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Fractionated heavy metal separation and ash utilization in biomass combustion and gasification plants



In order to establish a truly sustainable thermal utilization of biomass, it is necessary not only to close the CO2 cycle, which is possible through a sustainable wood culture, but also to close the natural cycle by recycling the ash. Previous
research has shown that total recycling of wood ash leads to environmental pollution by heavy metal deposition in the
forest ecosystem. Consequently, it is necessary to separate a small side stream, as rich in heavy metals as possible, in
order to be able to utilize the maximum amount of wood ash
produced. The more effectively this heavy metal fractionation works in practice, the higher the concentration of heavy
metals in the small separated ash fraction. Following this
approach, a stable closed-cycle economy could be realized.

Technical Approach

This project focuses on the following main points:
- Standardization of methods for taking ash and fuel samples, for sample preparation (homogenization)and for the complete
detection of elements in ashes and biofuels in order to
guarantee comparable results between the project partners.
- Test runs to investigate the potential of fractionated
heavy metal separation in two straw incinerators, one wood-fired CFB-combustion, one wood-fired downdraft gasification
and one bark-fired BFB-gasification plant.
- Basic research on modeling high temperature equilibrium and the condensation/aerosol formation of easily volatile heavy
metals in flue gases of combustion/gasification plants in
order to explain the experimental results and to derive
functional correlations. This modeling will be supported by
pilot plant experiments for parameter adaptation.
- Basic laboratory research on the potential for heavy metal recovery from highly contaminated fly-ash fractions by thermal ash treatment and bio-leaching in order to recycle raw
materials and to avoid ash disposal.
- Calculation of ecological and economical impacts for the
process of thermal energy utilization and for the ecosystems concerned by stream analyses for minerals and heavy metals and by using dynamic compartment models for the impact of ash
recycling on soils, plants and the ground water.

Expected achievements and exploitation

- Standardized sampling, sample preparation and analysis of trace elements in biomass fuels and ashes will be developed. - Techniques for fractionation of heavy metals in
combustion/gasification plant as well as the possibility of
treatment of highly contaminated ash fractions by thermal
treatment or/and bio-leaching will be suggested.
- Two partners (a SME and a large enterprise) will utilize
the results of this project for the construction of new
- Development of ready-to-use models for high temperature
equilibrium calculations for heavy metals and for particle and aerosol formation in combustion /gasification processes and
for the general economic and ecological evaluation of biomass ash utilisation.


Technische Universität Graz
8010 Graz

Participants (5)

Via Enrico Fermi 1, Tp 460
21020 Barasso
Technical Research Centre of Finland
02044 Espoo
Umeä Universitet
941 28 Piteå
Vattenfall Utveckling AB
99,Jaemtlandsgatan 99
162 87 Stockholm
dk-TEKNIK Energi & Miljø
15,Gladsaxe Moellevej 15
2860 Søborg