Coal Temperature Variation Mechanism during Gas Desorption Process

To further reveal the mechanism of coal gas migration, the reasons for coal temperature changes during the methane desorption process were analyzed from the aspect of molecular motion and the thermodynamic theory. .e temperature change mechanism was investigated, and the mathematical equation was established to describe the variation of temperature change during the methane desorption and diffusion process..e established equation was applied for the calculation of temperature change for two types of coal samples, and the measured and theoretical values of temperature changes were obtained. .e results show that the temperature changes in the coal gas desorption process are mainly caused by the heat adsorption. .e heat adsorption phenomenon was also caused by free gas expansion during the pressure relief process..e gas diffusion and work done for gas seepage also need heat adsorption..e temperature change is positively correlated to the coal gas pressure, quantity, and limit value of gas desorption volume. Due to the poor insulation in the test system, the difference between the theoretical and the measured temperature change values increase with the adsorption equilibrium pressure. It is helpful to further reveal the mechanism of coal and gas outburst. It also has an important reference value for controlling gas dynamic disasters in coal mines.


Introduction
Gas desorption is a dynamic process that changes over time.
e adsorption and desorption of gas in coal is considered to be a reversible physical process that happens with thermal effects.A large number of studies showed that the coal temperature is closely related to coal gas desorption.e higher the temperature, the larger the desorption rate [1][2][3][4][5][6][7][8][9][10][11].Temperature changes during gas adsorption and desorption were tested using a gas outburst simulator [12,13].It was found that the coal gas adsorption process is an exothermic process, whereas the gas desorption process is an endothermic process.
e hot-dip galvanizing high-sensitivity infrared temperature measurement technology was also applied in studying the temperature change law in the process of coal gas desorption [14].
Desorption heat can be reflected by the temperature changes in the system.e temperature variation during coal and gas outburst is studied by Guo and Jiang [15].A decrease in the temperature of working face and coal wall was observed before the outburst.Workers can even feel the cold currents of gas flow that spray and escape to the roadway, chamber, and working space during gas outburst.But coal and gas outburst is not only related to gas desorption.e gas outburst process includes three stages: inoculation, development, and occurrence.In each process, besides desorption and seepage of methane, the coal reservoir is also subjected to the surrounding ground stress and the erosion of the pore gas on the coal body.e coal body is squeezed and deformed and the internal energy is thus increased [16].Although the desorption process is a complex energy exchange process, the total energy of the desorption process is constant, following the law of conservation of energy.
At present, there is no unified understanding of the causes of temperature change in the process of gas desorption.Studying the mechanism of temperature change during coal gas desorption is of great significance for analyzing the mechanism of energy migration and transformation in coal mining.It is helpful to further reveal the mechanism of coal and gas outburst; therefore, it has important reference value in the prevention and control of gas dynamic disasters in coal mines.

Analysis for Heat Generation
Mechanism during Coal Gas Desorption Process

Microprocess of Gas Desorption.
e process of interaction between coal and gas is an integrated process of seepage-diffusion and adsorption-desorption.As we know, coal is a dual pore structure medium containing pore and fissure structures.Due to disturbance, the original pore distribution is destroyed and a large number of fissure structures are produced during the mine production.Pore structures are the main space for gas adsorption, whereas fractures are the main channels for gas seepage.Generally, the coal matrix is composed of several small matrix elements separated by cracks.e outer surface of each small matrix is a fissured channel with multiple macropore or micropore structures as shown in Figure 1.
Methane is absorbed on the surface of the fissures, the micropore, and the coal matrix.Once it gains enough energy to overcome the van Edward force between the coal molecules and the methane molecules, it passes through the gas film layer formed by methane adsorption.
e process that the adsorb separates into free gas and diffuses into the coal fracture network under the effect of gas concentration gradient is called the desorption process in the coal body.When the coal is suddenly exposed to the atmosphere, the methane gas which is ejected or poured out contains the desorbed methane gas and the original free gas in the fractured network.e movement of methane molecules require external energy support and there must be temperature variation in the process of coal gas desorption and diffusion.

Mechanism of ermal Change during the Coal Gas Desorption Process.
e gas in the coal starts to desorb due to the pressure gradient, temperature, and stress disturbance.It is an endothermic process due to the following reasons: (1) the desorption of the adsorbed gas is endothermic; (2) the free gas within the coal body absorb energy due to expansion; (3) the diffusion is endothermic; and (4) the work done due to seepage adsorb heat.

Heat Adsorption in the Gas Desorption Process.
In general, the undisturbed adsorbed gas in coal is in the state of adsorption equilibrium.
e methane molecules on the surface of coal particles are in a state of vibration, centered at the lowest energy equilibrium point.
e adsorption of methane by coal is a reversible physical adsorption process.e desorption heat of unit mass q can be calculated with where c denotes the mole ratio heat capacity of methane, kJ/(mol • K); m is the mole mass of desorbed methane, mol/g; and ΔT is the temperature difference before and after desorption, K.

Expansion Heat of Free Gas.
e expansion process of free gas can be regarded as an adiabatic process.When the gas expands, it expands outwards to do work.is energy comes from the internal energy of the gas.In order to expand continuously, energy will be absorbed from the surrounding area.
is part of energy will be supplied from coal.erefore, the temperature of coal will decrease due to energy loss.
According to the first law of thermodynamics, where w is the work done during expansion, kJ/kg; p 1 is the initial gas pressure, Pa; v 1 is the initial gas volume, m 3 ; p 2 is the gas pressure after expansion, Pa; v 2 is the gas volume after expansion, m 3 ; and k is the ratio of heat capacity at constant pressure to the heat capacity at constant volume of gas.
(3) erefore, the temperature change caused by gas expansion is where c is the specific heat capacity of coal, kJ/(kg•K).

Heat Adsorption due to Gas Diffusion.
When the adsorbed gas is desorbed, it starts to diffuse under the effect of concentration gradient.Assuming that the diffusion is one-dimensional in a square lattice, in which the methane molecules jump from one barrier to another, and the energy required is where ω 0 is the weak vibrational frequency of methane molecules at equilibrium positions; η is the friction coefficient in the diffusion of methane molecules; and E is the diffusion activation energy, J.

Heat Adsorption due to Gas
Seepage. e desorbed gas and original free gas increases the gas concentration, and the accompanying pressure gradient causes gas flow within the fracture network of the coal body.e kinetic energy of gas was provided by the coal body.According to the conservation of energy, where ΔU is the internal energy loss of the coal body, J; m is the mass of gas flow, g; and v is the velocity of gas flow, m/s.

Equations Used for Gas Desorption Temperature Change
Calculation.When studying the adsorption and desorption laws of gases, we usually refer to the knowledge of surface adsorption in physical chemistry, which regards desorption and diffusion as an entire process.When conducting experiments, the solid surface is usually exposed to the atmosphere.erefore, when studying the endothermic effect, the heat absorption in the process of seepage can be neglected.e diffusion heat effect and gas expansion heat effect can be treated as the two main heat sources in the gas desorption process.In order to study the heat change during desorption, the system is assumed to be adiabatic.

Desorption Temperature of Absorbed Gas.
Assuming the volume of desorbed gas is V t , differential desorption is dv t , and differential adsorption heat is dQ, we obtain According to previous studies, the differential adsorption heat and the gas pressure have the following relations: where A and B are 0.702 J/m 3 and 0.242 MPa −1 , respectively, and p is the pressure before desorption, Pa.
Combining Equations ( 7) and ( 8), we obtain where c 1 is the specific heat of coal, 1.46 kJ/(kg•K), and η � V t /V max is the gas emission rate.
According to Equation ( 9), the temperature decrease in the desorption process increases with the increase of the desorption rate, whose growth rate is accelerating.

Temperature Change of Free Gas due to Expansion.
Assuming the system is an adiabatic system and the expansion process of free gas is an isothermal process, the expansion energy of free gas can be thought that it comes from the coal.Assuming the gas pressure difference is dp in the expansion process and the volume of gas emit to the atmosphere is dV.e temperature of the coal body should decrease by where p is the gas pressure before desorption, Pa, and p 0 is the atmospheric pressure, Pa. e volume of gas within a coal seam can be calculated by V � α � � p √ [17], where α is the coal seam gas content coefficient, m 3 /(t•MPa 1/2 ).erefore, Bringing it into Equation (10), we get erefore,

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ΔT e � − where V 0 is the gas content before desorption, m 3 /t, and η is gas emission rate.erefore, the temperature change in the process of coal gas desorption and diffusion is e above analysis shows that the temperature decrease in the process of desorption is related to the gas desorption amount, the specific heat of the coal body, and the gas pressure.Large gas pressure and more desorption amount result in greater temperature change.

Calculation of Experimental Parameters of Coal Gas Desorption
Gas desorption experiments were conducted based on Malan 8# and Zhenxing2# coal samples.Gas desorption data was analyzed and calculated.Temperature changes under various experimental conditions were obtained.As shown in Table 1, ΔT total is the total temperature change of the coal body during the process of desorption, which is obtained from Equation ( 14); ΔT max measured is the maximum temperature drop of unit mass coal during the desorption experiment.
e above results show that the two temperature values increase with the increasing adsorption equilibrium pressure.However, when the adsorption equilibrium pressure is small, the theoretical temperature difference is close to the measured temperature difference.As the adsorption equilibrium pressure increases, the theoretical temperature difference is larger than the measured temperature difference.
e main reason is that the theoretical temperature difference is assumed to be an adiabatic process, which is not true in fact.When the desorption rate is greater, the rate of desorption and heat absorption is also faster.e heat at this time mainly comes from the coal body, and the measured temperature value should be closer to the theoretical value.But desorption rate decreases as time goes on, and the energy transfer between the environment and the system is difficult to eliminate, which results in larger error.
e value of heat change during the process of desorption was calculated with Equation (1), as shown in Figures 2-5.
e temperature change is the result of the energy change.In this paper, the change of heat, which can be regarded as the change of heat absorption, is obtained according to the experimental measurements and theoretical calculations.It is positively correlated with coal gas pressure, gas content, and desorption.Based on that, the temperature variation of coal can be used as a sensitive index for predicting coal and gas outburst.However, the temperature change in the desorption process is affected by many factors such as the stress formation, occurrence conditions, and metamorphic degree of coal.
e underground measurement method still needs to be studied from many aspects.e test and analysis of gas adsorption and desorption temperature of granular coal is feasible and can be used together with other outburst prediction sensitive indexes.
is method is feasible in theory, but the method of implementation and the determination of the critical value need further study.

Conclusions
(1) From the point of view of molecular motion, the cause of thermal change in the coal gas desorption process was analyzed combined with the theory of thermodynamics, including desorption heat of absorbed gas, expansion heat of free gas, diffusion heat, and heat adsorption due to gas seepage.(2) e temperature variation mechanism of the whole process of gas desorption was analyzed, and a mathematical equation describing the temperature variation was established.e temperature variation in this equation is related to parameters such as gas pressure, limiting desorption volume, and desorption rate.(3) rough the calculation and analysis of the experimental data of two types of coal samples, the measured values and theoretical values of temperature and thermal energy changes during gas desorption were obtained.e results showed both the temperature and heat values increase with the increase of adsorption equilibrium pressure.However, when the adsorption equilibrium pressure is small, the theoretical temperature difference is close to the measured temperature difference.As the adsorption equilibrium pressure increases, the theoretical temperature difference is larger than the measured temperature difference.(4) e temperature variation of coal gas desorption can be considered as a sensitive index for the prediction of gas outburst, but the relevant implementation methods and critical values still need to be studied in the future.Advances in Civil Engineering

Figure 1 :
Figure 1: Schematic diagram of the coal desorption process.

Table 1 :
eoretical and measured values of the two types of coal samples.