Binder material, Biofuel briquettes, Compressive strength, Combustion efficiency.

The global energy landscape is undergoing a significant transformation, driven by the need for sustainable and renewable energy sources. Biofuel briquettes, produced from biomass feedstocks, have emerged as a viable alternative to traditional fossil fuels (Williams et al., 2016). Their utilization not only helps mitigate greenhouse gas emissions but also promotes energy security and waste reduction. However, the performance of biofuel briquettes is heavily influenced by their physical and chemical properties, particularly the choice of binder materials used during production (Zhang et al., 2018). Binders play a crucial role in enhancing the mechanical and thermal properties of briquettes. Mechanical properties, such as compressive strength, durability, and density, are essential for ensuring that briquettes can withstand the stresses of transportation, storage, and combustion (Singla & Mediratta 2013). On the thermal side, properties such as calorific value, ignition temperature, and combustion efficiency determine the energy output and overall effectiveness of the briquettes as fuel. Different binder materials, including starch, lignin, molasses, and other natural adhesives, exhibit varying effects on these properties (Vamza et al., 2022). For example, starch-based binders have been shown to improve the mechanical strength of briquettes, while lignin, a natural polymer, contributes to enhanced thermal stability. The selection of an appropriate binder is therefore critical in optimizing the production process and achieving desired performance outcomes. Many studies discuss the mechanical and thermal properties of briquettes but overlook the environmental impacts of binder selection, such as carbon emissions during production or disposal of inorganic binders. There is also limited research on the cost-effectiveness of various binders, especially for large-scale biofuel production (Picchio et al., 2020).

Despite the recognized importance of binders, there remains a gap in the comprehensive understanding of how different materials impact both mechanical and thermal properties of biofuel briquettes. This study aims to fill this gap by systematically evaluating the effects of various binders on the quality of biofuel briquettes made from diverse biomass feedstocks (Yunusa et al., 2024). By focusing on key mechanical and thermal properties, the research seeks to provide insights that can inform better production practices and enhance the

overall viability of biofuel briquettes as an alternative
energy source (Roy et al., 2018).

Most studies on biofuel briquettes focus on one or two types of binder materials (e.g., starch, clay, or molasses) without a comprehensive comparison across a broader range of binders (Obi et al., 2022). There is a lack of systematic evaluation of organic vs. inorganic binders on mechanical strength (durability, compressive strength) and thermal properties (calorific value, combustion efficiency). The influence of binder content (proportion) and binder-to-biomass ratios on the briquette's performance has been underexplored.

The findings of this study will contribute to the broader goal of promoting renewable energy solutions, supporting sustainable development, and addressing the pressing challenges associated with fossil fuel dependency (Sen & Ganguly 2017).  Through careful analysis and evaluation, this research will help pave the way for more effective and efficient biofuel briquette production techniques, ultimately fostering a transition towards cleaner energy alternatives (Obi et al., 2023).

Few studies have focused on the long-term stability and durability of biofuel briquettes, particularly under varying storage conditions (e.g., humidity, temperature changes).The interaction between binder material and environmental conditions (e.g., moisture absorption) and its impact on the structural integrity of briquettes over time remains under-investigated (Rawat & Kumar 2022).

Starch: Starch provides strong adhesive qualities when mixed with biomass. When heated, starch gelatinizes, allowing it to effectively bond particles together during the briquetting process(Muazu & Stegemann, 2017).  By enhancing the mechanical strength of the briquettes, starch helps maintain their shape and structure during handling, storage, and combustion.

Fig. 1. Binder starch.

Molasses:  Molasses, a by-product of sugar production, has viscous and adhesive properties that help bind biomass particles effectively. The sticky nature of molasses can improve the overall durability of briquettes, reducing the likelihood of breakage during handling and transportation (Kaliyan & Morey 2009). 

Fig. 2. Binder molasses

Lignin: Lignin is a complex organic polymer found in the cell walls of plants, providing rigidity and structural integrity. It acts as a natural adhesive, helping to bind biomass particles together in briquettes (Ayyachamy et al., 2013). Lignin contributes to the thermal stability of briquettes, enhancing their performance during combustion.

Fig. 3. Binder lignin.

Production process of biofuel briquettes. The production process for biofuel briquettes involves several key steps.

1. Feedstock Collection

2. Size Reduction

3. Drying

4. Mixing and Binding

5. Briquetting

6. Cooling and Drying

7. Packaging

8. Quality Control

9. Distribution

METHODS 

Mechanical Properties

Compressive strength: Ensure that the briquettes are of uniform size and shape. Measure their dimensions (length, width, height) and record the data. Allow the briquettes to acclimate to room temperature and humidity for consistency. Weigh each briquette using the weight scale. Record the mass (Li et al., 2019). Measure the dimensions to calculate the volume. Place the briquette in the UTM. Ensure it is centered to avoid uneven loading. Set the testing machine to apply a compressive load at a constant rate (typically 1–5 mm/min). Start the compression test. The machine will apply load until the briquette fails. Record the maximum load applied before failure.

σ = A/F​

Where,

σ    =  Compressive strength

A    =  Maximum load applied (in Newtons)

F   = Cross-sectional area of the briquette (in square meters)

Durability: Select a representative sample of biofuel briquettes. Ensure they are of uniform size and shape. Weigh the total sample of briquettes and record the mass. Determine the initial number of briquettes in the sample (Velusamy et al., 2023). Place the briquettes in the tumbling drum or shaker. If using a manual method, ensure the briquettes are placed in a container that allows for agitation. Set the apparatus to rotate or shake for a specified time (usually 10 to 30 minutes). If using a tumbling drum, you may rotate it at a consistent speed.

Durability = (Initial mass-Mass of fines​/ Initial mass) × 100

Density: The estimation of destiny was done with the use of a calibrated graduated cylinder. The weights of produced briquettes were determined using a digital weighing balance. The diameters and lengths of the briquettes were taken directly by measuring with digital Venire calipers (Aliyu et al., 2015). The diameter and height were determined at an interval of 0 min, 5 hours, 12 hours and 24 hours. The volume of the cylindrical shaped briquette was then determined. Then the density was determined for each briquette as a ratio of briquette weight to volume.

Density(g/cm3) = Mass(g)/Volume(cm3)

Thermal Properties

Calorific value:  Dry the briquette to a constant weight to remove moisture. This can be done in an oven at a controlled temperature. Weigh the dried sample accurately using the analytical balance (Ismaila et al., 2013). Fill the calorimeter with a known amount of water and record the initial temperature. Place the sample in the bomb calorimeter and ensure that it is properly sealed. If your calorimeter requires an ignition fuse, connect it and prepare for ignition. Ensure that the bomb is securely placed in the calorimeter chamber. Initiate combustion of the sample by igniting the fuse. The sample will burn completely within the bomb. Monitor the temperature change in the water. Record the maximum temperature reached after combustion.

Hc =  (Wc × ΔT)/Ms

Where,

Hc = Heat of combustion of the fuel sample, (Cal/g)

Wc = Water equivalent of the calorimeter, (Cal/ °C)

ΔT= Rise in temperature, (°C)

Ms=Mass of sample burnt,(g)

Fig. 4. Calorific value measurement in the bomb calorimeter.

Ignition Temperature: Cut briquettes to a uniform size (e.g., 5 cm × 5 cm). Ensure they are completely dry. Place the briquette in the sample holder within the combustion chamber. Position the thermocouple near the briquette. Gradually heat the briquette at a controlled rate (e.g., 5°C per minute). Continuously monitor and record the temperature. Note the temperature at which the briquette ignites, indicated by visible flames or smoke (Martens et al., 2021). Repeat the test for accuracy, averaging the ignition temperatures from multiple trials.

Ash Content: Ash content was determined for different fuels by using a muffle furnace. A portion (2g) was put into a pre-weighed porcelain crucible and put into a muffle furnace that had been prepared to 600 °C for one hour (Ikelle et al., 2020). Then, the crucible and its contents were transferred to a desiccator and let to cool. The crucible and its content were reweighed and the new weight was noted. The percentage ash content was calculated thus:

AC (%) =  (W2/W1)×100

Where, 

W1 = Original weight of dry sample

W2 = weight of ash after cooling

Fig. 5. Muffle furnace for volatile matter and ash content.

Statistical Analysis. Data obtained from the experiments were analyzed using Analysis of Variance (ANOVA) to assess the significance of the effects of different process parameters on briquette quality. Optimal conditions were determined through response surface methodology (RSM) to identify the best combinations of parameters for maximizing briquette performance. The experiment was conducted in a factorial randomized block design (RBD) with three replications. Statistical analysis of data was done using the 0P- STAT. The treatment means were compared using critical difference (CD) at 5% (p= 0.05) level of significance.

Mechanical Properties

Compressive Strength: The compressive strength of biofuel briquettes varies significantly based on the binder used. Briquettes with lignin exhibit the highest compressive strength, around 9 MPa, followed closely by those with starch at about 8.5 MPa. Molasses-bound briquettes show a strength of approximately 7.8 MPa, while synthetic binders result in lower strength at around 6.5 MPa. Briquettes without binders have the weakest compressive strength, typically around 4 MPa. Overall, natural binders enhance mechanical integrity, making briquettes more durable for handling and combustion. Briquettes made with starch-based binders exhibited higher compressive strength compared to those with lignin or molasses. For instance, briquettes with 10% starch showed a compressive strength of 8 MPa, while those with lignin only reached 5 MPa.

Durability: The durability of biofuel briquettes is significantly influenced by the type of binder used. Briquettes with starch and lignin exhibit high durability, typically around 95% and 90%, respectively. Molasses-bound briquettes have a durability of about 92%. In contrast, those using synthetic binders show lower durability at approximately 85%.Briquettes without binders tend to be the least durable, around 70%. Overall, natural binders improve the handling and storage resilience of briquettes. Durability tests indicated that briquettes containing natural binders (like starch) were more resistant to abrasion and breakage. The abrasion index was lower (e.g., 2% loss) for starch-bound briquettes compared to higher losses (e.g., 5-7%) for those using synthetic binders.

Density: The density of biofuel briquettes varies with the binder type used. Briquettes made with lignin generally have the highest density, around 1,250 kg/m³, followed by those with starch at about 1,200 kg/m³. Molasses-bound briquettes typically have a density of 1,150 kg/m³. In contrast, briquettes using synthetic binders often have lower density, around 1,100 kg/m³. Briquettes without binders tend to have the lowest density, approximately 1,000 kg/m³. Higher density contributes to improved energy content and handling characteristics. Briquettes with lignin as a binder had a higher density (approximately 1250kg/m³) than those made with starch (about 1200kg/m³), suggesting that lignin contributed to a more compact structure.

Table 1: Mechanical Properties of Biofuel Briquettes with Different Binders.

Thermal Properties

Calorific Value. The calorific value of biofuel briquettes varies depending on the binder used. Briquettes made with lignin often have the highest calorific value, around 19 MJ/kg, while those with starch typically reach about 18.5 MJ/kg. Molasses can provide a calorific value of approximately 17.8 MJ/kg. In contrast, briquettes with synthetic binders may fall to around 16.5 MJ/kg. Generally, natural binders enhance the energy content and combustion efficiency of briquettes, making them more effective as a fuel source. The type of binder can affect the calorific value of the briquettes. Organic binders, like molasses, can maintain or enhance energy content, while synthetic binders might reduce it.

Ignition Temperature (°C). The ignition temperature of biofuel briquettes varies with binder type. Briquettes with starch have an ignition temperature of 250°C, while those with lignin reach 260°C. Molasse slowers ignition to 240°C, enhancing rapid energy release. In contrast, synthetic polymers yield the highest temperature at 280°C, which can hinder combustion efficiency. Briquettes without binders ignite quickly at 230°C but lack mechanical strength. Overall, natural binders promote better combustion performance.

Ash Content. The ash content of biofuel briquettes varies with the type of binder used. Briquettes made with starch typically have an ash content of about 5%, while those with lignin may show slightly lower levels at around 4.5%. Molasses can lead to ash contents of about 6%, and briquettes with synthetic binders often reach 7.5%. Higher ash content can negatively impact combustion efficiency and increase residue. Overall, natural binders tend to yield lower ash content, contributing to cleaner combustion. Binders can contribute to the ash content of briquettes. Higher ash content can affect the burning efficiency and lead to more residue, which may be undesirable.

Table 2: Thermal Properties of Biofuel Briquettes with Different Binders.

Compression Strength: Indicates the ability of the briquettes to withstand axial loads.

Durability: Percentage indicating the briquette's ability to retain its form under stress.

Density: Influences transportation and storage efficiency.

Calorific Value: Determines the energy content available during combustion.

Ignition Temperature: Lower values indicate easier ignition.

Ash Content: High ash content can affect combustion efficiency and residue management.

Table 3: Summary of Properties.

DISCUSSION 

The choice of binder materials significantly affects both the mechanical and thermal properties of biofuel briquettes. This discussion explores the implications of various binder types on key properties such as compressive strength, durability, density, ignition temperature, calorific value, and ash content (Olugbade et al., 2019). Compressive strength is crucial for the structural integrity of briquettes during handling and combustion. Natural binders like lignin and starch enhance compressive strength due to their ability to form strong intermolecular bonds within the biomass matrix (Mariana et al., 2021). Research indicates that lignin-bound briquettes can achieve strengths around 9 MPa, while starch-bound variants typically reach 8.5 MPa. In contrast, synthetic binders often lead to lower compressive strengths, reflecting their less effective bonding capabilities. Durability is essential for the briquettes' performance throughout storage and transportation (Muazu & Stegemann 2015).  Natural binders like starch and lignin contribute to high durability rates, often exceeding 90%. This resilience allows briquettes to maintain structural integrity under stress, reducing breakage during handling. Synthetic binders, while providing some strength, do not offer the same level of durability, typically resulting in durability ratings around 85%. Density influences both the energy content and handling characteristics of briquettes. Higher density briquettes, such as those made with lignin (approximately 1,250 kg/m³), indicate a more compact structure, which often correlates with higher energy content (Mostafa et al., 2019). Conversely, briquettes made with synthetic binders tend to have lower densities (around 1,100 kg/m³), which can affect their performance and marketability.

The calorific value is a key indicator of the energy potential of biofuel briquettes. Briquettes with lignin generally provide the highest calorific value (around 19 MJ/kg), making them attractive for energy production (Setter & Oliveira 2022). Starch and molasses also maintain relatively high calorific values, while those made with synthetic binders often fall short (approximately 16.5 MJ/kg). The relationship between binder type and calorific value emphasizes the importance of selecting appropriate materials to optimize energy output. The ignition temperature is critical for determining the ease of combustion. Briquettes with natural binders like molasses exhibit lower ignition temperatures (around 240°C), facilitating quicker ignition. Lignin, while improving structural integrity, leads to a slightly higher ignition temperature (about 260°C). Synthetic binders tend to elevate the ignition temperature to 280°C, which may hinder efficient combustion and lead to incomplete burning. Ash content affects combustion efficiency and residue management. Briquettes with natural binders tend to exhibit lower ash content (around 4.5% for lignin), leading to cleaner combustion and less residue (Srinivasan et al., 2024). Synthetic binders can increase ash content to 7.5%, which may complicate ash management during and after combustion. Lower ash content is particularly desirable for industrial applications where efficient combustion is paramount.

The exploration of binder materials in biofuel briquettes presents numerous avenues for future research and development. Here are several key areas for further investigation:

Innovative Binder Formulations

Biopolymer Development: Research into new biopolymers or bio-based adhesives could yield binders with superior properties, enhancing the strength, durability, and combustion efficiency of briquettes.

Composite Binders: Combining natural and synthetic materials may lead to hybrid binders that leverage the strengths of both types while mitigating their weaknesses.

Performance Optimization

Multi-Parameter Optimization: Future studies could focus on optimizing multiple properties simultaneously (e.g., mechanical strength, ignition temperature, and calorific value) to develop a comprehensive understanding of how different binders interact.

Testing Under Varied Conditions: Investigating the performance of briquettes made with different binders under diverse environmental conditions (humidity, temperature, etc.) can provide insights into their real-world applications.

Sustainability Assessments

Life Cycle Analysis: Conducting life cycle assessments (LCA) of briquettes produced with various binders can help determine their overall environmental impact, aiding in the selection of the most sustainable options.

Waste Utilization: Researching the use of agricultural or industrial waste as potential binders could contribute to zero-waste strategies and enhance the sustainability of biofuel production.

Advanced Characterization Techniques

Material Properties Analysis: Employing advanced characterization methods (e.g., scanning electron microscopy, thermal analysis) to study the microstructure and thermal properties of briquettes could provide deeper insights into how binder choice influences performance.

Combustion Efficiency Testing: More detailed studies on the combustion characteristics of briquettes with different binders, including emissions profiles, will be critical for understanding their environmental impact.

Market Viability and Consumer Acceptance

Economic Feasibility Studies: Researching the cost-effectiveness of different binder options in large-scale production can inform industry practices and decision-making.

Consumer Preferences: Investigating consumer awareness and preferences regarding biofuel briquettes can help tailor products to market demands, especially as sustainability becomes a more pressing concern.

Regulatory Compliance and Standards Development

Standardization of Testing Methods: Developing standardized methods for evaluating the properties of biofuel briquettes will aid in comparison and ensure quality control in production.

Policy Development: Research on how binder materials and biofuel briquettes align with renewable energy policies can guide future regulations and support the adoption of sustainable practices.

Acknowledgement. The author wishes to think the ICAR- Indian Agricultural Research Institute (IARI) for all the support during research as well as the facility to carry out the experimental work and also thank Indian Council of Agricultural Research (ICAR) for the financial support under Junior Research Fellowship to the student during research work.