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Article

Coffee Grounds as an Additive to Wood Pellets

by
Piotr Sołowiej
1,*,
Maciej Neugebauer
1 and
Ogulcan Esmer
2
1
Faculty of Technical Sciences, University of Warmia and Mazury in Olsztyn, 10-719 Olsztyn, Poland
2
Department of Agricultural Engineering & Technology, Faculty of Agriculture, EGE University, Bornova 35100, Izmir, Turkey
*
Author to whom correspondence should be addressed.
Energies 2024, 17(18), 4595; https://doi.org/10.3390/en17184595
Submission received: 10 July 2024 / Revised: 21 August 2024 / Accepted: 31 August 2024 / Published: 13 September 2024
(This article belongs to the Special Issue Biomass and Municipal Solid Waste Thermal Conversion Technologies II)
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Abstract

:
The immense popularity of coffee around the world generates significant amounts of coffee grounds. They are often improperly disposed of, which can have a negative impact on the environment. Due to their chemical composition and physical properties, coffee grounds are an excellent bioenergy material. This paper presents a study of the feasibility of using spent coffee grounds (CG) as an additive to pine sawdust (PS) pellets to improve their energy properties. The tests were carried out on samples of pellets consisting of 100% PS, 100% CG, and mixtures of 95% PS and 5% CG, 85% PS and 15% CG, and 70% PS and 30% CG. Physical and chemical analyses were carried out to determine the suitability of the obtained pellet as a biofuel in accordance with ISO 17225. Combustion tests were also carried out in a laboratory boiler to analyze flue gases and determine CO and NOx emissions in accordance with EN-303-5 for biomass boilers. The amount of emitted volatile organic compounds (VOCs) was also determined. Experimental results show that the addition of CG to PS reduces the durability of the pellets and increases CO and NOx emissions but increases their energy value and reduces the amount of VOC emissions. The requirements of both standards were fulfilled with a mixture of 95% PS and 5% CG. However, test results show that it is possible to add CG to PS in amounts up to 15%, although this will require additional research.

1. Introduction

Human activity, mainly through greenhouse gas emissions, has undeniably caused global warming. Between 2011 and 2020, the Earth’s surface temperature reached 1.1 °C above the 1850–1900 level. The dangers of global warming and the need to take appropriate action are described in a report by The Intergovernmental Panel on Climate Change [1]. One way to minimize the impact of human activity on the increase in global warming is to replace fossil fuels with biofuels. The most popular solid biofuel is wood in the form of wood chips, pellets, or briquettes. There is also a search for biological waste materials that can provide an alternative to wood. One such waste material may be coffee grounds (CG). Coffee is one of the most popular beverages in the world. In the 2021/2022 season, global coffee consumption was 175.6 million bags (10.536 million tons). The largest coffee consumer is Europe, which accounted for 32% of the global coffee consumption of the 2021/2022 season, i.e., 3.372 million tons [2]. In comparison, according to the “BIOENERGY EUROPE” report [3], consumption of wood pellets in 2022 in the European Union was 24.174 million tons. This shows the huge potential of CG as a biofuel. Utilizing bioenergetic waste such as coffee grounds is a good example of integrating systems for disposing of hazardous waste and producing high-energy solid fuels. This has significant implications for sustainable development of the bioeconomy in the energy sector. According to researchers, CG contain large amounts of organic compounds such as proteins, carbohydrates, fibers, polyphenols, lignin, and polysaccharides [4,5,6]. Phenolic compounds, caffeine, tannins, and antioxidants are some of the compounds that can be extracted from CG for composting or fertilizer materials and potential additives in the health and food industries [7,8,9,10]. After coffee brewing, CG still contain many valuable components that make them usable in many ways. The direct use of CG is in the production of biofuels, for example, biogas [11,12,13,14] in gasification [15,16,17], bioethanol [18,19], and biodiesel [7,20,21], and in composting [22,23,24,25]. CG are also considered as an agricultural fertilizer, or as a material for improving soil structure and properties [26,27,28,29]. However, the high carbon content (above 50%) and high calorific value (above 20 MJ/kg) [29,30] inspire researchers to look for opportunities to use CG as a fuel or to improve the energy properties of traditional biological fuels. Allesina et al. and Kang et al. presented the results of combustion studies of 100% CG. Unfortunately, excessive emissions of CO, NOx, and particulate matter disqualify neat CG as an environmentally friendly fuel [31,32]. Due to the physical properties of dried CG, a need exists to compact them to improve storage, transportation, and dispensing into the boiler. There are published studies on the possibility of using CG in briquette form in combination with fuels such as anthracite [33], crude glycerin [34], starch [35], sugar cane [36], beech chips [37], and many others.
Due to the simplicity of dispensing into boilers, packaging, and transportation, pelletization is the most common way of compacting biomass. Lisowski et al. [38] investigated the possibility of pelletizing CG and determined the properties of the pellets. There are many works describing the possibility of using CG as a fuel in combination with other biological materials such as pepper stalks [39] and rice husks [40]. Lachman et al. presented the results of a study of pellets made from spruce chips, CG, and their mixtures. They gave a definition of the mechanical durability of pellets as a measure of the resistance of compressed fuels to shock and abrasion as a result of handling and transportation, recognizing this parameter as one of the most important in describing the mechanical properties of pellets. As a result of their research, they found that the addition of CG to spruce chips significantly reduces the mechanical durability of pellets [41]. In addition, excessive NOx emissions due to fuel-bound nitrogen were observed. Atabani et al. [42] stated that combustion of 100% CG pellets is not possible due to high NOx emissions. In addition, when combustion is accompanied by a decrease in temperature due to incomplete combustion, it leads to a decrease in boiler efficiency and increased CO emissions. However, it indicates the possibility of using CG to valorize wood pellets with some limitations. Limousy et al. [43] presented the results of a study of combustion in a pellet boiler of 50% CG and 50% pine chips. He concluded that it is a good alternative to wood pellets.
Most of the authors dealing with the use of CG as a fuel for combustion in boilers used in their studies mixtures in which the CG content was from 30% upwards [31,33,34,40,41]. From a review of the literature, it can be concluded that the use of CG as fuels is significantly limited by excessive NOx and CO emissions [41,43]. Consequently, pure CG and biofuel blends with a significant CG content cannot be considered as a green fuel.
The purpose of the research presented in this article is to investigate the possibility of using CG as an additive (30% and less) to wood pellets to improve their energy value while respecting the limitations imposed by biofuel standards and certification parameters defined in European standards.

2. Materials and Methods

The testing methodology consisted of preparing the material for testing, determining the moisture content of the prepared samples, studying the particle size distribution of the prepared material, carrying out the granulation process of the prepared samples, and determining the mechanical durability, calorific value, and gas emissions during combustion in the boiler. The results obtained were compared with the requirements of standards for biofuel properties and biofuel combustion.

2.1. Preparation of Research Material

In the study, pine sawdust (PS) and coffee grounds (CG) were granulated. The pine sawdust was obtained from a wood furniture manufacturing plant. Coffee grounds were obtained from the authors’ households. The pine sawdust and coffee grounds were dried naturally to a moisture content of about 12%. Proximate and elementals analyses of the substrates related to the dry base are shown in Table 1.
For pelletization, pine sawdust and coffee grounds were prepared, as well as mixtures of sawdust with coffee grounds in the weight proportions shown in Table 2. The appearance of the made pellets is shown in Figure 1.

2.2. Moisture Content of Granulated Substrates

Moisture content was determined in accordance with the requirements of PN-G-04511:1980 [44] using an MAC 50 weighing machine with a moisture reading accuracy of 0.001% and a maximum drying temperature of 160 °C. Materials were sampled directly before granulation. The method of determining moisture content consisted of taking five measurements of the moisture content of the prepared substrates. Two extreme measurements were discarded, and an average was calculated from the remaining three, which is shown in Table 3.

2.3. Particle Size Distribution of Substrates

One of the most important parameters with a significant impact on the granulation process is the particle size distribution. The particle size distribution was determined on an EML 200 Premium Remote vibrating screen produced by MERAZET, Poznań, Poland using five sieves with diameters of 4 mm, 3 mm, 2 mm, 1 mm, and 0.25 mm. The tests were carried out according to the methodology recommended by the screen manufacturer.

2.4. Pelletization Process

The materials were thoroughly mixed before pelletization. Five samples of five kilograms each were prepared in the proportions shown in Table 2. The pelletization process was carried out in a wood pellet machine, the ZLSP 150B pelletizer produced by Anyang Gemco Energy Machinery Co., Ltd., Anyang, China, with a 5.5 kW motor. The pelletization process was carried out under laboratory conditions at an ambient temperature of 20 °C using the pelletizer settings recommended by the manufacturer. A die with a hole diameter of 6 mm was used. The pellets produced were stored under conditions simulating typical storage (20 °C and 50% relative humidity) for 48 h and then the moisture content of the pellets tested was determined.

2.5. Mechanical Durability

The high mechanical durability of the produced granules is one of the most important mechanical properties of this fuel. It has a significant impact on the durability of the pellets during the transportation process, dust emissions during handling, or on the correct functioning of the fuel feeding systems for boilers. Durability testing of the resulting pellets was carried out using the Holmen NHP100 Pellet Durability Tester produced by TEKPRO, North Walsham, UK in accordance ISO 17831-1:2015 [45].

2.6. Determination of Calorific Value

The calorific value was determined using a KL-12 Mn calorimeter produced by MERAZET, Poland in accordance with the technical conditions and standards of PN-ISO 1928:2002 [46]. Five measurements of the heat of combustion were made for each batch of material tested. Of each five measurements, the two extreme ones were rejected, and the average of the remaining three was considered the result. In some cases, more than five measurements had to be taken as the value of some measurements deviated too much from the others.

2.7. Gas Emissions

The analysis of gas emissions was determined using a laboratory biomass boiler with a nominal output of 5 kW. This is a boiler with a bottom-fired grate. Gas emissions were measured on the heated boiler at λ = 1.8. The measurement conditions for all samples were the same. The technical settings of the boiler were in accordance with the manufacturer’s recommendations for burning woody biomass. Flue gas analysis was carried out using a Horiba PG-250 Portable Gas Analyzer produced by HORIBA LTD, Kyoto, Japan under established conditions at the boiler’s nominal power. The flue gas was monitored for the density of such gases as CO, NOx, CO2, and O2. The HORIBA PG-250 uses the same measuring principles as the permanently installed CEMS. These include NDIR (pneumatic) for CO and SO2, NDIR (pyrosensor) for CO2, chemiluminescence (cross-flow modulation) for NOx, and a galvanic cell for O2 measurements. This analyzer meets the requirements established by agencies such as the EPA in the United States for portable or back-up continuous emission monitoring systems. Volatile organic compounds (VOCs) were also analyzed. The number of measurements was the same, and the determination of specific values was performed in the same way as for humidity measurements. In the case of results significantly deviating from the others, the tests were repeated.

3. Results and Discussion

The main objective of the study was to investigate the effect of CG as an additive to improve the energy qualities of pine sawdust pellets. The results of the study were considered with reference to International standard ISO 17225 “Solid biofuels” [47] and ISO EN 303-5+5A1:2023-05 [48] “Heating boilers—Part 5: Solid fuel heating boilers with manual and automatic fuel feeding with nominal power up to 500 kW—Terminology, requirements, testing and labeling”.

3.1. Particle Size Distribution of Substrates

The study showed that the particle size distribution of CG is quite different from the particle size distribution of PS—see Table 4. Due to the sieves available in the laboratory, we only separated two fractions of CG particles. We found no particles over 1 mm. In the 0.25–1 mm range, we found almost 70% of the particles, which does not contradict the results of other researchers.
The pelletizing process parameters specified by the pellet machine manufacturer indicate that the particle size should be 2 mm smaller than the desired pellet diameter. In our case, the granulation process produced pellets with a diameter of 6 mm. Most of the material particles used in our study (Table 4) were smaller than 4 or even 3 mm. However, this had no effect on the pelletization process and its quality. Bergström et al. [49] showed that the particle size distribution has an insignificant effect on the mechanical properties of the resulting granules. It is significant that they conducted their study on the same material. Therefore, the effect of adding CG to PS pellets on the properties of the resulting pellets may be due to the different chemical composition and other physical properties of the two components.
The particle size of the sawdust was similar to studies by other authors. Woo et al. [45] reported in their study that 58% of the pine sawdust particles were smaller than 2 mm. In our case, 61.7% of pine sawdust particles were smaller than 2 mm. The type of cutting tool used determines the size of the sawdust particles. In the case of CG, the particle size of the resulting grounds varies depending on the type of beverage brewed. In overflow coffee machines, coffee with larger particles is used. Espresso machines use finely ground coffee to bring out the aroma and flavor. Most of the literature reports particle sizes of less than 1 mm used in CG studies. Limousy et al. [43] used CG with a particle size of about 0.6 mm to produce pellets. Jezerska [50], in pelletizing and torrefaction of coffee grounds, garden chaff, and rapeseed straw, reported that most CG particles were in the 0.5–0.9 mm range. Woo et al. [51] reported that about 95% of the particles were in the 0.25–0.5 mm range.

3.2. Moisture Content

The moisture content of mixtures prepared for pelleting was determined immediately before the pelleting process. The moisture content of the granules was determined after cooling them to ambient temperature (20 °C).
A comparison of the moisture content of the prepared material as well as the resulting pellets is shown in Figure 2. The high temperature and pressure occurring during the granulation process had a significant effect on reducing the moisture content of the pellets, reducing it by an average of 4% for all samples. The largest difference in moisture content was observed for pine chips without CG, at 4.94%, and the lowest for pure CG, at 3%. This is probably due to the initial moisture content of the granulated substrates. In the case of PS and CG mixtures, the moisture contents of the granules were very similar. All types of granules tested had moisture contents below 10%, which meets the requirements of ISO 18134-1:2023-02 [52].
The moisture content of the pelletized material is one of the most important parameters affecting the pelletization process and the properties of the resulting pellets. Moisture can act as a binding medium, affecting mechanical durability and reducing bulk density, and as a grease lowering frictional forces extruded in the matrix of the pelletizing machine, which has an impact on reducing the energy intensity of the process [53]. In the case of wood pellets, the moisture content of the material in the range of 8–12% gives the possibility of producing a high-quality product [54]. Lisowski et al. [38] reported that, to obtain high-strength pellets, CG should have a moisture content of less than 20%. Limousy et al. [43], investigating the properties of pellets made from a mixture of CG and PS in a 50/50 weight ratio, used CG with a moisture content of 12% and PS with a moisture content of 10%.

3.3. Mechanical Durability

A study of the mechanical durability of the pellets produced has shown that the CG content has a significant effect on this very important property of the pellets (Figure 3). The ISO 17225-6:2021 standard [47] recommends a mechanical durability of at least 96%. Sample 2 with 15% CG content met this condition. Sample 3 was very close to the standard (95%). This may be related to the chemical composition and physical properties of CG. The mechanical durability of a pellet is defined also as the percentage loss of total fuel mass when the pellets wear against each other and their ability to remain intact [55]. It depends on factors such as friction, impact, compression, the chemical composition of the materials from which they are made, and their moisture content, among other things. Physical factors occurring during packaging, transport, and storage can contribute to the formation of dust suspensions which may pose an explosion hazard [56]. In addition, the powder produced by abrasion and reduction of pellet particles can adversely affect the operation of boiler fuel metering equipment.
The raw material from which the pellets are made also has a significant impact on the mechanical durability of the pellets. For example, a high oil content in the raw material weakens the intermolecular bonding forces, which significantly reduces the mechanical durability of the fuel [57]. The decrease in the mechanical durability of the tested pellets (Figure 3) may be due to the high oil content of CG [53], different particle size, and moisture content of CG and PS before granulation [41]. Some studies show that higher moisture content may have a negative effect on the mechanical durability of the pellets [58]. Kalian et al. and Tumuluru et al. [53,55,59] showed that the use of steam and the selection of appropriate process parameters, such as pressure, have a significant effect on pellet durability.
However, based on the results, it can be assumed that up to 15% CG can be added to PS for the resulting fuel to meet the ISO standard for pellet durability. To determine the exact limit of the addition of CG to PS, it is necessary to perform pellet durability tests for mixtures in the range of 5–15% CG content or to find better process parameters.

3.4. Calorific Value

Tests were carried out on the calorific value of pellet samples starting with PS and mixtures of PS and CG in fixed proportions - see Figure 4. It was shown that the calorific value increases as the CG content in the pellets increases. The addition of 5% CG to PS increased the calorific value of the pellets by more than 1.1 MJ/kg (5.6%), the addition of 15% CG increased it by 1.6 MJ/kg (8%), and the addition of 30% CG increased it by 1.8 MJ/kg (9%). The graph (Figure 4) shows that the largest increase in calorific value was observed for the 5% CG addition. For the subsequent samples, the increase in caloric value was not so dynamic, which is also confirmed by the results presented by other researchers [39,50,51].
CG have a high calorific value, making them suitable for use as a high-calorific fuel. In our study, CG reached a value of 23.957 MJ/kg. Similar calorific values of CG were reported by Lachman et al. [41]—22.83 MJ/kg, Lisowski et al. [38]—23.90 ± 0.10 MJ/kg, and Wo et al. [51]—23.8 MJ/kg. However, despite their high calorific value and low ash content, CG contain significant amounts of nitrogen. High nitrogen content is very often the cause of high NOx emissions, a factor that severely limits the use of CG as a fuel. A solution to this problem can be found in the co-firing of CG with other biofuels that have a low nitrogen content. Lachman et al. [41] presented the results of combustion tests performed on CG and spruce sawdust. They showed that the addition of CG to spruce sawdust increases the calorific value of the fuel. Nosek et al. [60], testing fuel samples consisting of CG and wood sawdust, showed an increase in calorific value from 17.15 MJ/kg for wood sawdust to 21.08 MJ/kg for wood sawdust with 30% CG addition. A similar situation occurs when CG is used as an additive to other biofuels. Park et al. [39] investigated the effect of adding CG to pepper stalks. They found that adding 10% CG to pepper stalks increased the calorific value of the fuel from 16.67 to 19.21 MJ/kg (15.2% increase). Further increases in the CG content of the samples did not result in such a significant increase in the calorific value of fuel. For 20% CG additive, the calorific value increased to 19.43 MJ/kg (16.5% increase), and, for 30% CG additive, the calorific value was 19.85 MJ/kg (19% increase).
As you can see, despite the three-times-higher proportion of CG in the fuel, the increase in calorific value was not as dynamic as it was at the beginning. Even after torrefaction, pellets made from rapeseed straw with the addition of CG show similar trends. The results of such a study were presented by Jezerska et al. [50]. They reported an increase in the calorific value of rapeseed straw pellets subjected to torrefaction from 16.34 MJ/kg to 19.13 MJ/kg after the addition of 25% CG (a 17% increase) and to 18.42 MJ/kg after the addition of 50% CG (a 12.7% increase).

3.5. Gas Emissions

The flue gas analysis (Figure 5) shows that, with the increase in CG content in the sample pellets, there was a dynamic increase in NOx content. For a sample of 30% CG, it was even higher than for 100% CG [366 mg/m3 NOx]. This could be due to the synergy between substances contained in CG and PS causing increased NOx emissions during combustion. CO content also increased, although not as dynamically. This is indicated by the high CO emissions when burning 100% CG—2093 mg/m3. The results obtained were related to the PN-EN-303-5+A1:2023-05 standard, where CO emissions are recommended to be ≤500 mg/m3 and NOx emissions ≤ 200 mg/m3.
In the case of CO emissions, all mixtures investigated showed emissivity below 500 mg/m3. In contrast, only the pellets with 5% CG addition fulfilled the NOx emission standard. The other mixtures significantly exceeded the 200 mg/m3 NOx value in the flue gas. The CG addition to PS showed a different trend for volatile organic compounds. The highest VOC content was obtained for pure PS pellets—315 mg/m3. The increase in CG content in the tested granules contributed to a decrease in VOC emissions to a level of 241 mg/m3 for a blend with 30% CG. This is a very positive trend due to the high toxicity of VOCs. After elementary analysis, Kang et al. [32] reported a nitrogen content of 1.45% by weight in dried CG. Bejenari [61] et al. reported 3.5 to 4.56 wt.% nitrogen in CG for different coffee grades. Compared to this, Nosek et al. [60] found that CG contained 17.78% nitrogen. They also investigated gas emissions when CG and pine sawdust samples were combusted at CG/PS ratios of 100/0, 30/70, 40/60, and 50/50. When 100% CG was burned, they obtained results indicating a low NO content. They explained this by the low combustion temperature, which resulted in incomplete combustion. In the other samples, due to the very low nitrogen content of the pine sawdust (0.03%), most of the nitrogen in the flue gas came from the CG combustion and was in the range of 180–200 mg/m3.
In the case of CO emissions, Nosek et al. [60] obtained inconclusive results because the 40/60 CG/PS sample showed the highest emissions of about 2200 mg/m3. The 50/50 sample showed CO emissions of about 1800 mg/m3, while the 30/70 sample showed just over 500 mg/m3. The latter result is comparable to the one obtained in our study (Figure 5).
When introducing new fuels, attention should be paid to the combustion parameters on which the composition of the emitted gases may depend. Roy et al. [62] compared the combustion results of wood pellets and grass pellets. They showed that NOx emissions are influenced by the nitrogen content of the fuel and the combustion temperature, while excess oxygen has no significant effect on this parameter. It turns out that the composition of the emitted gases depends not only on the combustion parameters, but also on the geometry of the main boiler components. Interesting research results were presented by Sungur and Basar [63]. They investigated the effect of the position of the air supply to the burner pots at different excess air ratios and heat outputs on the combustion characteristics and emissions in forced draft pellet stoves. They showed that increasing the excess air ratio increased CO emissions. In contrast, the distance of the supply air flow had no significant effect on NOx emissions, in contrast to the heat output. As the heat output increased, NOx emissions increased. On the other hand, if the fuel cannot mix homogeneously with the air, agglomeration occurs, which has a significant effect, resulting in higher CO emissions and lower efficiency. The above study shows that, in addition to the combustion and power parameters, the geometry of the basic boiler components should also be considered as it can have a significant effect on the composition of the emitted gases.
Due to increasingly stringent atmospheric emission standards, special attention should be paid to the composition of newly developed biofuels. CG are characterized by high nitrogen content. This leads to the formation of harmful and undesirable NOx compounds in the exhaust gas.

4. Conclusions

In the present study, the effect of adding CG to pine sawdust pellets was investigated. The results of the tests showed that, considering the requirements of ISO standards [47], only a 5% CG blend meets the standards. The addition of CG significantly reduces the mechanical durability of the pellets. Even at 15% CG, the mechanical durability dropped to 95% against the 96% required by the standard. It is possible that, if other granulation parameters were used, such as a higher granulation pressure [55] and a higher conditioning temperature [47], this mixture would achieve the required mechanical durability.
As for the results of emission tests, the CO content in the flue gas for all samples did not exceed the value recommended in the emission standard for boilers [48]. NOx emissions, on the other hand, were already exceeded for 15% CG content. However, if the calorific value of biofuels produced of CG with other types of biomasses is considered, care must be taken to ensure that the NOx and CO emission limits imposed by the relevant emission standards are not exceeded.
The results of the calorific value tests of the tested pellet samples show that the addition of CG to PS in the amount of 5% significantly increased the calorific value of the fuel. This confirms the thesis that the use of CG as an additive in the amount of 5–10% to other types of biomasses gives the result of increasing the calorific value of the fuel. This is also in accordance with the results of studies of other authors. In addition, there is no significant effect on the emission of NOx and CO, or on the mechanical strength of the produced pellets. An additional advantage of the CG as an additive is the reduction in VOCs in the flue gas. The resulting limit of 15% CG for both durability and NOx emission tests creates the need to test blends in the range of 5–15% CG. This would allow the determination of the optimal CG content that would raise the energy value of pine sawdust pellets with ISO requirements.
An important aspect of reducing emissions of harmful gases is the search for modern and innovative solutions in the design of basic boiler components such as burners and air injection. In combination with appropriately selected combustion parameters, they can influence the possibility of using fuels with higher nitrogen content without increasing NOx emissions.
The study presented here shows that CG, which are a harmful waste, can be treated as a valuable addition to wood pellets to improve their energy properties. The use of such mixtures can also contribute to more economical use of CG and reduce the consumption of firewood.

Author Contributions

Conceptualization, P.S.; methodology, P.S.; formal analysis, M.N.; investigation, P.S., M.N. and O.E.; resources, O.E.; writing—original draft preparation, P.S.; writing—review and editing, M.N.; visualization, O.E.; supervision, P.S. and M.N. All authors have read and agreed to the published version of the manuscript.

Funding

This publication was written as a result of the author’s internship in EGE University in Izmir, co-financed by the European Union under the European Social Fund (Operational Program Knowledge Education Development), carried as part of the Development Program at the University of Warmia and Mazury in Olsztyn (POWR.03.05. 00-00-Z310/17).

Data Availability Statement

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

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 17 04595 g001
Figure 1. View of samples of made pellets with different CG content.
Figure 1. View of samples of made pellets with different CG content.
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Figure 2. Comparison of the moisture content of samples before and after the pelletization process.
Figure 2. Comparison of the moisture content of samples before and after the pelletization process.
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Figure 3. Pellet durability depending on CG content in PS pellet.
Figure 3. Pellet durability depending on CG content in PS pellet.
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Figure 4. Pellet calorific value depending on CG content.
Figure 4. Pellet calorific value depending on CG content.
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Figure 5. Emissions of CO, NOx, and VOC depending on CG content.
Figure 5. Emissions of CO, NOx, and VOC depending on CG content.
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Table 1. Proximate and elementals analysis of the substrates.
Table 1. Proximate and elementals analysis of the substrates.
FertilizerProximate Analysis (wt.%db)Elementals Analysis (wt.%db)
AshVMFCCOHNS
PS0.3287.9311.7548.9145.355.590.120.03
CG1.9785.5212.5158.1130.578.232.890.20
Table 2. Weight proportions of samples prepared for pelletization.
Table 2. Weight proportions of samples prepared for pelletization.
Sample 1Sample 2Sample 3Sample 4Sample 5
PS100%95%85%70%0%
CG0%5%15%30%100%
Table 3. Moisture content of material prepared for pelletization.
Table 3. Moisture content of material prepared for pelletization.
Proportion by Weight [%]Moisture
Content [%]
PSCG
100012.94
95512.83
851512.68
703012.46
010011.02
Table 4. Particle size distribution of substrates.
Table 4. Particle size distribution of substrates.
Particle size (mm)≥43–<42–<31–<20.25–<1<0.25
Weight proportion (%)CG000068.5631.44
PS3.65.212.541.1827.4310.09
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Sołowiej, P.; Neugebauer, M.; Esmer, O. Coffee Grounds as an Additive to Wood Pellets. Energies 2024, 17, 4595. https://doi.org/10.3390/en17184595

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Sołowiej P, Neugebauer M, Esmer O. Coffee Grounds as an Additive to Wood Pellets. Energies. 2024; 17(18):4595. https://doi.org/10.3390/en17184595

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Sołowiej, Piotr, Maciej Neugebauer, and Ogulcan Esmer. 2024. "Coffee Grounds as an Additive to Wood Pellets" Energies 17, no. 18: 4595. https://doi.org/10.3390/en17184595

APA Style

Sołowiej, P., Neugebauer, M., & Esmer, O. (2024). Coffee Grounds as an Additive to Wood Pellets. Energies, 17(18), 4595. https://doi.org/10.3390/en17184595

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