Content, association and modes of occurrence of mercury in biomass ashes and its behaviour during biomass combustion

Toxic Hg species pose a global ecological threat due to the significant anthropogenic emissions of Hg to the atmosphere, particularly during coal combustion. An investigation about the content, association and modes of occurrence of Hg in diverse biomass ash (BA) varieties and its behaviour during biomass combustion was performed. These BAs belonging to woody, herbaceous, agricultural and aquatic biomass groups were examined with different chemical and mineralogical methods, as well as thermal and leaching procedures. The Hg contents in BAs are low and vary between 0.0032 and 0.0452 ppm (mean 0.0225 ppm). About 60–99% (mean 87%) and 63–100% (mean 91%) of Hg in biomass were volatilized at 500 °C and 700 °C, respectively, and only limited amounts of Hg were captured by BAs enriched mostly in salts such as carbonates, oxyhydroxides, phosphates, sulphates, and chlorides. The affinity of residual Hg in the BA system is towards relatively stable Fe-, P-, Ti- Al-, K-, and Si-bearing minerals (especially K aluminosilicates and Fe oxides), as well as less stable chlorides (particularly sylvite). The potential mode of Hg occurrence in BA is likely in the form of impurities in the above minerals. Alternative renewable fuels poor in Hg such as appropriate sustainable biomasses (0.001–0.043 ppm Hg) are suggested to partially or completely substitute the industrial coals enriched in Hg (0.14–0.57 ppm) and used in Bulgarian thermoelectric power plants and thus minimize the ecological problems related to this element.