Binderless Briquetting of Mixed Cassava Rhizome, Sugarcane Bagasse, and Sugarcane Straw for Producing Solid Biofuel with High Durability

This study aimed at investigating the binderless densification of biomass for the production of a durable solid biofuel by mixing cassava rhizome, sugarcane bagasse, and sugarcane straw. Six different treatments were applied with specific percentages of these three biomass feedstocks for the densification in a lab-scale press with heating of 120 °C. Three blend treatments and three 100% material treatments (without mix) were analyzed. Achieved briquettes were examined via various tests including proximate analysis, volumetric expansion, compression test, and energy index to assess the mechanical and energetic properties. Results indicated that the mixtures are suitable for the production of briquettes and treatment B2 was the best blend for briquette production. This study suggested that binderless densification of biomass can be achieved with the mixing of cassava rhizome, sugarcane bagasse, and sugarcane straw, which will support the development of qualified solid biofuels without the cost of binders.


Introduction
The economic development of a country is strongly linked to energy consumption and signifies industrialization [1,2]. The world economy was primarily based on fossil fuel energy matrices, such as oil and coal [3], which cause emissions of pollutants and greenhouse gases that have impacts, such as climate change and global warming [4][5][6]. The rethinking of alternative energy sources was necessary, and renewable energies have grown in recent years.
In 2019, the Brazilian Energy Balance (BEN) consisted of 45.3% renewable energy sources for domestic energy supply.
One of the most important renewable energy sources, sugarcane biomass (17.4%), surpassed the participation of hydraulic (12.6%) and natural gas (12.5%) in the energy matrix [7].
The primary source of biomass in Brazil used for electricity generation is sugarcane bagasse; the 2020/2021 harvest is projected to be 642 million tons [8]. Sugarcane straw and other agricultural residues have been studied and used for the generation of thermal or electrical energy [3,9,10].
One way to make better use of this waste for energy generation is to use densification, a process in which the material is compacted to obtain a product (briquettes or pellets) with higher density (kg/m 3 ) and higher energy density (kJ/m 3 ) than the initial material. This process is primary used for lowdensity materials, such as agricultural and agro-industrial residues, such as sawdust, rice husks, and sugarcane bagasse [11,12]. With the increase in density, the process also improves the transportation and storage of these materials [3,9,10,13].
Bajwa et al. [14] conducted a study on the production of briquettes for energy generation, found that production increased in the last decade and is expected to increase by 56% from 2010-2040, and pointed out that agricultural waste is an ideal candidate for densification because of the large amount of biomass produced. The low density of agricultural residue allows easy transport and handling. The use of densified agricultural waste for energy production can reduce dependence on fossil fuels, and densified products can be effectively used to produce heat and energy.
The pellets are formed by the extrusion process and have high density and a cylindrical size of 38 mm (1.5 in.) and diameter of 7 mm (0.3 in.). Briquettes can be made in several formats. In the case of the cylindrical format, S. Clarke et al. [15] showed that the Briquettes and pellets differ by size, where briquette presenting a diameter of approximately 25 mm (1 in.). The biomass material must be characterized for energy densification; characterization is primarily based on the following analyses: elementary chemical composition, immediate composition, and calorific power. The briquette quality can also be measured by mechanical resistance and durability tests.
Several papers on biomass characterization have been published to improve and optimize densification through briquetting [4,[16][17][18][19][20][21][22][23][24][25]. Felfli et al. [19] conducted a study on the state of biomass briquetting and its prospects in Brazil to determine the availability and characteristics of agro-waste products for briquetting. They found that wood residues, rice husks, and coffee husks are the most promising agro-residues for short-term briquetting in Brazil. L. Chen et al. [26] state that the moisture content for agro-industrial waste should be between 10 and 15% because higher moisture content values imply problems in burning and excessive use of energy for drying. Authors point ash that there is a variation among the wastes, and that for the agricultural waste, there is a large number of alkali metals from the soil.
Although densification began using residues from the wood industry [27], agricultural residues have also begun to be studied, and various blends (mixtures) have been tested. Silva et al. [6] analyzed four biomass types: sawdust from Eucalyptus sp. and Pinus sp., bagasse, and sugarcane straw, all of which were viable for the densification process, which was influenced by granulometry. Biomass briquettes have great potential for use as solid fuels. However, the formation of briquettes depends on the biomass characteristics. For example, some types of biomass (grass) are not easily compacted, and binders may be necessary for good formation. However, the use of a binder makes the process more difficult and expensive [24,[28][29][30][31][32][33].
The binder of briquettes plays a vital role in the briquette production process, serving to keep the particles together, making it better handling and transportation of briquettes. Thus, several binders are used for different types of briquettes being divided into three categories: organic binder, an inorganic binder, and composite binder. Organic binders are as follows: biomass binder, tar, pitch, and petroleum bitumen binder, lignosulphonate binder, and a polymer binder, while inorganic binders are as follows: clay lime, plaster, cement, sodium silicate, and sodium silicate. Composite binders are the combinations of two or more binders, which combine all the advantages of the different types of binders [33].
Aransiola et al. [34] studied the use of cassava starch as a binder at different concentrations, 10%, 20%, and 30% in the production of carbonized corncob briquette and concluded that the higher the concentration of binder, the better the briquettes, while higher compaction pressure will result in carbonized corncob briquette of higher quality for both storage and transportation.
Nurek et al. [35] and Granado et al. [36] analyzed the briquettes formation without binder addition in the material.
Nurek et al. [35] studied the effects of temperature and pressure on the manufacture of crushed wood residue briquettes and concluded that a useful compaction index and durability coefficient were obtained for a temperature of 73°C and moisture content of 10%. Granado et al. [36] analyzed the effect of pressure densification in raw cassava rhizome. They verified with time pressing of 120 s and pressure of 204 MPa; They obtain better higher durability (94.1%) for the briquette.
According to Gilvari et al. [37], density and durability are the most significant factors for determining the physical quality of densified materials.
This work aims to analyze the mixture of raw cassava rhizome, sugarcane bagasse, and sugarcane straw without binder addition and to evaluate the improvement in the mechanical and energetic properties.
After oven drying, all materials (bagasse, straw, and raw cassava rhizome) were kept at room temperature until moisture content equilibrium was achieved (10-12%). This range of moisture content was chosen because in literature is described that the natural moisture content of the material in the densification process should be between 10 and 15% [15]. Other authors, such as Castellano et al. [39], conducted a quality study of pellets made with wood and other biomass and mentioned that the moisture contents of the materials were between 10 and 12%. Wang et al.
[40] studied the briquetting process for rice straw with a moisture content of approximately 15%.
The dried material was ground in a knife mill before storage. The standard CEN/TS 15149-2 [41] was used to analyze particle size distribution. The particle size was chosen by the particles passed through a sieve with an aperture of 20 mesh (0.850 mm) and remained in a sieve with an aperture of 40 mesh (0.425 mm). After determining the granulometry, the materials were blended according to Table 1.

Bulk Density
The bulk density was determined for the bagasse, straw, and raw cassava rhizome materials. The E873-82 standard was used to analyze the bulk density [42]. A 1000-ml measuring cylinder was used to measure the sample volume, and a semianalytical balance was used to measure the corresponding mass to obtain the density by mass/volume fraction (Eq. (1)).
where ρ is the bulk density (kg/m 3 ); m is the mass (kg); and v is the volume (m 3 ).

Apparent Density
Apparent density was determined for the briquettes (after compaction of raw materials). Apparent density gives the ratio between mass and volume after briquette manufacturing (Eq. (2)).
where ρ a is the apparent density (kg/m 3 ), m is the mass (kg), and v is the volume (m 3 ).

Energy Apparent Density
Equation (3) gives the energy apparent density of the briquettes.
where ρ e is the energy apparent density (MJ/m 3 ); ρ a is the apparent density (kg/m 3 ); and HHVhigher heating value (MJ/kg).

Energy Bulk Density
The energy bulk density (raw material) was calculated according to Eq. (4).
where ρ ea is the energy bulk density (MJ/m 3 ), ρ is the bulk density (kg/m 3 ), and HHVis the higher heating value (MJ/kg).

Energy Comparison Index
The value of the energy in the briquette can be obtained by comparing the energy from the energy bulk density and the energy apparent density. This index is called the energy comparison index (Iρ e ) (Eq. (5)).
where ρ ea is the energy bulk density (MJ/m 3 ), and ρ e is the energy apparent density (MJ/m 3 ).

Briquettes: Volumetric Expansion and Physical Tests
Fifty grams of material was used per briquette for production. A lab-scale press model LB32 with a pressure of 100 bar, heating temperature of 120°C, pressing time of 5 min, and cooling time of 7 min was used. After the briquettes were made and cooled, they were stored in plastic bags to avoid humidity absorption. Fifteen briquettes were produced for each blend. Five pieces (briquettes) were used for the mechanical resistance test, and 10 briquettes were used for the durability test. Additionally, volumetric expansion was determined, in ambient temperature, by measuring the height and width of the briquettes with a caliper 1, 3, 5, 7, 24, 48, and 72 h after confection. The briquette volume was calculated using E. (6).
where V is the briquette volume (cm), r is the cylinder circumference radius (cm), and h is the briquette height (cm). The variation of this expansion is given by Eq. (7): where ΔV is the volumetric expansion of the briquettes (%), V 1 is the briquette volume immediately after compression (cm 3 ), and V 2 is the volume of the briquettes after a certain time after compaction (cm 3 ).

Resistance to Compression
After manufacturing, the mechanical resistance test was performed using the ABNT NBR 7222/2011 [43] standard. The test was performed in a universal machine of mechanical tests with a load cell of 5 kN (EMIC model DL30000) and load speed of 3 mm/min [39]. The maximum stacking height for each treatment was calculated from the test results. Equations (8) and (9) show the number of samples stacked (Nemp) and stacking height (Hemp) [6], respectively.

Durability
The durability test followed the standard CEN/TS15210-2 [41]. The equipment used was the roller-drum KT3010 with five repetitions. Two briquettes were used in the drummer, for a total of 100 g with 105 rotations per test. Finally, the briquette pieces were passed through a 6-mesh screen, and the retained pieces were weighed. The durability was calculated using Eq. (10): where DU is the briquette durability (%), m i is the initial mass of the briquettes before the test (g), and m f is the final mass of the briquette pieces after the test (g).

Fuel Properties and Compositions Analysis
Proximate Analysis and Higher Heating Value Proximate analysis and higher heating value (HHV) were determined for the raw materials. The analyses were performed using the American Society for Testing and Materials (ASTM). Proximate analysis determines the moisture content (ASTM D-3173 [38]), volatile matter (MV) -ASTM D3175 [40], ash content (AC) -ASTM D-3173 [38] and fixed carbon (FC) calculated by Eq. (11). The HHV analysis was performed using a calorimeter (IKA C200), following ASTM D2015-96.

Scanning Electron Microscopy and Energy Dispersive Spectroscopy
A scanning electron microscope (SEM) HITACHI model TM 3000 was used to analyze the ash from the blends. The chemical analyses were verified by X-ray energy dispersive spectroscopy (EDS) coupled to the equipment.

Statistical Analysis
The data obtained were subjected to ANOVA and Tukey's comparative test (with 95% significance) using the R statistical software.

Results and Discussion
Densification Experiment for the Blends Density Table 2 presents the material density and energy density data before the briquetting process and after the process application and the energy comparison index. The results showed that B2 briquettes with values of 1254 kg/m 3 (1.25 g/cm 3 ) and B5 briquettes with values of 1257 kg/ m 3 (1.26 g/cm 3 ) present apparent density higher than the values obtained by Banaag et al. [44] and Obi et al. [45]. Apparent energy density increased with respect to bulk energy density, because the presented energy comparison index (Iρ e ) varied from 6.20 to 8.25. The most significant increase for the 100% pure materials was obtained for B4, whereas the highest blended value was for B1.
The density of a material is linked to handling, transport, and storage. Thus, the briquetting process causes high density solids to be manufactured with these materials and improves the abovementioned variables (Bajwa et al., 2018, Gilvari et al., 2019). Banaag et al. [44] produced briquettes from sugarcane bagasse and rice bran at a temperature of 150°C during a pressing time of 30 min and a pressing ratio of 0.34 MPa/s and verified that the density was 1.194 g/cm 3 . Obi et al. [45] evaluated briquettes made with sawdust mixed with cassava starch as a binder prepared in proportions of 100:15, 100:25, 100:35, and 100:45 by weight and obtained a compacted density of 0.727 g/cm 3 . The optimal mixing ratio based on calorific power was reached at a mixture ratio of 100:35 with a compacted density of 0.703 g/cm 3 .
According to Yang et al. [46], the material moisture content used in densification can be varied between 5 and 28% because this amount of water acts as a lubricant and binder in this process. Kaliyan et al. [47] ensure that water in biomass mixtures containing soluble constituents such as starches and sugars helps in the briquetting process because water acts as a binder to strengthen the van der Waals force bonds between the particles. The heating in the briquetting process causes starch gelatinization, solubilization, and recrystallization of sugars and salts and improves the bonding between particles [47,48]. According to Mostafa et al. [49], temperatures between 70 and 150°C cause natural biomass binders such as lignin to be activated, and from 100 to 200°C provide starch binding. Thus, performing pelleting at these temperatures and then cooling them causes the lignin hardening and the proteins reabsorbed, thus increasing the strength and density of the pellets.
In this work, the biomass used was with moisture content between 10 and 12%, as shown in Table 1. In the briquetting process, the temperature used was 120°C. Thus, the action of temperature along with the level of moisture content considered good, and the mixtures of biomass provided that the natural constituents acted as natural binders in the briquetting process used here, forming a briquette with a high bulk density as shown in Table 2.

Volumetric Expansion
The volumetric expansions are presented in Table 3 and Fig. 1. Figure 1 shows that the briquettes of all blends presented a significant expansion between 7 and 48 h after their confection and tended to stabilize their volumetric variation after 72 h.
Treatment B5 presented the largest volumetric expansion, whereas the smallest expansion occurred for B6. The blends (B1-B3) showed a volumetric expansion of approximately 6%. This expansion did not affect briquette formation.

Compression Test
The compression test resistance results are presented in Table 4.
According to Gilvari et al. [37,50], a compression intensity is a way of verifying how the briquettes behave during transport and storage. Martinez et al. [51] studied the mechanical resistance of briquettes made with coffee and pinewood residue to obtain fracture limits from 415 to 569 kgf. Silva et al.  Table 4). The raw cassava rhizome (B6) material was the strongest treatment, followed by B2 (composition of 75% of raw cassava rhizome). Analysis of sugarcane bagasse and straw materials revealed lower resistances and addition of the raw cassava rhizome to blend B2 increased the resistance by 39% and 135% compared to that of sugarcane bagasse and sugarcane straw, respectively. Thus, the insertion of raw cassava rhizomes increases the resistance of briquettes composed of bagasse and straw, and raw cassava rhizome acts as a binder in the compaction process of the sugarcane bagasse and straw briquettes. Blend B2 had the highest stacking height.

Durability
Durability results are shown in Table 5. Different letters show significant difference between samples with 5% significance in Tukey test Durability measures how the fuel behaves throughout transport and handling [37]. Good fuel for transportation has a durability greater than 90% [14,57]. Gendek et al. [52] obtained a briquette durability of 97.87% for Picea abies. Table 6 shows that all briquettes had durability close to 97%. The values are high and guarantee the mechanical resistance of the briquettes during transport and handling.
The briquetting process described here used the temperature of 120°C, which promotes the action of the lignin and starch-like binders, constituting a solid bridge connection between the particles, as described by Kaliyan et al. [47]. Improving the fracture limit and the durability of the blended briquettes (Tables 4 and 5) demonstrate a higher performance of the blends studied here and that the use of raw cassava strain increases this performance.

Proximate Analysis and Higher Heating Value
The values of the volatile matter (TV), ash content (TC), fixed carbon content (TCF), higher heating value (HHV), and the comparisons with other studies are presented in Table 6.   The results for TV agree with the literature [6,18,53,54]. The difference for the TV of the raw cassava rhizome was 13.55% less compared to results presented by Veiga et al. [54]. It can be explained by the feedstock pre-treatment (wash) for the biomasses used by Veiga et al. [54]. In this study, washing was not used (only sieving). Thus, contaminants, such as soil, must have remained next to the materials and altered the values [55].
No significant change among values was obtained for blends B1, B2, and B3. The results showed that the addition of 50-75% raw cassava rhizome to the blends did not change the TV content.
The TC values obtained for B4 and B5 agree with the literature [6,37,53,56]. The TC of raw cassava rhizome (B6) was 3.1 times higher than that obtained by Veiga et al. [54], because the material was not washed. For the blends, the lowest TC was observed for B1. Blend B1 contains 50% raw cassava rhizome, whereas B2 and B3 contain 75% raw cassava rhizome. The results show that the blending process is efficient.
The TCF value obtained for straw (B5) agrees with Silva et al. [6] and Leal et al. [53]. For bagasse B4, the FC was lower than those found by [6,53], because of the difference in ash content. The impurities (sand and soil) in the material increased the ash content and influenced this value [55]. B6 presented a fixed carbon content larger than the value found by Veiga et al. [54], and indicates that this blend is very resilient during the combustion process.
The HHV is the amount of heat that the material provides. The values for B4 and B5 agree with those by Silva et al. [6] and Leal et al. [53]. The B6 value is 14% below that found by Veiga et al. [54]. The lack of a washing process should be responsible for decreasing the HHV. Blend 3 had the highest HHV of the blends.
According to the obtained results, it is possible to determine that blend 2 has the best composition for using the material as an energy source because of the high TCF value which characterizes a longer burning time and presents good TV, which characterizes good ignition power, burning, and HHV.

Scanning Electron Microscopy and Energy Dispersive Spectroscopy
Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) results are presented in Fig. 2.
According to Nakashima et al. [56], biomass has inorganic components in its composition, such as calcium, potassium, magnesium, phosphorus, iron, and zinc. But also, these and other minerals can be found during the biomass characterization, resulting in contaminants related to the soil. The inorganic elements present in biomass can cause incrustations and slags in equipment that use biomass. In Fig. 2, the EDS spectra identified the elements that compose each sample. The blends B1, B2, B3, and B6 presented a high amount of K and Ca, whereas B4 and B5 presented a high amount of Si. B1 presents K, Ca, and an indication of silica's presence in its composition because it presents 50% sugarcane bagasse and straw, which are materials that have more significant quantities of silica in the composition of their briquettes of 100% (B4 and B5). According to Nakashima et al. [56], silica is an element that appears in large quantities in sugarcane  [54] individuals. This element helps in its structure and gives rigidity to the plant; there is also a percentage related to contaminants due to the type of harvest, storage, and management of materials.

Conclusion
The objective of this study was to analyze binderless briquettes made with biomass blends. We obtained that the mixtures made with three different types of biomasses, cassava rhizome, sugarcane bagasse, and sugarcane straw, provided a good quality briquette because the increase of the energy was around 6.4 to 6.7 when compared with the energy existing in the material without densification. The high heat value obtained for all materials and blends was good, and blend 2 presented the best composition to use as an energy source due to the high TCF value and good TV. The blends gave 97% of durability, which shows excellent resistance to transport and handling. In addition, the use of raw cassava rhizome produced an increase in strength compared to briquettes made with 100% sugarcane bagasse or 100% sugarcane straw, and the briquettes made with 75% raw cassava rhizome were those that showed the best strength. We concluded that the mixture of biomass cassava rhizome, sugarcane bagasse, and sugarcane straw without using a binder is suitable for producing briquettes and presented promising results for immediate analysis and good mechanical strength. Data availability The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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