Pyrolysis and Combustion Characteristics of Two Russian Facemasks: Kinetic Analysis, Gaseous Emissions, and Pyrolysis By-Products

Round 1

Reviewer 1 Report

This paper conducted and analyzed the pyrolysis and combustion behaviors of two facemasks (Tissue and Surgical masks) which were commonly used in the Russian Federation. Lots of data were obtained. The work is meaningful. However, some places need to be improved.

1. What are the materials in the tissue mask and the surgical mask? What is the difference? Please indicate the specific components in the two masks respectively.

2. The paper described lots of phenomena without explanation. For example, in Figure 1, the maximum conversion rate of the Tissue mask appeared before 400 ℃, while the maximum conversion rate of the Surgical mask appeared after 400 ℃, why? In Figure 6, “The CO and CO₂ emissions obtained for the Tissue mask present two thin peaks”, why?

3. A lot of scattered research points were studied. However, this paper lacks a systematic analysis of the above research points. In the conclusions, please give a evaluation about the two masks.

Author Response

This paper conducted and analyzed the pyrolysis and combustion behaviors of two facemasks (Tissue and Surgical masks) which were commonly used in the Russian Federation. Lots of data were obtained. The work is meaningful. However, some places need to be improved.

1. What are the materials in the tissue mask and the surgical mask? What is the difference? Please indicate the specific components in the two masks respectively.

As indicated in Section 2.1, the Tissue mask is built with natural fibers (surely cotton and possibly linen). On the contrary, the Surgical mask is built with synthetic fibers (mainly polypropylene). This is added in Section 2.1. As already observed by different researchers and for a long time, the thermal degradation of these two base fibers highly differ, for example concerning the temperature ranges in which they are being degraded, see Table S1 in the Supplementary Material. Further and to complicate the analysis, the thermal degradation of these two masks slightly differ from that of the raw fibers they are built with, because of treatments the fibers are submitted to when building the masks.

 

2. The paper described lots of phenomena without explanation. For example, in Figure 1, the maximum conversion rate of the Tissue mask appeared before 400 ℃, while the maximum conversion rate of the Surgical mask appeared after 400 ℃, why? In Figure 6, “The CO and CO₂ emissions obtained for the Tissue mask present two thin peaks”, why?

Comparisons with results of the literature prove that the maximal height of the pyrolysis of face masks built with natural fibers is reached below 385 °C and that of face masks built with polypropylene fibers is reached above 420 °C, see Table S1 of the Supplementary Material. This is more explicitly indicated in the manuscript.

Concerning the CO and CO₂ emissions, such a situation with double thin peaks was already observed when submitting a facemask built with natural fibers, see ref. [8]. One reason could be that the devolatilization process occurs in mechanisms which release CO and CO₂ in two successive temperature ranges, although the mass decreases without any significant change of slope. The devolatilization process of cotton or other natural fibers indeed involve many reactions.

 

3. A lot of scattered research points were However, this paper lacks a systematic analysis of the above research points. In the conclusions, please give a evaluation about the two masks.

We hope that the many results contained in the manuscript are not too scattered, being organized in clearly defined sections and subsections.

A sentence concerning the treatment of these two masks is added in the Conclusions: Unless the purpose of the elimination of used facemasks is to directly produce energy through a combustion process, it would be better to collect separately the facemasks built with natural fibers and that built with synthetic fibers. Another way to valorize used facemasks consists to apply a pyrolysis process performed under an isothermal temperature. The obtained by-products depend on this temperature to be chosen depending on the desired by-products. In both cases, high isothermal temperatures should be applied to produce more by-products.

Reviewer 2 Report

Some background of this research is missing somehow, face masks are hazardous medical waste and purpose of this work should be its destruction by using pysolysis technology.  

Face masks are typical textile waste, why they are separated from the other medical waste of that kind, as clothes, shoes protection, etc.?

Please clarify a bit purpose of your research. 

I don't have a comment related to your research part, materials and methods, discussion of the results, and conclusions. 

 

Author Response

Some background of this research is missing somehow, face masks are hazardous medical waste and purpose of this work should be its destruction by using pysolysis technology.

The facemasks can be destroyed by combustion to directly produce energy or valorized through pyrolysis, which indeed leads to the production of valuable by-products. The study analyzes both processes for the two masks.

For sure, facemasks may be considered as hazardous medical waste, but they are not exclusively used in a medical context. People wear masks in the daily life, when necessary. This is indicated in the last line of the Introduction, which is now written as: “The present study thus gives a complete overview of the thermal degradations of the two Russian facemasks, whence proposes routes for their valorization through the direct production of energy by combustion, with the control of the associated gaseous emissions, or the production of valuable by-products by pyrolysis under an appropriate isothermal temperature”.

 

Face masks are typical textile waste, why they are separated from the other medical waste of that kind, as clothes, shoes protection, etc.?

Facemasks can be separated in two main families: those elaborated with natural fibers and those elaborated with synthetic fibers. The purpose of the present study is to analyze and compare the pyrolysis and combustion behaviors of two facemasks, which present differences in their composition, as they belong to the two above-indicated families. For sure, considering other medical waste is of great interest and some papers analyze the thermal degradations of such medical waste. However, this would introduce further heterogeneity.

 

Please clarify a bit purpose of your research.

As indicated in the answer to the preceding points, the purpose of the present study is to analyze and compare the pyrolysis and combustion behaviors of two facemasks which present differences in their composition. The study is not restricted to the observation of the thermogravimetric profiles and the corresponding kinetic modeling. It also determines the main gaseous emissions occurring during combustion experiments performed under a temperature ramp of 5 °C/min and the by-products obtained under pyrolysis experiments performed under isothermal temperatures. The last line of the Introduction makes this purpose more clear, see the answer to the first point above.

 

I don't have a comment related to your research part, materials and methods, discussion of the results, and conclusions.

Reviewer 3 Report

This thesis is a discussion of the combustion and pyrolysis aspects of two kind masks. The authors have obtained a detailed overview of the thermal degradation of two kind masks based on a detailed description and analysis of the combustion characteristics, kinetics, and gas emissions. The results of the article can be useful for thermal degradation. Here are the following points need to modify the article:

1. Why the model EIPR was chosen in the article and how it differs from other kinetic models. Other kinetic models can also be calculated to select the appropriate mechanism.

2. The abstracts in the articles are suggested to be re-edited and updated. For example, the abstract describes 5, 10, 15, or 20 °C/min, while the description within the article is 5, 10, 15, and 20°C/min. The conclusion description suggests re-editing.

3. What purpose does the references cited in the introduction serve for the article, and could the references be more relevant and justified? The references only states the other side's experimental conditions and procedure, not the conclusion of the experiment. Please complete the experimental conclusion.

4. A unique volatile fraction peak appears during combustion, please analyze in detail where it is unique. In the abstract it is stated that there is a small peak on the right side, can you analyze the reason for its appearance.

5. The mask contains a lot of metallic substances, does it have an effect on its combustion and pyrolysis. Can you make additions?

6. Two 3.3 sections in the article chapter.

Author Response

This thesis is a discussion of the combustion and pyrolysis aspects of two kind masks. The authors have obtained a detailed overview of the thermal degradation of two kind masks based on a detailed description and analysis of the combustion characteristics, kinetics, and gas emissions. The results of the article can be useful for thermal degradation. Here are the following points need to modify the article:

1. Why the model EIPR was chosen in the article and how it differs from other kinetic models. Other kinetic models can also be calculated to select the appropriate mechanism.

The EIPR model has proved its efficiency when simulating the pyrolysis or combustion of different materials. Further, it is based on characterizations of the material to be degraded. The purpose is not only to give values of the kinetic parameters associated to the pyrolysis or combustion processes for these masks, but also to propose reaction mechanisms (reaction functions involved in the model), and finally to prove that the triplets (kinetic parameters and reaction function) well simulate the thermal degradations. This is indicated in the Introduction. Other kinetic models such as a lumped one or the “original” Discrete Activation Energy Model (DAEM) are also known to propose simulations which well reproduce the experimental results. The EIPR model looks simpler to use.

 

2. The abstracts in the articles are suggested to be re-edited and updated. For example, the abstract describes 5, 10, 15, or 20 °C/min, while the description within the article is 5, 10, 15, and 20°C/min. The conclusion description suggests re-editing.

The “or” in the Abstract is changed to “and”. The Conclusion is completed with further guidelines for the choice of the valorization process to be applied to used facemasks.

 

3. What purpose does the references cited in the introduction serve for the article, and could the references be more relevant and justified? The references only states the other side's experimental conditions and procedure, not the conclusion of the experiment. Please complete the experimental conclusion.

As suggested, an analysis of the results obtained in each reference indicated in the Introduction is added. Thanks for the helpful suggestion.

 

4. A unique volatile fraction peak appears during combustion, please analyze in detail where it is unique. In the abstract it is stated that there is a small peak on the right side, can you analyze the reason for its appearance.

For the combustion of both masks, a unique peak appears before 400 °C. In the case of the Surgical mask, this peak is followed by a shoulder. In the case of the Tissue mask, a second much smaller peak occurs between 450 and 550 °C. Taking into account the composition of this Tissue mask (natural fibers), it is easy to assert that the first peak is a devolatilization one and the second a char combustion one, as indicated in the discussion following Figure 2. As the natural fibers used for the fabrication of the Tissue mask are almost totally composed of cellulose, the devolatilization process occurs in an almost unique stage. The Surgical mask is composed of synthetic fibers which may be treated in different ways before the fabrication of the mask, whence which are being degraded in slightly different temperature ranges.

This is added in the manuscript.

 

5. The mask contains a lot of metallic substances, does it have an effect on its combustion and pyrolysis. Can you make additions?

We suppose that you here refer to the minerals and metals contained in the masks and listed in Table 3, and not to the eventual small metallic piece, some face masks contain to stabilize them on the nose. For sure, such minerals and metals impact the degradation of the face masks. For example, potassium is known to enhance the thermal degradation. This is well documented in the literature, see for example the review [21], which deals for biomass. The purpose of the present study is to analyze the pyrolysis and combustion behaviors of “raw” and used facemasks, that is without any addition of elements enhancing their thermal degradations. This could be the purpose of the further study, with an experimental protocol consisting to add further concentrations of minerals or metals and to analyze the pyrolysis and combustion behaviors.

 

6. Two 3.3 sections in the article chapter.

Thanks for your careful reading. The mistake is corrected.