Study on microstructural changes and characteristics of low temperature oxidation of middle and high rank coal by microwave thermal radiation

2.1. Sample information

SM from Hongyang No. 2 Mine, a subsidiary of Shenyang Coking Coal Co., Ltd., and WYM from Ordos, Inner Mongolia, were selected as experimental samples. The microwave processing time of the two coal samples was 3 mins, and the power was 240W, 400W, 640W, and 800W, respectively. A sufficient amount of coal was put into the hammer crusher to break after natural cooling, and a total of 2000 g with particle size below 0.1 mm was screened by the vibrating screen machine, and mixed into the closed oxygen consumption experiment bottle to prepare for the closed oxygen consumption experiment. At the same time, four coal samples of less than 200 mesh of pulverized coal were screened respectively to prepare for other parts of the experiment, as depicted in Figure 1. The results of the industrial analysis of coal samples have been presented in Table 1. The coal specimens were created following GB474-2008 “Method for the Preparation of Coal Samples.”

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Figure 1.

Flow chart of coal radiated by microwave.

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The industrial analysis was tested at Hubei Sulin Technology Co., LTD. The industrial analysis of coal samples was conducted according to GB/T 212-2008 “Method for Industrial Analysis of Coal”, with the results presented in Table 1.

2.2. Low temperature nitrogen adsorption experiment

The JSM-7800F scanning electron microscope (SEM) (Hitachi, Japan) and the Autosorb-IQ gas adsorption analyzer (Quantachrome, USA) were employed to conduct low-temperature nitrogen adsorption experiments on coal samples at 77 K. All experiments were performed at Hubei Sulin Technology Co., Ltd. The microstructures of coal specimens were examined before and after exposure to microwave radiation at various power levels. The specific surface area and average pore diameter size were calculated via the Brunauer-Emmett-Teller (BET) model, while the volume of pores was determined using the Barrett-Joyner-Halenda (BJH) theoretical model.

2.3. Thermogravimetric analysis experiment

The experiment utilized a TA-Q600-SDT differential scanning calorimetry-thermogravimetric analysis (DSC-TGA) synchronous thermal analyzer by America TA, Instruments of America. The main components of the instrument include a heating furnace, a sensor, a high precision balance, a gas protection system, a programmed temperature control system, a vacuum system, a thermobalance constant temperature system, and an automatic recorder. Experimental process: the atmosphere was 20% O₂+80% N₂. The flow rate was controlled at 100 mL/min. A 10 mg coal specimen was weighed and positioned in the crucible. The rate of heating was adjusted to 10°C/min, with a temperature measurement range from 25°C to 800°C.

2.4. FT-IR experiment

The TENSOR27 Fourier transform infrared (FTIR) spectroscope from Germany Bruker was utilized for the infrared analysis. Experimental parameters: Temperature range was 25∼450°C, airflow velocity was 100 mL/min, atmosphere was 20% O₂+80% N₂, infrared spectrophotometer parameters were 4000cm^-1∼400cm^-1, infrared spectral resolution was 4cm^-1, infrared spectral scanning times were 32, KBr ratio was 1:200. Infrared radiation emitted by the spectrometer is directed onto the coal surface, where bonding atoms within the coal molecules absorb the energy and undergo vibrational transitions. These molecular changes are subsequently captured and visualized in the resulting infrared spectrum. Based on the peak intensities corresponding to distinct functional groups, their relative content was quantified. This allowed for the assessment of variation trends among different functional groups.

2.5. Closed oxygen consumption experiment

The experimental device is composed of an industrial computer, a coal sample tank, a thermostat, a closed circulation air pump, a gas concentration detector, a data acquisition module, a thermocouple, and a K-type contact thermometer. The connection of pipelines in the device has been depicted in Figure 2, where the pressure in the coal sample tank is standard atmospheric pressure. The relationship between oxygen consumption rate and oxygen volume fraction of coal under constant temperature condition can be obtained by the sealed oxygen consumption test. The coal sample tank is a metal tank with certain heat insulation and insulation, with a volume of 3L. The incubator, set to a constant temperature, houses the coal sample tank.

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Figure 2.

Diagram of the closed oxygen consumption experimental setup.

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Experimental preparation: The screened coal samples were broken down into 10 groups, for every group holding 3L. At 25°C, the raw coal, 240W, 400W, 640W, and 800W of the two coal samples were respectively divided into 10 groups for constant-heat testing. Coal specimens for experimentation were placed into the tank and preheated in the constant-temperature box. The preheating degree of the coal sample was measured by the contact thermometer. The coal specimens tank was sealed to ensure the air-tightness of the coal sample tank was linked to the experimental flow apparatus. Before the start of the experiment, the valve of the absorption tube should be opened first, and the two-way valve should be turned to the direction of the washing pipe, and the air pump should be started to wash the coal sample with air in a short time to remove the impact of other gases. When the O₂ volume fraction in the experimental system matches that in the air, the experimental pipeline should be re-closed, the valve of the absorption bottle should be closed, and the two-way valve should be turned to the experimental direction. Adjust the gas flow to 30 mL/min, perform the constant-temperature closed oxygen consumption test on coal samples under a steady airflow, run the thermostat until it is stabilized at the set temperature of 25°C, and start the industrial computer to open the data acquisition software to record the volume fraction value of O₂ and CO. The temperature control system adjusts the coal body temperature during the experiment. Stop the experiment when a change in the data curve becomes stable.

2.6. Oxidative heat release intensity

Figure 2 illustrates that the oxygen concentration $\text{c}\left(\tau \right)$ within the sealed tank, roughly follows a negative exponential function distribution, i.e.,

Where, $c_0$ is the starting volume fraction of oxygen, %; ${\lambda }_c$ is the rate of reduction in the oxygen volume fraction, s^-1; $c_b$ is the value of the oxygen volume fraction at the end of the experiment, %; $\tau$ is the experiment time, s.

The logarithm of Eq. (1) is taken to obtain an equation that can be easily used as a linear regression, and ${\lambda }_c$ is determined by regression analysis (Eq. 2).

Change in oxygen consumption, molar mass of the coal sample within the tank

Where, $\gamma$ corresponds to the coal sample's oxygen consumption volume rate in the closed oxygen consumption test, mol(cm³s)^-1

Substitute Eq. (1) into Eq. (3) to get $\gamma$ for oxygen volume fraction $\text{c}\left(\text{τ}\right)$

Formula (Eq. 4) demonstrates that the oxygen consumption rate in the tank containing the coal sample is linearly related to the oxygen volume fraction. This can be extended to infer that for coal in a loose packing state, the oxygen uptake rate is directly related to the fractional volume of oxygen.

Similarly, the distribution model of CO volume fraction during the steady-temperature sealed oxygen consumption study for coal is (Eq. 5)

There is a linear relationship between the CO generation rate and the speed of oxygen consumption. However, the CO generation rate by oxygen consumption of coal in goaf does not depend on the concentration of CO in the environment, but related to the oxygen concentration in the environment. Therefore, the connection between the rate of CO production and the CO concentration cannot be calculated without the pace of oxygen uptake. Instead, it is necessary to derive the link between the CO production rate and the concentration amount of ambient oxygen. The link between the CO production rate and the concentration amount of oxygen is as follows:

The volume formation rate of CO is Eq. (6).

Bring formula (2) into formula (5)

In Eq. (7), where, $\psi$ is the CO generation rate mol/(cm³·s); ${\phi }_{\left(\tau \right)}$ is the CO volume fraction at $\text{τ}$ time of the experiment, and the molar volume of 10-6 gas is 22400 mL/mol; $\mu$ is the CO volume fraction growth coefficient, min^-1; $A$ is the coefficient of regression that signifies the concluding CO volume fraction, 10^-6; B is the data deviation, which can be approximately regarded as the maximum CO volume fraction released by oxidation process of the coal specimens within the closed oxygen consumption channel, 10^-6.

It is assumed that the oxygen participating in coal oxidation reaction except CO is chemisorption, and the bond energy balance method [15-17] can be used to estimate the heat generation intensity of coal oxidation without considering other intermediate reactions during coal oxidation reaction (Eq. 8):

Where, $Q$ is the measure of heat release intensity in coal oxidation, J/(cm³_·s); $\Delta h_1$ is the heat released through chemical adsorption during the oxidation of coal, taking 58800 J/mol; $\Delta h_2$ is the heat generated during the formation of CO, 110540 J/mol.

3.1. Spontaneous combustion characteristic temperature of coal

To explore the impact of microwave exposure on the characteristic point temperature of coal, coal samples were analyzed using TG-DSC simultaneous thermal analysis under different microwave radiation power levels. The thermogravimetric analysis (TG) and DTG profiles for the coal specimens are depicted in Figure 5. During the oxidative self-ignition process, various structures of coal molecules engage in reactions with oxygen at specific temperatures [19]. Macroscopically, it appears as a change in the rate of weight loss of the sample. This temperature point is referred to as the temperature characteristic of the coal oxidation process. Therefore, the characteristic temperature points of oxidizing combustion of coal specimens can be determined through mass changes, these include the decomposition temperature (T1), peak mass temperature (T2), burning temperature (T3), maximum weight loss rate temperature (T4), and exhaustion temperature (T5).

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Figure 5.

(a-c) TG-DTG curve.

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As shown in Table 3, with the continuous improvement of microwave radiation power, T1 of SM advances 1.44 ∼ 18.53 °C, T2 advances 2.28 ∼ 18.26°C, T3 advances 0.77 ∼ 8.46°C, T4 advances 7.11 ∼ 10.81°C, and T5 advances 1.09 ∼ 6.47°C. T1 of WYM advanced by 0.8-9.94 °C, T2 advanced by 3.1-13.8 °C, T3 advanced by 5.26-15.59 °C, T4 advanced by 14.9-28.98 °C and T5 advanced by 5.42-24.35 °C. Relative to raw coal, the characteristic temperature of oxidation combustion in microwave-radiated coal is reduced, and microwave radiation coal shows a stronger tendency of spontaneous combustion. Because microwave radiation can destroy the original structure of coal, the cracks expand and increase after coal drying, the coal-oxygen contact area increases, and the reaction rate accelerates.

3.2. Activation energy

Activation energy refers to the minimal energy required by a reactive molecule to convert into an active molecule during a chemical reaction. The lower activation energy of the reaction, the faster the reaction will proceed. Based on the TG curve, the process of coal can be separated into four phases. The temperature boundary points of the divided stages are successively: the starting temperature point, the transition point from the conclusion of water loss to the onset of weight gain, and the transition point from the end of weight gain to the start of combustion weight loss, the point at which the mass of the end of combustion begins to remain unchanged, and the end temperature point. The corresponding activation energies are calculated as the activation energy for dehydration, ignition, and combustion [20-22].

Activation energy was calculated by the Coats-Redfern method. Following Eqs. (9) and (10) [23,24]:

Where: α is the rate of conversion; T is the temperature of the coal, K; β is the temperature increase rate; A indicates the pre-exponential factor, min^-1; E is the energy required for activation, J·mol^-1; R is the constant for molar gases; n is the reaction's order value; m₀ is the mass of coal at the beginning, g; m_b is the amount of coal present at time point b, g [25].

In Eq. (11), calculate the value of $\ln \left(\frac{dQ}{dT}\frac{\beta }{q{m}_0}\right)$ and T⁻¹ during the initial oxidation stage at low temperatures, the steepness of the calculated value was linearly fitted as -E/R, from which the reaction activation energy E and the pre-exponential factor A were derived, as shown in Figure 6.

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Figure 6.

Activation energy curve fit for raw coal T2-T5

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3.3. Content of functional groups in coal

To investigate the impact of microwave radiation of different powers on intermediate energy groups during coal oxidation, infrared spectrum tests were conducted on coal samples at various power levels. The infrared spectrum diagram has been shown in Figure 7.

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Figure 7.

(a, b) FTIR comparison of coal.

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The degree of the infrared absorption peak varies among coal samples measured at distinct wavelengths [26-28], -OH absorption range (3000–3600 cm⁻¹), aliphatic -CH₃ absorption range (2800∼3000 cm^-1), stretching vibration range of aromatic C=O compounds (1500∼1800 cm^-1) and alcohol molecules, the stretching vibration range for ethyl ether (-C-O-C) and phenol functional groups is 1000–1300 cm⁻¹. Sub-peak fitting was conducted using Peakfit in the ranges of 1500–1800 cm⁻¹, 1500–1800 cm^-1, 2800–3000 cm^-1, 3000–3700 cm^-1. The outcomes of sub-peak fitting have been illustrated in Figure 8, from which the locations and areas of absorption peaks were determined for each functional group.

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Figure 8.

(a, b) Peak fitting of three absorption bands of raw coal FTIR.

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The absorbance of the coal sample after microwave radiation has a significant change, and the absorbance of the coal sample after oxidation is generally higher than that of raw coal. With the increase of power, the absorbance of the coal sample also increases. The waveforms of infrared spectrum and the location distribution of characteristic peaks of coal samples treated by microwave radiation were basically the same as those of the control group, and there were differences only in the intensity between different spectra, which indicates that microwave radiation changes the content of functional groups but does not change the types of functional groups in coal.

Figure 9 shows that the abundance of functional groups with oxygen content in microwave-radiated coal exceeded raw coal, the result of a chemical reaction between O₂ and aliphatic hydrocarbon structures in coal. The higher the content of carbonyl and carboxyl groups, the more active substances in coal can be decomposed into CO and CO₂, and a lot of heat is generated at the same time. Under different microwave radiation powers, the hydroxyl content in SM and WYM coal is 2.75%–21.36% and 42.34%–69.28% higher than that of raw coal, respectively. The carbonyl content is 62.84%–228% and 34.39%–62.44% higher than the original coal, respectively. The carboxyl content is 6.6%–166% higher than that of raw coal, with a range of 2.55%–27.61%. Microwave exposure causes initial oxidative degradation in raw coal, triggering changes in its internal structure. This leads to the subsequent evolution of the coal matrix, increased crack formation, and improved interconnectivity of pore networks. These structural changes facilitate the cleavage of methylene (-CH2-) linkages in coal [29]. Functional groups such as -COOH and –OH became enclosed or trapped [30], thereby enhancing the coal's low-temperature oxidation reactivity and contributing to the enrichment of oxygen-containing functionalities [31].

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Figure 9.

(a, b) Area of oxygen-containing functional groups in coa.

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3.4. Change rate of coal oxidation gas

To further investigate the impact of microwave radiation on coal oxidation characteristics, based on experimental data collected, oxygen consumption rate and CO release rate of experimental coal sample at 20.9% oxygen volume fraction were calculated according to equations (4-7), and plotted as Figure 10. As shown in Figure 10, with a constant oxygen volume fraction, the oxygen consumption rate and CO release rate of coal increase exponentially with rising microwave power. Microwave-treated coal exhibits stronger oxygen consumption and CO release rates compared to raw coal, and the asymptotic oxygen concentration decreases.

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Figure 10.

(a-f) Changes of gas in coal sample closed oxygen consumption experiment.

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As can be seen from Figure 10(a), with the increase of microwave radiation power, the asymptotic oxygen concentration of SM decreases from 19.93% to 2.17%, and that of WYM decreases from 18.06% to 13.69%. It can be concluded that microwave radiation can change the natural tendency of coal through a closed oxygen consumption experiment. Owing to the low degree of metamorphism in SM, its properties are easier to change. According to ${\lambda }_c$ (800W) < ${\lambda }_c$ (800W), it is evident that as microwave radiation power increases, water will decrease and pore cracks will increase, resulting in easier contact with coal-like oxygen, thus consuming more oxygen. Due to the substantial increase in power, the carbonyl group will be greatly increased. Thus decomposed into more CO and CO₂, affecting the oxygen ratio in the experimental device so that ${\lambda }_c$ (800W) < ${\lambda }_c$ (640W). For WYM, its degree of deterioration is high, and its properties are not easily changed, but for WYM ${\lambda }_c$ has the same law as SM. Table 5 shows the relevant parameters for fitting the results of the closed oxygen consumption experiment of coal samples.

3.5. Coal oxidation heat release intensity

The heat released during coal oxidation is the primary heat source that induces spontaneous combustion in coal, and the intensity of heat release from coal oxidation is a crucial indicator of the heat release capacity of the oxidation reaction. Combined with equation (8), The oxidative heat release intensity Q of the experimental coal samples in each group was calculated at an oxygen volume fraction of 20.9%, and the results have been presented in Table 6.

As presented in Figure 11, as the power level rises, the oxidative heat emission intensity of SM and WYM conforms to the exponential growth trend. It can be seen from the calculation that with the increase of microwave radiation power, the intensity of heat release of SM increases from 9.53×10^-8 J/ (cm³·s) to 3.65×10-6 J/ (cm³·s), and the heat release intensity increases by 1.44∼3.73 times. The oxidative heat release intensity of WYM increased from 3.38×10^-7J/ (cm³·s) to 4.23×10^-6J/ (cm³·s), and the heat release intensity increased by 1.22 ∼ 4.13 times. With the increase of microwave radiation, the gas change rate is increasing, resulting in the increase of coal oxidation heat release intensity, which intensifies the risk of spontaneous combustion of coal.

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Figure 11.

Distribution of coal oxidative heat release intensity.

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