Geochemical Features of Unburned Coal Particles in Fly Ashes from Thermal Power Plants in Bulgaria

Abstract

Coal combustion in thermal power plants generates huge amounts of solid wastes, including fly ash (FA). The essays of its conversion into value-added products continuously increase. The aim of the study was: (i) to depict organic compounds (if any) present in industrial coal-derived FAs from four thermal power plants (TPPs) in Bulgaria; (ii) to assess the relationship of the compounds present with the feed coals using biomarker proxies; (iii) and, to appreciate biomarkers changes in distribution pattern caused by combustion. Average samples of FAs and samples from electrostatic precipitators of four TPPs feed by different rank coals were analyzed. The study was scheduled in comparison with parent feed coals. The yields of extractable organic matter (EOM) expressed in rel. % were low and comparable, ∼ 0.01%, but normalized to total organic carbon (TOC) depicted some differences. For lower rank coals FAs yields were higher, in the range 10-20 mg/g TOC, all dominated by polar components and an order of magnitude lower for higher rank coal FAs, < 2 mg/g TOC, with considerable portions of neutral compounds. After extensive fractionation of EOM from unburned coal particles in neutral fractions of FAs short chain n-alkanes (nC15-nC19) prevalence was distinguished. The feature was assigned to high temperature of combustion and thermal destruction of long-chain homologues (nC27-nC31). The distribution signatures for low rank coals kept some characteristics of parent coals while in the case of bituminous coal longer homologues were almost absent. The changes in n-alkanes distributions were rank depended and an attempt for explanation by FA particles porosity was done. For organic compounds registered in EOM of coal-derived FAs two provenances are supposed. The first group comes from the feed coals and the second one is formed in combustion processes or has changed distributions under the thermal treatment. Insofar as n-alkanes distribution signature was altered in combustion the origin should rather be assigned to the second group of components.