Combustion of Pelletized Coffee Residues for Bioenergy Valorization Within a Circular Economy Vision

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article concerns a very current topics related to the greatest possible waste management.

Unfortunately, I noticed some inaccuracies to which I would ask for the responded:

1) Table 1 in its present form is misleading. The reader browsing the table has the impression that there are no results for P7 and P9 samples. Only in the text there is information that the results are the same. Were the results actually the same? I would suggest changing the table layout and merge the cells so that this insertion can be seen.

2) The dependence shown in Figure 2 are illegible. The charts are very small. I would suggest each of the charts to put a drawing as the Dyży. The correlation coefficients later are included in Table 2. In the relationship, which is presented in Figure 2 does not seem to be needed and is unnecessary repetition.

3) Table 3 should be a legend, which means symbols A, B, X

4) whether the information contained from 255 to 274 lines is not more a study that matches the introduction and later only referring to this data during the results. Could it be combined, in the foundation of literature data, a biomase that could be used to dilute the nitrogen content in a pellet?

5) In conclusions, it would be worth giving examples of materials for use to dilute nitrogen in a pellet and not the name of the whole group.

Author Response

Unfortunately, I noticed some inaccuracies to which I would ask for the responded:

Table 1 in its present form is misleading. The reader browsing the table has the impression that there are no results for P7 and P9 samples. Only in the text there is information that the results are the same. Were the results actually the same? I would suggest changing the table layout and merge the cells so that this insertion can be seen.

Thanks for the comment, the table has been updated repeating the same values also for P7 and P9, as also suggested by second reviewer.

2) The dependence shown in Figure 2 are illegible. The charts are very small. I would suggest each of the charts to put a drawing as the Dyży. The correlation coefficients later are included in Table 2. In the relationship, which is presented in Figure 2 does not seem to be needed and is unnecessary repetition.

Thank you for the comment, we replaced the figure with another graph indicating the correlation matrix. We hope it will be clearer.

3) Table 3 should be a legend, which means symbols A, B, X

Thank you, adjusted. "A, B: pellet quality classes corresponding to the reference quality standards; X: pellet not corresponding to the reference quality standards". 

4) whether the information contained from 255 to 274 lines is not more a study that matches the introduction and later only referring to this data during the results. Could it be combined, in the foundation of literature data, a biomass that could be used to dilute the nitrogen content in a pellet?

Thank you for the comment several part of introduction and discussion were adjusted accordingly. Eg:

Introduction

“Other studies indicate that the use of blends with coffee grounds is suitable for energy valorization due to the high calorific value [54,55], optimizing both boiler efficiency and overall reduction of CO emissions [50]. In addition, Solowiej and Neugebauer (2016) [56] used blends with coffee grounds and rice straw, pine wood and tea grounds”.

Discussion

“Other authors studying the use of coffee grounds as fuel have utilized blends with coffee content varying from 30% upwards [85,88]. In this context, the contribution of Kougioumtzis et al. (2024) [38] enabled the evaluation of various mixtures of coffee residues with other residual biomasses such as forest residues, wood processing industry residues, urban pruning, corn residues, peach pruning, etc.. The authors highlighted that nitrogen content for 50% blending was between 1.83 to 2.3%. Our study is in line with these results, showed a value of nitrogen of 1,96% for 50% blending. In another study, Park et al. (2020) [88] produced pellets with 50% coffee grounds and 50% agricultural residues, examining various properties of the pellets and noting a nitrogen content ranging from 1.1% to 1.4%.

Nosek et al. (2020) [39] examined the influence of coffee grounds content on the elemental composition of pellets produced from pine sawdust. The results reported significant changes in the carbon, hydrogen, and nitrogen content, obtained by varying the pro-portions of coffee grounds in the pellets. Specifically, as the coffee grounds content in-creased from 30% to 70%, there was an increase in both carbon and hydrogen content of the pellets, while the nitrogen content showed a decreasing trend with higher coffee grounds content. In a study conducted by Allesina et al. (2017) [89], the use of coffee grounds in pellet production was examined, focusing on mixtures with spruce and pine sawdust. Pellets composed of a 50% coffee grounds and 50% spruce mixture showed a nitrogen content of 2.22% and an ash content of 2.2%.

5) In conclusions, it would be worth giving examples of materials for use to dilute nitrogen in a pellet and not the name of the whole group.

We added the following sentence:

“However, as in the literature presented in this paper, other biomasses such as forest residues, industrial residues, urban pruning, maize residues, peach pruning, etc. can be used successfully in similar blending percentages to reach acceptable N level."

Thank you for your suggestions.

Reviewer 2 Report

Comments and Suggestions for Authors

Review-Energies-3317666

In this research the authors explored the potential of utilizing pelletized coffee grounds, coffee waste, and wood-coffee mixtures as sustainable bioenergy sources within the circular economy. The investigations the combustion characteristics and energy output of these materials demonstrated their viability as alternative sources to fossil fuels. They indicated that coffee residues exhibited favorable combustion properties with respect to heating value, but also high concentrations of nitrogen compared to pellets obtained from other agricultural residues. This work presents interesting insights; however, needs some adjustments:

Abstract

1) The Abstract is well written; however, the authors do not make it very clear what the main differences or contributions/innovations of this study, considering other previously published studies;

2) A good Abstract should contain a brief contextualization of the subject to be addressed; general objective; methodology in a direct and succinct way; most relevant results of the research, and a catchphrase for closing/conclusion;

3) For making the Abstract more interesting, authors are encouraged to present the most relevant results of this study in the form of numerical values and/or percentages;

4) It is suggested that authors use different keywords of the Title to broaden the search for the paper, e.g., change/removal “coffee residues”, “circular economy”, “bioenergy” and “combustion” by other words related to the theme.

1. Introduction

1) lines 56-64, 72-74, and 85-88, add some references;

2) line 44, the authors reported "Numerous studies..." but only presented a single reference, add two or more references;

3) Within the Introduction, it is encouraged that the authors make a brief literature review of some works on use coffee residues for thermochemical processes (e.g., 10.1007/s40430-022-03993-y and 10.5380/reterm.v14i2.62135), to expand the contextualization of this study and form a database for comparative purposes;

4) The authors are encouraged to present some statistics data on the coffee production in Italy or even in Union European, as well as to compare this information with other countries that uses similar feedstocks;

5) The authors need to clarify for future readers of this paper what are the main differences or contributions/innovations of this study, when compared to other previously published studies.

2. Methods and Methods

1) lines 104-106, authors are invited to add the geographic location in terms of coordinates (latitude and longitude). If possible, also add the dates of coffee waste and eucalyptus sawdust collection and the weather conditions and/or seasons;

2) line 105-106, authors are encouraged to present the sample preparation steps; it is believed that perhaps a flowchart with this part would make the article much more interesting;

3) lines 107-111, authors should provide some more details about these pellets, as well as the measured weight for each of them. Why were these specific moisture contents chosen? What is the direct influence of this variable on the manufactured pellets?

4) line 144, the caption in Table 1 is very simplified, improve them;

5) line 117-118, in Figure 1 for better understanding, the authors could express the letters: a) and b), as well as in the caption. Also add the images of the other samples mentioned previously;

6) lines 122-127, authors could add a direct description of the standards being mentioned;

7) lines 135-141, for a better understanding by future readers of this article, the authors could briefly contextualize the Fisher’s, Welch-F, Games-Howell, Kruskal-Wallis, and Dwass-Steel-Critchlow-Fligner tests, and some references.

3. Results and 4. Discussion

1) lines 153-154, the caption of the Table 1 is very simplified, improve them, and standardize the number of decimal places;

2) in Table 1, why did the authors not present values for some variables of samples P7 and P9? It is recommended that in the revised version of the manuscript, for comparative purposes, these values be completed, even if they are repetitions for the P11 samples. Could the authors better explain the origin of the elements chlorine and sulfur in the samples studied?

3) line 157, it is necessary for the authors to present plausible justifications to explain the causes of these variabilities depending on the pellets type investigated;

4) line 218, in Table 3 authors must indicate in the footer what the letters A (green), B (yellow), and X (red) mean;

5) lines 224-226, how could researchers qualitatively assess that environmental aspects should prevail over purely mechanical considerations?

6) According to the main observations made by the authors, what are the main advantages of practical applications the results of this study?

7) Can the authors bring to readers the main limitations of this study? How can be remedy them? And what are the next steps in this research?

8) For better sustaining the results achieved, authors need to compare them with other studies already published. This part presents this limitation.

5. Conclusions

1) They are good, simple, and direct, presenting the main results of paper. Authors can also propose suggestions and/or innovations so that the most relevant results of this research can be applied to real thermochemical processes and/or more useful applications in people’s daily lives.

References

1) Observe the correct form (citations and references) in the journal template;

2) references [44], [48], and [55] are incomplete, without year/date and article title, respectively;

3) scientific names of the different biomasses must be written in italics and follow the rules of botany;

4) ≈ 38.24% of the references used in this paper have more than 5 years old (very high percentage), update them!

Author Response

In this research the authors explored the potential of utilizing pelletized coffee grounds, coffee waste, and wood-coffee mixtures as sustainable bioenergy sources within the circular economy. The investigations the combustion characteristics and energy output of these materials demonstrated their viability as alternative sources to fossil fuels. They indicated that coffee residues exhibited favorable combustion properties with respect to heating value, but also high concentrations of nitrogen compared to pellets obtained from other agricultural residues. This work presents interesting insights; however, needs some adjustments:

 

Abstract

 

1)The Abstract is well written; however, the authors do not make it very clear what the main differences or contributions/innovations of this study, considering other previously published studies;

The abstract has been modified including at the end the following sentence:

In general, this study confirmed the findings of previous scientific reports, highlighting that at least 50% blending with low-nitrogen biomasses is necessary to reach the marketability of the product. In addition, the study highlighted the criticality in terms of durability that these mixtures confer to the final product, emphasizing that future research should focus on optimizing the combination of these factors to improve the properties of the pellet.

2) A good Abstract should contain a brief contextualization of the subject to be addressed; general objective; methodology in a direct and succinct way; most relevant results of the research, and a catchphrase for closing/conclusion;

We updated the abstract as follow:

For Brief context.

Coffee is one of the most widely consumed beverages in the world; the European Union alone consumes about 2.5 million tons of coffee per year. Yearly, millions of tons of coffee waste are generated, becoming an attractive material for circular economy flows.

For general objective

This study explores the potential of utilizing pelletized coffee residues as sustainable bioenergy sources within the framework of a circular economy.

Methodology in a direct and succinct way

The coffee residues, obtained from damaged capsules and waste of cafeteria were utilized in pure form or blended with sawdust at different percentages, then analyzed with respect to physical and thermochemical parameters.

Most relevant results of the research

The results indicate that unblended coffee residues exhibit favorable combustion properties with respect to heating value (18,84 MJ*Kg-1), but also high concentrations of N (4.14%) compared to conventional pellets obtained from other agricultural residues. The blending with woody material negatively affects both durability and bulk density, but simultaneously promoted a reduction of ash content (3,09%) and N content (1,94%).

A catchphrase for closing/conclusion

In general, this study confirmed the findings of previous scientific reports, highlighting that at least 50% blending with low-nitrogen biomasses is necessary to reach the marketability of the product. In addition, the study highlighted the criticality in terms of durability that these mixtures confer to the final product, emphasizing that future research should focus on optimizing the combination of these factors to improve the properties of the pellet.

 

3) For making the Abstract more interesting, authors are encouraged to present the most relevant results of this study in the form of numerical values and/or percentages;

Thank you, changes applied.

 

4) It is suggested that authors use different keywords of the Title to broaden the search for the paper, e.g., change/removal “coffee residues”, “circular economy”, “bioenergy” and “combustion” by other words related to the theme.

Thank you, changes applied

 

1. Introduction

 

1) lines 56-64, 72-74, and 85-88, add some references;

Thank you for the comment, as you requested, we have added more citations

2) line 44, the authors reported "Numerous studies..." but only presented a single reference, add two or more references;

Thank you for the comment, as you requested, we have added more citations

3) Within the Introduction, it is encouraged that the authors make a brief literature review of some works on use coffee residues for thermochemical processes (e.g., 10.1007/s40430-022-03993-y and 10.5380/reterm.v14i2.62135), to expand the contextualization of this study and form a database for comparative purposes;

We followed your suggestion by introducing further bibliography on the subject. Thank you.

 

4) The authors are encouraged to present some statistics data on the coffee production in Italy or even in Union European, as well as to compare this information with other countries that uses similar feedstocks;

Thank you for the comment, regarding production, the EU is just importer. Some statistic has been added regarding the European imports of coffee.   

 

5) The authors need to clarify for future readers of this paper what are the main differences or contributions/innovations of this study, when compared to other previously published studies.

The final part of the intro has been modified as follows:

“Respect the available literature, deeper attention has been paid to the balancing between physical and chemical parameters, which is key to identify the ideal compromise to reach marketability standards. In addition, fresh wasted coffee powder was used for first time in its pure form and in blended solution with Ecalyptus spp.. The products obtained has been classified according European standards and has been compared with similar studies in order to enrich the scientific knowledge on coffee waste valorization perspectives.”

 

2. Methods and Methods

1) lines 104-106, authors are invited to add the geographic location in terms of coordinates (latitude and longitude). If possible, also add the dates of coffee waste and eucalyptus sawdust collection and the weather conditions and/or seasons;

Thank you for the observation, we have modified the text by adding the information you suggested. We can’t provide indications on the weather conditions because the raw material was stored in our laboratories in jute bags, as added in the text.

 

2) line 105-106, authors are encouraged to present the sample preparation steps; it is believed that perhaps a flowchart with this part would make the article much more interesting

Thank you a flowchart has been created.

3) lines 107-111, authors should provide some more details about these pellets, as well as the measured weight for each of them. Why were these specific moisture contents chosen? What is the direct influence of this variable on the manufactured pellets?

Thanks for the comment, we added the mean weight of the pellet in the results. Regarding the moisture content, 7% was the original moisture of the powder, we increased until 11 because 10/11% it is the optimal moisture content for pellet production. We added the following text:

The initial moisture content of the material was 7% (P7), and the first test was conducted on this pure feedstock. Subsequently, the moisture content was increased to 11% by uni-formly adding water, with samples left to stabilize for 24 hours. This percentage was cho-sen based on literature review, which indicates that a moisture content of 10-11% in bio-mass facilitates the densification during pelletizing, the efficiency combustion, the reduc-tion of emissions, and enhances the durability during storage. Additionally, this moisture level minimizes energy consumption during production by balancing the need for lubri-cation and binding properties of the material [57].

 

4) line 144, the caption in Table 1 is very simplified, improve them;

As you requested, we have provided further details in the caption.

 

5) line 117-118, in Figure 1 for better understanding, the authors could express the letters: a) and b), as well as in the caption. Also add the images of the other samples mentioned previously;

We have added additional pictures and inserted the letters as request.

 

6) lines 122-127, authors could add a direct description of the standards being mentioned;

Thank you for the comment, a direct description of the standards has been included in the text.

 

7) lines 135-141, for a better understanding by future readers of this article, the authors could briefly contextualize the Fisher’s, Welch-F, Games-Howell, Kruskal-Wallis, and Dwass-Steel-Critchlow-Fligner tests, and some references.

The paragraph has been revised as requested.

 

3. Results and 4. Discussion

 

1) lines 153-154, the caption of the Table 1 is very simplified, improve them, and standardize the number of decimal places;

We have added further information to the table description and standardized the number of decimals.

 

2) in Table 1, why did the authors not present values for some variables of samples P7 and P9? It is recommended that in the revised version of the manuscript, for comparative purposes, these values be completed, even if they are repetitions for the P11 samples. Could the authors better explain the origin of the elements chlorine and sulfur in the samples studied?

Thank you for the comment, all table was filled. Regarding the second part of the question, in general  terms, element such as Cl, S and N are analyzed because they are required from the standards for commercialization. In fact, the combustion of material containing Cl , S and N is risky because Chlorine can form hydrochloric acid (HCl) and dioxins, sulfur can form sulfur oxides (SOx), such as sulfur dioxide (SO2), while nitrogen can for nitrogen oxides (Nox) and CO. These compounds are harmful to human health, causing respiratory issues and other illnesses, but also risky for the environment. In addition, in case of coffee (a berry seed) it is important to evaluate such parameters because it is non-conventional biomass and because the process of manufacturing can vary from country to country with possible risks of contamination. This part has been added to the discussion section.

 

3) line 157, it is necessary for the authors to present plausible justifications to explain the causes of these variabilities depending on the pellets type investigated;

Thank you for the comment, the first part of the discussed was updated with the following:

The physical and chemical parameters of the pellets analyzed are indeed influenced by factors such as machinery type, biomass type, and initial moisture content. For in-stance, a study on the mechanical properties of Khaya senegalensis biomass energy pellets highlights how pelletizing temperature and pressure significantly affect pellet durability and compressive strength [73]. Another research article discusses the impact of physical and mechanical properties of biomass wood pellets on energy release and carbon emissions, emphasizing the role of moisture content and pellet dimensions [74].

4) line 218, in Table 3 authors must indicate in the footer what the letters A (green), B (yellow), and X (red) mean;

Thank you for the comment, a legend has been included

5) lines 224-226, how could researchers qualitatively assess that environmental aspects should prevail over purely mechanical considerations?

Thank you, upon you comment we actually preferred to remove the sentence as it is not properly correct.

6) According to the main observations made by the authors, what are the main advantages of practical applications the results of this study?

Thank you for the comment. The paper emphasize that crucial aspect to play with in using coffee residues is the balance between N content (chemical aspect) and durability (physical aspect). Indeed, it provides practical suggestion to focus future studies to this balance. Additionally, pure fresh coffee powder and blends with eucalyptus sawdust were never investigated providing further insights in the research. This was added in text.

 

7) Can the authors bring to readers the main limitations of this study? How can be remedy them? And what are the next steps in this research?

Thank you for the comment, we added the following part in discussion section.

The primary limitation of this study lies in the restricted number of blends and feed-stock types utilized for blending. This constraint was primarily due to the extensive number of analyses required and the associated costs. While the selected blends provided valuable insights, a broader range of feedstock could potentially yield more comprehensive results. Future research should aim to include a wider variety of blends to enhance the generalizability of the findings and provide a more robust understanding of the effects of different feedstock combinations.

 

8) For better sustaining the results achieved, authors need to compare them with other studies already published. This part presents this limitation.

Additional comparative references were added and discussed as also suggested by other reviewers.

A part of the discussion was rewritten as follow:

“Other authors studying the use of coffee grounds as fuel have utilized blends with coffee content varying from 30% upwards [85,88]. In this context, the contribution of Kougioumtzis et al. (2024) [38] enabled the evaluation of various mixtures of coffee resi-dues with other residual biomasses such as forest residues, wood processing industry residues, urban pruning, corn residues, peach pruning, etc.. The authors highlighted that nitrogen content for 50% blending was between 1.83 to 2.3%. Our study is in line with these results, showed a value of nitrogen of 1,96% for 50% blending. In another study, Park et al. (2020) [88] produced pellets with 50% coffee grounds and 50% agricultural residues, examining various properties of the pellets and noting a nitrogen content ranging from 1.1% to 1.4%.

Nosek et al. (2020) [39] examined the influence of coffee grounds content on the ele-mental composition of pellets produced from pine sawdust. The results reported signifi-cant changes in the carbon, hydrogen, and nitrogen content, obtained by varying the pro-portions of coffee grounds in the pellets. Specifically, as the coffee grounds content in-creased from 30% to 70%, there was an increase in both carbon and hydrogen content of the pellets, while the nitrogen content showed a decreasing trend with higher coffee grounds content. In a study conducted by Allesina et al. (2017) [89], the use of coffee grounds in pellet production was examined, focusing on mixtures with spruce and pine sawdust. Pellets composed of a 50% coffee grounds and 50% spruce mixture showed a ni-trogen content of 2.22% and an ash content of 2.2%.”

5. Conclusions

1) They are good, simple, and direct, presenting the main results of paper. Authors can also propose suggestions and/or innovations so that the most relevant results of this research can be applied to real thermochemical processes and/or more useful applications in people’s daily lives.

Thank you for the comment, the second part of the conclusion has been modified as follows:

Acceptable level of N (1.96%) can be reached by blending 50% the coffee with sawdust, but durability in such a condition is compromised, determining the unmarketability. On the other hand, durability is a parameter that in a practical way can be easily adjusted by us-ing some natural binding additives such as starch. In addition, as cited in the discussion section, other biomasses such as forest residues, industrial residues, urban pruning, maize residues, peach pruning, etc. can be used successfully in similar blending percent-ages to reach acceptable quantity of N.

 

References

 

1) Observe the correct form (citations and references) in the journal template;

Thank you, we double checked the reference chapter and made corrections as required by the journal template.

 

2) references [44], [48], and [55] are incomplete, without year/date and article title, respectively;

Thank you, we added the missing data

 

3) scientific names of the different biomasses must be written in italics and follow the rules of botany;

Thank you, in this regard we followed your suggestion

4) ≈ 38.24% of the references used in this paper have more than 5 years old (very high percentage), update them!

Thank you, we included more citations published over the last 5 years

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors,

After a rigorous reading of the new manuscript submitted to the journal by authors, and noting that these answered all the questions and made the changes proposed by me, corresponding to the myr expectations, I am in favor of accepting and publishing this article in this important journal. Congratulations for the excellent work!