Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Release of contaminants in the gasifier as a function of temperature and type of sewage sludge

Elements, such as Ba, Cu, Hg, Pb and Zn, can be present in sewage sludge at levels significant to the disposal of the residual streams and environmental emissions from a gasifier. The distribution of these elements to the coarser bed residue and the elutriated fine material will have implications for the need for additional gas cleaning systems and ash disposal.

At gasification temperatures below 900 degrees Celsius the majority of the trace metals present in the sludge feedstock were retained in the ash residues from the fluidised bed gasifier. At temperatures above 900 degrees Celsius, virtually all of the Hg, a substantial proportion of the Pb and rather less of the Zn were released was released to atmosphere; Ba was also released with specific sludges. The high Pb enrichment in the fly ash was shown to have a strong dependency on gasifier temperature indicating capture by chemical reaction and thermodynamic equilibrium modelling was used to provide a qualitative explanation for the volatility of Pb.

The extent to which this constitutes an emission problem will then depend on the effectiveness of the gas cleaning system at removing each volatile species from the fuel gas. The most important gas cleaning parameter is likely to be temperature. A wet scrubbing system should be effective for removing all of these elements from the gas, with the possible exception of Hg.

The experience of Hg removal by wet flue gas desulphurisation in coal-fired power stations is of variable performance dependent on coal chlorine content, poor removal generally being associated with low chlorine coals whose chlorine content is still higher than those typically found in sewage sludges. In a high chlorine system, much of the Hg will be oxidised and readily soluble Hg2+, whilst in a low chlorine system the less soluble Hgo will predominate. An activated carbon system for Hgo removal from the fuel gas may therefore be needed to supplement the inherent Hgo capacity provided by the residual carbon in the fly ash. The speciation of the elements found to be enriched in the fly ash (especially Pb) will be important in determining their leachability on disposal to landfill.

Reported by

Imperial College London
Prince Consort Road
United Kingdom
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