Gas diffusion coefficient in coal: calculation of tangent slope accuracy through the inflection point determination

This investigation aims to develop an accurate method to calculate the tangent slope (b) - a fundamental parameter to calculate gas diffusion coefficients under different pressures - using inflection point determinations. The authors also studied the different tangent slope behaviours depending on the experimental gas sorption used. The single Langmuir model for individual gases and the extended Langmuir model, for multicomponent gas mixtures were applied to fit experimental gas sorption isotherm data. Two coals were selected in order to minimize and/or avoid the maceral composition and vitrinite mean random reflectance effects. Samples were submitted to three different gas compositions, viz. 99.999% CH4; 99.999% CO2; and a gas mixture containing 74.99% CH4 + 19.99% CO2 + 5.02% N2. Results showed that the first and the second derivatives calculated to define the first inflection points represent exactly the final limit of tangent slopes.