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Article

Sustainable Production of Coffee Husk Pellets: Applying Circular Economy in Waste Management and Renewable Energy Production

by
Angélica de Cassia Oliveira Carneiro
1,
Antonio José Vinha Zanuncio
2,*,
Amélia Guimarães Carvalho
2,
Júlia Almeida Cunha Guimarães Jorge
1,
Raquel Julia Cipriano dos Santos
1,
Iara Fontes Demuner
1,
Letícia Costa Peres
1,
Shoraia Germani Winter
2,
Vinícius Resende de Castro
1,
Monique Branco-Vieira
3 and
Solange de Oliveira Araújo
4
1
Forest Sciences Department, UFV—Federal University of Viçosa, Av. Peter Henry Rolfs, Viçosa 36571-000, Minas Gerais, Brazil
2
Institute of Agricultural Sciences, Federal University of Uberlândia, Goiás Street, 2000, Vila Nova, Monte Carmelo 38500-000, Minas Gerais, Brazil
3
Julius Kühn Institute (JKI)—Federal Research Centre for Cultivated Plants, Institute for Strategies and Technology Assessment, Stahnsdorfer Damm 81, 14532 Kleinmachnow, Germany
4
Forest Research Centre (CEF), Associate Laboratory for Sustainable Land Use and Ecosystem Services (TERRA), School of Agriculture (ISA), University of Lisbon, Tapada da Ajuda, 1349-017 Lisbon, Portugal
*
Author to whom correspondence should be addressed.
Resources 2025, 14(2), 26; https://doi.org/10.3390/resources14020026
Submission received: 23 December 2024 / Revised: 21 January 2025 / Accepted: 24 January 2025 / Published: 31 January 2025
Browse Figure
Versions Notes

Abstract

:
Improper waste disposal is one of the leading causes of environmental pollution, impacting soil, water, and air quality. In coffee plantations, each kilogram of beans produced generates an equal amount of husk, emphasizing the urgent need for sustainable practices to process this residual biomass into valued products. This study aimed to evaluate the potential of coffee husks for pellet production. Three coffee husk types were selected with distinct chemical compositions and granulometries: I (>5.3 mm), II (>2.6 mm and <5.3 mm), and III (<1.77 mm). The biomass was characterized for elemental, structural, and proximate composition. Pellets were produced with two knife heights (15 and 20 mm) and assessed for moisture content, density, length, and mechanical resistance, which were compared with the EN 14691-6 standard (DIN, 2012). Pelletizer productivity was also evaluated. Pellets from biomass III had an ash content of 12.09%, exceeding the <10% requirement. Other treatments met the ash content standard, category B. Pellets from biomass I (17.55%) and II (18.1%) at 15 mm length did not meet the <15% moisture content standard. The remaining pellets met category B standards. Only pellets from origin III (1.62%) met the nitrogen content requirement for international trade (<2%). Pelletizer productivity was higher with smaller granulometry biomass. Coffee husk has demonstrated its potential for pellet production, highlighting the valorization and use of this waste for clean energy generation, contributing to greenhouse gas emissions mitigation, and strengthening circular economy.

1. Introduction

Brazil is the world’s leading coffee producer, with 3.3 million tons and 2.25 million hm2 dedicated to this crop [1]. Consequently, the country also generates significant amounts of coffee waste, especially husks. During processing, for every ton of coffee, one ton of coffee husk is produced [2]. The pollution caused by the improper disposal of this waste affects the quality of soil, air, and water, compromising these resources for current and future generations [2,3].
The utilization of coffee husks in another production chain generates economic gains through income opportunities and environmental benefits through proper waste valorization. This approach enhances a production chain where Brazil represents 33% of the world’s production [4]. In this context, energy generation from coffee husk pellet production shows potential due to the product’s quality and the sector’s demand [5,6], which is capable of consuming large quantities.
Pellets are small, compacted cylinders obtained through agglomeration and pressing processes, increasing their apparent and energy density [7]. This form facilitates storage, transport, and utilization [8], allowing pellets to be used in various segments, such as electricity generation and heating in homes, restaurants, hospitals, industries, and other facilities [9,10].
However, the pellets’ quality must adhere to strict production standards. The raw material must have a high calorific value, fixed carbon content, and lignin, as well as low ash, nitrogen, and sulfur content [11]. Additionally, the produced pellets must exhibit high mechanical strength and energy density [12,13]. Therefore, the final quality of the pellets depends on the quality of the raw material and the process, as these two factors must work together to produce a material of the highest possible quality [14].
To produce high-quality solid biofuel with minimal environmental impact, it is necessary to adapt the process to the quality of the raw material, maximizing economic gains while minimizing environmental impact. Researchers have found that pellets produced from coffee husks exhibit improved combustion and pyrolysis characteristics compared with unprocessed biomass, suggesting that pelletizing this waste material could contribute to more efficient energy production and energy efficiency [5,15]. Furthermore, studies conducted by Tesfaye et al. (2022) [16] showed that coffee husk briquettes are cost-effective and environmentally friendly compared with firewood, highlighting the potential use of this material as an alternative energy source.
Whereas previous research has primarily focused on the general characteristics of coffee husk as biomass [17,18], the objective of this study was to characterize coffee husk for energy generation and to test the quality of pellets created from three distinct granulometric fractions of the biomass and two length sizes. This study expands the existing knowledge on coffee husk pelletization by exploring the influence of different granulometric fractions and pellet sizes on the final product quality, density, mechanical strength, and gas emissions during combustion. Additionally, by comparing three distinct granulometric fractions, this study provides valuable insights into optimizing the pelletization process to maximize energy efficiency and minimize environmental impact, thereby contributing to the existing literature with more specific and applicable data for the industrial production of coffee husk pellets.

2. Materials and Methods

2.1. Biological Material

This study uses coffee husk residues from Coffea arabica plantations in the city of Viçosa, Minas Gerais state, Brazil (20°45′17″ S, 42°52′57″ W). The coffee husk samples were obtained directly from the producer, who employed a different processing method, resulting in varying particle sizes. This approach was taken to align the project’s results with the real-life practices of coffee producers. The coffee husk biomasses were: biomass I with a particle size greater than 5.3 mm, biomass II with a particle size between 2.6 and 5.3 mm, and biomass III with a particle size smaller than 1.8 mm. Each biomass originated from a different genetic material (Figure 1).

2.2. Biomass Characterization

The three residual biomass fractions from the coffee processing industry were characterized for their physical and chemical evaluation. Moisture content, on a dry basis, was determined according to the methodology described in the DIN EN 14774-1 standard [19]. Bulk density was determined according to the DIN EN 15103 standard [20].
The proximate composition was determined according to the ABNT NBR 8112 standard [21], for volatile matter, ash content, and fixed carbon content, on a dry basis. The elemental composition (carbon, nitrogen, hydrogen, and sulfur) was determined according to the DIN EN 15104 standard [22]. The equipment used was the Vario Micro Cube CHNS, Elementar®. The oxygen content was calculated by subtracting the sum of carbon, nitrogen, hydrogen, sulfur, and ash content from 100, as specified in the DIN EN 15296 standard [23].
The higher heating value (HHV) was determined using the methodology described in the NBR 8633 [24] standard and complementary standard NBR 6923 [25], employing an adiabatic calorimeter.
The extractives content was determined in duplicate according to the TAPPI 264 om-88 standard [26], with ethanol/toluene replacing ethanol/benzene. The insoluble lignin content was determined in duplicate using the modified Klason method [27]. Soluble lignin was determined by Goldschimid’s method [28]. The total lignin content was obtained by summing the values of soluble and insoluble lignin. The holocellulose content (cellulose and hemicellulose) was calculated by subtracting the sum of extractives and total lignin from 100.

2.3. Pellet Production and Characterization

The pellets were produced in three particle sizes and two lengths (15 and 20 mm), totaling six treatments. Three batches of 1.5 kg were produced per treatment, resulting in 27 kg of pellets.
The pellets were manufactured on a laboratory pellet mill with a horizontal circular die (Amandus Kahl, model 14-175). The pellet mill feeding system utilized an electric motor, a speed controller, and an auger.
The pelletizer productivity (kg h⁻1) was determined by weighing a mass of pellets collected over five minutes and extrapolating it to one hour. Moisture content on dry basis was measured according to the DIN EN 14774-1 standard [19], and density was determined according to the DIN EN 15103 standard [20].
The hardness of the pellets was determined through a compression test using a manual device called a hardness tester (Amandus Kahl brand). For each treatment, 30 pellets were individually placed in the hardness tester, applying increasing load until the sample broke. Thus, the maximum load that the pellet can withstand before breaking was recorded.
The diameter (mm) and length (mm) of the pellets were obtained following the EN 16127 standard [29].
The durability and fines percentage of the pellets were determined using Ligno-Tester equipment, according to the EN 15210-1 standard [30].

2.4. Statistical Analysis

The evaluated parameters were analyzed for variance homogeneity (Bartlett’s test, α = 0.05) and normality (Shapiro–Wilk test, α = 0.05) before undergoing an analysis of variance (ANOVA). Mean treatment differences were then assessed using Tukey’s test at a 5% significance level.

3. Results

3.1. Biomass Characterization

There were differences in the physical and chemical characteristics among the three fractions of residual coffee biomass particles (Table 1).

3.2. Pellet Characterization

The pellets’ characteristics showed significant variations depending on the knife height used during the process, clearly highlighting how the process influences the final product quality (Table 2).

3.3. Pellet Classification

All treatments met quality specifications regarding diameter, length, bulk density, mechanical durability, and fines for classification A of the marketing standard. However, the moisture content and ash content were not met for all treatments (Table 3).

3.4. Pelletizer Productivity

The pellet production process using the smaller particle size material showed higher productivity (Table 4).

4. Discussion

4.1. Biomass Characterization

The bulk density presented higher values for granulometry III. Using finer particles enhances packing efficiency within a container, minimizing empty spaces and maximizing mass within a given volume. Coffee husks are characterized by low bulk density; achieving higher bulk density values enables the transport of greater amounts of dry matter, thereby facilitating logistics and lowering transportation expenses. On the other hand, utilizing larger particles can lead to increased void volume within the resulting pellets, compromising their structural integrity and generating more fine particles (dust). Consequently, employing raw material with smaller particle sizes offers potential benefits in the production of coffee husk pellets.
The physical and chemical characteristics of the three fractions of residual coffee biomass particles were different. This variation occurs due to genetic material and environmental conditions such as soil, climate, occurrence of pests and diseases, among others [5,32].
The carbon and hydrogen contents varied between 45.41% and 48.8% and between 5.2% and 5.28%, respectively. Biomass I presented a higher carbon content, while the hydrogen content was similar among the three. These elements are directly proportional to the calorific value of biomass [33,34,35]. Both chemical elements are desirable for energy production, with hydrogen being more energetic. Thus, high values for the H/C ratio result in biofuels with high calorific value [36], a parameter for which all materials showed a value of 0.11. Oxygen, found between 34.75% and 36.93%, is undesirable in energy generation as it reduces the material’s calorific value [37].
All materials presented the same sulfur content, an undesirable component in energy generation. During combustion, sulfur oxide (SOx), an environmentally toxic component, is produced from the biomass nitrogen and atmospheric oxygen. Additionally, the low condensation point (≈150 °C) of these oxides forms acids that cause equipment corrosion [38].
The material from granulometry III exhibited the lowest nitrogen content values, resulting in a C/N ratio of 28.03, while the materials from granulometries I and II presented values of 17.55 and 16.99, respectively. Nitrogen is undesirable for energy production from biomass because it releases toxic gases, corrodes equipment, and does not contribute to calorific value, whereas the carbon content is directly related to energy generation potential. Moreover, high values for this parameter imply a rapid release of CO2 into the atmosphere during combustion, enhancing the negative effects of the greenhouse effect [39]. Therefore, a low C/N ratio, found for the material of granulometry III (<1.77 mm), results in high energy release with low emission of toxic gases and equipment wear. Finally, the low O/C ratios and high H/C ratios highlight the potential for using coffee husks for energy, as they contribute to increased calorific value [40].
Biomass I presented a higher fixed carbon content of 19.63%, while biomasses II and III presented similar fixed carbon contents to each other, presenting 16.86% and 16.02%, respectively. This parameter refers to the amount of material that burns in solid form, which is beneficial for energy production as it ensures prolonged and stable combustion, providing a constant energy flow [41]. The fixed carbon content generally varies between 10% and 30% for biomass [42]. For coffee husks, the values found were similar to those reported in the literature [5,43,44].
The volatile matter content varied between 71.29% and 75.26%, values similar to those reported for sugarcane bagasse [45] and wheat straw [46]. Volatile matter influences the combustion kinetics of energy biomass [47]. The high ash content, especially in the material from granulometry III (<1.77 mm), can limit its use, as ashes do not combust and reduce the calorific value of the material. Additionally, ashes cause equipment wear, leading to higher maintenance costs [48,49].
The coffee husk from granulometry III had a higher lignin content, while the materials from granulometries I and II had higher contents of extractives and holocellulose. Lignins have a carbon content exceeding 60% and strong bonds linking their monomers, which ensure a high fixed carbon content and greater energy release during combustion, thus increasing the calorific value of the material. Additionally, as a hydrophobic component, lignin does not adsorb moisture [50]. Lignins are also important in the biomass compaction process because heating lignocellulosic material softens its structure, allowing it to act as a binder, helping particles bond together to form pellets [51]. Holocellulose is a term that encompasses the combined presence of cellulose and hemicellulose, which are key components of wood. These substances are characterized by their high oxygen content and significant capacity for water adsorption. This high-water adsorption capacity can have a detrimental effect on the calorific value of the wood, as well as on the mechanical strength of pellets produced from it. Essentially, the presence of holocellulose can lead to a reduction in the energy efficiency and structural integrity of the pellets. Moreover, the extractives content in the wood plays a crucial role in the pelletization process. In the materials with granulometries I and II, the extractives content exceeds 30%, while in materials with granulometry III, it is over 22%. These extractives can act as natural lubricants during the pressing process, facilitating the formation of pellets. Additionally, they have the effect of lowering the ignition temperature of the biofuel, which can be advantageous in certain applications [52].
Although both holocellulose and extractives influence the properties of wood pellets, they influence them in different ways. Holocellulose tends to negatively impact the calorific value and mechanical strength, whereas extractives can support the pelletization process and reduce the ignition temperature of the resulting biofuel. This interplay of components is critical in determining the overall performance and suitability of wood pellets for various energy applications.

4.2. Pellet Characterization

Pellets produced with shorter knife heights (15 mm) showed higher moisture content. This occurs due to the shorter residence time of the particles in the pelletizing matrix, resulting in a lower exit temperature during production, thus reducing water loss during the process [53]. Additionally, the ends of the pellets absorb more moisture due to the larger surface area; these parts are more significant in shorter pellets, which increased the moisture content for this group [54].
The density of the pellets varied between 650.7 and 721.14 kg/m3. The highest densities were significantly observed in the pellets produced with granulometry III (<1.8 mm). The smaller particle size facilitates the filling of the matrix holes and reduces the free spaces inside, increasing compaction and, consequently, the bulk density of the pellets [38]. High density is desired in pellet production to increase the material’s energy density and reduce logistics costs, making the material more competitive [55]. The results are satisfactory concerning the EN 14961-6 commercial standard (DIN, 2012), being above the requirement of 600 kg/m3.
The adjustment in knife height was effective in regulating the size of the pellets produced during the process. These adjustments allowed all pellets to meet the standards established by the EN 14961 commercial standard [31], remaining within the required length range of 3.15 mm to 40 mm. By ensuring compliance with these specific size requirements, the knife height adjustment not only guarantees the quality of the produced pellets but also meets the market and regulatory standards established for this type of product.
The granule size and knife cutting height did not affect the mechanical durability of the pellets, which averaged 98.63%. During transport and storage, pellets are subjected to various mechanical stresses, so high values for this parameter are important. All treatments showed satisfactory mechanical durability and were above 97.5%, the minimum requirement set by the EN 14961-6 standard [31].
The pellets produced with a shorter knife height (15 mm) generated higher fines content, as the larger surface area of the pellets results in a greater number of friction points. For all treatments, the generation of fines was less than 2%, complying with the EN 14961-6 commercial standard [31]. The fines content and mechanical durability complement each other and determine the stability of the pellets. Mechanical durability refers to the pellets’ ability to maintain their physical integrity under various conditions and represents the deteriorated mass portion relative to its initial mass. Meanwhile, the percentage of fines represents the fraction of particle loss, in mass, compared with the final portion of the pellets [56].
Pellets produced from smaller biomass particle sizes exhibited higher hardness, which is related to their density [57]. Although durability is not standardized as a test for pellets, its assessment provides quick and accurate information on the product’s mechanical strength, proving particularly useful for quality evaluation in pellet production plants. Therefore, its inclusion as a supplementary evaluation metric can play a significant role in ensuring quality and process efficiency in pellet manufacturing [58].

4.3. Pellet Classification

All treatments met the quality specifications of classification A for diameter, length, bulk density, mechanical durability, and fines content, according to the commercial standard. However, moisture content and ash content requirements were not met for all treatments.
Treatments T2 and T4, corresponding to pellets with 15 mm length and granulometries I and III, respectively, do not meet the moisture content standards required by the standard. There is potential for compliance with the required moisture levels through oven drying of pellets in future productions and/or adjustment of the initial moisture content of the particles.
Treatments T5 and T6, corresponding to pellets produced with granulometry III, do not meet the ash content standards required by the standard. The ash content of granulometry III can also be adjusted by removing impurities from the drying process on drying yards that contaminate this fraction.

4.4. Pelletizer Productivity

The introduction of particles with a granulometry smaller than 1.8 mm has proven to be an effective strategy to significantly boost the productivity of the pelletization process, achieving an impressive increase of up to 117%. This phenomenon is primarily attributed to the ability of smaller particles to generate more intense friction among themselves, leading to a rapid rise in temperature and pressure within the pelletization system [59,60]. As a direct consequence of this increase in kinetic energy, there is a notable reduction in the time required for pellet production. This approach not only accelerates production but also has the potential to enhance the overall efficiency of the process, positively impacting both production costs and time.
The scalability of this process is a crucial consideration. The use of smaller particles can be effectively scaled up for larger production facilities, allowing for higher throughput and more efficient operations. This scalability is particularly beneficial for meeting the growing demand for biofuels and renewable energy sources.
From an environmental perspective, the increased efficiency and reduced production time can lead to lower energy consumption during the pelletization process. This reduction in energy use can contribute to a decrease in the overall carbon footprint of pellet production, aligning with broader environmental sustainability goals.
Furthermore, the economic viability of this approach for small-scale producers is significant. By adopting this strategy, small-scale producers can achieve higher productivity without the need for significant capital investment in new equipment. This can make pellet production more accessible and cost effective for smaller operations, fostering the growth of local biofuel industries and supporting rural economies.
Finally, the introduction of smaller particles in the pelletization process not only enhances productivity and efficiency but also offers broader implications for scalability, environmental benefits, and economic viability for producers of various scales.

5. Conclusions

The production of coffee husk pellets from various particle sizes and lengths demonstrated significant feasibility, meeting critical quality parameters outlined in the EN 14961-6 trading standard, including diameter, length, mechanical durability, fines content, higher heating value, and bulk density. However, some issues were observed that require further optimization. Pellets produced from particle size I and II, with a length of 15 mm, exceeded the maximum moisture content of 15%, indicating the need for improved drying techniques in future pelletization processes. Additionally, pellets produced from particle size III did not meet the required ash content threshold of 10%, highlighting the importance of proper management of coffee husk material to avoid contamination during production.
Despite these challenges, this study highlights the potential for coffee husk pellets to be developed for bioenergy applications, with further research focused on refining the production process. By addressing the moisture and ash content issues, these pellets could meet stricter market requirements, thereby enhancing their marketability and improving financial returns.
Moreover, the use of smaller particle sizes was found to boost pelletizer productivity, providing an opportunity for process optimization that can lead to more efficient and cost-effective production. Future studies that address these technical concerns could unlock new opportunities in the renewable energy sector, contributing to the circular economy by valorizing coffee husk waste into a valuable, sustainable biofuel.

Author Contributions

Conceptualization, J.A.C.G.J., A.J.V.Z. and A.d.C.O.C.; methodology, A.J.V.Z., V.R.d.C., R.J.C.d.S., I.F.D. and L.C.P.; formal analysis, V.R.d.C., S.G.W. and A.G.C.; writing—original draft preparation J.A.C.G.J., S.G.W., A.J.V.Z. and A.G.C.; writing—review and editing, A.J.V.Z., S.d.O.A., M.B.-V. and A.d.C.O.C. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful to “Fundação de Amparo à Pesquisa do Estado de Minas Gerais—FAPEMIG”, funding number APQ-04100-23; APQ-05311-24 and APQ-05431-24.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Resources 14 00026 g001
Figure 1. Samples of coffee husk with different particle sizes.
Figure 1. Samples of coffee husk with different particle sizes.
Resources 14 00026 g001
Table 1. Properties of residual coffee biomass in its different particle size fractions.
Table 1. Properties of residual coffee biomass in its different particle size fractions.
PropertiesUnitGranulometry
I (>5.3 mm)II (>2.6 mm e <5.3 mm)III (<1.77 mm)
Bulk Densitykg/m3272.96(3.6) a254.2(4.2) a313.5(4.8) b
C%48.8(2.3) a46.57(2.1) b45.41(1.0) c
N%2.8(4.6) a2.74(5.1 a1.62(5.3) b
H%5.26(4.3) a5.28(4.5 a5.2(4.8) a
O%34.75(3.3) a36.933.5 b35.1(4.5) a
S%0.6(5.2) a0.6(6.7) a0.58(6.3) a
O/C%0.71(4.3) a0.79(4.6) a0.77(5.6) a
H/C%0.11(6.2) a0.11(6.7) a0.11(6.2) a
C/N%17.55(5.4) a16.99(7.1) a28.03(5.6) b
Ashes%7.78(10.1) a7.88(11.5) a12.09(9.9) b
Volatile Matter%72.59(3.5) a75.26(4.2) b71.89(4.3) a
Fixed Carbon%19.63(4.3) a16.86(4.7) b16.02(4.3) b
Extractives%33.34(4.4) a30.34(5.4) b22.81(4.3) c
Total Lignin%25.87(4.3) a25.54(4.6) a31.14(4.1) b
Holocellulose%33.03(4.2) a36.42(5.8) b33.97(5.9) a
Higher Calorific ValueMJ/kg17.92(5.6) a15.89(6.3) b16.61(4.1) c
Mean values followed by the same letter (a, b, and c) do not differ according to the Tukey test at a 5% significance level. Values in parentheses represent the coefficient of variation.
Table 2. Pellets quality produced with different particle sizes and knife heights.
Table 2. Pellets quality produced with different particle sizes and knife heights.
ParameterKnife heightGranulometry
I (<5.3 mm)II (>2.6 and <5.3 mm)III (<1.8 mm)
Moisture15 mm14.91(2.3) bB15.31(2.2) aB15.25(2.6) abB
20 mm22.45(2.3) aA22.46(3.2) abA22.09(3.6) bA
Mean18.6818.8518.67
ParameterKnife heightGranulometry
I (<5.3 mm)II (>2.6 and <5.3 mm)III (<1.8 mm)
Density (kg/m3)15 mm680.3(5.6) bA665.8(4.5) bA721.59(4.6) aA
20 mm670.6(4.8) bA660.7(4.5) bA719.20(4.2) aA
Mean675.4653.2720.4
ParameterKnife heightGranulometry
I (<5.3 mm)II (>2.6 and <5.3 mm)III (<1.8 mm)
Pellet length (mm)15 mm14.91(3.2) bB15.31(3.4) aB15.25(4.3) abB
20 mm22.45(4.2) aA22.46(4.4) abA22.09(4.5) bA
Mean18.6818.8518.67
ParameterKnife heightGranulometry
I (<5.3 mm)II (>2.6 and <5.3 mm)III (<1.8 mm)
Mechanical durability (%)15 mm98.5(5.3) Aa98.7(5.2) Aa98.6(5.8) Aa
20 mm98.6(4.7) Aa98.5(4.9) Aa98.7(5.1) Aa
Mean98.5598.698.65
ParameterKnife heightGranulometry
I (<5.3 mm)II (>2.6 and <5.3 mm)III (<1.8mm)
Fines (%)15 mm0.21(3.2) bB0.19(3.3) abB0.15(3.6) aA
20 mm0.11(5.3) aA0.07(5.3) aA0.12(5.8) aA
Mean0.160.130.14
ParameterKnife heightGranulometry
I (<5.3 mm)II (>2.6 and <5.3 mm)III (<1.8mm)
Hardness (kgf)15 mm13.92(4.1) bA16.62(4.6) aA15.84(4.2) aA
20 mm11.13(4.8) aB9.55(5.3) bB10.94(6.2) aB
Mean12.5413.0913.39
Mean values followed by the same capital letters (A and B) between knife heights and lowercase letters (a and b) between particle sizes do not differ from each other, according to the Tukey test at a 5% significance level. Values in parentheses represent the coefficient of variation.
Table 3. Pellets classification according to the quality standard EN 14961-6 [31].
Table 3. Pellets classification according to the quality standard EN 14961-6 [31].
PropertiesUnitABT1T2T3T4T5T6
Diametermm25 ± 125 ± 1AAAAAA
Lengthmm3.15 < L < 4.03.15 < L < 4.0AAAAAA
Moisture%<12<15BXBXBB
Ashes%<5<10BBBBXX
Mechanical Durability%>97.5>96.0AAAAAA
Fines%<2.0<3.0AAAAAA
Higher calorific valueMJ/kg>14.1>13.2AAAAAA
Bulk Densitykg/m3>600>600AAAAAA
Nitrogen(%)<1.5<2.0XXXXBB
A: Treatment meets the requirements for classification A; B: treatment meets the requirements for classification B; X: treatment does not meet the EN 14961-6 standard [31].
Table 4. Average productivity (kg/h) of the pelletizer in different treatments.
Table 4. Average productivity (kg/h) of the pelletizer in different treatments.
Pellets LengthGranulometry IGranulometry IIGranulometry III
15 mm13.02 Bc11.64 Ab25.26 Aa
20 mm15.25 Ab11.67 Ac23.53 Ba
Mean values followed by the same capital letters (A and B) between knife heights and lowercase letters (a, b and c) between particle sizes do not differ from each other, according to the Tukey test at a 5% significance level.
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Carneiro, A.d.C.O.; Zanuncio, A.J.V.; Carvalho, A.G.; Jorge, J.A.C.G.; dos Santos, R.J.C.; Demuner, I.F.; Peres, L.C.; Winter, S.G.; de Castro, V.R.; Branco-Vieira, M.; et al. Sustainable Production of Coffee Husk Pellets: Applying Circular Economy in Waste Management and Renewable Energy Production. Resources 2025, 14, 26. https://doi.org/10.3390/resources14020026

AMA Style

Carneiro AdCO, Zanuncio AJV, Carvalho AG, Jorge JACG, dos Santos RJC, Demuner IF, Peres LC, Winter SG, de Castro VR, Branco-Vieira M, et al. Sustainable Production of Coffee Husk Pellets: Applying Circular Economy in Waste Management and Renewable Energy Production. Resources. 2025; 14(2):26. https://doi.org/10.3390/resources14020026

Chicago/Turabian Style

Carneiro, Angélica de Cassia Oliveira, Antonio José Vinha Zanuncio, Amélia Guimarães Carvalho, Júlia Almeida Cunha Guimarães Jorge, Raquel Julia Cipriano dos Santos, Iara Fontes Demuner, Letícia Costa Peres, Shoraia Germani Winter, Vinícius Resende de Castro, Monique Branco-Vieira, and et al. 2025. "Sustainable Production of Coffee Husk Pellets: Applying Circular Economy in Waste Management and Renewable Energy Production" Resources 14, no. 2: 26. https://doi.org/10.3390/resources14020026

APA Style

Carneiro, A. d. C. O., Zanuncio, A. J. V., Carvalho, A. G., Jorge, J. A. C. G., dos Santos, R. J. C., Demuner, I. F., Peres, L. C., Winter, S. G., de Castro, V. R., Branco-Vieira, M., & Araújo, S. d. O. (2025). Sustainable Production of Coffee Husk Pellets: Applying Circular Economy in Waste Management and Renewable Energy Production. Resources, 14(2), 26. https://doi.org/10.3390/resources14020026

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