Study on the Formation Mechanism of the Pyrolysis Products of Lignite at Different Temperatures Based on ReaxFF-MD.

The internal part of coal that is not in contact with oxygen will undergo pyrolysis reaction due to heat conduction, and the active groups generated can reverse-aggravate the degree of coal spontaneous combustion. At present, a few studies have been conducted on the pyrolysis mechanism of coal at different temperatures by using mutually validated experimental and simulation methods, resulting in the mismatch between the microscopic mechanism and macroscopic characteristics. In this paper, DH lignite is taken as the research object, and its macromolecular model is established. The pyrolysis reaction of lignite is studied by the experimental methods of coal pyrolysis index gas collection and detection experimental and thermogravimetric analyses and the simulation method of ReaxFF-MD. The influence of temperature on lignite pyrolysis is explored by analyzing the distribution of products at different temperatures and the formation mechanism of typical products, so as revealing the microscopic mechanism of lignite pyrolysis. The results show that 110-500 K of experimental temperature corresponds to 1400-2400 K of simulation temperature. CO2 and C2H4 are the main gas products during pyrolysis simulation. Carboxyl and ester groups are the main source of CO2, which gradually increases with the rise of temperature. Since CO2 can be reduced to produce CO, H2O, and C2H2O at high temperatures, the yield decreases when the temperature is higher than 2000 K. C2H4 is derived from the decomposition of long-chain aliphatic hydrocarbons, and its yield fluctuation rises with the rise of temperature. The formation of H2O and H2 mainly occurs in the secondary pyrolysis stage. When 1400 K < T < 2100 K, the primary pyrolysis is the main reaction, where the weak bridge bonds and macromolecular structure undergo cleavage to form gas products and tar free radical fragments. When T > 2100 K, the secondary pyrolysis reactions were significant. Tar free radicals and char undergo decomposition, hydrogenation, and polymerization reaction, gas products and tar free radicals increase, and the char yield decreases compared with the primary pyrolysis stage, so 2100 K is the key temperature of the pyrolysis reaction. The research is of great importance in improving the accurate control of coal spontaneous combustion.

Introduction

Coal is the main traditional energy source, which dominates the world’s energy consumption, especially in China.[1] Lignite is a kind of widely used low-rank coal that has large reserves, low cost, high chemical reactivity, and low pollution.[2,3] However, it also has some defects, such as low calorific value, high moisture content, and high spontaneous combustion tendency.[4] The process of coal spontaneous combustion is very complex. For a long time, scholars at home and abroad have carried out a lot of research on the mechanism of coal spontaneous combustion, and the coal oxygen composite theory has been most widely recognized.[5,6] However, the internal part of coal that is not in contact with oxygen will undergo pyrolysis reaction due to heat conduction, and the active groups generated can reverse-aggravate the degree of coal spontaneous combustion.[7−9] Therefore, it is necessary to conduct scientific research based on the inherent mechanism of pyrolysis. The analysis of the pyrolysis chain reaction process on this basis also plays an important role in improving the accurate control of coal spontaneous combustion disaster, which is the key to the safe production of coal mines.

Coal pyrolysis refers to a series of physical and chemical changes that occur when coal is heated under an isolated atmosphere or inert atmosphere. The organic matter in coal can be pyrolyzed to produce char, tar, and gas. The chemical process of coal pyrolysis is very complicated, mainly involving bond breaking, vaporization, and polyreaction. These reactions form three parallel reaction sequences: the thermal decomposition of surface functional groups forms gas; the degradation of macromolecules produces smaller fragments and then leads to evolution into tar; volatile substances and tar are transported to the outside of coal particles through the mass transport process,[10] and the remaining solid product is char. In the previous research on coal pyrolysis behavior, scholars have carried out a large number of experimental studies. Mandapati and Ghodke[11] evaluated the applicability of the commonly used pyrolysis kinetic model of lignite from India through mass loss experiments and gas evolution experiment under different conditions. Xu et al.[12] use a self-researched fixed bed reactor, TG, GC, GC/MS, BET, SEM, FTIR, and chemical means. The morphology and functional structure changes of semichar were investigated. They found the optimum reaction conditions for the formation of tar and pyrolytic gas. Liu et al.[13] carried out a pyrolysis test in a quartz tube reactor. The structural characteristics and thermal weight loss of coal were analyzed by elemental analysis, 13C-NMR, FTIR, and TG. They studied the relationship between structural characteristics and product distribution of lignite and concluded that the formation of pyrolytic products is related to the breaking of chemical bonds and the decomposition of functional groups. Scholars use TG, GC, and MS analysis equipment to analyze and study the weight loss characteristics and gas production law of pyrolysis reaction[14,15] through experimental research methods and use elemental analysis, 13C-NMR, FTIR, and other detection methods to study the structural change characteristics of coal samples in pyrolysis reaction.[16−18] In recent years, on the basis of macro-experiments, scholars have studied the micromechanism of pyrolysis behavior on the molecular scale through molecular dynamic simulation. ReaxFF is a force field[10,19−24] method developed in recent years to describe the activity of breaking bonds, bonding, and chemical reactions. It is an effective method to study complex phenomena such as complex chemical reactions under extreme conditions. It can almost reproduce the accuracy of quantum mechanics in the reaction system, including reactants, transition states, and products. Liu et al.[25] studied the pyrolysis process of perfluorinated ketones at different temperatures (300–5000 K) by using ReaxFF. Salmon et al.,[26] Yan et al.,[27] and Chen et al.[28] used ReaxFF to study the pyrolysis and oxidation of lignite. Xu et al.[29] used the ReaxFF-MD simulation method to study the characteristics of lignite pyrolysis products, main element transformation behavior, and pyrolysis mechanism from the perspective of clean utilization at 1600–3000 K. Zheng et al.[30] used the ReaxFF molecular dynamic method to study the product distribution and initial chemical reaction of Liulin coal during pyrolysis at 1000–2600 K. Gao et al.[31] discussed the dynamic migration mechanism of organic oxygen in the coal pyrolysis process by the ReaxFF molecular dynamic method. A simulation study on molecular scale enhances the reaction temperature to increase the reaction rate, resulting in a larger gap between the simulated temperature and the actual temperature. Liu et al.[32] studied the pyrolysis behavior of polycarbonate that was simulated by ReaxFF, verified by DFT calculation and experiment. At present, a few research studies have been conducted on the pyrolysis mechanism of coal at different temperatures by using mutually validated experimental and simulation methods, resulting in the mismatch between microscopic research and macroscopic characteristics.

In this paper, DH lignite was taken as the research object. The experimental analysis of coal samples was carried out by using experimental equipment such as a tube furnace, gas chromatograph, and synchronous thermal analyzer at 30–900 °C, and the ReaxFF-MD method was used to simulate the pyrolysis reaction at 1300–2400 K and 100 ps on the molecular model. The distribution of pyrolysis product content at different temperatures and times was analyzed by experiment and simulation. According to the decomposition and polymerization processes in the secondary pyrolysis process, the reaction pathways (RPWs) were predicted. In addition, the reaction mechanism of the main gas produced in the pyrolysis process was clarified, and the RPWs of the secondary reaction gas were discussed. The influence mechanism of temperature on the early stage of lignite pyrolysis and the microscopic mechanism of lignite pyrolysis were revealed. This study is helpful to find the key point of the primary pyrolysis reaction sequence of lignite molecules. Taking measures at this point reaction temperature can accurately cut off the chain reaction process of pyrolysis and hinder the further development of coal spontaneous combustion disaster. It is the basis for the development and utilization of a flame retardant and provides theoretical guidance for the accurate prevention and control of coal spontaneous combustion disaster.

Results and Discussion

Comparison between Simulation Results and Experimental Results

Figure a shows the concentration change curve of CO2, CO, and C2H4, which is from DH lignite pyrolysis and the molecular number of C2H4 from ReaxFF-MD. A large amount of CO2 and CO is detected in the temperature range of 40–440 °C, and the concentration is directly proportional to the temperature. C2H4 is produced at 110 °C, and the concentration is also directly proportional to the temperature. In the process of experiments, CO2 and CO can come from desorption and chemical reaction at the same time. Therefore, C2H4 is selected as the reference gas and the corresponding relationship between experiment and simulation temperatures is determined according to the temperature point generated. The temperature of coal pyrolysis experiment is usually less than 1300 K. However, the reaction rate is increased by selecting temperatures higher than normal experimental conditions according to the harmonic transition state theory and the principle of time temperature equivalence.[33,34] This is to observe the chemical reaction process more clearly in the simulation time. The ReaxFF-MD simulation temperature is increased to 1300–2400 K and C2H4 is formed at 1400 K. When it reaches 2400 K, the concentration of C2H4 increases continuously, and the change trend is similar to the experimental value. It can be preliminarily considered that the experimental and simulated temperature ranges correspond well.

Figure 1

Comparison of experiment and simulation data: (a) concentrations of CO2, CO, and C2H4 at 40–400 °C and C2H4 of simulation; (b) TG, DTG, and simulation data.

Figure b shows the comparison diagram of DH lignite pyrolysis TG curve and ReaxFF-MD results. TG shows the change curve of the thermal weight loss of coal sample under a N2 atmosphere of 30–900 °C and DTG reflects the change trend of TG. The TG decreases rapidly below 150 °C, which is mainly due to the evaporation of water and the desorption of adsorbed gas in the coal. When the temperature reaches 300 °C, it is in the active decomposition stage of pyrolysis reaction, and the thermogravimetric phenomenon is obvious. When the temperature is higher than 700 °C, the TG tends to be stable. To more accurately determine the corresponding relationship between simulation and experimental temperature thresholds, ReaxFF-MD simulation is also carried out at 2600, 2800, and 3000 K. It can be seen that the result of ReaxFF-MD at 1400–1800 K is higher than that of the TG, which is mainly due to the physical desorption of adsorbed gas in the drying and degassing stage of pyrolysis reaction, but there is only chemical reaction in the simulation results. In the range of 1800–2200 K, the simulated value is lower than that of experiment. At this time, the pyrolysis is in the decomposition stage and the chemical reaction increases. There are many influencing factors that can lead to the incomplete reaction in the experiment, such as particle size, heating conditions, and so on. When the simulation temperature is above 2300 K, the simulation results are in good agreement with the experimental results. Based on the above analysis, 110–500 °C of experiment corresponds to 1400–2400 K of simulation. Therefore, through the comprehensive comparative analysis of gas chromatography and thermogravimetric data, it can be considered that the ReaxFF-MD method is effective to analyze the reaction mechanism in the early stage of coal pyrolysis.

Pyrolysis Products of Lignite

The ReaxFF-MD pyrolysis simulation of the lignite molecular model can observe the structure change in the ps range and predict the chemical reaction process. The structure change and the crushing process of lignite in the early stage of pyrolysis were studied in this paper, and the evolution of gas, tar, and char was analyzed.

The pyrolysis products are mainly divided into gas, tar, and char.[34] According to the mass distribution of char and tar used in previous CPD models,[35] it can be divided into char with more than 40 carbon atoms, heavy tar with 14–40 carbon atoms, and light tar with 5–13 carbon atoms. In addition, the fragments with less than 4 carbon atoms are considered as gas in this case, divided into organic gas and inorganic gas. Later, the product evolution trend analysis and RPWs analysis of 2300 K are carried out, so 2300 K is also selected for product analysis. It can be seen from Table that there are 28 kinds of pyrolysis products and 68 molecular fragments under the environmental parameters of 2300 K and 100 ps. The mass fractions of inorganic gas and organic gas are 5.41 and 10.30%, respectively, mainly CO2, H2, H2O, and C2H4. The mass fractions of light tar and heavy tar are 5.88 and 38.84%, respectively. The char content is 39.57%. Some representative molecular diagrams of pyrolysis products are listed in Table .

Table 1

Summary of Main Products Obtained from ReaxFF Pyrolysis Simulations of the DH Lignite Model at 2300 K for 100 ps

inorganic gases		C1–C4 compounds		C5–C13 compounds		C14–C40 compounds		C40+ compounds
5	H2	1	CH4	2	C5H5ON	1	C16H23ON	1	C41H32O3
3	H2O	2	CH2O	1	C5H7	1	C24H12O3	1	C69H38O10
1	CO	1	C2H3	1	C6H8	1	C25H28	1	C160H38O13
11	CO2	21	C2H4	1	C7H8O	1	C26H14O5
		2	C2H2O	1	C9H11	1	C33H24O3
		1	C2H6			1	C29H17O
		2	C4H6			1	C34H29O5
			C4H5O2			1	C34H27O4
						1	C39H29O6

Table 2

Examples of Representative Pyrolysis Products Identified in ReaxFF Simulations of the DH Lignite Model at 2300 K for 100 ps

CO2 and C2H4 are the main gas products of DH lignite pyrolysis at 2300 K. Figure shows the generation of typical gas products in the initial stage of pyrolysis at 2300 K. A large amount of CO2 and C2H4 can be produced at the initial stage, which is closely related to the characteristics of lignite with more oxygen-containing functional groups (OCFGs) and a longer alkane chain structure. The main source of CO2 is the decarboxylation of carboxyl and ester groups. With the progress of pyrolysis reaction, the number of CO2 molecules decreases and then continuously increases. The mechanism will be analyzed in Section . The main source of C2H4 is the cleavage of the −C–O– or C=O bond in the side chain of fat and the formation of C=C. H2O and H2 are typical gas products of pyrolysis reaction at 2300 K. The formation of H2O is due to the fracture of −C–O– to form −C–O·, which combines with H· to form −C–OH and then hydrogenates to form H2O. The formation of H2 mainly comes from the H· of R–H and Ar–H.

Figure 2

Species analyses for ReaxFF-MD simulations of the initial stage of pyrolysis at 2300 K.

Effect of Temperature on Pyrolysis Products

Effect of Temperature on Char, Tar, and Gas Content in Pyrolysis

Figure shows the mass fraction distribution of char, tar, and gas products from DH lignite pyrolysis. It can be seen from the figure that no pyrolysis occurred in coal at 1300 K. When the temperature reaches 1400 K, coal molecules have been pyrolyzed to produce a small amount of inorganic gas and organic gas, such as CO2 and C2H4, and no tar. The mass fraction of char is about 98.88%. Heavy tar and light tar appeared at 1700 and 1800 K, respectively. Until the temperature rises to 2100 K, the yield of char decreases, but those of tar and gas increase. It can be considered that the coal is in the primary pyrolysis process, and the weak bridge bond and macromolecular structure in coal form gas and tar free radical fragments. When 2100 K < T < 2400 K, the content of char continues to increase, but the change of tar is on the contrary and the yield of gas changes little. This phenomenon shows that the tar polymerization occurs at this time and the pyrolysis is in the secondary stage.

Figure 3

Distribution of char, tar, and gas products in the pyrolysis of DH lignite at 1300–2400 K.

According to the results of product distribution, 2100 K may be the transition temperature between the primary pyrolysis and the secondary pyrolysis. To determine the turning point more intuitively, 41.12% of 2000 K and 39.57% of 2300 K with a similar char yield are selected to study the complex chemical reaction mechanism of lignite pyrolysis. Figure shows the variation trends of gas, tar, and char with those two temperatures. It can be seen from Figure a that the char at 2000 K shows a step-by-step downward trend, but the trends of tar and gas are on the contrary. However, as shown in Figure b, the char yield rapidly decreased by 90.35% and the decomposition is the main reaction within 60 ps at 2300 K. After 60 ps, the yield of char increased and that of tar decreased, but the yield of gas remained almost unchanged. It shows that polymerization is the main reaction type between char and tar.

Figure 4

Time evolution of pyrolysis products in ReaxFF-MD simulations: (a) 2000 K and (b) 2300 K.

Therefore, the ReaxFF-MD results show that the pyrolysis of DH lignite can be divided into two stages: (1) when 1400 K < T < 2100 K, the coal is in the primary pyrolysis process, and the weak bridge bond and macromolecular structure in the coal decompose to produce small molecular gas and tar free radical fragments; (2) when T > 2100 K, tar free radical fragments and char undergo decomposition, hydrogenation, and polymerization reactions. The small molecule gas and tar free radical fragments increase, and the char yield decreases compared with the primary stage. It is in the second pyrolysis reaction stage.

Effect of Temperature on Char Characteristics

The H/C and O/C of char can reflect the pyrolysis degree of coal to a certain extent.[36] It can be seen from Figure that the H/C and O/C of char from the pyrolysis of DH lignite at different temperatures continue to decrease with the increase in temperature, which is caused by the release of alkanes and the conversion of oxygen in OCFGs (such as carboxyl, ester, etc.) into tar and gas during pyrolysis. When 1400 K < T < 1900 K, H/C is basically unchanged compared with raw coal, while O/C is lower than that. It is indicated that the OCFGs in coal are easier to fracture than aromatic alkanes and aliphatic hydrocarbons. With the increase in simulated temperature, when 1900 K < T < 2100 K, H/C fluctuates and decreases in the range of 0.84–0.88, but O/C remains nearly constant. When T > 2100 K, those two kinds of parameters decrease rapidly, indicating that a high temperature makes char molecules release a large number of alkane chains and decompose stable OCFGs, and increase the aromaticity and polymerization degree. It conforms to the characteristics of the secondary pyrolysis reaction.

Figure 5

H/C and O/C curves of char with temperature.

Effect of Temperature on Typical Gas Distribution

Figure shows the distribution of typical gas in DH lignite pyrolysis, mainly CO2, C2H4, H2O, and H2. Because carboxyl and ester groups are easily decomposed at lower temperatures,[30,37] CO2 is the gas with the highest yield below 1900 K. With the increase in temperature, the yield of CO2 rises in a wavelike manner. When the temperature is above 2200 K, the yield of CO2 decreases, and the mechanism is analyzed in Section . C2H4 is the gas with the highest yield above 2000 K, and its formation is attributed to the fracture of a weak C–C bond in the alkane chain.[29,38] In the simulated temperature range, the yield of C2H4 fluctuates with the increase in temperature. In the pyrolysis process, H· reacts with hydroxyl to generate H2O.[29] Since the coal sample is deacidified and does not contain hydroxyl,[39−41] the output of H2O is low. The H2O generated at 2000 K comes from the rupture of an ether bond (−C–O−) to form hydroxyl. Some ester groups break and combine with H· to generate H2O, which is also related to the decrease in CO2 output at 2400 K. The formation of H2O mainly comes from the secondary pyrolysis stage. H2 is generated at 2100 K, and its production increased rapidly in the secondary pyrolysis stage.

Figure 6

Distribution of typical gas from pyrolysis at 1300–2400 K.

Formation Mechanism of Pyrolysis Products

Formation Mechanism of Char and Tar

Based on the above analysis and ReaxFF-MD pyrolysis simulation trajectory file, it can be judged that the primary pyrolysis process starts with the fracture of an alkyl ether bridge bond, marked as “①” in Figure . Then, the fracture of a C–O bond with a low bond dissociation energy (262.15 kJ/mol[42]) and a few C–C bonds (“②”) follows. Later, the decomposition of carboxyl and ester functional groups (“③”) promotes the formation of CO2 and generates H· (“④”). At the same time, the methylene radical (−·CH2) in the alkane chain decomposes to produce C2H4 (“⑤”). With the progress of the reaction, more C–C bonds break and form tar, char fragments, and gas.

Figure 7

Formation mechanism of the primary pyrolysis.

When T > 2100 K, the reaction is in the secondary pyrolysis stage. The polymerization reaction occurs between tar fragments, free radicals, and char. The new or larger volumes of char molecules and intermediate products are generated. The secondary RPWs of typical heavy tar and char at 2300 K are listed according to the ReaxFF-MD simulation trajectory file, as shown in Figure . P1–P3 is the chain RPW of char molecules during the secondary pyrolysis and the reaction process of decomposing tar molecules and gas small molecular fragments.

Figure 8

Secondary pyrolysis pathway of typical char and tar at 2300 K.

P1 reaction shows that the C=C and C–C bonds of the char molecule C92H77O11 (shown as bond 1-1, bond 1-2, and bond 1-3 in Figure ) are broken to form the methylene radical (−·CH2), heavy tar C26H17O4, and light tar C6H8O, respectively. Subsequently, the methylene radical (−·CH2) condenses with the methyl (−CH3) of the tar molecule of C38H32O3 (bond 1-4), forming new C98H83O9 and H·.

P2 reaction shows that the C–C bond of the heavy tar molecule C26H24O breaks (bond 2-1), C=C is formed, and then C3H4 is formed. The methyl (−CH3) on the benzene ring condenses with the methyl of C98H83O9 of the P1 reaction product to form C–C (bond 2-2), resulting in C115H95O10 and the hydrogen radical H·.

P3 reaction shows a polyreaction between the P1-decomposing product heavy tar C26H17O4, the P2 polyreaction product C115H95O10, and the heavy tar molecule C33H28O. First, the C–O bond (bond 3-1, bond 3-3, and bond 3-4) and C–C bond (bond 3-2) break to form ·C2H3, C9H16, C2H2O, and ·OH. Then, the C=C bond (bond 3-5, bond 3-6, and bond 3-7) breaks off and condensates with the methylene radical (−·CH2) to form the C–C bond (bond 3-8 and bond 3-9) to generate the char molecule C160H120O13 and H·.

P4 gives the RPW of the intermediate C2H2O in P3. It can be seen that the C–O bond (bond 4-1) in the heavy tar C26H17O4 breaks to form a new heavy tar C24H15O3, which is accompanied by the formation of acetylene alcohol (CH≡C—OH). But the acetylene alcohol is unstable and then transforms to more stable vinyl ketone (CH2=C=O) rapidly.

From the above analysis, it can be concluded that the reaction mechanism of a char molecule under the combined action of decomposition reaction and polymerization reaction is as follows: char + tar → char + tar + intermediate products + free radicals.

Formation Mechanism of Typical Gas

Formation of CO2

During the pyrolysis of DH lignite, a large amount of CO2 can be generated at the primary stage. Feng et al.[43] also found that CO2 was generated very early in the pyrolysis, mainly from the decarbonylation reaction of carboxyl and ester groups. The reaction equations are shown in formulas 1 and 2. The simulation in this study also verified the formation mechanism of CO2. Figure shows the process of decomposition of carboxyl and ester groups to form CO2. In Figure a, −COOH loses −H to form −COO·, and then decarbonylation occurs to generate CO2; in Figure b, the ester group first breaks the C–O bond to form −COO·, and then the reaction process is similar to that of the carboxyl group to produce CO2.

Figure 9

Snapshot of typical gas CO2 RPWs. (a) Formation of CO2 from −COOH; (b) formation of CO2 from −COO–.

It can be seen from Figures and 6 that CO2 partially disappears in the secondary pyrolysis stage after the rapid formation of it in the primary pyrolysis stage. Similar phenomena also appear in the articles of Zhao et al.[44] and Liu et al.[32] The continuous action of temperature causes the subsequent chemical reaction, which is the reason for the disappearance of CO2. By analyzing the trajectory file of pyrolysis simulation at 2300 K, CO2 can react with other products, lose an O atom, and convert to CO, as shown in Figure a; H· also participates in the attack and capture of O atoms in CO2, as shown in Figure b, forming carboxyl groups, which are further decomposed into unstable intermediate products acetylene alcohol (CH≡C—OH) and alcohol hydroxyl (−OH) or H2O and carbonyl (—C=O).

Figure 10

Snapshot of typical gas CO2 disappearance pathways. (a) CO2 converts to CO; (b) CO2 forms −COOH and then decomposes into CH≡C—OH and −OH or H2O and —C=O.

Formation of C2H4

As a common indicator gas for coal spontaneous combustion prediction, C2H4 is mainly generated through the decomposition of long-chain aliphatic hydrocarbons.[38] During the pyrolysis of DH lignite, −·CH2 is formed due to the fracture of the C–O bond, followed by decomposition reaction to generate C2H4, as shown in Figure , and the reaction equation is shown in formula 3.

Figure 11

Snapshot of typical gas C2H4 RPW.

Formation of H2O

From the pyrolysis simulation results of DH lignite at different temperatures, it can be seen that H2O appears in the pyrolysis products with T > 2000 K, which mainly belongs to the secondary pyrolysis stage. There are three RPWs of H2O. Figure a shows that −COOH breaks the C–O bond to form ·OH and combines with free H· to form H2O. After the ester group breaks C–O, it becomes −COO· and combines with free H· to form −COOH, which then breaks the C–O bond to form ·OH and combines with H· to form H2O, as shown in Figure b. The above two RPWs are similar to CO2 consumption pathways, which verifies the above view. Figure c shows that the breaking of the −C–O– bond to form phenolic hydroxyl (−OH), combined with H· to generate H2O, is the main RPW of DH lignite pyrolysis to generate H2O.

Figure 12

Snapshot of typical gas H2O RPWs. (a) Formation of H2O from −COOH; (b) formation of H2O from −COO·; (c) formation of H2O from −O–.

Formation of H2

H2 is produced at 2100 K, which belongs to the secondary pyrolysis stage. The generation of H2 comes from free H·. Two sources of H· are listed in Figure , which are R–H and Ar–H. H· interacts to form H2. It conforms to the characteristics of secondary pyrolysis reaction.

Figure 13

Snapshot of typical gas H2 RPWs.

Conclusions

To explore the structural change and crushing process of lignite in the early stage of pyrolysis at different temperatures, DH lignite is used as the research object. The pyrolysis reaction of lignite is studied by the experimental methods of coal pyrolysis index gas collection and detection experimental and TG analyses and the simulation method of ReaxFF-MD. The following conclusions are obtained:

The concentration changes of CO, CO2, and C2H4 in the range of 40–440 °C were determined by coal pyrolysis index gas collection and detection experiment. C2H4 was selected as the reference value and compared with the ReaxFF-MD simulation results. It was determined that 110–500 °C of experiment corresponds to 1400–2400 K of simulation.

The pyrolysis products of lignite can be divided into gas, tar, and char according to the number of C atoms. CO2, C2H4, H2O, and H2 are the typical gas products in pyrolysis of the DH lignite molecules. CO2 and C2H4 are the main gas products, among which the formation is closely related to the structural characteristics of more OCFGs and the alkane chain length of lignite. The formation of tar and char comes from C–O and C–C fracture with a low bond dissociation energy.

Temperature has a great influence on the type and content distribution of pyrolysis product. At 1400 K < T < 2100 K, DH lignite is mainly a primary pyrolysis reaction, which is characterized by the decomposition of weak bridge bonds and macromolecular structure in coal to produce small molecular gas products and tar free radical fragments. The secondary pyrolysis stage is after 2100 K. The tar radical fragments and char undergo decomposition, hydrogenation, and polymerization, the small molecular gas and tar radical fragments increase, and the char yield decreases compared with the primary pyrolysis stage.

The OCFGs in coal are easier to break than aromatic alkanes and aliphatic hydrocarbons. The change trend of H/C and O/C of char conforms to the reaction type law of the primary pyrolysis and secondary pyrolysis. The change trend of O/C shows that a high temperature decomposes the stable OCFGs.

CO2, C2H4, H2O, and H2 are typical gas products. Among them, the output of CO2 increases with the increase in temperature and decreases when the temperature is higher than 2200 K. C2H4 production keeps growing with the temperature increase. H2O and H2 appear at 2000 and 2100 K, which are mainly in the secondary pyrolysis stage.

In the second pyrolysis stage, the reaction mechanism of char molecules under the combined action of decomposition reaction and polyreaction is as follows: char + tar → char + tar + intermediate products + free radicals. Carboxyl and ester groups are the main sources of CO2 generation. CO2 disappears at a high temperature to form CO, H2O, and C2H2O. C2H4 comes from the decomposition of long-chain aliphatic hydrocarbons. The RPWs of H2O include carboxyl, ester, and phenolic hydroxyl formed by −C–O– bond breaking, which confirm the phenomenon of CO2 disappearance. The formation of H2 mainly comes from the H· of R–H and Ar–H.

This study is conducive to find the key points of the early pyrolysis reaction sequence of lignite molecules, accurately cut off its chain reaction process, and hinder the further development of coal spontaneous combustion disaster. Even more, it can provide theoretical guidance for the accurate prevention and control of coal spontaneous combustion disaster.

Experimental and Simulation Methods

Experimental Methods

Sample Preparation

DH lignite was selected as the research object, and large fresh coal samples were crushed and sieved to prepare two kinds of samples to be tested: (1) One hundred fifty grams of coal particles with five particle sizes of 1.25–1.6, 1.6–2, 2–3.5, 3.5–5, and 5–7 mm was screened and fully mixed into 150 g of coal samples to be tested, which were evenly divided into three groups for temperature-programmed and gas chromatography experiment. (2) Thirty milligrams of 60–80 mesh pulverized sample was screened for thermogravimetric test.

Experimental Details

Coal Pyrolysis Index Gas Collection and Detection Experiment

The experimental system is shown in Figure , including the gas supply system, gas generation system, and gas detection system. The gas supply system includes high-pressure gas cylinders for storing N2, H2, and automatic air source, which are supplied through the pyrolysis reaction process of a tube furnace and the detection work of a gas chromatograph. The gas generation system is mainly for the pyrolysis reaction of coal samples in the tube furnace and collects the pyrolysis gas. Fifty grams of particle coal sample was placed in an adiabatic reactor, and the N2 flow rate of the temperature programmed instrument was set at 60 mL/min. The temperature was kept at 30 °C for 30 min; after that, the furnace temperature was raised to 440 °C at 3 °C/min. The coal temperature was monitored in real time during the heating process. At the beginning of 30 °C, pyrolysis gas was collected into the gas collection bag every 20 °C until the coal temperature rose to 440 °C. The gas detection system was used to analyze the collected pyrolysis gas, and the gas analysis was carried out by a gas chromatograph, which was supplied with N2, H2, and air. The sampling time and the type and concentration data of index gas were recorded.

Figure 14

Coal pyrolysis index gas collection and detection experimental system.

Thermogravimetric Analysis

Thermogravimetric analysis (TG) was used to study the mass loss of coal during heating up. The thermogravimetric experiment was carried out in a N2 gas environment by using a synchronous thermal analysis instrument. Ten milligrams of pulverized coal samples was put into the crucible sample chamber in each group of experiments, the heating rates was set at 2 K/min, and the N2 flow rate was 60 mL/min in the range of 30–900 °C. According to the change of sample quality with temperature during heating up, the TG and DTG curve can be obtained.

Simulation Methods

Molecular Structure Model of Lignite

The organic compounds in lignite are composed of various aromatic units with alkyl side chains and OCFGs such as hydroxyl, methoxy, carbonyl, and carboxyl groups,[45−48] which are usually connected by oxygen bridges. In this paper, the macromolecular structure model of lignite constructed by our team was used for pyrolysis research,[39−41] as shown in Figure . The chemical formula of a single coal molecule in the model is C215H191O30N, which contains 24 OCFGs, including 2 carbonyls (—C=O), 16 ether bonds (−O−), 1 carboxyl (−COOH), and 5 esters (—C(=O)—O—). The unit cell size is 24.2 Å × 24.2 Å × 24.2 Å and the density of the system is 1.15 g/cm3.

Figure 15

(a) Macromolecular structure plan of lignite, (b) structural optimization model, and (c) macromolecular structure model of lignite after building the cell structure.

Simulation Parameter Setting

The molecular dynamic (MD) method was applied to simulate the pyrolysis process of the lignite molecular model at different temperatures, and ReaxFF was selected to keep the number of particles, volume, and temperature constant (NVT). The temperature was controlled from 300 K to 1300 K by a Berendsen thermostat with 0.1 ps damping constant[49] and a heating rate of 100 K/ps. It is worth noting that the simulation was carried out after the system was balanced for 20 ps. The total simulation time is 100 ps, the time step is 0.1 fs, and one frame is the output every 100 steps. To evaluate the relationship between the pyrolysis degree and the temperature, 11 temperature points were selected for simulation at 1300–2400 K with an interval of 100 K.[24]

To determine all possible RPWs in ReaxFF-MD simulation, each reaction system was simulated several times, and the trajectories of each atom (every 0.1 fs) were analyzed to find the corresponding relationship between the temperature and the number of fragments produced by pyrolysis and the molecular species. The intermediate configuration was deduced to obtain the most favorable RPWs.[50,51]