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

Abstract

Pyrolysis and combustion experiments were performed on two facemasks (hereafter named Tissue and Surgical) commonly used in the Russian Federation, first in a thermobalance and under four low-temperature ramps (5, 10, 15, and 20 °C/min). The pyrolysis mass rate curves present a unique devolatilization peak. The combustion mass rate curves present a unique devolatilization peak followed by a shoulder or a small further peak on its right-hand side. Both processes mainly occur between 200 and 500 °C. Simulations of these pyrolysis and combustion processes are performed with good agreement using the extended independent parallel reaction (EIPR) model. The gas chromatography technique was used to analyze the by-products of pyrolysis experiments performed under isothermal temperatures of 300, 400, and 500 °C. Combustion experiments were finally performed in a horizontal oven under a temperature ramp approximately equal to 5 °C/min to measure the main gaseous emissions: CO₂ emissions are the main emissions of the Tissue mask, while total hydrocarbons are the main emissions of the Surgical mask. Significant differences are observed between the results obtained for the two masks because of the fibers they are built with (natural or synthetic).

1. Introduction

Although the peak of the COVID-19 pandemic has now almost passed, people are encouraged and even required to wear facemasks in particular conditions and in many countries around the world. Masks built with natural or synthetic fibers may include different layers, and the fibers can be submitted to different treatments before manufacturing. Once they reach their usage limit, masks should be collected for further processing or valorization, which are complicated tasks due to their significant quantities, dispersion and composition [1,2]. In [3,4], the authors explored different potential solutions to valorize used masks, sometimes treated as medical waste. The main route for the valorization of used masks is thermal degradation, which should be carried out without undesirable impacts on people and the environment. It is therefore necessary to analyze the thermal degradations of masks and the resulting gaseous emissions. Different pyrolysis techniques that are applicable to masks built with polypropylene fibers and pyrolysis products were analyzed in the review [5]. The analysis of the available literature suggests that fast and catalytic pyrolysis significantly increases the quantity and quality of pyrolytic oil, but slow pyrolysis maximizes the yield of solid products (carbonaceous char and coke). In [6], the authors performed different pyrolysis techniques on the layers of surgical masks and characterized the resulting char and products. They observed that liquid fuel obtained after slow or medium pyrolysis was carbon- and hydrogen-rich, with a higher heating value of 43.5 MJ/kg. In [2], the authors analyzed the combustion characteristics of a surgical mask and an extra protective mask with a zeolite layer and silver ions. They performed combustion experiments on new and used masks under temperature ramps of 5, 10, 20, and 30 °C/min. They measured the emitted gases using mass spectrometry. Non-catalytic and catalytic pyrolysis experiments were performed in [7] on a complete surgical mask to determine the gaseous products. The pyrolysis and combustion of five masks (including tissue and surgical masks) used in France at the beginning of the COVID-19 pandemic were analyzed and compared in [8]. The main gaseous emissions occurring during combustion experiments were also determined in that study. In [9], the authors performed pyrolysis experiments on a surgical mask under temperature ramps of 15, 20, 25, and 30 °C/min. They determined the kinetic parameters associated with these pyrolysis experiments and analyzed the pyrolysis products using an online FTIR-MS technique. The values of the kinetic parameters and the main gaseous emissions determined in these papers are compared to those of the present study. In [10,11], the authors performed pyrolysis experiments on a surgical mask and on its layers, which were all built with polypropylene fibers. They performed a kinetic analysis and analyzed the pyrolysis products using the GC-MS technique. They found that propene, furan, 2,4-dimethyl-1-heptene, and isopropylcyclobutane were the major compounds. Thermal conversions of masks were also explored with other medical waste in [12,13], for example. This medical waste can be characterized by strong heterogeneity. Their thermal degradation can lead to emissions of toxic gases such as polycyclic aromatic hydrocarbons and hydrochloric acid. In [14], the authors performed hydrothermal liquefaction on surgical three-ply masks built with polypropylene and polyester. They observed that this thermal degradation simultaneously destroyed pathogens and recycled the different polymers present in the masks.

In the present study, two facemasks commonly used in the Russian Federation are first characterized. The results of the proximate and ultimate analyses, the higher and lower heating values, and the amounts of the main minerals and metals highly differ between the two masks. Pyrolysis and combustion experiments were performed on these two masks in a thermobalance under the temperature ramps of 5, 10, 15, and 20 °C/min. The thermogravimetric profiles slightly differ between the two masks, but also from those of quite similar masks previously analyzed in the literature. Kinetic modeling was performed on the pyrolysis and combustion of these two masks. The extended independent parallel reaction model was used to determine the pre-exponential factor and activation energy values related to the thermal degradation of these masks, together with the associated reaction functions. This model involves as many differential equations as the number of constituents to be degraded and is based on characterizations of the material. Its efficiency in simulating the mass and mass rate curves was already proven for different materials in [8,15], for example. Further pyrolysis experiments were performed on these two masks under isothermal temperatures of 300, 400, and 500 °C. The pyrolysis products were analyzed using the gas chromatography technique and assembled according to the IUPAC classes. The by-products obtained applying pyrolysis experiments performed under three isothermal temperatures highly differ between the two masks, and slightly differ from those obtained in the literature for quite similar masks. Finally, combustion experiments were performed on the two masks in a horizontal oven under a temperature ramp approximately equal to 5 °C/min to determine the main gaseous emissions (CO, CO₂, NO, NO₂, and total hydrocarbons (THCs)) occurring during such combustion experiments. The main gaseous emissions are CO₂ for the Tissue mask and total hydrocarbons for the Surgical mask. The results obtained in this study highly differ between the two masks due to the fibers (natural or synthetic) used for their fabrication. The present study thus gives a complete overview of the thermal degradations of the two Russian facemasks and proposes routes for their valorization by direct production of energy through combustion, with control of the associated gaseous emissions, or the production of valuable by-products with pyrolysis under appropriate isothermal temperature.

2. Materials and Methods

2.1. Facemasks

A Russian facemask built with natural fibers (mainly cotton), hereafter called the Tissue mask, and a surgical mask, hereafter called the Surgical mask, built with synthetic fibers (polypropylene) are considered in the present study. Photos of these two masks are presented in Figure S1 of the Supplementary Materials.

2.2. Characterizations

Different characterizations were first performed on the two facemasks: proximate and ultimate analyses, determination of their higher heating value (HHV) and lower heating value (LHV), quantification of the amounts of the main minerals and metals, and computation of their atomic H/C and O/C ratios, according to formulas given in [16], see the Supplementary Materials. The operating conditions used for these characterizations follow the appropriate and usual standards [17,18,19,20,21], as detailed in Section S2 of the Supplementary Materials.

2.3. Thermogravimetric Experiments

Pyrolysis (under pure nitrogen) and combustion (under synthetic air: 80% nitrogen and 20% oxygen) experiments were performed on the fabric part of each mask in a thermobalance TA Q500, TA Instruments. This fabric part was cut in pieces of approximately 5–6 mg, considering the fabric part as a whole even if the fabric part of the mask was composed of different layers. In the case of the Surgical mask, it was indeed difficult to separate the at most three layers building the mask. Thermogravimetric experiments were performed on these pieces, under four low temperature ramps (5, 10, 15, and 20 °C/min). A gas flow of 50 mL/min was injected. Three repetitions of each pyrolysis and combustion experiment were performed with good agreement.

2.4. Kinetic Modeling Using the EIPR Model

Simulations of the pyrolysis and combustion processes of the two Russian facemasks, which present successive stages, were performed by applying the extended independent parallel reaction (EIPR) model, described, for example, in [15]. Each stage of the overall thermal degradation of each mask is associated with the thermal degradation of a constituent of the mask. The EIPR model assumes that the thermal degradations of the constituents of the feedstock occur in an independent way. The number of constituents to be considered is deduced from the analysis of the conversion and conversion rate curves. The EIPR model is based on first-order differential equations that describe the evolution with respect to the time of the mass of the constituents. These differential equations involve unknown parameters and a reaction function, which were taken as the fourth-order Avrami–Erofeev function or Mampel’s first-order function; see Formula (S4) in Section S3 of the Supplementary Materials. In the pyrolysis case, the first-order differential equations of the EIPR model are provided in Section S3 of the Supplementary Materials. The unknowns of these equations are the activation energy and the pre-exponential factor associated with the degradation of each constituent and the initial proportions of the constituents in the mask; see Section S3 of the Supplementary Materials. In the combustion case, the first-order differential equations of the EIPR model slightly change; see Section S4 of the Supplementary Materials. In this combustion case, further unknowns of the EIPR model are the initial fractions of volatiles contained in the constituents of the mask.

To validate the simulations of the experimental conversion and conversion rate curves, relative errors $l_{\infty }$ and $l_2$ and determination coefficients $R_m^2$ for the mass, $R_{mr}^2$ for the mass rate, and $R_o^2$ for a combination of mass and mass rate are computed using Formulas (S9)–(S11) given in Section S3 of the Supplementary Materials.

2.5. Gas Chromatography Analysis

Pyrolysis experiments were performed on pieces of the fabric parts of the two masks in a capillary column HP-5MS. The temperature was increased from room temperature to the isothermal temperatures of 300, 400, and 500 °C. These three temperatures were chosen according to the combustion profiles of the two masks, as their degradation mainly occurs between 200 and 500 °C. The detailed pyrolysis conditions are indicated in Section S5 of the Supplementary Materials.

The gas chromatography technique was used to determine the pyrolysis products obtained during the pyrolysis experiments performed on the two masks. Complete lists of the chromatogram peaks were established depending on the final temperature. The 71 organic compounds found for the Tissue mask and the 89 organic compounds found for the Surgical mask were assembled according to the IUPAC classes and depending on the final isothermal temperature to compare the evolution of their peak areas with respect to the isothermal temperature. The main by-products (with peak area greater than 1000) were then finally analyzed.

2.6. Gaseous Emissions during Combustion Experiments Performed on the Two Masks in a Horizontal Oven

Combustion experiments were performed in a horizontal tubular Nabertherm oven under a temperature ramp approximately equal to 5 °C/min. The detailed experimental conditions are described in Section S6 of the Supplementary Materials. The main gaseous emissions (CO, CO₂, NO, NO₂, and total hydrocarbons (THCs)) were continuously measured. These combustion experiments were repeated three times for each mask with good agreement.

3. Results and Discussion

3.1. Characterizations

Table 1. Proximate analyses of the two masks. Higher heating value (HHV) and lower heating value (LHV).

Mask	Proximate Analysis (%) a				HHV (MJ/kg) b	LHV (MJ/kg) c
	Moisture	Fixed Carbon	Volatile Matter	Ash
Tissue	4.3	7.5	87.8	0.4	17.63	15.37
Surgical	0.1	2.1	97.7	0.1	46.25	43.29

^a as received; ^b on raw basis; ^c on dry and ash-free basis.

gathers the results of the proximate analyses performed on the two masks, and their higher and lower heating values.

Volatile matter has the highest percentage in both masks, being slightly higher in the Surgical mask. The ash percentage is very low in both masks, especially in the Surgical mask. The moisture content is forty times higher in the Tissue mask than in the Surgical mask. This is due to the presence of natural and hydrophilic fibers in the Tissue mask.

The ash content was found to be equal to 1.92% in the surgical masks analyzed in [14], which is slightly higher than that in the present Surgical mask. The percentage of volatile matter in polypropylene samples was found to be greater than 99% in [22] and evaluated at 99.73% in a polypropylene sample in [23]. In these studies, no ash was detected, and the moisture content was found to be very low.

The higher heating value of the Surgical mask is approximately three times that of the Tissue mask. It was found to be equal to 45 MJ/kg in [24] for polypropylene waste, which is very close to that indicated in Table 1 for the Surgical mask. In [22], the higher heating value of a polypropylene sample was found to be equal to 41.0 MJ/kg, which is lower than that indicated in Table 1 for the Surgical mask. In [23], the lower heating value of polypropylene powder was evaluated at 46.2 MJ/kg, which is higher than that indicated in Table 1 for the Surgical mask. In [14], the authors found an HHV of 43.12 MJ/kg for the surgical mask they analyzed. These differences may be the consequence of treatments applied to the fibers used for the fabrication of surgical masks.

Table 2. Ultimate analyses of the two masks and their H/C and O/C ratios.

Sample	Ultimate Analysis (%)					H/C Ratio	O/C Ratio
	C	H	O	N	S
Tissue	42.9	10.0	46.9	0.2	n.d.	1.84	0.82
Surgical	85.6	13.1	0.7	0.6	n.d.	2.33	0.01

n.d.: below detection.

gathers the ultimate analyses for both masks. The H/C and O/C ratios are computed from the characterizations presented in Table 1 and Table 2 using classical formulas; see Equation (S1) of the Supplementary Materials.

The carbon (C) percentage of the Surgical mask is approximately twice that of the Tissue mask. It is the main component of the Surgical mask. The hydrogen (H) percentages are approximately the same for the two masks. The oxygen (O) percentage is very low for the Surgical mask, although it is the main component of the Tissue mask. The nitrogen (N) percentages are very low for both masks, being slightly lower for the Surgical mask than for the Tissue mask. No sulfur (S) was detected in either mask.

The values of the ultimate analyses presented in Table 2 are quite close to those of similar masks analyzed in [8]. In [22], the authors found a C percentage equal to 85.7% and a H percentage equal to 14.3% in a low-density polypropylene sample. In [23], the C percentage of a polypropylene sample was found to be equal to 85.11 ± 0.06%. The H percentage was measured at 14.47 ± 0.07%. In these two articles, the authors found N, O, and S percentages equal to 0 or not detected. These values are quite close to those of the Surgical mask. In [14], the C, H, O, and N percentages were found to be equal to 76.21, 12.02, 7.77, and 2.08%, for surgical masks, which are slightly different from that of the present Surgical mask.

The H/C and O/C ratios of the Tissue mask are in good agreement with those of natural fibers, according to the van Krevelen diagram [25]. In [14], the H/C and O/C ratios were found to be equal to 1.892 and 0.077 for surgical masks built with synthetic fibers, different from that of the present study, due to the differences in the composition of the masks, whence in the ultimate and proximate analyses.

Table 3. Main minerals and metals present in the two masks (ppm).

Sample	Fe	Si	P	Cu	Ni	Mn	Cr	Al	Zn	Ca	K	Cl	Mg	Na	Ti
Tissue	14	28	13	7	5	<3	8	10	1	70	26	<50	3	45	395
Surgical	52	287	80	19	9	3	25	17	1	820	116	238	244	382	3

gathers the amounts (ppm) of the main minerals and metals contained in the two facemasks.

The Surgical mask contains a higher amount of each mineral and metal (except titanium) than the Tissue mask. The two masks present significant differences concerning the main minerals and metals they contain: titanium for the Tissue mask and calcium for the Surgical mask. The amount of titanium in the Tissue mask is higher than the sum of the amounts of all minerals and metals measured in this mask. This is not the case for calcium in the Surgical mask, as this mask also contains quite important amounts of silicon, chlorine, magnesium, and sodium. The presence of minerals and metals in a feedstock surely impacts its pyrolysis and combustion behaviors, see the recent review [26], for example.

3.2. Pyrolysis Experiments

Pyrolysis experiments were performed on small pieces (5–6 mg) of the fabric part of the two masks and under the temperature ramps of 5, 10, 15, and 20 °C/min. The conversion and conversion rate curves are gathered in Figure 1.

The only differences in the conversion curves appear in the temperature range during which the conversion increases very quickly, Figure 1a, c. For the Tissue mask, this temperature range is 300–400 °C, Figure 1a. A much slower increase in the conversion occurs at temperatures above 400 °C. The conversion of the Tissue mask reaches 1 only at the end of the experiment. The conversion of the Surgical mask increases quickly between 350 and 500 °C and reaches 1 at 500 °C, Figure 1c.

For the Tissue mask, the conversion rate curve remains almost equal to 0 (1/s) before 200 °C, Figure 1b. A similar situation occurs for the Surgical mask, but the conversion remains close to 0 until 350 °C, Figure 1d. This is partly the consequence of the small moisture amount contained in each mask, see Table 1.

A unique and thin devolatilization peak appears on the conversion rate curves of both masks, Figure 1b, d, with a very small shoulder on the left-hand side of the peak of the Tissue mask, Figure 1b. This is a consequence of the composition of the Tissue mask (natural fibers, mainly containing cellulose). This devolatilization peak is thinner and appears at higher temperatures for the Surgical mask than for the Tissue mask. Its height (highest conversion rate) is higher for the Surgical mask than for the Tissue mask. For both masks, by increasing the temperature ramp, the devolatilization peak slightly shifts to higher temperatures, and its height increases. Table S1 in the Supplementary Materials gathers details concerning the conversion and conversion rate curves for each mask, together with the final sample mass, for each temperature ramp. As the temperature ramp increases, the position of the devolatilization peak shifts to higher temperatures of 23.6 °C for the Tissue mask and 24.7 °C for the Surgical mask. The height of the unique devolatilization peak is multiplied by approximately 4 when increasing the temperature ramp from 5 to 20 °C/min. Table S1 in the Supplementary Materials proves that the maximal height of the devolatilization peak for cotton occurs at temperatures below 385 °C and that of polypropylene occurs above 420 °C. The final masses slightly decrease with respect to the temperature ramp. A reason that could explain the possible differences between the temperature peaks for the pyrolysis of polypropylene and that of the Surgical mask is the composition of the fibers building this mask or the treatment these fibers are submitted to for the elaboration of the mask.

The pyrolysis profile of the Tissue mask considered in the present study presents differences from that of the tissue (ECLT) mask, also built with natural fibers, considered in [8], as the position and height slightly differ from that observed for the cotton fabric sample. An explanation could be that the natural fibers that are used for the elaboration of the Tissue mask considered in the present study contain elements other than cotton.

The pyrolysis profile of the Surgical mask considered in the present study is quite close to that of the surgical (SGP2) mask considered in [8], both being built with polypropylene fibers.

3.3. Combustion Experiments

Figure 2 gathers the conversion and conversion rate curves for the two masks in the combustion case.

The conversion curves of the two masks have different shapes, Figure 2a, c. The conversion of the Tissue mask rapidly increases in the temperature range of 350–380 °C. This increase is much smaller up to 550 °C, where the conversion reaches the maximal value of 1, whatever the temperature ramp. The conversion of the Surgical mask increases rapidly between 200 and 300 °C. Then, the conversion increases much more slowly, and it reaches the maximal value of 1 at approximately 400 °C, whatever the temperature ramp, Figure 2c.

The conversion rate curves of the Tissue mask exhibit two well-separated peaks: a huge and thin peak between 250 and 450 °C and a much smaller one between 450 and 550 °C, Figure 2b. Because the Tissue mask is composed of natural fibers that almost totally contain cellulose, the first peak corresponds to the devolatilization process of the fibers, and the second one is associated with the char combustion stage. On the contrary, the conversion rate curves of the Surgical mask exhibit a unique peak, Figure 2d. A shoulder appears on the right-hand side of this peak, which is more pronounced for the two lowest temperature ramps of 5 and 10 °C/min. The Surgical mask is composed of synthetic fibers, which may be treated in different ways before the elaboration of the mask.

Table S2 in the Supplementary Materials gives the positions and heights of the peaks and the final masses for the combustion of the two masks under the different temperature ramps. By increasing the temperature ramp, the shift in the devolatilization peak is slightly higher in the combustion case (between 30.4 and 33.5 °C) than in the pyrolysis one.

The combustion profile of the Tissue mask considered in the present study slightly differs from that of the tissue (ECLT) mask considered in [8], although they are both built with natural fibers. The combustion profiles of these two masks built with natural fibers also present differences from those of cotton fabrics. This may again be explained by differences in the fibers building the masks or the cotton fabric, or in the treatment applied to these fibers.

The combustion profile of the surgical masks analyzed in [14] differs from that of the Surgical mask considered in the present study because the authors observed a main peak with a large shoulder on its right-hand side and a much smaller secondary peak. The combustion profiles of the Surgical mask considered in the present study slightly differ from that of the surgical (SGP2) mask considered in [8], although both masks are built with polypropylene fibers. These differences may again be explained by differences in the polypropylene fibers building these two surgical masks and in their treatment during the fabrication of the masks.

The pyrolysis and combustion profiles of the two masks considered in the present study are compared in Section S6 of the Supplementary Materials. The devolatilization peak of the conversion rate curve occurs at lower temperatures under air than under pyrolysis; this difference is approximately equal to 20 °C for the Tissue mask and approximately equal to 170 °C for the Surgical mask. An explanation is the presence of oxygen in the combustion experiments, which is known to enhance the degradation process.

3.4. Kinetic Modeling

The extended independent parallel reaction (EIPR) model is used to simulate the conversion and conversion rate curves presented in Figure 1 and Figure 2. This section presents the optimal values of the unknown parameters involved in this model, together with the different error measurements between the experimental and simulated mass and mass rate curves.

3.4.1. Determination of the Optimal Values of the Unknown Parameters and Error Measurements in the Pyrolysis Case

Table 4. Optimal proportions $c_i$ of the constituents in the masks (see Section S3 of the Supplementary Materials), optimal values of the kinetic parameters, absolute and relative differences between the experimental and simulated conversion rate curves, and determination coefficients in the pyrolysis case (see Formulas (S9)–(S11) of the Supplementary Materials).

Sample	Tissue		Surgical
$c_1$	0.98		1.00
$c_2$	0.02		-
$A_1$ (1/s)	58.4		1712.4
${Ea}_1$ (J/mol)	55,024.0		82,902.0
$A_2$ (1/s)	32.99 × 107		-
${Ea}_2$ (J/mol)	121,348.0		-
$l_{\infty }$	Abs. (-)	Rel. (%)	Abs. (-)	Rel. (%)
R5	3.0 × 10−4	13.4	6.0 × 10−4	27.9
R10	1.0 × 10−4	23.3	5.5 × 10−4	11.7
R15	1.5 × 10−3	23.8	6.1 × 10−4	9.0
R20	1.2 × 10−3	20.1	6.4 × 10−4	10.4
$l_2$	Abs (%/s)	Rel. (%)	Abs (%/s)	Rel. (%)
R5	1.1 × 10−3	15.7	1.7 × 10−3	22.4
R10	2.4 × 10−3	17.2	1.6 × 10−3	10.0
R15	3.6 × 10−3	17.0	1.8 × 10−3	8.0
R20	4.7 × 10−3	16.4	2.2 × 10−3	7.0
$R_m^2$
R5	0.994		0.996
R10	0.995		0.999
R15	0.995		0.999
R20	0.995		1.000
$R_{mr}^2$
R5	0.973		0.946
R10	0.968		0.989
R15	0.969		0.993
R20	0.971		0.995
$R_o^2$
R5	0.967		0.942
R10	0.962		0.988
R15	0.964		0.992
R20	0.965		0.995

gathers the optimal values of the unknown parameters associated with the pyrolysis of the masks together with values of the error measurements.

As already indicated in Section 2.4, the fourth-order Avrami–Erofeev reaction function was used to simulate the pyrolysis of the first constituent of the Tissue mask and that of the unique constituent of the Surgical mask because the unique devolatilization peak is thin and high in each case, see Figure 1b. The Mampel or first-order reaction function was used to simulate the pyrolysis of the second constituent of the Tissue mask, whose proportion $c_2$ is very small (equal to 0.02).

For each mask and temperature ramp, the determination coefficient $R_m^2$ for the conversion is greater than 0.994, which means that the conversion curve is very well-reproduced. Apart from the case of the Surgical mask and the temperature ramp of 5 °C/min, the determination coefficients $R_{mr}^2$ for the conversion rate take values between 0.968 and 0.995, and the overall determination coefficients $R_o^2$ take values between 0.962 and 0.995, which are very close to 1. The conversion rate curves are well reproduced in these cases. For the Surgical mask and the temperature ramp of 5 °C/min, the determination coefficients $R_{mr}^2$ and $R_o^2$ are relatively low (0.946 and 0.942) due to the thin devolatilization peak. Nevertheless, and because the determination coefficients are sufficiently close to 1 in all cases, the simulations can be accepted, with the numbers of constituents and the optimal values of the pre-exponential and activation energies indicated in Table 4. Figure 3 gathers the experimental and the simulated conversion and conversion rate curves for the pyrolysis of the two masks under the temperature ramp of 5 °C/min.

For the Tissue mask, the simulated conversion curve is slightly lower and then higher than the experimental conversion curve, Figure 3a. The maximum of the experimental conversion rate curve is not perfectly reached by the simulated conversion rate curve, Figure 3b. For the Surgical mask, a quite similar situation occurs for the experimental and simulated conversion curves, but the differences between the experimental and simulated conversion curves are smaller than in the case of the Tissue mask, Figure 3c. Again, the maximum of the conversion rate curve is not perfectly reached by the simulated conversion rate curve, Figure 3d. For both masks, the maximal relative difference $l_{\infty }$ between the experimental and simulated conversion rate curves is low, and the determination coefficients $R_{mr}^2$ and $R_o^2$ are sufficiently close to 1, suggesting that the simulations can be accepted, see Table 4.

For the other temperature ramps, similar observations are obtained (not presented here).

In [27], the authors performed pyrolysis experiments in a thermobalance on textile waste under the temperature ramps of 1, 5, 10, 20, 40, and 60 °C/min. They observed a unique peak with shoulders or small peaks on both sides. Using an independent reaction model, for the main stage, they obtained values of energy activation between 190 and 204 kJ/mol and of the pre-exponential factor between 4 × 10¹⁵ and 5 × 10¹⁶ 1/min, depending on the temperature ramp. In [8], an activation energy of 180.1 kJ/mol and a pre-exponential factor of 3.9 × 10¹² 1/s were obtained for the pyrolysis of a tissue mask. All these values are higher than those presented in Table 4 for the Tissue mask, which may be the consequence of the fibers used for the elaboration of the masks. In [28], the authors applied an isoconversional method to determine the kinetic parameters associated with the pyrolysis of pure polypropylene. Considering a reaction order equal to 1, they found activation energies lying in the range of 161–173 kJ/mol and pre-exponential factors in the range of 4.97 × 10⁹–4.97 × 10¹⁰ 1/s. In [22], the authors applied a Coats–Redfern method to determine the kinetic parameters associated with the pyrolysis of polypropylene. The activation energy was found to be equal to 219,073.9 J/mol, and the pre-exponential factor was equal to 4.349 × 10¹³ 1/s. The choices of the kinetic models or methods may explain the differences among the values of the kinetic parameters indicated in Table 4 and the literature.

3.4.2. Determination of the Optimal Values of the Unknown Parameters and Error Measurements in the Combustion Case

The combustion of both masks was simulated according to the procedure described in Section S5 of the Supplementary Materials. The same numbers of constituents to be considered in the combustion of the masks were used in the EIPR model as in the pyrolysis process. An Avrami–Erofeev reaction function of order four was used to simulate the combustion of the first constituent of the Tissue mask and the combustion of the unique component of the Surgical mask; see the thin peaks observed in Figure 4. The first-order reaction function was used to simulate the combustion of the second constituent of the Tissue mask.

Table 5. Optimal proportions $c_i$ of the constituents, optimal fractions of volatiles ${\tau }_{vol,i}$ in these constituents, optimal values of the kinetic parameters, absolute and relative differences between the experimental and simulated conversion rate curves, and determination coefficients in the combustion case.

Sample	Tissue		Surgical
$c_1$	0.98		1.00
$c_2$	0.02		-
${\tau }_{vol,1}$	0.98		0.85
${\tau }_{vol,2}$	0.44		-
$A_1$ (1/s)	9.8 × 105		2.2
${Ea}_1$ (J /mol)	98,884.9		32,173.9
$A_2$ (1/s)	1.2 × 105		-
${Ea}_2$ (J/mol)	82,073.9		-
$A_{comb}$ (1/s)	111,119.6		1.5 × 105
${Ea}_{comb}$ (J/mol)	165,999.8		136,000.0
$l_{\infty }$	Abs. (-)	Rel. (%)	Abs. (-)	Rel. (%)
R5	3.2 × 10−4	9.3	4.4 × 10−4	19.7
R10	1.2 × 10−3	18.6	4.7 × 10−4	16.9
R15	2.4 × 10−3	14.2	6.1 × 10−4	12.8
R20	1.5 × 10−3	13.3	9.4 × 10−4	16.3
$l_2$	Abs. (%/s)	Rel. (%)	Abs. (%/s)	Rel. (%)
R5	1.0 × 10−3	13.1	1.9 × 10−3	27.4
R10	2.3 × 10−3	14.2	2.0 × 10−3	16.6
R15	3.9 × 10−3	26.2	2.2 × 10−3	11.6
R20	3.3 × 10−3	10.8	3.6 × 10−3	14.5
$R_m^2$
R5	0.999		0.995
R10	1.000		0.999
R15	1.000		0.999
R20	1.000		0.999
$R_{mr}^2$
R5	0.982		0.918
R10	0.979		0.969
R15	0.971		0.985
R20	0.988		0.976
$R_o^2$
R5	0.980		0.913
R10	0.978		0.968
R15	0.971		0.984
R20	0.987		0.975

gathers the optimal values of the unknown parameters associated with the combustion of the two masks, together with values of the error measurements.

The relative differences $l_{\infty }$ and $l_2$ between the experimental and simulated conversion rate curves are slightly higher for the Surgical mask than for the Tissue mask.

For each temperature ramp and mask, the determination coefficient $R_m^2$ for the conversion is greater than 0.995, which proves that the conversion curve is well-simulated. For the Surgical mask, the determination coefficient $R_{mr}^2$ for the conversion rate takes a fairly low value under the temperature ramp of 5 °C/min (0.918). For the other temperature ramps and for the Tissue mask, the determination coefficients $R_{mr}^2$ for the conversion rate is close to 1 (higher than 0.969). The conversion rate curves are well-reproduced using the simulations performed with the numbers of constituents and the optimal values of the parameters indicated in Table 5.

The experimental and simulated conversion and conversion rate curves for the combustion of the two masks under the temperature ramp of 5 °C/min are gathered in Figure 4.

For the Tissue mask, the simulated conversion curve is slightly higher than the experimental conversion curve at the beginning and at the end of the rapid increase, Figure 4a. The simulated conversion rate curve decreases to 0 after the thin and high devolatilization peak, while the experimental curve does not, Figure 4b. For the Surgical mask, the shoulder on the right-hand side of the peak is not well-reproduced, Figure 4d. Nevertheless, the different measures of the differences between the experimental and simulated conversion rate curves indicated in Table 5 prove that the conversion and conversion rate curves are quite well-simulated.

For the other temperature ramps, similar situations are observed, and the determination coefficients are much closer to 1 (not presented here).

In [8], an activation energy of 249.6 kJ/mol and a pre-exponential factor of 3.7 × 10¹⁵ 1/s were obtained for the combustion of a surgical mask. These values are much higher than those presented in Table 5 for the Surgical mask considered in the present study. This may again be the consequence of the fibers used for the elaboration of the masks. In [29], the authors simulated the combustion of polypropylene using the reaction function $f\left(\alpha \right)={\left(1-\alpha \right)}^n$ and decomposing the combustion process into three stages. For the main stage and chosen reaction order n equal to 0.5, they found an activation energy equal to 230 kJ/mol and a pre-exponential factor equal to 2.0 × 10¹⁴ 1/s. These values are in good agreement with that of the kinetic parameters indicated in Table 5 for the combustion of the Surgical mask. The differences between the values gathered in Table 5 and those in the literature may be explained by the choice of kinetic models or methods.

3.5. Chromatogram Peaks

The species obtained from the pyrolysis experiments performed under the final isothermal temperature of 300, 400, and 500 °C are presented in Table S3 in Section S9 of the Supplementary Materials, for both masks. Organizing the 71 organic compounds obtained for the Tissue mask and the 89 organic compounds obtained for the Surgical mask, both presented in Table S4 of the Supplementary Materials, according to the IUPAC classes, leads to three histograms per mask, which are presented in Figure 5, corresponding to the three chosen isothermal temperatures. The class “Others” includes organic compounds with small peak areas such as nitrile, carboxylic acid, phenol, etc.

Increasing the isothermal temperature increases the peak areas, and the scaling of the vertical axes takes into account these variations in the peak areas. The highest chromatogram peaks (with an area greater than 1000) observed for the isothermal temperature of 500 °C are presented in Table S4 in Section S9 of the Supplementary Materials.

There are huge differences between the two masks, whatever the isothermal temperature. The major by-products obtained from the Tissue mask belong to the classes aldehyde, ketone, and sugar, while those obtained from the Surgical mask belong to the classes alcohol, alkane, and alkene. In the Tissue mask, there are almost no by-products in the classes alcohol, alkane, and alkene, while in the Surgical mask, there are almost no by-products in the classes aldehyde, ketone, and sugar. This is the consequence of the composition of these masks, as described in Table 1 and Table 2.

In total, 33 elements were found for the Tissue mask and 46 for the Surgical mask with a peak area greater than 1000, when submitted to the highest isothermal temperature of 500 °C. Under this isothermal temperature of 500 °C, the most important pyrolysis products obtained for the two masks are indicated in

Table 6. The five most important pyrolysis products obtained under the isothermal temperature of 500 °C.

Tissue		Surgical
Name	Relative Peak Area (%)	Name	Relative Peak Area (%)
D-Allose	51.0	2,4-Dimethyl-1-heptene	8.3
1,3-Diphenyl-1,3,5,5-Tetramethyl Cyclotrisiloxane	14.3	Cyclohexane, 1,2,3,5-tetraisopropyl-	7.1
2-oxo-propanoic acid, methyl ester	5.2	Tridecanol	5.9
2-Furancarboxaldehyde, 5-(Hydroxymethyl)-	4.1	1-Undecene, 8-methyl-	5.1
Acetic acid, methyl ester	2.4	Isotridecanol-	5.0

.

A review of thermal degradations (pyrolysis and combustion) of polypropylene under different experimental conditions was presented in [30], together with a toxicity analysis of the decomposition products. For each temperature (388, 414, 438 °C) imposed in that study, the major pyrolysis products were 2,4-Dimethyl-1-heptene, 2-pentene, 2,4,6-Trimethyl-8-nonene, and 3-Methyl-1-pentene. In [11], the authors performed pyrolysis experiments on surgical masks, and they measured the emitted by-products using the gas chromatography technique. They obtained slightly different pyrolysis products, as their major pyrolysis products were (with their relative peak area importance) 2,4-Dimethyl-1-heptene (32.32%), 2-Acetylcyclopentanone (6.58%), Cyclohexane, 1,1,3,5-tetramethyl-,cis- (5.46%), Ethanone, 1-cyclopentyl- (4.65%), and Toluene (4.48%). In [31], the authors performed experiments in a flash pyrolysis device until 900 °C on the fabric part of a mask that looked similar to a surgical one. They classified the pyrolysis products and obtained the following relative percentages: alkanes (17.95%), alkenes (22.75%), alcohols (6.40%), and esters (1.52%). The percentages are not easily comparable with those in Figure 5, as the pyrolysis experiments were performed under the high temperature of 900 °C. Potential chemical reactions occurring during the pyrolysis of surgical masks were described in [9].

3.6. Main Gaseous Emissions Occurring during Combustion Experiments

The main gaseous emissions (CO, CO₂, NO₂, and total hydrocarbons (THCs)) continuously measured during the combustion of the two masks performed in a horizontal oven are presented in Figure 6. They are represented in ppm per gram of material to compare the results obtained for the two masks.

The NO emissions are not presented in Figure 6 because they were lower than 18 ppm for the two masks and hence, certainly below or close to the detection limit. This is the consequence of the low nitrogen percentage in both masks; see Table 2.

The CO and CO₂ emissions obtained for the Tissue mask present two thin peaks, as shown in the black curves in Figure 6a,b. This was previously observed in [8] when submitting a facemask built with natural fibers to combustion. One reason could be that the devolatilization process occurs in steps that release CO and CO₂ in two successive temperature ranges, although the sample mass decreases without a significant change in slope. The CO and CO₂ emissions obtained for the Surgical mask present a unique peak, followed by a long tail on its left-hand side, as shown in the red curves in Figure 6a,b. For CO₂ emissions, the heights of the two peaks of the Tissue mask are five or six times higher than those of the Surgical mask. The emissions of total hydrocarbons (THCs) present a unique peak for both masks, Figure 6c. This peak is much bigger and higher for the Surgical than for the Tissue mask (approximately five times higher). For the two masks, the NO₂ emissions are much lower (at most 55 or 75 ppm/g) than the CO, CO₂, and THC emissions. The differences among these emissions may be explained by the differences in the fibers the masks are built with.

Further details concerning the position and the height of each peak of the gaseous (CO, CO₂, and THCs) emissions are presented in Table S5 in Section S10 of the Supplementary Materials.

Finally, the conversion rate curves associated with the combustion and the gaseous emissions are compared in Figure S4 in Section S11 of the Supplementary Materials.

4. Conclusions

Pyrolysis and combustion experiments were performed on two facemasks, commonly used in the Russian Federation and built with natural or synthetic fibers, in a thermobalance and under the low-temperature ramps of 5, 10, 15, and 20 °C/min. The thermogravimetric profiles differ between these two masks. Kinetic modeling was performed for the pyrolysis and combustion of the two masks using the extended independent parallel reaction model. The optimal values of the pre-exponential factor and activation energy obtained partly agree with those in the literature for quite similar masks. The main gaseous emissions were continuously measured during combustion experiments performed in a horizontal oven differ between the masks. The by-products of pyrolysis experiments performed under three isothermal temperatures and determined using chromatographic analyses also highly differ, belonging to completely different IUPAC classes: aldehyde, ketone, and sugar for the Tissue mask and alcohol, alkane, and alkene for the Surgical mask. The main gaseous emissions are CO₂ for the Tissue mask and total hydrocarbons for the Surgical mask, which is due to the high difference in the fibers used for the elaboration of the masks. Both facemasks could be considered for the production of valuable by-products or for energy production, provided they are collected separately. Unless the purpose of the elimination of used facemasks is to directly produce energy with a combustion process, it would indeed be better to collect separately those built with natural fibers and those built with synthetic fibers. Another way to valorize used facemasks is to apply a pyrolysis process performed under an isothermal temperature. The by-products obtained depend on this temperature, which can be chosen according to the desired by-products. In both cases, high isothermal temperatures should be applied to produce more by-products.