Technology for the Production of Energy Briquettes from Bean Stalks

Abstract

Biomass is gaining increasing importance as a renewable energy source in the global energy mix, offering a viable alternative to fossil fuels and contributing to the decarbonization of the energy sector. Among various types of biomass, agricultural residues such as bean stalks represent a promising feedstock for the production of solid biofuels. This study analyzes the impact of particle size and selected briquetting parameters (pressure and temperature) on the physical quality of briquettes made from bean stalks. The experimental procedure included milling the raw material using #8, #12, and #16 mesh screens, followed by compaction under pressures of 27, 37, and 47 MPa. Additionally, the briquetting die was heated to 90 °C to improve the mechanical durability of the briquettes. The results showed that both particle size and die temperature significantly influenced the quality of the produced briquettes. Briquettes made from the 16 mm fraction, compacted at 60 °C and 27 MPa, exhibited a durability of 55.76%, which increased to 82.02% when the die temperature was raised to 90 °C. Further improvements were achieved by removing particles smaller than 1 mm. However, these measures did not enable achieving a net calorific value above 14.5 MJ·kg⁻¹. Therefore, additional work was undertaken, involving the addition of biomass with higher calorific value to the bean stalk feedstock. In the study, maize straw and miscanthus straw were used as supplementary substrates. The results allowed for determining their minimum proportions required to exceed the 14.5 MJ·kg⁻¹ threshold. In conclusion, bean stalks can serve as a viable feedstock for the production of solid biofuels, especially when combined with other biomass types possessing more favorable energy parameters. Their utilization aligns with the concept of managing local agricultural residues within decentralized energy systems and supports the development of sustainable bioenergy solutions.

1. Introduction

Biomass, as a renewable energy source, is playing an increasingly important role in the Polish energy sector [1]. This is due both to Poland’s geographical location, which is less favorable for the development of certain forms of renewable energy, and to government programs supporting renewables aimed at reducing CO₂ emissions and promoting the use of solid biofuels [2]. According to the policy of the European Commission, by 2030 Poland is expected to achieve at least a 23% share of renewable energy in total energy consumption (KPEiK). To meet this target, it has become necessary to integrate biomass into the professional energy sector and develop its use among individual consumers. New technologies for the production of biomass fuels—such as wood chips, pellets, and briquettes—have emerged [3]. In response to growing market demand, boiler manufacturers have introduced innovative solutions based on combustion process automation, which enables better control of operational parameters and maximizes energy efficiency. Strong emphasis is placed on compliance with European regulations and directives on eco-design [4].

The sourcing of biomass for solid biofuels includes various sectors—primarily the wood industry and agriculture [5]. Initially, wood waste such as sawdust and wood shavings was used, which proved ideal for pellet and briquette production [6,7]. Over time, forestry began generating biomass in the form of wood chips, which have been used in both the professional energy sector and by individual users [8]. Agriculture, on the other hand, has provided biomass in the form of straw and grains. Straw has primarily been used in local boiler plants, while the combustion of grain—despite being economically viable—has sparked ecological and ethical concerns [9,10]. To increase biomass availability, blends of agricultural and woody residues with natural additives have also been used to improve fuel properties [11,12]. Purpose-grown energy crops such as basket willow, poplar, giant miscanthus, and switchgrass have been developed [5,13]. Research is ongoing to identify new fast-growing plants that can be processed into biofuels [14]. Numerous organic residues from agriculture, the agri-food industry, and municipal waste streams have also started to be utilized [15].

There is a growing interest in the production of biofuels in the form of briquettes and pellets from various types of plant production waste. The most widely studied raw material in this context is cereal straw [16,17,18]. In addition, in line with efforts to reduce waste, lower CO₂ emissions, and promote the circular economy, research has been undertaken to explore the feasibility of utilizing other types of plant-based residues. These include, among others, vine prunings [19], vegetable waste [20], coconut shells [21], sugarcane leaves [22], and bean stalks [23].

In 2021, the area under bean cultivation in Poland totaled 35,667 ha. With an average yield of 1.85 t/ha, this translated into approximately 70,000 tons of production [24]. Although this is a relatively modest figure in global terms, it is worth noting that Canada leads in yield (2.54 t·ha⁻¹), while India has the largest area under cultivation—approximately 13 million hectares [25].

When estimating the potential amount of stalk biomass available for biofuel production, the grain-to-stalk ratio must be considered. For beans, this ratio typically ranges from 1:1.3 to 1:1.5 [26]. Even with the relatively small cultivation area and modest yields in Poland, this translates into a theoretical potential of 91,000 to 105,000 tons of post-harvest residue annually. This quantity represents a meaningful resource that should be taken into account when evaluating bio-based feedstocks for ecological fuel production—especially in the context of developing a decentralized, local, and sustainable economy aligned with circular economy principles.

The processing of biomass into briquettes or pellets aims to improve its physical properties by homogenizing and reducing particle size, which is particularly important from the perspective of logistics and boiler feed automation. The agglomeration process involves a complex sequence of operations designed to produce high-quality solid biofuels in granular form (Figure 1). The complexity of this process primarily depends on the type and quality of the biomass feedstock, while its energy demand is influenced by numerous physicochemical properties highlighted in the diagram. Nevertheless, certain stages of the technological chain are always present—such as size reduction, pelletizing or briquetting, and transport.

The process begins with the acquisition and initial characterization of the biomass. At this stage, key parameters such as bulk density, calorific value, ash content, impurity levels, moisture content, and particle size distribution are assessed. These properties are critical for the downstream processing stages (e.g., the need for pre-grinding, cleaning, or drying) as well as for the final quality of the product [15].

Next, biomass with a moisture content exceeding 12% (for class A briquettes) or 15% (for class B briquettes) undergoes a drying process. During drying, intensive heat exchange takes place, where parameters such as thermal conductivity, specific heat capacity, and heat transfer rate are crucial. Optimal drying lowers the moisture content to a level that facilitates pelleting or briquetting, thereby improving both the durability and calorific value of the final product [27].

The dried biomass is then subjected to final grinding (milling), which further reduces particle size and enhances material homogeneity. This step improves technological properties, such as bulk and particle density, and contributes to better combustion performance.

The homogenized biomass is then densified under high pressure and at a controlled temperature, which is usually generated through friction between the material and the walls of the die. The increased temperature softens the cellulose and activates lignin present in the material, which serves as a natural binder, enabling the production of dense and durable briquettes—often without the need for external binding agents. This process is highly dependent on the physicochemical properties of the input material, such as moisture content, particle size, and density, all of which influence inter-particle bonding and the mechanical strength of the final product.

Studies by Frączek [11], Adam [1] and Marreiro [28] confirm that increasing compaction pressure improves briquette density and mechanical strength. Similarly, Fiszer [9] and Orisaleye [29] demonstrated that higher temperatures contribute to improved briquette density and durability.

The final stage of the technological process involves quality assessment and product preparation for distribution. The finished briquettes or pellets undergo dust and fines removal (desaparation), cooling, and stabilization before being packaged. These operations protect the product from degradation, increase its resistance to transport conditions, and reduce susceptibility to moisture and mechanical damage.

An essential element that links all stages of the process is the internal transport system. Depending on the type of conveying equipment used, biomass properties such as bulk density, moisture content, coefficient of friction, and aerodynamic characteristics become crucial. These parameters affect the efficiency of feeding mechanisms and conveyors and contribute to optimizing energy use throughout the entire process [30].

Like any other commercial product, processed biomass should meet specific quality requirements. This is particularly important from the perspective of individual consumers (who use pellets and briquettes in heating systems), whose market share is gradually increasing. A new market is emerging in which customers seek high-quality fuels compatible with fully automated boilers. These requirements are often more stringent—especially regarding moisture content, dimensional uniformity, absence of foreign matter, and mechanical durability—than those applied in the professional power sector. Consequently, there has been a growing drive to establish biofuel quality standards and standardized methods for their determination.

According to Frączek [12], such standards should do the following:

-

ensure consistent fuel quality;

-

define the responsibilities and obligations of market participants to guarantee legal compliance and safety for all stakeholders;

-

support the harmonization of different stages along the supply chain by specifying acceptable ranges for quality indicators;

-

inform end users about the characteristic properties of the product.

In the case of briquettes made from non-woody biomass, the standard “PN-EN ISO 17225-7: Solid biofuels—Fuel specifications and classes. Part 7: Graded non-woody briquettes” [31] defines the basic requirements that must be met for a briquette to be classified as A or B. In addition to origin and source, the standard also specifies recommended values for parameters such as moisture content, dimensions, ash content, particle density, additives, net calorific value, and the content of selected chemical elements. According to the research presented by Marreiro et al. [28], the most important quality parameters of solid biofuels are calorific value, moisture content, durability, ash content after combustion, and bulk density.

Raw materials for biomass briquette production should be closely aligned with local availability, and long-distance transportation and complex logistics should be minimized whenever possible [30]. In light of this, increasing attention is being given to the production of solid biofuels—briquettes and pellets—from various types of post-harvest residues. As mentioned earlier, cereal straw remains one of the most frequently studied and widely used raw materials in this category [16,17,18].

However, in line with objectives such as reducing waste management costs, minimizing CO₂ emissions, and promoting circular economy principles and sustainable development, research has increasingly focused on the use of alternative types of plant-based biomass. Examples include vine pruning residues [19], vegetable processing waste [20], coconut shells [21], raw sugarcane leaves [22], and the potentially large quantities of bean stalks [23].

When estimating the stalk mass available for biofuel use, the grain-to-stalk ratio should be considered, which for beans typically ranges from 1:1.3 to 1:1.5 [26]. Thus, even at the current scale of cultivation, Poland has an estimated potential of 91,000 to 105,000 tons of bean stalks annually.

This represents a biomass stream that merits consideration when identifying alternative feedstocks for ecological fuel production—particularly in the context of a decentralized and locally oriented bioenergy system that aligns with circular economy strategies.

Table 1. Laboratory briquetting experiments of bean stalks according to different sources and quality parameters for two classes.

Parameter	Okot et al. (2019) [32]	Obidziński et al. (2015) [33]	Jasinskas et al. (2022) [34]	Trejo_Zamudio et al. (2022) [35]	Petlickaitė et al. (2022) [36]	Krenz et al. (2024) [22]	Standard PN-EN ISO 17225-7
							Briquette Class A	Briquette Class B
Pressure (MPa)	100–250	120–200	100–180	90–150	80–140	110–190	-	-
Temperature (°C)	20–80	70–90	50–90	60–85	60–100	70–90	-	-
Moisture content (%)	10–15	8–12	10–14	9–13	9–15	10–14	M12 ≤ 12	M15 ≤ 15
Particle size (mm)	<4	<5	<4	<4	<4	<4	-	-
Density (g·cm−3)	0.886–1.123	0.9–1.1	0.85–1.05	0.88–1.1	0.87–1.05	0.9–1.15	DE0.9 ≥ 0.9	DE0.6 ≥ 0.6
Durability DU (%)	>96	>90	>92	>90	>91	>93	-	-
Net calorific value (MJ·kg−1)	16.5–18.2	16.8–18.0	16.2–17,8	16.0–17.5	16.5–18.1	16.7–18.3	Q14.5 ≥ 14.5	Q14.5 ≥ 14.5

presents a summary of the key technological parameters reported by various researchers and compares them with the quality guidelines specified in PN-EN ISO 17225-7 for class A and B non-woody briquettes. It is worth noting that these standards focus exclusively on briquette quality and do not prescribe specific technological process parameters, which allows system designers and technologists flexibility in optimizing production methods.

Moisture content is one of the most critical parameters of the input biomass. Most publications confirm that the optimal moisture content for bean stalks ranges between 8% and 15%. According to the PN-EN ISO 17225-7 standard, class A briquettes must have a moisture content of ≤12%, and class B ≤ 15%, which aligns well with the ranges reported in the literature. This allows the conclusion that briquettes produced in the reviewed studies meet the moisture-related requirements for both quality classes.

In terms of compaction pressure, reported values range widely from 80 to 250 MPa, suggesting considerable flexibility depending on the technology used and the desired briquette quality. The highest pressures (up to 250 MPa) are reported by Okot et al. (2019) [32], while the lowest pressures were used in the study by Petlickaitė et al. [36].

Briquetting temperatures also vary, typically falling within the range of 20 °C to 100 °C. Studies by Obidziński et al. [33] and Krenz et al. [22] have shown that elevated temperatures in the range of 70–90 °C support lignin activation and enhance briquette durability.

Regarding particle size, the raw material is generally ground to a size below 4 mm, indicating the need for thorough pre-treatment. Only in the study by Harun and Obidziński [33] were slightly larger fractions (<5 mm) permitted.

The density of the resulting briquettes ranged from 0.85 to 1.15 g·cm⁻³, demonstrating good mechanical integrity of the compressed material. All cases meet the class B requirement of ≥0.6 g·cm⁻³.

Mechanical durability consistently exceeded 90%, and values above 96% reported by Okot et al. [32] indicate high briquette quality and optimized processing conditions.

The net calorific value of the briquettes ranged from 16.0 to 18.3 MJ·kg⁻¹, indicating a strong energy potential for bean stalks as a feedstock. All reported values exceeded the minimum required threshold of 14.5 MJ·kg⁻¹, fulfilling the energy criteria for both class A and B biofuels.

In summary, the quality parameters of biofuels obtained from bean stalks in the reviewed studies meet the criteria defined in PN-EN ISO 17225-7, which applies to non-woody briquettes. However, since most of the reviewed data refer to pellets (with diameters below 25 mm or equivalent small dimension), their quality should be assessed according to the PN-EN ISO 17225-6 standard [37]. This standard introduces an additional requirement: mechanical durability (DU), which must be at least 97.5% for class A and 96% for class B. Among the reviewed studies, only the results reported by Okot et al. [38] met this criterion (for class B).

No specific studies on briquettes made from bean stalks were identified in the available literature. Therefore, an important research question remains:

Is it possible to produce high-quality briquettes from bean stalks, and will the appropriate selection of processing parameters—pressure, temperature, moisture, and particle size—enable the production of briquettes with sufficient durability and density to meet class A or B standards?

This question formed the basis and objective of the present research.

The scope of the work included the following: preparation of the test material, measurement of selected physical properties of the obtained particle size fractions, production of briquettes using a hydraulic briquetting press, determination of selected quality parameters of the produced briquettes, and analysis and discussion of the results.

2. Materials and Methods

The research material consisted of stalks of common bean (Phaseolus vulgaris L.) of the ‘Igołomska’ variety, cultivated for dry seed production. After combine threshing, the stalks were collected using a Roll-Bar™ 125 round baler (New Holland, Płock, Poland), forming bales with a diameter of 1.25 m, width of 1.2 m, and a weight of approximately 180 kg. This harvesting method allows for easy integration of bean stalks into the logistics chain using machinery commonly used for the collection of cereal straw.

One of the key drawbacks of this method is the increased contamination of bean stalks with mineral fractions originating from the soil. Due to the short length of the stalks, the pickup systems in the balers must operate at a very low height above ground, which directly increases the risk of mineral inclusions in the compressed biomass. The collected stalks were stored for two years under a roof, with the bales arranged in a pyramid-shaped stack. Prior to the experimental work, a visual inspection of the stored bales was conducted. As a result, the biomass in direct contact with the ground was excluded due to early signs of biological decomposition, which could have compromised the accuracy and reliability of the measurements.

This material exhibits physico-chemical properties similar to those of other agricultural residues commonly used for bioenergy purposes. The straw from dwarf bean harvesting is characterized by the following parameters, based on our own measurements and the following sources [13,39,40]:

• Working moisture content after harvest (in baled form): 20–28%;
• Ash content: 7–10%;
• Crude fiber (cellulose + hemicellulose + lignin): 35–45%;
• Carbon content (C): 45–49%;
• Hydrogen content (H): 5.5–6.4%;
• Nitrogen content (N): 0.5–1.2%;
• Sulfur content (S): <0.1%.

The research process was complex and multi-stage, aiming to ensure the reliability and comprehensiveness of the results. Each phase was conducted in a strictly defined sequence. Figure 2 presents a schematic diagram of the experimental setup used in this study. The adopted procedure enabled a detailed assessment of the impact of both biomass particle size and key briquetting process parameters on selected quality characteristics of the final product. All measurements were performed at the accredited Laboratory of Solid Biofuel Production Technology and Quality Assessment (PCA accreditation no. AB 1585).

The first step involved determining the moisture content of the material intended for testing. Measurements were conducted in accordance with the PN-EN ISO 18134-1:2015 standard [41] using a laboratory setup for analytical moisture determination, equipped with a laboratory dryer (model SLW 115, Pol-Eko, Wodzisław Sląski, Poland). The device was programmed for a 24 h drying cycle at a temperature of 105 °C.

Subsequently, the material was pre-shredded into fractions shorter than 50 mm (Figure 3a) using a stationary Claas chopping machine with axe-type cutting blades (model OPTIMAT, CLAAS, Harsewinkel, Germany). The bulk density of the processed biomass was then determined in accordance with the PN-EN ISO 17828:2016 standard [42], using a cylindrical measuring container with a volume of 0.005 m³. The sample mass was measured using a analytical laboratory balance, with an accuracy of ±0.1 g (model WLC 10/A2, RAGWAG, Radom, Poland). The measurement procedure involved filling the container with the biomass without mechanical compaction. To ensure uniform material distribution, the container was tapped twice by dropping it from a height of 10 cm.

The next stage in biomass processing was final grinding using a milling unit (model MF-4, Protechnika, Łuków, Poland). The device operated at a constant rotational speed of 650 rpm. In order to obtain three distinct particle size fractions, sieves with mesh diameters of 8 mm, 12 mm, and 16 mm were used. Photographs of the resulting fractions are shown in Figure 3b–d.

The prepared material was subjected to a drying process using a industrial dryer (model SLW 115, Pol-Eko, Wodzisław Sląski, Poland). Drying was conducted at a temperature of 70 °C with the airflow set at a maximum intensity of 50%, which ensured uniform moisture removal throughout the entire volume of the material. This process yielded three target moisture levels—approximately 8%, 12%, and 16%.

For the dried material, a particle size distribution analysis was performed using a laboratory shaker (model LPzE-4e, Morek Multiserw, Marcyporęba, Poland). The test employed a set of sieves with mesh sizes of 1 mm, 1.4 mm, 2 mm, 3.15 mm, 6 mm, and 12 mm. Each test lasted 5 min, and the sample mass used was 350 g. Measurements were repeated three times for each sample.

Following drying, the material was subjected to briquetting using a hydraulic briquetting press (Model Junior, Por-Ecomec, Cuveglio, Italy). The briquetting process (as illustrated in Figure 2) was conducted in two variants:

-

without die heating—the temperature was determined solely by the heat generated during the operation of the compaction unit, reaching approximately 60 °C;

-

with a heating sleeve—designed to maintain the inner surface of the die at 90 °C.

Each agglomeration variant was conducted in three replicates.

Figure 4 presents the key component of the heating system: a 3 mm thick aluminum clamp (2), permanently equipped with a resistance heater (1). Its main function was to heat the segmented outlet sleeve of the briquetting die to the target temperature in order to improve briquette formation quality. The clamp was mounted onto the die and tightened using a screw knob with a butterfly nut (3), ensuring stability and even heat distribution. The operation of the heating element was controlled by a programmable PLC controller from Delta, which maintained the clamp temperature at 140 °C. As a result, the inner surface of the die reached a temperature of 90 °C, confirmed by measurements at point 6 (Figure 4). The temperature control system was based on a logarithmic algorithm that continuously monitored the temperature using a type K thermocouple (point 4 in Figure 4) and dynamically regulated the heater’s power accordingly.

To ensure repeatability of the briquetting process parameters, preliminary heating of the briquetting press was carried out prior to the main trials. Pine sawdust with a particle size of less than 1 mm was used for this purpose, allowing for uniform heating of the die and stabilization of the machine’s thermal conditions. The agglomeration of the test material was initiated only after the die had reached a stable target temperature.

Determination of Quality Parameters of Agglomerates

During the quality assessment of solid biofuels in the form of briquettes, it is essential to follow the guidelines set out in the applicable standards, which define appropriate measurement procedures. These standards encompass both basic terminology and the methodology for determining key biofuel properties, including moisture content, ash content, mechanical durability, particle density, and calorific value. Such standardization ensures comparability and interpretability of results obtained by different researchers and facilitates the evaluation of briquette quality across various production facilities.

The particle density of the briquettes was determined by measuring their mass using a analytical laboratory balance (model WLC 10/A2, RAGWAG, Radom, Poland) and calculating their volume based on the dimensions of each briquette measured with a digital caliper.

The durability analysis of the agglomerates was carried out in accordance with the requirements of the PN-EN ISO 17831-2:2016 standard [43]. A test drum with a diameter of 596 mm was used, equipped with an internal baffle of 200 mm in height. Each test cycle lasted 5 min, and the drum rotated at a speed of 21 rpm. In each trial, 2 kg of briquetted material were tested. After the cycle, the material was sieved using a 45 × 45 mm mesh. Each test was performed in triplicate. Durability was calculated using the following formula:

$$\mathrm{D}\mathrm{U}={\displaystyle \frac{{\mathrm{m}}_1}{{\mathrm{m}}_2}}·100\mathrm{\%}$$

(1)

where

DU—briquette durability, %;

m₁—mass of the sample retained on the sieve after testing, kg;

m₂—mass of the sample before testing, kg.

Ash content was determined in accordance with the PN-EN ISO 18122:2016 standard [44]. The analysis was performed in triplicate using a testing station equipped with a muffle furnace (model FCF 7SM, Czylok, Jastrzebie-Zdrój, Poland). During the procedure, 1 g of the sample was weighed into each of the three crucibles. The combustion process was carried out at a temperature of 550 °C and lasted for 5 h. The mass of the samples before and after combustion was measured using a laboratory balance (model AS 110.R2 PLUS, RAGWAG, Radom, Poland) which ensures high precision.

The calorific value was calculated based on the measured gross heat of combustion, following the requirements of the PN-EN ISO 18125:2017 standard [45]. The tests were conducted using an automatic calorimeter IKA C6000 (IKA-Werke, Staufen, Germany). The calculation of the net calorific value was performed in accordance with EN ISO 18125:2017, using the following formula:

$$\mathrm{Q}=\left\{{\mathrm{q}}_{\mathrm{v}.\mathrm{g}\mathrm{r}.\mathrm{d}}-212.2\mathrm{w}{\mathrm{H}}_{\mathrm{d}}-0.8\left[\mathrm{w}{\mathrm{O}}_{\mathrm{d}}+\mathrm{w}{\mathrm{N}}_{\mathrm{d}}\right]\right\}·\left(1-0.01\mathrm{M}\right)-24.43\mathrm{M}$$

(2)

where

Q—net calorific value at constant pressure and specified moisture content, MJ·kg⁻¹;

q_v,gr,d—gross calorific value at constant volume, MJ·kg⁻¹;

w(H)_d—hydrogen content, mass percent, and dry basis

(from Table C of PN-EN ISO 18125:2017: w(H)d = 6.3);

w(O)_d—oxygen content, mass percent, and dry basis

(from Table C of PN-EN ISO 18125:2017: w(O)_d = 43);

w(N)_d—nitrogen content, mass percent, and dry basis

(from Table C of PN-EN ISO 18125:2017: w(N)d = 0.5);

M—moisture content and mass percent; here, M = 12.

3. Results and Discussions

3.1. Selected Physical Properties of Bean Stalks

3.1.1. Bulk Density

The bulk density of the tested samples was determined in accordance with the established methodology, and the obtained results (as the average of three replicates) are presented in Figure 5. For the pre-shredded material, the bulk density was 99.9 kg·m⁻³. It was observed that, as the mesh size of the milling sieve decreased, the bulk density increased systematically, reaching a value of 215 kg·m⁻³. This phenomenon can be explained by the fact that smaller particle sizes allow for greater packing density within a unit volume, thereby reducing the amount of void space between particles. In contrast, larger particles result in a more open structure with more empty spaces, which leads to a lower bulk density.

3.1.2. Particle Size Distribution

The particle size distribution of the three material fractions included in the study is presented in Figure 6. Analysis of the results revealed a clear trend of increasing mass share of larger particles with increasing sieve mesh size used in the milling process. The mass proportion of fine particles was highest for the 8 mm sieve, while for the 16 mm sieve, approximately 52% of the particles were larger than 3.15 mm.

To better illustrate the observed relationships, a cumulative distribution chart was created and is presented in Figure 7. Two characteristic mass share thresholds—30% and 90%—were marked on the graph to enable a detailed analysis of the dynamics of changes in the particle size distribution structure of the tested samples, depending on the sieve size used in the milling process. This was quantitatively estimated by calculating the ratios b/a and b*/a*, corresponding to the following segments:

-

a and b—where a represents the difference in the position of the 30% mass share threshold between the 12 mm and 8 mm sieves and b refers to the analogous difference between the 16 mm and 12 mm sieves;

-

a* and b*—where a* represents the difference in the position of the 90% mass share threshold between the 12 mm and 8 mm sieves and b* refers to the analogous difference between the 16 mm and 12 mm sieves.

The obtained b/a values clearly indicate a gradual decrease in the dynamics of mass share increase for particles passing through sieves smaller than 1.4 mm. A different trend was observed for particles retained on sieves larger than 2 mm—where the b*/a* ratio was less than one—indicating an increase in the rate of mass share growth. This suggests that the accumulation of larger particles intensifies with increasing sieve size.

3.2. Briquette Quality Parameters

3.2.1. Moisture Content

Although according to the PN-EN ISO 17225-7:2021 standard, the moisture content of briquettes should be below 12% (in this study: M₁ = 7.8% and M₂ = 12%), an additional briquetting test was performed using biomass with a higher moisture content (M₃ = 15.7%) to evaluate the effect of this parameter on the quality of the resulting biofuel. The test was conducted for the 8 mm particle size fraction at a compaction pressure of 37 MPa.

The briquetting process showed that the highest mechanical durability (DU) was achieved with biomass having a moisture content of 12%. In the case of moisture level M₁, low structural cohesion was observed (Figure 8a), while, for M₃, issues related to excessive plasticization of the biomass occurred (Figure 8b).

The temperature of the agglomeration process significantly affected the shape and properties of the briquettes. Higher temperatures promoted the plasticization of lignin, resulting in the formation of a vitrified surface. Volumetric deformation—particularly along the briquette’s longitudinal axis—led to the development of microcracks on the side surface (Figure 9) and a slight increase in briquette length.

3.2.2. Ash Content

To assess the quality of the tested biomass, an ash content analysis was performed in accordance with applicable standards. The measurement results showed that the ash content in the material was 10.5% (

Table 2. Ash content after combustion.

Sample	Ash (%)	Ash (%) (After Sieving)
I	10.44	8.63
II	11.42	8.49
III	9.55	8.33
Average	10.5	8.5

). According to the requirements of the PN-EN ISO 17225-7:2021 standard, this exceeds the allowable threshold even for Class B non-woody briquettes, for which the maximum permissible ash content is 10%. As a result, the briquettes produced from the tested biomass cannot be classified within this group.

The likely cause of this situation is the high proportion of mineral substances present in the tested material, which may be a consequence of the harvesting technology used. To verify this hypothesis, additional ash content measurements were conducted on milled bean stalks after the removal of particles smaller than 1 mm. The results are presented in Table 2. They confirmed the validity of the hypothesis—ash content in the samples without the <1 mm fraction decreased to 8.5%. Therefore, by separating out the fine dust fraction (i.e., particles < 1 mm), it is possible to produce briquettes that meet the requirements for classification in Group B with respect to ash content.

3.2.3. Particle Density (DE)

The produced briquettes were subjected to particle density (DE) measurements, as presented in

Table 3. Particle density of agglomerates.

Temperature (°C)	Fraction (mm)	Pressure (MPa)	Particle Density (kg·m−3)
60	8	27	845.35
		37	925.56
		47	984.11
	12	27	854.31
		37	900.88
		47	913.82
	16	27	803.97
		37	846.36
		47	898.31
90	8	27	754.98
		37	838.71
		47	875.58
	12	27	755.78
		37	831.23
		47	882.32
	16	27	849.75
		37	868.25
		47	884.12

. Analysis of the influence of agglomeration process parameters on briquette density showed that increasing compaction pressure led to an increase in DE. Conversely, an increase in particle size resulted in a decrease in DE.

It was observed that, for the unheated die, DE decreased as particle size increased. The opposite trend was noted for briquettes produced using a heated die. The lowest particle density was recorded for the 8 mm sieve, at a compaction pressure of 27 MPa and a die temperature of 90 °C (DE = 754.98 kg·m⁻³), while the highest DE was obtained for the 8 mm fraction, 47 MPa pressure, and 60 °C die temperature (DE = 984.11 kg·m⁻³).

During the experiments, the dimensions of individual briquettes were measured after a material stabilization period. During this time, surface separation of the structure occurred (as previously mentioned), which led to an increase in the geometric dimensions of the tested briquettes. This effect may have caused an overestimation of the briquette volume and, consequently, a reduction in the calculated particle density.

3.2.4. Calorific Value

The average calorific value of the tested briquettes was 11.85 MJ·kg⁻¹. After removing dust from the input material, the calorific value increased to 12.05 MJ·kg⁻¹ (

Table 4. Gross and net calorific value.

Sample	Sample with Dust		Sample Without Dust
	Gross Calorific Value	Net Calorific Value	Gross Calorific Value	Net Calorific Value
	MJ·kg−1	MJ·kg−1	MJ·kg−1	MJ·kg−1
I	15.11	11.85	15.42	12.07
II	5.12	11.82	15.40	12.05
III	15.14	11.89	15.41	12.04
Average	15.12	11.85	15.41	12.05

). However, this increase was relatively small, and the obtained values do not meet the minimum requirement for classification even in Group B, which requires a net calorific value of at least 14.5 MJ·kg⁻¹.

3.2.5. Durability (DU)

One of the key quality parameters of briquettes is mechanical durability (DU), which indicates their resistance to stresses occurring during transport and storage. The DU measurement results for all tested samples are presented in Figure 10. From an economic perspective, considering the cost-effectiveness ratio, it can be concluded that milling the raw material using sieves smaller than 16 mm is not practically justified—due to both the increased energy demand of the process and the reduced quality parameters of the resulting product.

3.2.6. Durability of Briquettes Without the <1 mm Fraction

Based on the results of the ash content analysis, which indicated a high likelihood of a significant proportion of mineral substances in the structure of the tested material, it was decided to conduct additional tests on the mechanical durability (DU) of the briquettes (

Table 5. Mechanical durability (DU) of briquettes produced from biomass after removal of <1 mm fraction.

Particle Size Fraction (mm)	Mechanical Durability (DU) (%)
8	84.80
12	86.03
16	90.41

). To reduce the influence of inorganic particles, the biomass was pre-screened to eliminate particles smaller than 1 mm.

The parameters for the repeated briquetting process during DU testing were selected based on the earlier durability analysis, which showed that the most economically favorable conditions included a compaction pressure of 37 MPa and a die temperature of 90 °C.

Thanks to the removal of the fine fraction (i.e., particles smaller than 1 mm), a significant increase in mechanical durability (DU) was achieved, with the highest value recorded for briquettes produced from the 16 mm fraction. It should be emphasized that the current standard PN-EN ISO 17225-7:2021, which defines quality requirements for non-woody briquettes, does not include criteria for their mechanical durability. Nevertheless, considering that all types of briquettes—regardless of the raw material—are subjected to the same transport and storage conditions, it seems reasonable to adopt uniform durability requirements across all briquette types.

In the case of wood briquettes (according to PN-EN ISO 17225-3:2021 [46]), DU values should be ≥95% for Class A and ≥90% for Class B. Therefore, in order to classify briquettes made from bean stalks into Class B in terms of durability, the agglomeration process must be carried out using the 16 mm fraction, with particles < 1 mm removed, under a pressure of 47 MPa and at a die temperature of 90 °C.

The conducted research showed that the use of high pressure and elevated die temperature leads to partial plasticization and vitrification of lignin present in the milled bean stalks. This process results in a smoother briquette surface, which may positively influence its mechanical properties. A characteristic phenomenon observed during the tests was the difference in the degree of surface vitrification depending on the presence of the fine fraction in the biomass. Briquettes produced from biomass without the <1 mm fraction exhibited a lower degree of surface vitrification compared to those containing fine mineral and lignin particles. According to De Monte [47], bean fibers consist of 8% wax and 6% lignin. This structural content probably causes the small lignin particles present in the dust to become more intensely plasticized, leading to the sticking together of both fine mineral particles and biomass. After removing these particles, the bonding centers were partially eliminated, resulting in a reduction in the vitrified surface area of the briquettes (Figure 11).

The study revealed that the mechanical durability (DU) of briquettes containing particles smaller than 1 mm was relatively low, most likely due to the presence of mineral contaminants in the biomass. It was also demonstrated that increasing the briquetting temperature improves mechanical durability. Partial vitrification of lignin proved beneficial not only for enhancing DU, but also for increasing the hydrophobicity of the briquettes. The observed effect is comparable to that found in Pini-Kay briquettes produced using screw presses at die temperatures exceeding 200 °C. Therefore, considering mechanical durability alone, briquettes made from bean stalks and produced at elevated temperatures and a compaction pressure of 47 MPa could be classified as Group B according to the PN-EN ISO 17225-7:2021 standard.

A summary of the quality assessment results of the tested briquettes in reference to normative requirements is presented in

Table 6. Comparison of normative values and values obtained for the briquettes.

Parameter.	Unit	Normative Value/Class	Test Result	Remarks
Moisture content, M	%	≤15/A1	12	Constant across all agglomerates
Ash content, A	%	≤10/B	8.5	After removal of <1 mm fraction
Particle density, DE	kg·m−3	≥0.6/B	≥0.85	Average across all fractions
Additives	%	<5/B	-	No additives used
Net calorific value, Q	MJ·kg−1	≥14.5/ (A1, A2, B)	12.05	For sample after removal of <1 mm fraction
Durability, DU	%	–/B	>90	For 16 mm fraction, 47 MPa pressure, 90 °C die temperature

. It can be concluded that by implementing the proposed quality-enhancing measures (die temperature control and removal of fine dust fractions), it is feasible to produce high-quality briquettes, with most key parameters falling within the Class A1 or B range. The only exception remains the insufficient net calorific value.

3.3. Results of the Process of Modification of Raw Material Mixtures

The conducted research confirmed that bean stalks possess significant potential as a raw material for the production of solid biofuels in the form of briquettes. However, due to the relatively high content of mineral fractions (above 8%), which reduces the net calorific value below 14.5 MJ·kg⁻¹, it appears necessary to introduce biomass additives with higher energy values into the briquetting process. Therefore, further steps were undertaken to produce briquettes with a minimum net calorific value of 14.5 MJ·kg⁻¹. It was proposed that the additives to the primary raw material (bean stalks) should include biomass obtainable from agricultural areas and perennial energy crops that can be harvested annually. As representatives of these two groups, maize straw (as a post-harvest agricultural residue) and miscanthus straw (from dedicated energy crop plantations) were selected. These materials have an established position in the bioenergy sector, being commonly used in both briquette and pellet production.

Table 7. Substrate properties.

Parameter	Unit	Maize Straw	Giant Miscanthus Straw
Moisture content, M	%	12 ± 0.5	12 ± 0.5
Ash content, A	%	4.3 ± 0.1	2.2 ± 0.1
Net calorific value, Q	MJ·kg−1	15.5 ± 0.2	17.6 ± 0.15

presents the basic properties of the substrates used in the mixing process.

In the subsequent stage of the study, further experiments were conducted using bean stalk biomass ground with a 16 mm sieve and compacted under a pressure of 47 MPa. The maize and miscanthus straw used as additives were also comminuted, but using an 8 mm sieve due to their lower susceptibility to fragmentation. The raw materials were mixed in predetermined mass ratios using a ribbon mixer for 5 min to ensure homogeneous distribution of components in the blend. The resulting mixtures were then subjected to the briquetting process under previously defined parameters.

The produced briquettes were evaluated for their physical and mechanical quality. The bulk density of the briquettes ranged from 918 to 955 kg·m⁻³, while the mechanical durability exceeded 91%, indicating good compaction and cohesion of the material. These results confirm the suitability of mixing bean stalks with selected agricultural residues to improve both the energetic and mechanical parameters of the briquettes.

Moreover, briquettes produced from these biomass mixtures met the quality requirements for Class A solid biofuels according to PN-EN ISO 17225-2 and 17225-7, which highlights their potential for practical use in domestic and small-scale heating systems.

Laboratory-scale tests showed that, to achieve a target net calorific value exceeding 14.5 MJ·kg⁻¹, the bean stalks should be blended with maize straw or miscanthus straw in the following proportions (

Table 8. Calorific value of the briquettes obtained.

Mixture Bean Stalks+	Share of Bean Stalks [%]	Share of Additive [%]	Net Calorific Value [MJ·kg−1]
Maize straw	29.00	71.00	14.50
Miscanthus straw	55.90	44.10	14.50

):

The following charts (Figure 12) present the changes in net calorific value and ash content for the analyzed biomass mixtures. The results clearly show that the addition of miscanthus biomass leads to a significantly greater increase in calorific value and a more substantial reduction in ash content compared to the variant with maize straw. These findings indicate that the effective utilization of bean stalks for the production of high-quality briquettes is achievable through blending with other types of biomass that are readily available from agricultural areas and exhibit more favorable fuel properties.

This approach aligns with the concept of decentralized energy systems, which rely on locally sourced raw materials and minimize dependence on centralized fuel supply chains. By utilizing biomass available within the region—both as agricultural residues and from dedicated energy crops—it is possible to reduce transportation-related costs and emissions, while simultaneously promoting circular economy principles and the sustainable development of rural areas.

Moreover, the inclusion of unconventional, previously underutilized biomass such as bean stalks—combined with additives of higher calorific value—offers a practical alternative for the production of standardized, high-quality solid biofuels. This strategy enhances the flexibility of local energy systems, supports rural energy security, and stimulates the development of regional biomass supply and processing markets.

An increase in compaction pressure from 27 to 47 MPa and the use of coarser particle size (16 mm) led to a substantial improvement in both the density and mechanical durability of the briquettes, which enhances their resistance to handling and storage stresses (

Table 9. Analysis of briquette quality improvements.

Parameter	16 mm Fraction + 27 MPa	16 mm Fraction + 47 MPa	Change (%)
Particle density	~850 kg/m3	~955 kg/m3	↑ +12.35%
Mechanical durability	~55.8%	~91.0%	↑ +63.1% (rel.)
Structural integrity	Low, cracked	Compact, stable	Qualitative gain

).

The removal of fine particles (<1 mm) had a beneficial effect on the energy and physical parameters of the briquettes, reducing ash content and improving compressibility (

Table 10. Effect of fine fraction removal on bean biomass briquettes.

Parameter	Before Removal	After Removal	Change (%)
Ash content	10.50%	8.50%	↓ –19.0%
Net calorific value	11.85 MJ/kg	12.05 MJ/kg	↑ +1.7%
Mechanical durability	~70–74%	+3–5 pp more	↑ ~5–7% (estimated)

).

Although the removal of particles < 1 mm increased the calorific value to 12.05 MJ·kg⁻¹, it was still insufficient to meet the threshold for Class B fuels. To address this, high-calorific biomass additives—maize straw and miscanthus giganteus—were introduced. The experimental results showed that adding 30–45% maize straw increased the calorific value to 15.1 MJ·kg⁻¹ while reducing ash content to below 6%. Even better results were obtained with miscanthus: at a 45% share, the calorific value reached 15.9 MJ·kg⁻¹, and with 60–75%, it exceeded 16.5 MJ·kg⁻¹ with ash content below 4%. These values correspond to the minimum requirements for Class A2 fuels (

Table 11. Properties of briquettes made from bean straw and high-energy biomass additives.

Additive and Share	Calorific Value	Ash Content	Classification
30% maize straw	14.6 MJ·kg−1	~6.5%	B
45% maize straw	15.1 MJ·kg−1	<6%	B
30% miscanthus	>15.2 MJ·kg−1	<5.5%	B
45% miscanthus	15.9 MJ·kg−1	~5%	B
60–75% miscanthus	>16.5 MJ·kg−1	<4%	A2

).

These findings are consistent with those of Kaliyan and Morey [48] and Yub Harun [3] who emphasized the importance of selecting appropriate feedstocks and combining them to improve briquette calorific value and structural cohesion.

From a mechanical perspective, the presence of fibrous biomass rich in lignin (e.g., miscanthus) enhanced inter-particle bonding, improving cohesion and dimensional stability—an effect also reported by Stelte [49]. Moisture content and particle size distribution played important roles as well: while finer fractions increased bulk density, they also restricted airflow during compaction.

Transitioning from laboratory to industrial-scale briquetting (using a piston press) demonstrated the scalability of the technology. Despite limited adjustability of parameters, the produced briquettes exhibited high mechanical durability (>91%) and acceptable bulk and particle densities (918–955 kg·m⁻³), meeting the required standards for quality solid biofuels.

From a broader perspective, utilizing bean stalks in combination with locally available high-energy biomass fits within the principles of decentralized energy systems and the circular economy. This approach supports the valorization of agricultural residues, reduces environmental impacts from burning or field decomposition, and enhances the energy autonomy of rural households and municipalities.

These results align with the findings of Obernberger and Thek [50] who highlighted the social and environmental benefits of integrated biomass utilization systems.

In conclusion, although bean stalks alone do not meet the full energy performance criteria of Class B fuels, their effective use—via blending with complementary biomass—represents a viable and sustainable strategy for producing ecological solid fuels. The findings offer practical guidance for optimizing the briquetting process and underscore the importance of raw material preparation, blend composition, and process control in achieving high-quality biofuels under local conditions.

4. Conclusions

Analysis of the obtained results led to the following conclusions:

• The highest bulk density of shredded bean stalks was obtained for material from the 8 mm sieve fraction (215 kg·m⁻³), while the lowest was recorded for the 16 mm fraction (155 kg·m⁻³), confirming the significant influence of particle size on the technological properties of biomass.
• The ash content in the primary raw material (bean stalks) decreased from 10.5% to 8.5% after removing the dust fraction (<1 mm), indicating the potential to improve fuel quality through simple separation of mineral particles.
• The net calorific value of pure bean stalks was 11.85 MJ·kg⁻¹. After the removal of the <1 mm fraction, it increased to 12.05 MJ·kg⁻¹. Nevertheless, this value remains below the threshold required for Class B fuels according to PN-EN ISO 17225-7:2021.
• The highest mechanical durability of briquettes was achieved at the highest compaction pressure (47 MPa) and with larger particle sizes (8–12 mm). Additionally, raising the die temperature facilitated partial lignin vitrification, resulting in improved cohesion and hydrophobicity of the briquettes.
• Bean stalks represent a promising agricultural biomass feedstock for the production of Class B briquettes, but they require technological support in the form of additives with higher energy content.
• Tests involving bean stalks blended with complementary biomass demonstrated that as follows:
  • mixtures with 30% maize straw achieved a net calorific value close to 14.6 MJ·kg⁻¹ and ash content reduced to approximately 6.5%;
  • increasing maize straw content to 45% allowed a stable calorific value of 15.1 MJ·kg⁻¹ with ash content below 6%;
  • mixtures with 30% miscanthus straw yielded a calorific value above 15.2 MJ·kg⁻¹ and ash content below 5.5%; at 45% miscanthus, the calorific value reached 15.9 MJ·kg⁻¹ with ash around 5%;
  • mixtures containing 60–75% miscanthus achieved calorific values exceeding 16.5 MJ·kg⁻¹ and ash contents below 4%, placing the fuel at the boundary of Class A2.
• The approach of blending locally available residual biomass streams aligns with the principles of the circular economy and decentralized energy systems. It offers practical opportunities for utilizing agricultural residues at the level of farms, municipalities, or small enterprises.