Reforming of CH 4 by partial oxidation: thermodynamic and kinetic analyses

Abstract

The traditional technology for synthetic-gas (H 2 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 CO 2, which could offer an advantage of vastly reduced energy requirement of the reforming process. This paper reports on a feasibility study of CH 4 partial oxidation into H 2 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 CH 4 oxidation. The effects of initial O 2/CH 4 ratio, temperature and pressure are examined. The thermodynamic analysis indicates that the synthetic-gas yields are strongly dependent on the initial O 2/CH 4 ratio with maxima occurring at an optimal initial O 2/CH 4 ratio varying with temperature. The optimal O 2/CH 4 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 CH 4 partial oxidation at temperatures greater than ca. 1273 K and O 2/CH 4 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 H 2O and CO 2 are the main reaction products. The second stage is a slow ‘conversion’ zone where steam and CO 2 reforming, water gas shift reaction as well as C 2H 2 coupling and C 2H 2 steam reforming takes place with H 2 and CO being the main products. Both thermodynamic and kinetic predictions of H 2 and CO yields compare well at high temperatures. The optimum operating conditions for CH 4 partial oxidation reforming are recommended at 0.5 O 2/CH 4 ratio, 1473 K and 1 atm.