Cominos, Vanya; (2000) Catalytic combustion of methane over axially non-uniform Pd catalytic monoliths prepared by chemical vapour deposition. Doctoral thesis (Ph.D.), University College London.

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Abstract

The escalating demand for electric power is expected to continue in the future. The need to satisfy this demand without sacrifice of environmental quality is a challenging problem facing all forms of power generation including the combustion-based power generating industry. Catalytic combustion is a prevention technology with the potential to achieve low emission gas turbines and thus comply with the advent stringent anti-pollution laws. This technology though continues to face problems due to poor catalyst performance at the high operating temperatures required by modem high efficiency gas turbines, as the catalyst systems are subjected to extreme temperature gradients which induce severe thermal stresses. Improved durability is possible for catalyst utilised in catalytic combustion through the alleviation of axial temperature gradients. There is therefore a need for new catalyst design. The aim of this work was to examine the potential of axially non-uniform catalyst distribution along monolith reactors for improving catalytic combustor performance. A two-dimensional mathematical model was formulated to investigate the effect of axially non-uniform catalyst distribution on temperature and concentration gradients in catalytic monoliths operating under both steady state and transient methane combustion conditions. It was observed that exponentially decreasing catalyst distributions initiated light-off at the entrance of the monolith. In effect, temperature gradients were lowered as compared to monoliths with uniform catalyst distribution under steady state conditions. Bulk fluid temperature suitable for homogeneous reaction initiation was reached further upstream. For transient conditions, non-uniform distributions alleviated axial temperature gradients but at the expense of large temporal gradients. It was thus concluded that non-uniform catalyst distributions could achieve lower thermal stresses under steady operation yet care should be exercised in their operation during transient conditions. The potential of chemical vapour deposition (CVD) for preparing axially non- uniform palladium catalytic monoliths was also investigated. The sublimation, deposition and reduction of the metal-organic precursor, palladium (II) acetylacetonate, was investigated through thermogravimetry and XRD. The temperature range for sublimation, avoiding thermal decomposition, was determined to be 100 - 160 °C and required to take place in the presence of an inert gas such as helium. The complex was found to be highly unstable in hydrogen. The Clausius-Clapeyron relation determined for the complex, InP = -96 x 10 3/RT + 29.26, is indicativeof the low vapour pressure of the precursor. Deposition of the metal-organic required the presence of hydroxyl groups. To obtain palladium metal particle deposits, the precursor was reduced under hydrogen at a subsequent stage. Pd on γ-Al2O3/cordierite catalytic monoliths were subsequently prepared in a CVD reactor. The effects of operating variables such as carrier gas flowrate, sublimation and deposition temperatures and process duration on the catalyst morphology were investigated. All catalyst prepared had exponentially decreasing distribution profiles and palladium particles obtained were small enough to warrant their use in catalytic applications. The performance of the catalysts was assessed under atmospheric methane combustion conditions. They showed alleviation of temperature gradients along monolith walls and improved methane conversions as compared to uniform catalysts prepared by impregnation. A mathematical model predicted the trends of wall temperature profiles satisfactorily. Non-uniform catalysts achieved lower temperature gradients and higher fuel conversions than uniform catalysts as high reaction rates were present further upstream. Hence, axial catalyst distribution was found to improve performance in the combustion of methane.

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