セキタン ノ クウキ ネンショウ オヨビ Oxy-Fuel ネンショウ カラノ ビリョウ キンゾク ノ ジョウハツ オヨビ フチャク キョドウ

博士(工学)中部大

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 oxyfiring 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 O₂/CO₂ mixture (27/73, v/v) versus air, the O₂/CO₂ mixtures doped with SO₂, 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 O₂. 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 degrees C inhibited the oxidation of Cr(IIII) to Cr(VI). Shifting the combustion gas from air to O₂/CO₂ exerted little effect on the Cr(VI) formation. Instead, the formation of iron chromite (FeCr₂O₄) was facilitated in O₂/CO₂ probably due to a strong reducing microenvironment formed by the CO₂ 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 SO₂ 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 alumina-silicate which is in the slagging phase

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

Condensation Behavior of Heavy Metals during Oxy-fuel Combustion: Deposition, Species Distribution, and Their Particle Characteristics

This study aimed to characterize the condensation behavior of two heavy metals, namely, Pb and Zn, during oxy-fuel combustion and to clarify and compare the differences in their behavior during oxy-fuel versus air-fired combustion. A lab-scale rotary quartz reactor with a multi-stage cooling zone was used to analyze the deposition content and species distribution of the condensed Pb and Zn vapors at different temperature ranges and/or points and to observe their particle characteristics in the simulated oxy-fuel flue gas (OFFG), air-fired flue gas (AFFG), oxy-fuel flue gas without steam (OFFGWS), and air-fired flue gas without steam (AFFGWS). The deposition content of the condensed Pb and Zn vapors in the AFFG was consistently higher than that of OFFG in the cooling zone from 800 to 100 °C. Moreover, the steam content had an obvious influence on the deposition content. The condensed Pb and Zn vapors were mostly deposited in the sulfates in OFFG at 600–300 °C, instead of in the chlorides in AFFG. The average diameter of particles that contain Pb and Zn increased as the temperature decreased, and their shape factor in both AFFG and AFFGWS was higher than that in OFFG and OFFGWS

Multiscale analysis of fine slag from pulverized coal gasification in entrained-flow bed

Abstract Fine slag (FS) is an unavoidable by-product of coal gasification. FS, which is a simple heap of solid waste left in the open air, easily causes environmental pollution and has a low resource utilization rate, thereby restricting the development of energy-saving coal gasification technologies. The multiscale analysis of FS performed in this study indicates typical grain size distribution, composition, crystalline structure, and chemical bonding characteristics. The FS primarily contained inorganic and carbon components (dry bases) and exhibited a "three-peak distribution" of the grain size and regular spheroidal as well as irregular shapes. The irregular particles were mainly adsorbed onto the structure and had a dense distribution and multiple pores and folds. The carbon constituents were primarily amorphous in structure, with a certain degree of order and active sites. C 1s XPS spectrum indicated the presence of C–C and C–H bonds and numerous aromatic structures. The inorganic components, constituting 90% of the total sample, were primarily silicon, aluminum, iron, and calcium. The inorganic components contained Si–O-Si, Si–O–Al, Si–O, SO4 2−, and Fe–O bonds. Fe 2p XPS spectrum could be deconvoluted into Fe 2p 1/2 and Fe 2p 3/2 peaks and satellite peaks, while Fe existed mainly in the form of Fe(III). The findings of this study will be beneficial in resource utilization and formation mechanism of fine slag in future

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 CO₂ 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 CrO₂Cl₂. Regarding the solid impurities including Na₂SO₄ and CaO, Na₂SO₄ 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 CaCrO₄ and Ca₃(CrO₄)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

Synergistic Mechanisms of CaCl 2 and CaO on the Vaporization of Cs from Cs-Doped Ash during Thermal Treatment

This study aimed to clarify the roles of CaCl2, CaO, and their mixture in the vaporization of Cs from Cs-doped ash during thermal treatment. In particular, potential mechanisms of the synergistic effect of the addition of a mixture of CaCl2 and CaO on Cs vaporization were investigated. Vaporization experiments were carried out in a lab-scale horizontal furnace at 900, 1000, and 1100 °C. The results indicated that adding a mixture of CaCl2 and CaO produced a synergistic effect on Cs vaporization when the reaction temperature was above 1000 °C and the vaporization ratio was noticeably increased in comparison to that when adding CaCl2 or CaO alone. The formation of wadalite (Ca6Al5Si2O16Cl3) and/or igumnovite [Ca3Al2(SiO4)2Cl4], derived from chemical reactions among CaCl2, CaO, and aluminosilicates in the Cs-doped ash, delayed the release of Cl during thermal treatment, thus extending the contact time of Cs and gaseous Cl. Furthermore, CaO destabilized the aluminosilicate structure, resulting in a higher volatility and reactivity of Cs, and thus, a reaction readily occurred between activated Cs and gaseous Cl released from the decomposition of wadalite and/or igumnovite

Vaporization Behavior of Cs, K, and Na in Cs-Containing Incineration Bottom Ash during Thermal Treatment with CaCl 2 and CaO

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