Green Hydrogen Blends with Natural Gas and Its Impact on the Gas Network
Round 1
Reviewer 1 Report
I wish to thank the authors for this work that is very interesting and well written.
I think the topic is really timely and all contributions can be essential for the energy transition. In the last month I happened to review a very similar article for another MDPI journal (article still under review) but I think the specific topics covered are different enough to allow this paper to be published as well. I believe the work can be accepted after a minor revision. in particular the manuscript should be revised with regard to language to correct some typos present. In addition, I believe that some graphs could be moved to supplementary materials in order to make the results more readable, which currently appear to be very heavy. In addition, I would suggest adding a paragraph related to hydrogen storage. Although it is desirable to produce and immediately transport hydrogen using the network that is already in place, it is possible to store hydrogen in the preliminary stage in different ways. How can storing hydrogen on solid hydrides affect its transport in pipeline gas with methane? Would large-scale storage using salt caverns, for example, be desirable? If so, how would bacterial contamination and conversion to methane of the hydrogen itself need to be dealt with? In this regard it might be interesting to consult and mention the work https://doi.org/10.3390/en14102856 (Figures 1 and tables). Good work to the authors !
Author Response
The Authors’ Responses:
We thank the reviewer for their time and valuable comments. The figures have been edited. Figure (12) is moved to the Appendix section. The language and typos are modified in this revised version.
This paper investigates operational variable changes after hydrogen injection through the gas network pipelines (transmission pipelines), and while storage is not the focus of this work, we thank the reviewer for their comment. We have included more discussion regarding storage in the introduction and referred to the suggested paper and its excellent work investing in hydrogen storage.
We added to the manuscript the following sentences:
One of the potential options to store green hydrogen is a salt cave. Portarapillo in [ 13 ] investigates how salt caves can be used as a solution for high-pressure hydrogen storage with minimum risk of leakage.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscrpit prepared by Ali Ekhtiari, Damian Flynn, and Eoin Syron presents an interesting research about bending of green hydrogen with natural gas. The studies are valuable and I recommend to publish the article after minor corrections:
1. The brackets in the keywords should be removed.
2. All abreviations should be explained when used for the first time.
3. Line 89: Italy instead of Itals.
4. In the introduction the Authors presnet the current research considering P2G concept studied in several countries, however the novelity of this work and a clear aim should be better stated.
5. The number of figure in the line 217 should be written.
6. The capital letter in "could" (line 221) is not necessary.
7. A dot in line 236 is missing.
8. The figures are not set in order as they are mentioned in the text. Authors first write about Fig. 10, then 8.
Author Response
The Authors’ Responses:
We thank the reviewer for taking the time to review the manuscript and for their valuable comments.
We addressed the comments as follows:
- The brackets from the keywords are removed.
- A list of Nomenclature and mathematical symbols has been added to the paper (after the abstract).
Also, in the main text body, the abbreviations have been explained.
- All typos are corrected.
- The literature review in the introduction section is revised to present a clearer vision of the research objectives. The following paragraphs have been added:
P2H (highlighted in violet colour in the figure) and P2G system.
Two important applications of P2G are:
- make use of excess renewable energy.
- Increase renewables share in an energy system to reduce air pollutants and GHG emissions.
Hydrogen or SNG can be stored by injection into (gas) pipelines (transmission/distribution) and mixing with the NG [ 10 ], [ 11 ], [ 12 ]. One of the potential options to store green hydrogen is a salt cave. Portarapillo in [13] investigates how salt caves can be used as a solution for high-pressure hydrogen storage with minimum risk of leakage. If hydrogen is injected directly into the NG network, then care must be taken regarding the permissible upper hydrogen limit. Hydrogen is lighter than air and, due to its small molecule size with high diffusivity, may increase leaks from the network [14] In addition, there are concerns about the risk of hydrogen fracture of metal pipes at elevated hydrogen concentrations [15]
- The figure numbers have been corrected
- The capital letter is corrected in line 221, and a dot is added in line 236.
- The ordering of figures has been corrected in the manuscript.
There are plans to increase the number and variety of these interconnections. In Europe, some ongoing projects are planned to investigate the impact of hydrogen in gas networks. Testing of hydrogen blends from 2\% to 20\% blends with natural gas is an experimental project in Network Innovation Centre in Ireland. This project investigates the admissible hydrogen limits within different end-user segments on the national scale [16]. The HyDeploy project was the UK’s first hydrogen blending project. The H2NG project in France studied the capabilities of domestic appliances in using hydrogen blend (20\% hydrogen blended with natural gas).
Ameli et al (2017), [20 ] investigated the potential role of battery storage or heat pumps [21] as an alternative option for renewable storage and P2G systems when power generation incorporated a considerable renewable capacity
Author Response File: Author Response.docx
Reviewer 3 Report
This paper deals with the impact of green hydrogen blends with natural gas and a modelling approach on this topic has been presented and discussed. The topic is worthy of investigation; however, some critical flaws are present and a careful major revision is suggested.
First, the manuscript is very difficult to read since abbreviation and symbols are not always described in the text and equations. A summary table providing all symbols and acronyms should be provided together with the related SI units.
On the technical hand, the injection of Hydrogen from a unique P2G point (i.e. 23 in Figure 2) should lead to not homogeneous concentrations of H2 in the network. Did the authors consider this issue in their simulation?
The novelty of this research should be better highlighted in the introduction section and some recent references should be also considered, as for example in terms of impact of hydrogen injection in natural gas measurements and SNG Generation via Power to Gas Technology.
As far as the numerical model is concerned:
- in Equation 4 pressure and temperature are nodal values? Are they evaluated by considering a spatial discretization?
- Section 2.1.1 What kind of numerical scheme has been adopted to solve the partial differential equations?
- Section 3.1. The authors state that the obtained results have been validated by SAInt software. Authors should better discuss what are the differences between the proposed model and the SAInt software?
Further punctual comments are listed below:
- Sentence in lines 41-43 should be better positioned in line 132
- Lines 48-49 Authors should briefly present the mentioned plans to improve interconnection between electrical and gas grids
- Figure 1 includes the scheme of the case study process, however the case study has not been introduced yet in the text
- Lines 93-96, this statement is ambiguous, there is not only wind within RES...
- What is the meaning of down2 in line 165?
- Figure 1 is not fully clear: blue and orange lines should be explained and curtailed wind should supply only the H2 production.
- Figure 2 and 3 have not been introduced in the text. Furthermore, Figure 2 is not fully clear: what is the meaning of “Residential and I/C”? Are these natural gas distribution networks or electrical? What is the meaning of the lines downstream a Power Plant (e.g. 13 and 14), do they are electrical lines or natural gas pipelines? The interconnection with electrical grid should be better highlighted
- Line 217: missing reference to figure
- Line 233 “63% efficiency” should be better than "63 efficiency"
- Eq.17 Authors should better highlight the physical meaning of the Flow rate of H2 in a network in which a mixture of NG and H2 is supposed to be flowing
- Line 242 a definition of “relative load factor of gas consumption” should be useful
- Figure 5: Generated H2 is plotted in volumetric units (Sm3/s), however the use of conservative quantities (i.e. mass or energy) should be preferable unless temperature and pressure conditions are clearly specified
- Table 1: Does only one type of gas circulate in Ireland? Are there no variations in the composition as a function of imports and national production? If so, this should be specified.
- Line 254. This statement is in contrast with the one in line 125
- Figure 10 is trivial. Differences with Figure 5 should be discussed, since trends are identical
- Figure 11 and Figure 12 seem trivial in terms of comparison between flow-rate with and without H2, since Figure 8 already presented these results. Authors should highlight additional differences, if any.
- Line 298 what unit is MMSCM? Please use SI units and apply related writing rules!
- Figure 13: the caption should specify this graph is related to Ireland. What is the meaning of figures of H2 and CH4 at the top of the graph? Title of Y axis should contain also reference to SNG.
Author Response
Authors: We Thank the reviewer for taking the time on the valuable comments to help improve the quality of the paper.
The authors’ responses are below:
This paper deals with the impact of green hydrogen blends with natural gas and a modelling approach on this topic has been presented and discussed. The topic is worthy of investigation; however, some critical flaws are present and a careful major revision is suggested.
First, the manuscript is very difficult to read since abbreviation and symbols are not always described in the text and equations. A summary table providing all symbols and acronyms should be provided together with the related SI units.
A list of abbreviations and technical/mathematical symbols has been added at the beginning of the manuscript on page 1.
On the technical hand, the injection of hydrogen from a unique P2G point (i.e. 23 in Figure 2) should lead to not homogeneous concentrations of H2 in the network. Did the authors consider this issue in their simulation?
Yes, the modelling approach has considered the variation in hydrogen concentration. To not repeat ourselves, we explained the equations and the taken approach, and then we refer to the complete explanation of the methodology in previously published papers:
- Investigation of the Multi-Point Injection of Green Hydrogen from Curtailed Renewable Power into a Gas Network (LINK)
- A Novel Approach to Model a Gas Network (LINK)
In the methodology section, the gas quality tracking has been explained in an itemised list on page 6
The novelty of this research should be better highlighted in the introduction section and some recent references should be also considered, as for example in terms of impact of hydrogen injection in natural gas measurements and SNG Generation via Power to Gas Technology.
The literature review is revised to refer to the renewable storage options and more details on green hydrogen with more updated and recent studies. The following paragraphs have been added:
P2H (highlighted in violet colour in the figure) and P2G system.
Two important applications of P2G are:
- make use of excess renewable energy.
- Increase renewables share in an energy system to reduce air pollutants and GHG emissions.
Hydrogen or SNG can be stored by injection into (gas) pipelines (transmission/distribution) and mixing with the NG [ 10 ], [ 11 ], [ 12 ]. One of the potential options to store green hydrogen is a salt cave. Portarapillo in [13] investigates how salt caves can be used as a solution for high-pressure hydrogen storage with minimum risk of leakage. If hydrogen is injected directly into the NG network, then care must be taken regarding the permissible upper hydrogen limit. Hydrogen is lighter than air and, due to its small molecule size with high diffusivity, may increase leaks from the network [14] In addition, there are concerns about the risk of hydrogen fracture of metal pipes at elevated hydrogen concentrations [15]
- The figure numbers have been corrected
- The capital letter is corrected in line 221, and a dot is added in line 236.
- The ordering of figures has been corrected in the manuscript.
There are plans to increase the number and variety of these interconnections. In Europe, some ongoing projects are planned to investigate the impact of hydrogen in gas networks. Testing of hydrogen blends from 2% to 20% blends with natural gas is an experimental project in Network Innovation Centre in Ireland. This project investigates the admissible hydrogen limits within different end-user segments on the national scale [16]. The HyDeploy project was the UK’s first hydrogen blending project. The H2NG project in France studied the capabilities of domestic appliances in using hydrogen blend (20\% hydrogen blended with natural gas).
Ameli et al (2017), [20 ] investigated the potential role of battery storage or heat pumps [21] as an alternative option for renewable storage and P2G systems when power generation incorporated a considerable renewable capacity
As far as the numerical model is concerned:
in Equation 4 pressure and temperature are nodal values? Are they evaluated by considering a spatial discretisation?
Thanks for the valuable comment. We explained more in the body of the manuscript about the average pressure in the compressibility factor equation. Equation 4 relates to the calculation of the compressibility factor through the pipelines. All pressures in this equation are average values between both ends of each pipe. The parameters and variables have been calculated and iterated for each time step to achieve the final result (for the compressibility factor) with a tolerance of 1x10-3 from the initial estimate.
The compressibility factor “Z” depends on the gas average pressure and temperature and was calculated using the PAPAY equation, which is applicable for high-pressure networks [36].
In equation (4), all pressures in this equation are average values between both ends of each pipe of the network.
Section 2.1.1 What kind of numerical scheme has been adopted to solve the partial differential equations?
The partial differential equations have been solved using the Newton-Raphson method. Further details are provided on page 6 of the updated paper, summarising the approach introduced in “paper details” by the same authors.
The following description has been added to the text:
The model outputs show that the iterative process in connection to the Newton-Raphson approach works well. This numerical method for solving gas networks’ equations was developed by Osiadacz [34] by linearising the differential equations.
Section 3.1. The authors state that the obtained results have been validated by SAInt software. Authors should better discuss what the differences between the proposed model and the SAInt software are?
We thank the reviewer for this comment.
As an example of how the proposed model works, a comparison of hydrogen concentration at node 5 between the SAInt model and the proposed model is illustrated in Figure A14. It is mentioned that there is less than 1.5% deviation between the results found using SAInt and the given approach.
We have added more explanations for this comparison in the manuscript as follows:
This deviation is due to using different numerical and finite difference methods to solve the PDE gas equations.
Further punctual comments are listed below:
Sentence in lines 41-43 should be better positioned in line 132
As suggested, the sentences have been moved to the recommended place.
Lines 48-49 Authors should briefly present the mentioned plans to improve interconnection between electrical and gas grids
The following paragraph is added:
“There are plans to increase the number and variety of the gas and renewable power interconnections. In Europe, projects are underway to investigate the impact of hydrogen in gas networks. Testing of hydrogen blends from 2% to 20% blends with natural gas is an experimental project for the Network Innovation Centre in Ireland, intending to investigate admissible hydrogen limits for different end-user segments at a national scale [16]. Similarly, the HyDeploy project was the UK’s first hydrogen blending project. The H2NG project in France studied the capabilities of domestic appliances using a hydrogen blend (20% hydrogen blended with natural gas).”
Figure 1 includes the scheme of the case study process, however the case study has not been introduced yet in the text
The explanation has been added:
P2H (highlighted in green colour in the figure). The orange line is the power carrier line, and the blue line represents the gas network as an energy carrier from suppliers to demand nodes.
Lines 93-96, this statement is ambiguous, there is not only wind within RES...
It is referred to Guandal study published in the journal of Hydrogen Energy (LINK) which that wind power supplies a higher renewable capacity for producing green hydrogen in Germany. The modified text from the paper is given below:
In addition, a comparison with the German case shows that a mainly wind-based power system in Germany provides a higher energy capacity for P2G; mainly, the potential of Germany's offshore wind contribution to produce hydrogen using P2G is substantial
What is the meaning of down2 in line 165?
The text has been corrected to dispatch down
Figure 1 is not fully clear: blue and orange lines should be explained and curtailed wind should supply only the H2 production.
It is replaced with an updated version.
Figure 2 and 3 have not been introduced in the text. Furthermore, Figure 2 is not fully clear: what is the meaning of “Residential and I/C”? Are these natural gas distribution networks or electrical?
What is the meaning of the lines downstream a Power Plant (e.g. 13 and 14), do they are electrical lines or natural gas pipelines? The interconnection with electrical grid should be better highlighted
Thanks for the comments. The following modifications have been made as follows:
Figure 2 has been updated, and Figure 3 has been introduced in line 173.
Residential, IC, Power plants, etc. are end-users types on the gas network. IC (Industrial and Commercial) has been added to the nomenclature section.
The figure’s legend (figure 2) explains that they are pipes and the grey boxes are pipe numbers. Each square represents the demand point with its end-user type.
The study does not consider a combined power and gas network, but the calculations only relate to the gas network.
The figure does not show the power grid to achieve a less cluttered representation of the gas network and the green hydrogen injection point.
Line 217: missing reference to figure
Line 233 “63% efficiency” should be better than “63 efficiency”
They have been addressed and edited
Eq.17 Authors should better highlight the physical meaning of the Flow rate of H2 in a network in which a mixture of NG and H2 is supposed to be flowing
It is just the pure volumetric value of hydrogen generated by a PEM electrolyser (in m3/sec) before blending with NG.
Line 242 a definition of “relative load factor of gas consumption” should be useful
The following definition has been added: The relative load factor is the proportion of hourly gas consumption and maximum hourly gas consumption.
Figure 5: Generated H2 is plotted in volumetric units (Sm3/s), however the use of conservative quantities (i.e. mass or energy) should be preferable unless temperature and pressure conditions are clearly specified
The units of Sm3/s for hydrogen are for standard conditions, which is 15 C and 101.325 kPa.
Table 1: Does only one type of gas circulate in Ireland? Are there no variations in the composition as a function of imports and national production? If so, this should be specified.
It is assumed that all gas sources supply the same gas specifications.
Line 254. This statement is in contrast with the one in line 125
The second statement has been clarified as follows:
“A distribution network can have up to 50% hydrogen and in some instances may contain 100% hydrogen [18]”
Figure 10 is trivial. Differences with Figure 5 should be discussed, since trends are identical
While both figures show results which are a consequence of hydrogen production, Figure 5 shows the hydrogen flow rate from the electrolyser, which is the inlet flow rate of pure hydrogen into node 5. Figure 10 shows the hydrogen concentration/fraction at the outlet streams of node 5, and, therefore, is also a function of the demand at node 5 and connected nodes.
Figure 11 and Figure 12 seem trivial in terms of comparison between flow-rate with and without H2, since Figure 8 already presented these results. Authors should highlight additional differences, if any.
Thanks for the comment. Figure 8 contains only the maximum increase in flow rate and pressure drop for each of the four scenarios, while Figure 11 presents a profile across 24 h for each case study, and the increase in flowrate and pressure are not constant across the day.
For maximum demand, the pressure drop and flow rate have been shown for the four scenarios, with the highest pressure drop increase is 1.4% and the flow rate increase is about 8.5% for pipe 10.
SThe. The flow rate increased by 2 sm3/s due to injecting hydrogen to compensate for the reduced energy density after blending H2.
Figure 12 has been moved to the Appendix.
Line 298 what unit is MMSCM? Please use SI units and apply related writing rules!
The text has been updated to specify units, as 26 million sm3
Figure 13: the caption should specify this graph is related to Ireland. What is the meaning of figures of H2 and CH4 at the top of the graph? Title of Y axis should contain also reference to SNG.
The figure has been edited to “Annual gas consumption in Ireland, and estimated hydrogen and synthetic gas methane (SNG) using P2G from renewable sources.”
Author Response File: 
Author Response.pdf
Round 2
Reviewer 3 Report
This reviewer very much appreciate the efforts of the authors in revising the manuscript according to the comments received. The manuscript is almost suitable for publication.
However, several comments have been only replied by the authors but they have not led to a revision of the corresponding part (or figure) of the manuscript (this applies manly to the punctual comments).
Therefore, authors are asked to review their manuscript according to the foreseen responses to the authors.
Author Response
We appreciate the reviewer for taking the time on reading the manuscript.
We considered the reviewer's comments. All new changes are highlighted in blue colour.
The article has been revised again. All punctuations, grammar and possible typos have been modified.
The revised version is attached.
Thanks again
Author Response File: Author Response.pdf
Round 3
Reviewer 3 Report
no further comments
