Characterization of ashes from biomass incineration and investigation of usability

The use of biomass incineration for the generation of heat and electricity has increased in recent years and this trend will continue also in the years and decades to come. Ashes are the inorganic residues of incineration. Apart from nutrients they also contain heavy metals and unburnt carbon. Bottom ashes have low levels of heavy metals and are therefore usually used as fertilizers in forests and on fields. Fly ashes are collected in a gas cleaning system as particles must not leave the incinerator and contaminate the environment - they must be properly disposed of. In view of the rising amount of biomass ashes, this doctoral thesis analyzes different ways for the utilization of biomass ashes – especially heavy metal contaminated fly ashes. Fly ashes could be used as valuable materials, costs of disposal be reduced. After a validation of the analytical method for the elements Al, As, B, Ba, Cd, Co, Cr, Cu, Fe, Mg, Mn, Na, Ni, Pb, Sb, Sr, Ti, V, and Zn, elemental concentrations of ashes from biomass incinerators and input materials were determined. Extraction experiments investigated the mobility of heavy metals and nutrients. Different durations and extraction media were tested. In acidic leaching experiments the best conditions for dissolving heavy metals from the ashes were analyzed. The aim was to determine whether the cleaned ashes could be used as fertilizing agent. The influence of fly ashes on plant growth was also researched. An air classification of the fly ashes investigated the elemental concentrations in different grain sizes. The possibility of separating the finest fly ash fraction with the highest amounts of heavy metals was investigated, so that the remaining ashes can be used as fertilizer. The possibility of using fly ashes as secondary material in the cement industry was also analyzed. Findings showed that elemental concentrations from different biomass incinerators vary depending on input material, location of biomass growth and the type of combustion system employed. Shorter extraction times with water or weak salt solutions release only low amounts of heavy metals from fly ashes. Nevertheless, longer leaching – especially under more acidic conditions – can set free higher amounts. Also an influence of fly ashes on plant growth was observed in a laboratory experiment. By leaching under acidic conditions, lactic acid released most of the harmful heavy metals, with the exception of cadmium, already at a moderate pH of 5. By separation of the finest parts of the fly ashes (< 30 μm), the cadmium concentration in the remaining ash fractions falls below the limit. Unfortunately, also high quantities of potassium are removed (about 75% by removing two thirds of the finest ashes). This result strongly depends on the original concentration of cadmium. In lower contaminated ashes, less fine ashes would be necessary for separation and more potassium would remain in the coarser part. A way to recapture potassium from the finest fly ashes is by leaching them with water or weak salt solutions. So, potassium can be dissolved whereas heavy metals remain in the ashes. Together with the coarser ash fraction the leachate can be used as fertilizing agent. A granulation of the ashes using the leachate for moistening the ash would be the most efficient way for using biomass fly ashes without exceeding limits of heavy metals. Recycling of fly ashes in the cement industry is possible, but high amounts of alkali and earth alkali metals and chloride can cause operation problems – so also here maximum limit values have to be recognized.