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

Abstract

This article examines the CO 2 adsorption−desorption kinetics of bituminous coal under low pressure injection (0.5 MPa) in the context of CO 2 sequestration in shallow level coal seams. This study used two different sizes of intact core samples of bituminous samples from seam no. 30 at the Experimental Mine Barbara (EMB) in Katowice, Poland. Manometric adsorption kinetics experiments were conducted on 50 mm dia. 60 mm long coal core samples (referred to as EMB1) and 50 mm dia. 30 mm long coal core samples (referred to as EMB2). The kinetics of adsorption at injection pressures ranging from 0.1 to 0.5 MPa were compared to those at elevated pressures ranging from 0.5 to 4.5 MPa. For the first time, intact sample adsorption−desorption data were fitted in pseudo first order (PFO), pseudo second order (PSO), and Bangham pore diffusion models. The PSO model fits the data better than the PFO model, indicating that bulk pore diffusion, surface interaction, and multilayer adsorption are the ratedetermining steps. Comparing the equilibrium amount of adsorbed (q e ) obtained for the powdered samples (9.06 g of CO 2 /kg of coal at 0.52 MPa) with intact samples (11.68 g/kg at 0.53 MPa and 7.58 g/kg at 0.52 MPa for the intact EMB1 and EMB2 samples) showed the importance of conducting experiments with intact samples. The better fit obtained with the Bangham model for lower pressure equilibrium pressures (up to 0.5 MPa) compared to higher pressure equilibrium pressures (4.5 MPa) indicates that bulk pore diffusion is the rate-determining step at lower pressures and surface interaction takes over at higher pressures. The amount of CO 2 trapped within the coal structure following the desorption experiments strengthens the case for intact bituminous coal samples' pore trapping capabilities.

Modeling Gas Adsorption–Desorption Hysteresis in Energetically Heterogeneous Coal and Shale

“…Gas storage by physical adsorption occurs in the coal matrices and shale formations, for example, , and the sorption hysteresis phenomenon has been observed extensively. ,,, Although the hysteresis phenomenon is evident in adsorbent–adsorbate systems, there is still no consensus on the origin of desorption hysteresis. In addition to the mechanisms mentioned above, pore deformation was also used to explain the occurrence of adsorption hysteresis, which has precedence in explaining the hysteresis behavior of gases on glassy synthetic polymers. , This explanation has recently been verified by Chen et al Through molecular simulations, it was found that the polymer swells to form water–polymer hydrogen bonds upon adsorption, while these hydrogen bonds do not break upon desorption at the same vapor pressure.…”

“…Experimentally measured adsorption isotherms do not coincide with corresponding desorption isotherms determined over the same pressure range on the same samples. This phenomenon, known as adsorption–desorption hysteresis, has been observed in many experimental studies. − …”

“…Higher CO 2 pressure that prevails in the vicinity of an injection well (during the injection stage) is expected to diminish until it reaches a steady state with respect to the pressure across the reservoir. The relaxation of pressure can drive a portion of the weakly sorbed CO 2 gas molecules to desorb into the free gas phase, and such a phenomenon was observed by Sadasivam et al in their laboratory-based (manometric) sorption studies involving intact bituminous coal samples. Wang et al also emphasized on the role of desorption hysteresis in assessing long-term containment and stability of stored CO 2 in coal deposits.…”

Low Subcritical CO2 Adsorption–Desorption Behavior of Intact Bituminous Coal Cores Extracted from a Shallow Coal Seam

“…Along with adsorption, CO 2 desorption behaviors are also important to assess containment of stored CO 2 at the postinjection stage . The CO 2 adsorption–desorption isotherm is not fully reversible .…”

“…Subcritical CO 2 adsorption in shallow coal seams is poorly understood. − CO 2 adsorption capacity of coal generally increases with pressures at subcritical conditions (<7.38 MPa and <304.15 K). ,− Micropores provide most of the surface area for gas adsorption, and it is higher in high-rank coals. However, the presence of channel-like pores and matrix swelling properties influence adsorption behavior of bituminous coals. − Ozdemir et al estimated the adsorption capacity of eight powdered Australian bituminous coal samples at 22 °C and pressures up to 4.0 MPa, and the reported values varied between 1.07 and 1.97 mmol/g of coal. Saghafi et al measured the adsorption capacity of 27 samples of crushed/granular bituminous and subbituminous coal at 39 °C and pressures below 6.0 MPa.…”