Ignition Process in Non-Conventional Combustion System
Picarelli, Antonio (2011) Ignition Process in Non-Conventional Combustion System. [Tesi di dottorato] (Inedito)
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The main pollutants responsible for the global and local environmental impact are species such as NOx, soot particles, CO2, SOx, polycyclic aromatic hydrocarbons (PAHs) formed during combustion processes. The interest of the scientific community is focused on identifying new technologies that enable more efficient energy production coupled with a simultaneous reduction of pollutants emission. In this background MILD combustion is one of the most promising new technology. It envisages the use of highly diluted and preheated mixtures. The modest temperature gradient due to high mixtures heat capacity implies that system working temperature is not crucial for the formation of pollutant species, while the high inlet temperatures and the uniform conditions in the combustion chamber insures high thermal efficiency. Mild combustion is a technology of interest in many applications, from industrial furnaces for the processing of raw materials to gas turbine combustors, but also in afterburners as a process of pollutants abatement. The high mixture inlet temperatures and dilution levels strongly affect the evolution of the combustion process. The massive employment of such technology in industrial plants is still not developed because of the lack of basic knowledge concerning the phenomenologies that accompanied such non conventional combustion conditions. In this background, it is necessary a study on the basic aspects of the process in consideration of the huge difference with traditional combustion system. In literature there are a lot of works relative to such new process, but they are realized on pilot or industrial facilities where the evolution of the combustion process is strongly affected by the interaction between kinetic and fluid-dynamic aspects. In this thesis the approach to the problem is original since model reactors have been used to characterize the novel technology. They allow to study the mere kinetic evolution of the oxidation process since the fluid-dynamic conditions are extremely simplified and they do not affect the oxidation process. The experimental work was carried out in a tubular flow reactor, at atmospheric pressure and several diagnostic techniques were used. The attention was devoted to the ignition process of methane and its derived fuel mixtures in such highly diluted and preheated conditions. It is worth noting that the thesis represents an important contribute since the lack of experimental data in operative conditions typical of Mild combustion in particular way at atmospheric pressure. A rich experimental database relative to auto-ignition times and reactivity maps at atmospheric pressure in such working conditions are the main results of such work. In parallel with the experimental work, a deepen numerical analysis was carried out by means of commercial software and kinetic detailed mechanisms in order to study the evolution of the oxidation process. It came out that they fail to correctly predict the main features of the combustion process under diluted and pre-heated mixture conditions. Experimental data clearly show that the competition between the oxidation and recombination channels is not properly described by the detailed kinetic schemes. In such context, it is worth noting that experimental database obtained in this work is a valid reference for tuning and updating the detailed kinetic mechanisms.
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