Development and Optimisation of Novel Fibrous Oxygen Carriers for Chemical Looping Reforming Processes

Aluminosilicate microfibers have been applied as structured catalyst supports for their specific use as oxygen carriers (OC’s) in the chemical looping steam reforming (CLSR) of methane for the first time. CLSR is a syngas and hydrogen production technology designed to intensify the traditional catalytic steam methane reforming process by improving the provision of heat to the endothermic steam reforming (SR) reaction whilst producing a non-nitrogen diluted syngas. It does so through the cyclical reduction and oxidation of a solid OC material. Application of fibrous structured OC’s to packed bed CLSR can, in principle, offer further intensification through their excellent heat and mass transfer properties and increased process and physical flexibility when compared to conventional OC’s.
Nine cobalt doped 18 wt% nickel-based OC’s each deposited on a Saffil® (Sf) aluminosilicate fibre support (~ 95 % γ-Al2O3, ~5 % SiO2, fibre width 3 - 4 μm and length ~ 5 mm) were produced using three methodologies; wet impregnation (WI), deposition precipitation (DP) and hydrothermal synthesis (HT). The Sf OC’s exhibited a deposited layer consisting of NiO with no evidence of bulk nickel aluminate (NiAl2O4) spinel phase. These materials were characterised and compared to a granulated 18 wt% NiO/α-Al2O3 OC (18NiO GR) prepared from bulk pellets in SR and CLSR experiments using a benchtop packed bed reactor and a N2 diluted CH4/H2O mixture feed (steam to carbon ratio 3:1).
Isothermal CLSR at 700 °C showed that once reduced to Ni by the CH4/H2O feed, the Sf OC’s increased the average CH4 conversion during SR when compared on an equal mass basis by up to ~ 10 percentage points (86 % to 96 %) in the 18Ni Sf-HT OC compared to the 18NiO GR OC. When compared to the 18NiO GR OC on an equal volume basis, the Sf OC’s produced only a small penalty to these factors above despite operating with 80% less mass of OC. These improvements in catalytic performance of the Sf OC’s are likely caused by a reduction in the effective diffusion length required for CH4 to transfer from the bulk gas to active Ni catalytic sites, and for reaction products to diffuse away from those sites into the bulk gas phase, thereby reducing the effects any diffusion limitation of the SR reaction. The Sf-DP and Sf-HT OC’s improved SR performance over the Sf-WI OC’s in both the equal volume and equal mass CLSR conditions. These results were similar when tested in SR experiments using an OC pre-reduced by a N2 diluted H2 feed.
In a series of kinetic investigations (450 - 700 °C), the reduction reactions of the Sf OC’s and the 18NiO GR OC during CLSR of methane were best described by the Avrami-Erofeyev nucleation and nuclei growth model. The Sf OC’s exhibited greater reduction reactivity, a higher peak rate of reduction, and a 40% decrease in the activation energy of reduction when compared to the 18NiO GR OC.