Use of Hexaaluminates in Catalytic Combustor for Gas Turbines

Different design concepts of catalytic combustors were proposed in order to meet the stringent requirements on emission control and on durability under the severe 

In the second configuration (hybrid combustor), only a portion of the fuel is fed to the catalyst section. The inlet air/fuel ratio is carefully controlled to limit the adiabatic reaction temperature typically below 1000 °C, and accordingly, to reduce the catalyst thermal stresses. The remaining amount of fuel is fed to a 

Effective preparation methods of hexaaluminates for catalytic applications, such as the hydrolysis of alkoxides and the co-precipitation in aqueous medium, ensure high interspersion of the constituents in the precursor. This allows the formation of single phase materials with layered-alumina structure at reasonably low temperature (1100-1200 °C) and with high surface area. The hydrolysis of alkoxides was extensively studied and used for the industrial scale-up in the production of catalysts in the monolith shape. However, the co-precipitation in aqueous medium has much potential in view of the possible commercialization of these materials due to its simplicity and low cost. 

The crystal structure and the sintering behavior of hexaaluminates was widely investigated. The relation of sintering resistance to anisotropic ion diffusion in the layered alumina phase was clarified to a large extent. Other evidence suggests that combustion activity is obtained through a redox mechanism associated with reversible variation of oxidation state of the transition metal ions in the structure. Mn was the best and most stable active component. However, fundamental and applied studies are needed to better clarify the redox mechanism of the reaction and how it is related to the chemical and structural features of the Mn-containing layered-alumina phase. This could also provide useful information for the development of an optimum catalyst composition, 

Prototype scale experiments demonstrated durability and efficiency of hexaaluminate type catalysts under the severe temperature conditions of industrial gas turbines fuelled by natural gas. A combination of very active Pd-based catalysts and very stable hexaaluminates and a proper catalyst aimed at minimizing the mechanical stresses due to thermal shocks was used to achieve this target. 

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