Abstract
Coal oxidation leads to coal fires prone to coal geohazard accidents. However, studies on carbon oxide gases being indicator gases in predicting spontaneous combustion of coal remain in the initial stage. To better investigate the stage characteristics of generating carbon oxide gases in the low-temperature oxidation process of coal, this study adopts STA-FTIR to experimentally determine the coal weight variation and exothermic condition and the generation law during the low-temperature oxidation process of coal (30–350°C). And, by employing the DFT method, the study determines the chemical reaction and activation energy generated by carbon oxide gases in coal molecules and ascertains the reaction characteristics at different temperatures. The research results indicate that the low-temperature oxidation phase of coal is divided into four main phases, namely, the physical-chemical adsorption phase, the desorption phase, the dynamic equilibrium phase, and the oxidative adsorption phase, each of which has different characteristics for the production of carbon-oxygen compounds, which is important for the prediction of coal spontaneous combustion and the prevention of coal geological disasters.
1. Introduction
In the process of coal extraction, transportation, and utilization, ever-deteriorating coal mine safety is being highlighted day by day, while is the major contributing factor in threatening work safety of coal mines [1, 2]. Coalfield fires caused by the spontaneous combustion of coal occur from time to time. When spontaneous combustion of coal occurs, the coal becomes loose and soft, which can easily cause geological damage, such as the coalfield fire areas in the large division of Xinjiang, where geological collapse and other disasters occur from time to time. In the spontaneous combustion process of coal, a lot of gases will be released, such as sulfur compounds (SO2, SO3, etc.), nitrogen oxides (NO, NO2, etc.) [3], oxycarbide (CO, CO2), and hydrocarbon (CH4, C2H6, etc.) [4]. Generally, carbon oxide serves as an indicator gas to predict the spontaneous combustion of coal [5]. Therefore, it is important to study the characteristics of low-order coal carbon and oxygen generation during the low-temperature oxidation stage to provide timely warning of the coal spontaneous combustion process and to prevent and forecast coal geohazards.
The low-temperature oxidation process of coal is accompanied by a series its variation. Among them, the release of gaseous products is the typical macroscopic characteristics during the oxidation process of coal [4, 6–8]. A study on the generation path of gaseous products of coal is of significance to reveal the spontaneous combustion of coal from oxidation both at the macro- and microlevel [9]. Based on the modern coal oxidation theory, coal oxidation reaction complies with double parallel reaction sequence, namely, chemical adsorption reaction sequence and direct burn-off reaction sequence [10–12]. When coal contacts with oxygen, an oxidation reaction occurs in the active groups of coal, producing unstable peroxides or hydroperoxide. Following a multistep reaction, ultimately gaseous products such as CO and CO2 are released [13]. By investigating the adsorption and desorption characteristics of gases during the coal oxidation process, it is found that, at the low-temperature oxidation stage, the initial temperature and rate of generating gaseous products in coal are different. More amount of CO2, CO, and CH4 is released [14], while less H2 and C2H4 is released. And, based on the variation law of gaseous products, the low-temperature oxidation process of coal is divided into three stages, oxygen-adsorption heat storage, autothermal oxidation, and accelerated oxidation [15–17]. The principal components of coal-adsorbed gases are CH4 and CO2. Some types of coal will adsorb little alkane gases, which will be gradually desorbed as coal temperature rises [18–21]. Gases from coal oxidation and decomposition are gaseous products generated by the oxidation and thermal decomposition reaction of carbon in coal, of which the main components include CO, CO2, CH4, C2H6, C2H4, C2H2, and H2. It can be found that CO2 and CH4 belong to coal-adsorbed gas [22–24] and also belong to coal oxidized/decomposed gases. This indicates the source of CO2 and CH4 cannot be fully ascertained. Meanwhile, CO2 and CH4 exist naturally in coal mines, which can be detected at a constant temperature. So, neither CO2 nor CH4 cannot serve as a single prediction indicator [25]. In addition, from the gas release amount point of view, the main gases released from coal are CO and CO2 [26], and the second ones are CH4, C2H4, H2, etc. [27]. Gas release amount is also a factor in the process of optimally selecting prediction indicators.
In the area of early warning of coal spontaneous combustion, Morrison applied the Semenov model to coal spontaneous combustion and used it to predict the critical temperature of coal. Jones et al. obtained kinetic parameters related to the oxidation process of coal based on the exothermic rate of coal during the warming process of the spontaneous combustion procedure and based on the transient method and proposed the method of heat release rate to determine the critical self-heating temperature of coal spontaneous combustion. Zhu et al. used TG analysis experimental tests to obtain nine characteristic temperature points for five coal samples in the low-temperature oxidation stage and the high-temperature spontaneous combustion stage through a volumetric resistivity temperature model of the coal samples and a complex medium thermostat model. Jian et al. used thermal analysis to obtain different characteristic temperatures through different heating rates and different oxygen concentration experimental tests. Deng et al. used elemental analysis, Fourier transforms spectroscopy and DTA-TG to investigate the mass, microscopic properties, characteristic temperature, and apparent activation energy of secondary oxide coals, which are prone to fire under the same conditions as the four first oxide coal samples. It can be seen that most of the above scholars have studied the natural processes of coal through TG experiments, FTIR experiments, and STA analysis.
In summary, the scholars had experimentally researched the release of gases in coal in the low-temperature oxidation process, but they failed in the detailed investigation on the release characteristics of carbon oxides, in particular, on the generation characteristics and mechanism of carbon oxides in coal at different stages of coal low-temperature oxidation. Hence, by STA-FTIR experiment, this study obtains the characteristic curve, dynamics characteristics, and coal-adsorbed oxygen amount at the stages of coal low-temperature oxidation and analyzes the release characteristics of carbon oxide. Based on this, the study adopts the DFT method and figures out the activation energy and enthalpy change in the key reaction steps of generating CO and CO2 in coal molecules and thus determines the type of reaction stage. The above research has led to the characterisation of the release of carbon and oxygen compounds at various stages during the low-temperature oxidation of coal, which will enable the prediction and early warning of coal spontaneous combustion and contribute to the prevention and prediction of coal geological hazards.
2. Coal Samples and Research Method
2.1. Selection of Coal Samples
This study selects samples at four different grades, which come from Hami and Wudong in Xinjiang, Chifeng in Inner Mongolia, and Yulin in Shaanxi, with a numbering of HM, WD, CF, and YL. The samples are grounded and sieved to the particles at a diameter of 0.15 mm. Then, the samples are stored in a dry and low-temperature environment for the following use. Table 1 lists the industrial analysis and element analysis of each of the samples.
2.2. STA-FTIR Equipment and Thermodynamic Calculation
The experiment employed STA-499-F3 simultaneous analyzer of German NETZSCH Co. Ltd. and TENSOR27 Fourier transform infrared spectrometer (FT-IR) of German BRUKER Co. Ltd., comprising a connection system, as shown in Figure 1. STA is connected to FTIR via a constant-temperature tube. The temperature in the constant-temperature tube and FTIR gas chamber is maintained at 200°C to prevent gas condensation, thus ensuring the accuracy of FTIR measuring results.
For the experiment, a 10 mg coal sample with a particle diameter of 0.15 mm is sieved and deposited in an Al2O3 ceramic crucible. The reaction gas is set at a pure oxygen atmosphere, in a flow of 50 mL/min. To ensure the stabilization of the temperature of the experimental system, after lasting 10 min at 30°C constant temperature, tests are separately made in the temperature from 30°C to 350°C at 5°C/min temperature-rise rate. Before each group of the experiment is conducted, an empty crucible is used for a separate blank test, for baseline correction. In thermal analysis, the gas products of coal are detected in real time by infrared spectrum spectroscopy. FTIR scans in the range of 400∼4000 cm−1, at a resolution of 4 cm−1. In each test, the sample is scanned for 32 s. After the completion of scanning, the results are synthesized into a spectrogram.
Under heating conditions, a carbon gasification reaction occurs between coal and oxygen, producing oxides and gases. The predecessors have conducted a lot of research on oxidation dynamics, achieving different methods for solving activation energy E and pre-exponential factor A. This study combines the Coats–Redfern integral method for calculation of oxidation dynamics [28] and adopts the Bagchi method is inferring reaction mechanism function.
Coats–Redfern integral method:where A is pre-exponential factor, min−1, E is apparent activation energy, kJ/mol, R is molar gas constant, 8.314 J/(mol·K)−1, β is constant temperature-rise rate, K/min, T0 is initial weight-loss temperature, K, ɑ is coal conversion rate at time T, , is the mass before weight loss, mg, and is the mass at time t, mg.
At constant temperature-rise rate, Coast–Redfern integral method is employed to obtain integration formula (1), where In [G(ɑ)/T2] constitutes straight line relationship with 1/T. The calculation results are fitted into a straight line, and by the slope −E/R and intercept In (AR/βE) of the line, activation energy E and pre-exponential factor A are solved.
2.3. DFT Calculation Method
In this study, quantum chemistry calculation based on density functional theory (DFT) in Gaussian 16A and Gauss view 6.0 software carries out structural optimization for each target system and performs calculation of resonance frequency, reactants, transition state, and product energy. The B3LYP method based on hybrid density functional theory is used to describe electronic interchange and its relevant functions. 6−31 G(d, p) base set level optimizes the geometrical structure of molecules. TS (Berny) is used to seek transition state. LQA algorithm-based intrinsic reaction coordinate (IRC) method is adopted to analyze reaction path. It is worth noting that calculation for the groups (free radical and oxygen) of which spin multiplicity is not 1 is performed in open shell environment.
Calculation of thermodynamic and kinetic parameters obtained from the DFT method is performed by the following method.
Thermodynamic parameter mainly includes enthalpy (H). At constant temperature and constant pressure, enthalpy change of chemical reaction (ΔHθ) is the difference between the enthalpy value of reactant and product. When ΔHθ ≥ 0, it is endothermic reaction, otherwise it is exothermic reaction.
Kinetic parameter is activation energy (ΔE), which is the difference between the energy of transition state and reactant. When activation energy ΔE < 40 kJ/mol, it suggests that reaction can be achieved at constant temperature. When ΔE ranges from 40 kJ/mol to 100 kJ/mol, it indicates that reaction can occur at 30°C to 70°C. When ΔE > 100 kJ/mol, it demonstrates that reaction can appear only at above 70°C [29].
3. Results and Discussion
3.1. Characteristic Curve of Coal at Low-Temperature Oxidation Stage
The low-temperature oxidation of coal, early on inferred from mass and temperature, changes in the phase change pattern of the coal warming and oxidation to the combustion process is important for the analysis of the kinetic and thermodynamic characteristics of spontaneous coal combustion, where the TG curve can be combined with the carbohydrate FTIR curve to analyze the stage.
Figure 2 shows TG curves and stage division of four kinds of low-rank coal. Analysis suggests that when temperature-rise rate remains unchanged, variation of coal’s characteristic temperature point shows certain law. Such points on TG curves include critical temperature T1, cleavage temperature T2, active temperature T3, rate-rise temperature T4, maximal mass temperature point T5, ignition temperature T6, maximal heat weight-loss rate temperature T7, and burnout temperature T8.
According to characteristic temperature point, the oxidation process of coal is divided into five stages: desorption stage (T0∼T2), dynamic equilibrium stage (T2∼T3), oxidation stage (T3∼T6), burn-off stage (T6∼T8), and burnout stage (>T8). Among them, the oxidation stage is further divided into the oxidation adsorption stage (T3∼T5) and the pyrolysis stage (T5∼T6). The characteristic temperature point and mass change quantity corresponding to each stage are listed in Table 2. This study focuses on analyzing the low-temperature oxidation stage at below 300°C. Hence, the study mainly analyzes the characteristics of three stages: desorption stage, dynamic equilibrium stage, and oxidation stage.
It is noted from the experimental results that, at different temperature-rise rates, there is little difference among the characteristic temperature points (T1∼T4) for the same coal samples at desorption stage, dynamic equilibrium stage, and oxidation stage. The mass change in such range is mainly related to the moisture desorption and coal oxidation in the samples.
It is seen from Table 2 that the characteristic temperature value of the four low-rank coal samples are roughly lignite < weak-caking coal < jet coal < gas coal. This is because that, in the process of burn-off of the samples, the more metamorphic degree of the sample is, the more heat is needed for burn-off, and the longer time lasts for pyrolysis. Meanwhile, they are also affected by moisture content, sulfur content, and volatile matter. Hence, the characteristic temperature point of the coal sample will shift to higher temperature.
The differential scanning calorimetry (DSC) experiment detects changes in the heat flow signal of a sample during combustion. Selecting the peak area in the characteristic temperature range of the sample DSC curve for integration is sufficient to calculate the heat given off per unit mass of sample during the thermal decomposition phase, i.e., the enthalpy. Figure 3 shows DSC curves and stage division of four kinds of low-rank coal. Downward represents heat release direction. The peak value of DSC curve registers maximal heat release rate. At different temperature-rise rates, maximal heat release rate and corresponding temperature of each sample are listed in Table 3. From the experimental results, it is known that the more metamorphic degree of the four low-rank coal is, the smaller maximal heat release rate DSCmax is, the higher the corresponding TDSCmax is, and the higher the enthalpy value of the coal is.
3.2. Dynamics Characteristics and Coal Oxygen-Adsorbing Amount of Coal at Low-Temperature Oxidation Stage
Coal gasification reaction order selected in this study is n = 2. Solutions of the dynamics parameters of the four kinds of coal sample at the three stages (moisture evaporation, oxygen-adsorption weight increase, and pyrolysis) in the process of temperature rise at different constant rates are obtained according to formula (1). The results for dynamics parameters of oxidation reaction of each sample, calculated at different temperature-rise rates and at different stages, are listed in Table 4. The results show that, with increase of temperature, the values of both activation energy E and pre-exponential factor A of coal at different oxidation stages tend to rise. As temperature-rise rate rises, the values gradually increase. An analysis of the metamorphic degree of the samples indicate that, with increase of the degree, E and A values of the samples at different stages tend to increase. This states that the low-rank coal with less metamorphic degree consumes less energy in its burn-off, and burn-off reaction is easier to occur.
On the TG curve, the added amount resulting from oxygen-adsorption weight adding peak at T3∼T5 is abbreviated as the oxygen-adsorption amount of coal. In order to study the oxygen-adsorption capacity of low-rank coal in a pure oxygen atmosphere, an investigation is conducted for the oxygen-adsorption amount of different coal samples at different temperature-rise rates, of which the results are listed in Figure 4. It is known from the experimental results that, at a pure oxygen atmosphere, the oxygen-adsorption amount of the four low-rank coal decreases with the increase of temperature-rise rate. This is possible because the lower temperature-rise rate and longer temperature-rise duration lead to the full combination of coal and oxygen and subsequent more oxygen-adsorption amount of coal. It is also observed that, in a pure oxygen atmosphere, weakly caking coal and lignite will lose their weight at the very beginning of temperature rise. Hence, the oxygen-adsorption weight adding peak is negative. This is possible because high oxygen concentration, lower metamorphic degree, more volatile matter of the samples, more alkyl side chains within coal molecule, and less benzene ring content cause a chemical reaction of alkyl side chains to occur by continuously adsorbing oxygen at low-temperature stages. Therefore, weakly caking coal and lignite samples lose their weight continuously in the temperature-rise process.
3.3. Infrared Spectrum Analysis of Gaseous Products
FTIR system will, according to the vibration change of different chemical bonds, analyze in real time the gases escaping in the entire temperature-rise process and forms its unique infrared spectrum. The spectrum can only perform qualitative analysis but not quantitative one. Figures 5 and 6 separately illustrate the results for CO and CO2 gas of the samples in the temperature-rise process. In this case, in order to observe the infrared characteristic peak intensity of CO and CO2 between 30 and 200°C, we zoomed in on the curves between these temperatures. Due to the large fluctuations in gas content between these temperatures, the data were processed for the characterisation of the generation carbon oxides release. It is noted from Figure 5 that, in the temperature-rise process, coal releases less gas amount at below 200°C, releases much more gas at 200°C–300°C, and releases gases of stable amount at above 300°C. If we scale up the curves of the gases at below 200°C, we can find further that, at below 70°C, the release amount of CO is less, the amount slowly rises at 70°C–120°C, and the amount is accelerated at above 120°C.
Figure 6 shows that, on the whole, the tendency of CO2 release amount of all the samples is close to that of CO. However, at below 200°C, difference exists between the CO2 infrared characteristic peak intensity of YL and CF samples and that of CO. CF sample releases less gases at below 150°C and more gases at above 150°C, while YL sample releases less gases at below 70°C, begins to release gases at above 70°C, and smoothes at above 150°C. Hence, CF sample, being lignite, mainly generates CO at the earlier spontaneous combustion of coal; YL sample, being weak-caking coal with higher development degree, releases less CO2 amount at the earlier spontaneous combustion of coal.
On the whole, CO remains unchanged at below 200°C. This is because, at the early stage of moisture evaporation and oxygen-adsorption weight adding, physisorption and chemisorption on coal molecule surface hinder the generation and release of CO at the earlier stage, resulting in less CO escaping amount. At 250°C, pyrolysis stage occurs, active groups in coal begin to participate in reaction, and CO output begins to increase. Up to 325°C, a stable stage is produced. And coal-oxygen chemical reaction arrives at an equilibrium state. Likewise, the coal samples release a little CO2 at the earlier desorption and oxidation, and up to 250°C, the pyrolysis stage begins and a lot of CO2 is generated.
3.4. Analysis of the Generation Process of Carbon Oxide
It is known from coal oxidation theory that oxygen-adsorption weight adding stage of coal is formed by superposing multiple parallel reactions such as formulas (2) and (3). These include dewatering reaction at dewatering stage, oxygen-adsorption reaction at oxygen-adsorption weight adding stage, and burn-off reaction at burn-off and pyrolysis stage. The best approach is to consider separately the parallel reactions.
Direct burn-off reaction:
Oxygen-adsorption and solid-state oxidant decomposition reaction:
Chemical reactions of generating CO and CO2 in coal are shown in Figure 7.
As can be seen in Figure 7, the production of CO and CO2 from coal in general requires the best efforts of the following four processes.(1)Oxygen chemisorption from coal: the seizure of an H atom from an aliphatic hydrocarbon in coal produces a chemical vacancy –CH2– in which oxygen is adsorbed before forming a peroxide compound (C–O–O·)(2)Formation of carbonyl radicals: subsequent formation of carbonyl radicals (C = O) after the oxygen atom therein has taken over the surrounding hydrogen atom(3)CO generation: carbonyl radicals decompose methyl radicals (C) and CO when there is insufficient oxygen or at low temperatures(4)CO2 generation: when oxygen is insufficient or at high temperatures, the carbonyl radical adsorbs oxygen to form a carboxyl radical (O = C = O), which decomposes into a methyl radical (C) and CO2
Here, we select three key reactions to analyze the key steps of generating CO and CO2 gas resulting from coal molecule oxidation and its activation energy and enthalpy, as shown in Table 5. It can be noted from Table 5 that the activation energy needed by oxygen molecule to seize ambient hydrogen atoms to generate water molecule and carbonyl free radical is 62.33 kJ/mol, suggesting that such reaction occurs at 30°C–70°C, while all subsequent reactions take place following this step. Thus, this step affects the entire process of generating CO and CO2 from coal molecule. In subsequent reactions, activation energy, needed by carbonyl-free radical to generate methyl-free radical and CO, is 34.25 kJ/mol, suggesting that this easily occurs at room temperature. So, we can find at low temperature, mainly CO gas is produced. Activation energy, needed by carboxyl-free radical to produce CO2 gas, is 78.11 kJ/mol, indicating that only at higher temperature can CO2 be generated, and recombined oxygen is necessary in the environment.
3.5. Stage Characteristics of Carbon Oxide at Low-Temperature Oxidation of Coal
By summarizing the experimental and simulation results above, we come up with the characteristics of generating carbon oxide at low-temperature oxidation stages of coal, as shown in Figure 8. We divide low-temperature oxidation of coal into four steps. Among them, 30°C–70 is the physisorption and chemisorption stage. At this stage, because oxygen is adsorbed physically and chemically on coal, a small amount of CO will be produced, and coal-oxygen-stabilized complex compound is generated and some heat is released. When temperature elevates to 70°C, coal-oxygen chemisorption intensifies while the moisture and gases in coal are desorbed. This stage is known as the desorption stage, at which the output of carbon oxides (CO and CO2) continuously increases but maintains at a tiny size. When temperature elevates to 120°C, a large amount of oxygen is adsorbed onto coal surface, but the rate of generating carbon oxides is slower than oxygen-adsorption rate. Hence, the coal weight maintains at an equilibrium stage, believed to be a dynamic equilibrium stage. However, the amount of carbon oxides continuously increases but at a slow pace. Ultimately, when temperature elevates to 200°C, because high-temperature oxidation adsorption intensifies, chemical reaction rate adds up to oxidation-adsorption stage, at which time a large amount of carbon oxides is produced. In the end, when temperature elevates to 270°C, the coal samples begin to burn, known as burn-off stage.
4. Conclusion
Through the STA-FTIR experiment, this study investigates the characteristic temperature points, heat release parameters, and oxygen-adsorption weight adding amount for different kinds of low-rank coal at the low-temperature oxidation stage and analyzes the carbon oxide gaseous products in their burn-off process. Furthermore, by the DFT method, the study calculates and obtains the activation energy and reaction enthalpy in the process of generating carbon oxides from coal molecules. The following conclusions are drawn.
Based on the STA-FTIR experiment and calculations by the DFT method, the low-temperature oxidation process of coal is divided into four stages: adsorption stage at 30–70°C, desorption stage at 70–120°C, dynamic equilibrium stage at 120–200°C, and oxidation stage at 200–270°C. The generation of carbon oxides at each stage has different characteristics. At the adsorption stage, CO gas is mainly produced; at above 70°C, CO2 gas begins to occur but at a small size; only at above 120°C, a large number of carbon oxides begin to be generated, and this trend quickly intensifies at above 200°C.
Our research further probes into the characteristics of generating CO and CO2 at the low-temperature oxidation process of coal and ascertains the characteristics of generating carbon oxides at each stage. This is of significance for researchers to determine the indicator gases of spontaneous combustion of coal and define the spontaneous combustion stage of coal-based on carbon oxides. Our findings are helpful to prevent and control the spontaneous combustion of coal.
Furthermore, our findings can help to further study the change of molecular structure of coal in its low-temperature oxidation process and chemisorption reactions from other many carbon oxide gases and ultimately to make clear the relationship between carbon oxide gases and low-temperature oxidation of coal.
Data Availability
No data were used to support this study.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was financially supported by National Key R&D Program of China (2021YFC3001300) and the National Natural Science Foundation of China (NSFC), under Grant nos. 52104225 and 51774234.