(280d) Evaluation of Iron Based Oxygen Carriers for Chemical Looping Dry Reforming

‘Chemical Looping Combustion’ (CLC) is an emerging clean combustion technology which offers an efficient and elegant route for fossil fuel combustion with inherent CO₂ capture. In CLC, an oxygen carrier (typically a metal) undergoes cyclic reduction and oxidation with fuel and air, respectively. By changing the oxidant, the process can be applied for purposes beyond combustion: Replacing air with CO₂ as oxidant results in dry reforming of the fuel, producing CO as result of the carrier oxidation. This process, Chemical Looping Dry Reforming (CLDR), hence results in utilization of CO₂, providing a potential alternate pathway to carbon mitigation via sequestration. However, since CO₂ is a weaker oxidant than air, significant challenges lie in the slow oxidation kinetics. While nano-sizing the oxygen carrier can be expected to increase carrier activity and result in higher carrier utilization, it also exacerbates the carrier stability issues at the demanding high-temperature cyclic operation conditions.

In the present study, iron was chosen as oxygen carrier for CLDR based on a prior thermodynamic evaluation of various metals. Different nanocomposite iron carriers, including Fe-BHA (barium hexaaluminate) and Fe@SiO₂ core-shell materials, were synthesized with the aim to combine high reactivity and high carrier utilization with good thermal stability. The carriers were characterized by TEM, SEM, XRD, and BET, and then evaluated in fixed bed reactor and TGA (thermogravimetric analysis) CLDR studies. Our results indicate that sufficiently small Fe nanoparticles show high CO₂ conversion activity down to temperatures well below 400^oC, but that the oxide matrix required to render the carriers thermally stable can result in significant mass transfer limitations at high-temperature conditions (~800^oC).  Overall, our results indicate that CLDR could offer a novel way for efficient CO₂ utilization over a broad temperature range.