Trace element concentrations and associations in some biomass ashes

The phase-mineral and chemical composition of biomass ashes (BAs) produced from 8 biomass varieties, namely beech wood chips, corn cobs, marine macroalgae, plum pits, rice husks, switchgrass, sunflower shells and walnut shells, was studied for the elucidation of trace element (TE) concentrations and their associations. For that purpose the contents of 60 major, minor and TEs in high-temperature BAs produced at 500 °C were determined by inductively coupled plasma and laser ablation mass spectrometry, as well as scanning electron microscopy equipped with an energy dispersive X-ray analyser. The phase-mineral composition of BAs was studied by light microscopy, powder X-ray diffraction, differential-thermal and thermo-gravimetric analyses plus some leaching, precipitation, ashing (500-1500 °C) and other procedures. The composition and properties of the biomass fuels studied and their ashes were characterized, and some general considerations about TEs in them are described. An elucidation of the TE contents in BAs and their ash types was conducted and a comparison with coal ashes was also performed. An explanation of the correlations and clarification of the associations of TEs with major and minor elements, as well as with different constituents (organic matter, inorganic matter, cellulose, hemicellulose, lignin, inorganic amorphous matter, silicates, phosphates, carbonates, oxides, hydroxides, sulphates and chlorides) in biomass and BAs are given. It was revealed that the greatest ecological challenges related to some TEs in biomass and BA include their: (1) high concentrations; (2) unfavourable modes of occurrence; (3) enhanced volatilization and limited retention and capture performance during biomass combustion; and (4) increased leaching behaviour during biomass and BA processing or storage.