Evaluation of the mercury adsorption from WFGD wastewater in coal-fired power plant using sulfur-containing activated carbon
Date Issued
2016
Date
2016
Author(s)
Chiou, Hsin-Jin
Abstract
Because thermal power generation is still extensively used globally, mercury (Hg) emissions from coal-fired power plants have been of greatest concerns to public. Among the available technologies for avoiding flue gas emissions, Wet Flue Gas Desulfurization (WFGD) has received considerable attention due to its capability to remove SO2 and Hg simultaneously. Under certain circumstances, however, oxidized mercury (Hg2+) captured by the WFGD system might be reduced by the reducing compounds, such as sulfites, and reemitted to the atmosphere in the form of Hg0 that causes secondary pollution and results in the lower efficiency of Hg removal by WFGD. Activated carbon (AC) containing sulfur is highly effective in adsorption of gaseous and aqueous Hg pollutants because of its suitable physical and chemical properties. In this study, a series of designed batch experiments were conducted to obtain the optimal adsorption conditions for removing the aqueous Hg from WFGD wastewater by using a sulfur-containing activated carbon (SAC). The test variables included temperature, pH value, SAC dosage, initial Hg2+ concentration, and the SO32- concentrations of WFGD wastewater. The adsorption isotherms and kinetics were subsequently obtained and better understood by using theoretical and empirical simulation models. The total surface area and sulfur content of SAC was measured to be 736.7 m2/g and 4.6%, respectively. The high microporosity of SAC made the adsorbent suitable for the adsorption of Hg. The experimental results indicated that the Hg adsorption capacity of SAC decreased with increasing pH value. Furthermore, Hg adsorption capacity was better fitted with linear adsorption isotherm model, which is mainly due to the low Hg concentration range tested in this study. By measuring the gaseous Hg concentration, the reemission of gaseous Hg was found to ascend with increasing the SO32- concentration from 5 to 100 mM, which may be resulted from Hg0 formation from Hg2+ reduction due to the presence of HSO3-. However, the reemission of reduced Hg was markedly decreased as increasing SO32- addition from 100 to 300 mM, which may stem from the formation of Hg(SO3)22- stably present in aqueous phase. Kinetic simulation showed that the fitting by pseudo-second order equation possessed a higher R2 compared to that by pseudo-first order equation. Thermodynamic parameter calculation concluded that △H°= -19.14 kJ/mole, △S°= -0.037 kJ/mole, and △G≒-30 kJ/mole. These analytical results indicate that Hg adsorption by SAC is thermodynamically spontaneous and exothermic.
Subjects
coal-fired power plant
wet flue gas desulfurization
mercury
sulfur-containing activated carbon
adsorption
re-emission
SDGs
Type
thesis
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