On the measurement of volatile metal species at elevated temperatures: Dissertation

Thermodynamic calculations were performed to find out the most relevant species in coal and waste combustion flue gases. According to thermodynamic calculations, vaporous cadmium, mercury, lead and zinc appear mainly in metallic form or as chlorides in flue gases when the atmosphere is oxidizing, and also as sulphides in the reducing atmosphere. Cadmium, lead and zinc will condense as sulphates in the oxidizing atmosphere and as sulphides or chlorides in the reducing atmosphere. According to thermodynamic calculations, mercury remains in the vapour phase at temperatures as low as 200 °C. The calculations were done using the measured data on coal and waste combustion processes. A vaporization/condensation method was developed to substitute inefficient and time consuming conventional liquid absorption methods. As this developed method is rather complicated and in order to get information on different oxidation states of volatile metals, methods based on diffusion and the subsequent chemical reaction were developed: a denuder tube coated with silver and a diffusion screen coated with gold can be used as molecular collectors of cadmium and mercury. A denuder coated with KCI collects selectively oxidized metal species, like HgCl2, whereas the Ag denuder and the Au screen collect both metallic and oxidized species. To investigate the performance of the developed volatile metal collectors in the laboratory, metal aerosol generators based on vaporization and diffusion, as well as an artificial laboratory stack, were developed and built. Mercury species were generated simply by vaporizing them in a temperature controlled diffusion cell and by leading the diffused species through heated tubes to a sampling device for testing. Other metal species (like Cd), which appear normally in the particulate phase at temperatures below 200 °C, were first allowed to nucleate homogeneously and then vaporized rapidly again in the artificial stack. When no water vapor was present the collection efficiency of the Ag denuder for metallic cadmium was high, over 95 % up to flow rates of 2 l/min. Water vapor seems to decrease the collection efficiency to some extent, 10-15 %. By cutting the denuder tube containing collected metal in several pieces and by using a modification of Gormley Kennedy equation, it is possible to determine the diffusion coefficient. According to this method a diffusion coefficient of (0.42 ± 0.04) cm²/s was obtained for metallic cadmium at 200 °C. A diffusion screen can operate at much higher flow rates (5-6 l/min) than denuders. The dependence of the collection efficiency on temperature for a gold coated diffusion screen was similar to dependencies obtained for denuders. The effect of SO2, H20 and NO on the collection efficiency was also tested, but no significant reduction in the collection efficiency could be observed. In the first field experiments, diffusion methods were compared with a conventional high efficiency liquid absorption system (HNo3/K2Cr2O7). At lower concentrations, e.g. mercury concentrations below 20 ~g/m³, the diffusion methods proved to be more efficient than the liquid absorption method Mercury concentrations as low as 0.5 mg/m³ can be easily measured using sampling times of 0.5 h. At higher concentrations the capacity of the diffusion screen used was too low. Based on the fact that the screen size used was so small, there should not be any difficulties in modifying the collector for higher capacities. A sequential combination of a KCI denuder and an Au-screen was used to collect oxidized and metallic mercury separately during the combustion of liquid waste. According to the results 0-51% of the total mercury was in the oxidized form.