Use of in-situ electron paramagnetic resonance to assess formation of free radicals and their role in the hydroliquefaction of coal

The depolymerization of coal is generally presumed to proceed via a free radical mechanism. A unique electron spin resonance cavity that allows the observation of free radical formation and behaviour under hydroliquefaction conditions (500 °C and 12.4 MPa) was designed to facilitate the study of the effect of process variables on the formation of free radicals. Parallel experiments were carried out in autoclaves, and the degree of conversion and quality of coal-derived liquids produced was assessed. Correlative statistical models were established that allow the quantitative determination of the effect of process variables, and their interactions, on the production and behaviour of free radicals and of coal-derived liquids produced. Strong evidence was obtained that free radical chemistry plays a key role in determining the degree of coal conversion and quality of products. The kinetics and mechanism of hydroliquefaction are addressed with the explicit inclusion of experimental free radical concentration in the kinetic expressions. Progressive and regressive reactions are discussed. Optimum combinations (‘processing windows’) of process variables can be identified.