Bench-scale Kinetics Study of Mercury Reactions in FGD Liquors

Abstract

This document is the final report for Cooperative Agreement DE-FC26-04NT42314, "Kinetics Study of Mercury Reactions in FGD Liquors." The project was co-funded by the U.S. DOE National Energy Technology Laboratory and EPRI. The objective of the project has been to determine the mechanisms and kinetics of the aqueous reactions of mercury absorbed by wet flue gas desulfurization (FGD) systems, and develop a kinetics model to predict mercury reactions in wet FGD systems. The model may be used to determine optimum wet FGD design and operating conditions to maximize mercury capture in wet FGD systems.Initially, a series of bench-top, liquid-phase reactor tests were conducted and mercury species concentrations were measured by UV/visible light spectroscopy to determine reactant and byproduct concentrations over time. Other measurement methods, such as atomic absorption, were used to measure concentrations of vapor-phase elemental mercury, that cannot be measured by UV/visible light spectroscopy.Next, a series of bench-scale wet FGD simulation tests were conducted. Because of the significant effects of sulfite concentration on mercury re-emission rates, new methods were developed for operating and controlling the bench-scale FGD experiments. Approximately 140 bench-scale wet FGD tests were conducted and several unusual and pertinent effects of process chemistry on mercury re-emissions were identified and characterized.These data have been used to develop an empirically adjusted, theoretically based kinetics model to predict mercury species reactions in wet FGD systems. The model has been verified in tests conducted with the bench-scale wet FGD system, where both gas-phase and liquid-phase mercury concentrations were measured to determine if the model accurately predicts the tendency for mercury re-emissions.This report presents and discusses results from the initial laboratory kinetics measurements, the bench-scale wet FGD tests, and the kinetics modeling efforts.v LIST OF TABLES EXECUTIVE SUMMARYThis document is the final report for DOE-NETL Cooperative Agreement DE-FC26-04NT42314, "Bench-scale Kinetics Study of Mercury Reactions in FGD Liquors." In the project, experimental methods were developed initially to use spectrophotometry to identify key reaction intermediate species in the reduction of oxidized mercury to elemental mercury under wet flue gas desulfurization (FGD) conditions, and to track their changes in concentration as the reactions proceeded. Separate but complementary methods for following the rate of evolution of mercury into the gas phase, and loss of mercury from the liquid phase were developed and applied. Using these methods, it has been determined that mercuric sulfite and chlorosulfite complexes form when oxidized mercury is absorbed by FGD systems, and correspondingly decompose to produce elemental mercury re-emissions. The rate of disappearance of the mercuric sulfite and chlorosulfite complexes and the corresponding rate of formation of elemental mercury were monitored by independent...

Control of Hg0 re-emission from gypsum slurries by means of additives in typical wet scrubber conditions

“…The results indicate that TMT has a lower chelating capacity than Na 2 S [24] and that the dose of TMT should be kept to the minimum in order to limit emissions of mercury since doses that are too high could lead to the reduction of mercury instead of its precipitation [25]. However, these results have not been demonstrated in wet scrubber conditions with limestone slurry as the sorbent.…”

Effects of Mg2+ on the bivalent mercury reduction behaviors in simulated wet FGD absorbents

“…Therefore, this was not the reason of magnesium promoting effect on bivalent mercury reduction. In the report of Blyth et al [46], it was indicated that the calcium ion produced a significant increase in mercury re-emissions at the base-case level of 15.0 mol/L sulfite and pH value of 5.0. They concluded that small gypsum like nucleation would form after Ca ion addition, which was recognized as CaSO 3 ·0.5H 2 O.…”

Influence of Flue-Gas Components on Mercury Removal and Retention in Dual-Loop Flue-Gas Desulfurization

“…ref 2. Multiligand complexes are suggested by Blythe et al 25 and Wo et al 26 They consist of Hg 2+ with both chloride and sulfite anions as ligands; see eq 2.5 and eq 2.6, and cf. ref 26 The reduction of mercury compounds such as HgCl 2 leads to a re-emission of elemental mercury.…”

“…Furthermore, additional S(IV) which is absorbed in the absorber could lead to the re-emission of mercury in the quencher by high S(IV) concentrations; cf. ref 25. The interactions of the removal rates are important, as the quencher removal rate defines the absorber raw gas concentration.…”