Direct determination of total CO2 uptake by coal: A new technique compared with the manometric method

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This paper reports a newly developed technique that directly measures total CO2 uptake of mm-sized coal matrix cylinders, without the application of the EoS for CO2 and without the need for swelling corrections or He-pycnometry. The technique makes use of a capsule composed of ductile metals (Au and In), pressure-fitted to the sample, which traps directly both adsorbed and free CO2 taken up by the sample upon exposure to CO2. We applied the method to samples of high volatile bituminous coal (Brzeszcze, Seam 364, Poland), saturated with CO2 at a fixed pressure between 0 and 18MPa at 40°C, and, as a function of pressure, yielded a Langmuir-like CO2 uptake curve (within a band less than 0.5mmolgcoal-1) with a maximum CO2 content of 4.08mmolgcoal-1 at 18MPa CO2 pressure. For comparison, manometric determinations were performed on a combined set of 8 samples, also at 40°C, which yielded a three-stage uptake curve showing lower uptake than the capsule-derived curve, showing 20–30% lower uptake at pressures above ∼9MPa. Allowing for worst case errors, the differences in CO2 uptake obtained using the two methods are attributed to (i) random errors and to uncertainties EoS in the manometric data set at low CO2 pressures (3–9MPa), and (ii) systematic errors, due to erroneous trapping of free CO2, dominating in the capsule data set high CO2 pressures (>9MPa). The capsule method proved reliable at CO2 pressures of 0–7MPa, while at pressures higher than 8 or 9MPa the manometric method was most reliable. Although improvement is needed to prevent erroneous trapping of free CO2 at pressures above 9MPa, our new encapsulation method has the potential to accurately determine the uptake of any adsorbate by any (swelling) adsorbent, e.g. CO2 uptake by shale and clay caprocks and is suitable for assessment of the effects of small-scale lithological differences in CO2 uptake. Use of the manometric method with sufficiently large samples, or the capsule method at P<9MPa, provides a reliable means of measuring the CO2 uptake capacity, yielding errors that are less than the effects of in situ stress on sorption.