Sigra performs laboratory derived sorption isotherms on coal samples utilising pure gases (normally methane and carbon dioxide, or other gases if they are present). A coal sample can either be provided by the client or more normally Sigra makes up a sample to represent the seam from core supplied as part of the gas content measuring process.

This sample is crushed, the moisture content determined, and the weighed sample is placed in a pressure vessel which is placed in an oil bath held at seam temperature to within 0.2oC.  The vessel is briefly evacuated and then filled with the test gas to a pressure greater than reservoir pressure. The pressure is allowed to reach equilibrium (usually a 12 or 24 hour step) and then gas is released and its volume measured. This process is continued down to near atmospheric pressure. The process may then be repeated for different gas types. Finally, helium is used in a similar manner to determine the dead volume in the pressure vessel. The vessel is then opened and a second sub sample taken for moisture and proximate analysis.

Sigra normally performs isotherms at as received moisture content basis. Some clients request and we can perform tests at a standardized moisture content using potassium sulphate to control humidity as per AS1038.17-2000. We do not however recommend this process as it does not relate to the in situ moisture content and it exposes the sample to a deterioration due to oxidation.

Where required, Sigra can calculate the mixed sorption isotherm of individual gas components. This calculation is based on ideal adsorbed solution theory. This service is provided at consulting rates.

Isotherm Equations

Sigra report isotherms on the basis of the measured behaviour, a Langmuir isotherm model fit (equation 1) and a logarithmic fit (equation 2) with an error estimate based on the difference between modelled and fitted behaviour. Coal sorption behaviours may sometimes be best expressed by using one or the other model. Reports are provided on an as received and mineral matter free basis.