Plant nutrients in different grain size fractions of biomass fly ashes
M. Kröppl, C. Lanzerstorfer - Plant nutrients in different grain size fractions of biomass fly ashes - XXth Slovak - Czech Spectroscopic Conference, Tatranska Lomnica, Slovakia, 2012, pp. 74
As biomass counts to the renewable energies, it is CO2-neutral when transportation and pre-treatment of the biomass are not taken into account. Biomass is therefore a good alternative to the usage of fossil based fuels. Especially wood is increasingly used for the generation of heat and electricity.
During the incineration process all the organic compounds are converted into gases. The remaining ashes (1-3% of the input biomass) contain nutritive elements for soils and plants (e.g. phosphorus, potassium, calcium and magnesium). The best way to recycle them is therefore by their usage as soil enhancers as it is often done with bottom ashes collected at the bottom of the incinerator.
But especially fly ashes collected in the gas cleaning system of the incinerator are often very loaded with harmful heavy metals which restrict in most cases the application of the fly ash on soils and so they are usually disposed.
In this investigation, fly ashes from an Upper Austrian biomass incinerator have been separated with a laboratory air classifier into different grain size fractions (2-55 µm). The distribution of nutrients in the fractions has been investigated. As it is known from other investigations that the biggest amounts of heavy metals are concentrated in the finer ash fractions, also the distribution of Cadmium was determined in the different grain size fractions as it is the most limited heavy metal in the Austrian recommendation for the usage of biomass ashes on soils with a limit of 8 mg/kg.
The original fly ash as well as the separated grain size fractions were digested in a microwave assisted digestion unit with hydrochloric acid and nitric acid at around 200°C. For the determination of metals ICP-OES (inductively coupled plasma optical emission spectroscopy) and IC (ionic chromatography) were used.
It could be found that most nutrients, as for example magnesium, are distributed homogeniously in the different grain size fractions of the fly ash. Only potassium showed higher concentrations in the finer ash fractions. Unfortunately – and as expected - the same could be found for cadmium.
Therefore, by separating the finest fly ash fractions and hereby removing high amounts of harmful heavy metals results in a cleaner and on fields recyclable coarser ash fraction but with a reduced content of potassium. Other nutrients, though, as magnesium, manganese or calcium, are still present in the coarser fly ash fraction which makes this ash still a good soil enhancer. The necessary amount of synthetic fertilizers and the costs for disposing of fly ashes can so be reduced.