Study on kinetics and thermsodynamics of municipal solid waste incineration fly ash in air and N2 atmospheres

Yegui Wang; Weifang Chen; Na Zhao; Yifan Chen; Baoqing Deng

doi:10.1371/journal.pone.0323729

Abstract

This study attempted to investigate the thermal behavior and reaction mechanisms of municipal solid waste incineration fly ash under air and N₂. Mass loss patterns at temperatures from 30ºC to 1100ºC were obtained through thermogravimetric analysis. Based on mass loss patterns, the behavior of fly ash under high temperature was divided into three stages. Mass loss in Stage I (30ºC-500ºC) amounted to 3.0%-6.2%. The majority of mass loss concentrated in Stage II (500ºC-800ºC) and Stage III (800ºC-1100ºC). Kinetic parameters of fly ash in Stage II and Stage III were evaluated using Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), and Friedman methods. By comparison, the iso-conversional FWO method exhibited the highest correlation coefficient with R² > 0.99. Activation energy (E) values in Stage II calculated via the FWO method indicate that reaction in air showed considerably higher hurdle (E = 171.11 kJ/mol) than reaction in N₂ (E = 124.52 kJ/mol). This difference was partly attributed to the presence of carbonation process in air. In contrast, E values in Stage III were similar with E of 373.38 kJ/mol in air and 382.25 kJ/mol in N₂. Mechanistic analysis via the Coats-Redfern (CR) model, employing 15 kinetic functions, identified dominant mechanisms of one-dimensional diffusion and contracting sphere for Stage II in air and N₂ respectively. At the same time, three-dimensional diffusion could best explain the reaction mechanism in Stage III in both air and N₂. Moreover, calculations of thermodynamic parameters (ΔH, ΔG, and ΔS) revealed that major reactions of fly ash during thermal treatment were endothermic and non-spontaneous, with Stage III exhibiting heightened complexity. This multi-stage characterization elucidates the degradation mechanisms of fly ash under varying thermal conditions and provides useful insight into the fly ash thermal treatment processes.

Citation: Wang Y, Chen W, Zhao N, Chen Y, Deng B (2025) Study on kinetics and thermsodynamics of municipal solid waste incineration fly ash in air and N₂ atmospheres. PLoS One 20(5): e0323729. https://doi.org/10.1371/journal.pone.0323729

Editor: Nor Adilla Rashidi, Universiti Teknologi Petronas: Universiti Teknologi PETRONAS, MALAYSIA

Received: January 20, 2025; Accepted: April 14, 2025; Published: May 14, 2025

Copyright: © 2025 Wang et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: The author(s) received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

1. Introduction

The global production of municipal solid waste has been growing steadily and is expected to reach 3.4 billion tons by 2050 [1]. Landfill and incineration are two of the most common methods of disposal. However, landfill is land-intensive while gaseous products and leachate from landfill facilities present great environmental challenges [2]. Incineration is renowned for its ability to effectively reduce mass (about 70%) and volume (about 90%) of municipal solid waste. But incineration also has its problems. One of them is the generation of fly ash. Fly ash is classified as hazardous waste in many countries due to its contents of toxic organics, heavy metals and salts [3]. On the other hand, fly ash could also be a valuable resource as it is rich in chemicals such as CaCO₃, SiO₂, and Al₂O₃. Therefore, the main focus on fly ash is effective treatment, safe disposal or cost-effective resource recycling.

Thermal treatment, stabilization/solidification, and separation are the main treatment methods for municipal solid waste incineration fly ash. Compared with the other two methods, thermal treatment has certain advantages in reducing volume, stabilizing heavy metals and decomposing toxic organics [4]. Studies have shown that thermal treatment around 400ºC can remove 95% of dioxins from fly ash [5], while treatment at 800ºC can effectively reduce heavy metal leaching [6]. Recent advancements in thermal treatment technologies have focused on the detoxification effects and resource utilization pathways. For instance, co-treating fly ash with iron ore has been shown to stabilize heavy metals [7], while high-temperature treatments enable the extraction of SiO₂ [8]. Additionally, fly ash has been successfully utilized in the preparation of cementitious materials [9] and ceramic products [10]. Due to its high efficiency and versatility, thermal treatment has gained considerable attention. Many studies have focused on the behavior of fly ash during thermal treatment.

Gu et al. [11] compared the effects of atmospheric conditions on thermal treatment. Their results showed that mass loss of fly ash followed the order of CO₂ > N₂ > air. Lane et al. [12] further investigated the relationship between the volatility of elements and atmosphere of thermal treatment. Elements such as K, Na, Pb, Cd, and Cu exhibited higher volatility in oxidative atmospheres. By comparison, Zn, Sb, Sn, and Bi were more readily released in reducing atmospheres. Importantly, the migration and transformation of heavy metals during thermal treatment are also strongly influenced by the choice of atmosphere [13]. These findings underscore the critical role of atmospheric conditions in determining the outcomes of thermal treatment, as they directly affect both the volatilization behavior of elements and the mass loss of fly ash.

Thermal treatment is a prevalent technology for fly ash management. Kinetic and thermodynamic analyses of these processes remain understudied. Kinetic and thermodynamic analyses enable the prediction of reaction feasibility and limitation, and mechanistic pathways [14,15], while providing essential guidance for reactor design optimization and industrial-scale parameterization [16]. Therefore, systematic investigations into fly ash’s thermal behavior under various conditions is necessary.

Thermogravimetric analysis (TG) stands as a pivotal tool in the investigation of the thermal behavior of solid materials [17,18]. By varying temperature, atmosphere, heating rate and reaction time, alterations in solid mass or conversion ratio could be obtained. Kinetic and thermodynamic parameters of solid materials during thermal treatment can then be computed by model-free and model-fitting methods. For instance, Yousef et al. [19] and Loy et al. [20] similarly applied model-free methods including Friedman, FWO and KAS to estimate the activation energy of mango seed shells and rice hull under high temperature. In addition, model-fitting methods such as the CR method can be employed to determine the reaction mechanism and pre-exponential factors based also on TG data [21,22]. That is, TG analysis results were versatile. They could be used to capture the thermodynamic and kinetic characteristics of the thermal behavior of solids. By fitting the data into various reaction mechanism models, they could also be used to elucidate the reaction mechanisms. For example, Chen et al. [23] utilized the determined kinetic parameters to compute enthalpy changes (ΔH), Gibbs free energy (ΔG), and entropy (ΔS) to assess the feasibility, direction, and energy requirements for thermal treatment of waste medical surgical mask rope. Despite these established applications in fuel and energy research, fly ash thermal treatment mechanisms remain insufficiently characterized. This deficiency not only limited a thorough understanding of fly ash thermal treatment mechanisms but also impeded the development and practical application of related technologies.

This study attempted to investigate the thermal behavior of fly ash in air and N₂ via thermogravimetric analysis. Three model-free methods, including the Friedman method, FWO method, and KAS method, were used to determine the reaction activation energy at different conversion ratios. One model-fitting method (CR method) was employed to determine the reaction-dominating model and pre-exponential factor. The accuracy of the obtained model was evaluated by comparing the calculated and experimental results. In addition, thermodynamic parameters like Gibbs free energy, enthalpy, and entropy were also calculated and discussed. This research endeavored not only to enhance the understanding of the thermal treatment of fly ash but also to contribute insights for process optimization.

2. Materials and methods

2.1 Raw materials

The fly ash used in this study was collected from a municipal solid waste incineration plant located in Zhejiang Province, China. To ensure sample homogeneity, composite sampling was carried out continuously for 14 days, and the collected samples were thoroughly mixed. The fly ash was then ground and sieved through a 100-mesh sieve then dried at 105ºC for 24 h before use.

2.2 Fly ash characterization

A TG/DSC simultaneous thermal analyzer (TG/DSC, PE STA8000, USA) was used to explore the thermal behavior of fly ash under air and N₂ atmosphere. About 10 mg of fly ash was calcinated from 30ºC to 1100ºC in an alumina crucible. The heating rates were set at 10ºC/min, 20ºC/min, and 30ºC/min, respectively. Gas flow rates were constant at 80 mL/min.

Proximate analysis of fly ash was obtained according to ASTM standard method E1131-08. X-ray Diffraction (XRD, Rigaku UItima IV, Japan) was employed to characterize the crystalline phases, with Cu-Kα radiation at 40 KV and 30 mA as 2θ was ranged from 5° to 90°. Chemical compositions and micromorphology of fly ash were determined via X-ray Fluorescence Spectroscopy (XRF-1800, Shimadzu, Japan) and Scanning Electron Microscope (SEM, Zeiss Gemini 300, Germany).

2.3 Kinetic and thermodynamics analyses

2.3.1 Estimation of activation energy.

TG/DSC results were used to determine the kinetic models and thermodynamic parameters of fly ash. Heterogeneous system kinetics (Eq. (1)) for solid decomposition reactions were selected.

(1)

where α is the conversion ratio and is calculated by Eq. (2); t is heating time; T is the reaction temperature and f(α) is the conversion function. k(T) stands for the reaction rate constant and can be represented by the Arrhenius equation as Eq. (3).

(2)

(3)

where m₀, m_t, and m_∞ are the initial mass, mass at time t, and final mass, respectively; A is the pre-exponential factor; E is the activation energy and R is the universal gas constant (8.314 J/mol/K).

At a constant heating rate (β=dT/dt), Eq. (1) can be rewritten as follows:

(4)

Friedman method, FWO method, and KAS method were employed to evaluate activation energy at different α [24].

Among them, the Friedman method is an iso-conversional method in differential form. In this method, Eq. (4) is converted to Eq. (5) by taking the logarithm of both sides.

(5)

E is calculated from the slope of a linear relationship between ln (dα/dt) and 1/T.

FWO and KAS are iso-conversional methods in integral form. Eq. (4) is rearranged to obtain Eq. (6) first. The integral form of f(α) is designated as g(α) to obtain Eq. (7). The equation is further transferred to Eq. (8) and Eq. (9) via mathematically simplified FWO and KAS methods respectively [25].

(6)

(7)

(8)

(9)

Here, E can be calculated from the slopes of Eq. (8) and Eq. (9) by calculating the linearity ln(β) vs. 1/T and ln(β/T²) vs. 1/T, respectively.

2.3.2 Determination of reaction mechanism function.

In Eq (7), g(α) is the integral form of f(α) and is related to the reaction mechanism [26]. Depending on the reaction mechanism, g(α) could be expressed in different functions. Table 1 lists fifteen of the most common mechanism functions of solid-state thermal reactions. These functions are divided into 5 categories: chemical reaction, phase boundary reactions, diffusion, random nucleation and nuclei growth, and exponential nucleation.

Download:

Table 1. Common kinetic mechanism functions of solid-state thermal reactions [27].

https://doi.org/10.1371/journal.pone.0323729.t001

In this study, the CR approximation method, a model-fitting method based on g(α), was used to determine the reaction model as well as the pre-exponential factor [28]. Via CR approximation, Eq. (7) is simplified to Eq. (10).

(10)

By comparing the goodness-of-fit between ln(g(α)/T²) and 1/T of the TG data with different g(α) functions, the best-fit model of the reaction mechanism was selected. E and A values were estimated from slope and intercept, respectively.

2.3.3 Estimation of thermodynamic parameters.

The thermodynamic parameters, ΔG, ΔH, and ΔS, were calculated from Eq. (11) to Eq. (13) based also on E values obtained and TG analysis results [29].

(11)

(12)

(13)

where T_p, K_B and h are the peak temperature of DTG curve, Boltzmann constant (1.381 × 10^-23 J/K) and Plank constant (6.626 × 10^-34 J ∙ s), respectively. T_α is the temperature at the α.

3. Results and discussion

3.1 Characteristics of fly ash

The results of proximate analysis and chemical composition of the fly ash are detailed in Table 2. Fly ash contained 3.1 wt% of volatile carbon and 9.9 wt% of fixed carbon, with the majority being ash. This was attributed to the high-temperature combustion in waste incineration plants where volatile carbon was effectively consumed. XRF results revealed that the relatively predominant elements were Ca, Cl, Na, K, and S. The high content of Ca was related to the CaO or Ca(OH)₂ sprayed to neutralize acidic gas for flue gas treatment [30]. The 12.2 wt% of Cl most likely stemmed from chlorine-containing plastic products or kitchen wastes [31]. In addition, there were 3.8 wt% of SiO₂, 3.3 wt% of Fe₂O₃, and 1.5 wt% of Al₂O₃.

Download:

Table 2. Proximate analysis and XRF results of fly ash.

https://doi.org/10.1371/journal.pone.0323729.t002

SEM and XRD analysis were next performed to further illustrate the micromorphology and crystalline phases of fly ash. Fig 1 (a) is the SEM image of the fly ash. Fly ash is composed of particles of different sizes and irregular shapes. XRD analysis (Fig 1 (b)) revealed the presence of calcite (CaCO₃), anhydrite (CaSO₄), calcium chloride hydroxide (CaClOH), portlandite (Ca(OH)₂), sylvite (KCl), halite (NaCl) and silica (SiO₂). Both XRF and XRD analysis showed that fly ash had an abundance of chemicals including Ca-compounds, salts of chloride, carbonate, sulfate and inorganic oxides. The complexity in composition may imply complex chemical reactions during thermal treatment.

Download:

Fig 1. (a) SEM image; (b) XRD pattern of fly ash.

https://doi.org/10.1371/journal.pone.0323729.g001

3.2 Thermogravimetric analysis

Thermogravimetric analysis was used to understand the thermal behavior of fly ash. Fig 2 showcases the TG, DTG, and DSC curves of fly ash with heating rates of 10ºC/min, 20ºC/min, and 30ºC/min under air and N₂ atmosphere, respectively. As shown in Fig 2(a) and (c), the total mass losses were 35.22%-36.28% and 36.83%-46.38% in air and N_2, respectively. Mass losses before 500ºC both in air and N₂ amount to only 3.36%-6.05% and 3.05%-6.22% indicating that changes in fly ash did not occur until relatively high temperature. The 1.92%-4.32% and 2.20%-4.38% mass loss in air and N₂ occurred at around 30ºC and 200ºC could be attributed to the loss of moisture [32]. In addition, a minor peak at 400ºC was observed from TG in air. This could be explained by the combustion of carbon to CO₂ [11]. The peak was absent from TG in N₂. The majority of mass loss was between 500ºC-1100ºC as manifested in two major peaks around 656ºC-710ºC and 973ºC-1031ºC in DTG curves.

Download:

Fig 2. (a) TG and DTG in air; (b) DSC in air; (c) TG and DTG in N₂ and (d) DSC in N₂ of fly ash at heating rates of 10ºC/min, 20ºC/min, and 30ºC/min.

https://doi.org/10.1371/journal.pone.0323729.g002

About 8%-11% of fly ash was lost between 500ºC and 800ºC. This loss was caused by the decomposition of compounds such as CaClOH and CaCO₃ [33]. Peaks in DSC curves in Fig 2(b) and (d) around 700ºC indicate that these reactions were endothermic. Mass losses of 21.54%-29.28% were observed at 800ºC-1100ºC. At this temperature range, it is believed that most of the original compounds were either decomposed, volatilized or melted. Zhao et al. [34] reported that reactions in this temperature range included volatilization of chloride salts, disintegration of sulfates, and production of silicates. Overall, TG and DTG patterns of fly ash in air and N₂ showed similar patterns. In light of the mass loss behavior, thermal processes can be divided into three stages, namely, Stage I (30ºC-500ºC), Stage II (500ºC-800ºC) and Stage III (800ºC-1100ºC).

TG results in Fig 2 further revealed that the heating rate also affected the thermal behavior. TG, DTG, and DSC curves tended to migrate to higher temperatures as the heating rate increased. This trend was ascribed to the fact that the system experienced reduced dwell time at higher heating rates, leading to insufficient thermal energy accumulation [24]. Consequently, a higher reaction temperature was required to compensate for the accelerated thermal input and to ensure enough heat energy for the chemical reaction. While variations in heating rates caused the peak temperature and reaction rate to change, they did not alter the thermal profile of the entire system [35].

3.3 Estimation of activation energy by model-free methods

Activation energy E represents an energy barrier, i.e., the minimum energy required for a reaction to occur. The lower the E value, the easier it is for the reaction to occur [23]. E values of fly ash at different stages of TG thermal behavior were calculated via FWO, KAS, and Friedman methods in this study. The α range was selected from 0.1 to 0.9. S1 and S2 Fig. in Supplementary Materials display the plots of lnβ, ln (β/T²), and ln (dα/dt) versus 1/T in air and N₂, respectively. Correlation coefficient R² ranged from 0.626–1.000 indicating good linearity at various α values. The values of E were calculated according to Eq. (5)-Eq. (9). Tables 3 and 4 are the E and R² values at Stage II and Stage III of TG analysis in air and N_2, respectively.

Download:

Table 3. The values of E and R² of fly ash thermal treated in air based on FWO, KAS, and Friedman methods.

https://doi.org/10.1371/journal.pone.0323729.t003

Download:

Table 4. The values of E and R² of fly ash thermal treated in N₂ based on FWO, KAS, and Friedman methods.

https://doi.org/10.1371/journal.pone.0323729.t004

As shown in Table 3 and Table 4, the thermal behavior of fly ash could be explained as single-step kinetic reactions because the variation of E at 0.1 ≤ α ≤ 0.9 was not significant [36]. In addition, the FWO model consistently showed higher correlations (R² values) in linearity than the KAS and Friedman methods at all α values, indicating that the FWO method explained the experimental data better. Based on the FWO method, the average E values in Stage II and Stage III of fly ash in air were 171.11 kJ/mol and 373.38 kJ/mol while those in N₂ were 124.52 kJ/mol and 382.25 kJ/mol, respectively.

Notably, in Stage II (500ºC-800ºC), the activation energy of fly ash in N₂ was lower than that in air. This could mean that reactions in N₂ were easier to happen and maybe simpler. For example, studies have shown that, in air, the carbon in fly ash was combusted to form CO₂ [24]. This is also true with the TG results in this study as shown in Fig 2 (a). Thus, CaClOH was carbonated to CaCO₃ first. CaCO₃ was next decomposed to CaO and CO₂ at temperatures between 500ºC and 800ºC [37]. In N₂, CaClOH underwent a simpler breakdown to CaO or CaCl at temperatures between 550ºC and 800ºC [38]. This is in accordance with the reaction temperature ranges in air (591ºC-711ºC) and N₂ (582ºC-752ºC). Because of the lack of carbonation, compounds such as CaClOH remained stable at lower temperatures and only decomposed at relatively higher temperatures.

Activation energies in Stage III were quite close between air and N₂. The same was true of the reaction temperatures range (862ºC-1035ºC in air, 857ºC-1036ºC in N₂). The similar E values and reaction temperatures indicated that fly ash may undergo similar thermal reactions at high temperatures. It was believed that this stage was characterized by the presence of complex reactions. These prominently involved the volatilization of chlorides and formation of complex compounds (such as mayenite ((CaO)₁₂(Al₂O₃)₇), chlorellestadite (Ca₁₀(SiO₄)₃(SO₄)₃Cl₂) etc.), as expressed in reactions (14)-(16) [39].

(14)

(15)

(16)

As shown in Fig 3, crystal structures of fly ash thermally treated at 1100ºC via XRD revealed significant alterations compared to the original fly ash. The peak intensity of NaCl and KCl dropped sharply and complex crystals like Ca₂SiO₄, (CaO)₁₂(Al₂O₃)₇, and Ca₁₀(SiO₄)₃(SO₄)₃Cl₂ emerged at 1100ºC. The XRD results also confirmed the occurrence of intricate reactions in fly ash in this study at elevated temperatures. Therefore, Stage III exhibited much higher E values compared to Stage II to achieve these complex reactions.

Download:

Fig 3. XRD patterns of fly ash before and after TG tests.

https://doi.org/10.1371/journal.pone.0323729.g003

3.4 The determination of reaction models by model-fitting method

Analysis of mass loss and calculations of activation energies above showed that thermal treatment of fly ash is a complex process involving various mechanisms such as evaporation, decomposition and nucleation [40]. Next, the CR method was used to determine the reaction mechanisms at each stage from TG data. In Eq. (10), g(α) in different forms were fitted with TG data. S1-S4 Tables list the goodness-of-fit (R²) and E values calculated from the slopes. TG and DTG results showed that mass loss patterns at different heating rates were quite similar. This is in accordance with the calculated E and R² values which varied little at different heating rates. The average values were used to judge the rationality of the model. A model with the highest R² was chosen as the reaction mechanism for fly ash in this study. Table 5 shows the mean values of kinetic parameters under the optimal reaction model.

Download:

Table 5. The averages E, R², and A were calculated under the best model.

https://doi.org/10.1371/journal.pone.0323729.t005

For Stage II in air, the highest average R² value of 0.98 was observed in the one-dimensional diffusion model. The integral form g(α)=α² and conversion function f(α)=1/2α are the most fitting mechanistic functions. The average E of 182.31 kJ/mol was close to the average value calculated by the FWO method (171.11 kJ/mol). The average A value of 5.37 × 10¹⁰ s^-1 was greater than 1 × 10⁹ s^-1, indicating the occurrence of simple complexation or interface reaction [41]. This finding aligns with previous studies on other fly ashes [24]. In addition, the E value is highly close to the activation energy of CaCO₃ decomposition of 180.2 kJ/mol [42]. It is likely that Stage II may involve mainly combustion of carbon, carbonation of CaClOH and decomposition of minerals such as CaCO₃. A different scenario was observed of Stage II in N₂. The contracting sphere model, g(α)=1-(1-α)^1/3, best characterized the decomposition process. This could be caused by the decomposition of CaClOH and CaCO₃. This is the same as in the study by Halikia et al. [43]. In addition, the average A was less than 1 × 10⁹ s^-1 indicating simpler reactions in this stage. This is consistence with results from kinetic analysis via model-free methods.

The three-dimensional diffusion model was identified as the optimal model for Stage III, with average E values of 386.92 kJ/mol (in air) and 372.08 kJ/mol (in N₂), which are close with those obtained from the FWO method. Notably, these E values are significantly higher than the value of 277.99 kJ/mol reported by Zhao et al. [39] for fly ash in the same stage, while showing better consistency with the range of 363 kJ/mol-367 kJ/mol documented by Wang et al. [33]. The variations in E values can be primarily attributed to the inherent complexity and compositional heterogeneity of fly ash. Moreover, TG and kinetic analyses revealed that the mass loss and activation energy requirements were notably increased during Stage III. This phenomenon was due to the decomposition and transformation of CaSO₄, CaCO₃, NaCl, and KCl, etc. into complex crystalline structures. That is, the chemical bonds of these inorganic salts were destroyed in Stage III, causing the migration of formed free atoms or ions via surface or volume diffusion to uphold system stability [33]. This is consistent with the three-dimensional diffusion process. Additionally, the average A values during Stage III were above 1 × 10¹⁴ s^-1 in both air and N₂. The high value of A indicates prolonged reaction times and accelerated molecular collisions [44]. This further explained the heightened mass loss and reaction energy during this stage.

Based on the CR method’s calculations, the thermal decomposition kinetics equations of fly ash in air and N₂ can be represented by Eq. (17)-(20).

Stage II in air:

(17)

Stage III in air:

(18)

Stage II in N₂:

(19)

Stage III in N₂:

(20)

Fig 4 compared the temperature-dependent relationships between calculated-α and experimental-α for each stage at heating rates of 10ºC/min, 20ºC/min, and 30ºC/min. Clearly, the fitting accuracy (R² > 0.97) was consistently high for both calculated-α and experimental-α within each model. The results demonstrated the applicability of the one-dimensional diffusion and contracting sphere models in expressing the thermal reaction mechanisms of Stage II in air and N₂, while the three-dimensional diffusion model was suitable for both Stage III.

Download:

Fig 4. Compared the experimentalα with calculated α via each model.

https://doi.org/10.1371/journal.pone.0323729.g004

3.5 Thermodynamic parameters analysis

Thermodynamic parameters such as ΔH, ΔG, and ΔS of fly ash were estimated at heating rates of 10ºC/min, 20ºC/min, and 30ºC/min and documented in S5-S7 Tables. The trends in the changes of ΔH, ΔG, and ΔS within the range of 0.1 ≤ α ≤ 0.9 remained unaffected by alterations in heating rate. Consequently, their average values could provide a viable approach for investigating the thermal reaction of fly ash in air and N₂. The variations of average ΔH, ΔG, and ΔS with α in the range of 0.1 to 0.9 were illustrated in Fig 5.

Download:

Fig 5. The relationship between conversion rate and the average values of

(a) ΔH, (b) ΔG, and (c) ΔS of fly ash.

https://doi.org/10.1371/journal.pone.0323729.g005

The parameter ΔH signifies the enthalpy difference between the products and reactants in a chemical reaction, with a positive value denoting an endothermic reaction [45]. For Stage II, the computed average ΔH values were 154.62 kJ/mol-181.43 kJ/mol in air and 79.88 kJ/mol-131.41 kJ/mol in N₂. Comparatively, Stage III demonstrated higher values, ranging from 344.89 kJ/mol to 384.05 kJ/mol in air and from 327.37 kJ/mol to 396.98 kJ/mol in N₂. These positive ΔH values were in accordance with the endothermic peaks observed in the DSC curves, indicating endothermic processes of both stages. The energy difference between the E and ΔH value (E-ΔH) is presented in S3 Fig. The average values of E-ΔH for Stage II were 7.73 kJ/mol and 7.81 kJ/mol in air and N₂. Correspondingly, E-ΔH in Stage III were 10.27 kJ/mol in air and 10.20 kJ/mol in N₂. The high energy barrier (>7 kJ/mol) observed in both stages may be attributable to the volatilization of elements and the decomposition of minerals, which induced changes in the system’s volume [39]. Moreover, Stage III necessitated a greater energy input than Stage II to overcome the activation barrier and achieve the desired chemical transformation.

The parameter ΔG, depicted in Fig 5 (b), serves as a thermodynamic indicator for assessing reaction directionality and feasibility. The ΔG values greater than zero signify non-spontaneous reactions [46]. Notably, ΔG values were stable across α values regardless of atmosphere suggesting an invariant reaction possibility throughout. The calculated ΔG ranged from 228.11 kJ/mol to 308.59 kJ/mol in air and 230.15 kJ/mol to 305.31 kJ/mol in N₂, confirming that all reactions were not spontaneous and required extra energy for reactions to happen. In addition, Fig 5 (c) elucidated the fluctuation in ΔS, with its value serving as a metric for assessing system degree of disorder and reaction activity [47]. The negative ΔS values for Stage II implied a reduced disorder among the products, hinting at the enhanced stability of the fly ash structure post-thermal treatment [48]. Conversely, Stage III manifested elevated ΔG and positive ΔS values relative to Stage II in both atmospheres. These are indicative of the more challenging reaction and the high reaction activity at elevated temperatures [49]. The positive ΔS values also suggested an increase in the degree of system disorder, which is attributed to the enhanced release of elements during the later stage [50].

4. Conclusions

The chemical composition of fly ash was complex, consisting mainly of CaCO₃, CaSO₄, CaClOH, Ca(OH)₂, KCl, NaCl, and SiO₂. Thermogravimetric analysis revealed the total mass loss of fly ash ranged from 35.22% to 36.28% in air and from 36.83% to 46.38% in N₂ within the temperature range of 30ºC to 1100ºC. The thermal decomposition process can be divided into three stages based on the mass loss: Stage I (30ºC-500ºC), Stage II (500ºC-800ºC), and Stage III (800ºC-1100ºC), with over 80% of the mass loss concentrated in the latter two stages.

Compared with KAS and Friedman methods, FWO method showed the highest correlation co-efficiency value explaining the kinetic data. Activation energies calculated based on FWO method were 171.11 kJ/mol for air and 124.52 kJ/mol in N₂ for Stage II. Activation energies for reactions in Stage III were much higher than those in Stage II but were similar in values regardless of atmosphere (373.38 kJ/mol in air and 382.25 kJ/mol in N₂). CR model further revealed that Stage II was controlled by one-dimensional diffusion (D1) in air and contracting sphere model (R3) in N₂. Three-dimensional diffusion (D3) predominated in Stage III in both air and N₂ probably because reactions such as volatilization of chloride salts and formation of complex compounds such as Ca₂SiO₄, (CaO)₁₂(Al₂O₃)₇, and Ca₁₀(SiO₄)₃(SO₄)₃Cl₂.

Thermodynamic analysis further revealed that the thermal reactions of fly ash were non-spontaneous and endothermic, with Stage III demonstrating higher complexity and reactivity compared to Stage II. These findings provided a rational explanation for the thermal behavior of fly ash in air and N₂ and could provide robust benchmarks to design and optimize thermal treatment processes.

Supporting information

S1 Fig. Kinetic plots of Stages II-III in air atmosphere by FWO, KAS, and Friedman (FR) methods.

https://doi.org/10.1371/journal.pone.0323729.s001

(TIF)

S2 Fig. Kinetic plots of Stages II-III in N₂ atmosphere by FWO, KAS, and Friedman (FR) methods.

https://doi.org/10.1371/journal.pone.0323729.s002

(TIF)

S1-S4 Tables. Values of E and R² via CR method based on 15 reaction models.

https://doi.org/10.1371/journal.pone.0323729.s003

(DOCX)

S5-S7 Tables. Thermodynamic parameters of fly ash thermal treated in air and N₂ atmospheres.

https://doi.org/10.1371/journal.pone.0323729.s004

(DOCX)

S3 Fig. The relationship between conversion rate and the difference between the E and ΔH (E-ΔH).

https://doi.org/10.1371/journal.pone.0323729.s005

(TIF)

Acknowledgments

The authors sincerely thank the University of Shanghai for Science and Technology for providing a safe experimental environment and advanced experimental facilities.

References

1. Kaza S, Yao LC, Bhada-Tata P, Van Woerden F. What a Waste 2.0: A Global Snapshot of Solid Waste Management to 2050. Washington, DC: World Bank. 2018. https://doi.org/10.1596/978-1-4648-1329-0
2. Gautam K, Pandey N, Yadav D, Parthasarathi R, Turner A, Anbumani S, et al. Ecotoxicological impacts of landfill sites: Towards risk assessment, mitigation policies and the role of artificial intelligence. Sci Total Environ. 2024;927:171804. pmid:38513865
- View Article
- PubMed/NCBI
- Google Scholar
3. He S, Lei X, Zhang H, Yu P, Zhou Y, Huang X. Durability and heavy metals long-term stability of alkali-activated sintered municipal solid waste incineration fly ash concrete in acidic environments. Construction and Building Materials. 2025;462:139990.
- View Article
- Google Scholar
4. Yuan Z, Cai G, Gao L, Wu M, Kong L, Bai J, et al. The physical encapsulation and chemical fixation of Zn during thermal treatment process of municipal solid waste incineration (MSWI) fly ash. Waste Manag. 2023;166:203–10. pmid:37182253
- View Article
- PubMed/NCBI
- Google Scholar
5. Li W, Li L, Wen Z, Yan D, Liu M, Huang Q, et al. Removal of dioxins from municipal solid waste incineration fly ash by low-temperature thermal treatment: Laboratory simulation of degradation and ash discharge stages. Waste Manag. 2023;168:45–53. pmid:37276633
- View Article
- PubMed/NCBI
- Google Scholar
6. Tang Y, Chen D, Feng Y, Hu Y, Yin L, Qian K, et al. MSW pyrolysis volatiles’ reforming by incineration fly ash for both pyrolysis products upgrading and fly ash stabilization. Chemosphere. 2023;313:137536. pmid:36528161
- View Article
- PubMed/NCBI
- Google Scholar
7. Fan X, Zhou Z, Huang B, Ji Z, Gan M, Sun Z, et al. Emission characteristics and control technology of heavy metals during collaborative treatment of municipal solid waste incineration fly ash in iron ore sintering process. J Iron Steel Res Int. 2024;31(11):2655–63.
- View Article
- Google Scholar
8. Jalil Z, Nurmalita N, Madjid SN, Setiawan A, Idroes R. Characteristics of Silica Powder Extracted from Fly Ash of Coal Fired Power Plant: Effect of Heat Treatment Process. J Ecol Eng. 2023;24(9):282–92.
- View Article
- Google Scholar
9. Gao Y, Liu F, Liu Z, Wang D, Wang B, Zhang S. Preparation and characterization of cementitious materials from dedioxinized and dechlorinated municipal solid waste incineration fly ash and blast furnace slag. Construction and Building Materials. 2024;432:136643.
- View Article
- Google Scholar
10. Chen J, Zhu W, Lin X, Li X, Liu S, Yan J. A novel method for MSWI fly ash resource based on calcium component regulation and CO2 mineralization. Chemical Engineering Journal. 2024;492:152320.
- View Article
- Google Scholar
11. Gu Q, Wu W, Jin B. Investigation of thermal characteristics of municipal solid waste incineration fly ash under various atmospheres: A TG-FTIR study. Thermochimica Acta. 2019;681:178402.
- View Article
- Google Scholar
12. Lane DJ, Sippula O, Koponen H, Heimonen M, Peräniemi S, Lähde A, et al. Volatilisation of major, minor, and trace elements during thermal processing of fly ashes from waste- and wood-fired power plants in oxidising and reducing gas atmospheres. Waste Manag. 2020;102:698–709. pmid:31794929
- View Article
- PubMed/NCBI
- Google Scholar
13. Jiao F, Ma X, Liu T, Wu C, Li H, Dong Z. Effect of Atmospheres on Transformation of Heavy Metals during Thermal Treatment of MSWI Fly Ash: By Thermodynamic Equilibrium Calculation. Molecules. 2021;27(1):131. pmid:35011362
- View Article
- PubMed/NCBI
- Google Scholar
14. Khan A, Naqvi SR, Zorpas AA, Juchelková D, Ali I. Thermokinetic study of tannery sludge using combustion and pyrolysis process through non-isothermal thermogravimetric analysis. Sustainable Chemistry and Pharmacy. 2024;41:101719.
- View Article
- Google Scholar
15. Shagali AA, Mostafa ME, Li H, Hu S, Xu J, Jiang L, et al. Pyrolysis characteristics and kinetic parameters assessment of typical agricultural residues using high heating photothermal TGA. Journal of Analytical and Applied Pyrolysis. 2023;174:106109.
- View Article
- Google Scholar
16. Singh B, Singh S, Kumar P. In-depth analyses of kinetics, thermodynamics and solid reaction mechanism for pyrolysis of hazardous petroleum sludge based on isoconversional models for its energy potential. Process Safety and Environmental Protection. 2021;146:85–94.
- View Article
- Google Scholar
17. Senneca O, Chirone R, Cortese L, Salatino P. Pyrolysis and combustion of a solid refinery waste. Fuel. 2020;267:117258.
- View Article
- Google Scholar
18. Wahab A, Sattar H, Ashraf A, Hussain SN, Saleem M, Munir S. Thermochemical, kinetic and ash characteristics behaviour of Thar Lignite, agricultural residues and synthetic polymer waste (EVA). Fuel. 2020;266:117151.
- View Article
- Google Scholar
19. Yousef S, Eimontas J, Striūgas N, Abdelnaby MA. Pyrolysis and gasification kinetic behavior of mango seed shells using TG-FTIR-GC–MS system under N2 and CO2 atmospheres. Renewable Energy. 2021;173:733–49.
- View Article
- Google Scholar
20. Loy ACM, Gan DKW, Yusup S, Chin BLF, Lam MK, Shahbaz M, et al. Thermogravimetric kinetic modelling of in-situ catalytic pyrolytic conversion of rice husk to bioenergy using rice hull ash catalyst. Bioresour Technol. 2018;261:213–22. pmid:29665455
- View Article
- PubMed/NCBI
- Google Scholar
21. Liew JX, Loy ACM, Chin BLF, AlNouss A, Shahbaz M, Al-Ansari T, et al. Synergistic effects of catalytic co-pyrolysis of corn cob and HDPE waste mixtures using weight average global process model. Renewable Energy. 2021;170:948–63.
- View Article
- Google Scholar
22. Fanezouné CK, Dhahak A, Peixinho J, El Bari H. Thermogravimetric analysis and kinetic modeling for empty fruit bunch date palm pyrolysis. Bioresource Technology Reports. 2024;27:101916.
- View Article
- Google Scholar
23. Chen R, Zhang D, Xu X, Yuan Y. Pyrolysis characteristics, kinetics, thermodynamics and volatile products of waste medical surgical mask rope by thermogravimetry and online thermogravimetry-Fourier transform infrared-mass spectrometry analysis. Fuel. 2021;295:120632.
- View Article
- Google Scholar
24. Zhao C, Zhao Y, Lin K, Wang Z, Zhou T. Comprehensive assessment of thermal characteristics, kinetics and environmental impacts of municipal solid waste incineration fly ash during thermal treatment. Process Safety and Environmental Protection. 2023;175:619–31.
- View Article
- Google Scholar
25. Cai H, Liu J, Xie W, Kuo J, Buyukada M, Evrendilek F. Pyrolytic kinetics, reaction mechanisms and products of waste tea via TG-FTIR and Py-GC/MS. Energy Conversion and Management. 2019;184:436–47.
- View Article
- Google Scholar
26. Huang J, Zhang J, Liu J, Xie W, Kuo J, Chang K, et al. Thermal conversion behaviors and products of spent mushroom substrate in CO2 and N2 atmospheres: Kinetic, thermodynamic, TG and Py-GC/MS analyses. Journal of Analytical and Applied Pyrolysis. 2019;139:177–86.
- View Article
- Google Scholar
27. Khawam A, Flanagan DR. Solid‐State Kinetic Models: Basics and Mathematical Fundamentals. ChemInform. 2006;37(47).
- View Article
- Google Scholar
28. COATS AW, REDFERN JP. Kinetic Parameters from Thermogravimetric Data. Nature. 1964;201(4914):68–9.
- View Article
- Google Scholar
29. Aprianti N, Faizal M, Said M, Nasir S, Fudholi A. Gasification kinetic and thermodynamic parameters of fine coal using thermogravimetric analysis. Energy. 2023;268:126666.
- View Article
- Google Scholar
30. Chen W, Wang Y, Sun Y, Fang G, Li Y. Release of soluble ions and heavy metal during fly ash washing by deionized water and sodium carbonate solution. Chemosphere. 2022;307(Pt 2):135860. pmid:35944671
- View Article
- PubMed/NCBI
- Google Scholar
31. Fan C, Wang B, Ai H, Qi Y, Liu Z. A comparative study on solidification/stabilization characteristics of coal fly ash-based geopolymer and Portland cement on heavy metals in MSWI fly ash. Journal of Cleaner Production. 2021;319:128790.
- View Article
- Google Scholar
32. Tan WF, Wang LA, Huang C, Green JE. Municipal solid waste incineration fly ash sintered lightweight aggregates and kinetics model establishment. Int J Environ Sci Technol. 2012;10(3):465–72.
- View Article
- Google Scholar
33. Wang X, Ji G, Zhu K, Li C, Zhang Y, Li A. Integrated thermal behavior and compounds transition mechanism of municipal solid waste incineration fly ash during thermal treatment process. Chemosphere. 2021;264(Pt 1):128406. pmid:33010627
- View Article
- PubMed/NCBI
- Google Scholar
34. Zhao C, Lin S, Zhao Y, Lin K, Tian L, Xie M, et al. Comprehensive understanding the transition behaviors and mechanisms of chlorine and metal ions in municipal solid waste incineration fly ash during thermal treatment. Sci Total Environ. 2022;807(Pt 2):150731. pmid:34634350
- View Article
- PubMed/NCBI
- Google Scholar
35. Huang X, Cao J-P, Zhao X-Y, Wang J-X, Fan X, Zhao Y-P, et al. Pyrolysis kinetics of soybean straw using thermogravimetric analysis. Fuel. 2016;169:93–8.
- View Article
- Google Scholar
36. Chen G, He S, Cheng Z, Guan Y, Yan B, Ma W, et al. Comparison of kinetic analysis methods in thermal decomposition of cattle manure by themogravimetric analysis. Bioresour Technol. 2017;243:69–77. pmid:28651140
- View Article
- PubMed/NCBI
- Google Scholar
37. Zhang S, Chen Z, Lin X, Wang F, Yan J. Kinetics and fusion characteristics of municipal solid waste incineration fly ash during thermal treatment. Fuel. 2020;279:118410.
- View Article
- Google Scholar
38. Yang G, Ren Q, Zhou L, Xu J, Lyu Q. Effect of Atmosphere on HCl Releasement during MSWI Fly Ash Thermal Treatment. J Therm Sci. 2023;32(6):2243–55.
- View Article
- Google Scholar
39. Zhao C, Zhao Y, Xie M, Xiao Y, Chen B, Wang Y, et al. Novel insight of the thermal degradation kinetics, environmental impact and transformation behavior of heavy metals and chlorine of municipal solid waste incineration fly ash in nitrogen. Fuel. 2023;353:129275.
- View Article
- Google Scholar
40. Huang B, Gan M, Ji Z, Fan X, Zhang D, Chen X, et al. Recent progress on the thermal treatment and resource utilization technologies of municipal waste incineration fly ash: A review. Process Safety and Environmental Protection. 2022;159:547–65.
- View Article
- Google Scholar
41. Maia AAD, de Morais LC. Kinetic parameters of red pepper waste as biomass to solid biofuel. Bioresour Technol. 2016;204:157–63. pmid:26773950
- View Article
- PubMed/NCBI
- Google Scholar
42. Li X-G, Lv Y, Ma B-G, Wang W-Q, Jian S-W. Decomposition kinetic characteristics of calcium carbonate containing organic acids by TGA. Arabian Journal of Chemistry. 2017;10:S2534–8.
- View Article
- Google Scholar
43. Halikia I, Zoumpoulakis L, Christodoulou E, Prattis D. Kinetic study of the thermal decomposition of calcium carbonate by isothermal methods of analysis. Eur. J. Min. Proc. Environ. Prot. 2001;1:89–102.
- View Article
- Google Scholar
44. Yuan X, He T, Cao H, Yuan Q. Cattle manure pyrolysis process: Kinetic and thermodynamic analysis with isoconversional methods. Renewable Energy. 2017;107:489–96.
- View Article
- Google Scholar
45. Barbanera M, Cotana F, Di Matteo U. Co-combustion performance and kinetic study of solid digestate with gasification biochar. Renewable Energy. 2018;121:597–605.
- View Article
- Google Scholar
46. Huang H, Liu J, Liu H, Evrendilek F, Buyukada M. Pyrolysis of water hyacinth biomass parts: Bioenergy, gas emissions, and by-products using TG-FTIR and Py-GC/MS analyses. Energy Conversion and Management. 2020;207:112552.
- View Article
- Google Scholar
47. Kaur R, Gera P, Jha MK, Bhaskar T. Pyrolysis kinetics and thermodynamic parameters of castor (Ricinus communis) residue using thermogravimetric analysis. Bioresour Technol. 2018;250:422–8. pmid:29195154
- View Article
- PubMed/NCBI
- Google Scholar
48. Naqvi SR, Tariq R, Hameed Z, Ali I, Naqvi M, Chen W-H, et al. Pyrolysis of high ash sewage sludge: Kinetics and thermodynamic analysis using Coats-Redfern method. Renewable Energy. 2019;131:854–60.
- View Article
- Google Scholar
49. Mumbach GD, Alves JLF, Da Silva JCG, De Sena RF, Marangoni C, Machado RAF, et al. Thermal investigation of plastic solid waste pyrolysis via the deconvolution technique using the asymmetric double sigmoidal function: Determination of the kinetic triplet, thermodynamic parameters, thermal lifetime and pyrolytic oil composition for clean energy recovery. Energy Conversion and Management. 2019;200:112031.
- View Article
- Google Scholar
50. Tian W-W, Xu F, Xing S-J, Wu R, Yuan Z-Y. Comprehensive study on the thermal decomposition process of waste tobacco leaves and stems to investigate their bioenergy potential: Kinetic, thermodynamic, and biochar analysis. Thermochimica Acta. 2023;723:179473.
- View Article
- Google Scholar

[ref1] 1. Kaza S, Yao LC, Bhada-Tata P, Van Woerden F. What a Waste 2.0: A Global Snapshot of Solid Waste Management to 2050. Washington, DC: World Bank. 2018. https://doi.org/10.1596/978-1-4648-1329-0

[ref2] 2. Gautam K, Pandey N, Yadav D, Parthasarathi R, Turner A, Anbumani S, et al. Ecotoxicological impacts of landfill sites: Towards risk assessment, mitigation policies and the role of artificial intelligence. Sci Total Environ. 2024;927:171804. pmid:38513865
View Article
PubMed/NCBI
Google Scholar

[3] View Article

[4] PubMed/NCBI

[5] Google Scholar

[ref3] 3. He S, Lei X, Zhang H, Yu P, Zhou Y, Huang X. Durability and heavy metals long-term stability of alkali-activated sintered municipal solid waste incineration fly ash concrete in acidic environments. Construction and Building Materials. 2025;462:139990.
View Article
Google Scholar

[7] View Article

[8] Google Scholar

[ref4] 4. Yuan Z, Cai G, Gao L, Wu M, Kong L, Bai J, et al. The physical encapsulation and chemical fixation of Zn during thermal treatment process of municipal solid waste incineration (MSWI) fly ash. Waste Manag. 2023;166:203–10. pmid:37182253
View Article
PubMed/NCBI
Google Scholar

[10] View Article

[11] PubMed/NCBI

[12] Google Scholar

[ref5] 5. Li W, Li L, Wen Z, Yan D, Liu M, Huang Q, et al. Removal of dioxins from municipal solid waste incineration fly ash by low-temperature thermal treatment: Laboratory simulation of degradation and ash discharge stages. Waste Manag. 2023;168:45–53. pmid:37276633
View Article
PubMed/NCBI
Google Scholar

[14] View Article

[15] PubMed/NCBI

[16] Google Scholar

[ref6] 6. Tang Y, Chen D, Feng Y, Hu Y, Yin L, Qian K, et al. MSW pyrolysis volatiles’ reforming by incineration fly ash for both pyrolysis products upgrading and fly ash stabilization. Chemosphere. 2023;313:137536. pmid:36528161
View Article
PubMed/NCBI
Google Scholar

[18] View Article

[19] PubMed/NCBI

[20] Google Scholar

[ref7] 7. Fan X, Zhou Z, Huang B, Ji Z, Gan M, Sun Z, et al. Emission characteristics and control technology of heavy metals during collaborative treatment of municipal solid waste incineration fly ash in iron ore sintering process. J Iron Steel Res Int. 2024;31(11):2655–63.
View Article
Google Scholar

[22] View Article

[23] Google Scholar

[ref8] 8. Jalil Z, Nurmalita N, Madjid SN, Setiawan A, Idroes R. Characteristics of Silica Powder Extracted from Fly Ash of Coal Fired Power Plant: Effect of Heat Treatment Process. J Ecol Eng. 2023;24(9):282–92.
View Article
Google Scholar

[25] View Article

[26] Google Scholar

[ref9] 9. Gao Y, Liu F, Liu Z, Wang D, Wang B, Zhang S. Preparation and characterization of cementitious materials from dedioxinized and dechlorinated municipal solid waste incineration fly ash and blast furnace slag. Construction and Building Materials. 2024;432:136643.
View Article
Google Scholar

[28] View Article

[29] Google Scholar

[ref10] 10. Chen J, Zhu W, Lin X, Li X, Liu S, Yan J. A novel method for MSWI fly ash resource based on calcium component regulation and CO2 mineralization. Chemical Engineering Journal. 2024;492:152320.
View Article
Google Scholar

[31] View Article

[32] Google Scholar

[ref11] 11. Gu Q, Wu W, Jin B. Investigation of thermal characteristics of municipal solid waste incineration fly ash under various atmospheres: A TG-FTIR study. Thermochimica Acta. 2019;681:178402.
View Article
Google Scholar

[34] View Article

[35] Google Scholar

[ref12] 12. Lane DJ, Sippula O, Koponen H, Heimonen M, Peräniemi S, Lähde A, et al. Volatilisation of major, minor, and trace elements during thermal processing of fly ashes from waste- and wood-fired power plants in oxidising and reducing gas atmospheres. Waste Manag. 2020;102:698–709. pmid:31794929
View Article
PubMed/NCBI
Google Scholar

[37] View Article

[38] PubMed/NCBI

[39] Google Scholar

[ref13] 13. Jiao F, Ma X, Liu T, Wu C, Li H, Dong Z. Effect of Atmospheres on Transformation of Heavy Metals during Thermal Treatment of MSWI Fly Ash: By Thermodynamic Equilibrium Calculation. Molecules. 2021;27(1):131. pmid:35011362
View Article
PubMed/NCBI
Google Scholar

[41] View Article

[42] PubMed/NCBI

[43] Google Scholar

[ref14] 14. Khan A, Naqvi SR, Zorpas AA, Juchelková D, Ali I. Thermokinetic study of tannery sludge using combustion and pyrolysis process through non-isothermal thermogravimetric analysis. Sustainable Chemistry and Pharmacy. 2024;41:101719.
View Article
Google Scholar

[45] View Article

[46] Google Scholar

[ref15] 15. Shagali AA, Mostafa ME, Li H, Hu S, Xu J, Jiang L, et al. Pyrolysis characteristics and kinetic parameters assessment of typical agricultural residues using high heating photothermal TGA. Journal of Analytical and Applied Pyrolysis. 2023;174:106109.
View Article
Google Scholar

[48] View Article

[49] Google Scholar

[ref16] 16. Singh B, Singh S, Kumar P. In-depth analyses of kinetics, thermodynamics and solid reaction mechanism for pyrolysis of hazardous petroleum sludge based on isoconversional models for its energy potential. Process Safety and Environmental Protection. 2021;146:85–94.
View Article
Google Scholar

[51] View Article

[52] Google Scholar

[ref17] 17. Senneca O, Chirone R, Cortese L, Salatino P. Pyrolysis and combustion of a solid refinery waste. Fuel. 2020;267:117258.
View Article
Google Scholar

[54] View Article

[55] Google Scholar

[ref18] 18. Wahab A, Sattar H, Ashraf A, Hussain SN, Saleem M, Munir S. Thermochemical, kinetic and ash characteristics behaviour of Thar Lignite, agricultural residues and synthetic polymer waste (EVA). Fuel. 2020;266:117151.
View Article
Google Scholar

[57] View Article

[58] Google Scholar

[ref19] 19. Yousef S, Eimontas J, Striūgas N, Abdelnaby MA. Pyrolysis and gasification kinetic behavior of mango seed shells using TG-FTIR-GC–MS system under N2 and CO2 atmospheres. Renewable Energy. 2021;173:733–49.
View Article
Google Scholar

[60] View Article

[61] Google Scholar

[ref20] 20. Loy ACM, Gan DKW, Yusup S, Chin BLF, Lam MK, Shahbaz M, et al. Thermogravimetric kinetic modelling of in-situ catalytic pyrolytic conversion of rice husk to bioenergy using rice hull ash catalyst. Bioresour Technol. 2018;261:213–22. pmid:29665455
View Article
PubMed/NCBI
Google Scholar

[63] View Article

[64] PubMed/NCBI

[65] Google Scholar

[ref21] 21. Liew JX, Loy ACM, Chin BLF, AlNouss A, Shahbaz M, Al-Ansari T, et al. Synergistic effects of catalytic co-pyrolysis of corn cob and HDPE waste mixtures using weight average global process model. Renewable Energy. 2021;170:948–63.
View Article
Google Scholar

[67] View Article

[68] Google Scholar

[ref22] 22. Fanezouné CK, Dhahak A, Peixinho J, El Bari H. Thermogravimetric analysis and kinetic modeling for empty fruit bunch date palm pyrolysis. Bioresource Technology Reports. 2024;27:101916.
View Article
Google Scholar

[70] View Article

[71] Google Scholar

[ref23] 23. Chen R, Zhang D, Xu X, Yuan Y. Pyrolysis characteristics, kinetics, thermodynamics and volatile products of waste medical surgical mask rope by thermogravimetry and online thermogravimetry-Fourier transform infrared-mass spectrometry analysis. Fuel. 2021;295:120632.
View Article
Google Scholar

[73] View Article

[74] Google Scholar

[ref24] 24. Zhao C, Zhao Y, Lin K, Wang Z, Zhou T. Comprehensive assessment of thermal characteristics, kinetics and environmental impacts of municipal solid waste incineration fly ash during thermal treatment. Process Safety and Environmental Protection. 2023;175:619–31.
View Article
Google Scholar

[76] View Article

[77] Google Scholar

[ref25] 25. Cai H, Liu J, Xie W, Kuo J, Buyukada M, Evrendilek F. Pyrolytic kinetics, reaction mechanisms and products of waste tea via TG-FTIR and Py-GC/MS. Energy Conversion and Management. 2019;184:436–47.
View Article
Google Scholar

[79] View Article

[80] Google Scholar

[ref26] 26. Huang J, Zhang J, Liu J, Xie W, Kuo J, Chang K, et al. Thermal conversion behaviors and products of spent mushroom substrate in CO2 and N2 atmospheres: Kinetic, thermodynamic, TG and Py-GC/MS analyses. Journal of Analytical and Applied Pyrolysis. 2019;139:177–86.
View Article
Google Scholar

[82] View Article

[83] Google Scholar

[ref27] 27. Khawam A, Flanagan DR. Solid‐State Kinetic Models: Basics and Mathematical Fundamentals. ChemInform. 2006;37(47).
View Article
Google Scholar

[85] View Article

[86] Google Scholar

[ref28] 28. COATS AW, REDFERN JP. Kinetic Parameters from Thermogravimetric Data. Nature. 1964;201(4914):68–9.
View Article
Google Scholar

[88] View Article

[89] Google Scholar

[ref29] 29. Aprianti N, Faizal M, Said M, Nasir S, Fudholi A. Gasification kinetic and thermodynamic parameters of fine coal using thermogravimetric analysis. Energy. 2023;268:126666.
View Article
Google Scholar

[91] View Article

[92] Google Scholar

[ref30] 30. Chen W, Wang Y, Sun Y, Fang G, Li Y. Release of soluble ions and heavy metal during fly ash washing by deionized water and sodium carbonate solution. Chemosphere. 2022;307(Pt 2):135860. pmid:35944671
View Article
PubMed/NCBI
Google Scholar

[94] View Article

[95] PubMed/NCBI

[96] Google Scholar

[ref31] 31. Fan C, Wang B, Ai H, Qi Y, Liu Z. A comparative study on solidification/stabilization characteristics of coal fly ash-based geopolymer and Portland cement on heavy metals in MSWI fly ash. Journal of Cleaner Production. 2021;319:128790.
View Article
Google Scholar

[98] View Article

[99] Google Scholar

[ref32] 32. Tan WF, Wang LA, Huang C, Green JE. Municipal solid waste incineration fly ash sintered lightweight aggregates and kinetics model establishment. Int J Environ Sci Technol. 2012;10(3):465–72.
View Article
Google Scholar

[101] View Article

[102] Google Scholar

[ref33] 33. Wang X, Ji G, Zhu K, Li C, Zhang Y, Li A. Integrated thermal behavior and compounds transition mechanism of municipal solid waste incineration fly ash during thermal treatment process. Chemosphere. 2021;264(Pt 1):128406. pmid:33010627
View Article
PubMed/NCBI
Google Scholar

[104] View Article

[105] PubMed/NCBI

[106] Google Scholar

[ref34] 34. Zhao C, Lin S, Zhao Y, Lin K, Tian L, Xie M, et al. Comprehensive understanding the transition behaviors and mechanisms of chlorine and metal ions in municipal solid waste incineration fly ash during thermal treatment. Sci Total Environ. 2022;807(Pt 2):150731. pmid:34634350
View Article
PubMed/NCBI
Google Scholar

[108] View Article

[109] PubMed/NCBI

[110] Google Scholar

[ref35] 35. Huang X, Cao J-P, Zhao X-Y, Wang J-X, Fan X, Zhao Y-P, et al. Pyrolysis kinetics of soybean straw using thermogravimetric analysis. Fuel. 2016;169:93–8.
View Article
Google Scholar

[112] View Article

[113] Google Scholar

[ref36] 36. Chen G, He S, Cheng Z, Guan Y, Yan B, Ma W, et al. Comparison of kinetic analysis methods in thermal decomposition of cattle manure by themogravimetric analysis. Bioresour Technol. 2017;243:69–77. pmid:28651140
View Article
PubMed/NCBI
Google Scholar

[115] View Article

[116] PubMed/NCBI

[117] Google Scholar

[ref37] 37. Zhang S, Chen Z, Lin X, Wang F, Yan J. Kinetics and fusion characteristics of municipal solid waste incineration fly ash during thermal treatment. Fuel. 2020;279:118410.
View Article
Google Scholar

[119] View Article

[120] Google Scholar

[ref38] 38. Yang G, Ren Q, Zhou L, Xu J, Lyu Q. Effect of Atmosphere on HCl Releasement during MSWI Fly Ash Thermal Treatment. J Therm Sci. 2023;32(6):2243–55.
View Article
Google Scholar

[122] View Article

[123] Google Scholar

[ref39] 39. Zhao C, Zhao Y, Xie M, Xiao Y, Chen B, Wang Y, et al. Novel insight of the thermal degradation kinetics, environmental impact and transformation behavior of heavy metals and chlorine of municipal solid waste incineration fly ash in nitrogen. Fuel. 2023;353:129275.
View Article
Google Scholar

[125] View Article

[126] Google Scholar

[ref40] 40. Huang B, Gan M, Ji Z, Fan X, Zhang D, Chen X, et al. Recent progress on the thermal treatment and resource utilization technologies of municipal waste incineration fly ash: A review. Process Safety and Environmental Protection. 2022;159:547–65.
View Article
Google Scholar

[128] View Article

[129] Google Scholar

[ref41] 41. Maia AAD, de Morais LC. Kinetic parameters of red pepper waste as biomass to solid biofuel. Bioresour Technol. 2016;204:157–63. pmid:26773950
View Article
PubMed/NCBI
Google Scholar

[131] View Article

[132] PubMed/NCBI

[133] Google Scholar

[ref42] 42. Li X-G, Lv Y, Ma B-G, Wang W-Q, Jian S-W. Decomposition kinetic characteristics of calcium carbonate containing organic acids by TGA. Arabian Journal of Chemistry. 2017;10:S2534–8.
View Article
Google Scholar

[135] View Article

[136] Google Scholar

[ref43] 43. Halikia I, Zoumpoulakis L, Christodoulou E, Prattis D. Kinetic study of the thermal decomposition of calcium carbonate by isothermal methods of analysis. Eur. J. Min. Proc. Environ. Prot. 2001;1:89–102.
View Article
Google Scholar

[138] View Article

[139] Google Scholar

[ref44] 44. Yuan X, He T, Cao H, Yuan Q. Cattle manure pyrolysis process: Kinetic and thermodynamic analysis with isoconversional methods. Renewable Energy. 2017;107:489–96.
View Article
Google Scholar

[141] View Article

[142] Google Scholar

[ref45] 45. Barbanera M, Cotana F, Di Matteo U. Co-combustion performance and kinetic study of solid digestate with gasification biochar. Renewable Energy. 2018;121:597–605.
View Article
Google Scholar

[144] View Article

[145] Google Scholar

[ref46] 46. Huang H, Liu J, Liu H, Evrendilek F, Buyukada M. Pyrolysis of water hyacinth biomass parts: Bioenergy, gas emissions, and by-products using TG-FTIR and Py-GC/MS analyses. Energy Conversion and Management. 2020;207:112552.
View Article
Google Scholar

[147] View Article

[148] Google Scholar

[ref47] 47. Kaur R, Gera P, Jha MK, Bhaskar T. Pyrolysis kinetics and thermodynamic parameters of castor (Ricinus communis) residue using thermogravimetric analysis. Bioresour Technol. 2018;250:422–8. pmid:29195154
View Article
PubMed/NCBI
Google Scholar

[150] View Article

[151] PubMed/NCBI

[152] Google Scholar

[ref48] 48. Naqvi SR, Tariq R, Hameed Z, Ali I, Naqvi M, Chen W-H, et al. Pyrolysis of high ash sewage sludge: Kinetics and thermodynamic analysis using Coats-Redfern method. Renewable Energy. 2019;131:854–60.
View Article
Google Scholar

[154] View Article

[155] Google Scholar

[ref49] 49. Mumbach GD, Alves JLF, Da Silva JCG, De Sena RF, Marangoni C, Machado RAF, et al. Thermal investigation of plastic solid waste pyrolysis via the deconvolution technique using the asymmetric double sigmoidal function: Determination of the kinetic triplet, thermodynamic parameters, thermal lifetime and pyrolytic oil composition for clean energy recovery. Energy Conversion and Management. 2019;200:112031.
View Article
Google Scholar

[157] View Article

[158] Google Scholar

[ref50] 50. Tian W-W, Xu F, Xing S-J, Wu R, Yuan Z-Y. Comprehensive study on the thermal decomposition process of waste tobacco leaves and stems to investigate their bioenergy potential: Kinetic, thermodynamic, and biochar analysis. Thermochimica Acta. 2023;723:179473.
View Article
Google Scholar

[160] View Article

[161] Google Scholar

Figures

Abstract

1. Introduction

2. Materials and methods

2.1 Raw materials

2.2 Fly ash characterization

2.3 Kinetic and thermodynamics analyses

2.3.1 Estimation of activation energy.

2.3.2 Determination of reaction mechanism function.

2.3.3 Estimation of thermodynamic parameters.

3. Results and discussion

3.1 Characteristics of fly ash

3.2 Thermogravimetric analysis

3.3 Estimation of activation energy by model-free methods

3.4 The determination of reaction models by model-fitting method

3.5 Thermodynamic parameters analysis

4. Conclusions

Supporting information

S1 Fig. Kinetic plots of Stages II-III in air atmosphere by FWO, KAS, and Friedman (FR) methods.

S2 Fig. Kinetic plots of Stages II-III in N2 atmosphere by FWO, KAS, and Friedman (FR) methods.

S1-S4 Tables. Values of E and R2 via CR method based on 15 reaction models.

S5-S7 Tables. Thermodynamic parameters of fly ash thermal treated in air and N2 atmospheres.

S3 Fig. The relationship between conversion rate and the difference between the E and ΔH (E-ΔH).

Acknowledgments

References

S2 Fig. Kinetic plots of Stages II-III in N₂ atmosphere by FWO, KAS, and Friedman (FR) methods.

S1-S4 Tables. Values of E and R² via CR method based on 15 reaction models.

S5-S7 Tables. Thermodynamic parameters of fly ash thermal treated in air and N₂ atmospheres.