Catalytic Combustion of Ventilation Air Methane Released from Coal Mines

Setiawan, Adi (2015) Catalytic Combustion of Ventilation Air Methane Released from Coal Mines. Doctoral thesis, University of Newcastle.

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Abstract

This thesis presents a series of investigation on catalytic combustion of lean methane mixtures emitted from coal mine ventilation air. The study involves catalyst preparation, catalytic activity and stability evaluation under simulated ventilation air methane(VAM) gas, and understanding the catalyst deactivation phenomena. The investigation on the chemical and physical properties of catalysts employed a number of techniques including nitrogen physisorption, hydrogen chemisorption, temperature-programmed desorption (TPD), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electronic microscopy and X-ray photoelectron spectroscopy (XPS) analyses. Investigation on the influence of pre-treatment conditions on Pd/Al2O3 catalysts discloses significant differences in the light off temperatures and the extent of coke deposition, depending on whether the catalysts were pre-treated under oxidising or reducing conditions. The oxidised palladium catalysts were reduced by methane under reaction conditions and exhibited similar activity compared to catalysts which were activated under hydrogen. The long-term stability tests suggest that the primary factor responsible for low temperature catalyst deactivation is the water vapour present in the feed stream. Although no palladium hydroxide phase was observed during short-term experiment, extended exposure to wet feed results in the formation of palladium hydroxide, which appears to match the progressive deactivation of the Pd/Al2O3 catalyst. Introducing VAM dust causes a variation in catalytic activity originating from coal-dust ignition and the effect of chloride on the surface of the catalyst. Nevertheless, in the presence of inhibiting agents, an average methane conversion of higher than 75 % over 1,100 h was achieved at reaction temperatures below 600 ºC. Although supported Pd catalysts are higher in activity, nano-sized Co3O4 catalysts exhibit excellent stability. No changes in oxidation/chemical states were observed from the Co3O4 catalyst after time on stream experiments. In contrast, the presence of strongly bonded hydroxyl species on the surface of Fe2O3 catalysts highlights the role of water vapour in catalyst deactivation. Oxygen TPD shows that higher oxygen surface coverage of Co3O4 is suggested to be responsible for a higher activity in comparison with Fe2O3 catalysts. Co-precipitating gold particles with cobalt oxide or iron oxide does not enhance the activity of the catalyst, which is most likely due to blocking the active site of support by the gold particles. Enhanced hydrothermal stability was observed over a novel Pd/TS-1 catalyst during 1,900 h time-on-stream experiments, where a 90 % methane conversion level was successfully maintained at temperature < 500 °C. Surface oxygen mobility and coverage plays a major role for the activity and stability of this catalyst in the presence of a large excess of water. It was identified that water adsorption and in turn hydrophobicity of the catalyst support was a major factor influencing the long term stability of combustion catalysts. The hydrophobicity and competitive adsorption of water with oxygen is suggested to influence oxygen surface coverage and in turn apparent activation energy over the catalysts. Catalyst characterization of Pd/Al2O3 confirms that the deactivation is due to palladium migration and particle growth and is the most prominent in the presence of water in the feed. The formation of α-Al2O3 during long-term stability tests explains the changes in pore structures which is responsible for the re-dispersion of palladium particles. Four accelerated ageing procedures were performed with a target of mimicking the properties of long-term used catalysts. Interestingly, no formation of α-Al2O3 phase was found from the aged catalysts suggesting that, the transformation of alumina phase occurs at a very slow rate. Among the four procedures, ageing under wet-oxygen in helium provides the most similarity to the properties of long-term used catalysts. Increasing the aging temperature up to 830 °C leads to depletion of surface palladium, which permanently reduces the performance of the catalyst.

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