Mercury Removal in the Flue Gas Scrubber of a Fluidized-Bed Sewage Sludge Incinerator

The fluidized bed sewage sludge incineration plant of the city of Hamburg started its operation in May, 1997. An acid scrubber was designed as the main sink of mercury. A first modification of the mercury removal concept was presented on the 16th Fluidized Bed Combustion Conference in 2001 [1]. Mercury compounds are now dissolved in the acid scrubber and precipitated with a solution of sodium dimethyldithiocarbamate. In the year 2002 the operation of the acid scrubber has been investigated to optimize the scrubber performance. Suspension pH, amount of precipitant and density of the scrubber suspension were varied to find the optimum operation parameters for the capture of mercury at minimum cost. It was found that with an increase of the density of the scrubber suspension the mercury removal efficiency was enhanced such that the dosage of the precipitant could be decreased significantly. After this optimization process the pollutant streams into the environment were minimized. The paper presents the influence of the scrubber operation parameters on the mercury capture using continuous mercury measurements in the flue gas upstream and downstream of the scrubber. The results of this investigation led to both a minimization of the pollutant streams into the atmosphere and a reduction of the operation cost.