Research and Application of CO₂ Fire Prevention Mechanism and Key Technologies in Mines: A Review

Comments 1: Proof-reading is recommended.

Response 1: Thank you for your recognition of this article. According to your suggestions, we carefully checked the full text, polished and proofread the language of the article, and corrected and supplemented the potential problems and deficiencies in the article in combination with the opinions of other reviewers to ensure the rationality, scientificity and readability of the content and structure of the article.

The specific amendments are as follows:

(1) Content supplement

In the part of 2.3 Coal mine CO₂ fire prevention technology, we have five kinds of devices of YZD and DQ, and introduce the two devices in different application scenarios. The specific modifications are shown in lines 130-138. In the 4.2 Downhole injection section, we supplement the composition and working principle of the CO₂ booster pump. The specific modifications are shown in lines 491-497. In the 5.Field application and effect section, we supplemented the application and effect of relevant regulations in the field practice, and the specific modifications are shown in lines 556-583. The specific modification content is marked in red font in the text.

(2) Potential format modification

We have standardized the units in the text, as shown in Table 1, Figure 3 and Table 3; Some of the fonts in the text have been modified and adjusted. The specific modifications are shown in Figure 10 and Figure 11; The description of the device mentioned in the article and the description of the device in the figure are unified. The specific modifications are shown in line 437,470,480,519, Figure 5, Figure 6, Figure 8, etc.in the 4.CO₂ fire prevention technology section. The specific modification content is marked in red font in the text.

(3) Reference format proofreading

We have proofread the format of the reference, and standardized the format of the reference again. See the References section for specific modifications.

Comments 1: In this paper, “China developed a fuel-inert gas fire-extinguishing technology that uses kerosene, removes oxygen by combustion, and produces inert gases, such as CO₂ and N₂. to protect the fire. Simultaneously, five specifications of DQ and YZD products were studied...”  fuel-inert gas s fire-extinguishing technology were reviewed. It is recommended to supplement the specific specifications of DQ and YZD devices and the differences between DQ and YZD to better reflect the comprehensiveness of the article.

Response 1: Thank you very much for your careful review! I fully agree with your precise insights and bring us inspiration. In response to your comments, we have supplemented the specific five specifications of the DQ and YZD devices developed by the fuel inert gas technology, and the two devices are suitable for different application scenarios. We have supplemented them.

The supplementary content is as follows:

In the late 1970s, China developed a fuel-inert gas fire-extinguishing technology that uses kerosene, removes oxygen by combustion, and produces inert gases, such as CO₂ and N₂, to extinguish the fire. Simultaneously, five specifications of DQ and YZD products were studied, DQ series has DQ-150, DQ-500 and DQ-1000 types, YZD series includes YZD-20/700 and YZD-20/5000 types, among which DQ series is mainly used in roadways and other disaster areas with small resistance. YZD series is mainly used in pressure in-jection fire prevention and extinguishing in goaf and other areas, field practices were car-ried out in the Xujiagou and Datong Dadougou coal mines in Henan Province, and a good fire extinguishing effect was achieved [36,37].

References are as follows:

[36] Wu, J.X. Current situation and prospect of inert gas fire extinguishing technology in coal mine in China. Saf. Coal Mines. 1998, 10, 20–22.

[37] Xiao, D.C.; Xu, Z.Y.; Wang, F.Q. DQ-500 type inert gas fire extinguishing device. China Saf. Sci. J. 1991, 3, 35–40.

See lines 130-138 of the 2.3 Coal mine CO₂ fire prevention technology section for additional details.

Comments 2: In the 4.CO₂ fire prevention technology section, lines 449-452 and 476-479 describe Figure 5 and Figure 6 in detail, respectively. Make sure that the name represented by (a) (b) in the description is consistent with the name in the figure.

Response 2: Thank you for your careful review! Thanks for your reminder, we modify the device names referred to in (a) (b) in Figure 5 and Figure 6 respectively to ensure that the (a) (b) device is consistent in both the figure and the text. The 4.1 Surface injection and 4.2 Downhole injection were examined. For the same problem, we unified the pressure meter (P), temperature meter (T), flow meter (Q) and other contents of the full text, including pictures, to ensure the standardization of the article. The specific revisions are shown in lines 437,470,480,519, Figure 5, Figure 6, and Figure 8. The revisions are marked with red fonts in the text.

Comments 3: In the 5 Field application and effect section, the relevant provisions such as ' Coal Mine Fire Prevention and Extinguishing Rules ' and ' Coal Mine Safety Regulations ' are mentioned. It is recommended to briefly describe the application of relevant provisions in field cases to provide support for them.

Response 3: Thank you very much for your careful review and valuable comments. We investigated and reviewed the safety problems in the process of using CO₂ fire prevention and extinguishing technology to prevent and control mine fires. In this paper, we supplemented the field case study and field application test of CO₂ fire prevention and extinguishing technology to explain in detail.

The supplementary contents are as follows:

Tang et al. [19] injected composite inert gas into the goaf of 5306 working face in Tangkou Coal Mine. At the same time, after injecting CO₂ composite inert gas into the goaf, the CO concentration in the return air flow and return air angle of the working face decreased rapidly from 14.9 ppm and 4821 ppm to 0.1 ppm and 21 ppm, respectively. During the gas injection process, the CO₂ concentration in the return air does not exceed 15 %, which ensures the safety of the working face personnel. Cao et al. [62] conducted a fire prevention and extinguishing experiment through an intelligent long-distance pressure-maintaining CO₂ conveying system using the Huoji soil well of Daliuta Coal Mine in Shendong Min-ing Area as an industrial test site. Practice has proved that the concentration of CO de-creased from 790ppm to 41ppm, indicating that liquid CO₂ has a significant effect on fire prevention. In order to explore the intelligent locking performance of CO₂ long-distance pressure-holding transportation, four observation points were set up in the working face in the field application. The CO₂ sensor at the observation point detects that the CO₂ con-centration is higher than 0.8 %, and the CO₂ pressure-keeping transportation system is in-telligently locked, and the ground control terminal of the system also issues an early warning in time. This effectively prevents the problem of CO₂ concentration exceeding the limit during transportation and prevents the occurrence of hypoxic asphyxiation acci-dents. Si et al. [86] established a physical model of goaf by combining numerical simula-tion with field test, and analyzed the safety of CO₂ injection volume to fire area, O₂ concen-tration distribution and CO₂ fire prevention and extinguishing technology. The relation-ship between the effect of CO₂ injection and safety production is balanced. The results show that the distribution of O₂ flow field in goaf changes with the change of CO₂ injection rate. The O₂ between the working face and the injection port is diluted, especially the O₂ at the air inlet is significantly reduced. Therefore, before using CO₂ fire extinguishing tech-nology, the amount of CO₂ injection should be calculated according to the actual situation of the mine, and the gas concentration should be observed at all times during the injection process to ensure the smooth progress of CO₂ fire extinguishing technology and the safety of personnel.

The detailed supplement is shown in 5.Field application and effect lines 555-582, and marked with red font in the text.

References are as follows:

[19] Tang, L.; Qin, Y.D.; Li, X.M.; Wang, J.Z. Coal fire prevention in large areas over long term with a composite inert gas-a case study in Tangkou coal mine, China. Energ. Source Part A. 2019, 1684600.

[62] Cao, N.F.; Liang, Y.T. Mechanism of fire prevention with liquid carbon dioxide and application of long-distance pressure-holding transportation technology based on shallow buried and near-horizontal goaf geological conditions. J. Chem. 2021, 2021, 5572963.

[86] Si, J.H.; Li, L.; Cheng, G.Y.; Shao, H.; Wang, Y.Q.; Li, Z.Q. Characteristics and Saf. of CO₂ for the fire prevention technology with gob-side entry retaining in goaf. ACS Omega. 2021, 6, 18518–18526.

Thank you again for your careful guidance and valuable advice!

Comments 4: On page 17, the name of Figure 11 and the font inconsistency in Figure 10, such as II020205, please correct and check the full text.

Response 4: Thank you very much for your careful review! According to your opinion, we normalized the name of the working face in Figure 10 and Figure 11, and modified the potential format of the text one by one to ensure the standardization of the article. The specific modifications are shown in Figure 10 and Figure 11, which are marked in red font in the text.

Comments 5: The overall format of the reference part of the article is standardized, but there are still some references that are not uniform, such as some of the volume numbers are not italicized and need to be corrected, such as reference 119 109, etc. Please check and modify the reference part comprehensively.

Response 5: Thank you very much for your careful review! According to your opinion, we have carefully checked the reference part. For the problem that some reference volumes do not have italics, we have modified the references [15,16,86,97,109,120]. In addition, we also modified the reference format such as [9,30,41,62,66,89]. The specific modifications are shown in the reference section, which is marked in red font in the text.

Thank you again for your careful guidance and valuable advice!

Comments 1: The author expounds the relevant rules and regulations and operation rules for the safety problems after CO₂ injection operation. It is suggested to further supplement the specific effect evaluation of these regulations after field implementation. It can be elaborated by supplementing relevant field case studies or field application tests. At the same time, it can be considered to supplement the application of the above regulations to improve the safety of operators.

Response 1: Thank you very much for your careful review and valuable suggestions. According to your suggestions, we supplement the field case analysis and practical application test results of CO₂ fire prevention and extinguishing technology for the safety of mine fire prevention and control by using CO₂ fire prevention and extinguishing technology. The detailed supplement is shown in 5.Field application and effect lines 555-582, and marked with red font in the text.

The specific contents are summarized as follows:

Tang et al. [19] injected composite inert gas into the goaf of 5306 working face in Tangkou Coal Mine. At the same time, after injecting CO₂ composite inert gas into the goaf, the CO concentration in the return air flow and return air angle of the working face decreased rapidly from 14.9 ppm and 4821 ppm to 0.1 ppm and 21 ppm, respectively. During the gas injection process, the CO₂ concentration in the return air does not exceed 15 %, which ensures the safety of the working face personnel. Cao et al. [62] conducted a fire prevention and extinguishing experiment through an intelligent long-distance pressure-maintaining CO₂ conveying system using the Huoji soil well of Daliuta Coal Mine in Shendong Min-ing Area as an industrial test site. Practice has proved that the concentration of CO de-creased from 790ppm to 41ppm, indicating that liquid CO₂ has a significant effect on fire prevention. In order to explore the intelligent locking performance of CO₂ long-distance pressure-holding transportation, four observation points were set up in the working face in the field application. The CO₂ sensor at the observation point detects that the CO₂ con-centration is higher than 0.8 %, and the CO₂ pressure-keeping transportation system is in-telligently locked, and the ground control terminal of the system also issues an early warning in time. This effectively prevents the problem of CO₂ concentration exceeding the limit during transportation and prevents the occurrence of hypoxic asphyxiation acci-dents. Si et al. [86] established a physical model of goaf by combining numerical simula-tion with field test, and analyzed the safety of CO₂ injection volume to fire area, O₂ concen-tration distribution and CO₂ fire prevention and extinguishing technology. The relation-ship between the effect of CO₂ injection and safety production is balanced. The results show that the distribution of O₂ flow field in goaf changes with the change of CO₂ injection rate. The O₂ between the working face and the injection port is diluted, especially the O₂ at the air inlet is significantly reduced. Therefore, before using CO₂ fire extinguishing tech-nology, the amount of CO₂ injection should be calculated according to the actual situation of the mine, and the gas concentration should be observed at all times during the injection process to ensure the smooth progress of CO₂ fire extinguishing technology and the safety of personnel.

References are as follows:

[19] Tang, L.; Qin, Y.D.; Li, X.M.; Wang, J.Z. Coal fire prevention in large areas over long term with a composite inert gas-a case study in Tangkou coal mine, China. Energ. Source Part A. 2019, 1684600.

[62] Cao, N.F.; Liang, Y.T. Mechanism of fire prevention with liquid carbon dioxide and application of long-distance pressure-holding transportation technology based on shallow buried and near-horizontal goaf geological conditions. J. Chem. 2021, 2021, 5572963.

[86] Si, J.H.; Li, L.; Cheng, G.Y.; Shao, H.; Wang, Y.Q.; Li, Z.Q. Characteristics and Saf. of CO₂ for the fire prevention technology with gob-side entry retaining in goaf. ACS Omega. 2021, 6, 18518–18526.

Comments 2: The basic supporting literature of Figure 5 and Figure 6 should be provided to ensure its reliability.

Response 2: Thank you very much for your careful review! According to the literature research, we supplemented the references in Figure 5 and Figure 6 to provide support for the pictures and enhance the readability and reliability of the article. Supplementary references are Ref [111,116], and marked with red fonts in the text. As shown below:

[111] Wang, S.M.; Shen, Y.J.; Sun, Q.; Liu, L.; Shi, Q.M.; Zhu, M.B.; Zhang, B.; Cui, S.D. Underground CO₂ storage and technical problems in coal mining area under the "dual carbon" target. J. China Coal Soc. 2022, 47, 45–60.

[116] Fan, S.X.; Wen, H. Chen, X.J.; Zhang, C.R.; Wei, G.M. Research and application of a complete set equipment of permeability enhancements induced by high-pressure L－CO₂ fracturing. J. China Coal Soc. 2020, 45, 801–812.

Comments 3: The overall format of the article is standardized, but there are still some formats to be further standardized. In order to ensure the overall rigor and professionalism of the article, it is recommended to make the following detailed modifications and adjustments: In Figure 2, the annotation of specific historical periods (such as the '19th century') is involved, and it is recommended to follow the upper-corner format in the text for unified processing.

Response 3: Thank you very much for your careful review! According to your opinion, we standardize the writing of the historical period in Figure 2 and unify the full text inspection. The specific modifications are shown in Figure 2.

Comments 4: Some font format editing errors should be modified. For the inconsistency of font format in the text, such as the font of '°C' in Table 1 does not match the text, it should be checked one by one and adjusted uniformly.

Response 4: Thank you very much for your careful review! According to your opinion, we standardize the ° C unit font in Table 1, which is consistent with the font format in the text, and check the full text. The specific modification contents are shown in Table 1, Figure 3 and Table 3. The modified contents are marked in red font in the text.

Comments 5: The standardization of references needs to be further improved, and the format of some references is not uniform. For example, some references lack volume numbers, such as [66], please check and modify according to the requirements of journals.

Response 5: Thank you very much for your careful review! According to your opinion, we have carefully examined the reference part and modified the reference. The specific modifications are shown in the reference [9,15,16,30,41,62,66,86,97,109,120] and so on. See the Reference part in detail, marked in red font in the text.

Comments 1: CSIRO, which appears in line 126 of the 2.3 Coal mine CO₂ fire prevention technology section, is an acronym for Commonwealth Scientific and Industrial Research Organisation. It appears for the first time in the text and should give its full name.

Response 1: Thank you very much for your careful review and valuable comments. As you said, Commonwealth Scientific and Industrial Research Organisation (CSIRO) first appeared in the text, should give full name. The supplementary content is shown in lines 126-127 of the 2.3 Coal mine CO₂ fire prevention technology section, which is marked in red font in this paper. Thank you again for your careful guidance and valuable advice!

Comments 2: Please check and ensure that the parameters, names, etc. appeared in the text are consistent in the full text. For example, in the 3.2 Mechanism underlying CO₂ fire prevention section, 203 lines of CO₂ are misspelled. It is recommended that the full text be checked uniformly.

Response 2: Thank you very much for your careful review! In view of your opinion, we corrected the spelling errors of 203 lines in the text, and modified the potential format one by one. The modified part is shown in line 207 and the Reference part, and marked in red font in the text. In addition, we normalize the unit fonts in Table 1, Figure 3 and Table 3, and the working face names in Figure 10 and Figure 11 to ensure the standardization of the article. The specific modifications are marked in red fonts in the text.

Comments 3: In the 4.2 Downhole injection section, ' The Xi'an University of Science and Technology developed China 's first mine used liquid CO₂ delivery pump, driven by electric power… ' It is recommended to supplement the composition and working principle of the delivery pump to increase the readability of the article.

Response 3: Thank you very much for your careful review and valuable comments! As you said, in order to improve the integrity and accuracy of the article, we supplemented the working principle and main components of the pump in the article.

The supplementary contents are as follows:

The pump is mainly composed of motor, pulley mechanism, cylinder block assembly, piston assembly, sealing assembly, inlet and outlet valve, etc. Its working principle is that the rotation motion of the motor is transformed into reciprocating motion, and the output power of the motor is transmitted to the cylinder block assembly. The motor is successfully converted by belt pulley, eccentric wheel, connecting rod and crosshead. Under this action, the liquid CO₂ in the storage tank can be pressurized by the conveying pump to achieve long-distance high-pressure transportation. The designed mine mobile liquid CO₂ storage tank comprises a liquid storage tank, self-pressurised control system, liquid pipe-line, gas pipeline, safety device, and control valve.

See lines 490-498 of ' 4.2 Downhole injection ' for more details.

References are given in:

[116] Fan, S.X.; Wen, H. Chen, X.J.; Zhang, C.R.; Wei, G.M. Research and application of a complete set equipment of permeability enhancements induced by high-pressure L－CO₂ fracturing. J. China Coal Soc. 2020, 45, 801–812.

Thank you again for your valuable suggestions to improve the readability of our manuscripts.

Comments 4: In Figure.9 and Figure.11, please standardize the CO concentration unit and check the full text.

Response 4: Thank you very much for your careful review and valuable suggestions. According to your comments, we standardize the units in Figure 9 and Figure 11. The main modifications are shown in Figure 9 and Figure 11.

Comments 5: References [15][16][97] and other formats are not unified with others. It is recommended to carefully check the reference format and unify it.

Response 5: Thank you very much for your careful review! According to your opinion, we have carefully examined the reference part, and modified the reference [15][16][97] and so on. The specific modification is shown in the Reference part, which is marked in red font in the text.

Thank you again for your careful guidance and valuable advice!

Comments 1: Partial figures are of poor quality, such as Figure 2. Hence, images of higher resolution should be offered.

Response 1: Thank you for your careful review! According to your opinion, we have reconstructed and modified Figure 2. History of CO₂ fire suppression. At the same time, the full-text pictures were also checked to meet the requirements of journal publishing. The specific modifications are shown in Figure 2.

Comments 2: Expressions in this work must be improved. Errors in format and syntax should be avoided, especially measuring units in Table 1. Punctuate treatment is essential for better understanding. In addition, active and passive voice were improperly utilized.

Response 2: Thank you for your comments! According to your opinion, we standardize the measurement units in CO₂ physical property parameters in Table 1. CO₂ physical property parameters. The potential format and sentence problems in the text are modified one by one and polished by professional academic English polishing institutions to ensure the standardization of the language in the text. The modified content is marked with red font in the text.

Comments 3: Chinese words in Figure 4 should be avoided.

Response 3: Thank you for your careful review! First of all, we are very sorry for our careless mistakes, thank you for your reminder. We modify the Figure 4. Illustration of the mechanism of CO₂ fire prevention, the specific modifications are shown in Figure 4.

Comments 4: Essential supporting literatures for Table 1-3 should be offered for guarantee of reliability.

Response 4: Thank you very much for your careful review! According to the literature research, we add references to Tables 1-3 to provide support for the table and enhance the readability of the article. In addition, we added references to Figure 1, Figure 3, and Figure 4 to ensure the reliability of the literature review. Supplementary references are Ref. [ 22,46,50,63,82,120], and marked with red fonts in the text. Additional references are listed below:

[22] Ding, Y.; Li, S.G.; Zhu, B.; Lin, H.F.; Zhang, J.F.; Tan, J.H.; Chen, W.B. Research on the feasibility of storage and estimation model of storage capacity of CO2 in fissures of coal mine old goaf. Int. J. Min. Sci. Techno. 2023, 33, 675-689.

[46] Chen, H.; Cao, Y.F.; Xing, X.S.; Zou, M.H.; Yu, J.F.; Du, X.Y.; Wang, Y.; Peng, J.L. Experimental study on the variation of physical property parameters of multi-component gases with high CO2 content. 2023 International Conference on Oil and Gas Exploration and Development, Wuhan, Hubei, China, 2023.

[50] Li, Y.F.; Dong, X.Y.; Wang, W.S.; Liu, L.; Zhao, Y.F. Numerical study on heat transfer of dry ice sublimation spray cooling on the surface of micro-ribbed plate. CIESC Journal, 2024, 75, 1830-1842.

……

See the specific supplementary content in the References section.

Comments 5: After injection of CO₂, how about the stability of the inert gases considering the unpredictable rock disturbance?

Response 5: Thank you for your careful review! With regard to the influence of unpredictable rock disturbance in goaf on the stability of CO₂, we can explain it in more detail from the following three aspects:

(1) Due to long-term mining activities, goaf, old goaf and other areas are less affected by mine pressure and mining disturbance, and the pressure situation in the area tends to be relatively stable. Therefore, after the injection of CO₂ gas in this area, the stability of the gas is less affected;

(2) The density of CO₂ is greater than that of air. When injected into the goaf, the CO₂ gas is affected by gravity and will be deposited in the roadway floor area. It is less affected by the wind flow and is not suitable for diffusion, so it has good stability;

(3) CO₂ fire prevention and extinguishing technology has been applied many times in mine fire prevention and control at home and abroad, and has achieved good application results. The relevant cases are listed in this paper. It can be seen that the '5.Field application and effect' part of this paper analyzes the effect of CO₂ fire prevention and extinguishing application cases.

In summary, from the pressure stability of the goaf or the old goaf, the characteristics of CO₂ itself and the field application effect, it is shown that CO₂ has good stability.

Thank you again for your careful guidance and valuable advice!

Comments 1: It is suggested to provide some information about global energy consumption in the introduction, and link it with China's energy consumption, and summarize the key points and difficulties of coal spontaneous combustion prevention and control, which will increase the readability of the article.

Response 1: Thank you for your comments. According to your suggestions, we supplement the global energy consumption information and the key and difficult points of coal spontaneous combustion prevention and control in the '1.Introduction' section. The supplementary part is on the first page 29-32 lines, 34-40 lines, and supplements the relevant references, see Ref. [2,5,6]. The following is added:

Coal remains one of the most important fossil fuels used in power engineering, metallurgy, the chemical industry, and other fields. Coal accounts for 68.8% and 57.7% of primary energy production and consumption structures, respectively. Coal is expected to become the primary energy source in China for a long time ……. Spontaneous coal combustion affects various countries, including Germany, the United States, Australia, South Africa, Poland, the Czech Republic, India, Pakistan, and Indonesia. This problem has long plagued the global coal industry. In Germany, the Ruhr industrial area suffers from approximately 10 mine fires caused by the annual spontaneous coal combustion. In India, spontaneous coal combustion fires account for 75% of all coalfield fires, and the fire situation in the Jharia Coalfield is particularly severe.

Please refer to '1.Introduction' for the additional part marked in red font.

Comments 2: Use a larger font to present your graphics, some words can not be seen clearly, such as Figure 2, it is recommended to deal with the whole manuscript uniformly.

Response 2: Thank you for your careful review! According to your opinion, we have reconstructed and modified Figure 2. History of CO₂ fire suppression. At the same time, the full-text pictures were also checked to meet the requirements of journal publishing. The specific modifications are shown in Figure 2.

Comments 3: In Table 1 CO₂ physical property parameters, the MPa unit style is not uniform enough, and it is recommended to check the full text uniformly.

Response 3: Thank you very much for your careful review! According to your opinion, we standardize the measurement units in Table 1. CO2 physical property parameters and check the full text.

Comments 4: Professional terms should be unified. gob and goaf mean the same in the article. It is suggested that they be handled uniformly throughout the manuscript, even in the charts.

Response 4: Thank you very much for your careful review, according to your opinion, we in the text for the goaf language expression has carried on the unification processing.

Comments 5: Syntax errors exist on page 4 (132-134) and page 12 (461-464).

Response 5: Thank you very much for your careful review. According to your opinion, we revise the grammatical errors on page 4 (132-134) and page 12 (461-464). The specific revisions can be found in the ' 3.1.1Physical and chemical properties of CO₂ ' section, lines 168-171, and ' 4.3Dry ice phase change fire prevention' section, lines 496-499. The modified part is marked in red font, and the full text language is rewritten and polished to ensure the standardization of the language. The specific modifications are as follows:

It does not react with other substances but can react with strong reducing agents at extremely high temperatures. In addition, from a combustion perspective, CO₂ itself cannot burn and does not support combustion; therefore, CO₂ is widely used as a fire-extinguishing agent in various fields.

Liquid CO₂ is unstable and, hence, can be easily gasified. Liquid CO₂ has high pressure and temperature requirements for the storage tank, whereas dry ice is stable and easy to transport. Therefore, some scholars have proposed placing dry ice in the goaf and using the characteristics of dry ice sublimation to prevent and extinguish fires.

Comments 6: In Table 2 Comparison table of fire prevention performance parameters of CO₂ and N₂, the last column of difference, the difference between the two lies in the comparison with which, it is recommended to verify and modify.

Response 6: Thank you very much for your careful review! In response to your comments, we have revised the table header of Table 2. Comparison of fire prevention performance parameters of CO₂ and N₂. Where there are objections, clarified the content of Table 2, and added references to Ref. [81,82], and marked it in red font in the text. The modified part is shown in Table 2.

Comments 1: The article concerns the technology and techniques of reducing spontaneous coal combustion in goafs of working faces and fighting fires in these goafs. The authors focus mainly on transport technology and the injection of CO₂ into goafs in various phases. They devote too little attention to the separation of prevention of coal self-heating during the advance of a working face and after stopping and sealing a working face as a result of a fire. These are two separate issues of technological use of inert gases in a coal mine.

Response 1: Thank you very much for your careful review and valuable comments. As you said, there are differences in the process layout of inert gas fire prevention and extinguishing technology during the advancement of the working face and the process of stopping and sealing the working face due to fire. It should be two independent issues, which we deeply agree with. However, considering the length of the article and the arrangement of the chapter setting, we take the development of CO₂ fire prevention mechanism and its technical process as the research focus of this paper. The application scheme and application effect of this technology in different scenarios are supplemented by the evaluation of the application effect of actual cases. This is because we believe that the principle and process of CO₂ fire prevention and extinguishing technology have good applicability in various application scenarios. Therefore, the above content is not shown in the text, leading to your questions. Specifically, we introduce three existing fire prevention and extinguishing processes at home and abroad in the text ' 4.CO₂ fire prevention technology', including ground pressure injection, cryogenic pressure injection, and dry ice phase-change systems. In the '5.Field application and effect' section of the paper, the field application case and result analysis of CO₂ fire prevention and extinguishing technology are introduced in detail. Thank you again for your careful guidance and valuable advice!

Comments 2: An additional weakness of the manuscript is the lack of emphasis on the safety of using CO₂ and its outflows from goafs of longwall faces, which is related to the geological conditions of the roof rocks and the technology of a working face. In the opinion of the reviewer, the authors pay too little attention to the safety associated with the concentration of CO₂ in the air at a corner of a working face. The authors rightly write in the conclusions that there still needs to be more understanding of the field personnel, which limits the wide application of CO₂ fire prevention technology. This view exists in mining in many countries around the world. It is justified.

Response 2: Thank you very much for your careful review! I fully agree with your precise insights and bring us inspiration. In response to your comments, we have insufficient attention to safety after CO₂ injection. Through research, we have supplemented the relevant regulations and rules of inert gas fire prevention and control in the text '5.Field application and effect'. For example, the " Coal Mine Safety Regulations " promulgated by the State Administration of Work Safety of China should continuously observe the parameters such as gas and temperature in the fire zone when applying CO₂ fire prevention and extinguishing technology, investigate the fire extinguishing effect, and improve the fire extinguishing measures until the fire zone reaches the extinguishing standard. During the perfusion, the CO₂ concentration is monitored at all times, and the carbon dioxide concentration in the inlet air flow exceeds 0.5 % or the carbon dioxide concentration in the return air flow exceeds 1.5 %, so as to ensure the safety of personnel. In addition, it is necessary to strengthen the training of coal mine workers on rescue plans, disaster avoidance routes and self-rescue and mutual rescue knowledge, and ensure that underground personnel are skilled in using self-rescue devices and emergency shelter facilities. The specific additions are as follows:

CO₂ concentration can be used as a key parameter to reflect the degree of risk to the goaf, corner of the working face, closed wall, and other areas. At the same time, it can be used as an important index to investigate the effects of fire prevention and extinguishing to effectively protect the safety of underground workers. In China, the 'Coal Mine Fire Prevention and Extinguishing Rules' issued by the National Safety Supervision Bureau stipulate that ‘When CO₂ fire prevention is adopted, the oxygen concentration in the return corner of the working face must be monitored. After the spontaneous combustion fire in the goaf of the working face is closed (or other closed areas where spontaneous combustion fire occurs), CO₂ is continuously injected into the closed area, and the concentration of carbon dioxide in the inlet and return air flow of the coal mining face must be monitored. When the carbon dioxide concentration in the inlet air flow exceeds 0.5% or the carbon dioxide concentration in the return air flow exceeds 1.5%, the perfusion must be stopped, the personnel must be evacuated, measures must be taken, treatment must be carried out, and the oxygen concentration in the closed area must not be greater than 5.0%.’

See line 518-544 on page 15 of '5.Field application and effect'. The additional content is marked in red font. See Ref. [121,122] for relevant references.

So, we conducted a survey on the safety issues after the application of CO₂ fire prevention and extinguishing technology in the field and the injection of CO₂:

Si et al. [1] analyzed the safety of CO₂ injection rate on fire zone, O₂ concentration distribution and CO₂ fire prevention and extinguishing technology by establishing a physical model of goaf by combining numerical simulation with field test. The relationship between CO₂ injection effect and safety production is balanced. The results show that the distribution of O₂ flow field in goaf changes with the change of CO₂ injection rate. The O₂ between the working face and the injection port is diluted, especially the O₂ at the air inlet is significantly reduced. Therefore, before using CO₂ fire prevention and extinguishing technology, the amount of CO₂ injection is calculated according to the actual situation of the mine, and the gas concentration is observed at all times during the gas injection process to ensure the smooth progress of CO₂ fire prevention and extinguishing technology and the safety of personnel.

Taking Huoji Tujing of Daliuta Coal Mine in Shendong Mining Area as an industrial test site, Cao et al. [2] carried out fire prevention and extinguishing experiments through an intelligent long-distance pressure-maintaining CO₂ conveying system. Practice has proved that the concentration of CO decreased from 790 ppm to 41 ppm, indicating that liquid CO₂ has a significant effect on fire prevention. In this paper, in order to explore the intelligent locking performance of CO₂ long-distance pressure-keeping transportation, four observation points were set up on the working face in the field application. The CO₂ sensor at the observation point detected that the CO₂ concentration was higher than 0.8 %, and the CO₂ pressure-keeping transportation system was intelligently locked, and the ground control terminal of the system also issued an early warning in time. This effectively prevents the problem of CO₂ concentration exceeding the limit in the transportation process and prevents the occurrence of low oxygen asphyxiation accidents.

The compound inert gas was injected into the goaf of 5306 working face of Tangkou Coal Mine by Tang [3]. At the same time, after the CO₂ compound inert gas was injected into the goaf, the CO concentration in the return air flow and return air corner of the working face decreased rapidly from 14.9 ppm and 4821 ppm to 0.1 ppm and 21 ppm, respectively. During the gas injection process, the CO₂ concentration in the return air flow never exceeded 15 % to ensure the safety of the working face personnel.

[1] Si, J.H.; Li, L.; Cheng, G.Y.; Shao, H.; Wang, Y.Q.; Li, Z.Q.; Characteristics and Safety of CO2 for the Fire Prevention Technology with Gob-Side Entry Retaining in Goaf. Acs Omega. 2021, 6, 18518-18526.

[2] Cao, N.F.; Liang, Y.T. Mechanism of Fire Prevention with Liquid Carbon Dioxide and Application of Long-Distance Pressure-Holding Transportation Technology Based on Shallow Buried and Near-Horizontal Goaf Geological Conditions. Int J Min Sci Techno. 2021, 5572963.

[3] Tang, L.; Qi, Y.D.; Li, X.M.; Wang, J.Z. Coal fire prevention in large areas over long term with a composite inert gas—a case study in Tangkou coal mine, China. Energ Source Part A. 2019, 1684600.

Therefore, in the process of using CO₂ fire prevention and extinguishing technology to prevent and control mine fires, it is necessary to continuously monitor the concentration of CO₂, O₂, CO and other gases in the fire area to ensure that the low oxygen environment sufficient to contain the fire is maintained. The oxygen content in the air in the working area (i.e., the working face) is above the safety threshold, so as to effectively avoid the low oxygen risk to the field personnel.

Thank you again for your careful guidance and valuable advice!

Comments 3: Therefore, it may be worth going back to the 1970s and 1980s to familiarize themself with the rich international literature on fighting fires using CO₂. The authors are encouraged to review conference materials on technologies for inerting the atmosphere in goafs using CO₂ and N₂. Unfortunately, the manuscript does not mention the widely developed technology of using CO₂ generated by jet engine inertisation units developed for use in mines, controlling and suppressing coal seam fires, and neutralizing firedamp situations. For example, the authors do not mention the GAG device at all, which was used in mining in many countries, e.g., Australia, New Zealand, or the USA, when extinguishing mine fires. A GAG unit was developed by the Queensland Mines Rescue Service, in association with the CSIRO. For example, GAG units have been used in Australia since 1998 and earlier in European countries. GAG has been withdrawn in some countries due to the threat of CO₂ leaks into mine ventilation airways. The beginning of the 21st century saw the development of technology for delivering inert gases through pipelines, primarily nitrogen.

Response 3: Thank you very much for your careful review and valuable comments! As you said, due to our literature research is not comprehensive enough, leading to the article for the international CO₂ fire prevention technology review is not comprehensive enough, we are deeply sorry for this. In order to improve the integrity and accuracy of the article, we investigated and reviewed the international literature and related materials of CO₂ fire prevention and extinguishing, and supplemented the related technologies and applications of CO₂ fire prevention and extinguishing in the article. Through the domestic and foreign literature and data research of GAG device, we supplement the principle, development process and related field application of GAG device, as well as the principle, equipment and field application of fuel inert gas device. The specific additions are as follows:

In addition, Poland developed a GAG system device in 1970, which has been applied to many coal mine fires and tests in Australia, the United States, and New Zealand, among other countries. Górnicze Aparaty Gaśnicze (GAG) uses military jet engines to produce high-humidity inert gases (a mixture of mainly CO₂, N₂, and water vapour) to reduce the oxygen concentration in the fire zone, thereby preventing and controlling mine fires. The Queensland Mine Rescue Service (QMRS) and CSIRO jointly developed a GAG device and applied it to mines, such as the Loveridge mine in West Virginia, the Pinnacle mine in West Virginia, the Loveridge mine in West Virginia, and the Pike River mine in New Zealand, and successfully extinguished mine fires. In the late 1970s, China developed a fuel-inert gas fire-extinguishing technology that uses kerosene, re-moves oxygen by combustion, and produces inert gases, such as CO₂ and N₂, to extinguish the fire. Simultaneously, five specifications of DQ and YZD products were studied, field practices were carried out in the Xujiagou and Datong Dadougou coal mines in Henan Province, and a good fire extinguishing effect was achieved.

The detailed supplementary content is shown in lines 120-134 on page 4 of '2.3 Coal mine CO₂ fire prevention technology'. The references can be seen in Ref. [31,32,33,34,35,36,37], and the supplementary part is marked in red font in this paper.

Through literature research, this paper gives examples of the application cases of CO₂ fire prevention and extinguishing technology in coal mines in the United Kingdom, the United States and other countries, and has achieved good results. The supplementary contents are as follows:

In the 1890s, the Sandwell Park and Abram coal mines in the United Kingdom used CO₂ to extinguish underground fires. The Westfield, Smith, and Brumaugh Bureaus of Mines used CO₂ to extinguish the fire broke out at the Valier mine in the United States in 1949 and successfully extinguished the fire by injecting CO₂ gas and liquid CO₂ into the fire area. In 1957, CO₂ in the form of dry ice in the Penokee mine successfully extinguished mine fire. In the 1959 fire that broke out at the Koehler mine in the United States, liquid CO₂ and dry ice were used to extinguish and successfully control the fire.

For details, see '2.3 Coal mine CO₂ fire prevention technology', page 4, lines 140-147, and additional references see Ref. [38,39,40,41,42].

Thank you again for your valuable advice, which has improved the readability of our manuscripts.

Comments 4: In the context of the history of nitrogen use technology, it is worth noting that before N₂ was used in the Roslyn Coal Mine in 1953, gaseous nitrogen was injected into the goaf in the Doubrava Dùl mine in the Czech Republic as early as 1949. The development of technology for using nitrogen to fight underground fires occurred in the 1940s and 1950s, not the turn of the 1960s and 1970s, as the authors write.

Response 4: Thank you very much for your careful review and valuable suggestions. First of all, I want to express my admiration for your deep professional knowledge in the field of coal spontaneous combustion prevention and control technology. Due to our lack of in-depth literature research, there are deficiencies in the review. Thank you for your reminding, let us find this error in time. Therefore, we investigated the historical background of nitrogen fire prevention technology and modified the text. The specific content of the modification is shown in lines 108-109 on page 3, and the supplementary references are Ref. [25]. The supplementary part is marked in red font in the text. The modification is as follows:

In 1949, the Doubravave coal mine in the Czech Republic first used N₂ to extinguish fires.

Thank you again for your careful guidance and valuable advice!

Comments 5: The article could be published, but the literature review in this area is poor and concerns only coal mining in China. Therefore, the article should be published in regional journals rather than global journals. In order to be published in international journals, one must have a much broader knowledge of the literature and a well-characterized background in the use of inert gases. It is also recommended that one review prestigious mining scientific journals on using inert gases.

Response 5: Thank you very much for your careful review and valuable suggestions. First of all, thank you very much for your interest in our research results, and pointed out the shortcomings of the article. We agree with your views and thank you for reminding us of this issue. According to your suggestion, we conducted a survey of international literature on different time periods. These literatures cover India, Germany, the United States, Australia and other countries, and are closely related to CO₂ fire prevention and extinguishing technology, including important technical data and research results. Through these literature reviews, the article is supplemented. The specific supplementary contents are marked in red fonts in the text. The supplementary literature is as follows:

[5] Biswal, S.S.; Gorai, A.K. Change detection analysis in coverage area of coal fire from 2009 to 2019 in Jharia Coalfield using remote sensing data. Int. J. Remote Sens. 2020, 24, 9545–9564.

[6] Muduli, L.; Jana, P.K.; Mishra, D.P. Wireless sensor network based fire monitoring in underground coal mines: a fuzzy logic approach. Process Saf. Environ. 2018, 113, 435–447.

[25] Mohalik, N.K.; Singh, R.V.K.; Pandey, J.; Singh, V.K. Application of nitrogen as preventive and controlling subsurface fire—Indian context. J Sci Ind Res India. 2005, 64, 273-280.

[27] Jürgen, F.B.; Saqib, A.S. Prevention of gob ignitions and explosions in longwall mining using dynamic seals. Int. J. Min. Sci. Techno. 2017, 27, 999–1003.

[31] Watkinson, M.; Liddell, K.; Muller, S.; Gido, M.; Nissen, J. A practical research study into the environmental and physical impacts during an underground mine inertisation with the GAG jet. Report C23006, ACARP; 2014.

[32] Bell, S.; Cliff, D.; Harrison, P.; Hester, C. Recent developments in coal mine inertisation in Australia. In: Coal 1998: Coal Operators’ Conference. University of Wollongong & the Australasian Institute of Mining and Metallurgy. 1998, 701–717.

…….

[38] Hatakeyama, T.; Aida, E.; Yokomori, T.; Ohmura, R.; Ueda, T. Fire extinction using carbon dioxide hydrate. Ind. Eng. Chem. Res. 2009, 8, 4083–4087.

[40] Westfield, J.; Brumbaugh, H.S.; Whittaker, R.W. Extinguishing fire with carbon dioxide in the Valier Mine, Valier Coal Co., Valier, Franklin County, Tllinois. USBM Information Cireular 1950, 7563.

[41] Haller, F.J.; Michels, F.G. Fighting a mine fire with carbon monoxide. Min. Cong. J. 1957.

[42] Moriss, R.E. A review of experiences on the use of inert gases in mine fires. Int. J. Min. Sci. Technol. 1987, 6, 37–69.

Specific references are Ref. [5,6,25,27,31,32,33,34,35,38,40,41,42,43].

Thank you again for your careful guidance and valuable advice!

Comments 6: First of all, English disqualifies this article in terms of language. The article must be rewritten linguistically correctly. Many terms in mining terminology need to be correctly formulated. Examples include unclear terms such as 'the ground may partially appear', 'very demanding and hard demand', 'support borehole', 'injection of liquid carbon dioxide into the goaf...through the return air lane', and many others. A total revision of the manuscript is required.

Response 6: Thank you very much for your careful review and valuable suggestions. It is an honor to get your guidance, which is of great help to improve my research quality and academic expression ability. I fully understand and accept your suggestion of rewriting the language of the article. I realize that clear and accurate English expression is very important for the spread and communication of scientific research. Therefore, according to your opinion, we have modified the language errors in the text one by one, and marked the modified part with red fonts. At the same time, it is handed over to the professional academic English polishing agency for polishing, followed by polishing proof. These changes will not affect the content and framework of the paper, we hope that the revised article can be accepted by you.