Investigation of Heavy Metal Removal from Ashes from the Gasification of Composted Sewage Sludge and Greenery Waste by Thermodynamic Equilibrium Calculations and Lab-Scale Tests
Hirt, H., Brunner, T., Obernberger, I.
Within the EU FP7¬project ENERCOM the polygeneration of heat, electricity, compost and pellets based on sewage sludge, greenery waste and biomass residues is demonstrated. One aspect of ENERCOM is that ashes from gasification of the input materials should be added to compost in order to recycle P and other relevant plant nutrients contained in the sewage sludge. Due to the high heavy metal contents of sewage sludge and consequently of the ashes from the gasification process, the limit values for heavy metals in the compost would be exceeded and therefore, a partial removal of the heavy metals from the ashes is needed. In this paper a thermo¬chemical heavy metal removal technology, which has been investigated within ENERCOM, is presented. Within this method the ashes are firstly mixed with a Cl¬containing additive and then are thermally treated in an appropriate reactor. Thereby heavy metals form volatile chlorides and are partly evaporated. Lab¬scale test runs as well as thermodynamic equilibrium calculations performed have revealed that for a real¬scale process CaCl2 should be used as additive with a ratio of 20 g Cl/kg dry ash and that a treatment temperature of 1,000°C at a residence time of 30 minutes should be applied. Under these constraints it should be possible to remove heavy metals from the ashes in a way that the limit values for the utilisation of biomass ashes as an additive in compost production according to the Austrian compost ordinance are kept.
composting, gasification, heavy metals, sewage sludge, ashes
R&D on Biomass Conversion Technologies for Heating, Electricity and Chemicals
Gasification for power, CHP and polygeneration
20th European Biomass Conference and Exhibition
875 - 883
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