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Introduction: Coal pyrolysis refers to a complex process of physical and chemical changes that occur when coal is heated to high temperatures in an air-free environment; this process has a significant impact on the gasification, liquefaction, and coking of coal. A deeper understanding of this process will help improve knowledge of coal utilization methods, and it holds significant practical importance, especially for the effective use of low-grade coal resources. The kinetics of coal pyrolysis are influenced by various factors, and there is extensive research on experimental and data processing methods as well as different types of models and their suitability. There are mainly three categories of models: (1) traditional single-reaction models, in which the reaction is typically represented as first-order or n-th order; the evolution of volatiles during coal pyrolysis is described using a first-order reaction kinetic model. This approach offers simple mathematical handling, but it is not suitable for non-isothermal processes ； (2) The overall kinetic model assumes that the chemical reactivity during pyrolysis is divided into several independent components that do not interact with each other, with the pyrolysis of each component being regarded as an independent first-order reaction ； (3) The Distributed Activation Energy Model (DAEM), which is an infinite parallel reaction model developed by Anthony et al. based on Pitts’ model. This model can effectively describe the coal pyrolysis process, enabling it to reflect both the changes in the overall conversion rate of a complex system and the changes in the yield of a specific reaction component within that system. However, the mathematical description of this model is very complex, and it is rarely used by domestic scholars at present. Most kinetic studies are conducted under isothermal conditions, while for the pyrolysis of typical coal types at high temperatures and under rapid reaction conditions, it is more appropriate to use non-isothermal thermal analysis methods. As a large-scale low-cohesion coalfield developed in recent years, Dongsheng coal is characterized by low ash and low sulfur content; however, its high volatile matter and low-cohesion properties limit its use in coking. Based on the application background of weak coking coal in the coking process, this paper uses non-isothermal thermogravimetry to study the pyrolysis behavior of Dongsheng coal and the methods for determining its kinetic parameters. 1 Experiment 1.1 Coal samples for the experiment The sample used in this experiment was obtained from Baosteel’s Dongsheng coal, and its industrial analysis and elemental analysis are listed in Table 1. http://www.nmtech.com.cn/jishuwang/upload1/0707091106594812.jpg 1.2 Thermal gravimetric experiment conditions and procedure for coal The pyrolytic weight loss experiment was carried out using a Shimadzu DTG-60 thermal analyzer, with a maximum heating rate of 50°C/min; the weight loss integration curve and thermal analysis curve were generated by computer. The amount of coal sample used is approximately 10 mg, with the particle size ground to below 0.074 mm. Using N2 as the carrier gas at a flow rate of 30 mL/min, heating was carried out at a constant rate with four different heating rates (5°C/min, 10°C/min, 30°C/min, 50°C/min) until the final temperature of 800°C was reached, after which the sample was held at this temperature for 10 minutes. 2 Results and Discussion 2.1 TG and DTG curves of Dongsheng coal and their analysis The thermogravimetric TG curves and the weight loss rate DTG curves of the coal sample at different heating rates are shown in Figure 1 and Figure 2, respectively. For the TG and DTG curves, 4 heating rates are used, and the TG curve is calculated on a dry basis. http://www.nmtech.com.cn/jishuwang/upload1/0707091107546579.jpg As can be seen from Figures 1 and 2, the weight loss decreases significantly when the final temperature of pyrolysis is reached. This is because, as coal is heated to high temperatures in an inert atmosphere, a series of complex physical and chemical changes occur, including the breaking of cross-linking bonds and secondary reactions, ultimately resulting in products such as gases (coal gas), liquids (tar), and solids (semi-coke or coke). As can also be seen from Figures 1 and 2, the pyrolysis process of coal is roughly divided into 3 stages: (1) The first stage ranges from room temperature to around 300°C; during this stage, drying and dehydration occur, as well as the removal of adsorbed gases and decarboxylation reactions. (2) The second stage is at 300°C to 550°C, during which intense decomposition reactions occur. The range of 300°C to 360°C constitutes a transitional phase, during which the coal loses less weight and the rate of weight loss is low; this phase can be regarded as the process of softening and melting of the coal ； The range of 360°C to 400°C corresponds to the stage of mild pyrolysis, during which small molecular side chains in the coal structure are primarily removed ； The temperature range of 400℃ to 550℃ corresponds to the stage of intense decomposition of coal, during which the heat loss weight is relatively high. This is because the side chains of the macromolecules begin to break apart and be removed. In this stage, depolymerization and decomposition reactions of coal dominate, resulting in the release of large amounts of gas and tar, and the coal transforms into semi-coke. (3) Above 550°C is the third stage, during which both the weight loss and the rate of weight loss change slowly; at this point, it is primarily semi-cocoon polycondensation that takes place. 2.2 Effect of heating rate As can be seen from Figures 1 and 2, as the heating rate increases, the weight loss increases. This is because the faster the heating rate, the greater the thermal shock experienced by the coal structure, which accelerates the breakdown of the coal’s macromolecular side chains and aromatic fused rings, while the condensation of pyrolysis products is relatively reduced. Furthermore, as can be seen from Figure 1 and the pyrolysis characteristic temperatures of Dongsheng coal, as the heating rate increases, not only does the initial pyrolysis temperature T0 increase overall, but the peak value of the weight loss rate also rises, with the peak temperature Tp corresponding to this peak weight loss rate shifting toward higher values as well. Since coal pyrolysis is primarily an endothermic reaction and coal has poor thermal conductivity, it takes some time for the reaction to proceed and for the products to be formed. As the heating rate increases, the pyrolysis time of tar decreases, resulting in an increase in the amount of tar. Some of the molecular structures do not have enough time to undergo pyrolysis, causing the products to escape and move towards higher temperatures, which leads to a lagging effect. Furthermore, the coking mechanism also suggests that the shift toward higher pyrolysis temperatures allows the coal to form a plastic mass (colloidal substance) with a higher liquid content prior to the formation of semi-coke, which helps improve the cohesiveness of the coal. This is particularly suitable for the Dongsheng coal, which has high volatility and low cohesiveness. 2.3 Pyrolysis characteristic temperature at different heating rates: To compare the thermal stability of the samples, the temperature corresponding to point C, where the line connecting points A and B – with conversion rate x reaching 2% at point A and 50% at point B – intersects the temperature axis, is defined as the initial pyrolysis temperature T0. Using the above method, the pyrolysis characteristic temperatures of Dongsheng coal at 4 different heating rates were determined (Table 2). http://www.nmtech.com.cn/jishuwang/upload1/0707091108334275.jpg As can be seen from Table 2, as the heating rate increases, the peak temperature at which weight loss occurs for Dongsheng coal, as well as the temperature at which the pyrolysis process comes to an end, also increase. Based on the data in Table 2, a regression analysis was performed between Tp and φ, yielding the relationship: Tp=1.124 4(15+426.95, r=0.993). Therefore, the φ value is a characteristic indicator of the peak temperature of the pyrolytic weight loss rate, indicating that Tp increases as the heating rate increases. 3 Kinetics of Pyrolysis Reaction of Dongsheng Coal 3.1 Mathematical Description of Reaction Kinetics The non-isothermal thermogravimetric method has been widely used to study the kinetics of solid-state pyrolysis reactions. For coal thermal decomposition, it can be expressed using the equation for the reaction rate of solid thermal decomposition: http://www.nmtech.com.cn/jishuwang/upload1/0707091109152267.jpg Where: x — conversion rate of coal thermal decomposition, % ； t—reaction time, min ； n—reaction order ； E—activation energy, kJ/mol ； T—absolute temperature, K ； A—prefactor, min —1 ； R—gas constant, kJ/mol·K ； W0—original mass of the sample, g ； W—mass of the sample at a certain moment, g ； Wf: the residual mass of the sample after pyrolysis to the specified end point, in grams ； △W—The weight loss of the sample at a certain moment, g ； △Wf—Weight loss of the sample at the specified end point of pyrolysis, in g. For non-isothermal processes, there is a linear relationship between temperature T and time t: T = T0 + φt, where φ is the rate of temperature increase. Substituting T into equation (1) and simplifying yields: http://www.nmtech.com.cn/jishuwang/upload1/0707091109599464.jpg The value of E is very large, so the term 2RT/E can be approximated as zero. If the reaction order is correct, there is a linear relationship between the left-hand side of equation (4) or (6) and the reciprocal of the temperature, 1/T. The activation energy E and the pre-exponential factor A of the reaction can be determined respectively from the slope and intercept of the resulting straight line. 3.2 Determination of pyrolysis kinetic parameters By substituting the x and T values obtained from the TG curves at different heating rates into Coasts-Redfern equations (4) or (6), the corresponding kinetic curves can be obtained. Taking the pyrolysis kinetic curve of Dongsheng coal at a heating rate of 50°C/min as an example (see Figure 3), where U = -ln/ln, and the reaction temperature is divided into three temperature ranges by three tangent lines. http://www.nmtech.com.cn/jishuwang/upload1/0707091110413430.jpg As can be seen from Figure 3, the pyrolysis of coal occurs in 3 stages: at temperatures below 400°C, reactions such as the breaking of hydrogen bonds and non-covalent bonds take place primarily ； In the range of 400℃ < T < 700℃, it is mainly the breaking of bridge bonds ； In the range where T > 700°C, polycondensation reactions primarily occur. According to literature reports, for Dongsheng coal, a fairly significant linear relationship can be obtained in the kinetic curves when the reaction order (n) ranges from 2 to 3. The value of the reaction order n is determined based on the linearity of the straight line and the error between the theoretical values and experimental values of the simple linear regression line. Based on the reaction order for which the linear correlation coefficient is closest to 1 and the error of the simple linear regression line is smallest, calculations were performed for different reaction orders (n=1, 2, 2.5, 3), and it was found that the linear relationship is most significant when n=3. Therefore, the kinetic parameters obtained at n=3 (as shown in Table 3) are used as the fundamental basis for analyzing the pyrolysis of Dongsheng coal under different heating rates. http://www.nmtech.com.cn/jishuwang/upload1/0707091111173769.jpg As can be seen from Table 3, the pyrolysis process of Dongsheng coal proceeds similarly at heating rates of 5°C/min and 10°C/min, both occurring in 2 stages; however, the temperature ranges for these reactions differ. Taking 5°C/min as an example, the range of 345°C to 453°C constitutes the drying and degassing stage. For Dongsheng coal, only limited thermal effects occur in this temperature range, primarily in the form of contraction ； In the temperature range of 453°C to 519°C, large amounts of volatiles (gas and tar) are generated and released, resulting in a primary degassing process of the coal. During this stage, the Dongsheng coal undergoes softening, melting, flowing, and expansion, forming an unstable intermediate phase in which gas, liquid, and solid phases coexist – namely a colloid. The quantity and quality of colloids determine the caking and agglomeration properties of coal, as well as its plastic behavior. The pyrolysis at heating rates of 30°C/min and 50°C/min, as well as that of Dongsheng coal at a heating rate of 10°C/min, all consist of 3 stages, but the temperature ranges for the reactions differ. It can also be seen from Table 3 that different activation energies exist at various stages. A higher activation energy is required during the intense pyrolysis stage; that is, the activation energy is highest during the plastic phase prior to the formation of semi-coke from coal pyrolysis, and this activation energy increases as the heating rate rises. This is because at lower temperatures, what occurs first is the breakdown of side chains and reactive groups with poor thermal stability; these have lower decomposition temperatures as well as lower activation energies for initiation. The plastic temperature range falls within the range of intense pyrolysis; at this stage, most chemical bonds, including those with high bond energies, begin to break, resulting in the formation of numerous active groups, and thus the apparent activation energy increases. Therefore, the plastic stage of the coal coking process requires a large amount of heat. During the subsequent pyrolysis process, polycondensation reactions mainly occur, with little weight loss; therefore, the activation energy decreases further. Once the heating rate φ is determined, the relationship between the kinetic parameters (activation energy E and pre-exponential factor A) is also established. Thus, its kinetic equation is obtained: http://www.nmtech.com.cn/jishuwang/upload1/0707091111534698.jpg 4 Conclusions 4.1 As can be seen from the pyrolysis curve, the pyrolysis process of Dongsheng coal can be divided into 3 main stages, and the peak temperature Tp of the weight loss rate is mainly in the range of 430°C to 480°C. 4.2 As the heating rate φ increases, the initial pyrolysis temperature T0, the temperature at which the weight loss rate reaches its peak Tp, and the temperature at the end of pyrolysis Tf all shift to higher values, simultaneously with an increase in the DTC peak. The relationship equation between Tp and φ, obtained from the regression equation, is: Tp=1.124 4φ+426.95, with r=0.993. 4.3 The activation energy for the coal pyrolysis process varies parabolically with temperature, requiring a higher activation energy during the intense pyrolysis stage. The plastic temperature ranges all fall within the range of intense pyrolysis temperatures. Through the calculation of kinetic parameters using linear regression, it was determined that the reaction order for Dongsheng coal is 3; the kinetic equation is: http://www.nmtech.com.cn/jishuwang/upload1/0707091112279001.jpg For a given φ, the activation energy E and the pre-exponential factor A can be determined from the relationship between the term on the left side of the equation and 1/T.