Abstract
Coal samples and by-products resulting from the combustion process collected from seven combined heat and power (CHP) plants and heating plants located in Upper Silesia, southern Poland, were subjected to petrographic analysis. The coal used as a fuel in these plants was collected from mines of the Upper Silesian Coal Basin; it occurs in a wide range of coalification and has variable quality parameters. The coal is dominated by macerals from the vitrinite group; however, a high content of macerals from the inertinite group has also been observed. Based on petrographic analysis of ash and slags, a highly variable mineral matter content, confirming variable combustion efficiency, has been found. In the case of samples collected from large CHP plants with pulverized and fluidized bed boilers, the mineral matter content is high. The share of mineral matter in combustion products clearly decreases in the case of smaller power plants and heating plants, especially those using grate boilers. The increased content of unburned coal can be explained by the fact that coals of higher rank are often used as fuels in the mentioned plants.The greatest diversity of char forms can be observed in the case of samples collected from small CHP plants. In the case of three samples of slag, collected from small heating plants using grate boilers, additional presence of coke, thermally altered, and unaltered coal has also been found.
Highlights
The combustion of coal generates by-products, including fly ashes, bottom ashes, slags, and flue gas desulfurization products
The presence of unburned carbon in the residues of the combustion process confirms the loss of fuel and reduced degree of efficiency of the process; in addition, it may prevent the use of ashes (Hower et al, 2017)
Bituminous coal used as a fuel in the analyzed combined heat and power (CHP) plants and heating plants is produced in mines exploiting coal deposits within the Upper Silesian Coal Basin
Summary
The combustion of coal generates by-products, including fly ashes, bottom ashes, slags, and flue gas desulfurization products. The bottom ashes in fluidized bed boilers and slags in pulverized and grate boilers are cooled with water and collected from the lower part of the boiler using a conveyor belt (Miller, 2010; Breeze, 2015). In the case of ashes collected from fluidized bed boilers, the amount of unburned carbon may be comparable or slightly lower (from a few to a dozen or more %) than in the case of pulverized boilers (Bartoňová et al, 2007; Jelonek and Mirkowski, 2015). There is a possibility of a reduction in the amount of unburned carbon in the ashes through the appropriate supply of coal and air to different parts of the boiler (Gao et al, 2013). In order to decrease the amount of unburned carbon, the used coal shall be ground to a finer size (Styszko-Grochowiak et al, 2004)
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