Renewable Energy 118 (2018) 43e55

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Renewable Energy
journal homepage: www.elsevier.com/locate/renene

Characteristics of briquettes developed from rice and coffee husks for

domestic cooking applications in Uganda
Michael Lubwama*, Vianney Andrew Yiga
Department of Mechanical Engineering, Makerere University, P.O. BOX 7062, Kampala, Uganda

a r t i c l e i n f o a b s t r a c t

Article history: The goal of this study was to develop briquettes from coffee and rice husks agricultural wastes as sus-
Received 26 December 2016 tainable fuel sources for domestic cooking applications. Clay and cassava starch were used as binders.
Received in revised form Physical properties of the coffee husks and rice husks as well as the developed briquettes were deter-
3 August 2017
mined using Thermogravimetric analysis. Higher heating value (HHV) results were determined using
Accepted 1 November 2017
bomb calorimetry. Drop test method was used to determine the mechanical and storage integrity of the
Available online 1 November 2017
developed briquettes. The results showed that the type of binder used in the development of the bri-
quettes significantly affected both their physical properties and calorific values. Average higher heating
Keywords:
Binder
values for briquettes developed with cassava starch binder ranged from 21.9 to 23.0 MJ/kg for coffee
Briquettes husks and 15.9e16.6 MJ/kg for rice husks. For coffee and rice husk briquettes developed with clay binder,
Coffee and rice husks average higher heating values ranged from 13.0 to 19.5 MJ/kg and 9.5e13.8 MJ/kg, respectively. Gener-
Physical properties ally, cassava starch binder imparted higher drop strengths (over 95%) onto the briquettes than clay
Drop strength binder material. The characteristics were influenced by the physical properties of the raw biomass
material as well as the high SiO2 ash in the clay binder.
© 2017 Elsevier Ltd. All rights reserved.

1. Introduction alternative fuels, especially for cooking so as to reduce deforesta-

tion as a result of trees being cut for both charcoal production and
Fuel for domestic cooking applications in Uganda, like most of firewood [7].
sub-Saharan Africa, is dominated by firewood (31.0%-Urban; 85.2%- One potential fuel source that is yet to be exhaustively tapped in
Rural) and charcoal (58.2%-Urban; 11.8%-Rural) (Uganda Bureau of Uganda are wastes from agricultural production and processing.
Standards [1]. This has resulted in a 46% loss in Uganda's forest These wastes can be used to produce modern energy of comparable
cover between 1990 and 2013 [2]. The effect of this has been a or better heating value than both charcoal and firewood (Govern-
change in the climatic pattern which has affected farming com- ment of Uganda [7,8]. A decade old Government of Uganda report
munities due to either extended droughts or excessive flooding and documents that over 1.2 million tons of agricultural wastes are
rainfall variability [3,4]. Uganda's deforestation rate now stands at generated annually [8]. This figure is expected to have increased as
1.8% per annum [2]. This implies that carbon dioxide (CO2) capture agricultural production has become both more commercialized and
and storage provided by forest cover will continue to substantially mechanized. Two agricultural wastes are the focus of this study: (1)
decline, hence promoting climate change and its negative impacts Rice husks; and, (2) Coffee husks. Rice has become a staple food in
[5,6]. Uganda's population is growing at a rate of 3% per annum. Uganda and coffee remains one of Uganda's main cash crops [2].
Demand for biomass energy is expected to increase in the short Therefore, their production is expected to increase for the fore-
term to match this growth. This increase in demand is typical for seeable future. Rice production has grown at a rate of over 9% per
developing countries where biomass represents 80% of the total annum [9]; whereas, coffee continues to play a leading role in the
energy supply mix. Therefore, there is an increasing need to source economy of Uganda, contributing 18% of export earnings between
the year 2000 and 2010 [10]. However, the disposal of rice and
coffee husks agricultural residues generated during processing is
problematic. The most common method of dipsosal is burning in
* Corresponding author.
open fields which has negative ecological impacts [11,12].
E-mail address: mlubwama@cedat.mak.ac.ug (M. Lubwama).

https://doi.org/10.1016/j.renene.2017.11.003
0960-1481/© 2017 Elsevier Ltd. All rights reserved.
44 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55

Therefore, the utilization of rice and coffee husks agricultural of bran and other forms of binders produced rice husk briquettes
wastes in the development of an alternative domestic cooking fuel with sufficient hardness and calorific value for cooking applica-
will: (1) reduce on the rate of deforestation for charcoal production tions. However, inclusion of clay binder during rice husk briquette
and firewood for domestic cooking applications; and, (2) enhance development had negative performance indicators including lower
waste management by the utilization of the rice and coffee husks calorific value, higher ash content, lower flame temperature and
[13]. These advantages in combination enhance climate change higher specific fuel consumption [14, 7]. investigated the effect of
mitigation and reduce environmental degradation and pollution binders, water content and bran content on physical properties of
[5,12]. Additionally, rice and coffee husks generated in Uganda have briquettes developed from rice husks. Briquettes made with rice
an energy potential of 0.58 PJ/year and 2.86 PJ/year, respectively [5]. dust (a waste by-product produced when rice kernals are milled to
Therefore, the development of rice and coffee husk briquettes produce flour) as binder had the highest durability and compres-
provides a sustainable way of utilizing this energy potential in the sive strength, while briquettes made with cassava starch as binder
production of a domestic cooking fuel that will provide potentially had the greatest density [7, 35]. characterized briquettes produced
more energy per unit volume when compared to both charcoal and from corncobs and rice husk residues. Results showed better per-
firewood [14]. formance for briquettes developed from corncobs compared to
The technological process involved in briquetting is relatively those developed from rice husks. Corncob briquettes had higher
well known [15]. Over the last decade a number of studies have volatile matter percentages (86.53) and higher heating values
demonstrated the development of briquettes from agricultural (20890 kJ/kg) whereas volatile matter and heating values for rice
wastes [16e41]. Even yet, these and more studies remain few given husk briquettes were 67.98% and 13389 kJ/kg, respectively [16].
the amount of wastes generated from agricultural production and developed activated carbon briquettes from wood and rice husks.
processing. Differences in hydrogeological conditions from one Addition of low quantities of rice husk were observed to improve
region to another imply that physical properties of agricultural the mechanical properties of the prepared briquettes. However,
wastes must be geo-specific [33,42]. Due to biomass variability, a thermogravimetric analysis indicated that the addition of rice
continuous effort must be applied to development and character- husks in briquette development decreased their combustibility [16,
ization of biomass briquettes for sustainable energy development 24]. developed briquettes from rice husk char using different
[33]. binders. Briquettes developed with starch binder showed good
Typical agrobased fibers like rice husks and coffee husks are hydrophobicity, but exhibited low volume density and mechanical
three dimensional bio-polymers composed mainly of cellulose, strength. Briquettes developed with sodium hydroxide (NaOH) as
hemicellulose and lignin [43,44]. Inorganic constituents of chemi- binder showed highest compressive strength and hydroscopicity.
cal ash compositions show the dominant presence of silicon diox- Briquettes developed with lignin and calcium hydroxide (Ca(OH)2)
ide (SiO2) and potassium oxide (K2O) [42]. A comparison of the exhibited more desirable characteristics for use as biofuels [24, 33].
organic and dominant chemical constituents between Rice husks developed composite briquettes from rice husks and corn cobs with
and coffee husks is shown in Table 1. Cellulose content is generally a mixture of starch and water as binder. Starch and water addition
higher for rice husks compared to coffee husks. Higher percentages were were required for attaining adequate briquette strength [20].
of cellulose is generally associated with a rigid structure and stiff- characterized briquettes made from rice straw with rice bran used
ness. Rice husk ash also have a higher percentage of SiO2 at 94.38% as binding material. An optimized development process using
compared to that of coffee husks at 14.65% [42]. Silica is also different types of binders has also been described [21, 29]. inves-
associated with the formation of rigid micro-structures that tigated the propertieds of briquettes made by mixing bamboo and
enhance structural stability and rigidness of the plant structure rice straw. With regards to briquettes made from coffee husks even
[44]. These differences imply that the properties of briquettes fewer studies have been done. The potential of coffee husk bri-
developed from rice and coffee husks will be affected by these quettes has been documented for countries in South America,
inherent differences in the raw materials under consideration. particularly, Brazil and Cuba [46,47]. However [48], noted that
More specifically, extremely few studies have been done on briquettes developed from coffee husks had the least durability of
briquettes developed from rice husks and coffee husks [45]. char- all the raw material sources used. These limited studies indicate
acterized and compared rice husk briquettes developed with cas- that further research should be done on briquettes developed from
sava peels and cassava starch as binders. Burning rate and water rice and coffee husks.
boiling test results indicated that rice husk briquettes combustion Therefore, in this study briquettes were developed from rice and
was improved with the use of both binders. However, properties of coffee husks using cassava starch and bentonite clay as binder
the rice husk briquettes made with cassava peels as a binder material. Cassava starch is composed of amylose and amylopectin
resulted in better performance [14]. produced briquettes from rice and both play a role in enhancing strength of briquettes following
husks in combination with rice bran using palm press fiber, palm gelatinization and retrogradation [33,49,50]. Previous studies have
press sludge and clay as binders. Briquettes with rice husks and rice utilized clay as a binder for the purpose of increasing the density
bran only were also developed. Results showed that incorporation and hardness of the developed briquette [14]. When dispersed in
water, bentonite clay breaks down into small plate-like particles
that become negatively charged on the surface and positively
charged on the edges. This unique ion exchange is responsible for
Table 1
Comparison between organic and dominant chemical ash constituents in rice husks their binding action [31]. The briquettes were developed using both
and coffee husks. low pressure after carbonization and high pressure development
techniques. Thermogravimetric analysis was used to determine the
Rice Husks Coffee Husks
physical properties and the weight loss-time profiles for the bri-
Cellulose 48.3a 19e26b
quettes. The higher heating values for the briquettes were deter-
Hemi-cellulose 31.6a 24e45b
Lignin 24.6a 18e30b mined using bomb calorimetry. Drop strength results were used to
SiO2 94.38a 14.65a evaluate the mechanical integrity and durability of the briquettes.
K2O 2.29a 52.45a The total cumulative time for ignition and water boiling were also
a
[42]. determined for application to domestic cooking.
b
[43].
 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55 45

2. Experimental strength, the briquettes were elevated up to 2 m and then dropped

onto a thick steel plate [14,56]. The ratio of the weight after drop-
2.1. Briquette development ping to the weight before dropping was recorded as the drop
strength. Drop strength is an indicator as to whether the briquettes
Coffee husks and Rice husks raw material were sun dried for will retain their form during packaging, storage and transportation
6e8 h. The dried coffee and rice husks were then fed into a car- [56,57].
bonizer. The carbonizer was made from a steel drum onto which
holes of 0.02 m diameter were inserted. The holes serve an air 2.4. Water boiling tests
regulation purpose. The carbonizer drum was of 200 L volume ca-
pacity with height 1 m and diameter 0.5 m. These drums are locally In order to evaluate the performance of the developed bri-
available on the market. Ignition of the waste raw material takes quettes for domestic cooking supply the total time taken to ignite
place from the top of the carbonizer drum after which the top of the the briquettes and boil 1 L of water using 200 g of briquettes was
drum was covered [32]. During the carbonization process these determined [32,58,59]. A traditional cook stove (locally called a
holes were covered with mud/clay to limit the amount of air ‘sigiri’ or ‘jiko’) was used.
available for complete combustion in the carbonizer as the waste
material reduces due to pyrolytic processes until bio-char was 3. Results and discussion
formed [17,24,32,51]. Schematic drawings of the briquetting pro-
cess have been presented elsewhere [32]. Structural characteristics between the pre-carbonized raw ma-
Bio-char was then measured into 1000 g portions which were terial and resulting bio-char after carbonization are very similar,
mixed with 30, 40, 50, 60 and 100 g of cassava starch binder; Bio- though more particle disintegration was observed for coffee husks
char was also measured into 1000 g portions which were mixed (See Fig. 1). This observation is not surprising due to the high
with 100, 200, 300, 400 and 500 g of clay binder. The starch binder reactivity reported for coffee during the mass loss stage charac-
was prepared by mixing 30, 40 50, 60 and 100 g of cassava flour in terized by oxidative degradation [48]. Also, coffee husks have
water and bringing to boil in order to obtain a uniform starchy higher percentages of volatile matter and lower percentages of ash
binder [7,32,52]. The clay binder was prepared by adding 100 g of content compered to rice husks which affects structural evolution
water to each of the grams of solid clay and stirring to obtain a during carbonization [42]. For rice husks similarity in the structural
uniform solution. The cassava starch and clay binder were then morphology is typical for carbonization processes for rice husk
mixed with the biochar and placed in cylindrical molds. The samples where the reaction ratio of pyrolysis is low [60].
resulting briquette was a black solid of 0.05 m diameter and 0.05 m Some samples of briquettes developed under both low pressure
height. after carbonization and high pressure techniques are shown in
For high pressure briquette development, rice and coffee husk Fig. 2. Only coffee husk briquettes were developed under high
raw material were first sun dried for 6e8 h. Materials were dried to pressure without any binding material. The development of rice
10%e15% moisture content before being fed into an automated feed husk briquettes under high pressure was unsuccessful. In previous
section that transferred it to the hopper by a screw mechanism run studies where rice husks briquettes were developed under high
by an electric motor [32]. From the hopper the raw material flows to pressure either a binder was used or heated water was applied
meet a shaft of 5.5 cm diameter and 40 cm length. The shaft is during the process [14,16,24,33]. The major limitation in the bri-
connected to a piston that gets mechanical energy from the rotary quetting of rice husks is the high ash content consisting of mainly
motion of wheels that are run by a motor at 1470 rpm. A SiO2 (see Tables 1 and 3). For the coffee husk briquettes developed
compaction pressure of 230 MPa was maintained throughout the under high pressure it was observed that the surface of the bri-
entire process. The formed briquette passed through a cooling quettes generally had a smooth texture but lateral cracks were
conveyor [32]. High pressure briquette development was used to evident along the circumferential diameter of the briquettes along
produce briquettes using coffee husks (1000 g) without any binder. their entire length. However a cross-sectional view of the coffee
Development of rice husk briquettes under high pressure husk briquettes developed under high pressure showed relatively
completely failed because of its high ash content. Processing pa- good internal bonding. The smooth surface finish, lateral cracking
rameters used in the development of rice husk and coffee husk notwithstanding, was expected as a result of the high pressure
briquettes are shown in Table 2. utilized during densification. Solid bridge bonding mechanisms are
expected to have occurred as a result of the influence of both high
2.2. Thermogravimetric analysis (TGA) and bomb calorimetry instantaneous pressure and temperature during the compression
stage of briquette development. Crystallization and chemical re-
An Eltra Thermostep Thermogravimetric analyzer was used to actions of the lignin in the coffee husk raw material due to
determine the physical properties of the developed briquettes. instantaneous high pressure and temperature ensure that solid
Thermogravimetric analysis (TGA) was used to determine the bridges are developed to permeate any gaps between the coffee
moisture content, ash content, fixed carbon and volatile matter husks [25,27]. However the formation of lateral cracks clearly in-
content for the developed briquettes [32,53]. TGA analysis was also dicates that the solidification of melted components during the
used to represent the weight loss, first derivative and temperature cooling phase was irregular from the center of the briquette to its
evolution profiles versus time for each of the rice and coffee husk circumferential exterior, hence the formation of the circumferential
briquettes developed [24,30,48,54,55]. Higher heating values for lateral cracks [27,61]. Briquettes developed after carbonization
the rice and coffee briquettes were determined using an oxygen generally had micro-pores across the entire circumferential area.
bomb calorimeter (IKA C 2000) [32]. This is expected because the pressures exerted during their for-
mation are low implying that mircro-spaces between the raw
2.3. Mechanical characterization materials are not completely filled with the binder material. This
bonding is characterized by short range forces due to intermolec-
Drop test method was used to determine the compaction ular hydrogen bonds between amylose and amylopectin compo-
integrity of the briquettes for the purpose of gaining an under- nents of starch, van der Waals' forces and mechanical locking
standing about their durability. In order to determine the drop [25,27,33,37,62].
46 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55

Table 2
Processing parameters used in the development of rice husk and coffee husk briquettes.

Raw material State of material at briquettes Briquette development pressure Binder Used Ratio of raw material to binder
development

Coffee husks Carbonized 7 MPa Cassava starch 100:3

Coffee husks Carbonized 7 MPa Cassava starch 100:4
Coffee husks Carbonized 7 MPa Cassava starch 100:5
Coffee husks Carbonized 7 MPa Cassava starch 100:6
Coffee husks Carbonized 7 MPa Cassava starch 100:10
Coffee husks Carbonized 7 MPa clay 100:10
Coffee husks Carbonized 7 MPa clay 100:20
Coffee husks Carbonized 7 MPa clay 100:30
Coffee husks Carbonized 7 MPa clay 100:40
Coffee husks Carbonized 7 MPa clay 100:50
Rice husks Carbonized 7 MPa Cassava starch 100:3
Rice husks Carbonized 7 MPa Cassava starch 100:4
Rice husks Carbonized 7 MPa Cassava starch 100:5
Rice husks Carbonized 7 MPa Cassava starch 100:6
Rice husks Carbonized 7 MPa Cassava starch 100:10
Rice husks Carbonized 7 MPa Clay 100:10
Rice husks Carbonized 7 MPa Clay 100:20
Rice husks Carbonized 7 MPa Clay 100:30
Rice husks Carbonized 7 MPa Clay 100:40
Rice husks Carbonized 7 MPa Clay 100:50
Coffee husks Non- carbonized 250 MPa e 1000:0

to the ash content of coffee and rice husks (See Table 3). A very
interesting trend in weight loss was observed for coffee husk bri-
quettes with cassava starch binder as shown in Fig. 4. The specific
weight loss during the volatization stage was much lower in
comparison to the mass loss for the coffee husk raw material.
Therefore, it is possible to attribute the reduction of volatile matter
in the briquettes to the carbonization pre-treatment process prior
to briquetting [29,67,68]. No significant difference in weight loss
was observed for coffee husk briquettes with 30e60 g of cassava
starch binder. However, for coffee husk briquettes with 100 g of
cassava starch binder a higher weight loss margin was observed
because of the lower ash content in these briquettes due to the
significance of the amount of binder present which reduces on the
ash content of these briquettes (See Table 3) [69].
The thermal degradation behaviour for rice husk briquettes with
cassava starch binder is shown in Fig. 5. The results clearly show
that the presence of cassava starch binder in the rice husk bri-
quettes and prior carbonization precesses reduce the devolatiza-
Fig. 1. Picture of rice husk and coffee husk raw material, before (a and c) and after (c tion stage significantly when compared to the thermal degradation
and d) carbonization. of rice husk raw material. However, when the thermal degradation
of the rice husk briquettes was compared to ascertain the influence
of increasing the cassava starch binder from 30 to 100 g very
In order to gain an in-depth understanding of the thermal minimal changes to weight loss were observed. This suggests that
degradation behaviour of the developed briquettes, TGA was used carbonization pre-treatment is highly responsible for the reduction
to obtain weight loss vs. time profiles as shown in Figs. 3e8. Results of weight loss during devolatilization. This behaviour is explained
for the weight loss for both coffee and rice husk raw materials are by the increase in ash content (see Table 3) as well as high level of
shown in Fig. 3a) and b) respectively. Initial weight loss of about SiO2 in rice husks inherently [42]. Heating rice husks may produce
13% and 11% were recorded for coffee and rice husk raw materials, several effects that hinder combustion such as formation of silica
respectively. This is attributable to the loss of adsorbed moisture ash, formation of silicon carbide and the strengthening of Silica-
related to the moisture content in the raw materials being expen- Carbon bonds [16].
ded [63]; [63e65]. The following rapid increase in weight loss The influence of clay binder on coffee husk and rice husk bri-
corresponded to initiation of thermal degradation of the raw ma- quettes is shown in Figs. 6 and 7, respectively. From Fig. 6 it is
terial. This weight loss corresponds to combustion of organic clearly observed that as the amount of clay binder is increased from
matter and burning of some residual char [64]. During this period 100 to 500 g, devolatization reduces. Clay itself is a mainly inor-
the drying phase changes to biomass volatization where volatile ganic material, which implies that the volatile content contribution
matter, due to hemicellulose (190  C e 320  C), cellulose (280  C e from the clay binder is very small [70]. It is also expected that the
400  C) and lignin (320  C e 450  C) decomposition, is released presence of ash in the briquettes will increase as the amount of clay
[53,54,66]. binder increases. This combination of factors combines to reduce
The measured weight losses during the develotilzation phase in on the weigth loss recorded for coffee husk briquettes with clay
Fig. 3a) and b) correspond exactly to the percentage of volatile binders during the devolatization stage. A similar trend is observed
matter in the coffee and rice husks. The residual mass corresponds for rice husk briquettes with clay binder. Devolatization is reduced
 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55 47

Fig. 2. Samples of briquettes developed under low pressure after carbonization (a) and non-carbonized under high pressure development.

further and ash content is expected to increase due to the combi- moisture adsorption which is a necessary aspect for increased shelf
nation of SiO2 in rice husks and clay binder [14,70]. The thermal life and storage of the briquettes by preventing rotting and
degradation results of the non-carbonized coffee husk briquettes decomposition [29,71]. Secondly, the developed briquettes have a
developed under high pressure (see Fig. 8) were very similar to the lower percentage of volatile matter compered to the coffee and rice
thermal degradation behaviour of coffee husk raw material. husk raw material. Low volatile matter implies that the ignitability
The results of the thermal degradation investigation tally very of the briquettes will be reduced, but once they ignite, then com-
well with the results obtained for the physical properties for rice bustion will produce little or no smoke with a clean flame [72].
and coffee husk briquettes shown in Table 3. A number of general Thridly, all of the developed briquettes had higher percentages of
observations can be made. Firstly, the moisture content in both the ash content compared to their respective parent raw material.
coffee and rice husk briquettes with both cassava starch and clay Fourthly, all of the briquettes developed after carbonization have
binder is lower than the moisture content percentage in coffee and higher fixed carbon percentages as a result of volatization processes
rice husk raw material. Carbonized briquettes adsorb less moisture that occur during pyrolysis and reactions between steam and car-
due to the destruction of hydroxyl groups, which are hydrophilic, in bon leading to elimination of heteroatoms and an increment in the
the carbonization process [29]. The carbonization process inhibits relative amount of ash [16]. These results are similar to results
48 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55

Table 3
Physical properties (Average ± standard deviation) for coffee and rice husk raw materials and coffee and rice husk briquettes with cassava starch and clay binder.

Sample Binder (Amount) % Moisture Content % Volatile Matter % Analytical ash % Volatiles on dry basis % Ash on dry basis % Fixed Carbon

Coffee Husks e 13.0 ± 0.6 65.4 ± 1.0 5.9 ± 0.3 73.9 ± 1.1 6.6 ± 0.4 15.7 ± 1.4
Rice Husks e 11.3 ± 0.0 56.4 ± 1.1 18.1 ± 0.1 62.8 ± 1.2 20.1 ± 0.2 14.2 ± 0.9
Coffee husks briquettes Cassava (30 g) 9.0 ± 0.5 29.6 ± 0.3 15.1 ± 0.6 32.2 ± 0.2 16.4 ± 0.6 46.4 ± 0.7
Coffee husks briquettes Cassava (40 g) 8.5 ± 0.9 30.8 ± 0.5 15.5 ± 0.6 33.4 ± 0.8 16.8 ± 0.5 45.2 ± 0.8
Coffee husks briquettes Cassava (50 g) 9.3 ± 0.2 31.3 ± 0.7 16.3 ± 1.1 34.2 ± 0.8 17.8 ± 1.2 43.1 ± 1.6
Coffee husks briquettes Cassava (60 g) 9.2 ± 0.3 30.9 ± 0.5 16.4 ± 2.8 33.8 ± 0.6 17.9 ± 3.0 43.4 ± 2.1
Coffee husks briquettes Cassava (100 g) 9.7 ± 0.0 36.9 ± 0.8 12.6 ± 0.7 40.5 ± 0.8 13.9 ± 0.8 40.7 ± 1.2
Rice husks briquettes Cassava (30 g) 5.8 ± 0.1 18.2 ± 0.7 38.7 ± 1.5 19.2 ± 0.7 40.9 ± 1.5 37.3 ± 0.7
Rice husks briquettes Cassava (40 g) 5.5 ± 0.0 21.2 ± 2.4 38.4 ± 2.3 21.6 ± 1.4 39.2 ± 0.5 38.3 ± 0.1
Rice husks briquettes Cassava (50 g) 6.2 ± 0.0 19.7 ± 0.5 36.6 ± 0.2 20.9 ± 0.5 38.9 ± 0.2 37.4 ± 0.3
Rice husks briquettes Cassava (60 g) 6.3 ± 0.1 20.9 ± 1.2 36.3 ± 0.1 22.3 ± 1.3 38.6 ± 0.1 36.4 ± 1.4
Rice husks briquettes Cassava (100 g) 6.1 ± 0.3 23.1 ± 0.7 36.9 ± 2.3 24.5 ± 0.8 39.2 ± 2.3 33.9 ± 1.3
Coffee husks briquettes Clay (100 g) 8.8 ± 0.6 30.2 ± 0.7 22.8 ± 1.3 32.9 ± 1.0 24.8 ± 1.5 38.1 ± 2.7
Coffee husks briquettes Clay (200 g) 8.2 ± 0.8 24.7 ± 0.9 32.2 ± 2.0 26.7 ± 1.0 34.8 ± 2.1 35.0 ± 1.1
Coffee husks briquettes Clay (300 g) 7.5 ± 0.2 23.3 ± 1.3 40.1 ± 3.6 25.1 ± 1.4 43.1 ± 3.8 29.0 ± 2.2
Coffee husks briquettes Clay (400 g) 6.5 ± 0.5 22.1 ± 1.7 47.4 ± 3.7 23.6 ± 1.7 50.5 ± 4.1 23.9 ± 4.7
Coffee husks briquettes Clay (500 g) 7.0 ± 0.4 21.8 ± 0.5 47.6 ± 2.2 23.3 ± 0.7 50.9 ± 2.1 23.7 ± 1.4
Rice husks briquettes Clay (100 g) 5.5 ± 0.6 15.7 ± 2.2 50.5 ± 2.3 16.6 ± 2.4 53.3 ± 2.2 28.3 ± 1.8
Rice husks briquettes Clay (200 g) 5.2 ± 0.0 14.4 ± 0.3 48.2 ± 1.0 15.2 ± 0.3 50.7 ± 1.1 32.2 ± 1.0
Rice husks briquettes Clay (300 g) 5.0 ± 0.2 14.0 ± 0.4 54.6 ± 2.7 14.6 ± 0.4 57.3 ± 2.7 26.5 ± 2.2
Rice husks briquettes Clay (400 g) 5.0 ± 0.2 13.7 ± 0.6 56.5 ± 4.2 14.4 ± 0.6 59.3 ± 4.4 24.8 ± 3.4
Rice husks briquettes Clay (500 g) 4.6 ± 0.2 12.7 ± 0.2 60.6 ± 1.3 13.3 ± 0.2 63.4 ± 1.3 22.1 ± 1.3

Fig. 3. Weight loss, first derivative and temperature build up for TGA analysis of coffee husk (a) and rice husk (b) raw material.

obtained in previous studies where rice and coffee husk briquettes obtained for coffee husk briquettes with cassava binder at 23 MJ/kg.
were developed [7,14,16,33]. The results suggest that as the amount of cassava starch binder is
More specific observations can be made from Table 3. Moisture increased to 100 g, the heating value drops to an average of 21.0 MJ/
content is lower for the rice husk briquettes compared to the coffee kg. This value is still higher than the heating value recorded for all
husk briquettes irrespective of the binder material used. Also, of the other briquettes. Cassava flour has low amounts of ash
volatile matter percentage was higher for coffee husk briquettes content [69]. However as the amount of cassava starch binder is
with both cassava starch and clay binder compared to rice husk increased then a cumulative increment in the amount of ash is
briquettes with cassava starch and clay binder. These results are expected. Hence the drop in heating value for coffee husk bri-
directly related to the moisture content and volatile matter in the quettes when 100 g of cassava starch binder is considered. Coffee
parent raw material as shown in Table 3. The percentage of ash is husk briquettes with 100 g and 200 g of clay binder had the next
highest for rice husk briquettes with clay binder. Although ash average higher heating values at 19.5 MJ/kg and 17.2 MJ/kg. These
content was also high for rice husk briquettes with cassava binder results imply that utilization of up to 200 g of clay binder in coffee
and coffee husk briquettes with clay binder. This is due to SiO2 ash husk briquettes could be acceptable for domestic cooking applica-
in both rice husks and clay binder [14,70]. The highest percentage tions with greater values of clay binder not very useful [70]. These
of fixed carbon was observed for coffee husk briquettes with cas- results were followed by rice husk briquettes with cassava starch as
save starch binder followed by rice husk briquettes with cassava binder. The lowest values for heating values were obtained for rice
starch binder. Briquettes developed with clay binder had lower husk briquettes with clay binder. This is due to the high levels of ash
percentages of fixed carbon. This result is similar to other studies content in these briquettes which negatively affects their energy
where clay binder was also used [70]. However, the presence of the content [14,16,70]. The HHV for non-carbonized coffee husk
clay binder may be advantageous in prolonging cooking time by its briquette developed under high pressure was 15.2 MJ/kg. This im-
low heat release and fuel saving effects [70]. plies that it is possible to develop low cost carbonized briquettes
The results for the heating values for the developed briquettes that have higher energy content than more costly high pressurized
are shown in Table 4. The highest heating value (HHV) results were briquettes. This result is of extreme importance in sub-Saharan
 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55 49

Fig. 4. Weight loss, first derivative and temperature build up for TGA analysis of coffee husk briquettes with 30 g (a), 40 g (b), 50 g (c), 60 g (d) and 100 g (e), of cassava starch binder.

Fig. 5. Weight loss, first derivative and temperature build up for TGA analysis for rice husk briquettes with 30 g (a), 40 g (b), 50 g (c), 60 g (d) and 100 g (e), of cassava starch binder.

countries where cost limitations and lack of data are noted as showed that there was a high level of significance (p < 0.05) in the
reasons for low uptake of briquette production [34]. relationship between binder proportion and most response factors.
Nested single factor analysis of variance (ANOVA) was used to This included results for the effect of binder proportion of fixed
determine the statistical significance of the effect of binder pro- carbon and volatile matter percentages in both coffee and rice husk
portion on the physical properties and HHV. Statistical analysis briquettes with cassava starch and clay binders where all P-values
50 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55

Fig. 6. Weight loss, first derivative and temperature build up for TGA analysis for coffee husk briquettes with 100 g (a), 200 g (b), 300 g (c), 400 g (d) and 500 g (e), of clay binder.

Fig. 7. Weight loss, first derivative and temperature build up for TGA analysis for rice husk briquettes with 100 g (a), 200 g (b), 300 g (c), 400 g (d) and 500 g (e), of clay binder.

were below 0.05 at 95% confidence interval (see Table 5). High binder; Ash content in rice husk briquettes with both cassava starch
levels of significance were also shown for the effect of binder and clay binder; and, HHV for coffee husk briquettes with clay
proportion on; Moisture content in both coffee and rice husk bri- binder. The lack of statistical significance in the effect of binder
quettes with clay binder; Ash content in coffee briquettes with clay proportion to HHV for all of the developed briquettes, except for
 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55 51

by the loss of the individual crystalline structure of the two com-

ponents [62]. This leads to the formation of a viscous solution that
undergoes retrogradation during cooling or storage. The viscosity
of hydrated starch increases its shear and tensile strengths and
gives it the ability to occupy void spaces present within and be-
tween biomass particles, thus forming solid briges that increase in
strength during air cooling and storage [33,62]. It is also observable
that the drop strength increases as the amount of clay binder is
increased in both coffee and rice husk briquettes. This was also
noted by Ref. [14]. However a marginally better drop strength
performance was observed for rice husk briquettes with clay binder
than coffee husk briquettes with clay binder. This is due to Silica in
the rice husk, which is also associated with the formation of rigid
micro-structures that enhance structural stability and rigidness of
the plant structure [44]. The drop strength for the non-carbonized
coffee briquette developed under high pressure was 90% and less
than the drop strength for the carbonized briquettes. This indicates
that the pressure applied during densification was inadequate for
the complete formation of solid bridges to occur. The circumfer-
ential crack observed on the surface of the non-carbonized bri-
quettes were obvious points for failure to occure once the drop test
Fig. 8. Weight loss, first derivative and temperature build up for TGA analysis of non- was applied [47]. developed coffee husk briquettes using a vertical
carbonized high pressure developed coffee husk briquettes. 10 tonne hydraulic press for applying a load to a heated die of
80 mm internal diameter and 140 mm length in which the
briquette was formed. This implies that if no binder is to be used
coffee husks with clay binder, is due to comparable energy content then considerations on application of heat, extremely high loads or
of the developed briquettes. There was no statistical significance for a combination of both have to be made.
the effect of binder proportion on moisture content for rice and The results for the total ignition and water boiling time for 1 L of
coffee husk briquettes with cassava starch binder as well as ash water using 200 g of developed briquette are shown in Table 4. The
content for coffee husks with cassava starch binder. This is similar tests were performed using a traditional cook stove because they
to results obtained by Refs. [59,73]. are majorly used for domestic cooking applications in Uganda due
The drop strength results for the developed briquettes are to the perceived notion that improved cook stoves are expensive
shown in Fig. 9. The results clearly indicate that briquettes devel- despite efforts to enhance their diffusion among rural communities
oped with cassava starch as binder had higher drop strengths (over [74]. Additionally, performance of field studies of improved cook
94%) than the briquettes developed with clay binder. This implies stoves are inconclusive mainly due to the numerous designs on the
that the increment of starch provided by the cassava starch binder market [75]. Lowest total times for ignition and boiling 1 L of water
allowed for better compaction. Starch is a polysaccharide and is a were observed for coffee husk briquettes with 60 g cassava starch
good binding agent due to its chemical and structural properties binder at 13 min and rice husk briquettes with 100 g clay binder at
[33]. Addition of water and heat to starch granules causes swelling, 15 min [14]. noted that 4 kg of rice husk char boiled 5 L of water but
which results in the formation of intermolecular hydrogen bonds were unable to boil 10 L even after 1 h [59]. recorded times of be-
between amylose and amylopectin components of starch, followed tween 31.5 and 52.5 min to boil 8e10 L of water in 2 L intervals

Table 4
HHV (Average ± standard deviation) for Coffee husk and rice husk briquettes with different amounts of cassava starch and clay binder.

Briquette Binder (Amount) Higher Heating Value (HHV) MJ/Kg Total Time taken to ignite and boil
1 l of water (minutes)

Coffee husks briquettes Cassava (30 g) 23.0 ± 0.8 40

Coffee husks briquettes Cassava (40 g) 23.5 ± 0.5 34
Coffee husks briquettes Cassava (50 g) 22.0 ± 0.1 20
Coffee husks briquettes Cassava (60 g) 22.6 ± 1.4 13
Coffee husks briquettes Cassava (100 g) 21.9 ± 0.9 17
Rice husks briquettes Cassava (30 g) 16.6 ± 0.0 23
Rice husks briquettes Cassava (40 g) 16.5 ± 0.1 35
Rice husks briquettes Cassava (50 g) 16.4 ± 0.0 45
Rice husks briquettes Cassava (60 g) 15.9 ± 0.9 32
Rice husks briquettes Cassava (100 g) 16.4 ± 0.1 18
Coffee husks briquettes Clay (100 g) 19.5 ± 0.6 26
Coffee husks briquettes Clay (200 g) 17.2 ± 0.4 15
Coffee husks briquettes Clay (300 g) 13.8 ± 1.4 29
Coffee husks briquettes Clay (400 g) 13.0 ± 1.3 25
Coffee husks briquettes Clay (500 g) 13.6 ± 0.9 24
Rice husks briquettes Clay (100 g) 12.0 ± 0.2 15
Rice husks briquettes Clay (200 g) 13.8 ± 0.3 36
Rice husks briquettes Clay (300 g) 12.0 ± 0.3 43
Rice husks briquettes Clay (400 g) 10.2 ± 0.8 44
Rice husks briquettes Clay (500 g) 9.5 ± 1.1 44
Non-carbonized coffee briquettes e 15.2 ± 0.4
52 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55

Table 5
Nested ANOVA for response factors including physical properties and HHV.

Type of briquette Type of binder Response Factor Source of Variation SS df MS F P-value

Coffee husks Cassava starch Fixed Carbon Binder proportion 71.411956 4 17.852989 6.126045363 0.009305478
Error 29.14276331 10 2.9142763
Total 100.5547193 14
Coffee husks Clay Fixed Carbon Binder proportion 539.796916 4 134.949229 19.29209234 0.000107824
Error 69.9505407 10 6.995054067
Total 609.747457 14
Rice husks Cassava starch Fixed Carbon Binder proportion 228.2362 4 57.05904 5.003889 0.01779
Error 114.0294 10 11.40294
Total 342.2656 14
Rice husks Clay Fixed Carbon Binder proportion 201.7832 4 50.4458 8.426443 0.003045
Error 59.86607 10 5.986607
Total 261.6493 14
Coffee husks Cassava starch Moisture Content Binder proportion 13.40067 4 3.350167 2.159718 0.14747
Error 15.51206 10 1.551206
Total 28.91273 14
Coffee Husks Clay Moisture Content Binder proportion 11.51441 4 2.878603 9.893125 0.001667
Error 2.9097 10 0.29097
Total 14.42411 14
Rice husks Cassava starch Moisture Content Binder proportion 0.765711 4 0.191428 2.994326 0.072652
Error 0.639302 10 0.06393
Total 1.405013 14
Rice husks Clay Moisture Content Binder proportion 3.095791 4 0.773948 10.11524 0.001531
Error 0.765131 10 0.076513
Total 3.860921 14
Coffee husks Cassava starch Volatile Matter Binder proportion 96.92809 4 24.23202 71.31192 2.60E-07
Error 3.398033 10 0.339803
Total 100.3261 14
Coffee husks Clay Volatile Matter Binder proportion 147.0785 4 36.76962 31.79872 1.16E-05
Error 11.56324 10 1.156324
Total 158.6417 14
Rice husks Cassava starch Volatile Matter Binder proportion 39.19587 4 9.798968 17.37456 0.000169
Error 5.639835 10 0.563984
Total 44.83571 14
Rice husks Clay Volatile Matter Binder proportion 33.62506 4 8.406266 21.00183 7.45E-05
Error 4.002635 10 0.400264
Total 37.6277 14
Coffee husks Cassava starch Ash Content Binder proportion 27.62444 4 6.90611 3.441818 0.051375
Error 20.06529 10 2.006529
Total 47.68974 14
Coffee husks Clay Ash Content Binder proportion 1322.798 4 330.6995 45.39514 2.22E-06
Error 72.84909 10 7.284909
Total 1395.647 14
Rice husks Cassava starch Ash Content Binder proportion 195.2793 4 48.81983 5.71265 0.011705
Error 85.45916 10 8.545916
Total 280.7385 14
Rice husks Clay Ash Content Binder proportion 371.5774 4 92.89434 13.77019 0.000448
Error 67.46048 10 6.746048
Total 439.0379 14
Coffee husks Cassava starch HHV (J/g) Binder proportion 6798209 4 1699552 2.400206 0.181436
Error 3540430 5 708085.9
Total 10338638 9
Coffee husks Clay HHV (J/g) Binder proportion 64106357 4 16026589 15.67081 0.004918
Error 5113515 5 1022703
Total 69219872 9
Rice husks Cassava starch HHV (J/g) Binder proportion 479637 4 119909.3 0.753082 0.596772
Within Groups 796123 5 159224.6
Total 1275760 9
Rice husks Clay HHV (J/g) Binder proportion 22952799 4 5738200 4.813435 0.057593
Within Groups 5960608 5 1192122
Total 28913406 9

using briquettes developed from matooke peels. The results ob- amounts of briquettes for cooking which translates into savings in
tained in the water boiling test were influenced by the amount of domestic energy use.
briquettes used that were less than what is described in other
similar studies. This meant that the total cumulative heat energy 4. Conclusions
generated was less. The use of a traditional cook stove in order to
align the study with what consumers actually use also affected the The utilization of agricultural wastes is very important for sus-
results because traditional cook stoves do not conserve heat and tainability of domestic cooking fuels in Uganda and sub-Saharan
heat loss due to conduction, convection and radiation occurs much Africa. This study investigated the physical properties, thermal
faster to the environment. However, the results for total times for degradation weight loss behaviour, heating values and drop
ignition and water boiling highlight the possibility of applying less strengths of briquettes developed from rice and coffee husk
 M. Lubwama, V.A. Yiga / Renewable Energy 118 (2018) 43e55 53

100

90

80

70

Drop Strength (%)

60

50

40

30

20

10

Fig. 9. Drop strength for different developed briquettes.

agricultural wastes with cassava starch and clay as binders. Thermal for ignition and boiling 1 L of water were observed for coffee husk
degradation results showed a decrease in mass loss for all of the briquettes with 60 g cassava starch binder and rice husk briquettes
developed carbonized briquettes. The physical properties showed with 100 g clay binder. The results for total times for ignition and
that the moisture content and volatile matter in the carbonized water boiling highlight the possibility of applying less amounts of
briquettes were lower than the moisture content percentage in briquettes for cooking which translates into savings in domestic
coffee and rice husk raw material. All of the carbonized briquettes energy use.
had higher percentages of ash content and fixed carbon compared
to their respective parent raw material. These results were due to Acknowledgements
devolatization processes having occurred during pyrolysis pre-
treatment prior to briquette development. The highest heating The authors would like to acknowledge the research grant
value results were obtained for coffee husk briquettes with cassava provided by International Foundation for Science (IFS) (Grant
binder at 23 MJ/kg. Also, as the amount of cassava starch binder is Number: C/5663-1) that facilitated the research presented in this
increased to 100 g, the heating value drops to an average of 21.0 MJ/ paper.
kg. Coffee husk briquettes with 100 g and 200 g of clay binder had
the next average higher heating values at 19.5 MJ/kg and 17.2 MJ/kg.
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