In chemical looping combustion, the combustion reaction is split into two sub-reactions linked by metal oxide oxygen carriers to transfer oxygen from the air to the fuel. The oxygen carriers play a crucial role in chemical looping processes and must maintain high performance during extended redox cycling at high temperatures. The preparation of mixed metal oxides (MMOs) via the calcination of layered double hydroxide (LDH) precursors has been shown to achieve a high degree of dispersion of active metal oxide within the support due to the high degree of mixing of metals in the LDH structure. In this thesis, CuO-based oxygen carriers derived from LDHs for chemical looping processes were evaluated.
Novel CuO-based oxygen carriers supported on Al2O3 and MgAl2O4 were developed by tuning the synthetic chemistry of the LDHs prepared via co-precipitation at constant pH. The choice of co-precipitating agent was found to significantly affect the morphology of the LDHs and the dispersion of active CuO in the aluminate support phase in the MMOs. The oxygen carriers supported on MgAl2O4 showed much higher rates of oxygen release and re-oxidation and higher chemical stability than those supported on Al2O3. The co-precipitation pH was determined to be an important parameter for tuning the mechanical properties of the MMOs. An increase in co-precipitation pH from 9.5 to 11 was found to decrease the porosity and increase the crushing strength of the MMOs. The higher-strength MMOs demonstrated near-constant conversion over extended redox cycling in a fluidised bed reactor.
For design, accurate knowledge of the intrinsic kinetics of the oxygen release reaction of the oxygen carriers is needed to scale up chemical looping reactors. The oxygen release kinetics were determined using an adapted effectiveness factor-based kinetic model. An activation energy of 51 ± 3 kJ mol−1 was calculated using an Arrhenius expression, which agreed well with values reported in the literature.
The formation mechanism of the Cu-Mg-Al LDHs was investigated by varying the co-precipitation pH value. The diameter and height of the LDH platelets and porosity of the bulk material were observed to generally decrease with increasing pH, except for LDHs synthesised at pH 10. The large LDH platelet diameters synthesised at pH 10 were attributed to an interplay of supersaturation, thermodynamic and electrostatic factors. The more porous MMOs synthesised at pH 9, 9.5 and 10 showed much higher rates of oxygen release than the less porous materials synthesised at pH 10.5, 11 and 11.5.
In this thesis, the LDH-MMO design strategy was shown to be effective for the development of CuO-based oxygen carriers with extremely high chemical stability and tuneable mechanical properties. The structural diversity of LDHs enables versatile combinations of metal ions to be highly dispersed in their structure, which could inspire the development of highly stable oxygen carriers for many emerging chemical looping processes.