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 (CaSO 4), calcite (CaCO 3), portlandite (Ca(OH) 2), and lime (CaO)); however, there are also significant amounts of silicon-containing minerals in bottom ash and fly ash (quartz (SiO 2), gehlenite (Ca 2 Al 2 SiO 7), and anorthite (CaAl 2 Si 2 O 8)). 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 (CaCO 3) 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.
The majority of the thin intercalated sterile layers, which are coexcavated with lignite in the Southern Field mine of Ptolemais, Northern Greece, are petrologically characterized as marly limestones. They consist of 90-95% calcite, 5-7% micas C clays, and < 4% feldspars. The coexcavation of lignite with marly limestones and/or carbonaceous marls increases a) the content of the carbonate minerals (already contained in lignite), b) the ash content, and c) the total and free CaO content of the lignite-ash and decreases the calori c value of the mined lignite. The increase of total CaO deteriorates the power plant operation due to fouling effects, while the increase of the CaO (free) content of the lignite ash results in an increase of the solid particle emissions from the electrostatic precipitators of Agios Dimitrios Power Plant. The temperature increase, which follows the hydration of CaO (free) , results in the evaporation of the ash water and the long-term damage of the ash transportation equipment as well. The concentration of CaO (free) in the lignite ash is estimated through a coef cient, which is obtained from the ratio of the CO 2 content (db) and the ash content (db) of
Emitted pollutants from the Agios Dimitrios lignite-fired power plant in northern Greece show a very strong linear correlation with the free calcium oxide content of the lignite ash. Dust (fly ash) emissions are positively correlated to free calcium oxide content, whereas sulfur dioxide (SO2) emissions are negatively correlated. As a result, at present, the Agios Dimitrios Power Plant operates very strictly within the legislative limits on atmospheric particulate emission. In the present study, the factors to be considered in assessing the impact of lignite combustion on the environment are presented and evaluated statistically. The ash appears to have a remarkable SO2 natural dry scrubbing capability when the free calcium oxide content ranges between 4 and 7%. Precipitator operating problems attributable to high ash resistivity can be overcome by injecting sulfur trioxide to reduce the ash resistivity, with, of course, a probable increase in operating costs.
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