Water-soluble chlorine and associated major cations from the coal and mudrocks of the Cannock and North Staffordshire coalfields

Samples of high-Cl coals and their immediate roof, floor and intraseam dirt bands were analysed for major water-soluble ions in order to predict Cl leachability. In organic-poor mudrocks the total CI is as NaCI groundwater solution which is 100% water-soluble. In the coals the absolute water-soluble CI levels are higher, but even with ultrafine grinding 100% extraction cannot be achieved. The relation of the water-soluble Cl to Na in the coarse fractions and Ca in the ultrafine samples suggests different mechanisms of Cl holding and release. Dull coal has a higher water-soluble Cl content that bright coal, which may be explained by the greater size of dull coal pores and greater accessibility of these pores to the groundwaters. The groundwater Cl is Na-associated and leached with relative ease from coarse samples; it accounts for approximately one-third of the total Cl. For ultrafine samples there is a close correlation between water-soluble Cl^?, Ca²⁺, HCO^?₃ and pH. This Cl^? is thought to be combined with the organic matter and released by ion exchange (OH^? for Cl^?), the acidity created being balanced by carbonate solution. Fine grinding is essential to create a maximum surface area for exchange reactions to take place. If the groundwater Cl concentrations are high enough, they may override the total Cl-coal type relationship, but if the water-soluble Cl is leached from coal thus affected, the ‘normal’ relationship of highest total Cl in bright coal and lowest in dull coal holds true.     

Combustion behaviour of Bowen Basin coals

Tightly constrained experimental thermogravimetry proves to be effective for characterising the effects of rank and maceral variations on Bowen Basin coal combustion behaviour. These coals show an increase in char burnout temperature with rank for both dull, inertinite-rich and bright, vitrinite-rich coals. Most dull coals have higher char burnout and peak combustion temperatures than their bright rank equivalents. The maximum rates of combustion for dull coals are lower than their bright counterparts, with the difference between the two varying with rank.     

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.     

Petrography and geochemistry of the Middle Devonian coal from Luquan, Yunnan Province, China

Coals from Luquan, Yunnan Province, China, have high contents of cutinite and microsporinite, with an average of 55 and 33.5 vol%, respectively, (on a mineral-free basis). The coals are classified as cutinitic liptobiolith, sporinite-rich durain, cutinite-rich durain, and sporinitic liptobiolith. These four liptinite-rich coals are often interlayered within the coal bed section and vary transversely within the coal bed. The vitrinite content varies from as low as 1.6-20.5% (mineral-free basis), and it is dominated by collodetrinite, collotelinite, and corpogelinite. The maceral composition may be attributed to the type of the peat-forming plant communities. Moreover, the Luquan coals are characterized by high contents of volatile matter, hydrogen, and oxygen, and the high values of the atomic hydrogen to carbon ratio as a result of the maceral composition. As compared with the common Chinese coals and the upper continental crust, the Luquan coals are enriched in Li, B, Cu, Ga, Se, Rb, Mo, Ba, Pb, Bi, and U, with averages of 99.9, 250, 111, 24.4, 4.55, 130, 58.8, 1276, 162, 3.85, and 34.1 μg/g, respectively. The SEM-EDX results show that V, Cr, Ga, and Rb occur mainly in clay minerals, and Cu and Pb are associated with clay minerals and pyrite, and Mo and U are mainly in clay minerals and organic matter. Barite and clay minerals are the main carrier of barium. The high B and U contents are probably resulted from deep seawater influence during coal formation.     

Trace element concentration reduction by beneficiation of Witbank Coalfield no. 4 Seam

South Africa remains the 5th largest producer and joint 4th largest exporter of coal in the world. It is also a major supplier of coal to the European Union. This is significant as the European Union has recently supported the environmental lobby that threatens the combined full scale use of coal in Europe and other first world countries. This promotes the development of clean coal technologies in order to counter the ever increasing number of environmental constraints threatening the export market. One critical development in clean coal technologies is coal beneficiation, which allows the reduction of mineral content. Permian coals from South Africa have characteristically high ash and inertinite contents and therefore require further beneficiation. The no. 4 Seam in the Witbank Coalfield is no exception, and it can be described as containing higher inertinite content and minerals compared to the no. 2 Seam in the same Coalfield. Beneficiation, therefore, is an important requirement for improving the quality of the coal, especially for export purposes.With the increase in environmental legislation and the drive towards “clean coal” a concern is raised in terms of the performance and marketability of export coal produced from the no. 4 Seam in the Witbank Coalfield. This seam is economically significant and remains an important source of export steam coal.Due to the nature and composition of the no. 4 Seam, coal beneficiation is essential to reduce the mineral content to be in line with export quality specification levels. This paper focuses on the association of the trace elements within the seam with the organic and inorganic components and possible methods of trace element reduction by removal using coal beneficiation techniques. The techniques investigated include reduction by dense medium beneficiation and flotation. The associated mineral reducibility investigations included mineralogical distribution and liberation analysis. By studying the relationship between coal mineralogy, petrography and trace element distribution, methods of optimum trace element reduction established. Furthermore, the distribution of mineralogical and organic components of sulphur in the Witbank Coalfield No. 4 seam was found to bear unique relationships with trace elements of special concern. By assessing the distribution and occurrence of trace element concentrations during washability and floatability tests, data was produced which could now be used in the promotion of saleable coal products from no. 4 seam in the Witbank Coalfield.     

Kinetics of swelling and plasticity of coal during rapid pressurized pyrolysis and hydropyrolysis

The change of vitrain and durain particles from three high volatile bituminous coals pyrolysed in a wire net apparatus has been filmed with a high speed camera. Results are given on swelling kinetics and plasticity data for a particle of size up to 1 mm, with pressures up to 10 M Pa of hydrogen and rates of heating, 55–900 K s⁻¹ in a helium atmosphere. Also, a correlation between swelling and tar formation is established, which can be used for predicting the change of coal particles in coal conversion reactors.     

Trace element geochemistry of the Obed Mountain deposit coals, Alberta, Canada

This study reports on the elemental concentrations and vertical variation of coal seams from the Obed Mountain deposit, Alberta Foothills, Canada. Results from two sections of Seam 1 show that the major elements (i.e. Al, Fe, Mg, K, Na, Ti, and Si) have high concentrations in intervals having high ash content, with the only exception of Ca. Similarities are apparent, in both sections, in the vertical variations of Th, U, Se, and Zn; Rb, Cs, and K; Sb, Mo, and W; Mn and Sr; and Ba, Cr, Co, Hf, and Sc. These similarities are also evident among the REEs, notably between Ce and La; also between Dy, Eu, and Sm. Most elements, with the exception of Ba and Sr are slightly more concentrated in Section 2 of Seam 1, located approximately 1.5Fig. km away from Section 1. Compared to Seam 1, Seam 2 has lower mean concentrations of elements. Boron in the coal ranges from 27 to 100 ppmw, though most values are less than 50 ppmw. Boron concentrations suggest a freshwater depositional environment. The element is depleted in the sedimentary partings (12–29 ppmw only) and is enriched in the coal interval near the roof and immediately beneath the partings. This enrichment shows possible downward mobilization of boron. Vertical variations of elements are helpful in delineating the boundaries between coal and sedimentary partings in the succession. The Obed Mountain coals are “clean” by world standards and their elemental concentrations are comparable with those in coals of a lower rank from the same coal formation used for power generation in Alberta. All sedimentary partings have low concentrations of Ba, Hf, Sc, Sr, Ta, Th, U, and REEs; this, along with the absence of an Eu negative anomaly suggests a non-volcanic origin for the partings.     

Carbons prepared from coals for anodes of lithium-ion cells

Carbons were prepared by pyrolysing eight different coals from the Argonne Premium coal sample bank at temperatures near 1000 °C. Electrochemical cells were made to study lithium insertion in these carbons. The electrochemical behavior and physical properties of these pyrolysed coals was compared to that of pyrolysed sugar, PVC and pitch which we have studied carefully before. Using powder X-ray diffraction and small angle X-ray scattering (SAXS), we show that pyrolysed coals which have the smallest fraction of parallel-stacked graphene layers and hence the largest number of nanoscopic pores per unit mass, have the largest capacity for lithium insertion. A reversible capacity of about 450 mAh/g was attained for lithium/carbon cells made from Blind Canyon Seam coal.     

Product compositions in rapid hydropyrolysis of coal

The rapid hydropyrolysis of a Montana lignite and a Pittsburgh Seam bituminous coal has been examined at a high heating rate (1000 °C s^?1). The results are interpreted in the light of those presented previously for pyrolysis of the two coals. A complex situation involving simultaneous chemical and mass transfer rate control is revealed, and it is apparent that the pyrolysis phase cannot be treated separately from the hydropyrolysis phase. Three key process variables, hydrogen pressure, temperature, and particle diameter, are seen to have major effects on the total yields of products. There appears to be an important tradeoff between high hydrogen pressures and high temperatures. Under the present conditions of almost zero vapour residence time at elevated temperatures, methane is the principal product of reaction between hydrogen and coal, and the yields of light aromatic liquids are small.     

The mode of occurrence of chlorine in high volatile bituminous coals from the Asturian Central coalfield

The occurrence of chlorine in two high volatile bituminous coals from the Asturian Central coalfield, with a chlorine content about 0.1 wt % has been discussed. The methodology used is based on obtaining the chlorine distribution in different densimetric fractions in gravimetric solutions free of chlorides. It allows the separation of elements that could be bonded to chlorine to study their likely association. Additional information has been obtained by carrying out studies on the solubility in water, and the relation of chlorine with alkaline and alkaline-earth elements. Direct evidence of the presence of chlorine and its chemical association has been gained by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). From the results it may be concluded that in the coals studied chlorine is mainly localized in the coal matrix associated to organic matter.     

Preliminary studies on the aromaticity of Australian coals: Solid state n.m.r. techniques

The aromaticities of samples from nine Australian coal seams, including pairs of hand-picked vitrains and durains, have been determined by ¹³C cross-polarization n.m.r. spectroscopy with magic angle sample spinning. The results clearly show that the aromaticity (f_a) of the coals increases with increase in vitrinite reflectance and carbon content and decrease in atomic H/C ratio. For a given coal seam, durain (inertinite-rich coal) has a higher f_a value than vitrain (vitrinite-tich coal). The trends for carbon content and atomic H/C are in good agreement with results from North American coals, although the aromaticities of Australian coals obtained in this study appear to be slightly lower than some of those reported for North American coals of similar carbon content.     

Change in physical characteristics of Roland Seam coal with progressive solvent extraction

The physical structure of Roland Seam coal was characterized by studying the surface areas, pore volumes, and pore distributions of raw and extracted coals. BET surface areas were measured by carbon dioxide adsorption at 195 K and nitrogen adsorption at 77 K. Pore volumes were calculated by the difference in the reciprocals of measured apparent and actual densities. Distribution curves were generated by measuring mercury penetration as a function of pressure. The data gathered suggest a micropore structure having a 3.4 nm average diameter and random constrictions of about 0.4 nm. Extraction generates new micropores and degrades the micropore structure for yields in excess of 30%. Changes in the pore distribution and the generation of new pores indicate that resistance to extraction due to the pore structure is minimal; pore-volume data confirm the preferential liquefaction of lower-molecular-weight material.     

Properties and rheology of coal-water mixtures using different coals

The properties, settling rates, and the rheology of coal-water mixtures (CWM) made up from different coals were investigated. Test suspensions containing different concentrations of suspended particles were made up from each of four different pulverized parent coals: a Pittsburgh Seam No. 8 coal, an Illinois coal, and the product and feed from a flotation process using an Illinois coal. All the parent coals had broad particle-size distributions. In addition, the Pittsburgh Seam No. 8 coal was classified into four different size fractions using sieving, resulting in three coarser, narrow particle size cuts and the fine residual fraction passing 325-mesh, which had a broad size distribution. The property characterizations for all coals included the following: elemental analyses, heating values, particle size analyses, particle surface areas and pore sizes, solid heat capacities, and thermal conductivities. Tests on the suspensions included detailed supernatant ion analyses, and measurements of pH values and zeta potentials. For each test coal maximum-packing volume fractions were estimated using centrifugation, and the settling rates and directly measured yield stresses, using the vane method, were determined for every concentration of coal-water mixtures (CWM) used. The shear-stress/shear-rate dependences of the test suspensions, covering the shear-rate range from 0.1 to 10⁵ s⁻¹, were determined using a capillary rheometer. For some of the test suspensions, correction for a pronounced wall-slip effect was required. Apart from the directly measured yield stresses using the vane method, yield stresses were estimated indirectly by extrapolation and rheological model-fitting. Extrapolation of directly measured yield stresses to infinite limit was used to estimate maximum packing for comparison with those determined from the irreducible sediment volumes using centrifugation. The two-parameter power-law, Bingham plastic and Casson empirical rheological models, and the three-parameter Herschel-Bulkley and Sisko models were used to fit the shear-stress/shear-rate data. In general, the shear-stress/shear-rate dependence was found to be shear-thinning, power-law, over the lower ranges of shear, and to tend to Newtonian limit at high shear; a dependence which is best described by the three-parameter Sisko model.     

Semi-quantitative ESCA examination of coal and coal ash surfaces

Four North American coals and their ashes were examined using the direct, surface sensitive technique of X-ray photoelectron spectroscopy (ESCA). Two US National Bureau of Standards reference coals (SRM 1632a, SRM 1635) and one ash (SRM 1633a) were used for instrument calibration. These results verified the semi-quantitative nature of ESCA and its previously determined element detection level of ≈ 10^?9 gm cm^?2 of surface (≈0.1 bulk wt%). Thus, major elements and surface concentrated trace elements were detectable for these samples. ESCA detected elements present in the coal and/or ash in different chemical environments, for example sulphur as the sulphide or sulphate and carbon as graphite, carbonyl, carboxyl or hydrocarbon. The results of this preliminary study indicate that ESCA is useful to elucidate element siting within coal; information that is difficult to determine by analytical techniques requiring sample destruction prior to analysis. Several raw coals contained fluorine and chlorine near the one per cent level. Ashing effectively concentrates the mineral fraction of a coal by an order of magnitude resulting in additional elements being detected by ESCA. The resultant detection level of elements in the original coal is thus lowered to ≈0.01 bulk wt% by ashing. It was found that fluorine and sulphur were highly concentrated on several coal ash surfaces possible indicating surface Sorption reactions during combustion. Application of ESCA in coal geochemistry and mineralogy is indicated by this study. ESCA has the potential to measure elements at the minor and possibly trace level and confirm element siting within coal and coal ash. In addition, it may be possible to identify coals from different deposits by their unique broad scan ESCA spectra fingerprints.     

Relations between chlorine in coal and the salinity of strata waters

Studies of coal and strata water analyses have established that the distribution of chlorine in coal seams is closely related to the occurrence of brines in the deeper Coal Measures basins. The sodium and chlorine present in the coals are believed to have been adsorbed from the brines. Higher-rank coals, i.e. over 86% carbon dmmf, appear to be unable to adsorb more than very small amounts of these ions, rarely over 0.2% chlorine, even when brines are associated with them. Hypersaline brines with concentrations of dissolved solids up to 200 g/l occur in several of the Northern and Midlands coalfields; strata waters from the Southern coalfields of Britain have significantly lower salinities. The apparently anomalous occurrence of high-chlorine coals closely below the Permian unconformity in south-east Durham may be due to the presence of secondary brines derived in part from the overlying Permian beds. Several lines of evidence suggest that the ingress of chloride salts followed the imposition of the rank patterns upon the British coalfields, probably during the Hercynian orogenic movements.     

Solvent effects in coal desulphurization by chlorinolysis near ambient temperature

The effects of solvent, temperature and reaction time on the desulphurization efficiency of the low-temperature chlorinolysis process were studied. Certain trends in sulphur reduction, chlorine uptake and extraction yield for coals undergoing slurry-phase chlorine treatment were established. Methanol-based solvent systems were found to display remarkable capabilities in coal chlorinolysis both in terms of selective sulphur extraction and high yields of coal extracts.     

Correspondence between the composition and plastic properties of British coals and the structural properties and reactivity of the corresponding cokes

The chemical composition and the physico-chemical properties of a suite of British coals, mainly of types 501/2 as defined by the NCB classification, but including some coals of higher and lower rank (NCB classification 204,401/2 and 601/2), were comprehensively investigated. Each coal was subsequently carbonized under controlled conditions of size distribution, bulk density and temperature on the 250 kg scale and the tensile strength, coke structure and coke reactivity were determined. This data highlighted the significance of the atomic ratio of the oxygen and carbon of the coals and the lack of relations between coal properties and coke structure and strength. As regards coke reactivity only complex relations with coal properties could be found, with coal rank, the oxygen/carbon ratio and the chlorine content being of greatest importance.     

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.     

The volatilization behavior of chlorine in coal during its pyrolysis and CO2-gasification in a fluidized bed reactor

The volatilization behavior of chlorine in three Chinese bituminous coals during pyrolysis and CO₂-gasification in a fluidized bed reactor was investigated. The modes of occurrence of chlorine in raw coals and their char samples were determined using sequential chemical extraction method. The Cl volatility increases with increasing temperature. Below 600 °C the Cl volatility is different, depending on the coal type and the occurrence mode of Cl. Above 700 °C, the Cl volatilities for the three coals tested are all higher than 80%. About 41% of the chlorine in Lu-an coal and 73% of that in Yanzhou coal are organic forms, and most of them are covalently-bonded organic chlorine, which shows high volatile behavior even at low pyrolysis temperatures (below 500 °C), while the inorganic forms of chlorine in two coal samples are hardly volatilized even at low pyrolysis temperatures (below 400 °C). The restraining efficiency of addition of CaO on chlorine volatility is greatly dependent on pyrolysis temperature. The optimal restraining efficiency can be obtained at temperature range from 450 to 650 °C during pyrolysis of Lu-an coal. The volatile behavior of Cl is mainly dependent on temperature. Above 700 °C high volatility of Cl is obtained in both N₂ and CO₂ atmospheres.     

Effect of impregnation with ZnCl2 and other catalysts on coal pyrolysis

Similarity of product distributions from the pyrolysis of ZnCl₂-impregnated coals and ZnCl₂-coal admixtures suggests that the catalyst does not distribute itself uniformly within the coal mass upon impregnation from solution. Elemental distribution maps for zinc and chlorine, obtained by electron probe microanalysis, confirm that the catalyst is deposited around the periphery of coal particles upon impregnation. Uniform distribution of the ZnCl₂ within the particles, however, appears to take place on heating to temperatures as low as 160 °C.