Abstract
The United States Environmental Protection Agency (EPA) has submitted that using
conventional energy (fossil fuels) has contributed to about 65 percent of worldwide greenhouse
gas emissions. Recently, the Intergovernmental Panel on Climate Change (IPCC) reported and
emphasized the urgent need to shift dramatically from conventional energy to environmentally
friendly energy to evade increasing global mean temperature and greenhouse gas emissions from
fossil fuel consumption. Renewable energy was suggested to mitigate climate change, and every
nation should actively and instantaneously key into it. Renewable energy sources include hydro,
wind, solar, geothermal, and biomass. The production of energy from biomass is the most
convenient and efficient way of providing power, even in developing nations. This is because of
the abundance of biomass feedstock for energy transformation processes and the favourable
economics offered by biomass. Biomass, such as agricultural residues, is promising for
sustainable energy generation. The agricultural residues are all biological byproducts generated
from farm activities. Agricultural residues generated are often dumped as wastes because of the
near absence of processing and ignorance of their energy potentials. The major challenges of
biomass for energy generation are related to feedstock properties, including particle sizes and
shapes and its thermal, chemical, and combustion characteristics. These challenges can be
mitigated through densification and thermochemical treatment such as torrefaction and
carbonization.
This research manufactured composite fuel briquettes from torrified agricultural residues as a
substitute for fossil fuels and waste management techniques for the people. The manufactured
briquettes were characterized to establish their physical, mechanical, thermal, and combustion
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properties. Their suitability for industrial and domestic applications was also analyzed. The
biomasses used for this research are corncob, rice husk, and banana stalk, while gelatinized
cassava starch was utilized as a binding agent. The feedstock was collected from Ilorin, Kwara
State, north-central Nigeria. The collected feedstock was sorted and sun-dried to remove surface
moisture. This was followed by preliminary characterization such as thermogravimetric,
proximate, ultimate, and calorific value analyses. The sorted feedstock was torrefied using
temperatures ranging from 200 to 380 oC and between 10-60 minutes of residence time. The
torrefaction solid yields were made to undergo densification at varying compaction pressure (50
kPa to 10 MPa) and dwelling time (2-4 minutes). The composite fuel briquettes were
manufactured at different blending ratios- 90% corncob: 10% rice husk, 80% corncob: 20% rice
husk, 70% corncob: 30% rice husk, 60% corncob: 40 % rice husk and 50% corncob: 50% rice
husk. Also, briquettes were produced at 90% corncob: 10% banana stalk, 80% corncob: 20%
banana stalk, 70% corncob: 30% banana stalk, 60% corncob: 40 % banana stalk, and 50%
corncob: 50% banana stalks. The particle size of the briquettes were 1.7-1, 1-0.5, and ≤0.5 mm.
The manufactured fuel briquettes were characterized for physical, mechanical, thermal,
chemical, and combustion properties. The raw feedstock's calorific value, moisture, ash, volatile
matter, and fixed carbon contents varied between 10.71-15.18 MJ/kg, 7.70-14.90%, 1.74-5.10%,
71.11-79.10%, 8.89-11.46%, respectively. Analyses of the results show that torrefaction yields
and their properties are influenced by torrefaction temperature and residence time. The
combustion and thermal properties increase with residence time and temperature. They varied
from 27.57-52.23%, 31.56 -44.78%, and 16.21-27.65%, respectively. Similarly, the solid, liquid,
and gas yields of the torrefied rice husk varied from 35.90-53.85%, 26.92-37.82%, and 19.23-26-
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28%. A kinetic decomposition study revealed that corncob and rice husk are thermally stable at
about 389 oC and 531oC temperatures, respectively. The feedstock torrefied at 260 oC
temperature, and 60 minutes of residence time displayed the optimum thermal and combustion
properties.
The characterization of the composite fuel briquettes revealed that torrefaction enhances the
combustion properties of the produced briquettes composite. However, the physico-mechanical
properties of the manufactured briquettes decrease with torrefaction temperature except for the
water-resistance property. It was discovered that the physico-mechanical properties of the
produced composite fuel briquettes majorly depend on the blending ratio and compaction
pressure. Higher compaction pressure gives better shatter index, compressive strength, water
resistance, durability, and high-density properties. Furthermore, results show that blending ratios
significantly influence the heating, proximate, and elemental properties of the produced
briquettes. Optimum physico-mechanical properties are achieved with briquettes made from 90%
corncob: 10% rice husk, and 90% corncob: 10% banana stalk. The empirical model developed in
this study agrees with experimental data and can be used to predict the mechanical characteristics
(compressive strength and shatter index) of composite fuel briquettes. The coefficient of
correlation 'r' of the model developed ranges from 0.961168 to 0.989456, which shows a stronger
linear relationship. The model can be used in biomass briquette design and production. The
composite briquettes manufactured in this study meet solid fuel requirements for domestic and
industrial applications.
Keywords: Agro-residues, Banana stalk, Combustion properties, Composite fuel briquettes,
Corncob, Densification, Gelatinized cassava starch, physico-mechanical properties, Proximate
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analysis, Ultimate analysis, Residence time, Rice husk, Thermal properties, Torrefaction,
Torrefaction temperature.