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.     

Distribution of inorganic and organic substances in the hydrocyclone separated Slovak sub-bituminous coal

A low-rank Slovak sub-bituminous coal from the Handlová deposit was physically treated by washing in a water-only cyclone with the goal to find the separation effect for inorganic (mainly Fe-bearing minerals) and organic substances (humic acids, diterpanes). A high-quality coal product with the ash content in the dry matter of 9.02% and carbon content of C^d = 68.12% at a mass yield of 29.51% was obtained using the water-only cyclone processing. At first, the physically treated coal samples were detailed characterized by XRD, ⁵⁷Fe Mössbauer spectroscopy, FT-IR and HR-TEM. In addition to non-crystalline organic coal components, inorganic compounds belonging to silicate minerals (kaolinite, muscovite and quartz) as well as to Fe-bearing sulphide minerals (pyrite) were identified in the sub-bituminous coal by XRD. ⁵⁷Fe Mössbauer spectroscopy detected the presence of iron carbonate (siderite), iron-containing clay mineral and two sulphur-containing minerals (pyrite, jarosite) in the untreated coal. On the other hand, only one Fe-bearing mineral, (pyrite) was found in the washed coal. Effect of the physical separation is also demonstrated in FT-IR spectra, where the peak at 1040 cm⁻¹ representing the silicate component in the untreated sample is not detectable in the washed coal sample. Presence of extractive organic substances, i.e. humic acids and tetracyclic diterpane (16α(H)-phyllocladane), in the hydrocyclone products is also evidenced. It was confirmed that the isolated diterpenoic compound is attendant in the washed product with the lowest ash content and it is assimilated with the organic part of coal. Surprisingly, humic acids were found in the highest concentration in the slurry that has the highest content of ash (63.14%).     

Microbial alteration of organic matter of humic coal during biological desulphurisation

The biodegrading influence of biological desulphurisation on bituminous coal and its density fractions was investigated using gas chromatography-mass spectrometry for organic matter alteration and atomic absorption for the assessment of changes in several trace element concentrations. Changes in extract group composition were assessed by comparing the contents of aliphatic, aromatic and polar compounds separated by preparative thin layer chromatography. Aliphatic compounds show extensive alteration due to biodegradation, mainly removal of n-alkanes and lighter acyclic isoprenoids from extracts. The sterane distribution was strongly affected while hopane/moretane distributions show minor changes. Aromatic hydrocarbons were less influenced but some changes were found. It can be assumed that the degree of biodegradation of density fractions increases with increasing concentration of mineral matter since density fractions with lower mineral concentration show smaller changes as a result of biodegradation than those with higher content of minerals. Reduction of concentrations of the following trace elements occurred: beryllium, chromium, zinc, gallium, cadmium, cobalt, lithium, manganese, copper, molybdenum, nickel, lead, and vanadium. The content decrease of an element is not influenced by its geochemical properties. An equally important factor seems to be bonding to organic and inorganic coal substances.     

Ash deposition of a Spanish anthracite: effects of included and excluded mineral matter

This study has been undertaken to provide an insight into the effects of mineral matter distributions in coal on the nature of boiler slags formed during the combustion of a high ash Spanish anthracite. Three density fractions were prepared, and the light and heavy fractions were combined to give a sample containing mainly excluded mineral matter. The mineral matter in the medium density fraction was predominantly included. Slag deposits were prepared in the laboratory from the whole coal, the light+heavy and medium density fractions. The coal with excluded mineral matter was found to produce a slag similar in nature and chemistry to the whole coal ash deposit. The coal with included mineral matter produced a vitreous, iron-rich deposit. The deposits were compared with slags, obtained from a Spanish power station boiler, burning a blend containing the same anthracite.     

Individual contributions to the overall microelement concentrations in coal

The microelement concentrations in coal represent the sum of contributions from the organic mass, the mineral impurities, and the initial plant matter. These contributions are mutually independent and may differ very considerably. Generally, the microelement concentrations in coal are determined by their content in the organic mass. Identification of the individual contributions to the overall microelement concentrations in coal is of interest in assessing the rare-metal content of coal and means of extracting microelements. Unsatisfactory and often conflicting results are obtained by existing methods of identifying those contributions (by separation of the coal into fractions of different density; by comparison with the abundance ratios in the aleuropelite fraction; by correlation with the ash content). A method is proposed for statistical resolution of the individual contributions to the overall microelement concentrations in coal.     

Structural characterization of filter cake solids from a solvent-refined coal process (SRC-1)

A study has been undertaken to characterize the structural aspects of the solids that have been recovered from solvent refined coal (SRC-I). Both the mineral matter as well as the organic materials are characterized through the utilization of various analytical techniques: chemical analysis, X-ray techniques, optical microscopy and electron microscopy studies, surface area measurements, density gradient separation, electron spin resonance, magnetic susceptibility and ESCA studies. A comparison is being made of the solids from this particular process with solids derived from another SRC process. Some of the main findings of this study include the following. Major minerals are pyrrhotite and α-quartz. Organic inerts consist of fusinite, semifusinite, micrinite and macrinite. The least dense fraction is essentially organic, while the high density fraction contains much of the pyrrhotite. The e.s.r.-free-radical measurements show that the solids contain few reactive species.     

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.     

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.     

铁钙比对煤灰中耐熔矿物生成的抑制机理研究

铁基助熔剂和钙基助熔剂能有效降低煤灰熔融温度,为了研究铁钙比(Fe2O3/CaO)对煤灰中耐熔矿物生成的抑制机理,根据煤灰化学成分组成,在三种不同系列的煤中加入含铁助剂,调整煤中的铁钙比,对煤灰进行灰熔融温度、煤灰成分分析,对还原性气氛下制备的煤灰渣进行X射线衍射分析(XRD).结果表明:加入含铁助剂可降低煤灰熔融温度,在相同铁钙比下,加入Fe助剂的煤灰熔融温度低于加入FeS2助剂的煤灰熔融温度,硫在煤灰中起增加煤灰熔融温度的作用;煤灰中铁钙比不同对高熔点矿物的生成影响不同,当铁钙比在1~2间时,灰渣中仅有钙长石,当铁钙比在3.5~5.5间时,灰渣中既有钙长石的也有耐熔矿物莫来石的存在,煤灰中铁质矿物和钙质矿物的含量对耐熔矿物的生成有很大影响.     

A mathematical model of ash formation during pulverized coal combustion

A mathematical model of ash formation during high-rank pulverized coal combustion is reported in this paper. The model is based on the computer-controlled scanning electron microscope (CCSEM) characterization of minerals in pulverized coals. From the viewpoint of the association with coal carbon matrix, individual mineral grains present in coal particles can be classified as included or excluded minerals. Included minerals refer to those discrete mineral grains that are intimately surrounded by the carbon matrix. Excluded minerals are those liberated minerals not or at least associated with coal carbon matter. Included minerals and excluded minerals are treated separately in the model. Included minerals are assumed to randomly disperse between individual coal particles based on coal and mineral particle size distributions. A mechanism of partial-coalescence of included minerals within single coal particles is related to char particulate structures formed during devolatilization. Fragmentation of excluded minerals, which is important particularly for a coal with a significant fraction of excluded minerals, is simulated using a stochastic approach of Poisson distribution. A narrow-sized sample of an Australian bituminous coal was combusted in a drop-tube furnace under operating conditions similar to that in boilers. The particle size distribution and chemical composition of experimental ash were compared to those predicted with the model. The comparisons indicated that the model generally reflected the combined effect of coalescence of included minerals and fragmentation of excluded minerals, the two important mechanisms governing ash formation for high-rank coals.     

Composition and variation of pulverized fuel ash obtained from the combustion of sub-bituminous coals, New Zealand

Twenty daily samples of pulverized fuel ash (PFA), obtained from the combustion of sub-bituminous Waikato coals, have been analysed. The contents of MgO, SO₃, B₂O₃ and total and uncombined CaO in the New Zealand material were high when compared with most PFA produced in Europe and the USA. The PFA was separated into three major components, namely a magnetic fraction, and nonmagnetic acid-insoluble and acid-soluble fractions. These three fractions were related to three major mineral suites found in the coal: respectively hydrated iron carbonates and oxides, quartz and aluminosilicates, and calcium minerals. The variable composition of the PFA arises from differences in the proportions of these three mineral suites in the original coal. The composition was also found to vary with particle size. It is suggested that this was due to differences in the particle sizes of minerals in the coal, and to increased thermal decomposition of minerals in the smaller particles of PFA. A low-density fraction was separated and found to be similar in composition to cenospheres found in British and American PFA. The decomposition and fusion of a mineral common to many coals may be the source of these cenospheres.     

Centrifugal float–sink separation of fine Turkish coals in dense media

Zonguldak bituminous coal, Tunçbilek and Soma–Merkez lignites were each separated into two sub-fractions, coal rich and mineral matter rich, using a centrifugal float–sink separation technique in heavy media. An isopropyl alcohol (IPA)–carbon tetrachloride (CCl₄) mixture and a zinc chloride (ZnCl₂) solution, with a specific gravity of 1.40 g cm⁻³ at 25°C were used as dense medium liquids. The addition of surface active agents (Triton X-100 and Brij-35) to the zinc chloride solution improved the removal of minerals. The recovery and purity of the final product (float) obtained from the heavy media separation depend on such parameters as the density of the medium, rotor speed and centrifugation time. The separation efficiency of each coal differed significantly. Particle size distributions of the coals and their float and sink fractions were analysed using a Laser Particle Size Analyser. A Scanning Electron Microscope (SEM) was used to interpret the liberation of minerals from the coal particles.     

Desulfurization of a Turkish lignite at various gas atmospheres by pyrolysis. Effect of mineral matter

An investigation was made of the removal of pyritic and organic sulfur by pyrolysis at ambient pressure of a Turkish lignite under nitrogen and carbon dioxide atmospheres and the effect of mineral matter on the sulfur removal in pyrolysis of HCl and HCl/HF-treated coal under carbon dioxide atmosphere. Results obtained indicated that both pyritic and organic sulfur removal increased with increasing pyrolysis temperature. The pyrolysis in carbon dioxide atmosphere had more effect on the organic sulfur removal at high temperatures. As a consequence of treatment of coal with HCl, pyritic sulfur removal increased but organic sulfur removal decreased. This implies that the removal of carbonates from coal negatively affects the organic sulfur removal. The observed decrease in organic sulfur removal may be related to the decrease in pyrolytic conversion. It was observed that HCl/HF treatment has an increased effect on the pyritic removal and organic sulfur removal during pyrolysis. The increase in organic sulfur removal after HF-treatment therefore might be due to the removal of clay minerals in the raw coal structure. In addition, it may be said that the presence of silicate minerals in the coal matrix can be induced that the easily removable organic sulfur compounds are converted to thermally stable and non-removable organic sulfur compounds (thiophenic or condensed thiophenic compounds) at these temperatures. Increase in the pyritic sulfur removal of HCl-treated and HCl/HF-treated coal samples may be attributed to the fact that increase of mass and/or heat transport in comparison with untreated coal as a result of elimination of mineral matter.     

Hydro-treatment of a direct coal liquefaction residue and its components

Direct coal liquefaction residue (DCLR) contains a significant fraction of heavy liquids. Better use of DCLR is an important task to overall economy of direct coal liquefaction. This paper studies hydro-treatment of a DCLR and its solvent-extraction components under typical direct coal liquefaction conditions. The DCLR is found hydro-treatable, resulting in more light products and less heavy products. In one case, 40 wt% heavy fractions in the DCLR are converted into light fractions. Hexane soluble fraction (HS) is the most stable while pre-asphaltene fraction (PA) is the most active during the hydro-treatment. Some synergetic effect can be observed when DCLR is treated as whole. The fractions of DCLR, before and after the hydro-treatment, are subjected to FTIR and NMR analyses, which show increase in hydrogen content and decrease in oxygen content in HS fractions after the hydro-treatment. GPC analysis reveals that the hydro-treatment results in slight decreases in molecular weight of HS fraction. The data seem to suggest that separation and hydro-treatment of liquid fractions from a DCLR is a viable option for high oil yield.     

Coal and coal ash characteristics to understand mineral transformations and slag formation

A knowledge of the composition and structure of minerals in coal is necessary in order to understand the mineral transformations and agglomerate or slag formation during combustion or gasification. Coal ash fusibility characteristics are difficult to determine precisely, partly because the ash contains many components with different chemical behaviours, and may vary from coal source to coal source.The first objective of this study was to determine if the most relevant characteristics of coal were representative of the typical coal from the South African Highveld region. Secondly, a detailed understanding of the coal and coal ash are needed in order to explain slag formation and mineral transformations.Based on standard coal properties, such as the ash content, volatile content, carbon content and maceral composition, it can be concluded that the coal sample used for this study was representative and comparable with the coal from the Highveld region.From the results obtained and the analysis done on the coal samples, it was observed that the mineral grains showed a wide range of types that ranged from pure coal to pure minerals. The types of mineral particles within the coal range from large irregular minerals to small irregular minerals on the edge of coal particles. Kaolinite and quartz can occur as fine inclusions in carbon rich particles or associated with mudstone, siltstone or sandstone, together with kaolinite infillings. The main minerals present in the coal feed are kaolinite, quartz, dolomite, calcite, muscovite, pyrite and microline. An abundance of calcium-rich particles, which are probably calcite and dolomite, were observed. These minerals are present throughout the coal structure and are not specific to one type of mineral grain or structure. An increase in Si and Al abundance in three different prepared coal fractions with increasing particle size distribution was observed the high density fractions are mainly situated in the coarser particles.After combustion or gasification, the major source of glass is derived from included minerals in carbon rich particles. It is clear that focus on the modification of the unclassified/amorphous phase, to increase viscosity (decrease slag formation or have a higher concentration of crystalline phases) at a certain temperature, or in general terms the ash fusion temperature of the coal, is important. Altering the ash chemistry involves the addition of a material to the coal to increase the viscosity.     

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.     

The nature of solids accumulated during solvent refining of coal

Solids accumulated in the reactor of a solvent-refined coal (SRC-1) pilot plant during processing of three coals were studied using optical microscopy and X-ray diffraction. A classification system was devised for each of the two groups of components: organic materials and mineral matter. The various organic components were classified by the extent of change from the original coal macerals, and by optical properties of different semi-cokes and other organic phases. Minerals were divided into four groups: those unchanged from the original coal; minerals which were physically degraded; minerals which were chemically or crystallographically transformed; and those minerals formed during processing of a subbituminous coal. Gold-tube carbonization experiments were performed on SRC to determine the conditions under which retrogressive reactions occur to form mesophase semi-coke. Autoclave experiments were designed to investigate the recrystallization of pyrite as pyrrhotites, and to determine the causes of carbonate-mineral formation in the reactor. Calcium carbonate was found to crystallize from the interaction of ion-exchangeable calcium and carbon dioxide, which are available when low-rank coals are processed.     

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.     

Inherent mineral matter in coal and its effect upon hydrogenation

The problems associated with assessing the catalytic effects of inherent mineral matter in coal during hydrogenation are discussed in detail. Preliminary results obtained from various relative density fractions of a South African high-mineral-matter coal are reported. Inert matter has been added in an attempt to minimize the complicating effects of agglomeration of the coal particles in order to gain a greater understanding of the relative importance of maceral versus mineral content of the coal during hydrogenation. It is difficult to determine any ‘catalytic effect’ of mineral matter in coal during hydrogenation since the mineral matter appears to be acting in a physical capacity as an internal diluent, reducing agglomeration of the coal particles. This decrease in agglomeration allows more effective diffusion of hydrogen to the sites of bond rupture and products away from the coal particles as well as maintaining a higher overall surface area for reaction.     

Trace element affinities in two high-Ge coals from China

The Lincang (Yunnan Province, Southwest China) and Wulantuga (Inner Mongolia, Northeast China) coal deposits are known because of the high-Ge content. These coals have also a high concentration of a number of other elements. To determine the mode of occurrence of the enriched elements in both coals, six density fractions from <1.43 to >2.8 g/cm³ were obtained from two representative samples using heavy-liquids. A number of peculiar geochemical patterns characterize these high-Ge coals. Thus, the results of the chemical analysis of these density fractions showed that both coals (very distant and of a different geological age) are highly enriched (compared with the usual worldwide coal concentration ranges) in Ge, As, Sb, W, Be, and Tl. This may be due to similar geochemistry of hydrothermal fluids influencing the Earth Crust in these regions of China. Moreover, Wulantuga coal (Early Cretaceous subbituminous coal) is also enriched in Ca, Mg, and Na, and Lincang coal (Neogene subbituminous coal) in K, Rb, Nb, Mo, Sn, Cs, and U. A group of elements consisting of Ge, W, B, Nb, and Sb mostly occur with an organic affinity in both coals. Additionally, Be, U, and Mo (and partially Mn and Zn) in Lincang, and Na and Mg in Wulantuga occur also with a major organic affinity. Both coals have sulfide-arsenide mineral assemblages (Fe, S, As, Sn, and Pb, and in addition to Tl, Ta, and Cs in the Lincang coal). The occurrence of Al, P, Li, Sc, Ti, V, Cr, and Zr in both coals, and Ba in Lincang, are associated with the mineral assemblage of silico-aluminates and minor heavy minerals. Furthermore, P, Na, Li, Sc, Ti, Ga, Rb, Zr, Cr, Ba, Th, and LREE (La, Ce, Pr, Nd, and Gd) in Lincang are associated with mineral assemblages of phosphates and minor heavy minerals. The two later mineral assemblages are derived from the occurrence of detrital minerals. Finally, the two coal samples have also the sulfate mineral assemblage (Ca and Sr) that probably occur as a consequence of a diagenetic oxidation and alteration of the coal seams. The enrichment of Ge in coal occurred when the organic matter was still reactive to trap Ge, but several features indicate that the enrichment was diagenetic.