Sulfur and carbon isotope geochemistry of coal and derived coal combustion by-products: An example from an Eastern Kentucky mine and power plant

The *³⁴S and *¹³C distribution in a mined coal, the pulverized coal fired into the boiler, and the coal combustion by-products, demonstrated that there is some fractionation of sulfur and carbon isotopes as a result of the combustion and ashcollection process. Overall, the changes in coal composition are indications of the spatial chemical differences in the coal and its heterogeneity at the mine. Coal at the mine shows a striking, stratigraphic upwards enrichment in ³⁴S, particularly notable in pyritic and organic S. The sulfur geochemistry of the coal bed is interpreted as having been influenced by late depositional or post-depositional incursion of sulfate-rich waters into the peat, indicating a change in depositional environment. The coal delivered to the power plant over two weeks does show variations in S composition. The coal delivered during week 1 had higher total sulfur and pyritic S contents and more enriched bulk *³⁴S values than the coal delivered during week 2. In contrast, the *¹³C of the coal delivered during week 1 is only slightly more enriched than the coal delivered during week 2, though with such a small sample set, this is probably not significant. The pulverized coal, which is the fuel injected into the boiler, has *³⁴S and *¹³C values similar to the average value of the power plant feed coal. The pulverizer reject has much higher pyrite content than the pulverized coal and more enriched *³⁴S values. With the exception of the last row of the ESP, which collects a fraction of a percent of the total fly ash, there is a slight increase in *³⁴s from the hotter end of the collection system to the cooler rows. With a decrease in temperature, therefore, there is increased fly ash enrichment in ³⁴S and flue gas depletion in ³⁴S. The observed fractionation in S isotopes is consistent with a study conducted at a power plant burning higher S coal than the plant in this study. Despite the significant difference in carbon content, only a small enrichment of 0.44 to 0.67%c in ¹³C in the ash relative to the coal is observed, demonstrating that fractionation of C isotopes in the boiler and convective passes, prior to the arrival of the fly ash in the ash-collection system, is minor. No significant difference in *¹³C values among the rows of the fly ash-collection system was detected, further confirming that the partitioning preceded arrival of the flue gas in the pollution-control system.