Study on the structure and gasification characteristics of selected South African bituminous coals in fluidised bed gasification

The gasification characteristics of three South African bituminous coals were investigated in a bubbling fluidised bed reactor. The three coals are similar in rank, but two are inertinite-rich coals and the third has a high vitrinite content. The microstructural characteristics of the parent coals and their resultant chars were determined using XRD, FT-IR, Raman and petrographic analysis. The microstructural changes that occurred in the organic (maceral) and the inorganic (mineral) fractions of the selected coals were evaluated. The change in the carbon structure was correlated to the proportions of inertinite and vitrinite macerals in the coals. High vitrinite content resulted in an increase in the order of the disordered carbon structure after gasification and this leads to greater graphitised ordered carbon structures. While a high inertinite content resulted in low or no structural transformation of the chemical structure. The transformation of inorganic mineral constituents of the coal was correlated to the amount of inertinite present in the selected coals. Higher proportions of inertinite macerals and inertinitic chars resulted in higher proportions of melted minerals. Char samples with low proportions of organic matter resulted in higher proportion of melted minerals covering the char surface.     

High-resolution density variations of coal macerals

A number of coals have been separated into their maceral components by density gradient centrifugation. Each maceral group exhibits a distribution of densities. In general, exinite distributions do not overlap with those of vitrinite. However, vitrinite density distributions overlap significantly with inertinite distributions. This behaviour places inherent limits on the ability to completely separate macerais using density measurements alone. The density distribution of vitrinites and probably exinites has been found to narrow as rank increases.     

陕北低阶烟煤显微组分结构特征及分子结构模型

为了更好地利用我国丰富的低阶煤资源,以变质程度较低的陕北低阶烟煤镜质组、惰质组富集物作为研究对象,采用常规的煤质分析、傅立叶变换红外光谱、高分辨固体13C核磁共振、X射线衍射等分析表征手段,对样品结构进行全面分析,得到了陕北低阶烟煤显微组分分子结构特征及结构参数,建立了镜质组和惰质组的部分分子结构模型。结果表明,陕北低阶烟煤镜质组和惰质组氧含量均较高,硫含量很低,所选镜质组的芳碳率为0.634,平均缩合环数约为2.98个;惰质组的芳碳率为0.734,平均缩合环数约为3.15个;相对于镜质组,惰质组的缩合程度更高,芳核尺寸更大,排列也更加规则有序。今后低阶烟煤加工转化工艺的研究应从分子层面揭示其不同显微组分结构特征的差异性,为低阶烟煤清洁高效利用新工艺的开发奠定基础。     

Thermochemical properties of coal fractions of different density: A review

Research is reviewed regarding the composition and thermochemical properties of coal fractions of different density. The properties discussed are the thermogravimetric parameters; the free-swelling index; the plastic-layer thickness; the dilatometric characteristics; the yield of fluid (mobile) components of the plastic mass. These properties are determined by the petrographic (maceral) composition of the fractions. With increase in density of the fractions, the liptinite and vitrinite content declines, while the inertinite content grows. Consequently, the chemical structure becomes less aliphatic and more aromatic, with corresponding deterioration in thermochemical properties of the fractions. The results show that fractionation of the coal prior to processing is potentially very effective.     

煤显微组分的性质及燃烧特性研究

本文根据元素分析、工业分析以及差热分析等数据研究了平朔煤、红阳煤、大同煤和东胜煤的显微组分的性质及燃烧特性。结果表明，壳质组具有最大Ｈ／Ｃ比和最高挥发分含量，镜质组的Ｈ／Ｃ比和挥发分含量高于隋质组；显微组分的粘结性，镜质组大于惰质组；镜质组和惰质组相比较，镜质组的着火点低、燃尽温度也低，具有较窄的燃烧区间；壳质组的着火点最低，但是燃尽温度要高于镜质组。     

The estimation of char reactivity from coal reflectogram

Measurements of the intrinsic reactivity of chars to oxygen are increasingly being sought as an indicator of the combustion potential of fuels. The coal reflectogram has been used to characterize the chemical properties of coal and its resultant char structure. In this study, six Australian coals varying in rank were separated using density separation technique to obtain vitrinite and inertinite rich fractions. Chars were obtained from these density fraction samples in a Drop Tube Furnace (DTF) at 1673 K. The reactivity of the chars was measured non-isothermally in a Thermal Gravimetric Analysis (TGA) in the temperature range of 573–1073 K. The results suggested that with the increase in the coal rank, the maximum reactivity of chars derived from vitrinite rich fractions decreases, while the reactivity of chars derived from inertinite rich fractions decreases with the increase in the inertinite content in samples and has no obvious relationship with rank. The kinetic parameters were derived using data from non-isothermal TGA after accounting for changing in surface area with conversion. The frequency factor is found to decrease with increasing coal FMR, defined as the summation of each reflectance value multiplied by its frequency, for a constant activation energy (E=146 kJ/mol). This suggests that the behavior of a maceral is characterized primarily by its reflectance distribution instead of the type of its parent coal.     

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.     

Coalification paths of exinite,vitrinite and inertite

Coalification is a burial metamorphic process fundamental to understanding the origin and nature of coals and petroleum. Accurate assessment of Coalification requires removal of the variation in coal properties owing to differences in coal type. This is best achieved by assessing the coal rank of the maceral groups exinite, vitrinite and inertinite (and ultimately the macerals within each group) in terms of maximum reflectance. The mean maximum reflectances of exinite (R?_E max), vitrinite (R?_v max) and inertinite (R?_I max) are highly correlated over the rank range, soft brown coals to low-volatile bituminous coals. A single exinite Coalification break is identified at 0.4 to 0.5% R?_V max. Thereafter, exinite coalification occurs at an increasing rate (R?_E max relative to R?_V max) over the rank range 0.5–2.0% R?_V max, crossing the vitrinite Coalification path at 1.55% R?_V max. A phase of rapid inertinite Coalification (0.7–2.0% R?_I max) occurs between 0.2 and 0.9% R?_V max. Since exinite is a major land-plant source of hydrocarbons, and R?_E max is a sensitive indicator of organic maturation over the range 0.2–1.1% R?_E max, which corresponds to the main phase of oil generation, R?_E max can be a most useful indicator of petroleum generation. The inertinite lnkohlungsprung represents a dramatic compositional change which should be taken into account when considering the origin of inertinite, its utilization properties and the general processes of rank change and organic maturation.     

The char structure characterization from the coal reflectogram

Coal is a heterogeneous substance and its heterogeneity is identified and characterized by variation in reflectance. The main objective of this paper is to characterize the heterogeneity of char and to correlate it with the coal reflectogram, which accounts for both rank and maceral composition effects. Chars from two density fractions in a set of coals were obtained in a Drop Tube Furnace (DTF) at 1400 °C in N₂ environment. The chars were examined under a Scanning Electron Microscope (SEM) and the morphology information was obtained from the image-processing technique. The average porosity of char changes systematically with the FMR of its parent coals (defined as the summation of each reflectance multiplied with its frequency). The char porosity increased with an increase in FMR up to a critical value around 98. With further increase in FMR, the corresponding char becomes dense. The char macro porosity distribution was found to be related to the coal reflectogram. In general, the char porosity distribution shows two peaks, which corresponds to the inertinite and vitrinite peaks in reflectogram. The intensity depends on the maceral content. The relationship between the char porosity and coal reflectance for this set of sample has been found, which is strongly dependent on the coal rank. However, these findings cannot be applied to coals with a strong maceral association (microlithotype).     

Chemical variations in coal macerals separated by density gradient centrifugation

Using density gradient centrifugation, 22 coals with varying maceral composition and rank have been separated. The carbon, hydrogen and nitrogen contents of selected density fractions have also been obtained for each coal. Examination of the data shows that the density gradient procedure provides information not only on the differences between maceral groups, but also within maceral groups. In general, as density increases the HC ratio was found to decrease across each maceral group density band. Although more variable, the OC ratio tended to increase across the maceral bands. In addition, the aromaticity and ring index calculated from the data show that, for many coals, both parameters increase through a given maceral distribution. From the data, it has been concluded that density gradient centrifugation can provide a more detailed insight into the nature of coal and coal macerals.     

神东煤显微组分与基本结构特征研究

对神东煤显微组分及其反射率分布特征进行了详细阐述．利用工业分析和元素分析结果及结构经验公式,研究了神东煤的结构特征与煤岩组成的关系．利用^13C-NMR对神东煤显微组分进行了分析测试．结果表明,神东煤显微组分及其亚组分具有各自不同的反射率分布范围,煤中低反射率的惰质组含量较高．挥发分、氢含量、氮含量和氢碳比随惰质组含量的增加而下降,碳含量、芳碳率和缩合环数随惰质组含量的增加而增加．^13C-NMR测得镜质组和惰质组的芳碳率分别为0．67和0．81．     

Dependence of carbon dioxide sorption on the petrographic composition of bituminous coals from the Czech part of the Upper Silesian Basin, Czech Republic

The effect of the rank and of the maceral composition of bituminous coal on carbon dioxide sorption capacity was studied on the basis of samples from two coal mines (Darkov, ?SM) from the Czech part of the Upper Silesian Basin. The samples from the two mines cover a small but very significant section of coalification within the transition zone between high-volatile bituminous A coal and medium-volatile bituminous coal, where porosity and sorption properties pass through their minima. The coal porous system was characterized by the micropore volume evaluated using the sorption isotherm of carbon dioxide and the volumes of meso-, macro- and coarse pores were determined by high-pressure mercury porosimetry. The micropore fraction in the coal porous system ranged between 53% and 75%. It was particularly high in coals with high vitrinite content, namely collotelinite, and also in coals with high inertinite content. The carbon dioxide sorption capacity was determined from the carbon dioxide sorption isotherms measured using a gravimetric sorption analyzer at 298 K until a relative pressure of 0.015 p/p_s, and was interpreted by characteristic parameters of the Dubinin and Langmuir equations. It was found that the adsorbed amount of CO₂ in the ?SM coal increases with the content of vitrinite and collotelinite, whereas no increase or only a slight increase was observed for the Darkov coal. The tendency of adsorption capacity to depend on maceral composition, and also to some extent on coalification, observed for the ?SM coal, may be related to higher microporosity due to the coalification process or oxidative processes leading to the formation of pseudovitrinite.     

Contribution of vitrinite macerals to the liquefaction of subbituminous coals

Four Alberta subbituminous coals were selected to investigate the contribution of vitrinite macerals to liquefaction. There are indications of a rectilinear correlation between conversion yields of different density fractions of coal and their vitrinite content, but it is too early to discern any conversion factor by which any maceral group may be used to predict the liquefaction behaviour of a subbituminous coal. Contrary to common belief, not all the vitrinite fraction of the feed coal is reactive. There seems to be a definite positive correlation between the percent vitrinite reacted and the liquefaction conversion yield. Much more research work is needed to further understand the contribution of vitrinite macerals to liquefaction of a subbituminous coal.     

Heavy media separation of SRC-II feedstocks: 1. Effect on coal properties

The objectives of this investigation were to determine the effects of coal preparation on the properties of Run-of-Mine (ROM) and washed Powhatan and Ireland Mine coals and to assess the potential effects on SRC-II liquefaction yields. The effect of washing on the two coals was found to be quite similar. For both coals, the properties were altered more significantly by changes in separation media gravity than by changes in the coal size. The elemental composition of the Powhatan and Ireland washed coals was correlated with carbon content. It was shown that both the hydrogen and oxygen levels increased linearly with the carbon content of the coal samples. However, the $\text{H}\text{C}$ and $\text{O}\text{C}$ ratios were not changed significantly by coal cleaning. Only small variations in the nitrogen and organic sulphur levels were observed while the sulphate sulphur and chlorine levels were not affected by coal cleaning. The major impact of the coal cleaning was to reduce the pyritic sulphur (and hence the total sulphur) content of the coals. Most of the pyritic sulphur was shifted into the middling coal and refuse fractions while the clean coals had much lower contents and the pyritic sulphur level decreased with increasing carbon content. Coal cleaning did not significantly alter the maceral contents of vitrinite, exinite, total reactive macerals (TRM), or the reflectance of vitrinite; all these parameters varied over a very narrow range, probably within the precision of the measurement technique.     

炼焦煤煤化度表征的研究

对8组挥发分相同或接近的不同矿点炼焦煤进行了实验,分别测定了8组煤的煤岩组分和镜质组平均最大反射率、软化起始温度,通过对比分析可知:具有相同或相近挥发分的煤,由于煤岩组分的差异,镜质组平均最大反射率的差值达到0.03%~0.24%;并不是惰质组含量的差值越大,镜质组平均最大反射率的差值就越大,惰质组含量差值8%~9%的炼焦煤,镜质组平均最大反射率的差值几乎一致。建议炼焦企业尽可能通过镜质组平均最大反射率来认识炼焦煤的煤化程度,从而提高煤质技术理论水平。     

中国西部弱还原性煤的结构特征初步研究

对中国西北地区弱还原性、高惰质组煤结构特征进行了初步研究．与同变质程度的其他成煤时代煤相比,中国西北侏罗纪弱还原性煤普遍具有较低的H／C原子比和较高的O／C原子比,同时其芳碳率和分子结构中环缩合程度相对较高．随惰质组含量增加,弱还原性煤芳碳率和平均缩合环数增高．X射线衍射（XRD）研究表明,惰质组芳环缩合程度更高,芳香层片在空间的排列更规则,相互定向的程度高于镜质组．核磁共振（^13C—NMR）研究表明,惰质组分子结构中苯环取代更多,脂肪部分所占比例少．对弱还原性煤中广泛分布的惰质组组分及其分子结构特征需进行深入研究．     

Dynamic nuclear polarization 13C n.m.r. of coal

A novel n.m.r. method for determining the aromaticity of coal has been evaluated. Polarization transfer from the paramagnetic electrons is used to enhance the intensity of the ¹³C n.m.r. spectrum. Analysis of five samples of Gondwana bituminous steam coals shows that the aromaticity of inertinite is far greater than that of vitrinite and that variation of maceral composition is a major factor determining coal aromaticity. This has been confirmed by the study of maceral concentrates.     

The variation of structural characteristics of macerals during pyrolysis

Vitrinite and inertinite were separated by DGC from Chinese Shenmu bituminous coal and the structural characteristics of the macerals, before and after pyrolysis, were analyzed by ultimate analysis, FTIR and ¹³C NMR. The results showed that vitrinite chars always had higher H and lower C content than inertinite char at the same pyrolysis temperature. The FTIR and ¹³C NMR indicated that vitrinite had more aliphatic C-H, hydrogen bonding and lower aromaticity. With increasing temperature, the aliphatic C-H decreased, aromatic C-H, aromaticity and H_ar/H_al ratio increased. At the same temperature, inertinite always had higher H_ar/H_al ratio than vitrinite, which is consistent with that inertinite had higher aromaticity than vitrinite. And the H_ar/H_al ratio was also related to the remainder volatile matter. With increasing H_ar/H_al ratio, the remainder volatile matter in vitrinite and inertinite decreased. The higher aromaticity and H_ar/H_al ratio and lower H content of the inertinite in all temperature range were correlated with its higher thermal stability and lower volatile yield than vitrinite.     

Proposal for coal classification

Several critiques to the ‘International Classification of Hard Coals by Type’ (ECE, Geneva, 1956) have been made in the past and a revision of its usefulness is currently being carried out by several investigators. The principal drawback of this classification is that it is not applicable to coals of variable maceral composition, especially those displaying a high content of inertinite. Furthermore, the parameters hitherto used in the International Classification and in some other national systems to define degree of coalification (rank), i. e. volatile matter and calorific value, are dependent on variable maceral composition. On the other hand, the parameters used in the International Classification to determine the agglutinating and coking properties of coals are competing parameters, instead of following a hierarchy. The proposed classification scheme is based on two primary parameters determined with microscopic techniques: (1) mean maximum reflectance of vitrinite, which is a good single measure of rank; and (2) petrographic composition (vitrinite and exinite) as an indication of the type of coal. A third parameter is chosen to qualify the different classes of coal: volatile matter for anthracitic coals; dilatation for semianthracitic and bituminous coals; and calorific value for subbituminous coals and lignites. The scheme is expressed by mean of a code number of four digits, which refers to the rank (first digit), type (second and third digits) and qualification (fourth digit) of coal.     

Dependence of coal liquefaction behaviour on coal characteristics. 2. Role of petrographic composition

The techniques used were the same as those used in Part 1 (p 34). Comparison of the liquefaction behaviour of two lithotypes from a Kentucky bituminous coal indicated that in this process pseudovitrinite is a reactive maceral. The hydrogenation of sets of maceral concentrates obtained from a New Mexico sub-bituminous and a Kentucky bituminous coal showed fair correlations between conversion and the total concentration of the presumed reactive macerals (vitrinite, pseudovitrinite and sporinite). Similar concentrates from a Montana lignite showed no such correlation; the one sample that showed a high conversion was a high-density fraction that had a high mineral-matter content and in which nearly all the pyrite in the coal had accumulated. Two samples that have boghead and cannel characteristics gave quite different results on hydrogenation. Both were highly aliphatic in structure and had unusually high hydrogen contents and volatile matter. One, which contained appreciable proportions of sporinite, alginite and resinite, gave essentially no conversion to oil. The other, predominantly vitrinitic but containing alginite as the second most abundant maceral, gave an excellent yield of an oil of low viscosity and aromaticity. It was concluded that although rank, petrographic composition and perhaps geological history are important factors determining liquefaction behaviour, there are other characteristics of coals that may at times override these basic parameters, and the composition of the inorganic matter may be the most significant of these other characteristics.