Mercury in Thermal Power Plants: Chemical Behavior and Possibilities for Separation

Abstract

Die weitere Absenkung der Emissionen von Quecksilber aus fossil befeuerten Kraftwerken erfordert eine Verbesserung der bestehenden Rauchgasreinigungssysteme oder den Einsatz von neuen oder adaptierten Prozessen. Dabei können spezielle Adsorbentien, zur Quecksilberoxidation verbesserte SCR-DeNOx-Katalysatoren oder Additive zur Quecksilberumwandlung und Abscheidung in nassen Rauchgasentschwefelungsanlagen eigesetzt werden. Da die chemischen Zusammenhänge der verschiedenen Quecksilberspezies von den Betriebsparametern der einzelnen Rauchgasreinigungsanlagen abhängen, ist ein detailliertes Verständnis der Vorgänge bei der Quecksilberrückhaltung von großer Bedeutung.The reduction of mercury emissions from fossil fired power plants is of major concern. The optimization and the improvement of the existing flue gas cleaning devices or the use of special equipment for mercury removal are possible ways.Therefore the use of sorbents, the optimization of SCR-DeNOx-catalysts and the utilization of additive to enhance the mercury oxidation and the separation in wet flue gas desulfurization are only some discussed technologies. As the behavior of all mercury species in flue gas depends on the operational parameters of the power plant and the special flue gas cleaning device, it is essential to understand the chemistry and the interaction of the different cleaning devices.

Mercury Emissions and Removal by Ash in Coal-Fired Oxy-fuel Combustion

“…10,30,31 With Cl available, surface carbon can be chlorinated, adsorb, and finally oxidize and chemisorb Hg by a mechanism proposed by Gale et al 32 Ashes with high unburned carbon (UBC) have generally higher Hg removal efficiencies. 11,12,18,32−34 Hg removal by the ash in filters decreases with increasing temperatures, 11,19,35 which can be explained by the complex sequence of sorption processes and reactions on the surface, with different temperaturedependent rate constants. 32,33 Direct capture of Hg 0 on fly ash is believed to be of minor importance, while the sorption of gasphase Hg 2+ or Hg 0 with subsequent oxidation to Hg 2+ and capture of the oxidized Hg on the surface seems to be more important for the removal of Hg on fly ashes in filters.…”

“…12 Removal efficiencies of Hg in flue gas cleaning equipment are affected by factors such as Cl and S contents of coal, composition of ash (e.g., UBC), degree of Hg oxidation at the cleaning equipment inlet, and type and operation mode of the applied flue gas cleaning equipment. 11,19,30 Generally, lowrank coals show lower Hg removal efficiencies than higher rank coals, 11,39 which is believed to be connected to the lower UBC generated during combustion of those fuels 11 and their Cl contents that are often lower than in high-rank coals. As shown by Pavlish et al, 39 the scatter of reported Hg removal efficiencies in different flue gas cleaning unit operations is high but significantly differing for the different coal types.…”

“…39 Besides in baghouse filter systems, Hg removal can take place in electrostatic precipitators (ESPs) and flue gas desulfurization (FGD) and can be promoted by activated carbon injection. 19,39 Overall, the most important factors influencing the concentration levels, speciation, and removal of Hg from flue gas are the Hg content of coal, Cl (and Br) content of coal, combustion conditions in the furnace, SO x concentrations and speciation, fly ash properties (UBC, Ca, and catalytically active compounds), residence time and temperature profile of the flue For oxy-fuel combustion, only limited information on the Hg emission behavior is available. The impact of the change from a N 2 to a CO 2 atmosphere was shown to have no considerable impact on equilibrium concentrations of Hg 0 and HgCl 2 .…”

“…Higher Cl contents in the fuel can lead to higher Hg oxidation . Hg oxidation can also occur via heterogeneous reactions, e.g., on fly ash (catalytically active unburned carbon and Fe 2 O 3 ,, ), injected activated carbon, , or selective catalytic reduction (SCR) catalysts. ,,,, The importance of the heterogeneous Hg oxidation however is varying for different plant layouts, plant loads, and fuels (e.g., Cl and S contents), − and some of the observations made in bench scale, such as the catalysis of the Hg oxidation by Fe 2 O 3 , could not be reproduced in technical- and industrial-scale facilities . The Hg 0 /Hg 2+ conversion in coal-fired systems, such as power plants, is not complete but kinetically limited , and depends upon the availability of Cl (and Br) and catalytically active materials.…”

“…Heterogeneous Hg oxidation can occur on entrained fly ash as well as ash deposited on filters. ,, With Cl available, surface carbon can be chlorinated, adsorb, and finally oxidize and chemisorb Hg by a mechanism proposed by Gale et al Ashes with high unburned carbon (UBC) have generally higher Hg removal efficiencies. ,,,− Hg removal by the ash in filters decreases with increasing temperatures, ,, which can be explained by the complex sequence of sorption processes and reactions on the surface, with different temperature-dependent rate constants. , Direct capture of Hg 0 on fly ash is believed to be of minor importance, while the sorption of gas-phase Hg 2+ or Hg 0 with subsequent oxidation to Hg 2+ and capture of the oxidized Hg on the surface seems to be more important for the removal of Hg on fly ashes in filters . If only little carbon or other sorption sites are available in the ash or available sorption sites are consumed by Hg 2+ or sulfur species, the chlorinated Hg generated on the ash may desorb and lead to an overall oxidation of gas-phase Hg without significant Hg removal. ,, The Hg oxidation on fly ash can be promoted by acid gases, such as HCl, SO 2 , or H 2 SO 4 , ,, while increased SO 3 /H 2 SO 4 are reported to decrease the potential for sorption of Hg 2+ , because both species adsorb on the same sorption sites on carbon surfaces. ,,,, Ca in the ash synergistically with carbon can affect the Hg capture beneficially, by offering sorption sites for chlorinated Hg .…”