Pyrolysis Reactive Behaviors and Kinetic Characteristics of HSW Vitrinite Coal based on Coats-Redfern and DAEM Models


 In this work, the weight loss behavior of vitrinite in hongshiwan coal at different heating rates was investigated by thermogravimetric mass spectrometry (TG-MS). Then Coats-Redfern and DAEM models were established to analyze the kinetics of coal pyrolysis. The results show that the weight loss rate of pyrolysis decreased with the increase of heating rate. When the pyrolysis temperature reaches 400–500°C, the weight loss rate reaches the maximum, which is 0.1593, 0.1539, 0.1478 and 0.1414%/°C respectively at the heating rates of 5, 10, 15 and 20°C/min, With the increase of heating rate, the corresponding temperature peaks of the five pyrolysis gases are shifted to the high temperature direction, and the amount of gas escaping is increasing. The trend of higher heating rate delayed the release of volatile compounds was consistent with TG-DTG results. Two kinetic models both prove that the activation energy of coal pyrolysis increases with the increase of temperature. The maximum activation energy occurs between 600 ℃ and 900 ℃, because the multi condensation of coal tar and the re solidification of semi coke will occur in this temperature range.


Introduction
Pyrolysis is the most important process in the current coal processing, and it is the core technology to realize the efficient and clean utilization of coal owing to its low production cost [1,2] . Therefore, in order to control the release of pollutants and use coal effectively, it is necessary to clarify the mechanism of coal pyrolysis. The kinetic model is used to study the pyrolysis process of coal, so as to obtain the basic data of pyrolysis rate, pre exponential factor, activation energy, etc. ,which provides a theoretical basis for the development of new conversion process and the realization of classification and quality conversion and clean utilization of new coal [3,4] .
There are many factors affecting coal pyrolysis, including coal rank, coal particle size, pyrolysis temperature, different heating rates and pressures, and different catalysts. At present, the common coal pyrolysis process is carried out under atmospheric pressure and inert atmosphere, which determines that the heating rate is one of the most important operation means. Through the study by thermobalance, Solomon [5] found that coal pyrolysis will show different characteristics at different heating rates. The thermal analysis of Zhu [6] results showed that four weight loss zones (<165, 165-400, 400-600, and >600 °C) were observed in TG curve, and a higher heating rate led to the increase of the maximum devolatilization rate and the higher temperature corresponding to the peak. Zou [7] used TG-MS technology to pyrolysis SM coal at 8, 10, 12, 15 and 20 °C/min. The results showed that the precipitation trends of H2, CH4, C2H2, CO and CO2 were significantly different at different heating rates. Jiang [8] used TG-FTIR and PY-GC/MS to explore the pyrolysis behavior and product distribution of SM coal at high heating rates of 100, 300 and 500 °C /min. The results showed that the initial pyrolysis temperature increased with the increase of heating rate, and the yield of solid semicoke decreased with the increase of heating rate, indicating that high heating rate is conducive to the formation of volatile substances. Yan [9] studied that DAEM could accurately predict the pyrolysis behavior of bituminous coals at very high heating rates, but the effectiveness of DAEM in modeling and predicting pyrolysis of lignite is less satisfying. Xu [10] studied that comparing with the single-step global model, the distributed activation energy model (DAEM) is more accurate to characterize the gasification kinetics for the studied cokes, and the activation energies are in the range of 159.80-254.10 kJ/mol. Dwivedi et al. [11] investigated the pyrolysis activity of three typical Indian low rank coals at 50, 100, 150 and 200 K/min by thermogravimetric analyzer, and fully revealed the relationship between pyrolysis behavior and activation energy of coal samples at different temperatures combined with Friedman kinetic model. HSW coal is a kind of coal in Western China. Due to its remote location and late development, there are few literatures on the pyrolysis mechanism of HSW coal. This study focuses on the influence of heating rate on the pyrolysis behavior of HSW coal, and then expounds the mechanism of coal pyrolysis through kinetic analysis [12,13] .
The first step of this work is to detect the chemical properties of HSW coal through industrial analysis and element analysis, which lays the foundation for the correlation between product distribution and chemical properties. Then the pyrolysis behavior and volatile release characteristics of HSW in different pyrolysis stages were studied by thermogravimetry mass spectrometry (TG-MS), and the kinetic analysis was carried out by Coats-Redfern model. Next, the effects of different heating rates on the pyrolysis process and the evolution of gaseous products were studied. Finally, the distributed activation energy model (DAEM) was used to calculate the distributed activation energy at different conversion rates to reveal the pyrolysis mechanism from the kinetic point of view. This study will provide useful guidance for the pyrolysis of other types of coal [14,15,16,17] .

Coal sample
In this work, the coal samples were obtained from Hongshiwan mining area in Ningdong. Firstly, zinc chloride was used as density medium for high-speed enrichment and separation. Then, the macerals obtained were screened to 200 mesh and vacuum dried at 105 °C for 24 hours to obtain high-purity macerals. The lithofacies analysis of coal samples is shown in Table 1

Experimental instruments and methods
The instrument used in this work is a combination of thermal analyzer (NETZSCH STA449F3) and quadrupole mass spectrometer (NETZSCH QMS403D).
The pyrolysis experiments were carried out at four heating rates of 5, 10, 15 and 20 °C /min. The vessel was an Al2O3 crucible, the shielding gas was argon, the flow rate was 30ml /min, and the initial temperature was 25 °C, while the final temperature was 1200 °C.
The gas curves of dry coal samples during pyrolysis were measured by mass spectrometry at heating rates of 5 to 20 °C /min, and the corresponding mass spectrum signals were recorded.

Pyrolysis kinetic analysis
The main task of coal pyrolysis kinetics analysis is to explore the macro phenomena in the process of pyrolysis reaction and reveal its reaction mechanism, understand the relationship between the chemical structure and reaction ability of coal samples, and provide theoretical basis for coal pyrolysis. In this work, a single reaction model (Coats-Redfern) and a distributed activation energy model (DAEM) are used.
Coats-Redfern model considers that the non-isothermal reaction process can be regarded as an isothermal reaction process in an infinitesimal period of time, so the coal pyrolysis process is assumed to be a first-order or n-order reaction, and a single reaction model can only obtain the average activation energy of coal pyrolysis reaction in a certain temperature range. The formula of the model can be simplified as follows: By making a straight line with and 1/T, the activation energy Ea and the pre-exponential factor A can be evaluated from the slope and intercept of the straight line.
DAEM model is mainly based on two hypotheses: infinite parallel reaction hypothesis and activation energy distribution hypothesis [18] : a) The activation energy obtained from each independent parallel first-order reaction conforms to the Gaussian function of continuous distribution; b) The activation energy of each independent parallel first-order reaction is different.
The activation energy obtained by DAEM is a function of conversion rather than a single value, which can truly reflect the variation of activation energy in the process of mixed combustion. The results show that DAEM has a good fitting correlation in the mixed combustion process at different heating rates, and its activation energy distribution can reflect the weight loss trend of DTG curve, which fully proves that DAEM can reflect the change of activation energy during coal pyrolysis.
The model expression is [19,20] Where: α is pyrolysis conversion; T is absolute temperature, K; A is pre exponential factor, min-1; β is heating rate, °C/min; Ea is activation energy, kJ/mol; R is gas constant, 8.314j/ (mol -1 ·K); f (Ea) is activation energy distribution function.
Where f(Ea) can be assumed to be a Gaussian distribution function with mean activation energy Ea and standard deviation σ, as follows: By introducing formula (1-4) into formula (1-3), we can get the following results: In order to obtain the kinetic parameters better, the formula (1-5) is simplified by Miura integral method: At the same pyrolysis conversion rate, the activation energy corresponding to each conversion rate can be calculated from the slope of the straight line of ln (β/T 2 ) and 1/T, whether it is temperature programmed or isothermal pyrolysis. It is generally believed that the pyrolysis process of coal can be roughly divided into the following three stages:

Drying and dehydration stage
The first stage is the drying and degassing stage, which is mainly the desorption process of water and adsorbed gas, and the temperature is before 340 °C [21,22] 。 Comparing the TG and DTG curves of HSW coal at different heating rates, it can be seen that the difference of TG curves of coal samples at different heating rates at the initial stage of pyrolysis is small, which indicates that there is little relationship between the rate of water evaporation and the heating rate of pyrolysis at the initial stage of pyrolysis.

Active decomposition stage
the second stage is the active decomposition stage, which is between 340-600 °C, HSW coal decomposes rapidly, and a large number of free radicals are generated by breaking bridge bonds. After further reaction, coal gas and tar are generated. In addition, hydrogen and CH4 are also produced in large quantities, and the weight loss rate reaches the maximum at 400-500 °C [23,24,25] 。With the increase of pyrolysis temperature, the weight loss rate of coal increases gradually. It can be seen that the higher the heating rate is, the lower the weight loss rate is. At the heating rates of 5,  [26] . With the increase of heating rate, DTG peak area and weight loss rate decrease, which indicates that higher heating rate makes HSW coal unable to fully react in the pyrolysis process, resulting in some structures unable to completely decompose.

Slow polycondensation stage
In the third stage, when the temperature exceeds 600 °C, the dehydrocondensation of aromatic structure is the main reaction in this stage, and the image tends to be flat, and the pyrolysis rate decreases [27] 。Many thermogravimetric experiments show that with the increase of coal metamorphism, the total weight loss rate of coal pyrolysis presents a decreasing trend, and the temperature of the maximum weight loss rate increased [28,29,30] . As a low rank bituminous coal, HSW coal selected in this work has a higher weight loss rate, because the carbon content in vitrinite is very low, only 79.15% [31] . Coal samples with low carbon content can be converted into low molecular products more quickly, especially the lower C/H in raw coal sample, the higher pyrolysis conversion rate of coal sample, and the corresponding increase of tar yield [32,33] .

Gas product analysis during pyrolysis
In order to explore the specific pyrolysis process of HSW coal, argon was used as carrier gas to transfer pyrolysis products from pyrolysis furnace to MS detector. Figure 2 is the mass spectrum curve with the heating rates of 10 °C/ min. Combined with mass spectrometer, it can be found that in the three stages of pyrolysis weight loss of HSW coal, the first stage is mainly the desorption process of water and adsorbed gas; the second stage is mainly the decomposition of macromolecules to produce water, hydrocarbons and carbon monoxide; the last stage is usually the condensation and aromatization reaction to produce hydrogen, methane, carbon dioxide and carbon monoxide. The formation process of H2,CH4, H2O, CO and CO2 was investigated at the heating rates of 5, 10, 15 and 20 °C/min, as shown in Figure 3.

CH4
It is generally believed that the formation of CH4 can be divided into three stages: CO and CO2. The fragment strength temperature with m/z=15 may be more suitable and effective for studying the evolution characteristics of CH4. Therefore, m/z=15 is selected in this study It can be seen from Fig. 3(a) that the faster the heating rate is, the higher the temperature peak corresponding to CH4 escaping will shift to the direction of high temperature, and the amount of gas escaping will continue to increase. The trend of higher heating rate delayed the release of volatile compounds was consistent with TG-DTG results.

H2O
The release of H2O is mainly due to the release of adsorbed water, the esterification and dehydration of phenolic hydroxyl groups, and the decomposition of oxygen-containing functional groups. It can be seen from the figure that water has been generated from about 400 °C to about 800 °C, which is mainly due to the different temperature required for cracking and decomposition of different oxygen-containing functional groups. As shown in Fig. 3(b), it can be found that the variation trend of H2O at different heating rates is similar to that of CH4, but different from one peak of CH4, there are three obvious peaks of H2O during pyrolysis.

H2
The production of H2 in the pyrolysis process can be divided into two stages: the first stage occurs between 400 and 600 °C, due to the condensation reaction between free radicals; the second stage occurs between 600 and 900 °C, which is mainly formed by condensation reaction (condensation and dehydrogenation reaction between aromatic lamellae) in the later stage of pyrolysis, that is, a small number of compounds change from aromatic rings with smaller ring number to aromatic rings with larger ring number. The H2 escape curve is shown in Fig. 3(c). It can be found that in the initial stage of the reaction, different heating rates have little difference in the production of H2. With the increase of heating rate, the corresponding temperature peak of H2 escape also shifts to the high temperature direction, and the gas escape shows a trend of increasing, but the peak temperature range is different under different heating rates.

CO2
The maximum emission peak of CO2 appears between 420 and 450 °C, which is considered to be the fracture of fat bond, part of aromatic weak bond and oxygen-containing carboxyl functional group in coal. A small peak appears at about 600 and 800 °C. The reason is that the HSW coal used in this study removed mineral impurities in the process of extracting vitrinite. As a result, the peak value between 420 and 450 °C becomes higher, the peak area becomes larger, and the peak value between 600 and 800 °C becomes smaller. It can be seen from Fig. 3(d) that the temperature range of CO2 generation from HSW coal is wide at different heating rates and CO2 is generated almost in the whole pyrolysis temperature range. It can be found that with the increase of heating rate, the first peak first decreases and then increases, while the second peak continues to increase with the increase of heating rate. This effect may be due to the high heating rate promoting the pyrolysis of coal molecular structure and the secondary reaction of tar. As a result, the level of pyrolysis gas increases. However, with the increase of heating rate, some pyrolysis reactions may occur at higher temperatures. Therefore, the release rate of pyrolysis gas decreases at low temperature.

CO
It can be seen that a large amount of CO is not produced until the temperature is higher than 350 °C. When the heating rate is 10 °C/min, three peaks of CO production appear at 430, 580 and 710 °C, respectively. It is generally believed that the peak at 430 °C is due to the cleavage of carbonyl group in coal, the peak at 580 °C comes from the decomposition of aliphatic ether structure and aromatic ether structure, and the peak at 710 °C comes from the pyrolysis of phenolic hydroxyl group and some oxygen-containing heterocycles in coal. Fig.   3(e) is the evolution curve of CO. By comparing the ionic strength at different heating rates, it is found that the increase of heating rate increases the initial peak temperature.
Although both carbon dioxide and carbon monoxide are oxidized, it is important that the initial temperature of carbon monoxide is higher than that of carbon dioxide, which indicates that carbon monoxide is produced by the pyrolysis of more stable oxygenated compounds [34] .   The maximum activation energy occurs between 600 and 900 °C, because the multi condensation of coal tar and the re solidification of semi coke will occur in this temperature range. In addition, some gases and liquids may undergo secondary decomposition to promote the formation of hydrogen, such as CH4, which can be decomposed into H2 and related low molecular hydrocarbon gases.

Pyrolysis kinetic analysis based on Coats-Redfern model
According to Guo [35] and other research results, the average activation energy of vitrinite in low rank coal is about 30.00 kJ/mol in the rapid pyrolysis stage, and the activation energy of vitrinite in HSW coal is lower than that of vitrinite in HSW coal.
This may be due to the fact that Vdaf of vitrinite coal sample selected in this study is 37.76%, while HSW coal has higher volatile content, Vdaf of coal is 39.17%.

Pyrolysis kinetic analysis based on DAEM model
The relationship between temperature and conversion rate in vitrinite pyrolysis is shown in Fig. 6 (left), in which the selected value range of coal pyrolysis conversion rate is 0.1-0.9, and the interval is 0.1. With the increase of temperature, the conversion rate increases. It can be found that the conversion rate of coal is higher at lower heating rate when the temperature is the same. This is because the lower heating rate leads to longer time to reach a certain temperature, which is equivalent to increasing the residence time.
Therefore, it is conducive to deepening pyrolysis. Figure 2-4 pyrolysis TG curve also shows this problem, that is, at the same temperature, the heating rate of 5 °C /min will lead to a greater weight loss rate of vitrinite. Wang et al. [36] investigated the pyrolysis activity of SX coal at three heating rates of 20, 30 and 40 °C/min, and also concluded that the coal sample had higher conversion at low heating rate. In order to obtain the kinetic parameters better, the Miura integral method [37]  The faster pyrolysis rate makes the conversion increase faster, so the required activation energy under different conversion rates has little difference, which is consistent with the change trend of TG/DTG curve in Figure 1 [38] . The pyrolysis activation energy of vitrinite increases with the increase of conversion.
With the increase of conversion, the coal pyrolysis process successively goes through drying dehydration, active decomposition, slow polycondensation and other stages. The energy required for the early dehydration of water and adsorption gas and the pyrolysis of functional groups such as weak fatty carboxyl group is less. With the progress of pyrolysis, the carbon skeleton structure of coal macromolecules and aromatic polycyclic groups with high thermal stability begin to decompose. The whole process requires a lot of energy, so the pyrolysis activation energy increases gradually [39] . This is consistent with the conclusion of Coats-Redfern model. By comparing Coats-Redfern model with DAEM model, it can be found that the kinetic parameters calculated by the two models are quite different, which is mainly due to the compensation effect of activation energy and pre-exponential factor. The compensation effect is a common problem in kinetic study, and the kinetic parameters are not unique for the same reaction process. The dynamic parameters of DAEM are optimized by the pattern search algorithm, and the process is random, and has a certain relationship with the selected initial value. Therefore, it is possible that the results obtained by DAEM and method are quite different from Coats-Redfern. It can be seen from table 5 that the correlation coefficient R 2 of DAEM fitting the weight loss curve data of pyrolysis process is higher, which can better reflect the weight loss trend of the curve and fully prove the change of activation energy during coal combustion. Therefore, DAEM is more suitable for the kinetic analysis of coal pyrolysis than Coats-Redfern model.

Conclusion
In this work, Ningdong HSW coal was selected as the sample, and the pyrolysis weight loss behavior and pyrolysis gas escape law of the coal vitrinite were investigated by pyrolysis experiments at different heating rates: (1) The higher the heating rate is, the lower the weight loss rate is, and the temperature corresponding to the maximum weight loss rate moves to higher temperature. With the increase of heating rate, DTG peak area and weight loss rate decrease, which indicates that higher heating rate makes HSW coal unable to fully react in the pyrolysis process, resulting in some structures unable to completely decompose.
(2) With the increase of heating rate, the corresponding temperature peak of five pyrolysis gas escape shifts to the high temperature direction, and the gas escape shows a trend of increasing. (4) The activation energy of vitrinite ranges from 59.85 kJ/mol to 328.24 kJ/mol when the pyrolysis conversion is 0.1-0.9 by using DAEM model. The kinetic parameters calculated by the two models are quite different, which is mainly due to the compensation effect of activation energy and pre-exponential factor. The correlation coefficient R 2 indicades that DAEM is more suitable for the kinetic analysis of coal pyrolysis than Coats-Redfern model.