Characterization of fly ashes from biomass combustion with focus on their utilization

Fly ashes from several biomass combustion plants using various types of biomass as fuel have been investigated in several studies. In three studies the main focus was on the chemical composition and the physical properties of the fly ashes. In another study the flowability of fly ashes from the combustion of forest residues was investigated in detail. The dependence of the concentration of various components on the particle size was investigated for one fly-ash. A two-stage leaching process for the removal of heavy metals from the fly ash was tested on bench scale. In the literature little data on the physical properties of fly ashes from biomass combustion is available. The measured physical properties provide an improved basis for the design of fly ash handling and storage equipment. The results showed that for reliable flowability results shear tests are required. The more easy-to-determine flow indicator angle of repose over-estimated the flowability of the fly ashes in several cases. The particle size was identified as the parameter with the highest influence on the flowability of the fly ashes. In the cyclone fly ash of a forest residue combustion plant, for many components a dependence of the concentration on the particle size was found. Some heavy metals (Bi, Cd, Cu, Hg, Pb and Zn) and nutrients (K and NO3-) were enriched in the smaller size fractions while others were depleted (Fe, Al, Si, Ti, As, Ba and V). For several other components no distinct dependence of the concentration on the particle size was found. In most fly ashes from the combustion of forest residues one or more heavy metal concentrations were above the limit concentrations for use as a soil conditioner for forest and agricultural land. Thus, treatment of these fly ashes for the reduction of the heavy metal content would be required. In contrast, the fly ashes from the combustion of straw and Miscanthus were only slightly contaminated with heavy metals and can be used as a soil conditioner without pre-treatment. For reduction of the heavy metal content of biomass combustion fly ashes a two-stage leaching process was investigated. The test showed the potential of this process to reduce the heavy metal concentrations while keeping the losses of the nutrients within an acceptable range. The application potential of biomass combustion fly ashes for construction materials seems to be quite limited. Considering the chloride content of the various biomass combustion fly ashes, only for cyclone fly ash from wood combustion might this be a feasible option.