Compressive Property of Biomass Briquettes Made of Hazelnut Shell and Peanut Shell

In Indonesia, energy consumption increases as with its population, while fossil energy sources keep running out. This condition forces the development of non-fossil energy sources. One promising alternative is biomass briquette. In this study a biomass briquette was made using hazelnut shell (limbah cangkang kemiri CK) and peanut shell waste (kulit kacang tanah KT) with various CK concentrations: 0/24, 4/24, 8/24, 12/24, 16/24, 20/24, and 24/24 (w/w). Initially, dried CK and KT waste were processed into charcoal powder with a particle size of ~ 0,25 mm. The briquette was prepared by densification of the two charcoal mixtures at a pressure of 29778,99 N/m2 for 15 min, followed by heating at 60 °C for 4 h. Due to storage and delivery process in the application, briquettes must have good compressive properties. In this study, compressive property of the produced briquette was characterized using a universal testing machine (UTM). The results showed that the values of ultimate strength, modulus of elasticity, and elongation at break of the CK-KT briquette increased with increasing CK concentration, with highest values of 5.69 MPa, 80 MPa and 0.146, respectively.


Introduction
In Indonesia, energy crisis is a crucial problem.The use of energy sources continues to increase with increasing population and economic growth.The use of energy to meet daily needs and production activities in the economic sector is one of the reasons for the higher consumption of fossil fuels [1].Dependence on fossil fuels can be reduced by utilizing biomass briquette as an alternative energy source.
In this study, we used peanut shell (KT) as the basic material for making briquettes because of its abundant availability.According to BPS (Biro Pusat Statistik) data, in 2021 the total area of peanut plantations, especially in the province of NTT has reached around 12,438 hectares [2].Then, in 2022 Central Java Province was 43,401.4hectares [3].However, KT is brittle [4], its brittle nature can reduce the strength of the briquettes.The characteristics of KT can be improved by adding hazelnut shells (CK) because CK is tough [5].The combination of these two materials is expected to increase the mechanical properties of briquettes so that they can withstand storage, transportation and marketing.
The main components of briquettes in this study were charcoal of CK and KT, therefore they can be considered as composite materials with these components.A composite material that consists of two component materials is developed to achieve better properties than each component [6].The mechanical property of a composite depends on the mechanical properties of and composition of its component.The mechanical properties of composite can be predicted using the rule of mixture.According to rule of mixture, the predicted highest and lowest values of modulus of elasticity of composites follows the formulas [7]: where E u , E l , E CK , E KT are upper bound, lower bound, CK, and KT modulus of elasticity.Vf is volume fraction.This research aimed to study the effect of briquette component composition on the compressive properties of briquettes.We also analyze the suitable of rule of mixture to experimental results of modulus of elasticity of the briquettes.

Materials and Methods
The raw materials used are CK/ the shells of candlenut (Aleuritas moluccana) obtained from Palembang, South Sumatra, Indonesia, KT/ the shells of peanut (Arachis hypogaea) obtained from local markets, and cassava starch (Rosebrand) from Indonesia is used as an adhesive in making briquettes.The dried CK and KT were then processed into charcoal by burning them in a closed container for 7 and 3.5 hours, respectively.The charcoal was crushed using a blender and filtered using a 60 mesh sieve to obtain fine charcoal powder (~0.25 mm).1.5 grams of cassava starch was added to 18 mL of water, then the mixture was stirred and heated using a candle flame until a clear and viscous liquid adhesive mixture was obtained.After the adhesive was produced, the CK and KT charcoal powders were added and stirred manually with the various compositions of materials.Various compositions of briquettes can be seen in Table 1.
The briquette mixture was put into the mold and then pressed with a pressure of 29778.99N/m 2 for 15 minutes.The briquettes were dried in an oven at 60℃ for 4 hours.Compression testing was carried out using a universal testing machine (Type Sinowon model SM-10) at a speed of 2 mm/min.The ultimate strength, elongation at break, and modulus of elasticity of the composite were obtained from compression testing.The elastic modulus of the upper bound (E u ) and lower bound (E l ) were also measured using the rule of mixture equations, as shown in equations 1 and 2. The volume fractions of CK (Vf CK ) and KT (Vf KT ) were calculated using equations 3 and 4, respectively.
The volume is calculated as mass divided by density, so the equation ( 3) and ( 4) can be re-write as follow: ρ KT ρ CK (6) where   dan   are the masses of CK and KT (grams), respectively.The density of CK and KT (kg/m 3 ) are represented by   and   , respectively.The density of CK and KT are obtained from briquettes made from pure CK and pure KT.After all the data is obtained, the composite elastic modulus, upper bound, and lower bound data are plotted together.

Physical Properties
The produced briquettes in this research are cylindrical, very dense, do not crack and have different heights, as shown in Figure 1 and the appearance of the briquettes from above is shown in Figure 2. The height of each briquette looks different even though the briquettes are made using the same pressure and adhesive mass.As the water content was removed during the drying process so that the only factor that influences the height of the briquettes is the composition of the material.The height of the briquettes increases as the CK concentration decreases.This means that CK charcoal is easier to compress than KT charcoal, thus affecting the volume and density of the briquettes.The density of the briquettes was in the range of (650.94 -939.17)kg/m 3 .The addition of CK causes an increase in density of the briquettes.The use of different raw material compositions can affect the density and compressive strength of briquettes [8].The density of briquettes is also closely related to the mechanical properties [9], raw material with higher density was more likely to possess a higher compressive strength than lower density [10].The density value of the produced briquettes already meets the SNI 01-6235-2000 standard for wood charcoal briquettes, which states that the minimum density value is 447 kg/m 3 .

Compressive Properties
Compressive strength analysis was carried out to study ultimate strength (US), elongation at break (EAB), modulus of elasticity (MOE), and the rule of mixture.A typical briquette stress-strain curve is shown in Figure 3.The shape of the curve produced after the compression test is S. The shape of the S curve was also found in research on briquettes from Palm Shells with the addition of plastic [11].The modulus of elasticity was taken from linear region and the ultimate strength was obtained from the maximum compressive strength that the briquette can withstand.The value of US, EAB, and MOE of the briquettes are presented in Table 2.One factor that greatly influences the compressive strength is the composition of the briquettes' raw material.The of the briquette compression test showed that the composition of CK-KT affected the US, EAB and MOE values.A more complete analysis will be presented in the next section.

Ultimate Strength and Elongation at Break
The US value is obtained in the range of 0.92 -5.91 MPa, as shown in Figure 4.The increase in US as CK content increases because the lignin content in CK is higher than in KT.The lignin content in CK is 54.46% [12] and KT is 36.1% [13].The lignin content in these briquettes can act as an internal binder during high briquetting pressure [14].The high lignin content in CK waste will increase the hardness of CK-KT briquettes [15].The compressive strength (US values) of all produced briquettes in this research met the standards.Namely, it must reach a minimum of 65 kgf/cm 2 (equivalent to 0.65 MPa) based on Minister of ESDM Indonesia Regulation no.047 of 2006.If we refer to the standard for charcoal briquettes from Japan, the minimum US values must reach 60 kgf/cm 2 (equivalent to 0.6 MPa).
The results also showed that EAB increases with the addition of CK concentration, as shown in Figure 5. EAB values for KT and CK-KT 4 briquettes only reached 0.077 and 0.082, respectively.This value is lower than EAB of the other briquettes.This indicates that KT and CK-KT 4 are more brittle than the others.The brittle nature indicates that the briquette will fracture before further deformation occurs under the applied compressive force [16].

The modulus of elasticity and Analysis using of The Rule of Mixture
In the range of CK concentrations from 0/24 -24/24 w/w, the MOE value increases, from 20 to 90 MPa, with increasing CK concentration.Higher MOE value indicates higher material rigidity [17].The rule of mixture was used to analyze the compressive properties of the briquettes by considering CK and KT as composite elements.It was noted that the adhesive mass was considered very small (6.25%) so it was ignored.The MOE of the briquettes was obtained from experimental results and the upper and lower bound of MOE were calculated using equation 5-6.All the MOE values were plotted together in Figure 6.The figure shows that the MOE values of the briquettes are outside the upper-lower bound area, precisely above the upper bound.This indicates that the bond strength between the particles in composites made from a combination of CK-KT is higher than pure CK and pure KT.Another study conducted by Zainuri et al. [18] reported that the MOE value of AI-SiC composites was also outside the upper-lower bound, precisely below the lower bound.

Conclusions
In this study, the briquette made from hazelnut shell and peanut shell waste with varied compositions have been successfully produced and analyzed in terms of their compressive properties.The results showed that the density, US, EAB, and MOE increased with increasing CK concentration.The rigid characteristic of CK has a good effect on increasing the compressive strength of CK-KT briquettes, so it can be concluded that the composition of the material greatly influences the compressive properties of the briquettes.From analysis using the rule of mixture, it was obtained that the MOE values of the produced briquette were higher than the upper bound, this indicated that the CK-KT composite has better bond strength between its particles than pure CK and KT.

Figure 2 .
Figure 2. The typical top view appearance of briquettes: CK-KT 12

Figure 3 .
Figure 3.Typical stress-strain curve of the briquettes obtained from compression testing (taken from CK-KT 20)

Figure 4 .
Figure 4.The US value of briquettes

Figure 5 .
Figure 5.The EAB value of briquettes

Table 2 .
Value of US, EAB, and MOE of briquettes.