The Influence Mechanism of Pore Structure of Tectonically Deformed Coal on the Adsorption and Desorption Hysteresis

Abstract

Pore is the main adsorption and desorption space of coalbed methane (CBM). Pore size configuration and connectivity affect the adsorption/desorption hysteresis effect. Using tectonically deformed coal (TDC) and original structure coal of medium- and high-rank coal as the research objects, through the N2/CO2 adsorption experiment to analyze the pore size distribution and connectivity of different scales. We investigate the control mechanism of heterogeneous evolution in the key pore scales against adsorption/desorption hysteresis characteristics during coal metamorphism and deformation by combining the CH4 isothermal adsorption/desorption experiment under 30°C equilibrium moisture. The findings indicate that the super micropores (<2 nm) are mainly combination ink bottle-shaped pores and have worse connectivity as the degree of metamorphism and deformation increases. The super micropores occupy the vast majority of pore volume and specific surface area; its pore size distribution curve change presents an “M” bimodal type and is mainly concentrated in two pore segments of 0.45–0.70 nm and 0.70–0.90 nm. The effect of ductile deformation exerts a significantly greater effect on super micropores than brittle deformation. The exhibited adsorption–desorption characteristics are the result of the combined effect of the unique pore structure of the TDCs and different moisture contents. The presence of a large number of super micropores is the most important factor influencing the degree of gas desorption hysteresis. The “ink-bottle effect” is the primary cause of gas desorption hysteresis. For CBM development, some novel methods to increase desorption and diffusion rate at the super micropores scale should be considered.

Effect of Physical Nature (Intact and Powder) of Coal on CO2 Adsorption at the Subcritical Pressure Range (up to 6.4 MPa at 298.15 K)

“…Methane desorption was widely focused on in previous works, − the CO 2 desorption data are less discussed in the literature, and it is important to investigate the reversibility of pore trapped CO 2 . The pore trapping mechanisms such as pore blockage, gas cavitation, adsorption induced deformation, and pore network (ink bottle effect) affect the adsorption–desorption hysteresis pattern and correlate with the coal rank . In general, a limited number of studies modeled the desorption kinetics. , Among the two dominant kinetic models, pseudo-first order (PFO) and pseudo-second order (PSO), the PSO model agreed well with the experimental results obtained using a manometric adsorption experimental set up for an intact and powdered bituminous coal sample, implying that CO 2 adsorption kinetics and hysteresis were determined by pore diffusion and condensation. , Njikam and Schiewer (2012) modified the commonly used adsorption kinetic models (PFO and PSO) to adopt the desorption process.…”

“…The pore trapping mechanisms such as pore blockage, gas cavitation, adsorption induced deformation, and pore network (ink bottle effect) affect the adsorption–desorption hysteresis pattern and correlate with the coal rank. 32 In general, a limited number of studies modeled the desorption kinetics. 32 , 33 Among the two dominant kinetic models, pseudo-first order (PFO) and pseudo-second order (PSO), the PSO model agreed well with the experimental results obtained using a manometric adsorption experimental set up for an intact and powdered bituminous coal sample, implying that CO 2 adsorption kinetics and hysteresis were determined by pore diffusion and condensation.…”

“… 32 In general, a limited number of studies modeled the desorption kinetics. 32 , 33 Among the two dominant kinetic models, pseudo-first order (PFO) and pseudo-second order (PSO), the PSO model agreed well with the experimental results obtained using a manometric adsorption experimental set up for an intact and powdered bituminous coal sample, implying that CO 2 adsorption kinetics and hysteresis were determined by pore diffusion and condensation. 34 , 35 Njikam and Schiewer (2012) 36 modified the commonly used adsorption kinetic models (PFO and PSO) to adopt the desorption process.…”

Kinetics of Gas Phase CO2 Adsorption on Bituminous Coal from a Shallow Coal Seam

“…However, the amount remaining adsorbed was greater than that of the corresponding equilibrium conditions of the adsorption because all the adsorbed CO 2 was not readily desorbed, or the process was not reversible, which further explains the better fit of the second-order kinetic model. The significant amount of residual CO 2 trapped in the coal samples (Figures –) was attributed to the pore trapping mechanisms such as pore blockage, gas cavitation, adsorption induced deformation, and pore network effect or ink bottle effect. − The results in Figure show that 17 g of CO 2 /kg of coal remained in the EMB1 coal core at the end of the desorption experiments. Similar CO 2 entrapment was observed during the lower pressure EXP2 and EXP3 tests on the intact EMB1 and EMB2 coal cores (Figures and ).…”

Adsorption-Desorption Behavior and Pesticide Bioavailability of Biochar in Soil

“…The adsorption-desorption hysteresis behavior of pesticides was more visible after adding black carbon to soil, and the higher the black carbon content, the more obvious the desorption hysteresis (45). The hypothesis of "micropore adjustment effect" explains desorption hysteresis (46). Accordingly, micropore adsorption is the direct cause of desorption hysteresis.…”