Dependence evaluation of factors influencing coal spontaneous ignition

Coal spontaneous combustion is determined by a variety of factors. Testing can describe the changes incoal spontaneous combustion with various factors, however, the dependence of spontaneous combustion on various factors is unclear. Therefore, reliability theory was used to deduce the functional relationship of the dependence of coal spontaneous combustion on various factors, and construct a model algorithm for predicting the probability of occurrence of coal spontaneous combustion, which is adopted to evaluate and rank the degree of influence of various factors on coal spontaneous combustion. Effective prevention methods are proposed by strengthening the role of the most important factors. The results show that, by taking the duration of coal heating to 150°C as the measurement standard of coal spontaneous combustion, the duration of the initial stage of coal heating increases linearly with the increase of specific heat capacity, thermal conductivity, and moisture content of coal. With the increase of oxygen concentration, oxidation rate, and initial temperature of the coal, the duration of the initial stage of coal heating decreases exponentially. With the increase of gas flow seepage velocity in the coal body, the duration of the initial heating stage changes in a parabolic manner. At the same time, the probability of spontaneous combustion decreases exponentially with the increase of specific heat capacity and moisture content of the coal. The probability of coal spontaneous combustion increases linearly with the increase of coal thermal conductivity, oxygen concentration, gas seepage velocity, and rate of oxidation. The dependence of coal spontaneous combustion probability on different factors can be expressed as follows: coal temperature > gas seepage velocity > specific heat capacity > oxidation rate > oxygen concentration > moisture content > thermal conductivity.

The coal industry is a powerful guarantor underpinning the rapid development of China's national economy, but its mining is restricted by gas, fire, water, dust, and other disasters. 1][4] Spontaneous ignition of coal depends on many factors; in recent years, scholars have conducted research into the factors affecting coal spontaneous ignition. 5,6Zhang et al. 7 and Wang et al. 8 analyzed the influence of metamorphism on oxidation and exothermic behavior of coal through spontaneous combustion test data pertaining to coal samples with different degrees of metamorphism.Wen et al. 9 and Xiao et al. 10 experimentally assessed the spontaneous combustion characteristics of coal under different oxygen volume fractions, and analyzed the characteristic temperature, mass loss, thermal effect, and thermal reaction kinetic parameters of coal sample oxidation.Pan et al. [11][12][13] carried out in-depth research on the thermal and microstructure characteristics of coal in the process of oxidation spontaneous combustion, and concluded that the thermal properties of coal spontaneous combustion have obvious segmental characteristics, which are not affected by metamorphism degree and heating rate, and the thermal dynamic balance is an important reason for the change of microstructure.In the oxidation process, the spatial arrangement of the molecular structure evolution of coal is reversible, and the -OH group is conducive to the oxidation of coal.In the low temperature oxidation stage of coal, the higher the preoxidation degree, the higher the wetting heat value and the higher the spontaneous combustion risk of coal.Xi et al. [14][15][16] studied the influence of air leakage on the spontaneous combustion process of coal, and described the trend in volume of gas generated by the reaction of coal and oxygen after changes in key parameters.Li et al. 17 and Wang et al. 18 simulated the temperature rise of oxidation of residual coal under high temperature in deep goaf to study the influences of different coal rock temperatures on the spontaneous combustion process thereof.By combining differential scanning calorimetry with programmed temperature rise, Zhang et al. 19 estimated the influences of different water contents on the parameters of coal sample absorption and heat release.Although beneficial results have been achieved in the research on the influencing factors of spontaneous coal combustion, these factors are of numerous types, complex structures, and interrelated influences, and the dependence studies on various factors influencing spontaneous combustion of coal are insufficient. 20,21n the measurement of the factors influencing coal spontaneous ignition, because the properties of the studied samples are different in each experiment, the resulting data are often significantly different, and the degree of influence of each factor cannot be described.Even if, on the basis of these data, the trend of the spontaneous combustion process of coal and the changes in the parameters affecting it can be obtained, it is difficult to rank them in order of importance or to describe mathematically the dependence of the process on each factor.Therefore, to evaluate the degree of influences of various factors on coal spontaneous ignition, the most effective method is to establish a mathematical model of the dependence of coal spontaneous ignition on various factors.][24][25][26] Based on the thermodynamic theory of gas flow in coal medium, the model of coal heat transfer and gas seepage was established, and the law of influence for influencing factors on coal spontaneous combustion was analyzed numerically.At the same time, the mathematical model for probability of coal spontaneous ignition was established to predict the dependence of coal spontaneous ignition on various factors.

| Basic theory and mathematical model
Coal is a porous medium, and its internal heat transfer, moisture transfer, and gas flow affect each other.Heat and water vapor transfer in a porous structure is a coupling phenomenon.In establishing transient heat transfer models in porous structures, the following basic assumptions should be made: (1) The structural properties of coal are uniform; (2) each phase is in thermal equilibrium; (3) The volume change of coal due to moisture and temperature is ignored; (4) Ignore the natural convection within the material.Then the basic WU ET AL.
| 3739 equation of one-dimensional heat transfer in porous media can be expressed as where φ is the porosity of porous medium, %; ρ the density of the porous medium, kg/m 3 ; C the heat capacity of the porous medium, J/(mol K); λ the thermal conductivity of the porous media, W/(m K); and q the variety of heat sources.
For the coal heat and moisture transfer model with internal gas seepage, factors such as oxidation heat, gas seepage convection heat transfer, and evaporation-driven heat absorption need to be considered.Then the coal heat and moisture transfer equation and oxygen concentration diffusion equation can be expressed thus [27][28][29] : In Formula (2), is the heat of oxidation; x denoted the heat transfer during gas seepage;   ρ C w w W t denotes the latent heat of vaporization of the water; ρ c is the density of coal, kg/m 3 ; C c is the heat capacity of the coal, J/(mol K); T is coal body temperature, °C; λ c is the thermal conductivity of the coal, W/(m K); Q is the thermal effect of the oxidation reaction, J/m 3 ; C is the oxygen concentration, %; η 0 denotes the rate of oxidation, m 3 /(kg•s); E is the activation energy, J/mol; R is the ideal gas constant, 8.314 J/(mol K); ρ g is gas density, kg/m 3 ; V is the gas seepage velocity, m/s; C g is the heat capacity of the gas, J/ (mol K); C w denotes the specific heat of evaporation of water, J/(kg K); W is the water content of the coal, %; ρ w is the density of water, kg/m 3 ; D is the molecular diffusion coefficient of oxygen, m 2 /s.

Initial boundary value conditions:
where T 0 is the initial temperature of coal, °C; α the heat transfer coefficient of the coal, W/(m 2 •K); α m the mass transfer coefficient of coal, s −1 ; C 0 the initial concentration of oxygen, %; and W 0 the initial moisture content, %; L is the length of the coal body, m.

| Factors influencing coal spontaneous ignition
There are many factors affecting coal spontaneous combustion, but the temperature change of coal spontaneous combustion process mainly depends on the thermal conductivity (λ c ), specific heat capacity (C c ), initial water content (W 0 ), oxidation rate (η 0 ), the initial temperature of the coal (T 0 ), gas seepage velocity (V), and initial oxygen concentration (C 0 ).Therefore, the influences of main factors on coal spontaneous ignition were analyzed when other parameters were fixed, as shown in Figure 1.
Other basic parameters of the coal body are: L = 3.0 m, ρ c = 1200 kg/m 3 , φ = 0.3, Q = 1.26 × 10 7 J/m 3 , E = 6.28 × 10 4 J/mol, C 0 = 21%, ρ g = 1.29 kg/m 3 , C w = 4200 J/kg K, C g = 1.003J/(mol K), ρ w = 1000 kg/m 3 , D = 1.5 × 10 −11 m 2 /s, 30 and α = 0.01 W/m 2 K. 31 To simplify the calculation, the temperature at which the water evaporates was set to 100°C, and the rate was chosen to maintain that temperature until the coal was completely dry.Here, the coal was kept at 150°C until spontaneous ignition, and the degree of influences of various factors on the time to spontaneous ignition was evaluated.

| Specific heat capacity of coal
The change of specific heat capacity of coal has a significant influence on spontaneous combustion (Figure 1A).With the increase of specific heat capacity of coal, the duration of the initial heating stage of coal increases linearly.For example, when the specific heat capacity of coal is increased by 1.42 times (from 1200 to 1600 J/kg K), the initial phase of coal heating will be prolonged by 1.39 times.Then the functional relationship between coal heating time and coal-specific heat capacity can be expressed as where C c is the specific heat capacity of coal, J/(kg K), t c is the time at 150°C, h.

| Thermal conductivity of coal
Although the time of the initial heating stage of coal increases linearly with the thermal conductivity of coal (Figure 1B), the change of thermal conductivity of coal has a weak influence on the duration of the initial heating stage.For example, a twofold increase in the thermal conductivity of coal (from 0.15 to 0.30 W/m K) only increases the heating time to the critical temperature by 1.27 times.The functional relationship between the heating duration of the region with a fixed temperature and the thermal conductivity of coal can be expressed as where λ c is the thermal conductivity, W/(m K). Figure 1A,B indicate that the increase of specific heat capacity and thermal conductivity of coal can delay coal spontaneous combustion, but cannot prevent it.

| Initial oxygen concentration
The increase of oxygen concentration reduces the initial heating time of coal (Figure 1C), that is, the decrease of oxygen concentration can significantly slow down or prevent spontaneous ignition.Numerical calculation shows that the properties of coal heating process can be significantly changed by the change of oxygen concentration in the gas stream.For the given coal Relationship between calculation results of influencing factors and heating time when coal heated to 150°C.(A) Specific heat capacity, (B) thermal conductivity, (C) oxygen concentration, (D) gas flow seepage velocity, (E) moisture content, (F) oxidation rate, and (G) initial temperature of coal.
reaction properties and oxygen concentration below 15%, spontaneous ignition will not occur within the calculated 750 h.Moreover, as the concentration of oxygen in the air stream decreases, so does the optimal air speed to ensure maximum temperature rise.When the oxygen concentration is reduced from 20% to 15% (i.e., the oxygen concentration is reduced by 25%), the time for spontaneous combustion to develop to the adopted critical temperature increases from 492.52 to 739.07 h (the initial heating time increases by 50.06%).This fully shows that this parameter has a significant influence on coal spontaneous combustion, because the decrease of oxygen content will lead to a sharp decline in heating rate.As a function of oxygen content in the air, coal heating time can be expressed as where C 0 is the oxygen concentration, %.The analysis of the effect of oxygen concentration on the initial heating time of coal shows that gas insertion in goaf is conducive to preventing the development of coal spontaneous ignition. 32,33However, experiments 34 show that the critical oxygen concentration to prevent spontaneous combustion of coal is a variable and depends on many other factors.The adsorption property of coal and gas penetration rate have the greatest influence on the critical oxygen concentration.For example, if the oxygen concentration is 15%, it is similar to this calculated result.If the coal adsorbability is increased by 1.5 times, the initial heating time is about 1000 to 1400 h, creating conditions for the formation of a high-temperature environment.The rate of gas penetration in coal produces a similar effect.For example, if the oxygen concentration is 15% and the other basic parameters of the coal pile remain unchanged, the gas seepage rate through the coal body will also cause coal spontaneous combustion within 700 to 800 h when it is reduced to 5 × 10 −4 m/s.Therefore, when gas inserted in goaf is implemented to prevent coal spontaneous combustion, oxygen concentration must be reduced for a long time, and a lot of inert gas is needed.

| Gas seepage velocity
The gas seepage velocity in coal is within the range of 10 −5 to 10 −2 m/s, and the influence of gas velocity (10 −4 to 10 −3 m/s) on coal spontaneous combustion was evaluated here (Figure 1D). Figure 1D indicates that the change in gas seepage velocity has an uncertain influence on coal spontaneous ignition.For each specific case, there is a maximum optimal rate of temperature rise, depending on a set of characteristics of the coal pile and environmental effects.That is, with the increase of the flow velocity in coal, the duration of the initial heating stage presents a parabolic change, which can be expressed as where V c is the gas seepage velocity, m/s.According to the analysis in Figure 1D, if the spontaneous combustion is to be prevented by reducing or increasing the air velocity penetrating into coal, the influence of uneven aerodynamic resistance in goaf on the change of air velocity in coal must be considered.Therefore, the change of air leakage through the goaf may save some parts from the optimal spontaneous ignition conditions, but may increase the possibility of internal fires in other parts.Due to technical and technological reasons, it is often difficult to prevent coal spontaneous combustion through different air leakage rates.

| Initial water content of coal
Figure 1E shows the influence of initial coal water content on the spontaneous ignition time of coal below 150°C.In the calculations, only the slowing down of heating due to heat loss from liquid phase evaporation at a fixed temperature of 100°C is considered.As can be seen from Figure 1E, increasing the water content in coal is an effective way in which to prolong the initial stage of spontaneous combustion.There is a linear relationship between the heating time to the adopted temperature and different water contents of coal, which can be expressed as where W 0 is the water content of the coal, %.
Figure 1E shows that a fourfold increase in coal water content (from 10% to 40%) results in a 1.32-fold increase in the time required to heat the coal to the critical temperature (150°C); however, delaying spontaneous combustion by increasing the water content of coal depends on many other factors.To a large extent, the delay time caused by water evaporation is affected by the chemical reactivity of coal, air penetration rate and so on.For example, the evaporation time of water increases as the chemical reactivity of coal decreases.Therefore, it is better to use heat-resistant sources that can increase the water content of the coal. 19,35

| Oxidation rate
The chemical reactivity of coal with oxygen is the number of interactions between oxygen molecules and the active center of coal, expressed here as the oxidation rate (η 0 ).Like the oxygen concentration, the chemical reactivity of coal with oxygen also has a significant effect on spontaneous ignition, as shown in Figure 1F.
In the equation, η 0 is the oxidation rate, m 3 /(kg s).
As can be seen from Figure 1F, with the increase of oxidation rate, the heating time at the initial stage of coal spontaneous ignition decreases exponentially, that is, with the decrease of oxidation rate, the heating time at the initial stage of coal spontaneous ignition increases exponentially.
This is because coal with an inhibitory anti-heat source changes the rate of oxidation, in particular because of the blocking effect of the film formed on the surface of the coal, which prevents or hinders the infiltration of oxygen molecules into the reaction center of the coal, since the anti-heat source reacts with the coal to deactivate it.

| Initial temperature of coal
Figure 1G shows the influence of the initial temperature of the coal on the duration of the initial heating stage.As can be seen from Figure 1G, as the initial temperature of coal mass increases, the duration of the initial heating stage decreases exponentially, that is, as the initial temperature of coal decreases, the duration of the initial heating stage increases exponentially.For example, if the initial temperature is reduced by a factor of 1.5 (decreased by 10°C from 30°C to 20°C), the initial heating period is doubled.The critical initial temperature of the coal to prevent the development of spontaneous combustion depends on the properties of the coal.The chemical reactivity of coal may change this parameter and the nature of the spontaneous combustion process to the greatest extent.With the increase of reactivity, the initial critical temperature to prevent spontaneous ignition of oxidized coal decreases.Reducing the initial temperature of coal is the best way to prevent endogenous fire.
where T 0 is the initial temperature of the coal, °C.
In conclusion, the change of initial temperature of the coal has the greatest influence on spontaneous ignition.The occurrence of coal spontaneous ignition can be prevented by reducing the temperature of factors inducing coal spontaneous ignition to the lowest level: the value of each parameter, however, corresponds strictly to a particular coal property and depends on a combination of its properties and environmental impacts.The combined action of several factors that independently prevent coal spontaneous ignition may enhance or weaken the overall preventive effect of the introduced interference.

| Theoretical derivation
In the case of spontaneous combustion of coal in the goaf of coal seams, it is necessary to estimate the possibility of a fire in a specific area at time t.According to the analysis of the degree of influence of the above factors on coal spontaneous ignition and the goaf model (Figure 2), reliability theory is adopted to establish the prediction model of the probability of occurrence of a risk of coal spontaneous ignition.
Based on the dynamic process of coal spontaneous combustion, the function relation of coal spontaneous combustion incidence is given as follows: where f(t) is the occurrence rate of spontaneous combustion of coal, which is a complex function of spontaneous combustion factors and process parameters of goaf.X* = (x 1 , x 2 , …, x n ) represents the spontaneous combustion factors; Y* = (y 1 , y 2 , …, y m ) represents the process parameters.These parameters were obtained by mathematical statistics test results; in engineering practice, it is difficult to obtain the initial information of coal spontaneous combustion and other complex processes in field tests, making it necessary to study the probability of coal spontaneous combustion to establish the mathematical equation of the process according to the physical law of coal spontaneous combustion.Based on the goaf layout, a model algorithm of coal spontaneous combustion probability is constructed.
When the coal reaches the critical temperature (T ct ) above (T > T ct ), the coal is subject to the possibility of combustion.Formula ( 12) is expressed as where F is the possibility of a fire occurring; T is the temperature of the coal, °C.In Equation ( 13), dF/dT can be assumed to follow the normal distribution rule when the number of samples is as large as possible, namely: where σ is the root-mean-square deviation between the critical temperature and its expected value under fixed conditions, °C.According to the heat balance equation, the change rate of temperature of the coal can be obtained [36][37][38][39] : where dT/dy is the temperature gradient along the air seepage direction, °C/m;  T 2 is the Laplace operator; H Γ( ) denotes the residual heat loss.
Equation (15) shows that the spontaneous combustion process depends on the heat exchange conditions in the coal, and the heat exchange problem is reduced to finding the temperature field in the goaf and the heat transferred by the coal and rock mass to the surrounding medium.According to the heat exchange conditions, the temperature field (T = T(x, y, z, t)) has a definite form in any given case.Here, the goaf can be considered as the temperature field of a plane with finite size.According to Veinik analysis, the approximate equation of the temperature curve can be expressed as where, T 0 is the initial temperature, °C; T s the surface temperature of coal rock mass at time t, °C; Y the depth of the heating layer at time t, m.By substituting Equation ( 17) into Equation ( 16), the equation of temperature curve is as follows: where the exponent n is determined according to the special conditions pertaining to the problem.
In addition, it is assumed that at time t = 0, a constant source of relative power q begins to act on the entire coal mass.In the absence of external heat exchange, the temperature at all points in the coal will change at the same rate.It can be found from the heat balance equation that temperature is time dependent.In time dt, the heat of coal body per unit volume is Due to heat release, the temperature of coal body increases by dT, then the heat of coal body per unit volume can be expressed as where C w is the specific heat of water.
According to Equations ( 19) and ( 20): By integrating (t = 0, T = T 0 ) against Equation ( 21), we can obtain: where q = Qη 0 Cρ c .If external heat exchange is superimposed during this process, some of the heat will be lost to the surrounding medium.According to Equation ( 22), the approximate equation of coal cooling can be expressed as where T Y is the mass temperature at depth Y from the surface.Due to thermal conductivity, the heat transferred through the surface of coal will be given by Fourier's law, namely: where S is the surface area of the coal, m 2 .In this case, the normal line of the surface lies in the same direction as the Y-axis, so the isothermal surface in the plane is parallel to the surface.Therefore, ( 24) can be expressed as Through Equation ( 25) pertaining to the temperature differential curve with respect to y, the temperature gradient can be obtained: For the surface of coal, y = Y, then: The heat lost by coal through conduction in time dt is The heat lost from the coal through convection is In Equations ( 23), (28), and (29), T h changes linearly with time.
By substituting T Y from Equation (25) into Equation ( 30) and setting T s = T 0 , we can get In Equation (31), Y is a function of time.The relationship between Y and t can be found from the heat balance equation, which can be obtained as follows: where Y 0 is half the size of the coal body in the direction of gas seepage, m.
By substituting Y in Equation (32) into Equation ( 31) and taking the derivative with respect to time t, the approximate equation of temperature of the coal field can be obtained: where Replacing the expression of temperature change rate obtained with that for the rate of occurrence of spontaneous ignition, that is, Equation ( 13), after mathematical transformation, the probability of spontaneous ignition of the coal is given by

| Numerical analysis of the probability of coal spontaneous ignition
According to the probability function of coal spontaneous ignition, MATLAB program can be used to estimate the probability of spontaneous ignition of coal in the goaf at any time t.At the same time, the probability of coal spontaneous ignition under the influences of different factors was estimated, and then the dependence of coal spontaneous ignition on the influencing factors was explored.The basic parameters of the goaf are: the density of the coal ρ c = 1200 kg/m 3 , initial oxygen concentration C 0 = 21%, the density of the gas ρ g = 1.29 kg/m 3 , the specific heat of water C w = 4200 J/kg K.The influences of thermal conductivity (λ c ), specific heat capacity (C c ), moisture content (W), oxidation rate (η 0 ), temperature of the coal (T), gas seepage velocity (V), and oxygen concentration (C) on the probability of coal spontaneous combustion are considered.

| Variations of coal spontaneous combustion with time
Under different conditions, time has an important effect on coal spontaneous combustion in goaf.The variations of the probability of coal spontaneous ignition with the duration under different influencing factors are shown in Figure 3: given thermal conductivity (λ c ), specific heat capacity (C c ), water content (W), oxidation rate (η 0 ), temperature of the coal (T), gas seepage velocity (V), and oxygen concentration (C), the probability of spontaneous combustion of goaf coal remains increases linearly with time.However, different factors have different influences on the probability of coal spontaneous ignition.For example, with the increase of the specific heat capacity of coal (C c ) and the water content of coal (W), the probability of coal spontaneous ignition decreases.With the increase of thermal conductivity (λ c ), oxidation rate (η 0 ), temperature of the coal (T), gas seepage rate (V), and oxygen concentration (C), the probability of coal spontaneous combustion increases.Different factors affect the probability of coal spontaneous ignition greatly.To describe the importance of these factors and rank them, this research selected the same time and changed the parameters, respectively to compare and analyze the changes in probability of coal spontaneous ignition.For example, at 240 and 480 h, the changes in the probability of coal spontaneous ignition caused by different factors are shown in Table 1.At 480 h, the specific heat capacity of coal increases from 1100 to 1500 J/kg K (an  With the increase of coal-specific heat capacity, the probability of coal spontaneous ignition decreases exponentially (Figure 4A).As thermal conductivity of coal increases, the probability of coal spontaneous ignition increases linearly (Figure 4B), but the increase rate is small, that is, the influence of this factor is small.With the increase of oxygen concentration, the probability of coal spontaneous ignition increases linearly (Figure 4C), and the rate of increase is large, that is, the influence of this factor is obvious.With the increase of gas seepage velocity, the probability of coal spontaneous ignition increases linearly (Figure 4D), and the increase rate is large, that is, the influence of this factor is significant.
With the increase of coal moisture content, the probability of coal spontaneous ignition decreases exponentially (Figure 4E).When the moisture content is small (less than 20%), the probability of coal spontaneous ignition is significantly affected; however, when the moisture content is large, the probability of coal spontaneous ignition is less significantly affected.With the increase of oxidation rate, the probability of coal spontaneous ignition increases linearly (Figure 4F), and the influence of this factor is relatively small.With the increase of temperature of the coal, the probability of coal spontaneous ignition can be divided into two stages (Figure 4G): the stage of very low probability of coal spontaneous ignition (the coal oxidation rate is very low, equivalent to the incubation period of spontaneous combustion) and the stage of sharply increasing probability of coal spontaneous ignition; in addition, the probability of coal spontaneous ignition also increases sharply with the increase of temperature of the coal (the self-heating stage), exerting a particularly significant influence.
To sum up, although the probability of coal spontaneous ignition is determined by a variety of factors, there are clear differences in the degree of dependence on different factors.According to the degree of influence, for example, for a certain kind of coal, the specific heat capacity, thermal conductivity, water content, and rate of oxidation change little, the ranking of important factors affecting the probability of coal spontaneous ignition is as follows: temperature of the coal > gas seepage velocity > oxygen concentration, and then the existing prevention methods of spontaneous ignition can be improved by strengthening the role of the most important factors.

| CONCLUSION
1. Taking the duration of coal heating to 150°C as the measurement standard of coal spontaneous ignition, the duration of coal heating at the initial stage was found to increase linearly with the increases of coalspecific heat capacity, thermal conductivity and water content.With the increase of oxygen concentration, oxidation rate and initial temperature of the coal, the duration of initial coal heating stage decreases exponentially.With the increase of the flow velocity in coal, the duration of the initial heating stage is parabolic.2. Based on the heat balance equation and reliability theory, the probability prediction model of coal spontaneous ignition was deduced theoretically, and the probability model algorithm of coal spontaneous ignition prediction was proposed.3. The probability of coal spontaneous ignition increases linearly with time for each influencing factor.Meanwhile, the probability of coal spontaneous ignition decreases exponentially with the increase of coal-specific heat capacity and water content.With the increase of thermal conductivity, oxygen concentration, gas seepage velocity, and rate of oxidation, the probability of spontaneous ignition of the coal increases linearly.With the increase in temperature of the coal, the probability of coal spontaneous combustion can be divided into two stages: the stage of very low probability of coal spontaneous combustion (coal oxidation rate is very low), and the stage of rapid increase of coal spontaneous combustion probability.4. The probability of coal spontaneous ignition depends on different factors, and the influences of different factors on coal spontaneous ignition are as follows: temperature of the coal > gas seepage velocity > specific heat capacity > oxidation rate > oxygen concentration > water content > thermal conductivity.5.One of the main advantages of this research method is that it can infinitely change any parameter of the spontaneous combustion process and any combination with other parameters to numerically analyze the probability of coal spontaneous combustion.The research provides a basis for improving the accuracy of predictions of the risk of spontaneous combustion of coal.

4
Variations of the probability of spontaneous combustion of coal.(A) Specific heat capacity, (B) thermal conductivity, (C) oxygen concentration, (D) gas flow seepage velocity, (E) moisture content, (F) oxidation rate, and (G) coal temperature.spontaneousignition with influencing factorsTo describe the changes in probability of coal spontaneous ignition when different influencing factors change, 120, 240, 360, and 480 h were selected, and the influencing factors were taken as variables.The curve of changes in the probability of coal spontaneous ignition is shown in Figure4.As illustrated in Figure4, the longer the time, the greater the probability of coal spontaneous ignition under different factors.At the same time, the probability of coal spontaneous ignition varies with the parameters.
Changes in the probability of coal spontaneous combustion.
T A B L E 1