Vaporization Behavior of Cs, K, and Na in Cs-Containing Incineration

Article Cite This: Energy Fuels 2017, 31, 14045−14052

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Vaporization Behavior of Cs, K, and Na in Cs-Containing Incineration Bottom Ash during Thermal Treatment with CaCl2 and CaO Facun Jiao,† Norikazu Kinoshita,‡ Masato Kawaguchi,‡ Motoyuki Asada,‡ Maki Honda,§ Keisuke Sueki,∥ Yoshihiko Ninomiya,*,† Dmitry Sergeev,⊥ Marc Blas̈ ing,⊥ and Michael Müller⊥ †

Department of Applied Chemistry, Chubu University, 1200 Matsumoto-Cho, Kasugai, Aichi 487-8501, Japan Institute of Technology, Shimizu Corporation, 3-4-17 Etchujima Koto-ku, Tokyo 135-8530, Japan § Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan ∥ Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan ⊥ Institute for Energy Research (IEK-2), Leo-Brandt-Strasse 1, 52425 Jülich, Germany ‡

ABSTRACT: The vaporization behaviors of the alkali metals Cs, K, and Na were investigated at 900, 1000, and 1100 °C in a lab-scale electrical-heating horizontal furnace using a Cs-doped ash with the addition of CaCl2 and/or CaO. Knudsen effusion mass spectrometry was employed to measure the vaporization of the alkali metals in the Cs-doped ash with CaO under a high vacuum. Molecular beam mass spectrometry was used online to measure their vaporization from the Cs-doped ash with either CaCl2 or a mixture of CaCl2 and CaO. The addition of CaO caused some vaporization of these elements, which was probably due to the replacement of the Cs+, K+, and Na+ cations in aluminosilicates with Ca2+ cations during the thermal treatment. The vaporization propensity of the three elements followed the sequence of Cs > K > Na. The vaporization of Cs, K, and Na were observed during a thermal treatment with CaCl2. An increase in the content of CaCl2 or the reaction temperature facilitated the vaporization of Cs, K, and Na. O2 and H2O in the reactant gas showed an inhibiting effect on the vaporization of Cs, K, and Na through accelerating the release of Cl from the decomposition of CaCl2. A synergistic effect was observed between the addition of CaCl2 and CaO on the vaporization of Cs, K, and Na because they delayed the release of Cl, which provided a longer contact time between the three metals and the gaseous Cl. Moreover, when the mixture of CaCl2 and CaO was used, the CaO produced unstable Cs, K, and Na that readily reacted with gaseous Cl, enhancing the vaporization of the alkali metals during thermal treatment. At 1100 °C, 93% of the Cs was vaporized from the Cs-doped ash with 5% CaCl2 and 20% CaO while the vaporization ratio of K and Na was 69% and 63%, respectively.

1. INTRODUCTION A huge amount of radiocesium-contaminated solid wastes were produced from the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) caused by the great earthquake in northeastern Japan on March 11, 2011.1−5 Incineration is a suitable method to treat such solid wastes due to its two primary advantages of energy recovery and volume reduction.6 However, Cs is concentrated in incineration residues such as bottom ash and fly ash after the raw wastes are incinerated; the concentration of Cs in the residues is several dozen times higher than that in the raw wastes.7 Therefore, the Cscontaining incineration ashes need to be further treated to reduce the Cs concentration and meet the storage guidelines specified by the Japanese government. Generally, CsCl, formed via chlorination during incineration, is a dominant species in fly ash8−10 and can be removed by water-extraction and subsequent capture by absorbents.11 In bottom ash, Cs mainly exists in the form of Cs-aluminosilicates that form from chemical reactions between Cs and various inorganic matter (e.g., SiO2 and Al2O3) at high temperatures;9,12 these forms of Cs are water-insoluble and cannot be removed by water extraction. According to our previous results, thermal treatment with CaCl2 can vaporize the Cs from bottom ash.13,14 By contrast, HCl gas was insignificant for the Cs vaporization.14 Two other alkali metals, K and Na, also are © 2017 American Chemical Society

usually present in incineration ashes and are expected to be vaporized with Cs during thermal treatments. When thermal treatment with CaCl2 is employed to vaporize Cs from the Cscontaminated incineration ash produced from the area affected by the FDNPP accident, it is important to understand the vaporization behavior of K and Na, as the formed gaseous KCl and NaCl might condense onto the surface of the heat exchanger when the flue gas temperature becomes lower than their dew point, which could corrode the heat exchanger by forming a eutectic.15−17 In the present study, the vaporization behaviors of Cs, K, and Na were examined through a thermal treatment with CaCl2 or a mixture of CaCl2 and CaO. CaO was used because it was thought to affect the decomposition of CaCl2 and thus influence the vaporization of Cs, K, and Na. The effects of the content of O2 and H2O in the reactant gas on the vaporization of the three metals were also investigated. In addition to the experiments conducted in a lab-scale electricalheating horizontal furnace, Knudsen effusion mass spectrometry (KEMS) was employed to clarify the vaporization behavior of the alkali metals in the ash with CaO, while molecular beam Received: September 28, 2017 Revised: November 16, 2017 Published: December 8, 2017 14045

DOI: 10.1021/acs.energyfuels.7b02930 Energy Fuels 2017, 31, 14045−14052

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Energy & Fuels mass spectrometry (MBMS) was used to characterize their vaporization behavior from the ash mixed with CaCl2 or a mixture of CaCl2 and CaO. These two analytical techniques (KEMS and MBMS) can online measure the intensity of vaporized ions over time and thus provide insights into the vaporization behavior of the elements under study. Due to the low concentration of Cs in raw bottom ash (only several parts per million), the signals recorded by KEMS or MBMS were relatively low. To obtain a strong signal-noise ratio, a Cs-doped ash was prepared and employed for all the vaporization experiments in this study. The vaporization mechanisms of Cs in the Cs-doped ash and the raw bottom ash during the thermal treatment with CaCl2 have been confirmed to be very similar.14 A thermodynamic equilibrium calculation was conducted using FactSage 7.0 to predict the vaporizations of Cs, K, and Na in the Cs-doped ash with CaCl2 and the effect of the O2 and H2O content in the reactant gas on the decomposition of CaCl2.

Figure 1. XRD pattern of the Cs-doped ash.

2. EXPERIMENTAL SECTION 2.1. Properties of the Cs-Doped Ash. The preparation of the Cs-doped ash has been described previously.13 Briefly, a coarse grained bottom ash was first pulverized to particle size less than 150 μm and then mixed homogeneously. The resulting powder ash was mixed with Cs2CO3 (purity >95%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) and then heated at 900 °C in a muffle furnace for 2 h. The temperature of 900 °C employed here is typical for solid waste incineration. The ash composition of the Cs-doped ash was determined by X-ray fluorescence (XRF) analysis and is shown in Table 1. Si and Al are the two major elements in the Cs-doped ash;

Figure 2. Schematic of the horizontal furnace.

of CaO was 20%. The content of CaCl2·2H2O (CaO base) and CaO added was calculated according to eqs 1 and 2, respectively.

Table 1. Composition of the Cs-Doped Ash, wt % elements

concentration

SiO2 Al2O3 Fe2O3 CaO MgO SO3 P2O5 TiO2 MnO2 Cl Cs K Na

47.82 36.07 5.6 2.47 0.86 0.63 0.76 3.12 0.028 0.044 0.416 0.332 0.750

CaCl 2·2H 2O, % =

amount of CaCl 2·2H 2O(MWCaO/MWCaCl2·2H2O) amount of ash

× 100 (1)

CaO, % =

amount of CaO × 100 amount of ash

(2)

where MWCaO and MWCaCl2·2H2O refer to molecular weight of CaO and CaCl2·2H2O (termed as CaCl2 hereafter). In eq 1, in order to estimate increase of the treated ash amount compared with that before thermal treatment, we used MWCaO/MWCaCl2·2H2O rather than MWCaCl2·2H2O in terms of a viewpoint that CaCl2 is decomposed to CaO remaining in the treated ash. The mixed ash sample was transferred into a combustion boat, which was then quickly moved into the middle of the reactor when its temperature increased to a given value. The reactant gas was continuously injected into the reactor to remove vaporized compounds. The reaction temperatures were 900, 1000, and 1100 °C. Apart from an air atmosphere (21% O2 and 79% N2), different O2 contents (0%, 10%, 20%, and 40%) and H2O contents (0%, 15%, and 30%) were studied to examine their effects on the vaporization of Cs, K, and Na. After a duration of 30 min, the treated ash was removed of the reactor and cooled in air. The treated ash was pulverized and then characterized by a series of analytical techniques. Each condition for the vaporization of Cs, K, and Na was repeated three times, and the error bars were used to represent the standard deviation of vaporization ratio of Cs, K, and Na. 2.3. KEMS Analysis. The ion currents of Cs, K, and Na in the Csdoped ash and its mixture with 20% CaO were measured using a modified CH5 magnetic KEMS (Finnigan MAT). The details of the KEMS instrument are reported elsewhere.18 In this study, approximately 70 mg of the Cs-doped ash sample was used in each experiment. Before each KEMS measurement, the samples were heated up to 1200 °C in a differential thermal analyzer to evaporate any CO2 and SO2 impurities. Vaporization of the samples from the Knudsen cell (made of iridium) was conducted under high-vacuum

their total content is higher than 80% (in the form of oxides). Mullite and quartz are the two dominant crystalline species while hematite and brookite are two minor compounds in the Cs-doped ash, according to the X-ray diffraction (XRD) analysis shown in Figure 1. In total, 1 kg of the prepared Cs-doped ash contains 4.16 g Cs, 3.32 g K, and 7.50 g Na, which were almost water-insoluble and were combined with aluminosilicates, as evidenced by a leaching test that only removed 0.31% of the Cs, 0.44% of the K and 0.53% of the Na from the Csdoped ash. The procedure for the leaching test has also been described previously.13 2.2. Experimental Setup and Methods. The thermal treatment of the Cs-doped ash was conducted in a lab-scale electrical-heating horizontal furnace, the schematic of which is shown in Figure 2. A detailed description of the furnace has been published elsewhere.13 Approximately 1.5 g of the Cs-doped ash was mixed with CaCl2·2H2O (purity: >99%, Kanto Chemical Co., Inc., Kyoto, Japan), CaO (reagent grade, Sigma-Aldrich Co., Germany), or a mixture of CaCl2·2H2O and CaO. The content of CaCl2·2H2O was 5% or 10%, while the content 14046


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Energy & Fuels conditions (10−11 atm) isothermally, i.e., at 1230 °C for 60 h. The ion currents of Cs+, K+, and Na+ were measured each hour. 2.4. MBMS Analysis. The release of Cs, K, Na, and Cl species from the Cs-doped ash with either 5% CaCl2 or a mixture of 5% CaCl2 and 10% CaO was measured using MBMS. The schematic representation of the reactor coupled to the MBMS system is shown in Figure 3. A detailed description of the MBMS system can be found

3. RESULTS AND DISCUSSION The vaporization of Cs, K, and Na was evaluated by the vaporization ratio defined in eq 3. m − m2 vaporization ratio, % = 1 × 100 m1 (3) where m1 and m2 refer to the amount of Cs, K, and Na in the Cs-doped ash before and after the thermal treatment, respectively. 3.1. Effect of the Addition of CaCl2 or CaO on the Vaporization of Cs, K, and Na. The vaporization of Cs, K, and Na in the ash with the addition of 5% CaCl2, 10% CaCl2 or 20% CaO as a function of temperature is illustrated in Figure 4.

Figure 3. Setup used for the release experiments.

in other reports.19−23 The reactor mainly consisted of a heated flow channel housed in a furnace with four independent heating zones. For the flow channel, a high-density alumina tube was used to prevent reaction of the tube walls with the released species. Helium was selected as the carrier gas because of its low atomic mass, which provides the highest signal intensities for MBMS.23 All parts of the reactor downstream from the reaction zone were kept at temperatures above the condensation point of the Cs, K, Na, and Cl species of interest, making it impossible for these species to condense in this region. Approximately 325 mg of ash was loaded into a sample boat and then inserted into the heated flow channel. A gas flow of 2.850 L/ min of He and 0.150 L/min of O2, corresponding to 95.0% He and 5.0% O2, was introduced into the reactor. The released gaseous products flowed to the end of the reactor where they were analyzed with MBMS. The monitored species included 23Na+, 35Cl+, 36HCl+, 39 + 39 K / NaO+, 58NaCl+, 74KCl+, 133Cs+, and 168CsCl+. 2.5. Characterization of the Ash Samples. The chemical composition of the Cs-doped ash was measured using XRF (Rigaku 2100). Crystalline compounds in the Cs-doped ash were analyzed using XRD (Rigaku RINT) with a Cu target (40 kV, 40 mA). The concentration of Cs, K, and Na in the Cs-doped ash and the treated ash after the experiment was determined by atomic absorption spectroscopy (AAS, Shimadzu AA6200) coupled with acid digestion, according to a microwave digestion procedure.14 2.6. Thermodynamic Equilibrium Prediction. Both the possible Cs-, K-, and Na-bearing species in the Cs-doped ash with CaCl2 and the effects of O2 content and H2O content on the decomposition of CaCl2 were predicted by FactSage 7.0 under a pressure of 1 atm. For the prediction of the three metallic species, the ash composition and the ratio of reactant gas to ash were crucial inputs whose values were set to be consistent with the values from the experiment of the Csdoped ash with 10% CaCl2. The calculation temperature ranged from 500 to 1200 °C at 50 °C intervals. To calculate the decomposition of CaCl2 in different concentrations of O2 (21% versus 0.1%) and H2O (30% versus 0%), both the direct and indirect decomposition (via chemical reactions with aluminosilicates) of CaCl2 were considered. The inputs of direct decomposition simulation included CaCl2, O2, N2, and H2O, while the inputs of indirect decomposition included CaCl2, SiO2, Al2O3, O2, N2, and H2O. The details of the calculation parameters are provided in section 3.2. The FactPS and FToxid thermodynamic databases used were databases were used. The output products consisted of gas, solid, and liquid phases.

Figure 4. Effects of the addition of CaCl2 or CaO on the vaporization of Cs, K, and Na at 900−1100 °C.

The addition of CaCl2 caused the vaporization of Cs, K, and Na during the thermal treatment. For example, 49% of the Cs, 17% of the K and 20% of the Na were vaporized at 1100 °C when 5% CaCl2 was added to the Cs-doped ash. An increase in the CaCl2 content from 5% to 10% resulted in the increase in the vaporization ratio at each temperature, indicating that Cl is a key factor in the vaporization of the three elements. Regardless of the amount of CaCl2 added, increases in the reaction temperature facilitated the vaporization of Cs, K, and Na. When 5% CaCl2 was added, the vaporization ratio of Cs was increased from 40% to 49% as the temperature shifted from 900 to 1100 °C, while the vaporization ratios of K and Na increased from 9% to 17% and from 7% to 20% under the same conditions, respectively. A thermodynamic equilibrium calculation was conducted to predict the distribution of Cs, K, and Na during the thermal treatment of the Cs-doped ash with 10% CaCl2. 14047


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Energy & Fuels The predicted results are shown in Figure 5. In general, chemical reactions between the gaseous Cl released from

Figure 6. Ion currents of Cs+, K+, and Na+ in the Cs-doped ash with and without CaO measured using KEMS at 1230 °C.

that of the Cs-doped ash without CaO after 12 h (Figure 6a), implying that the addition of 20% CaO improved the vaporization propensity of Cs in the Cs-doped ash. Similar results were observed for K and Na (Figure 6b and c). Such observation could be attributed to the replacement of monovalent metallic ions (e.g., Cs+, K+, and Na+) in aluminosilicates by divalent metallic ions (e.g., Ca2+) at high temperatures as Ca is a network modifier that can depolymerize the structure of aluminosilicates.26 As a result, the elements Cs, K, and Na were unstable in the ash and thus readily escaped from the ash particles under high vacuum. The final concentrations of the three elements in the Cs-doped ash with and without CaO after the KEMS experiments were determined and are illustrated in Figure 7. Cs, K, and Na had concentrations of 0.436%, 0.318% and 0.649%, respectively, which were very close to the concentrations before the KEMS experiment and indicated the stability of the three elements in

Figure 5. Prediction of the distribution of Cs, K, and Na during thermal treatment of the Cs-doped ash with the addition of 10% CaCl2.

decomposition of CaCl2 and the metallic compounds to form chlorides were one of the main routes governing the vaporization of the metals.24,25 Another route was suggested for the vaporization of Cs in which the compound CsCaCl3 (Figure 5a) was initially formed via a chemical reaction between the Cs in the ash and CaCl2, followed by its subsequent decomposition, as confirmed in our previous study.13,14 Similar predicted results can be observed for K (Figure 5b) indicating that the vaporization of K was partially attributed to the formation of KCaCl3 and its subsequent decomposition at increased temperatures. The vaporization mechanism of Na was probably only ascribed to the interaction of Na with gaseous Cl because the compound Na−Ca−Cl in the solid phase was not predicted by the thermodynamic calculations (Figure 5c). When 20% CaO was added, less than 10% of the alkali metals was vaporized at 1100 °C (Figure 4). To further reveal the role of CaO in the vaporization of Cs, K, and Na at high temperatures, a KEMS analysis of the Cs-doped ash and its mixture with 20% CaO was conducted at 1230 °C under a high vacuum. The ion currents of Cs+, K+, and Na+ as functions of the reaction time are shown in Figures 6. The ion current of Cs+ in the Cs-doped ash with 20% CaO was much higher than

Figure 7. Concentration of Cs, K, and Na in the Cs-doped ash with and without CaO after the KEMS experiment. 14048


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Energy & Fuels the Cs-doped ash at 1230 °C, even under a high vacuum. The addition of 20% CaO caused the concentration to decrease to 0.0109% for Cs, 0.029% for K, and 0.018% for Na, indicating that the presence of CaO facilitated the vaporization of these alkalis from the aluminosilicates. To compare the vaporization behavior of the three elements, the ion current ratios of Cs+/Na+ and K+/Na+ as a function of the reaction time are plotted in Figures 8. For the Cs-doped ash

Figure 9. Vaporization of Cs, K, and Na in the ash with the addition of 5% CaCl2 and 10% CaCl2 as a function of O2 content (a−c) and H2O content (d−f) at 1100 °C. Figure 8. Ion current ratios of Cs+/Na+ (a) and K+/Na+ (b) over reaction time.

rate of CaCl2, thereby altering the vaporization of Cs, K, and Na through their interactions with CaCl2 and/or gaseous Cl. Predictions of the decomposition of CaCl2 under different O2 content (21% versus 0.1%) and H2O content (0% versus 30%) were calculated using FactSage 7.0. Because the decomposition of CaCl2 was governed by two routes, direct decomposition and indirection decomposition, both were considered in the prediction. The predicted results are summarized in Figure 10. The inputs used in Figure 10a for evaluating the effect of O2 content on the direct decomposition are CaCl2 (0.00536 mol), N2 (0.4232 or 0.5351 mol), and O2 (0.1125 or 0.00057 mol). For Figure 10c, the inputs used to evaluate the effect of H2O

without CaO, the ion current ratio of Cs+/Na+ decreased from 1.5 to 0.5 (Figure 8a), while that of K+/Na+ remained almost constant at 0.6 for 35 h (Figure 8b), which indicated that Cs was readily vaporized relative to Na, while the vaporization of K and Na was similar. When 20% CaO was added into the Csdoped ash, the ratio of Cs+/Na+ was close to zero after 18 h (Figure 8a), suggesting that the vaporization of Cs was almost complete. Meanwhile, the ratio of K+/Na+ also exhibited a decreasing trend and reached approximately 0.1 after 35 h (Figure 8b). Accordingly, it can be concluded that the vaporization rate of the three elements followed the sequence of Cs > K > Na when 20% CaO was mixed with the Cs-doped ash. 3.2. Effect of O2 and H2O Content on the Vaporization of Cs, K, and Na. The effects of different levels of O2 and H2O in the reactant gas on the vaporization of the three metals was examined by conducting vaporization experiments at 1100 °C on the Cs-doped ash with 5% CaCl2 or 10% CaCl2. The vaporization results are shown in Figure 9. Irrespective of the CaCl2 content, when the O2 content was shifted from 0% to 20%, the vaporization ratios of Cs, K, and Na rapidly decreased (e.g., from 94% to 75% for Cs, from 71% to 37% for K, and from 50% to 24% for Na in the ash with 10% CaCl2), while a slow decrease was observed as the O2 content increased from 20% to 40% (Figure 9a−c). Injection of H2O into the reactant gas caused decrease in the vaporization ratios of Cs, K, and Na, as shown in Figure 9d−f. The vaporization ratios of Cs, K, and Na in ash with 10% CaCl2 decreased from 72%, 39%, and 26%, respectively, to 42%, 24%, and 16%, respectively, when the content of H2O increased from 0% to 30%. Changes in the content of O2 and H2O could have affected the decomposition

Figure 10. Prediction of CaCl2 in solid and liquid phases under different O2 content (a and b) and H2O content (c and d) by FactSage 7.0. 14049


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Energy & Fuels content on the direct decomposition are CaCl2 (0.00536 mol), N2 (0.4232 or 0.2962 mol), O2 (0.1125 or 0.07875 mol), and H2O (0 or 0.1607 mol). For the indirect decomposition of CaCl2, the inputs in Figure 10b for examining the effect of O2 content are CaCl2 (0.00536 mol), SiO2 (0.01196 mol), Al2O3 (0.0053 mol), N2 (0.4232 or 0.5351 mol), and O2 (0.1125 or 0.00057 mol), while the inputs in Figure 10d for the estimation of the effect of H2O content are CaCl2 (0.00536 mol), SiO2 (0.01196 mol), Al2O3 (0.0053 mol), N2 (0.4232 or 0.2962 mol), O2 (0.1125 or 0.07875 mol), and H2O (0 or 0.1607 mol). As shown in Figures 10a and 8b, the direct decomposition of CaCl2 changed only slightly between 0.1% O2 and 21% O2, whereas the indirect decomposition of CaCl2 changed substantially over that range. The Cl fraction in solid and liquid phases was much higher when 0.1% O2 was present in the reactant gas than when 21% O2 was present (Figure 10b). Such results indicated that O2 content greatly affected the release of Cl, mainly through the indirect decomposition of CaCl2. The content of H2O affected the release of Cl via the direct decomposition of CaCl2 (Figure 10c), and the addition of 30% H2O to the calculated system resulted in a much lower Cl fraction in the solid and liquid phases at temperatures of 900 °C−1100 °C relative to dry gas, indicating that the direct decomposition of CaCl2 was promoted by adding 30% H2O. In contrast, the effect of H2O on the indirect decomposition of CaCl2 was insignificant, as evidenced by the predicted results shown in Figure 10d, where the Cl fraction in the solid phases was quite low and almost the same between the dry and wet gases. According to these predictions, an increase in the content of O2 and H2O potentially promoted the decomposition of CaCl2 and in turn shortened the contact time between the three metals and gaseous Cl, resulting in a decrease in the vaporization ratios of Cs, K, and Na. 3.3. Effect of the Addition of a Mixture of CaCl2 and CaO on the Vaporization of Cs, K, and Na. The vaporization ratios of Cs, K, and Na in the Cs-doped ash mixed with 5% CaCl2 and 20% CaO and their dependence on the reaction temperature are shown in Figure 11. Additionally, the vaporization ratios of the alkali elements in the Cs-doped ash with 5% CaCl2 are shown here for comparison. Apparently, the vaporization ratios of Cs, K, and Na in the Cs-doped ash with a mixture of 5% CaCl2 and 20% CaO was higher than that in the Cs-doped ash with 5% CaCl2 regardless of the reaction temperature, indicating that the vaporization of the three elements was promoted by the addition of a mixture of CaCl2 and CaO. The difference between the vaporization ratios of these two cases of each metal increased as the temperature increased from 900 to 1100 °C, indicating that the synergistic effect of the mixture of CaCl2 and CaO on the vaporization of Cs, K, and Na was temperature dependent. To clarify the mechanisms of the synergistic effect of the mixture of CaCl2 and CaO, MBMS was used to online measure the ions 23Na+, 35Cl+, 36HCl+, 39K+/39NaO+, 58NaCl+, 74KCl+, 133 Cs+, and 168CsCl+ during the thermal treatment of the Csdoped ash with either 5% CaCl2 or a mixture of 5% CaCl2 and 10% CaO at 1100 °C. Here, 10% CaO was selected to confirm whether these promoting effects could be observed with a relatively low amount of CaO. The results are shown in Figure 12. The release of Cl (35Cl+ and 36HCl+) in the Cs-doped ash with 5% CaCl2 was almost finished 200 s, while it took approximately 400 s to completely vaporize the Cl from the ash sample mixed with 5% CaCl2 and 10% CaO. This result implied that the addition of a mixture of CaCl2 and CaO delayed the

Figure 11. Comparison of the vaporization of Cs, K, and Na between 5% CaCl2 and a mixture of 5% CaCl2 and 20% CaO at 900−1100 °C.

release of Cl compared to that of CaCl2 alone. The vaporization of Cs, K, and Na was also delayed by adding the mixture of CaCl2 and CaO, which was consistent with that of Cl, further confirming that the vaporization of Cs, K, and Na had a close relationship with the release of Cl. Since the intensity of the Cs, K, and Na signals measured by MBMS was normalized to one unit of ash, the peak area has a positive correlation with the amounts of the vaporized metals. The peak areas of the alkali elements were obtained through integration of the signal intensities over time, the results of which are illustrated in Figure 13. The peak areas for the three elements in the Csdoped ash mixed with 5% CaCl2 and 10% CaO was higher than those of the ash mixed only with 5% CaCl2, reflecting an increase in the total amount of the vaporized Cs, K, and Na. These observations from the MBMS analysis were in line with the vaporization results of the three elements based on the vaporization experiments conducted in the horizontal furnace (see Figure 11). From the results shown in Figures 12 and 13, it could be deduced that, relative to the addition of CaCl2, the addition of a mixture of CaCl2 and CaO delayed the release of Cl and thus provided a longer contact time between the Cs and the gaseous Cl, thereby facilitating vaporization of the three elements. Additionally, as mentioned in section 3.1, the addition of CaO decreased the stability of Cs, K, and Na, enabling them to readily associate with the gaseous Cl and thus enhancing the vaporization of Cs, K, and Na. In conclusion, K and Na exhibited a similar vaporization propensity as Cs during the thermal treatment of the Cs-doped ash mixed with CaCl2 and/or CaO. Adding a mixture of CaCl2 and CaO promoted the vaporization of Cs, K, and Na more than the addition of CaCl2 alone. Mixing CaCl2 and CaO into the ash during thermal treatments can reduce the amount of 14050


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4. CONCLUSIONS In this study, we examined the vaporization behavior of three alkali elements, Cs, K, and Na, from a Cs-doped ash with the addition of CaCl2 and/or CaO through a thermal treatment. Increases in the content of CaCl2 and the temperature facilitated the vaporization of Cs, K, and Na. Apart from chemical reactions between the three elements in the ash and the gaseous Cl released from the decomposition of CaCl2, the reaction between Cs or K and CaCl2 to form CsCaCl3 or KCaCl3 was partially responsible for the vaporization of Cs or K. The addition of CaO caused some vaporization of the three metals, probably due to the instability of Cs, K, and Na, resulted from their replacement in aluminosilicates by Ca. The vaporization propensity of the three elements followed the sequence of Cs > K > Na. Increases in the content of O2 and H2O prevented the vaporization of Cs, K, and Na by increasing the decomposition rate of CaCl2. The addition of a mixture of CaCl2 and CaO enhanced the vaporization of Cs, K, and Na from the Cs-doped ash more than the addition of CaCl2. Adding the mixture of CaCl2 and CaO delayed the release of Cl, which prolonged the contact time between Cs, K, and Na and the gaseous Cl. Additionally, the unstable Cs, K, and Na produced from the addition of CaO readily combined with gaseous Cl, thereby promoting the vaporization of the three metals. When the Cs-doped ash was mixed with a mixture of 5% CaCl2 and 20% CaO and treated at 1100 °C, 93% of the Cs was vaporized while the vaporization ratio of K and Na was only 69% and 63%, respectively.

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Figure 12. Ion intensity of alkali metals and chlorine as a function of time as determined by MBMS at 1100 °C.

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-568-51-9178. Fax: +81-568-51-1499. E-mail: [email protected]. ORCID

Yoshihiko Ninomiya: 0000-0002-3523-9666 Marc Bläsing: 0000-0002-6116-1604 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge financial support from Grant A of Chubu University, Japan, and JSPS KAKENHI Grant Numbers JP15F15051 and JP16H02983.

■

Figure 13. Peak area of each ion based on integrating the signal intensities over time.

REFERENCES

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CaCl2 needed to obtain the same vaporization ratio for Cs and, therefore, the environmental pollution caused by the emission of chlorine gas could be mitigated. However, special attention should be paid to monitoring the potential hot corrosion of heat exchangers during thermal treatments since the vaporization of K and Na was also enhanced. Even though the vaporization ratio of K and Na was lower than that of the Cs, the concentration of K and Na in flue gas was expected to be much higher than that of Cs due to a fact that real incineration ash contains higher content of K and Na than Cs. Therefore, a special design for the heat exchanger during thermal treatment with CaCl2 and CaO for the Cs removal from Cs-containing incineration bottom ash should be performed for corrosion resistance. 14051


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Vaporization Behavior of Cs, K, and Na in Cs-Containing Incineration

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