Review Reports
by
- Gal Hochman 1,* and
- Vijay Appasamy 2
Reviewer 1: Daeseung Kyung Reviewer 2: Grégoire Léonard Reviewer 3: Anonymous
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis study provides a clear overview of the carbon capture costs, specifically examining bioenergy carbon capture and storage (BECCS), carbon capture and storage (CCS), and direct air capture (DAC) technologies. The use of a meta-analysis perspective is emphasized, leveraging literature to understand how cost estimates compare with avoided costs. It addresses an important and timely topic, but some refinement in manuscript could enhance its clarity and impact.
1. It would be better to explicitly state the thesis or main argument of the paper to guide readers on what to expect. For example, the authors could clarify if the paper aims to explore the effectiveness of carbon-negative technologies in achieving net-negative emissions.
2. When introducing BECCS, CCS, and DAC technologies, it would be better to provide concise definition with conceptual figures for readers. This will enhance the understanding of the technologies deal with and main contents in the manuscript.
3. It is recommended that Table 1 be modified to look better by subdividing the referenced paper by technology.
4. In the "Summary Statistics" section, the authors present a box plot showing the distribution of avoided costs across three technologies (CCS, BECCS, and DAC). It would be beneficial to include a brief explanation of why avoided costs are a crucial metric and how they contribute to the overall economic viability of these technologies.
5. When discussing the standard deviations of avoided costs for each technology, the manuscript mentions the greater variation in DAC costs due to different assumptions about CO2 concentration. It would be helpful to elaborate on how these assumptions impact the overall reliability and comparability of the DAC cost estimates.
6. When discussing the models, provide a concise explanation of BMA (Bayesian Model Averaging) and WALS (Weighted Average Least Squares) tests for readers who may not be familiar with these methods. This will enhance the accessibility of the manuscript to a wider audience.
Comments on the Quality of English LanguageN/A
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThank you for offering to review this paper. However, the description of the followed methodology is not clear enough for the readers from the CCUS field to understand the modeling assumptions, leading to making this paper not exploitable as such.
The exact scope of the paper is also often unclear (CCS? CCUS? Carbon removal?), maybe evidencing a lack of technical understanding of these technologies by the author's article. Similarly, the authors often use the concept of "cost of CO2 avoided" in an apparently wrong way (definition of the concept done by the authors appears unclear, and in any case different from the commonly agreed definition). The conclusion along which CO2 capture costs are overestimated by the literature is also in contradiction with the consensus being build in the field (see for instance: https://oilprice.com/Energy/Energy-General/Carbon-Capture-Is-Coming-Under-Fire-For-Underperforming.html) .
As a conclusion, a major rewriting is needed if the authors would like to address the CCUS community with this paper.
In particular, following comments can be made:
Major comments:
- Lines 71 to 74: these conclusions about CO2 capture are still somehow questionned in the litterature, especially as only few real plants have been built (no large-scale oxyfuel capture exist as far as I know), and very few data about their costs and performances have been published. These lines thus need to better reflect the uncertainties of these technologies, and sources need to be mentioned.
Line 89: post-combustion capture does not necessarily involve absorption-regeneration loop and liquid solvents. Adsorbents (instead of solvents) could be used, as well as membranes, cryogeny, ... in post-combustion configuration. Please make sure to make a clear difference between configurations (post, pre, oxyy-combustion) and technologies (membranes, chemical/physical absorption, cryogenic separation, ...). The technologies listed later on lines 92-101 could also be applied to configurations such as pre-combustion or oxyfuel combustion.
- Line 94: aDsorption is the right name, not absorption.
- Line 122: what are the 108 variables? What is meant with variable? Please illustrate
- Table 1: most papers are very old for a field of research in rapid development. The most recent paper dates back to 2018 and most papers are dating back to 2010 on average, with some papers even from the last century. This is clearly a major shortcoming of this study.
- Line 291 mentions that most papers used for BECCS were published after 2017. This is however in contradiction with Table 1. Please clarify.
- Lines 137 and 138: the focus appears to be clearly set on CO2 capture for power production, which is not the main single objective of CCUS technologies anymore, although still relevant. CO2 capture from raw materials (cement, steel, lime...) or DAC is not discussed here, making the discussion sound outdated.
- Line 156: the definition of "avoided costs" seems very strange. The avoidance cost needs to consider the CO2 intensity of the energy source used for capturing the CO2, which is apparently not the case of the definition used in the submitted paper. I would suggest the use of the following paper:
https://www.tandfonline.com/doi/full/10.1080/17583004.2018.1553435
- Line 173: the second method for estimating avoided costs does not seem right, as it is based on CO2 tax values, which are incentives but cannot be assumed to break even with real CO2 capture costs.
- Line 365: "reports with more authors per paper": is that a variable of the model? What are the variables used in the model? The work is very difficult to understand to be honnest...
Minor comments:
- The introduction mentions about COP27 but not about COP 28. It would be more relevant to consider the most recent edition.
- The abstract mentions about CCS in coal power plants, while the introduction focusses on carbon removal technologies. Clean coal is not part of carbon removal technologies, this confusion should be avoided and the scope of the paper should be clarified.
- Section 2.1 starts with the wording "carbon sequestration" which is not covering the whole section, e.g. carbon use is not a sequestration option. The scope of section 2.1 needs thus to be clarified, is it about carbon removal (which may include mineralization for instance) or about geological sequestration only?
- Line 97: different types of membranes do exist, not only composite polymers. For instance, ceramic membranes. Idem, line 99: cryogenic separation may also work with a separation of CO2 in liquid phase, not solid.
- line 103: again, different ways of transporting CO2 exist, either in dense form (supercritical or liquid) or non-dense (compressed gas). There is not yet a consensus (at least in Europe) about the best way to transport it.
- 2.1.4: other potential re-use paths for CO2 are the production of e-fuels and the production of carbon-based materials (urea is already a large market, polycarbonates...). See for instance: https://www.nature.com/articles/s41586-019-1681-6
- Line 119: what is meant by "empirical studies"? Experimental studies?
- line 127: why working in 2016 dollars, and not more recent values?
- Equations 7.1 and 7.2: although I am not familiar with the statistical methodology used, I am skeptical about equations 7.1 and 7.2: how does is come that a same expression should be at the same time equal and different to zero?
- Line 289 mention a maximum cost of $2200 per ton of CO2 for DAc, while line 309 mentions $1000. Which one is correct?
- Table 3: coal, NG and air have indices attributed in the variable column. However, it is not clear where these indices have to be read?
- Table 3: the price of electricity should not depend so heavily (~600 vs ~1-5 $/kWh) on the capture vs avoided cost assumption.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors can please refer to the attached file.
Comments for author File: Comments.pdf
The authors can please refer to the attached file.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe reviewer would like to thank the authors for the improvements made, the article is now much clearer. However, it is still tainted with some flaws that decrease its relevance, and that have been already discussed, such as:
- lack of more recent references in the analysis, especially for DAC and BECCS for which the field of research has been booming in recent years
- conclusions that are far away from the current observations in the field (real-life installations) which are stating that the cost of carbon capture and storage is actually underestimated by scientific literature.
This being said, the article is a relevant meta-study of published works, which is well written and that brings some interesting insights into CCS cost estimates available in the literature.
Some minor comments can still be addressed before publication:
- line 214: "regression's sans" => what does "sans" mean?
- Is Figure 3 related to CCS (as in the caption) or to BECCS (as indicated in line 438)?
- Lines 491-492: please check grammar
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsA careful reading of the manuscript is required to check typos.
Author Response
We thank the reviewer for previous comments but could not find additional comments beyond those supplied during the first revision.
Please advise if we missed anything, and I am sorry for the confusion.