Performance of a low Ni content oxygen carrier for fuel gas combustion in a continuous CLC unit using a CaO/Al2O3 system as support

摘要

The behavior of a Ni-based oxygen carrier with low NiO content (11.8 wt.% NiO) prepared by impregnation on CaAl2O4 has been studied in a continuous CLC unit (500 Wth) using different gases as fuels (methane, H-2, CO, syngas, ethane and propane). More than 90 h of successfully operation at high temperature (1173 K) have been carried out analyzing the effect of the oxygen carrier-to-fuel ratio and fuel gas composition regarding combustion efficiency and product gas distribution. Using syngas, pure CO or H-2 as fuels, full combustion can be achieved working at oxygen carrier-to-fuel ratios, phi, higher than 1.2. Regarding methane, the maximum fuel combustion efficiency is reached in a narrow range of phi values close to 1 (1.0-1.2). An increase in the value of this parameter produces a decrease in the combustion efficiency. This behavior is different to that found using most of the Ni-based oxygen carriers present in literature, and can be attributed to the low global catalytic activity of the reduced oxygen carrier for reforming reactions. When light hydrocarbons are used as fuels, the oxygen carrier presents a similar behavior than in the case of methane combustion tests, reaching to the maximum fuel combustion efficiency at the same phi values. This fact also indicates that light hydrocarbons combustion mechanism is carried out through cracking reaction. The solids inventory needed to obtain a methane combustion efficiency of 99% is lower than 180 kg/MWth, which corresponds to a metallic Ni inventory around 17 kg/MWth. This value is the lowest referred in the literature for any kind of Ni-based oxygen carrier. This remarkable result is due to the low NiO content (11.8 wt.%) and the very high reactivity of this oxygen carrier because all nickel in the particle is present as free NiO, since the formation of less reactive nickel compounds is avoided using CaAl2O4 as inert support