Implementation of Formic Acid as a Liquid Organic Hydrogen Carrier (LOHC): Techno-Economic Analysis and Life Cycle Assessment of Formic Acid Produced via CO₂ Utilization

Round 1

Reviewer 1 Report

The authors assessed the potential of using formic acid as a liquid organic hydrogen carrier (LOHC). In addition, the updated technique was used to simulate a hydrogen distribution system using formic acid as the LOHC, and a series of industrialization processes are modeled and evaluated through technical and economic analysis (TEA) and life cycle evaluation (LCA). This could improve the use of formic acid for hydrogen transport in future. I think this could be published in Catalysts, but there are significant issues need to be improved by the authors.

1. In lines 20-24 of the abstract section, the author stated that the competitiveness of formate over methanol and toluene was low in the past. Are there any supporting materials?

2. The inlet CO₂ concentration in Table S4 is less than 5%, but inlet CO₂ concentration into the TREA base absorber column during the simulation process is not indicated, and will it affect the absorption of CO₂?

3. It is suggested that the full name of the abbreviation should appear as the abbreviation first appears. For example, CAPEX and OPEX first appear in 355 lines, the full name does not appear until 402 lines. I hope the authors can check it carefully.

4. Some fonts and lines in Figure 7 overlap, which can be optimized.

5. There are some formatting errors in the text, such as Table S1 subscript errors, which should be checked carefully.

6. Why the authors chose the formic acid as the liquid organic hydrogen carrier for the LCA? How about the LCA results about the methanol as the liquid organic hydrogen carrier? These publications on life cycle assessment or catalysis of CO₂-based chemicals are suggested to update in the introduction (Nature, 2019, 575, 87–97; Nature Communications, 2019, 10, 5698; Green Chem., 2017,19, 2244-2259).

Author Response

Thank you for your detailed and professional review on our manuscript. In the attached file, we enclosed the revised manuscript as well as the answers to reviews.  We believe that we have addressed all comments by the reviewers. We also confirm that all authors have checked the manuscript and have agreed to the revision. Thank you for your time and kind consideration. We are looking forward to hearing from you.

 

Sincerely

Ung Lee

Author Response File: Author Response.pdf

Reviewer 2 Report

The submitted manuscript by Changsoo Kim, Younggeun Lee, Kyeongsu Kim and Ung Lee reports a techno-economic analysis and life cycle assessment of formic acid produced via CO₂ utilization. The study is attractive due to the potential of the analyzed processes for CO₂ valorization. They studied different scenarios including catalytic and electrochemical conversion of CO₂ to formic acid, as well as the use of the produced formic acid as a liquid organic hydrogen carrier. The results are interesting, but I think that some details in the manuscript need to be improved before its publication.

Below are my recommendations:

The principal findings of the investigation are unclear in the abstract. I recommend including and highlighting the specific and successful results.

Between lines 78 – 92 the authors discussed a brief on the state-of-the-art about catalytic conversion of CO₂ to formic acid. I think that the reaction conditions of the cited studies such as temperature and pressure need to be included. In addition, the selectivity reported in those cited studies should be added, because is an important variable (even as the conversion) of the catalysts’ performance and can be critical for the viability of the processes.

I recommend adding a brief context and discussion of the thermodynamic properties of the reactions to evaluate the system’s viability.

In the introduction, the discussion of the reports related to economic, technical, and environmental analyses of formic acid production from CO₂ is too brief. I think that other paragraphs of the introduction can be reduced to give rise to this discussion. There are published many reports about this topic that are worth citing highlighting the differences and advantages of the author's proposal. See, for example,  https://doi.org/10.1039/C9GC01280J , https://doi.org/10.1016/j.ijhydene.2016.05.199 , https://doi.org/10.1016/j.apenergy.2020.114711 , https://doi.org/10.3390/app8060914

In the case of the article that the authors use as a reference for a model reactor (Park et al., 2020) the dimensions of the system are small-scale as commonly used in laboratory studies. The height of the packed bed is 65 mm, and the catalyst is used as a powder (diameter of 50-200 micrometers). However, these dimensions must change when the reactor is scaled to industrial size. I did not see in the manuscript or supplementary information the dimensions or general design of the reactor. In general, in industrial reactors, a matrix (packing) is used as support of the catalyst to stabilize it and avoid excessive pressure drops in the system. Were these details considered in the modeling by Aspen Plus?

The same comment in the case of the dehydrogenation reactor (Hong et al., 2019), I did not see in the manuscript or supplementary information the design of the reactor.

In addition, the articles used as references for the conversion of CO₂ and dehydrogenation of formic acid do not include kinetic models which are necessary for a proper reactor design. Although they report the reached conversion or yield at the specific reaction conditions, these values can change when the system is scaled up. Which were the kinetics models used for the reactors’ design?

 

Furthermore, the mass transfer in industrial reactors plays a very important role which can lead to reaching less conversion and selectivity compared to laboratory reactors. I think this must be cleared in the manuscript because the economic and environmental indicators could change due to these considerations.

Author Response

Thank you for your detailed and constructive review on our manuscripts. We enclosed the revised manuscript as well as the answers to you.  We believe that we have addressed all comments by the reviewers. We also confirm that all authors have checked the manuscript and have agreed to the revision. Thank you for your time and kind consideration. We are looking forward to hearing from you.

 

Sincerely

Ung Lee

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have addressed the comments very well, thus I suggest to acccept now.