Valorization of durian peel waste and sewage sludge as bio-briquette

Renewable energy is an alternative to fossil fuels due to the limited supply of fossil fuels and greenhouse gas emission contribution. Bioenergy is one of the renewable energies that is derived from biomass. Biomass waste, namely durian peel and sewage sludge, has the potential as raw material for bioenergy because of its energy content. Moreover, the carbonization of biomass water can improve the heating value of biomass material. This study aims to investigate bio-briquette properties from durian peel and sewage sludge with various biomass compositions and types of binder materials. Tapioca starch, rejected papaya, and cow manure were used as binding materials. The density of 1.01 g/cm3 and the 5288 kcal/gram calorific value were obtained from 90% durian peel and 10% tapioca starch binder. The bio-briquette with 90% durian peel has a moisture content of 10%, volatile matter of 21%, fixed carbon of 61%, and ash content of 18%. Although the calorific value and fixed carbon satisfy the quality standard of briquettes, the moisture content, volatile matter, and ash content still did not comply with the standard. The result shows that the rejected papaya is promising binder material that does not compete with the food commodity of tapioca flour.


Introduction
Indonesia is one of the largest producers of fossil fuels in the world, particularly in terms of coal and natural gas [1].Indonesia has a high commodity of fossil fuel resources and has exploited them for several decades to meet its domestic energy needs and export to other countries.According to the International Energy Agency (IEA), Indonesia was the world's largest exporter of thermal coal in 2020, with exports totalling 371 million tonnes [2].However, Indonesia has also recognized the need to diversify the energy supply and reduce its reliance on fossil fuels.
Renewable energy has emerged as a promising alternative to traditional fossil fuels, which are limited in supply and contribute significantly to global greenhouse gas emissions.Over the past few years, renewable energy utilization has grown worldwide [3] and Indonesia is no exception.Indonesia has set a target of generating 23% of its energy from renewable sources by 2025 to drive the need for clean, sustainable energy sources that do not harm the environment [4].Governments, businesses, and individuals increasingly invest in renewable energy technologies such as solar, wind, hydro, geothermal, and bioenergy to reduce fossil fuel carbon footprint and promote a more sustainable future.Bioenergy has the potential to become one of the clean energy choices and help the government achieve renewable energy targets in 2025.Bioenergy is renewable energy derived from organic materials, such 1239 (2023) 012018 IOP Publishing doi:10.1088/1755-1315/1239/1/012018 2 as plants, crops, and organic waste.Bioenergy is a sustainable and environmentally friendly alternative to fossil fuels [5], as it produces fewer greenhouse gas emissions [6,7] and reduces dependence on finite resources [8].Furthermore, the use of bioenergy can reduce waste and improve economic opportunities [9].Bioenergy has become an increasingly important energy source as the world seeks to transition to a low-carbon and sustainable energy future [7,10].
The most common bioenergy sources are agricultural waste [11] and municipal waste [12].One type of agricultural waste and urban waste is durian peel waste and sewage sludge.In theory, these wastes can be used as raw materials for bioenergy [13,14].Research on converting durian peel waste into bioenergy has been conducted to explore the feasibility and potential of using those waste as a sustainable energy source.A study from Zhao in 2017 investigated the use of durian peel waste for bioenergy production through anaerobic digestion.The study found that durian peel waste has a high potential for biogas production and can generate significant energy.The study also highlighted the importance of optimizing the anaerobic digestion process parameters to achieve maximum bioenergy production [15].Another study was investigated by Nuriana in 2014, and the study discovered that durian peel waste could be used as a renewable alternative fuel for bio briquettes with the addition of a carbonization process so that it can increase the heating value.The optimal composition was an average dry-weight ratio of peelto-peel of 1:3 [16].Research on the potential of durian peels as bioenergy was also carried out by Pusparizkita in 2022.The research showed the effect of drying time on the water content of durian peels as raw material for bio pellets.A 6 to 12 hours drying time is the best for bio-pellet raw materials [17].
Bioenergy from sewage sludge is a renewable energy source gaining popularity as an alternative to traditional fossil fuels.Sewage sludge is the solid residue left over after wastewater treatment processes, and it is rich in organic matter that can be used to produce biogas.Several studies have been conducted on the potential of sewage sludge as bioenergy.That research generally converted the gas produced into energy using anaerobic digestion [18] and pyrolysis [19].However, there is a need for innovation in using sewage sludge as an alternative energy considering its enormous potential.
Therefore, this research introduced alternative bioenergy in the form of bio briquettes made from a mixture of durian peel and sewage sludge.The objective of this study was to investigate the characteristics of bio-briquettes from the variation in raw material composition (durian peel waste and sewage sludge) and the type of binder.

Carbonization of durian peel and sewage sludge
Durian peel was collected from the local fruit store in Balikpapan, and sewage sludge was taken from a domestic wastewater treatment plant in Balikpapan.The durian peel was washed with tap water to remove dirt, cut, and dried using the sun drying method for three days.Finally, the durian peel was dried at the temperature of 105°C for approximately 1 hour to remove moisture.The sewage sludge also dried at a temperature of 105°C for 2 hours.The dry durian peel and sewage sludge were carbonized using a furnace at a temperature of 500°C for 5 minutes.The carbonized durian peel and sewage sludge were crushed and sieved into powder with a particle size of 100 mesh.The proximate analysis and calorific value were measured to determine the characteristics of raw material prior to briquetting.

Preparation of binder
The physical characteristics of briquettes are strongly affected by the type of binder and its concentration.Therefore, different types of binders have been used for briquette production.This research utilized tapioca starch, rejected papaya, and cow dung as binder material.The composition of binder and the mixture of durian peel and sewage sludge is 10%:90%, which leads to variation in briquette composition, as shown in Table 1.The tapioca starch binder was prepared by mixing 10% of the briquette mass with 250 ml hot water and was heated for 5 minutes until homogenous.Next, the rejected papaya was squashed until forming a slurry mixture, and the cow dung was directly used as it is.Once the binder was ready, each was combined with the raw material (sewage sludge and durian peel).

Briquetting
After preparing the binder, the next step is mixing all the raw material and binder, as stated in Table 1.Furthermore, the compression step using a manual briquette press was followed after filling the mixture into small rectangular casts (4 cm x 4 cm x 7 cm).After that, the drying process was completed for 5-7 days by exposing the briquettes to the sun until the binder was hardened due to moisture evaporation.Finally, the molded briquettes were tested for characterization, such as proximate and calorific value analysis.

Proximate analysis 2.5.1. Moisture content test
The moisture content was analyzed by oven-dry method.The briquette sample was kept and oven-dried at 105°C for 3 hours.Moisture content analysis was referring to SNI 06-3730-1995 technical activated charcoal.The moisture content test is defined using the equation ( 2).Moisture content Where, W1: mass of the sample before drying (grams) W2: mass of the sample after drying (grams)

Ash content test
Ash content is the residual material from the combustion process.Ash content was analyzed using a furnace.The sample was dried in a furnace at 900°C for 2 hours; then, the samples were cooled in a desiccator and weighed by analytical balance.Ash content analysis was according to SNI 06-3730-1995 technical activated charcoal.The ash content test is defined using the equation (3).Ash Content (%)= Where, W1: mass of the sample before drying (grams) W2: mass of the sample after drying (grams)

Volatile matters test
Volatile matters were analyzed using a furnace at 950°C for 7 minutes.Afterward, the samples were weighed after cooling in a desiccator-volatile matters by SNI 06-3730-1995 technical activated charcoal.The ash content test is defined using the equation (4).
Where, W1: mass of the sample before drying (grams) W2: mass of the sample after drying (grams)

Fixed carbon test
Fixed carbon content has a principle calculated from a 100% reduction of the part lost in the 950 o C heating process and the ash content.The calculation of the fixed carbon content according to SNI 06-3730-1995 is as follows: Fixed carbon (%)= 100%-(volatile matter %+ash content %) (5)

Calorific value analysis
The calorific value is measured by carrying out a combustion test using a bomb calorimeter.The measurement refers to ASTM D 5865-07 standard test methods to determine the gross calorific value of coal and coke by either an isoperibol or adiabatic bomb calorimeter.

Effect of binder type and composition on density of bio-briquette
Density is a mechanical strength indicator and bulk energy content in briquettes.Decent density also provides easy handling and low-space storage.Figure 1 demonstrates the impact of binder material and the composition of durian peel and sewage sludge on bio-briquette density.As can be seen on the graph, the tapioca starch and cow manure are binders that produce high-density briquettes of around 1-1.2 gr/cm 3 .Nevertheless, briquettes containing rejected papaya had a density of less than 1.0 gr/cm 3  In addition, the composition of durian peel and sewage sludge caused a minor effect on density alteration.The difference in density between the raw material ratio variation was around 0.1-0.2g/cm 3 .
Using tapioca starch as a binder, the briquettes with a 20% durian peel:70% sewage sludge ratio had a 1.13 g/cm 3 density.Decreasing sewage sludge to 0% and increasing durian peel caused a decreasing density of 1,01 g/cm 3 .The fine particle size of biomass of sewage sludge caused easy agglomeration of particles and produced highly dense briquettes [28].Therefore, reducing sewage sludge also reduces the density of briquettes.The 0.8 -1.0 g/cm3 density adequate for high-quality briquettes from biomass [29]. .

Effect of binder type and composition on moisture content of bio-briquette
Moisture content is a key parameter of energy source because it influences the bulk density and energy content for self-heating combustion [30].Figure 2 presents the effect of binder and raw material composition on the moisture content.The results show that bio-briquette with tapioca starch binder has the lowest moisture content among rejected papaya and cow manure.The lowest moisture content of 10% was obtained by combining 90% durian peel and 10% tapioca starch binder.In contrast to tapioca starch, all bio-briquettes with rejected papaya had the highest moisture content above 20%.The 90% sewage sludge and 10% rejected papaya binder reached a moisture content of 31%.The high-water content of papaya, approximately 85-90% [31], affected the moisture content of the final product of briquettes.
The composition of durian peel and sewage sludge also influenced the moisture content of the final product of bio-briquettes.In briquettes with tapioca starch, increasing the composition of durian peel and decreasing the sewage sludge resulted in the decline of the moisture content of briquettes.The blend of 20% durian peel, 70% sewage sludge, and 10% tapioca starch binder had a moisture content of 17%.With tapioca starch binder, the increasing composition of durian peel to 70% and 90% obtained 13% and 10% moisture content, respectively.The initial moisture of raw material affected the final product of bio-briquettes.The durian peel had lower moisture content than sewage sludge, so the higher durian composition lowered the moisture content of the briquette product.Meanwhile, the composition of durian peel and sewage sludge slightly affects the moisture content in cow manure-bonded briquettes.The finding shows that the briquettes with cow manure binder had around 16-24% moisture content.Because higher moisture decreases combustion temperature and negatively affects fuel combustion, briquettes should not have moisture contents exceeding 10-15% [32].Based on European and German standards of charcoal briquettes, bio-briquettes must have a moisture content of less than 8% and 6%, respectively [33].Thus, all produced bio-briquettes using durian peel and sewage sludge did not comply with European and German moisture content standards. .It is evident from the graph that all the briquettes had volatile matte under 40%.The highest volatile matter was obtained by bio-briquettes by cow manure, followed by tapioca starch.The bio-briquettes using rejected papaya had the lowest volatile matter.At the 90% durian peel ratio, briquettes with cow manure had just under 40% volatile matter, followed by tapioca starch and rejected papaya, which had around 30% and 20%, respectively.Past reports investigated that cow manure had a volatile matter of 50% [37].Therefore, the high volatile matter in cow manure affected the volatile matter of the final mixture.Using cow manure and tapioca starch binder, volatile matter gradually decreased with the increasing durian peel composition and decreasing sewage sludge.Sewage sludge had higher volatile matter (27%) than durian peels 22%, so the lowering sewage sludge decreased volatile matter.

Effect of binder type and composition on ash content of bio-briquette
Figure 4 presents the influence of binder type and raw material composition on ash content.Ash is an inorganic material that does not combust in briquettes.Therefore, the ash content considerably affected the heat transfer fuel and oxygen diffusion to the fuel surface of briquettes [38].On the other hand, the inert material in ash does not promote heat release during combustion and impacts the dust and toxic material in the air [39,40].
The briquettes with cow manure binder have the highest ash content, followed by briquettes with rejected papaya.On the other hand, the briquettes with tapioca starch have the lowest ash content.The briquettes with cow manure binder and rejected papaya reached ash content of just over 40% and above 30%, respectively.On the other hand, the briquettes with tapioca starch have an ash content of around 20%, except for the briquette, which comprises 90% of sewage sludge.
With tapioca starch as a binder, decreasing sludge composition from 90% to 0% significantly decreased the ash content from 32% to 18%.The ash content decreases from 36% to 24% when the sludge ratio decreases in briquettes with rejected papaya.It is evident that decreasing the sludge content in a mixture of bio-briquettes decreases ash content.This result agrees with the previous report on lowering the percentage of dried fecal sludge in the food market, and fecal sludge mixture results in lower ash content [41].The study of pelletizing fecal sludge with biowaste also reported similar results [42].The inorganic matter, such as silica in sewage sludge, resulted in high ash content when increasing sludge composition [43].%.All bio-briquettes with durian peel and sewage sludge did meet with European and German for ash content of less than 15% [33].

Effect of binder type and composition on fixed carbon of bio-briquette
Fixed carbon is available char for combustion once the volatile matter and some hydrocarbons are released from briquettes [44].Figure 5 shows the effect of binder type and composition on the fixed carbon of bio-briquettes.The briquettes with tapioca starch and rejected papaya binder have the highest fixed carbon content.The 90% durian peel composition and 10% tapioca starch binder achieved the highest carbon content of approximately 60%.The fixed carbon of 59% was obtained by a composition of 70% durian peel, 20% sewage sludge, and 10% rejected papaya binder.The briquettes with cow manure binder have the lowest fixed carbon, just under 20%.Only briquettes with a ratio durian peel of 90% had fixed carbon of 28%.The durian peel compositions significantly contribute to the fixed carbon in briquettes.Therefore, the briquettes experienced an increased fixed carbon when the compositions durian peel ratio was increased.For instance, with rejected papaya as the binder, increasing the durian peel ratio from 0% to 70 improved the fixed carbon from 43% to 59%.The increase of fixed carbon due to the initial durian peel had a fixed carbon of 35%, higher than sewage sludge fixed carbon.Biobriquettes with 90% of durian peel and 10% tapioca starch meet the European standard for fixed carbon of 60%.

Effect of binder type and composition on calorific value of bio-briquette
Figure 6 presents the influence of binder type and composition on calorific values of bio-briquette.It is evident that the briquettes with tapioca starch and rejected papaya have higher calorific value than cow manure-bonded briquettes.Briquettes with tapioca starch and rejected papaya have calorific values ranging from 3000-5000 kcal/gram.In the composition of durian peel from 0-70%, the bio briquettes with tapioca starch and rejected papaya have similar calorific values.When the composition of durian peel is 90%, the calorific value of briquettes with tapioca starch is higher than rejected papaya.The tapioca binder briquettes obtained the highest calorific value (5288 cal/gram), because tapioca starch has a higher calorific value (3574,47 cal/gram) than rejected papaya (435,02 cal/gram [45].Meanwhile, briquettes with cow manure have calorific varies between 1300-3500 cal/gram.The lower calorific value with cow manure was obtained due to the high ash content that reached more than 40%.The ash content is detrimental to the combustion process due to a reduction in the calorific value of the fuel

Conclusion
The result of this investigation recommended that durian peel is one of the options for biomass energy resources to produce bio-briquettes.The optimum composition is 90% sewage sludge and 10% tapioca starch binder and achieved the highest calorific values of 5288 kcal/gram.This optimum composition produces briquettes with a moisture content of 10%, volatile matter of 21%, fixed carbon of 61%, and ash content of 18%.Although the calorific value and fixed carbon meet the quality standard of briquettes, the moisture content, volatile matter, and ash content still did not comply with the standard.
The results also show that rejected papaya is potential binder material as a substitute for tapioca flour.

10 2. 4
Density measurementThe density was measured from mass to volume ratio of the briquette sample-the measurement referring to SNI 1973:2016 about the density measurement methods.The density calculation is defined using the equation (

Figure 1 .
Figure 1.Effect of binder type and composition on the density of bio-briquette (with 10% of binder)

Figure 2 .
Figure 2. Effect of binder type and composition on the moisture content of bio-briquette with 10% of binder 3.4 Effect of binder type and composition on volatile matter of bio-briquette Figure 3 illustrates the influence of binder type and the ratio of raw material on the volatile matter of bio-briquette.Volatile matter indicates the long and short-chain of hydrocarbon, hydrogen, and oxygen in biomass that evaporate during combustion [34].The highly volatile matter was preferable because it initiated faster ignition and combustion [35].However, high volatile content releases much volatile gas such as methane ad hydrocarbon[36].It is evident from the graph that all the briquettes had volatile matte under 40%.The highest volatile matter was obtained by bio-briquettes by cow manure, followed by tapioca starch.The bio-briquettes using rejected papaya had the lowest volatile matter.At the 90% durian peel ratio, briquettes with cow manure had just under 40% volatile matter, followed by tapioca starch and rejected papaya, which had around 30% and 20%, respectively.Past reports investigated that cow manure had a volatile matter of 50%[37].Therefore, the high volatile matter in cow manure affected the volatile matter of the final mixture.Using cow manure and tapioca starch binder, volatile matter gradually decreased with the increasing durian peel composition and decreasing sewage sludge.Sewage sludge had higher volatile matter (27%) than durian peels 22%, so the lowering sewage sludge decreased volatile matter.

Figure 3 .
Figure 3.Effect of binder type and composition on the volatile matter of bio-briquette with 10% of binder

Figure 4 .
Figure 4. Effect of binder type and composition on ash content of bio-briquette with 10% of binder

[
46,47]Therefore, like fixed carbon, the durian peel compositions significantly contribute to the calorific value of briquettes.Increasing sewage sludge content in briquettes decreased calorific values.For example, in briquettes with tapioca starch binder, increasing sewage sludge from 0% to 90% decreased calorific value from 5288 to 2988 cal/gram.The briquettes with the composition of durian peel 90%, and 10% tapioca starch binder had 5288 cal/gram and already satisfied the Indonesian standard of 5000 cal/gram calorific value in briquettes[48].

Figure 5 .Figure 6 .
Figure 5.Effect of binder type and composition on fixed carbon of bio-briquette with 10% of

Table 2 . Proximate analysis of carbonized durian peel and sewage sludge
Table1presents the proximate analysis results of carbonized durian peel and sewage sludge.Carbonized sewage sludge and durian peel have a 5% and 3% moisture content, respectively.The moisture content of durian peel already meets the standard of 5-10% of moisture content for biomass briquettes[20].Durian peel had a volatile matter of 22%, and sewage sludge contained a volatile matter of 27.5%.The previous study reported that the briquette from durian peel has a comparable volatile matter of 21.13%[21].Carbonization lowers the volatile matter value in biomass because the volatile particle is vapored at the temperature of 500°C[22].For instance, the past report revealed that the volatile matter of durian peel decreased from 68% to 48 after carbonization at a temperature of 380°C for 1 hour[23].This study used a temperature of 500°C, so the higher temperature decreased the volatile matter in durian peel.Durian peel had high fixed carbon of 35% that supported a high calorific value of 3.590 cal/gram.