Towards High Efficiency CO₂ Utilization by Glow Discharge Plasma

Round 1

Reviewer 1 Report

In the present Communication, authors describe a device for the CO₂ splitting reaction by plasma elaborated on basis of an APGD reactor. The design and operation of the device are described in detail. However, the rest of the manuscript is not written in sufficient detail. Overall, the manuscript appears to be part of the content in a more wide scale project.

I have some comments and questions as listed below.

1. Authors recently presented a very similar reactor, using a direct current APGD, which delivered interesting results [https://doi.org/10.1016/j.jcou.2020.101322]. What are the main differences between the present device and device presented in previous work of authors (https://doi.org/10.1016/j.jcou.2020.101322)? What are the unique features of the reactor compared to other similar reactors developed in the past? These need to be discussed in the manuscript.
2. Authors wrote that the efficiency of CO₂ conversion and its energy efficiency observed in present study were compared with other technologies. But, despite the large amount of literature data, the authors use them too little (about ten sources). It is known that the efficiency of CO₂ conversion is strongly dependent on the electrode material, CO₂ content in the gas mixture, etc. Efficiency of the presented device must be discussed in the perspective (1) of other APGD reactors, (2) of other types of plasma reactors and (3) of other CO₂ conversion technologies. It is incorrect to state that "The presented plasma reactor provides competitive CO₂ conversion and energy efficiency" based on a small literature review.
3. Figure captions not detailed enough in the manuscript. Captions to figures should provide information sufficient for understanding of the figure. All abbreviations and symbols used in figures (as well as in equations and formulas) have to be explained in the legend.
4. Despite the fact that in general the article is written in an understandable way, the writing style is very difficult to perceive.

The submitted manuscript contains very little information on how the presented results were obtained:

1) What was the initial concentration of carbon dioxide in the stream? Or was it pure gas?

2) How was the gas flow rate measured (specify the analogue gas flow meter model)?

3) How was the composition of exhaust gas measured? Indicate on the diagram the location of the sensor and it models.

4) Indicate the model of the chromatograph, the type of used columns, etc. (as well as all other devices used in the work).

Line 47. What are “best measurements”?

Line 9. What are “X processes”?

Research may be important for development of "ideal" device for CO₂ splitting. However, the manuscript is prepared somewhat superficially and does not allow the reader to imagine an overall picture. The reader will have to guess many things. This manuscript cannot be published as presented. Major revision of the manuscript is required before acceptance.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In the manuscript, the authors investigated the conversion of CO₂ by atmospheric pressure glow discharge. It provides just a recent progress, so the length of the manuscript is adequate for the Communication type of manuscript.

The manuscript is well written, easily, and clearly understandable. In general, the experiment is done at a good level with a clear statement of the problem. I have some comments that can improve the information content and overall quality of the manuscript. I recommend only a minor revision.

Comments:

P1 L20, 21: „…plasma catalysis – the latter has the smallest TRL but also offers a large potential for future improvements [2].“ This point deserves adding many other related references, not only one reference.

P1 L22, 23: „...plasma reactors: Dielectric barrier discharges (DBDs), gliding arc (GA), atmospheric pressure glow discharges (APGD) and microwave (MW) plasmas.“ Could the authors be more specific and add related references? It would be more informative for the reader.

P1 L33, 34:  „This communication aims to give an update into the ongoing design process for the ideal plasma reactor for the CO₂ splitting reaction.“ I am not really sure about the word „ideal“ that authors used in this sentence. In which sense the authors mean the word „ideal“? The reactor used by the authors and the efficiency of the investigated process is still far from „ideal“ situation...

P1,2 L41-44: The authors described reasons for the utilisation of catalyst packed bed. However, is there some specific reason for the utilisation of zirconia (ZrO₂) material?

P2 L53: Is the applied power “P_el” the power dissipated in the discharge, i.e. discharge power? In addition, the authors use terms “applied power” and “input power” throughout the entire manuscript for the same thing. This is quite confusing.

P2 L61: How was the “steady state” in exhaust gas concentrations determined?

P2 L66, 67: “The achieved CO₂ conversion X and energy efficiency η for different applied powers P_el and gas flow rates V in is shown in figure 2.“ Here, the authors denoted P_el as applied power, however, in the Figures 2, the results are presented as a function of energy density (in J/SCC) and not discharge power. This is also confusing.

P3 Figures 2: Why did the authors use the unit J/SCC when the flow rate was in SLM? In the field of plasma catalysis, the energy density in J/L is commonly preferred. 

P4 Figure 3(a): Perhaps, it is a good idea to indicate the types of discharges used by other authors in the legend of Fig. 3 (a).

P4 L119: “Conversion efficiency can even be superior.“ I think, this is a quite bold statement. In my opinion, there is no evidence for this statement in the manuscript.

P4 Conclusions: Real conclusions of the presented results are contained only in the first sentence of the Conclusions. Other sentences of this part are focused on comparing the plasma and other technologies as well as on major obstacles of CO₂ conversion systems that must be challenged. Future goals may be involved in the Conclusions, however, this part should mainly involve a brief overview of the most important obtained results.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

1. The full name for TRL should be provided in the introduction. 2. In addition to the obstacles for the industrial plasma, the recent progress of CO2 utilization by plasma should also be summarized. 3. In the results, the effect of axial magnetic field in improving the CO2 conversion performance should be discussed. This can further improve the quality of this work. 4. How about the energy cost and efficiency of CO2 utilization by plasma in comparison with the thermochemical processes and electrolysis? After adding important details and addressing the above comments, it can be published

Author Response

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Reviewer 4 Report

In the present study, CO₂ conversion to CO using glow discharge plasma with permanent magnets was investigated. Results show that high energy efficiency of up to 45% and high CO₂ conversion efficiency were achieved. The magnetic field in reactor setup force the plasma into a large disc-like volume, which suppress the glow-to-arc transition. 

So, I think this work will prompt the development of CO₂ conversion to CO using glow discharge plasma. Based on the important topic of the manuscript, it can be accepted for publication after the minor revision. However, the following issues should be addressed and there are some errors and inaccuracies about the grammar, structure in this paper, which should be revised carefully. 

Specific Comments:

Title

Please revise this title of “Recent progress in high efficiency CO₂ utilization by glow discharge plasma”. It looks like a review at first sight.   

Abstract

There are some suggestions/questions below, please revise them carefully.

Page 1, line 7: This sentence really hard to understand, revised it.

Page 1, line 9: What is the “X processes”? Please descript clearly so that it is easy to understand for different reader.

Page 1, line 9: What kind of problems this reaction poses in practice? Even through this work is a communication, some important information should be given clearly.

Page 1, lines 9-10: If the magnetic field in your study is critical role, it should be involved. 

Keywords

Again, If the magnetic field is a key role, please set it as a keyword, which will increase the citing.

1. Introduction
   page 1, line 30: For “e.g. [6]”, please revise it because it is unnormal.

Please compare the different CO2 conversion or utilization methods which are widely reported in literature (A review of carbon dioxide sequestration by mineral carbonation of industrial byproduct gypsum, etc.). 

2. Materials and methods

Page 1, line 36: Revise 38mm to 38 mm.

Page 1, line 37: please check this sentence of “One is a ring, the other a ring”.

Page 2, line 45: Please supply the mode and factory of the flow meter.

Page 2, line 46: Please supply the detailed information of the Non-dispersive infrared sensors. Same issue, like GC, should be revised.

Page 2, line 62: Please rewrite this sentence because it is hard to read.

4. Discussion

Page 3, lines 81-82: Please revise this sentence of “However, the conversion does not increase with power indefinitely- at the highest applied power of P_el = 192W, conversion reduces.”

Page 3, lines 83-84: cite a reference to prove your assumption or give solid evidence.

Figure 3b: Please make it easy to read. If possible, give every technology name rather that the reference.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors did a good work, made corrections to improve the manuscript, and answered questions from the reviewers.
The reviewer has no additional questions or comments.
However, a short note about the potential ecological and/or technological significance of the obtained results should be included.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

1. What is the institute information for all the authors? chair for electrical energy storage systems??? Please give the detailed institute address.
2. In Fig. 3, are all the reference numbers still the same???
3. Please improve the format of the references carefully.
4. Is the newly added sentence "This approach also gained popularity in gliding arc plasma reactors." related to the paper topic? It is mentioned in Introduction. Please remove these contents.

Author Response

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Author Response File: Author Response.pdf

Round 3

Reviewer 4 Report

Ok to be accept