“…The global rate models are useful only if the specific area and the effective diffusivity can be related to conversion. In the case of spetifi~ area, correlation has been achieved either empirically (8,48,71) or with the aid of theoretical models (78,38,62 17, 25, 37, 38, 40, 41, 54, 60, 62,.69, 87, 90, 98}. The most convenient of these equations express a relationship between reaction rate and the fraction of coal char.ga~ified •.…”