(691d) Effect of the Flue Gas Composition On Mercury Speciation Under Homogeneous and Heterogeneous Conditions
Mercury is commonly found in the flue gas derivated from the combustion of fossil fuels such as coal and is considered a hazardous pollutant for humans and the environment. In flue gas, mercury can be present in both elemental and oxidized forms. The oxidized form is water soluble and can generally be collected by existing particulate and SO2 control devices. In contrast, elemental mercury is not soluble in water and is highly volatile at typical combustion temperatures. These factors make it difficult to capture by conventional control devices; hence, considerable attention has been given to finding ways to oxidize elemental mercury. Complex mechanisms for heterogeneous oxidation have been proposed. A better understanding of mercury speciation and oxidation mechanisms is essential for the development of air pollution control strategies and devices.
The focus of this work is to study how the composition of the flue gas components (HBr, SO2, and HCl) can affect mercury speciation under homogeneous and heterogeneous conditions. Experimental data were obtained with two reactors: a 300-W, methane-fired, tubular, quartz-lined reactor for studying homogeneous oxidation reactions and a fixed-bed reactor, also of quartz, for studying heterogeneous reactions. The latter was attached to the exit of the former to provide realistic combustion gases. The fixed-bed reactor was packed with one gram of coconut-shell carbon and kept at a temperature of 150oC. All methane, air, SO2, and halogen species were introduced through the burner to produce flue representative of real combustion systems. Speciated mercury measurements were performed using a Tekran 2537A Analyzer coupled with a wet conditioning system.
Gas phase measurements showed that in presence of bromine the observed mercury oxidation ranged from 28% at 18 ppm bromine (as HBr equivalent) to 80% at 55 ppm bromine as (HBr equivalent), while values of mercury oxidation were 2% with 100 ppm chlorine (as HCl equivalent) and 6% with 500 ppm chlorine (as HCl equivalent). In general, the SO2 showed an inhibitory effect on the mercury oxidation obtained by either bromine or chlorine, with mercury oxidation values of 25 % for the mixture 45 ppm bromine (as HBr equivalent), 500 ppm SO2 and 5% for the mixture 500 ppm chlorine (as HCl equivalent) and 500 ppm SO2 . Under heterogeneous conditions, the coconut-shell carbon showed a mercury capture efficiency of about 20% but in presence of bromine or chlorine this uptake increased up to 90%. In the absence of halogens, SO2 increased the mercury adsorption efficiency to up to 30 percent. The extent of adsorption decreased with increasing SO2 concentration when halogens were present.
A model of fixed-bed capture of mercury that incorporates Langmuir adsorption kinetics was developed to predict mercury adsorption and the effect of the flue gas components. The model neglects intraparticle diffusion resistances and is only applicable to pulverized carbon adsorbents. The model roughly describes experimental data from the literature. The current version includes the ability to account for competitive adsorption between mercury, SO2, and NO2. Future work will focus on the development of surface reactions between mercury and halogens.