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Previous research has shown that recycling of biomass ash cannot be recommended due to environmental pollution by heavy metal depositions in the forest ecosystem, via air and rain. It is necessary to separate a small side stream, rich in heavy metals, in order 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 must be separated.
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 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 evaluation of an international Round Robin on biomass fuel and ash analyses. This Round Robin provided the basis for the correct representation of analytical results and allowed for reliable comparison of the data obtained by the project partners.
The experimental results were accompanied by aerosol measurements and investigations concerning aerosol formation, as well as 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 most environmentally relevant heavy metal, Cadmium (Cd), is depleted in the bottom ash in all processes investigated, and is mainly found in fly ashes precipitated downstream. This indicates that Cadmium (Cd) is vaporised at high temperatures and under reducing conditions present in and around burning fuel particles and then undergoes condensation on, or reactions with, the surface of fly ash particles.
Aerosol measurements and modelling indicated that the vaporised heavy metals mainly undergo surface reactions with coarse fly ash particles at high temperature prior to condensation. This makes high temperature separation of heavy metals from fly ash particles in the combustion plant itself difficult at temperatures below 850 degrees Celcius. Most biomass-fired CFBC units operate at or below 850 degrees Celsius, therefore, a useful fractionation within CFBC seems difficult. In grate-fired biomass combustion units the combustion temperature is higher and the bottom ash is not diluted with bed material (like in CFB plants), which creates a very high potential for fractionated heavy metal separation. Based on these results, several possibilities of utilising and enhancing the fractionation in biomass-fired grate combustion units were derived.
Moreover, laboratory research on the possibilities and potential of heavy metal recovery from highly contaminated fly ash fractions, by thermal ash treatment and biochemical leaching, have revealed promising results and offer possibilities for heavy metal recovery from contaminated biomass fly ashes, completely avoiding ash disposal.
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