Investigation of the mobility of ions in fly ash and bottom ash from wood combustion by extraction experiments

Biomass incinerators produce heat by burning plant materials as wood, straw or hay. During the incineration process the organic input material is decomposed and ashes remain as inorganic residues. The coarse ash (bottom ash) is collected at the bottom of the incinerator. After leaving the incinerator together with the off-gas the fines (fly ash) are collected in the gas cleaning system. The ash fractions contain valuable nutrients for soils like calcium, magnesium, potassium, nitrate and phosphate. Ashes from biomass incineration can therefore be valuable in terms of recycling these components to the soil when the ash is spread on greenlands. In this way the cycle of nutrients for biomass growth can be closed. In this study the mobility of the mentioned nutritive ash components into the soil was investigated in order to determine the amount of elements set free by humidity and precipitation and getting so available for plant growth. Extraction experiments on ashes were done using different extraction solutions according to a six step extraction procedure described in a literature where the mobility of cadmium in fly ash from biomass combustion was investigated. The first extraction medium was deionised water, the last extraction step was a digestion of ash components with diluted nitric acid. All leachates were analysed by ion chromatography (IC). The measured ions were Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, PO43- and SO42-. In order to determine the availability of the elements, also their total concentration in the ashes was analysed by IC after a microwave digestion procedure. It was found, that most of the investigated elements could already be extracted in the first extraction step – the highest amounts being found for potassium, calcium, chlorine and sulphate. This is a satisfying result as these elements are important for the growth of biomass.