Highly Efficient CO2 Capture and Utilization of Coal and Coke-Oven Gas Coupling for Urea Synthesis Process Integrated with Chemical Looping Technology: Modeling, Parameter Optimization, and Performance Analysis
Round 1
Reviewer 1 Report
This work developed a new concept for urea generation, where the CUT, CLAS, CLH are combined to get a good system efficiency. ASPEN software was used for process analysis, comparison, and techno-economic analysis. Sufficient data was obtained and used for the comaprison of various integrations. In general, this is a good paper, well arranged and written. I think this work can be accept for publication. Below are minor comments the author may think when revising.
- English language should be improved.
- More literatues can be refered for hydrogen generation from CLH technology. E.g. https://doi.org/10.1016/j.jpowsour.2008.11.038, https://doi.org/10.1016/j.fuproc.2021.107088
- May be it can present better the CLAS and CLH technologies if the authors include a schematic description of these two technologies
- 830℃ was selected for CLAS, based on thermodynamics, but the authours should be aware this temperature might be too low for oxidization and oxygen release in practical CLAS.
- Mn2O3 and Fe2O3 may be not suitable for CLAS, as you cannot expect siginificant O2 release from them.
Author Response
This work developed a new concept for urea generation, where the CTU, CLAS, CLH are combined to get a good system efficiency. ASPEN software was used for process analysis, comparison, and techno-economic analysis. Sufficient data was obtained and used for the comparison of various integrations. In general, this is a good paper, well arranged and written. I think this work can be accepted for publication. Below are minor comments the author may think when revising.
1- English language should be improved.
Answer: Thanks for your comments. The English language has been improved, including the grammatical errors and the incomprehensible sentence.
2- More literatures can be referred for hydrogen generation from CLH technology. E.g. https://doi.org/10.1016/j.jpowsour.2008.11.038, https://doi.org/10.1016/j.fuproc.2021.107088.
Answer: Thanks for your suggestion. We have referred more relevant literatures for CLH technology in the modeling section.
3- May be it can present better the CLAS and CLH technologies if the authors include a schematic description of these two technologies.
Answer: Thanks for your comments. We have added the schematic description of the CLAS and CLH in the revised paper.
4- 830℃ was selected for CLAS, based on thermodynamics, but the authors should be aware this temperature might be too low for oxidization and oxygen release in practical CLAS.
Answer: Thanks for your comment. A higher temperature, e.g., 840 ℃, will keep increasing the promotion of the conversion rate of Mn2O3. We added the case of 840 ℃ in the Figure 11. As the temperature changes from 800 to 840 ℃, the steam/Mn2O3 ratio will change from 0.25 to 0.6 to maintain the Mn2O3 conversion at 95%. In order to realize the 100% of the Mn2O3 conversion and lower steam consumption, 830 ℃ and 0.3 are chosen for reaction temperature and steam/Mn2O3 ratio.
5- Mn2O3 and Fe2O3 may be not suitable for CLAS, as you cannot expect significant O2 release from them.
Answer: The CLAS is mainly a thermo-chemical process that can be carried out at biospheric pressure. In addition, CuO/Cu2O, CoO/Co3O4, MnO2/Mn2O3 and Mn2O3/Mn3O4 perform well as intermediate oxygen carriers in CLAS. Among these carriers, Mn2O3/Mn3O4 exhibits a high reaction rate under mild conditions (Shah et al., 2012). Therefore, in this paper, we used Mn2O3/Mn3O4 as oxygen carrier for CLAS.
Shah, K.; Moghtaderi, B.; Wall, T. Selection of suitable oxygen carriers for chemical looping air separation: a thermodynamic approach. Energy & Fuels 2021, 26, 2038-2045.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this paper, the authors proposed a novel process for urea synthesis. It is interesting to integrate the chemical looping technology with the urea synthesis process. However, some parts still need to be clarified more clearly, so I suggest a major revision. The main points I am concerned about are as follows for reference in the revision.
1. Figures 2 and 3: CLAS should be difficult to produce pure O2 and N2 at the same time, so the N2 line should be oxygen-lean air. Please make a clarification.
2. Line 402 and Figure 11: It is not clear why you chose 830C as the reaction temperature. I guess a higher temperature, e.g., 840C, will keep increasing the promotion. If yes, please clarify the selection reason. If not, plot the case of 840C.
3. CLHU will inevitably have some coke formation in the reduction step, leading to CO formation in the H2 production step. Would CO affect your following ammonia and urea synthesis?
4. Both Figures 14 and 15 use the symbol of COG/CG, but they have different meanings. Please change the wording to avoid confusion.
5. Line465 and Fig.15: why a higher COG/CG value (>1.2) does not improve energy efficiency?
6. For the total production cost analysis, it is better to have a sensitivity analysis of key parameters.
Author Response
In this paper, the authors proposed a novel process for urea synthesis. It is interesting to integrate the chemical looping technology with the urea synthesis process. However, some parts still need to be clarified more clearly, so I suggest a major revision. The main points I am concerned about are as follows for reference in the revision.
- Figures 2 and 3: CLAS should be difficult to produce pure O2 and N2 at the same time, so the N2 line should be oxygen-lean air. Please make a clarification.
Answer: Thanks for your comment. The N2 line of the CLAS is O2-lean air. The content of O2 in N2 line can be controlled at a small amount by adjusting the circulating and supplementary volume of oxygen carrier. The trace O2 content has no effect on ammonia synthesis reaction. Similar situation for the coal-to-ammonia, the cryogenic air separation technology is used for separation of N2 and O2 by air condensation liquefaction and distillation separation. The purity of N2 and O2 is controlled by adjusting the operating parameters of the distillation tower. The N2 line of the cryogenic air separation contains trace O2 and the N2 is then used for ammonia synthesis.
- Line 402 and Figure 11: It is not clear why you chose 830C as the reaction temperature. I guess a higher temperature, e.g., 840C, will keep increasing the promotion. If yes, please clarify the selection reason. If not, plot the case of 840C.
Answer: Thanks for your comment. A higher temperature, e.g., 840 ℃, will keep increasing the promotion of the conversion rate of Mn2O3. We added the case of 840 ℃ in the Figure 11. As the temperature changes from 800 to 840 ℃, the steam/Mn2O3 ratio will change from 0.25 to 0.6 to maintain the Mn2O3 conversion at 95%. In order to realize the 100% of the Mn2O3 conversion and lower steam consumption, 830 ℃ and 0.3 are chosen for reaction temperature and steam/Mn2O3 ratio.
- CLHU will inevitably have some coke formation in the reduction step, leading to CO formation in the H2 production step. Would CO affect your following ammonia and urea synthesis?
Answer: Thanks for your comment. After washing and desulphurization, the crude syngas, including CO, H2 and CH4, enters the FR, where the syngas is almost completely oxidized to CO2 and H2O by Fe2O3. Trace residual CO is negligible and there is no effect on the following ammonia and urea synthesis. The reactions occurring in the FR and simulation results are shown as follows.
Fe2O3 + CO → 2 FeO + CO2 (1)
Fe2O3 + H2 → 2 FeO + H2O (2)
4 Fe2O3 + 3 CH4 → 8 Fe + 6 H2O + 3 CO2 (3)
FeO + CO → Fe + CO2 (4)
FeO + H2 → Fe + H2O (5)
- Both Figures 14 and 15 use the symbol of COG/CG, but they have different meanings. Please change the wording to avoid confusion.
Answer: Thanks for your suggestion. This is a mistake. The correct symbol is COG/CG (kmol/kmol). We have corrected this in Figures 14 and 15.
- Line 465 and Fig.15: why a higher COG/CG value (>1.2) does not improve energy efficiency?
Answer: Thanks for your comment. When COG/CG value is more than 1.2, the increase of energy efficiency tends to be constant, which mainly because the introduction of COG will generate more H2, which indirectly increases the yield of urea. In addition, the processing capacity of each unit will increase with the increase of COG/CG, which means that the energy consumption of the whole integrated process will also increase.
- For the total production cost analysis, it is better to have a sensitivity analysis of key parameters.
Answer: Thanks for your suggestion. We have added the sensitivity analysis of the total production cost in the revised paper. For the total production cost analysis, e.g. COG-CTUCLAS&H, the raw material cost, utilities cost, operating & maintenance cost, depreciation cost, plant overhead cost, and general expenses respectively account for 69%, 13%, 0.8%, 12.3%, 0.6% and 4% of the total production cost. The price of raw coal, steam price, and total capital investment have the greatest impact on the total production cost. Therefore, in this paper, we conduct sensitivity analysis of the effect of coal price, steam price, and total capital investment on the total production cost.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The authors have well addressed my comments. I can recommend it to be published now.
