Ash Deposition in a Pulverized Coal-Fired Power Plant after High-Calcium Lignite Combustion

Abstract

The fabric, mineralogy, and chemistry of high-calcium ash deposits from the Agios Dimitrios pulverized-lignite power plant in northern Greece have been investigated as a key to the ability to improve the performance of the power plant. All inner surfaces in the boiler are covered with a layer of particulate matter showing various degrees of agglomeration or with fouling deposits. Agglomerated fabric occurs in the lower and upper furnace sections, whereas sintered textures are mainly observed in the air heater sections. Particles in the size range of 10−50 μm with no sign of large agglomeration constitute the fly ash. Calcium mineral phases are dominant in deposits (anhydrite (CaSO4), calcite (CaCO3), portlandite (Ca(OH)2), and lime (CaO)); however, there are also significant amounts of silicon-containing minerals in bottom ash and fly ash (quartz (SiO2), gehlenite (Ca2Al2SiO7), and anorthite (CaAl2Si2O8)). Gas−solid reactions control deposition, without the participation of alkaline element compounds in the buildup of deposits. Technological modifications in the facility cannot improve the situation significantly, because of the fact that these effects are the consequence of the thermal decomposition of the high content of calcite (CaCO3) in the feed lignite, coming from the lignite seams and the intermediate sterile layers co-excavated with the lignite. Only through improvement of the quality of the lignite during mining can the power plant improve its operations.