From Biogas to Hydrogen: A Techno-Economic Study on the Production of Turquoise Hydrogen and Solid Carbons

Round 1

Reviewer 1 Report

Dear authors, please find attached my comments on the manuscript. I hope they can help to improve the quality of your work.

Comments for author File: Comments.pdf

Author Response

Thank you for taking the time to read our manuscript and suggest revisions which helped us improve the quality of our paper. We greatly appreciate it.

• The references have been significantly extended to take into account extra literature
• The values reported in Table 1 have been referenced.
• Technology for biogas upgrading has been mentioned.
• Reference to methanol has been removed – our error – thank you for picking it up
• Reference to ‘h’ has been made rather than ‘hr’
• More discussion on Figure 5 has been made.
• Assumed reaction conditions and equations have been made – please refer to the revised manuscript.

General comments and Corrections:

• Improvements in the language and grammar have been done.
• Consistency in the various scenario labelling has also been effected.
• An update on all figures needed to be done based on a change in assumptions in Table 1. This was done based on new literature and hence an increase in the process efficiencies.
• Figures, tables and reported values were updated accordingly.
• The introduction section was expanded to include a paragraph on current existing technologies for making hydrogen from methane.
• It has also been expanded to identify why we are looking at biogas, and not just methane from natural gas.
• The conclusions were aligned with the discussions in the document.
• A paragraph was added to include future work plans.

Importantly, the following suggested correction has been effected.

• The plateau identified by the reviewer was initially thought to just be an Excel flaw. However, the cause has been identified: At scales above 300 m³/h (720kW), the cost estimate for the upgrading plant changes. This is based on empirical data from operational plants internationally as well as in South Africa. While costs are spread, two sections of estimates were used.
• All other items raised were addressed as per the general comments above.

Reviewer 2 Report

Page- part- line	Reviewer comment
Abstract	It can be added with numerical findings as well. Straight to the point
Introduction	Short- Could be longer and more thorough
Results and methodology	Results and discussion paragraphs are well presented and elaborated, but they seem as if they do not align with what is presented in the conclusions
Conclusions	Lacks future perspectives of the project
Figures	Need to be more aligned and separated from the writings

The topic is interesting, the work structure and the scientific content of the paper is good, the academic level of the paper must be improved, the conclusions are not justified properly.  Some of the comments are given as above.

 

Author Response

We greatly appreciate your time to read and review our submission. We have gone an extra mile to not only address your suggested changes but also to improve the manuscript a great deal. 

General comments and Corrections:

• Improvements in the language and grammar have been done.
• Consistency in the various scenario labelling has also been effected.
• An update on all figures needed to be done based on a change in assumptions in Table 1. This was done based on new literature and hence an increase in the process efficiencies.
• Figures, tables and reported values were updated accordingly.
• The introduction section was expanded to include a paragraph on current existing technologies for making hydrogen from methane.
• It has also been expanded to identify why we are looking at biogas, and not just methane from natural gas.
• The conclusions were aligned with the discussions in the document.
• The paragraph was added to include future work plans.

Reviewer 3 Report

The manuscript deals on the techno-economic assessments of the production of turquoise hydrogen and carbons from biogas. It is original and has some interesting findings under different assumed scenarios, but this research needs to be further explained and improved. This manuscript may be recommendable for the publication after majorly modification based on following comments.

1) In Table 2, the authors assumed different biogas digester scenarios, such as scenario 1, 2 and 4. Is there scenario 3 because of the discontinuous numbering?

2) The rationality of economic parameters is very important for this study. Whether the selected parameters in Table 3 are representative and widely applicable or not? In particular, I noticed that many parameters were based on an investigation about producing dimethylether from biogas, and it seemed irrelevant to your study.

3) It is recommended to add a mass-flow diagram and a cash-flow diagram, which contain all the progresses from biogas to products and by-products including anaerobic digester, biogas cleaning, biogas upgrading and methane cracking.

4) The cost and efficiency of different biogas cleaning and upgrading technologies varied significantly, for example, water scrubbing, amine scrubbing and membrane separation. In the absence of a specific technology, the economics of this aspect did not make much sense.

5) In the assumption of this paper, the process efficiencies of CH₄ removal and CO2 removal from biogas were 90%. Did the authors consider the unremoved or unutilized portion (10%)? After all, this portion of CH₄ and CO₂, if released directly into the environment, would make the whole technology far less economical and environmental-friendly.

 

Author Response

We appreciate your time to review our manuscript and recommend changes. As highlighted in the revised version (uploaded), we have addressed these changes. In addition, we would like to highlight the following.

• The scenario numbering was corrected.
• The use of the article focussing on DME as a base refers. That article also looked at the use of biogas, and had enough information to help with building the TE model. In our case, we looked at different product streams, however, our model can also be expanded to go to downstream chemicals.
• The upgrading technology has been mentioned now in the table now. This is also based on data from upgrading facilities in operation above (see comments for reviewer 1).
• The released CO2 based on the inefficiencies has been calculated and mentioned in the paper.

We would also like to draw your attention to the general comments and corrections:

• Improvements on the language and grammar has been done.
• Consistency in the various scenario labelling has also been effected.
• An update on all figures needed to be done based on a change in assumptions in Table 1. This was done based on new literature and hence an increase in the process efficiencies.
• Figures, tables and reported values were updated accordingly.
• The introduction section was expanded to include a paragraph on current existing technologies for making hydrogen from methane.
• It has also been expanded to identify why we are looking at biogas, and not just methane from natural gas.
• The conclusions were aligned with the discussions in the document.
• A paragraph was added to include future work plans.

Reviewer 4 Report

The manuscript deals with experimental activities and an overview of From biogas to hydrogen: a techno-economic study on the production of turquoise hydrogen and solid carbons. The topic is well aligned with the scope of SUST journal. The authors should improve the readability and scientific soundness, the manuscript cannot be accepted in this present form, please carefully revise it to improve the quality, the reviewer has been highlighted several open points below:
1. INTRO: This section should be extended, please consider focus also on the exciting technologies/reserch that can be used as a bridge between the short and mid-period, for example, the role of another ions in the he anaerobic baffle reactor. Updated and complete literature review should be conducted to present (last 5 years) the state-of-the-art and knowledge gaps of the research with strong relevance to the topic of the paper. In the Introduction section should be contained the information on biogas and biomethane that its utilisation needs pre-cleaned. Consider recent published papers: https://doi.org/10.1016/j.jclepro.2021.127404  and https://doi.org/10.37256/sce.3120221015 
2. The abstract should be about 150-250 words. Answer the questions: What problem did you study and why is it important? What methods did you use? What were your main results? And what conclusions can you draw from your results? Please make your abstract with more specific and quantitative results while it suits broader audiences. 
3. Highlight the novelty aspect in coatrast to existing work. Add one chapter in the end of Intro section.
4. M&M: Alanytical methods are presented without details. Extend the section and add the proper data. 
5. Results:
The results section should be improved reporting more detailed analysis, the reviewer suggestions are to perform detailed statistical analysis of presented results. The ststitsic evaluation is essential.

 

Author Response

Thank you for the review comments. We have taken note and embedded all the suggested corrections in the attached revised manuscript. Additionally, we would like to hight the following;

1. We have extended the introduction sections to capture the state-of-the-art and knowledge gaps of the research with strong relevance to the topic of the paper.
2. We have refined the abstract.
3. The result section has been refined.

Also, please also see the general corrections below.

• Improvements on the language and grammar has been done.
• Consistency in the various scenario labelling has also been effected.
• An update on all figures needed to be done based on a change in assumptions in Table 1. This was done based on new literature and hence an increase in the process efficiencies.
• Figures, tables and reported values were updated accordingly.
• The introduction section was expanded to include a paragraph on current existing technologies for making hydrogen from methane.
• It has also been expanded to identify why we are looking at biogas, and not just methane from natural gas.
• The conclusions were aligned with the discussions in the document.
• Paragraph was added to include future work plans.

 

Round 2

Reviewer 2 Report

Dear editor/Authors,

              Greetings! The authors have improved a lot in the revised version of the manuscript. However I would like to suggest some of the general comments to be addressed before its publication.

* Original Experimental Set up image must be added 

* Pictures quality can be improved

* References should be taken care and checked again.

* Overall drafting of the article must be improved.

  

Author Response

Reviewer 2 – Second round review

* Original Experimental Set up image must be added 

This was a desktop study with a model developed in Excel. There was no experimental study and hence no image to be included.

* Pictures quality can be improved

Pictures have been imported from Excel and saved in the best format available.

* References should be taken care and checked again.

References was updated even according to other comments from other reviewers where extra content needed to be included (kindly see the attached revised manuscript)

* Overall drafting of the article must be improved.

Further improvements were made to the layout, grammar and English use (kindly see the attached revised manuscript).

......................

Below was also what had been addressed for Reviewer 2 – First round review

• Introduction: Short- Could be longer and more thorough

The introduction was expanded to include other relevant literature. Based on other reviewers comments, this was further expanded to include other components and suggestions as well.

Results and methodology

• Results and discussion: paragraphs are well presented and elaborated, but they seem as if they do not align with what is presented in the conclusions

Paragraphs were re-written to align with the conclusions and better merging of the discussions

• Conclusions: Lacks future perspectives of the project

Together with the rewriting of the results and discussions to align with the conclusions, we have included a future works perspective too

• Figures: Need to be more aligned and separated from the writings

Figures have been moved to be separated from the text.

Reviewer 4 Report

Dear Authors,

I do not see the significant improvements on this work. Revision was made as a cosmetic changes. 

Authors did not revised manuscript accordingly my comments, therefore I cannot recommend it for publication. Sorry that my decision cannot be more favorable.

Author Response

Reviewer 4 – Second round review

Dear Authors,

I do not see the significant improvements on this work. Revision was made as a cosmetic changes. 

Authors did not revised manuscript accordingly my comments, therefore I cannot recommend it for publication. Sorry that my decision cannot be more favorable.

We had made significant changes after the first round of reviews because of changes to the input parameters. As such, some of the discussions needed to be changed as well as some figures. We might have missed some of the suggestions made by the reviewer initially, but all of his suggestions have now been incorporated. Kindly see attached manuscript as confirmation that we have acted on all suggestions. Below are more details on what was done based on Reviewer 4 – First round review

............................

Details on improvements based on Reviewer 4 – First round review

1. INTRO: This section should be extended, please consider focus also on the exciting technologies/reserch that can be used as a bridge between the short and mid-period, for example, the role of another ions in the he anaerobic baffle reactor. Updated and complete literature review should be conducted to present (last 5 years) the state-of-the-art and knowledge gaps of the research with strong relevance to the topic of the paper. In the Introduction section should be contained the information on biogas and biomethane that its utilisation needs pre-cleaned. Consider recent published papers: https://doi.org/10.1016/j.jclepro.2021.127404 and https://doi.org/10.37256/sce.3120221015 

The introduction was expanded to include the role of biogas in the circular economy, the needs for biogas cleaning as well as a paragraph on the biogas upgrading technologies. The reviewer has asked about the role of other ions in the AD reactor, and we feel that this is not relevant for the discussions, and have for those reasons excluded it from the introduction as well.

2. The abstract should be about 150-250 words. Answer the questions: What problem did you study and why is it important? What methods did you use? What were your main results? And what conclusions can you draw from your results? Please make your abstract with more specific and quantitative results while it suits broader audiences.

The abstract has been re-written to be more concise as well as including the results from the study as suggested by the reviewer.

3. Highlight the novelty aspect in coatrast to existing work. Add one chapter in the end of Intro section.

This has been alluded to in the last chapter of the introduction why this work was being undertaken. The work has been done when we use biogas as a feedstock, whereas most other work has focussed on using natural gas as a feedstock.

4. M&M: Alanytical methods are presented without details. Extend the section and add the proper data.

The techno-economic method has been explained. Because most TE models are very dependent on the assumptions made, we have tried to clarify all assumptions used, as well as the references given for why we have used certain values. We hope that his answer the concern.

5. Results:
   The results section should be improved reporting more detailed analysis, the reviewer suggestions are to perform detailed statistical analysis of presented results. The statistic evaluation is essential.

There is no need to perform a statistical analysis on the results because they are based on a techno-economic model. However, to test certain assumptions, we have performed a sensitivity analysis by looking at the income streams and how that would affect the feasibility.

 

Round 3

Reviewer 4 Report

Dear Authors,

Changes in MS are insufficient and responses for reviewer comments does not allow to accept the particle for potential publication. The overlook of MS is nice, however, the merit is still missing. 

Author Response

Dear Reviewer

We have made effort to proof-ready the entire manuscript afresh, checked for spelling and grammar mistakes, and have also included extra wording where a sentence/statement might have been unclear. 

Attached is the latest updated version. We hope that it now satisfactory to you since we had previously made effort to provide all the point-by-point responses that you pointed out.

Regards

Nicholas Musyoka

Author Response File: Author Response.docx