Microwave-Assisted Pyrolysis of Polyethylene and Polypropylene from End-of-Life Vehicles: Hydrogen Production and Energy Valorization

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Very good work, but I felt something basic was missing. When the material comes from post-consumer sources, it is usually colored—what effect does the dye have in this process?

I also noticed that the author does not cite any references throughout the discussion—is there a reason for such an approach?

The author used a material as a catalyst—how was this material reused, considering that as a catalyst, it is not consumed by the process?

When the author states, "Gross heating values in the literature 307 are identified," what literature is being referred to?

According to the author, GC and GC-MS analyses were used, yet no chromatogram was presented—why?

It is common for the equations used to be named throughout the text, followed by their numbering, but the author does not do this—what is the reason for that?

What is the effect of the PE:PP ratio on the final product?

Author Response

Thank you for your suggestions that allowed the article to be improved. All additions are underlined with yellow;

Comments 1: Very good work, but I felt something basic was missing. When the material comes from post-consumer sources, it is usually colored—what effect does the dye have in this process?

Response 1: All the materials used were black, the existence of the dye cannot be highlighted because the solid residue is also black.

Comments 2: I also noticed that the author does not cite any references throughout the discussion—is there a reason for such an approach?

Response 2: More references have been added in the chapter Results and discussions

Comments 3: The author used a material as a catalyst—how was this material reused, considering that as a catalyst, it is not consumed by the process?

Response 3: Experiments with the recirculation of the AC catalyst have been carried out, the data are added to the table ...

Comments 4: When the author states, "Gross heating values in the literature 307 are identified," what literature is being referred to?

Response 4: The data regarding the "Gross heating value" presented in the table .. have been completed with the bibliographic reference

Comments 5: According to the author, GC and GC-MS analyses were used, yet no chromatogram was presented—why?

Response 5: In subchapter 2.3.2. GC and TGA-DTG Analysis, examples of chromatograms of gas and liquid samples were also presented

Comments 6: It is common for the equations used to be named throughout the text, followed by their numbering, but the author does not do this—what is the reason for that?

Response 6: Additional explanations have been added regarding the equations used

Comments 7: What is the effect of the PE:PP ratio on the final product?

Response 7: Considering that the aim is to obtain especially the gaseous fraction with high hydrogen content, the effect of the PE vs PP composition is visible only in the composition of the liquid fraction, but since it is obtained in small quantities, we consider that the effect of the PE:PP composition is insignificant.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript addresses a global issue. There are no new insights from this study, as the results align with expectations. 

A brief description of ELV is needed in the introduction. If possible, add statistical data too. 

Microwave-assisted pyrolysis is not reviewed in the introduction. Add the recent literature on the same specifically to the intended work here. 

There are several polymers mentioned in the manuscript in abbreviated forms. A few of them are included in the list of abbreviations at the end. But the majority is missing from that list. 

How did you confirm the wastes are exactly PP-W and mixed PP-PE-W from the car components? Of course, there are TGA data in the manuscript. Did you try FTIR? From FTIR data, we may get information related to the additives in the waste materials. I strongly recommend adding FTIR spectra and XRD data as the characterization of the waste materials. 

What is Ci in equations 4 and 5?

Line 352: Figure 2, it is Figure 3. 

What is the unit in Figure 3? Only numbers like 50, 43, 65, etc. are given. 

Table 2: PP_T_03 and PP_T_05 are similar in amounts in the configuration. Also, PP_T_04 and PP_T_06. But the conversion% % and H2% are different. How?

Why the GHV values of PP_W and PP_T are different? Need to understand the difference in structure by any method, and compare the results with the structure difference. 

Line 576: Table 1? 

 

 

Author Response

Thank you for your suggestions that allowed the article to be improved. All additions are underlined with yellow. Changes in English expression are underlined with light blue.

Comments 1: A brief description of ELV is needed in the introduction. If possible, add statistical data too. 

Response 1: In the introduction, new information about the composition of the ELV has been introduced (rows 55-65.

 

Comments 2: Microwave-assisted pyrolysis is not reviewed in the introduction. Add the recent literature on the same specifically to the intended work here. 

Response 2:In the introduction (rows 119-137) new information about the use of microwaves in pyrolysis has been introduced

Comments 3: There are several polymers mentioned in the manuscript in abbreviated forms. A few of them are included in the list of abbreviations at the end. But the majority is missing from that list. 

Response 3:The list of abbreviations has been updated

Comments 4: How did you confirm the wastes are exactly PP-W and mixed PP-PE-W from the car components? Of course, there are TGA data in the manuscript. Did you try FTIR? From FTIR data, we may get information related to the additives in the waste materials. I strongly recommend adding FTIR spectra and XRD data as the characterization of the waste materials. 

Response 4:Additional FTIR/DSC/XRD analyses were performed, results and comments were included in the text

Comments 5: What is Ci in equations 4 and 5?

Response 5:Ci represents the volumetric concentration of each compound, this was mentioned in the raw 338

Comments 6: Line 352: Figure 2, it is Figure 3. 

Response 6:Bug fixed

Comments 7: What is the unit in Figure 3? Only numbers like 50, 43, 65, etc. are given. 

Response 7:Additional explanations have been added to the legend of the figure....

Comments 8: Table 2: PP_T_03 and PP_T_05 are similar in amounts in the configuration. Also, PP_T_04 and PP_T_06. But the conversion% % and H2% are different. How?

Response 8:Samples PP_T_03 and PP_T_05 have different configurations, figure 6a has been corrected to show these differences.

Comments 9: Why the GHV values of PP_W and PP_T are different? Need to understand the difference in structure by any method, and compare the results with the structure difference. 

Response 9:The GHV values presented in Table 9 are calculated according to the composition of the gaseous and liquid fractions obtained from the pyrolysis of different raw materials. In the experiments PP_T and PP_W slightly different compositions of these fractions have been obtained, they may be due to specific additives existing in the ELV plastic.

Comments 10: Line 576: Table 1? 

Response 10:Bug fixed

Reviewer 3 Report

Comments and Suggestions for Authors

1. In the title the full stop at the end looks unnecessary. It should be removed for better formatting.
2. The abstract of a good journal paper always ends by outlining the benefits of the study findings and recommendations as a way forward. The manuscript is missing such 1-2 lines in the abstract.
3. Although the introduction section is built very nicely, but I think it contains too much information. For example, on page 2, paragraph 5, it contains extra information on catalysts types. It is highly recommended to short/reduce the introduction and leave only discussion that is more relevant to the manuscript's scope and aims.
4. In methodology, it is also recommended to provide a schematic diagram of the process used. Although it is a much-generalized process, but it will give a good impact on the manuscript.
5. Figures are informative, however Figure 2 (TGA/DTG) and Figure 3 (reactor layering) would benefit from higher resolution and clearer labeling. Table numbers seem inconsistent in places.
6. In results section, the role of activated carbon (AC) as both a catalyst and hydrogen donor is acknowledged, but its surface chemistry (acidic/basic sites, functional groups) is not characterized. Since the study centers on its catalytic impact, consider including FTIR or Boehm titration results to explain activity variation between microwave and conventional treatments.
7. The energy content calculations (GHV, CO₂ emissions, etc.) are strong. However, a brief techno-economic discussion (e.g., cost of microwave energy vs. yield) would enhance the manuscript’s practical relevancy.
8. It is much recommended to provide a comparative studies table (literature review table), having similar factors/parameters as have been used in this study.
9. It is recommended to discuss and explain what the appropriate policies should be based on the findings of this study. In addition, the results should be further elaborated to show how they could be used for real
10. It is strongly recommended to add a subsection, 'practical implications of this study,' outlining the challenges in the current research, future work, and recommendations, before the conclusion.
11. The conclusions section repeats points already discussed extensively in the discussion. Condensing this section would improve focus.
12. Be consistent with PP_T vs. PP_T_09 and other sample codes across text and tables.
13. Use standard SI spacing (e.g., “45.2 wt.%” instead of “45.2wt.%”).
14. Proofreading should be conducted to improve both language and organization

Author Response

Thank you for your suggestions that allowed the article to be improved. All additions are underlined with yellow. Changes in English expression are underlined with light blue.

Comments 1: In the title the full stop at the end looks unnecessary. It should be removed for better formatting.

Response 1: The title of the paper has been changed according to the recommendations of another reviewer

Comments 2:The abstract of a good journal paper always ends by outlining the benefits of the study findings and recommendations as a way forward. The manuscript is missing such 1-2 lines in the abstract.

Response 2:The abstract has been improved

Comments 3:Although the introduction section is built very nicely, but I think it contains too much information. For example, on page 2, paragraph 5, it contains extra information on catalysts types. It is highly recommended to short/reduce the introduction and leave only discussion that is more relevant to the manuscript's scope and aims.

Response 3: A series of literature information regarding different catalysts used in the pyrolysis of plastics has been removed, information regarding the catalysts used in the article has been preserved.

Comments 4:In methodology, it is also recommended to provide a schematic diagram of the process used. Although it is a much-generalized process, but it will give a good impact on the manuscript.

Response 4: The working methodology has been introduced, in figure 1

Comments 5:Figures are informative, however Figure 2 (TGA/DTG) and Figure 3 (reactor layering) would benefit from higher resolution and clearer labeling. Table numbers seem inconsistent in places.

Response 5: The figures have been reworked and their resolution has been improved

Comments 6:In results section, the role of activated carbon (AC) as both a catalyst and hydrogen donor is acknowledged, but its surface chemistry (acidic/basic sites, functional groups) is not characterized. Since the study centers on its catalytic impact, consider including FTIR or Boehm titration results to explain activity variation between microwave and conventional treatments.

Response 6: These characterizations are the subject of future research when the catalyst will be verified and reused several times.

Comments 7:The energy content calculations (GHV, CO₂ emissions, etc.) are strong. However, a brief techno-economic discussion (e.g., cost of microwave energy vs. yield) would enhance the manuscript’s practical relevancy.

Response 7: The technical-economic analysis will be possible only on a pilot plant, which is to be built. We have given details in the subchapter Limitations and prospects

Comments 8: It is much recommended to provide a comparative studies table (literature review table), having similar factors/parameters as have been used in this study.

Response 8: Results from the literature were presented, in table 7

Comments 9: It is recommended to discuss and explain what the appropriate policies should be based on the findings of this study. In addition, the results should be further elaborated to show how they could be used for real

Response 9: A new chapter Limitations and prospects has been introduced

Comments 10: It is strongly recommended to add a subsection, 'practical implications of this study,' outlining the challenges in the current research, future work, and recommendations, before the conclusion.

Response 10: A new chapter Limitations and prospects has been introduced

Comments 11: The conclusions section repeats points already discussed extensively in the discussion. Condensing this section would improve focus.

Response 11: Some of the conclusions have been reformulated in order to increase the clarity of the conclusions

Comments 12: Be consistent with PP_T vs. PP_T_09 and other sample codes across text and tables.

Response 12: An additional explanation of this coding has been added, line 459

Comments 13: Use standard SI spacing (e.g., “45.2 wt.%” instead of “45.2wt.%”).

Response 13: The necessary corrections have been made

Comments 14: Proofreading should be conducted to improve both language and organization

Response 14: The article has been corrected, for expression in English, all corrections are marked with light blue

Reviewer 4 Report

Comments and Suggestions for Authors

The work addresses an important aspect of plastic waste valorization via microwave-assisted pyrolysis, a promising alternative to traditional thermal methods.

There is a strong experimental design and comparison between MW and conventional heating, however there is some redundancy in methodology/objectives, it needs more discussion on the scalability and environmental impact and some sources could be better cited.

Line 2–3: Title is clear but I suggest you to revise it: “Microwave-Assisted Pyrolysis of Polyethylene and Polypropylene from End-of-Life Vehicles: Hydrogen Production and Energy Valorization”
Line 33: You could replace “significantly lower hydrogen yields” with specific % value to make comparison more striking if it is possible.

Line 73–84: The list of processes is useful but too dense. I suggest you to format this part as a table or bullet points with target plastic types, temperature, and key outputs.

Line 114–119: The sentences are too long and complicated, you can make it as below:

“AC’s high surface area and porosity promote secondary reactions. Its thermal stability and hydrogen-donor properties further enhance light hydrocarbon formation.”

Line 273–279: Microwave setup is impressive. I suggest you to add a table as the one below:

Parameter	Value
Microwave frequency	2.45 GHz
Max Power	700 W
Cavity type	Mono-mode
Reactor Material	Quartz

Line 338–341: Interesting additive analysis. You might speculate on what kind of filler could resist decomposition (e.g., talc, glass fiber?). Because this was not clear to me. What is this component?

Line 490–504: Excellent comparison. I suggest you to  highlight the Environmental takeaway: MW pyrolysis → 25–35% lower CO₂/kJ than fossil fuels & include a bar graph comparing specific CO₂ emissions of samples vs natural gas, diesel, etc.

Line 580–587: You can consider showing % surface area reduction (e.g., “AC lost 26.8% surface area post-MW pyrolysis”) for better reader impact.

Lines 611–614: The “conclusion” section should briefly mention the environmental benefits (lower emissions, less char) and some scalability considerations (e.g., feedstock purity, cost of microwave system).

Line 638–639: You can also add here: “Future work should focus on regenerating spent catalysts to improve long-term viability.”

Line 729: Ensure all references are current and correctly DOI-linked. A this specific reference I think something is missing.

Author Response

Thank you for your suggestions that allowed the article to be improved. All additions are underlined with yellow. Changes in English expression are underlined with light blue.

Comments 1: There is a strong experimental design and comparison between MW and conventional heating, however there is some redundancy in methodology/objectives, it needs more discussion on the scalability and environmental impact and some sources could be better cited.

Response 1: A new subchapter has been added: 4. Limitations and prospects; Data from other results presented in the literature for microwave pyrolysis of plastics have been added

Comments 2: Line 2–3: Title is clear but I suggest you to revise it: “Microwave-Assisted Pyrolysis of Polyethylene and Polypropylene from End-of-Life Vehicles: Hydrogen Production and Energy Valorization”

Response 2: Title corrected
Comments 3: Line 33: You could replace “significantly lower hydrogen yields” with specific % value to make comparison more striking if it is possible.

Response 3: Corrected, row 34

Comments 4: Line 73–84: The list of processes is useful but too dense. I suggest you to format this part as a table or bullet points with target plastic types, temperature, and key outputs.

Response 4: It was added table 1 Overview of plastic waste recycling and recovery processes applied in Europe

Comments 5: Line 114–119: The sentences are too long and complicated, you can make it as below:

“AC’s high surface area and porosity promote secondary reactions. Its thermal stability and hydrogen-donor properties further enhance light hydrocarbon formation.”

Response 5: Bug fixed

Comments 6: Line 273–279: Microwave setup is impressive. I suggest you to add a table as the one below:

Parameter	Value
Microwave frequency	2.45 GHz
Max Power	600 W
Cavity type	Mono-mode
Reactor Material	Quartz

 

Response 6: Table 2 has been added with this information

 

Comments 7: Line 338–341: Interesting additive analysis. You might speculate on what kind of filler could resist decomposition (e.g., talc, glass fiber?). Because this was not clear to me. What is this component?

Response 7: Car manufacturers do not present the precise composition of the plastics used. The analysis methods used (TGA/FTIR/XRD) showed the presence of some additives but it was not possible to identify them precisely.

 

Comments 8: Line 490–504: Excellent comparison. I suggest you to  highlight the Environmental takeaway: MW pyrolysis → 25–35% lower CO₂/kJ than fossil fuels & include a bar graph comparing specific CO₂ emissions of samples vs natural gas, diesel, etc.

Response 8: Figure 10 has been added.

Comments 9: Line 580–587: You can consider showing % surface area reduction (e.g., “AC lost 26.8% surface area post-MW pyrolysis”) for better reader impact.

Response 9: Added the proposed text in lines 667-668

Comments 10: Lines 611–614: The “conclusion” section should briefly mention the environmental benefits (lower emissions, less char) and some scalability considerations (e.g., feedstock purity, cost of microwave system).

Response 10: The conclusions have been completed

Comments 11: Line 638–639: You can also add here: “Future work should focus on regenerating spent catalysts to improve long-term viability.”

Response 11: Text has been added

Comments 12: Line 729: Ensure all references are current and correctly DOI-linked. A this specific reference I think something is missing.

Response 12: The list of references has been updated and corrected

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The comments were addressed seriously and the manuscript is improved a lot.