The mechanism of hydrogenation of coal to methane

Abstract

It has been shown that the overall kinetics of hydrogenation of coal can be explained on the basis of a two-stage reaction mechanism. The first stage involves the very rapid reaction of hydrogen with groupings in the coal which are associated with oxygen, and can be considered to be instantaneous in practical systems. At temperatures in excess of 700º the yield of hydrocarbons from the groups in coal which contain oxygen is a function of the hydrogen partial pressure, which in turn is limited by equilibrium of the water gas shift reaction.    Hydrogenation of the volatile matter results in the formation of a less readily hydrogenated structure which is predominantly carbon. The rate of hydrogenation of the residual carbon can be expressed in the form which was proposed for the system carbon-hydrogen-methane : (see formula in article) where k_l, k₂, k₃, and k₄ are constants, and p_l, p₂ are the partial pressures of hydrogen and methane respectively. It has been shown that the β-graphite equilibrium applies to the system char-hydrogen-methane and is independent of the reactivity of the char. This fact can be used to simplify the rate expression and write the constants in terms of ratios of quantities which can be readily measured. The initial high reactivity noted by the authors and other workers can be produced repeatedly by adsorbing hydrogen on the surface of the char at low temperature and desorbing this hydrogen as methane at high temperature. This behaviour was predicted from the proposed kinetic scheme.