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

Almolliyeh, Maram 2022. An experimental investigation of adsorption and desorption characteristics of coal and rock. PhD Thesis, Cardiff University. Item availability restricted.

Preview	PDF (PhD thesis) - Accepted Post-Print Version Download (6MB) | Preview
	PDF (Electronic Theses and Dissertations Publication Form) - Supplemental Material Restricted to Repository staff only Download (118kB)

Abstract

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 (<6.1 MPa) for both intact and powdered samples. At near critical pressures (6.1 to 6.4 MPa), the BET model (multilayer) was fitted well for the intact samples, indicating different mechanisms of adsorption occur. The kinetic model fitting (PFO, PSO and Bangham pore diffusion kinetic models) showed that surface interaction and pore diffusion mechanisms are the rate-determining mechanisms of CO2-coal adsorption processes. The experimental data for CO2 adsorption on coal were well fitted with characteristic curves based on surface potential and adsorbed phase density. Rock samples with higher calcium and iron contents absorbed more CO2, emphasizing the significance of caprock chemical composition in CO2 adsorption. Wet and Bacillus mojavensis-loaded sand samples had higher CO2 adsorption capacities than dry sand, indicating the possibility of a chemical reaction (bio-mineralization). At equilibrium pressures of 3.6 MPa, MX80 bentonite exhibited a greater CO2 adsorption capacity than Speswhite kaolin, and both dry and wet sand samples. Experiments on CO2 adsorption and modelling of kinetics showed that physical adsorption occurs with dry sand and clays, while mineralization and surface interactions occur when sand is mixed with biofilm or water.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Engineering
Uncontrolled Keywords:	Subcritical-CO2 , Adsorption-desorption , Kinetics , Coal , Wet-Coal , Caprock
Date of First Compliant Deposit:	26 July 2022
Last Modified:	13 Feb 2023 12:12
URI:	https://orca.cardiff.ac.uk/id/eprint/151467

Actions (repository staff only)

Edit Item