Opportunities and Challenges of High-Pressure Fast Pyrolysis of Biomass: A Review

Round 1

Reviewer 1 Report

This article gives an understanding of high-pressure fast pyrolysis of biomass from classification of pyrolysis; reactors used in fast pyrolysis; heat transfer in pyrolysis; feedstock; fast pyrolysis parameters; properties/yields of fast pyrolysis products; high pressure on pyrolysis process; catalyst types and their application; and problems to overcome in the pyrolysis process. However, the influence and theory of high pressure on biomass fast pyrolysis process is not discussed and summarized in detail. I suggest that there are a number of issues as below.

 

1. In Table 1, the gap of each line should be larger for reading clearly.

 

2. In Section 3, the authors classified the pyrolysis as four types: conventional, flash, fast and slow. Then, authors thought the conventional was he same as slow. In my view, the statement about “pyrolysis comprise four (diverse) process types, namely: conventional, flash, fast and slow” is repetitive and useless.

 

3. In Section 3.2, authors stated that “The fast pyrolysis would produce 60–75% of the biooil along with a range of 15% and 13% of biochar and pyrolytic gas residues, respectively.” The values of product yield were different from those in the Table 2. What is the reason for the difference?

 

4. In Lines 153 and 154, it mentioned that Particles are usually heated with very high rate of heating between 10 to 200 K/s in the discussion of the flash pyrolysis. Is there a clerical error?

 

5. In Fig. 1, the units of Y are needed.

Author Response

The responses to Reviewer comments are in the attached file.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper deals with an interesting topic. The discussion is attractive and acceptable.

I recommend to publish the article as it is.

Author Response

The responses to Reviewer comments are in the attached file.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript presents a mini-review on high-pressure fast pyrolysis of biomass.

The authors present in-depth information about the classification of pyrolysis, reactors used in fast pyrolysis, heat transfer in pyrolysis feedstock, fast pyrolysis parameters, properties of fast pyrolysis products, the effect of high pressure, catalyst types and their application. The problems that have to be overcome in the pyrolysis process are also considered.

The manuscript is based on 87 published papers, which are enough for a mini-review. I find Table 1, dedicated to previously published reviews to be very useful. I suggest to the authors add an additional column, showing the year of the review, to avoid continuous scrolling to the end of the paper.

There is a wrong numeration starting from point 8. There are two 8.1 subpoints (Biooil and Biochar). I suggest omitting the numeration 8.1.1 Potential application of biooil and  Potential application of biochar, as it is not followed by 8.1.2.

The same is valid for 9.1. Impact of high pressure on quantity and quality of the products - it is not followed by 9.2.

Author Response

The responses to Reviewer comments are in the attached file.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments on energies-1341566

 Major comments:

The study is focused on High – Pressure Fast Pyrolysis of Biomass. The research is very limited in the aspects of the study and covers only a few points of the whole study. The literature study is very weak and outdated, and there is no proper linkage between different sections of the papers. The language of the paper also needs further improvements as it contains several mistakes. The equations should be written using equation writer instead of the same word format and references should be divided properly and should not be generic. The headings also need changes as they should be based on obtained results instead of the equipment used. The English language used in the manuscript needs improvements, as there are some punctuation and grammatical mistakes throughout the manuscript. Sentences need more clarity and better construction. Some figures need more transparency; special focus is required in labelling the axis and titles. Overall, the paper needs some structural and literature revisions to meet the requirements of the journal. It is obvious the quality of the manuscript does not meet the standards of energies journal, therefore should be rejected in its present form or needs major revisions.

 Introduction:

The introduction is very short and too general, therefore need more specific information related to the present research. It should be more focused on the topic of interest. The literature in the introduction needs more updates and the latest literature should be used for the studies. The introduction needs to be more emphasized on the research work with a detailed explanation of the whole process considering past, present, and future scope. The conventional fuels and technologies need to be explained well to indicate the relevance of the research work. It needs to be strengthened in terms of recent research and updated literature review in this area with possible research gaps. It is strongly recommended to add a recent literature survey about different biomasses, renewable fuels, climate change, recent global warming trends and the role along with the wide range of applications. How do these sustainable fuels affect the current levels of CO₂ and alarming global warming issues? Research gaps should be highlighted more clearly and future applications of this study should be added.

 Specific comments:

1. The title is not appropriate, authors are advised to revise the tile, which should be comprehensive and novel.
2. Abstract: It is suggested to add some background with few objectives and possible applications of this study and highlight the novelty of this work clearly. The abstract only contains some parameters without any process conditions or key values from results, which is insufficient to delineate the whole pictures of contribution and possible application of this study.
3. Revise keywords add more specific and novel keywords with broader meanings (5-7 words).
4. Most of the information presented in the introduction is too general and very common. Therefore, should be omitted and replaced by some latest research and their findings.
5. Introduction: Background is weak, no information is provided about the CO₂ emission from fossil fuels (Coal etc.) the major contributor to global warming, therefore, the authors are advised to read and add CO₂ emission levels from the following studies: International Journal of Chemical Reactor Engineering, 2019; 17(11). Journal of Analytical and Applied Pyrolysis, 2020;150:104897. International Journal of Chemical Reactor Engineering, 2019; 17(4).
6. Major formatting changes including proper paragraphing is required throughout the report.
7. The figures and tables numberings must be consistent, you have used Fig. and Figure at different places?
8. Most of the headings and subheadings need to be revised in a professional way and for better understating.
9. The figure/graphs are very dim and low quality, the axis units are missing in some figures, need to be revised. The authors can use Origin Lab software to draw good qulity graphs.
10. Most of the information presented in the discussion is refereed to your own findings/values, please discuss this critically with the literature and other published word.
11. The tables/ figures inserted are not explained or discussed well in the text please discuss critically/explain all tables/figures in the text wherever possible.
12. The graphs throughout the manuscript are not consistent, some coloured, some black and white with blurry resolution. Revise all graphs with high-quality images and keep all figures as coloured with consistent fonts. The units stated in the graphs axis need to be double-checked.
13. Please round off all numbers up to two decimal places throughout the text in the entire manuscript.
14. The obtained values in the results are just stated in the text without explaining them. Explain the reasons behind your trends/values and discuss them critically with literature.
15. Revise figures in the manuscript. Draw all figures in high-quality figures should be coloured and attractive.
16. Each Table and Figure should be cross-referenced atleast twice in the text for the beeter understanding of the results presents.
17. More recent research about types of renewable fuels including biomass, CO₂ reduction methods and sustainable energy development is suggested to be added to make the background and discussion more strong: Journal of CO2 Utilization, 2020; 40:101193. Sustainable Energy Technologies and Assessments, 2019; 36:100540. Energy, 2020; 209:118444. ACS Sustainable Chemistry& Engineering, 2020;8(34):12877-90. International Journal of Ambient Energy, 2019; 40(1):86-95.
18. All figures numbering needs to be carefully checked and revised.
19. All figures captions should be revised in a more meaningful way.
20. Avoid an abundance of references do not cite more than 2 references in a single place. Correct all these types of references throughout the manuscript.
21. The conclusions only talk about some studied parameters, which is insufficient to depict the whole picture of the contribution of this study. The authors are advised to write the conclusions in a comprehensive way and should contain key values, suitability of the applied method, the major findings, contributions and possible future outcomes.
22. References: The authors are advised to revise this section, including the latest reference. Please see some suggestions in the specific comments and in the ‘introduction’ section.

Author Response

The responses to Reviewer comments are in the attached file.

Author Response File: Author Response.docx

Round 2

Reviewer 4 Report

The authors have addressed most of the comments; they have also tried to make changes according to the reviewers’ suggestions. After revisions, the quality of the manuscript has been adequately enhanced. Therefore, the manuscript could be considered for the publication in the Journal. However, there are still some editing/ syntax errors present in the manuscript which need to be corrected, hence the publishing team is advised to read the manuscript carefully before publishing.

Author Response

The response to the reviewer is in the attached file.

Author Response File: Author Response.docx