(622c) Reducible Supports for Nickel-Based Oxygen Carriers in Chemical Looping Combustion

‘Chemical looping combustion’ (CLC) is an emerging technology for clean energy-production from fossil and renewable fuels. In CLC, an oxygen carrier (typically a metal) is first oxidized with air. The hot metal oxide is then reduced in contact with a fuel in a second reactor, thus combusting the fuel. Finally, the reduced metal is transferred back to the oxidizer, closing the materials “loop”. In this way, CLC produces sequestration-ready CO₂-streams without expensive air separation and hence without significant energy penalty. Combined with sequestration, CLC thus allows high-efficiency, CO₂ emissions-free combustion of fossil fuels, or combustion processes with negative CO₂-footprint from biomass-derived fuels.

To-date, various oxygen carriers have been investigated to maximize carrier and fuel conversion while making the carriers more thermally and mechanically stable to obtain sustained performance at demanding operating conditions of CLC. Potential oxygen carrier metals viz. Ni, Fe, Cu, Co and Mn have been studied extensively. But these metal oxides have been investigated only on non-reducible supports like Al₂O₃, SiO₂, TiO₂, etc. some of which are currently considered to be most viable supports for CLC.

Here, we report on comparative evaluation for performance of Ni-based oxygen carriers supported on reducible supports (CeO₂, La₂O₃) and non-reducible supports (Al₂O₃, SiO₂). Most of the published work to-date has been focused on using only non-reducible supports. But using reducible supports not only leads to understanding their effect on reactivity of supported metal in redox operation, but also allows estimation of added contribution of support for oxygen carrying capacity of carrier. 

In this work, we synthesize Ni-based carriers (40 wt% Ni) by simple incipient wet impregnation technique on as synthesized and/or commercial supports. These as synthesized and post reactive tests carrier samples are characterized by TEM, XRD, BET, and EDX. Initially, the reducible supports CeO₂ and La₂O₃; along with Ni-based carriers were tested in thermogravimetric analyzer (TGA) at 800^oC for their activity in CLC with H₂ as a model fuel. Although CeO₂ and La₂O₃, themselves do not contribute significantly to overall oxygen carrying capacity. More importantly, Ni-CeO₂ and Ni-La₂O₃ show complete carrier utilization in TGA, whereas Ni-SiO₂ and Ni-Al₂O₃ shows partial utilization of oxygen carrying capacity (~91% and ~81% respectively). With increase in operating temperature, contribution from reducible support shows a marginal increment and non-reducible support show improvement in carrier conversion.

Ni-CeO₂ and Ni-Al₂O₃ are then down selected based on parameters like reducibility, thermal stability and cost for further evaluation with CH₄ as fuel in TGA and fixed-bed studies at 800^oC. The experiments were aimed at limiting/avoiding coking on the carriers, critical for their long term reactivity in CLC, by means of limited CH₄ exposure for reduction half cycle. In TGA studies, Ni-CeO₂ again shows complete carrier utilization and thus outperforms Ni-Al₂O₃ (~77% carrier conversion). Post use characterization of carriers by TEM, XRD, and BET does not reveal any significant sintering and spinel formation for the two carriers. Furthermore, these carriers are then evaluated in fixed-bed reactor, wherein the reduction period was limited to complete combustion of CH₄. Ni-CeO₂ not only shows a longer period of reduction half cycle indicating better carrier performance, but also >90% fuel conversion and thus is a promising candidate for efficient CO₂ capture via CLC. Thus our results show, reducible support based carriers can be a new class of cheap and efficient carriers for CLC operation.