Process Concept of a Waste-Fired Zero-Emission Integrated Gasification Static Cycle Power Plant

Round 1

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors The authors set the stage to describe a layout of a urban waste fired zero emission power plant. This layout integrates gasification with a power generation section and implements two parallel conversion processes, one supplied by the heat of the gasifier, consisting of a thermoacoustic-magnetohydrodynamic (TA-MHD) generator, while the second one the syngas is treated in order to obtain pure hydrogen. The resulting plant avoids any type of emissions in the atmosphere, increases mechanical efficiency due to absence of moving parts, resolving at its root the ever-increasing waste-related pollution problems. The mixed municipal waste of 100 kg contains 19.55 MJ/kg of energy content which is capable of generating 14.71 MW of electric power. This study exemplifies the convergence of sustainability and innovation, however there are some issues that have to be addressed.    1) In introduction, the authors should mention similar (literature-based) urban waste fired zero emission power plants and compare their layouts and efficiency with their own study (in a table probably)   2)  Have the authors tested their system under different operating conditions (e.g. S/C ratio, Voltage or current density of the fuel cell)?   3) Have the authors calculated economic metrics (e.g. NVP, LOCE)? 

Author Response

Comment 1: In introduction, the authors should mention similar (literature-based) urban waste fired zero emission power plants and compare their layouts and efficiency with their own study (in a table probably)   

Response 1: At the best of our knowledge, there aren’t other works which address the issue of emissions due to thermal treatment of waste. To deal with this problem, the usual approach is to reduce the waste, and for the residual part, to reach a balance between emitted and re-absorbed CO2 by plants. This is a partial solution, because it addresses only CO2 emissions, but they are not the only ones caused by combustion. This is the reason why we referred to zero-emission coal fired power plants, which specifically deal with emissions

Comment 2: Have the authors tested their system under different operating conditions (e.g. S/C ratio, Voltage or current density of the fuel cell)?

Response 2: This work concerns the conceptual design of the cycle. The sensitivity analysis, as well as the optimization are beyond the scope of the study. On the other hand, the aim of the proposed technology is not to produce energy, but rather to find an economically and environmentally sustainable solution for thermal waste treatment. The target of the study at this stage is just to assess theoretically the feasibility of the process. 

Comment 3: Have the authors calculated economic metrics (e.g. NVP, LOCE)? 

Response 3: At the present stage of the study any economic metric would be premature. The economic analysis performed in the Section 6 just considers the existing WtE plants and the contribution to the total cost of the various components, especially those which are eliminated in the proposed plant. This is necessarily a coarse analysis, as the overall cost is not the sum of the single components costs, but a relevant quote is due to the way they are assembled. Another important aspect, which cannot be precisely estimated a priori, is the scale of the plant. Finally, an economic comparison with power plants supplied with different fuels would be not much significant, because here the objective is the treatment of non-recyclable waste, rather than the production of energy, which here is a by-product which allows to reduce the management costs

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

Weaknesses and Suggestions for Improvement

1. Abstract and Clarity

• Issue: The abstract is dense and does not clearly foreground the novelty or performance metrics.
• Suggestion: Emphasize in the abstract that the system is completely static, achieves 52.39% efficiency and delivers 7.71 MW net power with zero emissions.

1. Figures and Visual Aids

• Issue: Some diagrams (e.g., Fig. 2 – system layout) are overly dense, with small fonts and unclear flow paths.
• Suggestion: Enhance clarity with larger labels, consistent arrow directions and visual grouping by subsystem (e.g., fuel conversion, TA-MHD, H₂ processing).

1. Economic Feasibility

• Issue: The cost analysis is preliminary and lacks detail, particularly regarding emerging components (TA-MHD, high-temp fuel cells).
• Suggestion: Include sensitivity ranges or cite benchmark costs from similar technologies to support investment viability.

1. Language and Grammar

• Issue: There are grammatical inconsistencies and some unclear phrases (e.g., “main quote of electricity”).
• Suggestion: The manuscript would benefit from a professional language review to improve flow and scientific tone.

1. Assumptions and Limitations

• Issue: The model assumes ideal component behavior (e.g., perfect gas separation and 100% conversion in WGSR and fuel cells).
• Suggestion: Acknowledge technical limitations more explicitly and discuss potential efficiency losses or component constraints in real-world settings.

1. Literature Support and Citations

• Issue: Some references are outdated or incomplete in format (e.g., lacking full journal name or DOI).
• Suggestion: Update and standardize citations per MDPI formatting guidelines.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

There are grammatical inconsistencies and some unclear phrases (e.g., “main quote of electricity”). The manuscript would benefit from a professional language review to improve flow and scientific tone

Author Response

Comment 1:  Abstract and Clarity

• Issue: The abstract is dense and does not clearly foreground the novelty or performance metrics.
• Suggestion: Emphasize in the abstract that the system is completely static, achieves 52.39% efficiency and delivers 7.71 MW net power with zero emissions.

Response 1: The Abstract has been re-formulated to meet the suggestion of the Reviewer

Comment 2: Figures and Visual Aids

• Issue: Some diagrams (e.g., Fig. 2 – system layout) are overly dense, with small fonts and unclear flow paths.
• Suggestion: Enhance clarity with larger labels, consistent arrow directions and visual grouping by subsystem (e.g., fuel conversion, TA-MHD, H₂ processing).

Response 2: The images have been modified to meet the suggestion of the Reviewer

Comment 3: Economic Feasibility

• Issue: The cost analysis is preliminary and lacks detail, particularly regarding emerging components (TA-MHD, high-temp fuel cells).
• Suggestion: Include sensitivity ranges or cite benchmark costs from similar technologies to support investment viability.

Response 3: 

At the best of our knowledge, there aren’t other works which address the issue of emissions due to thermal treatment of waste. To deal with this problem, the usual approach is to reduce the waste, and for the residual part, to reach a balance between emitted and re-absorbed CO₂ by plants. This is a partial solution, because it addresses only CO₂ emissions, but they are not the only ones caused by combustion. This is the reason why we referred to zero-emission coal fired power plants, which specifically deal with emissions. However, at the present stage of the study any economic metric would be premature. The economic analysis performed in the Section 6 just considers the existing WtE plants and the contribution to the total cost of the various components, especially those which are eliminated in the proposed plant. This is necessarily a coarse analysis, as the overall cost it is not the sum of the single components costs, but a relevant quote is due to the way they are assembled. Another important aspect, which cannot be precisely estimated a priori, is the scale of the plant. Finally, an economic comparison with power plants supplied with different fuels would be not much significant, because here the objective is the treatment of non-recyclable waste, rather than the production of energy, which is proposed as a by-product which allows to reducing the management costs

Comment 4: Language and Grammar

• Issue: There are grammatical inconsistencies and some unclear phrases (e.g., “main quote of electricity”).
• Suggestion: The manuscript would benefit from a professional language review to improve flow and scientific tone.

Response 4: The article has been thoroughly revised to improve the language

Comment 5: Assumptions and Limitations

• Issue: The model assumes ideal component behavior (e.g., perfect gas separation and 100% conversion in WGSR and fuel cells).
• Suggestion: Acknowledge technical limitations more explicitly and discuss potential efficiency losses or component constraints in real-world settings.

Response 5: 

The hypothesis of an ideal functioning of all products reflects the fact that the study is still at a preliminary stage. Any more realistic hypothesis should refer to devices available on the market. This type of analysis goes beyond the scope of this work. However, some considerations have been added that help to frame the problem. In any case, these are consequences of an economic nature, in particular affecting the quantity of energy produced, while the no-emission characteristic is still preserved.

Comment 5: Literature Support and Citations

• Issue: Some references are outdated or incomplete in format (e.g., lacking full journal name or DOI).
• Suggestion: Update and standardize citations per MDPI formatting guidelines.

Response 5: The format of references is generated automatically by Mendeley. Unfortunately not all documents have a DOI number

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

The comments are placed in the attachment. 

Comments for author File: Comments.pdf

Author Response

Comment 1: The introduction mentions 'drawing on coal-fired power plant technologies,' but there are significant differences between the feedstock characteristics of waste gasification and coal. Please explain how the traditional process can be adjusted to accommodate waste treatment.

Response 1: in the new version the two types of system are compared, highlighting the main differences due to the different type of fuel, as well as the use in the proposed system exclusively of static devices 

Comment 2: In section 3.8, it is experimental data in the existing literature on the frequency matching of similar coupled systems? Please provide theoretical derivations or reference supporting data (such as specific conclusions from references [59, 62-64]).

Response 2: References 59, 62-64 have been further explained, indicating the results obtained.

Comment 3: In Section 4 of the article, the synthesized gas after conversion enters the absorption tower, where CO2 is separated from H2 and stored in liquid form at room temperature (80 bar). The power consumption for compressing CO2 can be limited by operating both the WGSR and absorption at high pressure simultaneously, as pressure has minimal impact on the conversion reaction, whereas high pressure benefits the absorption process. How significant is the energy consumption associated with compression in the overall system efficiency?

Response 3: 

In the hypothesis of carrying out the entire process at high pressure, the pumping cost is negligible, because the pressurization always occurs in the liquid phase. Even the CO2, which is in the gaseous state at the exit of the absorption column, can be easily condensed by lowering the temperature, and then pressurized when it is in the liquid state. It was not considered appropriate to describe these details because in the engineering phase the adopted solutions can vary greatly, while it was preferred to focus attention on the general aspects of the process. These considerations have been added in the new version of the paper.

Comment 4: In Section 4 of the article, the temperature of the syngas at the gasifier outlet reaches 1400° The article mentions the use of ceramic foam or metal foam, but does not specify the exact material types or their long-term corrosion resistance at 1400°C. It is recommended to include the rationale for material selection or experimental testing results.

Response 4: In the new version of the paper, the maximum temperature of the cycle has been lowered to a more suitable vale. However, a few indications have been added in Section 4 concerning materials

Reviewer 4 Report (New Reviewer)

Comments and Suggestions for Authors

This manuscript introduces a novel zero-emission waste-to-energy power plant concept. While promising, it needs clearer validation, realistic assumptions, and simplification to support practical feasibility and technical credibility.

1. The idea of a zero-emission plant with zero moving parts is wonderful; however, what really is new here compared to past WtE and IGCC systems should be explicitly highlighted.
2. The entire system is an ideal one. There is no lab test, no model, no sim. If you could somehow improve through at least a little experiment or maybe just by referencing some tested models, your claims would, I think, really stand stronger.
3. You propose a comparatively novel energy combination: thermoacoustic plus magnetohydrodynamics. It's a good idea—however, this coupling has not been proven by any real-world apparatus. Please provide some references or discuss the challenges related to linking these two.
4. The energy content of 19.55 MJ/kg for municipal wastes looks quite exaggerated. Is that a typical value? Add some data on local composition or real-world waste compositions to back this claim.
5. The economic analysis is interesting but rather vague. Certain key costs (NaK, cooling, pressure systems) have been either absented or underestimated. Please provide clarity on the true costs involved, especially in relation to novel components such as TAR and MHD.
6. The plant design is a bit too complex. Can that really be applied to a small developing area like southern Lebanon? Discuss the requisite technical capacity or infrastructure for this system to actually operate.
7. The paper is a deep dive into theoretical development—perhaps too deeply at points. Remove some of the repeated equations, and bring to the front some insight at a system level. The diagrams themselves could also be improved to be more intuitive.
8. The zero emissions claim is way too strong. They do capture CO₂, yes—but what about the other gases or any hazards posed by chemicals such as NaK? A realistic account of emissions and safety should be inserted.
9. This comprensivereview is solid, but it could use more up-to-date sources from 2023–2025. New studies on hydrogen production, WtE pilots, and hybrid conversion systems should be included.

Author Response

Comment 1: The idea of a zero-emission plant with zero moving parts is wonderful; however, what really is new here compared to past WtE and IGCC systems should be explicitly highlighted.

Response 1: The abstract and introduction have been reworded to better highlight the strengths of the work

Comment 2: The entire system is an ideal one. There is no lab test, no model, no sim. If you could somehow improve through at least a little experiment or maybe just by referencing some tested models, your claims would, I think, really stand stronger.

Response 2: The system is proposed as a conceptual design. On the other hand, the single parts which made up the system are well-established processes. Based on such partial results we tried to hypothesize the performance of the entire plant, but it is clear that such hypotheses have a large margin on uncertainty. To corroborate the assumptions made in the study, in the new version of the paper the experimental aspects of the references have been better highlighted.

Comment 3: You propose a comparatively novel energy combination: thermoacoustic plus magnetohydrodynamics. It's a good idea—however, this coupling has not been proven by any real-world apparatus. Please provide some references or discuss the challenges related to linking these two.

Response 3: The findings of the experimental works in the References have been better highlighted

Comment 4: The energy content of 19.55 MJ/kg for municipal wastes looks quite exaggerated. Is that a typical value? Add some data on local composition or real-world waste compositions to back this claim.

Response 4: The study assumes that the WtE plant is implemented within the management system described in [69]. The high value of the energy content derives from the fact that the waste separation occurs in a capillary way, so that all fractions with zero energy content, such as glass and metals, are separated at the source. Furthermore, the organic fraction, essentially the one contaminated by heavy metals and therefore not compostable, is subjected to dehydration, with the use of solar energy or residual heat from the plant. Since the pre-treatment of waste was not the specific object of the study, it was not considered appropriate to report information of this nature in the paper, referring to bibliography for further information.

In any case, a comment was added to highlight the reference.

Comment 5: The economic analysis is interesting but rather vague. Certain key costs (NaK, cooling, pressure systems) have been either absented or underestimated. Please provide clarity on the true costs involved, especially in relation to novel components such as TAR and MHD.

Response 5: Some devices, such as TAR and MHD generator, are not established in the market yet, even if they reached a high TRL, so that by assuming the current market prices could yield to overestimate the costs. Some references are reported to give an insight of the possible CAPEX/OPEX costs of the liquid metal MHD generator. Some other have been added concerning TAR. Anyway, as specified in Section 6, the TAR-MHD section contributes for a 5% of the total power, therefore in case the real cost should reveal to be excessive, the option to make simply a quench of the syngas is always possible. More in general, in this stage of the study the uncertainty about the scale of the single components, as well as supports, piping and facilities, makes unreliable a detailed estimate. Therefore, rather than to give a precise estimate, in this study we assumed to refer to the certificated costs of similar plants and to indicate the main aspects which could affect the costs of the proposed plant.

Regarding the power for pressuring, as this is performed on the liquid phase, it can be considered negligible in a preliminary study. Concerning the cooling, there is no special need in the proposed plant, therefore this cost can be embedded in the cost of similar plants.

Comment 6: The plant design is a bit too complex. Can that really be applied to a small developing area like southern Lebanon? Discuss the requisite technical capacity or infrastructure for this system to actually operate.

Response 6: There are several elements that lead to optimism. First of all, with 65 tons/day the plant is practically on a pilot scale. Secondly, all the components work without solid moving parts, which makes their management simple. Moreover, the largest devices, such as the gasifier, the absorption column, the fuel cell, are conventional devices, which therefore require skills that are easily available on the job market. The more innovative components, such as the TAR and the MHD generator, require personnel with research experience, who can be hired from the Lebanese academic environment, which boasts numerous excellences in this field. Finally, the plant does not require infrastructures that are not already available in the territory under study, essentially roads for the transport of materials, water network, power lines, data networks.

Comment 7: The paper is a deep dive into theoretical development—perhaps too deeply at points. Remove some of the repeated equations, and bring to the front some insight at a system level. The diagrams themselves could also be improved to be more intuitive.

Response 7: We worked to dry up the text and make the figures more intuitive.

Comment 8: The zero emissions claim is way too strong. They do capture CO₂, yes—but what about the other gases or any hazards posed by chemicals such as NaK? A realistic account of emissions and safety should be inserted.

Response 8: The aim of this work is only to present a concept design, therefore other issues which affect real plants, such as leakages and safety, are not considered here. What we want to remark is that, at least from an ideal point of view, no material is released in the atmosphere. Most part of pollutants and particulate present in the syngas are removed by means of conventional processes (Claus-Scot, scrubber, quench). Some residuals, such as Nitrogen oxides, can be separated from the Hydrogen after the CO2 has been condensed, but it depends on the purity requirements of the fuel cell.

Concerning safety for the use of Sodium, the quantity required for the considered level of power is below the threshold for adopting special measures. Anyway, other options, less critical, are available, such as GalInStan.

As we have done in other parts of this work, we have left out some aspects, even if important, to focus on the ideal cycle, which in itself involves several issues that deserve special attention.

Comment 9: This comprensivereview is solid, but it could use more up-to-date sources from 2023–2025. New studies on hydrogen production, WtE pilots, and hybrid conversion systems should be included.

Response 9: In the analysis of the state-of-the-art, we had to review a very large number of publications. This was inevitable, because the study covered various areas, from waste management to coal-fired power plants, conversion processes, gas treatment, economic analysis. Rather than giving an exhaustive overview of the publications of interest on related topics, we preferred to select the articles directly related with the work of the papers. This led us, in many cases, to prefer an older publication, but for this very reason already known to the Scientific Community, to more recent publications, which were not functional to clarify the discussion. The references of the years suggested by the Reviewer refer mainly to the economic or system aspects, which are subject to rapid changes, rather than to technological solutions, which instead require longer times.

Round 2

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors

-

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper presents the concept of an emission-zero integrated gasification static cycle power plant that can reduce pollutant emissions and achieve the goal of zero emissions. The article also has the following issues that need to be revised:

1.     The introduction should list previous articles on waste incineration treatment for comparison to highlight the innovation of this paper.

2.     The introduction draws on some technologies that have been developed for coal handling, and it is necessary to indicate exactly what they are and what advantages they offer.

3.     Chapter 2, Materials and Methods, is too long and diffuse, and it is recommended that it be streamlined to highlight the main points.

4.     The main application of power plants in the article is urban, but why were the villages chosen as the target for the case study in Chapter 4?

5.     The conceptual design in the article seems to lack experimental validation. It is recommended that the authors conduct experimental tests or simulations to verify the performance and efficiency of the proposed system.

6.     The main feature of the power plant in this paper is that it does not emit any greenhouse gases into the atmosphere, but the economic cost of the process is not discussed, and it is recommended that a related economic analysis be done.

 

7.     The formatting of references in the article needs to be adjusted, and there are several places where reference sources are not found.

Comments on the Quality of English Language

can be improved

Author Response

please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Although the subject is interesting, this paper should be vastly improved by following the reviewers observations. In this current state I recommend resubmitting the paper, after all these issues are covered.

The abstract lacks numerical values, which are essential for any scientific paper in engineering and energy conversion. These types of studies must emphasize the numerical results obtained to provide a clear context for the research.

The first part of the introduction, up to line 55, requires significant revision. It should be rewritten with your own interpretation and expressions, supported by more recent references to ensure the work is up to date.

Presenting only a table with three references does not provide sufficient info regarding the context. This section should be vastly expanded to include a  discussion of prior research in the field. Additionally, it is necessary to present the novelty of your study and how it contributes to advancing the current state of art.

The subsections 2.1 to 2.3 primarily discuss chemical reactions that are  found in standard chemistry handbooks. Unless there is a compelling justification for including these formulas in your paper, I strongly recommend removing them, as they do not appear to add substantial value. The same advice applies to subsection 2.5 with the formulas part of electrical energy handbooks.

Regarding MHD generators, while they are an interesting concept for converting energy into electricity, their practical application remains limited due to economic and material challenges. In the current context, they are more of a research interest than a commercially viable technology. For your study, all theoretical discussions about MHD generators should be complemented by a cost-benefit analysis of the proposed plant and an LCA  to ensure practicality and relevance.

Lines 406 and 407 contain ERRORS that must be addressed and corrected.

The scheme provided appears to be a theoretical amalgamation of various components.

No details are given regarding how the values were calculated or obtained. 

Finally, the Conclusion section is far too brief and does not present original, credible results. It requires significant expansion to include detailed insights, a summary of key findings, and their implications for future research or practical applications.

Comments on the Quality of English Language

English should be improved with more common expressions.

Author Response

please see the attachment

Author Response File: Author Response.pdf