The Role of Catalysts in Life Cycle Assessment Applied to Biogas Reforming

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

 

This paper provides us with an overview of catalyst life cycle analysis in biogas reforming. The discussion is interesting and I’m happy to recommend publication if the following questions are well addressed.

 

1. The format of the reference lists is not consistent. i.e.,  the abbreviation of the journal name, bold format of year, and the format of volume and number.
2. In Equation (7)-(11), there is a misplaced box or symbol on the top of the arrow signs. It seems that the authors forgot to remove the placeholder box when formatting the equation by word.
3. Could the authors consider combining the figure 4 and Figure 6 into a single figure? Figure 6 seems to be just the further explanation of Figure 4, but they all represent the procedure of the LCA.
4. In Table 6, Could the authors also include the performance (i.e., conversion, selectivity, and stability) of some state-of-art catalysts with these active phases?
5. In Figure 5 (b) and (d), the bar charts represent the percentage of research distribution for LCA and biogas reforming. Could the authors mention the year of these data? Maybe the data is calculated by the average of the time period between 1995-2025. Also, in Figure 5d, around 40% of the papers are from mathematics, and physics and astronomy field. It’s hard to believe they are related to the biogas reforming. Maybe these papers utilized the ML method to predict or analyze the biogas reforming related data. Could the authors comment on that?

Comments on the Quality of English Language

language needs to be improved.

Author Response

Dear reviewer,

First of all, thank you so much for your effort and help during the review of this work. We are sure that your advice has considerably contributed to the improvement in this article. Our answers to your questions or queries are the following:

This paper provides us with an overview of catalyst life cycle analysis in biogas reforming. The discussion is interesting and I’m happy to recommend publication if the following questions are well addressed.

 Thank you for your encouraging feedback.

1. The format of the reference lists is not consistent. i.e.,  the abbreviation of the journal name, bold format of year, and the format of volume and number.

Thank you for your remark. We have checked the format of all references, including bold format for all years and the format of volume and number when possible. Also, the abbreviations of some journals were corrected, whereas some journals seem to offer the complete name (especially in journals with one single word). In any case, if the article is finally accepted for publication, we will check further these issues with the editorial team during the proofreading process. It should be noted that these changes were not pointed out in red in the final text, as they were numerous.

1. In Equation (7)-(11), there is a misplaced box or symbol on the top of the arrow signs. It seems that the authors forgot to remove the placeholder box when formatting the equation by word.

You are absolutely right. In any case, this was the only option in our Word version. Nevertheless, this problem is not evident in final PDF version, as it happened in previous accepted papers (you can check it if you wish). Nevertheless, we will check this issue with the publishing team if the article is finally accepted, just in case. Thank you for your observation.

1. Could the authors consider combining the figure 4 and Figure 6 into a single figure? Figure 6 seems to be just the further explanation of Figure 4, but they all represent the procedure of the LCA.

This is an interesting question. You are right about the fact that these figures are similar, but we wanted to point out in Figure 4 the different possibilities of different boundaries that can be established in LCA applied to biogas production and reforming, whereas Figure 6 is focused on the specific steps during biogas production (which is one of the boundaries pre-established in Figure 4), although there are references to further steps included in other boundaries. In other words, the focus in Figure 4 and 6 is different and important to follow the reasoning of this work. We understand your point of view in order to simplify the figures and the content of this work, but these figures are relevant to further explanations, as observed during the whole article.

1. In Table 6, Could the authors also include the performance (i.e., conversion, selectivity, and stability) of some state-of-art catalysts with these active phases?

A new column with catalytic performance results was added (with CH4 conversion), including comments in the final text about stability. It should be noted that the works considered for this estimation were carried out under different operating conditions, so these ranges should be taken as a proximate idea about the catalytic performance of these different active phases. These comments were added in the final text after Table 6 (changes in red).

1. In Figure 5 (b) and (d), the bar charts represent the percentage of research distribution for LCA and biogas reforming. Could the authors mention the year of these data? Maybe the data is calculated by the average of the time period between 1995-2025. Also, in Figure 5d, around 40% of the papers are from mathematics, and physics and astronomy field. It’s hard to believe they are related to the biogas reforming. Maybe these papers utilized the ML method to predict or analyze the biogas reforming related data. Could the authors comment on that?

The period when these data were collected was 1998-2023, which can be considered the last 25 years. This period was selected due to the data availability and to provide a wide scope of article publication (even though our review was focused on a shorter period of time, as explained in the introduction). The period is included in Figure 5 caption. Regarding the field distribution, there was a terrible mistake. We have changed the colors in the Figure, and now both the comments and the figures are correctly related. In other words, the majority field for Figure 5d is Energy and not Mathematics, Physics or Astronomy. Also, the color corresponding to Environmental Science has been changed to avoid confusion. Sorry for the inconvenience.

 

Again, thank you for your attention and sincerely,

Dr. Sergio Nogales Delgado.

Reviewer 2 Report

Comments and Suggestions for Authors

 Thank you for the opportunity to review the manuscript “The role of Catalysts in LCA Applied to Biogas Reforming,” which emphasizes the importance of life cycle assessment (LCA) in biogas reforming. Although the topic is relevant, I recommend its publication in Catalysts only after major revisions. The manuscript is repetitive, addressing various fields superficially, and the sections on catalysis are too basic. I believe the manuscript could be significantly more informative and focused.

The author discusses the challenges of applying LCA to biogas reform, including issues with catalyst recovery, by-products, separation requirements, and potential H2 leakage. However, every transformation will inevitably produce some level of pollution. Therefore, the focus should be on developing tools to minimize by-product generation. Additionally, before implementing any process, whether at the bench or industrial scale, operational guarantees must be ensured, such as preventing H2 leakage. It is also essential that the catalyst be optimized for its intended purpose.

The manuscript provides an extensive yet disconnected discussion of reform reactions. Given its generic nature, I believe it would be more suitable for a journal with a broader scope than Catalysts. While I do not discredit the article entirely, it falls short in terms of catalysis and is tedious due to excessive repetition of information.

Author Response

Dear reviewer,

Firstly, thank you for your review and point of view about our review article. Our replies to your interesting comments are the following:

Thank you for the opportunity to review the manuscript “The role of Catalysts in LCA Applied to Biogas Reforming,” which emphasizes the importance of life cycle assessment (LCA) in biogas reforming. Although the topic is relevant, I recommend its publication in Catalysts only after major revisions. The manuscript is repetitive, addressing various fields superficially, and the sections on catalysis are too basic. I believe the manuscript could be significantly more informative and focused.

Thank you for your opinion. Regarding your comments, we consider the following:

• Even though the manuscript might be repetitive, it is important to bear in mind different factors:
  • We consider that the basic foundations of LCA should be addressed, as there are not many review works about its application to catalyst performance and an introduction to this concept is essential. Then, when talking about the specific use of LCA to catalytic biogas reforming, some references to this part are necessary.
  • Also, LCA in general, as well as biogas reforming operating conditions, present a strong influence on catalysts, but the latter, in turn, also have a strong influence on LCA and biogas reforming. That is the reason why some concepts seem to be repetitive. For instance, the use of high temperatures has a negative effect in LCA and catalytic performance, whereas the use of catalysts presents a positive effect on LCA and biogas operating conditions, allowing a decrease in reaction temperature. Although this reasoning seems to be similar, it is not the same, as pointed out in the text. The same happens with other concepts in this work.
• We have tried to deepen in some fields that can be superficially covered, especially regarding catalyst, in order to make this work more suitable for its specific publication in this journal (you can see changes in red throughout the text, where some tables have been extended and more reasoning have been added in some parts of this review). Thus, the work is more focused on catalysts thanks to your comments and those required by the rest of reviewers.

 

The author discusses the challenges of applying LCA to biogas reform, including issues with catalyst recovery, by-products, separation requirements, and potential H2 leakage. However, every transformation will inevitably produce some level of pollution. Therefore, the focus should be on developing tools to minimize by-product generation. Additionally, before implementing any process, whether at the bench or industrial scale, operational guarantees must be ensured, such as preventing H2 leakage. It is also essential that the catalyst be optimized for its intended purpose.

You are right about this point, and it is one of the main conclusions of this work. It can be considered an “endless story”, where the implementation of new technologies should be thoroughly studied by using LCA, in order to firmly consider its inclusion in a biorefinery (for instance). In this sense, as explained in the final text, the use of technologies to valorize the wastes generated during biogas production or reforming seems to be the most suitable ones from an environmental point of view. We have added this new conclusion (changes in red), which was commented in some parts of this review.

• Thus, one interesting conclusion was that, both in general and when applied to biogas reforming, the implementation of new technologies normally results in more complex LCA, with possibly new challenges derived from these new technologies applied. In this sense, the suitable choice of solutions is essential to avoid shifting the environmental problem. That is the reason why some steps related to the valorization of wastes generated during biogas production could be an interesting alternative, as in the case of active carbon production from digestate that can be used to adsorb H₂S, with the subsequent improvement in catalytic performance during biogas reforming. Even for this choice, there are alternatives such as pyrolysis or HTC, with the latter being more appropriate due to the high moisture of this waste.

 

Also, the optimization of the catalyst is essential, to provide a good performance and to avoid environmental problems during its production, as explained in the text (changes in red):

In this sense, the optimization of catalyst production and performance is essential to contribute to the abovementioned conditions for a positive LCA. With this regard, and considering the previous reasoning, the following points should be considered:

 

• The right selection of the support, allowing a high surface area once the final catalyst is generated, along with other properties such as high thermal and mechanical resistance, strong interaction between the support and the active phase, etc.
• The optimum combination, quantity and proportion of metallic active phases in order to boost the efficiency and durability of the process, avoiding the release of unreacted products. Also, the lowest amount of active phase, keeping its activity as much as possible, is desirable to reduce the impact during its production.
• Also, the optimization of the process according to a specific catalyst is essential, trying to reduce energy costs but, in turn, enhancing the activity and durability of the synthetic catalyst.

 

For this reason, multiple optimization studies have been carried out, considering different catalyst supports [90,100,113], bi or trimetallic catalysts [91,99], or variable operating conditions [69,71,72].

 

The manuscript provides an extensive yet disconnected discussion of reform reactions. Given its generic nature, I believe it would be more suitable for a journal with a broader scope than Catalysts. While I do not discredit the article entirely, it falls short in terms of catalysis and is tedious due to excessive repetition of information.

As previously commented, we think that there is not excessive repetition of information, but related ideas that can should be expressed in both senses (from LCA and biogas reforming applied to catalytic biogas reforming and viceversa). In this way, different nuances can be found, especially concerning the influence of the general process on catalysts and the effect of catalysts on the general process. Also, the concept of catalysis is highly present throughout the text, especially after the introduction of new paragraphs about this subject in the final text. In this sense, we think that this article could be suitable for this journal. Also, there are specific works, some of them published in catalysts, where the use of catalysts in biogas reforming is thoroughly covered (which are highly recommended and cited in this text). However, in this work, the specific relationship between LCA, biogas reforming and catalysts is pointed out, which can lead to the idea that it is not specially focused on catalysts. In any case, this is just our opinion, and we leave this decision to the editor and the opinion of the reviewers, of course.

 

Again, thank you for your time and attention, and sincerely,

Dr. Sergio Nogales Delgado.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

 There are some typographical errors in the equations. Once these errors are corrected, the papers will be ready for publication.

 Kind regards.

Author Response

Dear reviewer,

First of all, thank you so much for your advice. We are glad that most of our changes have been suitable according to your criteria. Regarding your new comment, our answer is the following:

 

Dear Authors,

 There are some typographical errors in the equations. Once these errors are corrected, the papers will be ready for publication.

 Kind regards.

We have corrected these typographical errors.

 

Thank you for your comment and sincerely,

 

Dr. Sergio Nogales Delgado.