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OCCURRENCE OF MERCURY IN BIOMASS AND BIOMASS ASH

Gergana Velyanova, Stanislav Vassilev, Christina Vassileva, Mariya Georgieva, Svetoslav Georgiev

Abstract

Mercury is among the most hazardous trace elements released during fuel combustion, yet its occurrence in biomass and biomass ash remains insufficiently constrained. This paper presents an integrated assessment of published data on Hg contents, associations, and probable modes of occurrence in eight biomass types and their ashes generated during combustion, with emphasis on Hg volatilization, retention in biomass ash, and comparison with industrial coals used in Bulgaria. Published chemical, mineralogical, thermal, and leaching data were combined to assess Hg distribution in biomass, as well as its volatilization behavior and capture in ash during combustion. The investigated biomasses contain low Hg concentrations, ranging from 0.003 to 0.043 ppm (mean 0.013 ppm), which is about one order of magnitude lower than the Clarke value of Hg for coal. Mercury in biomass is associated with both organic and inorganic matter, particularly hemicellulose and water-soluble Cl-, S-, N-, and Na-bearing phases, which are unstable during biomass combustion. Reported biomass ashes produced at 500 C also contain low concentrations of Hg, namely 0.0032–0.0452 ppm (mean 0.0225 ppm). Most Hg is released during biomass incineration as its volatilization behavior is 60–99% at 500 C and 63–100% at 700 C. As a result, only limited amounts of Hg are retained in ashes enriched in carbonates, oxyhydroxides, phosphates, sulphates, and chlorides. Residual Hg in biomass ash shows affinity to relatively stable Fe-, P-, Ti-, Al-, K-, and Si-bearing minerals, especially K aluminosilicates, Fe oxides, and sylvite, where it probably occurs as impurities in such mineral species. These results show that the renewable and sustainable biomass is markedly poorer in Hg than the industrial coals used in Bulgaria (0.14-0.57 ppm) and may serve as a cleaner alternative for reducing mercury-related environmental risks.

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