Deep Low-Carbon Economic Optimization Using CCUS and Two-Stage P2G with Multiple Hydrogen Utilizations for an Integrated Energy System with a High Penetration Level of Renewables

Comments 1: This paper develops a carbon capture, utilization, and storage (CCUS) method that integrates CCUS into IES with a high proportion of renewable energy to construct electric-thermo-hydrogen-gas IES. On this basis, a deep low-carbon economy optimization scheme is proposed, which considers carbon trading, coal consumption, renewable energy abandonment penalty and natural gas purchase cost. Renewable energy can be fully utilized to expand the use of hydrogen, and the proportion of renewable energy consumption in IES can reach 69.23%. There are obvious shortcomings in this paper that need to be improved, there are questions to be addressed:

Response 1: Thank you for your helpful comments. We are very grateful for your critical comments and thoughtful suggestions.

Comments 2: Avoid repeating keywords, abstract and paper titles. keywords need to be filtered and cannot be highlighted at this time.

Response 2: Thank you for your critical comments. According to your comment, the paper title is revised as “Deep low-carbon economic optimization using CCUS and two-stage P2G with hydrogen multiple utilizations for an integrated energy system with high-proportion renewable energy” to avoid repeating keywords of hydrogen. Also, the abstract is further condensed. The keywords is revised as follows:

Keywords: integrated energy system; flexibility; renewable energy; carbon capture, utilization, and storage (CCUS); two-stage P2G with hydrogen multiple utilizations

Comments 3: Nomenclature is where? And nomenclature must be in alphabetic order and the Greek symbols need to be treated separately.

Response 3: Thank you for your critical comments. We have added the Nomenclature in the revised manuscript. Also, we have already processed the nomenclature and Greek symbols.

Comments 4: Check the equations for the full text.

Response 4: Thank you for your valuable and helpful comments. We have checked the equations of the full text.

Comments 5:  Figures 2 are too monotonous and should be improved.

Response 5: Thank you for your valuable and helpful comments. We have modified the details of Figure 2 as follows:

Comments 6: The discussion of Figures 6 and 7 should be intensified.

Response 6: Thank you for your valuable and helpful comments. We have intensified the discussion of Figures 6 and 7 in the revised manuscript. The related changes are as follows:

To further analyze the flexible operation characteristics of the CCS in IES, this paper analyzes the power distribution of a coal-fired power plant with CCS in Case4, as shown in Figure 6. It is shown that during low load periods with a relatively high wind energy output, the carbon capture power plant maintains a minimum total output of 150 MW. This results in an increased energy consumption of carbon capture and an elevated net output level of the coal-fire power unit. Additionally, during peak load hours with relatively lower wind power, the carbon capture power plant achieves flexible regulation of the net power output by reducing the energy consumption of carbon capture. At the same time, a minimum total output is maintained. Moreover, Figure 6 shows that the amounts of carbon dioxide treated by the absorption tower and regeneration tower are not equal in all dispatch periods. In peak load hours, the absorption tower captures and stores all the generated carbon dioxide in the rich liquid tank without regeneration to increase the net output of the coal-fire power unit to supply the load. Nevertheless, in periods with abundant wind energy, the excess wind energy is utilized to regenerate both the stored carbon dioxide in the rich liquid tank and the newly captured carbon dioxide from the absorption tower. It is worth noting here that in Case 2, the energy consumption for the carbon capture is restricted by the coupling of the absorption tower and regeneration tower. In contrast, the addition of a liquid storage tank in Case 3 eliminates the space limitation. Thus, it can flexibly adjust the net output of the carbon capture power plant. The carbon capture power plant increases the net output at the peak load to reduce the output of the hydrogen-blended CHP, which reduces the net output of the coal-fire power unit during the low load period to increase the consumption of the wind power and reduces the carbon emissions while improving the consumption of wind power. As a result, it is reported that the gas purchase of the IES is reduced from 2901.9 thousand yuan in Case2 to 2560.5 thousand yuan in Case3, improving its economic efficiency.

Secondly, we modify the discussion and analysis of Figure 7 as follows:

The optimal operation results of power, heat, natural gas, and hydrogen for each de-vice in case 4 are shown in Figure 7(a). This shows that the electrical load and electrical consumption of the electrolyzer are met by the coal-fired power unit, the hydrogen-blended CHP unit, wind power, and the hydrogen fuel cell. It is highlighted that 00:00-07:00 and 22:00-24:00 are the two periods with curtailment of wind power. At this time, the coal-fired power unit maintains a low net output to increase the accommodation of wind power. This process is equivalent to using part of curtailed wind to supply carbon capture energy consumption, reducing carbon emissions while consuming wind power and enhancing the low-carbon economic performance of IES. Additionally, the electrolyzer converts excess wind power into stored hydrogen and cooperates with the coal-fired power unit to achieve the deep accommodation of wind power. When the peak hours of electrical load with insufficient wind power, the power supply-demand balance is achieved by increasing the output of the coal-fired power unit, hydrogen-blended CHP unit, and hydrogen fuel cell to meet the electrical demand. This ensures the stable electrical operation of the IES. In addition, the heat optimization results of Case 4 are presented in Figure 7(b). It is shown that the required heat load is supplied by the hydrogen-blended CHP unit, GB, and TES. In this case, the hydrogen-blended CHP unit primarily meets the heat load of the IES. This is supplemented by the GB when additional heat is needed. Additionally, excess heat is stored in the TES to be released later during periods of high heat demand. Furthermore, it improves the thermoelectric coupling characteristics of CHP and the operation performance of the system.

The natural gas optimization results of Case 4 are presented in Figure 7(C). During the period 04:00-06:00, with curtailment of wind power, it is shown that methane reaction converts surplus wind power into natural gas. This gas is then used as a resource for the hydrogen-blended CHP units and for satisfying the gas load while consuming carbon dioxide from the coal-fired power unit and hydrogen-blended CHP unit. Overall, carbon emissions are reduced and excess wind power is accommodated, enhancing the low-carbon performance of the IES.

Moreover, the hydrogen optimization results of Case 4 are presented in Figure 7(d). In the wind-abundant periods of 00:00-07:00 and 22:00-24:00, it is shown that the hydrogen produced by the electrolyzer using excess wind power is converted into natural gas by the methanation device along with the carbon dioxide captured CCS. In this case, the produced natural gas can be harnessed to satisfy the gas load or stored in the HST. Most of the hydrogen produced by the electrolyzer is sent to the HST, and only a small portion is sent to the methane reactor. The hydrogen storage tank supplies most of the hydrogen to the hydrogen fuel cell during peak load periods. Because hydrogen fuel cells directly generate electricity, resulting in higher energy utilization efficiency. At the same time, hydrogen fuel cells produce clean electricity, the system will prioritize the allocation of hydrogen energy to the hydrogen fuel cells. However, it is foreseeable that with the progress of gas hydrogen blending technology, the economic and environmental benefits brought by hydrogen blending will be further improved. As a result, it is reported that the gas purchase of the IES is reduced from 2779.1 thousand yuan to 2312.8 thousand yuan. This decreases the operating cost of the IES by 1042.6 thousand yuan, improving its economic efficiency.

Comments 7: The current feasibility analysis of the optimization schemeis not enough.

Response 7: Thank you for your valuable and helpful comments. We have added an analysis the impact of different carbon trading prices on total costs and carbon emissions to increase the feasibility of the optimization scheme. The related changes are as follows:

According to the previous analysis, the carbon trading price significantly influences the optimization results, and different carbon trading price results will get different optimization results. This paper analyzes the total cost and net carbon emissions under different carbon trading prices in Case4, and the results are shown in Figure 8. The results presented that as the carbon trading price continues to increase, the total cost and net carbon emissions continue to decrease. Among them, when the carbon trading price is less than 60 yuan, a low carbon trading price cannot stimulate CCUS to capture CO₂, resulting in almost unchanged net carbon emissions. When the carbon trading price is between 60 yuan and 160 yuan, carbon emissions show different degrees of reduction, and carbon emissions are stable at 140 yuan. In conclusion, the pricing of carbon trading can effectively steer the IES towards carbon reduction and facilitate the operation of a low-carbon economy.

Figure 8. Total cost and net carbon emissions under different carbon trading prices in case 4.

Comments 8: The full text repetition rate is as high as 25% and must be reduced.

Response 8: Thank you for your valuable and helpful comments. We reduced the full text repetition rate form 25% to 20%. By checking the duplicate report, the repetition parts in the revised manuscript are mainly some paper information (e.g., Citation, Author's institution, and Finding) and proper nouns. The repetition rate of research content is relatively small and meets the requirements of the journal.

Comments 9: A detailed description of future research directions, particularly on how to address issues not covered by the current study, should be provided.

Response 9: Thank you for your valuable and helpful comments. In the conclusion, we have added a detailed description of the future research direction and the problems not involved in the current research. The corresponding revisions are shown as follows.

This paper mainly considers the four heterogeneous energy sources of electricity-heat-hydrogen-gas, while ignoring the cold energy. In the future research, cold energy is added to enrich the model. In addition, source-load uncertainty is also considered into the optimization. (in conclusions)

Comments 1: It is important that the authors clearly establish the objective of the paper and the implications of the main results in the abstract.

Response 1: Thank you for your helpful comments. We are very grateful for your critical comments and thoughtful suggestions. The objective of the paper is proposed, (in the abstract). We put forward two questions in the abstract. The first is the full capture CCS of flue gas exists a strong coupling between lean and rich liquor of carbon dioxide liquid absorbent；The second is the integrating split-flow CCS of flue gas will bring a short capture time to give priority to renewable energy. To address these limitations, this paper develops a carbon capture, utilization, and storage (CCUS) method, where the storage tanks for lean and rich liquor and a two-stage power-to-gas (P2G) system with multiple utilization of hydrogen including a fuel cell and hydrogen-blended CHP unit are introduced.

Comments 2: I also consider that it should be mentioned that the case study was carried out (where) briefly in the summary.

Response 2: Thank you for your critical comments. We have briefly described the results of the case analysis in the abstract. The corresponding results are shown as follows.

The results show that the proposed method allows for a significant reduction in both carbon dioxide emissions and total operational costs. It outperforms the IES without CCUS with an 8.8% cost reduction and 70.11% emission elimination. Compared to the IES integrating a full capture CCS, the proposed method yields reductions of 6.5% in costs and 24.7% in emissions. Furthermore, the addition of two-stage P2G with multiple utilization of hydrogen further amplifies these benefits, cutting costs by 13.97% and emissions by 12.32%. Besides, integrating the CCUS into IES enables full consumption of renewable energy and expands hydrogen utilization and the consumption proportion of renewable energy in IES can reach 69.23%.

Comments 3: It is important to expand the importance and implication of the research in the introduction. How would this contribution help? How does it contribute to literature?

Response 3: Thank you for your critical comments. In the second half of the introduction, we have explained the shortcomings of the previous literature and given the contribution of this article. The corresponding contributions are shown as follows.

(1) A CCUS method incorporated an integrated flexible CCS with the storage tanks for lean and rich liquor and a two-stage P2G with hydrogen multiple utilization including hydrogen fuel cells and blending units is developed to realize deep low-carbon and flexible operation of electricity-heat-hydrogen-gas IES.

(2) A deep low-carbon economic optimization for an IES with a high penetration level of renewables, which considers stepwise carbon trading, coal consumption, renewable curtailment penalty, and gas purchasing costs, is proposed to quantify the operational performance benefits.

(3) Taking an IES with high-proportion renewable energy in northern China as an object of investigation, the impacts of integrated flexible CCUS, P2G, and stepwise carbon trading on operational performance results are evaluated through a case-comparison analysis.

Comments 4: I consider it important that the authors explain why this new method is better than the existing ones. What are the advantages and disadvantages?

Response 4: Thank you for your valuable and helpful comments. We have summarized the advantages and disadvantages of the two methods (in the introduction). Most of the existing studies have integrated various kinds of CCS (e.g., full capture of flue gas, split flow of flue gas, flexible operation, and integrated flexible) to realize the deep de-carbonization and some flexible equipment (e.g., electric boiler, thermal energy storage, and P2G) to enhance the flexibility of IES. However, they pay little attention to integrating two-stage P2G with multiple utilization of hydrogen into the integrated flexible CCS to develop the CCUS technology and incorporating this CCUS to build an electricity-heat-hydrogen-gas IES with a high proportion renewable energy for its deep low-carbon and flexible operation.

Comments 5: I consider that the case study must be explained and justified.

Response 5: Thank you for your valuable and helpful comments. In the case study, we have set up four scenarios to highlight the superiority of the proposed method (Case 4). We first obtain the optimization results of the model as shown in Table 2, and analyze the changes in various costs, wind power utilization rate, and net carbon emissions.

Then, we use the electricity-heat-gas energy scheduling diagram (Figure 7) and the accommodation level of wind power (Figure 5), and the power distribution and carbon trace of CCUS (Figure 6) to verify the results.

Comments 6: Perhaps a comparative table with the main characteristics between some of the existing methods and the new one can help the reader to understand better.

Response 6: Thank you for your valuable and helpful comments. we have set up four scenarios: Case1: the IES integrated without a CCUS; Case2: the IES integrated with a split-flow CCS of flue gas; Case3: the IES integrated with an integrated flexible CCS; Case4: the IES integrated with a CCUS. We can see from the total cost, Wind power utilization rate, net carbon emissions, and other indicators in the results that Case4 has better economy and operating performance.

Comments 7: In the conclusions it is important that the authors explain the implications of the contribution, the limitations and future lines of research.

Response 7: Thank you for your valuable and helpful comments. We have given the limitations and future lines of research. The corresponding revisions are shown as follows:

This paper mainly considers the four heterogeneous energy sources of electricity-heat-hydrogen-gas, while ignoring the cold energy. In the future research, cold energy is added to enrich the model. In addition, source-load uncertainty is also a key issue that we need to solve.

Comments 8: Also in the conclusions, it would be important to add that the case study was carried out, under what conditions and where.

Response 8: Thank you for your valuable and helpful comments. We add the background in the conclusion. In fact, we have given the background and conditions in the case study.

To validate the effectiveness of the proposed method and the corresponding established model, a case study of an industrial park IES in Baoding, Hebei, is considered for demonstration and evaluation. The typical wind power profile and the gas, heat, and power load curves of the park are presented in Figure 4. Additionally, the parameters of various facilities in IES with a high proportion of renewable energy are shown in Table 1. The lower heating values of natural gas and hydrogen in the park are considered as 39MJ/m³ and 11MJ/m³, respectively. The power-to-heat conversion coefficient is set at 3600MJ/MW·h, and the purchase price of natural gas is set at 3.79 Yuan/m3.