Effect of Atmospheres on Transformation of Heavy Metals during Thermal Treatment of MSWI Fly Ash: By Thermodynamic Equilibrium Calculation.

The vaporization behaviors of eight heavy metals (Pb, Zn, Cu, Cd, Cr, Co, Mn, and Ni) in municipal solid wastes incineration (MSWI) fly ash during thermal treatment under air atmosphere (21% O2/79% N2), an inert atmosphere (100% N2), and a reducing atmosphere (50% CO/50% N2) were evaluated based on a thermodynamic equilibrium calculation by FactSage 8.1. The results show that the reducing atmosphere promotes the melting of MSWI fly ash, resulting in a more liquid phase than in air or an inert atmosphere. Except for Cd, the formation of liquids can dissolve heavy metals and reduce their vaporization ratio. In the air and inert atmospheres, Pb, Zn, Cu, Co, Mn, and Ni vaporize mainly in the form of metallic chlorides, while Cd volatilizes in the form of metallic Cd (g) and CdO (g). In the reducing atmosphere, Co, Mn, and Ni still vaporize as chlorides. Zn and Cd mainly vaporize in the form of Zn (g) and Cd (g), respectively. In terms of Pb, in addition to its chlorides, the volatiles of Pb contain some Pb (g) and PbS (g). Cr has a low vaporization ratio, accounting for 2.4% of the air atmosphere. Cr, on the other hand, readily reacts with Ca to form water-soluble CrCaO4, potentially increasing Cr leaching. Except for Cd, the results of this study suggest that the reducing atmosphere is used for the thermal treatment of MSWI fly ash because it promotes the melting of fly ash and thus prevents heavy metal vaporization.

Use of thermal treatment with CaCl2 and CaO to remove 137Cs in the soil collected from the area near the Fukushima Daiichi Nuclear Power Plant.

A massive amount of soils and inflammable materials of plants etc. contaminated by radiocesium are generated from decontamination work in the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident affected area. In present study, the removal experiments of 137Cs in a soil collected from the FDNPP accident affected area were carried out in a lab-scale electrical heating horizontal furnace through thermal treatment with CaCl2 addition over a temperature of 900 - 1300 °C. The results indicated that the average radioactive concentration of 137Cs in the soil was 52.8 Bq/g. The removal ratio of 137Cs in the soil treated at 1300 °C was 96.3 % when 20 % CaCl2 was added. The addition of CaCl2 and CaO mixture exhibited a synergistic effect on the removal of 137Cs, relative to the addition of CaCl2 alone. Accordingly, the addition of CaCl2 or its mixture with CaO during thermal treatment is suggested to remove 137Cs in the soil collected from the FDNPP accident affected area. Additionally, segregation of the soil sample to fine and coarse fraction and then treated individually are also recommended.

Vaporization Mechanisms of Water-Insoluble Cs in Ash During Thermal Treatment with Calcium Chloride Addition.

The vaporization mechanisms of water-insoluble Cs in raw ash and Cs-doped ash during thermal treatment with CaCl2 addition was systematically examined in a lab-scale electrical heating furnace over a temperature range of 500-1500 °C. The results indicate that the water-insoluble Cs in the ash was associated with aluminosilicate as pollucite. Addition of 10% CaCl2 caused the maximum vaporization ratio of Cs in the raw ash to reach approximately 80% at temperatures higher than 1200 °C, whereas approximately 95% of Cs was vaporized at temperatures higher than 1300 °C when 30% CaCl2 was added. The formation of an intermediate compound, CsCaCl3, through the chemical reaction of Cs with CaCl2 was responsible for Cs vaporization by means of the subsequent decomposition of this intermediate upon the increase in temperature. The indirect chlorination of Cs by the gaseous chlorine released from the decomposition of CaCl2 was insignificant. A high CaCl2 content in the resulting annealed products with 30% CaCl2 addition delayed the decomposition of CsCaCl3 and thus lowered the Cs vaporization ratio compared to that with 10% CaCl2 addition at 900-1250 °C. Thermal treatment with CaCl2 addition is a proposed method to remove Cs from Cs-contaminated incineration ash.

Elucidating the mechanism of Cr(VI) formation upon the interaction with metal oxides during coal oxy-fuel combustion.

The thermodynamics underpinning the interaction of Cr-bearing species with basic metal oxides, i.e. K2O, Fe2O3, MgO and CaO, during the air and oxy-fuel combustion of coal have been examined. The synchrotron-based X-ray adsorption near-edge spectroscopy (XANES) was used for Cr speciation. For the oxides tested, Cr(VI) formation is dominated by the reduction potential of the metals. The oxides of Ca(2+) with high reduction potential favored the oxidation of Cr(III), same for K(+). The other two basic metals, Fe2O3 and MgO with lower reduction potentials reacted with Cr(III) to form the corresponding chromites at the temperatures above 600°C. Coal combustion experiments in drop-tube furnace have confirmed the rapid capture of Cr vapors, either trivalent or hexavalent, by CaO into solid ash. The existence of HCl in flue gas favored the vaporization of Cr as CrO2Cl2, which was in turn captured by CaO into chromate. Both Fe2O3 and MgO exhibited less capability on scavenging the Cr(VI) vapor. Particularly, MgO alone exhibited a low capability for capturing the vaporized Cr(III) vapors. However, its co-existence with CaO in the furnace inhibited the Cr(VI) formation. This is beneficial for minimizing the toxicity of Cr in the coal combustion-derived fly ash.

Use of synchrotron XANES and Cr-doped coal to further confirm the vaporization of organically bound Cr and the formation of chromium(VI) during coal oxy-fuel combustion.

Through the use of synchrotron XANES and Cr-doped brown coal, extensive efforts have been made to clarify the volatility of organically bound Cr during oxy-fuel combustion and the mode of occurrence and leachability of Cr in resulting fly ashes. As the continuation of our previous study using raw coal, the Cr-doped coal has been tested in this study to improve the signal-to-noise ratio for Cr K-edge XANES spectra, and hence the accuracy for Cr(VI) quantification. As has been confirmed, the abundant CO(2) as a balance gas for oxy-firing has the potential to inhibit the decomposition of organically bound Cr, thereby favoring its retention in solid ash. It also has the potential to promote the oxidation of Cr(III) to Cr(VI) to a minor extent. Increasing the oxygen partial pressure, particularly in the coexistence of HCl in flue gas, favored the oxidation of Cr(III) into gaseous Cr(VI)-bearing species such as CrO(2)Cl(2). Regarding the solid impurities including Na(2)SO(4) and CaO, Na(2)SO(4) has proven to preferentially capture the Cr(III)-bearing species at a low furnace temperature such as 600 °C. Its promoting effect on the oxidation of Cr(III) to Cr(VI), although thermodynamically available at the temperatures examined here, is negligible in a lab-scale drop tube furnace (DTF), where the particle residence time is extremely short. In contrast, CaO has proven facilitating the capture of Cr(VI)-bearing species particularly oxychloride vapors at 1000 °C, forming Ca chromate with the formulas of CaCrO(4) and Ca(3)(CrO(4))(2) via a direction stabilization of Cr(VI) oxychloride vapor by CaO particle or an indirect oxidation of Cr(III) via the initial formation of Ca chromite. The fly ash collected from the combustion of Cr-doped coal alone has a lower water solubility (i.e., 58.7%) for its Cr(VI) species, due to the formation of Ba/Pb chromate and/or the incorporation of Cr(VI) vapor into a slagging phase which is water-insoluble. Adding CaO to coal increased the water-solubility of both Cr(VI) and Cr(III) by forming Ca chromite and chromate, respectively.

Synchrotron-based XANES speciation of chromium in the oxy-fuel fly ash collected from lab-scale drop-tube furnace.

Speciation of chromium (Cr) in the fly ash collected from oxy-firing of Victorian brown coal has been reported for the first time to address the potential formation of toxic Cr(VI) and the variation of the quantities of Cr(III)-bearing species with flue gas composition. Synchrotron-based X-ray absorption near-edge structure (XANES) was employed for Cr speciation. Apart from a pure O(2)/CO(2) mixture (27/73, v/v) versus air, the O(2)/CO(2) mixtures doped with SO(2), HCl, and steam individually or together to simulate real flue gas have also been tested. Under all of the conditions tested here, the fractions of Cr(VI) in the fly ashes are insignificant, constituting no more than 5% of the total Cr. The test of Cr-doped brown coal in pyrolysis further confirmed that the Cr(VI) formation preferentially occurred through a local oxidation of Cr(III) at the oxygen-containing functions sites within coal matrix, rather than through an oxidation by external bulk O(2). This reaction is also highly temperature-dependent and slower than the interaction between Cr(III) and other metals such as iron oxide. Increasing temperature to 1000 °C inhibited the oxidation of Cr(IIII) to Cr(VI). Shifting the combustion gas from air to O(2)/CO(2) exerted little effect on the Cr(VI) formation. Instead, the formation of iron chromite (FeCr(2)O(4)) was facilitated in O(2)/CO(2), probably due to a strong reducing microenvironment formed by the CO(2) gasification reaction within the char matrix. The accumulation of HCl in flue gas favored the vaporization of chromium as gaseous chloride/oxychloride, as expected. The coexistence of SO(2) inhibited this phenomenon by promoting the formation of sulfate. The presence of steam was even beneficial for the inhibition of water-soluble Cr sulfate through stabilizing the majority of Cr into alumino-silicate which is in the slagging phase.