An experimental investigation of adsorption and desorption characteristics of coal and rock

The adsorption characteristics of coal and the associated caprock system are the key aspects of carbon sequestration in un-mineable coal seams. The effects of injection pressure, fabric, presence of water, biogeological conditions of coal, and compositions of caprock systems tend to affect the CO2 adsorption-desorption behaviour. Adsorption-desorption behaviour of an anthracite coal (Aberpergwm) and a bituminous coal (Big Pit) from South Wales Coalfield, a sedimentary rock from East Irish Sea, a sand and two clays (Speswhite kaolin and MX80 bentonite) were studied at sub-critical (up to 6.1 MPa) and near-critical (6.1 MPa and 6.4 MPa) injection pressures at a temperature of 298.15 K. A state-of-the-art manometric gas adsorption experimental apparatus was used for this purpose. The water retention characteristic curves of the coals were established using the chilled-mirror dew-point technique for a large range of suctions. For the coals studied, both powdered and intact samples were tested at various water contents. The rock samples tested were from two different depths and had different chemical compositions. The sand was tested under dry, wet and Bacillus mojavensis (bacteria) loaded conditions. The clays were tested in powder form. Existing Langmuir and Brunauer-Emmett-Teller (BET) adsorption isotherm models, pseudo-first-order (PFO), pseudo-second order (PSO), and Bangham pore diffusion kinetic models, and characteristic curves based on potential theory of adsorption and adsorbed phase density were used to evaluate the experimental data. At the subcritical pressure region, the adsorption characteristics of powdered coal samples differed from those of intact coal samples, highlighting the significance of the fabric and structure of the sample in relation to the coal rank. Similarly, at near-critical pressure ranges, the CO2 adsorption isotherm pattern of intact coal was significantly different from that of a powdered sample of the same coal rank (anthracite). The CO2 adsorption capacities of wet intact coal samples were found to be higher than those of dry samples for anthracite coal. Wet powdered coal samples, on the other hand, had a lower adsorption capacity than dry samples. The CO2 adsorption-desorption hysteresis of a wet powdered anthracite coal sample demonstrated that CO2 molecules interact with coal differently than those of dry samples. The findings show that at lower injection pressures (<2 MPa), CO2 molecules must compete with water molecules, but at higher pressures, CO2 molecules can replace the water present in the pores and occupy the water-activated sites. The findings from the water retention behaviour and pH buffering capacity of coal supported the need to investigate the CO2 adsorption capacity of wet coal. Bacillus mojavensis bacteria was found to grow well on coal samples, implying that CO2 biomineralization should be taken into account and emphasised the importance of examining intact samples with site conditions. CO2 adsorption experimental data were better fitted with the Langmuir model (monolayer) at lower pressures (