Integrated Approach for Biomass Conversion Using Thermochemical Routes with Anaerobic Digestion and Syngas Fermentation
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis review paper (sustainability-3564957) provides a comprehensive overview of the integration of thermochemical and biochemical processes for biomass conversion, highlighting the potential of hybrid systems to maximize resource recovery and improve energy efficiency. The paper effectively summarizes various integrated approaches, such as anaerobic digestion combined with pyrolysis, hydrothermal liquefaction, and hydrothermal carbonization, as well as gasification and hydrothermal liquefaction combined with syngas fermentation/biomethanation.
This study covers a wide range of thermochemical and biochemical processes and their integration, providing a holistic view of the field. In this sense, one of strength point is balanced discussion of the advantages and challenges associated with both individual conversion processes and integrated systems, especially in section 2 (Hybrid thermochemical and biochemical systems). Another positive point is the clear structure of the paper, with a clear progression from individual conversion processes to integrated systems, economic and environmental impacts, challenges to make industrial process and future perspective; making it easy for readers to follow the logic and grasp the key concepts.
Even thought, the authors effectively emphasize the technical and economic synergies of integrating diverse technologies, highlighting the benefits of hybrid systems in terms of resource recovery, energy efficiency, and economic viability, a more in-depth economic analysis, including specific cost breakdowns and comparisons with conventional technologies, would further strengthen the review. Other point to improve is the environmental benefits of integrated systems that need to be expanding the discussion with more specific data on greenhouse gas emission reductions and other environmental indicators would add more value to the paper.
Therefore, I recommend to reject the manuscript that a minor revision is warranted. The manuscript presents valuable insights into thermochemical and biochemical processes for biomass conversion, highlighting the potential of hybrid systems to maximize resource recovery and improve energy efficiency but little changes is necessary in economic and environmental analysis.
Author Response
Comment 1: "Even thought, the authors effectively emphasize the technical and economic synergies of integrating diverse technologies, highlighting the benefits of hybrid systems in terms of resource recovery, energy efficiency, and economic viability, a more in-depth economic analysis, including specific cost breakdowns and comparisons with conventional technologies, would further strengthen the review."
Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have expanded “Section 3 Economic and environmental impact analysis” (line 413 to 426 and 510 to 578) by incorporating a dedicated economic comparison of hybrid versus conventional biomass conversion systems. This includes detailed cost breakdowns and unit treatment costs for AD+HTC, AD+HTL, and SCWG+biomethanation configurations. These are now directly compared with standalone AD and incineration, strengthening the review's techno-economic dimension.
Comment 2: "Other point to improve is the environmental benefits of integrated systems that need to be expanding the discussion with more specific data on greenhouse gas emission reductions and other environmental indicators would add more value to the paper."
Response 2: Thank you again for this observation. We also agree with this comment. Therefore, we have expanded Section 3 Economic and environmental impact analysis (lines 580 -605) by incorporating a summary of environmental indicators associated with hybrid biomass conversion systems.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsFig. 1 is superfluous, contributes nothing to the review
Line 47 the breakdown of C, H & O bonds is imprecise – which specific bonds is involved in the thermochemical processes
Line 52 what do mean by grindability of char?
Line 116, 117 The main processes among the biochemical methods of biomass conversion mentioned by authors are anaerobic digestion (AD) and syngas digestion (SNF). However, we have many other bioprocesses in biochemical conversion of biomass like ethanol fermentation, butanol fermentation, biohydrogen production and bioelectrochemical processes, that should also be mentioned. And the Table1 extended.
General remarks:
There are no references to the various lignocellulosic biomass feedstocks, yet the type of biomass largely determines the efficiency of the process. Authors do not specify which type of biomass is suitable for which processes.
Lack of DOI in references, some of papers are or available at all
Author Response
Comment 1: "Fig. 1 is superfluous, contributes nothing to the review"
Response 1: Thank you for your observation. We agree that Figure 1 does not contribute technical content to the review. However, we believe that its inclusion enhances the document's visual appeal and helps to engage readers by contextualizing the conventional approach to biomass utilization, which contrasts with the advanced integrated systems discussed throughout the paper. For this reason, we would prefer to retain the figure, provided the editorial team agrees with its relevance in terms of presentation and readability.
Comment 2: "Line 47 the breakdown of C, H & O bonds is imprecise – which specific bonds is involved in the thermochemical processes"
Response 2: Thank you for this observation. We agree that the original phrasing lacked chemical precision. Therefore, we have revised the sentence in line 47 to specify the types of covalent bonds involved. The new sentence reads: “Thermochemical conversion processes release the chemical energy stored in biomass by breaking covalent bonds such as C–C, C–H, and C–O present in carbohydrates, lignin, and other organic macromolecules formed during photosynthesis” (lines 58-59) This change improves the scientific accuracy of the description.
Comment 3: "Line 52 what do mean by grindability of char?"
Response 3: Thank you for your comment. We acknowledge that the term “grindability” required clarification. Therefore, we have revised the sentence in line 52 to specify that it refers to the ease of particle size reduction, which is relevant to downstream processing. The updated sentence now reads: “These methods improve the ease of particle size reduction (grindability), enhance storability by reducing moisture and biological degradation, and increase the energy density of biomass” (lines 65-66).
Comment 4: "Line 116, 117 The main processes among the biochemical methods of biomass conversion mentioned by authors are anaerobic digestion (AD) and syngas digestion (SNF). However, we have many other bioprocesses in biochemical conversion of biomass like ethanol fermentation, butanol fermentation, biohydrogen production and bioelectrochemical processes, that should also be mentioned. And the Table1 extended."
Response 4: Thank you for this comment. We agree with the observation. Although our review focuses primarily on gas-generating pathways, we acknowledge the broader spectrum of biochemical biomass conversion technologies, including ethanol and butanol fermentation, biohydrogen production, and bioelectrochemical systems. Therefore, we have clarified this scope in lines 132-136 by specifying that the main biochemical processes discussed are those leading to gaseous products. Additionally, we have updated the title of Table 1 to: “Overview of thermochemical and gas-generating biochemical processes for biomass conversion” to reflect this focus.
Comment 5: "There are no references to the various lignocellulosic biomass feedstocks, yet the type of biomass largely determines the efficiency of the process. Authors do not specify which type of biomass is suitable for which processes."
Response 5: We agree that the nature of lignocellulosic biomass feedstocks plays a key role in determining the efficiency and suitability of conversion processes. To address this point, we have expanded the discussion in the Introduction (lines 40–51) to include a general overview of the most common types of biomass and their relevance to specific thermochemical and biochemical technologies. Although a detailed mapping of feedstocks to specific processes is beyond the scope of this review, we now highlight that factors such as moisture content, ash composition, biodegradability, and lignin content play a key role in determining the most suitable conversion pathway.
Comment 6: "Lack of DOI in references, some of papers are or available at all"
Response 6: We would like to clarify that the reference style used throughout the manuscript strictly follows the abbreviated format required by the journal, which excludes the inclusion of DOIs. Accordingly, no DOIs have been added to ensure consistency with the journal’s referencing guidelines.
Author Response File: Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsComments for the authors
The article addresses a very relevant and topical issue, especially in the framework of the energy transition and the circular economy.
The approach of reviewing hybrid systems that combine thermochemical and biochemical processes is well-founded, and the manuscript has a clear structure, with a generally careful and accessible wording.
In addition, it is supported by recent and relevant bibliography, which reinforces its value as a review article.
I would like, however, to suggest some small improvements that could help round off the work:
- Reviewing citations: There are some references that are reused in different contexts (e.g., [10]) and can be confusing if they are not more precisely adjusted to each specific statement. It would be useful to review the assignment of citations to ensure that each one corresponds exactly to the idea it supports.
- Avoid repetition: Similar ideas are repeated in some sections, especially in relation to the use of biochar in AD systems. I would recommend simplifying or condensing some of these sections to improve the flow of the text.
- Table 2: The table is very useful, but it could benefit from a clearer or more visual format. Making the first row different, as has been done in Table 1. Perhaps you could adjust the alignment or introduce some shading or block separation.
- Deepening the challenges: Although some limitations of hybrid systems are mentioned, I think that a somewhat more critical reflection could be included on the main technical and economic challenges presented by their industrial scaling.
Small writing adjustments: Some paragraphs are somewhat long and could be divided to make it easier to read. Adding a Dot and Aside. There are also phrases that, with a somewhat more direct wording, would gain in clarity. In short, I consider that the manuscript offers a good contribution to the field and that, with these small revisions, it will be ready for publication.
Author Response
Comment 1: "Reviewing citations: There are some references that are reused in different contexts (e.g., [10]) and can be confusing if they are not more precisely adjusted to each specific statement. It would be useful to review the assignment of citations to ensure that each one corresponds exactly to the idea it supports."
Response 1: Thank you for this observation. We acknowledge that some references are cited in multiple contexts. This is intentional, as these sources provide comprehensive insights that are relevant to more than one section of the manuscript. We have ensured that each use of a reference remains appropriate to the specific statement it supports. Therefore, we have opted to maintain the current citation structure.
Comment 2: "Avoid repetition: Similar ideas are repeated in some sections, especially in relation to the use of biochar in AD systems. I would recommend simplifying or condensing some of these sections to improve the flow of the text."
Response 2: Thank you for your helpful suggestion. We agree that some redundancy existed regarding the role of biochar in AD systems. As a result, we have revised Section 1.2 (lines 151-155) to streamline the introductory overview and removed the comment in Section 2.1 to avoid repetition (line 243). However, we have retained the references to hydrochar and syngas-derived carbon products in Sections 2.2 and 2.3, respectively, as these are discussed in the specific context of HTL and SCWG hybrid systems. Each mention serves to illustrate distinct valorization strategies relevant to the corresponding conversion pathway.
Comment 3: "Table 2: The table is very useful, but it could benefit from a clearer or more visual format. Making the first row different, as has been done in Table 1. Perhaps you could adjust the alignment or introduce some shading or block separation."
Response 3: Thank you for your comment. We have introduced visual adjustments to Table 2 at this stage of the revision to improve its readability, including clearer header formatting and alignment improvements. However, we understand that the final layout and formatting will be handled by the journal during the production stage, and we trust the editorial team will apply the appropriate visual style for publication.
Comment 4: "Deepening the challenges: Although some limitations of hybrid systems are mentioned, I think that a somewhat more critical reflection could be included on the main technical and economic challenges presented by their industrial scaling."
Response 4: Thank you for this comment. We agree that a more critical reflection on the industrial scalability of hybrid systems was needed. Therefore, we have expanded the end of Section 3. Economic and environmental impact analysis (lines 494–505) to include a dedicated paragraph addressing the main technical and economic limitations that hinder their commercial implementation. This addition highlights barriers complementing the previously added techno-economic discussion without altering the overall structure or tone of the section.
Comment 5: "Small writing adjustments: Some paragraphs are somewhat long and could be divided to make it easier to read. Adding a Dot and Aside. There are also phrases that, with a somewhat more direct wording, would gain in clarity."
Response 5: We have reviewed the manuscript to improve clarity and readability. Several long paragraphs have been split, and minor writing adjustments have been made to enhance conciseness and flow. These changes are reflected throughout the text, especially in Sections 2 and 3, without altering the technical content or structure of the review.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsI agree with the explanations given by the authors of the manuscript. However, if this mini-review is limited to "gas-producing routes", then the title should be changed to reflect this limitation of the biochemical processes to anaerobic digestion and syngas fermentation only. e.g:
Integrated approach for biomass conversion using thermochemical with anaerobic digestion and/or syngas fermentation: a mini-review.
I will accept this manuscript for publication provided that the title is changed as the current one is misleading to the reader.
