Reforming of CH4 by partial oxidation: thermodynamic and kinetic analyses

The traditional technology for synthetic-gas (H2 and CO) production from natural gas is steam reforming. A major drawback of this technology is the intensive energy requirement due to the high endothermicity of the reforming reactions. A possible alternative is methane partial oxidation, or in a combination with steam or CO2, which could offer an advantage of vastly reduced energy requirement of the reforming process. This paper reports on a feasibility study of CH4 partial oxidation into H2 and CO by means of thermodynamic and kinetic analyses. The thermodynamic analysis has been performed using the Gibbs free energy minimisation method, and the kinetic modelling has employed the CHEMKIN package incorporating the GRI 1.2 mechanisms of CH4 oxidation. The effects of initial O2/CH4 ratio, temperature and pressure are examined. The thermodynamic analysis indicates that the synthetic-gas yields are strongly dependent on the initial O2/CH4 ratio with maxima occurring at an optimal initial O2/CH4 ratio varying with temperature. The optimal O2/CH4 ratio decreases with increasing temperature and approaches 0.5 at temperatures greater than 1073 K. The synthetic-gas yields also increase with increasing temperature but with decreasing pressure, yet high temperature can suppress the pressure effect. The GRI mechanisms are found to be adequate for the CHEMKIN simulations of CH4 partial oxidation at temperatures greater than ca. 1273 K and O2/CH4 ratio greater than 0.5. The CHEMKIN simulations suggest that two distinct stages exist during the partial oxidation. The first stage is a rapid ‘oxidation’ zone where H2O and CO2 are the main reaction products. The second stage is a slow ‘conversion’ zone where steam and CO2 reforming, water gas shift reaction as well as C2H2 coupling and C2H2 steam reforming takes place with H2 and CO being the main products. Both thermodynamic and kinetic predictions of H2 and CO yields compare well at high temperatures. The optimum operating conditions for CH4 partial oxidation reforming are recommended at 0.5 O2/CH4 ratio, 1473 K and 1 atm.