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Article

Fuel Pelletization of Digestate: A Pathway to Renewable and Sustainable Energy Sources

by
Iryna Vaskina
1,
Roman Vaskin
2,
Ruslan Ostroha
3,
Mykola Yukhymenko
3,
Maksym Skydanenko
3,
Karol Kupryaniuk
1,
Lenka Demkova
4,* and
Serhii Sydorenko
2
1
Department of Biosystems Engineering, Faculty of Environmental and Mechanical Engineering, Poznan University of Life Sciences, Wojska Polskiego Str. 50, 60-627 Poznan, Poland
2
Department of Ecology and Environmental Protection Technologies, Faculty of Technical Systems and Energy Efficient Technologies, Sumy State University, Kharkivska Str. 116, 40007 Sumy, Ukraine
3
Department of Chemical Engineering, Faculty of Technical Systems and Energy Efficient Technologies, Sumy State University, Kharkivska Str. 116, 40007 Sumy, Ukraine
4
Department of Ecology, Faculty of Humanities and Natural Sciences, Presov University in Presov, 17 November Str. 15, 08001 Presov, Slovakia
*
Author to whom correspondence should be addressed.
Fuels 2025, 6(2), 32; https://doi.org/10.3390/fuels6020032
Submission received: 6 March 2025 / Revised: 27 March 2025 / Accepted: 27 April 2025 / Published: 2 May 2025
(This article belongs to the Special Issue Feature Papers in Fuels)
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Versions Notes

Abstract

:
Digestate as a by-product of biogas production requires appropriate utilization methods to convert it into a valuable resource. This study investigated the feasibility of using digestate from a biogas plant as a sustainable feedstock for fuel pellet production. Digestate from an agricultural biogas plant was dried and pelletized, both with and without the addition of biochar. The resulting pellets were analyzed for their physicochemical properties, elemental composition, and calorific value. Samples of pellets were examined using a calorimeter and XRF analyzer. Results showed that digestate pellets exhibited promising fuel characteristics comparable to traditional wood pellets (17.07–17.11 MJ/kg). However, the addition of biochar, while increasing calorific value, led to high ash content and elevated concentrations of Cl, S, N, Ni, Zn, exceeding acceptable limits defined by ISO 17225-6. Consequently, biochar addition is not recommended due to potential environmental concerns upon combustion. The findings highlight that digestate with initial moisture content of 7–7.5% is the most suitable for pelletization in terms of mechanical durability and strength quality. Further research is recommended to fully assess the environmental and economic viability of digestate-based fuel pellets. This approach addresses two issues: it enables waste utilization and produces a valuable resource.

Graphical Abstract

1. Introduction

At the current stage of technological progress worldwide, the sustainable management of organic waste, as well as increasing renewable energy use has become a key issue. Anaerobic digestion, a technology increasingly adopted globally, offers an effective and promising solution for waste utilization by generating valuable products like biogas and digestate. Hence, biogas plants focusing on the bioconversion process of various organic materials through their methane fermentation have become widespread [1,2,3,4]. This aligns with circular economic principles and contributes to renewable energy production. During the processes of organic matter decomposition, mineralization and biogas releasing (about 10% of the total biomass), digestate is formed (about 90% of the total biomass) [5]. The composition of biogas and digestate depends on the type of feedstock used in the process of methane fermentation [6]. Digestate is a very promising product that can be used as a nutrient both in raw form and as a base for new types of organic fertilizers in granular form [7]. Digestate, with its valuable physicochemical and microbiological properties, has proven effective as a fertilizer in Europe. Digestate is rich in nutrients like nitrogen, phosphorus, potassium and contains organic matter and causes challenges in managing in large quantities. Converting it to other forms (like pellets) reduces storage volume and simplifies transportation [8].
There is currently growing interest in digestate as a solid biofuel. This application of digestate helps bridge the gap between currently unmatched challenges: energy generation and waste disposal in agriculture [9]. The effective utilization of digestate from biogas plants presents a significant challenge across Europe. In temperate climates with seasonal variations, direct field application of digestate is limited for 5 months a year (autumn and spring). During other periods, storage in lagoons or specialized containers (tanks, silos, digester bags, etc.) becomes necessary, posing environmental risks such as nutrient leaching and greenhouse gas emissions [10]. Processing digestate into fuel pellets offers a viable solution, simplifying storage and providing a valuable fuel source [11]. It should be noted that, for example, in Ukraine, most biogas plants (about 40%) are associated with sugar factories, utilizing sugar beet pulp as a primary feedstock [12]. On the other hand, they do not have available agricultural lands which makes the use of digestate difficult. At the same time, such an enterprise always needs energy to ensure the technological process, so pellets from digestate can provide a sustainable on-site energy source. Fuel pelletization of digestate offers a promising pathway to valorize this byproduct, transforming it into a solid biofuel suitable for combustion and potentially gasification [13]. This approach not only addresses waste management concerns but also contributes to a circular economy by creating a renewable energy source from an existing waste stream.
Weaknesses of using fuel pellets or granules from digestate and their mixtures include higher ash content in the burnt residues, as well as nitrogen, sulfur and chlorine compounds compared to conventional biomass such as wood [14,15,16]. High ash content can cause fouling, slagging and corrosion on the surface of furnace structures [17,18], and high levels of nitrogen, sulfur and chlorine compounds can lead to an increase in the content of their oxides in flue gas emissions, which can cause problems with air quality in the environment [19,20].
The role of digestate as an alternative source of thermal energy is increasingly emphasized [21,22]. The advantage of the solid fraction of digestate is its calorific value. The calorific value of solid biofuels obtained from the solid fraction of fermented pulp is comparable to the calorific value of biofuels obtained from wood sawdust [21]. Pulp processed into fuel pellets can be considered as an alternative to wood fuel in solid biofuel technologies. Some researchers [23,24] also note that the calorific value of poultry manure is equivalent to the calorific value of low-grade coal. So, burning the solid form of digestate or co-burning it with other biomass materials can become a practical method of energy production.
The chemical composition and physical properties of digestate fuel pellets depend on the type of feedstock used for biogas production. Kratzeisen et al. [25] conducted experiments on the combustion of digestate pellets to clarify the process conditions and estimate emissions. It was found that the calorific value of digestate pellets ranged from 15.8 MJ/kg to 15.0 MJ/kg with a water content of 9.2% to 9.9%. The ash content ranged from 14.6% to 18.3%, with a softening temperature of 1090 °C to 1110 °C. Therefore, according to the authors, the digestates investigated in this experiment were recommended as fuels for combustion.
Chen et al. [26] investigated the production of pellets from agricultural waste (digestate, chicken manure) and their potential as solid biofuels comparing them to wood. It is noted that fuel pellets from digestate from grass silage mixing with cow manure provided the highest combustion temperature. This was followed by pellets obtained from chicken manure and digestate mixed with wood waste. It was concluded that such fuels meet the energy requirements of biomass fuels, but combustion characteristics and gas emissions can cause problems due to the increased ash content in the burned residues. The impact of feedstock composition on pellets combustion characteristics was also studied by Rhen et al. [27].
The solid fraction of digestate can be used for producing granules and pellets of various shapes. The last one is of practical interest as fuel. Post-production waste generated at agricultural and food processing enterprises is also used as fuel. The ash content of digestate pellets is relatively high [28] and its composition needs to be carefully managed to ensure it is suitable for use as a fuel. It also should be mentioned that the calorific value of digestate pellets could be lower than other biomass fuels, impacting their economic viability. High moisture content and variable composition of digestate depending on the feedstock also create challenges [29,30].
The work [31] presents an analysis of mixtures of different digestates to assess their suitability for combustion. A digestate based on corn silage and apple pomace was used as a test fuel. The analysis showed that these materials are a valuable source of energy, but combustion must be carried out in special installations. Increased moisture and ash content (up to 12%) can lead to sintering of ash particles with each other and, accordingly, clogging of structural elements of fuel boilers. However, Pedrazzi et al. [32] indicated that the reduction of the ash sintering effect is facilitated by obtaining fuel pellets consisting of 50% digestate and 50% wood. They found that these fuel pellets provided a high calorific value, equal to 16.6 MJ/kg.
The technology of fuel pelletization also significantly affects the final product characteristics. Andersone et al. [33] investigated the potential of fuel pellets produced from lignocellulosic agricultural residues (sea buckthorn fruit and branches) using a granulator with a flat matrix. Obidziński et al. [34] examined fuel pellets made from onion peel waste with the addition of potato pulp. This mixture was granulated in a matrix granulator at different matrix rotation speeds. The highest quality granulate was obtained from a mixture containing 10% potato pulp, which was compacted on a matrix at 170 rpm. The quality of the pellets was assessed by burning them in a boiler.
Several studies have explored waste agglomeration using a matrix granulator [35,36,37]. Pretreatment of organic waste before granulation and its effect on the mechanical and fuel characteristics of fuel pellets are described in the works [37] (drying), [38] (addition of a binder) and [39,40] (hydrothermal treatment). Abdoli et al. [41], Muramatsu et al. [42] and Mostafa et al. [43] provide further insights into pellet production variables and their effects on pellet quality.
Thus, the solid fraction of digestate in pure form or with additives of biomass is recommended as a fuel. However, there are certain features of the use of digestate depending on the specific type of feedstock origin. The main quality indicators of pellets are ash content and moisture content. To justify using digestate for producing fuel pellets, it is necessary to analyze the technological parameters of the process of burning the solid fraction of digestate and fuel pellets from various raw materials, ash content, the qualitative properties of fuel pellets (mechanical durability and calorific value) depending on the type of feedstock.
Therefore, the aim of this work is to fill the gap by exploring the pelletization process and studying physicochemical properties of the solid phase of digestate, fuel characteristics of the pellets, their strength properties and chemical composition in order to determine the most economically feasible parameters of pellets production considering requirements of ISO 17225-6:2021 [44] for non-wood pellets.

2. Materials and Methods

2.1. Materials

The solid phase of digestate from the biogas complex of the Teofipol Energy Company (Teofipol, Khmelnytskyi region, Ukraine) was used in this study. The biogas complex uses sugar beet pulp, corn straw and cattle manure as the feedstock. The feedstock composition of digestate is presented in Table 1. Two types of digestates resulting from fermentation of different feedstock were used for studies. Digestate D1 is the regular digestate obtained at this biogas plant. Digestate D2 is obtained from an auxiliary technological line that operates for several months a year. Digestate D1 was used to study its granulometric composition, producing pellets and investigation of mechanical properties, chemical composition, and the impact of initial humidity on the mechanical properties of the pellets. Digestate D2 was used for producing pellets that were used as comparative ones during the study of their physical and mechanical properties and chemical composition. In addition, the mixture (Mix DBC) of digestate 1 (D1, 90%) and biochar (10%) was used in order to check how it impacted the calorific value of digestate pellets.
As an additive to digestate that could lead to increasing the calorific value of digestate pellets, the biochar obtained from sunflower husks harvested in the Sumy region was used. The characteristic of biochar is presented in Table 2. Such an additive was chosen among other possible additives due to its calorific value (higher than wood), low price and wide availability.
The selection of feedstocks took place after the preliminary separation of the solid and liquid phases on the separator. The digestate was previously dried in ambient conditions without exposure to sunlight, using the free convection method (Figure 1a). The obtained dry digestate was crushed (Figure 1b) using the CHOPPER—400 hammer crusher. The particle size distribution of the crushed digestate is presented in Figure 2. No digestate particles were observed on a sieve with a hole diameter of 3 mm and above. Pellet production was performed on a matrix granulator. It is widely used equipment and is most suitable for processing material with a size of up to 2 mm [45].
To study the effect of initial moisture content on the quality of the resulting pellets, five batches of pellets from D1 with different moisture contents were produced. Five batches of pellets were produced to study the impact of the initial moisture content of digestate on pellet durability. The total content of carbon, hydrogen, and nitrogen in pellets was determined according to the requirements of standard ISO 16948:2015 [46]. Analysis of the carbon, hydrogen, and nitrogen in pellets was performed using a Flash2000 analyzer (Thermo Fisher Scientific, Cambridge, UK). Samples were combusted at 950 °C in inert conditions. After the combustion (pyrolysis), gas mixtures CO2, H2O, and NO2 were separated on the gas chromatography column. The calorific value of pellets was determined according to the requirements of the standard ISO 18125:2017 [47]. An automatic bomb calorimeter IKA C6000 (IKA-Werke GmbH & Co. KG, Baden-Württemberg, Germany) was used.

2.2. Methods

The determination of the particle size distribution of the adulterated solid phase of the digestate was carried out in accordance with ISO 2591-1:1998 [48]. Methods using control sieves according to SIST EN 15149-1:2011 [49], on an OCTAGON 200 (OCTAGON, Barcelona, Spain) sieve analysis apparatus. 100 g of dried crushed material was poured onto the upper sieve, and the sieve analysis apparatus was turned on for 5 min. After the time elapsed, the residue from each sieve was weighed on an analytical balance.
Before pelletizing, the material was tested for moisture content in accordance with SIST EN ISO 18134-1 [50] in 3 batches of 5 samples with a mass of 10 g each. The bulk density of the materials tested was determined in accordance with SIST EN ISO 17828:2016 [51]. The digestate fuel pellet durability index (PDI) was determined using a ligno tester LT-1 (ABC Tech, Pisochyn, Ukraine) in accordance with SIST EN ISO 17831-1:2016 [52].
To determine the ash content, the method in [53] was followed. For each sample, 1 g was placed in ceramic containers and put into a stove for 1 h at 250 °C and then for 5 h at 550 °C. The ash content was calculated as follows:
%   a s h = m f m i · 100 % ,
where mi is the weight before the samples were introduced in the oven and mf is the weight after samples were removed from the oven.
The GCV (Gross Calorific value) was examined using KL–12Mn calorimeter according to the norm established by ISO 18125:2017 [47]. The measurement of the GCV was based on complete combustion of the substrate sample placed in a calorimeter.
The GCV was investigated in the three samples: the D1, D2 and Mix DBC (Table 1).
In order to convert the GCV into NCV (where Net Calorific value is GCV reduced by the value needed for water to evaporate), the measurement of the humidity has been conducted by using the laboratory dryer in accordance with the norm of SIST EN ISO 18134-1:2022 [50]. Values of the energetic parameter were determined using (2) and (3),
Q s a = C D t − k − c · m − 1
where:
  • Q s a —combustion calorific of the analysed fuel (GCV) (MJ/kg),
  • C—calorimeter heat capacity (J/°C),
  • Dt—the overall temperature rise of the main period (°C),
  • k—correction for environmental heat exchange (°C),
  • c—correction sum for the additional heat effects (J),
  • m—fuel sample mass (g).
Q w a = Q s a − 24.42 W a − 8.94 H a
where:
  • Q w a —calorific value of the analysed fuel in the analytical state (NCV) (MJ/kg),
  • Q s a —combustion calorific of the analysed fuel (GCV) (MJ/kg),
  • Wa—moisture content of test sample (%),
  • Ha—hydrogen content of test sample (%).
The process of pelletizing was conducted to obtain PDI of more than 97.5%. The study of the process of pelletizing was carried out on a matrix granulator GKP-7.5 (Figure 3). The granulator consists of a fixed flat matrix of 30 mm length and with holes of 6 mm, and two compacting rollers that rotate and move along the matrix to compact the raw material. The speed of rotation of the rollers is about 150 rpm. The pellets leave the working system of the granulator through the discharge pipe. The experimental installation is equipped with a power consumption meter METROL KWS 1083 (Metrol, Zielona Gora, Poland).
Tests for the density of digestate with different initial moisture content were carried out with the following initial parameters:
  • do = 6—diameter of matrix holes (mm);
  • Qm = 50—consumption of mixture (kg/h);
  • nr = 150—rotational frequency of the compaction roller systems (rpm);
  • hr = 0.2—gap between the rollers and the matrix (mm).

3. Results

3.1. Qualitative Parameters of Produced Pellets

All pellets from digestate were produced specifically for research purposes (see Section 2.2). At the beginning, experiments to determine the optimal moisture content of the digestate for pellets production were conducted as the initial moisture content affects the strength of the pellets. Digestate 1 (D1) was used for these studies. The physical characteristics of the resulting pellets are presented in Table 3 and its images are in Figure 4. The diameter of the produced pellets was about 6 mm, and the length varied from 7.5 to 28.5 mm. PDI of resulting pellets varied from 97.67% at initial humidity of the digestate of 7.35% to 76.5% at humidity 10.2%.
Based on the analyses, the digestate pellets were specified according to the pre-standard ISO 17225-6:2021 [44], as shown in Table 4. The average dimensions of the pellets, diameter/length, are 6.0/28.5 mm for D1, 8/29.4 mm for D2 and 8.4/30.2 mm for Mix DBC. Bulk density of three digestates was more than 700 kg/m3.
The ash content was high only in Mix DBC—9.05%. For D1 and D2 it was less than normative—4.4% and 5.2%, respectively. Moisture content in pellets was low for all 3 samples: 6.16% (D1), 6.35% (D2), and 5.85% (Mix DBC). Average dry weight for individual pellets: 93.84% (D1), 93.65% (D2) and 94.15% (Mix DBC).

3.2. Calorific Value of Pellets

The calorific values of the studied digestate pellets are shown in Table 5. The net calorific value of digestate 2 resulted in 17.11 MJ/kg at a water content of 6.35%. Pellets of digestate 1 showed a marginally lower net calorific value with 17.07 MJ/kg at a water content of 6.16%. Fuel pellets produced from mix digestate and biocoal with water content of 5.85% show a higher net calorific value with 18.86 MJ/kg.
Table 6 shows the ultimate analysis of the digestate fuel pellets in comparison to threshold values according to ISO 17225-6:2021 [44]. Remarkable are the high contents of nitrogen, sulfur and chlorine for Mix DBC. Each of these elements is responsible for the formation of noxious emissions during combustion. For Mix DBC, nitrogen content exceeds the threshold value by a factor of 2.2. Overstepping of sulfur was approximately 2 times for Mix DBC, and chlorine content oversteps the threshold value by 3.3 times. Zn and Ni content was 1.6 and 2 times higher than the threshold value for Mix DBC.

4. Discussion

The effect of raw material moisture on pellet density varies across different moisture levels [54]. Higher moisture content might initially enhance binding and reduce inter-particle friction, leading to denser pellets. However, excessive moisture could create voids within the pellet structure during drying, ultimately decreasing density. The initial moisture, which acts as a lubricant and binder in the agglomeration process, is considered the most important factor affecting the quality of pellets in terms of pellet density [55,56,57]. In this study, we investigated parameters of pellets with moisture levels: 10.2%, 9.5%, 8%, 7.45% and 7.35%. With the initial moisture range of 7–7.5%, pellets with the highest strength class for non-wood were obtained (PDI is 97.67% and 95.07%, respectively). For 8% of initial moisture, PDI is also high—88.63%. Its level is comparable to that for wood pellets. This is possible when using digestate containing lignocellulose as the raw material. For other types of digestates, further research is needed. At low initial moisture content, energy costs for the granulation process are excessive and make the process economically unprofitable [58,59]. With increasing moisture content to 7–8%, energy costs for granulation were reduced and sufficient strength of pellets, which meets the standard for non-wood materials, was achieved. Excess moisture can hinder the granulation process, leading to poor granule formation, reduced durability, and difficulty in handling [60]. Moisture can also promote microbial growth, affecting storage stability.
Therefore, the optimal moisture content of digestate range for granulation is 7–8%. These data are confirmed by other authors. Mani et al. [61] claim that pellets made from corn with a lower moisture content (5–8%) have a higher density than pellets with a moisture content of 12–15%. This is confirmed by Shaw and Tabil [62], who claim that reduced moisture content of the compacted material increases the density of the resulting granulate. The complex interplay of factors affecting pellet formation, including the specific material being pelletized, the compaction process used, and the interaction between moisture and other process parameters such as temperature and pressure, need also be considered. Karamchandani et al. [63] supports the importance of durability for pellet quality, and Tulumuru et al. [58] emphasizes that moisture content interacts with other process variables to influence density and durability.
Pellets made from digestate (D1 and D2) did not exceed the standard for non-wood pellets in terms of ash content (Table 4). At the same time, results showed significant impact of feedstock composition and biochar addition on the ash content of digestate pellets. D2, derived from plant residues fermented with cow manure, has a higher ash content of 5.02% vs. 4.4% for D1. This indicates a greater proportion of inorganic components and aligns with the general understanding that animal manure can contribute significantly to ash content in resulting products [64,65]. The increase in ash content can also be caused by the fermentation inoculant used for biogas production, as noted by Waliszewska et al. [66]. The addition of biochar (Mix DBC) demonstrated a significant increase in ash content compared with D1 and D2 (more than twice)—4.4% for D1 and 9.05% for MixDBC. While biochar has several potential benefits as an amendment, its high ash content makes it problematic for pellet production. Adding even 10% of biochar (mix) already has a negative impact on the quality of pellets when targeting low ash standards. Similar results were obtained by Ogwang et al. [67] studying briquettes from digestate based on animal feedstock. At the same time, the authors’ results also demonstrate that digestate compositions (animal manure in that case vs. plant residues in studied D1) significantly affect the pellet properties.
The calorific value of digestate fuel pellets is comparable to those of wood. The studied pellets from digestate indicate it at the level of 17.07 MJ/kg (D1), and 17.11 MJ/kg (D2), which have no significant difference to those from beech 17.74 MJ/kg [68,69] or hardwood 17.6 MJ/kg [70].
Low calorific value of the digestate was also indicated by Jenkins et al. [14]. They also discussed the combustion of biosolids and the importance of analyzing exhaust gases, providing a relevant framework for further studies on digestate pellets.
NCV of MixDBC is higher than D1 and D2 and exceeds standard levels. But due to the levels of other indicators (ash content, Cl, S, N, Ni, Zn), it would not be recommended for use as fuel pellets. These factors outweigh the benefit of the increased calorific value, making MixDBC unsuitable due to potential emissions and operational challenges during combustion [71].
Elemental composition of the digestate fuel pellets (Table 6) shows a significant difference in characteristics of samples D1 and D2 (including only digestate) and MixDBC (with addition of biochar). Moreover, content of Cl, S, N, Ni, Zn in MixDBC exceeds threshold values according to ISO 17225-6:2021 [44] by a few times. It is likely that the separation of the liquid fraction contributes to the reduction of heavy metal concentrations in the solid digestate. A few studies show the same effect [72,73,74,75]—the level of heavy metals in most samples of digestate from agricultural biogas plants does not exceed the limits for non-wood pellets. It is justified for digestate from cow manure and corn silage [72], as well as for chicken manure and sunflower [76].

5. Conclusions

This study investigated the potential of digestate as a sustainable feedstock for fuel pellet production. Analysis of different digestate pellets revealed key findings regarding their physicochemical properties and elemental composition. Digestate for pelletization should have initial moisture content of 7–7.5% to follow the mechanical durability and strength quality and energy consumption for granulation. Digestate pellets exhibited calorific values comparable to those of conventional wood pellets, indicating their potential as a viable alternative fuel source. However, the addition of biochar, while increasing the calorific value, introduced elevated concentrations of Cl, S, N, Ni, and Zn, exceeding the standards thresholds. Such pellets do not meet the standard requirements in most respects, so they cannot be used for heating utility rooms. Pellets made from digestate obtained from agricultural biomass can be recommended for fuel application, especially close to the point of origin. It should be mentioned that further research is needed to provide comprehensive analysis of the exhaust gases produced during the combustion of the proposed digestate pellets. This analysis, along with an economic feasibility assessment, is crucial for a complete evaluation of the potential of digestate pellets as a sustainable fuel source.

Author Contributions

Conceptualization, I.V. and R.V.; methodology, M.S., I.V.; software, S.S.; validation, R.O., L.D. and I.V.; formal analysis, I.V. and R.V.; investigation, R.V., K.K. and L.D.; resources, I.V., M.S. and R.V.; data curation, S.S., M.Y. and R.V.; writing—original draft preparation, I.V., R.V., R.O., K.K. and M.S.; writing—review and editing, R.V., L.D. and I.V.; visualization, K.K., M.S. and M.Y.; supervision, I.V. and L.D.; project administration, R.V.; funding acquisition, R.V. and R.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research was provided within the project “Implementation of the tasks of the perspective plan for the development of the scientific direction “Technical Sciences” of Sumy State University” (state registration number 0121U112684), ordered by the Ministry of Education and Science of Ukraine in cooperation with project funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 871072.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

Authors also appreciate The National Scholarship Program of the Slovak Republic and Presov University for supporting Roman Vaskin in this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Fuels 06 00032 g001
Figure 1. Solid phase of digestate (D1): (a)—during drying at ambient conditions; (b)—in crushed and dried form.
Figure 1. Solid phase of digestate (D1): (a)—during drying at ambient conditions; (b)—in crushed and dried form.
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Fuels 06 00032 g002
Figure 2. Granulometric composition of crushed digestate (D1).
Figure 2. Granulometric composition of crushed digestate (D1).
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Fuels 06 00032 g003
Figure 3. Scheme of the experimental stand.
Figure 3. Scheme of the experimental stand.
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Figure 4. Samples of pellets with different initial moisture content of the digestate 1 (D1): (a)—10.2%; (b)—9.5%; (c)—8%; (d)—7.45%; (e)—7.35%.
Figure 4. Samples of pellets with different initial moisture content of the digestate 1 (D1): (a)—10.2%; (b)—9.5%; (c)—8%; (d)—7.45%; (e)—7.35%.
Fuels 06 00032 g004
Table 1. Feedstock composition of digestates used as test fuels (% of fresh matter).
Table 1. Feedstock composition of digestates used as test fuels (% of fresh matter).
NameFeedstock ComponentsContent (%)
Digestate 1 (D1)Maize silage60
Sugar beet pulp40
Digestate 2 (D2)Cattle manure10
Sugar beet pulp60
Maize silage30
Digestate (D1) + biochar (Mix DBC)Maize silage54
Sugar beet pulp36
Biochar10
Table 2. Biochar Characteristics.
Table 2. Biochar Characteristics.
ParameterMoisture (%)Ash (%)C (%)H (%)N (%)GCV (MJ/kg)NCV (MJ/kg)
Biochar (sunflower husk)13.25.274.03.60.825.823.2
Table 3. Results of fuel pellet (D1) strength tests depending on the initial moisture content.
Table 3. Results of fuel pellet (D1) strength tests depending on the initial moisture content.
No. of the BatchPDI (%)Moisture (%)Consumed Power (kWt)
№197.677.356.764
№295.077.456.232
№388.638.005.586
№478.409.505.320
№576.5010.205.120
Table 4. Characteristics of digestate fuel pellets according to standard (ISO 17225-6:2021 [44]).
Table 4. Characteristics of digestate fuel pellets according to standard (ISO 17225-6:2021 [44]).
ParameterUnitD1D2Mix DBCISO 17225-6:2021
Diametermm6.006.006.006.00
Lengthmm7.50–28.508.00–29.408.40–30.203.15–40.00
Moisture content%6.166.355.85≤12.00
Dry weight%93.8493.6594.15–
Ash content%4.405.209.05≤6.00
Bulk densitykg/m3710.00706.00730.00≥600.00
Mechanical durability%97.6797.7497.45≥97.50
Note: Values in bold exceed threshold.
Table 5. Gross and net calorific value of digestate fuel pellets.
Table 5. Gross and net calorific value of digestate fuel pellets.
Moisture Content (%)Calorific Value (MJ/kg)
GrossNet
D16.1618.6517.07
D26.3518.7117.11
Mix DBC5.8520.4418.86
ISO 17225-6:2021 ≥14.5
Table 6. The physicochemical parameters of the digestate fuel pellets and threshold values according to ISO 17225-6:2021 [44].
Table 6. The physicochemical parameters of the digestate fuel pellets and threshold values according to ISO 17225-6:2021 [44].
Element(%) Dry Matter(mg/kg) Dry Matter
CNOHPSKClCaAsCdCrCuPbHgNiZn
D143.41.635.45.30.11470.13030.2960.07000.305 32 344
D245.21.826.45.10.12300.13300.2850.06500.280 41 252
Mix DBC62.83.418.85.80.52000.44941.3200.33771.810 134 21160
ISO 17225-6:2021 ≤1.5 ≤0.2 ≤0.1 ≤1≤0.5≤50≤20≤10≤0.1≤10≤100
Note: Values in bold exceed threshold.
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MDPI and ACS Style

Vaskina, I.; Vaskin, R.; Ostroha, R.; Yukhymenko, M.; Skydanenko, M.; Kupryaniuk, K.; Demkova, L.; Sydorenko, S. Fuel Pelletization of Digestate: A Pathway to Renewable and Sustainable Energy Sources. Fuels 2025, 6, 32. https://doi.org/10.3390/fuels6020032

AMA Style

Vaskina I, Vaskin R, Ostroha R, Yukhymenko M, Skydanenko M, Kupryaniuk K, Demkova L, Sydorenko S. Fuel Pelletization of Digestate: A Pathway to Renewable and Sustainable Energy Sources. Fuels. 2025; 6(2):32. https://doi.org/10.3390/fuels6020032

Chicago/Turabian Style

Vaskina, Iryna, Roman Vaskin, Ruslan Ostroha, Mykola Yukhymenko, Maksym Skydanenko, Karol Kupryaniuk, Lenka Demkova, and Serhii Sydorenko. 2025. "Fuel Pelletization of Digestate: A Pathway to Renewable and Sustainable Energy Sources" Fuels 6, no. 2: 32. https://doi.org/10.3390/fuels6020032

APA Style

Vaskina, I., Vaskin, R., Ostroha, R., Yukhymenko, M., Skydanenko, M., Kupryaniuk, K., Demkova, L., & Sydorenko, S. (2025). Fuel Pelletization of Digestate: A Pathway to Renewable and Sustainable Energy Sources. Fuels, 6(2), 32. https://doi.org/10.3390/fuels6020032

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