Supercritical CO2 extraction of organic matter from coal based on CO2 sequestration in deep coal seams

To evaluate the potential for mobilizing organic matter in coal during CO2 sequestration into deep coal beds, three coal samples of different rank were extracted with supercritical CO2 (Sc-CO2) using a self-assembled experimental device. The results showed that the extract yields decrease with the increase in coal rank, extraction temperature or coal particle size. Sc-CO2 extraction yields of lignite, bituminous and anthracite coal sample with a size less than 0.2 mm were 749, 218 and 201 mg/kg on a dry, ash-free basis at 40 degrees C respectively. Most of the small molecular organics in coal could be extracted at 40. with Sc-CO2, especially for low-rank lignite and high-rank anthracite. Although Sc-CO2 has a strong dissolution effect on small coal molecules, medium and high pressure (> 7 MPa) did not favor the diffusion of the extract from coal into the CO2-free phase. The main control factors in Sc-CO2 extraction of different rank coals may be different, e.g., the dissolved-organics quantity for lignite, the extraction temperature for bituminous coal, and the pore structure of coal for anthracite. These results demonstrate that Sc-CO2 dissolved small organic molecules trapped within coal will be mobilized with CO2, which is important to evaluate CO2 sequestration into deep coal seams.