Kinetic investigation of solar chemical looping reforming of methane over Ni–CeO2 at low temperature

Abstract

Leveraging solar thermal energy to drive the chemical looping reforming of methane (CLRM) is a promising method of efficiently and selectively reforming methane to produce syngas using renewable energy. In this work, the role of catalytically active nickel in reaction kinetics, conversion, selectivity, and total syngas production during CLRM over Ni–CeO₂ is investigated. Through thermogravimetric analysis (TGA), metallic nickel is shown to help enhance partial oxidation of methane (POM) reaction rates as a result of a lower activation energy reaction mechanism at all oxygen nonstoichiometries, compared to CeO₂. For example, reduction rates of Ni–CeO₂ at 700 °C are comparable to CeO₂ at 900 °C, and no reaction is observed for CeO₂ at 700 °C. Further, extended cycling with Ni–CeO₂ demonstrated stable reaction rates and yields during CLRM at 700 °C, and S_CO remained above 0.98 for the duration of experimentation. Utilizing a larger-scale packed-bed reactor system, Ni–CeO₂ also demonstrated comparable methane conversion, syngas production and selectivity to CeO₂, but at notably lower operating temperatures, i.e., T ≤ 800 °C. Higher rates of coking were observed during POM over Ni–CeO₂; however, all carbon was removed in the subsequent step and accumulation was not observed during extended cycling. A parametric study of gas velocity, temperature, and inlet partial pressure of methane is also presented to examine the effect these operating conditions have on conversion, selectivity, and syngas production. Notably, a tradeoff between conversion and the quantity of syngas produced was observed as gas velocity increased; however, time response of conversion indicates an ideal reaction cut-off time exists where high rates of syngas production can be achieved simultaneously with near complete methane conversion.