The relationship between excluded mineral matter and the abrasion index of a coal

Predictions of the wear rates of components in grinding mills at pulverised coal-fired power stations are currently made using empirical relationships based on the ash content of the coals. However, modern coal characterisation techniques now allow the mineral inclusions in a coal that are responsible for the abrasive nature of the coal to be accurately characterised. Hence, there is scope to make improved predictions of wear based on a detailed knowledge of the mineral matter in a particular coal. It is first necessary, however, to understand the nature of the minerals and properties of the minerals in a coal that would contribute to abrasive wear. In this study known quantities of quartz, pyrite and slate have been added to a washed coal and the Abrasion Indices of the coal/mineral mixtures have been measured. The results show how the size, shape and hardness of excluded mineral matter contribute to the abrasive properties of a coal.     

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.     

煤磨碎性能与煤质特性的相关性研究

采用多元统计方法对煤质特性与可磨性HGI的相关性进行了研究.结果表明,水分和挥发分越高,HGI越低;灰分和固定碳含量越高,HGI越高.但HGI与煤质特性存在明显的非线性.国外文献提供的非线性预测方程不适合我国煤种.仅从煤的工业分析等化学组分出发,将其看作一种均匀的物质,不能科学地反映煤的可磨性.煤的显微组分、矿物质类型、颗粒大小和分布以及煤种的显微构造等物理参数是决定煤可磨性能的重要因素.     

Factors governing reactivity in low temperature coal gasification. Part II. An attempt to correlate conversions with inorganic and mineral constituents

Links between extents of coal gasification and the amounts and compositions of mineral components in coals have been investigated. The influence of demineralisation and impregnation with various inorganic components on the pyrolysis and CO₂-gasification behaviour of two coals have been examined at 0.1 and 1 MPa. The effect of mineral matter on pyrolysis and gasification behaviour has also been examined by correlating actual conversions of a calibration set of 23 coal samples with the mineral matter-related bands of their FT-IR spectra. Whilst mineral matter contents clearly affect conversions during gasification, results from this work show that it is difficult to find systematic patterns, regarding the effect of specific inorganic components in different coals. The prediction of catalytic activity from amounts and compositions of particular inorganic components appears unlikely to be feasible. These findings confirm the difficulty of relating information on original structural features of coals to weight loss during gasification.     

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.     

Radiometric assessment of abrasiveness of coal in pulverizers

A laboratory method is described for the measurement under water, of the abrasiveness of a coal as it affects wear in pulverizers. The coal is ground by a rotating blade containing radioactive material. The determination of the level of radioactivity on the ground material provides an indication of the amount of material abraded from the blade. A coal which caused excessive wear in commercial grinding in a pin-disc mill gave an extremely high ‘abrasive index’ when subjected to this laboratory test. Abrasive indices have been determined for a series of New Zealand coals. High values correlate with increased ash level, with a high silica-to-alumina ratio in the ash and especially with a high free-silica content of the coal.     

The effects of morphological changes and mineral matter on H2S evolution during coal pyrolysis

The H₂S release profiles of five important Colombian coals have been monitored using temperature programmed pyrolysis. It was found that there was no correlation between the amount of H₂S and the sulphur content of the original coals. Coals which had been treated to remove all of the mineral matter and inorganic sulphur showed a good correlation with the free swelling index of the coals. This was explained by the physical trapping of H₂S in closed porosity formed during pyrolysis. A similar result was found for demineralised coals with pyrite present. The H₂S for untreated coals showed no systematic variation with rank, the coal sulphur content or free swelling index. This was because certain coals were rich in finely dispersed calcium which could chemically prevent H₂S release.     

The roles of lime and iron oxide on the formation of ash and deposits in PF combustion

This paper presents the results of a study to assess the slagging propensities of a suite of UK, Spanish and South African coals, ranging from lignites to anthracites. Laboratory deposits were collected on ceramic deposition probes at gas temperatures of ∼1250°C, using an entrained flow reactor that simulates the time-temperature conditions experienced by pulverised coal particles in a large utility boiler. The degree of sintering and consolidation of the deposits would not have been predicted from bulk ash chemistry, indicating the importance of mineral matter distributions in the pulverised coal. Deposits with similar base to acid ratios and Fe₂O₃ contents displayed a range of slagging propensities on CCSEM analysis, consistent with the visual ranking. CCSEM analysis of the fly ashes collected from the combustion gases revealed a similar chemical composition to the coal ash and ash collected at the base of the EFR. CaO was observed to have readily assimilated into the aluminosilicate fly ash particles. On deposition, the CaO distribution largely remained unchanged. Fe₂O₃ was redistributed on forming a deposit possibly aided by CaO already dissolved in the aluminosilicates. The study provides an insight into the observations made by boiler operators burning coals with high CaO and Fe₂O₃ ashes.     

Studies on the combustion behaviour of blends of Indian coals by TGA and Drop Tube Furnace

Combustion behaviour of blends of two Indian coals of same rank with wide variation in mineral matter content were studied using Thermogravimetric Analyzer (TGA) and Drop Tube Furnace (DTF). The characteristic TGA parameters determined from the burning profiles showed both additive and non-additive behaviour. The burnout temperature and peak temperature showed a linearly decreasing trend with the increasing proportion of the high ash coal. Deviation from the linear trend was observed in the case of the reactivity parameter. The high ash coal showed better TGA reactivity than the low ash coal. The higher TGA reactivity could arise from the combined effect of mineral matter and the nature and distribution of the macerals, particularly those of the inertinite group.The burnout in DTF showed a nonlinear effect. The burnout behaviour of the coals and their blend observed in DTF was not similar to that reflected in TGA. Coal blends with less than 50% of high ash coal showed better burnout than the individual coals.     

Influence of petrographic and mineral matter composition of coal particles on their combustion reactivity☆

Combustion at programmed temperature in a thermobalance is a rapid technique, which monitors coal burning characteristics and has shown its utility to classify coals according to their combustion performance. However, combustion profiles are affected by different coal properties and characteristics such as particle size, rank, maceral composition and mineral matter content, whose separate effects are difficult to determine. The objective of this work was to ascertain the influence of coaly and mineral matter composition and distribution on burning profiles as determined by thermogravimetric analysis, by using coals of different rank, and fractions of these coals obtained by density separation. Five coals ranging in rank from lignite to anthracite and with variable mineral matter content and composition were used in this study. Density fractions were separated from each coal to obtain samples with different organic/mineral matter proportions. Some of the factors influencing coal combustion profiles are widely recognised as the negative effect of increasing both rank and inertinite content on the reactivity. The favorable effect of mineral matter content on the reactivity has shown to be related to the maceral size within the density fractions and the intimate association organic/mineral matter that favors the diffusion of the reacting gas. Catalytic effects of the mineral matter could not be demonstrated.     

The characterization of surface properties and steam reactivities of two Spanish coals of high ash content

Studies were made on two Spanish coals of high as content (a semi-anthracite and a high volatile bituminous coal) and on the coals after heat treatment and on acid demineralisation in HCl and HF. X-ray diffraction revealed that the mineral matter content of the coals included quartz, siderite and aluminosilicate minerals; siderite and the aluminosilicates were decomposed by heat treatment. Mineral matter content was substantially reduced by acid treatment, but the metallic element content, as revealed by EDAX, remained similar to that of the raw coals. N₂ and CO₂ adsorption isotherms and mercury porosimetry show that the coals contain mainly micropores and macropores, the semi-anthracite having the greater microporosity. For bituminous coal, macro- and micro-porosity increase substantially upon heat treatment; for anthracite there is a smaller increase in macroporosity and a decrease in microporosity. Microporosity in both coals is unaffected by acid demineralisation, but macroporosity is increased. The steam reactivity of the bituminous coal char is greater than that of the anthracite coal char, and demineralisation of the coals increases steam reactivity. N₂ surface areas and steam reactivities for both coal chars follow trends with parent coal rank previously established for US coals.     

用于混煤燃烧控制砷排放模型研究

混煤掺烧是控制燃煤砷排放的有效方式,但由于缺乏相关的配煤模型,限制了该技术的应用。根据燃煤过程中砷的挥发释放机制,提出一种采用砷的释放指数P表征煤燃烧过程砷的释放特性的配煤模型。该模型综合考虑煤的灰分、灰中主要矿物元素含量、各矿物元素对砷的固定系数以及煤中砷含量等因素。研究结果表明,随着煤灰固定系数由23.12增至50.90,煤灰的气相砷吸附量由3.39 mg/g增至6.14 mg/g;随着释放指数P增大,砷的固定率减小,且随着温度升高,两者相关性由900℃的0.67增至1 300℃的0.86。根据P值筛选煤种进行掺烧,当掺混煤种P值差异较大时,掺烧低P值煤种不仅可降低混煤中的砷含量,还能促进高P值煤的砷在灰中富集,促进率达77.14%;掺混煤种P值差异较小时,掺烧低P值煤种会促进砷的释放。本模型可较好地筛选煤种,为混煤掺烧控制砷等痕量元素的排放提供了新的思路。     

Mobilisation of trace elements from as-supplied and additionally cleaned coal: Predictions for Ba, Be, Cd, Co, Mo, Nb, Sb, V and W

The partitioning of the elements antimony, barium, beryllium, cadmium, cobalt, molybdenum and vanadium between the products of combustion of coals containing them burnt as pulverised fuel in excess air has been modelled using the MTDATA thermodynamic equilibrium package with data from the MTOX silicate melt model added to the standard database and trace element data added where necessary. The coals examined were Gascoigne Wood, ElCerrejon and Harworth coals as normally supplied (washed) and after additional washing, and Binungan low ash coal only as normally supplied, represented by the analyses for coal, coal mineral and trace elements obtained in a study of the partitioning carried out in a pilot scale pf combustor by PwerGen on behalf of the United Kingdom DTI. Excess air levels were 1% for all coals and 3% in addition for Harworth. The equilibrium amount of silicate melt was predicted to fall more rapidly with falling temperature for additionally washed than for normally washed coals. It was also predicted that Ba and Co would be almost immobile, Be and V would be relatively immobile, and Sb, Cd and Mo would be mobile. Additional calculations were carried out for niobium and tungsten as trace elements in the coals, and Nb was found to be relatively immobile and W mobile. The mobilities of Ba, Be, Co, Cd, Mo, Sb and V were in agreement with those implied by the ratio of bottom ash to fly ash concentrations found in experimental investigation.     

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.     

Total chlorine in coal seam profiles from the South Staffordshire (Cannock) coalfield

In a coalfield of noted high-chlorine coals, seam profiles were taken from two collieries. Three seam profiles from each colliery were analysed for total chlorine to try to correlate chlorine level with position in the seam. Moisture contents and ash yields were determined for four of the seams. Total chlorine is highest in the coals, reaching 1 wt%, and low in the mudrocks, down to 0.08 wt%. Intermediate chlorine values are associated with dirty coals and carbonaceous mudrocks. Water centrifuged from an intraseam dirt band showed that the moisture held the total chlorine content. Similar element ratios were achieved from water centrifuged from coal. These results, together with National Coal Board groundwater data, suggest that the majority of chlorine in coal has been introduced by chloride-rich groundwaters which evolved during diagenesis, as has been suggested for Illinois and Nottinghamshire coals. There is a very significant positive correlation between coal moisture and total chlorine contents. Total chlorine levels are thus thought to be controlled by groundwater concentration levels and coal porosity, hence the positive relationship between organic carbon and chlorine concentrations. Total chlorine levels vary through the seams, but there is not a consistent change with position in the seam. There exists a correlation between coal type and total chlorine content. Bright coals hold significantly more chlorine than dull coals, owing to different porosities of the coal macerals. Departures from this relationship with coal type may be explained by mineral matter dilution or the ingress of recent groundwaters, overriding the original coal type-chlorine relationship.     

Desulfurization of model coal sulfur compounds by coal mineral matter and a cobalt molybdate catalyst—I.: Thiophene

A continuous-flow differential reactor was used to obtain rate equations for the reaction of thiophene and hydrogen over coal mineral matter and a commercial cobalt-molybdate catalyst (Nalcomo 471). Coal mineral matter in its least altered state was obtained by low temperature ashing of Western Kentucky ?9 and ?11 coals in an oxygen plasma. Conversion was determined from the C₄ gases separated by gas chromatography. Pretreatment of both coal mineral matter and catalyst with hydrogen sulfide increased thiophine reaction rates considerably, but introduction of the gas with reactants markedly decreased the rate.     

The Application of High Field and High Gradient Methods to the Magnetic Separation of Mineral Matter from Micronized Coal

Abstract

In exploratory laboratory measurements, high field high gradient magnetic separation (HGMS) has removed as much as 74% of the mineral matter and 99% of the pyritic sulfur from micronized coals with mineral contents up to 16.39 wt%. Magnetic cleaning methods are limited by the fact that not all mineral matter in coal is magnetic. HGMS methods are further restricted when mineral matter levels generally exceed about 2 to 3% because of excessive capture-matrix loading which leads to poor clean coal weight yields. The use of selective flocculation of coal mineral matter and processing at high flow velocities (made possible with the use of high field superconductive magnet technology) offer hope for overcoming these process limitations and for extending use of HGMS technology to preparation of low ash and low sulfur coal-slurry fuels. Measurements of mineral matter and sulfur removals achieved in high field HGMS processing of water slurries of dispersed micronized coals are presented. Field strengths up to 15 Tesla, flow velocities up to 3 cm/s, and slurry solids up to 38.4% were investigated. The use of models of the magnetic capture mechanism for scaling laboratory data to commercial applications is discussed.     

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 X-ray powder diffraction analysis of clay minerals in Australian coals using Rietveld methods

The mineralogy of clay-rich mineral matter isolated from a range of Australian bituminous coals has been evaluated in quantitative terms from X-ray powder diffraction (XRD) patterns using a Rietveld-based data processing technique. The chemical composition of coal ash derived from this mineral matter has been calculated and compared to the directly determined composition of the ash prepared from the same coal samples. Although there are some minor differences due in part to uncertainty regarding the actual composition of several minerals, the compositions indicated by the two methods show a relatively high correlation, suggesting that the Rietveld technique provides mineralogical analyses that are consistent with independently determined chemical data. Comparison of the normalised clay mineral percentages from the Rietveld analysis to quantitative interpretations based on a peak intensities in glycolated and heat-treated oriented aggregates of the respective clay fractions also shows a high correlation, confirming mutual consistency of the two different mineralogical analysis methods. Such quantitative mineralogical data are significant to a range of coal exploration, mining and utilization activities, including seam correlation, material handling and ash and slag formation in combustion processes.     

Correlation between the sudden and regular releases of coal-bed methane and the structures of the organic matter of natural coals

A correlation between the structure of the organic matter of coal and the ability of coal to release gases upon thermal and mechanical actions was considered. It was found that coal-bed methane released in different modes (regular and sudden) during coal-bed mining is of mechanochemical rather than sorption nature. Relationships were proposed to determine the natural methane content of coals and the sorption capacity of coals from a metamorphic series for methane using a structure-chemical parameter: the concentration of paramagnetic centers calculated from a broad line of the EPR signal. The use of spectroscopic techniques provides an opportunity to considerably simplify and make cheaper a very labor intensive technology for the determination of natural gas content. The regulations of methane safety in coal mines should be corrected based on the original concepts of the nature of coal-bed methane.