THE CHEMISTRY OF METHANE REFORMING WITH CARBON-DIOXIDE AND ITS CURRENT AND POTENTIAL APPLICATIONS

The reforming of CH4 with CO2 produces synthesis gas with a lower H-2/CO ratio than that generated by the widely employed steam/CH4 reforming reaction. The two reactions have similar thermodynamic characteristics except that in the case of CO2/CH4 reforming there is a greater potential for carbon formation, primarily due to the lower H/C ratio of this system, Thermodynamic analysis of the CO2/CH4 reforming reaction system shows that carbon formation is possible over a wide range of reaction conditions of possible commercial interest, Whilst technology has been developed to enable CO2/CH4 and steam/CH4 reforming to be carried out simultaneously, the former reaction has to date had no significant commercial application by itself. However, there is now renewed interest in C-1-chemistry to produce chemicals and fuels requiring synthesis gas with a 1/1 H-2/CO ratio. Conducted in the absence of steam/CH4 reforming, CO2/CH4 reforming has a number of major advantages over alternative chemical reactions for the thermochemical storage and transmission of renewable energy sources such as solar energy. Hence it is likely to become an increasingly important industrial reaction in the future. A review of the literature on the catalysis of CO2/CH4 reforming shows that Group VIII metals, when distributed in reduced form on suitable supports, are effective catalysts for this reaction. Rh appears intrinsically to be the most suitable, and considering the relative material costs, Ni catalysts deserve closer attention. In the latter case the emphasis should be on developing catalysts which are capable of carbon-free operation under practical reaction conditions. Of the various supports studied to date, alumina and magnesia or combinations thereof are most promising. Analysis of the reaction mechanism indicates that the effective catalysts are those metal-support combinations which actively dissociate CH4 into CHx residues including carbon, whilst at the same time also activating CO2 to generate CO and an adsorbed O species on the catalyst surface. The O thus produced is consumed in the conversion of CH2 and C to CO. Net carbon formation becomes a problem when the CH4 dissociation and CO2 activation steps are out of balance. Considering the current status of catalyst development and the likely future large-scale applications for CO2/CH4 reforming, significant scope exists for further work in optimising both catalysts and reactor design for this reaction.