Conversion of Tannery Solid Wastes into Fuel Briquettes Using Wastepaper as a Binder

The disposal of tannery solid waste (TSW) and the need for clean and affordable energy are two pressing issues. Converting TSW into briquettes could be a solution to both problems. This paper focuses on preparing and characterizing fuel briquettes from TSW using a wastepaper as a binder. Raw TSW samples were obtained from the nearby leather industry, sun-dried, treated, carbonized


INTRODUCTION
Leather tanning is a major industry in many countries, including Ethiopia, where it contributes significantly to export earnings and employment.The process is complex and involves several physical, biological, mechanical, and chemical transformations to convert raw animal hides and skins into durable and versatile materials.However, despite its proven worth and long history, leather tanning is a highly polluting industry that generates large amounts of solid and liquid waste [1][2][3].Depending on the leather processing technologies installed in the tannery, approximately 750-800 kg of solid waste and 30 m 3 of wastewater are generated from the processing of one metric ton (MT) of raw hides https://doi.org/10.31881/TLR.2023.124and skins that produce 200-250 kg of leather [4,5].Globally, leather processing generates 6 million MT of solid waste annually with 23.33% and 2.50% contributions coming from China and India, respectively [2,4,5].The improper disposal of tannery solid waste (TSW) remains a major environmental problem worldwide, especially in developing countries where chromium-based tanning is prevalent and wastes are often discharged untreated [6,7].Sustainable waste-to-energy (WTE) technologies can help mitigate the environmental impacts of TSW and maximize its benefits.So far, several studies have explored different valorization technologies for converting TSW into valuable products.Research has been reported on the use of TSW for compost [8], packing materials [9], adsorbent [10], and enzyme [11] productions.Moreover, TSW's high heating value makes it a promising source for biofuels (biogas, biodiesel, bio-oil and bio-char) productions [12][13][14][15][16].While those studies are promising, several challenges must be overcome before their findings can be applied in practice, especially in low-income countries like Ethiopia, where it is essential to utilize waste in economically feasible and sustainable ways.
Currently over 30 tanneries are in operation in Ethiopia, most of which use the conventional chromium-based tanning method [6,17].It was reported that the country generates 70,104 MT of pretanned leather waste annually, disposing of it using conventional methods such as landfills and incineration [18].Additionally, Ethiopia faces a shortage of clean and affordable energy alternatives as the population and economy grow [19].In 2020, the energy mix was heavily reliant on traditional solid biomass fuels (87.1%), followed by imported gasoline (10.0%), and a small fraction (2.9%) of electricity [20].The vast majority of households use traditional wood biomass, which is inefficient and polluting, with negative impacts on health, the environment, and the economy.In addition, despite its rich biomass waste potential, only 0.6% (25 MW) of Ethiopia's total electricity generation capacity comes from municipal solid wastes-based WTE technologies [21,22].Hence, Ethiopia needs to modernize and diversify its energy mix to meet its growing energy needs.
To this end, converting TSW into fuel briquettes can help manage tannery waste, generate affordable energy, and reduce reliance on wood fuel, all of which are important goals for sustainable development.Briquetting biomass sources requires the addition of strong, non-polluting, and thermally neutral binding materials to hold the briquettes together during transportation, forming, and storage [23].Despite numerous investigations, researchers continue to seek the ideal binding material for briquetting biomass sources.
Therefore, this study aims to produce cost-effective and environmentally friendly briquettes using wastepaper as a binder from TSW sourced from a local leather industry in Ethiopia.To the authors' understanding, briquette production from TSW remains scarcely explored, and no study has investigated briquetting TSW with wastepaper binder (WPB) to produce briquette fuel.In one study, starch has been investigated as a binder for TSW briquettes [24], but its high cost and impact on the https://doi.org/10.31881/TLR.2023.124food supply make it an impractical option.In this regard, despite its poor fuel properties, wastepaper is a viable binder for biomass-based fuel briquettes due to its adhesive properties [25], low sulfur contents and nitrogen oxide emissions [26,27], and abundance.Hence, this study aims to explore the briquetting of TSW using wastepaper as a binder with desirable physical and thermal properties for alternative energy use in low-income communities.The briquettes produced in this study were compared to other reported briquettes.

EXPERIMENTAL Raw materials collection and preparation
The solid wastes were collected from the post-tanning operation units (tanning, buffing, and shaving) of Sheba Leather Industry PLC in Wukro town, eastern Tigray, Ethiopia.As shown in Figures 1a, b, and   c, mixture samples in equal amounts from each operation unit were treated by sorting to remove any trace of impurities (bones, metal, and wood) and other unwanted materials, followed by sun-drying and oven-drying at 105 °C to reduce moisture content [24].Next, the raw material was analyzed for proximate analysis (volatile matter, fixed carbon, ash content) and calorific value.For the TSW carbonization, a kiln drum fabricated at Mekelle University was used.The drum has a chimney on the top with holes at the bottom.The treated TSW were placed inside the drum, burned from the bottom and waited until the smoke from the drum stopped.Then, the carbonized sample was reduced to a uniform size of less than 2 mm (Figure 1d) using a grinder by passing through a series of sieves arranged vertically.
On the other hand, WPB samples were collected from Mekelle University's student residence and staff office bins located in Mekelle City, Tigray, Ethiopia.About 500 g wastepaper sample was manually shredded into small pieces, mixed, and soaked in a small bath jar containing 1.5 L water for three days, followed by maceration to form a uniform slurry as displayed in Figure 1e.

Briquette preparation
In this study, a manual press moulding machine designed to have 5 MPa [26] pressure with a cylindrical mould, having a 7 cm outer diameter and 20 cm height was used.Briquette preparations were conducted at the laboratories of the Department of Chemical Engineering, and the School of Mechanical and Industrial Engineering at Mekelle University.A total of five briquettes were developed using a total weight of 400 g of combined raw materials at different proportions of TSW:WPB (100:0, 80:20, 60:40, 40:60, 20:80).After mixing, the raw materials were then placed in the mould and hand pressed for six min.After the production process, the briquettes were carefully ejected from the mould and sun-dried for one week to reduce their moisture content.Finally, the briquettes were ready for further characterization, as shown in Figure 1f.

Briquette characterization
The proximate analysis test was conducted in accordance with the ASTM standards outlined in Afsal et al. [28].By adhering to the ASTM standards, the physical properties (density, porosity index, shatter resistance and combustion rate) were evaluated to measure the quality and durability of the produced briquettes.Additionally, the moisture, volatile matter, fixed carbon, and ash contents were determined to assess the quality of the briquettes.Furthermore, the calorific value of the briquettes was measured using bomb calorimetry at the laboratory of Messebo Cement Factory PLC, Mekelle, Ethiopia.Below are detailed explanations of physical properties, proximate analysis, and calorific value determinations.
Density: The density of briquette samples was determined by dividing the sample mass (m) by the mould volume (V) [24].
Porosity index: Each briquette sample was immersed in water to determine the percentage of porosity index (PI) by calculating the difference between its mass after immersion and its initial dry weight.
where, ΔW and Wd are the weight difference after immersion and dry weight briquette samples, respectively.https://doi.org/10.31881/TLR.2023.124 Shatter resistance: The shatter resistance (SR) value was estimated by weighing the briquette sample before shattering (Wb) followed by dropping it from a constant 2 m height and recording the weight after shattering (Wa).Then, SR was obtained as follows.
Combustion rate: The combustion rate (CR) is a crucial parameter that measures the time taken for a specific amount of briquette sample to burn in the air.To determine this rate, 100 g of each briquette sample was placed separately on a traditional burner for 30 min.Afterwards, the retained sample was weighed to calculate CR as follows.
where mb is the mass of burned briquettes and t is the time of burning.
Moisture content: The percentage of moisture content (MC) was determined by measuring the difference between the initial weight before drying (W1) and the weight of the briquette after oven drying (W2) at 105 °C for 2 hr.
Volatile matter content: For the volatile matter content (VMC) determination, a 2 g sample of ovendried briquette was placed in a furnace at a temperature of 550 °C for 10 min.Then, the VMC percentage was calculated from weight loss during heating divided by the initial weight of the sample using the following equation.
where, W2 and W3 are the weight of oven-dried briquette sample and weight after furnace drying at 550 °C for 10 min., respectively.

Ash content:
The percentage of ash content (AC) was determined by taking 2 g of oven-dried briquette sample with known weight (W2) and furnace drying at 550 °C for 4 hours.The retained ash was then weighed, and the percentage of AC was calculated using the following equation.where W4 is the weight of the sample after furnace drying at 550 °C for 4 hours.

𝐴𝐶 (%) =
Fixed carbon content: The percentage of fixed carbon content (FCC) was calculated by subtracting the total percentages of moisture, volatile matter, and ash contents from 100, as follows.

Proximate analysis of raw tannery solid waste sample
Proximate analysis is a useful tool for assessing a fuel's performance and characteristics.From the experimental analysis, the density of the raw TSW sample was determined to be 0.40±0.04g/cm 3 using equation 1, and the content of moisture was found to be 19.50±0.50%,indicating that moisture content reduction is necessary before carbonization or densification.Additionally, the volatile matter was determined to be 46.37±0.33%using equation 6, and the ash content was calculated to be 5.10±0.10%by dividing the weight of ash by the weight of the dry sample using equation 7. Table 1 presents the results of the proximate analysis conducted on the TSW sample material.These findings suggest that TSW possesses properties that fall within the range of biomass raw material standards suitable for briquette fuel production.The results also highlight the importance of continued research in this field to identify and develop TSW-based alternative energy as viable sources of renewable energy that can help mitigate the impact of industrial wastes.

Physical properties of the TSW-WPB-based briquettes
The briquette density was determined from the mass-to-volume ratio.The variation in the composition of the briquettes affects their homogeneity and size.Figure 2 shows the results of the measured density of the TSW-based briquettes.As displayed in Figure 2, the density of the briquette samples ranged from 0.34±0.03 to 0.62±0.03g/cm 3 , which is in the range of agro-wastes and wood residues-based briquettes [29].The highest density was observed in the sample containing 80% TSW and 20% WPB, which could be attributed to the amount of adhesive that meets the void ratio formed by the particle size [30].From the highest point onward, the density of the briquettes decreased with the wastepaper amount.This can be associated with the fact that wastepaper is made up of lightweight materials such as cellulose that can easily compressed.
As shown in Table 2, the PI value of the briquettes produced ranged from 25.10±0.17 to 34.50±1.32%.
Adding binder content decreased the PI values, likely because the porous cellulose in the wastepaper fills the void spaces in solid waste.The large decrease in PI for the sample with 20% WPB suggests that this proportion of binder could be optimal.Furthermore, the SR ranged from 87.94 to 93.46%.The SR values increased to a maximum at 40% binder (93.2±0.26%) and then decreased with increasing binder content, likely due to the weak properties of the wastepaper.In addition, the rate of combustion https://doi.org/10.31881/TLR.2023.124increased with wastepaper addition (Table 2).Longer burning time is reported to be an indication of high briquette fuel density [29].The produced TSW-based briquettes were generally found to have satisfactory physical properties, with WPB percentage influencing them.These findings provide valuable insights into the potential use of TSW briquettes as a sustainable alternative to traditional fuels.Details of these results are given in Table 2.

Proximate analysis of TSW briquette samples
Proximate analyses were performed on the tannery wastes and wastepaper-based briquette.When it comes to briquettes, proximate analysis can provide insight into the chemical and physical properties of the briquette in terms of its moisture, volatile matter, ash and fixed carbon contents.Table 3 summarizes the results in comparison to values reported in the literature.

Moisture content
Briquette fuels are hygroscopic in properties meaning can easily absorb moisture from surroundings [37].Hence, the value of water content needs to be determined as it affects the thermal efficiency and burning rate of the briquettes.The MC as a percentage of the initial weight of a briquette is shown in Figure 3.The MC of the TSW-based briquettes varied from 1.30±0.01 to 5.30±0.10%,which is within the range of reported biomass-based briquettes (Table 3).The highest moisture content was obtained for the briquette sample with 80% WPB.The moisture content increased as the percentage of wastepaper increased, likely due to the hydrophilic behaviour of the wastepaper.As higher moisture can reduce the calorific value of material and make it more difficult to burn, lower MC of a fuel source is good for improving its combustion efficiency and cleanliness.This is because moisture acts as a natural fire retardant, hindering the combustion process and producing more smoke and pollutants [24].A further disadvantage of high moisture content is the facilitation of a breeding ground for fungi and other microorganisms [33].

Ash content
Ash is the amount of inorganic material that is left behind after the briquette is burned.High ash concentration lowers the calorific value of the fuel since the ash does not participate in the combustion [29].The ash content for the produced briquettes ranges between 2.80±0.04 to 5.50±0.13%(Figure 3).
A lower percentage of ash content was obtained for the briquette with no WPB and higher for the briquette sample with 80% WPB (Figure 3).The ash content of the briquette sample increases with increasing the amount of wastepaper, likely due to inorganic contaminants in the wastepaper that remain after incineration.A lower percentage of ash content has been obtained in this study in comparison to different biomass-based briquettes, which shows the produced briquettes have good quality [31,32,35,36].Elevated levels of ash content can have a detrimental impact on the combustion behaviour of fuel, leading to the creation of hazardous chemicals that pose a threat to the environment.It is imperative to maintain low levels of ash content to ensure optimal combustion efficiency and minimize the release of harmful pollutants.

Volatile matter and fixed carbon contents
The percentage of volatile matter in a biomass source is a mixture of short and long-chain hydrocarbons such as combustible or incombustible gases or a combination of both mainly comprises carbon, hydrogen, and oxygen elements [29].The lower the volatile matter the higher will be the energy value.On the contrary, with increasing the percentage of fixed carbon calorific value increases.
A high percentage of fixed carbon is an indication of a high heating value of the briquette [34].Figure 4 shows the variation of volatile matter and FCC of the produced TSW-based briquette fuels.
As shown in Figure 4, the volatile matter was obtained in the range between 4.01±0.09to 10.21±0.18%.
The briquette sample with no WPB has shown a lower percentage of volatile matter and higher for the briquette sample having 80% WPB.Percentages of VMC increased with the increasing amount of WPB.
On the other hand, a lower percentage of fixed carbon (79.00±0.54%)was found for the briquette sample with 80% WPB and higher FCC was obtained on pure TSW briquette (91.90±0.36%).As illustrated in Figure 4, increasing the amount of wastepaper resulted in a decrease in the percentage of fixed carbon.A lower percentage of the volatile matter was obtained in this study, in comparison to other briquette materials produced from different biomass sources; such as a mixture of 25% vegetable market waste and 75% sawdust (71.72-83.20%)[28], a mixture of biogas digestate and wastepaper (38.57-61.19%)[31], disposed wood waste with paper (11.30-26.00%)[35], and rice husk (63.80-71.60%)and banana leaf (77.40-79.00%)[36].This is an important consideration in the fuel production process as the energy value of a substance increases as its volatile matter decreases.This means that the less volatile matter a substance contains, the more energy it can provide.https://doi.org/10.31881/TLR.2023.124

Calorific value
Calorific or heating value is a measure of heat energy present in a material that depends on its chemical composition and can vary widely between different substances.The calorific value for the produced briquettes ranged between 20.48±0.08 to 21.09±0.04MJ/kg (Figure 5).As displayed in Figure 5, the highest calorific value was obtained for the briquette sample with 80% TSW and 20% WPB (21.09±0.04MJ/kg), and a lower value (20.48±0.08MJ/kg) was obtained for briquette sample with 80% of WPB.
The calorific value of TSW briquette increases up to 20% of wastepaper admixture and gradually decreases with increased wastepaper.The results suggest WPB has a significant effect on the heating value of the TSW-based briquettes, and about 20% adhesive is sufficient to fill the voids between the particles.A briquette sample with a higher heating value is more acceptable.The calorific values obtained in this study were better than some reported biomass-and waste-based briquettes [27,28,31,33,36], suggesting a more efficient and effective option for briquette fuel production, and in agreement with elsewhere reported TSW-based biomass briquette with cassava starch binder [24].https://doi.org/10.31881/TLR.2023.124All briquette samples in this study had sufficient calorific values to meet the energy requirements of household cooking and other small commercial applications [28].TSW-derived briquettes can directly replace wood biomass in household cooking stoves, offering several advantages, such as higher energy density, lower emissions, and convenience.They can also be used for various commercial applications, such as fueling brickmaking kilns, boilers, crop dryers, and electricity generators.In addition, the briquettes are produced with lower moisture content, which is important for storage and handling.
Briquettes with higher moisture content spoil and grow mould more easily, and are heavier and more likely to stick together, making them difficult to transport and handle.Moreover, using TSW-based briquettes can help to mitigate the environmental impacts of TSW while maximizing its benefits.
Furthermore, it creates jobs and boosts the local economy.

CONCLUSION
This study confirms that briquettes produced from TSW have great potential for use as a viable and economical domestic fuel.The study presents an opportunity to combine TSW and wastepaper to create briquettes that can be used as a clean and cost-effective energy source in rural communities and small businesses.The briquettes showed calorific values that are significantly high and comparable to those of agricultural waste briquettes.The briquettes displayed an acceptable moisture content that had minimal impact on their calorific value.The relatively high shatter index of the produced briquettes indicates that they can be handled and transported with minimal risk of disintegration.

Table 3 .
Proximate and heating value characteristics of tannery solid wastes and wastepaper-based briquettes in comparison with values reported in the literature