Effect of air classification on utilization potential of fly ashes from co-combustion
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Ilmaluokituksen vaikutus yhteispolton lentotuhkien hyötykäyttökelpoisuuteen
Circular economy is an upward trend that applies also to ashes. It will be increasingly important to change the status of ash from a waste that has to be landfilled to a useful product. The main driving force behind this change are the high landfilling costs but utilization of ash saves also natural resources. However, the Finnish legislation sets limit values for heavy metals and other harmful substances for several utilizations. High heavy metal content can therefore prevent the utilization of some ashes. Options for these ashes are disposal of at a landfill or a refining process to reduce the heavy metal content. The aim of this thesis was to examine the functionality of air classification as an ash refining technology. A test run was conducted with an air classifier pilot equipment to investigate the effect of air classification on the utilization potential of fly ashes. The fly ashes used in the experiments were received from five power plants combusting different mixtures of fuels to cover an extensive section of ashes produced in Finland. These power plants used either bubbling or circulating fluidized bed technology. In air classification process, ash feed was divided into three size fractions. The basic idea was that the heavy metals enrich in the fine ash fractions so the coarse fraction was more easily utilized due to the reduced heavy metal concentrations. This study is limited to the utilization of ashes as fertilizers and in earth construction since there is exact legislation for the properties of ash used in these applications. The thesis attained its aim of evaluating the functionality of the air classifier pilot. Mechanically the classifier functioned adequately. However, air classification had no effect on the utilization potential of the test ashes despite the lowered heavy metal concentrations in the coarse ash fractions. This is because, most of the test ashes were already utilizable as fertilizers or in earth construction so there was no need for the refining process. One of the test ashes was originally hazardous waste but the air classification could not improve its properties enough to remove the status of a hazardous waste. The profitability analysis revealed that air classification would be profitable if ash could be transferred from a landfill to utilization. Therefore, a specific type of ash should be found for the air classification to be feasible. Overall, the low availability of suitable ashes and the limitations in profitability restrict the use of air classification technology in the refinement of fly ash.