Optimisation of Wet Hg Capture from Flue Gases of Coal-fired Power Plants in the Case of Co-combustion of Mercury-rich Secondary Fuels

Project Number 313

Municipal waste is increasingly utilised as additional fuel in coal-fired power plants. Against the background of the sometimes elevated mercury (Hg) content of such secondary fuels, it is necessary to optimise wet flue gas desulphurisation (FGD) with regard to mercury removal.

In the model studies of the preliminary project ( 293 ), factors influencing the Hg chemistry in scrubbers and the subsequent FGD-washing water treatment previously unknown could be identified. A large number of potentially influencing factors a few proved to be particularly relevant parameters:

  • Gypsum / pH-value,
  • Iodide,
  • Iron hydroxide,
  • Humic acid/formic acid.

Besides the experimental validation of some of these effects (better reproducibility), in this follow-up-project  the effectivity of commercially available additives to keep the oxidised Hg in the FGD-washing suspension as well as the  influence of these additives on discharge of mercury out of the system is to be investigated. In addition, reagents for the separation of elemental mercury by FGD are to be tested and rated.

The transferability of the results from the synthetic FGD-washing suspensions to full scale FGD scrubbers is to be validated by experiments with “real” scrubbing solutions.

The project will lay the foundations for an economic and reliable reduction of mercury emissions from coal-fired power plants with use of secondary fuels. To this end subsequently the procedural system parameters will be determined to ensure that

  • oxidized Hg is to a large extent, elemental mercury is at least partly removed from the raw gases in the FGD-scrubber,
  • the Hg separated does not concentrate in the FGD-gypsum, but is discharged to the wash-water treatment plant,
  • Hg from the wash-water treatment sludge will be separated and concentrated in a small Hg-rich sludge fraction, resulting in mercury-free clean wastewater and low-Hg in the main sludge fraction.

The project results can be implemented immediately in plant operation.

The project is realised from July 2008 to December 2009 at the Institute for Environmental Technology of the Martin-Luther-University Halle-Wittenberg and directed by Prof. Dr. Heinz Köser. A group of members of the VGB Working Panels Flue Gas Cleaning Technology , Chemistry of Water Treatment and of the Technical Committee Power Plant By-products accompanies the investigations.