Assessment of Co-Gasification Methods for Hydrogen Production from Biomass and Plastic Wastes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The research work is well structured, the topics are adequately developed and the references cited are relevant. The limitations of the study are stated in point 3.5. However, in the development of the issues, despite setting the concepts and referring to the sensitivity analysis, incorporating quantitative information could provide greater solidity to the statements and partial conclusions. It is understandable that within the framework in which the work is carried out (four methods and six case studies are pretty broad), it remains for the most part quite conceptual.

The presentation of a synoptic table of the cases presented will probably serve as a basis for subsequent quantitative validation studies.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Thanks to the authors for their proposition. The paper evaluates the heat and mass balance numerically  for electricity and hydrogen cogeneration, using municipal solid wastes (MSW) with a high energy density and comparing the performances in terms of hydrogen and energy productions with more mature technologies as coal autothermal gasification or steam methane autothermal reforming, this latter is taken as a reference as it's the main technology nowadays for hydrogen production, although catalytic steam methane reforming has the big share. MSW is used in classical way as for coal and natural gas and also in an innovative process based on gasifier in-situ CO2 adsorption by calcium-based sorbent (SEWGS for Sorbent Enhanced Water Gas Shift reactions). There are however some points that should be clarified in this study before publication:

1. Page 3, line 88: the statement that 'Steam gasification uses supercritical steam ...' is maybe a typographical error, usually surperheated steam is sufficient? Please confirm that the gasifier pressure is 50 bars (page 3, line 76)! And also that the feedstock is in slurry form in order to be compressed to such high pressure.

2. Page 3: it's not very clear how the steam is integrated to the process for the cases 1, 2 and 3. Cases 4 and 5 clearly use external boiler while for cases 1 to 3, one understands that steam for gasification and WGS is provided internally from the process as there is no value for this parameter in Table 3 for Cases 1, 2 and 3 (line Steam Firing Duty). However, the balance of Table 2 shows equality of HRSG steam flow rate condensed by cooling water (line HRSG to Cooling Water of Table 2) and the HRSG feed water rate (line HRSG Water Feed, please correct to HRSG Feed Water as for other feed fluxes). If the steam of gasification and WGS is integrated to the process, some amount of steam exported from HRSG is converted to hydrogen in the gasifer and in the WGS units, other part of steam is blowdown as water in syngas cooling system, in other hand an amount of steam could be produced by the WGS unit. Steam which could be produced by the WGS unit needs feed water and doesn't balance necessarily the steam exported from the HRSG to both the gasifier and the WGS units. Please clarify the feed water and steam balance for Case , 2 and 3.

3. Page 4, line 159: the statement 'However, to model the liquid-liquid gas equilibria in the SELEXOL ...' is not clear. As I know, Selexol solvent is used in pure form and not in aqueous one, so the liquid-liquid separation is not applicable here neither the electrolyte reactions as the absoprtion is of physical nature and not of the chemical one. NRTL is however very powerfull for determining activity coefficients of chemical species in liquid phase, allowing accurate prediction of vapor-liquid equilibria.

4. Page 6, line 259: This auxiliary consumption is given in different unit than input heat, it should be noticed out that it's of the same order of magnitude than the heat provided by the feedstock, around 1200MWth, which explain the higher production of the concerned cases!

5. Page 8, lines 318-319: the statement 'using such a physisorptive method for acid gas removal can adequately capture almost all the carbon dioxide generated during gasification' maybe is exaggerated, the capture rate in this type of pre-combustion process depends on WGS conversion rate and Selexol-based process capture rate, these parameters should be over 97% to achieve around 94% global capture rate with however significant energy consumption (steam) to attain the respective 97% rates. 94% CO2 global capture rate could be considered as a limit beyond which the energy consumption grows rapidly.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

This paper presents a comprehensive thermodynamic model and life cycle assessment of integrated gasification combined cycle systems for the treatment of mixtures of biomass and plastic wastes. Some necessary modificataion are listed below:

In the abstract Key findings on steam gasification, in-situ CO2 capture using sorbents, and integration with renewable energy could be highlighted more prominently.

In the introduction, overall the literature review does not indicate the necessity to conduct the presented research. Therefore, as a reader, it is difficult to draw a conclusion from them as to why this study has been carried out.

In methodology the model validation is not discussed.

Many assumptions are made throughout the simulation models which can influence the results and conclusions. These should be clearly validated.

Only one feedstock combination is analyzed (corn stover + plastic). Varying shares and types of feedstocks may give different results.

No uncertainty analysis is done. Sensitivity to model parameters is not fully assessed.

Long term dynamics like sorbent recyclability, carbonate utilization options are not considered.

The length of the discussion could be shortened in areas by moving repetitive interpretations to the conclusion. 

Author Response

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Author Response File: Author Response.docx