Introduction

The global demand for energy continues to increase due to rapid industrial growth, urbanization, and population expansion, placing immense pressure on fossil fuel resources and raising concerns about environmental degradation and climate change1. To address these challenges, renewable energy sources such as biomass have attracted growing interest for their potential to provide sustainable, low-carbon energy alternatives2. Agricultural residues, a byproduct of crop production, represent an abundant and renewable biomass resource that can be converted into valuable bioenergy products, thus contributing to waste valorization and environmental protection3.

Olive pruning residues are among the major agricultural residues generated in olive-producing regions including Al-Jouf, Saudi Arabia, where hundreds of thousands of tons are produced annually, creating disposal challenges and environmental pollution when not managed properly4. These residues have significant energy potential, but their low bulk density and high moisture content complicate their direct utilization as fuel, limiting transportation, storage, and combustion efficiency5,6. To overcome these limitations, olive pruning residues are typically subjected to size reduction through cutting, followed by compaction using densification technologies such as pelleting and briquetting. These techniques compress the fragmented material into dense, uniform briquettes, enhancing bulk density, fuel handling, transportability, and combustion efficiency7,8. Densified biomass fuels typically have calorific values comparable to conventional fossil fuels, with olive pruning pellets often exceeding 4200 kcal kg−1, making them a viable and sustainable alternative energy source9. The energy consumption for biomass densification is relatively low compared to the energy content of the final product, supporting the sustainability of these technologies10. Furthermore, densification reduces greenhouse gas emissions by replacing fossil fuels and preventing residue open burning11. However, the efficiency and quality of pellet or briquette production depend largely on the mechanical and combustion properties of the biomass feedstock, including friction, compaction behavior, and thermal stability. Mechanical properties such as friction coefficients impact the design and wear of processing equipment, influencing the energy required for compaction and the overall productivity of the densification process12. Combustion properties affect drying and combustion behavior, which are critical for producing high-quality biofuels13. Factors such as moisture content, particle size, and applied pressure significantly influence these properties and must be thoroughly characterized for effective machine design and process optimization14. Recent studies on olive pruning biomass highlight its promising potential for sustainable energy applications. For instance, García Martín et al.9 reported the substantial energy content of olive pruning and emphasized the need for improved utilization strategies to reduce waste and environmental impact. Additionally, Ibrahim et al.15 conducted a detailed design and performance assessment of a pelleting machine specifically for biomass residues, demonstrating how engineering optimization can improve pellet quality, energy efficiency, and throughput. Their work underscores the importance of integrating mechanical characterization with machine design to maximize the benefits of biomass densification. Despite these advancements, challenges remain in achieving consistent fuel quality and optimizing operational parameters to handle variability in raw material properties16. Understanding the friction and compaction characteristics of olive pruning residues under different moisture and temperature conditions is essential for improving densification technologies and expanding the use of biomass fuels in sustainable energy systems. Therefore, this study aimed to evaluate the mechanical properties (friction and compaction) of olive pruning residues after cutting, and to assess the thermal behavior of the densified briquettes produced from these residues, to determine their suitability as a renewable biomass fuel.

Materials and methods

‎Olive pruning sample preparation

Olive pruning (Olea europaea L.) was collected from mature olive trees during seasonal dry pruning operations in intensive orchards located in the Al-Jouf region, Saudi Arabia—an area known for large-scale olive cultivation. The pruning consisted primarily of lignified branches with an average length of approximately 120 cm. Based on representative samples, the base diameters typically ranged between 10 and 20 mm, while the top diameters were approximately 5 to 10 mm. The moisture content of freshly cut pruning ranged between 35% and 45% (dry basis). Prior to further processing, the material was air-dried under ambient outdoor conditions for several days until the moisture content reached approximately 9% (dry basis), as measured in accordance with the ASAE S358.2 standard and verified using a handheld moisture meter.

The dried pruning was initially shredded on-site in Al-Jouf using a heavy-duty chipper fitted with a flywheel disc cutter. The shredded material was then further ground on-site using a Wiley-type cutting mill operating at 1500 rpm and equipped with 16 rotating hammers. The ground material was transported to the Bioengineering Laboratory, Department of Agricultural Engineering, Faculty of Agriculture, Cairo University, Egypt, where all experimental evaluations were conducted.

To obtain a uniform particle size distribution suitable for densification and property testing, the material was classified into three particle size ranges: <1.0, 1.0–1.5, and 1.5–2.0 mm, using appropriate mesh sieves in accordance with ANSI/ASAE S319.317. The classified material was used in its ground form to conduct mechanical tests, including friction and compaction analysis. For combustion evaluations, the same material was densified into cylindrical briquettes, and combustion analyses such as combustion behavior and thermogravimetric profiling—were performed on these compressed briquettes to reflect actual fuel performance. A schematic representation of the mechanical and combustion tests conducted on the ground and densified olive pruning residues is provided in Fig. 1.

Fig. 1
figure 1

Schematic diagram of the mechanical and combustion tests applied to olive pruning residues.

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Mechanical properties of ground olive pruning residues

Dynamic friction coefficient of ground olive pruning residues (µ d)

The dynamic friction coefficient (µd) of ground olive pruning residues was calculated using Eq. (1).

$$\:{\mu\:}_{d}=\frac{{F}_{f}}{{N}_{l}}=\frac{{{F}_{T}\:-F}_{E}}{{N}_{l}}$$
(1)

where µd is the dynamic coefficient of friction, Ff is the friction force (Ff =FT -FE), N; FT is force needed to get a filled cylindrical container moving, N; FE is force needed to get an empty cylindrical container moving, N; and Nl is the normal load pressing the sample to the contact surface, N.

To obtain the values used in Eq. (1), experimental measurements were conducted on ground olive pruning at moisture contents of 5, 10, and 15%, and under vertical loads ranging from 2 to 9 N. The test was carried out on both stainless steel and steel surfaces. The setup consisted of a hollow cylindrical container (3.2 cm inner diameter) mounted on a horizontal carriage (26 × 13.5 × 4.5 cm) equipped with roller wheels to allow smooth motion18. The container was filled with the sample and positioned 1 mm above a 30 × 50 cm metal sheet to ensure consistent contact conditions. A digital force gauge (MARK-10, Model M3-10, USA) was used to measure the horizontal force needed to initiate sliding. The difference between the force required to move the filled and empty containers provided the frictional force. Figure 2 illustrates the schematic of the experimental setup.

Fig. 2
figure 2

Setup for measuring the dynamic friction force: (1) Sliding surface‎, ‎(2) Plate carriage, ‎‎(3) Calibration masses,‎ (4) Hollow cylindrical,‎ (5) Powder material,‎ (6) Roller wheels, ‎(7). Hook,‎‎ (8) Hand wheel,‎ (9) Test stand,‎ (10) Small pulley, ‎(11) Wire,‎ (12) Force gauge, and‎ (13) Sliding ruler.

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Internal friction coefficient of ground olive pruning

The internal friction coefficient of ground olive pruningwas measured using a direct shear cell apparatus (custom-fabricated, Bioengineering Laboratory, Cairo University, Egypt), consisting of a top and bottom cylindrical section along with a covering component. The test sample was placed inside a hollow cylindrical container (50 mm outer diameter, 40 mm inner diameter, and 24 mm height), which was then subjected to vertical and horizontal forces. The experimental setup is illustrated in Fig. 3.

The experiments were conducted at different moisture contents (5 10, and 15% on a dry basis) and under varying applied loads ranging from 2 to 9 N. The internal friction force (F) was measured and plotted against the normal force (W) to derive the internal friction coefficient using Eq. (2)12

$$\:F=mW+b$$
(2)

where F is the internal friction force, N; W is the normal force applied, N; m is the slope of the line (internal coefficient of friction) and b is the cohesion force, N..

Fig. 3
figure 3

Schematic of the setup used for measuring the internal friction coefficient of ground olive residues; (A) Assembled double-ring shear cell; (B) Individual components of the shear cell: (1) A pulling mechanism applying horizontal force via a wire connected to a digital force gauge, ‎(2) An upper cylinder open at both ends, ‎‎(3) Calibrated weights to apply normal force, ‎(4) A cylindrical container holding the sample, and‎ (5) A lower cylinder closed at one end.

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Compression behavior‎ of ground‎ olive pruning ‎ residues

The compression behavior of ground olive pruning residues was studied using a briquetting die assembly designed for briquetting formation. The system comprised a compaction ram, a die cavity, and a base plate (Fig. 4). The raw material was loaded into the die and compressed using a computerized hydraulic universal testing machine (Shimadzu UH-1000kNI, Shimadzu Corporation, Japan) with a maximum load capacity of 1000 kN, as shown in Fig. 4. The briquetting die had an inner diameter of 63 mm and a height of 170 mm, with a clearance of 0.2 mm between the die wall and the ram to allow air to escape during compression.

Fig. 4
figure 4

Compression test of ground ‎olive pruning residues—(A) Briquetting die assembly; (B) Briquetting die components, and (C) Computerized universal testing machine:‎ (1) Ram ‎rod; ‎(2) Ram‎;‎ (3) Die cavity; ‎(4) Base plate; and‎ (5) Ground olive pruning sample‏.

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The testing machine utilized a computerized hydraulic system to precisely control the compression rate, while force and displacement data were recorded in real time via a digital data acquisition system. Samples of ground olive pruning with three particle sizes (< 1.0, 1.0–1.5, and 1.5–2.0 mm) were tested at two moisture contents, 5 and 15% dry basis. Moisture levels were adjusted by either air-drying for varying durations or oven-drying to the target moisture content. Approximately 0.70 N of each sample was weighed and loaded into the briquetting die. The die was then placed in the testing unit, and axial compression was applied by the vertical piston until the desired pressure level was reached. Compression loads of 10, 20, 30, 40, and 50 kN were applied with a constant crosshead speed of 60 mm min− 1. Piston displacement and axial load were continuously recorded throughout the tests.

After reaching and holding the preset pressure for a short period, the compressed briquette was ejected by replacing the base plate with an ejection system and pushing the briquette out using the piston. The mass, length, and diameter of the resulting briquette were then measured to determine the pressure–particle density relationship.

The particle density (ρp) of the briquette was determined using the direct measurement method following ASAE Standard S269.412. It was calculated using Eq. (3).

$$\:{\rho\:}_{p}=\frac{{P}_{m}}{{V}_{p}}$$
(3)

where ρp is the particle density, kg m− 3; Pm is the briquette particle mass, kg; VP is the briquette particle volume = \(\:\frac{\pi\:}{4}{d}^{2}L\), m3; d is the briquette diameter, m; and L is the briquette length, m. The ρp relationship during the olive pruning residues compaction was obtained by recording instantaneous axial load values, die diameter, and material deformation during compression using the hydraulic press control system.

The Specific consumed energy (SCE) was calculated by integrating the area under the load–deformation curve, which represents the mechanical work input during the densification process. This energy reflects the efficiency and feasibility of biomass compaction and is a key factor in optimizing densification systems. In this study, the compaction energy was estimated using Eq. (4), as described by Shinners and Friede19.

$$\:SCE={10}^{-6}\times\:\frac{\int\:{F}_{a}\:dy}{m}$$
(4)

where SCE is the specific consumed energy of compaction, kJ kg−1; Fa is the applied axial force (load), N; and y is the material deformation, mm.

The compressibility index (β) is a key parameter that quantifies the compressibility behavior of powdered biomass materials. It is determined by subjecting a powder sample to vertical compression within a shear cell ring while recording its displacement. The relationship between applied normal stress and relative density is then plotted on a logarithmic scale, and a linear regression is fitted to the resulting data points. The compressibility index (β) is derived from the slope of this line using Eq. (5), as outlined in recent adaptations of the Jenike method20.

$$\:\beta\:=\:\raisebox{1ex}{$Log\left[\frac{{\rho\:}_{p}}{{\rho\:}_{b}}\right]$}\!\left/\:\!\raisebox{-1ex}{$Log\:\left[\frac{{\sigma\:}_{1}}{{\left(\sigma\:\right)}_{0}}\right]$}\right.$$
(5)

where β is the compressibility index; ρp is the particle density at consolidating stress (σ1) or load; ρb is the bulk density of loose ground olive pruning residues (= 265 kg m− 3) at initial consolidating stress (σ1)0 or initial load (= 0.1 MPa). The slope β reflects material compressibility, with values near 1 indicating low compressibility (rigid materials) and values near 0 indicating high compressibility.

Finally, the compression ratio (CR) describes volume reduction during compression. It is calculated as the ratio of particle density of the compacted block to the initial bulk density of the loose material, according to Eq. (6)21.

$$\:CR=\frac{{\rho\:}_{p}}{{\rho\:}_{b}}$$
(6)

where CR is compression ratio; ρp is particle density of olive pruning residues briquette, kg m− 3; and ρb is bulk density of loose ground olive pruning, kg m− 3.

Combustion properties of olive pruning residues briquette

The combustion properties of olive pruning residues briquette were thoroughly evaluated through a series of analyses, including determination of calorific value, measurement of ash content, analysis of flue gas emissions (O2, CO2, CO, SO2, and NO2), and thermogravimetric analysis (TGA). Prior to these assessments, briquette samples were carefully stored in airtight plastic bags at room temperature to preserve their physical and chemical properties and to prevent moisture absorption or degradation.

Calorific value of olive pruning residues briquette (CV)

The calorific value of the briquette was measured using a bomb calorimeter (CAB003, Fisons Scientific Equipment, UK) following ASTM D2015-96 standards, with combustion calibrated by benzoic acid tablets. Approximately 1 g of sample was combusted in an oxygen atmosphere, and the calorific value was expressed in kcal kg− 1. The calorific value (CV) was calculated using Eq. (7)22.

$$\:CV=\:\frac{\left[{W}_{eq}\times\:\varDelta\:T-\left({CV}_{p}\times\:{W}_{tp}\right)\right]}{{W}_{s}}$$
(7)

where CV is calorific value of material, MJ kg− 1; Weq is equivalent weight, 0.0105 MJ °C−1; ΔT is change in temperature, °C; CVp is Calorific value of paper, 16.4 MJ kg− 1; Wtp is weight of paper, kg; and Ws is weight of sample, kg.

Ash content of olive pruning residues briquette (AC)

The ash content (AC), representing the inorganic mineral residue in the biomass, was determined according to ASTM D3174-97. Approximately 1.0 g of briquette sample was placed in a pre-weighed porcelain crucible and incinerated in a muffle furnace (Thermo Scientific, USA) at 575 ± 25 °C for at least 4 h. The crucibles were then cooled in a desiccator and reweighed until a constant mass was achieved.

To ensure complete combustion and removal of volatile matter, the heating procedure followed a programmed sequence: from room temperature to 105 °C with a 12-min hold, then to 250 °C at 10 °C min−1 for 30 min, and finally to 575 °C at 20 °C min−1 for 180 min. After ashing, the final weight was recorded, and the ash content was expressed as a percentage of the dry sample weight using Eq. (8)23.

$$\:AC=\:\frac{Mass\:of\:ash}{Mass\:of\:biomass}\times\:100\:\:\:,\:\%$$
(8)

Flue gas emissions of olive pruning residues briquette

Flue gas emissions were analyzed to evaluate the gaseous byproducts generated during the combustion of olive pruning briquettes. The samples were combusted in a custom-designed briquette stove (Fig. 5), where, following ignition and stabilization of steady-state combustion, the concentrations of major gases—namely oxygen (O2, %), carbon monoxide (CO, ppm), carbon dioxide (CO2, %), nitrogen oxide (NO, ppm), nitrogen oxides (NOx, ppm), and sulfur dioxide (SO2, ppm)—were measured using a multi-gas analyzer (MRU DELTA 1600 V, Germany).

The gas analyzer probe was inserted into a hole drilled approximately 1 m from the stove’s boiler outlet and aligned with the center of the flue gas stream to ensure representative sampling. For each sample, three consecutive readings were taken at one-minute intervals. Prior to each test, the combustion chamber and fuel tank were thoroughly cleaned to prevent cross-contamination and to ensure consistent results.

Fig. 5
figure 5

The briquetting stove. (1) fuel chamber, (2) viewing door, (3) air inlet, (4) exhaust pipe, and (5) emission measurement probe.

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Thermo gravimetric analysis (TGA) of olive pruning residues briquette

Thermogravimetric analysis (TGA) was conducted to investigate the thermal degradation and combustion characteristics of the olive pruning residues briquette. The analysis was performed using a DTG-60 H thermobalance (Shimadzu, Japan) equipped with a high-temperature furnace and a controlled nitrogen atmosphere. Approximately 10 mg of finely briquette sample was placed in an alumina crucible and heated from ambient temperature to 800 °C at a constant heating rate of 10 °C min−1 under a nitrogen flow of 50 ml min−1 to maintain an inert environment24. The mass loss of the sample was continuously recorded throughout the heating cycle using a high-precision electronic balance, allowing the determination of thermal stability, decomposition stages, and combustion profiles of the biomass material.

Statistical analysis

Statistical analyses were performed to determine the significance of the effects of moisture content, particle size, and compression pressure on both the mechanical and combustion properties of olive pruning residues. One-way and two-way analyses of variance (ANOVA) were employed to evaluate the influence of individual factors and their interactions. When significant differences were detected (p < 0.05), Tukey’s post hoc test was used to compare treatment means, particularly for parameters such as the compressibility index. Statistical significance was consistently reported across the results to substantiate trends observed in frictional behavior, compaction characteristics, and combustion-related emissions.

Results

Particle size distribution

Table 1 presents the particle size distribution and bulk density of ground olive pruning residues. The data indicate a clear predominance of fine and medium-sized particles in the sample. Particles smaller than or equal to 1.0 mm constitute 45.5% of the total, forming the largest fraction. These fine particles contribute to a smoother overall texture and reduced inter-fiber mechanical interlocking, which in turn decreases the frictional resistance when sliding over solid surfaces, such as stainless steel or galvanized steel. Medium-sized particles in the range of 1.0–1.5 mm account for 39.0% of the material. This size range offers a balance between compactness and particle mobility, promoting moderate dynamic friction due to stable contact and controlled movement under shear. Coarser particles (1.5–2.0 mm) make up the remaining 15.5% of the ground biomass. Although present in smaller proportions, these particles tend to form irregular agglomerates that can enhance mechanical interlocking and increase localized resistance during movement. The bulk density of the ground olive pruning residues varied with particle size, ranging from 240 ± 2.7 kg m−3 for the coarser fraction (1.5–2.0 mm) to 281 ± 6.2 kg m−3 for the finer fraction (≤ 1.0 mm), while the intermediate fraction (1.0–1.5 mm) exhibited a bulk density of 265 ± 5.1 kg m−3. Finaly, this particle size distribution suggests favorable characteristics for handling, compaction, and thermomechanical behavior in bioenergy and material processing applications.

Table 1 Particle size distribution and bulk density of ground olive pruning residues.
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Mechanical properties of ground olive pruning residues

Dynamic friction

Figure 6a,b illustrate in detail the dynamic frictional response of ground olive pruning residues when tested against stainless steel (SS) and steel (St) surfaces under different normal loads (2–9 N) and moisture contents (5–15%). In Fig. 6a, a clear increasing trend in friction force is observed with rising load and moisture content on both surfaces. For example, under a low load of 2 N and 5% MC, the friction force measured 1.1 N on stainless steel and 1.5 N on steel, while at the highest applied load of 9 N and 15% MC, the values increased to 4.3 N and 5.2 N, respectively. This demonstrates that both load and moisture content exert a strong influence on the magnitude of frictional resistance. The friction force of ground olive pruning residues on stainless steel (SS) and steel (St) surfaces was consistently increased by increasing the typical load and moisture content. On St, where the increase in friction from wetness was greater for all applied stresses, the effect was more noticeable. In comparison to 15–18% on SS, friction increments on St were around 25% on average when the moisture content was increased from 5 to 15%. This suggests that SS showed relatively lesser sensitivity to moisture-induced interfacial interactions, whereas steel’s increased roughness and surface energy exacerbated them. These results confirm that steel surfaces consistently exhibit higher friction force than stainless steel, especially under moist and heavily loaded conditions. Statistical analysis confirmed that friction force was significantly affected by load, moisture content, and surface type (p < 0.01), with steel showing higher values. Interaction effects were also significant, and increases at 15% MC were notable under loads ≥ 4 N.

Figure 6b presents the variation of the dynamic friction coefficient (µd) with moisture content on stainless steel (SS) and steel (St) surfaces. At 5% MC, µd was approximately 0.66 on SS and 0.69 on St. Increasing the moisture content to 10% raised µd to 0.71 for SS and 0.78 for St. At 15% MC, µd reached 0.745 on SS and 0.825 on St, corresponding to increases of 12.8% and 15.9%, respectively, compared to the values at 5% MC. Across all moisture levels, the steel surface consistently exhibited higher µd values than the stainless-steel surface. Statistical analysis indicated a significant effect of moisture content on µd (p < 0.01), with one-way ANOVA confirming that differences among moisture levels were statistically significant (p < 0.05), particularly between 5% and 15% MC.

Fig. 6
figure 6

Frictional response of ground olive pruning: (a) Influence of load and moisture content on friction force against steel (St) and stainless-steel (SS) surfaces. (b) Coefficient of friction with moisture content for both surfaces.

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When comparing both figures, it becomes evident that moisture content plays a dual role: it increases the friction force through enhanced adhesion and mechanical interlocking (Fig. 6a), and it elevates the µd (Fig. 6b), particularly on rougher surfaces like steel. The rough texture and higher surface energy of steel enhance the retention and interaction of moist biomass particles, leading to greater resistance compared to stainless steel. For instance, at 10% MC, the dynamic friction coefficient for steel was about 0.07 higher than for stainless steel, reinforcing the observation that steel promotes stronger bonding under moist conditions. Both applied load and MC significantly influence the tribological behavior of ground olive biomass, with steel surfaces showing consistently higher frictional resistance than stainless steel. Moisture amplifies these effects by increasing adhesion and deformation, especially on rougher steel surfaces.

Internal frictional behavior

The internal frictional behavior of ground olive pruning residues under mechanical stress was assessed through two complementary tests: internal shear testing (Fig. 7a) and internal friction and cohesion parameters (Fig. 7b).

In Fig. 7a, the relationship between internal friction force (F) and applied normal load (W) is presented at three moisture contents: 5, 10, and 15%. For each moisture level, a clear linear relationship was observed, confirming the applicability of the classical friction model expressed by Eq. (2), F = mW + b, where m represents the internal coefficient of friction and b the cohesion. At 5% moisture, the measured friction force ranged from approximately 1.5 N at a 2 N load to 5.6 N at a 9 N load. As the MC increased to 10%, the friction force rose to about 1.7 N at 2 N and 6.3 N at 9 N. At the highest moisture level of 15%, the friction force further increased, reaching approximately 1.9 N at 2 N and 6.8 N at 9 N. These results demonstrate a progressive increase in frictional resistance with moisture due to enhanced contact area, adhesion, and surface interlocking within the biomass particles. The increase in slope observed in the linear regression lines across the moisture levels confirms that m increases with moisture, indicating moisture-enhanced shear resistance. Statistical analysis confirmed that both load and moisture content significantly affected internal friction force (p < 0.01), with clear linear trends at each moisture level. The slope (m) of the regression lines increased with moisture, indicating enhanced shear resistance. Differences between moisture levels were statistically significant according to one-way ANOVA (p < 0.05), particularly between 5% and 15% MC.

Figure 7b summarized the extracted values of the internal friction coefficient and cohesion b from the regression models fitted to the data in Fig. 7a. The internal coefficient of friction increased from 0.54 at 5% MC to 0.67 at 15% MC, corresponding to a relative increase of approximately 24%.

Fig. 7
figure 7

Internal frictional response of ground olive pruning residues at different moisture contents, (a) Normal load-friction force relation. (b) Internal coefficient of friction and cohesion.

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This reflects the intensification of shear resistance as moisture promotes closer packing and stronger particle interaction. In contrast, cohesion values showed a decrease from 0.80 N at 5% MC to 0.68 N at 15% MC, representing a 15% reduction. This inverse relationship suggests that water disrupts the internal binding forces within the material, weakening its capacity to resist deformation when no normal pressure is applied. This dual trend, an increasing slope and decreasing intercept, has direct implications for predicting material behavior under different moisture regimes and can be utilized in mechanical modeling of flow and compaction processes. Statistical analysis showed a significant increase in the internal coefficient of friction with moisture content (p < 0.01), while cohesion values decreased significantly (p < 0.05). The opposing trends were confirmed by linear regression analysis, indicating a moisture-driven shift in shear behavior. These changes were statistically meaningful and aligned with known physical mechanisms affecting particulate materials. The data presented in Fig. 7a and b form a cohesive framework for designing and modeling biomass handling systems. The increasing m with moisture suggests that equipment must exert greater mechanical force when processing wet biomass, especially during mixing, extrusion, or briquetting. Conversely, the decreasing cohesion at higher moisture levels indicates a lower likelihood of material arching and bridging in storage bins and hoppers conditions that often impede flow. The linear friction model, validated across moisture conditions, enables engineers to predict shear behavior using simple parameters. In conclusion, the integration of shear test data (Fig. 7a) and regression-derived parameters (Fig. 7b) provide a robust mathematical and physical description of how ground olive pruning respond to mechanical stress. The observed moisture-dependent trends in internal friction and cohesion can be directly applied to optimize processing, reduce mechanical wear, and enhance the efficiency of biomass systems operating under varying environmental and moisture conditions.

Moisture content and surface type significantly influence the frictional behavior of ground olive pruning residues. Higher moisture increased sliding and internal friction due to greater adhesion and particle bonding. Steel Internal friction experiments showed that increasing moisture increased shear strength but decreased cohesiveness, while steel showed greater resilience than stainless steel. This demonstrates how important moisture is to mechanical handling and processing.

Compression behavior‎‎

Compression–particle density relationship

Figure 8a presents the effect of compression pressure on the particle density of ground olive pruning as influenced by particle size (1.5–2.0 mm, 1.0–1.5 mm, and < 1.0 mm) and moisture content (5% and 15%). A consistent trend across all treatments was that increasing the pressure from 3 to 16 MPa produced higher particle densities, reflecting reduced inter-particle voids and improved bonding under elevated pressure. The densification effect was more pronounced at lower moisture content, with all particle sizes showing higher densities at 5% MC compared to 15% MC. Particle size also played a clear role, as finer particles (< 1.0 mm) consistently resulted in greater densities than larger ones, indicating the advantage of increased surface area and better packing efficiency. At higher pressures, particularly for samples at 15% MC, the densification curves began to plateau, suggesting a compaction limit beyond which further pressure produced minimal gains. Statistical analysis confirmed significant effects (p < 0.05) of pressure, particle size, and moisture content, as well as their interactions, on particle density. Thus, the suitable densification of olive pruning residues can be achieved at pressures above 10 MPa, using particles smaller than 1.0 mm, and maintaining moisture content close to 5%.

Specific consumed energy during compression (SCE)

Figure 8b presents the variation in specific compaction energy (SCE) as a function of applied pressure (3–16 MPa) for ground olive pruning residues at three particle sizes (1.5–2.0 mm, 1.0–1.5 mm, and < 1.0 mm) and two moisture content levels (5% and 15%). A consistent trend was observed across all treatments: SCE increased steadily and nearly linearly with increasing pressure. At 3 MPa, values were generally below 10 kJ kg−1 with only minor differences among particle sizes and moisture levels. At 16 MPa, values exceeded 40 kJ kg−1, with finer particles (< 1.0 mm) at 5% MC showing slightly higher energy demand than coarser fractions. Particle size influenced SCE in a clear but secondary way. At all pressure levels, finer particles (< 1.0 mm) consistently showed marginally higher energy consumption than coarser particles (1.5–2.0 mm), particularly at higher pressures. This can be explained by their larger surface area and higher number of contact points, which increase internal friction and demand more energy for rearrangement and compaction. While finer particles promote higher packing density, this comes at the expense of increased compression energy.

Moisture content also played a measurable role. Samples with 5% moisture required slightly more energy than those with 15%, especially at pressures above 10 MPa. Although higher moisture generally makes biomass more plastic and easier to deform, lower moisture increases stiffness and reduces elastic recovery, which enhances resistance to compaction and raises energy requirements. These results observed that dry, rigid biomass requires greater force to achieve permanent deformation during compression.

Fig. 8
figure 8

(a) Effect of pressure, moisture, and particle size on particle density. (b) Specific energy consumed under varying pressure, moisture, and particle size.

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Despite these variations, the most dominant factor affecting SCE was the applied pressure, with a statistically significant effect (p < 0.05). Both particle size and MC contributed to the total energy demand, but to a lesser extent. For instance, at 16 MPa, the difference in SCE between the < 1.0 mm and 1.5–2.0 mm particle sizes was less than 1.2%, illustrating that while size and moisture matter, the energy demand is primarily governed by pressure.

The compressibility index (β)

Table 2 shows the compressibility index (β) of ground olive pruning residues at three particle size ranges (1.5–2.0 mm, 1.0–1.5 mm, and < 1.0 mm) and two moisture levels (5% and 15%), determined using the pressure–density relationship (Eq. 5).

Table 2 Compressibility index (β) values for ground olive pruning residues at different particle sizes and moisture contents.
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For coarse particles (1.5–2.0 mm), β slightly decreased from 0.2876 at 5% MC to 0.2715 at 15%, suggesting modest improvement in compressibility due to moisture-enhanced plasticity and reduced internal friction. Mid-size particles (1.0–1.5 mm) exhibited a similar trend, with β dropping from 0.2911 to 0.2751, indicating that this fraction benefits most from moisture by balancing deformation and flowability.

Interestingly, the finest particles (< 1.0 mm) had the highest β values (0.2944 at 5% and 0.2784 at 15%), reflecting lower compressibility. This may result from agglomeration, increased inter-particle bonding, and mechanical entanglement, all of which hinder rearrangement under pressure. These observations indicate that while moisture generally enhances compressibility, very fine particles can counteract this effect due to their physical characteristics.

In general, olive pruning residues show moderate compressibility due to their woody structure. Medium-sized particles at 10–15% moisture may offer the best performance in densification processes. Statistical analysis revealed that both particle size and moisture content had significant effects on β values (p < 0.05). Two-way ANOVA showed a notable interaction between these factors, confirming that the compressibility response depends on their combination. Tukey’s test indicated that the < 1.0 mm group differed significantly from larger sizes, especially at 5% MC, emphasizing the reduced efficiency of overly fine particles.

The compression ratio (CR)

Compression ratio (CR) is a critical indicator of the compaction efficiency of biomass during densification. Figure 9 illustrates the variation in CR for ground olive pruning residues across different particle sizes (1.5–2.0 mm, 1.0–1.5 mm, and < 1.0 mm), moisture contents (5% and 15%), and applied pressures (3–16 MPa).

A general trend is observed wherein CR increases with applied pressure and decreases with particle size. At 5% moisture content, the highest CR of 3.905 was achieved with particles smaller than 1.0 mm under 16 MPa, followed by 3.79 for 1.0–1.5 mm particles. The coarsest particles (1.5–2.0 mm) exhibited the lowest CR, with 3.773 under the same pressure. This reflects the superior packing and void-filling capacity of finer particles, which can more effectively fill interstitial spaces and achieve higher volume reduction during compression.

Moisture content exerted a consistent influence. For all particle sizes, samples with 5% MC produced slightly higher CR values than their 15% MC counterparts. At 10 MPa and < 1.0 mm particle size, CR was 3.88 at 5% MC compared to 3.585 at 15% MC. The higher CR at lower moisture is attributed to reduced elastic recovery and increased stiffness of dry fibers, which promotes permanent deformation. While the elevated moisture increases biomass plasticity and particle cohesion, which may facilitate deformation but also encourage elastic rebound, limiting the final degree of compression. Compression surface plots (Fig. 9) showed that the steepest increases in CR occurred between 3 and 6 MPa, after which the rate of increase gradually plateaued. Much of the structural rearrangement and void reduction thus occurs at moderate pressures. From 10 to 16 MPa, for instance, the CR of < 1.0 mm particles at 5% MC rose only from 3.88 to 3.905, confirming the diminishing returns at higher pressures.

Fig. 9
figure 9

Compression ratio of ground olive pruning residues at different particle sizes and pressures at 5% and 15% moisture content (MC).

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The compression behavior of ground olive pruning residues is clearly influenced by particle size, moisture content, and applied pressure. Smaller particle sizes and lower moisture levels consistently led to higher CR, while the effect of pressure became less significant above 10 MPa. Statistical analysis confirmed that particle size, moisture content, and pressure had significant effects on CR (p < 0.05). The interaction between particle size and pressure was particularly notable, as finer particles exhibited a more pronounced increase with applied pressure. However, the difference in CR between 13 MPa and 16 MPa was not statistically significant, supporting the observed plateau effect. These results suggest that sutable briquetting performance can be achieved using moderately fine particles (1.0–1.5 mm), low moisture content (around 5%), and pressure levels between 6 and 13 MPa, thereby balancing densification efficiency, product quality, and energy cost crucial factors in the development of sustainable biomass fuel systems.

Combustion behavior of densified olive pruning briquettes

The olive pruning residues briquettes exhibited a gross heating value of 4250 kcal kg−1 and an ash content of 2.4%, as measured without any additives, thereby reflecting their inherent lower heating value. The densification process resulted in briquettes with high energy content and relatively low ash fraction. These measured properties confirm their effectiveness as biomass fuel. The low ash content recorded in the samples reduces the likelihood of slagging, fouling, and mineral deposition on heat exchange surfaces, thereby supporting higher thermal efficiency during combustion25.

Flue gas emissions of olive pruning residues briquettes‎

Figure 10 illustrates the dynamic evolution of major flue gas components during the combustion of compressed olive pruning residues over a 720-second interval. The monitored gases included oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), nitric oxide (NO), and total nitrogen oxides (NOx), providing comprehensive insights into combustion kinetics, pollutant release, and the stages of thermal degradation of this lignocellulosic biomass fuel.

During the ignition stage (0–120 s), O2 concentration decreased from 4.2% at 0 s to a minimum of 2.5% at 60 s (≈ 40% reduction), before slightly recovering to 3.0% at 120 s. In parallel, CO emissions rose sharply from 1200 ppm initially to a maximum of 3300 ppm at 60 s, before declining to 2800 ppm at 120 s. CO2 increased from 8.0% at ignition to a peak of 10.5% at 60 s, then slightly decreased to 10.0% at 120 s. SO2 exhibited a continuous but gradual rise, ranging from 20 ppm to 70 ppm across this period. NO concentrations showed only minor variation, decreasing slightly from 92 ppm to 90 ppm, while NOx decreased from 96 ppm to 94 ppm26.

In the flaming stage (180–480 s), O2 stabilized around 4.0–4.2%, indicating a balanced oxygen supply. CO2 remained almost constant in the range of 9.0–9.5%. CO declined steadily from 2400 ppm at 180 s to 2000 ppm at 300 s, and then remained close to 2200 ppm up to 480 s. SO2 increased continuously during this period, rising from 80 ppm at 180 s to 350 ppm at 480 s. NO and NOx displayed modest reductions, with NO decreasing from 89 ppm to 84 ppm and NOx declining from 93 ppm to 88 ppm27.

Fig. 10
figure 10

Variation of flue gas emission values of olive pruning residues briquette.

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During the burnout stage (540–720 s), O2 increased markedly from 6.0% at 540 s to 18.0% at 720 s, representing a threefold rise as combustible material was exhausted. In contrast, CO2 fell from 8.0% to 1.0%, representing a ≈ 91% reduction. CO emissions dropped sharply from 1800 ppm at 540 s to 500 ppm at 720 s, reflecting an ≈ 85% decrease relative to peak values. SO2 continued to rise steadily, reaching 500 ppm by the end of the test, equivalent to a 25-fold increase compared with the initial value of 20 ppm. NO declined gradually from 83 ppm at 540 s to 80 ppm at 720 s, while NOx decreased from 87 ppm to 84 ppm over the same period28.

The maximum CO2 concentration observed was 10.5% at 60 s, which falls within the range reported for efficient combustion of other lignocellulosic residues (10–18%)27. The lowest CO value recorded was 500 ppm at 720 s, highlighting near-complete carbon oxidation. The final SO2 concentration of 500 ppm and NOx values remaining above 80 ppm underscore the environmental significance of these emissions29.

Generally, the flue gas profiles verify three separate phases of combustion: a volatile-driven ignition phase with rapid O2 depletion and a CO surge, a stable flaming phase with effective CO oxidation and consistent CO2 production, and a burnout phase characterized by char oxidation and delayed sulfur release. The effectiveness of olive pruning residues as a solid biofuel and the environmental cost of their sulfur and nitrogen emissions are both highlighted by these results. In large-scale applications, post-combustion cleaning systems or optimization of combustion parameters such as staged air supply, excess oxygen control, and sufficient residence time are crucial for balancing high conversion efficiency with emission reduction. These results reinforce the necessity of integrating emission control strategies into biomass energy systems to ensure both sustainable energy production and environmental protection.

Thermogravimetric analysis (TGA) of Olive pruning residues briquettes

Figure 11 illustrates the mass decomposition (TG) of olive pruning residues briquette during TGA and DTA analyses, conducted under a nitrogen atmosphere with a flow rate of 50 ml·min⁻¹ and a constant heating rate of 10 °C·min⁻¹. The combustion degradation of compressed olive pruning residues briquettes occurs in three distinct stages, clearly reflected in thermogravimetric analysis (TGA) and differential combustion analysis (DTA) curves. Each phase corresponds to specific physical or chemical transformations occurring during the combustion process. The first stage, known as the dehydration stage, occurs from room temperature up to around 150 °C. This phase is characterized by a small weight loss of approximately 6%, mainly due to the evaporation of moisture, including free, bound, and crystalline water.

The second stage, known as the active devolatilization phase, spans from 150 to 350 °C and is associated with the most significant mass loss around 48% of the initial weight. This stage is primarily driven by the thermal decomposition of hemicellulose and cellulose, the main structural components of olive pruning residues. The TGA curve shows a steep decline in this region, while the DTA curve presents a strong exothermic peak, indicating intense combustion activity and the formation of a carbon-rich char.

Fig. 11
figure 11

Thermo gravimetric analysis (TGA) and differential combustion analysis (DTA) ‎of olive pruning residues briquette.‎.

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Hemicellulose begins to decompose at around 234 °C, while cellulose reaches its maximum decomposition rate between 300 and 350 °C30. The third and final stage, ranging from 350 to 800 °C, represents the slow combustion degradation of the char residue, which consists mainly of lignin. This stage contributes to an additional 20–22% weight loss. The TGA curve gradually levels off near 800 °C, indicating the completion of pyrolysis and the retention of only ash and mineral content. Lignin, known for its thermal stability, decomposes over a broad temperature range (150–900 °C), which justifies the extended mass loss during this phase. Studies confirm that under nitrogen and air atmospheres, lignin degradation in olive pruning residues proceeds slowly, contributing to a more gradual TGA curve tailing31. Comparable mass loss behavior has been recorded for olive pruning and sunflower stalk, where lignin degradation extended beyond 750 °C with residual masses of 18–22%32. The combustion behavior of olive pruning residues briquettes is influenced by their chemical composition, particularly the relative content of cellulose, hemicellulose, and lignin. These components decompose at different temperature intervals, resulting in the three-stage thermal profile commonly seen in TGA analysis.

Thus, the thermal and emission characteristics of olive pruning residues briquettes indicate efficient energy conversion and manageable combustion behavior. The sharp CO₂ peak (17%) and low final CO (300 ppm) confirm complete oxidation, while the three-phase TGA profile with 48% devolatilization and low ash residue—reflects high thermal reactivity. However, the progressive rise in SO2 (to 550 ppm) and persistent NOx emissions (above 80 ppm) highlight environmental concerns.

Discussion

This study comprehensively evaluated the mechanical and combustion properties of olive pruning residues to assess their potential as a solid biofuel in densified form. The mechanical properties investigated included frictional behavior (both dynamic and internal) as well as compression characteristics, while combustion properties encompassed gross calorific value, flue gas emissions, and thermogravimetric behavior. Importantly, the experimental design differentiated between the form of biomass used for each group of tests. Specifically, frictional properties were assessed using the ground olive pruning residues in loose particulate form, simulating their behavior during storage, handling, and feeding. In contrast, all compression-related and combustion tests were conducted on the briquetted (densified) samples, allowing for an accurate evaluation of the material’s performance as a compacted biofuel.

Within the mechanical properties, frictional behavior was highly influenced by moisture content, surface type, and the structural configuration of the particles. As moisture content increased from 5 to 15%, the dynamic friction coefficient significantly increased from 0.66 to 0.745 on stainless steel and from 0.69 to 0.825 on steel surfaces. This increase is attributed to enhanced interfacial adhesion, moisture-induced capillary forces, and greater contact area due to partial softening of fiber surfaces. These effects have also been observed in other lignocellulosic residues where water films facilitate inter-particle bonding and adhesion under sliding conditions12,33. These observations are consistent with those of Mohsenin12 and Zhang et al.33, who emphasized the role of moisture in enhancing interfacial bonding and frictional resistance in agricultural residues. Similarly, Jan et al.34 reported comparable behavior for fibrous residues interacting with rough metallic surfaces, highlighting that while stainless steel provides lower and more stable friction values, steel tends to respond more dramatically to increasing moisture. This makes the choice of contact material critical in processing equipment where moisture levels vary.

Internal shear tests performed on the ground material confirmed that increasing moisture enhanced internal resistance. The internal coefficient of friction rose from 0.54 to 0.67 as moisture increased from 5 to 15%, whereas cohesion decreased from 0.80 N to 0.68 N. This dual effect higher shear resistance but reduced cohesion is consistent with previous results and reflects the role of moisture in promoting particle bonding under motion while weakening static bridges under no load34. Such behavior also supports the physical interpretation proposed by Mohsenin12, where water films increase friction via capillary action but reduce cohesion through interference with interparticle bonds. These results are critical for understanding the material’s flow behavior in hoppers, conveyors, and densification equipment. It should also be noted that rolling friction was not evaluated in this study, as the most relevant resistance mechanisms in practical densification are sliding and internal shear.

The compaction-related mechanical properties were determined using briquetted olive pruning residues, highlighting how structural densification is governed by particle size, moisture content, and applied pressure. Briquette density increased steadily with pressure, especially at 5% MC and for finer particles. At 16 MPa, briquettes from particles < 1.0 mm reached 1180 kg m−2, compared to 1140 kg m−2 for coarser particles. The enhanced packing of finer, drier particles is attributed to their superior rearrangement ability and reduced elastic recovery, confirming that structural compaction is more effective under low-moisture conditions35. These trends align with previous reports for other biomass types such as corn stover and wheat straw36. The compression–particle density relationship observed here also mirrors the behavior noted for various lignocellulosic residues36.

The specific consumed energy (SCE) for briquetting followed a nearly linear trend with pressure. Values ranged from 7.79 to 42.11 kJ kg−1, with finer and drier samples requiring more energy due to increased internal resistance and stiffness. These observations are consistent with research by Li et al.36, who demonstrated a similar linear relationship between compression pressure and SCE during densification of herbaceous biomass. He et al.37 and Ahmed et al.38 who observed similar energy-demanding behavior in dry herbaceous biomass.

The compressibility index (β), calculated from pressure–density curves, ranged from 0.2715 to 0.2944 and showed slight decreases with increased moisture content. Finer particles exhibited slightly higher β values, possibly due to agglomeration and fiber entanglement that restrict rearrangement. These trends confirm that olive pruning residues, due to their woody and semi-rigid nature, exhibit moderate compressibility, with the best performance observed in medium-sized particles at controlled moisture levels. This observation is in line with Eling et al.11 and Malik et al.16, who reported similar limitations for overly fine particles in biomass briquetting. The compression ratio (CR) also served as a clear indicator of densification efficiency. It increased with pressure and decreased with particle size, with a maximum value of 3.905 for < 1.0 mm particles at 5% MC and 16 MPa. However, CR gains plateaued beyond 13 MPa, suggesting that further pressure increases offer diminishing returns. While finer particles enable higher volume reduction, the marginal improvements may not justify the additional preprocessing energy. Thus, medium particles (1.0–1.5 mm) compacted at moderate pressure (10–13 MPa) and low MC (~ 5%) appear optimal for balancing mechanical performance and energy cost39,40.

The combustion properties of the briquetted residues further confirmed their suitability as biofuel. A gross calorific value of 4250 kcal kg−1 and ash content of 2.4% place these briquettes among the higher-performing agricultural residues in terms of fuel quality. Low ash content reduces fouling and maintenance demands in combustion systems, consistent with characteristics of other woody biomass fuels9,25. Flue gas emissions, measured during controlled combustion of briquettes, showed clear combustion stages. CO2 concentrations peaked at 10.5%, and CO level decreased by approximately 500 ppm by the burnout phase, indicating efficient carbon oxidation. In contrast, SO2 emissions increased steadily, reaching 500 ppm, and NOx emissions remained above 80 ppm, highlighting potential environmental concerns. These emissions originate from fuel-bound sulfur and nitrogen compounds, especially in the lignin structure, as discussed by Bashir et al.28. Emission control strategies such as staged combustion or flue gas cleaning are recommended for large-scale applications. The thermogravimetric analysis (TGA) of briquetted samples revealed a three-stage thermal decomposition process: initial drying, volatile release (150–350 °C), and final char combustion up to 800 °C. The DTG peak at 327 °C represents maximum devolatilization, while the final 20–22% mass loss corresponds to lignin degradation. These results align with those reported for similar biomass fuels such as bagasse, sunflower stalks, and cotton residues41,42.

The mechanical properties (frictional and compressive) and combustion behavior of olive pruning residues, evaluated in both particulate and briquetted forms, demonstrate strong potential for bioenergy applications. Frictional tests clarified the handling and flow characteristics of the raw material, while compaction and combustion tests on briquettes highlighted their favorable densification and combustion profiles. With optimal particle size, moisture content, and compression parameters, olive pruning residues can serve as a reliable and sustainable fuel in briquetted form. Beyond technical performance, their utilization as a biofuel also provides environmental and economic advantages offering a sustainable pathway for managing agricultural waste, reducing greenhouse gas emissions compared to fossil fuels, and generating added value for olive cultivation through rural development and enhanced energy security in regions with significant olive production.

Conclusions

This study assessed the mechanical and combustion properties of compacted olive pruning residues to evaluate their suitability as a renewable biomass fuel. The mechanical properties, including friction and compaction behavior, were analyzed on ground olive pruning residues prior to briquetting, while thermal performance was evaluated on the resulting briquettes. Frictional analysis revealed that increasing moisture content from 5 to 15% significantly raised both dynamic and internal friction coefficients, indicating enhanced adhesion and particle interlocking. Meanwhile, cohesion values decreased, facilitating improved material flow under static conditions. Compaction results showed that fine particles (< 1.0 mm) at low moisture (5%) achieved the highest briquette density (up to 1180 kg m−3) under 16 MPa. Specific consumed energy increased with pressure, reaching 42.11 kJ kg−1, particularly in drier, finer biomass. The compressibility index (β) was moderate and slightly reduced with moisture. Compression ratios exceeded 3.9 under optimal conditions, reflecting effective volume reduction and structural stability. Combustion evaluation confirmed strong fuel potential. Olive pruning briquettes exhibited a high calorific value (4250 kcal kg−1) and low ash content (2.4%). Flue gas analysis showed high CO2 output (10.5%) and low residual CO (≈ 500 ppm), but elevated SO2 and NOx emissions that require mitigation in large-scale systems. Thermogravimetric analysis revealed a three-stage decomposition pattern, with 48% mass loss due to hemicellulose and cellulose breakdown, confirming high combustion reactivity. In conclusion, compacted olive pruning residues demonstrate excellent potential for sustainable bioenergy. They combine good compaction behavior, high energy density, and manageable emissions.

Future studies should focus on scaling up compaction systems, optimizing combustion for cleaner emissions, and evaluating economic feasibility for broader implementation.