Mobilisation of trace elements during thermal conversion of algae cultivated in ash dam water

The work presented here assesses the potential for the mobilisation of 11 trace elements (As, Be, Co, Cu, Mn, Ni, Pb, Sb, Se, V, Zn) during the thermal conversion of micro- and macroalgae that were cultivated in ash dam water. The volatility of the trace elements was quantified by mass balances based on elemental analyses of char and ash residues. The residues were prepared in a laboratory-scale fixed-bed reactor at a range of different temperatures (500-1100 °C) and gas atmospheres (N2, 2% O2 and CO2) to simulate pyrolysis, combustion and gasification processes. The results showed high volatilities for Se (∼79-97%) and As (∼51-79%) below 500 °C. Zn, Pb and Sb were mainly volatilised above 700 °C. The different gas atmospheres had little influence on the volatility of these elements, which increased sharply to more than 90% with increasing temperature from 700 to 1100 °C. Volatilities for V, Mn, Cu, Co, Ni and Be were relatively minor over the full range of investigated operating conditions. Samples of each alga and their thermal conversion residues were subject to batch leaching in water. All of the tested trace elements, except for Pb and Be, were partially leached from the algae. Vanadium was up to 4-5 times more leachable in the combustion residues than in the algae. The other trace elements were generally less leachable following thermal conversion. The trace elements were more stable in residues prepared under pyrolysis and gasification conditions than in residues prepared under combustion conditions at the same temperature.