The Effect of Cherry Stone Addition to Sawdust on the Pelletization Process and Fuel Pellet Quality

Abstract

This study presents the results of research on the pelleting process of pine sawdust with the addition of cherry stone waste, which was carried out using a flat-die pellet press in the context of fuel pellet production. The findings indicate that increasing the proportion of crushed cherry stones in the sawdust mixture from 10% to 20% reduced the pelletizer’s power demand by approximately 14% (from 3.35 to 2.86 kW) and by around 24% (from 3.79 to 2.86 kW), compared with the compaction of sawdust alone. The incorporation of 10% crushed cherry stone waste into pine sawdust slightly improved the kinetic strength of the pellets, increasing it by about 2% (from 94.6 to 96.60%). However, raising the cherry stone content further to 20% resulted in a moderate decrease in kinetic strength, by approximately 5% (from 96.60 to 91.37%). A similar trend was observed for pellet density: the addition of cherry stones (10–20%) slightly reduced the density by about 5.5% (from 1312.02 to 1241.65 kg·m⁻³), accompanied by a small decrease in bulk density. This study also confirmed the high calorific potential of crushed cherry stones, with a heat of combustion of 24.418 MJ·kg⁻¹ (dry basis) and a net calorific value of 22.326 MJ·kg⁻¹. Their incorporation at levels of 10–20% into sawdust mixtures increased the heat of combustion of the pellets by 0.42–0.84% (from 19.959 MJ·kg⁻¹ for sawdust alone at 15% moisture content to 20.042 MJ·kg⁻¹ with a 10% addition and 20.126 MJ·kg⁻¹ with a 20% addition). Moreover, the inclusion of cherry stone waste in the mixture had a beneficial effect on combustion performance, lowering emissions of harmful compounds such as CO, NO, and SO₂, due to the higher combustion temperature achieved. Consequently, the use of cherry stone waste as an additive to sawdust not only enhances the energetic and environmental performance of pellets but also provides an effective pathway for the management of large quantities of fruit industry residues.

1. Introduction

The obligation to support the development of renewable energy sources results from the provisions of EU directives, including Directive 2023/2413 (RED III) on the promotion of electricity produced from renewable sources in the internal electricity market [1]. Pursuant to the Directive, Member States are required to increase the share of renewable energy sources in final energy consumption to at least 42.5% by 2030, with an additional indicative target of 45%. This Directive also introduces new targets for individual economic sectors in terms of obtaining energy from renewable sources. In Poland, the use of waste for the production of energy from renewable sources is regulated primarily by the Act of 20 February 2015 on renewable energy sources (Journal of Laws 2024, item 1361) [2], which defines plant and animal waste from all industrial sectors, including the agri-food industry, as biomass, which can be used as fuel for the production of electricity, heat or biogas/biomethane.

According to Bioenergy Europe, domestic pellet production in the European Union (EU27) increased from 19.83 million tons in 2021 to over 20.5 million tons in 2022 [3]. Despite numerous challenges, including a reduction in heating demand due to climate change, Europe remains the world’s largest region producing and using wood pellets. Currently, over 80% of the raw materials used in European pellets are sawmill residues, including sawdust and shavings. It is becoming necessary to use other plant materials to maintain the economic attractiveness of this fuel and meet the growing demand. In addition to wood waste, other waste materials are increasingly being used in pellet production, including fruit and vegetable industry waste, which, according to Żary-Sikorska [4], is generated in Poland annually at approximately 10 million tons. These wastes come mainly from the processing of potatoes, apples, and sugar beets. Flejszman et al. [5] assumed that at least 20% to 50% of the plant raw material entering production processes becomes waste, which should then be managed. Waste from the fruit and vegetable industry is used, among others, to produce biogas and high-quality biofertilizers, compost, biopolymers, biodegradable packaging, and bioactive compounds [6,7,8,9].

One method of managing waste is to process it through pressure agglomeration (pelletization, densification) into pellets or briquettes [10,11,12,13]. Waste pelletization facilitates transport and storage of the material by increasing its bulk density. This process also allows for the use of the obtained fuel in installations with an automatic feeding system. Furthermore, densification enables the development of a product composed of two or more components, providing considerable flexibility in shaping the properties of the final fuel [14].

Arulprakasajothi et al. [10] conducted research on the production of pellets from biomass derived from fruit and vegetable market waste. The obtained pellets meet ecological fuel standards in terms of both physicochemical properties and energy suitability. Additionally, lower pellet production costs provide a potential and environmentally friendly fuel for energy generation. In a study conducted by Gholami Banadkoki et al. [15], pelletization of leaves from six vegetable crops, including tomatoes, eggplants, zucchini, cucumbers, corn, and soybeans, was investigated to assess their potential as bioenergy raw materials. The physicochemical properties of these biomasses, such as particle size and shape, lignin content, and elemental composition, were examined to assess their effect on pellet density and strength [15]. Obidziński et al. [14] assessed the effect of adding potato pulp to pine sawdust on the pelletization process and the properties of the obtained fuel pellets, taking into account both laboratory and industrial tests. It was found that potato pulp acts as a natural binder, improving the pelletization process (lower energy, higher strength). Chojnacki et al. [16], producing pellets from post-industrial waste generated during the production of apple, carrot, and beetroot juice, noted the effect of these wastes on the pellet mass density, pellet hardness, ash content and calorific value. Additionally, they found that higher proportions of additives and higher moisture content in the raw materials influenced the hardness and density of the obtained pellet. Hejft et al. [17] presented the results of research on the pelletization process of oat bran and apple pomace. The study determined a significant effect of the apple pomace content (10%, 15%, and 20%) in the mixture with oat bran and the effect of the pomace type on the energy consumption of the device and the kinetic stability of the obtained pellets. In turn, Kraiem et al. [18] conducted experiments on the densification and combustion of pellets from sawdust with the addition of tomato and grape marl production residues (in a 1:1 ratio). They observed a high calorific value of this fuel and increased NO_x and SO_x emissions for the combustion technology used. Kang et al. [19] investigated characteristics of spent coffee ground as a fuel and combustion characteristics in a small boiler system, such as CO, NO_x, O₂ and heating characteristics of heating boiler. The high quality of coffee grounds as a fuel is also confirmed by Allesina et al. [20], giving a high heating value.

Poland is a leading cherry producer in the European Union. According to Nowicka et al. [21], on average 70% of the domestic cherry harvest goes to processing plants, where it is used to produce frozen foods, concentrates, juices, nectars, jams, preserves, and alcoholic beverages. Considering that cherry stones constitute from 8% to 15% of the total fruit weight, depending on the variety [22], tens of thousands of tons of this type of waste are generated worldwide each year, which is currently used to a small extent. Cherry stones are extensively studied, among other things, as a raw material for construction materials [23] and filtration materials [24,25], as well as an antioxidant product that can be used in healthcare, cosmetics, and packaging applications [26]. Kniepkamp et al. [27] showed that cherry stone waste could also serve as feedstock for lipid extraction. This paper presents the results of an assessment of the potential of agri-food waste, specifically cherry stones, as raw material for the production of high-quality wood pellets. The aim of the study was to assess the impact of varying the proportion of cherry stones on the pressure agglomeration process of pine sawdust and on the quality of the resulting pellets. This work is innovative because, for the first time, it allows for the determination of the optimal cherry stones content in solid biofuels produced from wood waste, taking into account the preservation of their physical (mechanical) and energy properties, while also potentially reducing the costs of electricity generation.

2. Materials and Methods

2.1. Materials

The primary raw material used in this study consisted of former production residues (sawdust, shavings, and cuttings) obtained from the “DAK-POL” plant company and crushed cherry stones supplied by C.K. Frost Sp. z o.o. (Śmiłowice, Poland) (Figure 1).

2.2. Moisture of the Tested Raw Materials

The moisture content of the raw materials—sawdust and mixtures of sawdust with crushed cherry stones—was determined prior to the densification process in accordance with the PN-EN ISO 18134-3:2023-12 standard [28]. Measurements were carried out using an AXIS ASG laboratory moisture analyzer (AXIS, Gdańsk, Poland).

2.3. Granulometric Distribution of Tested Raw Materials

The granulometric distribution of the raw materials (sawdust and crushed cherry stones) was analyzed using a Multiserv Morek LPz-2e programmable sieve shaker equipped with a set of sieves (Multiserv Morek, Marcyporęba, Poland), in accordance with the PN-R-64798:2009 standard [29]. A series of nine sieves with square mesh sizes of 8.0, 4.0, 2.0, 1.0, 0.5, 0.25, 0.125, and 0.063 mm was employed.

2.4. Elementary Composition Analysis of Raw Materials

The elemental composition of the waste materials was determined using a CHN628 analyzer (St. Joseph, MI, USA). The carbon and hydrogen contents in dry biomass were measured by high-temperature combustion with infrared (IR) detection, while nitrogen was quantified using the katharometric method, in accordance with PN-EN ISO 16948:2015-07 standard [30], based on a 0.1 g sample. Sulfur content was determined by high-temperature combustion with IR detection according to PN-EN ISO 16994:2016-10 standard [31], using a 0.3 g sample. The instrument operates on a combustion principle and reports results in weight percent or parts per million (ppm).

2.5. Density and Bulk Density of the Tested Raw Materials and Pellets

The density of the pellets was determined 24 h after removal from the compaction chamber. For this purpose, the height and diameter of 15 randomly selected pellets were measured using a caliper with an accuracy of ±0.02 mm, while their mass was determined with an OHAUS AX324M laboratory balance (OHAUS Europe GmbH, Nanikon, Switzerland) with an accuracy of ±0.0001 g. Pellet density was calculated as the ratio of mass to volume.

The bulk density of sawdust, crushed cherry stones, and pellets was determined using a metal cylinder with a volume of 407.5 cm³, a laboratory balance (AX324M, OHAUS Europe GmbH, Nanikon, Switzerland), and a steel scraper. The procedure was carried out in accordance with the PN-EN ISO 17830:2016-07 standard [32].

2.6. Pelleting Process of Sawdust and Crushed Cherry Stones

Following the determination of the physicochemical properties, mixtures of sawdust and crushed cherry stones were prepared, containing 10%, 15%, and 20% cherry stone content. After adjusting the moisture content to 16%, the mixtures were placed in sealed containers and stored for 24 h to prevent water evaporation.

Pelletization was carried out using an SS-4 system with a P-300 pelletizer (Protechnika, Łuków, Poland), equipped with a Y132M electric motor (7.5 kW, 1440 rpm) and roller compactors, according to the methodology described in previous authors study [33].

The pelleting system was fitted with a flat die with 6 mm diameter holes and a working gap of 0.4 mm between the rollers and the die socket. The pellets obtained were cooled under ambient conditions for 24 h before further testing. The tests were carried out at a rotational speed of the compaction roller system of 270 rpm and a mass flow rate of the raw material through the working system of 50 kg·h⁻¹.

2.7. Kinetic Strength of Pellets

Twenty-four hours after exiting the pelletizing system, the kinetic strength (durability) of the pellets was determined using a Holmen tester. The tests were carried out in accordance with PN-EN ISO 17831-1:2016-02 standard [34] and following the methodology described in previous study [33]. In the Holmen method, a 100 g pellet sample is introduced into the test chamber, where it is set in motion by an air stream, causing circulation within the chamber and repeated collisions with the perforated metal walls of the tester. The pellets remained in the test chamber for 60 s. After this period, the remaining material was collected, sieved, and weighed. The kinetic durability was calculated as the ratio of pellet mass after the test to the initial sample mass.

2.8. Calorific Value and Heat of Combustion

The calorific value and heat of combustion of the raw materials were determined in accordance with PN–EN ISO 1928:2002 standard [35] and following the methodology described in previous authors work [33], using a KL–12M calorimeter (PRECYZJA-BIT, Bydgoszcz, Poland). The calorific value (Q^a_s), taking into account the contents of volatile matter, moisture, ash, and sulfur entered into the calorimeter’s software.

2.9. Combustion Tests of Produced Pellets

Combustion tests were performed using a 10 kW automatic-feed boiler designed for wood pellets and equipped with a refractory steel retort burner with multi-level air inlets to improve combustion efficiency. The fuel was transported from the hopper to the burner by a screw feeder, while the flue gases passed through a heat exchanger to the chimney, assisted by a fan supplying combustion air. The boiler controller automatically adjusted the fuel and air supply based on oxygen concentration measurements obtained from a lambda probe.

The tests were conducted under nominal operating conditions following a one-hour preheating period. The flue gas composition at the boiler outlet was measured using a SIEMENS analyzer system: ULTRAMAT 23 (for CO, CO₂, SO₂, NO, NO₂, and HCl, by the IR reference method) and OXYMAT 5 (for O₂, paramagnetic reference method, 0–25% range). Continuous sampling was performed using a heated probe with a ceramic filter, a heated hose, and a gas conditioning unit.

Additional tests were carried out in a 5th-class retort grate boiler under manual control, where both pellets and cherry stones were combusted. The fuel feed rate was maintained at 2.0 kg·h⁻¹, with constant airflow to the combustion chamber. The boiler water temperature was kept within the range of 65–70 °C, in accordance with standard biomass combustion testing procedures.

The concentrations of the analyzed flue gas components (CO₂, CO, NO, SO₂, and HCl) were normalized to 10% oxygen (O₂) content using Equation (1) [33]:

$$Z_{s2}={\displaystyle \frac{21-O_2^{\prime }}{(21-O_2^{″})}}·Z_{s1} [\%, \mathrm{mg}·{\mathrm{m}}^{-3}]$$

(1)

where

Z_s1—actual content of the chemical compound in the exhaust gas [%, mg·m⁻³],

Z_s2—content of the chemical compound in the exhaust gas for a given oxygen content [%, mg·m⁻³],

O₂′—set oxygen content in the exhaust gas [%],

O₂″—actual oxygen content in the exhaust gas [%].

2.10. Statistical Analysis

The results of the raw material tests are presented as mean values with standard deviations, calculated from five replicates. The Shapiro–Wilk test was applied to verify the normality of data distribution, and all analyzed variables met the assumption of normality. Pearson correlation coefficients were then calculated for the entire dataset to identify relationships among the analyzed variables. Statistical significance was assumed at p < 0.05. The results are presented in the form of a correlation matrix and a principal component analysis (PCA) plot. Correlation coefficients greater than 0.8 or lower than −0.8 were considered strong correlations.

Additionally, the sum of the absolute values of the correlation coefficients was calculated to provide an aggregated measure of the overall influence of each variable on the others in the dataset. PCA was performed to model the relationships among variables, and the final biplot was constructed based on two principal components (PC1 and PC2), which together explained 99.0% of the total variance. All statistical analyses and visualizations were carried out using Statistica 13.3 software (TIBCO Software Inc., Palo Alto, CA, USA).

3. Results and Discussion

3.1. Moisture of the Tested Raw Materials

Table 1. Results of moisture determination for raw materials designated for pelleting process.

Raw Material	Moisture ± SD [%]
Cherry stones	4.32 ± 0.09
Pine sawdust	8.35 ± 0.10

presents the moisture content of pine sawdust and cherry stones, determined using an AXIS ASG laboratory moisture analyzer.

The average moisture content of cherry stones was approximately 4.3%, while pine sawdust exhibited an average moisture content of about 8.4%. From the perspective of the pelletization process, the moisture content of sawdust was too low. Therefore, additional water or another moisture-enhancing component is required when processing sawdust of such low humidity. The inclusion of crushed cherry stones did not increase the overall moisture content of the mixture, indicating the necessity of external supplementation with water or another component that raises the moisture level.

According to Serrano et al. [36], the optimal initial moisture content for pellet production from barley straw should be in the range of 19–23%, while the recommended value after pelletization is 6–8%.

Jiang et al. [37] investigated the effect of moisture content on pellet formation and properties using three different biomass types (pine sawdust, corn straw, and peanut shell). They found optimal values of 12% for both pine sawdust and corn straw, and 14% for peanut shells.

Similarly, Matkowski et al. [38], studying pelletization of wheat straw and its mixtures with cassava starch or calcium carbonate (using a flat die with an open chamber and 8 mm opening diameter), reported that the highest single-pellet density was achieved at a mixture moisture content of 23% (w.b.).

3.2. Granulometric Distribution of Tested Raw Materials

The granulometric distribution of sawdust and cherry stones particles obtained from sieve analysis is presented in Figure 2. The dominant fraction in the sawdust was 0.50 mm, accounting for 30.64% of the total. Slightly lower proportions were observed for the 1.00 mm (26.47%), 0.25 mm (17.35%), and 2.00 mm (11.11%) fractions. The smallest proportion, 0.09%, corresponded to the undersized fraction below 0.063 mm.

The predominant fraction in the cherry stones was 1.00 mm, which represented 61.75%. Smaller contributions were recorded for the 0.50 mm (13.47%), 0.25 mm (12.45%), 2.00 mm (7.14%), and 0.125 mm (4.71%) fractions. The smallest fraction, 0.1%, was observed at 0.063 mm. Owing to the oil content of cherry stones, clogging of the analyzer sieves was observed, which prevented the detection of undersized particles smaller than 0.063 mm. Figure 3 illustrates the morphology of the individual sawdust fractions, while Figure 4 presents the morphology of the respective crushed cherry stone fractions after sieve analysis.

3.3. Elementary Composition Analysis of Raw Materials

The results of the elemental composition analysis of sawdust and crushed cherry stones are summarized in

Table 2. Elementary composition of raw materials.

Raw Material	C ± SD [%]	H ± SD [%]	N ± SD [%]	S ± SD [%]	O* [%]	Cl [%]	Ash [%]
Cherry stones	52.51 ± 0.05	6.48 ± 0.02	1.31 ± 0.001	0.105 ± 0.001	38.20	0.001	1.40 ± 0.001
Pine sawdust	48.86 ± 0.05	6.65 ± 0.01	0.135 ± 0.001	0.024 ± 0.001	44.24	0.004	0.89 ± 0.02

O*—calculated by difference.

.

The conducted research revealed that sawdust contains 48.86% carbon, while crushed cherry stones contain a slightly higher amount—52.51%. Cherry stones, on the other hand, have a slightly lower hydrogen content (6.48%) compared to sawdust—6.65%. In terms of nitrogen content, sawdust contains 0.135% nitrogen, while crushed cherry stones contain a slightly higher amount—1.31%.

According to Piskowska-Wasiak [39], nitrogen in biomass is primarily associated with proteins, DNA, RNA, chlorophyll, alkaloids, and porphyrins. Its presence in thermally processed biomass is undesirable due to the potential formation of nitrogen oxides (NO_x).

Most standards for wood pellets [40,41] require pellets to contain less than 0.3% nitrogen. The EN Plus A2 certificate (PN-EN ISO 17225-2:2021-10 standard [42]) allows for a maximum of 0.5%, whereas the EN Plus B certificate permits up to 1.0%. Therefore, pellets produced with up to 20% cherry stones comply with the EN Plus B requirements. Sulfur content was also higher in cherry stones (0.105%) than in sawdust (0.024%). According to the ÖNORM M7135 standards [40], the sulfur content in wood pellets should not exceed 0.04%. As noted by Obernberger [43], a sulfur content exceeding 0.2% may significantly increase SO_x emissions. Nevertheless, sulfur in biomass can have a positive effect by mitigating the risk of chloride-induced corrosion, as chloride ions in alkali chlorides (NaCl and KCl) may be converted into more stable sulfates [44]. Hardy et al. [45] report that the risk of chloride corrosion is negligible if chlorine content in biomass does not exceed 0.02%.

3.4. Bulk Density of the Tested Raw Materials

Table 3. Results of bulk density tests of sawdust and cherry stones.

Raw Material	Bulk Density ± SD [kg·m−3]
Pine sawdust	100.08 ± 4.48
Cherry stones	561.02 ± 3.01

presents the bulk density results of sawdust and cherry stones.

Sawdust exhibited a relatively low bulk density of approximately 100 kg·m⁻³, whereas cherry stones showed a significantly higher value of about 561 kg·m⁻³. For comparison, Stasiak et al. [46] reported a bulk density of 143 kg·m⁻³ for sawdust at 10% moisture content, while Netinger Grubeša et al. [47] recorded 472.88 kg·m⁻³ for cherry stones at 9.30% moisture content.

3.5. Pelleting Process of Sawdust and Ground Cherry Stones

Table 4. Effect of crushed cherry stone content in sawdust mixtures on the pelleting process (pelletizer’s power demand and energy consumption), as well as on the kinetic strength, density, and bulk density of the produced pellets.

Pellet Quality
Cherry Stones Content [%]	Pelletizer’s Power Demand [kW]	Energy Consumption [kW·h·t−1]	Kinetic Strength [%]	Density [kg·m−3]	Bulk Density [kg·m−3]
0	3.79	75.8	94.64	1312.02	675.75
10	3.35	67.0	96.60	1255.43	604.59
15	3.05	61.0	96.20	1249.55	606.36
20	2.86	57.2	91.37	1241.65	634.36

summarizes the results of the study investigating the influence of crushed cherry stones content in mixtures with sawdust on the pelleting process. Specifically, it presents the pelletizer’s power demand during pellet production, as well as the effects on the kinetic strength, density, and bulk density of the resulting pellets.

Figure 5 presents the effect of crushed cherry stone content in sawdust mixtures on the pelleting process, expressed as the pelletizer’s power demand during densification.

Based on the results presented in Table 4 and Figure 5, the content of crushed cherry stones in the sawdust mixture was found to have a significant influence on the pelleting process, particularly on the pelletizer’s power demand.

An increase in the proportion of crushed cherry stones from 10% to 20% (at a mixture moisture content of approximately 16%) reduced the pelletizer’s power demand by about 14% (from 3.35 kW to 2.86 kW). Compared with the pelletization of pure sawdust, this reduction reached approximately 24% (from 3.79 kW to 2.86 kW).

The tests confirmed that the addition of crushed cherry stones significantly enhanced the compressibility of the mixture, as evidenced by the marked decrease in pelletizer power demand with increasing cherry stone content.

The recorded values during the compaction of the sawdust–cherry stone mixtures in a pelletizer with a flat die further confirm the beneficial effect of cherry stone addition on reducing energy consumption. Under the same moisture conditions (≈16%), the pelletizer’s power demand for mixtures was substantially lower than for sawdust alone, which required 3.79 kW.

The relationship between the cherry stone content in the sawdust mixture and the pelletizer’s power demand N_g is described by Equation (2):

N_g = −0.31·z_p + 4.03

(2)

where

z_p—content of crushed cherry stones [%],

N_g—pelletizer’s power demand [kW].

According to Obidziński et al. [48], the addition of herbaceous elderberry waste at levels ranging from 10% to 20% did not significantly affect the pelletizer’s power demand, which nonetheless decreased compared to the pelletization of pure pine sawdust. The reduction relative to pure sawdust amounted to approximately 10%.

Similarly, increasing the potato pulp content in mixtures with sawdust from 10% to 25% reduced the pelletizer’s power demand by about 20% (from 7.35 kW to 5.92 kW) [14].

Obidziński et al. [49] further reported that the inclusion of post-flotation dairy sludge in sawdust mixtures had a significant effect on the pelletizer’s power demand. Increasing sludge content from 10% to 20% decreased the power demand by approximately 26.8% (from 3.92 kW to 2.87 kW). Compared with sawdust alone, the reduction in pelletizer’s power demand for the 20% sludge mixture reached 32%.

3.6. Kinetic Strength of Pellets

Figure 6 presents the results of tests on the effect of crushed cherry stones in the mixture with sawdust on the kinetic strength of the obtained pellets.

Based on the conducted research (Figure 6), it was found that increasing the content of crushed cherry stones in sawdust mixtures from 10% to 20% resulted in a slight decrease in pellet kinetic strength, by approximately 5% (from 96.60% to 91.37%). The kinetic strength of pellets with a 10% addition of crushed cherry stones was slightly higher than that of pellets produced from pure sawdust under the same moisture conditions (16%), which reached 94.64%.

The addition of crushed cherry stones, combined with water (increasing the mixture moisture from approximately 8% in raw sawdust to about 16% during pelletization) or with steam supplied in the process, promotes the formation of a binder (sticky gel) at elevated temperatures. This binder enhances interparticle bonding, leading to the formation of durable agglomerates with high kinetic strength after drying.

The effect of crushed cherry stone content z_p in the sawdust mixture on pellet kinetic strength P_dxg is described by Equation (3):

P_dxg = −1.69·z_p² + 7.47·z_p + 88.77

(3)

where

z_p—content of crushed cherry stones [%],

P_dxg—kinetic strength of pellets [%].

Research conducted by Cwalina et al. [50] showed that the kinetic strength of pellets decreased by 0.7% (from 98.21% to 97.56%). In another study [33], it was reported that pellets produced from hemp shives with a 10% addition of potato pulp exhibited a kinetic strength of 98.1%. A further increase in potato pulp content led to only a slight reduction in strength, with a minimum value of 96.42%.

Dorokhov et al. [51] investigated pellet compositions based on wood sawdust with the addition of coal slime, peat, and a mixture of straw and rice husk. Their findings indicated that the kinetic strength coefficients were 6–32% higher compared to pellets made from pure sawdust.

3.7. Density and Bulk Density of Pellets

Figure 7 presents the effect of crushed cherry stone content in sawdust mixtures on the physical density (Figure 7a) and bulk density (Figure 7b) of the produced pellets. As shown in Figure 7a, increasing the proportion of crushed cherry stones from 10% to 20% slightly reduced pellet density, from 1255.43 kg·m⁻³ to 1241.65 kg·m⁻³. At each level of cherry stone addition, the pellets exhibited lower density compared to those made from pure sawdust at the same moisture content (16%), which reached 1312.02 kg·m⁻³.

The measured pellet densities (all exceeding 1000 kg·m⁻³) for mixtures containing 10–20% crushed cherry stones confirm that the pellets are of very high quality and can be considered a fully valuable and innovative biofuel. These values meet the requirements of the current PN-EN ISO 17225-2:2021-10 standard [42]. Consequently, the produced pellets can be used as a full-value fuel both in professional power plants equipped with high-efficiency boilers (with strict fuel quality requirements) and in individual biomass heating systems.

The relationship between the bulk density of pellets and the proportion of crushed cherry stones in the sawdust mixture is clearly evident (Figure 7b). An increase in the share of crushed cherry stones from 10% to 20% results in a rise in bulk density, from 604.59 kg·m⁻³ to 634.36 kg·m⁻³. This increase is accompanied by a reduction in pellet length as the proportion of crushed cherry stones grows (Figure 8). The observed effect is attributed to the higher oil content of cherry stones, which promotes the formation of shorter pellets.

The effect of crushed cherry stones z_p in the mixture with sawdust on the pellets density ρ_g and on the pellets bulk density ρ_ug is described by Equations (4) and (5):

ρ_g = −12.17·z_p² − 82.56·z_p + 1379.80

(4)

ρ_ug = 24.79·z_p² − 136.19·z_p + 784.82

(5)

where

z_p—content of crushed cherry stones [%],

ρ_g—pellets density [kg·m⁻³],

ρ_ug—bulk density of pellets [kg·m⁻³].

The conducted research indicates that the optimal addition of crushed cherry stones, in terms of pellet quality, ranges from 10% to 15%. Within this interval, the produced pellets exhibit favourable properties, including high kinetic strength and increased bulk density. Moreover, this level of additive reduces the energy demand of the pelleting process compared with the energy required for compacting sawdust alone. Paczkowski et al. [52] reported that increasing the proportion of black locust sawdust in the mixture to 75% decreased bulk density from 716 kg·m⁻³ to 662 kg·m⁻³. Similarly, the incorporation of potato pulp reduced pellet density from 1290 kg·m⁻³ to 1230–1260 kg·m⁻³, demonstrating the influence of organic additives on density. The inclusion of 10% bakery waste in the form of wholegrain bread and pumpkin bread also lowered the bulk density of pellets by approximately 11–16% [53].

3.8. Calorific Value and Heat of Combustion

Table 5. Test results of the calorific value and heat of combustion of sawdust and cherry stones.

Raw Material		Heat of Combustion [MJ·kg−1]		Calorific Value [MJ·kg−1]
	Analytical Moisture ± SD [%]	Dry Matter ± SD	Analytical Moisture ± SD	Dry Matter ± SD	Analytical Moisture ± SD
Cherry stones	8.35 ± 0.09	23.499 ± 0.045	21.528 ± 0.050	22.022 ± 0.046	19.974 ± 0.046
Pine sawdust	15.00 ± 0.082	4.418 ± 0.022	20.792 ± 0.026	22.326 ± 0.087	19.004 ± 0.038

presents results of tests on the calorific value and heat of combustion of sawdust and crushed cherry stones.

Based on tests conducted for dry sawdust (0% moisture) and for sawdust at 8.35% moisture, the effect of sawdust moisture content on the heat combustion of sawdust Q^a_s obtained during combustion in a calorimetric bomb was determined using the following Equation (6):

Q^a_s = −0.242·w_p + 24.418

(6)

The effect of sawdust moisture content wt on the calorific value Q^a_i using the following Equation (7):

Q^a_s = −0.222·w_p + 22.326

(7)

Based on the dependence of the heat of combustion and the calorific value of sawdust and crushed cherry stones on moisture content, the heat of combustion and calorific value of the tested raw material mixtures (at 15% moisture content) were determined for different proportions of cherry stones in the mixture with sawdust.

Table 6. The effect of crushed cherry stones in the mixture with sawdust on the calorific value and combustion heat.

Cherry Stones Content [%]	Heat of Combustion [MJ·kg−1]	Calorific Value [MJ·kg−1]
0	19.959	18.323
5	20.001	18.357
10	20.042	18.391
15	20.084	18.425
20	20.126	18.459
25	20.167	18.493

presents the results of the study on the effect of crushed cherry stone content in the mixture with the raw material on its calorific value and heat of combustion at analytical moisture (15%).

The obtained values of the heat of combustion and calorific value indicate that the addition of crushed cherry stones enhances the energy performance of the produced pellets. For instance, incorporating 10% crushed cherry stones into sawdust increased the heat of combustion and calorific value by 0.081 MJ·kg⁻¹ (approximately 0.42%), while a 20% addition resulted in an increase of 0.167 MJ·kg⁻¹ (approximately 0.84%).

According to Chojnacki et al. [16], who investigated the effects of adding fruit pomace and vegetable residues to barley straw, the incorporation of pomace slightly reduced the calorific value of pellets compared with pellets produced from straw alone. A 30% pomace addition decreased the calorific value by less than 7% relative to pure barley straw pellets. Similarly, Jekayinfa et al. [54], who pelletised rice bran using a screw-type pelleting machine with starch as a binder, reported heating values in the range of 16.54–17.30 MJ·kg⁻¹. Ferreira et al. [55] characterised pellets produced from residual biomass derived from the processing of Dinizia excelsa and Manilkara elata wood, in combination with Eucalyptus spp. They obtained a heating value of 20.39 MJ·kg⁻¹ for pellets produced from a 50:50 mixture of Dinizia excelsa and Manilkara elata.

3.9. Combustion Tests of Produced Pellets

Table 7. Exhaust gas composition from the combustion of pellets made from used sawdust and a mixture of sawdust and crushed cherry stones.

Parameters	Pellets Without Cherry Seeds		Pellets with 10% Cherry Seeds		Pellets with 15% Cherry Seeds		Pellets with 20% Cherry Seeds		Ecodesign Limit Value (at 10% O2)
	Actual Concentration	At 10% O2	Actual Concentration	At 10% O2	Actual Concentration	At 10% O2	Actual Concentration	At 10% O2
CO2 [%]	6.07	7.86	6.30	7.77	6.25	7.54	7.41	7.51	-
CO [mg·Nm−3]	439.38	569.27	382.51	471.83	368.66	445.14	417.54	423.31	500
NOx [mg·Nm−3]	208.24	269.81	153.29	189.04	139.95	168.99	149.81	151.88	200
SO2 [mg·Nm−3]	24.88	32.23	17.88	22.05	17.42	21.03	22.53	22.84	-
HCl [mg·Nm−3]	1.98	2.56	2.22	2.74	2.19	2.64	2.76	2.79	-
O2 [%]	12.51	-	12.08	-	11.89	-	10.15	-	-
Excess air coefficient λ	2.39		2.28		2.23		1.89
Flue gas temperature in the boiler flue [°C]	160		170		175		190

presents the combustion results (exhaust gas composition) obtained during the combustion of sawdust pellets and pellets made from a mixture of sawdust and crushed cherry stones.

Analysing the combustion performance of the produced pellets, it was observed that increasing the proportion of cherry stones from 10% to 20% raised the exhaust gas temperature in the boiler flue from 160 °C (reference sawdust pellets) to 170 °C (10% cherry stone addition) and further to 190 °C (20% addition). Under identical boiler operating parameters, this increase in flue gas temperature reflects the higher calorific value of the fuel, confirming that cherry stones act as an additive enhancing the heat of combustion of the pellets. Elevated combustion temperatures contributed to a reduction in carbon monoxide and nitrogen oxide emissions, which for pellets with cherry stone additions remained within the limits set by the EU Ecodesign Directive [56], i.e., below 500 mg·Nm⁻³ for CO and 200 mg·Nm⁻³ for NO_x. Under the tested biomass combustion conditions, the main source of nitrogen oxides was fuel-bound nitrogen, as the combustion temperature did not exceed 1300 °C. Consequently, the Zeldovich mechanism was not expected to occur, and due to the high excess air coefficient (λ), so-called “Prompt NO” formation, i.e., the reaction of atmospheric N₂ with hydrocarbons in fuel-rich zones, was also negligible.

In all cases, the excess air coefficient remained at a level acceptable for the tested unit. Notably, the slight reduction in λ observed with increasing cherry stone content indicated improved contact between combustible particles and the oxidising agent (air).

During combustion, most of the chlorine present in biomass is released as hydrogen chloride (HCl), which may subsequently react with other exhaust gas components to form dioxins. As the flue gases cooled outside the combustion chamber, a considerable share of chlorine condensed as salts on heat exchanger surfaces or on fly ash particles, leading to high-temperature chloride corrosion of boiler components. Although the Ecodesign Directive does not specify maximum HCl concentrations in flue gases from small-scale boilers, German standards set a permissible limit of <5 mg·m⁻³. In the present study, HCl concentrations in the flue gases were comparably low, reflecting the low chlorine content of the raw materials used for pellet production.

Sulphur oxide emissions depended primarily on the sulphur content of the fuel. During combustion, biomass sulphur was oxidised mainly to sulphur dioxide (SO₂), with minor amounts of sulphur trioxide (SO₃), subsequently forming alkalis and sulphates. The significance of sulphur lies less in its emission levels and more in its role in corrosion processes. High SO_x concentrations in exhaust gases contribute to the sulphurisation of alkalis and alkali metal chlorides, reducing the flue gas dew point and promoting chlorine release. Certified wood pellets burned in the same installation generated sulphur dioxide emissions of approximately 23 mg·m⁻³, a level similar to that observed for the tested pellets.

For comparison, combustion tests carried out using uncrushed cherry stones in the same boiler installation showed significantly poorer emission characteristics and less favourable combustion behaviour compared with pellets. The values reported in

Table 8. Composition of exhaust gases from the combustion of whole cherry stones.

Parameters	Value	Ecodesign Standard [56]
CO2 [%]	7.78	-
CO [mg·Nm−3]	731.86	500
SO2 [mg·Nm−3]	58.49	-
NO [mg·Nm−3]	381.83	200
HCl [mg·Nm−3]	9.04	-
Actual oxygen content in exhaust gases [%]	14.34	-
Excess air coefficient λ [-]	3.00	-
Flue gas temperature in the boiler flue [°C]	160	-

are expressed relative to 10% oxygen content in the exhaust gases.

In summary, the incorporation of cherry stones into wood sawdust improved pellet combustion performance by reducing emissions of harmful CO, NO, and SO₂ compounds, primarily as a result of the higher combustion temperature. The observed reduction in the excess air coefficient (λ) indicated enhanced fuel–air mixing conditions within the combustion chamber, thereby decreasing heat losses through the exhaust gases. Importantly, the CO and NO emissions from the combustion of wood–sawdust pellets enriched with cherry stones in a Class 5 boiler with a retort grate complied with the requirements of the Ecodesign Directive of the European Parliament and Council [56], which has been in force since 1 January 2020.

3.10. Statistical Analysis

The proportion of cherry stones in the sawdust mixture exhibited the highest sum of absolute correlation values and was strongly associated with the physical and energy parameters of the pellets (Table S1). This factor had a significant effect (r > 0.8 or r < −0.8) on seven of the 13 variables analysed, with the strongest correlations observed for the heat of combustion and the calorific value of the pellets. In contrast, kinetic strength had the weakest influence on pellet quality parameters, with correlation coefficients ranging from −0.8646 (CO₂ emissions) to 0.8269 (O₂ emissions). Apart from these two parameters, kinetic strength showed no strong correlations with the other variables and yielded a relatively low sum of correlation values (Σ|r| = 6.0079) (Table S1).

Principal component analysis (PCA) confirmed these results, indicating that the proportion of crushed cherry stones, heat of combustion, and calorific value were characterised by the highest negative loadings, ranging from −0.9921 (heat of combustion) to −0.9951 (cherry stone content), with respect to the first principal component (Figure 9). By contrast, kinetic strength was positively loaded (0.9303) on the second component.

4. Conclusions

Based on the conducted research, it was found that the sawdust was characterised by excessively low moisture content and an excessive fine fraction, which adversely affected the pelleting process and the quality of the pellets. The addition of crushed cherry stones to the mixture proved to be an effective method of improving the raw material properties. It enhanced compactibility, reduced the energy consumption of the process by more than 20%, and enabled the efficient management of the problematic fine fraction. At the same time, the incorporation of cherry stones reduced wear on the pelletizer operating system and improved overall process efficiency.

The analysis of pellet physical properties demonstrated that the addition of cherry stones caused only minor changes in strength and density while maintaining values compliant with market requirements and the PN-EN ISO 17225-2:2021-10 standard. From an energy perspective, the produced pellets exhibited higher heat of combustion and calorific values, which, combined with lower emissions of CO, NO, and SO₂, translated into improved combustion quality and reduced heat losses. The cherry stone pellets also complied with EN Plus B, DIN-Plus, and ÖNORM M7135 requirements for nitrogen and sulphur content, and their gaseous emissions met the limits of the Ecodesign Directive.

The resulting product, with a density exceeding 1000 kg·m⁻³, represents a high-quality solid fuel with favourable performance parameters. Its production process can be regarded as both innovative and energy-efficient. An additional advantage lies in the utilisation of large quantities of cherry stone waste from the fruit processing industry, in line with EU waste management policies promoting prevention, recycling, and reuse. Consequently, the use of cherry stones in pellet production may be considered both technologically advantageous and environmentally sustainable within the renewable energy sector.

The proportion of cherry stones in the sawdust mixture exerted a decisive influence on pellet physical and energy parameters, particularly heat of combustion and the calorific value. By contrast, kinetic strength played only a marginal role, showing significant correlations mainly with gas emissions. Principal component analysis (PCA) confirmed the key importance of cherry stone content and energy parameters, whereas kinetic strength was associated primarily with the second component, underscoring its lesser relevance in assessing pellet quality.

Using cherry stones and sawdust for pellet production not only provides an alternative source of renewable energy but also helps reduce the burden of waste management in the agri-food sector. The valorization of these residues through pellet production enhances environmental sustainability and makes a significant contribution to the development of circular bioeconomy strategies.