Pyrolysis and combustion kinetics of disposable surgical face mask produced during Covid-19 pandemic

ABSTRACT In order to prevent the spread of Covid 19, most countries have made face masks compulsory. Millions of face masks are disposed of daily in the community. Therefore, the aim of the present research work is to carry out the pyrolysis and combustion process of the face mask in a thermogravimetric machine. The kinetic parameter activation energy was calculated using model-free methods (Flynn-Wall-Ozawa, Kissinger-Akihara-Sunose, and Starink) at four different heating rates (5, 10, 15, and 20°C/min). Results have shown that face masks decompose in the temperature range of 320–480°C during pyrolysis with a maximum derivative weight loss of 2.5%/°C. Combustion took place between 200°C and 370°C with a maximum derivative weight loss of 1.25%/°C. The residue char at 500°C for pyrolysis and combustion was in the range of 1.74 to 2.73 wt.%. The average activation energies calculated using model-free methods for pyrolysis and combustion were ~135 kJ/mol and ~65 kJ/mol, respectively. In conclusion, combustion process could be the immediate solution to dispose of the face mask due to lower activation energy and decomposition temperature and low emissions as compared to pyrolysis process.


Introduction
The outbreak of the SARS-CoV-2 epidemic and its rapid spread throughout the world has had a catastrophic impact on the social life, economy, and health of the community. At a global level, this deadly virus has taken more than 2.0 million human lives, with active cases reaching more than 87 million, with third and fourth waves in some countries (World Health Organization 2020). Figure 1a shows the number of confirmed Covid-19 cases across the world (WHO Coronavirus Disease 2020). Scientists are exploring cures and medicines with some success for Covid-19 pathogenesis, genetic evolution, and prevention. Face masking and physical distancing are the most appropriate control measures to reduce the severity of disease (Gandhi and Rutherford 2020). On January 29, 2020, the WHO advised using masks in the community to avoid contamination and the spread of this deadly virus. Therefore, most countries across the world made it compulsory to wear face masks (FMs) in public or crowded places.
Thus, the demand for personal protective equipment (PPE), including FM, has increased markedly. The global market for disposable masks was worth $75 billion in the first quarter of this year and is expected to grow at a rate of over 50% for the next 7 years (World Economic Forum 2020). The surge in demand for FM for both health care public use has put a strain on the global nature of mask production supply chains and distribution systems (Lee et al. 2020a). For instance, Sangkham (2020) reported that approximately 4.6 billion FM units were used in 49 Asian countries, with the highest use in China (989,103,299 units), followed by India, Indonesia, Bangladesh, Japan, Pakistan, Iran, the Philippines, and Vietnam. Although the use of FMs plays an important role in a crisis like Covid-19, its significance will remain even after life returns to normalcy.
The continual and mass use of FM during the pandemic has increased the environmental consequences related to littering, waste, and unsafe disposal, as shown in Figure 1c. However, the potential impacts of FM are far overshadowed by the more urgent health issues (Klemeš et al. 2020). The unpredictable environmental aspect of Covid-19 has increased medical waste, including surgical FMs, by a considerable degree (Saadat, Rawtani, and Hussain 2020). Currently, millions of contaminated surgical FMs and other PPE materials related to the Covid-19 virus are becoming infectious waste (Sangkham 2020) that can cause serious environmental and health problems if not handled properly (Nzediegwu and Chang 2020). Poor solid waste management in most countries will impose a major threat of contamination in the general community, particularly for countries that still rely on open landfills for their waste management.
Thermochemical methods such as pyrolysis and combustion are promising processes that can dispose of waste materials as well as convert them into valuable products. Meanwhile, thermochemical and kinetic studies of biomass and waste materials are available in the literature. For instance, municipal solid waste and bituminous coal (Ding et al. 2021), wood (Barzegar et al. 2020), cotton stalk (Gupta, Thengane, and Mahajani 2020), Azadirachta indica seeds and Phyllanthus emblica (Indian gooseberry) (Mishra and Mohanty 2020), pineapple, orange, and mango peel wastes, and agroindustrial by-products, rice husk, and pine wood (Arenas, Navarro, and Martínez 2019), electronic waste (Krishna, Damir, and Vinu 2021), sewage sludge, and low-density polyethylene (Zaker et al. 2021), animal manure (Fernandez-Lopez et al. 2016). So far, few studies Jung et al. 2021;Yousef et al. 2021) are available on the kinetic data for the FM. None have presented and compared the kinetics of pyrolysis with combustion process in the literature. Recently, Jung et al. (Jung et al. 2021) used nitrogen and carbon dioxide environments in a thermogravimetric analyzer (TGA) from 35°C to 900°C with a constant heating rate of 10°C/min. They also carried out one-stage, twostage, and catalytic pyrolysis of FMs in a tubular reactor and analyzed the chemical compounds of the gaseous product. This study was limited to pyrolysis with only one heating rate. Attempts were made recently to recycle the FM as fiber reinforcement to improve the mechanical characteristics of fat clay (Ur Rehman and Khalid 2021). However, it is difficult to recycle FMs (Jung et al. 2021) because FM materials are made up of multi-layered non-woven polypropylene (PP) fabric ( Figure 1b) and have a high probability of cross-contamination. Another problem is the prolonged storage of Covid-19 FM waste for disinfection (Ur Rehman and Khalid 2021). Very recently (Alcaraz et al. 2022), researchers washed the used FM ten times with warm water, sterilized in autoclaves, and then irradiated with beta and gamma radiation. All these operations might take a high amount of resources (water) and energy to pre-treat or disinfect the FM before reusing it for domestic or commercial applications. Recently,  Chen et al. (2021) published an article on the pyrolysis characteristics, kinetics, thermodynamics, and volatile products of waste medical surgical mask rope made up of elastic. An even more detailed study (Yousef et al. 2021) on the pyrolysis of 3 ply face masks was published using TG-FTIR-GC-MS system. Clearly, the research work published till date on the valorization of FM is solely focused on the pyrolysis process and none have compared it with combustion kinetics. Thus, kinetic analysis provides crucial information on the decomposition behavior of any feedstock, mechanism of reaction that can help to design the reactor (Arenas, Navarro, and Martínez 2019), optimization of the process, and scale-up (Arenas, Navarro, and Martínez 2019;Dhyani and Bhaskar 2018). Therefore, the novelty of our research work is the comparison of two thermo-chemical (pyrolysis and combustion) processes with kinetics calculations that can be useful to design and optimize the process at large scale.
Therefore, the objective of the present study was to compare the pyrolysis and combustion behavior of FM. The kinetic analysis was performed at four different heating rates (5, 10, 15, and 20°C min −1 ). The temperature was varied from room temperature to 750°C. The activation energy was calculated using three model-free methods (FWO, KAS, and STK). This study provides useful data for disposing of FM waste materials in a controlled manner.

Materials
New disposable surgical FMs obtained from the local market were used in this study due to safety and health concerns of the students and staff. In actual conditions, the waste FM generated from both domestic and medical facilities can be sorted and collected with great care and can then be sent for thermo-chemical processing. FM samples were manually hand-cut into small particles in the size range of 1-3 mm and kept in airtight sealed bags for further analysis. It should be noted that the combination of three layers of FM (inner, outer, and middle) was utilized in the analysis as received. Table 1 presents the proximate and ultimate analysis of FM samples. The carbon content in FM is about ~180%, ~75%, and ~40%, higher than the sewage sludge (Zaker et al. 2021), variety of biomasses (Arenas, Navarro, and Martínez 2019), and bituminous coal (Ding et al. 2021), respectively. However, it is almost similar to electronic keyboard key waste (Krishna, Damir, and Vinu 2021), and low-density polyethylene (Zaker et al. 2021). The chemical composition of FM clearly indicates its potential to convert it into energy, fuels, and chemicals with minimal environmental impact since the sulfur and nitrogen contents are very low.

Pyrolysis and combustion of FM using TGA machine
The experiments were conducted in a TGA (TA Instrument Q50, USA). In each experiment, about 5-6 mg of FM samples were placed in a ceramic pan and loaded into the TGA. The temperature was ramped from room temperature to 750°C at different heating rates (β) of 5, 10, 15, and 20°C min −1 in an inert (nitrogen) and oxygen (air) environment under a constant gas flow rate of 60 ml/min. The effect of different β on the thermal behavior was examined, and the kinetic parameter was calculated for the decomposition temperature range.

Kinetics analysis
The activation energy (E a ) was calculated based on the TG data using model-free (FWO, KAS, and STK) methods. In the present study, the E a was calculated in the main decomposition region. The integral form of a solid-state reaction rate is typically expressed as where p x ð Þ is an approximation equation, with x ¼ E=RT. Several approximations will eventually lead to the following final equations.
The plot of ln β T 2 � � against 1=T in the KAS method (Dhyani and Bhaskar 2018) shows a linear relationship, and the slope of the straight line is equal to À E a =R. Similarly, in the FWO method (Flynn and Wall 1966;Ozawa 1965), a linear relationship is established by plotting ln β ð Þ against 1=T, and the resulting slope will be equal to À 1:052E a =R. Finally, a linear regression-line plot of ln β T 1:92 � � versus 1=T in the STK method (Starink 1996), provides a slope that will be equal to À 1:0008E a =R.

Pyrolysis of disposable surgical FMs
Pyrolysis TG and DTG profiles of the FM samples showed one major decomposition region, as presented in Figure 2. There was no weight loss below 300°C in both TG and DTG profiles, indicating a total absence of moisture in the FM sample. Typically, PP non-woven textile fabrics have the capability to retain and quickly transfer heat and moisture away from the FM for higher levels of wear comfort (Lee et al. 2020b). Apparently, new FMs do not contain moisture as they are manufactured in a controlled environment and packed to avoid contamination. However, used FMs may contain some moisture, which is absorbed by the inner layer of the FM (Figure 1b) from the exhaled air due to respiration (Roberge, Kim, and Coca 2012). The moisture can be absorbed by the FM fabric in two ways: either by diffusion, and/or by wicking by capillary transport (Premkumar and Thangamani 2017). The presence of a small amount of moisture in the used FM does not affect thermochemical processing. The major weight loss of the FM sample occurred during the main pyrolysis stage within a temperature range of 320-500°C. Only one major derivative peak was detected in this temperature range for FM pyrolysis, as seen in Figure 2b (DTG curve), unlike biomass materials (Mishra and Mohanty 2020;Sait et al. 2012;Salema, Ting, and Shang 2019), which showed two or more peaks. The initial and final degradation temperature of the FM in the main pyrolysis region was delayed, with an increase in heating rate as presented in Table 2. This is due to a slower decomposition process at higher heating rates, as the heat transfer is not as effective at higher heating rates compared with lower heating rates (El-Sayed and Khairy 2015). The sample heats gradually at lower heating rates and thus has more effective and improved heat transfer.
In the present study, only one sharp DTG peak (Figure 2b) in the range of 315°C to 490°C was observed that clearly indicated the presence of PP ((C 3 H 6 ) n consisting of a thermoplastic polymer chain that is usually crystalline) as the major constituent in the FM. This agrees with the TG and DTG profiles of PP studied by other researchers (Rex, Masilamani, and Miranda 2020;Yousef et al. 2021). Moreover, industrial nonwoven fabrics are embedded with cellulose and are used for manufacturing cosmetics, masks, medical accessories, and other things (Futamura 2020). A high degradation temperature (>300°C) during pyrolysis designates a higher thermal stability of the FM materials.
Another prominent region was located above 400°C (Figure 2), also known as the char region, where no significant weight loss was observed. No peak was noted beyond 450°C in the DTG curves, which indicates a complete degradation of the sample with very minimal leftover residue at a temperature of 500°C (Table 2). It should be noted that FM is a very light (low density) material and the remaining residue or char (left after TGA analysis) might fly away with the purge gas.

Combustion of disposable surgical FMs
The combustion behavior of FM is different from the pyrolysis process (Figure 3). For instance, the initial degradation temperature during combustion was much lower (200°C to 240°C) than pyrolysis. There was no obvious weight loss below 200°C, thus showing an absence of moisture content as during the pyrolysis process (Section 3.2). This would be quite beneficial to the combustion process, as it can save the energy needed to evaporate the water content and can cause earlier ignition of the sample (Reis et al. 2019). The second event or major combustion happened in the temperature range of 200°C to 370°C. The ignition temperatures were 218, 252, 260, and 280°C at 5, 10, 15, and 20°C/min, respectively, according to the intersection method applied on the TG curves (Lu and Chen 2015). These ignition temperatures are much lower than anthracite (~800°C), bituminous (~600°C), and brown (~450°C) coals, but are comparable to some biomass such as date palm seed (200°C) (Sait et al. 2012), and amazon biomass (140-200°C) (Reis et al. 2019). It was reported by Rybak et al. (Rybak, Moroń, and Ferens 2019) that ignition temperature is related to the volatile matter content and the chemical composition of the material. For instance, the ignition temperature decreases when the volatile content is high. The volatile matter of the FM sample was found to be higher than 90 wt.% (TGA curve), which attributes to lower ignition or combustion temperature. The ignition temperature of pure PP material is ~270°C at a 10°C/min heating rate (Fina, Cuttica, and Camino 2012), which is somewhat higher than that of the FM material.
The DTG thermograms of the combustion process for FM samples showed only one peak (Figure 3b) similar to pyrolysis (Figure 2b), however the maximum degradation rate (DTG max ) and peak temperature (T max DTG ) ( Table 2) are different. Furthermore, the presence of small peaks between 400°C and 450°C during the combustion process, as seen in the DTG profile ( Figure 3b) is due to the oxidation of char to ash. The maximum degradation rate (from the DTG curve) of FM samples at 5, 10, 15, and 20°C/min was 1.04, 1.20, 1.31, and 1.35 wt.%/°C, respectively. This signifies that the rate of combustion is faster than the rate of pyrolysis (Gunasee et al. 2016). The oxygen environment facilitated the early decomposition of carbon, and oxygen content in the sample. It is worth noting that the remaining residue (1.3 to 3.5 wt.% in Table 1) at 500°C was almost similar for both the pyrolysis and combustion processes. Notably, the FM char was difficult to breakdown in the combustion process, which might be due to the presence of inorganic and mineral components that might have limited the further oxidation and thus the residue leftover is comparable to the pyrolysis. FM contains majorly (~97 wt.%) of carbon ( Table 1) that reacts with available oxygen during combustion process to produce gases and ash residues.
Typically, PP materials contain a high amount of volatile matter (~99 wt.%) and low ash (~0.15 wt.%), as reported in (Gunasee et al. 2016). Therefore, they produce very minimal char at the end of a thermo-chemical process. The decomposition process was almost finished at 500°C, as the residual weight at 500°C and at 600°C were similar. The degradation temperature, peak degradation temperature, and residue left over from the present FM sample was close to that of pure PP material studied by previous researchers (Gunasee et al. 2016). This proves that the thermal behavior and stability of the FM sample resembles that of PP materials, which are the primary material for manufacturing the FM.

Conversion degree of FM samples during pyrolysis and combustion
The conversion degree shows only one principal stage of reaction consistent with TG and DTG profiles, as presented in Figures 2c and 3c. Surprisingly, it took a long time (60 min at 5°C/min) to start the initial pyrolysis degradation of FM sample, as compared with 30 min at 10°C/min, 21 min at 15°C/ min, and 16 min at 20°C/min. In comparison, the initial combustion degradation of the FM sample started at 35 min for 5°C/min, 18 min at 10°C/min, 14 min at 15°C/min, and 10 min at 20°C/min. The FM samples took 25 min at 5°C/min to completely pyrolyze, 13 min at 10°C/min, 9 min at 15°C/min, and 7 min at 20°C/min, whereas it took 45 min at 5°C/min, 23 min at 10°C/min, 13 min at 15°C/min, and 12 min at 20°C/min for the complete combustion of FM samples. The maximum conversion of the FM sample during pyrolysis and combustion shows great potential for fuel and chemical applications (Jung et al. 2021).

Kinetics analysis
The data from the TGA were used to calculate the activation energy (E a ) for all the kinetic methods using linear regression, as mentioned earlier in Section 2.3. The parallel linear regression lines are attributed to similar kinetic behavior and reaction mechanisms (Çepelioğullar, Haykırı-Açma, and Yaman 2016). Table 3 shows the coefficient of determination (R 2 ) obtained for each kinetic method is commonly greater than 0.9. The models used to calculate E a were applied to the main decomposition region (320-480°C for pyrolysis and 200-360°C for combustion). Remarkably, the average E a values for FWO, KAS, and STK were quite comparable and those for KAS and STK were similar. This is because Starink (1996) presented a more accurate approximation of the temperature of the FWO and KAS equations.
In the case of model-free methods, E a is calculated by building a relationship between the heating rate and decomposition temperatures at the conversion degree, and thus a series of E a at a different conversion degree can be obtained. Very recently, Singh and Sawarkar (2020) found the average E a for garlic stalk to be 95.11 and 94.54 kJ/mol for the KAS and FWO methods, respectively. A higher E a will slow the reaction rate, and vice versa. The average value of E a for cotton stalk was found to be in the range of 100 to 120 kJ/mol for the KAS, FWO, and CR methods (Gupta, Thengane, and Mahajani 2020). It is also reported that the particle size of the biomass sample could play a role in the variation in E a values. Further, the difference in E a may also be attributed to the operating conditions of the pyrolysis process, heating conditions, and the chemical composition of the FM.
The distribution of E a against the FM conversion degree with model-free methods is compared in Figure 4. This provides an important investigation of the change in activation energy with the varying conversion degree (Çepelioğullar, Haykırı-Açma, and Yaman 2016). As can be observed, the E a profiles are quite similar to each other, except that the value of E a obtained from the FWO values *FWO -Flynn-Wall-Ozawa, @ KAS -Kissinger-Akihara-Sunose, $ STK -Starink was slightly higher than those from the KAS and STK values. The FWO method is simply based on the heating rate and involves no temperature function. The variation in E a distributions clearly indicates the complex multi-step reactions as discussed previously (Arenas, Navarro, and Martínez 2019). Moreover, FM is composed of different materials (non-woven fabric and melt blown filter) which might vary the E a as a whole, since the E a of each FM component differs in value (Yousef et al. 2021). In general terms, E a changes with the conversion degree. Interestingly, the profile of E a for pyrolysis is opposite to that of the combustion conversion process. The E a of the present study during pyrolysis of FM was lower than reported by recent study (Yousef et al. 2021). This could be due to physio-chemical content, experimental and data analysis methodologies that can cause substantial variations.
An abrupt increase in the E a (from 95 kJ/mol to 140 kJ/mol) was observed in the case of FM pyrolysis when the FM sample was converted from 10% to 40%, with a brief increase (from 140 kJ/mol to 155 kJ/mol) from 40% to 90%. This is because the thermal degradation rate is high from 10% to 40%, as seen in the DTG curve corresponding to temperatures between 300°C and 450°C. Moreover, a sharp increase in the E a for a 10% to 40% conversion degree depicts multi-step reactions due to the mixture of FM layers (non-woven PP fabrics with cotton filter fabrics). This specifies that it may be difficult for the FM material to decompose during the initial pyrolysis process. Hence, it takes a higher temperature to initially degrade the FM sample (Figure 2), as evident from the TG and DTG profiles.
A relatively short variation in E a after 40% conversion shows a change in kinetic mechanism, with reactions moving to a single step. Comparison of the obtained E a shows that KAS and STK methods provided results similar to the FWO for an α between 10% and 90%. The profile of E a for FM pyrolysis conversion was quite unique when compared with other biomass (Arenas, Navarro, and Martínez 2019;Gupta, Thengane, and Mahajani 2020;Singh and Sawarkar 2020), and waste materials (Çepelioğullar, Haykırı-Açma, and Yaman 2016;Chen et al. 2021). The model-free methods also prove that the reaction mechanism of the FM lies in the major conversion region (0.3 to 0.7) as reported in (Yousef et al. 2021). The average value of E a for the FWO, KAS, and STK methods during the combustion of the FM sample was ~71.0, 64.6, and 65.5 kJ/mol, respectively.
The E a increased slowly in the case of combustion conversion from 58 to 68 kJ/mol from 10% to 60% conversion, and after that it increased rapidly until a conversion of 90% to a maximum of 95 kJ/mol. The profile of E a during FM combustion conversion somewhat resembled that of rice husk and pine wood biomass samples (Arenas, Navarro, and Martínez 2019), whereas the E a was stable within a wide conversion range (10% to 80%) and increased at a high conversion (around 80%). Two main decomposition regions (devolatilization and char oxidation) can be observed from the E a profiles of the combustion process (Fernandez-Lopez et al. 2016). Devolatilization occurring between 10% and 60% conversion and char oxidation between 60% and 90% conversion corresponds to a temperature of approximately 200°C to 320°C and 280°C to 350°C, respectively. These were in close agreement with (Fernandez-Lopez et al. 2016). A high E a above 80% conversion at temperatures higher than 350°C is related to the decomposition of the strongest bonds in the sample, which is usually difficult to degrade (Arenas, Navarro, and Martínez 2019). Another fact is that the heat transfer inside the particle is enhanced as the heating rate increases, and consequently so is the reactivity of the sample (Mishra and Mohanty 2020). Similar to pyrolysis, the values of E a obtained from the FWO method were slightly higher than those obtained from KAS and STK. The values of E a for combustion of the FM samples are lower than the values reported in the literature for the combustion of pine wood chips (Barzegar et al. 2020) and manure (Fernandez-Lopez et al. 2016). However, the present result of E a during combustion is in line with the combustion values of cotton stalk (Gupta, Thengane, and Mahajani 2020). A lower E a during the combustion process is desirable so that the reaction initiates more quickly. This could be due to the presence of an oxygen environment in the combustion process, whereas with pyrolysis, a higher E a delays the initiation of reaction due to the inert environment. Another disadvantage of pyrolyzing the FM would be the energy recovery since the input energy to an overall system would be difficult to compensate in exchange of energy recovered from the pyrolysis product, unless all the products are utilized. Among the thermo-chemical conversion, pyrolysis of FM might affect the circular economy as it would create products (bio-oil and gases), which could turn the process into linear economy. However, this requires further research work.

Conclusions
The study compared and presented the pyrolysis and combustion kinetics of FM samples. The profiles of pyrolysis and combustion of FM showed only a single degradation peak (320-480°C and 200-370°C, respectively). The average E a values obtained from the FWO, KAS, and STK methods during pyrolysis were ~139, 135, and 134 kJ/mol, respectively, whereas for the combustion process, the values were ~70, 64, and 64 kJ/mol, respectively. Interestingly, the residual char obtained after the pyrolysis and combustion process of the FM was minimal (in the range of 1.35-3.50 wt.%). The thermochemical conversion processes, such as pyrolysis and combustion have the potential to convert FM waste into energy in addition to offering safe disposal. The immediate solution to dispose the FM would be through co-combustion process, because most of the power plants are based on combustion process that uses solid fuels such as coal, biomass, waste, etc. In the future, thermochemical conversion of FM will be carried out in a lab-scale with further analysis on solid, liquid, and gas products.