Analyzing the Formation of Gaseous Emissions during Aluminum Melting Process with Utilization of Oxygen-Enhanced Combustion

Round 1

Reviewer 1 Report

The issues of the article is important and valid. It is noteworthy that the authors designed and constructed a laboratory-scale tilting rotary furnace and performed combined experimental-modeling analysis of greenhouse gas (GHG) emissions produced during the combustion of natural gas at higher oxygen concentrations in the oxidizer. Two variants of burner design utilizing oxygen-enhanced combustion (OEC) in the aluminum melting process in a rotary tilting furnace were studied. The results they achieved are satisfactory because using the modified burner along with OEC led to around 10 % lower CO2 emissions and around 15% lower total GHG emissions, compared to using a standard air/fuel burner.

The research and calculations carried out by the authors have a high cognitive value. Minor editorial errors do not influence the high rating of the article.

I kindly request the authors to take into consideration comments and answer the questions that arise when reading the article, namely:

• The authors used K-type thermocouples (NiCr-NiAl) PTTK-TKb-60-2-SP 99 to measure temperature at 16 points in the furnace. At what distance from the burner individual thermocouples were placed? The authors should consider to put this information in section 2.
• Line 119 - there is a double comma instead of single one
• Line 165 - in the sentence „Where:?? (…)” there is a lack of space between „where:” and „??”
• The graph in Figure 10 is not legible. The font is too small. I recommend to correct this graph.

The article should be published after considering the comments and answering the question presented in this review.

Comments for author File: Comments.pdf

Author Response

We would like to convey many thanks to the respected reviewer for proposed suggestions and comments from which we could benefit and following which we could improve the quality of our manuscript. We endeavored to implement them properly and a step-by-step response is provided below. All changes in the manuscript resulting from their implementation were documented by the “Track changes” function.

Conclusions: Thank you very much for the Results improvement suggestion. We performed changes in the revised manuscript, please see Results and Discussion sections (4,5) where several figures were substituted or newly added. The corresponding discussion was enriched. Doing this we closely followed the recommendation of the other reviewer as well.

Comments and Suggestions for Authors

The issues of the article is important and valid. It is noteworthy that the authors designed and constructed a laboratory-scale tilting rotary furnace and performed combined experimental-modeling analysis of greenhouse gas (GHG) emissions produced during the combustion of natural gas at higher oxygen concentrations in the oxidizer. Two variants of burner design utilizing oxygen-enhanced combustion (OEC) in the aluminum melting process in a rotary tilting furnace were studied. The results they achieved are satisfactory because using the modified burner along with OEC led to around 10 % lower CO2 emissions and around 15% lower total GHG emissions, compared to using a standard air/fuel burner.

The research and calculations carried out by the authors have a high cognitive value. Minor editorial errors do not influence the high rating of the article.

I kindly request the authors to take into consideration comments and answer the questions that arise when reading the article, namely:

• The authors used K-type thermocouples (NiCr-NiAl) PTTK-TKb-60-2-SP 99 to measure temperature at 16 points in the furnace. At what distance from the burner individual thermocouples were placed? The authors should consider to put this information in section 2.

Answer: Following your recommendation, part 2 has been enriched, please see Figure 2 and Table 1 in the revised manuscript.

• Line 119 - there is a double comma instead of single one

Answer: Thank you for careful reading, issue solved in revised manuscript.

• Line 165 - in the sentence „Where:?? (…)” there is a lack of space between „where:” and „??”

Answer: Thank you for careful reading, issue solved in revised manuscript.

• The graph in Figure 10 is not legible. The font is too small. I recommend to correct this graph.

Answer: Following the recommendation of both reviewers, Figure 10 was deleted, and a textual assessment was included instead.

The article should be published after considering the comments and answering the question presented in this review.

Answer: Thank you for your favorable opinion on our manuscript.

Reviewer 2 Report

Dear Authors,

Your paper is good, however, major improvements are necessary. For reference, see the following comments.

• The English of the paper is understandable, but a significant style update is required. Ask a native in this field.
• L18: Two burner designs; later: baseline and modified burner
• L39: OEC is not a novel technology; it is several decades old.
• The burner geometry and its boundary conditions should be included in this paper.
• The term mathematical model frequently used instead of the proper terms. The geometry in CFD is not a mathematical model. You can omit this term in several cases.
• A standard could be cited instead of Section 3; it can be fully omitted. See, e.g., 10.1016/j.enconman.2017.03.064 Also, always provide the reference O2% when discussing emissions.
• L182: air flow rate
• Fig. 2 and the corresponding discussion can be omitted. This is the direct consequence of Tables 1 and 2.
• Measurement uncertainty is missing. Do not mix it with the accuracy, usually given by the manufacturer of the sensor. See, e.g., 10.1002/9780470485682. Also, add error bars to the diagrams.
• If you are unable to validate the temperature, it is recommended to entirely omit Subsection 4.2 since neither this nor the CO emission was directly compared to the experimental results.
• Please, cite the appropriate references containing the CFD setup. Only discuss your modifications. Otherwise, it is enough to refer to the ANSYS theory and user guides.
• The bottom part of Fig. 10 is not readable. However, the mesh independence study would tell more about your mesh quality.
• Please, compare the temperature data to the numerical results. It is good that the emissions match, however, compare as much data as possible.
• L393: it is enough to refer back to your previous studies in the introduction and shortly summarize the results.
• L419: Your burner is more efficient; it can be quantified by the transferred heat to the ingot. It is not a deNOx technique since you avoid the NOx emission. The deNOx techniques convert NO to N2 and O2.
• Table 4 and Fig. 15 are redundant.
• Please, show the velocity fields in greater detail and discuss the differences.
• Rewrite the conclusions part. You have to include only the results of the current paper. The descriptive nature of this part is rather an introduction-like text. Highlight only the scientific contribution of the present paper, including both CFD and experimental results. Hence, it would be only 1/3 of the current length, but there is no need for more.

Author Response

We would like to convey many thanks to the respected reviewer for proposed suggestions and comments from which we could benefit and following which we could improve the quality of our manuscript. We endeavored to implement them properly and a step-by-step response is provided below. All changes in the manuscript resulting from their implementation were documented by the “Track changes” function.

English language and style

(x) Extensive editing of English language and style required
( ) Moderate English changes required
( ) English language and style are fine/minor spell check required
( ) I don't feel qualified to judge about the English language and style

Answer: The original submission as well as the revised manuscript has been thoroughly checked with regard to the quality of the English language by a native speaker with twenty years of experience in proof-reading scientific research papers in various fields including metallurgy, namely Andrew Jonathan Billingham, BA Dunelm, Joint Hons in Modern Languages, PGCE TESL London, RSA TOEFL London.

Answer: We value the recommendations and comments of the respected reviewer and would like to thank for the presented positive overall impression on our manuscript. Regarding the recommendations for improvement of individual manuscript parts, we provide the following answers:

Introduction: several terms were clarified

Research design: Experimental equipment description was enriched.

Methods description: This manuscript part has been modified according to your recommendation, shifting the generally known and/or less important parts to appendix.

Conclusions: This manuscript part was substantially shortened according your recommendation.

Please see the answers to individual queries provided below for further details.

Comments and Suggestions for Authors

Dear Authors,

Your paper is good, however, major improvements are necessary. For reference, see the following comments.

• The English of the paper is understandable, but a significant style update is required. Ask a native in this field.

Answer: The original submission as well as the revised manuscript has been thoroughly checked with regard to the quality of the English language by a native speaker with twenty years of experience in proof-reading scientific research papers in various fields including metallurgy, namely Andrew Jonathan Billingham, BA Dunelm, Joint Hons in Modern Languages, PGCE TESL London, RSA TOEFL London.

• L18: Two burner designs; later: baseline and modified burner

Answer: Thanks for making us aware of it. We unified this throughout the manuscript and use base and modified burner terms.

• L39: OEC is not a novel technology; it is several decades old.

Answer: Apologies for this mistake. Given sentence was omitted in the revised manuscript.

• The burner geometry and its boundary conditions should be included in this paper.

Answer: Key information has been added in the revised manuscript in form of Table 1 and Figures 2,3.

• The term mathematical model frequently used instead of the proper terms. The geometry in CFD is not a mathematical model. You can omit this term in several cases.

Answer: We decided to use “simulation model” instead of “mathematical model” where suitable. Thank you for this remark.

• A standard could be cited instead of Section 3; it can be fully omitted. See, e.g., 10.1016/j.enconman.2017.03.064 Also, always provide the reference O2% when discussing emissions.

Answer: Following your recommendation we included reference to legislation that sets the general frame for related calculations. Nevertheless, this explanatory manuscript part is important for general audience with less experience with combustion processes and the related technical calculation in our opinion. After careful consideration we decided to leave this manuscript part as it was in the original submission.

Regarding the O2 % in the flue gas, adequate information is already provided in the manuscript: oxidizing agent flow rate was set with respect to fuel flow rate as to reach oxygen coefficient of excess of 1.1. (see Table 2). Air infiltration in the furnace increased the final value to 1.2 as discussed in part 4.2., which corresponds to nearly 4 % vol. O2 in dry flue gas.

• L182: air flow rate

Answer: It has been corrected in the revised manuscript.

• Fig. 2 and the corresponding discussion can be omitted. This is the direct consequence of Tables 1 and 2.

Answer: We value your point of view. Nevertheless, this artwork and the related discussion is important for general audience with less experience with combustion processes and the related technical calculation in our opinion. After careful consideration we decided to leave this manuscript part as it was in the original submission.

• Measurement uncertainty is missing. Do not mix it with the accuracy, usually given by the manufacturer of the sensor. See, e.g., 10.1002/9780470485682. Also, add error bars to the diagrams.

Answer: We understand the reviewer’s point of view. Given the comparatively large size of the furnace and the associated thermal inertia, only one experiment per day could be performed. Completing the experimental measurements in the scope as described in the manuscript took almost one month. Thus, for practical reasons, experiments weren’t repeated. Relevance of the obtained experimental results was checked by their comparison with CFD modelling results.

• If you are unable to validate the temperature, it is recommended to entirely omit Subsection 4.2 since neither this nor the CO emission was directly compared to the experimental results.

Answer: We included the subsection 4.2 in the manuscript because considering the CO emissions released from a non-adiabatic system just as a function of combustion temperature is insufficient. The presented calculation model yielded combustion temperatures in OEC conditions (Figure 6) for adiabatic system; providing a preliminary explanation of the observed CO concentration increase. However, fuel consumption decreased due to higher burner efficiency at the same time. As to provide additional evidence and support for this subsection, we included temperature data measured by thermocouples for basic and modified burner in Appendix A.   

• Please, cite the appropriate references containing the CFD setup. Only discuss your modifications. Otherwise, it is enough to refer to the ANSYS theory and user guides.

Answer: Following your recommendation majority of this manuscript part was moved to Appendix B.

• The bottom part of Fig. 10 is not readable. However, the mesh independence study would tell more about your mesh quality.

Answer: We agree. Given figure was deleted and a concise mesh independence study is presented in textual form. Please refer to lines 466-468in the revised manuscript.

• Please, compare the temperature data to the numerical results. It is good that the emissions match, however, compare as much data as possible.

Answer: Thank you for this suggestion. Comparison of measured temperatures by thermocouples with those obtained from ANSYS is provided in Appendix A.

• L393: it is enough to refer back to your previous studies in the introduction and shortly summarize the results.

Answer: We value your opinion. After a thorough consideration and mutual discussion, we decided to leave this manuscript part unchanged.

• L419: Your burner is more efficient; it can be quantified by the transferred heat to the ingot. It is not a deNOx technique since you avoid the NOx emission. The deNOx techniques convert NO to N2 and O2.

Answer: Thank you for your opinion. We decided to omit the given statement.

• Table 4 and Fig. 15 are redundant.

Answer: Following your comment, we decided to shift given table to Appendix C.

• Please, show the velocity fields in greater detail and discuss the differences.

Answer: New figures 15 and 16 showing the flue gas flow patterns in more detail were created and included in the revised manuscript. The related discussion was enriched, please see lines 554-566 in the revised manuscript.

• Rewrite the conclusions part. You have to include only the results of the current paper. The descriptive nature of this part is rather an introduction-like text. Highlight only the scientific contribution of the present paper, including both CFD and experimental results. Hence, it would be only 1/3 of the current length, but there is no need for more.

Answer: Following your kind recommendation the Conclusions part was shortened with the introductory text being omitted.

Round 2

Reviewer 2 Report

The present reviewer accepts the answers.