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Physico-mechanical Properties of Composite Briquettes from Corncob and Rice Husk

  • Densification of agricultural residues into briquettes as the alternative renewable feedstock can improve their physico-mechanical and storage properties as solid fuels. This paper presents the physico-mechanical properties of the composite briquettes made from corncob and rice husk. Raw samples of corncob and rice husk were collected, sorted and pulverised into fines of 0.25, 1.00 and 1.75 mm particle sizes. The fines were blended at mixing ratios of 80:20, 70:30, 60:40, and 50:50, bonded with 5% starch on weight percentage basis and compressed at compaction pressures of 25, 50, and 65 kPa to produce the briquette samples. The briquette made from 80:20 ratio of corncob to rice husk, 0.25 mm particle size and 65 kPa pressure exhibited the highest compressive strength of 111 kN/m2 and the least of 39 kN/m2 from briquette with 50:50 ratio of corncob to rice husk, 1.75 mm particle size and 25 kPa pressure. The briquette made from 50:50 ratio of corncob to rice husk, 0.25 mm particle size and 65 kPa pressure had the highest water resistance capacity, and the least from briquette of 80:20 ratio of corncob to rice husk, 1.75 mm particle size and 25 kPa pressure. The resulting physico-mechanical qualities of the produced corncob and rice husk briquettes suggested that they could be used as the solid fuels for domestic and industrial applications.
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Physico-mechanical Properties of Composite Briquettes from Corncob and Rice Husk

    Corresponding author: H A AJIMOTOKAN, hajims@unilorin.edu.ng
  • 1. Department of Mechanical Engineering, University of Ilorin, Ilorin, Nigeria
  • 2. Department of Materials and Metallurgical Engineering, University of Ilorin, Ilorin, Nigeria

Abstract: Densification of agricultural residues into briquettes as the alternative renewable feedstock can improve their physico-mechanical and storage properties as solid fuels. This paper presents the physico-mechanical properties of the composite briquettes made from corncob and rice husk. Raw samples of corncob and rice husk were collected, sorted and pulverised into fines of 0.25, 1.00 and 1.75 mm particle sizes. The fines were blended at mixing ratios of 80:20, 70:30, 60:40, and 50:50, bonded with 5% starch on weight percentage basis and compressed at compaction pressures of 25, 50, and 65 kPa to produce the briquette samples. The briquette made from 80:20 ratio of corncob to rice husk, 0.25 mm particle size and 65 kPa pressure exhibited the highest compressive strength of 111 kN/m2 and the least of 39 kN/m2 from briquette with 50:50 ratio of corncob to rice husk, 1.75 mm particle size and 25 kPa pressure. The briquette made from 50:50 ratio of corncob to rice husk, 0.25 mm particle size and 65 kPa pressure had the highest water resistance capacity, and the least from briquette of 80:20 ratio of corncob to rice husk, 1.75 mm particle size and 25 kPa pressure. The resulting physico-mechanical qualities of the produced corncob and rice husk briquettes suggested that they could be used as the solid fuels for domestic and industrial applications.

1.   Introduction
  • Fossil fuel has been largely utilized since its discovery as a major energy source for the global energy demands (Olorunsola, 2007). The uses of these fuels like coal, crude oil and natural gas, have increased rapidly to satisfy the rising energy demand due to the global population growth, rapid industrialization and urbanization (Olorunsola, 2007; Ajimotokan, 2014; Obi et al., 2016). With the growing concern over the Earth's limited fossil fuel reserves and its market volatility, greenhouse gas emission and environmental challenges (Ajimotokan, 2014; 2015), there are renewed interests in the alternative renewable energy sources for domestic and industrial applications. Among these, biomass is the promising sustainable energy source that can serve as a substitute to the non-renewable fossil fuel due to their global availability and renewability nature (Soponpongpipat et al., 2015), which contributes to about 14% of the world energy consumption (Rabiu et al., 2018). Biomasses, in particular, agricultural residues such as bagasse, husks, stalks, shells, and cobs have gained prominence as one of the widely utilized renewable energy sources and they contributed to the affordability and security of energy supply (Szyszlak-Barglowicz et al., 2012; Eddine et al., 2017).

    As an alternative energy source, agricultural residues could be used either directly as solid fuel through combustion, or harnessed and transformed through densification for domestic and industrial applications (Ibitoye, 2018). However, only a small proportion of these agricultural residues are being used as solid fuel in sustainable energy solutions since they are bulky and contain high moisture, which make them difficult to use in their raw form (Wilaipon, 2007; Ibitoye, 2018). These inherent properties make agricultural residues not readily available as the excellent sources of solid fuel, thus their densification; that is, briquetting, palletization or cubing into solid fuel to improve their densities, handling, storage and combustion properties, and overcome the logistic economics in sustainable energy solutions (Kaliyan et al., 2009; Amoo et al., 2012; Rabiu et al., 2018). The densification process that is concerned with increasing the density of biomass residues to nearly 1000–1200 kg/m3 of loose biomasses, reducing the volume by 8–10 times is known as briquetting (Wakchaure et al., 2009; Obi et al., 2016). This densification process of making briquettes demonstrates the potential of appropriate technology for producing solid fuel from forest and agricultural residues (Akowuah et al., 1996; Dasappa et al., 2011). For domestic and industrial heating purposes, the practice includes the use of briquettes of biomass residues from agricultural produces, food industry, or combinations of different types of plant residues with other additives (Gil et al., 2010; Stelte et al., 2011; Raslavičius, 2012; Mitchual et al., 2014; Oladeji, 2015). Among the alternative renewable energy sources, agricultural residues from all renewable energy sources are expected to be one of the utmost beneficial energy sources in the near future as studies have shown that the world production of agricultural residues is projected to be approximately 2x1012 t per year (Asonja et al., 2017).

    Literature is replete with several research findings on briquetting of corncob or rice husk while little published research on composite briquettes was made from a blend of both corncob and risk husk. For instance, Oladeji (2010) investigated the comparative characterisation of the corncob and rice husk briquettes. The properties of the produced briquette samples were investigated to evaluate which of the two agricultural residues can be used to gain comparative advantages as solid fuel. The briquettes produced from each of these two residues were reported to have good properties as solid fuels, but the corncob briquette has better physico-mechanical properties than the rice husk briquette because it has a relatively moderate moisture content, higher density and lower relaxation ratio among others. Obi et al. (2016) characterized briquettes from blends of rice husk and palm oil mill sludge (POMS). The increase in the percentage of the POMS in the blends was reported to influence the physical and combustion properties of the briquettes significantly. However, there is a need to carry out physico-mechanical characterization of briquettes made from biomass residues and their blends to determine their suitability for production of solid fuels, which can comparatively replace charcoal and wood that their use is leading to deforestation and erosion. Thus, this study investigated the physico- mechanical properties of composite briquettes made from blends of the corncob and rice husk. Selected fundamental physico-mechanical properties of briquettes such as compressive strength, durability, and green and relaxed density among others were evaluated.

2.   Methodology
  • Raw samples of corncobs were obtained from the corn farms at the University of Ilorin Teaching and Agriculture farm, and the rice husk was obtained from the rice farms at Ganmo, Ilorin, Nigeria. The starch used as the binder was obtained from the cassava processing factory at Gaa- Akanbi, Ilorin, Nigeria. The raw samples of the corncob and rice husk were sorted and pulverized into fines of 0.25, 1.00 and 1.75 mm particle sizes. The fines were blended at mixing ratios of 80:20, 70:30, 60:40, and 50:50 respectively, and stored separately. Afterwards, the blended fines were bonded with 5% starch on weight percentage basis and compressed at compaction pressures of 25, 50, and 65 kPa respectively to produce the briquette samples, and their physico-mechanical properties were determined.

  • The raw samples of the corncobs and rice husks were sorted to remove every form dirt such as sand, stone, and plant residues, and then sun-dried for three days to reduce the moisture content to about 12% (Asamoah et al., 2016). The raw samples were then pulverized into fines and screened to 0.25, 1.00 and 1.75 mm particle sizes respectively in accordance with the BS EN 15149-2 standard (BS EN 15148. Solid biofuels. Method for the determination of the content of volatile matter, 2009), followed by pulverization using a 3730 W hammer mill. Afterwards, the processed biomass was stored separately in a zip-locked polythene bag for later processes to prevent them from absorbing atmospheric moisture. The 270 mL of boiling portable water was employed to gelatinize a thoroughly mixed 130 g of starch with 180 mL portable water to form a uniform jelly-like starch gel.

  • The corncob and rice husk fines were weighed using the citizen electronic weighing balance (Model MP5000 with 0.001 g resolution) and blended at different mixing ratios of 80:20, 70:30, 60:40, and 50:50, respectively. The blends of the corncob and rice husk were mixed with the binder (starch gel) using an electric mixer at 100 r/min. The weight percentage of the binder in the briquette samples was 5%, which was constant at different mixing ratios and particle sizes. The feedstock was then poured into prepared moulds after blending. The compaction of the feedstock was done using 1560 kN hydraulic jack machine (Model: EL31 072) at the Department of Civil Engineering, University of Ilorin, Ilorin, Nigeria. The holding time of each briquette was 120 s and the samples were produced in duplicate at three different compaction pressures. Immediately, after the ejection of the briquettes from the moulds, the mass and dimensions were taken using a digital weighing balance and vernier caliper, respectively. The briquettes were then sun-dried for five days to remove moisture and then cured at room temperature. Figure 1 shows the samples of the produced briquette from different biomass particle sizes.

    Figure 1.  Produced briquettes samples, showing briquette samples of particle size 0.25 mm (a), 1.00 mm (b), and 1.75 mm (c)

  • The physical and mechanical properties investigated include the compressive strength, green and relaxed density, porosity index, durability and water resistance.

  • The compressive strength or cold crushing strength of the produced briquettes; i.e., the maximum crushing load the briquettes can withstand before failure, was determined by using a universal strength-testing machine of 100 kN capacity (Model: Testometric FS5080) with standard method ASTM D2166-85 (ASTMD2166, Test Method for Unconfined Compressive Strength of Cohesive Soil, 2016). The test was carried out 21 d after briquetting when the briquettes must have attained their maximum strength. The peak stress (compressive strength) displayed at the end of each test was taken. For each of this test, the experiment was conducted in duplicate.

  • The green or compressed density was determined immediately after the produced briquette samples were ejected from the mould. The mass and dimensions were measured using a digital weighing balance and vernier caliper, respectively. The measured values obtained immediately after the ejection of briquettes were used to determine the green density, while the values obtained after being sun-dried for 30 d were used to determine the relaxed density of the briquettes following the ISO 3131 standard (ISO 3131, International Standard Test Method for Density of Regular Solids, 1975). The density of the produced briquette samples is expressed as Equation (1):

    where m is the mass of the produced briquette and v is the volume of the produced briquette. The volume v of the produced briquette is expressed as Equation (2):

    where h is the height of the produced briquettes, R is the external radius of the briquettes and r is the internal radius of the briquettes.

  • The relaxation ratio is the ratio of maximum density to relaxed densy and it is expressed as Equation (3):

  • Durability or shattering index, a parameter that indicates the toughness of the produced briquette during storage, handling and transportation, was measured by dropping the briquette from a 1.85 m height onto a flat steel plate for four times. The percentage durability is the ratio of the final weight of the produced briquette retained after four drops to the initial weight of the sample (Odusote et al., 2013). The durability of the produced briquette is as follows:

  • The water resistance capacity of the produced briquette was determined by immersing the briquette in a container filled with cold tap water, and the time taken to collapse was recorded (Demirbas, 1999).

3.   Results and Discussion
  • The physico-mechanical properties of composite briquettes made from blends of the corncob and rice husk were investigated. Figure 2 shows the effects of variations in particle size, mixing ratio and compaction pressure on the compressive strength of the produced briquettes. The compressive strength of the produced briquettes varied from 39 kN/m2 to 111 kN/m2. The briquette made from 80:20 mixing ratio of the corncob to rice husk, 0.25 mm particle size and 65 kPa compaction pressure exhibited the best compressive strength and the least from briquette with 50:50 ratio of corncob to rice husk, 1.75 mm particle size and 25 kPa compaction pressure. The briquettes in the present study possess better strength than those reported by Andres et al. (Andres, 2016), with 0.24 kPa and 1.13 kPa for briquettes from carbonized rice hull and corncob respectively. However, Bianca et al. (2014) reported compressive strength of 360 kPa for the briquettes produced from banana leave residues. The variation might be due to the nature of the biomass and methods used in producing the briquettes. The variations in the particle size, mixing ratio and compaction pressure have considerably influenced the compressive strength. The compressive strength of the produced briquettes increased with the decrease in particle size. The briquettes produced from the corncob and rice husk of finer particle sizes are less porous due to the stronger inter-molecular bond between the particles, which in turn increased the strength of the briquettes. In addition, it was found that as the percentage of rice husk in the blends increased, the compressive strength of the produced briquette decreased. It suggested that the corncob played a prominent role in improving the compressive strength of composite briquette. The compaction pressure is another critical factor that contributes to the strength properties. The compressive strength increased with the increase in compaction pressure. It can be attributed to the pressure applied to achieve the compactness that enhances the inter-molecular bonding property of the briquette particles, hence improves the strength property.

    Figure 2.  Effect of variations in mixing ratio, particle size and compaction pressure on compressive strength

    Figure 3 shows the effects of variations in particle size, mixing ratio and compaction pressure on the green density of the produced briquettes. It was found that the green density of the produced briquette varied from 1.1 g/cm3 to 1.86 g/cm3. The briquette made from 50:50 mixing ratio of corncob to rice husk, 0.25 mm particle size and 65 kPa compaction pressure had the highest green density and the least from briquette with 80:20 mixing ratio of corncob to rice husk, 1.75 mm particle size, and 25 kPa compacting pressure. The findings obtained in this study were higher than those reported by Oladeji et al. 2012), which ranged from 0.533 g/cm3 to 0.980 g/cm3 for briquette made from corncob residues. It was observed that the green density increased with a decrease in particle size, and also, it increased as the percentage of rice husk in the blends increased and with an increase in compaction pressure.

    Figure 3.  Effect of variations in mixing ratio, particle size and compaction pressure on green density

    Figure 4 shows the effects of variations in particle size, mixing ratio and compaction pressure on the relaxed density of the produced briquettes. The relaxed density of the produced briquette varied from 0.42 g/cm3 to 0.78 g/cm3. The briquette made from 50:50 mixing ratio of corncob to rice husk, 0.25 mm particle size and 65 kPa compaction pressure had the highest relaxed density and the least from briquette with 80:20 ratio of corncob to rice husk, 1.75 mm particle size, and 25 kPa compacting pressure. The relaxed density increased with an increase in particle size and it increased with an increase in the percentage of rice husk in the aggregate and an increase in compaction pressure. In addition, the relaxed density increased with the increase of compaction pressure because the increase in compaction pressure resulted in a decrease in volume at constant mass (Gino et al., 2015).

    Figure 4.  Effect of variations in mixing ratio, particle size and compaction pressure on relaxed density

    Figure 5 shows the effects of variations in mixing ratio, particle size and compaction pressure on the relaxation ratio of the produced briquettes. The relaxation ratio varied from 2.21 to 2.94. The briquette made from 60:40 mixing ratio of corncob to rice husk, 1.75 mm particle size and 65 kPa compaction pressure had the highest relaxation ratio and the least from briquette with 80:20 mixing ratio of corncob to rice husk, 0.25 mm particle size, and 25 kPa compacting pressure. It was observed that the relaxation ratio increased with an increase in compaction pressure, which is similar to the finding reported by Gino et al.(2015). However, as the particle sizes decreased, the relaxation ratio decreased. It indicates that briquettes produced from particle size 0.25 mm and 1.00 mm are more stable than briquettes from 1.75 mm. These values compared favorably well with the findings by Oladeji (2010), who reported the minimum and maximum relaxation ratio of 1.33 and 2.89 respectively for briquettes produce from corncob. The values of the relaxation ratio of this study exhibited better characteristics than that of Oladeji et al. (2012), which can be attributed to the nature of the biomass used in their studies. The obtained values of relaxation ratio indicated that the produced briquettes from the finer particles were more stable than those produced from the coarse particles. In addition, high relaxation ratio implied more void in the compressed materials and small value indicated more volume displacement, which was an important property for packaging, storage and transportation. Above all, it was an indication of stable and good quality briquettes (Nilson, 2012).

    Figure 5.  Effect of variations in mixing ratio, particle size and compaction pressure on relaxation ratio

    Figure 6 shows the effects of variations in mixing ratio, particle size and compaction pressureon the durability of the produced briquettes. The durability varied from 32.22% to 99.13%. The briquette made from 50:50 mixing ratio of corncob to rice husk, 0.25 mm particle size and 65 kPa compaction pressure had the highest durability and the least from briquette with 80:20 mixing ratio of corncob to rice husk, 1.75 mm particle size, and 25 kPa compaction pressure. The durability increased with the increase in the percentage of rice husk in the aggregate, which implied that the rice husk played a prominent role in enhancing the durability of the blend. The durability of the briquettes increased with a decrease in particle size of the produced briquettes. Because the smaller particle sizes had better inter-molecular bonding due to less pore space between the particles, hence the adhesive force between the blended particles was high, which made the particle interlocked and bonded together, thereby improving the durability property. In addition, the durability of the briquettes increased with the increase in compaction pressure of the produced briquette and this was because pressure enhanced the intermolecular bonding property of particles.

    Figure 6.  Effect of variations in mixing ratio, particle size and compaction pressure on durability

    Figure 7 shows the effects of variations in mixing ratio, particle size and compaction pressure on the water resistance capacity of the produced briquettes. The water resistance capacity varied from 480 s to 972 s. The briquette made from 50:50 ratio of corncob to rice husk, 0.25 mm particle size and 65 kPa pressure had the highest water resistance capacity, taking 972 s to collapse when immersed in water and the least from briquette of 80:20 ratio of corncob to rice husk, 1.75 mm particle size and 25 kPa pressure, taking 480 s to collapse in water. Briquettes with smaller grain sizes; higher percentage of rice husk and greater compaction pressure resisted water penetration better. This was because that the briquettes with smaller particle sizes had fewer pore spaces, which reduced water percolation and capillary action in those briquettes. In addition, higher compaction pressure improved the water resistance capacity because particles were more compacted at higher pressure, giving rise to fewer pore spaces and hence difficult for such briquette to absorb water when exposed to the humid environment.

    Figure 7.  Effect of variations in mixing ratio, particle size and compaction pressure on water resistance capacity

4.   Conclusions
  • This study investigated the physico-mechanical properties of composite briquettes made from blends of the corncob and rice husk. Selected physical and mechanical properties of the briquettes such as compressive strength, durability, and green and relaxed density among others were evaluated. The variations in the densification process parameter significantly influenced the investigated physico-mechanical properties of the produced briquettes. The briquette made from 80:20 mixing ratio of corncob to rice husk, 0.25 mm particle size and 65 kPa compaction pressure had the highest compressive strength and the least from briquette with 50:50 ratio of the corncob to rice husk, 1.75 mm particle size and 25 kPa compaction pressure. The green and relaxed density increased with an increase in the percentage of rice husk in the produced briquette samples while the durability of the briquettes increased with a decrease in the particle size and an increase in the percentage of rice husk. The briquette made from 50:50 ratio of the corncob to rice husk, 0.25 mm particle size and 65 kPa pressure had the highest water resistance capacity, and the least from briquette of 80:20 ratio of corncob to rice husk, 1.75 mm particle size and 25 kPa pressure.

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