Co-Combustion of Dried Sewage Sludge and Coal The Fate of Heavy Metals

Paper in proceeding, 2001

The fate of some selected heavy metals has been investigated during co-combustion of sewage sludge and coal in two fluidised bed combustors: the 12MWth circulating fluidized bed (CFB) boiler at CTH and the laboratory scale unit at IVD. The total and species balances were, in general, fairly well closed, and the increased amounts of heavy metals originating from the input sludge were also found in the exit flows. Volatile species such as Hg and Cd are sensitive to the temperature conditions. Thus, almost no mercury was found in the bottom ash, and a cyclone at 350 °C captured little Hg in comparison to a cyclone and filters at 150 °C. The high particle loading in a CFB appears to contribute essentially to the reduction of the flue gas mercury concentration; the emission is reduced by orders of magnitude compared to a plant with low particle concentration. In most cases investigated the emissions of heavy metals were below the limits related to co-combustion set by the European Union.