Risk Assessment of the Large-Scale Hydrogen Storage in Salt Caverns

Round 1

Reviewer 1 Report

The article is well written, contains interesting research results and, above all, an interesting analysis of underground hydrogen storages. Below I present some small remarks.

1. To the data contained in Table 1, it should be written where the analyzes were performed and who performed them.
2. In the second Chapter, it should be written a few sentences about the dimensions of the salt cavern for hydrogen storage (length, width, height).
3. In the third Chapter, it should be added two or three sentences why years 2.5, 21 and 30 were selected.
4. Line 231, please use the same pressure units.
5. In the conclusions, please add a few sentences directly related to the research results, with an indication for risk assessment.

Author Response

Reviewer: 1

The article is well written, contains interesting research results and, above all, an interesting analysis of underground hydrogen storages. Below I present some small remarks.

Answer of Reviewer #1

We wish to thank Reviewer #1 for her/his helpful comments and for the time spent on revising the manuscript. All the Reviewer’s suggestions have been taken into account, and some corrections have been made to the manuscript.

1. To the data contained in Table 1, it should be written where the analyzes were performed and who performed them.

The data contained in Table 1 were obtained from NIST database. To clarify it, the citation to the database has been added in the table caption.

2. In the second Chapter, it should be written a few sentences about the dimensions of the salt cavern for hydrogen storage (length, width, height).

Some information about typical dimensions of salt caverns were given in  section 1 (Lines 40-44). In Section 2, we defined the dept (820 m) and we added the volume of the salt cavern of Bad Lauchstadt.

3. In the third Chapter, it should be added two or three sentences why years 2.5, 21 and 30 were selected.

We modified the text to clarify our choice and inserted few sentences in Section 2 (Lines 132-137 of the marked manuscript):

“These times were chosen in order to have three different scenarios. At low times, the higher hydrogen content is present in the stored gas. After 21 years, the hydrogen content is equal to the methane content while, after 30 years, the maximum contents of methane and, above all, hydrogen sulfide are present. H₂S introduces toxic release problems both as it is but also oxidized to SO₂.”

4. Line 231, please use the same pressure units.

Done.

5. In the conclusions, please add a few sentences directly related to the research results, with an indication for risk assessment.

We thank the reviewer for the suggestion and modified the Conclusions section.

Author Response File: Author Response.docx

Reviewer 2 Report

Major Comments:

In this study, the authors have carried out a quantitative risk assessment on salt caverns used for hydrogen storage. The research project is very well structured, the ideas are clear, and the writing is concise and argumentative. The language used in this paper was technical and terms were explained clearly. The literature is outdated for the paper and should be from the recent 5-10 years. The models used in this study are not an exact fit for the hydrogen data. The models such as Van Buijtenen’s model are not exactly developed for the hydrogen release and might be inaccurate over a long period of 30 years. A mixture of other gases also forms by the reaction of hydrogen results in making the models more inaccurate. The assumptions were used in the worst-case scenario that only counts for the maximum mass flow rate. In the results and discussion section, the results of models are finely divided into sections. The few improvements needed in results are the validity of the models, verification of the data, assumptions validity, and accuracy of the conditions. The assumptions in models can make them very inaccurate as it prolongs over a period of 30 years making them highly inaccurate. There is a huge gap in the prediction of parameters as after the prediction of 2.5 years, the models directly jump to 21 years. The overpressure cannot be predicted by only 3 values with such gaps as shown in Figure 3. The main problem in hydrogen storage can also include the pressure gaps and irreversible stresses that will also have a great impact on safety. It is obvious that this study needs more accurate predictions and improvements before it could be considered for publication in Energies Journal.  

Introduction:

The introduction is lacking sufficient background information that is unable to give the reader detailed background knowledge and possible wide application of this study. The introduction needs to be more emphasized on the research work with a detailed explanation of the whole process considering past, present, and future scope. The conventional fuels and technologies need to be explained well to indicate the relevance of the research work. It needs to be strengthened in terms of recent research and updated literature review in this area with possible research gaps. It is strongly recommended to add a recent literature survey about renewable fuels, climate change, recent global warming trends and the role along with the wide range of applications. How these sustainable fuels affect the current levels of CO₂ and alarming global warming issues? Research gaps should be highlighted more clearly and future applications of this study should be added.

 Methodology:

The methodology needs some changes, and the current data is limited in some respects. The data was obtained from Hemme and van Berk, some other source could also be used to make a good comparison as a single source is not viable for models. A lot of assumptions and theoretical values are used for the calculations like valve closure duration of 1 min and adiabatic expansion. For the calculation of radiant flux, the values of parameters should be given. Van Buijtenen’s model is used for the depiction of the release of TNT cloud, and will not depict the explosion of hydrogen as both have different release patterns.

Specific Comments:

1. Please combine the results and discussion section under one heading “Results and Discussion” for a better understanding of readers.
2. Please remove “I”, “we”, ours and “us” words from all manuscript. The revised manuscript should be read and corrected by a native English speaker before resubmission.
3. Page 1, Line 34: “costs per megawatt-hour of storage, low leakage rates, big storage volumes” Why are assessments needed if salt caverns have low leakage rates, also mention its details?
4. Page2, Line 50: “The mechanisms of gas pollution in salt caverns are mainly caused by the presence of the bacteria”. Please explain the mechanism of gas pollution.
5. Page 2, Line 51: “They live in the sump at the bottom (up to 30% of the total cavern volume)”. How can bacteria convert hydrogen to hydrogen-sulfide without the presence of catalyst?
6. Page 2, Line 69: “no previous work has been published on the risk analysis of salt caverns used for hydrogen storage”. Kindly recheck https://doi.org/10.1016/j.rser.2019.01.051 for confirmation.
7. Page 2, Line 82: “we considered the data obtained by Hemme and van Berk”. Please use some other data also instead of a single source for comparison.
8. Page 2, Line 84: “hydrogen stored at 160 atm and 80 °C”. Are bacteria active to perform a reaction in these conditions?
9. Page 3, Line 101: “In this work, we considered a duration of 1 minute”. Please specify why 1-minute duration was chosen?
10. Page 4, Line 142: “In case of no-ignition, the outcome is the toxic release due to the presence of H2S”. Is H2S is the only gas playing role for toxic release in case of no-ignition?
11. Page 5, Line 150: “The explosion effects were modeled through the TNT equivalency model”. Does the TNT equivalency model can accurately predict the explosion of hydrogen?
12. Page 5, Line 153: “For the toxic release of H2S and SO2 (obtained for complete combustion of H2S), the Britter-McQuaid Model for dense gas dispersion was used”. Does Bitter-McQuaid model is good to predict H₂S and SO₂ release?
13. Page 6, Line 194: “the extension of the zone in which the cloud concentration is within the flammability limits decreases”. Please specify how the flammability limit decreases.
14. Page 7, Line 209: “To quantify the SO2 mass flow rate, we considered the complete fuel combustion.”. It does not make the process ideal?
15. More recent research about types of renewable fuels, hydrogen, CO₂ reduction methods and sustainable energy development is suggested to be added to make the background and discussion more strong: ACS Sustainable Chemistry & Engineering, 2020;8(34):12877-90. Energies, 2020;13(15):3783. Journal of Energy Chemistry, 2021;52:421-7. Energies, 2020;13(19):5080.
16. Page 8, Line 219: “we calculated the individual risk, and we built the effect zones at 50 % fatalities level”. Is there a specific reason for calculations at 50% fatalities?
17. Table 2: Kindly use some other data also to include more years to form a good prediction of results.
18. Figure 3: How overpressure trend as a function of the distance downwind was drawn? Please show the source of data.
19. The conclusions only talk about some studied parameters, which is insufficient to depict the whole pictures of the contribution of this study. The authors are advised to write the conclusions in a comprehensive way and should contain key values, suitability of the applied method, the major findings, contributions and possible future outcomes
20. The authors are advised to revise references, including the latest references. Please see some suggestions in the comments for the ‘introduction’ section.

Author Response

Reviewer: 2

In this study, the authors have carried out a quantitative risk assessment on salt caverns used for hydrogen storage. The research project is very well structured, the ideas are clear, and the writing is concise and argumentative. The language used in this paper was technical and terms were explained clearly. The literature is outdated for the paper and should be from the recent 5-10 years. The models used in this study are not an exact fit for the hydrogen data. The models such as Van Buijtenen’s model are not exactly developed for the hydrogen release and might be inaccurate over a long period of 30 years. A mixture of other gases also forms by the reaction of hydrogen results in making the models more inaccurate. The assumptions were used in the worst-case scenario that only counts for the maximum mass flow rate. In the results and discussion section, the results of models are finely divided into sections. The few improvements needed in results are the validity of the models, verification of the data, assumptions validity, and accuracy of the conditions. The assumptions in models can make them very inaccurate as it prolongs over a period of 30 years making them highly inaccurate. There is a huge gap in the prediction of parameters as after the prediction of 2.5 years, the models directly jump to 21 years. The overpressure cannot be predicted by only 3 values with such gaps as shown in Figure 3. The main problem in hydrogen storage can also include the pressure gaps and irreversible stresses that will also have a great impact on safety. It is obvious that this study needs more accurate predictions and improvements before it could be considered for publication in Energies Journal.  

Answer of Reviewer #2

We wish to thank Reviewer #2 for her/his helpful comments and for the time spent on revising the manuscript. All the Reviewer’s suggestions have been taken into account, and some corrections have been made to the manuscript.

Introduction:

The introduction is lacking sufficient background information that is unable to give the reader detailed background knowledge and possible wide application of this study. The introduction needs to be more emphasized on the research work with a detailed explanation of the whole process considering past, present, and future scope. The conventional fuels and technologies need to be explained well to indicate the relevance of the research work. It needs to be strengthened in terms of recent research and updated literature review in this area with possible research gaps. It is strongly recommended to add a recent literature survey about renewable fuels, climate change, recent global warming trends and the role along with the wide range of applications. How these sustainable fuels affect the current levels of CO₂ and alarming global warming issues? Research gaps should be highlighted more clearly and future applications of this study should be added.

We modified the introduction, following the reviewer’s suggestions.

 Methodology:

The methodology needs some changes, and the current data is limited in some respects. The data was obtained from Hemme and van Berk, some other source could also be used to make a good comparison as a single source is not viable for models.

We thank the reviewer for his/her comments. We agree with him/her that other source could be used for the sake of comparison but unfortunately, we did not find them. Practical experience of underground hydrogen storage is still rare and even rarer are the analyses on gases stored in salt caverns [1]. However, especially in recent years, the important effect of bacterial activity on the purity of underground stored hydrogen has been recognized [2–4]. Therefore, the present study stands in this background as a preliminary tool for the quantification of risk over the time and, consequently, as the characteristic composition of the stored gas varies. For this purpose, the data obtained in the study from Hemme and van Berk [5] were very adequate as time variables and moreover, among the cases proposed by the authors, the worst case was chosen, i.e. in which the H₂S content is the highest ever at 30 years.

We modified the text as in following (Lines 126-130 of the marked manuscript):

“It is worth underlining that there are several studies on the effect of bacterial activity on the composition of underground stored gas [16,25,29–31]. Unfortunately, in none of the other studies, the composition of the stored gas was detailed as reported in Hemme and van Berk work [28], nor were there any data that vary over time.”

 A lot of assumptions and theoretical values are used for the calculations like valve closure duration of 1 min and adiabatic expansion.

Analysis of the worst-case scenarios associated with salt cavern storage need to consider release from the surface for a fracture of the riser pipe. This is generally vertically orientated, and the flow rate of such releases is however limited by the small diameter (250 mm or less). Modern plant design includes a subsurface valve, which should prevent complete loss of pressure in a cavern from any event at the surface, and in addition multiple valves at the wellhead Christmas tree. The duration of the outcomes depends on the time taken for automatic closure of the valves in the system. Compared with cross country pipelines, the isolatable inventories tend to be small, and if the valves operate correctly, any jet flame or gas release should last only a few minutes [6]. Consequently, we set a duration time of 1 minute. In the case of long pipes (in this case 820 m) and large pressure differences, the adiabatic expansion represents the worst scenario (i.e. highest mass flow rate) [7,8].

For the calculation of radiant flux, the values of parameters should be given.

In Table 5, all the parameters used for the calculation of radiant flux are reported.

Van Buijtenen’s model is used for the depiction of the release of TNT cloud, and will not depict the explosion of hydrogen as both have different release patterns.

In this work, we considered neutrally buoyant models (Gaussian dispersion model) as a good approximation of the behaviour of any vapor cloud provided that it is at a certain distance downwind from its release. Particularly, the Pasquill-Gifford plume model was used. To calculate the amount of gas in the explosive region of a continuous plume, Van Buljtenen carried out a three-dimensional integration of the Gaussian plume model equation [9]. Given the derivation of the formula for the calculation of the flammable mass in the cloud, this can be used for any species for which the Gaussian dispersion model is suitable or, as in our case, at long distance from the source where the passive dispersion prevails over the buoyancy. Moreover, there are applications of this model in the case of hydrogen in risk analysis manuals [10,11].

Several techniques are available for predicting the strength of the blast wave produced by an unconfined vapor cloud explosion (UVCE), as a function of distance from the vapor cloud. In recent works, the TNT model, TNO model, Baker-Strehlow- Tang model (BST) and Dorofeev model are used to predict the strength of hydrogen unconfined cloud explosion. The results indicate that the TNT model predict higher explosion overpressures at the same distance than the others [12–15]. To verify it, the TNO model was applied as reported below in the specific comment.

Given that, this work provides a preliminary tools for understanding the risk associated with underground hydrogen storage, we have chosen to apply the TNT method in order to obtain the most conservative assessments.

We modified the text to include it (Lines 204-215 of the marked manuscript):

“Several techniques are available for predicting the strength of the blast wave produced by an unconfined vapor cloud explosion (UVCE), as a function of distance from the vapor cloud. In recent works, the TNT model, TNO model, Baker-Strehlow-Tang model (BST) and Dorofeev model are used to predict the strength of hydrogen unconfined cloud explo-sion. The results indicate that the TNT model predict higher explosion overpressures at the same distance than the others [39–42].

Regarding the van Buijtenen’s model, it was derived through a three-dimensional in-tegration of the Gaussian plume model equation [38]. Given the derivation of the formula for the calculation of the flammable mass in the cloud, this can be used for any species for which the Gaussian dispersion model is suitable or, as in our case, at long distance from the source where the passive dispersion prevails over the buoyancy. Moreover, there are applications of this model in the case of hydrogen in risk analysis manuals [43,44]”

Specific Comments:

1. Please combine the results and discussion section under one heading “Results and Discussion” for a better understanding of readers.

We modified the text, following the reviewer’s comment.

1. Please remove “I”, “we”, ours and “us” words from all manuscript. The revised manuscript should be read and corrected by a native English speaker before resubmission.

Thank you for the suggestion. The manuscript language quality was improved.

1. Page 1, Line 34: “costs per megawatt-hour of storage, low leakage rates, big storage volumes” Why are assessments needed if salt caverns have low leakage rates, also mention its details?

We thank the reviewer for his/her comment.

Compared to the quantity of hydrogen stored through the use of salt caves, the leakage rates are limited [16] but, as evidenced by the preliminary risk analysis conducted in this study, the effects of the release are indeed critical. The health safety and environmental issues involved in the operation of future hydrogen underground storage include emissions during construction and the unhindered escape of the stored gas in case of a blow-out. Different methodologies through groundwater control and/or permeability control  were developed to reduce and control the risk of leakage. However, it is not possible to completely exclude a potential leakage of stored gas from caverns sealed using the water management technology [16,17].

1. Page2, Line 50: “The mechanisms of gas pollution in salt caverns are mainly caused by the presence of the bacteria”. Please explain the mechanism of gas pollution.

In respiratory metabolism, various microorganisms use hydrogen as the electron donor and four essential biotic reactions can be distinguished:

• Methanogenesis from hydrogen and CO₂ or hydrogen and CO induced by methanogenic Archaea

 (Sabatier’s reaction)

The optimal pressure and temperature for growth of these bacteria are 90 bar and 30–40 °C (mesofils), but they are capable of growing even at 97 °C. The CO₂ needed for these reactions in UHS is frequently present in the injected gas (town gas), found in the carbonaceous reservoir rocks and/or generated through the decomposition of acetate produced by hydrogenotrophic bacteria (following reaction).

• Acetogenic bacteria convert hydrogen and CO₂ into acetate:

Acetogenic bacteria have a maximal activity at the same pressure and temperature as methanogens.

• Archaeoglobus bacteria promotes sulfate (from rocks) reduction and prefers high temperatures (higher than 92 °C)
• Geobacter metallireducens and Shewanella putrefaciens provide to transform Fe³⁺ (into reservoir rocks) into Fe²⁺

Moreover, the abiotic redox reactions induced by hydrogen are significant at temperatures below 100 °C without special catalysts. For instance, the reaction rate of the reduction of pyrite rocks (FeS₂) into pyrrhotite (FeS_1+x) and hydrogen sulfide remains significant even at 50 °C [12].

 (0<x<0.125)

All these reactions provide to the degradation of hydrogen and the enrichment in methane and H₂S [18].

We modified the text to clarify the mechanism (Lines 73-91 of the marked manuscript):

“In particular, in situ generation of methane from hydrogen and CO₂ or CO, according to the Sabatier’s reaction, can occur at high temperatures (800°C) in the presence of a catalyst (nickel), or at low temperatures (30–40°C) in the presence of microorganisms (Methanogen-ic Archaea) [10]. Moreover, acetogenic bacteria convert hydrogen and CO₂ into acetate. The CO₂ needed for these reactions in UHS is frequently present in the injected gas (town gas), found in the carbonaceous reservoir rocks and/or generated through the decomposition of acetate produced by hydrogenotrophic bacteria. The optimal pressure and temperature for the growth of methanogenic and acetogenic bacteria are 90 bar and 30–40 °C, but they are capable of growing even at 97 °C [10] and 750 bar [17,18]. Moreover, the abiotic redox reactions induced by hydrogen are significant at temperatures below 100 °C without special catalysts. For instance, the reaction rate of the reduction of pyrite rocks (FeS₂) into pyrrhotite (FeS_1+x) and hydrogen sulfide remains significant even at 50 °C [19]. H₂S is also produced by sulfate (from rocks) reduction promoted by archaeoglobus bacteria. The reduction is favorited by high temperatures (higher than 92 °C) [10]. Due to the critical influence of bacteria metabolism on hydrogen stored purity, the general recommendation is to analyze the relevant microbiological characteristics of each underground site (salt caverns, deep aquifers and oil-/gas reservoirs has) to establish a good monitoring of potential microbial side effects [20].”

1. Page 2, Line 51: “They live in the sump at the bottom (up to 30% of the total cavern volume)”. How can bacteria convert hydrogen to hydrogen-sulfide without the presence of catalyst?

Bacteria can convert hydrogen into H₂S through two different reactions: the former is a biotic one and consists of the rock sulphate reduction at high temperature while the latter is an abiotic reaction between rocks (such as pyrite) and hydrogen. The specific reactions are reported in the previous answer. These reactions take place without the presence of external catalysts inside the salt caverns but, actually, the bacteria act as biological catalysts in these processes favouring the mentioned reactions at low temperatures [18].

1. Page 2, Line 69: “no previous work has been published on the risk analysis of salt caverns used for hydrogen storage”. Kindly recheck https://doi.org/10.1016/j.rser.2019.01.051 for confirmation.

Thank you for your suggestion. The review [19] you recommended to check what we said in the sentence " no previous work has been published on the risk analysis of salt caverns used for hydrogen storage" confirmed what we knew. In fact, in this study, as in other recent studies [2,20,21], all the potentials, limits and risks of underground storage are highlighted. Particularly, key issues addressed in these papers are the change in capacity and efficiency of UGS associated with the blending of hydrogen in the stored natural gas, the geological integrity of the reservoir and caprocks, the technical integrity of gas storage wells, durability of the materials used for well completions, corrosion, leakage and environmental risks associated with the products of microbial metabolism. In none of these works, nor in the others studied, there is a quantification of the risk in case of leakage or in any other accidental scenario.

We modified the text in order to clarify it (Lines 101-108 of the marked manuscript):

“Indeed, in recent studies [23–26], all the potentials, limits and risks of underground stor-age are highlighted. Particularly, key issues addressed in these papers are the change in capacity and efficiency of UGS associated with the blending of hydrogen in the stored natural gas, the geological integrity of the reservoir and caprocks, the technical integrity of gas storage wells, durability of the materials used for well completions, corrosion, leakage and environmental risks associated with the products of microbial metabolism [23–26]. In none of these works, there is a risk assessment in case of leakage or in any other accidental scenario.”

1. Page 2, Line 82: “we considered the data obtained by Hemme and van Berk”. Please use some other data also instead of a single source for comparison.

Thank you for your suggestion. It would have been very interesting to compare the effect of different compositions to verify the validity of our evaluations but unfortunately, we have not found other data available in the literature.

It is worth underlining that there are many studies on the effect of bacterial activity on the composition of underground stored gas [1,20,22–24]. In none of the other studies, the composition of the stored gas was detailed as reported in Hemme and van Berk paper [5], nor were there any data that vary over time.

Only in the work of Šmigáň et al. (1990) [1], there are experimental data about the composition of town gas stored in an underground reservoir artificially created in water-saturated strata of an anticlinal structure near Lobodice, Czechoslovakia. The composition of the gas 7 months after the gas injection is comparable to that used in our work at 21 years and, therefore, we felt it was useless to repeat the calculations in this case. Moreover, in this work [1], the presence of hydrogen sulphide the gas was neglected.

We modified the text as in the following (Lines 126-130 of the marked manuscript):

“It is worth underlining that there are several studies on the effect of bacterial activity on the composition of underground stored gas [16,25,29–31]. Unfortunately, in none of these studies, the composition of the stored gas was detailed as reported in Hemme and van Berk work [28], nor there were data of composition as function of time.”

1. Page 2, Line 84: “hydrogen stored at 160 atm and 80 °C”. Are bacteria active to perform a reaction in these conditions?

Thank you for the question. Surprisingly, bacteria can survive at these extreme conditions and act as biological catalysts for the biotic or abiotic reactions mentioned above [25,26]. More precisely, the optimal pressure and temperature for growth of these bacteria are 90 bar and 30–40 °C (mesofils), but they are capable of growing even at 97 °C [18] and 750 bar [25,26].

We included it in the text (Lines 80-82 of the marked manuscript):

“The optimal pressure and temperature for the growth of methanogenic and acetogenic bacteria are 90 bar and 30–40 °C, but they are capable of growing even at 97 °C [10] and 750 bar [17,18].”

1. Page 3, Line 101: “In this work, we considered a duration of 1 minute”. Please specify why 1-minute duration was chosen?

We thank the reviewer for his/her question.

Analysis of the worst case scenarios associated with salt cavern storage need to consider release from the surface for a fracture of the riser pipe. This is generally vertically orientated and the flow rate of such releases is however limited by the small diameter (250 mm or less). Modern plant design includes a subsurface valve, which should prevent complete loss of pressure in a cavern from any event at the surface, and in addition multiple valves at the wellhead Christmas tree. The duration of the outcomes depends on the time taken for automatic closure of the valves in the system. Compared with cross country pipelines, the isolatable inventories tend to be small, and if the valves operate correctly, any jet flame or gas release should last only a few minutes [6]. Consequently, we set a duration time of 1 minute.

We clarify this issue in the text (Lines 150-152 of the marked manuscript):

“If the valve operates correctly, any jet flame or gas release should last only a few minutes [22]. Consequently, in this work, a duration of 1 minute was used.”

1. Page 4, Line 142: “In case of no-ignition, the outcome is the toxic release due to the presence of H2S”. Is H2S is the only gas playing role for toxic release in case of no-ignition?

Thank you for the question. In case of no-ignition, we considered H₂S as the only specie involved in toxic release. Only at very high concentrations in air (indoor scenario), hydrogen is a simple asphyxiant gas because of its ability to displace oxygen and cause hypoxia [27]. Also, methane gas is relatively non-toxic; indeed, it does not have an OSHA Permissible Exposure Limits Standard.

1. Page 5, Line 150: “The explosion effects were modeled through the TNT equivalency model”. Does the TNT equivalency model can accurately predict the explosion of hydrogen?

Several techniques are available for predicting the strength of the blast wave produced by an unconfined vapor cloud explosion (UVCE), as a function of distance from the vapor cloud. In recent works, the TNT model, TNO model, Baker-Strehlow- Tang model (BST) and Dorofeev model are used to predict the strength of hydrogen unconfined cloud explosion. The results indicate that the TNT model predict higher explosion overpressures at the same distance than the others [12–15]. Given that this work provides the preliminary tools for understanding the risk associated with underground hydrogen storage, we have chosen to apply the TNT method in order to obtain the most conservative assessments. 

To verify the validity of what is reported in the literature, we carried out the calculations of the effects of the VCE also applying the TNO Multi-Energy method. By way of example, we report the example of the account in the case of VCE relating to the case at 2.5 years. As reported in the literature, the evaluation obtained with the TNT method is more conservative. The same trend was found for all times.

We modified the text to include it (Lines 204-209 of the marked manuscript):

“Several techniques are available for predicting the strength of the blast wave produced by an unconfined vapor cloud explosion (UVCE), as a function of distance from the vapor cloud. In recent works, the TNT model, TNO model, Baker-Strehlow-Tang model (BST) and Dorofeev model are used to predict the strength of hydrogen unconfined cloud explosion. The results indicate that the TNT model predict higher explosion overpressures at the same distance than the others [39–42].”

1. Page 5, Line 153: “For the toxic release of H2S and SO2 (obtained for complete combustion of H2S), the Britter-McQuaid Model for dense gas dispersion was used”. Does Bitter-McQuaid model is good to predict H₂S and SO₂ release?

The Britter-McQuaid model was successfully obtained through a rigorous dimensional analysis of the fundamental mass balance equation in order to provide a simple but effective correlation for modelling dense gas releases. Detailed comparisons of model predictions with field test data were performed over the years and the Britter-McQuaid model produces remarkably good results with predictions outperforming many more complex models for many chemicals (also for H₂S and SO₂) [28,29]. Given the simplicity of the model, it is widely used for the preliminary assessment of the risk of dispersal of these species in several technical reports [30,31].

We modified the text to include it (Lines 217-222 of the marked manuscript):

“Detailed comparisons of dense gas dispersion model predictions with field test data were performed over the years and the Britter-McQuaid model produces remarkably good re-sults with predictions outperforming many more complex models for many chemicals (also for H2S and SO2) [46,47]. Given the simplicity of the model, it is widely used for the preliminary assessment of the risk of dispersal of these species in several technical reports [48,49].”

1. Page 6, Line 194: “the extension of the zone in which the cloud concentration is within the flammability limits decreases”. Please specify how the flammability limit decreases.

Thank you for the question. As can be seen in Figure 1, with increasing time, the hydrogen content is reduced due to the action of bacterial metabolism. Hydrogen has the widest flammability limits compared to the other considered species (40000-750000 ppm). Since the flammability limits of the mixture have been calculated using the Le Chatelier rule, the lower the hydrogen content in the mixture, the lower the impact of its flammability limits on those of the mixture itself. In particular, with the increase of the time, LFL increases while the UFL decreases, approaching the values of methane (50000-150000 ppm).

1. Page 7, Line 209: “To quantify the SO2 mass flow rate, we considered the complete fuel combustion.”. It does not make the process ideal?

We thank the reviewer for his/her comment. We considered an ideal case but thanks to this hypothesis we have the maximum flow rate of SO₂ produced.

1. More recent research about types of renewable fuels, hydrogen, CO₂ reduction methods and sustainable energy development is suggested to be added to make the background and discussion more strong: ACS Sustainable Chemistry & Engineering, 2020;8(34):12877-90. Energies, 2020;13(15):3783. Journal of Energy Chemistry, 2021;52:421-7. Energies, 2020;13(19):5080.

We modified the introduction by adding more recent studies.

1. Page 8, Line 219: “we calculated the individual risk, and we built the effect zones at 50 % fatalities level”. Is there a specific reason for calculations at 50% fatalities?

For a more conservative calculation of the effect zones (i.e., to consider the largest possible isorisk zones), the risk analysis manuals recommend using the probit value (defined for each outcome effect) relative to the probability of fatality equal to 50% [7,8].

1. Table 2: Kindly use some other data also to include more years to form a good prediction of results.

Thank you for your suggestion. It would have been very interesting to compare the effect of different compositions to include more years and to compare the obtained results. Unfortunately, we did not find any work similar to Hemme and van Berk one. Particularly, no one shows in detail the composition of the stored gas as a function of time.

There are several studies on the effect of bacterial activity on the composition of underground stored gas [1,20,22–24]. Only in the work [1], there are experimental data about the composition of town gas stored in an underground reservoir artificially created in water-saturated strata of an anticlinal structure near Lobodice, Czechoslovakia. The composition of the gas 7 months after the injection of the gas is comparable to that used in our work at 21 years and therefore we felt it was useless to repeat the calculations in this case. Moreover, in this work [1], the presence of hydrogen sulphide inside the gas was neglected.

1. Figure 3: How overpressure trend as a function of the distance downwind was drawn? Please show the source of data.                   

We thank the reviewer for his/her question. The overpressure trend as a function of the distance downwind was obtained by using the TNT equivalence method. In particular, for each distance, the scaled distance was calculated and, through the digitalization and the regression of the blast chart, the overpressure was evaluated. In the following figure, the digitalization of the blast chart was shown while the source data are given in the table.

1. The conclusions only talk about some studied parameters, which is insufficient to depict the whole pictures of the contribution of this study. The authors are advised to write the conclusions in a comprehensive way and should contain key values, suitability of the applied method, the major findings, contributions and possible future outcomes

We thank the reviewer for the suggestion and we accordingly modified the Conclusions.

1. The authors are advised to revise references, including the latest references. Please see some suggestions in the comments for the ‘introduction’ section.

Done, following the reviewer’s suggestions.

References contained in the answers to Reviewer 2’ comments

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2. Zivar, D.; Kumar, S.; Foroozesh, J. Underground hydrogen storage: A comprehensive review. Int. J. Hydrogen Energy 2020, doi:10.1016/j.ijhydene.2020.08.138.
3. Dopffel, N.; Jansen, S.; Gerritse, J. Microbial side effects of underground hydrogen storage – Knowledge gaps, risks and opportunities for successful implementation. Int. J. Hydrogen Energy 2021, 46, 8594–8606, doi:10.1016/j.ijhydene.2020.12.058.
4. Strobel, G.; Hagemann, B.; Huppertz, T.M.; Ganzer, L. Underground bio-methanation: Concept and potential. Renew. Sustain. Energy Rev. 2020, 123, 109747, doi:10.1016/j.rser.2020.109747.
5. Hemme, C.; van Berk, W. Hydrogeochemical modeling to identify potential risks of underground hydrogen storage in depleted gas fields. Appl. Sci. 2018, 8, 1–19, doi:10.3390/app8112282.
6. Tyldesley, A. Major hazards of natural gas storage. Inst. Chem. Eng. Symp. Ser. 2011, 361–366.
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8. Crowl, D.A.; Louvar, J.F. Chemical and Process Safety; 2002; ISBN 0130181765.
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10. Lees, F.P. Loss Prevention in Chemical Process Industries; Butterworths: London, 1996;
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12. Lobato, J.; Cañizares, P.; Rodrigo, M.A.; Sáez, C.; Linares, J.J. A comparison of hydrogen cloud explosion models and the study of the vulnerability of the damage caused by an explosion of H2. Int. J. Hydrogen Energy 2006, 31, 1780–1790, doi:10.1016/j.ijhydene.2006.01.006.
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15. Casal, J. Evaluation of the Effects and Consequences of Major Accidents in Industrial Plants: Second Edition; Elsevier Inc., 2017;
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17. Kjørholt, H.; Broch, E. The water curtain-a successful means of preventing gas leakage from high-pressure, unlined rock caverns. Tunn. Undergr. Sp. Technol. Inc. Trenchless 1992, 7, 127–132, doi:10.1016/0886-7798(92)90042-G.
18. Panfilov, M. Underground and pipeline hydrogen storage; Elsevier Ltd., 2016; ISBN 9781782423621.
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21. Amid, A.; Mignard, D.; Wilkinson, M. Seasonal storage of hydrogen in a depleted natural gas reservoir. Int. J. Hydrogen Energy 2016, 41, 5549–5558, doi:10.1016/j.ijhydene.2016.02.036.
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23. Panfilov, M. Underground Storage of Hydrogen: In Situ Self-Organisation and Methane Generation. Transp. Porous Media 2010, 85, 841–865, doi:10.1007/s11242-010-9595-7.
24. Cord-Ruwisch, R.; Kleinitz, W.; Widdel, F. Sulfate-Reducing Bacteria and Their Activities in Oil Production. JPT, J. Pet. Technol. 1987, 39, 97–106, doi:10.2118/13554-PA.
25. Sinha, N.; Nepal, S.; Kral, T.; Kumar, P. Survivability and growth kinetics of methanogenic archaea at various pHs and pressures: Implications for deep subsurface life on Mars. Planet. Space Sci. 2017, 136, 15–24, doi:10.1016/j.pss.2016.11.012.
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Author Response File: Author Response.docx

Reviewer 3 Report

The paper needs substantial changes 

1-The sentence

"The storage of hydrogen is very thought-provoking due to its very low density: indeed, 1 kg of hydrogen gas occupies over 11 m3 at room temperature and atmospheric pressure"

kindly rephrase the sentence as it's not of technical clarity. Also, provide a reference

2-Kindly provide some significance of Hydrogen technology in the Introduction part; can refer to the following paper

https://www.mdpi.com/1996-1073/13/22/5879

3-In 37 "kWhel m-3" kindly rearrange the unit, same in 38 and 39

4-In 46 change change "behavior" to "storage"

5-In table 3-how the relativity humidity of 50% is arrived.

6-Any insight on the H2 purity?

7-Can make the Results and Discussions into a single section

8-Numerous grammatical errors, kindly fix it

Author Response

Reviewer: 3

 Answer of Reviewer #3

We wish to thank Reviewer #3 for her/his helpful comments and for the time spent on revising the manuscript. All the Reviewer’s suggestions have been taken into account, and some corrections have been made to the manuscript.

The paper needs substantial changes 

1-The sentence

"The storage of hydrogen is very thought-provoking due to its very low density: indeed, 1 kg of hydrogen gas occupies over 11 m3 at room temperature and atmospheric pressure" kindly rephrase the sentence as it's not of technical clarity. Also, provide a reference

We modified the text as reported in the marked manuscript.

2-Kindly provide some significance of Hydrogen technology in the Introduction part; can refer to the following paper

https://www.mdpi.com/1996-1073/13/22/5879

We accordingly modified the introduction, by including the suggested interesting paper.

 3-In 37 "kWhel m-3" kindly rearrange the unit, same in 38 and 39

Done: we changed it into “kWhel/m³”.

4-In 46 change "behavior" to "storage"

Done.

5-In table 3-how the relativity humidity of 50% is arrived.

We thank the reviewer for his/her question. In this work, we referred to some data available for the Bad Lauchstadt salt cavern (such as pipe length, diameter and salt cavern volume). Consequently, as far as the humidity value is concerned, we have chosen the average value registered in Germany (Leipzig).

6-Any insight on the H₂ purity?

We thank the reviewer for his/her question. Four types of underground hydrogen storage can be distinguished depending on the H₂ purity and its final destination.

• Underground pure hydrogen storage: the final destination of this hydrogen is for use in fuel cells
• Underground storage of natural gas and hydrogen (6–15%): the mixture is used as fuel
• Underground storage of rich hydrogen mixture with CO, CH₄, and CO₂ (syngas or town gas):the mixture is used as electricity (through thermo-mechanical conversion in gas turbines) and/or as fuel
• Underground methanation reactor (UMR): the objective of such storage is to enrich the energy potential of the gas by transforming the mixture of H₂ and CO₂ into methane through the action of methanogenic bacteria [18]

We modified the text including this clarification (Lined 47-58 of the marked manuscript):

“It is worth saying that four types of underground hydrogen storage can be distinguished depending on the H2 purity and its final destination:

• Underground pure hydrogen storage: the destination of this hydrogen is for use in fuel cells
• Underground storage of natural gas and hydrogen (6–15%): the mixture is used as fuel
• Underground storage of rich hydrogen mixture with CO, CH4, and CO2 (syngas or town gas):the mixture is used as electricity (through thermo-mechanical conversion in gas turbines) and/or as fuel
• Underground methanation reactor (UMR): the objective of such storage is to enrich the energy potential of the gas by transforming the mixture of H2 and CO2 into me-thane through the action of methanogenic bacteria [10].”

7-Can make the Results and Discussions into a single section

Done.

8-Numerous grammatical errors, kindly fix it

Thank you for the suggestion. We improved the English quality and fixed all the errors.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have responsed well to those proposed questions by reviewers and carefully revised the whole paper according to the suggestions of reviewers step by step. Therefore, the quality of this paper has elevated greatly, hence this paper is now suitable to be published in the journal.

Reviewer 3 Report

With the substantial changes made, the paper can be accepted;

However comprehensive proofreading is mandatory.