Experimental Research on an Afterburner System Fueled with Hydrogen–Methane Mixtures

Round 1

Reviewer 1 Report

Authors experimentally studied the afterburner system fueled with hydrogen methane mixtures. Overall, it is poor. It is difficult to understand.

1. Authors used in papr[x], it is strange. Too many

2. Besides，the abstract is poor without any results.

3. In Line 69, “In paper [8] were investigated” what is the meaning?

4.In L89, “in paper [11] were experimental” what is it? Sorry , too many, I cannot catch the meaning.

5. In figs. 4-8, it looks like no meaning.

6. In Figs. 8 and 9, I cannot see the color bar. What is the meaning?

7. Overall, this paper reads as a technical report without any scientific results. You just describe what parameter increases or decreases, but it is not enough. You should explain or discuss the mechanism behind it.

Therefore, it cannot be accepted. Sorry.

Authors experimentally studied the afterburner system fueled with hydrogen methane mixtures. Overall, it is poor. It is difficult to understand.

1. Authors used in papr[x], it is strange. Too many

2. Besides，the abstract is poor without any results.

3. In Line 69, “In paper [8] were investigated” what is the meaning?

4.In L89, “in paper [11] were experimental” what is it? Sorry , too many, I cannot catch the meaning.

5. In figs. 4-8, it looks like no meaning.

6. In Figs. 8 and 9, I cannot see the color bar. What is the meaning?

7. Overall, this paper reads as a technical report without any scientific results. You just describe what parameter increases or decreases, but it is not enough. You should explain or discuss the mechanism behind it.

Therefore, it cannot be accepted. Sorry.

Author Response

First, thank you for your time and suggestion. Regarding the comments I present the following answers:

1. Authors used in papr[x], it is strange. Too many

The paper presents results obtained after an experiential campaign involving: test rig adaptations, instrumentation control, testing, data acquisition and analysis. All authors took part in the experimental campaign. Each member of the team had his specific role in the experimental campaign.

2. Besides the abstract is poor without any results.3. In Line 69, “In paper [8] were investigated” what is the meaning?4. In L89, “in paper [11] were experimental” what is it? Sorry, too many, I cannot catch the meaning.

Newer and more influential references are included in the introduction to properly position this work in the current state of the field. The literature survey was reviewed and was included more citations to articles published in the last 5 years.

3. In figs. 4-8, it looks like no meaning.

In figure 4.1 are presented images during the testing. In Figure 4.2 is presented the testing rig diagram. In Figures 5-7 are presented the measured CO₂, CO and NO concentration levels (the vertical bars) and the measured temperatures (the orange and blue lines) in the 5 measuring points, for both prototypes, for the 3 used fuels. This was chosen in order to better observe the gases concentrations behaviour function of the temperature trend. Figure 8 presents the average velocity field for prototype P1, measured using PVI technique.

4. In Figs. 8 and 9, I cannot see the color bar. What is the meaning?

The figures are replaced with increased font. Figure 8 and 9 presents the average velocity field for prototype P1 and P2, measured using PVI technique.

4. Overall, this paper reads as a technical report without any scientific results. You just describe what parameter increases or decreases, but it is not enough. You should explain or discuss the mechanism behind it.

The aim of the research is to design, manufacture and perform experiments on an afterburner system fueled with pure hydrogen, as well as Hydrogen-Methane mixtures (80 % by volume, respectively 60 % by volume) in order to evaluate the flame stability.  The research fits with the objectives and intelligent specializations defined in the current Romanian National Hydrogen Strategy and Action Plan 2023-2030[1]: “New CCGT capacities” where “CCGT capacities are planned to be deployed between 2022 and 2030, using hydrogen to comply with the maximum permissible CO2 emissions according to the European Union Or “Developing projects, and subsequently, based on confirmed economic results, industrial deployment for applications of hydrogen blended into with natural gas (at least 50% by volume hydrogen admixture) or 100% hydrogen, in natural gas combined cycle power plants (CCGT) or cogeneration plants. The final goal is the possibility of integrating this system into a cogeneration plant.

Reviewer 2 Report

The design of the work is interesting and innovative. It should be noted that the article is written substantively, reliably, and with particular scientific accuracy. The data presented in the manuscript are interpreted appropriately and coherently, and the conclusions are consistent with the evidence and arguments presented.

However, the manuscript in its current version cannot be accepted and requires several corrections:

1. The abstract should contain the most important achievements of the article. You should summarize the obtained results in a few sentences.

2. Newer and more influential references should be included in the introduction to properly position this work in the current state of the field. Many of the works cited are quite old. More citations to articles published in the last 5 years would strengthen the literature review.

3. The novelty of the article should be highlighted more precisely.

4. Fig. 1 c - the readability of dimensions should be improved.

5. Line 202 - please specify which fig the description applies to.

6. Fig. 4.2b - too small fonts make the figure illegible.

7. Figure 5-12 – indexes should be used for CO2, CH4, H2

8. Fig. 8 – 9 - illegible figures. The fonts should be increased

9. Line 418 - The analyzes show that NO was measured, not NOx

10. What further improvements in the process of stable afterburner operation do the authors plan to consider in subsequent works? How can these improvements help improve process efficiency?

Author Response

First, thank you for your time and suggestion. Regarding the comments I present the following answers:

1. The abstract should contain the most important achievements of the article. You should summarize the obtained results in a few sentences.

The abstract was reviewed and replaced with the following text: A new afterburner installation is proposed, fuelled with pure Hydrogen(100%H2) and hydrogen-methane mixtures (60%H2+40%CH4, 80%H2+20%CH4) for use in cogeneration applications. Two prototypes (P1 and P2) with same expansion angle (45 degrees) are developed and tested. P1 is manufactured by the classic method and 1 by additive manufacturing both manufactured from Inconel 625 as material. During the tests, analysis of flue gases (CO2, CO and NO concentration), PIV Experiments and Noise measurements was conducted.  The flue gases analysis, emphasizes that the behaviour of the two tested prototypes is very similar. For all 3 used fuels, the amount of CO2 concentration levels are slightly lower in the case of the additive manufactured prototype (P2). The CO levels are significantly higher in the case of the additive manufactured prototype (P2) when 60% H2/ 40 % CH4 and 80% H2 / 20% CH4 mixtures are used as fuel. When pure H2 is used as fuel, the measured data suggests that no additional CO is produced during the combustion process, the level of CO being similar to the one coming from the Garrett micro gas turbine in all 5 measuring points. The NO emissions gradually decrease as the percentage of H2 in the fuel mixture increases. The NO concentration is significantly lower in the case of the additive manufactured prototype (P2) in comparison with the classic manufactured prototype (P1). Examining the data obtained from PIV experimental measurements of the flow within the mixing region shows the highest axial velocity component value on the centreline is measured for P1 prototype. The acoustic measurements showed that a higher H2 concentration leads to a reduction in noise of approximately 1.5 dB for both afterburners prototypes. The outcomes reveal that the examined V-gutter flameholders prototypes flow is smooth, without any oscillations perpendicular without chaotic motions and turbulent oscillations to the flow direction across all tested conditions by keeping constant thermal power.

2. Newer and more influential references should be included in the introduction to properly position this work in the current state of the field. Many of the works cited are quite old. More citations to articles published in the last 5 years would strengthen the literature review.

The literature survey was reviewed and was included more citations to articles published in the last 5 years.

3. The novelty of the article should be highlighted more precisely.

The following text has been added in the paper: Earlier studies (e.g. HSE Research Report RR1047, 2015), indicate that the addition of up to 20% hydrogen by volume is unlikely to present significant changes to any risks already associated with natural gas delivery. 20% is the level at which it is expected that gas customers supply and usage will not be affected by the change in gas composition. The aim of the research is to design, manufacture and perform experiments on an afterburner system fueled with pure hydrogen, as well as Hydrogen-Methane mixtures (80 % by volume, respectively 60 % by volume) in order to evaluate the flame stability.  The research fits with the objectives and intelligent specializations defined in the current Romanian National Hydrogen Strategy and Action Plan 2023-2030[1]: “New CCGT capacities” where “CCGT capacities are planned to be deployed between 2022 and 2030, using hydrogen to comply with the maximum permissible CO2 emissions according to the European Union Or “Developing projects, and subsequently, based on confirmed economic results, industrial deployment for applications of hydrogen blended into with natural gas (at least 50% by volume hydrogen admixture) or 100% hydrogen, in natural gas combined cycle power plants (CCGT) or cogeneration plants. The final goal is the possibility of integrating this system into a cogeneration plant.

4. Fig. 1 c - the readability of dimensions should be improved.

Picture replaced with higher readability

5. Line 202 - please specify which fig the description applies to.

Picture replaced and text added “Along the axial direction (Figure 4.2a) downstream of the flameholder a ruler was mounted where the 5 sampling distances for the gas composition and temperature were marked.”

6. Fig. 4.2b - too small fonts make the figure illegible.

Picture replaced with higher readability/Fonts

7. Figure 5-12 – indexes should be used for CO2, CH4, H2

 Indexes are added for CO2, CH4, H2

8. Fig. 8 – 9 - illegible figures. The fonts should be increased

Picture replaced with higher readability/Fonts

9. Line 418 - The analyzes show that NO was measured, not NOx

The mistake was corrected.

10. What further improvements in the process of stable afterburner operation do the authors plan to consider in subsequent works? How can these improvements help improve process efficiency?

The following text has been added: For the further improvements (Figure 14) in the process of stable afterburner operation the testing of two other flame holder prototypes (P3 and respectively P4) is planned. Using the same experimental testing rig, the same instrumentation and fuel blends, two new V-gutter flameholders with the same expansion angle (60 degrees) will be tested. The P3 flame holder has 40 holes of Ø1mm diameter, while P4 flame holder has 40 holes of Ø3mm diameter through which the fuel is injected. The experimental models have already been manufactured. Thus, the experimental measurements will start in the shortest possible time. The results of these researches will be published in a future work.

Reviewer 3 Report

The submitted manuscript deals with the highly topical topic of implementing hydrogen into the combustion process in an afterburner system on two prototypes.

From the point of view of the analysis of the investigated parameters, I positively evaluate the performed experimental measurements and their visualization in the form of graphs and graphical representations of the velocity fields.

Despite the very detailed analysis, I have a few professional and formal comments:

1) What key was used to select the composition of gaseous fuel for additional combustion? I consider 80 vol.% by volume and 60 vol.% by volume of pure hydrogen in the fuel mixture to be high concentrations. Why were lower hydrogen concentrations not also investigated? There are currently discussions about adding hydrogen to natural gas distribution networks, and some limit values of up to 20 vol.% hydrogens in the mixture are being considered.

2) Why was only one turbine power setting used for testing? It is not clear whether the turbine is used in several modes or whether only one turbine power was deliberately used.

3) In several places in the manuscript, when analyzing CO and CO2 emissions, the authors mention carbon molecules in the mixture as the reason for lower emissions of these gases. How is it meant? Carbon itself does not form molecules. Here I recommend improving the description of this statement. I believe that it is more about the proportion of carbon atoms in the gaseous fuel.

4) The authors' claim regarding CO emissions that the roughness of the material affects it is somewhat misleading, but this factor cannot be ruled out. The amount of CO emissions is more related to the kinetics of chemical reactions and the conditions in the combustion chamber. It is a more complex problem and therefore I would suggest improving the description of the formation of CO emissions.

5) The analysis of measurements describes the consequences of changing the composition of gaseous fuel in additional combustion but does not talk about recommendations for further research or how the combustion process can be optimized. I assume that further research will continue in this direction.

Experimental research is still a powerful tool in research works and therefore I wish the authors a lot of positive energy and new ideas for the continuation of the research.

Author Response

First, thank you for your time and suggestion. Regarding the comments I present the following answers:

1) What key was used to select the composition of gaseous fuel for additional combustion? I consider 80 vol.% by volume and 60 vol.% by volume of pure hydrogen in the fuel mixture to be high concentrations. Why were lower hydrogen concentrations not also investigated? There are currently discussions about adding hydrogen to natural gas distribution networks, and some limit values of up to 20 vol.% hydrogens in the mixture are being considered.

Answer to the question 1:

The following text has been added: Earlier studies (e.g. HSE Research Report RR1047, 2015), indicate that the addition of up to 20% hydrogen by volume is unlikely to present significant changes to any risks already associated with natural gas delivery. 20% is the level at which it is expected that gas customers supply and usage will not be affected by the change in gas composition. The aim of the present research is to design, manufacture and perform experiments on an afterburner system fueled with pure hydrogen, as well as hydrogen-methane mixtures (H₂ 80 % by volume, respectively 60 % by volume) in order to evaluate the flame stability.  The research fits with the objectives and intelligent specializations defined in the current Romanian National Hydrogen Strategy and Action Plan 2023-2030[1]: “New CCGT capacities” where “CCGT capacities are planned to be deployed between 2022 and 2030, using hydrogen to comply with the maximum permissible CO2 emissions according to the European Union Or “Developing projects, and subsequently, based on confirmed economic results, industrial deployment for applications of hydrogen blended into with natural gas (at least 50% by volume hydrogen admixture) or 100% hydrogen, in natural gas combined cycle power plants (CCGT) or cogeneration plants. The final goal is the possibility of integrating this system into a cogeneration plant.

[1] https://energie.gov.ro/wp-content/uploads/2023/05/EN-Proiect-National-Hydrogen-Strategy.pdf (page 71)

2.Why was only one turbine power setting used for testing? It is not clear whether the turbine is used in several modes or whether only one turbine power was deliberately used.

The following text has been added: The functioning regime of the gas turbine have been kept same in order to notice if there are eventually discrepancies between the prototypes in terms of combustion stability, flame length and geometry functionality (especially for the P2 prototype if it melts).

3) In several places in the manuscript, when analyzing CO and CO2 emissions, the authors mention carbon molecules in the mixture as the reason for lower emissions of these gases. How is it meant? Carbon itself does not form molecules. Here I recommend improving the description of this statement. I believe that it is more about the proportion of carbon atoms in the gaseous fuel.

As you have suggested we have replaced carbon molecules with carbon atoms in the text.

4) The authors' claim regarding CO emissions that the roughness of the material affects it is somewhat misleading, but this factor cannot be ruled out. The amount of CO emissions is more related to the kinetics of chemical reactions and the conditions in the combustion chamber. It is a more complex problem and therefore I would suggest improving the description of the formation of CO emissions.

The following text has been added in the paper: From a very simplistic point of view, hydrocarbon combustion can be characterized as a two-step process: 1) breakdown of fuel to CO, and 2) oxidation of CO to CO₂. The CO + OH → CO₂ + H reaction is the key step in CO oxidation. In Figure 6a, the maximum concentration of CO recorded is situated 80 mm downstream of the flame holder. In Figure 6b, the corresponding peak is observed at a distance of 60 mm. The shorter flame length observed at higher hydrogen percentage in the fuel mixture is attributed to the heightened reactivity of hydrogen, leading to increased OH radical concentrations within the flame. This heightened reactivity accelerates the combustion rates, thereby shifting the reaction zone towards the upstream vicinity of the flame holder, where the inlet fuel orifices are located. On the other hand, in the case of 100% H₂, the only source of CO and CO₂ is the Garrett gas turbine. Because of the locally increase of temperature due to higher calorific power of the hydrogen gas (reported as mass unit) the only mechanism involved in this situation is the dissociation of the CO₂ into CO and O (CO₂ ↔CO + O). For hydrogen burning in air at 1 atmosphere the Adiabatic Flame temperature is 2,400 K. This leads to obtaining in several local points, in the nearby of the flame holder surface, the temperature needed for this phenomenon to occur.

5) The analysis of measurements describes the consequences of changing the composition of gaseous fuel in additional combustion but does not talk about recommendations for further research or how the combustion process can be optimized. I assume that further research will continue in this direction. Experimental research is still a powerful tool in research works and therefore I wish the authors a lot of positive energy and new ideas for the continuation of the research.

The following text has been added: For the further improvements (Figure 14) in the process of stable afterburner operation the testing of two other flame holder prototypes (P3 and respectively P4) is planned. Using the same experimental testing rig, the same instrumentation and fuel blends, two new V-gutter flameholders with the same expansion angle (60 degrees) will be tested. The P3 flame holder has 40 holes of Ø1mm diameter, while P4 flame holder has 40 holes of Ø3mm diameter through which the fuel is injected. The experimental models have already been manufactured. Thus, the experimental measurements will start in the shortest possible time. The results of these researches will be published in a future work.

Round 2

Reviewer 1 Report

Although authors tried to improve this paper, I cannot see any improvement in the novelty or any technical point. Please re-arrange the whole story instead of only modify the sentence. Besides, please polish the English, too many errors.

Although authors tried to improve this paper, I cannot see any improvement in the novelty or any technical point. Please re-arrange the whole story instead of only modify the sentence. Besides, please polish the English, too many errors.

Author Response

Although authors tried to improve this paper, I cannot see any improvement in the novelty or any technical point. Please re-arrange the whole story instead of only modify the sentence. Besides, please polish the English, too many errors.

Thank you for your time and suggestions. Regarding the comments

I did my best to correct the grammatical mistakes.

Reviewer 2 Report

Thank you for your answers and corrections.

Author Response

Thank you for your time and suggestions!

Reviewer 3 Report

After editing the manuscript and clarifying the controversial issues by the authors, I consider the edited manuscript valuable. The manuscript provides valuable information regarding the use of hydrogen in the combustion process of gas turbines. Despite the fact that the authors have presented their future research, I have a few comments that are not intended to reduce the scientific level of the manuscript.

1) From the comparison of the graphic display of figures 5c) and 6c), the influence of the reaction kinetics on the reduction of the amount of CO2 and the increase of the amount of CO can be seen. This is under the conditions of burning 100%H2 in an afterburner. I would recommend the authors also focus on the chemistry of the reactions due to the effect of hydrogen on the exhaust gases from the turbine combustion chamber.

2) It is not clear from the presented images 4.1 and 4.2 whether the research took place in the closed space of the combustion chamber or whether these images are presented as a result of visualization. In the combustion chamber, combustion kinetics can show different results.

I have no further comments. It can be seen that the authors already have a vision for future research and I wish them many new ideas and positive energy in their further research work.

Author Response

• From the comparison of the graphic display of figures 5c) and 6c), the influence of the reaction kinetics on the reduction of the amount of CO2 and the increase of the amount of CO can be seen. This is under the conditions of burning 100%H2 in an afterburner. I would recommend the authors also focus on the chemistry of the reactions due to the effect of hydrogen on the exhaust gases from the turbine combustion chamber.

Thank you for your suggestions, due to limited time, I will focus on the chemistry of the reactions in my next paper.

• It is not clear from the presented images 4.1 and 4.2 whether the research took place in the closed space of the combustion chamber or whether these images are presented as a result of visualization. In the combustion chamber, combustion kinetics can show different results.

Line 226- The afterburner assembly has been mounted downstream of a Garrett GTP 30-67 micro gas turbine engine, in the exhaust gas flow. The combustion tests have been conducted at atmospheric pressure. Images 4.1 and 4.2 are acquired during experiments on real time .