Behaviour of inorganic matter during heating of Bulgarian coals: 2. Subbituminous and bituminous coals

Bulgarian subbituminous (Pernik, Bobov Dol) and bituminous (Balkan) coals were gradually heated under air from 100 °C to their fluid ash-fusion temperatures (1400–1600 °C) via 100 °C intervals and the behaviour of their inorganic matter (IM) was studied. The original minerals and newly formed inorganic phases in the oxidation and combustion products (OCPs) of these coals were identified and the behaviour of 33 minerals and phases was described. The coals studied reveal high detrital abundance and low authigenic mineralization with sulphide–sulphate, carbonate or mixed sulphide–sulphate and carbonate tendencies. The IM of coals is composed mainly of quartz, kaolinite, illite + muscovite, feldspars, pyrite, and calcite, while the other minerals identified have subordinate occurrence. The IM of OCPs includes various pre-existing minerals and newly formed phases. The latter phases are glass, quartz–cristobalite–tridymite, mullite, amorphous clay material, hematite–magnetite, anhydrite, and others originating from the heating of these coals or storage of their OCPs. The physico-chemical processes and temperatures that result in the formation of new phases in OCPs are described. The relationships between the ash-fusion behaviour and chemical and mineral composition of the coals are also discussed. A systematization of the physico-chemical transformations and some comparative characterizations, as well as prediction of certain technological and environmental problems related to the behaviour of IM during heating of Bulgarian lignites, subbituminous and bituminous coals are also described and summarized.     

Depolymerization of oxidized bituminous coals

Certain high-sulphur-containing, high-volatile bituminous (hvb) coals have been converted into products which are almost completely soluble in chloroform. These products were obtained by a series of reactions involving initial oxidation with 2N nitric acid, depolymerization with boron trifluoride-phenol, and acetylation. Solubility of the depolymerized coal decreased sharply when the nitric acid reaction was eliminated or was replaced with a hydrochloric acid treatment. The extent of oxidation by the nitric acid treatment is discussed. Also included in this investigation are a low-sulphur-containing hvb coal and a low-volatile bituminous coal. After depolymerization, the solubilities of these two coals in chloroform were comparatively low.     

Storage of bituminous coals under argon

Two bituminous coals, crushed to < 0.32 cm ($\text{1}\text{8}$ in), have been stored for 455 days under argon at room temperature and examined for indications of oxidation. The only changes noted over this time were a 20–30% reduction in maximum fluidity from the Gieseler plastometer test for both coals, and conversion of 5% of the pyrite to iron oxyhydroxide in one coal. Although minor, these changes do indicate that more stringent conditions are necessary to minimize oxidation during long-term storage (>2 years).     

Rheological properties of selected bituminous coals

The rheological behaviour of several bituminous coals has been evaluated by means of a constant torque Gieseler plastometer, utilizing both steadily increasing (3 °C min⁻¹) and isothermal (410 °C) temperature conditions. The results indicated that under either thermal treatment, the coals investigated showed predominantly pseudoplastic behaviour. The greatest degree of non-Newtonian behaviour occurred at or near the temperature of maximum fluidity for coals heated at 3 °C min⁻¹, or within the early stages of melting when isothermal heating was used. Empirical relationships relating the Gieseler fluidities to apparent viscosity were derived from the data.     

Decaking of bituminous coals by alkaline solutions

Experimental results are reported concerning the decaking effect of alkalies on two Western Kentucky coals. Decaking treatment with dilute sodium hydroxide solutions was the most effective of the various methods examined. Caking and agglomerating character were destroyed when about 1–2 wt % (dry basis) sodium hydroxide was retained by the coal following the solution treatment. The free-swelling index was considerably reduced for both seam coals. In contrast, 20 wt % of sodium hydroxide was required in a dry mixing method. The amount of decaking reagent required for effective solution treatment varied with seam of origin but the difference in particle size range and/or mineral content may have been significant. Independent effects of these variables were not determined. Experimental results indicate that the decaking reactions occur at temperatures above 300 °C, but the actual mechanisms are not known. The alkaline-solution decaking treatment was conceived as a possible step in coal preparation for a coal-gasification process. Various important questions remain for continued study.     

Thermogravimetry and differential scanning calorimetry of Kentucky bituminous coals

A detailed investigation has been made of the thermal characteristics of six Kentucky bituminous coals undergoing pyrolysis in an inert atmosphere at three different heating rates. The specific heats of the coals and the enthalpy changes characterizing their thermal degradation were measured by differential scanning calorimetry. Thermogravimetry was employed to measure the attendant weight changes, which were used to normalize the heat flow data to unit sample weight, enabling a quantitative comparison of the thermal behaviour of the several coals. The specific heats of the dry coals lie in the range 1.21–1.47 J gK^?1, 100–300 °C. The exothermic heat flow from 300 to 550 °C, where the major weight loss occurs, has been associated with the primary carbonization process, the development of the plastic state, and the onset of secondary gasification, which is responsible for coke formation. In the high pyritic sulphur coals, the endothermic pyrite/pyrrhotite transformation at ≈580 °C is clearly defined. A preliminary global kinetic analysis of the thermogravimetric data has been made, using a modified Kissinger equation at the maximum rate of weight loss. Activation energy and pre-exponential factor values of the order 198–220 kJ mol ^?1 and 2–85×10¹²s^?1 have been obtained. These compare well with the limited published values for similar coals.     

Comparison of solvent-refined lignite with solvent-refined bituminous coals

Comparison of laboratory-deashed samples of solvent-refined lignite, subbituminous and bituminous coals on the basis of gross combustion analysis, acid and basic titres, molecular weight, nuclear magnetic resonance, ultraviolet and electron spin resonance spectra including various derived parameters indicated that there is relatively little difference between the range of values for lignite- and bituminous coal-derived solvent-refined products. The probability that coal nitrogen content is reflected in the solvent-refined product was noted.     

Hydrothermal dewatering of lower rank coals. 3. High-concentration slurries from hydrothermally treated lower rank coals

A novel method of producing a product of low intra-particle porosity (<1 μm pore radius) from highly porous Latrobe Valley raw brown coals uses a combination of hydrothermal and evaporative drying. Low porosity coal was made in three different batch autoclave systems at 320 °C for residence times as low as 10 min. Higher temperatures (up to 350 °C) increased porosity slightly but the water vapour pressure and the loss of organic material were significantly increased. Although the low and high porosity products differed dramatically in appearance and hardness, other chemical and spectroscopic properties were similar with the exception of pyrolysis-gas chromatography-mass spectrometry patterns.The relationship between intra-particle porosity and the maximum wt% dry solids concentration of aqueous slurries (for a viscosity of less than 1000 mPa s), ?_max, established by earlier workers for hydrothermally treated brown coals was found to hold for the new products and was extended to a wider range of porosities and a range of mean particle sizes (mps) (20-100 μm). A range of surfactants (anionic, cationic and neutral), which led to an increase of up to 7% in ?_max for a bituminous, Blair Athol coal, increased ?_max for products of hydrothermal or the new treatment by only 2-4%. This small increase resulted, however, in the formation of slurries of the low porosity products with ?_max's of up to 64%, similar to those obtained with high rank coals, and considered to be of commercial interest.     

Flash pyrolysis of coals. Devolatilization of bituminous coals in a small fluidized-bed reactor

The devolatilization behaviour of ten bituminous coals was investigated under rapid heating conditions using a small-scale fluidized-bed pyrolyser. The pyrolyser operated continuously, coal particles being injected at a rate of 1–3 g h^?1 directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C₁–C₃ hydrocarbon gases, and total volatile-matter and an agglomeration index are reported for all coals. Maximum tar yields were obtained at about 600 °C and were always substantially higher than those from the Gray-King assay. Total volatile-matter yields were also substantially higher than the proximate analysis values. The maximum tar yields appear to be directly proportional to the coal atomic $\text{H}\text{C}$ ratio. The elemental analysis of the tar is strongly dependent on pyrolysis temperature. The tar atomic $\text{H}\text{C}$ ratio is proportional to that of the parent coal. The effect on the devolatilization behaviour of two coals produced by changes in the pyrolyser atmosphere and the nature of the fluidized-bed material were also investigated. Substituting an atmosphere of hydrogen, helium, carbon dioxide or steam for nitrogen, has no effect on tar yield and, with one exception, little effect on the hydrocarbon gas yields. In the presence of hydrogen the yield of methane was increased at temperatures above 600 °C. Tar yields were significantly reduced on substituting petroleum coke for sand as the fluid-bed material. A fluidized bed of active char virtually eliminated the tar yield.     

Hydrogasification characteristics of bituminous coals in an entrained-flow hydrogasifier

The characteristics of hydrogasification to generate substitute natural gas (SNG) by using various bituminous coals such as Alaska, Cyprus, Curragh, and Datong have been determined in an entrained-flow hydrogasifier (0.025 m I.D.×1 m high) with high pressure coal feeder and data acquisition system. The effects of reaction pressure (60–80 atm), reaction temperature (600–800 °C) and H₂/coal ratio (0.3–0.5) on composition of product gas and carbon conversion have been determined. The concentration of SNG and carbon conversion increased with an increasing of reaction pressure and temperature, but the carbon conversion and concentration of each bituminous coal were quite different because of different coal properties. Also the H₂/coal ratio affected the carbon conversion and the concentration of SNG.     

Comparative characteristics of vitrinites of Carboniferous coals of the Ukraine and Jurassic coals of Siberia

The investigation of vitrinites of various ages was carried out using phyteral analysis and detailed separation of coal. Jurassic vitrinites appeared to be more diverse both in phyteral composition and chemical properties. The great variety of Jurassic vitrinites and their chemical characteristics are governed by the influence of the initial plant material and conditions of its accumulation.     

Formation of carbonaceous cenospheres during fluidized-bed combustion of bituminous coals

The discovery of cenospheres in fly ash from fluidized-bed combustors is reported and differences between the fluidized bed and more conventional combustors which are likely to affect cenosphere formation are indicated. Morphological studies and qualitative chemical analyses obtained by electron microscopy/energy dispersive X-ray specttometry (SEM/EDS) techniques show differences in composition between the ‘window’ areas of cenospheres and their ‘skeletons’.     

Relationship between carbon aromaticities and H/C ratios for bituminous coals

Carbon aromaticities for a selection of 19 bituminous coals and vitrinite concentrates have been obtained by solid-state ¹³C n.m.r. using the inherently quantitative single pulse excitation technique. Within experimental error, the aromaticity values in the range 0.75–0.90 correlate extremely well with the H/C ratios (A² = 0.93); the 11 samples investigated with H/C ratios between 0.74 and 0.77 all have aromaticities in the range 0.75–0.81. Furthermore, the correlation holds for the coals investigated with high inertinite contents.     

Structure in cokes from coals of different rank

A range of bituminous coals has been carbonized to 1273 K. Polished surfaces of the solid products, carbons or cokes, are examined for optical texture by optical microscopy. Fracture surfaces of the carbons are examined by scanning electron microscopy (SEM). The carbon from the lowest rank coal (NCB Code No. 702) is isotropic and fracture surfaces are featureless. Carbons from coals of ranks 602, 502 are optically isotropic but fracture surfaces are granular (size 0.1–0.2 μm), indicating small growth units of mesophase. In the carbon/coke from a 401 coal, the anisotropic optical texture and grain size are both ≈0.5–10 μm diameter. Coke from a coking coal (301a, 301b) has a layered structure extending in units of at least 20 μm diameter with sub-structures ~ 1.5 μm within the layers, indicating perhaps that the bedding anisotropy of these coals is not totally lost in the fluid phase of carbonization. The carbons from the higher rank coals have the bedding anisotropy of the parent coal. The combined techniques of optical microscopy and SEM (both before and after etching of the fracture surfaces of coke in chromic acid solution) reveal useful detail of structure in carbons/cokes and of the mechanism of carbonization of coking coals.     

Dependence of coal liquefaction behaviour on coal characteristics. 7. Structural differences between coals of different rank and the asphaltenes derived from them

Aslphaltenes are difficult to characterize by any procedure that involves chromatography. In an attempt to obtain useful information on unfractionated asphaltenes and their structural relations to their parent coals, asphaltenes from a set of eight coals of hvA rank and different coalification history have been studied by ¹ H n.m.r. In addition, structural features in the coals and their asphaltenes were compared by g.c.-m.s. analysis of the products of oxidation by pertrif luoroacetic acid. Protons α to a benzene ring are the most abundant in the asphaltenes, and their relative concentration changes little with the rank of the parent coal. Protons in the β-position are abundant, however, and may be associated with aliphatic junctions linking two or more aromatic ring systems. Whereas malonic and ethane-trioic acids are the major aliphatic di-and tri-acids from oxidation of the coals, succinic and propane-or butane-tricarboxylic acids are the corresponding products that dominate the distributions from the asphaltenes. These and other differences help to define the structural changes caused by liquefaction. In addition, the results indicate the nature of structural changes with increasing rank, which are manifest in the oxidation products from the asphaltenes as well as from the coals.     

Increase in extractability of bituminous coals caused by Friedel-Crafts acylation

Four bituminous coals of different rank were acylated with aliphatic acyl chlorides using Friedel-Crafts catalysts. A dry steam coal and a coking coal showed particularly good reactivity, five acyl groups being inserted per 100 carbon atoms. Compared with the initial coal, the acylated coals were distinguished by considerably higher amounts soluble in pyridine and other solvents. 85% of the coking and dry steam coal substance became soluble when acylated with lauric acid chloride or acyl chlorides having still longer chains. The extractability of the coals after acylation depends on the chain length of the inserted acyl groups. To all appearances, the extracts are dispersed in the solution in the form of molecules. It is supposed that the good extractability of acylated coals is mainly due to depolymerization of the coal substance during acylation, disappearance of hydrogen bridges by esterification of the phenolic OH groups, and neutralization of inter-molecular forces between the aromatic structures by insertion of long-chain substituents.     

Solubilisation of some bituminous coals in quinoline by ultrasonic irradiation

The solubilisation of 4 hvab and 1 hvbb coals by ultrasonic irradiation of coal-quinoline systems at ambient temperature was investigated. The amount of coal solvated was found to be a function of time and particle size. The use of a char prepared at the temperature of maximum coal fluidity increased the amount of material solubilised. There was little difference in the amounts of coal solubilised at 36° and 80°. There was a decrease in the sulphur and ash content of the solubilised coal compared with the starting coal. Quinoline can be re-used as a solvent with no decrease in solubilisation efficiency. Based on high-resolution mass spectrometric and infra-red spectroscopic investigations and particle size measurements of the solubilised material, it appears that the increased solubilisation of coal in quinoline results from a grinding action and not a chemical action.     

Solubilization of bituminous and lignite coals by chemically and biologically synthesized surfactants

Tween 80 (polyoxyethylene sorbitan monooleate), Triton X-100 (nonaethylene glycol octylphenol ether), and SDS (sodium dodecyl sulfate) solubilized several components of Mississippi Wilcox lignite and Illinois No. 6 bituminous coal. Tween 80 extraction of alkali-soluble Ugljevik lignite resulted in an increase in hydrogen content and decreases in the nitrogen and sulfur contents of undissolved coal solids. Biosurfactant-containing cell-free extracts of Bacillus licheniformis solubilized a 53 000 Da coal component of Mississippi Wilcox lignite. The results suggest that solubilization of coal organic components by chemically- and biologically-synthesized surfactants has potential in terms of upgrading coals.