. Materials and methods
according to [16], with a total production of 28.38 million tons, of which
11.36 million tons (40 %) were produced in Africa. Nigeria produces 2.1. Materials
about 40% of Africa's millet (4.53 tons, with the northern part of the
country producing more than 80% of the millet [17]. Nigeria has been In this study, Millet hull was obtained from a farm while gum Arabic
ranked as the world's second-largest millet producer (Food and Agricul- from Dadin Kowa market both in Maiyama Local Government Area of
tural Organization of The United Nations: Economic and Social Depart- Kebbi State, Nigeria. The millet hull was then crushed into different
ment). The Statistics Division 2007 Millet hulls are a fibrous by-product particle sizes (0.3mm, 0.4mm, 0.6mm and 1.7mm) based on the BS1377-
of millet grain dehulling. As at 2020, two million metric tons of millet 2 [27]. The samples were then stored in zip lock plastic bags ready for
was produced in Nigeria, about 40% of the harvested millet weight is analysis. The sample and Gum Arabic were sent to the Chemical Engi-
extracted as hulls after harvest [18,57]. . Many consumers decorticate neering department Ahmadu Bello University Zaria, Kaduna State for
(dehull) the kernel before grinding it in to various particle sizes for use as analysis to determine their physical properties, chemical composition
different products. Pearl millet is usually decorticated by washing the and calorific value (Table 1). The pearl millet hulls of different particle
clean grain in water. The water is removed and the grain is crushed using sizes (0.3, 0.4, 0.6 and 1.7 mm) were mixed with gum Arabic (an extract
a stone mortar and wooden pestle [59]. The hull is removed by washing from acacia Senegal L.) at binder concentration of 25%, 30%, 35% and
or winnowing the sun-dried crushed material, and collected in polythene 40% by weight respectively before introduction into the mould by hand
bags for other purpose or are discarded in field where they are being and compacted at different pressures (10, 15, 20 and 25 MPa). A total of
burnt. sixty-four (64) different samples were prepared at four levels of the three
Some researchers have used millet by-products with some success factors investigated. Each sample was replicated thrice. The briquettes
[19]: Predicted the performance of millet bran briquettes (using a Neural were formed in a cylindrical mould of diameter 65 mm mm and 75 mm
Network Approach) and concluded that ANN can be used as a fast and long (with a plunger of 64.6mm diameter and 80mm long). The mould
versatile modeling method for any non-linear problem involving biomass was filled with the mixtures and densified at different pressures, with a
briquettes with a higher degree of accuracy. The combustion character- manually operated 20-tonne air hydraulic piston press (KENNEDY Model
istics of briquettes fuels made from sorghum panicle – pearl millet with HBP020, UK). The hydraulic press was held at the respective compacting
cassava starch as binder were investigated by [20], and it was discovered pressures for 45 s (dwell time) before release and extrusion from the
that sorghum panicle–pearl millet briquettes have a higher calorific value mould. A stop watch was used for the purpose of timing. Drying the
and produce higher heating values than pongamia–tamarind shell bri- briquettes to a moisture content of 8.5% at an average relative humidity
quettes [21]. investigated the combustion properties of briquettes made 75% and temperature of 28
C took between 18 and 21 days. Proximate
from different particle sizes of finger millet straws, and it was discovered and ultimate analyses were conducted on the produced briquettes using
that milled finger millet straws had a high volatile matter content with ASTM E870 – 82 (2015) [28] and ASTM D3178, D3179, D3177 standards
low moisture and ash content, indicating that finger millet straws are a respectively [29]. Calorific values were determined using ASTM
potential renewable energy source. From the works of [19–21], different astm:D2015 – 00 standard procedures [30,31]. The moisture content was
residue components of millet bran, finger, and stalks were used as bri- determined using ASTM D3173 standard procedure (ASTM, 1993) [32].
quetting material. [22], in their work, considered maize and millet husks The moisture content reported in this article are on wet mass basis unless
blends as briquetting material in certain ratios and used starch paste as otherwise stated.
binder [23] determined the optimum conditions for preparing from The initial density of the mixture of millet and gum Arabic mixture
foxtail millet by investigating the effects of moisture content, tempera- were determined using the formula
ture and applied pressure on the briquette quality using response surface
methodology. Mass ðkgÞ
ρ¼ (1)
Gum Arabic is a leguminous tree species that is well adapted to Sudan Volume ðm3 Þ
and Sahalian agro-ecology of Africa. In Nigeria, estimated hectarage of
Each particle size was taken in turn and used to produce briquettes
gum Arabic both cultivated and the wild form (forest reserves) is put at
with respective binder concentration and compaction pressure. . The
2.5 million (ha) [24]. Production of Gum Arabic is in the northern region
gum Arabic was weighed in a digital balance to determine its percentage
of Nigeria which coincides with the region where pearl millet is pre-
by weight and then dissolved in water and was allowed to gelletize before
dominantly cultivated in Nigeria [25] examined the effect of binder type
mixing with the hull [33]. The samples were conditioned to moisture

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A. Kuhe et al. Heliyon 7 (2021) e07802

Table 1. Proximate and ultimate analyses of millet husk and gum Arabic.

Parameter Proximate Analysis Ultimate Analysis

Millet Husk Gum Arabic Millet Husk Gum Arabic

Moisture Content (%) 8.20 6.63
Ash Content (%) 6.50 5.05
Fixed Carbon (%) 16.50 23.05
Volatile Matter (%) 68.80 64.26
Higher Calorific Value (MJ/kg) 15.27 12.24
Carbon (%) 56.80 61.21
Hydrogen (%) 5.31 4.42
Oxygen (%) 35.00 28.31
Nitrogen (%) 0.86 0.97
Sulphur (%) 0.30 0.21

 
content of 15 % by weight to make the mixture suitable for briquetting Bulk Density
Potosity ¼ 1  (5)
[34]. True Density

2.3. Thermal characteristics of the briquettes

2.2. Physical and mechanical properties of briquettes
2.3.1. Ignition time
2.2.1. Compressed density and relaxed density In order to determine ignition time, each briquette was oven dried at
Compressed density of the briquettes was determined immediately 105  3
C to constant masses in 24 hours to drive off moisture in the
after the briquette was removed from the mould using Eq. (1), while the briquettes for effective combustion. A dried briquette was then ignited at
relaxed density (RD) was determined 12–19 days after removal from the the edge with a Bunsen burner and the time taken for each briquette to
press and sun – drying according to ASTM D5373 standard procedures start burning was recorded in seconds [39].
[25,35].
distance burnt ðmmÞ
108; 000  M ðgÞ Ignition time ¼ (6)
Relaxed Density ¼ (2) total time taken ðsecÞ
π ðd1 þ d2 þ d3 Þ  ðl1 þ l2 þ l3 Þ
2

2.3.2. Burning rate

where M (g) is the mass of briquette while di and li ði ¼ 1; 2; 3Þ are the Burning rate is the mass loss per unit time due to combustion. In order
diameters (mm) and lengths (mm) respectively measured at three to determine the burning rate of the briquettes; a briquette was placed on
different points on the briquettes. a steel wire mesh grid resting on three supports to allow free flow of air.
Now the whole system was placed on a digital mass balance. The
2.2.2. Impact resistance index briquette was ignited from top and mass loss data was taken at intervals
The impact resistance index (IRI) was determined by adopting ASTM of 10 s [40].
astm:D440 standard methods for drop shatter for coal [35]. Each
briquette sample was repeatedly dropped from a height of two meters on mass of total consumed ðgÞ
Burning rate ¼ (7)
a concrete floor [36]. The number of drops (N) taken by each briquette to total time taken ðminÞ
break into pieces was recorded. Impact resistance index was then
calculated from the formula 2.3.3. Water boiling test
Since the briquettes are going to be used for domestic cooking, their
N
IRI ¼ X 100 (3) suitability is been accessed by water boiling. 83 grams of each briquette
n
sample was measured and put on a metal domestic briquette stove to boil
where n ¼ number of pieces that weigh up to 5 % or more of the initial 1 L of water using aluminum pot the process used as outlined by [41].
mass of the briquette after N drops. The measured time taken to boil the water, the temperature, the
remaining ash and briquette left are used for the calculation.
2.2.3. Compressive strength
The Compressive strength of briquettes was determined using a uni- 2.4. Economic analysis
versal testing machine (INSTRON 3382) with a load cell capacity of 50
kN and a cross-head speed of 1 mm/min in accordance with ASTM Cost analysis was determined to access whether briquette produced
D2166-85 until the structure of the briquette failed. The maximum force from millet hull is economically viable to fuel wood used in this part of
the briquette was able to withstand was then recorded and used to the country for domestic cooking. . The aim is to determine how much of
determine the compressive strength of the briquette with the help of the fuel wood can be substituted by millet hull and its monetary value. The
formula [37]. millet hull was considered a waste that is free and the cost of processing it
as briquette was assumed to be of no effect in the analysis since a similar
Force ðNÞ cost exists also in wood processing and is captured in the price. The pa-
Compressive Strength ¼ (4)
Area of briquette ðcm2 Þ rameters used in this analysis include:

2.2.4. Porosity of the briquettes - Average annual millet production for Nigeria (Mt), kg
Porosity is a measure of the void spaces in a material and is a fraction - Husk ratio in millet (W), kg
of the volume of voids over the total volume; it generally lies between 0 – - Average calorific value of millet hull briquette (Cm ), MJ/kg
1. It determined using the formula [38]. - Average calorific value of wood (Cw ), MJ/kg

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A. Kuhe et al. Heliyon 7 (2021) e07802

- Average Price of fuel wood (Pw), N densities increased with increase in binder concentration. This agrees
with results obtained by [33] while working on fonio husk, a plant in the
Two kilograms of millet was threshed manually by pounding in a same family with millet.
mortar. The hull was separated by winnowing. The husk ratio in millet,
W was determined using the formula
3.2. Handling characteristics of millet husk briquettes
Mh
W¼  100 % (8)
Mm 3.2.1. Compressed density and durability of the briquettes
The effect of binder concentration and compaction pressure on the
where. Mh ¼ Mass of husk; Mm ¼ Mass of whole millet ð2 kgÞ compressed density of millet husk briquette is shown in Table 3. It shows
The average annual husk produced in Nigeria, Mt ¼ W Mt , kg (9) that the compressed density of the briquettes ranged between 438 kg/m3
and 669 kg/m3, while impact resistance varied between 70 % and 93 %
HCm
∴Annual wood equivalent; Wm ¼ (10) which is in line with the range found by [47]. According to the table, an
Cw
increase in compaction pressure and binder concentration resulted in an
In monetary terms, cash saved due to fuel wood substitution with increase in the compressed density and durability (Impact Resistance) of
millet husk briquette, the briquettes. These densities are higher than the initial densities of the
uncompressed mixture, thereby making briquetting a worthy venture
Nw ¼ Pw  Wm ; naira (11) [48]. Briquettes made with a hydraulic piston press are typically less than
As part of the economic analysis, a survey was conducted on the 1,000 kg/m3 and have a density of between 300 and 600 kg/m3, ac-
potential acceptability of the briquettes and existing potential uses of the cording to the findings of the study by [49,50].
millet husk that are likely to compete with briquetting for fuel. Two
questions were asked in a structured questionnaire distributed to 50 3.2.2. Relaxed density of the briquettes
millet farming households in some communities in Kebbi state, Nigeria. The effects of Compaction Pressure, Particle size and binder con-
centration on the relaxed density of the millet husk briquettes are shown
i. State any existing use(s) of millet husk that in your opinion may in Figure 1. It shows that the relaxed density of the briquettes varied
reduce the quantity of husk available for briquetting. between 383 kg/m3 and 411 kg/m3, which were observed to be higher
ii. From your cooking (or other uses) experience with the briquettes, will than the initial density of unprocessed mixture of millet husk and binder,
you be willing to use it as replacement for wood if available? ranging between 167 kg/m3 and 213 kg/m3. The relaxed density
increased with increase in compaction pressure and decreases with par-
3. Results and discussion ticle sizes at 25 % binder concentration as shown in Figure 1a.
These results suggest that the relaxed density of the briquettes pro-
3.1. Proximate and ultimate analysis of briquettes duced increases with increasing compacting pressure level with smaller
particle size are likely to have higher relaxed density than those with
The results of proximate and ultimate analyses of millet husk larger particle size. This result confirms that of other researchers [51,52].
briquette with gum Arabic as binder are presented in Table 1. It shows The reason for this trend is that increasing the compacting pressure will
that the millet husk and gum Arabic have moisture contents of 8.20 % lead to the particles of biomass material being closely packed due to
and 6.63 % respectively, which is below the moisture content required reduction of void ratio and plastic deformation of the millet husk parti-
for briquetting by [41], but within the minimum ranges of 8–12 % as cles, therefore leading to increased density of the briquettes [53].
reported by [42] for agricultural waste. The table further shows that the Additionally, if the raw material is finer, it gives a larger surface area for
millet husk and gum Arabic, have calorific values of 15.27 MJ/kg and bonding which results in the production of briquette with higher density.
12.24 MJ/kg respectively, which is the most important of fuel charac- Figure 1b, shows that the relaxed density increased with binder con-
teristics [42]. The obtained values are lower than the average value range centration at different particle sizes of 1.7, 0.6, 0.4 and 0.3mm respec-
of 18–20 MJ/kg reported by [43], but greater than minimum require- tively. These results agree with the findings of [33] for briquettes made
ment (14.5 MJ/kg) recommended by [44]. This can be attributed to the from fonio husk. The increase in relaxed density with compaction pres-
fair amount of fixed carbon and hydrogen contents, which are the major sure and binder concentration can be attributed to possible increase in
sources of heat in a material during combustion. The high oxygen con- compactness due to increased plasticity and decrease in the size of voids
tents of 35 % and 28.31 % for millet husk and gum Arabic shows that the due to decrease in particle size [33]. It was observed that from the work
briquettes will require less oxygen from the atmosphere and it sometimes of [54] the highest compressed density of 1437 kg/m3 was obtained for
as a result of high degree of inherent moisture. The ultimate analysis briquettes made of cassava binder at 30% binder concentration under
shows that the sulphur contents are 0.30 % and 0.21 %, and falls within 150 kPa compacting pressure, while the lowest value of 952.46 kg/m3
safe limits for indoor fuels respectively [44]. The volatile matter is the was obtained from gelatin binder at 10% binder concentration under 50
amount of fuel that is release as gases when the densified biomass is kPa compacting pressure. It was observed that the compressed density of
heated. The higher this is, the faster the fuel burns; its reaction rate is the briquettes increased with increase in binder ratio and compaction
more and it is easily ignited [44]. For biomass, a typical range is from 65 - pressure.
85 % wt which is in agreement with 76 % volatile matter for the millet
husk [45], with a low ash content of 6.5 % which is a common charac- 3.2.3. Compressive strength of the briquettes
teristic of biomass residues [46]. The initial densities of millet husk/- The Compressive strength varied between 0.67 N/cm2 and 8.2 N/
2
binder mixtures are shown in Table 2. The table shows that the initial cm . It increased with increasing compaction pressure, but decreased
with increase in particle size. This trend can be attributed to the fact that

Table 2. Initial densities of millet husk/binder mixtures.

Binder Concentration (%) 25 30 35 40

Initial Density (kg/m3) 167 181 201 213

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Table 3. Effects of binder concentration and compaction pressure on density and durability of millet husk briquettes.

Binder Concentration (%) Compaction Pressure (MPa) Compressed Density (kg/m3) Impact
Resistance Index (%)
25 10 434 70
15 455 74
20 503 77
25 507 79
30 10 438 73
15 467 75
20 525 78
25 569 80
35 10 458 73
15 481 75
20 537 85
25 662 90
40 10 473 83
15 493 85
20 545 91
25 669 93

Figure 1. (a) Effect of variation of compaction pressure on relaxed density of briquettes at 25% binder concentration (b) Effect of variation of binder concentration on
relaxed density of briquettes (c) Effect of particle size on porosity index of briquettes made at 10 MPa compaction pressure (d) Effect of compaction pressure on
porosity index of briquettes made at 25% binder concentration. Values are means  standard deviation (n ¼ 3).

the smaller particle sizes are likely to bond better with smaller voids, size. The significance of this result is that higher binder concentrations
forming stronger solids than the larger particles. The compressive and smaller particle sizes make the briquettes stronger, thus easier to
strengths are however generally good and likely to enable easy handling handle. This finding supports the claims of [49,51] and [52] that pellet
of the briquettes [48]. On the other hand, the compressive strength of the durability (mechanical strength) is inversely proportional to particle size
briquettes was observed to vary from 6.7 N/cm2 and 9.2 N/cm2 with because smaller particles have more surface area for moisture addition
variation in binder concentration and particle size. It increased with during steam conditioning, resulting in improved starch gelatinization
increasing binder concentration but decreased with increasing particle and better binding.

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A. Kuhe et al. Heliyon 7 (2021) e07802

3.2.4. Porosity of the briquettes Figures 2c and 2d. The burning rate ranged between 0.09 g/s and 0.18 g/
The effects of various parameters on the porosity index of the bri- s. The burning rate increased with increase in particle size as shown in
quettes are shown in figure 1c and d. The porosity index ranged between Figure 2c.
0.19 and 0.27. It increased with increasing particle size, but decreased From the study of [56], the obtained briquette burning rate values
with increasing binder concentration and compaction pressure. decreased as the binder percentage was increased. The implication of this
This trend shows that increasing particle size increases the likelihood observation is that with the briquettes produced, more fuel may be
of larger pores, while increasing compaction pressure and binder con- needed for cooking. The briquettes made with 50 % binder had the
centration means more plastic deformation and bonding among the slowest rate of combustion. As a result, briquettes without a binder
particles, hence smaller pores and less porosity. This result indicates that burned more quickly than those with a binder [56]. The burning rate
higher binder concentration, compaction pressure and smaller particle decreased with increasing compaction pressure and binder concentra-
size result in better briquettes; in terms of porosity; on which water ab- tion. See Figures 2c and 2d.
sorption of the briquettes depend. The optimum values of these param- Increase in density of briquettes occasioned by increase in binder
eters may however require to be determined in future studies for concentration and compaction pressure results in a decrease in the
optimum performance. burning rate of the briquettes due to smaller pores, thereby allowing little
air supply into the briquettes for combustion [56]. The influence of the
3.3. Thermal properties of the briquettes particle size on the burning rate of the briquettes might be due to the fact
that the bigger particle sizes could have more pores in-between them
3.3.1. Proximate and ultimate analysis of the millet husk briquettes than smaller particle sizes, thus, increasing porosity index of the
The results of the proximate and ultimate analysis of the millet bri- briquette. The high porosity index reduces the time taken for adjacent
quettes are presented in Table 4. It shows that the volatile matter for particles to ignite and burn due to increased air flow to support com-
millet husk alone is 51 %, moderate for ease of ignition and mild emis- bustion [56]. This trend agrees with the findings of [33] while studying
sion. The calorific value is 16.13 MJ/kg, close to the average value for briquettes made from fonio husk.
wood, 18.8 MJ/kg [55], making it a good substitute for wood especially
as it is derived from waste. The nitrogen and sulphur contents are low, 3.3.4. Water boiling test
indicating low NOx emission and corrosion rate of combustion and ash The results of the water boiling test of the millet husk briquettes with
handling equipment. The high oxygen content of 37.10 % means less air Gum Arabic as binder are shown in Figure 3. The raw millet husk bri-
will be required for its combustion than wood. According to [66], bri- quettes particle size at 10MPa with 25% binder composition took 5 min
quettes from agricultural residues showed less average CO emissions to boil one liter of water while with 40% binder compositions took 11
than that of fuel wood briquettes while for NOx the reverse is the case, min to boil one liter of water. This indicates that boiling time increase
but for SO2 emissions are very similar for both depending on their with increase in binder composition and decreases with increase in
Sulphur content. Although agro-based briquettes have higher potassium particle size as for the same binder composition. From the result obtained
and ash content, it did not show higher particulate matter emissions than [58], it was observed that the water heated with rice husk - starch
fuel wood [59]. briquette took 15 min to boil 2 L of water compared to fire wood that
took 21 min to boil the same quantity of water. This implies that millet
3.3.2. Ignition time husk briquettes in this study can conveniently substitute fuel wood.
The influence of the various parameters on the ignition time of the
briquettes is shown in Figures 2e and 2f. It shows that the ignition time
3.4. Economic analysis
ranged between 109 s and 140 s. The ignition time decreased with
increasing particle sizes for all binder concentrations. According to [56],
Substituting the values of the parameters in equations 9 – 12 yielded
the lowest ignition time values were attributed to high porosity (increase
the results in Table 5. It shows that utilizing all the average quantity of
in particle size) between inter and intra-particles, which allows for easy
millet husk produced annually in Nigeria will allow about one billion,
oxygen percolation and outflow of combustion briquettes due to low
three hundred and twenty-two million, five hundred and fifty-two
bonding force.
thousand, seven hundred and fifty-two kilograms of fuel wood to be
Figure 2a, shows that the ignition time increased with increasing
saved. This will translate into monetary savings of about nine billion, two
binder concentration and compaction pressure for all particle sizes a
hundred and fifty-seven million, eight hundred and sixty-nine thousand,
shown in Figure 2b. Increased in compaction pressure automatically
two hundred and sixty-eight naira only annually. The results of the po-
increased the density of briquettes and consequently, delayed the igni-
tential acceptability survey shows that more than 90 % of the re-
tion time of the briquettes. Furthermore, briquettes compressed to a
spondents were willing to accept millet husk briquettes as replacement
higher density will tend to have a lower porosity, and thus elongate the
for wood if made available.
ignition time [56].
This trend shows that briquettes made with large particle size millet
husk will ignite faster, but those made at high compaction pressures and Table 4. Proximate and ultimate analyses of millet husk briquettes.
binder concentrations will take more time to ignite [55]. This can be
attributed to the fact that larger particles leave larger pores in the Parameter Proximate Analysis
briquette, allowing more air through it to support combustion. On the Proximate Ultimate
other hand, high compaction pressures and binder concentrations entail Moisture Content (%) 6.40
more plastic deformation of the particles and more bonding, allowing Ash Content (%) 5.20
little room for air passage to support combustion. The values obtained Fixed Carbon (%) 37.30
from this study are within the range of 19–186 s for bio-coal briquettes Volatile Matter (%) 51.10
made by mixing the materials at various concentrations of 10–50% with Higher Calorific Value (MJ/kg) 16.13
coal [39]. Briquettes for domestic use should have a low porosity index,
Carbon (%) 58.20
low volatile content, and low ash content [57].
Hydrogen (%) 6.21
Oxygen (%) 37.10
3.3.3. Burning rate
Nitrogen (%) 0.85
The influence of particle size, compaction pressure and binder con-
Sulphur (%) 0.3 0
centration on the burning rate of the millet husk briquettes are shown in

6
A. Kuhe et al. Heliyon 7 (2021) e07802

Figure 2. (a) Effect of particle size and binder concentration on the ignition time of briquettes made at 10MPa compaction pressure. (b) Effect of compaction pressure
variation on the ignition time of briquettes produced with 25% binder concentration. (c) Effect of binder concentration variation on the burning rate of briquettes
produced at 10MPa compaction pressure. (d) Effect of compaction pressure variation on briquettes burning rate. Values are means  standard deviation (n ¼ 3).

Figure 3. Effect of Binder composition on boiling time of millet husk Briquettes of different particle sizes at 10 MPa compaction pressure. Values are means 
standard deviation (n ¼ 3).

7
A. Kuhe et al. Heliyon 7 (2021) e07802

Table 5. Possible fuel wood and monetary savings due to millet husk utilization.

Mm Mh Mt Cm Cw Pw W H wm Nw
2 kg 0.36 kg 7905  106 kg*** 16.13 MJ/kg 18.8 MJ/kg* N7/kg** 19.50 % 1541475000 kg 1322552752.66 kg N9257869268.62 ($25716303.52)
***
CEIC, 2017.
*
Gunther, et al., 2020.
**
Kuhe, et al., 2017.

If implemented, this will reduce deforestation in Nigeria while Additional information

improving the income of the millet farmers. The waste will be reduced
and green-house gas emission due to the open rotting of the wastes will No additional information is available for this paper.
also be mitigated. The resulting widespread briquette making will pro-
vide jobs for the youth, giving room for more economic development. References

[1] P. Wilaipon, K. Trirattansirichai, K. Tangchaichit, Moderate die-pressure banana-

4. Conclusion and recommendations
peel briquettes, J. Renew. Energy Smart Grid Technol. 2 (1) (2007) 50–55.
[2] B. Sajjakulnukit, R. Yingyuad, V. Maneekhao, V. Pongnarintasut, S.C. Bhattacharya,
The study used millet husk with gum Arabic as binder to produce P. Abdul Salam, Assessment of sustainable energy potential of non-plantation
briquettes and determined the effect of particle size, compaction pressure biomass resources in Thailand, Biomass Bioenergy 29 (2005) 214–224.
[3] M.U. Garba, S.U. Gambo, U. Musa, K. Tauheed, M. Alhassan, O.D. Adeniyi, Impact
and binder concentration on the physical, mechanical and combustion of torrefaction on fuel property of tropical biomass feedstocks, Biofuels 9 (3) (2018)
properties of the produced briquettes. The result reveals that it was 369–377.
possible to produce briquettes from millet husk with considerably fuel [4] International Energy Agency, Africa Energy Outlook 2019 World Energy Outlook
Special Report, 2019. https://www.iea.org/t%26c/. (Accessed 20 November 2019).
properties for use as domestic fuel for cooking. Particle size, compacting [5] O. Oroka, T.E. Akhihiero, Production of fuel briquettes and biogas from water
pressure and binder concentration affect the density, compressive hyacinth- cow dung mixture for domestic and agro- industrial application in
strength, impact resistance index, porosity, ignition time and burning Nigerian, in: A.A. Galadima (Ed.), Compendium of Oil and Gas Research. Abuja,
Nigeria, Petroleum Technology Development Fund, 2003, pp. 458–523.
rate of the briquettes produced thereby influencing their quality. [6] C. Antwi-Boasiako, B.B. Acheampong, Strength properties and calorific values of
The briquettes produce enough energy for cooking as compared to sawdust-briquettes as wood-residue energy generation source from tropical
wood fuel used predominantly in Northern Nigeria. Also, they are hardwoods of different densities, Biomass Bioenergy 85 (2016) 144–152.
[7] A. Yank, M. Ngadi, R. Kok, Physical properties of rice husk and bran briquettes
considered as carbon neutral because when growing the biomass under low pressure densification for rural applications, Biomass Bioenergy 84
removes as much CO2 as is emitted into the atmosphere from its com- (2016) 22–30.
bustion and based on the toxic emission of atmospheric pollutants which [9] K.H. Ayuba, A. Dami, Environmental Science. An Introductory Text. A Revised and
Enlarged Edition, Apani Publications, 2011, pp. 72–76. No 27, Bagaruwa Road,
are responsible for an appreciable number of health problem from the use
Costain, Kaduna.
of fuel wood, the briquettes produced are renewable, environmentally [10] S.C. Bhattacharya, H.L. Pham, R.M. Shrestha, Q.V. Vu, CO2 emissions due to fossil
friendly, economically viable and generally willing to be acceptable by and traditional fuels, residues and wastes in Asia, in: Workshop on Global
the dwellers of this region. Warming Issues in Asia, Asian Inst. of Technol., Bangkok, 1993.
[13] Food, F.A.O. Agriculture Organisation, World Millet Production, Economic and
It is recommended that further studies be carried out to determine Social Department, The Statistics Division, United States, 2007.
optimum values for relevant parameters for the production of the [14] O.A.U. Uche, M. Adamu, M.A. Bahuddeen, Influence of millet husk ash (MHA) on
briquettes. the properties of plain concrete, J. Epistem. Sci. Eng. Technol. 2 (2) (2012) 68–73.
[15] A.B. Akande, Economic analysis of the relationship between drought and millet
production in the arid zone of Nigeria: a case study of borno and yobe state,
Declarations J. Agric. Soc. Sci. 2 (3) (2002) 112–121.
[16] G. Kumar, G. Thampi, P.K. Mondal, Predicting performance of briquette made from
millet bran: a neural Network Approach, Adv. J. Grad. Res. 9 (1) (2021) 1–13.
Author contribution statement [17] S. Velusamy, A. Subbaiyan, R.S. Thangam, Combustion characteristics of briquette
fuels from sorghum panicle–pearl millets using cassava starch binder, Environ. Sci.
Aondoyila Kuhe: Conceived and designed the experiments; Analyzed Pollut. Control Ser. (2021) 1–15.
[18] Ayub, H. R., Bichang’a, D. O., Kipsanai, J. J., Kegesa, D. A., & Osumo, B. N.
and interpreted the data; Wrote the paper. Combustion Properties of Briquettes Produced from Finger Millet Straws of
Achirgbenda Victor Terhemba: Performed the experiments; Contrib- Different Particle Sizes.
uted reagents, materials, analysis tools or data. [19] A.A. Hammajam, Z.N. Ismarrubie, S.M. Sapuan, Millet husk fibre filled high density
polyethylene composites and its potential properties, Int. J. Eng. Tech. Res. (IJETR)
Humphrey Iortyer: Analyzed and interpreted the data; Wrote the (2) (2014).
paper. [20] S.M. Batagarawa, Extraction of cellulose from waste millet husk and its hydrolysis
Umar Abacha: Performed the experiments; Contributed reagents, to glucose using solid acid catalyst, Katsina J. Nat. Appl. Sci. 4 (2) (2015).
[21] S.M. Auta, A.J. Shiwua, T.Y. Tsado, Compressive strength of concrete with millet
materials, analysis tools or data. husk ash (MHA) as a partial replacement for cement, Magaz. Civil Eng. 7 (59)
(2015) 79.
[22] S.B. Oyeleke, N.M. Jibrin, Production of bio-ethanol from Guinea corn husk and
Funding statement millet husk, Afr. J. Microbiol. Res. 3 (4) (2009) 147, 142, http://www.academ
icjournals.org/ajmr.
[23] D.S. Senchi, A.A. Lawal, B. Ibra him, I.D. Kofa, Biomass power critical tool in
This research did not receive any specific grant from funding agencies fighting climate change, J. DOI 6 (12) (2020).
in the public, commercial, or not-for-profit sectors. [24] T. Mustapha, The economic, social and environmental relevance of gum Arabic with
a focus on the major challenges and opportunities for producing countries of Africa:
a case study of Nigeria, in: United Nations Conference on Trade and Development.
Data availability statement Round Table: the Economics of Gum Arabic in Africa, 2018, 27 April 2018, Geneva.
[25] O.A. Sotannde, A.O. Oluyege, G.B. Abah, Physical and combustion properties of
briquettes from sawdust of Azadirachta indica, J. For. Res. 21 (1) (2010) 63–67.
Data will be made available on request. [27] ASTMTest Methods for Analysis of wood Fuels, ASTM International, West
Conshohocken PA, 2015. www.astm.org.
[28] ASTM D3177 – 89, Standard Test Methods for Total Sulfur in the Analysis Sample of Coal
Declaration of interests statement and Coke, ASTM International, West Conshohocken PA, 2002, 2002, www.astm.org.
[29] ASTM 02015 – 00, Standard Test Methods for Gross Calorific Value of Coal and
Coke by Adiabatic Bomb Calorimeter, ASTM International, West Conshohocken PA,
The authors declare no conflict of interest. 2015. www.astm.org.

8
A. Kuhe et al. Heliyon 7 (2021) e07802

[30] W. Bandara, P. Kowshayini, Evaluation of the performances of biomass briquettes [45] C.A. Garcia, A. Pena, R. Betancourt, C.A. Cardona, Energetic and environmental
produced with invasive eichorniacrassipes (water hyacinth), wood residues and assessment of thermochemical and biochemical ways for producting energy from
cow dung for small and medium scale industries, J. Fund. Renew. Energy Appl. 8 agricultural solid residues: coffee cut-Stems case, J. Environ. Manag. 216 (2017)
(1) (2017) 247, 154. 160–168.
[31] ASTM D3173, Standard Test Methods for Moisture Content in the Analysis Sample of [46] T.A. Omwando, Investigating the Combustion Behaviour of Selected Agricultural
Coal and Coke, ASTM International, West Conshohocken PA, 1993. www.astm.org. Residues Available in Kenya in a Fluidized Bed Combustor, Doctoral dissertation,
[32] A.L. Yaumi, A.M. Murtala, H.D. Muhd, F.M. Saleh, Determination of physiochemical Moi Univesity, Kenya, 2006.
properties of Gum Arabic as a suitable binder in emulsion house paint, Int. J. [47] O.O. Fasina, Physical properties of peanut hull pellets, Bioresour. Technol. 99
Environ. 5 (1) (2016) 67–78. (2008) 1259e1266.
[33] D.Y. Bisu, Y.H. Kwala, S.I. Ige, Handling and thermal characteristics of briquettes [48] A.N.E. Rahman, M.M. Aziz, C.S.N. Prasad, M. Venkatesham, Influence of size and
made from fonio husk, CARD Int. J. Eng. Emerg. Scient. Discov. 2 (2) (2017) 68–90. shape on the strength of briquettes, Fuel Process. Technol. 23 (1989) 185–195.
[34] ASTM D5373 – 16, Standard Test Methods for Determination of Carbon, Hydrogen [49] J.S. Tumuluru, C.T. Wright, J.R. Hess, K.L. Kenney, A review of biomass
and Nitrogen in Analysis Samples of Coal and Carbon in Analysis Samples of Coal densification systems to develop uniform feedstock commodities for bioenergy
and Coke, ASTM International, West Conshohocken, PA, 2016. www.astm.org. application, Biofuel. Bioprod. Bioref. 5 (6) (2011) 683–707.
[35] ASTM D440-86, Standard Test Method of Drop Shaltter Test for Coal, ASTM [50] E.E. Saeidy, Technological Fundamentals of Briquetting Cotton Stalks as a Biofuel,
International, West Conshohocken, PA, 1986. www.astm.org. 2004.
[36] G.C. Wakchauere, I. Mani, Thermal and storage characteristics of biomass [51] S. Eriksson, M. Prior, The Briquetting of Agricultural Wastes for Fuel (No. 11), Food
briquettes with organic binders, J. Agric. Eng. 48 (4) (2011) 43–53. and Agriculture Organization of the United Nations, 1990.
[37] ASTM International, Standard Test Method of Compressive Strength of wood, ASTM [52] P. Krizan, Research of factors influencing the quality of wood briquettes, Acta
standard D2166-85, West Conshohocken, 2008. Montan. Slov. Rocn. 12 (3) (2007) 223–230.
[38] J.T. Oladeji, Fuel characterization of briquettes produced from corcob and rice husk [53] J.A. Lindley, M. Vossoughi, Physical properties of biomass briquets, Trans. ASAE 32
residues, Spec. J. Sci. Technol. 2 (1) (2010) Pp101–106. (2) (1989) 361–366.
[39] T.U. Onuegbu, U.E. Ekpunobi, I.M. Ogbu, M.O. Ekeoma, F.O. Obumselu, [54] E.F. Aransiola, T.F. Oyewusi, J.A. Osunbitan, L.A.O. Ogunjimi, Effect of binder type,
Comparative studies of ignition time and water boiling test of coal and biomass binder concentration and compacting pressure on some physical properties of
briquettes blend, IJRRAS 7 (2) (2011) 153–159. carbonized corncob briquette, Energy Rep. 5 (2019) 909–918.
[40] T. Rajaseenivasen, K. Scrithar, Performance investigation on solar still with circular [55] B. Gunther, K. Gebaner, R. Barkowski, M. Rosenthal, C. Bues, Calorific value of
and square fins in basin with co2 mitigation and economic analysis, Desalination selected wood species and wood products, Eur. J. Wood Prod. 70 (2012) 755–757.
380 (2016) 66–74. [56] R.M. Davies, D.S. Abolude, Ignition and burning rate of water hyacinth briquettes,
[41] J. Werther, M. Saenger, E.U. Hartge, T. Ogada, Z. Siagi, Combustion of agricultural J. Scient. Res. Rep. (2013) 111–120.
residues, Prog. Energy Combust. Sci. 26 (1) (2000) 1–27. [57] V.I.O. Ndirika, Evaluation of a baking oven using charcoal as source of energy,
[42] S. Eriksson, M. Prior, The Briquetting of Agriculture of Agricultural Wastes for Fuel, Development and Performance,. Nig. J. Renew. Energy 12 (1998) 83–91.
Food and Agricultural Organization Publication, 1990, 1990. [58] D.B. Yahaya, T.G. Ibrahim, Development of rice husk briquettes for use as fuel, Res.
[43] T. Chungcharoen, N. Srisang, Preparation and characterization of fuel briquettes J. Eng. Appl. Sci. 1 (2) (2012) 130–133.
made from dual agricultural waste: cashew nut shells and areca nuts, J. Clean. Prod. [59] D.S.C. Spencer, M.V.K. Sivakumar, Pearl Millet in African Agriculture, in:
256 (2020) 120434. Proceedings of the International Pearl Millet Workshop, 7-11 April 1986, ICRISAT
[44] ISO 17225 7, 2014. In: Solid BiofuelsdFuel Specifications and ClassesdPart 7: Center, India, 1986 (Accessed 5 July 2021).
Graded Non-woody Briquettes. ISO, Geneva, Switzerland. .