The potential of industrial sludge and textile solid wastes for biomass briquettes with avocado peels as a binder

Producing biomass briquettes from industrial solid wastes is a more environmentally friendly way to provide alternative energy and is essential for Ethiopia to satisfy its growing energy needs while also ensuring efficient waste management in the expansion of industrial parks. The main objective of this study is to produce biomass briquettes from a mixture of textile sludge and cotton residue using avocado peels as a binder. Textile solid waste, avocado peels, and sludge were dried, carbonized, and turned into powder to make briquettes. Briquettes made from the mixture of industrial sludge and cotton residue were combined in various ratios: 100:0, 90:10, 80:20, 70:30, 60:40, and 50:50 with the same amount of the binder. Briquettes were then made using a hand press mold followed by sun-drying for two weeks. The moisture content, calorific value, briquette density, and burning rate of biomass briquettes ranged from 5.03 to 8.04%, 11.19 to 17.2 MJ/kg, 0.21 to 0.41 g/cm3, and 2.92 to 8.75 g/min, respectively. The results revealed that the briquette produced from a 50:50 ratio of industrial sludge to cotton residue was the most efficient. The inclusion of avocado peels as a binder enhanced the briquette's binding and heating properties. Thus, the findings suggested that mixing various industrial solid wastes with fruit wastes could be an effective means of making sustainable biomass briquettes for domestic purposes. Additionally, it can also promote proper waste management and provide young people with employment prospects.


Introduction
The majority of the world's growing energy demand is still being supplied by fossil fuel energy sources, specifically coal, crude oil, and natural gas (Tipantuna et al. 2021). As a result, reserves are rapidly depleted, bringing the time when they run out closer. They have also made a variety of environmental problems worse during the past few decades. Climate change and global warming, caused by CO 2 and other greenhouse gases (GHGs), have become an international concern in recent years (Martin et al. 2021). Several countries, including Ethiopia, have boosted their use of renewable energy sources, notably hydropower, wind, and solar energy, for electricity generation over the last decade Benti et al. 2022), wind (Asress et al. 2013, geothermal (Benti et al. 2023), and bioenergy (Degfie et al. 2019;Erchamo et al. 2021;Tiruye et al. 2021;Benti et al. 2022). Currently, firewood, agricultural waste, dung, and charcoal make up the majority of Ethiopia's energy use, which accounts for over 87% of the nation's energy needs to be met by conventional biomass, with the remaining 13% coming from the national electricity grid and petroleum products (Wolde et al. 2020;Tiruye et al. 2021). The heavy dependence on traditional biomass in Ethiopia is leading to different environmental, health, and economic problems, including deforestation, soil erosion, water contamination, and indoor air pollution (Wassie 2020).
The number of industries in Ethiopia has been rapidly increasing in recent years as a result of the growth of industrial parks; consequently, much industrial waste is dumped into the environment and causes environmental degradation.

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To alleviate this environmental issue, wastes must be transformed into various value-added products, such as biomass briquettes and brick production (Beshah et al. 2021). Trees are the principal CO 2 absorbers and help mitigate the effects of climate change. Unsustainable usage of forests as wood fuel, such as firewood and charcoal, should be avoided because they emit pollutants when burned (Yang et al. 2022). As a result, sustainable fuel charcoal development is necessary, and briquettes may be a viable solution to these problems.
Binders are also the most crucial element in the densification of briquettes. The type of binder has an impact on both the quality and performance of briquettes. To firmly bind the substrates and raise the calorific content of the produced briquettes, various types of binders are usually applied. The three types of binders utilized in the briquetting process are organic, inorganic, and composite. Organic binders have several great benefits, including strong bonding, efficient burning, and little ash. Likewise, inorganic binders have numerous advantages, including a wide supply, low cost, outstanding thermostability, and high hydrophilicity. However, a key issue brought on by the use of inorganic binders is related to the considerable increase in ash. The composite binders are made up of at least two binders, each of which serves a particular purpose (Zhang et al. 2018).
As a renewable energy source, the densification of biomass into high-density, energy-efficient briquettes has shown the potential to alleviate cooking energy poverty and offer environmental and economic benefits in many developing countries (Akowuah et al. 2012;Hakizimana and Kim 2016;Kpalo et al. 2020Kpalo et al. , 2021. If briquettes are produced at a cheap cost and are easily accessible to customers, they could be widely used as an energy source in place of firewood and charcoal for residential cooking and small industrial operations. Additionally, briquettes are advantageous over fuel wood in terms of heat output, cleanliness, and suitability and require less space for storage than fuel wood (Yulei et al. 2020). Biomass briquette production can be implemented at a small scale and will be a business idea for micro and small enterprises. Therefore, in addition to environmental benefits, similar studies can contribute greatly to economic growth and social harmony by creating job opportunities for many jobless youths in Ethiopia.
In this study, we showed the potential of solid organic wastes from the textile industry (sludge and cotton residue) for the production of high-energy biomass briquettes that can effectively satisfy both domestic energy needs and environmental concerns. Moreover, avocado peels are added to textile solid wastes to boost their calorific content and capacity to bind substrates. In addition to the produced briquettes, the physicochemical characteristics of the raw sludge, cotton-based textile waste, and avocado peels were also analyzed using various methods.

Sample collection
Sludge samples were collected from Hawassa Industrial Park. The cotton residue was collected from the leftover textile products. Each day, on average, 1200 kg of cotton waste is generated, which is a sustainable resource for making biomass briquettes. In addition, avocado peel wastes were collected from selected juice houses in Addis Ababa city.

Proximate analysis i) Moisture Content (MC)
The moisture content (MC) is the amount of water present in the sample. The moisture content of 2 g of each sample was determined using (ASTM 1992). Each measurement was taken in triplicate. Samples were dried in an oven set at a temperature of 110 °C for 4 h and cooled in desiccators for half an hour to prevent further absorption of moisture. The samples were then weighed. The moisture content of each sample was calculated using Eq. (1).
where W 0 is the weight of the sun-dried sample (g), and W 1 is the weight of the oven-dried sample (g). (1) The volatile matter (VM) of each sample was determined using the ASTM method (ASTM 1992). Each measurement was taken in triplicate. Two grams of each sample was placed in a muffle furnace set at a temperature of 550 °C for 10 min and then weighed after cooling in a desiccator for half an hour. The volatile matter was estimated using Eq. (2).
where W 1 is the weight of the oven-dried sample (g) and W 2 is the weight of the sample after furnace heating (g).

iii) Ash Content (AC)
The ash content of each sample (2 g) was determined using (AOAC 1999) in triplicate. The samples were placed in a muffle furnace and heated at 550 °C for 4 h. Then, the crucibles were removed from the furnace and placed into the desiccators to cool for an hour. After cooling, the weight was used to determine the ash content and was calculated using Eq. (3).
where W 1 is the weight of an oven-dried sample (g) and W 3 is the weight of ash (g).

iv) Fixed Carbon Content (FC)
Fixed carbon is the total amount of carbon used for the production of heat energy of the fuel during the combustion of the biomass briquette. The percentage of fixed carbon was calculated using Eq. (4) by deducting the sum of percentages of moisture content (MC), volatile matter (VM), and ash content (AC) from 100% (ASTM 1992).

Ultimate analysis i) Heavy metal determination of feedstocks
Samples that had been sun-dried (sludge, avocado peels, and cotton-based textile residue) were then dried in an oven at 110 °C until they attained a constant weight, ground into fine particles for homogenization, and passed through a 90 µm sieve. The sample preparation procedure for heavy metal analysis was carried out according to EPA 3050B (USEPA 1996). Twenty milliliters of aqua regia (HNO 3 : HCl = 1:3 volume ratio) was used to digest 5 g of the dried sample over the course of 24 h. A sample was cooked for 2.5 h in 500 mL of the solution prepared by adding up to 400 mL of distilled water. After that, the solution was filtered using Whatman No. 1 filter paper. The filtrates were then collected to determine the concentration of heavy metals (Pb, Cd, Cr, Ni, Cu, Zn, and Cd). Heavy metal analysis was performed using a 4200 MP-AES (microwave plasma atomic emission spectrometry) instrument.

b) Elemental analysis of samples
The elemental analysis was carried out utilizing the EA 1112 Flash CHNS/O-analyzer after the samples were fully dried. The sun-dried samples were subjected to oven drying at 110 °C for 4 h in GX-65B followed by grinding using an RRH-200 grinder. The samples were then sieved with a 90 µm sieve to make them ready for analysis. All samples were evaluated for their carbon, hydrogen, and nitrogen (CHN) content following ASTM D3176/D5372 methods (ASTM 2002).

Briquette production
The collected industrial solid waste sludge, cotton residue, and avocado peels were sun-dried for one week in open air. The process of carbonization was carried out in oxygen-limited conditions inside the furnace (Nazari et al. 2020). After being fully dried, the sample was carbonized in a muffle furnace for 30 min at 500 °C. After cooling, the carbonized samples were ground using an RRH-200 grinder. Avocado peels were employed to strengthen the binding and improve the biomass briquette's compactness, strength, and calorific value. Sludge-to-cotton residue compositions were prepared at weight percent ratios of 100:0, 90:10, 80:20, 70:30, 60:40, and 50:50. The same amount of binder, 50 g of avocado peels, was added to each mixture. A binder was used in a 1:5 ratio to sludge and cotton residue to produce a total mass of 300 g. The three raw materials were mixed manually, and a slurry was formed using 150 mL of water. Then, a slurry was transferred into a briquette machine (hand press molder) to produce a cylindrical beehive briquette with 12 holes at the center that were 13 cm in diameter and 4 cm in length. It was then sun-dried for two weeks. To examine its potential as a biofuel for heating and cooking, the dried biomass briquettes were subjected to proximate analysis, ultimate analysis, and combustion testing. Figure 1 shows a flow chart of briquette production from biomass sludge, cotton residue, and avocado peels.

Briquette density determination
The density depends on the geometry, size, surface properties, and measurement method (Attarzadeh et al. 2020). Denser items are preferred in terms of handling, storage, and transportation. In comparison to briquettes with lower bulk densities, those with higher densities can produce more heat energy. The particular briquette density was determined after two weeks of drying in open air using the density formula.

Calorific value determination
The calorific value is the quantity of heat released by burning a small mass of material in an oxygen environment. It was determined using the Parr 6200 isoperibol oxygen bomb calorimeter, which is commonly used for calorific testing of this type of material (Thapa and Engelken 2020). One gram of sample was transferred into an ignition cup, wrapped with a fuse wire using cotton thread, and sealed before entering the bomb calorimeter. The sample was ignited inside a bomb in a high-pressure oxygen-rich atmosphere. The bomb calorimeter was placed in a static jacket containing 2 L of water and 30 atmospheres of oxygen before being connected to the energy source. After approximately 15 min of combustion, the calorific value was calculated internally and displayed on the calorimeter's screen in MJ/kg units.

Ignition time
The dried briquettes ignited when placed straight on the fired electrical stove. The electrical stove was left open from the beginning of ignition until it entered its steady-state burn phase. Compared to highly compacted briquettes, less compacted briquettes are more likely to ignite (Yulei et al. 2020).

Burning time
Depending on the nature of the sample, some samples burn in a short time, but others may require a longer time to complete their combustion. The burning time was estimated by keeping track of the interval between the starting and ignition ending times using a stopwatch.

Burning rate
The burning rate determines the rate at which a certain mass of fuel briquette is combusted in a given time interval (Kpalo et al. 2020). The fuel-burning rate was determined according to (Ikelle et al. 2020). A briquette sample of known weight was placed on wire gauze (briquette stove) for ignition. The briquette's burning rate was monitored every five minutes during the combustion process until it was completely burned and a constant weight was obtained. The burning time was calculated using Eq. (5).  where BR is the burning rate (g/min), W i is the initial weight of fuel before cooking (g), W f is the final weight of fuel after cooking (g), and t is the total burning time (minute).

Emission test
The pollutant emissions from raw materials (sludge and cotton waste), binder (avocado peels), and briquettes made from these solid wastes were measured. The emission tests of ignited raw materials and produced briquettes were examined using a gas flow meter within a hood. The average emission value was obtained for each sample (10 g) based on 30 measurements collected every minute.

Proximate analysis of raw biomasses
As shown in Fig. 2, the moisture content of textile sludge was higher than that of cotton residue and avocado peels on a dry basis of biomass. This indicated that the amount of water in textile sludge was higher than that in cotton residue and avocado peels. Thus, this implies that removing the moisture from textile sludge takes more time and energy than it does from cotton residue and avocado peels. The volatile matter of cotton residue was greater than that of textile sludge and avocado peels on the dry basis of biomass. This indicates that compared to industrial sludge and avocado peels, gas emissions from cotton residue were higher, and when a briquette containing more cotton residue burns, more gases are released into the environment. Textile sludge had a larger ash content than cotton residue and avocado peels. This indicates that a large portion of the textile sludge is inorganic and is not converted into energy during the combustion of the biomass briquette. To limit excessive ash generation and produce the desired briquette, a much greater amount of cotton residue was added to the mixture. Figure 3 and Table 1 present the proximate analysis of the briquettes produced from different ratios of sludge to cotton residue at a fixed amount of binder. The moisture contents of the raw sludge were higher than those of the biomass briquettes produced at different ratios, as shown in Fig. 2. The carbonization process that took place during briquette production is what caused the decrease in moisture levels of the formed briquettes. The moisture content in the briquette decreases as the composition ratio of sludge decreases from 100 to 50%, while the composition ratio of cotton residue increases from 0 to 50% in the fixed mass of avocado peels. As shown in Fig. 2, the raw materials, sludge, avocado peels, and cotton residue contain considerable amounts of volatile substances, making the briquettes made from them highly reactive fuel with a rapid combustion rate. Among raw materials, the cotton residue has higher volatile matter than the others, indicating that it can easily ignite when combusted. The amount of volatile matter can be reduced significantly by carbonization (Marreiro et al. 2021). Similarly, the carbonization process decreases the volatile matter of the produced briquettes more than that of the raw materials. As a result, briquettes offer increased density and heating values compared to raw materials. The ash content influences heat transfer and oxygen diffusion to the surface during char combustion (Gruber et al. 2021). Fuel with low ash content is better suited for thermal Proximal analysis utilization than fuel with high ash content. The ash content is highest in the briquette with the highest ratio of industrial sludge to cotton residue (100:0) compared to the other ratios. This implies that both the combustible content of sludge and its level of energy output are quite low. The briquette formed at a 50:50 ratio contained a lower ash content-it provides a greater amount of energy than other proportions.

Proximate analysis of produced biomass briquettes
Fixed carbon is the total amount of carbon used for the production of heat energy in the fuel during the combustion of biomass. The fixed carbon content increases when the composition changes from 100:00 to 50:50. Briquettes with more fixed carbon can offer more calorific value. Generally, the ratio of 50:50 of sludge to cotton residue in the fixed mass of avocado peels as a binder is preferable. It has a lower moisture content, lower ash content, higher fixed carbon, and lower volatile matter. As shown in Table 1, modest standard deviation values were found in all of the proximal analyses, demonstrating that the triplicates were fairly similar.

Heavy metal analysis of the raw materials
Heavy metals are threats to human beings when they are present in briquettes (Lau and Tsai 2022). If their concentration in the briquettes exceeds the standard limit, it causes environmental problems such as damaging vegetables, water bodies, and plants and human health complications such as respiratory problems, heart case problems, and some allergic problems on the skin. The heavy metals in the sludge come from the dyeing of cloths and cotton residue, and avocado peels absorb from the soil using their roots. As seen in Table 2, the concentration of heavy metals (Pb, Cr, Cu, Zn, and Ni) present in raw materials is below the standard limit (US-EPA 1994), and even for some of them, the value is below the instrument's detection limit. These heavy metals, therefore, have no harmful effects on the environment or human health. Fig. 3 Proximate analysis of briquettes made from various ratios of sludge to cotton residue at a fixed amount of avocado peels (binder). AC refers to ash content, VM refers to volatile matter, FC refers to fixed carbon, MC refers to moisture content, S refers to industrial sludge, and CR refers to cotton residue

Elemental analysis of raw materials
The chemical composition of raw materials and produced biomass briquette samples was determined. The percentages of carbon (hydrogen) in the sludge, cotton residue, and avocado peels were 15.67 (3.77), 43.71 (6.91), and 56.02 (7.10), respectively. The amount of heat energy produced is directly proportional to the amount of carbon content in the briquette. The carbon content present in the cotton residue is far higher than that in the sludge and binder.

Briquette density determination
Briquette density is a measurement of the weight of solid fuel briquettes per unit volume that can be used to determine how much heat or strength they can provide (Tumuluru and Fillerup 2020). The briquette with the highest density should be chosen. As shown in Fig. 4, the briquette density increases as the cotton residue content increases at a constant amount of binder. Due to the fiber character of cotton residue, in addition to being used as a primary component, it also helps to improve the binding of briquettes. The briquette (50:50) achieved the highest density and the longest burning time. Figure 4 demonstrates that briquettes with higher density (50:50) burn at lower rates, indicating that fewer briquettes will be used per minute and will last longer when used. The calculated standard deviations in density measurements are less than 0.05 for all tested composition ratios.

Calorific value determination
The calorific values of raw sludge, cotton residue, and avocado peels were found to be 6.74, 16.70, and 24.6 MJ/ kg, respectively. Industrial sludge has a lower calorific value than cotton residue and avocado peels. When examined in terms of calorific value, avocado peels have an average calorific value that is significantly higher than that of cotton waste and industrial sludge. In this work, biomass briquettes are produced using a waste-driven binder and a variety of industrial solid wastes as source materials. Additionally, a binder made from avocado peels raises the energy content of the briquettes. The energy value will rise if the briquettes include more sludge and less cotton residue while maintaining the same amount of binder. Table 3 shows the calorific value of briquettes made with various ratios of sludge and cotton waste with a fixed content of avocado peels (50 g) as the binding agent. The briquette produced from 100% industrial sludge was   (Erol et al. 2010;Yin 2011). According to Table 3, the composition ratio of 100:00 has the highest standard deviation (1.58), while the standard deviations of the other composition ratios are all less than one.

Combustion test
The ignition time indicated how long it takes for the briquette to ignite during steady-state combustion (Yulei et al. 2020). It takes longer to ignite the combustion if the briquette's particles are tightly packed (Olugbade et al. 2019). Table 3 shows that the igniting time for briquettes made from various compositions ranges from 7.02 min (100:0) to 12.15 min (50:50). As the quantity of cotton residue increases, the briquette density increases, and the particle arrangement becomes more compact, which results in an increase in the ignition time. For example, the 50:50 briquette needs more time to begin combustion than the 100:0 briquette. As shown in Table 4, the burning time of briquettes ranges from 35 to 95 min in the ratio of 100:0 to 50:50. This suggests that higher cotton residue mix (50:50) briquettes burn longer when used, for example, for cooking purposes. Cotton residue raises the briquette's density, which lengthens the burning time. The boiling test of water utilizing waste-derived briquettes is comparable to other energy sources. On a 50:50 briquette, one liter of water was boiled for 17 min. It took 16 min to boil the same quantity of water using the same amount of regular charcoal.
According to Oladeji reports, rice husks that had been bound with glue and starch took 28 min to boil water, but sawdust briquettes bound with glue and starch took 26 and 28 min, respectively, to reach the boiling point. Melon shell briquettes that were bound with glue and starch took 22 and 24 min, respectively, to boil the water, but cassava peel briquettes that were bound with glue and starch took 20 and 22 min, respectively. To boil the same volume of water, kerosene, and firewood took 14 and 18 min, respectively. Thus, earlier findings demonstrated that biomass briquettes are reliable substitutes for kerosene and firewood (Oladeji 2013).
The burning rate is an indicator of the intensity of combustion, i.e., the rate of fuel combustion influences the rate at which energy is delivered (Ikelle et al. 2020). Biomass briquettes will be consumed quickly because of the shorter burning time caused by the greater burning rate. As Table 4 indicates, the burning rate of the briquette decreases as the content of cotton residue increases while the amount of industrial sludge decreases. This shows that the interconnection of particles increases. The 50:50 ratio of sludge and cotton residue took more time to burn than the 100:0 ratio.

Emission test
As shown in Table 5, the produced briquettes are also tested for CO and NO emissions. The results revealed that the briquettes produced from sludge and cotton residue in the presence of avocado peels as binders do not release any NO gas during combustion. However, the raw cotton residue, sludge, and avocado peels released 2, 1, and 0.8 ppm of NO emissions, respectively.
According to the U.S. National Ambient Air Quality Standards, the usual levels of CO in homes with gas stoves typically range from 5 to 30 ppm, whereas in the outdoor air, it is  Table 3, the amount of CO released during combustion is within the limit of the U.S. National Ambient Air Quality standards. Although the briquettes emit CO at levels of approximately 20 ppm, it is still advisable to use them in an open environment at least until their ignition reaches a steady state. Pilusa et al. and Suryaningsih et al. reported 74 ppm and over 600 ppm of CO, respectively, for briquettes produced from eco-fuel biomasses and a mixture of rice husk to corn cob (Pilusa et al. 2013;Suryaningsih et al. 2019). Furthermore, the level of carbon dioxide in the produced briquettes is less than 3200 ppm, which is low and does not cause notable health problems.

Conclusion
This study revealed that biomass briquettes produced from a mixture of industrial sludge and cotton residue using avocado peels as binders were identified as potential substitutes for charcoal, firewood, and petroleum-based fuels for domestic uses as a source of energy in Ethiopia. The briquette made from a higher proportion of cotton residue (50:50) was found to be the most effective composition over other proportions considered in the study. Therefore, as the composition of cotton residue in the proportion increases, the calorific value, fixed carbon content, briquette density, and burning time of the briquette increase.
Moreover, the addition of avocado peels as a binder significantly improved the briquette's binding property and heating content. Using industrial wastes as a source for biomass briquettes offers alternative household energy while lowering indoor air pollution that causes diseases. Hence, household energy constraints could be resolved by providing alternative sustainable energy in the form of biomass briquettes made from industrial sludge, textile solid waste, and fruit waste. Additionally, it provides more employment possibilities in the country.
Authors' contributions All authors contributed to the study's conception and design. Material preparation, data collection, and analysis were performed by Eyasu Derbew Demeke, Mekonnen Abebayehu Desta, and Yedilfana Setarge Mekonnen. The first draft of the manuscript was written by Eyasu Derbew Demeke, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Funding The authors would like to acknowledge the financial support from the thematic research project (Grant No: TR/036/2020) of Addis Ababa University. YSM acknowledges support from the ICTP through the Associates Programme (2020-2025).
Data availability All data generated or analyzed during this study are included in this published article.

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Ethics approval and consent to participate Not applicable

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Competing interests The authors declare that they have no competing.