CFD Simulation of Moving-Bed Pyrolizer for Sewage Sludge Considering Gas and Tar Behavior

Round 1

Reviewer 1 Report (Previous Reviewer 1)

Comments and Suggestions for Authors

Dear Authors

The document has improved a lot with respect to the one presented initially, however there are details that should be completed in the attached document you will find my specific recommendations.

greetings

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report (Previous Reviewer 2)

Comments and Suggestions for Authors

The article has been corrected, my comments have been taken into account. The article can be accepted for publication in the journal "Sustainability "

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

Green hydrogen (H2) is considered environmentally friendly, and this study focuses on using a small-scale gasifier to produce green H2 from sewage sludge. The study addresses the issue of tar condensation in the pyrolizer, which can reduce the efficiency of H2 production, and aims to improve tar decomposition by increasing reaction temperatures and analyzing temperature distribution using computer simulations. The research suggests that higher heat carrier temperatures are beneficial for reducing tar issues, but improving heat exchange efficiency is crucial to enhance overall system performance.

In the introduction, the authors justify the importance of the study focused on hydrogen production, show the investigated process in the Advanced  Gasification Module and the problem of tar condensation in it, and emphasize the aspects of the study, methods and objectives. The study is based on numerical modeling, validated with experimental data. The article is written at a sufficient level and has a clear logic of presentation. The terminology corresponds to the field of study.

The revisions made to the article have notably improved its clarity and structure. The authors have done a good job of improving the article  according to previous comments, in particular:

1    Moving the subsection “1.3 CFD simulation” to section “2. Materials and Methods” has enhanced the logical flow.
2    Clarifying the temperature intervals related to Figure 2 provides better context for the peaks of H2 production mentioned in the text.
3    Defining the symbols ε and μ after formulas (4-1) and (4-2) has improved understanding of the parameters used.
 Specifying that the chemical process is multiphase due to the presence of solid (SS), gas, and tar, along with a description of interfacial interactions, adds depth to the analysis.
4    The expanded details about the computational grid, turbulence model, solver settings, and boundary conditions enrich the CFD results section.
5    Finally, specifying a preferable higher HC temperature with an actual value strengthens the conclusions of your findings.

These changes have significantly enhanced the manuscript's quality. Paper is ready for publication.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report (Previous Reviewer 1)

Comments and Suggestions for Authors

Dear

Authors, the new version of manuscript its OK.

Regards

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Editor

The article is significantly lacking in the methodological section, omitting key topics of CFD modeling that are essential for the replication of the study, such as the details of the preprocessing. It is recommended that a more detailed description of the preprocessing steps, boundary conditions, and simulation parameters be included to ensure that other researchers can replicate and validate the study.

In addition, the results section should be improved in terms of discussion and literature support. It is crucial to contextualize the findings with respect to previous studies and provide adequate references to support the claims made. The validation of the numerical model should also be more robust, including more detailed comparisons with experimental data and statistical analyses that reinforce the reliability of the model.

Finally, the graphs presented in the article need to be improved in terms of clarity and presentation. It is suggested to use clearer formats and to add annotations that facilitate the interpretation of the results. These improvements will contribute to a better understanding of the study and increase its impact and validity in the scientific community.

Regards

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The content of the article corresponds to the theme of the journal "Sustainability". At the same time, it is necessary to more clearly formulate the scientific novelty of the data obtained within the framework of the tasks set by the authors, what distinguishes this work from the works of other authors and its contribution to the area of ​​study. Below are comments and additions, the article can be improved and published in the journal.

1. In the introduction, only 12 sources of literature are considered. It would be necessary to add the experience of other researchers in the field of reducing resin formation during pyrolysis, methods for increasing hydrogen yield.

2. Based on what considerations was the size of the zones in the pyrolyzer selected during modeling?

3. It is necessary to compare the results obtained with other authors.

4. In the formula = (6) HCl, what does it mean and should it be in this formula? 5. From the review of the available studies it is known that the maximum amount of hydrogen in biomass pyrolysis can be obtained at a temperature of about 600 degrees, and by increasing the temperature in order to reduce the resins in the apparatus, the amount of hydrogen can decrease in the direction of forming other components. What other methods can be used to achieve optimal conditions for hydrogen production and resin reduction, in addition to increasing the temperature? This can be discussed in the results and general recommendations.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Green hydrogen (H2) is considered environmentally friendly, and this study focuses on using a small-scale gasifier to produce green H2 from sewage sludge. The study addresses the issue of tar condensation in the pyrolizer, which can reduce the efficiency of H2 production, and aims to improve tar decomposition by increasing reaction temperatures and analyzing temperature distribution using computer simulations. The research suggests that higher heat carrier temperatures are beneficial for reducing tar issues, but improving heat exchange efficiency is crucial to enhance overall system performance.

In the introduction, the authors justify the importance of the study focused on hydrogen production, show the investigated process in the Advanced Gasification Module and the problem of tar condensation in it, and emphasize the aspects of the study, methods and objectives. The study is based on numerical modeling, validated with experimental data. The article is written at a sufficient level and has a clear logic of presentation. The terminology corresponds to the field of study.

There are comments to the article that require major revision:

1. To improve the logic of the article, the subsection “1.3 CFD simulation” would be better moved to section “2. Materials and Methods”.
2. On page 4, line 127. Authors mentioned: “There were three peaks of H2 production; the first one was between 200 and 400 °C, the second was between 600 and 800 °C, and the third was above 900 °C.” Is this related to Figure 2? If yes, indicate the temperature intervals in Figure 2.
3. Clarify the meaning of the symbols ε and μ after the formulas (4-1), and (4-2).
4. Is the chemical process described as single-phase or multiphase? If multiphase (it should be multiphase because SS, gas, tar...), the interfacial interaction must be described in numerical simulation.
5. 5.   Since the paper is mainly based on CFD results, it is worth being more specific in the description. Add information about the computational grid, turbulence model (if any), solver settings, boundary conditions and so on.
6. In conclusion, the authors mentioned: “From this result, a higher HC temperature is preferable.” Specify a value of temperature.

The article needs a major revisions.

Author Response

Please see the attachment.

Author Response File: Author Response.docx