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Abstract

We propose an experimental adsorption device, imitating the environment of a coal-mine goaf and the composition of the flue gas in Tashan Mine Power Plant. The characteristics of the coal adsorbing flue gas were studied with the atmospheric volumetric method. The factors affecting the seal of CO₂ were analyzed and the effect of power plant flue gas on fire prevention in the goaf was investigated at normal temperature and pressure. It can be inferred from the experiment that N₂, SO₂, and H₂O can also reduce CO₂ adsorption capacity. The increase or decrease in pH can increase the adsorption capacity of CO₂, which is apparently larger when the pH is decreasing than when the pH is increasing. The O₂ adsorption capacity can evidently be reduced when the power plant flue gas is injected into the goaf. The activation energy of coal burned in air is greater than that of coal burned in flue gas, indicating that the use of power plant flue gas, with N₂ and CO₂ as the main components, to replace the traditional inert gas can not only save N₂ generation cost, but also reduce the emission of greenhouse gases, while the power plant flue gas can be adsorbed by coal.

Keywords

Coal spontaneous combustion power plant flue gas seal activation energy adsorption

Introduction

Spontaneous combustion of coal seriously affects the safety of coal mining and pollutes the environment. It starts with the physical adsorption of oxygen in coal at a lower temperature. With the progress of adsorption, physical adsorption gradually transforms into chemical adsorption, eventually leading to the spontaneous combustion of coal (Deng et al., 2008; Liang et al., 2005; Wang et al., 2007). The fire prevention technology using inert gas is effective in the inertization of the goaf and preventing spontaneous combustion of coal (Liu et al., 2014; Zhu et al., 2013). The use of power plant flue gas, in which the main components are N₂ and CO₂, to replace the traditional inert gas can save the cost of generating nitrogen, as well as reduce the emission of greenhouse gases while the power plant flue gas can be absorbed by coal (Jia et al., 2014, 2015; Shang, 2009). In January 2008, China signed the cooperation project of “Technology of Mining Coal Bed Methane after Injecting/Burying Carbon Dioxide in Deep Coal Beds” with Canada. The experiment on the sequestration of CO₂ in the coal seam and enhancement of the coal bed methane yield conducted by China and Canada has achieved remarkable success. At present, carbon capture and storage (CCS) technology is still in the R&D stage. The storage of CO₂ in unminable coal seams or goafs has become a potentially leading technology method. Actually, before CCS technology was proposed, the theoretical analysis and experimental study of injecting CO₂ into the coal seam has been of significant concern among researchers (Clarkson and Bustin, 2000; Jiang et al., 2010; White et al., 2005; Zhou et al., 2011). Moffat and Weale (1955) studied the adsorption of CO₂ by anthracite under the condition of high pressure, which was the beginning of research on CO₂ sequestration. Reucroft and Patel (1986) studied the change in coal volume after CO₂ where inert gas was adsorbed by different coal types. Nikolai and Andreas (2007) studied the adsorption behavior of supercritical CO₂ for different coal samples at 450°C and a maximum pressure of 20 MPa. However, most of the current research has been focused on the competitive adsorption of CH₄/CO₂ using the high-pressure capacity method, and the research on competitive adsorption of CO₂/N₂ and CO₂/O₂ under atmospheric temperature and pressure is still scarce. As research progressed further, people gradually realized that the experimental environment should simulate the actual conditions in the goaf. Accordingly, this study used an in-house-developed experimental absorption device, while simulating the environment of the coal seam in the goaf and the composition of flue gas in Tashan Mine Power Plant, to study the adsorption characteristics of CO₂ in coal, which is the main component of power plant flue gas. The factors affecting the sequestration of CO₂ were analyzed, and the effectiveness of power plant flue gas in preventing spontaneous combustion of coal in the goaf was investigated at normal temperature and pressure.

Experiments and methods

Preparation before the experiment

The coal samples collected from Tongxin mine, Tashan mine (from Shan Xi), and Gaohai mine (from Liao Ning) were of belonging to bituminous coal. The coal samples were ground to a particle size of 0.15 mm by ball milling and put into a vacuum oven at 60°C to prevent the coal sample from being oxidized.

The mixed gases were prepared in advance as the adsorption gas for experiments and purge gas for thermogravimetry. The composition of the mixed gas was used to simulate the air environment of the goaf and the composition of the power plant flue gas. The main components and contents of the power plant flue gas are 79% N₂, 16.5% CO₂, 4.5% O₂; in addition, it contains a small amount of air pollutants such as SO₂, NO₂, and CO.

Experimental apparatus and method were designed for measuring the amount of gas adsorbed by a large amount of coal at atmospheric pressure, including the pipeline, gas supply device, sealed adsorption cylinder, vacuum pump, and data collection/detection device. The volume of the adsorption cylinder was approximately 55 l. The experimental setup is shown in Figure 1. The adsorption process was as follows: a large amount of pulverized coal was placed in the sealed adsorption cylinder, which was evacuated with a vacuum pump; the experiment gas then was injected into the adsorption cylinder through the gas supply device until the pressure of the cylinder reached approximately 0.1 MPa; the initial volume fraction of the experiment gas inside the cylinder was measured by using gas chromatography; after adsorption for a period of time, the volume fraction of the experimental gas was measured once again, and the adsorption amount of each gas component was calculated by the following formula (equation (1))

V = \frac{(n i - n i') V_{m}}{m} \times 10^{3}

n_{i} = \frac{φ_{i} P_{K} V_{i}}{R T}, n_{i}' = \frac{φ_{i}' P_{K}' V_{i}'}{R T}

(1)

where V is the amount of gas adsorbed by coal (cm³/g); n_i and n_i_′ are the molar mass of the gas components before and after adsorption in the sealed cylinder (mol), respectively; V_m (24.5 l/mol) is the molar volume of the gas at normal temperature and pressure ; m is the quality of coal (g); Φ_i and φ_i′ are the volume fraction of the gas components in the sealed cylinder before and after adsorption, respectively (%); V_i and V_i′ are the volume of the gas components in the sealed cylinder before and after adsorption, whose value is 55 × 10⁻³ m³; P_K and P_K′ are the pressure of the sealed cylinder before and after adsorption (Pa), respectively; T is the temperature whose value is 293.15 K; R is the gas constant of ideal gas (8.314 J/mol K).

Study on the adsorption and storage of power plant flue gas

CO₂ and SO₂ are the main greenhouse gases in power plant flue gas, among which CO₂ has a high concentration and SO₂ is in trace amount, so the effect of N₂, SO₂, H₂O, and pH on CO₂ adsorption was investigated using Tashan and Tongxin coal samples. Each adsorption experiment required 5 kg of coal samples. To study the adsorption capacity of CO₂ and the effects of N₂ and SO₂ on CO₂ adsorption, the experimental composition of the gas was used to simulate the power plant flue gas, in which CO₂: 16.5%, N₂: 79%, SO₂: 0.0049%, and Ar are used as the balance gas because it is not adsorbed by coal. To study the effect of H₂O molecules on CO₂ adsorption in coal molecules, the coal samples were ground and sealed without drying. In the experiment for studying the effect of pH on CO₂ adsorption, two equal-quality coal samples were immersed in water to form two suspensions: 3 mmol/l H₂SO₄ was added in one of the suspensions until the pH of the solution was equal to 2; 10 mmol/l NaOH was added to another suspension until the pH was equal to 12, then the two coal samples were filtered separately and placed in a vacuum drying oven at 60°C for more than 12 h. Since the physical adsorption is essentially saturated in 12 h, the volume fraction of CO₂ was measured by gas chromatography every 2 h until 12 h. The adsorption capacity of CO₂ under different gas atmospheres and chemical environments is given in Table 1.

Table 1.

Adsorption capacity of CO₂ under different physical and chemical conditions.

Adsorptioncapacity(cm³/g)Time (h)	CO₂		N₂+CO₂		SO₂+CO₂		H₂O+CO₂		pH = 2		pH = 12
Adsorptioncapacity(cm³/g)Time (h)	Tongxin	Tashan	Tongxin	Tashan	Tongxin	Tashan	Tongxin	Tashan	Tongxin	Tashan	Tongxin	Tashan
2	0.8280	0.8002	0.7960	0.7832	0.8113	0.7666	0.8045	0.7635	0.8627	0.8361	0.8443	0.8172
4	0.9131	0.8752	0.8765	0.8559	0.8950	0.8451	0.8840	0.8349	0.9429	0.9087	0.9328	0.8923
6	0.9507	0.9080	0.9213	0.8938	0.9287	0.8753	0.9292	0.8725	0.9777	0.9415	0.9675	0.9264
8	0.9748	0.9317	0.9447	0.9201	0.9530	0.8969	0.9500	0.8986	1.0029	0.9652	0.9926	0.9511
10	0.9924	0.9448	0.9693	0.9310	0.9665	0.9106	0.9635	0.9092	1.0166	0.9784	1.0062	0.9647
12	0.9992	0.9516	0.9787	0.9382	0.9736	0.9201	0.9710	0.9165	1.0243	0.9851	1.0138	0.9719

Study on the effect of power plant flue gas on prevention of fire

The experiment analyzing the condition under which the power plant flue gas restricted the adsorption of O₂ on coal and increased the activation energy of coal was conducted using the in-house-developed adsorption device and the thermogravimetric (TG) analysis. The coal samples were from Tongxin and Gaohai mines. Three bottles of the mixed gas were prepared for the experimental gas to study the adsorption of O₂ and spontaneous combustion of coal under N₂ atmosphere and flue gas atmosphere, respectively: one was a mixture of 20% O₂ and 80% N₂, one was a mixture of 20% O₂ and 80% flue gas, and the other was the comparison gas whose components were 20% O₂ and 80% Ar, since the concentration of O₂ in goaf air is approximately 20%. The O₂ adsorption capacity is given in Table 2. NETZSCH-STA449C synchronous thermal analyzer was used for the TG experiment, the mass of the sample was 15 mg, the purge gas was air or flue gas, the gas flow was 30 ml/min, experiment temperature was 20–800°C, and the heating rate was 5°C/min. The TG-DSC figure is shown in Figure 2.

Figure 1.

System diagram of the coal adsorption device.

Table 2.

The adsorption capacity of O₂ under different atmosphere.

Adsorptioncapacity (cm³/g)Time (h)	20%O₂+80%Ar		20%O₂+80%N₂		20% O₂+80% flue gas
Adsorptioncapacity (cm³/g)Time (h)	Tongxin	Gaohai	Tongxin	Gaohai	Tongxin	Gaohai
2	0.1179	0.1227	0.0995	0.1068	0.0943	0.0960
4	0.1540	0.1577	0.1239	0.1314	0.1130	0.1083
6	0.1761	0.1813	0.1379	0.1426	0.1245	0.1145
8	0.1864	0.1929	0.1432	0.1493	0.1317	0.1184
10	0.1923	0.1983	0.1476	0.1537	0.1360	0.1224
12	0.1964	0.2016	0.1520	0.1581	0.1389	0.1250
24	0.2043	0.2109	0.1625	0.1699	0.1482	0.1342
36	0.2130	0.2170	0.1722	0.1770	0.1553	0.1412

Figure 2.

TG-DSC figure of Tongxin and Gaohai coal under different gas atmosphere. TG: thermogravimetric.

Results and discussion

The influence of N₂, SO₂, H₂O, and pH on CO₂ sequestration

To analyze the influence of 79% N₂, 0.0049% SO₂, H₂O and pH on CO₂ sequestration thoroughly, the CO₂ adsorption capacity curve at each time point is shown in Figure 3. The result indicates that there is no change in the trend of CO₂ adsorption after introducing N₂, SO₂, H₂O in the experiment or changing the pH; the adsorption capacity of CO₂ increases with time and reaches saturation at 12 h. Despite the high concentration of N₂, it can only reduce the adsorption capacity of CO₂ by 1.4–2.1%. On one hand, it competes with CO₂ to occupy the pore structure of coal. On the other hand, N₂, as a nonpolar molecule, has only nonpolar bonds and CO₂ has polar covalent bonds. According to the basic theory of adsorption, polar adsorbents tend to adsorb polar adsorbates, so the attraction between CO₂ and coal is greater than that between N₂ and coal, considering coal is a polar macromolecule.

Figure 3.

Effect of N₂, SO₂, H₂O, and pH on sequestration of CO₂ for each coal sample.

SO₂ can reduce the adsorption capacity of CO₂ by 2.3–3.3%, and SO₂ could be entirely sealed after the adsorption is completed; this is because in the competitive adsorption of SO₂ and CO₂ molecules, the power between polar molecules, SO₂, and coal is greater than that between nonpolar molecules, CO₂ and SO₂, as an acid gas could react with the alkaline minerals in coal to produce heat, which can restrain the adsorption process. H₂O can reduce the adsorption capacity of CO₂ by 2.8–3.7%; this is because during the competitive adsorption between H₂O and CO₂ molecules, hydrogen bonds could be found between hydrophilic groups of coal and H₂O molecules, which can result in the reduction of CO₂ adsorption. From the above data, it can be seen that the adsorption capacity of CO₂ will be reduced when SO₂, N₂, and H₂O exist in the adsorption system, due to the competitive molecular adsorption.

However, it can be found that the adsorption capacity of CO₂ can be increased by changing the pH of the adsorption system. The change in pH (increase or decrease) can increase the adsorption capacity of CO₂ by 1.5–3.8%, and the adsorption capacity of CO₂, when the pH is decreasing, is evidently larger than the adsorption capacity of CO₂ when the pH is increasing. The reason for this phenomenon is probably due to the type of the minerals in coal. Raman spectroscopy, XRD spectroscopy, and XRF spectroscopy have revealed many types of minerals, such as calcite, rutile, dolomite, anatase, magnetite, and pyrite, in coal (Gao et al., 2016). These minerals reside in the pore structure of the coal and most of the minerals could react with acids to form soluble salts, as shown in equation (2), so the minerals are entirely left in the solution after the coal is washed with H₂SO₄, and the removal of the acid-soluble minerals will increase the effective pore volume of the coal, making it easier for the adsorption of gas, thus increasing the adsorption capacity of CO₂

C a C O_{3} + 2 H C l \to C a C l_{2} + C O_{2} + H_{2} O

T i O_{2} + 4 H C l \to T i C l_{4} + 2 H_{2} O

C a M g {(C O_{3})}_{2} + 4 H C l \to C a C l_{2} + M g C l_{2} + 2 C O_{2} + 2 H_{2} O

(2)

F e_{3} O_{4} + 8 H C l \to F e C l_{2} + 2 F e C l_{3} +4 H_{2} O

F e S_{2} + 2 H C l \to F e C l_{2} + H_{2} S + S

It has been reported that the ash content of coal with an ash content of 15% would be reduced by 13–11.8% after washing with acid, while the ash content of coal is only reduced by 0.2% after washing with alkali, which indicates that most of the minerals in the coal were not removed after washing with alkali. However, from the experiment in the present study, it could be found that the adsorption capacity of CO₂ is also significantly increased when the coal is washed with alkali; the only explanation could be that NaOH is excessively retained on the surface of the coal after washing with alkali, and the residual NaOH reacts with CO₂, which results in the increase in CO₂ adsorption capacity.

The effect of power plant flue gas on prevention of spontaneous combustion of coal

The adsorption capacity of O₂ curve at each time point is shown in Figure 4. A conclusion could be obtained that adsorption capacity of O₂ is different under different gas atmospheres. The coal sample has the largest O₂ adsorption capacity under the O_2/Ar atmosphere because Ar is not adsorbed by coal. The O₂ adsorption capacity decreases by 18–19% under the O₂/N₂ atmosphere when N₂ is competitively adsorbed with O₂; it is least under the O₂/flue gas atmosphere, while it could be decreased by 27–35%. The above data show that both N₂ and flue gas can play a role in inhibiting the adsorption of O₂. While the effect of flue gas on inhibiting adsorption is better than that of N₂, it shows that the flue gas is more effective than N₂ in preventing the spontaneous combustion of coal.

Figure 4.

Adsorption capacity of O₂ under different atmospheres for each coal sample.

The kinetics of the TG reaction was studied according to the results of TG experiments; the Coats–Redfern approximate integral was derived from the Arrhenius equation, as given by the following formula (equation (3))

\ln [\frac{G (α)}{T^{2}}] = \ln (\frac{A R}{β E}) - \frac{E}{R T}

(3)

where

G (α)

is the most probable mechanism function; the mechanism function

{[- \ln (1 - α)]}^{3 / 2}

is used as the most probable mechanism function of coal oxidation reaction according to the double extrapolation method; α is the conversion percentage, T is the temperature, A is a prefactor, R is molar gas constant, β is the constant heating rate, and E is activation energy. The curve was obtained from

\ln [G (α) / T^{2}]

and

1 / T

, and the activation energy of the oxidation reaction of coal can be obtained from

- E / R

, which is the slope of the curve. The activation energy of each coal sample under different gas atmospheres is given in Table 3.

Table 3.

Activation energy of each coal sample under different gas atmospheres.

Coal	Tongxin		Gaohai
Coal	Air	Flue gas	Air	Flue gas
k	−14,963.39	−17,561.90	−17,623.99	−18,077.13
E (kJ/mol)	124.41	146.01	146.53	150.29

The activation energy of coal is the energy barrier that coal needs to overcome in order to absorb O₂ and react with it, and the smaller the activation energy, the easier spontaneous combustion of coal. It can be seen from the absolute values that the activation energy of the coal burned in the air is smaller than that of the coal burned in the flue gas, which means that coal requires more heat to burn in the flue gas than in air. The presence of flue gas increases the difficulty of spontaneous combustion of coal, and the result illustrates that the flue gas has an inhibitory effect on the spontaneous combustion of coal from the dynamic point of view.

Conclusions

In this study, factors affecting the CO₂ seal in goafs were analyzed and the effect of flue gas on fire prevention in goafs was investigated at normal temperature and pressure. N₂ can reduce the adsorption capacity of CO₂ by 1.4–2.1%, SO₂ can reduce the adsorption capacity of CO₂ by 2.3–3.3%, H₂O can reduce the adsorption capacity of CO₂ by 2.8–3.7%, while the change in pH (increase or decrease) can increase the adsorption capacity of CO₂ by 1.5–3.8%. The adsorption capacity of CO₂, when the pH is decreasing, is obviously greater than the adsorption capacity of CO₂ when the pH is increasing. When the power plant flue gas is injected into the goaf, the adsorption capacity of O₂ can be reduced by 27 and 35% for Tongxin and Gaohai coal, respectively, and the activation energy of the coal burned in air is greater than that of the coal burned in the flue gas. It shows that power plant flue gas can replace conventional inert gases to prevent the spontaneous combustion of coal effectively.

Footnotes

Authors’ contributions

The paper was a collaborative effort among the authors. FG conceived and designed the experiments, wrote the paper; CD proposed the idea; XL performed the experiments; XW and FD analyzed the data.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (51874161) and Youth Fund of Liaoning Province Education Administration (17-1164).

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Experimental study on adsorbing of flue gas and its application in preventing spontaneous combustion of coal

Abstract

Keywords

Introduction

Experiments and methods

Preparation before the experiment

Study on the adsorption and storage of power plant flue gas

Study on the effect of power plant flue gas on prevention of fire

Results and discussion

The influence of N2, SO2, H2O, and pH on CO2 sequestration

The effect of power plant flue gas on prevention of spontaneous combustion of coal

Conclusions

Footnotes

Authors’ contributions

Declaration of Conflicting Interests

Funding

References

The influence of N₂, SO₂, H₂O, and pH on CO₂ sequestration