Carbon Capture and Storage (CCS) in Coals

Geological formations are suitable locations for CO2 sequestration, and among them, coals are excellent targets because of their nanoporous structure, which leads to a high gas adsorption capacity. In this thesis, a variety of techniques were employed on coal samples to investigate their general properties and gas storage capacity. Initially, a comprehensive literature review was performed to gain more information regarding coal properties and to figure out what factors influence gas storage capacity in coals and what kind of techniques can be applied in pore structural characterization. This thorough literature review led to the publication of a review paper (publication 1). Coal samples from the Fohnsdorf Basin in Austria were studied by methods including Rock-Eval pyrolysis (RE), organic petrography, and low-pressure (LP: N2, CO2) sorption tests. To make a comparison and assess gas sorption capacity, coals from the Leoben Basin in Austria were also examined by gas sorption measurements (publication 2). However, the study focused primarily on Fohnsdorf coal, as Leoben coal reserves were mostly depleted from past mining activities. The pyrolysis Tmax (temperature at peak hydrocarbon generation) and vitrinite reflectance values indicated that both Fohnsdorf and Leoben coals are subbituminous low-rank. At 273 K and ~1 bar, the average CO2 adsorption was highest for Fohnsdorf low-lying mire coals (~0.8 mmol/g). The theoretical CO2 storage potential of the remaining unmined Fohnsdorf coal reserves was estimated at 4.65 million tons. Additionally, their gas-rich composition, containing roughly 1.2 billion m3 of CH4 in place, implied significant potential for enhanced coalbed methane production. Furthermore, coal samples from the D6 coal seam in Karaganda Basin in Kazakhstan were studied using the same approach, and including high-pressure (HP: CO2, CH4) sorption tests (publication 3). Vitrinite reflectance and Tmax values showed that seam D6 reached the medium-volatile bituminous rank. Relatively high ash yield (>8%) and very low sulfur contents (<0.6%) suggested that the coals were deposited in a freshwater low-lying mire. N2 sorption measurements classified the coals as meso- to macroporous. The CBM potential of seam D6 was predicted at 9 billion m3 initial gas and 360 million m3 producible gas in place. Ultimately, the adsorptive and total CO2 storage capacities were estimated at 1.1 gigatons (Gt) and 3.6 Gt, respectively. With this substantial total storage capacity, the D6 seam could store Kazakhstan's current annual CO2 emissions for approximately 14 years. A hysteresis loop was observed between the CO2 adsorption and desorption isotherms for all coal samples studied in this thesis. To determine if this loop resulted from high binding energies between the CO2 molecules and coal matrix, the isosteric heat of adsorption (Qst) was measured for representative samples (publication 4). High Qst values (>40 kJ/mol) in low-rank coals indicated chemical adsorption (chemisorption), while lower Qst values (<40 kJ/mol) in medium-rank coals suggested only physical adsorption of CO2 molecules. Overall, the achievements of the thesis could be summarized as below: The review paper provided a comprehensive discussion on the advantages and disadvantages of methods that can be used for the assessment of gas adsorption capacity in coals as well as factors that influence gas adsorption capacity. Regional case studies indicated high capacity of coals for CO2 sequestration and CH4 production. In particular, D6 coal seam in the Karaganda Basin showed a storage capacity in gigatons scale, thus could be considered as a high-potential carbon sink in Kazakhstan. Furthermore, it was highlighted that CO2, at low-pressure conditions in which it is in subcritical phase, can be adsorbed on coals chemically (although via week bonds) as high heat of ads