Chemical looping beyond combustion: production of synthesis gas via chemical looping partial oxidation of methane

Abstract

The recent surge in natural gas reserves has revived interest in the development of novel processes to convert natural gas into valuable chemical feedstocks. In the present work, we are applying “chemical looping”, a technology that has found much attention as a clean combustion technology, towards selective partial oxidation of methane to produce synthesis gas (CLPOM). By tailoring the composition of Ni_xFe_1−x–CeO₂ oxygen carriers and carefully controlling the supply of oxygen, i.e., the extent of the carrier reduction and oxidation in redox cycles, the reactivity and selectivity of these carriers for partial oxidation was optimized. Addition of a small amount of Ni to iron oxides allowed the combination of the high reactivity of Ni for methane activation with the good syngas selectivity of iron oxides. An optimized carrier with the composition of Ni_0.12Fe_0.88–CeO₂ demonstrated excellent stability in multi-cycle CLPOM operation and high syngas yields with a H₂ : CO ratio of ∼2 and minimal carbon formation. Finally, a simplified fixed-bed reactor model was used to assess the thermal aspects of operating the process in a periodically operated fixed-bed reactor. We found that the process is highly sensitive to the degree of carrier utilization, but that maximum temperatures can be easily controlled in CLPOM via control of the active metal content and oxygen utilization in the carriers. Overall, chemical looping partial oxidation of methane emerges as an attractive alternative to conventional catalytic partial oxidation, enabling the use of low-cost transition metal oxides and air as oxidant, and resulting in inherently safe reactor operation by avoiding mixed methane/air streams.