Comprehensive Effect of Oxidant Addition in an FGD Slurry on the Removal and Distribution of Selenium: A Field Study.

Flue gas desulfurization (FGD) scrubbers capture selenium in coal-fired power plants, leading to a high concentration of selenium in the slurry. This research proves that SO32- is preferentially oxidized compared to SeO32- by S2O82-. With the increase in the oxidation-reduction potential (ORP) caused by S2O82- addition, the conversion rate of SO32- increased and the size of gypsum grains grew from 31.2 to 34.6 µm. SeO32- migrates into gypsum grains during the growth of CaSO4·2H2O, leading to selenium fixation in gypsum. In a field study of a 350 MW unit, the ORP increased from 142 to 450 mV when Na2S2O8 was fed into the FGD slurry. With the addition of the oxidant, 65.1% of selenium in the liquid phase migrated into gypsum. The concentration of selenium in the leachate of gypsum after oxidant addition decreased by 68.0%. A 2.34% increase in the selenium removal rate was observed in the scrubber. This study focuses on the migration and conversion of selenium in an actual FGD slurry via a field test. The results found in the 350 MW unit are consistent with laboratory results. The change in ORP has been proven to be effective in adjusting the selenium distribution in the FGD slurry.

Selenium migration mechanism in wet FGD slurry: Experimental and DFT analysis.

Selenium (Se) is one of the hazardous trace elements emitted from coal-fired power plants. The Se migration behavior in wet flue gas desulfurization (FGD) slurry is still unclear, and the species of Se in FGD gypsum remains controversial. In this research, the bubbling experiments using simulated slurry with/without gypsum crystallization process were conducted. The experimental results indicated that pure gypsum has poor capability to capture Se components, and only selenite could be trapped in gypsum during its crystal growth stage. Furthermore, the DFT calculation was conducted to provide the microscopic information of Se adsorption and substitution characteristics during gypsum crystallization process. The research findings of this study could help understand the mechanism of Se migration process in FGD slurry, and facilitate the development of effective Se emission control technologies in the future.

The distribution and conversion of selenite and selenate with the bubbling of simulated flue gas in simulated WFGD slurry.

Selenium is one of the hazardous trace elements emitted from coal-fired power plants. The distribution of selenium in Wet Flue Gas Desulfurization (WFGD) process is still unclear and even in controversial, impeding the development of selenium removal technologies. This research has found that the selenite in simulated slurry could be reduced by SO2 while selenate has not been affected. Characterization methods including X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to provide an evidence that the product of the reduction reaction is amorphous elemental selenium. Meanwhile, the influences of other gaseous components, pH, temperature and S2O82- in simulated slurry has also been considered in this research. It is found that with the increase of SO2 concentration in flue gas, the reduction of selenite increased and the reduction reaction is an exothermic reaction. Meanwhile, the oxidation effect of S2O82- competes with the reduction effect of SO2. This study introduced the influence of flue gas into the research of the conversion of selenium in FGD slurry and indicate the effect of flue gas on the potential emission treatment techniques of selenium in FGD slurry.

Effects of O2, SO2, H2O and CO2 on As2O3 adsorption by γ-Al2O3 based on DFT analysis.

In order to reveal the affecting mechanisms of flue gas on As2O3 adsorption by γ-Al2O3 and to enhance the adsorbing capacities of γ-Al2O3, the influences of flue gas constituents on As2O3 adsorption on γ-Al2O3(0 0 1) surface are investigated theoretically via density functional theory (DFT) in this study. The flue gas constituents selected include O2, H2O, SO2 and CO2. O2 converts nearly all of the physisorption structures into chemisorption structures except one structure, in which the O2 electron cloud does not interact with As2O3 molecule and therefore does not enhance the capture of As2O3. For the effects of H2O, SO2 and CO2, they behave almost the same as those of O2, but the physisorption structures vary from different constituents. The difference of stable adsorption structures of O2, H2O, SO2 and CO2 on the surface of γ-Al2O3 and their corresponding properties are the main reason for variance of positions and quantities of As2O3 physisorption structures. Results of this study could provide useful information for enhancing capture capacities of γ-Al2O3 under actual flue gas environments.

Research on the removal of As2O3 by γ-Al2O3 adsorption based on density functional theory.

In order to protect selective catalytic reduction (SCR) catalysts for flue gas denitration in coal-fired power plants, the adsorption of As2O3 on γ-Al2O3(0 0 1) surface is investigated theoretically through density functional theory (DFT) in this study. The adsorption sites, adsorption structures, adsorption energies, electronic clouds, transition processes, and intermediate and transition structures are investigated. The theoretical results indicate that the adsorption of As2O3 molecule on the surface of γ-Al2O3(0 0 1) could be either physical or chemical, depending on the sites the molecule hangs over. Compared with the experimental results from other researchers, this study unveils that, although the apparent adsorption of As2O3 molecule on γ-Al2O3(0 0 1) surface is physical, some of the sites on γ-Al2O3(0 0 1) surface presents strong chemical affinity towards As2O3 adsorption. Further, this study depicts the adsorption process to clarify the reason of the net effect of As2O3 adsorption on γ-Al2O3 being physical. Meanwhile, the study also reveals that apparent physical adsorption of As2O3 on γ-Al2O3(0 0 1) surface is due to the high energy barrier that prohibits the transformation of physical adsorption to chemical adsorption. The research results provide useful information for exploiting γ-Al2O3 as a potential metal oxides sorbent.

Theoretical Study of As2O3 Adsorption Mechanisms on CaO surface.

Emission of hazardous trace elements, especially arsenic from fossil fuel combustion, have become a major concern. Under an oxidizing atmosphere, most of the arsenic converts to gaseous As2O3. CaO has been proven effective in capturing As2O3. In this study, the mechanisms of As2O3 adsorption on CaO surface under O2 atmosphere were investigated by density functional theory (DFT) calculation. Stable physisorption and chemisorption structures and related reaction paths are determined; arsenite (AsO33-) is proven to be the form of adsorption products. Under the O2 atmosphere, the adsorption product is arsenate (AsO43-), while tricalcium orthoarsenate (Ca3As2O8) and dicalcium pyroarsenate (Ca2As2O7) are formed according to different adsorption structures.

The promotion effect of manganese on Cu/SAPO for selective catalytic reduction of NO x with NH3.

The activity and hydrothermal stability of Cu/SAPO and xMn-2Cu/SAPO for low-temperature selective catalytic reduction of NO x with ammonia were investigated. An ion-exchanged method was employed to synthesize xMn-2Cu/SAPO, which was characterized by N2 adsorption, ICP-AES, X-ray diffraction (XRD), NH3-temperature programmed desorption (NH3-TPD), NO oxidation, X-ray photoelectron spectrum (XPS), UV-vis, H2-temperature programmed reduction (H2-TPR) and diffuse reflectance infrared Fourier transform spectra (DRIFTS). 2Mn-2Cu/SAPO and 4Mn-2Cu/SAPO showed the best SCR activity, in that at 150 °C NO conversion reached 76% and N2 selectivity was above 95% for the samples. NO oxidation results showed that the 2Mn-2Cu/SAPO had the best NO oxidation activity and the BET surface area decreased as manganese loading increased. XRD results showed that the metal species was well dispersed. NH3-TPD showed that the acid sites have no significant influence on the SCR activity of xMn-2Cu/SAPO. H2-TPR patterns showed good redox capacity for xMn-2Cu/SAPO. UV-vis and H2-TPR showed that the ratio of Mn4+ to Mn3+ increased as manganese loading increased. XPS spectra showed a significant amount of Mn3+ and Mn4+ species on the surface and addition of manganese increased the ratio of Cu2+. The promotion effect of manganese to 2Cu/SAPO comes from the generation of Mn3+ and Mn4+ species. Deduced from the DRIFTS spectra, the Elay-Rideal mechanism was effective on 4Mn-2Cu/SAPO.