Causes and consequences of errors in determining sorption capacity of coals for carbon dioxide at high pressure

Recent comparisons of CO₂ sorption by coals at high pressures have shown major differences between the results obtained by different laboratories. These need to be resolved for laboratory estimation of CO₂ sequestration in coal seams to be useful. A compilation of potential sources of error in determination of sorption characteristics and their impact on sorption measurements is provided here. A series of tests is also provided that can be used to identify and reduce such errors in measurement. For example, an error in temperature produces a characteristic distortion of the sorption curve for carbon dioxide, which can be corrected to some extent. A negative value for excess sorption at high pressure is almost certainly diagnostic of either a cell volume that has been overestimated or that some part of the substrate that is inaccessible to the gas is accessible to helium. The major source of variation between results from the different laboratories that supplied the closest sorption values was found to be variations in the assumed free space volume, which could be due to discrepancies in determined helium density or measured cell volume. Including a term in the sorption model that is proportional to gas density will markedly reduce the influence of such errors in estimating sorption capacity or heats of sorption. The influence of swelling and moisture on sorption isotherms is also quantified here. Correction for swelling of coals in carbon dioxide changes the estimated sorption capacity by less than 1%, if a term that is proportional to gas density is included as a free parameter in the model fitting the isotherm.