BeschreibungBiomass (e.g. wood pellets or wood chips) is increasingly used as fuel for generation of heat and electricity. The main reason for this development is its CO2-neutrality (the absorption of CO2 when biomass is growing is as high as the liberation of CO2 when the biomass gets naturally degraded). In this calculation transportation and pre-treatment of the biomass is not taken into account. When the biomass is harvested and used as fuel, the organic material gets decomposed to nearly 100% in the incineration process, whereas the inorganic components remain as ashes. By burning wood normally the ash fraction accounts to 1-3%. In small furnaces usually only one ash fraction is produced; in bigger incineration plants (> 200 kW) two ash fractions are collected. The coarse fraction (about 70-80% of the whole mass of ashes) remains at the bottom of the incinerator (bottom ash), the fly ashes are collected in a gas cleaning system with cyclones and E-Filters (a cyclone is no must). The two ash fractions (fly ashes and bottom ashes) can be recycled in different ways. As biomass ashes contain valuable plant nutrients (especially phosphorus, potassium, calcium and magnesium), it is an efficient and environmentally positive way that they are reused as fertilizers on soils to return the nutrients to the ground for the growth of new biomass. This is mostly done with bottom ashes as they are not so much loaded with heavy metals. For fly ashes often only deposition is possible as the heavy metal concentrations in this fraction are above certain limit values. In Austria these limit values are given in a recommendation for the use of biomass ashes on agricultural fields and grassland from 1998 by the Austrian Ministry of Agriculture. In the present investigation, fly ash-fractions from different biomass incineration plants were collected in the summer of 2010. After separation of the fly ashes in an air classifier into different grain size fractions the fractions were analyzed with SEM-EDX (scanning electron microscopy - energy dispersive X-ray spectroscopy) and XRD (X-ray diffraction). Furthermore all ash fractions were brought into solution using a microwave assisted digestion method by adding hydrochloric acid and nitric acid and heating up the samples to around 200°C in closed Teflon vessels. In all solutions the metal concentrations (heavy metals listed in the Austria recommendation and nutrients as e.g. potassium and magnesium) were determined with ICP-OES (inductively coupled plasma optical emission spectroscopy), the concentration of ions (e.g. phosphate) was determined with IC (ion chromatography). It could be found that the concentrations of most heavy metals is increased in the finest ash fractions whereas in the coarser fractions the concentrations are below the limit concentrations given by the Austrian Ministry for Agriculture. Most nutrients are enriched in the coarse fractions - unfortunately these coarse fractions are depleted in potassium. The results can be interesting for district heating distributors using biomass as fuels. By finding an economical way to separate the fractions with high concentrations of heavy metals the minor contaminated fractions could be used as fertilizers which would decrease the need of synthetic fertilizers and the cost for the waste removal of the fly ash fractions.
|Zeitraum||28 Apr 2011|