Name: a uniaxial compression experiment with co2-bearing coal using a visualized and constant-volume gas-solid coupling test system
Uploaded: 2019-06-12T13:00:00
Description: Watch this Scientific Journal Video about A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System at JoVE.com

This work was supported by the China National Major Scientific Instruments Development Project (Grant No. 51427804) and the Shandong Province National Natural Science Foundation (Grant No. ZR2017MEE023).

1. Sample preparation
<ol>
	<li>Collect raw coal blocks from the 4671B6 working face from the Xinzhuangzi coal mine. Note that, due to the low strength and looseness of the structure, the raw coal is broken and probably mixed with impurities. To avoid the influence of these internal and external factors, as well as reduce the inhomogeneity of coal as much as possible, select large coal blocks (about 15 cm long, 10 cm wide, and 10 cm high).</li>
	<li>Use a tweezer to remove impurities mixed in the coal and scrub the cru...

<ol>
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Considering the danger of high-pressure gas, some critical steps are important during the test. The valves and O rings should be inspected and replaced regularly, and any source of ignition should not be allowed in the laboratory. When using the manual pressure-regulating valve, the experimenter should twist the valve slowly to make the pressure in the visualized vessel increase gradually. Do not disassemble the vessel during the test. When the experiment is finished, the back door of the vessel should be opened after th...

The increasing concentration of CO2 in the atmosphere is a direct factor causing the global warming effect. Due to the strong sorption capacity of coal, CO2 sequestration in a coal seam is regarded as a practical and environment-friendly means to reduce the global emission of greenhouse gas1,2,3. At the same time, the injected CO2 can replace CH4 and result in gas production promotion in coalbed methane recovery (ECBM)4,5,6. The ecological and economic prospects of CO2 sequestration have recently attracted worldwide attention among researchers, as well as among different international environmental protection groups and governmental agencies.
Coal is a heterogeneous, structurally anisotropic rock composed of a pore, fracture, and coal matrix. The pore structure&#160;has a large specific surface area, which can adsorb a large amount of gas, playing a vital role in gas sequestration, and the fracture is the main path for free gas flow7,8. This unique physical structure leads to a great gas adsorption capacity for CH4 and CO2. Mine gas is deposited in coalbed in a few forms: (1) adsorbed on the surface of micropores and larger pores; (2) absorbed in the coal molecular structure; (3) as free gas in fractures and larger pores; and (4) dissolved in deposit water. The sorption behavior of coal to CH4 and CO2 causes matrix swelling, and further studies demonstrate that it is a heterogeneous process and is related to the coal lithotypes9,10,11. In addition, gas sorption can result in damage in the constitutive relation of coal12,13,14.
The raw coal sample is generally used in coal and CO2 coupling experiments. Specifically, a large piece of raw coal from the working face in a coal mine is cut to prepare a sample. However, the physical and mechanical properties of raw coal inevitably have a high dispersion degree due to the random spatial distribution of natural pores and fractures in a coal seam. Moreover, the gas-bearing coal is soft and difficult to be reshaped. According to the principles of the orthogonal experimental method, the briquette, which is reconstituted with raw coal powder and cement, is regarded as an ideal material used in the coal sorption test15,16. Being cold-pressed with metal dies, its strength can be preset and remains stable by adjusting the quantity of cement, which benefits the comparative analysis of the single-variable effect. Additionally, although the porosity of the briquette sample is ~4-10 times, that of the raw coal sample, similar adsorption and desorption characteristics and stress-strain curve have been found in the experimental research17,18,19,20. In this paper, a scheme of a similar material for gas-bearing coal has been adopted to prepare the briquette21. The raw coal was taken from the 4671B6 working face in the Xinzhuangzi Coal Mine, Huainan, Anhui Province, China. The coal seam is approximately 450 m below ground level and 360 m below sea level, and it dips at about 15&#176; and is approximately 1.6 m in thickness. The height and diameter of the briquette sample are 100 mm and 50 mm, respectively, which is the recommended size suggested by the International Society for Rock Mechanics (ISRM)22.
The previous uniaxial or triaxial loading test instruments for gas-bearing coal experiments under laboratory conditions have some shortages and limitations, presented as fellows23,24,25,26,27,28: (1) during the loading process, the vessel volume decreases with the piston moving, causing fluctuations in gas pressure and disturbances in gas sorption; (2) the real-time image monitoring of samples, as well as circumferential deformation measurements in a high gas pressure environment, is difficult to conduct; (3) they are limited to stimulation of dynamic load disturbances on preloaded samples to analyze their mechanical response characteristics. In order to improve the instrument precision and data acquisition in the gas-solid coupling condition, a visualized and constant-volume test system29 has been developed (Figure 1), including (1) a visualized loading vessel with a constant volume chamber, which is the core component; (2) a gas filling module with a vacuum channel, two filling channels, and a releasing channel; (3) an axial loading module consisting of an electro-hydraulic servo universal testing machine and control computer; (4) a data acquisition module comprised of a circumferential displacement measurement apparatus, a gas pressure sensor, and a camera at the window of the visualized loading vessel.
The core visualized vessel (Figure 2) is specifically designed so that two adjusting cylinders are fixed on the upper plate and their pistons move simultaneously with the loading one through a beam, and the sectional area of the loading piston is equal to the sum of that of the adjusting cylinders. Flowing through an inner hole and soft pipes, the high-pressure gas in the vessel and the two cylinders is connected. Therefore, when the vessel-loading piston moves downward and compresses the gas, this structure can offset the change in volume and eliminate pressure interference. In addition, the enormous gas-induced counterforce exerting on the piston is prevented during the test, significantly improving the safety of the instrument. The windows, which are equipped with tempered borosilicate glass and situated on three sides of the vessel, provide a direct way to take a photograph of the sample. This glass has been successfully tested and proved to resist up to 10 MPa gas with a low expansion rate, high strength, light transmittance, and chemical stability29.
This paper describes the procedure to perform a uniaxial compression experiment of CO2-bearing coal with the new visualized and constant-volume gas-solid coupling test system, which includes the description of all pieces that prepare a briquette sample using raw coal powder and sodium humate, as well as the successive steps to inject high-pressure CO2 and conduct uniaxial compression. The whole sample deformation process is monitored using a camera. This experimental approach offers an alternative way to quantitively analyze the adsorption-induced damage and fracture evolution characteristic of gas-bearing coal.

<table>
	<tbody>
		<tr>
			<td>3Y-Leica MPV-SP photometer microphotometric system</td>
			<td>Leica,Germany</td>
			<td>M090063016</td>
			<td>Used for vitrinite 
			reflectance measurement</td>
		</tr>
		<tr>
			<td>Automatic isotherm adsorption instrument</td>
			<td>BeiShiDe Instrument Technology (Beijing)CO.,Ltd.</td>
			<td>3H-2000PH</td>
			<td>Isothermal adsorption test</td>
		</tr>
		<tr>
			<td>Electro hydraulic servo universal testing machine</td>
			<td>Jinan Shidaishijin testing machine CO.,Ltd</td>
			<td>WDW-100EIII</td>
			<td>Used to provide 
			axial pressure</td>
		</tr>
		<tr>
			<td>Gas pressure sensor</td>
			<td>Beijing Star Sensor Technology CO.,LTD</td>
			<td>CYYZ11</td>
			<td>Gas pressure monitoring</td>
		</tr>
		<tr>
			<td>Gas tank(carbon dioxide/helium)</td>
			<td>Heifei Henglong Gas.,Ltd</td>
			<td></td>
			<td>Gas resource</td>
		</tr>
		<tr>
			<td>high-speed camera</td>
			<td>Sony corporation</td>
			<td>FDR-AX30</td>
			<td>Image monitoring</td>
		</tr>
		<tr>
			<td>Incubator</td>
			<td>Yuyao YuanDong Digital Instrument Factory</td>
			<td>XGQ-2000</td>
			<td>Briquette drying</td>
		</tr>
		<tr>
			<td>jaw crusher</td>
			<td>Hebi Tianke Instrument CO.,Ltd</td>
			<td>EP-2</td>
			<td>Coal grinding</td>
		</tr>
		<tr>
			<td>Manual pressure reducing valve</td>
			<td>Shanghai Saergen Instrument CO.,Ltd</td>
			<td>R41</td>
			<td>Outlet gas pressure adjustment</td>
		</tr>
		<tr>
			<td>Proximate Analyzer</td>
			<td>Changsha Kaiyuan Instrument CO.,Ltd</td>
			<td>5E-MAG6700</td>
			<td>Coal industrial analysis</td>
		</tr>
		<tr>
			<td>Resistance strain gauge</td>
			<td>Jinan Sigmar Technology CO.,LTD</td>
			<td>ASMB3-16/8</td>
			<td>Poisson ratio measurement</td>
		</tr>
		<tr>
			<td>Sieve shaker (6,16mesh)</td>
			<td>Hebi Tianguan Instrument CO.,Ltd</td>
			<td>GZS-300</td>
			<td>Coal powder shelter</td>
		</tr>
		<tr>
			<td>Soft pipe</td>
			<td>Jinan Quanxing High pressure pipe CO.,Ltd</td>
			<td></td>
			<td>Inner diameter=5 mm 
			maximal pressure=30 MPa</td>
		</tr>
		<tr>
			<td>Standard rock sample circumferential deformation test apparatus</td>
			<td>Huainan Qingda Machinery CO.,Ltd</td>
			<td></td>
			<td>Circumferential deformation 
			acquisition</td>
		</tr>
		<tr>
			<td>Strain controlled 
			direct shear apparatus</td>
			<td>Beijing Aerospace Huayu Test Instrument CO.,LTD</td>
			<td>ZJ-4A</td>
			<td>Tensile strength, cohesion, internal friction 
			angle measurement</td>
		</tr>
		<tr>
			<td>Vaccum pump</td>
			<td>Fujiwara,Japan</td>
			<td>750D</td>
			<td>Used to vaccumize the vessel</td>
		</tr>
		<tr>
			<td>Valve</td>
			<td>Jiangsu Subei Valve Co.,Ltd</td>
			<td>S4 NS-MG16-MF1</td>
			<td>Gas seal</td>
		</tr>
		<tr>
			<td>Visual loading vessel</td>
			<td>Huainan Qingda Machinery CO.,Ltd</td>
			<td></td>
			<td>Instrument for sample 
			loading and real-time monitoring</td>
		</tr>
	</tbody>
</table>

a uniaxial compression experiment with co2-bearing coal using a visualized and constant-volume gas-solid coupling test system

Injecting carbon dioxide (CO2) into a deep coal seam is of great significance for reducing the concentration of greenhouse gases in the atmosphere and increasing the recovery of coalbed methane. A visualized and constant-volume gas-solid coupling system is introduced here to investigate the influence of CO2 sorption on the physical and mechanical properties of coal. Being able to keep a constant volume and monitor the sample using a camera, this system offers the potential to improve instrument accuracy and analyze fracture evolution with a fractal geometry method. This paper provides all steps to perform a uniaxial compression experiment with a briquette sample in different CO2 pressures with the gas-solid coupling test system. A briquette, cold-pressed by raw coal and sodium humate cement, is loaded in high-pressure CO2, and its surface is monitored in real-time using a camera. However, the similarity between the briquette and the raw coal still needs improvement, and a flammable gas such as methane (CH4) cannot be injected for the test. The results show that CO2 sorption leads to peak strength and elastic modulus reduction of the briquette, and the fracture evolution of the briquette in a failure state indicates fractal characteristics. The strength, elastic modulus, and fractal dimension are all correlated with CO2 pressure but not with a linear correlation. The visualized and constant-volume gas-solid coupling test system can serve as a platform for experimental research about rock mechanics considering the multifield coupling effect.

The average mass of the briquette sample was 230 g. Depending on the industrial analysis, the briquette exhibited a moisture content of 4.52% and an ash content of 15.52%. Furthermore, the volatile content was approximately 31.24%. As the sodium humate was extracted from the coal, the components of the briquette were similar to raw coal. The physical characteristics are displayed in Table 2.
The comparison of th...

Watch this Scientific Journal Video about A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System at JoVE.com

A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System

This protocol demonstrates how to prepare a briquette sample and conduct a uniaxial compression experiment with a briquette in different CO2 pressures using a visualized and constant-volume gas-solid coupling test system. It also aims to investigate changes in terms of coal&#8217;s physical and mechanical properties induced by CO2 adsorption.

A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System

Injecting carbon dioxide (CO2) into a deep coal seam is of great significance for reducing the concentration of greenhouse gases in the atmosphere and ...

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