Adsorption Effect of Overlying Strata on Carbon Dioxide in Coalfield Fire Area

2.1 Samples

Wuda region locates at ~E39°26′51″, N106°42′46″, 3.5 km away from the Yellow River in the east, 2km away from Wuhushan hill in the west. It is the intersection of Kubuqi, Maowusu and Wulan deserts. Experimental samples are taken from No.3 coalmine of Huayin Company, located in Wuda, Inner Mongolia. The samples are obtained from four stratums (sandy soil, sandstone, mudstone and shale, from surface to coalbed), which are outcrop obviously (See Figure 1). The samples collected in situ are put in sealed containers and taken to the laboratory. Then, they are smashed to particles and are sieved to 60 meshes, which are put into wide mouth bottle for experiment. The sample mass used in each adsorption test is 120 g. The basic parameters of the experimental samples are listed in Table 1.

Table 1. Basic parameters of samples

1.png

Figure 1. Stratum of the sampling site at coal fire area

2.2 Equipment and method

Selection of experimental pressure and temperature. Burning depth in the coalfield fire in north China are range from dozens of meters to over two hundred meters14 -16, but not more than 250m usually. Normal formation pressure gradient is 9.8-10.496 KPa/m 17. Therefore the experimental pressure range is set as 0.5-3.5Mpa with an interval of 1Mpa (namely, there are four pressure equilibrium points, i.e. 0.5Mpa, 1.5Mpa, 2.5Mpa and 3.5Mpa). The research and survey of coal spontaneous combustion in north China 18 showed that temperature of overlying strata ranged from normal to 300^oC at different stages of coal fire. Thus, the experimental temperature is selected as 20-300^oC. During the experiment, gas analysis is conducted every 20 ^oC (there are 15 temperature equilibrium points).

Determination of the proportion of absorbed gas. CO₂ has the largest proportion in all of GHGs during coal fire burning, followed by CH₄ 19. Meanwhile, there also exists N₂ and other micro gases (such as CO) in the environment of coal fire. Our objective is to investigate the characteristics of overlying rock (soil) adsorbing CO₂. Thus, here ignores the influence of micro gases and observes the law of rock (soil) adsorbing CO₂ when there exist CH₄ and N₂. The adsorption experiment is preceded in two steps: (1) isothermal adsorption of pure CO₂, N₂, and CH4; and (2) adsorption of CO₂/N₂/CH₄ ternary mixed gas. For CO₂/N₂/CH₄ ternary mixed gas adsorption test, the volume proportion of N₂, CH₄ and CO₂ is set as: (1) N₂: CH₄:CO₂=20:20:60; (2) N₂: CH₄:CO₂= 50: 10:40; (3) N₂: CH₄:CO₂= 20:60:20%; (4) N₂: CH₄:CO₂=50:40:10.

The experimental system is shown in Figure 2. Adsorption experiment is finished by ISO-300 gas adsorption analyzer, made in TerracTek Company of USA. During experiment, open the control valve and the reference cylinder valve, and charge CO₂ and other gases into the experimental system; adjust the temperature of the reference cylinder to target temperature. When the temperature reaches the target and remains steady, the adsorption test procedure is started to collect time, pressure, temperature and other relevant data in the sample cylinder and the reference cylinder automatically. And the data are recorded as a file. According to the maturity and the quality of samples, the adsorption equilibrium time is determined as no less than 12 hours.

2.png

Figure 2.  Gas adsorption analyzer system

4.1 CO₂ adsorption equation

During the process of coal fire developing, the adsorption of rock (soil) on burning products (CO₂, CH₄, N₂, etc.) belongs to gas-solid adsorption issue, which has the most commonly applied theory - Langmuir adsorption theory. According to Langmuir theory, the adsorption equation of rock (soil) on single component gas 25 can be described as follows

$V=V_{L} \cdot \frac{P}{P_{L}+P}$    (1)

Where V is the adsorptive capacity, m³/t; P is the adsorption pressure, MPa; V_L is Langmuir volume coefficient; P_L is Langmuir pressure coefficient.

The experimental results of coal fire in situ and in laboratory 1213 show that CO₂ had the largest proportion in the GHGs produced by coal fire, followed by CH₄, while N₂ has the largest proportion among all the gas components. Our experimental results conclud that CH₄ and N₂ reduce the quantity of CO₂ adsorbed by the overlying strata because of the adsorption competition. Thus, Langmuir single component adsorption equation do not suitable for predicting the adsorption effect of the overlying rock (soil) on CO₂ directly. Extended Langmuir model retains all the consumed conditions of traditional Langmuir adsorption theory and considers that each component will conduct competitive adsorption at each active center. Therefore, here adopts the extended Langmuir model to investigate the adsorptive capacity of rock (soil) on CO₂ when there exist CH₄ and N₂. The adsorption model based on extended Langmuir equation is

$V_{i}=\frac{V_{L i} b_{i} P_{i}}{1+\sum_{i=1}^{n} b_{i} P_{i}}$      (2)

Where V_i is the adsorptive capacity of component i, m³/t; V_Li is Langmuir volume coefficient of component i；b_i is Langmuir adsorption coefficient of component i, equals the reciprocal of Langmuir pressure coefficient, 1/P_Li; P_i is the partial pressure of component i, MPa.

According to law of partial pressure, in the mixed gas, the partial pressure (P_i) of component i equals to the total pressure multiplied by the volume fraction (or mole fraction) of component i. Then, based on Eq. (2), the equation of calculating CO₂ adsorptive capacity of the overlying rock (soil) in the coal fire area is

$V_{C O 2}=\frac{V_{L, C O_{2}} y_{C O_{2}} P / P_{L, C O_{2}}}{1+\frac{y_{C O_{2}} P}{P_{L, C O_{2}}}+\frac{y_{C H_{4}} P}{P_{L, C H_{4}}}+\frac{y_{N_{2}} P}{P_{L, N_{2}}}}$       (3)

Where V_CO2 is CO₂ adsorptive capacity at multi-components, m³/t; V_L,CO2 is Langmuir volume coefficient of pure CO₂; P_L,CO2, P_L,CH4, P_L,N2 are Langmuir pressure coefficient of pure CO₂, pure CH₄ and pure N₂; P is total pressure, MPa; y_CO2, y_CH4, y_N2 are the volume fraction of CO₂, CH₄ and N₂ in the mixed gas.

It is needed to be explained that V_L and P_L in Eq. (3) have definite physical meanings. V_L represents the volume coefficient, and the smaller it’s fitting value, the less the gas saturation adsorption capacity. P_L stands for the pressure coefficient, whose value relates with gas adsorption heat. The larger the P_L value, the smoother the isothermal adsorption curve of the gas. However, if achieving the real physical meanings of V_L and P_L, the pressure adopted in the adsorption test should reach a rather large range. It needs change from very low range of Henry’s Law to the adsorption limit pressure of the gas. The pressure doesn’t reach the state of adsorption saturated pressure in our study, for this reason, it is unnecessary to emphasize the physical meanings here. V_L and P_L are regarded as empirical constants merely.

According to the experimental data and Eq. (1), we regress the adsorptive capacity of pure CO₂, pure CH₄ and pure N₂ at different temperatures to acquire the V_L and P_L of single component (see Table 3). It showes that the fitting results are high correlation with the experimental results (R² were above 0.98). From Table 3, it can be seen that P_L around one certain constant with a small variation range with the rise of the temperature, while C_L decreases. Therefore, we apply nonlinear fitting to obtain the computational equation of C_L and regard P_L as a constant. The expression of C_L and the mean value of P_L regressed from the experimental results are all listed in Table 4. According to Table 4, the expression of V_L fitted from the four samples can be expressed as

$V_{L}=a_{1} \ln t+a_{2}$    (4)

Where a­₁ and a₂ are the two coefficients in fitted equation of V_L in Table 4.

Taking Eq. (4) into Eq. (3), the volume quantity of CO₂ absorbed by unit mass rock (soil) at standard state is

$V_{C O 2}=\frac{\left(a_{1} \ln (t)+a_{2}\right) \cdot y_{C O_{2}} \cdot P / P_{L, C O_{2}}}{1+\frac{y_{C O_{2}} P}{P_{L, C O_{2}}}+\frac{y_{C H_{4}} P}{P_{L, C H_{4}}}+\frac{y_{N_{2}} P}{P_{L, N_{2}}}}$    (5)

Table 3. V_L and P_L of the shale

Table 4. C_L,CO2 expressions and mean value of P_L to pure gas adsorption

Table 5. Comparison of predicted value and experimental value of CO₂ (CO₂:CH₄:N₂=40:10:50, shale)

In order to observe the accuracy of Eq. (5), here selects the predicted value of CO₂ absorbed by rock (soil) to compare with the experimental value at CO₂:CH₄:N₂= 40:10:50 (see Table 5). The results show that the predicted value of CO₂ as the ternary adsorption are very close to the experimental value. These mean that the extended Langmuir equation can realize the computation of CO₂ adsorption capacity of rock (soil) well.

We regard the earth surface pressure as 0.1MPa and the formation pressure gradient as 9.8KPa/m, then the equilibrium pressure in Eq. (5) can be expressed as P=0.1+0.0098x (x is the depth away from earth surface, m). So the quantity of CO₂ adsorbed by single overlying stratum of the coal fire area is

$G_{C O 2}=S \cdot \rho \cdot h \cdot

\int_{x_{1}}^{x_{2}} \frac{1964 \times\left(a_{1} \ln t+a_{2}\right) y_{1}(0.1+0.0098 x) / P_{L, C 02}}{1+\frac{y_{C O 2}(0.1+0.0098 x)}{P_{L, C O 2}}+\frac{y_{C H 4}(0.1+0.0098 x)}{P_{L, C H 4}}+\frac{y_{N 2}(0.1+0.0098 x)}{P_{L, N 2}}} d x

$    (6)

Where G is the mass of CO₂ adsorbed by the stratum, g; t is temperature, ^oC; x is the depth of the stratum away from the earth surface, m; h is tthickness of the stratum, m; S is the projected area of the fire area on the surface, m²; ρ is the density of the rock (soil), g/cm³; 1964 is the coefficient of the volume of CO₂ converting into quality,  the mole number of 1m³ gas is 22.4Kmol at the standard state, then the quality of 1m³ CO₂ at the standard state is 1000/22.4×44=1964g.

4.2 Emission coefficient of CO₂

Coal measures of Wuda coalfield is rather steady inclined strata with 23 coal seams (8 seams in Zhiluo formation, i.e 1#, 2#, 3#, 4#, 4_l#, 5#, 6#, and 7#, with the total thickness of 16.6 m and the coal bearing ratio of 15.4%; 15 seams in Yan’an formation, i.e. 8#, 9#, 10#, 11#, 12_u#, 12#, 13_u3#, 13_u2#, 13_u1#, 13#, 14#, 15#, 16#, 17# and 18#, with the total thickness of 20.7m and the coal bearing ratio of 9.4%). Most of roofs and floors of the coal seam are leveling. In 2012, the area of Wuda coal field fire area was about 239.6 hm² 26. The densities of shale, mudstone, sandstone and sandy soil overlying the coal bed are 2.5g/m³, 2.55 g/cm³, 2.6 g/cm³ and 1.6 g/cm³, respectively. The average thickness of shale, mudstone, sandstone and sandy soil are about 40m, 40m, 20m, and 1m. According to the observation value of surface temperature and coal spontaneous combustion temperature 2728 and the stratum heat transfer coefficient 29 30, the temperatures of shale, mudstone, sandstone and sandy soil strata are about 100 ^oC, 80 ^oC, 50 ^oC and 20 ^oC. The laboratory test of coal spontaneous combustion showes that the volume concentration of CO₂ generated by spontaneous combustion could reach as high as ~15% when coal fire reaches fully developing stage (~800^oC 28). Correspondly, the volume concentration of N₂ is ~77% and that of CH₄ as high as 4%. Besides CO₂, N₂ and CH₄, during coal fire reaction, there also includ other gases, such as O₂, CO, etc, which are included in N₂ component in our paper. Thus, here selectes CO₂:CH₄:N₂=15:81:4. According to Eq. (6), the CO₂ adsorption quantity of the different overlying strata in Wuda fire area are shale with 55606t, mudstone with 16424t, sandstone with 5040t and sandy soil with 12t. Therefore, the total quantity of CO₂ adsorbed by the overlying strata is 77082t.

In the 23 coal beds of Wuda coalfield, 1#, 2#, 4#, 6#, 7#, 9#, 10#, 12# has coal fire. Based on the data of thickness of coal beds 31, the average thickness of coal fire existing is 7.06m 32. Based on the data of combustion efficiency of industrial bituminous coal 3334, the average combustion efficiency is estimated as 76.5%. The average density of coal in Wuda fire area is 1.53g/m³ 35, while the carbon content is 0.72 and the average coal remaining rate is 60%. And so, the CO₂ released by the coal in Wuda coalfield fire scope is calcu1ated as 313617t based on coal complete combustion law. Then, the proportion of CO₂ from coal combustion fixed by the overlying rock(soil) is 77082/313617=25% in the maximum. Namely, if we ignore CO₂ absorbed by coal beds and underground water, the emission coefficient of CO₂ released in Wuda coal field fire area is ~75%.