Trace element behavior in wood-fueled heat and power stations in terms of an urban mining perspective

Abstract

Trace elements (TE) occur in natural and waste wood in a broad variety at considerable concentrations. Those TEs end up in different ash fractions (bottom-, fine-, and fly ashes) after combustion in furnaces at heat and power plants (HPP). The ash fractions differ significantly in TE concentrations and are therefore of variable quality. On the one hand they can be a matter of cost intensive disposal due to their toxicity. On the other hand they are a resource of valuable elements demanded by future technologies in an urban mining perspective. By understanding the individual element behavior during combustion and in the flue gas cleaning facilities, it is possible to predict element concentrations in the ash fractions. The results show the differences of TE concentrations in wood fuels as well as in the corresponding ash fractions precipitated in the air pollution control devices (APCDs) of four HPPs. The results from the mass balances show that the behavior of 23 investigated elements in wood fired stations is comparable to the behavior in coal fired stations. The individual element concentrations in the ash fractions are determined by different parameters, i.e. the element concentration in the fuel, the element volatility, and the operation of available flue gas cleaning facilities. The concentration of TE in bottom and especially in fly ashes of wood fired HPP are comparable to those found in economic important ores such as magnetite. The ashes from industrial wood combustion systems can therefore be considered as a valuable resource instead of a hazardous substance.