Thermal behaviour of mineral fractions separated from selected American coals

The thermal behaviour of four samples of mineral matter, whose quantitative mineralogical compositions had previously been determined, was investigated using thermogravimetric and derivative thermogravimetric analysis methods. The mineral fractions were initially separated from the coal substance by a low-temperature ashing technique. The decomposition of the individual mineral species associated with coals is briefly reviewed. A synthetic standard mineral mixture was examined under various combustion atmospheres. X-ray diffraction analysis was utilized to study the high-temperature phases yielded by the mineral fractions. Ash-fusion data were also obtained, and can be understood in relation to the corresponding mineralogical analyses. No evidence was found of solid-state interactions below 1100 °C.     

Oxygen in coal ash: a simplified approach to the analysis of ash and mineral matter in coal

Oxygen was determined accurately in eight U.S. Bureau of Mines coal ash samples A, B, D, F, G, I, and J, NBS coal fly ash 1633 reference material, and two low-temperature ashes (LTA) from lllinois State Geological Survey. The method uses fast-neutron activation (FNA) analysis employing a dual counting and irradiation system which is essentially free from interferences. The stoichiometric balance based on analyses of the ashes performed by the USBM is calculated and summations given in oxide and element percent. Excellent agreement is found with the chemical data obtained by classical silicate analysis methods. Accurate oxygen determination for coal ash and LT-ash (or mineral matter) is important for calculation of data in the ultimate analysis of coal as such. Knowledge is required for recalculation of the data on a dry and dry-ash-free basis. The routinely used ‘oxygen by difference’ values are inadequate for accurate work. In order to determine the organic oxygen in coal one also has to correct for oxygen in mineral matter and oxygen in the water removed as moisture. The Parr formula and other methods of empirical estimation are inadequate and may be replaced in some cases by the oxygen determination. The complete data provide a quantitative basis for stoichiometric interpretation of coal analyses. It was found that the eight coal-ash samples analysed contained 45.5 ± 3% oxygen. Since these ashes represent a large variety of U.S. coals, this figure can be used as an estimate for recalculation and evaluation of the proximate and ultimate coal analyses. It is better, however, to use values actually determined in ash by the rapid fast-neutron activation method. This permits a better estimation of the sum of cations plus sulphates in the ash.     

Behaviour of coal mineral matter in sintering and slagging of ash during the gasification process

The mineral matter in typical feed coals used in South African gasification processes and the ash derived from gasifying such coals have been investigated using a variety of mineralogical, chemical and electron microscope techniques. The mineral matter in the feed coals consists mainly of kaolinite, with minor proportions of quartz, illite, dolomite, calcite and pyrite plus traces of rutile and phosphate minerals. The calcite and dolomite occur in veins within the vitrinite macerals, and are concentrated in the floats fraction after density separation. Some Ca and Ti also appear to be present as inorganic elements associated with the organic matter.Electron microscope studies show that the gasification ash is typically made up of partly altered fragments of non-coal rock, bonded together by a slag-like material containing anorthite and mullite crystals and iron oxide particles, with interstitial vesicular glass of calcic to iron-rich composition. Ash formation and characteristics thus appear to be controlled by reactions at the particle scale, allowing the different types of particles within the feed coal to interact with each other in a manner controlled mainly by the modes of mineral occurrence. Integration of such techniques provides an improved basis for evaluating ash-forming processes, based on quantitative phase identification, bulk and particle chemistry, and the geometric forms in which the different phases occur.     

A new approach for the prediction of ash fusion temperatures: A case study using Turkish lignites

Prediction of ash fusion temperatures by using the chemical composition of the ash has previously been conducted only with linear correlations. In this study, a new technique is presented for predicting the fusibility temperatures of ash. Non-linear correlations are developed by using the chemical composition of ash (eight oxides) and coal parameters (ash content, specific gravity, Hardgrove index and mineral matter content). Regression analyses are conducted using information for Turkish lignites. Regression coefficients and variances of non-linear and linear correlations are compared. The results show that the non-linear correlations are superior to linear correlations for estimating ash fusion temperatures.     

Mechanistic prediction of ash deposition in a pilot-scale test facility

A mechanistic approach has been used for assessing the ash deposition tendency and has been used for predicting the ash deposition behavior of Australian bituminous coals in a pilot-scale test furnace. The detailed analysis of mineral matter in coal determined by QemSCAN analytical technique was incorporated into an ash formation model to estimate the character of ash particles. The detailed chemical composition and particle size distribution data of ash particles from the ash formation model were incorporated into a computational fluid dynamics (CFD) code to get the arrival rate and retention ability of each ash particle onto the heat transfer surfaces.The CFD code along with the ash character from the ash formation model was able to simulate trends in ash deposition along the furnace length similar to those in the test furnace. The approach was also able to distinguish coals with high ash deposition potential from low ash deposition problems and the results agreed with those in test furnace.     

A formula for the mineral matter to ash ratio for low rank coals

Existing formulae relating mineral matter to ash yield of coal, which assume that all the mineral matter is separate from the coal, do not apply to low rank coal because part of the mineral matter is bound in the coal substance. A mineral matter/ash formula is derived for low rank coal that allows the calculation of the ratio of mineral matter to ash for a sample of any coal the basic properties of which have been determined. The formula requires a modification to the procedures previously developed for calculating basic properties of low rank coals. Basic properties and the parameters determining the mineral matter to ash ratio are presented for New Zealand sub-bituminous Waikato coals.     

Correlation relations between mineralogical components in ash from Kaa-Khem coals

Regression analysis was used to study correlation relations between the mineral components of coals. Regularities in the variability of the concentrations of individual ash-forming elements with changing ash contents of coals and changing seam depth were found. The X-ray diffraction characteristics of coal ashes and the qualitative composition of their mineralogical components are presented.     

The nature of mineral matter in a coal and the effects on erosive and abrasive behaviour

The Yancey, Geer and Price (YGP) abrasion index has been used for many years to assess the abrasive nature of a coal. The index is clearly related to the mineral matter in the coal, but a direct correlation with the ash content is poor. A better correlation is achieved by relating the wear of coal grinding mills and pf pipe work to the quartz and pyrite contents of the coals, but the data still shows significant scatter and is of limited use for predicting component wear rates. This paper presents the results of a study aimed at relating the nature of the mineral matter in a range of UK and world-traded coals with their abrasive and erosive wear as measured by the Mitsui Babcock mini-mill test and pulverised coal erosion studies. The mineral matter in the coals has been characterised by computer controlled scanning electron microscopy (CCSEM). The results from the mini-mill and erosion tests have given linear correlations between the mineral matter in the coal and the abrasive and erosive wear by considering only the excluded mineral occurrences in the pulverised coal that that are harder than steel and have a size that is >25 μm.     

Mineral matter-organic matter association characterisation by QEMSCAN and applications in coal utilisation

The association of mineral matter with organic matter is extremely important for coal utilization process such as pf coal combustion. With the development of advanced analytical instruments such as QEMSCAN, it is now possible to measure directly the mineral matter-organic matter association on a particle-by-particle basis. The mineral matter and mineral-organic associations of a suite of fourteen CCSD coal bank coals (as pf) have been determined by QEMSCAN. An interface program was developed to make QEMSCAN data compatible with the CCSEM-based ash formation model developed previously in CCSD. Size and chemistry of flyash was predicted by a partial coalescence sub-model for included mineral grains, and a fragmentation sub-model for excluded mineral grains, respectively. The size and chemistry of predicted flyash was estimated on a particle-by-particle basis, and was used to rank the ash effect on heat transfer reduction for all the CCSD coals using the CCSEM-based model, in which coal property, furnace geometry and operational conditions have been taken into account. Other applications and further developments of the technique are also outlined.     

混煤燃烧过程中矿物质的形态变化及相变

引　言炉内受热面结渣被认为是电站锅炉中最难以处理的一个问题 ,特别在我国长时间内煤粉锅炉作为主要的能源供应手段 .近年来 ,国内有许多电站采用混煤燃烧的方法以适应不同的要求 ,如减少结渣和ＳＯｘ 的排放 .然而 ,实际上如果混煤中设计使用的煤和实际使用的煤特性不一样 ,混煤燃烧可能会导致严重的结渣 .对单一煤种燃烧过程中的矿物质行为已经有了广泛研究[1～ 4 ] ,但对混煤很少有研究[5,6] .本文主要研究混煤燃烧后矿物质行为特性的变化 ,并用ＸＲＤ分析矿物质形态随温度的变化 ,从矿物学的观点来解释混合煤灰熔融特性 ,以揭示混煤结…     

Fourier Transform Infrared study of mineral matter in coal. A novel method for quantitative mineralogical analysis

A novel method for the quantitative determination of mineral matter in coal is reported. The low-temperature ash of coal is analysed by means of absorbance spectral subtraction of individual components. The spectra of individual minerals, stored in digital form on computer memory, are multiplied by appropriate weighting factors and subtracted from the spectrum of the low-temperature ash, so that the characteristic bands of the mineral are removed. Provided that the weight of each mineral in the infrared beam is known then the weight fractions can be determined from the weighting factors. Successive subtraction starting with the most strongly absorbing components reveals the minor or less strongly absorbing species, which could not previously be determined by infrared spectroscopy. The analysis of several mixtures and of the low-temperature ash of various coal samples is reported.     

Utilization of by-products from western coal combustion in the manufacture of mineral wool and other ceramic materials

The ash by-products from combustion or gasification of western U.S. coals have chemical and mineralogical characteristics that lend themselves to utilization in ceramic materials. Laboratory and pilot-scale fabrication of four such materials has been studied. Cyclone slag from four lignite-fired power plants and a dry scrubber ash have been fabricated into mineral wool insulation in a pilot-scale cupola. Extruded and fired mixtures of fly ash, clay and ground glass have produced ceramics with extraordinary high flexural strength. Ceramic glazed wall tile that utilize fly ash in place of clay have been prepared and shown to meet most specifications for fired clay tile. Both fired and unfired dry-pressed brick containing 100% western fly ash have met ASTM specifications for fired clay brick.     

Graphical methods for determination of the mineral-matter-free properties of coal and the mineral matter/ash ratio

Mean values of the mineral-matter-free specific energy, volatile matter and moisture content of coal, together with the mineral matter/ash ratio and the mineral matter moisture content, can be obtained from linear regression analyses of the appropriate analytical quantities against ash yield for grouped coal samples from geographical areas. The results can be used to assess rank and quality variations and the determined mineral matter/ash ratio can be used to correct individual analyses to a mineral-matter-free basis.     

Influence of weathering process on the flotation response of coal

The floatability of a Spanish coal from a seam (Adolfa) which is simultaneously surface mined (ASM) and underground mined (AUM) has been studied. The mineral composition of AUM and ASM coal samples, as well as changes in both mineral and organic constituents due to chemical weathering, were characterized by X-ray diffraction, i.r. and Mössbauer spectroscopies and X-ray fluorescence spectrometry. The effect of pH and surface charge on the floatability of AUM and ASM samples was investigated. The formation of humic substances in the surface of ASM coal and the presence of Fe(III) species in its mineral matter, both a consequence of weathering, together with the high content of mineral impurities (especially aluminosilicates) provide an explanation for the flotation behaviour of this coal. Basic pHs that led to humic acids dissolution and to iron(III) hydroxide precipitate desorption from the surface improved ASM coal floatability up to values similar to those obtained with unoxidized AUM coal. The formation of humic acids-mineral impurities complexes at basic pHs renders clay particles hydrophobic. As a consequence, the improvement of ASM coal floatability at pHs higher than neutral was accompanied by a low ash rejection. Mineral impurities had a large influence on the surface charge of ASM coal.     

Glow discharge excited low temperature ashing: A new technique for separating mineral matter of coals

Low temperature ashing techniques are widely used to determine mineral matter content and in the analysis of the composition of the inorganic matter in coal and coal products. This Paper presents a new technique which makes it possible to ash coals in 4 (low-rank coals) to ≈8 (high-rank coals) h per gramme. The problem of the formation of LTA artefacts is described and differences between authigenic and artefactic minerals are indicated.     

Quantitative determination of the mineral-matter content of coal by a radiofrequency -oxidation technique

The development of a low-temperature (≈150 °C) radiofrequency-oxidation technique as a routine laboratory method for the quantitative determination of the mineral-matter content of coal is described. The main advantage of the method over air-oxidation (370 °C) and acid-extraction methods is that isolation of the unaltered mineral matter permits a more accurate expression of coal analyses on a dry mineral-matter-free basis. Comparison with the air-oxidation method has demonstrated the superiority of the radiofrequency-oxidation method for retention of carbonate and sulphide minerals; reproducibility is generally similar. The radiofrequency-oxidation method yields higher results for the percentage of mineral matter than the air-oxidation method. The differences between the combined water contents of the mineral-matter samples prepared by the two methods indicate that the air-oxidation method partly dehydrates clay minerals and that the radiofrequency-oxidation method produces results closer to the true mineral-matter contents. Other advantages — independence of extra analytical determinations, lower elapsed time and labour costs, applicability to a wider range of coals — are detailed in the Conclusions.     

Evolution with Temperature of Crystalline and Amorphous Phases in Porcelain Stoneware

Porcelain stoneware tile is a ceramic building material characterized by high technological properties, especially regarding water absorption, chemical and frost resistance, bending strength and abrasion resistance. Because mineralogy is one of the main factors affecting the mechanical properties of porcelain stoneware, a complete determination and quantification of the mineral and amorphous phases is of special importance in the study of porcelain stoneware tiles. In the present work, a reference industrial composition (50% kaolinitic clay, 40% feldspar, and 10% quartz) of porcelain stoneware tiles fired at different temperatures (400°–1400°C) was characterized by X-ray powder diffraction combined with quantitative full-phase analysis using the Rietveld method, including amorphous content. The green composition contained albite, microcline, and muscovite as fluxing agents, which start to decompose at low temperatures (400°–800°C range) and are completely dissolved above 1200°C. The mullite phase is formed from 1100° to 1230°C and at the latter temperature, quartz particles start to dissolve. Studies of mineralogical evolution have revealed that the high heating rate (45°–50°C/min) required in ceramic tile manufacture leads to significant differences in comparisons with whiteware ceramics fired at a lower heating rate (10°C/min). Thus, the formation of mullite in porcelain stoneware occurs at higher temperatures (1100°C) whereas the transformation of β-quartz to β-cristobalite does not take place. The experimental results of this study show that qualitative mineralogical analysis, based on the intensity of a particular diffraction peak for each crystalline phase, is a suitable methodology to obtain preliminary knowledge of mineralogical changes with temperature.     

Correlation between the content of volatile matter and mineral matter in hard coals

If coal is heated to 900 °C, in the absence of air the organic coal substance and the associated mineral matter are decomposed. The analytically determined volatile-matter yield includes the volatile decomposition products of both the coal substance and the minerals. The correlation between mineral matter and ash and the volatile-matter yield is derived, and its accuracy shown by evaluation of test results. Methods are proposed for calculating a value for the volatile matter dmmf for coals of a particular mine. Various formulae for calculating the volatile matter of coals to dmmf basis are critically considered. Finally, a generally applicable equation for calculating dmmf volatile matter is derived which can be used for classification.     

Analysis of pulverized coal particles (10–100 μm) for Fe, S, Ca, Si and Al on a particle-by-particle basis

A computer controlled scanning electron microscope fitted with soft X-ray fluorescence analysed thousands of pulverized coal particles in the size range 10 to 100 μm for amounts of Al, Si, Ca, S and Fe in each of the particles. Two coals were used, namely, subbituminous C, Comanche-Wyoming and subbituminous B, Rosebud-Montana. In pulverized coal combustion Rosebud coal has a bad slagging record not predictable from conventional slagging indices based on the elemental analyses of the total coal ash. The coal particles were placed into one of four groups: little mineral matter; low melting point inorganic composition; high melting point compositions; and large amounts of iron or pyrite. On this basis, Rosebud coal had 47% of its mineral matter in groups likely to cause slagging compared with 12% for Comanche coal, in agreement with the slagging record. It is concluded that although the particle-by-particle analysis technique used here is in a crude state of development, it can give a correct indication of the possible slagging behaviour of a pulverized coal in cases where conventional slagging indices based on the analysis of the total mean coal ash fail to give the correct indication.     

Mineralogical and elemental composition of fly ash from pilot scale fluidised bed combustion of lignite, bituminous coal, wood chips and their blends

The chemical and mineralogical composition of fly ash samples collected from different parts of a laboratory and a pilot scale CFB facility has been investigated. The fabric filter and the second cyclone of the two facilities were chosen as sampling points. The fuels used were Greek lignite (from the Florina basin), Polish coal and wood chips. Characterization of the fly ash samples was conducted by means of X-ray fluorescence (XRF), inductive coupled plasma-optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), particle size distribution (PSD) and X-ray diffraction (XRD). According to the chemical analyses the produced fly ashes are rich in CaO. Moreover, SiO₂ is the dominant oxide in fly ash with Al₂O₃ and Fe₂O₃ found in considerable quantities. Results obtained by XRD showed that the major mineral phase of fly ash is quartz, while other mineral phases that are occurred are maghemite, hematite, periclase, rutile, gehlenite and anhydrite. The ICP-OES analysis showed rather low levels of trace elements, especially for As and Cr, in many of the ashes included in this study compared to coal ash from fluidised bed combustion in general.