Self-Heating Risk of Coals and Metal Powders: A Comparison
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe topic of the article is current and important, especially from the point of view of industrial application. The authors performed a number of experiments on real industrial samples and obtained original results, which they tried to correctly interpret and generalize. Which is not easy, especially for samples of metal powders.
However, I recommend considering some minor modifications in the Results and Discussion section:
- The Mg sample behaves quite differently compared to the other metals (see q30 wetted state values in Tab.1). The text does not explain why. Is it because of the much larger surface area of ​​the Mg sample compared to the other metal samples? Or was the Mg sample more oxidized in the as-received state?
- It would be appropriate to discuss more the influence of the particle size (and thus the surface) on the results of the oxyreactivity of the analyzed metal powders. The particle diameter (d50) differs by an order of magnitude, will this difference not affect the resulting q30 values?
- In my opinion, the statement "Tentatively, the threshold for high self-heating risk is assumed to be a q30 value of 2 W kg-1 and an R ratio of approximately 10x." it is too bold and premature. Especially in connection with metal powders.
The article lacks the Conclusion chapter. Authors should summarize the main conclusions of their research in the Conclusion chapter.
Author Response
- The Mg sample behaves quite differently compared to the other metals (see q30 wetted state values in Tab.1). The text does not explain why. Is it because of the much larger surface area of ​​the Mg sample compared to the other metal samples? Or was the Mg sample more oxidized in the as-received state?
Our reaction: Unfortunately, information on the Mg sample is limited. Our (admittedly speculative) opinion is that prior surface oxidation mainly influenced the sample's behavior. Nonetheless, the significant increase in oxidation heat in the wetted sample is intriguing, which is why we included it in the series of industrial metals.
- It would be appropriate to discuss more the influence of the particle size (and thus the surface) on the results of the oxyreactivity of the analyzed metal powders. The particle diameter (d50) differs by an order of magnitude, will this difference not affect the resulting q30 values?
Our reaction: In the revised version, new text has been added discussing the importance of and relationship between particle size and surface passivation on oxidation heat (Lines 155–161). From this, it can be deduced that the effect of surface passivation on q30 values prevails over that of grain size. Similarly, the section addressing the synergistic effect of water has been expanded, with an additional sentence highlighting the relatively less significant impact of grain size (Lines 195–196).
- In my opinion, the statement "Tentatively, the threshold for high self-heating risk is assumed to be a q30 value of 2 W kg-1 and an R ratio of approximately 10x." it is too bold and premature. Especially in connection with metal powders.
Our reaction: We greatly appreciate the reviewer’s positive approach. In the revised version of the manuscript, the conclusion of Section 3.3 was modified: the original statement was removed (Lines 259–260), and only the final sentence was retained.
The article lacks the Conclusion chapter. Authors should summarize the main conclusions of their research in the Conclusion chapter.
Our reaction: We had indeed considered including a Conclusion section in the original manuscript. However, due to the concise nature of the contribution and the informative abstract, we ultimately decided against it. This option is explicitly mentioned in the Instructions for Authors. Therefore, we would prefer to maintain the absence of a Conclusion section in the revised manuscript as well.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsPlease see attached.
Comments for author File: Comments.pdf
None.
Author Response
You have mentioned regarding “contamination of the sample by workplace impurities” in the sample. Line 129). Have you studied how impurities will impact the q30 separately for metals and coals? (Line 129)
Our reaction: We did not perform such an investigation. In fact, we regard impurities in coal (ash) to be integral part of the given sample. As to metal powders, the impurities are usually SiO2 in nature, which can be considered more like an inert part of the sample.
How does the proposed q30W indicator compare to other risk assessment methods used in industry? What advantages or limitations does it have? ((Line 238)
Our reaction: The q30W indicator takes into account separately the oxyreactivity of the sample and the effect of water on the oxyreactivity. This way, it assesses the self-heating risk of the material in a dry versus wet/corrosive environment. To the best of our knowledge, a similar indicator has not yet been proposed, with other indicators evaluate “only” oxyreactivity.
Have you considered to propose a mathematical model to predict the self-heating behaviour based on moisture content and material properties based on the results you have provided comparing q30 and q30W?
Our reaction: We really thank for the idea, which we consider inspiration for further work. However, to date (partly due to relatively limited series of samples) no mathematical model has been proposed for this purpose.
Reviewer 3 Report
Comments and Suggestions for AuthorsOverview comment of the study and MS:
Much as the topic is interesting, the manuscript needs to be beefed up for most of the sections.
Specific comments.
1. For the abstract to be complete, I suggest that last two sentences report the major results (synergistic effects) and the recommendation. I think writing these two sentences in a general way take away the scientific rigor of the paper.
2. Introduction section needs more information to be added. For example, information on what causes and factors that lead to metal powder ignition/burning should be thoroughly discussed in the introduction. Additionally, previous studies on this subject matter should briefly reviewed.
3. Line 67: If the characterization results of the sample (Fig. 1) are presented here then you might as well present particle size analysis of metal power somewhere in Section 2.
4. Lines 84 to 85: You need to explain further how you carried out crushing and sieving in airtight to help someone who would want to repeat your experiments in future.
5. Bullet points in Section 2.3 are unnecessary.
6. Line 96: Was this drying procedure for coal or for metal powder?
7. Characterization of surfaces of particles of metal powder is missing in this paper. This is important to ascertain the level of surface oxidation which affect the very ignition this MS is trying to communicate.
8. Comparison could have made sense if particle size parameter was the same for coal and metals powders.
9. Detailed mechanisms are missing in this MS.
Comments on the Quality of English LanguageIt needs minor corrections and proofreading
Author Response
- For the abstract to be complete, I suggest that last two sentences report the major results (synergistic effects) and the recommendation. I think writing these two sentences in a general way take away the scientific rigor of the paper.
Our reaction: To respect the reviewer´s comment, the end of the abstract was modified. Penultimate sentence of the Abstract was removed, leaving only the last sentence.
- Introduction section needs more information to be added. For example, information on what causes and factors that lead to metal powder ignition/burning should be thoroughly discussed in the introduction. Additionally, previous studies on this subject matter should briefly reviewed.
Our reaction: We are aware of rather concise style of the Introduction section, and we would prefer to preserve the style even in the revised version of the manuscript. In order to respect the reviewer´s comment, however, the text of the Introduction was at least slightly modified, and a new sentence was appended (Lines 29-31).
- Line 67: If the characterization results of the sample (Fig. 1) are presented here then you might as well present particle size analysis of metal power somewhere in Section 2.
Our reaction: To respect the reviewer´s comment, figure of particle size distribution of metal powders (Fig. 2) was moved to Section 2. Simultaneously, the caption of figure 2 was amended.
- Lines 84 to 85: You need to explain further how you carried out crushing and sieving in airtight to help someone who would want to repeat your experiments in future.
Our reaction: Crushing and sieving of coal samples were performed in air. To avoid potential misunderstanding, this section of the text was slightly modified. In the revised version, it now reads (Lines 86–87): "The crushing and sieving of coals were carried out in air as quickly as possible...".
- Bullet points in Section 2.3 are unnecessary.
Our reaction: The bullet points were removed from Section 2.3 in the revised version.
- Line 96: Was this drying procedure for coal or for metal powder?
Our reaction: The drying procedure was applied both for coals and metals. To avoid possible misunderstanding, the text in the part was slightly modified. In the revised version, one can now read (Line 98): “Drying of each sample was performed under vacuum …”.
- Characterization of surfaces of particles of metal powder is missing in this paper. This is important to ascertain the level of surface oxidation which affect the very ignition this MS is trying to communicate.
Our reaction: We acknowledge this fact. In the manuscript, we try to reflect character of the particles surface implicitly using measured values of the oxidation heat. To the new version, text concerning importance and relation between surface passivation and particle size of the sample on the oxidation heat was newly appended (Lines 155 -161).
- Comparison could have made sense if particle size parameter was the same for coal and metals powders.
Our reaction: We completely agree with this perspective. However, it is practically hardly possible to obtain metal samples of the same grain size from different industrial processes. Moreover, the manuscript addresses also other aspects (such as surface passivation and the effect of water) that render the influence of grain size less significant (Lines 155-161, 195-196).
- Detailed mechanisms are missing in this MS.
Our reaction: We acknowledge this fact. However, since our contribution primarily addresses the practical implications of our findings, we chose to report “only” the fundamental mechanistic view on the chemical nature of the described phenomena (Lines 203–211). For detailed information, references to the original sources are provided. We would like to maintain this concise style in the revised version of the manuscript as well.
Reviewer 4 Report
Comments and Suggestions for AuthorsConsidering the European trend towards green energy, it is clear that coal is increasingly undervalued, but it is necessary for research to be continued in the event that coal reserves are again evaluated for energy. Such works on research on the self-ignition of flammable substances are extremely necessary because there are many substances (raw materials, waste) in the industry that must be checked to ensure safety conditions. The work represents a plus for the specialized literature in the field.
Author Response
Considering the European trend towards green energy, it is clear that coal is increasingly undervalued, but it is necessary for research to be continued in the event that coal reserves are again evaluated for energy. Such works on research on the self-ignition of flammable substances are extremely necessary because there are many substances (raw materials, waste) in the industry that must be checked to ensure safety conditions. The work represents a plus for the specialized literature in the field.
Our reaction: We really thank the reviewer for such a nice evaluation of our work.
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors have decided to maintain the introduction in its current form. My view is that it has insufficient background, problem statement, and niche.
If detailed mechanisms cannot be explained in a paper then why write it in the first place?
The reasons for not carrying out surface characterization of the metals powder/particles are not convincing. I think this is important to look at the proposed study. You cannot rely on other people's studies whose sample may be way different from yours. I feel characterization of the particle before and after could improve the quality of the results and manuscript.
Comments on the Quality of English Language
English language is fine.
Author Response
The authors have decided to maintain the introduction in its current form. My view is that it has insufficient background, problem statement, and niche.
Our reaction: In order to respect the reviewer´s comment, the actual text of the Introduction section was (at least slightly) modified, and a new sentence was appended (Lines 31-32).
If detailed mechanisms cannot be explained in a paper then why write it in the first place?
Our reaction: We are convinced that information about the basic mechanistic view of the described phenomena should be available already when reading the text. Links to the original sources are then attached for possible additional details.
The reasons for not carrying out surface characterization of the metals powder/particles are not convincing. I think this is important to look at the proposed study. You cannot rely on other people's studies whose sample may be way different from yours. I feel characterization of the particle before and after could improve the quality of the results and manuscript.
Our reaction: We certainly agree with the reviewer and see the characterization of the particle before and after the tests as motivation for our further investigation in the field. However, in the context of this manuscript, we still consider it appropriate to reflect the character of the particles surface implicitly using the measured values of the oxidation heat.
