A review on thermogravimetric analysis‐based analyses of the pyrolysis kinetics of oil shale and coal

Solid fossil fuels still support the development of the energy and chemical industry. The clean and efficient utilization of solid fossil fuels can effectively alleviate the problems of energy shortage and environmental pollution. An in‐depth understanding of the kinetic process, models, and mechanism of pyrolysis of solid fossil fuels is also of great significance for the exploration and utilization technologies of petroleum resources. Thermogravimetric analysis is an effective method to study the pyrolysis process and kinetics with the advantages of high sensitivity, repeatability, and reliability for data acquisition. Researchers have conducted a lot of research on the thermogravimetric kinetics of solid fossil fuels in recent years, but there is a lack of systematic summary and comparison of kinetic methods, research progress, and trends. Therefore, a comprehensive study on pyrolysis kinetic principles, methods, and models of solid fossil fuels by thermogravimetric analysis is carried out in this paper. The influencing factors of pyrolysis kinetics of solid fossil fuels using thermogravimetry are elaborated and discussed in detail. This study also provides progress in recent years and prospects of thermogravimetric kinetics of solid fossil fuels. It is expected that this paper can provide certain technical support and theoretical guidance for the research on pyrolysis kinetics based on thermogravimetric analysis, and the exploration and development, clean, and efficient utilization of solid fossil fuels.

In the pyrolysis kinetic analysis of solid fossil fuels based on thermogravimetry, the equation for reaction rate of nonisothermal pyrolysis kinetics can be presented as follows: where α is the conversion rate of solid fossil fuels; k is the rate constant; A refers to the pre-exponential factor, min −1 or s −1 ; E is the apparent activation energy of the reaction, kJ/mol; R is molar gas constant, R = 8.314 J/ mol/K; T is the absolute temperature, K; f(α) is a function base on that reaction mechanism.α can be obtained by the following formula: where w 0 is the initial mass (g) of the sample, w t is the actual mass (g) of the sample at time t, and w f is the final mass (g) of the sample after the reaction.The final mass is generally defined for given conversion degree s below 100%. 21,22, f(α), and α in Equation ( 1) are collectively referred to as kinetic triplets.16 The process of studying the pyrolysis kinetics in solid fossil fuels can be divided into four steps as illustrated in Figure 1.The kinetic parameters were determined from the mass loss diagram (TG) and the weight loss rate graph (DTG) at different heating rates.

| Calculation of activation energy
The calculation methods of activation energy mainly include isoconversional methods, which can be applied to multistep reactions, and reveal the relationship between activation energy and conversion degree.Table 1 summarizes the formulas used by different methods to calculate activation energy based on TGA.

| Isoconversional methods
Isoconversional methods can be divided into the linear model-free method, model method, and nonlinear method.The linear model-free and model method can estimate the activation energy via linear regression by analyzing the slope of the fitting line.
The linear model-free methods, including Friedman, Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), Starink, and so on, do not need to assume the reaction model (reaction mechanism function).The reaction rate is only a function of temperature, and the activation energy is independent of the reaction mechanism.
Friedman adopts the differential form of the rate equation without mathematical approximation.The differential method's advantage is its straightforward and precise calculation, although it does have a systematic error in the reaction heat change with heating rate.Additionally, the experimental data is liable to noise. 36WO, KAS, and Starink are integral methods, and the following formula can be obtained by combining F I G U R E 1 (A) TG and DTG curves of pyrolysis at a heating rate of 20 K/min (redrawn from Kuang et al. 23 ).(B) Relationship between temperature and conversion or conversion rate at 30°C/min (redrawn from Montiano et al. 24 ).(C) In(β/T 2 ) − 1/T plots pyrolysis at different heating rates in the case of KAS (redrawn from Yan et al. 25 ).(D) The theoretical master plots of different reaction mechanisms and experimental data (redrawn according to Chen et al. 26 ).DTG, weight loss rate graph; KAS, Kissinger-Akahira-Sunose; TG, mass loss diagram.
With u = E/RT, p u ( ) is the integral of temperature,  ( ) . p u ( ) has no analytical solution, which needs to be approximated.The above formula can be expressed in logarithmic form as FWO and KAS formulas are obtained by using different approximate methods to p u ( ). 29,32 The Starink formula (Table 1) can be obtained through the improvement of FWO and KAS methods. 30ccording to the graph at different heating rates, the activation energy at a certain conversion rate can be calculated.However, considering the approximate solution of the temperature integral in the derivation process, there are some errors in the solution of activation energy. 37Some researchers have improved the accuracy of approximate solution by iterative mathematical calculation in recent years. 38ccording to different mathematical treatment methods, nonlinear methods can be divided into nonlinear integral isoconversional method (NL-INT) 34,39 and nonlinear differential isoconversional method (NL-DIF), and the specific equation is shown in Table 1.The error produced by these two nonlinear methods is very small because the approximate temperature integral used is more accurate, which is no limit to the value of u = E/RT.

| Model fitting method
The model fitting method mainly includes Coats-Redfern and direct Arrhenius method. 40,41The following formula of Coats-Redfern method can be obtained by integrating the rate equation 42


At the usual pyrolysis temperature, 1 ≪ , it can be ignored.
The direct Arrhenius method shifts the terms on the left and right sides of the Friedman formula.
4][45][46][47] Therefore, this method is not recommended for the evaluation of kinetic triplets by TGA. 48A B L E 1 Calculation method of activation energy.
According to the form of activation energy distribution function, activation energy can be distributed according to Gaussian, Weibull, gamma, Rayleigh, and discrete distribution functions.Gaussian distribution is the most widely used to calculate pyrolysis kinetics parameters based on TGA, 49 and discrete distribution is widely used to calculate kinetic parameters of oil and gas generation in closed systems (such as golden tubes) or other open systems. 50,51Miura et al. 31 brought the probability density function of the Gaussian model into the distributed activation energy equation of the first-order reaction for optimization and integration and obtained the Miura method (Table 1).With the deepening of research, researchers found that the DAEM fitting effect of reaction order (n ≠ 1) is better than that of n = 1, 52 and other function models may be better than Gaussian models by TGA. 53ultiple methods for calculating activation energy are frequently employed concurrently to enable comparative analyses. 54By doing so, we can improve the accuracy of the inferred activation energies while allowing exploring the relative validity of each calculation method.The Coats-Redfern method can be used in addition to model-free method to determine the reaction model. 55he pyrolysis of organic matter in source rocks is a complex process in which multiple parallel or competing reactions occur, resulting in multiple reaction products that undergo secondary or further reactions.Thus, to increase the precision of calculating kinetic parameters, researchers often assume numerous reactions in the pyrolysis process of organic matter when using the isoconversional method.The pyrolysis peak of the TGA curve is separated is separated using Gaussian, double Gaussian, or three Gaussian model.The activation energy for each reaction is then determined using either isoconversional or alternative methods 56,57 2.2 | Determination method of reaction mechanism function

| Reaction mechanism function
The reaction mechanism functions are mainly used to determine the reaction model and mechanism and are divided into the following models in Table 2. 58 The differential form f(α) and integral form g(α) of common kinetic reaction mechanism functions are summarized in Table 2. 59,60

| Calculation method of reaction mechanism function
As mentioned earlier, the model fitting method for calculating activation energy can be used in combination with the model-free method to determine the reaction mechanism function.In addition, there are several commonly used fitting methods for determining the mechanism function of the pyrolysis reaction of solid fossil fuels, mainly including Popescu, master plots, and the nonlinear least-squares method.
Popescu method is used to study reaction mechanisms by measuring conversions at the same temperature for different heating rates. 61Because this method is insensitive to the reaction mechanism function, 23 it is less used today in pyrolysis kinetics of solid fossil fuels.
Master plot methods are divided into two types, one is based on the integral and differential forms of kinetic data, and the other is based on the integral form.
Master plot methods based on differential and integral forms are as follows 62 : Set the conversion rate α = 0.5 as the reference point, and define the function as For the selected α, T corresponding to different heating rates is brought into the above formula, and the experimental diagram of ( ) with α is drawn.Similarly, the theoretical graph of versus α is drawn.The higher the degree of overlap between the experimental graph and theoretical graph (Figure 1D), the selected f(α) is the most suitable reaction mechanism function.
Master plot methods based on integral form 63 operate by p u ( ).It can be obtained by Formula (4) From the experimental data obtained at different heating rates, p u p ( ) (0.5) can be obtained, which is the measured experimental value.The different reaction mechanism functions are brought into g α g ( ) (0.5) , namely, the theoretical value.
T A B L E 2 Main kinetic reaction mechanism functions and their differential forms (f (α)) and integral forms (g (α)) in solid-state pyrolysis process.

Model
Symbol Reaction-order model F Nonlinear least-squares analysis 64 uses the error function to judge the fitting effect by matching the prediction results of the model with the measured data.The diversity of organic matter and inorganic minerals in solid fossil fuels leads to the difference in the pyrolysis process and kinetic parameters.Therefore, a single reaction mechanism function may not be able to accurately characterize the pyrolysis mechanism of solid fossil fuels, and the reaction mechanism function needs to be revised.Málek 65 proposed two special functions to revise the reaction mechanism: The shape and value of the local maximum function of y α ( ) and z α ( ) (α m and α p  , respectively) can determine the reaction mechanism function by Málek. 66Different models are used to correct the reaction mechanism according to different α m values and the curve trend of y α ( ). 67 When α 0.633 m ≪ , Sestak-Berggren proposed a model function to revise the reaction mechanism 22 : , the following equations are used to modify the reaction mechanism 68 : follows the reaction-order model. 69In a word, the least-squares method is an effective method to identify the most probable reaction mechanism function because of the insensitivity to the estimation methods of heating rate and activation energy, but the calculation process is cumbersome.Master plots methods require that the reaction mechanism function and activation energy remain unchanged in the whole conversion range, so it may not be suitable to describe the pyrolysis process of solid fossil fuels in some cases. 23

| Heating rate
The heating rate affects heat transfer behavior during the pyrolysis of solid fossil fuels, thus affecting the composition and quantity of pyrolysis products and pyrolysis kinetic parameters.The appropriate heating rate facilitates enhancing the pyrolysis reaction's effectiveness, increasing the pyrolysis oil's yield, 70 and regulating the distribution and composition of pyrolysis products.The higher the heating rate, the higher the temperature corresponding to the maximum weight loss rate, and the lower the weight loss in the stage of generating hydrocarbon by pyrolysis. 71Al-Harahsheh et al. discovered that with the increase of heating rate, the content of aliphatic compounds in shale oil increased, while the content of aromatic compounds decreased. 72Pan et al. studied the pyrolysis characteristics of Jimsar oil shale and found that increasing the heating rate can increase the ratio of light alkanes to heavy alkanes, unsaturated hydrocarbons to their respective alkanes, and benzene to alkyl aromatics. 73arly researchers found that the calculated activation energy can change as the heating rate varied. 74,75With the deepening of research, researchers found that heating rate is insensitive to the activation energy when applying the model-free methods.
It is worth noting that one of the main drawbacks of TGA analysis is that solely low heating rate measurements can be performed.One must exercise caution when utilizing TGA-derived kinetic parameters, as industrial high-speed gasification and combustion reactors often entail substantially greater characteristic temperatures and heating rates. 76,77][80][81] Therefore, although TGA is a very useful tool to study the relative impact of the ambient atmosphere and of the composition of a given fuel on its pyrolysis behavior, the validity of the kinetic triplets inferred using such a technique when simulating data obtained over an extended range of operating conditions (heating rate) can be truly ill-adapted.

| Final pyrolysis temperature
The final pyrolysis temperature of pyrolysis has an impact on the pyrolysis process of solid fossil fuels, the degree of organic matter decomposition, product distribution, and characteristics of the product, as well as the yield of oil.Additionally, it also influences the secondary pyrolysis of pyrolysis products.Na et al. studied the influence of the final pyrolysis temperature on the yield and quality of shale oil in the western United States.The results show the yield of shale oil increases with the increase of temperature below 500°C but decreases after 500°C. 82

| Particle size of solid fossil fuel
The size of particles has a significant impact on mass transfer, heat transfer, and secondary reactions during the pyrolysis of solid fossil fuels. 83Smaller particle size makes the pyrolysis reaction more uniform, reduces the formation of slagging, and has higher pyrolysis efficiency, but the oil yield will be lower due to the decrease of organic matter proportion.Excessive particle size can strengthen the secondary reaction, produce more coke, and reduce the oil yield. 84Jayaraman et al. found that with the increase of particle size, the average activation energy and pre-exponential factor of high ash coal also increase. 85

| Catalysis
Catalysis encompasses the catalysis of intrinsic minerals and external materials.Minerals in solid fossil fuels are cross-linked with organic matter, and inherent minerals can promote or inhibit the pyrolysis of organic matter.Minerals in solid fossil fuels mainly include carbonate (kaoline, calcite), silicate (quartz, clay), pyrite, and so on.These minerals often contain some alkali metals and alkaline earth metals, which also have a certain impact on the pyrolysis of organic matter. 86,87Ballice et al. found that carbonate had a catalytic effect on the pyrolysis of organic matter, and silicate had an inhibitory effect on pyrolysis.However, the inhibitory effect surpasses the catalytic effect. 88Chang et al. discovered that inherent carbonate can promote the formation of shale oil, while silicate plays an inhibition role.Inherent silicate can promote the cracking and aromatization of aliphatic hydrocarbons, while carbonate can inhibit the cracking of long-chain aliphatic hydrocarbons. 89Gai et al. found that when the pyrolysis temperature is lower than 540°C, the inherent pyrite can accelerate the pyrolysis rate of oil shale and increase the shale oil yield, but reduce the pyrolysis gas yield.The sulfur in the pyrite is believed by the author to be a possible explanation. 90Hetényi et al. studied the effects of calcite and montmorillonite on the pyrolysis of different types of kerogens.The investigation revealed that calcite's catalytic effect is not apparent with a high montmorillonite content.For type I kerogen, montmorillonite showed obvious catalytic activity, and the extraction rate of soluble kerogen is higher than that of kerogen without montmorillonite.For type III kerogen, the converse phenomenon was observed. 91 addition to the catalysis of inherent minerals, researchers added catalysts for catalytic pyrolysis to reduce the activation energy of solid fossil fuel pyrolysis reactions and change reaction paths.The pyrolysis of solid fossil fuels often employs catalytic materials such as metal or molecular sieve catalysts.Metal catalysts can be categorized further into transition metals, alkali metals, and alkaline earth metals.Wang et al. summarized the research advances in the fields of biomass pyrolysis and biomass/coal co-pyrolysis.Metal additives have been found to reduce the decomposition temperature of biomass and increase the yields of char and gas.In addition, alkali and alkaline earth metals have been shown to induce a synergistic effect that leads to a decrease of the apparent activation energy of the pyrolysis reaction. 87Jiang et al. discovered that the addition of CoCl 2 •6H 2 O can improve the oil yield of pyrolysis and significantly promote the formation of aromatics in shale oil, resulting in an increase of approximately 18% compared to noncatalytic pyrolysis. 92u et al. discovered that Co can accelerate the cleavage of C-C bonds in organic compounds, improve the selectivity of aromatic hydrocarbons and promote the aromatization of olefins, thus increasing the content of olefins and aromatic hydrocarbons. 93Wang et al. added CaCO 3 and Fe 2 O 3 to kerogen pyrolysis and found that Fe 2 O 3 can increase shale oil yield and reduce gas phase product yield and CaCO 3 can catalyze the secondary cracking of products. 94Williams and Chishti found that the addition of ZSM-5 increased gas production of oil shale pyrolysis, but the yield of shale oil decreased.The content of polycyclic aromatic hydrocarbons in the oil phase increased, while the content of N and S decreased. 95

| Other factors
Pyrolysis time, pyrolysis atmosphere, pressure, and other factors also have some influence on pyrolysis kinetics.Noble et al. discovered that with the increase of pressure, the total output of shale oil decreases, aromatic products increase, and the olefin products decrease, while the saturated hydrocarbon and weakly polar compounds remain relatively stable. 96The atmosphere mainly affects the secondary reaction of pyrolysis products.Nazzal and Williams discovered that after nitrogen in the pyrolysis atmosphere was mixed with a certain amount of steam, the presence of steam could improve the yield of tar. 97he pyrolysis of solid fossil fuels, based on TGA, is a complex process.The type and chemical composition of solid fossil fuels, as well as the experimental conditions employed in the thermogravimetric analyzer, have an impact on the pyrolysis kinetics.It is essential not only to recognize their impact on kinetic parameters but also to examine the reaction process, reaction mechanism, and product alterations.Furthermore, it is imperative to conduct a detailed investigation of the catalytic mechanism and reaction process of pyrolysis at the molecular level for effective catalytic pyrolysis.

| RESEARCH PROGRESS ON PYROLYSIS KINETICS OF SOLID FOSSIL FUELS BASED ON TGA
The research on pyrolysis kinetics of solid fossil fuels by TGA mainly focuses on oil shale and coal.Based on these two types, the research on pyrolysis kinetics by TGA in recent years is reviewed.

| Research on pyrolysis kinetics of oil shale
With the rapid consumption of oil and other conventional energy sources, the development and research of oil shale, an unconventional new oil and gas resource, becomes particularly important.Oil shale is a type of combustible organic rock with high ash content, consisting of different inorganic minerals and organic material.Shale oil can be produced from it through pyrolysis, and it can also be reprocessed to manufacture liquid fuels.In recent years, in situ exploitation of oil shale has been paid more and more attention by researchers.By studying the pyrolysis kinetics of oil shale and the structure-activity relationship between the organic structure of oil shale and pyrolysis products, the process and mechanism of oil generation by oil shale pyrolysis can be further clarified, guiding the exploration and development of oil shale and in situ exploitation, improving the comprehensive utilization efficiency of oil shale and reducing the environmental pollution.
The pyrolysis process of oil shale includes the removal of volatile components such as water, pyrolysis of organic matter, and decomposition of minerals.The pyrolysis of organic matter is the main process of oil generation from oil shale, mainly including the pyrolysis of kerogen to produce asphalt, gaseous hydrocarbons, and oil, the further pyrolysis of asphalt to produce gaseous products, oil, and small molecular organic matter, and polycondensation to produce aromatic compounds. 70Qing et al. 21studied the pyrolysis mechanism of Fushun oil shale and kerogen by structural characterization and pyrolysis experiments.The authors divided the pyrolysis of organic matter into two stages: the first stage was that chemical bonds break to form aliphatic hydrocarbon structures, such as normal alkanes, isoparaffins, olefins, and so on.The second stage involved aromatization reaction.The specific possible cracking path is shown in Figure 2.
In the early study of oil shale pyrolysis kinetics, the activation energy determination method, such as Coats-Redfern, was usually adopted and the reaction mechanism model was assumed to be the first-or n-order reaction.However, oil shale pyrolysis is a very complicated process involving many reaction mechanisms.With the deepening of research, researchers use various methods to improve the accuracy of calculation and further explore the mechanism of oil shale pyrolysis reaction.
F I G U R E 2 Mechanism of Fushun oil shale kerogen pyrolysis rearranged from Qing et al. 21hang et al. 22 divided the pyrolysis reaction rate curve of Beizao oil shale into four stages by the K-K method to calculate the second derivative of conversion rate and double Gaussian model (Figure 3A,B), and then determined the kinetic parameters of each stage by FWO method (Figure 3C).Additionally, the researchers utilized the Coats-Redfern technique to ascertain the reaction model and discovered that these four stages followed the mechanism of nucleation growth, reaction sequence, geometric shrinkage, and reaction sequence, respectively.The adjustment function was used to modify the reaction mechanism function, which further improved the accuracy of the prediction of the reaction mechanism function.Similarly, Bai et al. 56 divided the pyrolysis rate peak of oil shale organic matter into two subpeaks by double Gaussian model, and determined the activation energy by Friedman, FWO, KAS, and Coats-Redfern methods, and further judged the reaction mechanism by master plots method.It is found that the two subpeaks in the pyrolysis stage of organic matter follow R 3 (geometric shrinkage model) and A 5/6 (nucleation growth) reaction mechanism respectively, while the decomposition of minerals follows A 2 (nucleation growth) reaction mechanism.Kuang et al. 23 used Friedman, FWO, KAS, NL-INT, and NL-DIF to study the pyrolysis kinetics of Green River oil shale.The activation energy obtained by these five methods had a good fitting effect, and the author put forward the early first-order reaction mechanism function and the late second-order reaction mechanism function by the leastsquares method to account for the pyrolysis process of organic matter in oil shale.Bouamoud et al. 99 employed the FWO and KAS iteration methods to study the activation energy parameters of type I oil shale rich in fatty chains in the Congo area.The accuracy of activation energy can be effectively improved by iterative processing.Bai et al. 100 employed Friedman, Starink, Miura, and FWO iteration methods to study the pyrolysis kinetics and mechanism of Huadian, Fushun, and Nongan oil shale in Northeast China.The author pointed out that the differences in pyrolysis behavior and kinetic parameters of the three samples are related to the diversity of organic matter and mineral composition of oil shale, among which Huadian oil shale has the lowest activation energy, and Nongan oil shale has the highest activation energy because of its high ash content.The F I G U R E 3 (A) Determination of peak locations at 40 K/min by the K-K method (redrawn according to Zhang et al. 22 ).(B) Bi-Gaussian multipeak fitting for Beizao oil shale at 40 K/min (redrawn according to Zhang et al. 22 ).(C) The activation energy E of subreaction by FWO method (redrawn according to Zhang et al. 22 ).(D) E versus α curves of the FS type Ⅰ and DCG type Ⅱ kerogens calculated via the Friedman and KAS methods (redrawn according to Bai et al. 98 ).DCG, Dachanggou; FS, Fushun; FWO, Flynn-Wall-Ozawa; KAS, Kissinger-Akahira-Sunose.
results showed that activation energy calculated by Friedman, DAEM, and FWO iteration methods were similar, with Friedman's computations exhibiting larger values.Many researchers also found that activation energy calculated by the Friedman method is greater than that calculated by other model-free methods when studying the pyrolysis kinetics of oil shale (Figure 3D). 56,98,101ased on the above investigation and literature summary on oil shale pyrolysis kinetics, the following methods were mainly employed by researchers to enhance calculation and simulation accuracy.First, they compared and studied the calculation methods of activation energy.Second, the reaction process is further refined by function fitting.Third, with regard to the calculation method of activation energy based on the integral method, researchers can improve the calculation accuracy of activation energy through iterative processing.Concerning the study of reaction mechanism function, researchers found that the pyrolysis reaction mechanism of oil shale is not only the n-order reaction but also the nucleation growth model and geometric shrinkage model.
Researchers also studied the pyrolysis kinetics of different types of oil shale.Han et al. 102 studied Huadian type I oil shale and Fushun type II oil shale by the DAEM method, discovering that the activation energy of type I oil shale is lower than that of type II oil shale, possibly due to the higher concentration of aromatic organic compounds present in type II oil shale.Similarly, Bai et al. 98 researched the pyrolysis kinetics of four oil shale kerogen and found that the content of aliphatic and aromatic carbon in kerogen had a significant influence on the pyrolysis characteristics and kinetic parameters.Saturated bonds and oxygencontaining bonds were easy to crack, and the activation energy of the reaction was lower than that of aromatic bonds (Figure 3D).
Table 3 summarizes the reaction conditions, kinetic parameters, and reaction models of oil shale pyrolysis in different regions.Owing to variations in the composition and organic content of oil shale across different regions, applied kinetic methods and experimental conditions differ.Hence, the kinetic parameters obtained vary.Table 3 illustrates that early research on oil shale pyrolysis kinetics using TGA may have contained inaccuracies in experimental results due to the limitations of kinetic methods.However, as research on oil shale pyrolysis kinetics has progressed, activation energy calculation methods have become increasingly widespread, resulting in improved fitting accuracy.Additionally, more attention has been paid to the determination and correction of the reaction model.
In a word, regarding the research on the kinetics of oil shale pyrolysis by TGA, researchers strive to enhance the precision of kinetic parameters, including activation energy, using various mathematical and kinetic techniques.This enables the investigation of oil shale pyrolysis mechanism and process for more accurate theoretical support toward resource utilization and ecofriendly conversion of oil shale.Furthermore, through an investigation into the pyrolysis kinetic parameters of various types and maturity levels of oil shale using TGA, it is possible to infer the oil production potential of oil shale, providing valuable theoretical insight for exploring and producing oil shale in situ.

| Study on coal pyrolysis kinetics
Coal can be utilized to produce electricity, chemicals, and liquid fuels through thermochemical conversion.Coal pyrolysis mainly includes the decomposition of organic matter, volatilization of light components, condensation of residues, secondary decomposition, and recondensation of condensation products, and so forth.The simplified process is shown in Figure 4A.Understanding the structural evolution of different rank coal from coal to char by thermogravimetric kinetics analysis is essential for the development of advanced coal utilization technologies.
Yan et al. 112 applied Miura, FWO, and Friedman methods to expound the pyrolysis process and kinetics of lignite and bituminous coal.The results indicate that the activation energy obtained by the three methods was consistent (Figure 4B), and that activation energy increased with the increase of reaction degree.In addition, the author employed a three-Gaussian function model (DAEM-G3) to fit the conversion rate versus temperature graph for kinetic modeling (Figure 4C) and summed up the pyrolysis of lignite and bituminous coal into three stages combined with previous studies.The first stage involved the release of bound water and carboxylic acid, followed by the cleavage of covalent bonds between C and C, H, O, N (hydrocarbon generation stage) in the second stage and the third stage encompassed the cleavage of bonds between C and C, O and S, carbonate decomposition, and aromatic ring condensation.However, the author found that the activation energy distribution obtained by DAEM-G3 kinetic modeling (Figure 4D) was not completely consistent with the activation energy distribution calculated by Miura, FWO, and Friedman methods.The author believed that the DAEM-G3 model was suitable for describing the pyrolysis of low-and middle- | 341 rank coal.In addition, the author found that with the increase in coal rank, the temperature corresponding to the maximum weight loss rate increased.Yu et al. 114 also discovered that oxygenates in coal were the first species to participate in the pyrolysis reaction through tube furnace pyrolysis.As the temperature increased, there was a rise in the quantity of aromatic hydrocarbons, whilst the amounts of aliphatic hydrocarbons and oxygenates decreased.Yan et al. 77 applied the Miura method to analyze the pyrolysis kinetics of lignite and bituminous coal with different volatile content.With a higher coal rank, it was discovered that aliphatic carbon was gradually transformed into aromatic carbon with a gradual shortening and elimination of the aromatic side chain.The distribution range of activation energy also gradually narrowed.In addition, due to the wide distribution of lignite carbon types and the complex chemical structure, DAEM is less effective than bituminous coal in simulating and predicting the pyrolysis process of lignite.Yan et al. 25   (F ) [116]   Bituminous and lignite 3

419-1173
Coats-Redfern Anthracite: [ 115] Low volatile bituminous: In A 3 = 2.9 (F model-free methods to calculate the activation energy of four kinds of middle-and low-rank coal pyrolysis.It is discovered that the E values given by KAS, FWO, and Starink methods are similar in the whole conversion range, while the Friedman method has a larger deviation.Casal et al. 115 used the Coats-Redfern method to study four kinds of coal (anthracite, bituminous coal, and lignite) with different chemical structures.The result showed that the average activation energy of lignite was the lowest (73 kJ/mol), that of bituminous coal was between 97 and 117 kJ/mol, and that of anthracite coal was the highest (138 kJ/mol).In addition, the author discovered that with the increase in volatile content of bituminous coal, the average activation energy gradually decreased.Ma et al. 113 improved the DAEM model for the lack of a reasonable method to obtain the pre-exponential factor in the DAEM method.
It is assumed that the coal pyrolysis process consists of n consecutive parallel reaction sequences, each of which was controlled by an n replaced by a first-order reaction mechanism.Simultaneously, the authors improved the calculation accuracy of activation energy through the six-Gaussian function model (DAEM-G6, as shown in Figure 4E).The pyrolysis reaction of coal was categorized into six subreactions, namely, the decomposition of peroxides and carboxylic acids at low temperature, aliphatic hydrocarbons at medium temperatures, the formation of aromatic hydrocarbons and phenols, and the secondary reactions of aldehydes, ketones, and alcohols at high temperatures.The results proved that the model proposed by the authors provided more accurate predictions for the coal pyrolysis process, and the authors also proved that the n-order reaction model was more accurate than the first-order reaction.Many researchers also unearthed that the fitting effect of reaction order n ≠ 1 is better than that of n = 1 when studying the mechanism of coal pyrolysis reaction. 75,116,117able 4 summarizes the reaction conditions and kinetic methods and parameters of different types of coal pyrolysis based on TGA in recent years.Various calculation methods of kinetic parameters have been applied in the kinetic analysis of coal pyrolysis, among which the DAEM method is the most widely used, which is related to the pyrolysis process of coal with a better fitting effect.With the deepening of research, the Gaussian model is also continuously improved, ranging from one Gaussian function model to six Gaussian function model.In addition to the Gaussian model, other distribution models, 123 such as Weibull, gamma, and Rayleigh distribution, also have certain potential application in the coal pyrolysis process to improve the fitting effect.The content in parentheses after the pre-exponential factor is the reaction model.

| Research progress on pyrolysis kinetics of materials related to solid fossil fuels
In addition to oil shale and coal, the research on the pyrolysis kinetics of other solid fossil fuels and related materials is also involved.This includes unconventional fossil fuels such as asphalt and oil sands, pyrolysis intermediates (asphaltene, oil, and gas), and components of solid fossil fuels (aliphatic hydrocarbons, aromatic hydrocarbons, etc.).Light oil products can be efficiently and manageably produced from unconventional heavy oils through pyrolysis processes.In this way, the pyrolysis mechanism and process of solid fossil fuels can be more deeply understood and the gap between energy supply and demand can be filled.Shin et al. 124 employed Friedman and DAEM methods to estimate well the kinetic parameters of oil sand bitumen and its components.Ding et al. 125 used the Gaussian model to divide the pyrolysis process of oil shale semicoke and extracted bitumen into several bondbreaking reaction processes, namely, the breaking of The possible mechanism of n-alkanes and carbon monoxide rearranged from Jiang et al. 127 LI ET AL.
| 345 T A B L E 5 Pyrolysis reaction conditions and kinetic parameters of other solid fossil fuels and related materials.researched the pyrolysis kinetics of saturated components in coal at different heating rates by the Coats-Redfern method and speculated the reaction mechanism of saturated components (as shown in Figure 5).With the increase in heating rate, the activation energy of saturated components increased gradually.

Type of sample
The pyrolysis reaction conditions and kinetic parameters of materials related to solid fossil fuels based on TGA are summarized in Table 5.The research is still in its early stages, and the kinetic techniques employed were rather uncomplicated and did not encompass the investigation of mechanistic or phenomenological models.With the continuous improvement of analytical technology, model software, and calculation ability, there will be more and more research types related to pyrolysis kinetics, research content will become more and more detailed, and there will also be a growing awareness of structural complexity.The research scope has gradually expanded from the pyrolysis kinetics of bulk solid fossil fuel to the pyrolysis kinetics of specific components or intermediate products, aiming to gain a deeper F I G U R E 6 Distribution of activation energy for co-pyrolysis of (A) dewatered sewage sludge (DS) and (B) hydrothermal char (HC) blended with moderate-rank coal (coal 3) at three blending ratios according to a discrete distributed activation energy model (redrawn according to He et al. 141 ).(C) Carbon numbers distribution of aliphatic hydrocarbons in the pyrolysis oil of Changji oil shale (CRJ), Manasi coal (MNS), and their blends (redrawn according to Lu et al. 142 ).(D) E values of individual and mixed fuels at different conversion rates during pyrolysis were obtained by the Starink method (redrawn according to Lu et al. 142 ).
T A B L E 6 Reaction conditions and kinetic parameters of co-pyrolysis of solid fossil fuels.understanding of the pyrolysis mechanism of solid fossil fuels and examine the effects of different components on the contribution of pyrolysis products.

| Study on co-pyrolysis kinetics related to solid fossil fuels
Co-pyrolysis technology is such an environmentally friendly and highly efficient technology for enhancing the quality and chemical characteristics of fuel. 135In the research of pyrolysis of solid fossil fuels by TGA, the co-pyrolysis technology between solid fossil fuel and other materials, such as biomass, 20,136 algae, 137 polymer, 138 or different types of solid fossil fuels, can not only improve the selectivity of fuel pyrolysis products but also reduce their apparent activation energy. 139In addition, it has great potential in slowing down energy depletion, dealing with environmental problems, and recycling resources.Dai et al. 137 researched the co-pyrolysis behavior of oil shale and spirulina, discovering that oil shale delayed the thermal decomposition process of organic matter in spirulina.With the increase of the oil shale mixing ratio, the average activation energy (calculated by the FWO method) of mixed materials decreased from 278.84 to 192.53 kJ/mol.Lin et al. 140 studied the co-pyrolysis kinetics of oil shale and sludge by KAS and Starink methods.The study indicates that an optimal sludge ratio of 10% leads to a significant improvement in oil shale pyrolysis performance, and the average activation energy of oil shale after mixing (determined via the Starink method) decreases from 285.3 to 253.6 kJ/mol.He et al. 141 studied the co-pyrolysis kinetics of dewatered sewage sludge (DS) and hydrothermal char (HC) with three different rank coal by the DAEM method.With the increase of the ratio of coal to DS or HC, the activation energy gradually increased from 175 to 198 kJ/mol (as shown in Figure 6A,B).Furthermore, it was found that the activation energy increases with higher fixed carbon content in both HC and coal.Additionally, increasing the coal/DS ratio was found to promote the formation of CH 4 whilst decreasing the concentration of CO 2 .Notably, high-rank coal exhibits a significant synergistic effect on the yield of light hydrocarbons.Lu et al. 142 studied the co-pyrolysis kinetics of Manas coal and Changji oil shale in Xinjiang province.The results showed that Manas coal is abundant in alkaline earth metals, which can significantly improve the oil and gas yield of oil shale.Moreover, the relative content of shortchain aliphatic hydrocarbons in pyrolysis oil of mixed materials increases, as shown in Figure 6C.When the The content in parentheses after the activation energy is the reaction model.

LI ET AL.
| 349 conversion rate exceeded 0.5, the activation energy of coal/oil shale mixture at 5:95 and 10:90 was lower than that of monomer material, as demonstrated in Figure 6D, which indicated that there was synergy and catalysis between them.Zhai et al. studied the pyrolysis characteristics, kinetics, and gaseous products of co-pyrolysis of corn stalk and oil shale.The results found that the interaction between oil shale and corn stalk was the most significant with the lowest apparent activation energy when the blending ratio of corn stalk is 50%. 143ecent studies on co-pyrolysis of solid fossil fuels based on TGA are summarized in Table 6.Different mixing ratios affected the reaction process and kinetic parameters.The investigation into the co-pyrolysis of solid fossil fuels is still in its early stages, and it is possible that the two chosen materials will not have a substantial combined impact during the co-pyrolysis process.Further research is needed in the future to improve the synergistic effect by modifying materials or incorporating new materials.In addition, the co-pyrolysis research on solid fossil fuels mainly focuses on reducing activation energy, obtaining more valuable products, and reducing environmental pollution.The kinetic method used is uncomplicated, and there may be certain errors and limitations.The mechanism of co-pyrolysis synergy between the two materials and the factors affecting the reaction process and products need to be further explored at the molecular level.

| CONCLUSION
In this paper, the research progress of pyrolysis kinetics of solid fossil fuels based on TGA was summarized in two parts.First, this section introduces the principle of TGA kinetics and the influencing factors.In the second part, according to the classification of solid fossil fuels, the research on pyrolysis kinetics of solid fossil fuels by TGA was introduced.For different activation energy calculating methods, except the Coats-Redfern method, which was widely used in the early kinetic studies and which shows certain errors and limitations, other methods mentioned in this article have a wide range of applications.However, the Coats-Redfern method is often employed in conjunction with model-free models to determine the reaction mechanism function.Different activation energy calculation methods may have different results due to different principles.For the pyrolysis kinetics of oil shale and coal, in recent years, researchers have used a variety of kinetic methods to determine the activation energy and correct the reaction mechanism.More mathematical processing and kinetic methods will be employed to further improve the accuracy of the kinetic parameters.For other related materials and copyrolysis research, researchers are currently paying more attention to the development of related solid materials and co-pyrolysis materials, but the application of kinetic methods is relatively simple, and more kinetic methods need to be employed in the future to study the pyrolysis mechanism more deeply.
Regarding the research status of pyrolysis kinetics of solid fossil fuels by TGA, the following aspects may require further research: First, concerning the study of influencing factors, pyrolysis kinetics of solid fossil fuels mainly focused on experimental conditions, catalysis, and other factors, while the influencing factors such as type, maturity, and organic matter composition of solid fossil fuels need further discussion and research, which is of great significance for judging the oil generation potential and exploration and production of solid fossil fuels such as oil shale by pyrolysis.In addition, for catalysis, it is also necessary to investigate additional catalytic materials to improve the oil yield and selectivity of pyrolysis and to further study the catalytic mechanism at the molecular level.Second, researchers could consider enhancing the experimental equipment associated with thermogravimetry, such as improving instrumentation to extend the range of experimental conditions (faster heating rate), combining with more instruments and expanding instrument functionality, continuously improving the experimental and calculation methods, and accurately distinguishing the weight loss process of pyrolysis hydrocarbon generation from other weight loss processes, which can significantly expand the application scope and value of TGA of solid fossil fuels.Finally, with the continuous improvement of molecular simulation technology in recent years, the structure and pyrolysis process of solid fossil fuels can be simulated through software simulation. 154,155Combined with the pyrolysis process and calculated kinetic parameters, the structure and pyrolysis kinetics can be more deeply understood, which is also an important direction for future research.
In a word, the research in this paper is expected to provide theoretical references for the study of pyrolysis kinetics of solid fossil fuels and provide some research ideas and technical support for future research and application.

T A B L E 3
Abbreviations: DAEM, distributed activation energy method; FWO, Flynn-Wall-Ozawa; KAS, Kissinger-Akahira-Sunose. a E n refers to the activation energy of nth stage of pyrolysis classified in the literature.b E is the average activation energy calculated in the literature.

T
Abbreviations: DAEM, distributed activation energy method; FWO, Flynn-Wall-Ozawa; KAS, Kissinger-Akahira-Sunose. a E n refers to the activation energy of nth stage of pyrolysis classified in the literature.b E is the average activation energy calculated in the literature.
Abbreviations: DAEM, distributed activation energy method; FWO, Flynn-Wall-Ozawa; KAS, Kissinger-Akahira-Sunose. a E n refers to the activation energy of nth stage of pyrolysis classified in the literature.b E is the average activation energy calculated in the literature.
Abbreviationsa E n b E is the average activation energy calculated in the literature.c