Assessing the Kinetics and Capacity of Gas Adsorption in Coals by a Combined Adsorption/ Diffusion Method

Storage of carbon dioxide in geological formations is for many countries the only option to reduce greenhouse gas emissions and thus to satisfy the Kyoto agreements. The CO ₂ storage in unminable coal seams has the advantage that it can sequestrate CO ₂ emissions from industrial processes and be used to enhance coal bed methane recovery (CO ₂-ECBM). For this purpose, the storage capacity of coal is an important economical parameter, An experimental set-up has been developed to determine the rate of adsorption of gases such as carbon dioxide and methane in coal. Sorption kinetic experiments were carried out at various pressures and on coal particles with varying grain sizes (40μm-2mm diameter). The particles were considered to consist of aromatic aggregates embedded in amorphous aliphatic coal, intersected by cleats. The characteristic time of adsorption increases with increasing pressure and increasing particle diameter. However the characteristic time increases only for very small particles. This is attributed to the presence of cleats inside the larger particles. From the experiments it appears that water inhibits diffusion due to its presence in the small particles or in the small cleats. The results of the diffusion experiments can be described by an upscaled diffusion -adsorption model. Based on this model and the experiments an accurate assessment of the storage capacity can be achieved.