Coals and shales exhibit slow desorption behaviour that is related to diffusion. The exact nature of the diffusional process is complicated. The diffusional rates are important from the viewpoint of analysing gas release. This might be from broken coal off the shearer or from fragmented material in an outburst.
The simplest mathematics is that used to describe Fickian diffusion along a concentration gradient. Its use tends to lead to varying diffusion coefficients on different size particles with the larger particles exhibiting higher diffusion rates. Whether this is a function of microfracturing within, or odder shaped larger particles, is uncertain.