Reduction of Heavy Metals in Wet Flue Gas Scrubbers
Raiko, Mikko Juhani
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Raskasmetallien sitoutuminen märissä savukaasupesureissa
Heavy metal emissions are a significant pollutant in both flue gas and wastewater in power plants. Due to their wide range of health and environmental hazards, research into their behavior in flue gas and reduction in wet scrubbers is of vital importance. In order to understand heavy metal reduction, knowledge has to be supported by speciation and phase behavior in flue gas. In the EU ever increasing emission limits are the driving force behind improvements in heavy metal emission control. The general aim of this thesis was to investigate heavy metal reduction behavior and efficiencies in wet flue gas scrubbers. This investigation was divided into three different target goals: modeling tool investigation, data survey, and measurement campaign. Modeling tool review was conducted through three approaches: commercial modeling tools, specific and detailed thermodynamic modeling tool, and finally lumped model –tool. The initial idea was to examine heavy metal behavior and sensitivity in flue gas from the furnace outflow to the stack, but due to multiple issues this approach was simplified to only investigate some reduction phenomena. Data survey was performed on existing data, such as publications and previous measurement reports. The idea behind data survey was to find information and measurement data on heavy metal reduction that would support findings from the measurement campaign. Finally, the measurement campaign was conducted in order to examine reduction and heavy metal behavior in a proper, controlled environment and find patterns in behavior and reduction phenomena. Calculation tools failed to provide a comprehensive modeling tool to investigate heavy metal behavior in flue gas, but some advances were made in reduction prediction. Commercial modeling programs proved to be lacking in heavy metal species and scrubber type data. Thermodynamic and kinetic modeling was incompatible and too complex for the scope of this study, although some useful calculation methods were identified and refined. Lumped model –tool was created to predict heavy metal reduction with data received from data surveys and measurement campaign. Data surveys managed to find some data, which lead to determining average reduction efficiencies for most investigated heavy metals and the data was also used to evaluate heavy metal speciation and phase in flue gas. Data surveys suffered from typical issues, such as measurement inaccuracies, data availability, and comparability. Finally, the measurement campaign was conducted on two combined heat and power plants and five individual cases were measured. Measurement campaign data used failed to provide accurate mass balance evaluations, process parameter sensitivity or cut-off concentrations, but some reduction rates were calculated and heavy metal class behavior identified.