Effect of Chemical Compositions on Ash Fusibility Characterization of a Jincheng Anthracite during Combustion and Gasification

The ash melting temperature of coal ash has an important effect during the fluidized bed combustion and gasification process, which affects the slagging and deposition characteristics of the boiler. Experiments on the effects of chemical components on the ash fusion behaviors have been completed on the ash fusion temperatures (AFTs) analyzer under typical gasification and combustion atmospheres. Meanwhile, calculations on the variation of minerals in ash with ash composition were conducted using the FactSage software. The results indicated that the AFTs under gasification were a little higher than those under the combustion atmosphere. On increasing the Fe₂O₃, CaO, and Na₂O contents under the combustion and gasification atmospheres, the four temperatures deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT) decreased dramatically and the generation and transformation of minerals occurred. The iron-containing minerals, such as hercynite and fayalite, formed with increase in the content of Fe₂O₃; the Ca-bearing feldspar minerals, like gehlenite and anorthite, started appearing on increasing the CaO content, and the Na-containing feldspar minerals, like carnegieite, were detected as the Na₂O was increased. These three minerals can form low-temperature eutectics, decreasing the fusion temperature.     

Correlation between the ash composition and melting temperature of waste incineration residue

The correlation between the ash composition of various incinerated waste residues and their melting temperatures was examined by using their chemical composition parameters. There was a low correlation between the melting temperatures and the acidic oxide content in the ashes. However, the composition parameters derived from the basic oxides showed a good correlation with the ash melting temperature. The composition parameter, P₇, which is defined as the ratio of basic oxides (CaO+MgO+K₂O+Na₂O) to acidic oxides (SiO₂+Al₂O₃+Fe₂O₃), showed a strong correlation with the ash melting temperature. By fitting the composition parameter to the experimental data, the correlation equation for the half fluid temperature (HFT) was found to be HFT=426.77P ₇ ² ?736.76P₇+1592.3 with a correlation coefficient of 0.91. The correlation equation could be used to predict the melting temperatures of various waste incineration residues. The relative error between the measured and predicted melting temperature was approximately 5%. Overall, these parameters and correlation equations can be used to predict and reduce the melting temperature of incineration residues.     

Study on the ash fusion temperatures of coal and sewage sludge mixtures

The coal, sewage sludge, water and chemical additives are milled to produce coal-sludge slurry as a substitute for coal-water slurry in entrained-flow gasification, co-gasification of coal and sewages sludge can be achieved. The ash fusion temperature is an important factor on the entrained-flow gasifier operation. In this study, the ash fusion temperatures (DT, ST, HT and FT) of three kinds of coals (A, B and C), two kinds of sewage sludges (W1 and W2) and series of coal-sewage blends were determined, and the mineral composition during the ash melting process was analyzed by X-ray diffraction (XRD). The results showed that the ash fusion temperatures of most coal-sewage blends are lower than those of the coals and sewage sludges. The ashes have different mineral composition at different temperature during the heating process. It was found that the mineral composition of AW1 blend ash is located in the low-temperature eutectic region of the ternary phase diagram of SiO₂-Al₂O₃-CaO. The minerals found in BW1 blend ash are almost the same as those in B coal ash. Kyanite is detected in CW1 blend ash, which results in the ash fusion temperatures of CW1 blend ash higher than those of C coal. We found that sodium mineral matters are formed because of NaOH added to W2, which can reduce the ash fusion temperature of coal-sewage blends.     

Effect of different reaction atmospheres on the sintering temperature of Jincheng coal ash under pressurized conditions

The sintering temperature of coal ash is studied to further understand ash behavior. The objective of this study is to obtain a detailed understanding of the effect of the reaction atmospheres on the sintering temperature under elevated pressure. A series of experiments and analyses have been completed using a pressurized pressure-drop measuring device and X-ray diffractometer (XRD) analyzer. The results show that the sintering temperatures decline markedly under all reaction atmospheres with the rise in pressure. The pressure influences the sintering temperatures by affecting the reaction rate and the mineral transformations undergone by the coal ash, as observed from the XRD patterns. The sintering temperatures measured under the reducing reaction atmospheres are lower than those for oxidizing atmospheres. The sintering temperature under N₂ is lower than those under other oxidizing atmospheres. The sintering temperature under the gasification atmosphere is close to those under H₂ and CO atmospheres, whereas the sintering temperature under a H₂ atmosphere is lower than that under a CO atmosphere.     

Investigation of the high-temperature behaviour of coal ash in reducing and oxidizing atmospheres

The high-temperature behaviour of ashes from a suite of coals exhibiting a wide range of mineralogies has been investigated. Phase analysis of ash samples quenched from various temperatures under either a reducing (60% CO/40% CO₂) or an oxidizing (air) atmosphere was performed by Mössbauer spectroscopy, scanning electron microscopy (SEM)/automatic image analysis (AIA), and X-ray diffraction (XRD). It was found that significant partial melting of the ashes occurred at temperatures as low as 200–400 °C below the initial deformation temperature (IDT) defined by the ASTM ash cone fusion test. Melting was greatly accelerated under reducing conditions, for which the percentage of melted ash increased rapidly between 900 and 1100 °C, saturating at temperatures above ≈ 1200 °C. The observation of such phases as wustite (FeO), fayalite (Fe₂SiO₄), hercynite (FeAl₂O₄), and ferrous glass in samples quenched from 900 to 1200 °C indicates that ash melting in a reducing atmosphere is usually controlled by the iron-rich corner of the FeO-Al₂O₃-SiO₂ phase diagram. Ashes rich in CaS are an exception to this rule, for large quantities of iron sulphide are formed and the melting behaviour is controlled in part by the FeO-FeS phase diagram. Under oxidizing conditions, potassium appears to be the most important low-temperature fluxing element, as the percentage of glass in samples quenched from temperatures below 1100 to 1200 °C was proportional to the amount of the potassium-bearing mineral illite contained in the coal. Above 1200 °C in air, calcium and, to a lesser extent, iron became effective as fluxing elements; melting accelerated between 1200 and 1400 °C, and was near completion between 1400 and 1500 °C for most ashes. To retard ash melting, it is generally concluded that aluminium is the most desirable constituent of ash, whereas the most undesirable constituents are iron, calcium, and potassium.     

Influence of the composition of coal ash on its flow temperature and on the <Emphasis Type="Italic">CSR</Emphasis> and <Emphasis Type="Italic">CRI</Emphasis> values of the coke produced

Two sets of experimental data characterizing the chemical composition of coal ash, its flow temperature t _c, and the CSR and CRI values of the coke from such coal are analyzed. The results indicate that the content of four oxides (SiO₂, Fe₂O₃, CaO, and Na₂O) may be used to predict t _c and also CSR and CRI in mathematical models. Models of the form y = a(I ₄) ^b , where y = t _c, CSR, or CRI and I ₄ = SiO₂/(Fe₂O₃ + CaO + Na₂O), adequately describe the experimental data within the range considered.     

煤灰熔融黏温特性及对气流床气化的适应性

以21个中国典型煤样为研究对象,根据煤灰中CaO和Fe₂O₃含量,将之分为低钙低铁类、中钙中铁类、中钙高铁类、高钙低铁类、高钙高铁类等类别。利用高温黏度计测量煤灰熔渣黏温特性,并利用计算软件FactSage对煤灰熔融状态进行热力学平衡计算,研究了液相熔渣及固体矿物质结晶与熔渣黏度的关系,分析整理了煤灰最初硅铝比(SiO₂/Al₂O₃)、固体结晶物以及液相熔渣组成3个因素对煤灰熔融特性和熔渣黏温特性的影响,为根据煤灰组分分析来预测不同煤的熔渣黏温特性及对气流床气化的适应性提供了一个简单而实用的判断方法。对气流床气化液态排渣的适应性从高到低依次为:中钙高铁类、高钙低铁类、中钙中铁类、低钙低铁类和高钙高铁类。     

Ash deposition characteristics of Moolarben coal and its blends during coal combustion

We report a systematic and comprehensive laboratory investigation of the ash deposition behavior of Moolarben (MO) coal, which has recently begun to be imported into Korea. Ash deposition experiments were conducted in a drop tube reactor, and a water-cooled ash deposit probe was inserted into the reactor to affix the ash. The tests were conducted using five types of single coals (two bituminous and three sub-bituminous, including MO coal) and blended coals (bituminous coal blended with sub-bituminous coal). Two indices represent ash deposition behavior: capture efficiency and energy-based growth rate. A thermomechanical analysis evaluated the melting behavior of the resulting ash deposits. The MO coal had the least ash deposition of the single coals due to its high melting temperature, indicated by high ash silica content. Indonesian sub-bituminous coals formed larger ash deposits and were sticky at low temperatures due to relatively high alkali content. However, blends with MO coal had greater ash deposition than blends with other bituminous coals. This non-additive behavior of MO coal blends is likely due to interactions between ash particles. Coals with higher silica content more effectively retain alkali species, resulting in lower melting temperatures and larger ash deposits. Therefore, we recommend that when blending in a boiler, silica-rich coals (SiO₂>80%, SiO₂/Al₂O₃> 5) should be blended with relatively low-alkali coals (Na₂O+K₂O<3%), and the blending ratio of the silica-rich coals indicates less than 10%, which can safely operate the boiler.     

Effect of atmosphere control on magnetic properties of CaO-Al2O3-SiO2-Fe3O4 glass ceramics

The influences of atmosphere during processes of melting and heat treatment, heat treatment temperature, Fe₃O₄ content and basicity on the magnetic properties of magnetite-based glass ceramics were investigated. For sample containing 20 % Fe₃O₄ melted in different atmospheres, the highest saturation magnetisation was realized in 20vol% air + 80 vol% Ar, due to the fact that ratio of Fe³⁺ to Fe²⁺ in melt obtained in this atmosphere was close to 2. However, it was found that the coercivity of glass ceramics was not affected by the melting atmosphere. A high sintering temperature led to the decrease of saturation magnetisation and the increase of coercivity. As increasing Fe₃O₄ content, the main crystal phase transformed from CaSiO₃ to CaFe_0.6Al_1.3Si_1.08O₆ and finally to magnetite phase, accompanied by the increase of saturation magnetisation and coercivity. In addition, the increase of basicity caused the decrease of saturation magnetisation and the increase of coercivity.     

Influences of minerals transformation on the reactivity of high temperature char gasification

Two Chinese coals were used in this study and coal chars were prepared at different temperatures. High temperature gasification of coal chars with CO₂ was investigated in a bench scale fixed-bed reactor and the transformations of minerals from these two coals were also studied from 1100 to 1500 °C. Mineral matters produced at different temperature and ash generated after gasification were collected and analyzed by XRD and FTIR. It was found that the iron oxides were only catalytic mineral matters existing at high temperature. And gasification behaviors above ash melting temperature were different for different mineral composition, especially the content and form of iron oxide, which not only accelerates the gasification reaction, but also reduces the influence caused by melting minerals.     

Interactions of high-chromia refractory materials with infiltrating coal slag in the oxidizing atmosphere of a cyclone furnace

The mechanism of high-chromia refractory failure in the oxidizing atmosphere of cyclone furnaces differs from the reducing atmosphere in gasifiers. In this paper, postmortem analysis was conducted to investigate the changes in the microstructures of exposed high-chromia refractory caused by its interaction with infiltrating coal slag under cyclone furnace conditions. The effects of the temperature level and viscosity of the molten slag were also investigated. Postmortem analysis confirmed that the form of Fe found in the slag in an oxidizing atmosphere was Fe₂O₃ rather than FeO, the phase present in a reducing atmosphere of gasifiers. Furthermore, the higher melting temperature of Fe₂O₃ weakened the slag penetration and chemical corrosion in an oxidizing atmosphere. As coal slag infiltrated a high-chromia refractory, Fe₂O₃ in the slag reacted with Cr₂O₃ until Fe₂O₃ was depleted in the penetrating slag. Cr₂O₃ was dissolved in the slag because of the permeation of the slag in large pores of the refractory. The depth of the slag penetration increased as the temperature increased because of its lower viscosity at higher temperature.     

Coal fouling characteristic to deposit probe with different temperatures under the gasification condition

Coal gasification was carried out to verify the coal fouling characteristic in a drop tube furnace (DTF). Four pulverized coal samples, in the range of bituminous and sub-bituminous, were used. To analyze the fouling characteristic by different temperature of deposit probe, a two-stage deposit probe was used in the experiment. Ash deposition rate was at upper deposit probe higher than at lower one. The X-ray fluorescence (XRF) results indicated that coal fouling included acid minerals such as SiO₂ and Al₂O₃ at upper deposit probe more than that at lower deposit probe. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicated that the fouling particles at high deposit temperature were agglomerated more than those at low deposit temperature. And the convective heat transfer efficiency was reduced by ash deposition on probe. Especially, the convective heat transfer coefficient substantially declined with small particle size of fouling and Fe₂O₃, CaO, and MgO.     

Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures

The present paper was addressed to mineral transformations and ash formation during O₂/CO₂ combustion of pulverized coal. Four Chinese thermal coals were burned in a drop tube furnace to generate ashes under various combustion conditions. The ash samples were characterized with XRD analysis and ⁵⁷Fe Mössbauer spectroscopy. The impacts of O₂/CO₂ combustion on mineral transformation and ash formation were explored through comparisons between O₂/CO₂ combustion and O₂/N₂ combustion. It was found that, O₂/CO₂ combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. The differences observed in the ashes formed in two atmospheres were attributed to the impact of the gas atmosphere on the combustion temperatures of coal char particles, which consequently influenced the ash formation behaviors of included minerals.     

Chemical fractionation tests on South African coal sources to obtain species-specific information on ash fusion temperatures (AFT)

Coals from the different sources used by Sasol vary substantially in terms of chemical and physical properties and directly relates to gasifier behavior. Due to the large variation in coal properties from various sources, detailed coal and feedstock characteristics are essential to predict gasification performance when a specific coal source is to be gasified. One property that specifically gives detail information on the suitability of a coal source [Alpern B, Nahuys J, Martinez L. Mineral matter in ashy and non-washable coals—its influence on chemical properties. Commun Serv Geol Portugal 1984; 299–317.] for gasification purposes is the ash fusion temperature (AFT). The AFT of a coal source indicates the extent to which ash agglomeration and ash clinkering are likely to occur within the gasifier. Ash clinkering inside the gasifier can cause channel burning, pressure drop problems and unstable gasifier operation. The principle aim of this paper is to obtain mineral species-specific information on ash properties and the specific affect on AFT. Chemical fractionation treatment resulted in coals having different mineral properties that can be used to explain the affect of specific minerals on the AFT of coal. The highest concentration and species of minerals were removed from the coal by acid leaching (HCl and HNO₃) where Al, Ca, Mg, Na and Fe were removed in high concentrations from the coal. An interesting finding in the ash composition of the coal after leaching was that the SO₃ concentration decreased from >2 mass % in the original coal sources to <0.3 mass % after the acid leaching. The AFT of coal after leaching increased to >1600 °C. Based on the 95% confidence intervals depicted the following components can be highlighted as having a statistical significant effect on the AFT: Al₂O₃, Fe₂O₃, CaO, MgO, P₂O₅ and SiO₂–Al₂O₃ combination. When mineral ratio was used, the best correlation coefficient (R) with AFT was obtained with the dolomite ratio. This is in agreement with the results obtained from the correlations between the AFT and the ash composition where CaO and MgO resulted in the best correlation with AFT. Although the correlation (R) of 0.81 is fairly similar to that of the individual correlations with CaO and MgO, the dolomite ratio also includes Fe, Na and K, which can have mineral interaction with the Ca and Mg and thus may be included in the ratio. Results presented in this paper again highlights the fact and confirmed work from other researchers [Slegeir WA, Singletary JH, Kohut JF. Application of a microcomputor to the determination of coal ash fusibility characteristics. J Coal Quality 7: 248–54.] that ash composition (elemental analyses) on its own does not explain AFT behavior or commercial performance of coal accurately.     

Aerosols over melts of a corium–sacrificial materials system

The behavior of a melt of a UO₂–ZrO₂–SiO₂–Fe₃O₄–CaO–Al₂O₃ system that is formed in the case of the dilution of corium (a melt of a UO₂–ZrO₂–Zr system) with sacrificial materials (a mixture of aluminum and iron oxides with Portland cement) has been studied. It has been found that, as a result of the evaporation of oxides, spherical aerosol particles are formed, the sizes of which have a trimodal distribution (1, 5, and 11 μm) and the composition of which depends on the nature of the atmosphere over the melt: 90.8UO₂ · 5.2SiO₂ · 4.0Fe₃O₄ in the atmosphere of nitrogen and 68.9U₃O₈ · 6Al₂O₃ · 2.5Fe₃O₄ · 22.5Na₂O in air.     

Crystallization behavior of iron- and boron-containing nepheline (Na2O·Al2O3·2SiO2) based model high-level nuclear waste glasses

This study focuses on understanding the relationship between iron redox, composition, and heat-treatment atmosphere in nepheline-based model high-level nuclear waste glasses. Glasses in the Na₂O–Al₂O₃–B₂O₃–Fe₂O₃–SiO₂ system with varying Al₂O₃/Fe₂O₃ and Na₂O/Fe₂O₃ ratios have been synthesized by melt-quench technique and studied for their crystallization behavior in different heating atmospheres—air, inert (N₂), and reducing (96%N₂–4%H₂). The compositional dependence of iron redox chemistry in glasses and the impact of heating environment and crystallization on iron coordination in glass-ceramics have been investigated by Mössbauer spectroscopy and vibrating sample magnetometry. While iron coordination in glasses and glass-ceramics changed as a function of glass chemistry, the heating atmosphere during crystallization exhibited minimal effect on iron redox. The change in heating atmosphere did not affect the phase assemblage but did affect the microstructural evolution. While glass-ceramics produced as a result of heat treatment in air and N₂ atmospheres developed a golden/brown colored iron-rich layer on their surface, those produced in a reducing atmosphere did not exhibit any such phenomenon. Furthermore, while this iron-rich layer was observed in glass-ceramics with varying Al₂O₃/Fe₂O₃ ratio, it was absent from glass-ceramics with varying Na₂O/Fe₂O₃ ratio. An explanation of these results has been provided on the basis of kinetics of diffusion of oxygen and network modifiers in the glasses under different thermodynamic conditions. The plausible implications of the formation of iron-rich layer on the surface of glass-ceramics on the chemical durability of high-level nuclear waste glasses have been discussed.     

Effect of Al2O3/CaO on the melting and mineral transformation of Ningdong coal ash

Coal ash melting characteristics has a direct impact on the smooth operation of entrained gasifier. Mineral conversion of coal ash is very significant to be investigated, because the mineral can affect the melting temperature and viscosity under high temperature conditions. In this paper, the effects of different Al₂O₃/CaO on the mineral conversion, melting temperature and viscosity of Ningdong coal ash are studied by the combination of experiment and simulation. The trend of melting temperature decreases firstly and rises with increasing Al₂O₃/CaO. The ash melting point reached to the lowest when the ratio is 1.23. XRD and Factsage software are used to analyze crystallization behavior of samples. The results show that the content of anorthite, albite and corundum increases and subsequently decreases, while the content of mullite decreases firstly and then rises with increasing Al₂O₃/CaO. High content with CaO can contribute to form albite and anorthite of low-melting. Besides, high content with Al₂O₃ can tend to produce mullite of high-melting. The results of experimental and simulation are basically the same, which lays a foundation for the melting characteristics of Ningdong coal ash and can provide technical support for the smooth operation of the entrained-gasifier.     

Thermodynamic calculation of the equilibrium composition of the gasification products of oil shale from the Kendyrlyk deposit

Based on the thermodynamic calculations of the equilibrium composition of the gasification products of shale from the Kendyrlyk deposit (Republic of Kazakhstan) (air blast coefficient α = 0.3; pressure, 0.1 MPa) with consideration for the chemical composition of the ash of mineral components (SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, K₂O, and Na₂O) and the concentrations of trace elements (Cu, Sr, Zn, Cr, Ti, Mn, and Ni) in it, the constituents of a gas phase (the most probable of 500 substances) were determined depending on the process temperature. The equilibrium compositions of gas and condensed phases at a temperature of 1185.65 K were calculated using the HSC Chemistry software.     

AGGLOMERATION BEHAVIOR IN A BUBBLING FLUIDIZED BED AT HIGH TEMPERATURE

Minimum fluidization velocity and agglomeration behavior were investigated at high temperature in an 80?×?30?mm two-dimensional quartz fluidized bed and in an 82?mm i.d. circular fluidized bed. Bed materials tested were two sizes of glass beads as well as three sizes of fluidized bed combustor (FBC) ash. The minimum fluidization velocity decreased with increasing bed temperature, whereas the minimum sintering fluidization velocity increased with the bed temperature. The sintering of glass beads belongs to visco plastic sintering, the first type. FBC ash agglomerate has higher amounts of SiO₂, Al₂O₃, Na₂O, K₂O, and SiO₂ than in the original ash, indicating that low melting eutectics were formed and that the liquid phase in a silicate system was formed. The agglomeration of FBC ash belongs to the second type, an excessive quantity of liquid being formed by melting or chemical reaction.