A Kinetic Investigation of As and Se Speciation within Coal Combustion Flue Gases using ab initio Methods

Urban, David Raymond

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A Kinetic Investigation of As and Se Speciation within Coal Combustion Flue Gases using ab initio Methods

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In the technologically driven information age, the consumption of power is as vital to daily life as food and shelter. The generation of that power comes from a variety of sources of which coal is the predominant provider of electrical energy. Coal combustion is a well-known technology and the United States possesses the most abundant coal deposits on Earth, however, the drawback accompanying this process is the significant emissions which are released during combustion. Over the years, much effort has gone into reducing the emissions of majority constituent elements CO2, CO, NOx, SOx, etc. but it is only in the last decade or so that much attention has been given to the trace metals present within coal. Most of the work into examining these trace metals has been upon Hg and how it speciates within the flue gas in order to determine the most effective means of removal. In this study, the trace metals arsenic (As) and selenium (Se) will be investigated in a similar manner to evaluate the speciation of these elements. While previous experimental work has been performed in this area, it has been limited to thermodynamic studies which determine the speciation after equilibrium has been reached, this ignores the fact the residence times within the flue are often only several minutes during which time rapid quenching is taking place. This study takes a different approach by examining the speciation using computational chemistry which affords the advantage of being able to perform a kinetic study which is more useful in creating a flue gas model. Using ab initio the properties of various As and Se species can be evaluated compared to existing experimental data for validation. After which, a number of reactions may be selected and the structure of the transition state for each identified. Once the properties of the transition structure are known, the appropriate kinetic model, be it Transition State Theory, RRKM Theory, etc. can be applied and the rate constant determined. It is by the determination of these rate constants that the kinetic model of the flue gas can be improved and a more accurate depiction of the speciation of these race metals created.

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