Review Reports - Parameters Affecting the Explosion Characteristics of Hybrid Mixtures Arising from the Use of Alternative Energy Sources

Round 1

Reviewer 1 Report (Previous Reviewer 2)

Comments and Suggestions for Authors

The authors have finally addressed all my comments. The paper could be now considered for publication as it is.

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

The article deals with a very interesting and current topic. In fact, hybrid mixtures are not very well studied, therefore the results obtained in this paper are welcome for establishing a good literature base in this field. Hybrid mixtures are very dangerous due to enhanced explosive parameters. The more they are studied, the more awareness can be spread about the dangers and protective measures that can be taken. On the other hand, there are only a few research collectives worldwide that can perform such experiments with hybrid mixtures, so the experimental results are welcome. The paper is well written and should be easily published.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper investigated the explosion characteristics of hybrid mixtures arising from the use of alternative energy sources. I consider this to be very topical and important for practice and topic is relevant to the journal Fire, but the paper requires extensive English editing.

1. The terminology used is not correct, giving the manuscript a sense of non profesionality because these parameters are widely used in the field e.g. solid dispersion, pressure wave transmission in explosions, agitation of the dust dispersion explosion wave growth rate ...

2. Authors in the article use term 0.02 m3 chamber instead of 20L chamber - which is commonly used and is in also the international standard for dust explosion testing. 3. 51 – 56 - Computer modeling is not the subject of the authors' research, there is no need to include this information in the introduction

4. 177 – correct a typo in the text „at at atmospheric“

191 - the volume is approximately 0.02 m3 – Please explain why approximately? 222 - Table In Table 1, chemical initiator is incorrect terminology, it is a chemical igniter Fig. 7-10 different font styles are used in the description, they should be unified Fig. 7,8 - the word “jiskra” is used - needs to be corrected

355-359 - It is difficult to understand the context. Maybe break the sentence, the way it is now it is not easy to understand.

Comments on the Quality of English Language

The paper requires extensive English editing.

Author Response

Dear reviewer,

the authors thank you very much for all your comments to the article. They tried to incorporate your comments into the article and at the same time, they added the explanations to the individual comments.

Sincerely,

Authors

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents the results of comparative experimental studies of the constant-volume explosion characteristics of some selected hybrid mixtures composed of gas (syngas, hydrogen, or methane) and dust (a solid byproduct of biomass gasification (char), or corn starch), using different initiation sources (spark discharge or chemical igniter) and initial temperatures (from normal to 100 C). It fits the scope of Fire but must be rejected in the present form. My detailed comments are given below.

(1) The experimental paper must contain the error analysis, which is completely missing.

(2) In addition to Fig.3 showing the particle size distribution for corn starch, the authors must present a similar figure for the biomass char dust. Also, it would be instructive to have the elemental composition of char, moisture, etc. (i.e., its ultimate and proximate analyses) to estimate its heating value.

(3) The given composition of syngas (8 % CO2, 10 % H2, 29 % CO, 4 % CH4, 45 % N2, 1 % O2) is lacking some 3% of other substances. What are they? Is it the original syngas or its surrogate? If original, how is it produced and what is exactly the feedstock?

(4) For the sake of clarity, the authors must provide the definitions of Pmax, (dP/dt)max, and K, as well as the exact way they determine these values. For this purpose, they must include the primary pressure records (time histories) in the manuscript for all three types of explosions, gaseous, dust, and hybrid. It is always important to see the primary data and the way they are processed.

(5) It could be also instructive to see the snapshots/frames of the hybrid flame appearance, once the authors mention the optical access to the explosion chamber. It could help understanding the hybrid mixture uniformity, flame shape, etc.

(6) The authors must explain how they measure the ignition energy of the chemical igniter (50 kJ) and the spark discharge. What is their estimate for the energy consumed by ignition itself?

(7) It is not clear why the authors plot the MEC (minimum explosion concentration) along the X-axis of Figures 5 to 12. It must be something else, like mass concentration of dust or so.

(8) In the literature, there are many data on Pmax, (dP/dt)max, and K for gaseous hydrogen and methane. The authors must compare their own data with those available in the literature to see the difference. For example, when I look on the data obtained for methane - air explosions in the 20-l bomb at normal conditions, I see a very large difference between the present authors' results and the results by Pekalski and many other researchers (see Fig. 2 in doi:10.1205/psep.04211), in particular for methane concentrations larger than 16 vol.%. At [CH4] = 16%, the present authors surprisingly measure 5.2 < Pmax <6.8 bar, whereas all other researchers measure Pmax = 1 bar. Moreover, the maximum of Pmax in the measurements of other researchers is attained for the near-stoichiometric mixture with [CH4] = 9-11%, whereas the present authors have it surprisingly at [CH4] =13-15%. What is the reason for such a large difference? This also applies to hydrogen to some extent if one compares the available data obtained by other researchers with the data of the present authors.

(9) The legend "jiskra" in Figures 7 and 8 must seemingly be replaced by "spark"?

(10) The conclusions look not much substantiated by the data presented by the authors.

Author Response

Dear reviewer,

Sincerely,

Authors

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This work deals with hybrid mixtures. It is a topic that deserves research and the results are interesting. However, significant improvement is needed:

1.- Further information about the Materials and Methods is required:

- Type of injection nozzles for dust in the 1 m3 and the 0.02 m3 vessels.

- Type of pressure sensors (model, manufacturer) in both vessels and their pressure range and error.

- Pressure and volume of air used for dust injection into the vessels.

- Indicate ignition delay in gas/dust/hybrid experiments and for both volumes.

- Provide some information about the tested materials from the gasification technology. Was it input or output material or material from some intermediate phase? What was the original material?

2.- Lines 366-372 needs further explanation, which should be linked with the ignition delay values to be reported (see comment 1 above). Are the levels of turbulence equivalent between the dust/gas/hybrid explosions? This aspect is key and discussion should be improved.

3.- There is a lack of recent relevant references and this affects the quality of introduction and discussion sections.

- Improve Introduction by adding recent studies, both in hybrid and in dust/gas explosions (see also comment 5)

- Improve discussion by comparing results and trends with previous works by other researchers. In this sense, statement in lines 417-418 should be moved to the Discussion, should be enlarged and should be supported by references.

4.-It is not really clear why, according to authors, the 1.00 m3 explosion chamber is more suitable for hybrid explosions. The issue of turbulence is not clear (see ignition delay above), especially considering that neither the turbulence level nor the dust/gas concentrations in the vessels have been measured.

5.- Other comments:

- EN standards in Abstract are wrong.

- Though reference [5] is interesting, it is obsolete when referring to computational modelling.

- Lines 52-56, "Validation of the various models...": Indicate which models and provide references. Again, "However, the degree of general validity of the model...", indicate which model and provide reference.

- "autoclave" is a term that is not used in this science field. Use vessel, chamber, equipment, etc.

- Lines 163-165, "The explosion chamber with a volume of 1.00 m3 is one of the superior ones at the level of the Czech Republic and the European Union". What is the meaning of "superior"? Authors does not mention that there exist other much larger vessels and vessels with other interesing features in Europe that have been used for explosion research. Most of them can be found in scientific literature. It is expected that some of them appear in the Introduction.

- Quality of Fig. 1 (resolution of text in boxes) should be improved.

- Particle size distribution of corn starch is quite unusual, with several peaks and an abnormal part on the left side of the graph. This needs further comment.

- Fig. 5 and following figures, correct "concetration". Detail if this concentration axis only applies or not to the gas explosions' results.

- Fig. 7 "jiskra"?

- Avoid abbreviations "pc" and "pcs"

- Review statements in lines 350-353

Author Response

Dear reviewer,

Sincerely,

Authors

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Author Response

Dear reviewer,

Sincerely,

Authors

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors The authors have made all required corrections to the article, so I recommend the article for publication in the Fire journal.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed most of my comments but the manuscript still contains dubious results and must be rejected at this stage. The problem is that the authors do not explain large differences with the measurements of multiple other researchers for the seemingly simplest experiments with pure gaseous mixtures. The authors explained these differences by the error in the fuel concentration scale. However, this explanation does not improve much the correlation of their results with the results of others. I treat this point as a lack of validation of the authors’ approach. In view of it, all other results presented by the authors, in particular those for complex hybrid mixtures, look dubious. To go further, the authors must carefully reproduce the results of other researchers obtained in the same apparatus at the same conditions. This will provide the grounds to trust their measurement results.

Also, despite the authors have made many changes in the revised manuscript, some of the deficiencies mentioned in my previous comments were not corrected completely (jiskra, etc.). Regarding the measurement errors, I am not satisfied with the authors response. Instead of providing long explanations regarding the existing standards, the authors could simply provide the data on the reproduction of their results for the same experimental conditions to see the scatter and to estimate the experimental uncertainty. This is not done: the authors seem to provide data only for individual experiments.