A retrospection on the content, association, and significance of mercury in coals and coal ashes from Bulgarian thermoelectric power stations

A short overview on the content, association, and significance of toxic Hg in 9 coal types and their fly ashes (FAs) from 12 Bulgarian thermoelectric power stations (TPSs) was conducted by a compilation of reference and our own data obtained by a combination of different chemical and mineralogical analyses, and separation procedures. The Bulgarian and Ukrainian coals studied are enriched in Hg (0.14-0.57 mg/kg) occurring in both organic and inorganic associations. The most abundant coals in Hg have higher S contents and ash yields, and are enriched in Fe sulphides, calcite, and Ca and Fe sulphates, as well as some clay minerals and feldspars. The dominant quantity (about 50-98%) from the fuel Hg was not captured by the coal ashes in TPSs. The significant Hg capture potential (38-50%) show FAs enriched in char, Ca and Fe sulphates and oxides, and Ca carbonates. It was found that the Hg concentrations in some FA water leachates are significantly higher in comparison with the Clarke values for fresh water and could provoke environmental risks. Alternative and sustainable biomass poor in Hg is suggested to substitute totally or partially the industrial coals used in Bulgarian TPSs to avoid the Hg problems.