Analysis of a Newly Developed Afterburner System Employing Hydrogen–Methane Blends

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

I am jumping directly to the point, so, please find below, some of my specific comments/questions or brief remarks on both the form and the substance of your manuscript:

 

- In the abstract you are using an acronym, CHP, without adding its significance; please, add it

- In the introduction:

- “Hydrogen is renowned for its cleanliness as a fuel, emitting only water vapors and heat”; adding “heat” in this formulation is without added value as it goes without saying the any fuel combustion means heat..

- “blended hydrocarbons/hydrogen” and then, suddenly you are introducing “H2 and CH4” without mentioning any other example of hydrocarbons used with hydrogen; then, by comparison, why using CH4 would be more beneficial?... Please, develop a bit

- at the end, even though I fully appreciate the fact that you identified a so-called gap after the literature review, I fear that the formulation “many aspects have not been covered in the literature like the effect of using hydrogen blends as fuel on bluff body.” “Many aspects” = “the effect of using H2 blends…” What do you mean by that? Different blends of H2 and CH4? If yes, please, be more specific and try to prove your statement, I mean, really, are no other studies on different blends of H2 and CH4?

- equally at the end of the introduction, you mentioned “The present work continues the investigation of an after-burner system fueled with hydrogen/methane mixtures started in [24].” So, am I to understand that actually, the current study is another iteration of the same approach on the same study? In fact the only change is “Two new V-gutter flameholders (P1 and P2)”… I am simply wondering if any iteration on a project could produce a scientific article ?…In other words, considering figure 8 from the conclusions, am I to understand to see another article on this?

- usually, figures are introduced in the order they are referenced: I mean not figure 2 before figure 1…

- at section 3, page 11: “Thus, the heat loss due to the radiation mechanism will be diminished at higher hydrogen content in the fuel mixtures” – you mentioned that in the context of justifying the lower NO; however, lower heat loss means higher temperature in the combustor, am I right, which means higher NO… I understand the other explanation you provided, however, in my opinion, the statement quoted here was given without discussing it in connection with the other explanation… Could you please reformulate so that the flow of explanation to be more comprehensible?

 

-        “ These data reveal high gradients in flow velocity near …” What do you mean by “high gradients” Are you sure it is the right formulation? If yes, please, add the necessary details, as in my opinion they are needed…

 

All in all, before the acceptance the manuscript, I think the authors should clearly specify in what regard this manuscript brings really new important contribution with respect to the previous? “Two new V-gutter flameholders… » Personally, I wouldn’t encourage publishing an article after (about) each minor iteration on the same project. Instead, it is perhaps better to wait to have all the research ended in order to perform a complete analysis/comparison to be presented in a solid article etc… But that’s just my point of view. About the methodology used by the authors, I think it’s acceptable, the paper is rather well structured and written, the results are well explained….

So, my recommendation is not related to the quality of the manuscript but to this particular aspect. Anyway, if the editor decides in the favor of the authors, I would kindly ask the authors to consider my comments.  

 

 

Author Response

First of all, thank you for your time and patience!

 

Dear Authors,

I am jumping directly to the point, so, please find below, some of my specific comments/questions or brief remarks on both the form and the substance of your manuscript:

 

- In the abstract you are using an acronym, CHP, without adding its significance; please, add it

In the abstract for the CHP acronym, was added its significance.  So that the paragraph is presented in the following form (The modification is implemented in the text and marked with a red color):

A considerable number of Combined Heat and Power (CHP) systems continue to depend on fossil fuels like oil and natural gas, contributing to significant environmental pollution and the release of greenhouse gases.

 

- In the introduction:

- “Hydrogen is renowned for its cleanliness as a fuel, emitting only water vapors and heat”; adding “heat” in this formulation is without added value as it goes without saying the any fuel combustion means heat..

 

Thank you for your suggestion the paragraph was rephrased and added in the text (page 1 marked with red colour):

Hydrogen is renowned for its cleanliness as a fuel, emitting only water vapours and releasing energy when is used in combustion, making it a carbon-free energy carrier thus, the process has no byproducts that are harmful to the environment.

 

- “blended hydrocarbons/hydrogen” and then, suddenly you are introducing “H2 and CH4” without mentioning any other example of hydrocarbons used with hydrogen; then, by comparison, why using CH4 would be more beneficial?... Please, develop a bit

Thank you for pointing this out, therefore a new paragraph was inserted in the text (page 2 marked with red colour):

Blended hydrocarbons and hydrogen systems, often discussed in the context of Combined Heat and Power (CHP) applications, typically involve mixing hydrogen (H₂) with various hydrocarbons to optimize performance and environmental benefits. Among these, methane (CH₄) stands out as particularly beneficial. Methane, the primary component of natural gas, is abundant, inexpensive, and widely available. It has a high hydrogen-to-carbon ratio, leading to more efficient combustion with a lower carbon footprint compared to longer-chain hydrocarbons like ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). When blended with hydrogen, methane's combustion characteristics are enhanced, resulting in more stable and efficient energy production with reduced emissions of NOₓ and CO₂. Additionally, methane’s compatibility with existing natural gas infrastructure makes it a cost-effective and practical solution for cleaner energy transitions. These attributes make methane a preferred choice over other hydrocarbons for optimizing the performance and environmental impact of CHP systems when blended with hydrogen.

 

- at the end, even though I fully appreciate the fact that you identified a so-called gap after the literature review, I fear that the formulation “many aspects have not been covered in the literature like the effect of using hydrogen blends as fuel on bluff body.” “Many aspects” = “the effect of using H2 blends…” What do you mean by that? Different blends of H2 and CH4? If yes, please, be more specific and try to prove your statement, I mean, really, are no other studies on different blends of H2 and CH4?

Agree. We have revised the paragraph to emphasize this point. The paragraph was replaced with the following text:

Even though there is extensive literature on Combined Heat and Power (CHP) systems and hydrogen blends, there still exists gaps in the detailed effects of different H₂/CH₄ blends on bluff body combustion. Even though some studies have focused on the general performance of hydrogen methane blend during combustion, they rarely look at the influence of variations in the ratio of H₂/CH₄ blends to dynamics, stability and emissions characteristics when used with bluff body configurations. This is an important gap because many studies have concentrated on hydrogen-enriched natural gas for its overall combustion properties without addressing intricate interactions and flame behaviours in bluff body scenarios. This void cannot be ignored as bluff body stabilization is widely used in industrial burners and gas turbines hence specific impacts of H₂/CH₄ blends can allow for optimized designs resulting in better performance. Therefore, our research will attempt to bridge this gap by conducting a systematic study of different ratios of H₂/CH₄ blend,” contributing to a better understanding of fuel behavior within these applications.

 

 

- usually, figures are introduced in the order they are referenced: I mean not figure 2 before figure 1…

Regarding the comments I present the following answers:

The sentences have been changed so that the figures are in the order of the introductions. The new paragraph marked with red colour is:

Figure 1a shows the diagram of the experimental test rig. The material used for manufacturing both prototypes, shown in Figure 2, was INCONEL 625.

- at section 3, page 11: “Thus, the heat loss due to the radiation mechanism will be diminished at higher hydrogen content in the fuel mixtures” – you mentioned that in the context of justifying the lower NO; however, lower heat loss means higher temperature in the combustor, am I right, which means higher NO… I understand the other explanation you provided, however, in my opinion, the statement quoted here was given without discussing it in connection with the other explanation… Could you please reformulate so that the flow of explanation to be more comprehensible?

The paragraph was replaced with the following text:

Since carbon atoms are major contributors to the radiative heat loss in hydrocarbon flames, their reduction means less energy is lost through radiation. Hydrogen atoms, in contrast, do not significantly contribute to radiation. As a result, increasing the hydrogen content reduces radiative heat loss. Despite the reduced radiative heat loss, flames with higher hydrogen content still exhibit higher temperatures. This occurs because hydrogen has a higher flame temperature than many hydrocarbons, including methane, a phenomenon confirmed by the much higher LHV (lower heating value) expressed in units of mass (LHV_H2=120.1 MJ/kg and LHV_CH4=50 MJ/kg). Hydrogen combustion produces water vapour (cp=1,996 kJ/kg·K), which has a higher heat capacity than carbon dioxide (cp=0,844 kJ/kg·K), resulting in more heat being absorbed by the combustion products. Additionally, hydrogen burns more quickly and completely, releasing energy more efficiently. These factors contribute to maintaining higher flame temperatures, especially in the nearby of fuel injection ports, even as radiative heat loss diminishes. Consequently, hydrogen-enriched methane mixtures can achieve higher temperatures, improving combustion efficiency and performance.

 

 

-        “ These data reveal high gradients in flow velocity near …” What do you mean by “high gradients” Are you sure it is the right formulation? If yes, please, add the necessary details, as in my opinion they are needed…

Thank you for your observation. The paragraph was replaced with the following text and marked with red in the paper:

The data in the graph reveal significant changes in axial velocity near the flow separation point, evidenced by steep gradients in the velocity profiles of both P1 and P2. These steep gradients indicate rapid changes in velocity over a short distance, which are prominent near the bluff body. As we move downstream, these rapid changes gradually smooth out, resulting in more uniform velocity distributions further away from the separation point. This behaviour is characteristic of the wake behind a bluff body, where the initially separated flow starts to reattach and stabilize. The sharp velocity gradients near the separation point are critical as they influence the wake formation, turbulence intensity, and the overall aerodynamic behavior of the flow around the bluff body.

 

- equally at the end of the introduction, you mentioned “The present work continues the investigation of an after-burner system fueled with hydrogen/methane mixtures started in [24].” So, am I to understand that actually, the current study is another iteration of the same approach on the same study? In fact the only change is “Two new V-gutter flameholders (P1 and P2)”… I am simply wondering if any iteration on a project could produce a scientific article ?…In other words, considering figure 8 from the conclusions, am I to understand to see another article on this?

-All in all, before the acceptance the manuscript, I think the authors should clearly specify in what regard this manuscript brings really new important contribution with respect to the previous? “Two new V-gutter flameholders… » Personally, I wouldn’t encourage publishing an article after (about) each minor iteration on the same project. Instead, it is perhaps better to wait to have all the research ended in order to perform a complete analysis/comparison to be presented in a solid article etc… But that’s just my point of view. About the methodology used by the authors, I think it’s acceptable, the paper is rather well structured and written, the results are well explained….

So, my recommendation is not related to the quality of the manuscript but to this particular aspect. Anyway, if the editor decides in the favor of the authors, I would kindly ask the authors to consider my comments.  

 

One of the most important indicators for each national/European project is the publication of an article for each phase of the project. Practically, if you do not publish, is the failure to achieve the indicator/objectives which is translated into a budget cut. From my experience, no one expects the results to be published after finishing the whole project or project phase. Also, the previous research primarily focused on comparing flame holder prototypes, having the same geometry, but being manufactured using two methods: the classic one and, respectively, additive manufacturing, emphasizing their respective advantages and disadvantages. In contrast, the current study represents a distinct advancement in the investigation of afterburner systems fueled with hydrogen/methane mixtures. In this stage of the project, the V-gutter flame holder expansion angle has been increased from 45 degrees to 60 degrees and the inlet fuel hole area has been varied. While the experimental setup and fuel blends remain consistent for continuity, this study aims to enhance the operational stability and performance of the afterburner system. The new prototypes (P1 and P2 at 60 degrees) contribute to a deeper understanding of combustion dynamics, stability, and emissions characteristics in this context, thereby justifying their exploration as a new scientific article. Regarding, Figure 8, it is included as a result of previous experience where reviewers encouraged/ asked me to publish plans and discuss further research. The final goal of the project is to develop a stable functioning flame holder fuelled with H₂/Ch₄ mixtures, up to 100% H₂, that can be incorporated in a combined heat and power system, thus contributing to the reduction of the energetic system fossil fuel dependence.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Please refer to attached document for review conclusion

Comments for author File: Comments.pdf

Author Response

 This paper has important qualities:

-the topic is pertinent in the perspective of the decarbonization of combustion equipment (here a GT)

-the bibliographic section is well detailed

-the experimental work is significant and professionally conducted

-the interpretation is well elaborated

 

Thank you for your time and patience.

 

However, I detected several defects in this draft paper:

1-the nature of the primary fuel (that burned in the GT) should be mentioned: natural gas or diesel oil?

 

The paragraph about GT (Garrett GTP 30-67 micro gas turbine engine) was rephrased and primary fuel(kerosene) was added in the text (page 4 marked with yellow colour):

The afterburner assembly was installed downstream of a Garrett GTP 30-67 micro gas turbine engine, within the exhaust gas flow, to supply burned gases to the post-combustion system. Using kerosene as fuel (Mass flow: 15.9 kg/h), the gas turbine engine can achieve an outlet temperature of 600 K. During the combustion tests conducted at atmospheric pressure, the Garrett micro gas turbine operated consistently at an engine speed of 52,800 ± 200 rpm (idle, no load). The exhaust gas temperature and pressure at the afterburner inlet were maintained at 550 K and 1.05 bar absolute, respectively.

 

 

2-The ratio between the primary and secondary fuel (that burned in the tested post-combustion device) is an important parameter as well as the O2 concentration at GT outlet. They should be indicated in each configuration.

 

Thank you for the observation, the following text and table are added on page 7 and marked with yellow colour:

The table below (Table 1) presents the ratio of primary fuel (Gas Turbine- GT) to secondary fuel (post-combustion) and the corresponding flue gas composition at the exit of the GT. As the hydrogen percentage in the fuel mixture increases, there is a noticeable decrease in the secondary fuel flow, leading to an increase of the primary fuel/secondary fuel ratio as can be seen in Table 1. This trend can be attributed to hydrogen's higher calorific value compared to methane. Thus, in order to obtain a temperature of 1000 C in the first monitoring point (at 60 mm from the flame holder), a lower mass flow of secondary fuel is needed.

 

	Mass flow kerosene /Mass flow H2/CH4			Flue gases GT(Garrett)
	60%H2+40%CH4	80%H2+40%CH4	100%H2	NO [ppm]	CO [ppm]	CO2[%]	O2[%]
P1	6.20	6.77	11.58	0.8	260	1.25	19
P2	6.88	8.15	13.33

                                Table 1- Ratio between the primary fuel (GT) and secondary fuel (Afterburner)

 

 

3-Device P2 differs from P1 by the larger fuel inlet holes (3 mm dia. versus 1 mm dia.) while the air inlet holes are identical in both configurations. Therefore, the overall fuel richness in P2 is higher than in P1.

This probably explains:

-(1) the lower CO2 content at P2 exhaust (in the combustion gas of P2 there is more H2O coming from the presence of more H2 in the overall balance)

-(2) the lower NOx produced by P2 (richer combustion in P2’s flame front).

Therefore, the analysis of the results should be revisited taking into account of this effect.

 

Thank you for the suggestion the following text has been added to the text marked with yellow color for CO₂ and NO:

 

The CO₂ content in P2 is consistently lower than in P1 at various distances from the flame holder. This is likely due to the presence of more H2 in the case of prototype P2. Thus, more hydrogen is converted to water vapours, rather than methane is converted to carbon dioxide.

 

Richer combustion in P2, due to larger fuel inlet holes, likely results in lower NOx emissions. In rich combustion zones, there is less oxygen available, which reduces the formation of NOx. This is because NOx formation is highly dependent on the availability of oxygen and the temperature of the flame front. The reduced oxygen in the richer P2 combustion environment leads to lower NOx production, as observed in our data.

 

 

Additional remarks:

-The fuel feed to the GT should be drawn in figure 1a.

The Kerosene Fuel Tank was drawn in Figure 1a

-the sizes of figures should be reduced to have the legend appearing in the same page

The paper was formatted so that the figures have the legend appearing on the same page

-Explain what means: “fuel inlet surface”

The expression” fuel inlet surface” was replaced with “Larger inlet fuel holes area” and marked with yellow on the text

-figure 8, as it is, is too poorly detailed and has not much interest. It should be more explicit: the inlets of fuel and air should be represented.

Figure 8 - replaced with a new figure with the inlets of fuel and air represented.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors, thank you for considering my comments!

As for your final remark, “One of the most important indicators for each national/European project is the publication of an article for each phase of the project. Practically, if you do not publish, is the failure to achieve the indicator/objectives which is translated into a budget cut.”, I understand the pressure but, in the same time, in my opinion, that does not imply that any (not major) increment should be published…. But, as I already said, your paper is rather well structured and presented and if the editor considers it’s worth being published as it is (and the editor decided as such since I received the invitation to analyse your modifications), then, I have no other criticism.

All the best!

 

Reviewer 2 Report

Comments and Suggestions for Authors

After the latest changes made by authors, I consider the manuscript worth for publication.