Enhancing the Fuel Properties of Spent Coffee Grounds through Hydrothermal Carbonization: Output Prediction and Post-Treatment Approaches

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The aim of this interesting paper is to assess the performance of spent coffee grounds as a potential solid fuel through a set of well-planned experimental works. Results have been well presented and discussed using necessary instrumental analyses. I really enjoyed the paper as it has been well decorated with deep discussion. So, the paper can be suggested for publication after considering some minor improvements as follows:

1. Authors may rethink about the title. It is too long.

2. Please enrich the quality of abstract by adding more numerical results.

3. Literature review is acceptable. However, please avoid chain citation. Addressing many references for a short sentence is not logical! I think you can remove unnecessary ones.

4. There are many nonlinear trends in results. Could you please provide more clarifications?

4. Conclusion section must be edited. You may find a bullet-wise style more useful.

5. Do you have any suggestion for future works? If so, please add in conclusion section.

Good luck,

Author Response

Dear Reviewer 1,

Please see the attachment.

Best regards,

Chau Dang

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This work addresses the potential for reusing spent coffee grounds as fuel through hydrothermal carbonization (HTC) and water washing and agglomeration post-treatment methods. The topic appears to fall within the aim and scope of the journal of Sustainability. However, my main concern is the limited discussion on the novel aspects of this manuscript and the original contributions. Moreover, although the manuscript is overall well-written, I recommend to have it checked before final production. Accordingly, minor revisions are required to make this paper qualified for publication in the journal of Sustainability. The suggested revisions are outlined below:

1.      In my opinion, the title is excessively long and may be improved. Here is a suggestion: “Enhancing the Fuel Properties of Spent Coffee Grounds through Hydrothermal Carbonization: Output Prediction and Post-Treatment Approaches”, or “Enhancing the Fuel Properties of Spent Coffee Grounds through Wet Torrefaction: Output Prediction and Post-Treatment Approaches”

2.      Please, clearly define and highlight the innovative nature of the research carried out. Are there similar works in the scientific literature? There are many articles about process water recirculation back to HTC process, as well as the effects of process parameters on the HTC performance. From my perspective, it's advisable to significantly expand the discussion concerning existing gaps in scientific understanding. This may be achieved by offering pertinent background information and addressing fundamental questions, including: What existing knowledge is available in the literature? What areas require further investigation (i.e., research gaps)? What actions are necessary, and why, along with the approach to address these gaps? Moreover, the distinct and innovative contributions made by this manuscript to address these research deficiencies should be explicitly highlighted and positioned toward the conclusion of this section.

3.      The manuscript lacks sufficient clarity regarding the types of impurities or soluble substances generated during the carbonization process. It is crucial to provide a more detailed and specific description of the nature of these substances for a better understanding of the carbonization process and its potential implications, as well as the need for their removal in a subsequent washing process.

4.      In figure 3, the combined effect of operating temperature and reaction time may be better represented by the severity factor.

5.      In Figure 5, there is a noticeable discrepancy where the severity factor for coal-like hydrochars appears to be lower than that of peat-like hydrochars. The authors should provide a thorough explanation for this observation in the caption or main text to enhance the clarity of the presented data. Additionally, it is recommended to consider including error bars in Figure 5. The inclusion of error bars can provide valuable insights into the variability or uncertainty associated with the severity factor measurements.

6.      It is reported that research design techniques and statistical verification of the obtained results were used in this work. Accordingly, in all tables and charts (for instance, table 3), at least standard deviations should be included. This would allow the reader to assess the range of variability of the obtained results.

7.      In my opinion, the conclusions should be more insightful and briefly discuss the outcomes of this work with respect to the existing literature. I also recommend to focus on the value of this research work for the scientific community and suggest future works.

8.      Finally, authors are encouraged to consider providing their figures in high-quality vector graphic formats like EPS or PDF, if feasible. This transition from raster formats would significantly enhance the overall quality of the figures.

Other suggested minor revisions are listed below:

Hydrothermal carbonization (HTC) is also termed by some sources in the scientific literature as wet torrefaction (WT). Although these two terms are sometimes used interchangeably, it is noteworthy that the ultimate goal of WT is upgrading solid biomass fuels for energy applications such as combustion, gasification and pyrolysis only; whereas HTC mainly aims to produce charcoal that can be used not only as fuel, but also for other applications such as activated carbon, soil enhancer or fertilizer [Source: 10.1016/j.renene.2019.07.142]. Given that the authors report in the title that the product from hydrothermal carbonization of SCG is intended for solid fuel applications, the use of the term wet torrefaction may be more appropriate in this context. If the term hydrothermal carbonization is preferred, the alternative term should at least be mentioned in the manuscript. Clarifying this distinction will enhance the precision of the terminology used in the manuscript.

The manuscript should adhere to the practice of defining abbreviations upon their first usage. This is particularly relevant for the abbreviation "GP," and it is recommended to provide its definition when it is first introduced in the text.

The use of the abbreviation "HC" for "hydrochar" is unnecessary and inconsistent throughout the manuscript. Please maintain a consistent approach regarding the use of abbreviations and consider using the full term "hydrochar" for clarity.

There are some format errors in the article, such as time (t), or temperature (T), solid content (So), etc. These parameters, as physical quantities, should be italicized.

Comments on the Quality of English Language

Abstract, lines 398 and 677: energetic properties -> energy properties.

Lines 83 and 289: a period is missing

Line 223, 397 and 398: energetic characteristics -> energy characteristics

Line 237: The correlations for HTC outputs from DoE model and GP models listed in Table 2 -> The correlations for HTC outputs from DoE model and GP models are listed in Table 2

Lines 344, 364, 465 and 467: roasting process -> torrefaction process

Author Response

Dear Reviewer 2,

Please see the attachment.

Best regards,

Chau Dang

Author Response File: Author Response.pdf