278425 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 800oC 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 800oC. 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.
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