High-Pressure Redox Behavior of Iron-Oxide-Based Oxygen Carriers for Syngas Generation from Methane

Abstract

For gas to liquid (GTL)-type applications, partial oxidation of methane (CH4) is a viable route for conversion of natural gas to valuable chemicals. The oxygen for this partial oxidation process can be supplied using solid oxygen carriers, which can be single or mixed metal oxides. A particular partial oxidation scheme for CH4 conversion consists of two reactors, using Fe-based oxygen carrier particles, which circulate within the two units and undergo cyclic reduction–oxidation (redox) reactions. The solid carriers, therefore, serve as a vehicle for oxygen between the units, enabling clean conversion of the fossil fuel with high-purity product streams generated. Unlike the conventional combustion and/or gasification, the gaseous products of the two reactors are inherently separated. This allows for minimization of downstream processing and gas separation, making it a highly efficient energy conversion process. For applications involving high-pressure downstream processing (such as producing syngas as an intermediate feedstock for liquid fuels and chemical synthesis), it is advantageous to operate this gas–solid partial oxidation system at elevated pressures. Thus, it is desirable to study the effect of the pressure on the reaction kinetics of the various reactions involved. In this study, the high-pressure experiments were conducted for reduction and oxidation of oxygen carrier particles in a specialized thermogravimetric analyzer. The relative reaction rates are computed for all experiments conducted in the range of 1–10 atm. Specifically, the rate of reduction under H2 is observed to double when the pressure was increased from 1 to 10 atm compared to a 5-fold increase in the reduction rate under CH4. By comparison, the oxidation reaction rate under air is observed to increase by ∼50%. The reduced and oxidized samples are analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) techniques to determine the role of pressure in producing a more reactive particle, which explains the superior reaction rates observed at elevated pressures.