Round 1

Reviewer 1 Report

The authors propose a comparative study of four CO2 capture technologies applied to a part of syngas produced by coal gasification in order to manufacture Ethylene Glycol which needs a high ratio of hydrogen over carbon monoxide.The paper is clearly presented in terms of exposed hypothesis and the approach for benchmarking the different technologies. There are however some enhancement necessary before its publication :

1. Please evaluate more precisely how cold could be recovered from ASU to be used in Rectisol: is there sufficient cold (cryogenic temperature range) available in ASU allowing to down the methanol temperature to less than 55°C?
2. The english writing should be enhanced little bit, please pay attention to according the verbs to the subject as in line 201, page 5 where the sentence 'As a result, the CO2 and H2S of syngas is sufficiently absorbed' should be 'As a result, the CO2 and H2S of syngas are sufficiently absorbed' or in Line 235, page 7, the sentence 'Due to DMC has a high solubility of CO2 and a low toxicity' could be rephrased in 'Due to high solubility of CO2 in DMC' etc ... in may places where 'Due to ...' is used.

Thank you for the effort.

Author Response

Reviewer #1:

The authors propose a comparative study of four CO₂ capture technologies applied to a part of syngas produced by coal gasification in order to manufacture Ethylene Glycol which needs a high ratio of hydrogen over carbon monoxide. The paper is clearly presented in terms of exposed hypothesis and the approach for benchmarking the different technologies. There are however some enhancement necessary before its publication:

1. Please evaluate more precisely how cold could be recovered from ASU to be used in Rectisol: is there sufficient cold (cryogenic temperature range) available in ASU allowing to down the methanol temperature to less than 55 °C?

Answer: Thank you for your comments. We are very sorry for the above misunderstanding. What we emphasize is the use of the cooling capacity of low-temperature nitrogen of the air separation unit. Due to the cold nitrogen is at very low temperature and has a relative high cooling ability, the cold utility of the Rectisol and dimethyl carbonate processes can be greatly reduced by integrating with the cold nitrogen stream of the air separation unit, which has also been widely used in industry; but it still needs other coolants to cool the methanol.

2. The English writing should be enhanced little bit, please pay attention to according the verbs to the subject as in line 201, page 5 where the sentence 'As a result, the CO₂ and H₂S of syngas is sufficiently absorbed' should be 'As a result, the CO₂ and H₂S of syngas are sufficiently absorbed' or in Line 235, page 7, the sentence 'Due to DMC has a high solubility of CO₂ and a low toxicity' could be rephrased in 'Due to high solubility of CO₂ in DMC' etc ... in may places where 'Due to ...' is used.

Answer: Thank you for your comments. We have corrected these poor language in the revised paper. In addition, we have checked the grammar errors, corrected these incomprehensible language carefully and re-edited the manuscript.

Reviewer 2 Report

The authors present an interesting comparative analysis of carbon capture technologies applied to coal to ethylene glycol process. However, the overall structure of the paper is quite confusing and does not allow for a complete understanding of the methodology followed from a process simulation point of view. I suggest major revisions before considering the publication of this manuscript. In the following some detailed comments:

• Using acronyms in the abstract is quite confusing. Please consider explicitly reporting the meaning of the acronyms
• The references from line 101 to 120 don't seem to be pertinent to the manuscript topic. Can the authors explain? Is the reason for citing them, to justify using a techno-economic analysis?
• Line 148: is 95% molar or mass base?
• I could not access the supporting information. As a general comment, it is necessary to provide the detailed process information of each unit in figure 2.
• Table 1 is very confusing. There are no units for some variables, which have not been defined. The manuscript needs a nomenclature and explanation of each variable and symbol
• It is very difficult to understand the validation process. No information is given about the reference cases and sizes of the plants considered. This part needs to be improved to allow for a good understanding of the methodology
• Line 299: the authors do not defined anywhere any Key Performance Indicators. There is no definition of the number reported here.
• Some sentences are very vague. See for example line 312: "..., which need lots of steam..." This is way of writing in not scientifically sound

Author Response

Reviewer #2:

The authors present an interesting comparative analysis of carbon capture technologies applied to coal to ethylene glycol process. However, the overall structure of the paper is quite confusing and does not allow for a complete understanding of the methodology followed from a process simulation point of view. I suggest major revisions before considering the publication of this manuscript. In the following some detailed comments:

1. Using acronyms in the abstract is quite confusing. Please consider explicitly reporting the meaning of the acronyms.

Answer: Thank you for your comments. We have explicitly described the meaning of the acronyms in the abstract. The MEA, CAP and DMC are CO₂ capture technologies, which mean mono-ethanol amine, chilled ammonia process and dimethyl carbonate, respectively.

2. The references from line 101 to 120 don't seem to be pertinent to the manuscript topic. Can the authors explain? Is the reason for citing them, to justify using a techno-economic analysis?

Answer: Yes, you are right. In the revised paper, the references from line 101 to 120 are removed to make the structure of the revised paper more reasonable.

3. Line 148: is 95% molar or mass base?

Answer: Thank you for your comments. The 95% refers to molar base. We have corrected this mistake.

4. I could not access the supporting information. As a general comment, it is necessary to provide the detailed process information of each unit in figure 2.

Answer: Thank you for your comments. Considering the length of the paper, the detailed process information (parameters, modelling and simulation) of each unit is placed in the supporting information. Sorry for our negligence, we did not attach the supporting information when we submitted the manuscript. The supporting information will be submitted together with the revised paper.

5. Table 1 is very confusing. There are no units for some variables, which have not been defined. The manuscript needs a nomenclature and explanation of each variable and symbol.

Answer: Thank you for your comments. In table 1, △P is the pressure drop, Q is the heat load, η is the mechanical efficiency of pump, 35% is the molar concentration of the lean MEA. We have added a nomenclature with explanation of each variable and symbol in the revised paper.

6. It is very difficult to understand the validation process. No information is given about the reference cases and sizes of the plants considered. This part needs to be improved to allow for a good understanding of the methodology.

Answer: Thank you for your comments. The selected reference CtEG plant is a successful operated plant in Inner Mongolia. The production scale of all cases is assumed to be the same, i.e., 300,000 tons/year. The method in this part is relatively simple, which is to compare the simulation results with actual industrial or literature data. In order to facilitate readers' understanding, we have carefully modified this part.

7. Line 299: the authors do not defined anywhere any Key Performance Indicators. There is no definition of the number reported here.

Answer: Thank you for your comments. In this study, the energy consumption, capital investment, CO₂ capture cost, production cost, and internal rate of return are conducted to compare the techno-economic performance of the CTEG process with Rectisol, MEA, CAP and DMC technologies. We have supplied these indicators and their definition in the revised paper. Eq. (14) is used for energy consumption calculation, Eq. (15) and (16) for capital investment calculation, Eq. (17) - (21) for production cost calculation, and Eq. (22) for internal rate of return calculation.

8. Some sentences are very vague. See for example line 312: "..., which need lots of steam..." This is way of writing in not scientifically sound

Answer: Thank you for your comments. We have checked the grammar errors, corrected these incomprehensible language carefully and re-edited the manuscript.

Reviewer 3 Report

This manuscript compares different CO2 capture technologies for a specific industrial process. This paper is well structured and the key points have been presented clearly. The following minor points should be addressed: 

The abstract should be able to be read alone. This is not possible with too many abbreviations in your abstract. 

Line 212: eNRTL is the "thermodynamic method" not "physical method

The source of the parameters in Table 1, 2, & 5 etc must be cited

The numbering in the conclusion section creates confusion as the heading itself is number 4. Use roman numerals or bullet points

Author Response

Reviewer #3:

This manuscript compares different CO₂ capture technologies for a specific industrial process. This paper is well structured and the key points have been presented clearly. The following minor points should be addressed:

1. The abstract should be able to be read alone. This is not possible with too many abbreviations in your abstract.

Answer: Thank you for your comments. In order to conveniently describe the comparison of the techno-economic performance of the CTEG process with Rectisol, MEA, CAP and DMC technologies and reduce the length of the abstract, we use abbreviations in abstract. We have explicitly described the meaning of the acronyms in the abstract. The MEA, CAP and DMC are CO₂ capture technologies, which mean mono-ethanol amine, chilled ammonia process and dimethyl carbonate, respectively.

2. Line 212: NRTL is the "thermodynamic method" not "physical method.

Answer: Thank you for your comments. We have corrected this mistake in the revised paper.

3. The source of the parameters in Table 1, 2, & 5 etc must be cited.

Answer: Thank you for your comments. We have supplied the cited references in Table 1, 2, & 5 etc.

4. The numbering in the conclusion section creates confusion as the heading itself is number 4. Use roman numerals or bullet points.

Answer: Thank you for your comments. We have corrected this with roman mumerals in the revised paper.

Round 2

Reviewer 2 Report

The authors have addressed most of the comments. I would still kindly ask the authors to tidy up the tables in the revised manuscript and add a "Variables" section in the Nomenclature to clarify definitely what N, P, T, ΔP, etc are and their units of measure. 

Also, in one of the answers, the authors report that the size of the plant is 300,000 ton/year, but this number is not reported in the text. The validation reports 37.5 t/h. What is the equivalent operating hours per year of the plant?

Given the extensive thermodynamic simulations that the authors have done for the different plant configurations, I would expect that it should be possible to identify Key Performance Indicators to assess the quality of the process on a relative basis (percentage), so that a comparison with other studies and plants is easier. Can the authors define an efficiency measure for the production of ethylene glycol that can take into account of the different efficiencies of the process and the different CO2 emissions? An example, quite used nowadays in the literature for CCS processes, is the SPECCA index. You can find and example of its use in this recent publication: https://doi.org/10.1016/j.jpowsour.2019.227461

Having a thermodynamic way to compare your cases I think is important.

Author Response

1. The authors have addressed most of the comments. I would still kindly ask the authors to tidy up the tables in the revised manuscript and add a "Variables" section in the Nomenclature to clarify definitely what N, P, T, ΔP, etc are and their units of measure.

Answer: Thank you for your comments. We have added a variables section in the Nomenclature in the revised paper. Please see in the Nomenclature.

2. Also, in one of the answers, the authors report that the size of the plant is 300,000 ton/year, but this number is not reported in the text. The validation reports 37.5 t/h. What is the equivalent operating hours per year of the plant?

Answer: Thank you for your comments. The production scale of ethylene glycol of all cases is assumed to be 300,000 tons/year. The equivalent operating hours per year of the plant is 8000 h. The production of ethylene glycol per hour is 37.5 ton.

3. Given the extensive thermodynamic simulations that the authors have done for the different plant configurations, I would expect that it should be possible to identify Key Performance Indicators to assess the quality of the process on a relative basis (percentage), so that a comparison with other studies and plants is easier. Can the authors define an efficiency measure for the production of ethylene glycol that can take into account of the different efficiencies of the process and the different CO₂ emissions? An example, quite used nowadays in the literature for CCS processes, is the SPECCA index. You can find and example of its use in this recent publication: https://doi.org/10.1016/j.jpowsour.2019.227461

Answer: Thank you for your comments. For different CO₂ capture technologies used for coal to ethylene glycol processes, we defined energy efficiency index other than energy consumption to express the CO₂ capture energy consumption and the efficiencies of different situations of coal to ethylene glycol processes. Detailed method definition, description and discussion, please see in lines 12-13, page 17; lines 2-11, page 18; lines 20-28, page 19. In addition, Fig. 12, the overall performance comparison, has been changed according to these analyzed indexes.

4. Having a thermodynamic way to compare your cases I think is important.

Answer: Thank you for your comments. In this paper, we have added energy efficiency index to analyze and compare their thermodynamic performance of different CO₂ capture technologies for coal to ethylene glycol processes. Detailed description and comparison, please see in lines 12-13, page 17; lines 2-11, page 18; lines 20-28, page 19.