Community Research and Development Information Service - CORDIS


Previous research has shown that an entire recycling of biomass ash cannot be recommended due to environmental pollution by heavy metal depositions in the forest ecosystem via air and rain. Consequently, it is necessary to separate a small side stream as rich in heavy metals as possible in order to be able to utilise the largest amount of biomass ash produced. The more effectively this heavy metal fractionation works in practice, the higher is the concentration of heavy metals in the small ash fraction that has to be separated.
Consequently, comprehensive full-scale and bench-scale test runs were performed in three straw incinerators, one wood-fired CFB combustion unit, one wood-fired fixed-bed gasification unit, one bark-fired CFB gasification plant, and a bench-scale BFB combustion reactor in order to investigate the possibilities and the potential of the technique of fractionated heavy metal separation. Prior to the test runs, standard methods for sample taking, sample preparation and analyses of ash-forming and trace elements in biomass fuels and ashes were established by the performance and the evaluation of an international Round Robin on biomass fuel and ash analyses. This Round Robin provided the basis for correct and representative analytical results and allowed for reliable comparison of the data obtained by the project partners.
The experimental results achieved were accompanied by aerosol measurements, investigations concerning aerosol formation as well as by chemical equilibrium mode calculations for heavy metals and ash-forming elements in biomass combustion and gasification plants.
The results from elemental mass balances revealed that the environmentally most relevant heavy metal, Cd, is depleted in the bottom ash in all processes investigated and is mainly found in fly ashes precipitated downstream the boiler section. This fact indicates that Cd is vaporised at high temperatures and under reducing conditions as present in and around burning fuel

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