Physical modification law and dust reduction effect of liquid CO2-water circulation on coal

Abstract

Coal seam water injection is a proactive and curative measure to prevent coal mine dust hazards and also one of the essential means to prevent and control gas protrusion, impact ground pressure, and other disasters. However, many coal seams in China are characterized by high ground stress, low porosity, and low permeability, and the traditional methods and techniques face some bottlenecks such as complex water injection, long water injection cycle, and pseudo-wetting of coal. To this end, using liquid CO2 with excellent characteristics such as low-temperature freezing, high permeability, phase change self-pressurization, acidification, and unblocking, the authors put forward a new idea of fracturing and wetting coal by liquid CO2-water circulation, developed an automatic control test system of cold dipping by liquid CO2 circulation. The influence law of liquid CO2-water cycling on the pore structure of coal was investigated using low-field nuclear magnetic resonance (LF-NMR) instrumentation, and the mechanism of change in the physical properties of coal by cyclic action was investigated using an electro-hydraulic servo-pressure experimental machine. The dust production characteristics of cyclic action on the coal crushing process were investigated using the coal rock-cutting dust production experimental system. The results show that the liquid CO2-water circulation increased the effective porosity (\begin{document}${\varphi _{{\mathrm{NF}}}} $\end{document}) of the coal, and the increase was positively correlated with the number of circulations. The bound fluid inside the coal become less, the free fluid increased, and the T2cutoff value decreased. Combined with the fractal theory, it was found that based on the fractal dimensionality, Ds of seepage porosity had prominent fractal characteristics. The primary pores of the coal underwent the “enlargement” process. Microfracture and original fracture formed a through, and the circulation increased the connectivity of the effective seepage channel and optimized the coal’s porosity network and seepage conditions. With the increase in the number of cycles, the coal’s maximum stress value (\begin{document}${\sigma _{\mathrm{c}}} $\end{document}) decayed exponentially to 4.93 MPa and the strain value (\begin{document}${\varepsilon _{\mathrm{c}}} $\end{document}) increased linearly to 2.29%, the compressive strength of the coal weakened, the deformation capacity increased, the cyclic action changed the state of the coal matrix interconnection, and the resulting freezing and expansion forces exerted extrusion on the coal exacerbated the expansion of the fissures, and the brittleness index B5 decreased by the maximum amount of 34.71%, which significantly weakened the brittleness of coal, and had a better resistance to dynamic load or impact. The weakening of the storage capacity of the coal body for the applied energy changed the form of coal destruction in the cutting process, and the whole dust production rate of the coal body cutting under the test conditions of this paper was reduced by 74%. The percentage of respiratory dust was reduced to 2%, drastically weakening the dust’s hazardousness.

Keywords

• liquid co2-water
• circulation effect
• pore structure
• mechanical properties
• physical modification
• dust reduction effect