KINETICS OF THE THERMAL DECOMPOSITION OF HYDRATED DOLOMITIC LIME AND SINTERING OF NASCENT CALCINE

A study is reported of the controlled thermal decomposition of hydrated dolomitic lime originated from a high-grade dolomite. Both increasing and constant temperature weight-loss methods have been employed to measure the rate of thermal dehydration of hydrated dolomitic lime at 305-400^°C. The amassed data have been correlated with the use of an Arrhenius-type rate equation. Changes in the specific surface area of calcines due to sintering phenomena have also been studied at 400-600^°C. An empirical, fractional-order kinetic relationship has been developed for surface area reduction as a function of temperature and elapsed time of sintering. Presented findings make it possible to specify the operation conditions needed for the preparation of reactive dolomite-derived sorbents.     

Effectiveness of SO2 sorbents by thermogravimetry-combustion with coal

A new experimental method is described for non-isothermal thermogravimetry (TG) involving combustion of mixtures of sieved coal with sieved calcium-containing sorbents. This rapid TG method utilizes a baseline for TG combustion of coal alone, derives an equation that gives a semi-quantitative measure (±10% repeatability) of the coal's reactive sulphur retained by the sorbent, the extent of retention of S0₂ generated in situ during combustion varying with different sorbents. The method permits direct variation in separate experiments of the calcium-to-sulphur ratio during combustion and relative ranking of different sorbents by retention of reactive sulphur in combustion. Relative rankings are presented for three pre-calcined natural stones (two limestones and one dolomite); these results correlate with relative rankings from another TG method reported in the literature. It is suggested that this new method is useful for pre-screening the effectiveness of S0₂ sorbents considered for use in fluidizedbed combustion of coal.     

HYDROTHERMAL REACTION OF FLY ASH/HYDRATED LIME: CHARACTERIZATION OF THE REACTION PRODUCTS

Class F coal fly ash was slurried with hydrated lime at 90°C in 1/3, 5/3, 9/3, and 15/3 weight ratios and for 3, 5, 7, and 9 hours of hydration, in a process to prepare sorbents for SO₂ removal. The amounts of aluminum, silicon, and calcium in the product of the pozzolanic reaction were determined in order to study the evolution of product composition with the initial raw materials ratio and hydration time and to relate this composition to the desulfurization capability of the material. Al, Si, and Ca were present in the solid product for any raw materials ratio and hydration time, showing that calcium silicates, calcium aluminates, and/or calcium aluminum silicates were obtained simultaneously. The products formed show a nearly constant molar ratio of Al₂O₃/SiO₂ independent of the experimental conditions tested and similar to the Al₂O₃/SiO₂ ratio in the fly ash. The SiO₂/CaO molar ratio in the products decreased as the initial fly ash/Ca(OH)₂ ratio decreased, being approximately constant for each ratio with respect to hydration time after 5 hours of hydration. The maximum moles of CaO, SiO₂, and Al₂O₃ per gram of sorbent in the reaction product were found for any hydration time for the 5/3 sorbents, meaning that at this initial ratio the pozzolanic reaction takes place at the highest rate. The capacity of the sorbent for SO₂ removal depends not only on the amount of products produced by the pozzolanic reaction but also on the specific surface area of the sorbent.     

不同钙基吸附剂捕集CO2后的硫酸化反应特性研究

钙基吸附剂进行多次CO₂捕集后,碳酸化效率会大幅衰减,此时的吸附剂能否高效脱硫利用是值得重点关注的问题。鉴于此,筛选了高性能合成钙基吸附剂和天然石灰石吸附剂,通过热重分析仪分析对比其在多循环CO₂捕集后的碳酸化和硫酸化反应性能,采用微粒模型研究其硫酸化反应动力学特征。结果发现,高性能合成钙基吸附剂的碳酸化反应速率和CO₂吸附能力明显高于石灰石吸附剂。在长达500循环的CO₂捕集试验后,高性能合成钙基吸附剂的CO₂吸附能力比石灰石高10倍以上,其SO₂吸附能力相较于石灰石提升约40%。经历多次CO₂捕集反应循环后,2种吸附剂的硫酸化能力均有提升:其中,石灰石吸附剂的提升幅度更大,硫酸化转化率从26%提升到35%,而高性能合成钙基吸附剂的硫酸化转化率则从38%提升到43%。通过微粒模型计算发现,2种吸附剂的硫酸化反应均是与SO₂浓度相关的一级反应,多循环捕集CO₂反应后,石灰石吸附剂的硫酸化反应活化能下降接近30%,而高性能合成钙基吸附剂的硫酸化反应活化能只下降了5%。研究结果说明2种不同钙基吸附剂在进行循环CO₂捕集后,脱硫能力得到了不同程度的提高,且均可以较好地应用于SO₂的脱除。     

Hydrothermal synthesis, characterization and structure refinement of chlorate- and perchlorate-sodalite

Chlorate- and perchlorate-sodalites were synthesized hydrothermally in the temperature range of 160°C to 500°C. IR absorption bands indicate the enclathration of NaClO₃ (624 cm⁻¹) and NaClO₄ (624 cm⁻¹ and 2050 cm⁻¹) in the sodalite cages. The thermal decomposition has been characterized by simultaneous thermal analysis (TG, DTG, DTA) and high temperature X-ray powder diffraction. The total collapse of the sodalite framework structure and the formation of nepheline could be observed at 750°C and 1100°C for chlorate-sodalite and perchlorate-sodalite, respectively.

The crystal structure has been determined for NaClO₄-sodalite showing cubic symmetry (a₀=9.071 Å, SG P 3n) and complete ordering of Si and Al in the silicate framework; the Si-O-Al angle is 146.7°. The Cl atoms of the enclathrated perchlorate are located at the central positions of the sodalite cages. For the oxygen atoms of the Cl₄⁻ anions the structure refinement led to orientationally disordered sites having a close resemblance to the well-known O(2) positions of basic sodalite Na₈[AlSiO₄]₆(OH)₂·2 H₂O.     

Sol-gel synthesis of BaTiO3 and Ba1−x−yCaySrx(ZryTi1−y)O3 perovskite powders

A BaTiO₃ powder has been prepared by the sol-gel process from the hydrolysis of a solution of barium acetate and titanium ethylate in the presence of acetic acid as a catalyst. Supplementary constituents in the form of Ca(CH₃COO)₂, Zr(OC₃H₇)₄, Sr(NO₃)₂ also can be used. Intermediate phases of barium acetate and barium carbonate have been identified by differential thermal analysis, X-ray diffraction, infra-red and scanning electron microscopy. BaTiO₃ with perovskite structure synthesizes in the temperature range from 600 to 1000°C.     

高温下改性钙基吸附剂固定床反应器从烟气中捕捉二氧化碳(英文)

Four kinds of Ca-based sorbents were prepared by calcination and hydration reactions using different precursors: calcium hydroxide, calcium carbonate, calcium acetate monohydrate and calcium oxide. The CO2 absorption capacity of those sorbents was investigated in a fixed-bed reactor in the temperature range of 350-650℃. It was found that all of those sorbents showed higher capacity for CO2 absorption when the operating temperature higher than 450℃. The CaAc2-CaO sorbent showed the highest CO2 absorption capacity of 299mg·g-1. The morphology of those sorbents was examined by scanning electron microscope (SEM), and the changes of composition before and after carbonation were also determined by X-ray diffraction (XRD). Results indicated that those sorbents have the similar chemical compositions and crystalline phases before carbonation reaction [mainly Ca(OH)2], and CaCO3 is the main component after carbonation reaction. The SEM morphology shows clearly that the sorbent pores were filled with reaction products after carbonation reaction, and became much denser than before. The N2 adsorption-desorption isotherms indicated that the CaAc2-CaO and CaCO3-CaO sorbents have higher specific surface area, larger pore volume and appropriate pore size distribution than that of CaO-CaO and Ca(OH)2-CaO.     

Environmentally friendly solid acid catalyst prepared by modifying TiO2 with cerium sulfate for the removal of volatile organic chemicals

An environmentally friendly solid acid catalyst, Ce(SO₄)₂/TiO₂ was prepared simply by modifying TiO₂ with Ce(SO₄)₂ for acid catalysis of volatile organic chemicals, 2-propanol and cumene. The characterization of prepared catalysts was performed using FTIR, XRD and DSC. The surface area of 7-Ce(SO₄)₂/TiO₂ calcined at 300 °C was very high (206.0 m²/g) compared to that of unmodified TiO₂ (115.2 m²/g) due to the interaction between Ce(SO₄)₂ and TiO₂. 7-Ce(SO₄)₂/TiO₂ containing 7 wt% Ce(SO₄)₂ and calcined at 300 °C exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acid amounts of catalysts measured by an ammonia chemisorption method. The role of Ce results in an increase in the thermal stability of the surface sulfate species and consequently the acid amount of Ce(SO₄)₂/TiO₂ is increased. The asymmetric stretching frequency of the SO bonds for Ce(SO₄)₂/TiO₂ catalysts was related to the acidic properties and to the catalytic activity for acid catalysis to remove volatile organic chemicals, 2-propanol and cumene.     

Effect of promoters including tungsten and barium on the thermal stability of V2O5/sulfated TiO2 catalyst for NO reduction by NH3

The effect of tungsten and barium on the thermal stability of V₂O₅/TiO₂ catalyst for NO reduction by NH₃ was examined over a fixed bed flow reactor system. The activity of V₂O₅/sulfated TiO₂ catalyst gradually decreased with respect to the thermal aging time at 600 °C. The addition of tungsten to the catalyst surface significantly enhanced the thermal stability of V₂O₅ catalyst supported on sulfated TiO₂. On the basis of Raman and XRD measurements, the tungsten on the catalyst surface was identified as suppressing the progressive transformation of monomeric vanadyl species into crystalline V₂O₅ and of anatase into rutile phase of TiO₂. However, the NO removal activity of V₂O₅/sulfated TiO₂ catalyst including barium markedly decreased after a short aging time, 6 h at 600 °C. This may be due to the transformation of vanadium species to inactive V–O–Ba compound by the interaction with BaO which was formed by the decomposition of BaSO₄ on the catalyst surface at high reaction temperature of 600 °C. The addition of SO₂ to the feed gas stream could partly restore the NO removal activity of thermally aged V₂O₅/sulfated TiO₂ catalyst containing barium.     

PREPARATION OF PEROVSKITE 0.75Pb(Ni1/3Nb2/3)O3-0.25PbTiO3 CERAMICS USING SEMICHEMICAL METHODS AND RELATED REACTION MECHANISMS

This article discusses a mechanism for preparing perovskite powders, 0.75Pb(Ni_1/3Nb_2/3)O₃-0.25PbTiO₃ (PNN-PT), using a semichemical method (SCM).Precursors were prepared by adding aqueous Ni(Ac)₂ solutions to an alcohol slurry of PbO, Nb₂O₅, and TiO₂. The TG-DTG and DSC analysis of the precursors and XRD analysis of the powders at different thermal treatment temperatures showed that the reaction mechanisms in this method differ from those in the conventional mixed-oxide method. The aqueous Ni(Ac)₂ solution reacted with PbO to form Pb(Ac)₂ · Pb(OH)₂ · H₂O and Ni(OH)₂, which decomposed to form nascent PbO and NiO, thereby improving the reactivity and distribution of PbO and NiO. Pb₃Nb₂O₈ and NiNb₂O₆ formed and were easily converted into the perovskite phase during the thermal treatment process. At a thermal treatment temperature of 850°C, the content of the perovskite phase reached 98%. Pyrochlore-free PNN-PT ceramic was obtained after 2 h of sintering at 1100°C, and its dielectric properties were found to be excellent at temperatures ranging between -55 and 120°C.     

Physicochemical investigation of the decomposition products of ammonium metal carboxylates: ammonium ferric citrate hydrate

The thermal decomposition up to 400 °C of ammonium ferric citrate hydrate, of unknown structure and formula weight, was studied by thermogravimetry, differential thermal analysis, infrared (IR) spectroscopy and X-ray diffractometry. The possible identities of the formula weight and the intermediate products of calcination are discussed. The results revealed that the parent material is amorphous and contains two moles of water and two moles of ammonia. Decomposition takes place via six weight-loss processes, three endothermic (90–230 °C) and three exothermic (240–298 °C), leading eventually to the formation of Fe₂O₃. The intermediate solid products are mainly unstable amorphous oxycarbonates, as indicated by X-ray and IR spectroscopies. The gas-phase decomposition products identified by IR spectroscopy are NH₃, CO₂, CO, CH₃COCH₃, CH₄ and NH₄OH. Surface area measurement and scanning electron microscopy showed that Fe₂O₃, the final product at 400 °C, hada surface area of 40 m²/g and good crystalline and porous character.     

二氧化碳浓度对石灰石分解反应动力学的影响

基于石灰石热重-差示扫描量热（TG-DSC）分析结果,分别在不同浓度的二氧化碳（二氧化碳与空气的混合气）气氛下对石灰石进行热分解特性研究。TG和DSC分析结果表明,碳酸钙分解是吸热反应,反应温度范围为750~950 ℃;提高升温速率,反应进程加快,TG曲线向高温区移动,DSC曲线吸热峰和吸热面积明显增大;反应气氛中二氧化碳浓度提高,TG曲线稍微向高温区移动,反应起始温度相同,反应终止温度相差约20 ℃。在高浓度二氧化碳气氛下,石灰石分解遵循随机成核和随后生长模型。此研究结果可为进一步优化石灰石煅烧工艺以及煅烧炉的设计提供理论支持。     

Water-induced bulk Ba(NO3)2 formation from NO2 exposed thermally aged BaO/Al2O3

Phase changes in high temperature treated (>900 °C) 8 or 20 wt% BaO supported on γ-Al₂O₃ model lean NO_x trap (LNT) catalysts, induced by NO₂ and/or H₂O adsorption, were investigated with powder X-ray diffraction (XRD), solid state ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and NO₂ temperature programmed desorption (TPD) experiments. After calcination in dry air at 1000 °C, the XRD and solid state ²⁷Al MAS NMR results confirm that stable surface BaO and bulk BaAl₂O₄ phases are formed for 8 and 20 wt% BaO/Al₂O₃, respectively. Following NO₂ adsorption over these thermally treated samples, some evidence for nanosized Ba(NO₃)₂ particles are observed in the XRD results, although this may represent a minority phase. However, when water was added to the thermally aged samples after NO₂ exposure, the formation of bulk crystalline Ba(NO₃)₂ particles was observed in both samples. Solid state ²⁷Al MAS NMR is shown to be a good technique for identifying the various Al species present in the materials during the processes studied here. NO₂ TPD results demonstrate a significant loss of uptake for the 20 wt% model catalysts upon thermal treatment. However, the described phase transformations upon subsequent water treatment gave rise to the partial recovery of NO_x uptake, demonstrating that such a water treatment of thermally aged catalysts can provide a potential method to regenerate LNT materials.     

A DRY SCRUBBING MODEL FOR SO2 REMOVAL

The use of dry injection of sorbents for control of S O₂ in conjunction with a fabric filter is currently undergoing evaluation at the large pilot plant level, and is being considered for full scale application. This paper discusses a recently developed model for predicting the removal efficiencies of S O₂ as a function of system operating parameters, and utilizing a semi-emperical pore plugging experience to account for the observed reaction die off behavior. The model assumes that the reaction between the gas and the solid occurs in two phases; (1) in a transport-line section where the finely powdered solid is injected and mixed with the flue gases, and (2) across the filter cake which develops at the fabric filter. In the current development of the model, isothermal conditions are assumed. Comparisons between predicted and reported values of S O₂ removal are given for the Na₂C0₃—S O₂ system     

CO2 absorption and regeneration of alkali metal-based solid sorbents

Potassium-based sorbents were prepared by impregnation with potassium carbonate on supports such as activated carbon (AC), TiO₂, Al₂O₃, MgO, SiO₂ and various zeolites. The CO₂ capture capacity and regeneration property were measured in the presence of H₂O in a fixed-bed reactor, during multiple cycles at various temperature conditions (CO₂ capture at 60 °C and regeneration at 130–400 °C). Sorbents such as K₂CO₃/AC, K₂CO₃/TiO₂, K₂CO₃/MgO, and K₂CO₃/Al₂O₃, which showed excellent CO₂ capture capacity, could be completely regenerated above 130, 130, 350, and 400 °C, respectively. The decrease in the CO₂ capture capacity of K₂CO₃/Al₂O₃ and K₂CO₃/MgO, after regeneration at temperatures of less than 200 °C, could be explained through the formation of KAl(CO₃)₂(OH)₂, K₂Mg(CO₃)₂, and K₂Mg(CO₃)₂·4(H₂O), which did not completely converted to the original K₂CO₃ phase. In the case of K₂CO₃/AC and K₂CO₃/TiO₂, a KHCO₃ crystal structure was formed during CO₂ absorption, unlike K₂CO₃/Al₂O₃ and K₂CO₃/MgO. This phase could be easily converted into the original phase during regeneration, even at a low temperature (130 °C). Therefore, the formation of the KHCO₃ crystal structure after CO₂ absorption is an important factor for regeneration, even at the low temperature. The nature of support plays an important role for CO₂ absorption and regeneration capacities. In particular, the K₂CO₃/TiO₂ sorbent showed excellent characteristics in CO₂ absorption and regeneration in that it satisfies the requirements of a large amount of CO₂ absorption (mg CO₂/g sorbent) and fast and complete regeneration at a low temperature condition (1 atm, 150 °C).     

Raman spectroscopic studies on the sulfation of cerium oxide

In the present study, we have examined sulfation of cerium oxide via impregnation of (NH₄)₂SO₄, followed by heating in the temperature range of 220–720°C, using Raman Spectroscopy. Based on the SO and SO stretching frequencies in the range of 900–1400 cm⁻¹, a wide range of surface oxysulfur species and bulk cerium-oxy-sulfur species are identified. At 220°C, a mixture of (NH₄)₂SO₄ crystals, SO²⁻_4(aq) and HSO¹⁻_r(aq) is found to have formed on ceria's surface, whereas complete conversion of (NH₄)₂SO₄ to SO²⁻_4(aq) and HSO¹⁻_4(aq) occurs at 280°C. At 350°C, formation of a mixture of surface pyrosulfate S₂O²⁻_7(surf.0, consisting of two SO oscillators and a bulk type compound identified as Ce(IV)(SO₄)_x(SO₃)_2−x (0 < x < 2) have been observed. Upon introduction of moisture, the former transforms to HSO¹⁻_4(surf.), whereas the latter remains unchanged. At 400°C, only the bulk type compound can be observed. At 450°C, only Ce₂(SO₄)₃ is generated and remains stable until 650°C. Further increase in the temperature to 720°C results in the formation of CeOSO₄. The present study not only provides a more thorough understanding of the sulfation of cerium oxide at a molecular level, but also demonstrates that Raman spectroscopy is a highly effective technique to characterize sulfation of metal oxides.     

Microstructure and phase development in NiMn2O4 spinel ceramics during isothermal sintering

Studies of the sintering behaviour of NiMn₂O₄ semiconducting ceramics at 1100°C under oxygen atmosphere show the advantage of a powder prepared by thermal decomposition of oxalate mixed crystals NiMn₂ (C₂O₄)₃. 6H₂O at 450°C for the formation of dense ceramics with homogeneous microstructure. The microstructure of the semiconductor ceramics was established by image analysis. SEM and EDX indicate a phase separation related to partial oxygen loss as observed by thermogravimetry and redox analytic measurements. Reoxidation to a single-phase homogeneous microstructure by annealing at 800°C is possible only in porous samples. NiMn₂O₄ is not stable in oxygen at 1100°C. The spinel decomposes into NiO and a Mn-rich spinel matrix Ni_x^IIMn_1−x^IIMn₂^IIIO₄. For producing a reproducible semiconducting ceramics it is necessary to stimulate sintering by separation of NiO. A one-phase spinel is obtained by reoxidation of long duration.     

The storage of nitrogen oxides on alumina-supported barium oxide

The storage and release of NO₂ on alumina-supported barium oxide has been studied with particular attention to the stoichiometry of the two processes. At 400 °C the storage process is characterised by a short period of complete uptake, possibly as nitrito or nitro species, followed by a slower partial uptake in which approximately one NO is released for every three NO₂ lost. The latter reaction appears to supply the oxygen necessary to store NO₂ as nitrate ions. Molecular O₂ has little direct involvement even if in large excess. The second storage reaction also occurs, but to a much lesser extent, with Al₂O₃ alone. During temperature programmed desorption, release of NO_x from Al₂O₃ peaks at 430 °C with evolution of NO₂ and some O₂. Release from BaO/Al₂O₃ exhibits an additional peak near 520 °C corresponding to formation of NO and a higher O₂ concentration. The NO may arise from NO₂ since BaO/Al₂O₃ has activity for NO₂ decomposition by 500 °C. Although CO₂ at low concentration is rapidly taken up by BaO/Al₂O₃ at 400 °C it is displaced by NO₂ and does not interfere with storage. Thermodynamic calculations show that the formation of Ba(NO₃)₂ by the reaction of NO₂ with bulk BaCO₃ under the conditions used here is more favourable above 380 °C if NO is evolved than if O₂ is consumed.     

Pt-Ba/alumina NOx storage-reduction catalysts: Effect of Ba-loading on build-up, stability and reactivity of Ba-containing phases

A series of Pt-Ba/Al₂O₃ catalysts with Ba-loadings in the range 4.5–28 wt.% has been prepared by wet impregnation of Pt/Al₂O₃ with barium acetate (Ba(Ac)₂) as Ba precursor. The build-up and thermal stability of the deposited Ba-containing species was followed by means of XRD and thermogravimetry (TG) combined with mass spectroscopy (MS). Samples were characterized before and after thermal treatment (calcination). The study showed that the thermal stability of the Ba-containing phases depends on their interaction with the alumina support and the presence of dispersed platinum. In calcined catalysts, three different Ba-containing species could be distinguished based on their crystallinity and thermal stability. The relative concentration of these species varied with the Ba-loading. The first layer of Ba-containing species, corresponding to saturation of the alumina surface with Ba(Ac)₂, contained up to 12.5 wt.% of Ba in the form of amorphous BaO. Increasing the Ba-loading further resulted in 5–6 wt.% of Ba in the form of amorphous carbonates with relatively low thermal stability (LT-BaCO₃). At Ba-loadings higher than about 16 wt.%, crystalline barium carbonate became discernible which exhibited remarkably higher thermal stability (HT-BaCO₃). NO_x storage tests accomplished by exposing the catalysts to pulses of NO in oxygen containing carrier gas at 300 °C indicated that from all characterized Ba-containing phases, LT-BaCO₃ possesses the highest reactivity for NO_x storage, i.e. LT-BaCO₃ is transformed most rapidly to Ba(NO₃)₂.     

Al18B4O33 aluminium borate: A new efficient support for palladium in the high temperature catalytic combustion of methane

Well crystallised aluminium borate Al₁₈B₄O₃₃ has been synthesised from alumina and boric acid with a BET area of 18 m²/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al₁₈B₄O₃₃ was prepared by conventional impregnation of Pd(NO₃)₂ aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al₁₈B₄O₃₃ in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al₂O₃. Pd/Al₁₈B₄O₃₃ exhibited a much lower activity than Pd/Al₂O₃ when treated in hydrogen at 500 °C or aged in O₂/H₂O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al₁₈B₄O₃₃ which became as active as Pd/Al₂O₃. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al₂O₃ and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al₁₈B₄O₃₃. The existence of two distinct types of PdO species formed on Al₁₈B₄O₃₃ and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O₂.