Chemical Behavior of Mercury in Wet Flue Gas Desulphurisation Systems

Dissertation von Carolina Acuna Caro
Universität Stuttgart, 2013

Mercury is a toxic element present naturally in trace amounts on the earth’s crust. However, industrialization has contributed to increase the natural concentrations of this metal in the environment. Within the major anthropogenic sources of mercury are fossil fuel combustion for power and heating generation, followed by artisanal and small-scale gold production, metal production, cement production and waste incineration among others. The high vapour pressure, low water solubility, and chemical inertness of elemental mercury (Hg0) are responsible for its long residence time in the atmosphere, constituting a global contamination issue. The mercury species that are released from fossil fuelled power plants are gaseous elemental mercury (Hg0), gaseous compounds containing the mercuric ion (Hg2+) and a small share of mercury adsorbed by fine dust particles (Hgp). These species are transferred to different ecosystems by wet/dry deposition. In these systems bacteria carry out the conversion of inorganic mercury species into organic compounds. Methyl mercury ion (CH3Hg+) is known as the main organic species that can be easily assimilated by fish in water bodies or bioaccumulated in grains cultivated in contaminated sites. Hence, the most important routes of human exposure are diet and respiratory via.
 
Since fossil fuel combustion has been identified as an important point source of gaseous mercury compounds in the United Stated of America (USA), in the European Union (EU) and respectively in Germany regulations have been established to reduce mercury emissions from these facilities. In order to achieve the required levels of emission, the necessity to gain a deep knowledge about the behaviour of mercury along the flue gas path constitutes a major issue. Until now besides the incorporation of additives (e.g. activated carbon, bromine salts), a suitable possibility for mercury capture could rely on the existent air pollution control devices (APCDs). It is known that a significant share of Hgp remains at the electrostatic precipitator (ESP), while depending on the catalyst utilised for nitrogen oxides reduction at the selective catalytic reduction unit (SCR-DeNOx), the oxidation of Hg0 may occur. The latter could enhance the share of water-soluble Hg2+ species increasing the mercury capture at wet flue gas desulphurisation units (FGD). Nevertheless, even though that behaviour has been observed, the technology has not been yet improved, since most of the chemical mechanisms involved are still not well understood.
 
For these reasons, the study of mercury behaviour from a chemical point of view is the general goal of the present work. Since wet FGD plants could offer a real potential for mercuric species removal from flue gas, and because of the variety of chemical processes occurring in this system, it is of top priority to gain a deep knowledge on the chemical mechanisms that affect and/or improve mercury capture in this system. Thereby, the investigation of the main flue gas and scrubber solution components as well as wet FGD operating factors that influence the chemistry of mercury in both gaseous and aqueous phase are the main objectives of this work. Through a literature survey information about flue gas desulphurisation technologies is presented as well as the chemical and physical properties of the element mercury together with the relevant reactions that take place along the flue gas path. Additionally, thermodynamic equilibrium calculations are performed considering typical flue gas and scrubber solution compositions. Based on the method of the free Gibbs energy minimisation, wet FGD conditions are simulated considering two streams: the flue gas entering the system and the scrubber liquid. Data collected from seven different limestone wet FGD installations are used as input values. Temperature and pH of the scrubber solution as well as chlorine (HCl (g) and Cl- (aq)), oxygen and sulphur dioxide concentrations in flue gas are varied. The outcomes obtained from these calculations showed that the combination of high temperature and low chloride concentration increases elemental mercury release from the sump. However, if the shares of hydrochloric acid and oxygen are augmented, the temperature influence is no longer observed and tetrachloro complex of mercury results as the main species. Regarding the relation between oxygen and chlorine species effect, if the amount of O2 (g) is greatly enhanced, it prevails over chlorine. Nevertheless, at normal levels of oxygen the influence of an augmentation on chlorine concentration demonstrates its significance for mercury capture. On the other hand, as the share of sulphur dioxide raises the amount of mercuric compounds that remain in solution decreases as a consequence of the reducing effect of SO2. Meanwhile, variations on the pH seem to favour the formation of different mercury species in complete absence of chlorine.
 
Laboratory-scale experiments follow the hints gained from these calculations. The influence of different acidic components on the conservation of Hg2+ in aqueous solution is analysed. Heterogeneous interactions are also considered through the addition of solid compounds, e.g. CaSO4, CuCl among others, in order to understand their role on mercury adsorption/absorption on gypsum material. The construction of a test rig that allows simulating wet FGD operation conditions is performed, constituting the core of the experimental investigations presented in this work. Batch tests are carried out by means of a synthetic flue gas generator, a reactor vessel that contains CaCO3 slurry and appropriate measurement equipment to control inlet and outlet concentrations of mercury, SO2 and O2. The main goal of these tests is to validate the parameters considered for thermodynamic equilibrium calculations that have shown an impact on mercury oxidation/reduction processes. Thereby, oxidation tests included the study of chloride and oxygen concentrations along with pH changes in a wide range, using different acid compounds. In general, results indicate chlorine and oxygen as the key chemical partners to capture mercury in solution. Even the O2 added through aeration seems to promote Hg0 retention at the sump. Furthermore, temperature changes in the reactor solution do not affect significantly the amount of Hg2+ (aq) at high Cl- (aq) and O2 (aq) concentration. Nevertheless, at low concentration of chloride and oxygen, a temperature of about 70ºC causes an increase on mercury emissions. Regarding pH variations, the influences seem to be directly related with the nature of the ionic species that induce a change on pH value, like HNO3, H2SO4, H2SO3, SO2. The availability of chlorine species affects also in this case the behaviour of the measured mercury emissions, as well as the share of Hg0/Hg2+ entering the system. Meanwhile, reduction of Hg2+ (aq) is observed when SO2 is added in absence of O2 (aq) and Cl- (aq) indicating the importance of these two species on the retention of mercury in solution.
 
From these results it is concluded that mercury capture in wet FGD systems depends mainly on the share of Hg2+ entering the system, but it can also be improved by increasing the amount of chlorine species in solution as well as strong aeration. Nevertheless, an increment of chlorine may produce side effects such as corrosion, fouling, formation of other toxic chlorinated compounds and it can affect the desulphurisation performance. Therefore, pilot- and full-scale research shall be accounted as the next step, to prove the feasibility and the limitations of such solutions. 
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