Experimental Study of the Pore Structure and Gas Desorption Characteristics of a Low-Rank Coal: Impact of Moisture

Abstract

Coal–water interactions have a prominent impact on the prediction of coal mine gas disasters and coalbed methane extraction. The change of characteristics in the microscopic pores of coal caused by the existence of water is an important factor affecting the diffusion and migration of gas in coal. The low-pressure nitrogen adsorption experiments and gas desorption experiments of a low-rank coal with different equilibrium moisture contents were conducted. The results show that both the specific surface area and pore volume decrease significantly as the moisture content increases, and the micropores (pore diameter <10 nm) are most affected by the water adsorbed by coal. In particular, for a water-equilibrated coal sample at 98% relative humidity, micropores with pore sizes smaller than 4 nm as determined by the density functional theory model almost disappear, probably due to the blocking effects of water clusters and capillary water. In this case, micropores with a diameter less than 10 nm still contribute most of the specific surface area for gas adsorption in coal. Furthermore, the fractal dimensions at relative pressures of 0–0.5 (D 1) and 0.5–1 (D 2) calculated by the Frenkel–Halsey–Hill model indicate that when the moisture content is less than 4.74%, D 1 decreases rapidly, whereas D 2 shows a slight reduction as the moisture content increased. In contrast, when the moisture content exceeds 4.74%, further increases in the moisture content cause D 2 to decrease significantly, while there is nearly no change for D 1. The correlation analyses show that the ultimate desorption volume and initial desorption rate are closely related to the fractal dimension D 1, while the desorption constant (K t) mainly depends on the fractal dimension D 2. Therefore, the gas desorption performances of coal have a close association with the pore properties of coal under water-containing conditions, which indicate that the fluctuation in moisture content should be carefully considered in the evaluation of gas diffusion and migration performances of in situ coal seams.

Effect of Water on Effective Pore Structures for Medium-Rank Coal: Based on the Prefreezing Nitrogen Adsorption–Desorption Experiment

“…It can be observed that water primarily penetrates into pores with d < 10 nm. This phenomenon is attributed to the fact that the small pores of coal samples are more susceptible to capillary condensation of water . These pores do not completely disappear under the influence of capillary condensation, which is different from the result of former research studies that micropores will be vanished completely .…”

“…The fractal dimension D 1 decreased first and then increased with the increase in RH, indicating that the inner surface of the pores is smooth and then becomes rougher with the increase of water content. The reason for this phenomenon is that hollows and tiny pores will be filled by water molecules during monolayer adsorption, , which then makes the pore walls smoother. Then, as the RH increases, the uneven adsorption of water molecules on the pore wall leads to an increase in irregularity and disorder in the coal surface morphology.…”

Experimental study on the outburst potential energy caused by added water under stepwise depressurization

“…Outburst potential energy caused by added water low-pressure nitrogen adsorption and desorption experiments of different moisture, and confirmed that the added water can change the microscopic pore characteristics of coal, which in turn affects the gas desorption and the initial desorption rate. 24 Zhang et al experimentally verified the obstruction of water molecules to gas desorption path and the existence of limit water content, and concluded that the added water had a significant effect on the average desorption rate of coal samples at the initial stage of water injection. 25 Ding et al simulated the coal and gas outburst through experiments, illustrated the influence of moisture on outburst and pointed out that the critical gas pressure of high moisture briquette is much higher than that of low moisture one.…”

“…Chen et al. carried out low‐pressure nitrogen adsorption and desorption experiments of different moisture, and confirmed that the added water can change the microscopic pore characteristics of coal, which in turn affects the gas desorption and the initial desorption rate 24 . Zhang et al.…”

Characteristics and Influence Factors of Natural Desorption in Coal Bodies from Fukang Mining Area, Xinjiang, China

“…Clearly, the higher the coal rank, the slower the desorption rate decays and the longer the desorption time. The former studies showed that the reservoir temperature, , pressure, and internal factors, such as coal metamorphism, structure, rock, quality, , and pore structure, , are dominant factors that could influence the desorption characteristics.…”