Catalytic sulphation of limestone/lime with platinum

The effects of the presence of a Pt catalyst on the limestone/lime sulphation process were investigated by thermal analysis methods to provide a better understanding of the factors limiting gas desulphurization when Ca-based sorbents are used. It was found that for the Pt-catalysed sulphation of precalcined limestone the weight increase is above 100% higher under isothermal and dynamic conditions (up to 830°C). These results are direct evidence that Pt catalyses the CaO-SO₂-O₂ reaction. It can be presumed that the process proceeds through a gaseous intermediate, SO₃, a highly reactive gas, which explains the increased rate of sulphation. SO₃ then reacts with CaO to form CaSO₄ directly, in contrast with the non-catalysed oxidation of SO₂ to SO₃, where CaSO₃ formation is the most probable early stage of sulphation. The proposed mechanisms were supported by the phase identification of the products.     

Application of thermoanalytical methods to the study of limestone sulphation

The role that can be played in the elucidation of the limestone sulphation mechanism by thermal analysis methods with some specific procedures is discussed. Contrasting examples of applications of thermoanalytical techniques using the variable conditions are provided. These examples deal with the programmed thermal analysis using different gas sequences, the influence the calcination and sulphation conditions on the capture of SO₂, the effect of catalysts on limestone sulphation and the thermal stability of CaSO₃. Two proposed mechanisms were supported by the phase identification of the solid products.     

Sulphur capturing by an inertinite rich high ash bituminous coal during conversion in a pilot packed bed reactor

Sulphur is liberated from the coal structure and released in various forms during coal thermal processing. The possibility of sulphur capture, through injection of SO₂ into a packed coal bed in a pilot packed bed reactor operated under controlled conditions, was investigated. Results showed that SO₂ injection into a packed coal bed leads to sulphur capturing mainly in the coal mineral matter. Mineralogical analysis (XRD) of the ash samples obtained from the experiments indicates that the sulphur-capture products that are formed include FeS, CaS and small amounts of organically associated sulphur. Troilite (FeS) was observed in the SO₂ treated samples, while no troilite was observed in the reference samples. Calcite and dolomite are transformed into CaO and other calcium-containing compounds in the pyrolyses zone, with some CaS being formed in the gasification zone via the reaction between SO₂ and CaO in the presence of CO from the gasification reactions. CaO formed at the high temperatures in the combustion and ash zone is transformed into CaSO₄ upon reaction with SO₂ as an oxidizing atmosphere prevails in this zone. The existence of these compounds is dependent on the extent of oxidising or reducing conditions during the process, with CaS favoured under reducing conditions and CaSO₄ favoured under oxidising conditions.     

The thermal behaviour of compounds in the Ca-S-O system

Simultaneous thermogravimetry and differential thermal analysis up to ca. 1400C of the thermal stability of CaSO₃, CaS, CaSO₄ and physical mixture of CaS and CaSO₄ (molar ratio = 13) both in air and nitrogen atmosphere has allowed overlapping reactions to be defined. The thermal decomposition of CaSO₃ in air does not result in CaO and SO₂ either, but after disproportionation reaction they are formed via the system: 3CaSO₄ + CaS 4CaO + 4SO₂. The oxidation reactions of CaSO₃ and CaS should also be taken into account. The data from the X-ray diffraction analysis of the stable solid intermediates and from the polythermal X-ray diffraction studies provide supporting evidence for the proposed reaction mechanisms.The author thanks Prof. Dr. D. Knöfel of the Universität Gesamthochschule in Siegen and Prof. Dr. W. H. Hoffmann of the Westfälische Wilhelms-Universität in Münster for making the facilities of their laboratories available to her. This work was performed under a UN-Fellowship.     

Decarbonization of Natural Lime-containing Materials and Reactivity of Calcined Products Towards SO2 and CO2

The results obtained by studying decarbonization of different samples of Estonian limestone and dolomite and the following sulphation or carbonation of calcined products to estimate their SO2 and CO2 binding ability were presented. Experiments were carried out with thermogravimetric equipment(Q-Derivatograph, MOM and Labsys™, SETARAM) – calcination of the samples in the atmosphere of air with the heating rate 10 K per minute using multiplate crucibles, the following sulphation or carbonation of the calcined products after cooling to the fixed temperature (temperature range 400–900°C) under isothermal conditions in the flow of air-SO₂ or air-CO₂ mixture. Chemical, X-ray, BET nitrogen dynamic desorption, etc. methods for the characterization of the initial samples, intermediate and final products were used. In addition, the possibilities of recurrent use of oil shale ashes taken from different technological points at operating thermal power plants (Estonian and Baltic TTPs, Estonia) as sorbents for SO₂ binding from gaseous phase were studied, as well as the possibilities of activation of these ashes towards SO₂ binding. The results of these studies confirmed the high reactivity of Estonian limestone and dolomite towards SO₂ and CO₂. Dependence of SO₂ binding mechanism on the SO₂ concentration has been established. Modelling of SO₂ capture of dolomite and limestone was carried out to establish the kinetic parameters of these processes. The possibilities of activation of oil shale ashes and their effective recurrent use for binding SO₂ and CO₂ from gaseous phase were confirmed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermogravimetric studies of the synthesis of cas from gypsum, CaSo4·2H2O and phosphogypsum

Using a heating rate of 2°C min^–1, CaS reacts with oxygen in air from 700°C to form CaSO₄, with a complete conversion at 1100°C. Synthesis of CaS from the reaction between CaSO₄ containing compounds and carbon compounds in air would not be possible, as the carbon reacts from 600°C with oxygen in the air to give CO₂. Heating stoichiometric amounts of carbon and pure CaSO₄, synthetic gypsum or phosphogypsum in a nitrogen atmosphere, results in the formation of CaS from 850°C. Using a heating rate of 10°C min^–1, the formation of CaS is completed at 1080°C. Addition of 5% Fe₂O₃ as a catalyst lowers the starting temperature of the reaction to 750°C. Activation energy values at different fraction reaction values () differ between 340 and 400 kJ mol^–1. The relationship between the activation energy values and conversion () indicates that the reaction proceeds via multiple steps.     

Synthesis and characterisation of K–Ag/Al2O3 catalysts for CH4-SCR of NOx: effect of SO2

This paper characterizes the integrated activity of fresh and used catalysts on the selective catalytic reduction of NOx using CH₄. The synthesised K–Ag/Al₂O₃ catalysts exhibited a promotional effect on deNOx activity in the presence of SO₂. In addition, 130 h of time-on-stream reactions demonstrated the thermal and mechanical stability of the synthesised materials. A TEM analysis and diffraction patterns demonstrated the sintering of finely dispersed particles to ~0.5 micron size clusters by sulphation. Furthermore, under reaction conditions, the de-sulphation initiated the re-dispersion of the Ag clusters to different sized particles. The TPD studied demonstrated the strong adsorption sites for methane and formation of R–SOx compounds. This surface modification in the SO₂ feed stream is considered to be the reason for the promotional effect on the deNOx reaction.     

Effect of acid on morphology of calcite during acid enhanced defluoridation

Fluoride removal from water by lime materials is a promising defluoridation process. Acid enhanced limestone defluoridation (AELD) technique involves precipitation of CaF₂ as well as adsorption of fluoride on the surface of limestone which is capable of reducing fluoride concentration to below the WHO guideline value of 1.5 mg/L. Acids such as acetic acid and citric acid are added to the fluoride water before filtration through limestone column to enhance the Ca²⁺ activity in solution for precipitation of fluoride as CaF₂. This paper describes the effects of these acids on the quality of the limestone during the AELD process, which has been studied to evaluate the reusability of the limestone. The reaction products that formed during the AELD process have also been analyzed. The detail study of the morphology of the limestone before and after use have been done using various analytical techniques, viz., X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy combined with energy dispersive X-ray spectroscopy. The study reveals that the limestone degrades to some extent in the process due to dissolution of calcium carbonate by the acids and adsorption of fluoride by the limestone. While appreciable quantity of the citrate salt of calcium was formed in the column, the acetate salt mostly remained dissolved in the water. Since mainly the surface of the limestone particles take part in the reaction, the limestone particles can be reused for the defluoridation process after cleaning the outer surface. The limestone after use remains also suitable as raw material for cement.     

Statistical evaluation of potash-lime-silica glass weathering

Two potash-lime-silica model glasses with compositions similar to those of medieval stained glass were exposed at 26 test sites all over Europe and in North America for 3–6 years. The objectives of this large-scale field exposure programme were: (a) a qualitative analysis of the weathering products formed on the surface of the samples in the (environmental) scanning electron microscope in combination with energy dispersive X-ray microanalysis (ESEM/EDX or SEM/EDX, respectively); (b) a statistical evaluation of the weathering phenomena by applying a multiple linear regression (MLR) analysis to find correlations between the degree of weathering, which was measured in terms of surface coverage with reaction products, and environmental parameters such as the concentration (c) of the acidifying gases SO₂, NO₂ and O₃, the temperature (T) and the relative humidity (RH); and (c) the calculation of a time-dependence function of the weathering process of these glasses. Mainly sulfates of calcium and potassium such as gypsum (CaSO₄·2 H₂O), arcanite (K₂SO₄) and syngenite (CaSO₄·K₂SO₄·H₂O) could be identified in the SEM and ESEM. Carbonates, nitrates and many particles deposited on the glass surface were found as well. MLR calculations exhibit significant dependencies of the degree of weathering on T, RH, c(NO_x), c(SO₂) and c(O₃). Applying a time-dependence function of the general form y=a·t^b (t=time) results in a value of approximately 0.42 for the exponent b, which comes close to values expected from various studies in the literature.     

Heterogeneous reactions of gaseous methanesulfonic acid with NaCl and sea salt particles

Methanesulfonic acid (MSA) has been identified as one of the most important intermediate products of DMS reactions in the atmosphere. Although considerable amounts of MSA have been found in the marine boundary layer, little is known about the interaction of gaseous MSA with sea salt particles. To understand the fate of MSA in the atmosphere and its potential importance in atmospheric chemistry, the heterogeneous reactions of gaseous MSA with micron-scale NaCl and sea salt particles were studied using diffuse reflectance infrared Fourier transform spectrometry, X-ray photoelectron spectroscopy, and scanning electron microscopy. The CH₃SO₃Na and CH₃SO₃ ⁻ were the major products of the condensed phase of the reaction of gaseous MSA with NaCl and with sea salt particles. The steady-state uptake coefficient was determined to be (5.94±2.32)×10⁻⁷ (1 σ) for the reaction of gaseous MSA with NaCl particles and (2.23±1.25)×10⁻⁷ (1 σ) for the reaction of gaseous MSA with sea salt particles. The heterogeneous reaction of MSA with NaCl particles was found to be first-order for MSA. The reaction mechanisms were discussed. Supported by the National Natural Science Foundation of China (Grant No. 40490265) and the National Basic Research Priorities Program (Grant No. 2002CB410802)     

惰性载体对CaSO4载氧体化学链燃烧性能的影响

为提高化学链燃烧中CaSO₄载氧体的机械强度和反应活性,采用机械混合法制备了负载甘油、硅溶胶以及拟薄水铝石惰性载体的CaSO₄载氧体,并对其机械强度、反应特性等进行了实验研究。结果表明,甘油和硅溶胶的加入可显著提高CaSO₄载氧体的机械强度,而拟薄水铝石作用较小。热重还原实验表明,甘油和硅溶胶的加入可加快载氧体与甲烷的反应速率,缩短反应时间。XRD分析表明,惰性载体的加入不会影响CaSO₄向CaS转化,CaSO₄接近完全转化。     

Peculiarities of Interactions in the CaCO3/CaO-SO2/SO3-Air System A review

The reactions occurring between CaCO3/CaO and SO2/SO3 in oxidizing atmospheres are discussed. Calcination and sulphation were carried out in a thermobalance under conditions relevant to atmospheric fluidized bed combustion. It is suggested that the quality of limestones as potential SO2 sorbents can be assessed on the basis of tests carried out in a TG apparatus adapted for use with corrosive gases. Limestone calcined under different conditions including the treatment with NaCl leads to changes in surface texture. Sulphated samples were examined in a SEM by energy dispersive X-ray and backscattered electron imaging. Two sulphur-bearing solids (CaSO4 and CaS) were identified. The sulphation rate and extent are determined by the product layer diffusion. The amount of CaS present at the end of the process depends on the particle size of the sorbent. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sinterization and hydration of synthesized cement clinker doped with sulfates

This paper aims to evaluate the influence of three kinds of sulfates from the green production of cement on its sintering and hydration. The properties of clinker and hydration were monitored by thermogravimetric and differential thermal analysis (TG–DTA), X-ray diffraction, X-ray fluorescence and isothermal conduction calorimeter. Results indicate that gypsum lowers the decomposition temperature of CaCO₃ and all these Sulfates will enhance the solid-phase reaction but increase melting temperature. Sulfates reduce the content of C₃S, but K₂SO₄ and 2CaSO₄·K₂SO₄ is conducive to the formation of β-C₂S. The hydration induction period is shortened by the sulfates. K₂SO₄ and 2CaSO₄·K₂SO₄ improve the early hydration of clinker, but gypsum may lightly reduce the hydration reactivity of clinker in acceleration period. 2CaSO₄·K₂SO and K₂SO can significantly accelerate the compressive strength development of cement clinker before 3 d; by contrast, gypsum is detrimental for that. The precipitation of hydration products (CH and C–S–H) in clinker with sulfates is more than that of clinker without sulfates at 9 h. K₂SO₄ can accelerate the hydration of clinker without forming ettringite.

     

Study of clinker dopped with P and S compounds

The sintering and the structure of clinkers, modified by the introduction of different ionic forms of sulfur and phosphorus into the raw mix, were examined. One reference synthetic mixture and 25 modified mixtures were prepared by mixing the reference sample with 0.5, 1.0, 1.5, 2.0 and 2.5%w/w of chemical grade CaSO₄, CaS, Ca₃(PO₄)₂, CaHPO₄ and Ca(H₂PO₄)₂. Free lime content in all samples was measured. The sintering reactions in samples were recorded by means of differential thermal analysis. The texture of the clinkers was examined using a scanning electron microscope and EDX. It is concluded that, despite of their relatively low doping concentration in the raw mix, P and S affect considerably its reactivity and the texture of the clinker. The various ionic forms of the same element (SO₄ ^2-, S^2- for S, PO₄ ^3-, HPO₄ ^2- and H₂PO₄ ^- for P) exhibit a different and unequivocal effect on the reactivity of the synthetic raw mix and on the texture of the corresponding clinkers. S (in both forms) and P (added as HPO₄ ^2-) are mainly dissolved in the melt and they have a positive effect on the burnability of the raw mix. P (added as PO₄ ^3- or H₂PO^4-) is preferentially accumulated in belite. In this case further stabilization of β-C₂S occurs and the binding of the free lime is hindered. This revised version was published online in August 2006 with corrections to the Cover Date.     

THERMAL REACTIONS IN SYNTHETIC APATITE–AMMONIUM SULFATE MIXTURE

The thermal reactions in the mixtures of hydroxylapatite or fluorapatite and (NH₄)₂SO₄up to 500°C were studied with the purpose of elaborating the conditions of obtaining calcium–ammonium cyclophosphate that could be used as fertilizer. Thermal analysis with a simultaneous FTIR analysis of the evolved gases as well as the analyses of chemical and phase composition of solid products were performed. The thermal changes in the mixtures could be divided into three steps: (1) decomposition of (NH₄)₂SO₄and reactions of apatite with these products at 250–420°C, (2) calcium ammonium polyphosphate formation at 290–450°C, and (3) reaction of CaSO₄with CaNH₄P₃O₉at 320–500°C. Higher concentrations of NH₃in the gas phase promote the formation of CaNH₄P₃O₉and increase its stability. Calcination at temperatures above 350°C causes decomposition of CaNH₄P₃O₉with a decrease in the content of water-soluble phosphorus and evolvement of SO₂.     

Thermal analysis of ternary gypsum-based binders stored in different environments

Although gypsum belongs to the low-energy environmentally friendly binders, its wider applications in building constructions are limited due to the negative effect of moisture on its mechanical properties. When calcined gypsum (CaSO₄·1/2H₂O) transforms into its hydrated form (CaSO₄·2H₂O), it is partially soluble in water and it has a relatively low strength. This problem can be resolved when gypsum is used as a part of binary or ternary binders. In this paper, a system consisting of calcined gypsum, lime, and silica fume is presented as a functional solution for a wider utilization of gypsum in wet environments. For this purpose, the newly designed materials were stored in different environments (laboratory conditions in air or water) up to 182 days. The effect of silica fume on the hydration process and the growth of the main products is evaluated by using differential scanning calorimetry and thermogravimetry in the temperature range from 25 to 1000 °C with a heating rate of 5 °C min^?1 in an argon atmosphere. The carbonation level of studied materials is also evaluated. Besides this, the information about the thermal stability of studied materials is provided. These results are supported by evolved gas analysis, X-ray diffraction, and scanning electron microscopy. The basic physical and mechanical properties are determined to provide more detailed information about the behavior of the designed materials under various conditions at selected days of hydration. The addition of silica fume to the gypsum–lime system activates the pozzolanic reaction of the analyzed pastes, which is proved by the presence of the CSH phase and by the consumption of portlandite in the mixtures. Wet environment speeds up the hydration processes and prevents samples from carbonation.     

Zur Darstellung und Charakterisierung von Calciumhydrogensulfaten

Preparation and Characterization of Calcium Hydrogen Sulfate CaSO₄ · H₂SO₄ was identified as calcium hydrogen sulfate whereas CaSO₄ · 3 H₂SO₄ is an adduct of CaSO₄ with H₂SO₄. Depending on the excessive amount of H₂SO₄ both compounds exist side by side up to a temperature of 343 K, whereas above this temperature only Ca(HSO₄)₂ is stable. The DTA curve of Ca(HSO₄)₂ shows two maxima at 488 K and 523 K, according to the separation of H₂O under formation of pyrosulfate and decomposition of this compound under elimination of SO₃. In comparison with other hydrogen sulfates Ca(HSO₄)₂ shows a considerable increased O? H distances. The d-values of Ca(HSO₄)₂ are calculated and represented.     

Sulfation of Polysaccharides using Monomethyl Sulfate

ABSTRACT

The polysaccharides, curdlan, starch and dextran were sulfated when heated in DMSO with sodium methyl sulfate and a catalytic amount of H₂SO₄ or with pyridinium methyl sulfate. Use of diminished pressure and anhydrous CaSO₄ as a desiccant improved the degree of sulfation and recovery. Under conditions using sodium methyl sulfate, H₂SO₄ and CaSO₄ in vacuo, sulfation at O-6 was predominant in the cases of curdlan and starch, while sulfation at O-2 and O-3 was preferential in the case of dextran.     

The role of MgO in the binding of SO<Subscript>2 </Subscript>by lime-containing materials

Summary The results of investigation of MgO participation in the binding of SO₂ with lime-containing materials as sorbents are presented. Experiments of SO₂ binding into solid phase using model samples of reactive grade MgO and CaO varying the mole ratio of MgO/CaO from 9:1 to 1:9 were carried out. Besides, dolomite and limestone samples with different MgO/CaO mole ratio (from 1.24 to 0.13) and samples of ashes formed at combustion of Estonian oil shale (containing 35-40% of carbonates) and its semicoke were studied Initial samples, intermediate and final products were subjected to chemical, IR-spectroscopy, X-ray and BET specific surface area analyses. The results of the present study confirmed the active participation of MgO in the binding of SO₂ into the solid phase. In addition to CaSO₄ the formation of Ca,Mg-double sulphate CaMg₃(SO₄)₄ and -MgSO₄ was observed. The presence of CaMg₃(SO₄)₄ was fixed in a large temperature range 400-900°C and that of -MgSO₄ in between 500-700°C. The optimum temperature range for formation and durability of CaMg₃(SO₄)₄ was 700-800°C.     

SO2 binding into the solid phase during thermooxidation of blendsestonian oil shale semicoke

Approximately one million tons of semicoke (SC) is formed and stored in open air dumps every year in the production of shale oil by processing Estonian oil shale (OS). The content of different harmful compounds as sulphides, PAH, phenols, etc. in SC make these dumps one of the most serious sources of environmental contamination. The aim of this work was to study the behaviour of sulphur compounds in OS and its SC, formation of SO₂ and possibilities of binding it into the solid phase during thermooxidation of fuel blends based on SC. Blends modified with SC ash addition were studied as well. It was determined that SO₂ emission in thermooxidation of SC samples started at 280-300°C and proceeded with a steady speed up to 580-600°C and the amount of sulphur evolved was 5-10% from the total content of sulphur in the sample. The amount of SO₂ emitted decreased depending on the mass ratio of the composite fuels from 49-56 to 15-35% during thermooxidation of OS samples studied or their blends with SC, respectively, from 43-80% for coal samples to 13-60% for their blends with SC and to 2-13% during thermooxidation of these blends modified with SC ash addition. In the products of thermooxidation formed at 800-900°C the only sulphur containing phase was CaSO₄, at 650°C also traces of CaS and CaMg3(SO₄)₄ were fixed. This revised version was published online in July 2006 with corrections to the Cover Date.