Design and System Evaluation of Mixed Waste Plastic Gasification Process Based on Integrated Gasification Combined Cycle System

Round 1

Reviewer 1 Report

The manuscript is very interesting. It deals with the important issue of the management of plastic wastes. The Authors proposed three different waste plastic gasification processes, taking into account their advantages and disadvantages. Below I am sending some comments/questions to the article.

1. A little text editing is necessary.
2. Please, improve Table 4.
3. Please improve quality of Figures 12 and 13, x axis and y axis values are poorly visible.
4. The proposed gasification processes are well described. But there is no information on the costs of these designs. Please explain it.

Author Response

1. A little text editing is necessary.

• We have checked the text format and adjusted the editing of the text.

2. Please, improve Table 4.

• We have adjusted table 4 to make it more aesthetically pleasing.

3. Please improve quality of Figures 12 and 13, x axis and y axis values are poorly visible.

• We have reformatted the diagram so that it can look clearer

4. The proposed gasification processes are well described. But there is no information on the costs of these designs. Please explain it.

• Regarding the three designs in this paper, the feasibility of gasification of waste plastics is verified more from theoretical point of view. The evaluation and validation of the system we will provide the economic performance of the system under the emission trading market in the subsequent work.

Author Response File: Author Response.docx

Reviewer 2 Report

1. First two paragraphs discussing the general plastic waste recycling methods and its importance can be reduced to make it more concise.
2. The literature survey presented is very limited, I only see four references (7 – 10) that briefly discuss the literature. In my opinion, more recent research studies should be added to present the state-of-the-art work.
3. What is the difference between MPW composition given in Table 2 and Table 4?
4. How the specifications of SEP2 block shown in Figure 6 are determined?
5. Validation shown in Table 3 are for the same MSW or different conditions? It needs more description to clarify it.
6. How the oxygen flow to the gasifier is selected for the base case design?
7. I don’t think its fair to compare design 1,2 with design 3, as the product is different for the two cases, and different metrics must be selected for the evaluation of process performance. How do the authors justify comparing two different systems?
8. For design 1 and 2, what kind of units are employed for the methanol production? The syngas coming from the upstream gasification unit does not seem appropriate in terms of composition for optimized methanol production.
9. Detailed material balance for the three designs should be provided in the supplementary material.

 

Author Response

1. First two paragraphs discussing the general plastic waste recycling methods and its importance can be reduced to make it more concise.

• We have made appropriate reductions in the repetitive and less important parts.

 

2. The literature survey presented is very limited, I only see four references (7 – 10) that briefly discuss the literature. In my opinion, more recent research studies should be added to present the state-of-the-art work.

• We have added more references (11-13) to illustrate the recent and related work.

 

3. What is the difference between MPW composition given in Table 2 and Table 4?

• Table 2 is the composition comparison between plastic, straw and coal given in the literature. However, Table 4 is the measured values of collected MPW as gasification feedstock in this work.

 

4. How the specifications of SEP2 block shown in Figure 6 are determined?

• We established a mechanism model of waste plastic gasification in bubbling fluidized bed by Aspen. All the modules in Figure 6 represent a structural simulation of a bubbling fluidized bed. SEP2 is not a device in a specific process, so there are no technical specifications.

 

5. Validation shown in Table 3 are for the same MSW or different conditions? It needs more description to clarify it.

• In order to verify the validity and generality of the Aspen model, we have used the same temperature, pressure and feedstock composition of the waste plastic gasification experiments in the literature as the input conditions of this model. Then the simulation results of the model are compared with the experimental results of the literature. This is further explained in our paper.

 

6. How the oxygen flow to the gasifier is selected for the base case design?

• We have used sensitivity analysis tool in Aspen to find the most appropriate oxygen quality. Through the influence of oxygen quantity on the composition change of syngas and the efficiency of cold gas in the system shown in Figures 7 and 8. At the same time, we have considered the influence of oxygen quality on gasification temperature and water quantity. Considering all the conditions, we finally decided that the oxygen feed rate was 1814kg/h in Design 1. Due to the reduced oxygen purity of designs 2 and 3, the feed rate was 2135 kg/h.

 

7. I don’t think its fair to compare design 1,2 with design 3, as the product is different for the two cases, and different metrics must be selected for the evaluation of process performance. How do the authors justify comparing two different systems?

• Design 1 and 2 are mainly to investigate the possibility of gasification of waste plastics and the impact of changes in oxygen production process on energy consumption. The biggest advantage of Design 3 is its carbon capture rate, which increases its energy consumption and outputs cleaner hydrogen as a product. Instead, we evaluate the three systems in terms of carbon capture rate, cold gas efficiency, and power output. We can still calculate the calorific value of different products to measure the superiority of the system. We may give different weights to different evaluation indicators. For example, we may give higher importance to carbon capture rate. In future work, we will compare the three systems using economic calculations.

 

8. For design 1 and 2, what kind of units are employed for the methanol production? The syngas coming from the upstream gasification unit does not seem appropriate in terms of composition for optimized methanol production.

• We have re-added the product flow in Table 7 to make it clearer, where the unit is kg/s. Due to the small batch size of our plant design, the total amount of syngas is not large. We know that the production ratio of H₂/CO is about 2 is the most suitable production ratio for methanol production. In the article, we consume a lot of energy in the process because of the raw material. We don't need methanol as the only output, cogeneration is also important to power the system itself.

 

9. Detailed material balance for the three designs should be provided in the supplementary material.

We have reworked the conditions for the process operations of three designs in Table 5. Also, we have provided a supplemental material with added flowcharts of simulation details for HRSG and MeOH.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors present process modeling of a plastics as fuel  to IGCC. The topic is timely, and of interest to the field. IT gives useful incised into modeling that are of value in other modeling efforts. The authors have omitted some tedious, but necessary details to  give confidence in the modeling and make the work reproducible. If these are addressed the article will be suitable for publication.

1. The authors do an excellent job of lit review with respect to plastics as a feed. However, given the more general a brief description of what IGCC is in the introduction is needed. Furthermore the literature review should put the current modeling in the context of previous research on IGCC based around coal and/or biomass.
2. “VPSA has the advantage of energy consumption, water consumption….,” VPSA is is generally reported as comparable to CSA. It is more energy efficient than cheaper PSA units, but its main benefit over CSA is its typically cost at small scales. That the simulation shows much better than CSA efficiency requires comment and citation support by the authors to give confidence in the simulation. The “water consumption” advantage is a non-sequitur as presented. Further explanation is needed.
3. “Chemical looping combustion is a new type of chemical conversion”. Chemical looping for CO2 capture has been studies for 40 years, and the general concept is well over 100 years old. The authors likely intended to convey that it is an emerging technology with relatively few commercial implementations to date.
4. The placement of the reaction equations in figure 5 are likely to be confusing to a non-expert.
5. The authors did not appear to report the thermodynamics/equation of state packages used in ASPEN. “Compr” efficiencies (e.g. Isentropic efficiency: 0.90) seem to only be reported for select compressors/turbines.
6. The authors introduce methanol synthesis and combined cycle power generation to the process model without much description of either process. This combined with the general the lake of tables of equipment conditions (e.g. temperature pressure ) undermine the reproducibility of the work.  A supplemental with such details should be included.

Author Response

2.1 The authors do an excellent job of lit review with respect to plastics as a feed. However, given the more general a brief description of what IGCC is in the introduction is needed. Furthermore, the literature review should put the current modeling in the context of previous research on IGCC based around coal and/or biomass.

• We have inserted a brief introduction to the IGCC system in the Introduction. We have added more references (11-13) to illustrate the recent and related work, which is mainly based on biomass and plastics of IGCC.

 

2.2 “VPSA has the advantage of energy consumption, water consumption….,” VPSA is is generally reported as comparable to CSA. It is more energy efficient than cheaper PSA units, but its main benefit over CSA is its typically cost at small scales. That the simulation shows much better than CSA efficiency requires comment and citation support by the authors to give confidence in the simulation. The “water consumption” advantage is a non-sequitur as presented. Further explanation is needed.

• We have re-examined ‘water consumption’ and found that statement is indeed inappropriate. It was deleted from the original Article. We have added references 22、23 to the article to provide more arguments for VPSA. In this paper, the waste plastics are treated on a smaller scale, so the VPSA can be adapted to the requirements of the process. Design 1 uses CAS and Design 2 uses VPSA, and we calculated the total and net power of the system for both processes through simulation. The simulation results show that the energy consumption of VPSA is lower, as shown in Table 7.

 

2.3 “Chemical looping combustion is a new type of chemical conversion”. Chemical looping for CO2 capture has been studies for 40 years, and the general concept is well over 100 years old. The authors likely intended to convey that it is an emerging technology with relatively few commercial implementations to date.

• With more stringent requirements for carbon emissions, chemical-looping combustion can well help achieve carbon capture. We've fixed how it's presented in the article.

 

2.4 The placement of the reaction equations in figure 5 are likely to be confusing to a non-expert.

• We have re-adjusted and modified Fig.5 to make it look more concise and easy to understand.

 

5.The authors did not appear to report the thermodynamics/equation of state packages used in ASPEN. “Compr” efficiencies (e.g. Isentropic efficiency: 0.90) seem to only be reported for select compressors/turbines.

• The physical property method used globally is PR-BM, which we have added in article.

 

6.The authors introduce methanol synthesis and combined cycle power generation to the process model without much description of either process. This combined with the general the lake of tables of equipment conditions (e.g. temperature pressure) undermine the reproducibility of the work.  A supplemental with such details should be included.

• We re-add details of the operating conditions for the steam combined cycle and methanol synthesis sections in Table 5. At the same time, we provide a supplementary material containing the simulation flowcharts of HRSG and MeOH.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

All the comments raised in the previous review have been addressed. 

Reviewer 3 Report

The authors have adequately addressed the major issues.  Formatting improvement is needed on Table 5 to make it easier to read.