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.

Mineral Carbonation of Biomass Ashes in Relation to Their CO2 Capture and Storage Potential.

Short-term stored, long-term stored, and weathered biomass ashes (BAs) produced from eight biomass varieties were studied to define their composition, mineral carbonation, and CO2 capture and storage (CCS) potential by a combination of methods. Most of these BAs are highly enriched in alkaline-earth and alkaline oxides, and the minerals responsible for CCS in them include carbonates such as calcite, kalicinite, and fairchildite, and to a lesser extent, butschliite and baylissite. These minerals are a result of reactions between alkaline-earth and alkaline oxyhydroxides in BA and flue CO2 gas during biomass combustion and atmospheric CO2 during BA storage and weathering. The mineral composition of the short-term stored, long-term stored, and weathered BAs is similar; however, there are increased proportions of carbonates and especially bicarbonates in the long-term stored BAs and particularly weathered BAs. The carbonation of BAs based on the measurement of CO2 volatilization determined in fixed temperature ranges is approximately 1-27% (mean 11%) for short-term stored BAs, 2-33% (mean 18%) for long-term stored BAs, and 2-34% (mean 22%) for weathered BAs. Hence, biomass has some extra CCS potential because of sequestration of atmospheric CO2 in BA, and the forthcoming industrial bioenergy production in a sustainable way can contribute for decreasing CO2 emissions and can reduce the use of costly CCS technologies.