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

Summary

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’s physical and mechanical properties induced by CO₂ adsorption.

Abstract

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.

Introduction

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 gas^1,2,3. 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 ....

Protocol

1. Sample preparation

1. 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).
2. Use a tweezer to remove impurities mixed in the coal and scrub the crusher chamber with absorbent cotton and acetaldehyde.
3. Smash the coal blocks into small pieces with a jaw crusher, and shelter them in a sieve....

Results

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.......

Discussion

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.......

Materials

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