Evolution of pore structure and changes in oxidation characteristics of coal after heating freezing treatment

2.1 Coal sample preparation

Long-flame coal was chosen as the experimental coal sample. Upon collection at the site, the coal sample is promptly sealed and dispatched to the laboratory. In the lab, the coal undergoes crushing and screening. Specifically, it is screened using a grinder and a vibrating screen to achieve a mesh size of 120–200 mesh (equivalent to 0.0075–0.125 mm). Subsequently, the coal is placed in a vacuum drying oven and allowed to dry at room temperature for 24 h. Table 1 shows the parameters of coal sample.

2.2 Experimental device

2.3 Experimental process

The coal sample was subjected to oxidation and pyrolysis experiments through a programmed temperature device, and the preheating temperature (ST) was set at 70, 120, 170, 230℃. After reaching the target temperature, the temperature was constant for 1 h, and then the coal sample was taken out and put into a Dewar bottle, and liquid nitrogen was quickly added. After volatilization of LN₂, the coal sample was taken out for experiment. Raw coal is labeled R, and raw coal plus LN₂ is labeled L_R. The labels of coal samples under other conditions are shown in Table 3.

3.1 Influence of LN₂ on pore structure of coal

In the process of inert gas inert coal spontaneous combustion, due to the low-temperature characteristics of LN₂, it is necessary to analyze the pore and surface change characteristics of the quenched-coal.

3.2 SEM analysis

To obtain the influence of LN₂ on the surface morphological characteristics of spontaneous combustion coal, SEM test was used to magnify the coal samples under different conditions by 10,000 times, as shown in Fig. 1. Due to space limitations, only test results at 70° C and 230° C are displayed.

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Fig. 1

SEM images of coal samples under different conditions.

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As can be seen from Fig. 1, with the increase of ST, the surface roughness of preheated coal is significantly higher than that of raw coal. Under oxidation and pyrolysis conditions, surface pores and cracks increase with the increase of temperature, but the pores of O-R coal are more than those of P-R coal. This is because the coal oxygen reaction increases the surface roughness of coal. After the treatment of LN₂, the pore structure of coal surface is more developed. With the increase of temperature, the influence of LN₂ on the surface roughness of pyrolysis coal is greater than that under oxidation condition.

The damage degree of LN₂ to the quenched-coal surface structure increases with the increase of preheating temperature, which is because the coal body expands under heat, and the addition of LN₂ makes the coal body shrink sharply, forming temperature stress and forming a certain stress field on the surface and interior of the coal pore. When the temperature stress reaches a certain level, it is damaged by the tensile stress, and the pore and fracture structure of coal develops (Liu et al., 2021). In order to further analyze the effect of LN₂ on the pore structure distribution of quenched-coal, the fractal characteristics and pore parameters of coal are discussed by low temperature nitrogen adsorption method.

3.2.1

3.2.1 Fractal dimension analysis

The fractal characteristics of coal have previously been shown to follow the power function of the isothermal adsorption curve, as described by the Frenkel-Halsey-Hill (FHH) model (Li et al., 2019; Han et al., 2020; Mahamud and García, 2018). The equation for this relationship is as follows:

(1)

$$\mathrm{l}\mathrm{n}(\mathrm{V})=\mathrm{K}\mathrm{l}\mathrm{n}(\mathrm{l}\mathrm{n}({\mathrm{P}}_0/\mathrm{P}))+\mathrm{C}$$ Where, V is the gas adsorption capacity under relative pressure, C is the constant, P is the equilibrium pressure, P₀ is the saturated vapor pressure of the gas, and K is the slope of the linear fitting equation (1).

When solid–gas interactions dominate, equation (1) can be written as follows:

(2)

$$\mathrm{K}=\left(\mathrm{D}-3\right)\bullet 3$$

As the relative pressure of the adsorbed gas increases, subsequent multilayer molecular adsorption gradually begins until capillary condensation occurs (Han et al., 2020; Li et al., 2019). The following equation can be used:

(3)

$$\mathrm{K}=\mathrm{D}-3$$ Where, D is the fractal dimension based on the FHH pattern.

According to P/P₀, the fractal dimension of coal pores can be divided into two parts: the low pressure area (P/P₀ < 0.5) and high pressure zone (P/P₀ > 0.5), corresponding to fractal dimensions D1 and D2, respectively. D1 is the fractal feature of microhole, D2 is the fractal feature of transition hole, middle hole and large hole. The pores in the low pressure area are mainly adsorption pores, which mainly depend on van der Waals force. The pores in the high-pressure zone are mainly porous and mainly depend on capillary cohesion (Li et al., 2019).

Based on the nitrogen adsorption and desorption curves of each coal sample under LN₂ suppression, the low pressure region and high pressure region were fitted respectively. lnV was taken as the ordinate, ln[ln(P₀/P)] as the horizontal coordinate, and K as the slope, the fractal dimensions D1 and D2 of the two regions were calculated.

Fig. 2 shows the fractal dimension fitting curves of coal samples under different conditions obtained under the FHH model. The fractal dimension calculation results and pore parameters of the low-temperature nitrogen adsorption test are shown in Table 4. The fractal dimension is between 2 and 3, which conforms to the fractal theory. Fractal dimension D1 characterizes pore roughness in coal samples. The larger D1, the coarser the pore surface of the coal sample. The fractal dimension D2 characterizes the complexity and heterogeneity of pores in coal samples. The larger the D2, the higher the heterogeneity of the pores (Nie et al., 2015; Liu et al., 2022).

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Fig. 2

Fractal dimension fitting diagram of coal samples under different conditions.

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Table 4 Pore parameters of coal samples under different conditions.

Coal Sample	BET specific surface area(m2 /g)	BJH adsorption internal surface area (m2 /g)	BJH pore Volume (cm3 /g)	Average pore size (nm)	D1	D2
R	1.5479	1.1226	0.0060	13.7012	2.5355	2.4848
LR	1.5857	1.2882	0.0066	14.1072	2.5522	2.5069
O-R70	1.7208	1.3116	0.0067	14.7046	2.5485	2.4893
O-L70	1.7473	1.3321	0.0069	15.0680	2.5600	2.5114
O-R120	1.7915	1.4822	0.0068	14.8935	2.5502	2.5127
O-L120	1.8665	1.5681	0.0070	15.0361	2.5699	2.5230
O-R170	1.9334	1.5754	0.0068	15.4050	2.5705	2.5167
O-L170	1.9827	1.5896	0.0071	15.7988	2.5839	2.5276
O-R230	2.0044	1.7611	0.0073	15.7909	2.6026	2.5237
O-L230	2.0962	2.0259	0.0075	16.0349	2.6211	2.5302
P-R70	1.7391	1.3159	0.0066	14.6137	2.5500	2.5066
P-L70	1.8654	1.5004	0.0074	15.1460	2.5522	2.5226
P-R120	1.8715	1.7656	0.0070	15.3252	2.5654	2.5237
P-L120	1.9525	1.8112	0.0072	15.3600	2.5848	2.5302
P-R170	1.9900	1.8613	0.0078	15.4166	2.6059	2.5402
P-L170	2.2402	1.9595	0.0079	15.8156	2.6119	2.5513
P-R230	2.1693	1.8910	0.0078	16.5634	2.6142	2.5327
P-L230	2.2874	2.0650	0.0080	17.0172	2.6340

It can be seen from Table 4 that with the increase of ST, the fractal dimension D1 and D2 of quenched-coal both increase. Compared with raw coal, when ST is 230℃, O-L coal D1 and D2 increase by 3.37 % and 1.35 % respectively; P-L coal D1 and D2 increased by 3.88 % and 1.56 %, respectively. The results show that the pore surface roughness of coal treated with LN₂ is greater than that of raw coal.The results of fractal dimension D1 and SEM experiments confirm each other, the higher the preheating temperature of coal, the more serious the damage of coal after adding LN₂, and the more obvious the increase of surface pores and cracks. The increase of D2 indicates that LN₂ makes the micropores in coal expand into mesoporous, which makes the pore structure of coal more complex. D1 is greater than D2, which indicates that the pore surface of coal micropores is more complex.

Compared with raw coal, the specific surface area, pore volume and average pore diameter of quenched-coal have increased in different degrees. Under oxidation conditions, when LN₂ was added at ST 70, 120, 170 and 230℃, the specific surface area increased by 12.88 %, 20.58 %, 28.08 % and 35.42 % respectively; The pore volume increased by 15.55 %, 17.18 %, 17.71 % and 24.24 %, respectively; The average aperture increased by 9.97 %, 9.74 %, 15.30 % and 17.03 %, respectively. Under pyrolysis conditions, when LN₂ is added to ST at 70, 120, 170 and 230℃, the specific surface area increases by 20.51 %, 26.13 %, 44.72 % and 47.77 % respectively; The pore volume increased by 23.09 %, 19.21 %, 31.95 % and 32.47 %, respectively; The average aperture increased by 10.54 %, 12.10 %, 15.43 % and 24.20 %, respectively. By analyzing the fractal dimension and pore parameters of the pores of quenched-coal treated with LN₂, it can be concluded that pyrolysis coal is most affected by LN₂ as the temperature increases.

3.3 Low-temperature oxidation results of coal samples treated with LN₂

To obtain the spontaneous combustion characteristics of coal quenched by LN₂ in goaf, the gas products of coal samples during low-temperature oxidation are compared and analyzed. Fig. 3 shows the changes of CO and CO₂ concentrations produced by coal samples under different conditions.

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Fig. 3

Changes of CO and CO₂ concentrations of coal samples with temperature.

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The experimental results clearly demonstrate that preheating temperature significantly influences the release of CO and CO₂ during the coal oxidation process. As the temperature (ST) increases, the release of CO and CO₂ gradually escalates during coal oxidation. Notably, this effect catalytically impacts the release of CO₂ and CO at 60 °C and 80 °C, respectively. This is because with the increase of ST, the oxidation degree of O-R coal goes deeper, the number of active functional groups greatly increases, and P-R coal also increases with the desorption active site of gas, resulting in a stronger reaction rate of preheated coal than raw coal at the low-temperature stage.

After the addition of LN₂, the quenched-coal shows a stronger reaction ability. At the initial stage of oxidation, the content of CO and CO₂ released by the reaction of quenched-coal is not significantly different from that of the preheated coal, which is because the desorption of nitrogen does not completely affect the oxygen reaction of coal at low-temperature. As the oxidation temperature increases, quenched-coal exhibits a greater reaction rate. When ST is 70℃, when O-L coal and P-L coal react to 200℃, CO release increases by 124.44 ppm and 258.77 ppm compared with preheated coal. When ST is 230℃, when O-L coal and P-L coal react to 200℃, CO release increases by 419.35 ppm and 838.37 ppm compared with preheated coal. It shows that the higher the preheating temperature of quenched-coal in goaf, the stronger the secondary oxidation capacity. Comparing the gas release of coal under oxidation and pyrolysis conditions, the release of CO and CO₂ at ST of 230℃ is 8123.65 ppm, 29717.03 ppm, 9353.76 ppm and 39006.79 ppm, respectively. P-L coal shows a stronger secondary oxidation reaction capacity, which is due to the more developed pore cracks in coal under pyrolysis conditions, providing more active sites for coal-oxygen reaction.

3.4 DSC analysis

Due to the fact that the continuous heat storage capacity of goaf coal is the most important indicator affecting coal spontaneous combustion, analyzing the influence of LN₂ on the heat release characteristics of quenched-coal is of great significance. As shown in Fig. 4, there are significant differences in the heat flux curves (DSC) of coal samples under different conditions. In order to better reflect the heat release of quenched-coal at low-temperature, the maximum endothermic temperature (T1), initial exothermic temperature (T2) and their reaction process on the DSC curve of the oxidation process were selected for analysis. The three stages are: 27 ℃ - T1 for Stage I, T1-T2 for Stage II, and T2-250 ℃ for Stage III. The heat flow rate variation of DSC curve integration is shown in Table 5.

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Fig. 4

DSC curves of coal samples under different conditions.

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It can be seen from Table 5 that the characteristic temperature of quenched-coal is lower than that of preheated coal. When ST is 70℃, the characteristic temperature points T1 and T2 of O-L coal and P-L coal decrease by 2.74 %, 3.06 % and 9.51 %, 2.74 %, respectively. When ST rises to 230℃, the characteristic temperature points T1 and T2 of O-L coal and P-L coal decrease by 0.85 %, 0.45 % and 2.58 % and 4.48 % respectively. Although the quenched-coal is affected by nitrogen desorption in the low temperature oxidation stage, the increase of specific surface area makes the coal adsorb more oxygen, resulting in the coal entering the exothermic stage in advance.

As can be seen from Table 5, the total heat of preheated coal shows an increasing trend with the increase of ST. The heat absorption of raw coal is 91.29 J/g. When ST is 70, 120, 170, 230, the total heat absorption of O-L coal and P-L coal is 70.44, 58.22, 41.61, 37.47 J/g and 65.03, 50.93, 39.13, 32.72 J/g, respectively. Since P-L coal will absorb more oxygen at low temperatures, the adsorption heat generated is much greater than O-L coal, so the total heat absorption of P-L coal is smaller.

The reaction mechanism function was determined by Coats-Redfern method and the actual apparent activation energy was calculated for further analysis. The Coats-Redfern method is calculated as follows (Zhang et al., 2024; Wang et al., 2023; Zhang et al., 2023):

The relationship between coal quality change and heat release change is as follows:

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Fig. 5

Changes of activation energy of coal samples under different conditions.

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According to the above calculation, the stage I and II of the temperature rise process of coal belong to the endothermic process. The absorption of heat mainly comes from the desorption of gas in coal and the evaporation of water. With the increase of temperature, the heat released by oxidation adsorption and reaction of coal increases, resulting in the maximum heat absorption rate in coal (T1). As the temperature continues to rise, the heat transfer rate of the coal-oxygen reaction begins to be greater than the heat absorption rate, showing that the heat absorption decreases. Finally, when the heat discharge is greater than the heat absorption (T2), the coal begins to enter the heat release stage. Therefore, the reaction capacity of coal at Stage I and Stage II directly affects the heat release of coal-oxygen reaction.

The determination of apparent activation energy can provide insights into the challenges associated with spontaneous coal combustion. During the endothermic stage, higher activation energy corresponds to lower heat absorption, leading to increased oxygen adsorption and reaction rates for coal (Zhang et al., 2023). As can be seen from Fig. 5, the activation energy of coal under LN₂ quenched-coal is higher than that of preheated coal at the same stage. In the stageⅠ, when ST is 70, 120, 170, 230℃, the activation energy of O-L coal and P-L coal increases by 6.00, 7.33, 8.81, 9.85 kJ/mol and 9.12, 10.34, 11.84, 15.96 kJ/mol, respectively, compared with that of preheated coal. In the stageⅡ, when ST is 70, 120, 170, 230℃, the activation energy of O-L coal and P-L coal increases by 3.73、4.98、7.24、10.50 kJ/mol and 1.44、2.10、5.62、6.85 kJ/mol, respectively. This is because LN₂ increases the coal oxygen reaction rate in the low-temperature endothermic stage of preheated coal, and the oxidation energy barrier decreases, which is manifested as the apparent activation energy increases in the endothermic stage. With the increase of ST, liquid nitrogen has a greater influence on the activation energy of coal at low-temperature endothermic stage. Comparing the effect of liquid nitrogen on the activation energy of preheating coal at different stages, it is found that the activation energy of P-L coal increases more at Stage I. This is because the pyrolysis coal adsorbs a large amount of oxygen at the beginning of the coal-oxygen reaction and releases the adsorption heat. In Stage II, the increment of activation energy of O-L coal is greater than that of P-L coal, and more active groups undergo oxidation exothermic reaction, resulting in a higher apparent activation energy of endothermic reaction in this stage.

3.5 FTIR analysis

The types and contents of different functional groups in coal under different conditions were determined by FTIR method in order to analyze the reason why coal was prone to spontaneous combustion after LN₂ injection. The FTIR spectra of coal samples under different conditions are shown in Fig. 6.

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Fig. 6

FTIR curves of coal samples under different conditions.

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The infrared absorption peak intensity of coal samples at different wavelengths is different, and the vibration intervals with great differences are as follows (Deng et al., 2023; Zhang et al., 2019; Zheng et al., 2020): –OH absorption stretching vibration range (3000 ∼ 3600 cm⁻¹), aliphatic –CH₃ absorption stretching vibration range (2800 ∼ 3000 cm⁻¹), aromatic -C = O compounds stretching vibration range (1500 ∼ 1800 cm⁻¹) and alcohol, phenol and ether -C-O-C stretching vibration range (1000 ∼ 1300 cm⁻¹). Peakfit was used to perform sub-peak fitting for 1500–1800 cm⁻¹, 2800–3000 cm⁻¹ and 3000–3600 cm⁻¹, and the sub-peak fitting results were shown in Fig. 7. According to the peak fitting results, the changes of aliphatic hydrocarbon and oxygen-containing functional groups were analyzed semi-quantitatively. The main functional groups analyzed are as follows: –CH₂/–CH₃ (2922/2955 cm⁻¹), -C = O/-C = C (1653/1610 cm⁻¹), –COOH/-C = C (1702/1610 cm⁻¹), –OH/-C = C (3423/1610 cm⁻¹) (Lu et al., 2021; Li et al., 2019; Wang et al., 2022). The results of functional group semi-quantitative analysis of coal samples under different conditions are shown in Fig. 8.

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Fig. 7

Peak fitting curve of raw coal.

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Fig. 8

Changes in functional groups of coal samples under different conditions.

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As can be seen from Fig. 8, with the increase of preheating temperature, the oxygen-containing functional groups of O-R coal show an increasing trend, while –CH₂/–CH₃ gradually decrease. The content of oxygen-containing functional groups of O-L coal is much higher than that of preheated coal. When ST is 70℃, the content of -C = O, –COOH and –OH in O-L coal increases by 2.69 %, 6.12 % and 5.74 % respectively. When ST rises to 230℃, the -C = O, –COOH and –OH in O-L coal increase by 6.35 %, 13.30 % and 10.95 % respectively, which is due to the continuous consumption of oxygen-containing functional groups in the cooling process of O-R coal. However, LN₂ blocked the oxidation reaction of O-R coal, and a large number of oxygen-containing functional groups could not carry out further reaction. When in the spontaneous combustion process again, these active groups show a stronger reaction capacity than the raw coal. However, the aliphatic hydrocarbon ratio in quenched-coal continues to decrease with the increase of ST compared with that in preheated coal, mainly because LN₂ aggravates the crushing of high-temperature coal, resulting in the aliphatic hydrocarbon bridge bond –CH₂- fracture in coal.

With the increase of preheating temperature, a large number of active groups in P-R coal are released. The content of oxygen-containing functional groups and aliphatic hydrocarbons in coal decreased gradually compared with that of raw coal. The content of -C = O, –COOH and –OH decreased by 34.05 %, 37.93 % and 29.24 % respectively when ST was 230℃. The content of oxygen-containing functional groups in P-L coal is higher than that of P-R coal, which may be due to the continuous release of desorption gas during the cooling process of pyrolysis coal. The content of –CH₂- also decreased with the continuous increase of pyrolysis temperature.

Comparing the effects of LN₂ on the active groups of coal in the two reaction processes, it was found that LN₂ had the greatest influence on the content of –COOH in oxidation coal, followed by –CH₂-. This is due to the continuous release of CO₂ during the cooling process of oxidation coal. The LN₂ has the greatest influence on the content of –CH₂- in pyrolysis coal. With the increase of ST, LN₂ causes serious damage to the coal body, and the pore structure of the coal body is further developed, which aggravates the fracture of the bridge bond.

3.6 ESR analysis

Free radicals are generally considered as the active sites on the surface of coal, and the active sites can be converted into oxygen-containing functional groups under certain conditions (Guanhua et al., 2019; Li et al., 2019). g factor and free radical concentration (Ng) in coal under different conditions were determined by ESR. ESR spectra of coal samples under different conditions are shown in Fig. 9.

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Fig. 9

ESR curves of coal samples.

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It is generally believed that the g factor of carbon atom center radical is close to that of free electron 2.0029, the g factor value of aromatic structure radical is 2.0032, the g factor of carbon atom center radical with oxygen atom nearby is 2.0034–2.004, and the g factor of oxygen atom center radical is greater than 2.004 (Duan et al., 2023; Li et al., 2019). It can be seen from Fig. 10 that with the increase of ST, the g factor and free radical concentration of O-R coal gradually increase, indicating that the proportion of alkoxy radicals in the total free radicals increases. When ST is 70 °C, 120 °C, 170 °C and 230 °C, the g-value of O-L coal increases by 4.28E-06, 9.24E-06, 1.45E-05 and 2.34E-05, respectively, compared with that of raw coal. The g-value of P-L coal decreased by 4.14E-06, 5.92E-06, 8.13E-06 and 1.12E-05, respectively, compared with that of raw coal, and the proportion of alkyl free radicals in total free radicals increased. This is due to the inerting of LN₂ and the sealing of oxygen-containing functional groups in O-R coal. The temperature stress caused by the freezing of P-R coal by LN₂ also causes the bridge bond to break, and more alkyl radicals are formed. It can be seen from the change of free radical concentration that the addition of LN₂ increases the concentration of free radicals in coal, provides more active sites for coal-oxygen reaction under low-temperature conditions, and promotes coal spontaneous combustion.

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Fig. 10

Changes in g-values and free radical concentrations of coal samples.

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3.7 Mechanism

After injecting liquid nitrogen (LN2) to quench the fire in a spontaneous combustion coal seam, the primary coal undergoes damage during the initial oxidation and quenching process. This leads to secondary structural development within the coal, as depicted in Fig. 11. Compared with raw coal, the fissure of preheated coal increases and the active site increases. Oxygen-containing functional groups of O-R coal increase. Oxygen-containing functional groups of P-R coal decrease. Preheated coal has larger cracks and more active sites than raw coal. The oxygen-containing functional groups of O-R coal increase. The oxygen-containing functional groups of P-R coal are reduced. After adding LN₂, the fracture of coal increases, the average pore diameter of coal is greatly increased by temperature stress, and the pore connectivity of coal is enhanced, which is beneficial to the adsorption of oxygen. In terms of chemical structure changes, after LN₂ injection, the broken coal body leads to the fracture of aliphatic bridge bond –CH₂- in coal, the increase of carbon free radical concentration and the increase of active sites. Oxygen-containing functional groups such as –COOH and –OH are sealed, which promotes the low-temperature oxidation activity of coal.

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Fig. 11

Mechanism diagram.

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Therefore, after injecting LN₂ into the goaf for fire prevention and quenching, it is essential to enhance vigilance regarding the reburning of the old goaf and the management of fire-prone areas. Implementing effective measures will help mitigate the risk of coal spontaneous combustion recurring.