Transitioning Hochschule Geisenheim University: A Shift from NET Source to NET Sink Regarding Its CO2 Emissions
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript is very interesting, scientifically sound and well written. It also touches upon a relevant topic, i.e. the achievement of negative emissions within individual organisations.
From the technical point of view the authors should be more detailed in the description and mass balance of the pyrolysis process, and more explicit about the biogenic CO2 emissions generated by the combustion of the syngas. Of course their GWP is zero, but I would like them to be quantified and separately reported.
From a more general point of view, the manuscript strictly refers to the specific case of Geisenheim University, where the high generation of biomass waste facilitates the achievement of negative emissions. In order to make the results more replicable to other universities, the authors should extend their final discussion, trying to suggest possible alternative solutions for other types of universities in less favourable situations.
More detailed remarks are reported directly in the annotated pdf.
Comments for author File: 
Comments.pdf
Author Response
Summary: Dear Reviewer! Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions and correction in the track changes of the re-submitted file. I thank you for improving my piece.
Comment 1: "CH4 is generally negligible in the syngas"
Response 1: Thank you for this important observation. I added reference [24] (Khlifi, Pozzobon et al. 2024) which demonstrates that certain combinations of process parameters can indeed lead to minimal CH4 production, but as well indicates that in most pyrolysis configurations, CH4 concentrations typically exceed 5% of the syngas composition. Given this significant presence in common applications, I believe it is important to include CH4 in the list of primary syngas components. I have modified the text to better reflect the variable nature of syngas composition through the added qualifier 'in varying ratios, depending on feedstock, heating rate, temperature and residence time' p.2 line 92ff
Comment 2: strictly speaking it gets oxidated to CO, so please rephrase
Response 2: Thank you for this observation. While some carbon is partially oxidized to CO using oxygen from the biomass itself, the dominant process is aromatization and condensation. I have revised the text to: 'As no external oxygen is supplied to the process, most carbon undergoes aromatization and condensation rather than oxidation, resulting in stable carbonaceous structures.' and hope you find it more accurate. p.2 line 94
Comment 3: Wood chip combustion (figure)
Response 3: Thank you for this remark. Totally agree, changed p. 3 figure 1
Comment 4: GB or PB? Please use the same acronym throughout the paper
Response 4: Thank you for noting this inconsistency, changed in all occurences. p.5 figure 2
Comment 5: explain the purpose of the transformer stations
Response 5: Thank you for this question. I expanded the description to better explain the purpose: 'The university operates four transformer stations that step down the incoming medium voltage to low voltage levels required for building operations and laboratory equipment:' p.5 line 160
Comment 6: please comment on the fact that assessing the Scope 3 emissions in percentage term is very uncertain, since it mainly depends on the entity of Scope 1 and 2 ones.
Response 6: Thank you for this important methodological observation. You are correct that expressing Scope 3 emissions as a percentage can be misleading since the percentage heavily depends on the magnitude of Scope 1 and 2 emissions. I modified the text by adding a note about this limitation in reporting Scope 3 emissions. 'However, such percentage estimates of Scope 3 emissions should be interpreted with caution as they are highly dependent on the magnitude of Scope 1 and 2 emissions at individual institutions.' p.6 line 191
Comment 7: whats the mix?
Response 7: Thank you for this critical question. The "mix" applied is manure/maize I clarified it in the table 1
Comment 8: is this just a water loss? Please be more explicit
Response 8: Thank you for your comment regarding line 203-207. To clarify: The 19% loss refers to physical collection losses during mechanical harvesting and baling operations, not water loss. All values are reported on a dry matter basis. I revised the text accordingly to make this explicit. 'The theoretical potential of biomass was determined based on dry matter (DM) by combining literature values and on-site pruning residue harvest determined in long term trials by the Institute of Viticulture and the Institute of Plant Nutrition and Soil Science of Geisenheim University [38–42]. Technical potential was calculated accounting for physical collection losses of 19 percent during mechanical harvesting and baling operations (e.g., material left on the ground, losses during baling process)' p.6 l 216
Comment 9: please be more explicit on the fact that the syngas gets combusted, producing energy and releasing biogenic CO2 emissions
Response 9: thank you for pointing out the neccessity to be more clear. The text has been revised to explicitly describe the combustion of syngas and the associated biogenic CO2 emissions during the pyrolysis process. I added details about the carbon conversion pathways and included a reference supporting these technical aspects. The revised text now clearly explains how the partial oxidation during pyrolysis leads to both heat generation and carbon stabilization in biochar. p7 line 239
Comment 10: All units of measure are missing! p.9 line 342
Response 10: Thank you. Added units
Comment 11: which unit of measure? Years? p. 12 table 6, line 421
Response 11: Yes, years, fixed.
Comment 12: plant 2 is not existing in this scenario, please cross check
Response 12: Thank you very much for finding this major mistake. The Table shouldn't appear in the article because it did not represent the final version. Therefore Table 6 has been updated and the paragraph has been rephrased accordingly.
'The comparative analysis of different heating systems revealed distinct performance profiles (Table 6). All pyrolysis-based systems demonstrated significant carbon dioxide removal potential, though their economic performance varied. The reference fossil gas and biomethane (BM) systems, despite being technically simple, showed LCOE of 188 and 197 €/MWh respectively. Pure wood chip firing (WC) achieved 198 €/MWh, while pure pyrolysis (PY) reached 287 €/MWh but provided the highest carbon removal at -36,376 t CO2e.
Hybrid systems combining pyrolysis with wood chip boilers showed improved performance characteristics. The smaller hybrid configuration (PY10/WC35) achieved carbon removal of -11,619 t CO2e but at an above-average LCOE of 225 €/MWh. The larger hybrid system (PY20/WC25) emerged as optimal, removing -32,929 t CO2e while maintaining an LCOE of 198 €/MWh when partially supplied with residual biomass - comparable to the pure wood chip system.
The economic performance improved substantially when operating with locally sourced biomass (PY20/WC25 II), reducing LCOE to 131 €/MWh. This configuration requires approximately 2,600 t of annual biomass input to produce 638 t of biochar, aligning with the identified local biomass availability.'
Comment 13: BC acronym was not introduced
Response 13: fixed in line 69
Comment 14: please round all economic figures!
Response 14: Thank you for your valuable comment regarding the readability. I improved the readers experience by rounding to zero digits and hope you agree with my decision to leave the two digits in table 6.
Thank you for your time to improve my paper. Please see the attachment for my response to your remarks. If you have issues accessing the calculation file, you can download it here: https://hgucloud.hs-gm.de/s/iAgi3XifjMyFbRn
Using the password: c-Sink1!HGU
Best regards!
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for Authors1. How did the author calculate the energy and cost of the pyrolysis system? Pyrolysis is an endothermic process, meaning it requires energy input. Although theoretically, it can be used for heating. It is challenging in practical application. The manuscript claims that the system can provide sufficient heat for the university, but it does not fully explain the calculations. Please include all the references, calculations, and assumptions for the numbers used in the Pyrolysis part.
2. How about the scalability of this strategy? Can it be applied to other HEIs? It would be great if the authors could discuss this.
3. Please check reference errors, for example, L 365 in Section 3.3.
4. It is suggested to explain some concepts/ technical terms in the introduction section, such as bivalent heating systems and C-sink certificates.
Author Response
Dear reviewer!
Thank you for your time to improve my paper. Please see the attachment for my response to your remarks. If you have issues accessing the calculation file, you can download it here: https://hgucloud.hs-gm.de/s/iAgi3XifjMyFbRn
Using the password: c-Sink1!HGU
Best regards!
Author Response File: Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsTitle: Transitioning Hochschule Geisenheim University: A Shift from NET Source to NET Sink regarding its CO2 emissions
Authors: Georg Ardissone-Krauss, Moritz Wagner, Claudia Kammann
Journal: Sustainability
Review
The study addresses a relevant and innovative topic within the transition to carbon neutrality in higher education institutions (Hochschule Geisenheim University). Its focus on the use of waste biomass and pyrolysis systems as a COâ‚‚ removal mechanism (CDR) is innovative.
However, there are some topics that should be addressed before publication, e.g.:
- How are uncertainties in biomass availability and quality handled?
Perhaps, some of these values are in the Supplementary Materials, as stated in lines 324- 325 “The complete analysis methodology and parameter boundaries are documented in the Supplementary Materials”, but in the version submitted for review, the Supplementary Materials could not be accessed.
- How to handle variations in factors such as biomass availability, pyrolysis system efficiency or biochar market costs, for example, a supply of pruning residues is mentioned, but there is no analysis of seasonality or variations in biomass production.
- What is the experimental evidence on the performance of the proposed system, has it been implemented at pilot scale or is it only a theoretical projection? If based on models, have these models been validated with data from real systems?
Minor changes
- Line 112, Rheingau region (Table 4). Mentioned very distant from where it is, even 3 tables appear first.
- Revise the format of tables, top and bottom lines.
- Improve quality of figures, e.g., Figure 4.
- Revise the style of references, e.g. Refs. 1, 22, 26. 1, 22, 26.
MDPI style
“- References to books should cite the author(s), title, publisher, publisher location (city and country), publication year, and page:
Smith, A.B. Textbook of Organic Chemistry; D. C. Jones: New York, NY, USA, 1961; pp. 123-126.
- In referring to a book written by various contributors, cite author(s) first:
Winstein, S.; Henderson, R.B. In Heterocyclic Compounds; Elderfield, R.C., Ed.; Wiley: New York, NY, USA, 1950; Volume 1, Chapter 1, p. 60.”
Author Response
Dear reviewer!
Thank you for your time to improve my paper. Please see the attachment for my response to your remarks. If you have issues accessing the calculation file, you can download it here: https://hgucloud.hs-gm.de/s/iAgi3XifjMyFbRn
Using the password: c-Sink1!HGU
Best regards!
Author Response File: Author Response.docx
Reviewer 4 Report
Comments and Suggestions for AuthorsThis is a very well-designed and conducted research. The writing and presentation technique is satisfactory. The topic itself has scientific and social justification. I suggest that the manuscript be accepted after the following minor changes and additions to the manuscript are made.
1) The abstract must be written in one paragraph.
2) The abstract should state what the aim of the research was
3) Do not write the manuscript in personal pronouns (we…..) and possessive adjectives (our… ). The manuscript should be written in the third person and in the past tense.
4) Line 164: Use the unit (ha) for – hectares.
5) Line 206: Use the symbol (%) for word– percentage.
6) Lines 223-225: This sentence is unclear. It should be noted that biomass ash is being referred to here.
7) The introductory chapter is very well written. I suggest that the introduction be expanded with the following statement: “In agriculture, a large amount of biomass is produced annually, such as: corn stalks, corn cobs, straw, sunflower stalks, sunflower husks, fruit pits, fruit tree and grapevine pruning residues, etc., forming a relatively easily usable form of energy.” (Consult and add the following research: https://doi.org/10.2298/TSCI231223104E).
8) Line 266: It is unclear specifically for which biomass the price is given. 0.05 €/kWh.
9) Line 365: There is a printing mistake.
10) In Table 4, the potential of biomass from vineyards is shown. It remains unclear whether this potential is calculated with a sustainability factor of 1, which would mean that the entire biomass from vineyards is used for energy purposes. Based on the values shown in Table 4, a conclusion can be drawn, but this should be specified in the paper.
11) The Methodology of the paper should state which scientific methods were used in the research.
12) If the authors have data on the average yield biomass per vine - (kg/vines), it would be good to present it.
13) Table 6 should not be presented in the form of a Figure.
14) Line 453: Use the unit (€/t) for - € per ton.
15) Lines 473 and 487: Use the unit (t) for - tons.
16) The discussion is very well and clearly written.
17) The conclusion is clear.
18) The choice of references is good.
Author Response
Dear reviewer!
Thank you for your time to improve my paper. Please see the attachment for my response to your remarks. If you have issues accessing the calculation file, you can download it here: https://hgucloud.hs-gm.de/s/iAgi3XifjMyFbRn
Using the password: c-Sink1!HGU
Best regards!
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors addressed all my comments comprehensively. The quality of this manuscript has improved a lot.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe author has improved the presentation and has responded to the concerns raised, for my part, there are no problems if the document is accepted.
Comments on the Quality of English LanguageNo comments
