Examination of the Factors Inhibiting CO₂ Adsorption on Coal: A Case Study from Shallow-Depth Low-Rank Coal Seams

Understanding the inhibitory factors affecting the adsorption of CO₂ on low-rank coal from shallow-depth coal seams is essential to identify potential coal seams for CO₂ sequestration. The CO₂ adsorption capacity of shallow-depth coals was measured at a low pressure on raw and dry coals. The samples were also prepared for organic analyses, scanning electron microscopy analyses, and low-temperature nitrogen adsorption analyses to evaluate the CO₂ adsorption and identify the inhibitory factors. An investigation was conducted to determine how CO₂ adsorption occurs on coal by fitting experimental data to adsorption isotherm models, followed by analyzing the results based on the statistical analysis. In addition, this study used Henry's law, surface potential, and Gibbs free energy to identify the adsorption inhibitor between CO₂ and coal. The CO₂ adsorption experiment was conducted on raw coal with a moisture content of 15.18-20.11% and dry coal with no moisture. The experimental data showed that the CO₂ adsorption capacity in dry coal was 1.6-1.8 times greater than that in raw coal. A fitting graph between the adsorption data and the isotherm model indicated that CO₂ adsorption on coal occurred on monolayers and multilayers under raw and dry conditions. Statistical evaluation of the adsorption isotherm models showed that the Langmuir and Freundlich models aligned more closely to the experimental data. According to this result, low-pressure adsorption of CO₂ on coal occurred in monolayers and multilayers under raw and dry conditions. Coal containing a high huminite content had a higher potential for CO₂ adsorption, and the drying increased the positive relationship. On the other hand, coal containing high inertinite content inhibited CO₂ adsorption onto the coal, but the drying process did not adversely affect CO₂ adsorption. Furthermore, coal with high moisture and inertinite content inhibited the affinity, accommodation, and spontaneous CO₂ adsorption onto the coal. CO₂ adsorption could lead to swelling, but moisture loss opened more sites and micropores, resulting in the swelling effect not closing all micropores in dry coal. Based on these results, coal seams with low moisture and inertinite content are the most promising for CO₂ adsorption. Altogether, this study provides an understanding of the percentage of inhibitor factors that affects CO₂ adsorption on low-rank coal from shallow depths, which may lead to different CO₂ adsorption capacities.