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.     

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.     

工业煤粉中矿物质的分布规律

采用浮沉实验和激光粒度法研究了工业煤粉中矿物质的分布规律,考察了原煤的变质程度、原煤的灰分高低以及煤粉制备工艺对分布结果的影响.结果表明,工业煤粉都呈现出灰分随着煤粉粒度增加而降低的趋势,并且随着变质程度和原煤灰分的提高,灰分随煤粉粒度增加降低的幅度增大,而煤粉制备工艺对该规律的影响不明显.     

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.     

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.     

A new rheometer for direct measurement of the flow properties of coal ash at high temperatures

Development of more effective technologies of utilising low rank coals for power generation has been driven by a demand for higher efficiency, low capital costs and minimal environmental impacts. Fluidised bed systems are regarded as one of the more promising alternative technologies for power generation to overcome the disadvantage of the existing pulverised coal burning power generation plants for low rank coals. However, ash deposition and bed agglomeration are potential problems in fluidised bed processing of coals with high alkali and sulphur contents. In order to gain a better understanding of the mechanism of agglomeration in fluidised beds, a good knowledge of the rheological behaviour of coal ash deposits at high temperatures and under the processing conditions is necessary. Rheological characterisation of materials at high temperatures is difficult due to lack of standard instruments and reliable measurement techniques. We have recently developed a rheometer that has the capability of measuring the rheological properties of coal ash slag over a wide range of temperatures from 600 to 1300°C and under different processing atmospheres. In this paper the features of this unique instrument are described and the experimental technique developed for flow property measurement is outlined. Some typical measured rheological properties of coal ashes from different Australian low-rank coals are presented and discussed to illustrate the potential applicability of the rheometer for high-temperature rheological characterisation. Furthermore, by means of the experimental results obtained it is demonstrated that the alkali content of the coal ash plays a significant role in controlling the rheological characteristics of the ash deposit, which in turn has an important implication on agglomeration in fluidised bed combustion processes.     

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.     

水煤浆气化原料的成浆性研究

在实验室条件下研究了从低煤化度烟煤到高煤化度无烟煤,以及石油焦等不同气化原料煤的成浆性.为提高低煤化度烟煤的成浆浓度,在保证其混合原料灰熔融特征温度满足液态排渣前提下,将低煤化度烟煤与一种或两种煤化度较高的煤或者石油焦配比,考察了它们的成浆性.结果表明,煤化度适中的QD煤单独制浆浓度达到70%,黏度536mPa.s,流动性为A;通过不同煤种的级配,三种原料配合的料浆浓度为62%时,黏度在340mPa.s～550mPa.s之间,可以获得符合液态排渣气化要求的混合料水煤浆,扩大了气化原料来源.     

Formulas for calculating the calorific value of coal and coal chars: Development,tests, and uses

A new formula for calculating the calorific value of coal from its ultimate analysis has been obtained by regression analysis of a data bank consisting of data on 775 samples of U.S. coals of all ranks. It yielded zero average difference between observed and calculated values and a standard deviation of 129 Btu/lb (300 kJ/kg). Neither average difference nor standard deviation varied much with rank of the coal. The Dulong, Mott-Spooner, Boie, and Grummel-Davies formulae were also tested with the data but gave substantially poorer results. The new formula has been substantiated with data from other laboratories and with data on chars. The distribution of variance, attributable to variability of mineral matter, variability of coal organic matter not related to rank, and variance of laboratory determinations was estimated. Use of the formula to monitor performance of a laboratory is illustrated, as is modification to obtain maximum precision in heat balance calculations on a coal conversion process.     

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.     

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.     

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。     

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.     

Exploring the possibilities of using Brazilian subbituminous coals for blast furnace pulverized fuel injection

The current study investigates the combustion and blast furnace injection performance of three Brazilian subbituminous coals (Mina do Recreio) and their beneficiation products using laboratory scale combustion tests. The coals have relative high ash yields (up to 40 wt%) that were reduced stepwise to levels as low as 12 wt%, dry basis. The reduction of ash yields is paralleled by a significant decrease in sulphur and inertinite contents.The combustion tests were performed in a drop tube reactor operating at 1300 °C using two different atmospheres (2.5 and 5% O₂). The chars exhibited preferentially rounded shapes with thick walls and abundant secondary porosity for the 2.5% O₂ chars, whereas the 5% O₂ chars showed very thin walls as a consequence of extensive burnout. The intrinsic reactivities of both set of chars were similar. The differences in conversion between the two working atmospheres were 24-37% and roughly tend to increase with increasing mineral matter content. Conversions as high as 76-81% were reached operating under 5% O₂ indicating that the coals are easy to burn. The small differences in burnout among the coals and their beneficiation products cannot be clearly attributed neither to mineral matter or inertinite content. A rough inverse relationship was found between the intrinsic reactivity of the chars and the inertinite content of the parent coal indicating that the char material derived from inertinite was intrinsically less reactive than that derived from vitrinite. These differences were no longer relevant at high temperature.Blast furnace injection performance was studied through thermobalance experiments using CO₂ atmosphere and 1050 °C temperature. It is apparent that the beneficiation process has no effect on the reactivity of the coals from Recreio Mine. The only exception is the low ash coal-2-LabB (11.5 wt%), for which a higher reactivity is indicated. The reactivity tests show also that the coals have adequate properties to be used together with imported coal blends in pulverized coal injection in the blast furnace (PCI).     

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.     

气化条件下煤灰熔融性研究

以CaO和Fe2O3为助熔剂,分别与7种煤样进行不同比例的混合,在气化条件下进行煤灰熔融性实验,降低煤灰熔融性温度,为生产合成气用煤的选择提供科学依据。实验结果表明,7种煤样的流动温度均能降至1400℃以下,煤灰添加CaO助熔剂时的灰熔融性温度变化比较稳定,而对Fe2O3助溶剂都较为敏感,仅在很小的含量范围内能达到最低点,而且规律性较差。     

Microstructural changes in coal during low temperature ashing

Samples of two US coals were oxidized for 72 hours in a low temperature asher. Scanning transmission electron microscopy and automated electron microprobe analysis were employed to observe the mineralogical changes produced in the coal by the ashing process. It was found that the microstructure of the low temperature ash was strongly influenced by the mineral matter which was originally chemically-bound in the organic matrix of the starting coal. The partial oxidation of pyrite during the ashing was also observed. The results reveal some of the physical mechanisms involved in the low temperature ashing process.     

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.     

Assessment of quality of New Zealand coals from borecore analyses

Methods are needed for predicting the properties of as-mined coals from analyses carried out on borecores. Statistical techniques are described for isolating the mean moisture, coal and mineral properties from sets of laboratory analyses carried out on borecores. Comparisons between mine and borecore samples from the same area can be used to derive general methods for predicting properties of mined coal. This Paper deals with the derivation of moisture, ash, voltaile matter, specific energy and elementary composition of coal and minerals for mined coal from borecore analysis results.     

由脱灰煤制备富勒烯

以 3种高变质程度的中国煤为原料 ,用电弧等离子体蒸发法制备富勒烯 .富勒烯产品的表征采用质谱、紫外光谱和红外光谱等技术 .采用碱法脱灰技术对 3种煤进行脱灰处理 ,考察了脱灰前后煤样的灰分和相应炭棒中碳含量对富勒烯产率的影响 .结果表明 ,由实验所用煤种均可制得富勒烯 .原料煤本身的化学结构和组成对富勒烯的产率有明显影响 ,表现为富勒烯的产率随炭棒中碳含量的增加而增大 .煤中的矿物质对富勒烯的形成有很强的抑制作用 ,表现为富勒烯的产率随原料煤中灰分的增加而降低 ,这一变化趋势通过对介休焦煤进行不同程度的脱灰处理而得到进一步证实.