Thermal Degradation Characteristic and Kinetic Analysis of the Solid Byproducts Recovered from Vacuum Pressurized/Friction Thermal Sterilization of Hospital Solid Wastes

Abstract

We investigated the TFSS byproduct’s thermal degradation and kinetic characteristics from hospital waste’s thermal frictional sterilization (TFSS) using isothermal and advanced nonisothermal model formulas. We studied TFSS byproducts degradation characteristics using TGA from room 28 to 1000°C. We used Kissinger, Kissinger–Akahira–Sunose, Flynn–Wall–Ozawa, and advanced Vyazovkin model formulas to derive the activation energies. We also analyzed conventional proximate/ultimate and calorific values. Gross calorific value was 5.730 kcal/kg, followed by a net calorific value of 4.038~4.133 kcal/kg because of the high carbon content, 70% volatile content, 2.6~2.8% moisture and 8~13% fixed carbon content. Nonlinear lateral shift in peak temperatures were observed in the first–third degradation zone, but the nonlinear lateral shift had minimal influence on the values for all temperature rates. Activation energy for Kissinger model was 172.25 kJ/mol. Average activation energy derived using Kissinger–Akahira–Sunose, Flynn–Wall–Ozawa, and advanced Vyazovkin model formulas were 176.65, 177.22, and 177.16 kJ/mol, respectively. Fluctuations in activation energies from for 3 model formulas was because of the disruption of hydrogen bonds and covalently bonded chemicals. Activation energy values had an exponential increase from for three model formulas. Results showed that TFSS process generates heterogeneous byproducts with high calorific values and predictable activation energy values.

INTRODUCTION

Current statistics have shown that the emergence of pandemics and related emergencies has increased hospital solid waste (HSW) generation around the world [1–3]. The COVID-19 pandemic has widened the sources of HSW generation to include residential houses, public facilities, and, in some cases, on the streets [4–6]. Like many other hazardous wastes in Korea and the People’s Republic of China, hospital solid wastes (HSW) require specialized treatment facilities and use incineration systems on many occasions [7, 8]. However, a sharp increase in the HSW generation has overwhelmed the operations of the existing incineration facilities [2, 9]. The World Health Organization (WHO) published a recent report highlighting the unprecedented situation with pandemic-generated HSW and the urgent need to solve the HSW accumulation issue [10]. Such a situation and prospects of a new pandemic have created an opportunity for other approved HSW treatment systems to complement the current incineration systems to solve the emerging HSW accumulation problem.

WHO has approved several HSW treatment systems that could be used as stand-alone systems or complement the current incineration systems; one is the thermal frictional sterilization system [7, 8]. The thermal frictional sterilization system (TFSS) uses high-speed friction-attrition phenomena at high temperatures (135~150°C) and vacuum conditions to destroy pathogens and biological wastes; the HSW applicable could be infectious or general [7, 8]. Therefore, the HSW fed into the system will be comminuted and sterilized, achieving 75% volume and 25% mass reduction. Italian companies have developed such systems and successfully used them in hospitals with up to 600-bed capacity [7, 8]. Once the TFSS system could be integrated with the incineration system, the HSW treatment process could be decongested and optimized, especially during pandemic situations where HSW treatment time is an essential variable (Fig. 1). Preliminary material flow and cost-benefit analysis show that hospitals that would integrate the HSW treatment systems as shown in Fig. 1 would expect a 30~60% cost saving when 32~51% of material (i.e. TFSS byproduct) is recovered during routine and emergency operation period (Fig. 1).

Fig. 1.

Model concept for integrating the incineration systems with the thermal frictional sterilization systems.

The sterilized waste from the TFSS is a calorific byproduct because a greater portion of the HSW contains combustible waste such as plastics, textiles, paper etc. [7, 10, 11]. Preliminary calorimetric analysis from the sample we obtained from one of the treatment systems showed that the TFSS byproduct has enough calorific value to be used as fuel or other thermal conversion processes. Dharmaral et al. [13] highlighted that the HSW could be pyrolyzed into sustainable energy because the dominant component in the HSW is a combustible waste. Thus, since the integrated HSW treatment system model and the respective cost-benefit analyses are under consideration, it would be imperative to determine the thermal kinetic properties of the TFSS byproducts.

Studies and review papers published hitherto have focused on the thermal conversion process of HSW generated during the pandemic because of the high plastic PPE composition [13–15]. In other instances, thermal decomposition and kinetic characteristics of HSW or the respective components have been studied, and results presented. Qin et al. [16] studied the thermal degradation and reaction pathways of medical plastic waste, and they used a mixture of PP and PS in medical infusion bags and plastic bottles. The kinetic characteristics derived are based on the model-free methods. Chen et al. [17] used the model-free methods to derive the kinetic characteristics of the surgical mask ropes. Gorbovskiy et al. [18] used the nonisothermal conditions to investigate an open system.

All the HSW or model HSW used in the thermal degradation studies uses raw HSW, representative HSW (mainly plastic), and not a mixture of materials with very different properties. Many hospitals have/have not successfully sorted and classified the HSW into different categories recommended by WHO and the International Committee of the Red Cross because the sorted HSW is based on the nature of contamination [19–21]. Therefore, we expect that the resulting HSW will be a mixture of different material components. The TFSS byproduct is a mixture of sterilized solids from plastics, textiles, metals, latex gloves, PPE plastics, paper, etc. For example, the sample we obtained could be a mixture of blood-coated textiles, bandages, cotton yarn, general HSW, etc. Furthermore, the pandemic period might require that the waste be treated and disposed of on an as-it-is basis. Thermal degradation and kinetic studies of TFSS byproducts have not been presented hitherto. Therefore, it would be essential to discover the thermal degradation characteristics of the TFSS byproduct, the impact and dominant mechanism behind the TFSS process.

This study presumed that the TFSS byproducts could undergo independent and simultaneous volatile and char decomposition processes; such degradation mechanisms would be characteristics in a mixture. For example, cotton-based textile will have a higher oxidation onset temperature than single-use plastic-based PPE [22]. Consequently, a 30°C difference in oxidation onset temperature could be observed between uncoated and coated cotton yarn [22, 23]. Therefore, it would be essential to use the conventional model-free and advanced Vyazovkin methods recommended by the ICTAC. The objective of this study will be to describe and discover the thermal degradation characteristic of the TFSS byproducts, which are not known or investigated before. Data obtained from the study will complement existing and future studies in the age of an unprecedented generation of HSW during the pandemic. This manuscript describes the material preparation process, analytical procedure, the kinetic model formulation, and the results emanating from the formulation thereof.

MATERIALS AND METHODS

Material preparation. The sample material (TFSS byproduct) was obtained from one of the thermal frictional sterilization facilities installed. According to the HSW treatment procedure, the HSW generated from the healthcare facility was sorted into recommended categories and subjected to thermal frictional sterilization process at 145°C for 45 min. Later on, the samples were cooled down to room temperature, after which the sample TFSS frictional byproduct was dried. Later on, the sample TFSS byproduct obtained was crushed further using a 4-blade high-speed blender to reduce the particle size to <5 mm. The proximate and ultimate analysis was conducted and the results shown in Table 1. We analyzed moisture content using halogen lamp moisture analyzer (Precisa XM60).

Table 1.  .

Ultimate and Proximate analysis of the TFSS byproduct

Ultimate analysis, %
Carbon	Hydrogen	Nitrogen	Sulfur	Oxygen	Inert††	Total
51.68	7.68	0.33	0	34.08	6.23	100
Proximate analysis, %
Moisture		Volatile		Ash content		Total
9.41		72.13		18.46		100

†† Obtained by difference.

Calorific value calculation. The sample TFSS could be classified as solid waste, and we determined the gross calorific values using semi-automated oxygen bomb calorimeter developed by IKA (model C 200) fitted with the appropriate propriety decomposition vessel and the oxygen filling stations. We determined the gross calorific value based on ASTM D5468-95 settings. We calculated the net calorific values empirically based on the formulations below [24–26]. We used these empirical formulations because they are based on experimentally determined values:

1

2

In the Eqs. (1), (2) above, , , and represents the ash content, moisture content and gross calorific value (kcal/kg) of the TFSS byproduct, respectively.

TGA analysis. We used approximately 100 ± 2 mg TFSS byproduct was used in the TGA analysis. The TFSS byproduct was fluffy and maintained some level of elasticity. Thus, we used a tablet pill die mold (ø7 mm) to modify the shape of the sample TFSS byproduct before placing it into the sample pan. 10 minutes waiting period was observed after placing the sample into the pan to determine if the elastic and fluffy nature of the sample would affect the sample positioning in the sampling pan. Auto-TGA from TA instruments (model Q500) was used in the thermal decomposition process. We used nitrogen gas to create the inert conditions at a flow rate of 50 mL/min. The heating rates were 5, 10, 20, 50, and 90°C/min, and the temperature diapason was from room temperature to 1000°C.

Kinetic characteristics. As mentioned before, the TFSS byproduct contains several materials which could undergo simultaneous solid-state decomposition processes. Therefore, the main deriving equation rate could be derived as follows [27]:

3

In the equation above, T and represents the absolute temperature and the degree of conversion, respectively. Since is the Arrhenius constant, the nth Arrhenius plot could be derived as follows:

4

5

Further derivation of the equation above to formulate the appropriate isoconversion model formulas have been described in many literatures [28–30]. This study used the Flynn–Ozawa, Kissinger–Akahira–Sunose and Kissinger model formulas as the conventional model-free isothermal model formulas. The formulas are described in Table 2. In addition, we used the nonisothermal advanced Vyazovkin model formula [31, 32], which is a nonlinear algorithmic model used to complement the model-free methods, especially if and when thermal effect could deviate the sample’s temperature from an expected heating program [33]. Different authors have derived the advanced Vyazovkin model formula from the first principles [33–35]. Other researchers have simplified the process by developing software tools for computation [36, 37]. Thus, we used a software tool developed by Drozin et al. [36] to compute the activation energy of the samples at different temperature rates (. We also used the computation module developed by Joseph et al. [37] to compute and compare the activation energies at different conversion factors based on advanced Vyazovkin model.

Table 2.  .

Model-free kinetic model equations used in the analysis

Model	Equations
Flynn–Wall–Ozawa
Kissinger–Akahira–Sunose
Kissinger

RESULTS AND DISCUSSION

Calorific values. The gross and net calorific values of the TFSS byproduct are shown in Table 3 and Fig. 2. The gross calorific value (determined experimentally) was approximately 5.730 kcal/kg, which is within the range of many refuse-derived fuels derived from MSW [37–39]. We believe that the thermal frictional system was instrumental in sterilization and removal of other compounds which could be impurities and aid calorific loss. Therefore, we concluded that the TFSS byproduct from the reference Hospital components was enough to be used as fuel or for thermochemical conversion. Moreover, the ultimate analysis revealed the presence of carbon and oxygen content as the dominant products and fewer impurities (sulfur and nitrogen-based compounds) from the high-temperature treatment of HSW.

Table 3.  .

Gross and net calorific values of the TFSS byproduct

Particular	Deriving formula	Value (kcal/kg)
Gross calorific value	None (derived using bomb calorimeter)	5730
Net calorific value		4038.25
		4133.05

All the values were computed based on dry material basis.

Fig. 2.

Characterization of the calorific values and losses.

The net caloric value was reduced by approximately 1597~1692 kcal/kg based on the formulas used in the calculation; there was a 95 kcal/kg difference between the net calorific values computed using the Ilinykh and ASTM E955 empirical formulas. The differences in the calorific losses due to moisture and ash content are displayed in Fig. 2. As it will be observed, there was a minor difference (51.3 kcal/kg difference) in the calorific value losses from moisture content for the two empirical formulas used (i.e., Ilinykh and ASTM E955). The dominant difference in the calorific value losses was in the ash content (983 kcal/kg difference). Empirical formulas are developed based on several variables and limitations, such as the region, weather seasons, and composition among other entities [38, 39]. We concluded that the calorific value, low moisture and ash content (Table 1) could be useful for thermal processes.

TGA characteristics. The TGA results Figs. 3, 4 and Tables 4, 5 show the thermal decomposition characteristics of the TFSS byproduct at different temperature rates. We observed four distinct zones from the thermal decomposition process, and the first zone was the moisture evaporation zone (Fig. 3, Tables 4, 5). The overall mass loss in the first zone ranged from 2.6~2.8% for all the temperature rates (. The moisture difference between the TGA results and the proximate analysis conducted by the halogen lamp moisture analyzer could be because of the surface characteristics and methodological/instrumental error. The TGA instrument required a far lesser sample (mg level) than the requirements for the halogen lamp moisture analyzer (g level).

Fig. 3.

TGA curves of the TFSS byproducts.

Table 4.  .

Analysis of the zone and their respective peaks at different heating rates

Zone	Heating rate, °C/min	, °C	DTG, %/min	Mass loss, %
Zone 1	5	60.33	–0.0257	1.330
	10	68.70	–0.0641	1.335
	20	74.38	–0.1141	1.507
	30	89.59	–0.1574	1.308
	50	85.47	–0.3150	1.013
	90	93.72	–0.4641	1.005
Zone 2	5	322.69	–0.6819	33.633
	10	336.46	–1.6153	38.335
	20	347.29	–2.8573	35.347
	30	355.55	–4.5725	36.517
	50	363.80	–7.2583	35.190
	90	377.73	–12.1326	36.720
Zone 3	5	451.82	–0.4368	68.356
	10	465.69	–0.6427	69.630
	20	478.33	–1.3977	67.191
	30	486.59	–2.1070	68.748
	50	492.00	–2.5348	68.181
	90	505.94	–5.7811	67.353
Zone 4	5	628.22	–0.0473	80.421
	10	654.51	–0.0969	80.214
	20	675.41	–0.1683	78.448
	30	703.00	–0.3470	80.658
	50	703.00	–0.4123	78.863
	90	731.90	–0.7227	79.169

Therefore, we expect that the difference and sampling accuracy would have affected the results. Further, the sterilization process results in structural, morphological, and interfacial properties' changes in the HSW [40, 41]. Therefore, we assumed that the resulting interfacial properties could be attractive sites for moisture vapor which could have been instrumental in the increase in the moisture content when analyzing with the halogen lamp moisture analyzer (Table 1). Our day-to-day handling of the TFSS byproducts could have either increased or decreased the moisture content. We observed a nonlinear lateral shift in peak temperature from 60°C at to 93.72°C at . The nonlinear lateral shift in the peak temperature could be because of the evolution of chemically bonded moisture and some volatile compounds. The nonlinear characteristics were also observed in the DTG values (Table 3). The mass losses at each differed either because of the possible structural changes in the TFSS byproduct or on the heterogenenous character of the TFSS byproduct during analysis. The FTSS byproduct is a heterogeneous material generated through partial degradation, melting and granulation.

The second region, associated with the first devolatilization zone, has peak temperatures ranging from 322.69 to 373.73°C for all the used in the analysis (Fig. 4, Tables 4 and 5). This region could be characterized at thermal cracking and decomposition of the solid TFSS byproducts. This zone’s rapid decomposition characteristic was observed because of the decomposition of the volatile carbonaceous compounds generated from the sterilization process. In the first devolatization phase, other processes commence prior to pyrolysis [42]. In light of the TFSS operating conditions (vacuum, high temperature, steam), we assumed that the dominant process in this zone could have been the disruption of the remaining hydrogen bonds and the transport of bonded molecular phase [42, 43]. The bonded molecular phase could have been hemicelluloses remaining after the sterilization process. Furthermore, the ultimate analysis (Table 1) showed that the dominant components in the TFSS byproducts are carbon, oxygen, and hydrogen, and the carbon content is almost similar to the composition presented in other literature [43].

Fig. 4.

dα/dT versus T curves at different heating zones.

As mentioned before, sterilization coupled with the high-temperature friction process processes influences the materials' structural, morphological, and interfacial properties. From the reactivity point of view, the thermal degradation process conducted at exhibited higher reactivity relative to the rest of the samples; such an observation could be observed in such case scenarios where heterogeneous mixtures such as municipal solid wastes are tested [26]. The mass losses in this zone’s peak were in the range of 33~38%. We could not establish a correlation between the mass losses with from the first zone and the second zone because of the heterogeneous structure of the material. We observed a lateral shift in the peak temperature and the onset temperature with the increase in (Tables 4 and 5). Of peculiar interest is the versus T in this zone (Fig. 4). We observed similar values (0.01~0.013 min) irrespective of the heating rates (with an exception of ) because of the generation of partially molten compounds, disruption of the remaining hydrogen bonds and the transport of bonded molecular phase.

The third zone was considered to be a continuation of the previous zone, and primary degradation of carbonaceous molecules could be characteristic in this zone. Carbonaceous, carbon-based functional compounds and the unreacted noncovalent bonded molecular compounds from the previous zone would be further degraded, leaving fixed carbon. The corresponding mass losses in these zone’s peak were in the range of 67~69%. We could not establish a correlation between the mass losses with . Conversely, we observed a lateral shift in the peak temperature with the increase in (Table 4). The reaction in the second and the third zone could be attributed to the structure of the TFSS byproduct after the thermal frictional sterilization process. The thermal frictional sterilization process embodies the degradation of organic materials from human tissues, partial melting, coating, and granulation of the mixed material to form a brand-new byproduct with calorific value (thanks to the carbon content) that is inert in terms of the sanitary conditions (Fig. 5). Therefore, we expect that part of the degraded organic materials in the HSW that have coated the plastic and textile materials would valorize at an irregular rate from the second and probably in the third zone prior to the degradation of the other carbonaceous compounds in the TFSS byproducts [44]. Also, we expect the valorization composition to be very high as observed in Table 5: the volatile matter (zone 2 + zone 3) was approximately 71~73% of the TFSS byproduct. We observed a lateral shift in the peak temperature and the onset temperature with the increase in (Tables 4 and 5). Also, we observed values in the 0.005/min region for heating rates (Fig. 4). In overall, we considered that the third zone was in part, a continuation of the second zone followed by pyrolysis of the other compounds.

Fig. 5.

Thermal frictional sterilization process mechanism.

Table 5.  .

TFSS byproduct’s thermal degradation properties

, °C/min	, °C	, °C	Moisture, %	Ash, %	Volatile matter, %	Fixed carbon,%
5	154.87	657.07	2.605	10.601	73.595	13.199
10	161.82	674.63	2.709	13.793	72.978	10.520
20	180.65	708.43	2.742	16.996	70.264	9.998
30	193.03	731.90	2.690	15.781	72.406	9.123
50	198.45	746.86	2.820	17.647	71.657	7.876
90	208.25	756.66	2.614	17.979	71.356	8.051

The final zone was characterized with the final condensation of the fixed carbon matrix. In this case, the remaining fixed carbon content (Table 5) would dissociate to generate gases. We observed a decrease in the carbon content with an increase in the (Table 5) which could be associated with the diffusion processes associated with the thermal degradation processes. In general, we concluded that the dominant zones were the second and the third zone, where over 70% of the volatile compounds were generated. The TFSS process was inherent in transforming a considerable section of the organic materials and plastic into unified, heterogeneous material with very high volatile content, as shown in Fig. 5 [44]. In the TFSS process, the organic materials in the HSW undergo partial degradation and melting/hydrolysis process in the presence of water vapor, infused into the process, thereby generating the wetting film. The partial degradation is due to the temperature increase from the inbuilt heater and the “thermal catalytic effect” from the friction process [44]. The friction blades in the reaction vessel rotate at very high rpm and comminute the other solid fraction of fibrous and polymeric nature to small fractions. Some solid fractions would partially degrade to form volatile compounds, which would combine with the blends from organic materials to create a kind of film on top of the solid fractions [45]. Thus, we believe the coated solid fractions would have nonlinear degradation characteristics since the coating would not be even on all situations. Furthermore, the fibrous materials have microstructural characteristics which would also play a dominant role in the nonlinear degradation characteristics.

Kinetic description. We observed the necessity to define the crucial kinetic characteristics based on the conventional isothermal and nonisothermal model formulas from the nonlinear characteristics described in the previous section. The results are shown in Figs. 6–8 and Tables 6–8. We observed a similar trend in the activation energy for the four models used (Table 8). First of all, the activation energy derived using the Kissinger model formula was 172.15 kJ/mol which was relatively higher than the rest of the model formula at and the value was similar for all the zones studied (Fig. 6 and Table 8). Activation energy derived using the Flynn wall-Ozawa model formula showed a slight decrease at (151.86 kJ/mol) relative to the Kissinger model followed by a 21.26 kJ/mol increase at .

Fig. 6.

Kissinger plot.

Table 6.  .

Linear expression for each α based on Flynn–Wall–Ozawa model

α		R 2	E α
0.05		0.9636	151.86
0.1		0.9886	173.12
0.15		0.9916	173.92
0.2		0.998	161.47
0.25		0.9949	177.12
0.3		0.9956	178.47
0.35		0.9954	179.75
0.4		0.9943	181.83
0.45		0.9905	184.22
0.5		0.9758	190.93
0.55		0.9941	170.02
0.6		0.9652	170.38
0.65		0.9006	168.16
0.7		0.9527	211.85
0.75		0.993	324.01
0.8		0.9903	285.51
0.85		0.9758	310.46
0.9		0.8608	291.19

Table 8.  .

Comparison of the activation energies

α	Flynn–Wall–Ozawa	Kissinger–Akahira–Sunose	Kissinger	Vyazovkin
0.05	151.86	150.22	172.25	151.31
0.1	173.12	171.78	172.25	172.98
0.15	173.92	172.35	172.25	173.56
0.2	161.47	191.28	172.25	175.07
0.25	177.12	175.37	172.25	176.58
0.3	178.47	176.66	172.25	177.87
0.35	179.75	177.89	172.25	179.15
0.4	181.83	179.97	172.25	181.23
0.45	184.22	182.36	172.25	183.68
0.5	190.93	189.25	172.25	190.70
0.55	170.02	167.13	172.25	186.63
0.6	170.38	167.62	172.25	13.94
0.65	168.16	168.22	172.25	11.50
0.7	211.85	210.95	172.25	137.16
0.75	324.01	328.54	172.25	285.18
0.8	285.51	298.16	172.25	314.28

⟡ The comparison was limited to because of the recommendations provided by Drozin et al. [35]

Fig. 8.

Kissinger–Akahira–Sunose plot.

Table 7.  .

Linear expression for each α based on Kissinger–Akahira–Sunose model

α		R 2	E α
0.05		0.9611	150.22
0.1		0.9877	171.78
0.15		0.9907	172.35
0.2		0.8845	191.28
0.25		0.9945	175.37
0.3		0.9952	176.66
0.35		0.9951	177.89
0.4		0.9939	179.97
0.45		0.9899	182.36
0.5		0.9738	189.25
0.55		0.9966	167.13
0.6		0.9702	167.62
0.65		0.9686	168.22
0.7		0.9477	210.95
0.75		0.9924	328.54
0.8		0.9622	298.16
0.85		0.9765	312.62
0.9		0.7238	334.62

We observed an upward-downward drift in the activation energies (12.578 kJ/mol standard deviation) in the second and third zone ( which could be attributed to continuous generation of volatile products from the TFSS byproducts (Fig. 7, Tables 6, 8). Similar observations were made for the activation energies derived using Kissinger–Akahira–Sunose model formula, where average activation energy of 177.22 kJ/mol was recorded (. We also observed an upward-downward drift in the activation energies (14.0768 kJ/mol standard deviation) in the region , an observation similar to the Flynn–Wall–Ozawa formula. In the region corresponding to , the highest activation energy values were observed in the second zone () because of the disruption of the remaining hydrogen bonds and the transport of bonded molecular phase, which could have required more activation energy. The peak in activation energies were followed by low activation energy demand (α = 0.1~0.15; (). The possible reason for such upward-downward shift in the activation energies could be attributed to the nature of the coated TFSS byproduct.

Fig. 7.

Flynn–Wall–Ozawa plot.

When we analyzed the activation energies derived using the advanced Vyazovkin model, the initial value at α = 0.05 was similar to that obtained from Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose model formulas. We observed an increase from to . Even though there was a sharp decrease at , the overall trend was an exponential increase in the activation energy (upto ), a characteristic observed both in the Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose model formulas.

By observing the activation energies from the versus T curves, we could assume that the activation energies will be similar irrespective of the model formula used in the computation. The thermal degradation process had similar characteristics because of the phase transition characteristics in the second and the third zone.

CONCLUSION

Thermal degradation and kinetic parameters of thermal degraration of the TFSS byproduct was investigated using different models according to thermogravimetric data and the following conclusions were made:

1. Gross calorific value was approximately 5.730 kcal.kg and the net calorific values ranges from 4.038~4.133 kcal/kg.

2. Moisture evolution zone had 2.6~2.8% mass loss with a nonlinear lateral shift in the peak temperatures.

3. Onset temperatures increased from 154.87 to 208°C with an increase in temperature rates, while the offset temperature shifted from 657.07 to 756.66°C with an increase in temperature rates.

4. Over 70% volatile matter and 10~17% ash content was observed. The fixed carbon content was approximately 8~13% of the TFSS byproducts.

5. values ranged from 0.008~0.013 min in the second zone, 0.0041~0.0055 min in the third zone and 0.0007~0.013 min in the fourth zone.

6. Average activation energy values was in the 170 KJ/mol range irrespective of the model formula used.

7. Lowest and highest activation energy values we recorded in and respectively.

This study investigated the kinetic characteristics of the TFSS byproducts alone and revealed the essence of understanding the TFSS process and the thermal coating processes that will result in similar activation energy values. Therefore, future studies will look at the mechanism inherent in the TFSS process.

NOTATIONS

TFSS	thermal frictional sterilization system
HSW	hospital solid wastes
QN	net calorific values, kcal/kg
QG	gross calorific value, kcal/kg
AS	ash content, %
XS	moisture content, %
	degree of conversion
T	temperature, °C
E	activation energy, kJ/mol
	temperature rates
t	time, min

FUNDING INFORMATION

This project research was financed in part by the Study of Comprehensive Treatment and Countermeasures Support Fund (Grant no. BK202012).

CONFLICT OF INTEREST

The authors declare that they have no conflicts of interest.