Carbonization and liquid-crystal (mesophase) development. Part 4. Carbonization of coal-tar pitches and coals of increasing rank

Coal-tar pitches, from coals of different rank and with various quinoline-insoluble contents, were carbonized under pressure (67 to 200 MN m⁻²) to maximum temperatures of 923 K. The resultant cokes were examined by optical and scanning electron microscopy in terms of size and shape of anisotropic structures within the coke. Natural quinoline-insolubles and carbon blacks both destroyed growth of the mesophase and development of anisotropy. Graphite particles (<10 μm) promoted growth and coalescence of the mesophase. Fourteen coals, of carbon content 77 to 91 wt%, VM 41 to 26%, were similarly carbonized under pressure. In the lower-rank coals no microscopically resolvable anisotropic mesophase was produced, but at a carbon content of 85% anisotropic units 1–2 μm in diameter were detected, increasing in size at a carbon content of 90% to 5 μm diameter. Results are discussed in terms of the origins of anisotropic mosaics observed in cokes, their variation in size with coal rank, and their significance in the carbonization of coal.     

Carbonization and liquid-crystal (mesophase) development. 7. Optical textures of cokes from acenaphthylene and decacyclene

Optical textures of cokes prepared by carbonizing acenaphthylene, decacyclene and mixtures thereof at selected values of heat-treatment temperatures and soak time have been compared. Optical textures are assessed using polished surfaces and reflected-polarized-light microscopy in conjunction with a half-wave plate. The acenaphthylene is chemically more reactive than the decacyclene which is itself formed during the carbonization of acenaphthylene. Products of carbonization of acenaphthylene can influence rates of carbonization of the decacyclene. Similar optical textures in cokes cannot be formed by compensating low heat-treatment temperatures with long soak periods. In addition to chemical rate-controlling processes, the physical properties of the system must be acknowledged, in particular the viscosity. Very large non-coalesced growth units of mesophase (800 μm diameter) have been observed. Pre-alignment of growth units of mesophase may occur prior to coalescence.     

Carbonization and liquid-crystal (mesophase) development. 5. Importance of eutectic zones formed from nematic liquid crystals

The development of anisotropy in carbons arises from growth of nematic liquid crystals from the plastic phase of carbonization systems. Mixed, nematic liquid crystals form eutectic zones stable at lower temperatures than the single components. This phenomenon may have relevance to co-carbonization of mixtures of organic compounds and to coal blending procedures. A lowering of the temperature of onset of anisotropy may be beneficial to both graphite and coke quality. In the co-carbonization of carbazole and pyromellitic dianhydride to form anisotropic carbons, such a concept may be relevant. However, in the co-carbonization of two coals, although some lowering occurred in the temperature of formation of anisotropy, other interfacial phenomena are observed.     

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.     

煤碳化成焦机理的研究进展

从煤碳化的中间相机理、不同煤阶煤的成焦过程、气体压力的生成,以及碳化过程中所发生的化学反应和焦炭微织态的形成过程等方面综述了近年来在煤的碳化成焦机理研究方面的新进展,指出今后应加强对煤碳化机理的研究,以利于指导炼焦生产．     

Carbonization and liquid-crystal (mesophase) development. 6. Effect of pre-oxidation of vitrinites upon coking properties

The effects of atmospheric oxidation at 378 K upon the carbonization of a coking and a caking vitrinite have been examined in terms of the origins and extents of development of anisotropic material. The vitrinites, oxidized from 1 to 40 days, were carbonized to temperatures between 618 and 878 K, in open boats under nitrogen at atmospheric pressure, and in sealed gold tubes at maximum pressures of 140 to 310 MPa. Optical microscopy was used to observe, qualitatively, changes in reflectance and in shape and size of the anisotropic material of the carbonized product; morphological changes were monitored by scanning electron microscopy. For both vitrinites, whereas one day of oxidation destroyed coking properties and almost all of the anisotropic development in the open-boat carbonizations, the pressure carbonizations were not significantly affected until after five days of oxidation. Anisotropy still developed by mesophase growth from the plastic phase of carbonization, to produce the shaped, botryoidal material characteristic of pressure carbonizations. Thereafter, although in the pressure carbonizations the particles of coking vitrinite only fused slightly at interfaces to form a coherent product, marked increases occurred in reflectance and in observed anisotropy, showing maxima at nine days of oxidation. Results are interpreted on the assumption that oxidation cross-links the macromolecular structure of the vitrinite substance. The effect of high pressure during carbonization after five days of oxidation is to preserve and perfect the original basic anisotropy of the vitrinites initially stabilized by the cross-linkage of oxygen atoms.     

Optical anisotropy of carbonized coking- and caking-coal vitrains

The purpose of this work was to characterize in detail the optical anisotropy formed during carbonization of the range of coals used in the coking industry, the ultimate objective being to attain a better understanding of the coking process. Vitrains hand-picked from a series of coking and caking coals were carbonized to various temperatures between 380 and 1000 °C. The semicokes and cokes so produced were examined by polarized-light microscopy to determine the proportions of the different types of optical anisotropy developed during carbonization. The results demonstrated that coals normally grouped within one class of the coal classification system used by the National Coal Board can lead to cokes which are significantly different in terms of their optical anisotropy. The process of the anisotropic development during carbonization can be explained generally in terms of loss of volatile matter, variations in viscosity of the plastic mass, and distortion of ordered phases by the pressure of evolving gases. Differences in carbonization behaviour as judged by the coke anisotropy can be attributed to differences in the ‘molecular-structure’ of the parent coal. In this respect the oxygen in the coal is considered to be of primary significance.     

Carbonization and liquid-crystal (mesophase) development. 13. Kinetic considerations of the co-carbonizations of coals with organic additives

Five coals, of rank from an anthracite to a non-caking coal, have been carbonized singly and also cocarbonized with decacyclene, mixing ratio 7:3, in the temperature range 648 K to 823 K, heating at 10 K min^?1, with various soak times. The objective of the study is to derive the basic factors which influence the kinetics of formation of mesophase and anisotropic coke. Accordingly, resultant cokes were polished and surfaces examined by reflected polarized light in an optical microscope. The size, shape and extent of anisotropic development is discussed in terms of the conditions of carbonization and the rank of coal. In these systems a somewhat larger optical texture results in cokes produced at the higher carbonization temperatures. The temperature of onset of growth of anisotropic carbon in co-carbonizations was below that of either the coal or the decacyclene. Reactivities are evidently modified. The origins, growth and coalescence of growth units of anisotropic carbon in these cocarbonizations of coals with decacyclene are demonstrated.     

Carbonization of coal blends: mesophase formation and coke properties

Laboratory investigations of strength of cokes from blends of coals incorporating pitch were supported by 7 kg trials. The stronger cokes showed a greater interaction between coal and pitch to produce an interface component of anisotropic mozaics which is relatively resistant to crack propagation. The process whereby coal is transformed into coke includes the formation of a fluid zone in which develop nematic liquid crystals and anisotropic carbon which is an essential component of metallurgical coke. Strength, thermal and oxidation resistance of coke can be discussed in terms of the size and shape of the anisotropic carbon which constitutes the optical texture of pore-wall material of coke. Coals of different rank form cokes with different optical textures. Blending procedures of non-caking, caking and coking coals involve the interactions of components of the blend to form mesophase and optical texture. Petroleum pitches used as additives are effective in modifying the carbonization process because of an ability to participate in hydrogen transfer reactions.     

Carbonization and liquid-crystal (mesophase) development: Part 1. The significance of the mesophase during carbonization of coking coals

Recent concepts of the growth processes of liquid-crystal structures, also called mesophase systems, during the carbonization of pitch substances is extended to coal carbonization. A basic model is formulated to explain the coal and coal-blend carbonization processes leading to metallurgical coke. This model explains the differences observed by optical microscopy in the size and shape of anisotropic structures seen in cokes in terms of liquid-crystal growth processes. These processes are considered to be restricted by chemical heterogeneity within the plastic phase, or to be influenced by the presence of solid surfaces (inerts) within the carbonizing system.     

Microscopic and chemical study of anisotropic mesophase possessing novel features

Anisotropic mesophase possessing novel features obtained from acenaphthylene was studied by optical and scanning electron microscopy. Variations in refluxing temperature affected the shapes of resultant anisotropic mesophase, suggesting that growth mechanisms were influenced. Polarized-light microscopy at high magnification revealed several structural types indicating that the single mesophase unit contains two domains with quite different lamellar molecular orientations. The novel mesophase, isolated by quinoline extraction, was also examined microscopically. It possessed shapes similar to those observed in dendritic growth of some graphitic substances. The quinoline-soluble fraction associated with this novel mesophase is assumed to be a dehydrogenated linear polymer of acenaphthylene, whereas the soluble matter associated with usual mesophase spheres is assumed to be decacyclene as previously reported. The mechanism of formation of this novel mesophase during refluxing is discussed in these terms.     

Co-carbonization of hard coals and coal extracts 2. The co-carbonization of medium- and high-rank coals with extracts

Studies on the influence of anthracene coal extracts on the carbonization process of medium- and high-rank coals were undertaken. Extracts from flame coal (Int. Class. 900) and gas-coking coal (Int. Class. 632) were used as additives. The blends prepared from the examined coals and the extracts exhibited better coking properties than the parent coals. The addition of extract to the coals gave an increase in the microstrength of the resultant cokes. The effects of co-carbonization of coking coals with extracts were increases in the size of the optical texture as well as in the degree of structural ordering of cokes. In the co-carbonization of semicoking coal with addition of coal extracts, a reduction in the size of the anisotropic units and a decrease in the crystallite height of cokes were observed. No modification of the basic anisotropy of coke from anthracite by coal extract was observed. With increasing extract content in anthracite/extract blends there was an increase in the degree of structural ordering of co-carbonization products. Extract addition was unable to modify the behaviour of fusinite. Based on the results of investigation of the influence of coal extracts on the carbonization of different-rank coals, a division of coals according to the modification of the optical texture of coke is given.     

Development of optical anisotropy in vitrains during carbonization

The purpose of this work was to examine the possible significance in the formation of metallurgical coke of the anisotropic spherical mesophase exemplified by that found during the carbonization of pitch-like materials, and to ascertain if the various types of optical anisotropy found in coke could form a basis for the characterization of cokes produced from different coals. Vitrains from a wide range of coals were carbonized at temperatures from 370 to 1000 °C and the types and amounts of optical anisotropy in the resulting semi-cokes and cokes were determined from microscopic examination, the anisotropic components being classified according to grain size of the granular mosaics and appearance. The anisotropy developed directly from the isotropic phase, appearing initially as a fine-grained mosaic. With increasing carbonization temperature, this fine-grained mosaic was transformed into progressively coarser-grained anisotropy, the extent of this transformation depending on the rank of the vitrain. It is therefore concluded that the formation, growth and coalescence of anisotropic spherical bodies, such as occurs during the carbonization of pitch, is not a necessary precursor of the mosaic anisotropy in coke. The type and amount of anisotropy developed provide a quantitative means of characterising different cokes.     

Formation of anisotropic mesophase from various carbonaceous materials in early stages of carbonization

The low-temperature carbonization of pitches, reduced crudes, synthetic polymers and various ranks of coals has been studied by polarized-light microscopy, X-ray diffraction and solvent extraction. Formation of anisotropic mesophase is a function of heat-treatment temperature and residence time for those materials that pass through a fusion stage in the temperature range from 350 to 500 °C. The nucleation of spherules from the pitches and reduced crude during low-temperature carbonization is proportional to the amount of β-resin, which is benzene-insoluble and quinoline-soluble, and the rate of its formation. Addition of sulphur to the pitch turns the texture of mesophase spherules to that of a fine mosaic when the mixtures are heat-treated in the temperature range of 350–500 °C. Methylation of the hydroxyl groups in 3,5-dimethyl phenol formaldehyde resin, by reacting with dimethyl sulphate, gives an anisotropic texture which consists actually of tiny spherules together with parent matrix; the original resin gives the typical mosaic texture. The molecular forces in the decomposing carbonaceous materials during low-temperature carbonization are among important factors governing the mesophase transformation. A common mechanism of formation of various anisotropic textures can explain the behaviour of coal-cokes, pitch, crudes and polymers.     

Carbonization and liquid—crystal (mesophase) development. 19. Co-carbonization of oxidized coals with model organic compounds

This study examines further the phenomena of the modification of coal carbonizations by organic additives. Anthracene, pyrene and chrysene modify the carbonization in a closed system of coking coals as observed from increases in the size of optical textures of resultant cokes. Weakly caking coals are unaffected. Chrysene is the most efficient modifier probably because of its lowest calculated free valence. The co-additives tetralin and hydrogenated anthracene oil further enhance the modification processes so obviating the necessity to use hydrogenated additives. Co-carbonizations of oxidized coking and caking coals with decacyclene are effective in removing the effects of mild oxidation. Increased rates of carbonization enhance the sizes of optical textures of resultant cokes.     

Characteristics and carbonization behaviors of coal extracts

N-methyl pyrrolidone (NMP) raw coal extract (EXT), hydrogenated coal extract (HEXT) and the blend of EXT and HEXT in NMP (BLD), from two bituminous coals, were studied. The extracts were carbonized in both tube-bomb and a temperature programmable furnace. Elemental analysis, FTIR spectroscopy and optical microscopy techniques were employed to characterize the extracts and the carbonization residues. It was found that the extracts resembled petroleum-derived pitches in the hydrogen content and (C/H)_atomic ratio. Higher oxygen and nitrogen contents differentiated the coal extracts from commercial petroleum pitch. More carbon and hydrogen, and lesser oxygen and sulfur differentiated HEXT from EXT. The ratios of integrated IR band intensity for aromatic and aliphatic CH stretching indicate that the relative content of aliphatic hydrogen in EXT is higher than in HEXT. HEXT contains comparatively more aromatic hydrogen, a feature necessary for thermal stability and fluidity during carbonization. BLD materials are at a place somewhere in between. Kinetic modeling of the aliphatic hydrogen change during carbonization reveals that EXT has high carbonization rate and low apparent activation energy. This can be related to the optical texture size of carbonization residues. The residues made from EXTs exhibited fine mosaic optical texture and limited mesophase development. HEXTs were readily converted into highly anisotropic coke. BLDs produced carbonization residues with intermediate properties. Extracts with similar activation energies produced similar residues in the same coal series. The degree or extent of anisotropy displayed by the carbonization residues was found to be dependent on the relative distribution of aromatic and aliphatic hydrogen.     

Investigation of the chemical structures of coking and non-coking coals in original and reductively alkylated solid states via 13C n.m.r. and i.r. spectroscopies

Chemical structures of four Turkish coals in original and reductively alkylated forms were investigated in the solid state by CP/MAS ¹³C n.m.r. and i.r. spectroscopies. Dilatation properties of these samples were also determined. It was observed that, while the aliphatic parts of the coking coals are mainly composed of short and straight alkyl groups and alicyclic structures, the aliphatic parts of the non-coking coals are to a larger extent composed of branched chains and/or alicyclic structures. It was concluded that ether bonds linking aromatic units are more prominent in the coking coals, whereas in the non-coking coals aromatic-O-aliphatic and/or alicyclic ethers dominate. The degree of condensation of aromatic structures was found to be higher in the coking coals, and the extent of reductive alkylations was higher in comparison with the non-coking coals. Non-coking coals were found to be alkylated preferentially at their phenolic oxygens. Difference spectra were found to be very useful in following organic chemical structural changes that accompany reductive alkylation.     

The effects of impregnation of coal by alkali salts upon carbonization properties

The co-carbonization of coking and caking coals with potassium and sodium salts destroys the coking and caking capacities of the coals. Further, the resultant char is of high surface area and exhibits a high chemical reactivity to oxidizing gases because of the catalytic influence of potassium retained within the char.This article attempts to explain the above phenomena, i.e. the loss of coking mechanisms, the development of high surface areas and the retention of the potassium. Initially, current theories are outlined of coking mechanisms which establish the anisotropic, carbonaceous structural units within resultant cokes. These structural units are best observed as isochromatic areas in colours of blue, yellow or purple, using a polarized light microscope and a half-wave retarder plate in conjunction with polished surfaces of coke. When a coking coal is carbonized, it first softens and melts to form an isotropic, pitch-like fluid. On further heating, anisotropic units of irregular shape develop from within this fluid phase. In coal systems, these units grow to about 0.5–5.0 μm at which stage they join or fuse together but do not coalesce. Their identity is maintained, and they establish what are termed finemozaics. At the same time, the macro-properties of coke, e.g. porosity, are established.The formation of these anisotropic mozaics occurs via the growth of lamellar nematic liquid crystals containing stacked lamellar molecules. The liquid crystals possess the crystalline order which is transferred to the solid coke substance. It is the plasticity of the liquid crystals which allows the growing anisotropic units to fuse together, and the introduction of disclinations which impart desirable properties to the coke substance.The addition of potassium salts to coking coals is thought to reduce the fluidity of the coals primarily by increasing the number of cross-links which normally exists between the aromatic and hydroaromatic constituent molecules (building blocks) in the coal. Such an increase results, in turn, in an increase in molecular weight of the coal, decrease in its fluidity upon heat treatment, and the consequent decrease in mobility of planar regimes preventing their alignment to form the liquid crystals and then the anisotropic mozaics. It is suggested that the presence of potassium results in a higher oxygen content being present in the coal upon heating, either by reducing the rate of oxygen evolution from the coal as CO, CO₂ and/or water or by acting as an intermediate to extract additional oxygen from the steam added as a reactant to the system (that is, steam gasification). Thus, an increased oxygen content results in more cross-linking in the structure probably via ether linkages between aromatic and/or hydroaromatic regimes. This increase in cross-linkage creates the isotropic carbon of the char, the spaces between the cross-linked constituent molecules being microporosity responsible for the high surface area of the char. The potassium could be retained within the microporosity by being bonded to the oxygen attached to the carbon.     

Some aspects of the role of coal thermoplasticity and coke structure in coal gasification: 3. The effect of rank,pitch and sodium carbonate on Brabender plastometry parameters

The thermoplastic properties of coal are a significant factor in determining gasification behaviour in most gasification processes. This investigation has involved the use of constant shear rate (Brabender) plastometry to study the plastic properties of a wide range of coals varying from 101 to 902 in the NCB Classification. The results obtained show more complex behaviour during the carbonization process than reported previously and are compared with ASTM Gieseler plastometry and BS dilatometry results. Some coals have two peaks in the caking region of the plastometry curve. This has been attributed to an initial peak corresponding to softening of the coal followed by caking due to active decomposition. The phenomenon was observed for coals with ranks 301a to 602, the temperature difference between the peaks being largest for the prime coking coals. It has been shown that this technique produces much additional information about the initial phase of the carbonization process. The effect of pitch and sodium carbonate addition on the plastometry parameters has also been studied. The results show that small amounts of these additives can radically change the plastometry curves. Pitch increases the plastic range whereas sodium carbonate reduces it. The addition of sodium carbonate tends to modify the plastometry curves so that they are similar to coals of lower rank whereas pitch addition, in general, tends to increase fluidity and, in the case of low-rank coals, produces plastometry curves similar but not identical to higher-rank coals.