Phase changes in mineral matter of North Dakota lignites caused by heating to 1200 °C

Changes in structures of minerals taking place in lignitic coals during combustion were investigated by first concentrating the mineral matter in the coal by low-temperature ashing and then heating the mineral matter at 100 °C intervals from 200 °C–1200 °C and analysing the major mineral phases by X-ray powder diffraction. Samples of high and low sodium contents were analysed to determine differences in mineral phases with varying sodium contents. Quartz and bassanite were identified as major phases in the low-temperature mineral matter of all ten lignite samples, and pyrite and calcite were identified in eight of the ten samples. Kaolinite was the only clay mineral identified and appeared in nine of the ten samples. Those samples with a sodium oxide content of 8.56 wt % or greater, showed sodium nitrate as a major mineral phase in the low-temperature mineral matter. When the mineral matter was heated to higher temperatures, quartz was a major phase at 1200 °C in five of the samples, and was stable to 1000 °C in all of the samples. Anhydrite was a major mineral phase in all samples from 600 °C–800 °C, appearing in some of the samples as low as 200 °C, and persisting to 1100 °C in some samples. Hematite was found to be a major phase in seven of the ten samples, having an overall temperature range from 300 °C–1000 °C. Magnetite was detected in the range from 800 °C–1200 °C with hercynite forming as a major mineral phase, after magnetite, in two of the samples at 1200 °C. The solid solution series gehlenite-akermanite was found in all ten samples from 1100 °C–1200 °C although they appeared in some samples at 900 °C. Samples of high sodium content formed sodium sulphates at intermediate temperatures and sodium silicates at higher temperatures. Low Sodium samples formed bredigite, a calcium silicate, at higher temperatures.     

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.     

Determination of oxygen in coal and coke using a radio-frequency heating method

Rapid procedures are described for the direct determination of organic oxygen in coal and coke at 1950 °C. Particular features include reduction—fusion of the sample in a radio-frequency heated, carbon-saturated iron bath, ease of attaining a steady blank rate, transference of the evolved carbon monoxide through a combined oxidation—purification system with argon carrier gas, and a gravimetric determination. The procedures are applicable to both demineralized and undemineralized samples, with a wide range of mineral matter contents. Results obtained for organic oxygen in a range of demineralized samples were in agreement with results obtained by the Unterzaucher—Oliver procedure. With coals of less than 5% mineral matter satisfactory agreement was achieved for demineralized or undemineralized samples or ‘by difference’; with coals of more than 5% mineral matter satisfactory agreements were obtained provided that a range of correction techniques for inorganic oxygen were used. Satisfactory results were obtained also for demineralized coke samples. Results for all techniques showed similar precision throughout the range of contents studied, with a standard deviation (1 s) of ca. 0.1% absolute.     

Catalytic activity of coal minerals in water vapour gasification of coal

Possible catalytic influences of coal minerals during water vapour gasification of coal have been studied by kinetic measurements and microanalytical methods. A bituminous coal without and with various pretreatments and also model chars synthesized from PVC and PVC-sulphur mixtures were used as raw materials. Kinetic measurements were performed in a fixed-bed flow reactor at pressures between 0.2 and 2 MPa and temperatures from 880 to 1010 °C using hydrogen/water vapour mixtures as gasification agents. It was found that coal gasification at and beyond 880 °C can be decisively catalysed by the iron as constituent of mineral matter. Preconditions are elimination of inorganic sulphur and reducing atmosphere to stabilize elemental iron. The optimum pressure is in the range of 0.5 to 1 MPa. Scanning electron microscopy and electron probe microanalysis confirm that catalytic gasification starts as soon as the iron is free of sulphur. The organic sulphur of coal does not prevent but lowers the catalytic activity of iron.     

Catalytic activity of coal minerals in hydrogasification of coal

Hydrogasification of six bituminous coals was studied in a fixed-ped flow reactor at pressures up to 2 MPa and temperatures from 790 to 960 °C. Ranges of distinct methane formation are found with all coals between 500 and 600 °C, 750 to 800 °C and >850 °C. The reactions in the first two ranges are determined by the molecular structure of coal and are not affected by catalytic activities of constituents of coal minerals. In the third range, >850 °C, iron as a constituent of mineral matter of coal can accelerate methane formation significantly if the pressure is sufficiently high. Thermodynamic calculations indicate, and were verified by thermogravimetric studies, that iron disulphides in original coals can be desulphurized during gasification. Alkali and alkali earth oxides and carbonates can act as sulphur scavengers via an exchange reaction and thus accelerate the desulphurization of iron sulphides.     

Mineral matter effects on air-oxidation of cokes from a coking coal

Indigenous mineral matter in coal affects the chemical reactivity of resulting cokes through both catalytic graphitization and catalytic gasification. The significance of both catalytic effects on air-oxidation was examined using cokes from a medium-volatile bituminous coking coal with 9 wt% mineral matter. Catalytic graphitization by mineral matter enhanced the reactivity of the resulting coke in spite of the formation of highly crystalline carbons. This effect, however, was less than that of catalytic gasification by mineral matter. The coke from the acid-treated predemineralized coal exhibited no catalytic effects but was the most reactive. The implications of these results are discussed in detail.     

Mineralogy of ash of some American coals: variations with temperature and source

Ten samples of mineral-matter residue were obtained by the radio-frequency low-temperature ashing of subbituminous and bituminous coals. The low-temperature ash samples were then heated progressively from 400 °C to 1400 °C at 100 °C intervals. Mineral phases present at each temperature interval were determined by X-ray diffraction analyses. The minerals originally present in the coals (quartz, kaolinite, illite, pyrite, calcite, gypsum, dolomite, and sphalerite) were all altered to higher temperature phases. Several of these phases, including kaolinite, metakaolinite, mullite, anhydrite, and anorthite, were found only in limited temperature ranges. Therefore the temperature of formation of the ashes in which they occur may be determined. Mineralogical differences were observed between coal samples from the Rocky Mountain Province, the Illinois Basin, and the Appalachians; and as a result of these mineralogical differences, different high-temperature phases resulted as the samples were heated. However, regional generalizations cannot be made until a greater number of samples have been studied.     

Measurement of volatile matter in Victorian brown coal

The effects of using one-stage and two-stage heating, using air-dried coal and coal oven-dried under nitrogen, and using final temperatures of 850°C and 900°C on the measured volatile matter for twelve Victorian brown coals have been examined by the use of a statistically controlled experiment. The repeatability of the two-stage method was found to be significantly better than that of the one-stage method (0.7% abs. compared to 0.9% abs.). The volatile matter of coals at the front of the furnace was found to be significantly different from that of coals at the back of the furnace for the one stage method but not for the two-stage method. A two-stage heating method (7 min at 400°C and then 7 min at 900°C) using oven-dried coal is recommended for the routine determination of volatile matter in brown coal.     

Catalytic action of minerals in the low temperature oxidation of coal

The mild oxidation of coal by air in a fluidized-bed reactor has been studied by utilizing X-ray powder diffraction, scanning electron microscopy accompanied by energy dispersive X-ray analysis and Mössbauer spectroscopy. These techniques show that some of the highly dispersed minerals in coals undergo changes concurrently with the reactions of the coal matter. An oxidation treatment at a reactor temperature of 350 °C transforms most of the pyrite (FeS₂) into hematite (Fe₂O₃) andthecalcite (CaCO₃) into anhydrite (CaSO₄). Pronounced pitting of the coal surface was observed in the vicinity of mineral crystallites and particles. Calcium was present as a dominant element in those minerals that caused the oxidative pitting to occur most rapidly.     

Associations and parageneses of microelements in coal

Microelements present in coal may be chemically bound with organic matter or with mineral impurities (clastic and authigenic minerals). Analytical data present the total (gross) content of microelements in coal. Such microelements are generally present in associations. In addition, some chemical elements accumulate in the coal on account of their content in organic matter; they constitute a paragenesis. In predicting the content of rare metals in coal deposits and in their extraction from the coal, it is important to establish the parageneses of the microelements—in other words, to distinguish the parageneses from associations of microelements. Such microelements may create anomalous concentrations in the coal. The present work establishes the characteristics of associations and parageneses of microelements in coal and proposes a statistical method for distinguishing parageneses from associations. The method is illustrated for data regarding Baikal coal deposits; it proves very effective.     

Removal of mineral matter from bituminous coals by aqueous chemical leaching

Most of the ash-forming minerals in bituminous coals are insoluble in water or acids, but a high proportion (more than 90%) can be extracted with aqueous sodium hydroxide solutions at 200–300°C under pressure. The major minerals extracted —silica and kaolin — are converted into sodium alumino-silicates (sodalites) which form a separate insoluble phase in contact with water or alkali, but which are readily soluble in aqueous acids. Minor mineral components are also partly removed. The spent leachants can be regenerated for recycling; the extracted minerals form a solid by-product. The chemical extraction steps may be coordinated with conventional coal-cleaning procedures which use physical separation techniques. Experiments with three Australian coals from the Liddell and Lithgow seams (NSW) and the Theodore seam (Queensland), and a vitrinite concentrate from Liddell, gave beneficiated products with ash yields of 0.25–0.75%. Demineralized coal may have applications as a low-ash fuel or carbonaceous raw material.     

Changes in state of combination of inorganic constituents during carbonization of Victorian brown coal

The inorganic constituents of low-rank Victorian brown coal, which are mainly present as inherent inorganic combinations attached to the coal molecule, are different from the minerals present in higher-rank coals. Changes in the state of combination of the inorganics in the chars of these coals have been studied by determining the minerals formed when chars are prepared at various temperatures. It is shown that when brown coals containing inorganic carboxylates are carbonized, the reactive functional groups which are present will begin to decompose below 400 °C and their decomposition is completed by 600 °C. The inorganic elements released will form mineral combinations depending on the carbonizing temperature and the reduction potential of the metal. Sodium carbonate, calcium oxide, magnesium oxide (periclase) and iron oxide (magnetite) are usually the chief minerals that may be formed. Mineral constituents present in the original coal may also change during carbonization. As the temperature increases, hydrated oxides of iron, clay and aluminium will lose water of crystallization and become dehydrated, forming magnetite, dried clay and alumina. Pyrite will lose up to half its sulphur at 400–450 °C. If the carbonization temperature is over 600 °C, metallic iron may be formed from the magnetite, and sodium chloride (and later sodium carbonate) will volatilize. Quartz remains unaltered.     

Characterization of low-temperature coal ash behaviors at high temperatures under reducing atmosphere

The coal ash obtained at 815 °C under oxidizing atmosphere was further treated at 1300 °C and 1400 °C under reducing atmosphere. The resultant ashes were examined by XRD, SEM/EDX and FTIR. The results show that the residence time of coal ash at high temperatures has considerable influences on the compositions of coal ash and little effect on the amounts of unburned carbon. The amorphous phase of mineral matters increases with the increasing temperature. The FTIR peaks due to presence of different functional groups of minerals support the findings of XRD, and supply additional information of amorphous phase which cannot be detected in XRD. The ash samples generated from a fixed bed reactor during char gasification were also studied with FTIR. The temperatures of char preparation are responsible for the different transformation of minerals during high temperature gasification.     

Effects of remained catalysts and enriched coal minerals on devolatilization of residual chars from coal liquefaction

Devolatilization behaviour of residual chars from coal liquefaction was investigated using thermogravimetric analysis. Effect of remained and enriched minerals on devolatilization of residual chars was mainly concerned. By analysing TG/DTG profiles, it was found that the measured volatile matter (VM) includes not only the decomposed volatile products of unreacted organic substances of coal but also that of the inorganic materials, such as enriched mineral matters, remained liquefaction catalyst, as well as by-products formed in catalyst preparation. Therefore, the VM measured by the standard proximate analysis (up to 950 °C) should preferably be called ‘total volatile matter’ (TVM), which does not reflect the organic properties of the residue as one would expect for VM. Therefore, the VM released from 110 to 700 °C was named organic VM, which shows a better linear relationship with H/C ratios of the residual char and characterises the reactivity of the residue more rationally than TVM does.     

Hydrogen-transfer catalytic activity of minerals common to coal

The hydrogenation/dehydrogenation catalytic activity of minerals commonly present in coal was investigated. The extent of reaction undergone by the model hydrogen-transfer system tetralin/1,2-dihydronaphthalene/naphthalene at 400 °C in the presence of these minerals was used to measure the catalytic activity. For the four most active minerals, the order of catalytic activity per gram of mineral, for a given range of particle size, is limonites > pyrites > diaspore > magnetites. The pure chemical analog of the limonites, Fe₂O₃, was found to have catalytic activity similar to that of the limonites. Heating the minerals at 400 °C before running the reaction was found to decrease these catalytic activities, despite the surface-area increases resulting from the annealing process. The catalytic activity per unit area of mineral was determined and used as an index of the catalytic activity of the reactive sites for these heterogeneous systems. The order of decreasing site catalytic activity for the more active minerals is pyrites > magnetites > diaspore > limonites. The observed mineral/ chemical compound catalytic activities indicate that the more reactive mineral hydrogenation/ dehydrogenation catalytic sites are composed of iron compounds.     

An XPS study of the oxidation of pyrite and pyrites in coal

X-ray Photoelectron Spectroscopy (XPS) was used to study mineral, synthetic and coal-associated pyrites, oxidized for various time intervals at low temperatures with humid air or oxygen. This was done to find out if XPS could detect, monitor and clarify pyrite surface-oxidative changes that influence surface-dependent coal-cleaning methods such as froth flotation, and could provide a means of directly analysing coal sulphur, by determining if oxidizing conditions existed which would effectively eliminate the surface pyrite whose XPS peak may occur at the same energy as the organic sulphur peak of coal. The conditions of study were as follows: a mineral and two coals containing pyrite were exposed to air at 24 ± 3 ° C and 33 ± 8% relative humidity up to 600 h; two mineral pyrites were exposed to oxygen at 100% relative humidity and 35 ° C for up to 200 h; and the two mineral and a synthetic pyrite were exposed to oxygen at 100% relative humidity and 55 ° C for up to 300 h and at 72°C for 25 h. The results indicated that the XPS S2p pyrite peak at ≈169 eV and the surface-oxidation-product(s) peak(s) at ≈163 eV could be detected and followed with XPS, although no conclusions could be made about the oxidation mechanism. The pyrite XPS peak became small compared to that of its oxidation products when the synthetic and mineral pyrites were exposed to 55 ° C oxygen at 100% relative humidity for 300 h. These conditions may prove useful in trying to determine directly the organic sulphur in coal.     

Co-gasification behavior of meat and bone meal char and coal char

The co-gasification behavior of meat and bone meal (MBM) char and two types of coal (Jincheng anthracite (JC) and Huolinhe lignite (HLH)) char was investigated using a thermogravimetric analyzer (TGA). The effects of coal type, mineral matter in MBM, gasification temperatures and contacting conditions between MBM char and coal char on the gasification behavior were studied. The results show that the gasification behavior of MBM char and HLH char can be well described by ash diffusion controlled shrinking core model, while that of JC char can be described by chemical reaction controlled shrinking core model. The co-gasification rate of MBM/JC chars at 950 °C is approximately 1.5 times faster than that calculated from independent behavior. The mineral matter in MBM may play as a catalyst during co-gasification. However, the analogous effect observed in the blends of HLH/MBM chars is smaller, suggesting that the coal types play a great role. Furthermore, as the gasification temperature increased from 850 to 1000 °C, the maximum synergistic effect is observed at 900 °C. The lower temperature is not conducive to transferring the mineral matters of MBM to the coal char, while the higher temperature makes Na and Ca react with minerals of coal, leading to a loss of catalytic activity.     

Identification of clay minerals in coal and wastes from main coal washery, Zonguldak, Turkey

Identification of clay minerals present in coal and washery wastes is important in cleaning fine coal by froth flotation and in flocculation and dewatering. Therefore samples of wastes from jigs and the flotation cell at the Zonguldak main coal washery were collected and analyzed petrographically for their mineral matter content and by X-ray diffraction for their clay content. The “loss on ignition” method was carried out to determine their organic carbon and carbonates. The waste samples contain 48–68% clay minerals in addition to silicates, carbonates, sulfides and coal. Three clay minerals were identified, namely illite, kaolinite and chlorite. Illite seemed to be the dominant clay mineral in washery wastes. Loss on ignition indicated high percentages of organic matter in the fine jig tailings (21%) and flotation tailings (33%). 3%–6.5% of carbonates have also been found.     

Transformation of mineral and emission of particulate matters during co-combustion of coal with sewage sludge

Co-combustion of coal with sewage sludge was carried out in laboratory-scaled drop tube furnace to understand the interaction between different fuels. The combustion conditions were selected as follows: the raw material feeding rate was 0.2-0.3 g/min, temperature was 1200 °C, the atmosphere of 10% O₂ and N₂ being balance was used to guarantee an air ratio of 1.5, and the residence time varied from 0.6 to 2.4 s. The coal/sewage sludge is kept at 50:50 (wt% to wt%), four fuel pairs were selected with respect to the mineral association within individual fuel. The results showed the obvious interaction between coal and sewage sludge during their co-combustion. For the carbon conversion, the devolatilization of mixing fuel occurred quickly; the combustion of both char and evolved volatile progressed almost completely. As a result, the unburnt carbon was almost zero in the fly ash. In addition, the evolution of both mineral and PM varied with the association of minerals in raw fuels. For both coal and sewage sludge rich in included minerals, they combusted separately in the furnace, less interaction occurred accordingly. Conversely, for both them rich in excluded minerals, the minerals reacted with each other to form much agglomeration, and therefore, the particle size of the fly ash was increased, while the amount of PM was decreased, which changed as the coarse fly ash particles. Finally, for the case of coal rich in excluded mineral and sludge rich in included mineral, their co-combustion led to the interaction of their minerals. As a result, more the fine particles were formed, which in part changed into PM. For the vaporized trace elements, they were adsorbed by the melt CaPO₄/Al-Si in the ash and accordingly, their contents in the particulate matter were reduced whereas their particle size distribution shifted to the large value.