Dynamisches verhalten von Hg an SCR-DENOX-Katalysatoren

The existing assumption of a quasi-continuous emission of mercury from waste incineration units is based on results from discontinuous measurements performed with state of the art techniques. The launch and application of Hg-CEMs (continuous emission monitors for mercury) recently has demonstrated that the emission behaviour of mercury is subjected to conspicuous temporal fluctuations depending on the applied air pollution control devices and a number of plant parameters. At the Geiselbullach waste incineration unit fuel related mercury peaks are avoided by a controlled overdosage of the sorbent (activated carbon or activated lignite). Thus after this cleaning stage mercury concentrations are achieved that are far below the existing limiting value. However, with the retrofit of Hg-CEMs after the last cleaning stage mercury peak emissions at different intervals were detected exceeding the half hour limiting value. The significant curve shape of the peaks seem to be implausible to originate neither from a fuel related breakthrough nor from impurities in the sampling system. A test program had been performed to clarify the origin of the mercury peak emissions but did not turn out with a sufficient explanation. In another attempt to find an explanation for the phenomenon the observations made at Geiselbullach plant were compared with results from a laboratory scale catalytic unit at the Institute of Process Engineering and Power Plant Technology of the University of Stuttgart. Extensive investigations in laboratory scale have identified an adsorptive behaviour of mercury at the applied catalyst in an investigated temperature range between 280 and 340 °C. This adsorption is HCI dependent and for HCI-concentrations below 10 mg/m 3 , typical for a tail-end application of SCR-DENOX catalysts, particularly distinct. Even a temporary increase of the HCI-concentration leads to a sudden release of previously adsorbed mercury. That implies that SCR-DENOX catalysts, installed after a more or less complete separation of acid gas components especially HCl, and in tail-end application act as a buffer for mercury. Thus the catalyst is charged with mercury during a period of steady operation of the air pollution control devices. Up to now the increase of the HCl-concentration, e.g. caused by a HCl breakthrough or a breakdown of the combined HCl/SO 2 cleaning stage, is clearly identified in laboratory scale as a trigger mechanism.