Chemical and Biomolecular Engineering - Theses

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    Oxidation of rice husk, palm fibre and palm fibre char
    Alias Ales, Azil Bahari ( 2012)
    The selection and design of any biomass combustion system is primarily determined by the combustion characteristics of the fuel. It is the thermal energy released by the combustion reactions which drives the entire process. Thus, understanding of the combustion characteristics or oxidation behaviour of the biomass may lead to successful biomass combustion process at larger scale. This study investigates the oxidation of rice husk, palm fibre and palm fibre char using an evolved gas analysis (EGA) technique. This EGA technique involves heating the biomass and char from 100°C to 500°C at a controlled rate with a constant flow of air. The evolved gas was continually analysed for its oxygen, carbon monoxide and carbon dioxide contents. A series of EGA experiments was carried out using rice husk, palm fibre and palm fibre char. By studying how the evolved gas composition varies with temperature, the many oxidation reactions occurring may be grouped into just three simultaneous and competing reaction regimes known as low, medium and high temperature oxidation. The different reaction regimes may be decoupled and the key kinetic parameters such as activation energy for each regime were determined. These not only demonstrated the potential of the EGA technique as a tool to study the biomass oxidation but also provide insights into the effects of variations in pressure, oxygen partial pressure and heating rate for both of the biomass and biomass char. The oxidation behaviour of both the rice husk and palm fibre was found to be substantially different to that observed for the palm fibre char. The principle difference observed was the number of peaks present in a plot of oxygen consumption versus temperature - two peaks were formed for the both of biomass but EGA experiments consistently shows only one peak for palm fibre char. It was found that the oxygen partial pressure of the oxidising gas in the combustion cell significantly influences the oxidation behaviour. Increasing the partial pressure of the oxygen typically moved the reactions to lower temperatures. The activation energies and peak oxygen consumption temperatures were all found to be linear functions of the oxygen partial pressure of the gas supplied to the reactor. Varying the total pressure of the reaction vessel while keeping the oxygen partial pressure constant did not lead to any significant variations in the reactions parameters.