HEAVY METALS BEHAVIOUR DURING MONO-COMBUSTIONAND CO-COMBUSTION OF SEWAGE SLUDGE

摘要

This paper presents the study of the combustion of granular dry sewage sludge performed on a pilotfluidized bed system. The results of mono-combustion of sludge and co-combustion with coal were comparedwith those of coal combustion for ash partitioning, the formation of gaseous pollutants and heavy metalsbehaviour. It was found that the mineral matter of sludge was essentially retained as bottom ashes. Theproduction of fines ashes was small during the mono-combustion but was high during co-combustion due tothe tendency of coal to produce fine ashes which also contained unburned char. The degree of heavy metalvolatilization was found to be slightly higher during co-combustion than in mono-combustion, however, mostof them were retained in ashes and their emissions were found to be below the regulated levels. Hg wascompletely volatilized, however during combustion trials involving coal it was captured by cyclone ashes attemperatures below 300oC. During sludge mono-combustion the retention of Hg in cyclone ashes containinglow LOI was not enough to decrease emissions below the regulated levels, hence it is necessary to installdedicated flue gas treatment for Hg removal.The leachability and ecotoxicity of sludge and ashes was compared with the new regulatory limits forlandfill disposal in the EU. It was found that the release of organic matter and heavy metals, found in thesludge, was low from granular bed ashes, hence, except for sulphate release, bed ashes were converted intoinert and non ecotoxic materials. Ashes from tests with limestone and cyclone ashes seemed to be moreproblematic because of pH effects and contamination with steel corrosion products. The recovery andreutilization of sludge bed ashes could, therefore, be possible, as long as the release of sulphate do not interferewith the process.