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

The isotopic compositions of S (834 S) and C (delta C-13) were determined for the coal utilized by a power plant and for the fly ash produced as a by-product of the coal combustion in a 220-MW utility boiler. The coal samples analyzed represent different lithologies within a single mine, the coal supplied to the power plant, the pulverized feed coal, and the coal rejected by the pulverizer. The ash was collected at various stages of the ash-collection system in the plant. There is a notable enrichment in S-34 from the base to the top of the coal seam in the mine, with much of the variation due to an upwards enrichment in the delta S-34 values of the pyrite. Variations in delta S-34 and in the amount of pyritic S in the coal delivered to the plant show that there was a change of source of coal supplied to the plant, between week one and week two of monitoring, supporting a previous study based on metal and sulfide geochemistry for the same plant. The fly ash has a more enriched delta S-34 than the pulverized coal and, in general, the delta S-34 is more enriched in fly ashes collected at cooler points in the ash-collection system. This pattern of delta S-34 suggests an increased isotopic fractionation due to temperature, with the fly ash becoming progressively depleted in S-34 and the flue gas S-containing components becoming progressively enriched in (34)s with increasing temperatures. Substantially less variation is seen in the C isotopes compared to S isotopes. There is little vertical variation in delta C-13 in the coal bed, with delta C-13 becoming slightly heavier towards the top of the coal seam. An 83-93% loss of solid phase C occurs during coal combustion in the transition from coal to ash owing to loss of CO2. Despite the significant difference in total C content only a small enrichment of 0.44-0.67 parts per thousand in delta C-13 in the ash relative to the coal is observed, demonstrating that redistribution of C isotopes in the boiler and convective passes prior to the arrival of the fly ash in the ash-collections system is minor. (C) 2007 Elsevier Ltd. All rights reserved.