(649a) Effects of Coal Interaction with Supercritical CO2: Chemical Structure

Interactions between CO2 and coal have been gaining interests due to its application in geologic sequestration of CO2 and methane recovery in coalmines. Recently, we reported the changes in combustion behaviors, including NO emissions and carbon burnout, and physical structures of coals after treatment. The objective of the current work is to investigate the changes in chemical structure of coals after supercritical CO2 treatment. Lignite and bituminous coal are treated by supercritical CO2 in a batch reactor at 80 and 130 °C, at 2000 to 8100 psig, and with 48 h contact time. The as-received, dried (under vacuum at 100 °C overnight) and treated coals are characterized by temperature-programmed pyrolysis (TPP) with 5 °C/min heating ramp up to 1650 °C followed by online analysis by gas chromatography / mass spectrometer (GC/MS). These coals are also characterized by Fourier-transform infrared spectrometer (FTIR). All raw, dried and treated bituminous coals emit little physically bound CO2 below 200 °C during TPP. All of them emit CO2 with peaks at about 400 and 650 °C with dried and raw bituminous coal treated at 8100 psig emitting highest amount CO2. All lignite samples emit similar amount of CO2 at about 450 °C, which is about ten times that from the bituminous coal samples. CO emission from treated bituminous coal increases dramatically with increasing treatment temperature and pressure during TPP. But such pressure effect is not observed for treated lignite, that emit double the amount of CO that their bituminous coal counterparts do. Drying induces little effect on CO emission during TPP for either bituminous coal or lignite.

FTIR spectra suggest that both drying and treatment significantly enhance the alcoholic/phenolic groups for both bituminous coal and lignite. Both drying and CO2 treatment reduce the ether linkages in bituminous coal and lignite. Carboxyl groups in bituminous coal decrease after drying, but they increase with treatment pressure. For lignite, carboxyl groups do not change significantly with drying and treatment conditions. Carbonyl groups only appear in bituminous coal and they increase significantly after drying and treatment. Drying creates polycyclic aromatic structure in both coals; interestingly, but the newly created polycyclic aromatic structure does not remain in the treated coals.