Determination of the leaching yield of selected elements from wood ashes

The use of wood as one representative of biomass for heat and electricity generation has been rising in the last years and will increase even more in the near future. As biomass is considered as CO2-neutral (it absorbs CO2 when it is grown and sets the same amount of CO2 free when it is burnt) and thus as one of the combustibles for the coming years. During combustion of biomass the organic material is converted into CO2; the inorganic components remain as ashes. These ashes contain on the one hand valuable plant nutrients like N, P, K, Ca, Mg, S, which makes them interesting for the use as soil enhancers. But on the other hand they also contain heavy metals, which can harm the environment, animals or men consuming the plants or lead to minor plant growth. This limits the use of biomass ashes on soils and thus its usage as fertilizer is regulated with maximum allowable concentrations. In a biomass incinerator different ash fractions are generated. One is the coarse ash that remains at the bottom of the furnace (bottom ash) and is mostly not so much contaminated with heavy metals. The fine ash fraction however, being collected from the flue gas (fly ash) in a dedusting device, contains usually heavy metals in concentrations beyond the limit values for reusing these ashes in agriculture. These limit values for harmful elements vary in different countries (e.g. the limit concentration for the use of biomass ashes on forest soil for Cd in Austria 8 is mg/kg, in Denmark 0.4 mg/kg and in Sweden even 30 mg/kg). One of the important factors for deciding what maximum concentrations of elements can be allowed is the knowledge about their solubility and mobility. These parameters strongly depend on the chemical forms in which they are brought into the soil, on the pH, on the soil matrix and also on the precipitation length and intensity. The answer for the limits is therefore very complex. In this study the influence of different leaching media and time periods have been investigated for different ash fractions. As leaching solvents distilled water and salt-solutions (NaAc at pH 8 and NaAc/HAc at pH 5) have been used. Coarse ash, e-filter-ash and ash from the cyclone have been investigated. The elemental concentrations in the ashes have been determined after a microwave assisted digestion with HNO3 and HCl and an analysis with inductively coupled plasma optical emission spectroscopy (ICP-OES). The leaching experiment has been done by adding 1,000 ml of extraction media to 100 g of each ash sample. The leaching time took 1.6 hours, 16 hours, 160 hours, and 1,600 hours. After the leaching all extracts were preconcentrated through evaporation in order to more precise data especially for the ultra trace elements. Then these samples were analyzed with (ICP-OES). The leaching yield at different extraction conditions (media and time) was then calculated. The results for the different ash fractions are compared.