SORPTION BEHAVIOR OF HELIUM AND NITROGEN IN UNCRUSHED COAL CORES UNDER CONTROLLED PRESSURE AND TEMPERATURE

Coal can adsorb carbon dioxide (CO₂), a known greenhouse gas and a cause of global warming, while simultaneously desorbing methane (CH₄), a clean-burning natural gas. This process, known as Enhanced Coal Bed Methane (ECBM), has the potential to store large amounts of anthropogenic CO₂ emissions while producing natural gas. However, coal typically contains nitrogen (N₂), which might affect the overall sorption behavior.  For this reason, it is important to determine the sorption behavior of different gases in coals and the efficiency of coalbeds for long-term storage of greenhouse gases.

A pressure reactor with temperature and pressure controls was constructed to simulate underground coal seam conditions (600 to 700 psi, and 14 to 18 ºC).  More importantly, however, is that the reactor has the ability to analyze whole coal cores, rather than crushed coal, allowing for more realistic analyses of gas sorption and desorption.  The reactor consists of a pressure vessel in which the coal core is housed in a rubber sleeve and suspended by a gas feeder device.  The core is isolated from the reactor walls by the annular space filled with gas. The interaction between feed gas composition, coal, and product gas composition can be monitored within the pressure reactor.  Initial pressure tests with different gas combinations (helium He, nitrogen N₂) indicate that N₂ is preferentially sorbed over He.  While running the system with He, no changes in pressure were observed up to 60 hours of experiment duration.  However, when the system was run with N₂, there was a marked exponential decrease in pressure during the first few hours, followed by a slower decrease as the N₂ diffused into the coal.  After initial saturation occurred, changes in N₂ pressure were minimal.  These results indicate that helium is better than N₂ for testing purposes since helium does not interact with coal structure.  Further tests will compare the sorption behavior of CO₂ and CH₄ versus N₂ in uncrushed coals to help in predicting CO₂ capture and storage in coalbeds containing either a single gas (i.e., CH₄) or a gas mixture (i.e., CH₄, CO₂, N₂).