Chaotic behaviour of acoustic emission induced in hard coal by gas sorption-desorption

This paper presents study of non-linear dynamics of acoustic emission (AE) generated in coal samples subjected to gas sorption-desorption. Carbon-dioxide and methane were used as sorbats. Experimental facilities used in high pressure sorption of CO₂ and/or CH₄ on coal comprised a pressure vessel and associated pressurisation and monitoring systems. Tests were conducted on medium-rank coal obtained from the Upper Silesia Basin. Several approaches to the treatment of experimental results are proposed in order to detect and characterize deterministic chaos: (1) analysis of fractal/multifractal character of AE energy rate, using fractal generalised dimensions D _q (q); (2) analysis of temporal changes of AE energy rate and its fractal correlation dimension D ₂; and (3) evaluation of attractor dimension within the reconstructed phase space from experimental time series. It was shown that AE generated during CO₂ sorption on medium-rank coal is a more heterogeneous and lower dimensional process in comparison with AE induced by CO₂ desorption. Yet, the AE associated with desorption of CO₂ exhibits higher heterogeneity than the AE generated during desorption of CH₄. There are certain similarities between changes of D₂ during desorption of CO₂ as well desorption of CH₄. However, dynamics of these changes and character of time distributions of D₂ differ, depending on a sorbate. We do not know the precise reason for observed differences, but we presume that the carbon-dioxide molecules dissimilarity to methane molecules can account for them.