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. 9. The co-carbonization of vitrains with Ashland A200 petroleum pitch

Vitrains from a wide range of ranks of coals were carbonized singly and also co-carbonized (HTT 1273 K) with 25% of Ashland A200 petroleum pitch. Polished surfaces of the resultant cokes were examined for optical texture in a polarizing-light optical microscope using a half-wave retarder plate to produce interference colours. For the anthracites, there is no modification of either component during co-carbonization. The growth of optical texture from the A200 pitch is not affected. For all caking vitrains the optical texture of coke from the blend system is extensively modified when compared to the optical texture of coke from the vitrain. For the low-rank non-caking vitrains the isotropic coke becomes totally or partially anisotropic in co-carbonization. The mechanism of modification of the optical texture of resultant cokes is related to the formation of nematic liquid crystals, mesophase and the semi-coke. It is not considered that the chemistry of pyrolysis is modified on cocarbonization of the vitrain and pitch.     

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.     

Carbonization and liquid-crystal (mesophase) development. 22. Micro-strength and optical textures of cokes from coal-pitch co-carbonizations

This study examines the micro-strength and optical textures of a laboratory coke from a base-blend of Freyming and Pocahontas coal (wt ratio, 1:1) and of cokes from the co-carbonization of the blend, with each of five petroleum pitches in various proportions. Coke pieces, 212–600 μm, from the micro-strength test are assessed in terms of origin and propagation of cracks induced by the test. Always, the addition of pitch to the base-blend improves the strength of the resultant cokes, the pitches behaving differently. A qualitative, subjective appraisal of results indicates that increases in coke strength are associated with relative abilities of pitches to interact with the coals to produce a fluid phase, of solution of coal in pitch, which gives an ‘intermediate’ coke with an optical texture of mozaics. This intermediate coke strengthens the bonding at interfaces. Cracks originate predominantly from the shrinkage cracks in the domains of Pocahontas coke. Mozaic structures tend to resist crack propagation. The coal/pitch system may flow around coal particles so containing incipient crack formation in resultant coke particles.     

Carbonization and liquid crystal (mesophase) development. 14. Co-carbonization of coals with A240 Ashland petroleum pitch: effects of conditions of carbonization upon optical textures of resultant cokes

This study examines the effect of pitch concentration, rate of heating, soak temperature and time of soak upon the optical texture of cokes prepared from the co-carbonizations of a coal (Oxcroft-Clowne, NCB Rank 802) and three vitrains of NCB Rank 204, 801, 902 with Ashland A240 petroleum pitch. Using the coal (Rank 802) with 10 wt % and 25 wt % additions of pitch caused progressive penetration of the pitch into the coal with a resultant development of a mozaic anisotropy in the coke to replace partially the original coke isotropy. With 50 wt % addition of pitch almost all of the coal particles, 600 to 1100 μm in size, were modified during carbonization. Some pitch coke was formed. For the coal and three vitrains with increasing rates of co-carbonization from 0.5–10 K min^?1 to 1200 K, using 25 wt % of A240 pitch, resultant cokes showed progressively increased extents of modification. For the two vitrains (Rank 801, 902) soaking at temperatures of 650–690 K caused a decrease in the extent of modification of isotropic coke when compared with the coke of HTT 1200 K. Evidently fast heating rates create the conditions of fluidity necessary for the pitch to modify the coal leading to growth of mesophase and anisotropic coke.     

Binder—filler bonding in anode carbon Influence of filler coke surface

Bonding between binder and filler cokes in an experimental anode carbon, made using a coal-derived filler coke and a normal electrode-binder pitch, was investigated using scanning electron microscopy. The surface of each filler coke particle was found to contain rough and smooth areas formed respectively by breakage of the coke matrix during crushing and devolatilization pore formation during coal carbonization. Good bonding, indicated by continuity of structure, could be achieved between the fracture surfaces and granular or suitably aligned lamellar binder coke. Inadequate binder-filler bonding at smooth pore surfaces was evident by shrinkage fissures at the interface. The findings are explained in terms of the structures of the various types of carbon and the availability of edge carbon atoms for bond formation.     

炭化条件对针状焦结构的影响

以除去喹啉不溶物的中温煤沥青为原料,分别在不同的反应温度和保温时间下制备了中间相炭微球(MCMB);在磁场条件下制备了有序结构针状焦;通过扫描电镜(SEM)考察了不同反应条件下中间相炭微球和针状焦的形貌,讨论了中间相形成影响因素对针状焦结构的影响.结果表明,中间相形成阶段的反应温度、保温时间和体系黏度对针状焦的结构和性能具有重要影响,磁场对针状焦的流线型结构有促进作用.     

Carbonization and liquid-crystal (mesophase) development. 16. Carbonizations of soluble and insoluble fractions of coal-extract solution

A coal-extract solution prepared by extraction of a coking coal (CRC 301a) with anthracene oil by the National Coal Board is separated into fractions using solvents of increasing solvent power. These fractions are carbonized to 823 K and the optical textures of resultant cokes are assessed. The objective of the study is to examine the role of the molecular components of the coal-extract solution including the residual anthracene oil in mechanisms of formation of the optical texture of the anisotropic coke. Generally, the low-molecular-weight fractions of the coal-extract solution produce cokes with larger sized optical textures than the coke from the parent coal-extract solution. The higher-molecular-weight fractions produce cokes with smaller sized optical textures. Isotropic coke is produced from material which is not soluble in benzene and tetrahydrofuran. Within this parent-coal-extract solution it would appear that the dominant partner effect is influential over the size of the optical texture of coke from the coal-extraction solution, that is the minor component of smaller molecules controls the necessary growth of liquid crystals. Also, the presence of anthracene oil augments the size of optical texture of resultant cokes by providing the necessary physical fluidity of the system and possibly some chemical stability.     

The microstructure of nuclear graphite binders

The microstructures of the binder in two grades of nuclear graphite, Gilsocarbon graphite and Pile Grade A graphite, were characterized by light microscopy, transmission electron microscopy and high resolution transmission electron microscopy. A variety of structures of carbon were observed, including a well-graphitized structure, nanosized graphite particles, quinoline insoluble (QI) particles, chaotic structures and non-graphitizing carbon. QI particles were observed on the surface of mesophase spheres as well as inside mesophase spheres. The aggregation of QI particles on the surface of mesophase spheres, or very near the surface in mesophase spheres, appears to have a significant influence on the development of the mesophase structure, resulting in refinement of the mesophase spheres before their coalescence. The chaotic structure, which is turbostratic and isotropic, is suggested to have developed from the isotropic pitch remaining between mesophase spheres. The non-graphitizing carbon consisted of flat and curved single layer graphene fragments with a size typically less than 1 nm. The formation of the various structures in the two graphites is attributed to the introduction of different pitches at stages of the graphite manufacture.     

Growth and coalescence of mesophase in pitches with carbon black additives and in coal

The techniques of polatized light optical microscopy and of washing the surfaces of solid pyrolysis products with chloroform prior to SEM examination are used to monitor the growth and coalescence of growth units of mesophase in a petroleum pitch, a coal extract and a caking coal. Additions of 1 wt% of carbon black retard growth and coalescence and promote clustering of these units because of adhesion of carbon black particles. This has the effect ultimately of reducing the size of the optical texture in coke from the coal extract, but not with coke from the petroleum pitch which has lower viscosity. With the coal, mesophase growth units tend to form clusters and do not coalesce. Mesophase can form an anisotropic skin on the inside of developing pores (bubbles) in the fluid phase and this may limit their growth.     

Carbonization and liquid-crystal (mesophase) development. 17. Co-carbonization of a solvent-refined coal with petroleum pitches and hydrogenated coal-extract solution

This study characterizes the optical textures of cokes prepared by the carbonization of Ashland petroleum pitches, of non-hydrogenated and hydrogenated coal-extract solutions (CES) and of blends of non-hydrogenated CES materials with the petroleum pitches and hydrogenated CES materials. At an HTT of 823 K petroleum pitches produce cokes with large sized optical texture of flow-type anisotropy characteristic of needle-cokes. The non-hydrogenated CES materials produce cokes with optical textures of mozaics, 2–10 μm. However, following hydrogenation the CES materials carbonized to cokes all of which possess considerable large sized optical textures which for some materials resemble that of needle-cokes by possessing strong flow-type anisotropy, > 100 μm. Hydrogenation of the CES materials evidently facilitates the physical and chemical requirements for growth and coalescence of lamellar nematic liquid-crystals and mesophase from the fluid phase of carbonization leading to anisotropic carbon. Co-carbonizations of the non-hydrogenated and hydrogenated CES materials exhibit the dominant partner effect and are comparable in behaviour with Ashland petroleum pitches which are known to produce needle-cokes on carbonization.     

Carbonization and liquid-crystal (mesophase) development. 11. The co-carbonization of low-rank coals with modified petroleum pitches

Coals of rank ranging from medium quality coking to non-caking, non-fusible, have been co-carbonized with Ashland petroleum pitches A170, A240 and A200 as well as pitches modified by heat-treatment with aluminium chloride using A170, and by reductive hydrogenation of the A200. The mixing ratio was 7:3, the final HTT was 873 K, heating at 10 K min^?1 with a soak time of 1 h. The optical texture of the resultant cokes is assessed using polished surfaces and a polarized-light microscope using reflected light and a half-wave plate. The changes in optical texture are studied from the point of view of using coals of low rank in the making of metallurgical coke. The optical texture of resultant cokes is modified by co-carbonization and the mechanism involves a solution or solvolysis of the non-fusible coals followed by the formation of nematic liquid crystals and mesophase in the resultant plastic phase. The modified A170 pitch is more effective in modifying optical texture than the A170 because of an increase in molecular weight. The hydrogenated A200 is a very reactive additive probably because of an increased concentration of naphthenic hydrogen. The hydrogenated A200 can modify the optical texture of cokes from the organic inerts of coals and from oxidized, non-fusible coals.     

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.     

Structural study of cokes using optical microscopy and X-ray diffraction

Cokes exhibiting a range of optical texture from isotropie to anisotropic domains > 60 μm diameter were examined by X-ray diffraction. The variation of an optical texture index (OTI) with crystallite height and interlayer spacing was studied. The OTI varies little with the X-ray parameters for cokes whose optical texture is larger than medium-grained (1.5–5.0 μm) mosaic anisotropy. For cokes of smaller optical texture there is a sharp decrease in crystallite height and an increase in interlayer spacing. These results are discussed in terms of fluid mesophase removing defects in cokes of optical texture of size of coarsegrained mosaics and larger. The cokes of smaller optical texture are formed from less fluid mesophase which does not coalesce. Defects therefore remain in this anisotropic carbon of the coke so reducing crystallographic order.     

Carbonization behavior of hydrogenated ethylene tar pitch

Carbonization behavior of ethylene tar pitch has been studied with respect to mesophase formation by means of modification of the chemical composition of the starting materials. The hydrogen treatment of ethylene tar pitch has been carried out over the temperature range from 473 to 673 K under a pressure of 10 MPa without catalyst. Then, the hydrogenated ethylene tar pitches were carbonized at 723 K and the optical texture of the resultant cokes were assesed by optical microscopy. It was revealed that the carbonization of the ethylene tar pitch hydrogenated at 673 K gives a coke of optical texture with enlarged flow-domain. The hydrogen-transfer ability of the ethylene tar pitches during the temperature range of mesophase formation was estimated by the method of 9,10 dihydroanthracene (DHA) formation through co-carbonization of the pitch with anthracene. It was recognized that the larger the amount of conversion of DHA, the better is the development of optical texture.     

The role of low-molecular-weight components in the pyrolysis of pitches

Two pitches of different volatile contents, obtained from the same coal tar by different procedures, were characterized by elemental analysis, solubility, ¹H n.m.r., donor—acceptor ability, FT-i.r., extrography and size-exclusion chromatography. Their pyrolysis behaviour was followed by hot-stage microscopy and thermogravimetric analysis. The pitches were carbonized in a horizontal furnace and the resulting cokes were characterized by optical microscopy. Three partly devolatilized pitches were prepared by thermal treatment of the two pitches and their pyrolysis behaviour was compared in terms of mesophase development and optical texture of coke. The results indicate a significant influence of low-molecular-weight components of pitch on the development of mesophase but no effect on the subsequent optical texture of the coke.     

酚醛树脂改性煤焦油沥青中制得的中间相球体的结构特征

Mesocarbon microbeads （MCMB） were prepared from coal tar pitch modified by phenolic resin and from the same pitch modified by phenolic resin and hexamethylenetetramine at 440℃ for lh. By investigating the morphology of mesophase spheres and the structure of the MCMB carbonized at 1000℃ for lh using scanning electron microscope （SEM） and XRD, it was found that phenolic resin accelerated the formation and coalescence of mesophase spheres. Some of the obtained MCMB were hi- or tri-spheres with the distorted microtextural carbon layers. Hexamethylenetetramine in the pitch modified by phenolic resin accelerated the condensation of phenolic resin and consequently expedited the combination of mesophase spheres, which was proved by the formation of some tetra-spheres. Owing to the cross-linkage of the additives, MCMB with complex structure were obtained.     

Carbonization and liquid-crystal (mesophase) development. 10. The co-carbonization of coals with solvent-refined coals and coal extracts

Coals (NCB rank 102 to 902) were co-carbonized with solvent-refined coals and coal extracts, mixing ratio of 7:3, to 873 K, heating at 10 K min^?1 with a soak period of 1 h. Resultant cokes were examined in polished section using reflected polarized-light microscopy and optical textures were recorded photographically. These optical textures were compared to assess the ability of the additive pitch to modify both the size and extent of optical texture of resultant cokes. The objective of the study is to provide a fundamental understanding of the use of pitch materials in co-carbonizations of lower-rank coals to make metallurgical coke. A Gulf SRC was able to modify the optical texture of cokes from all coals except the anthracite. Soluble fractions of this Gulf SRC were less effective than the parent SRC. A coal extract (NCB D112) modified coke optical texture, the extent being enhanced as the rank of coal being extracted was increased. Hydrogenation of the coal extract increased the penetration of the pitch into the coal particles but simultaneously reduced the size of the optical texture relative to the non-hydrogenated pitch. This indicates a positive interaction of pitch with coal in the co-carbonization process. The optical texture of the cokes from the hydrogenated coal extract in single carbonizations was larger than that from the non-hydrogenated material. Mechanisms explaining these effects are briefly described.     

Unusual structures in mesophase from a petroleum pitch

Cokes from the carbonization of a petroleum pitch possess an optical texture containing anisotropic spherules with an unusual structure. These spherules (10 to 20 μm diameter) were found to be present in the original pitch and could be removed as a quinoline-insoluble fraction. This suggests that the spherules are formed during the modification of the pitch after initial fractionation of the petroleum feed-stock. The optical texture of these spherules, examined by polarized light optical microscopy, reveals a complicated ‘rosette’ structure quite unlike the structure of the spheres of mesophase now known as ‘Brooks and Taylor’ spheres. An EDAX analysis of polished segments of the spherules indicated no unusually high concentration of inorganic matter, e.g. vanadium, suggesting that the spherules did not originate from asphaltenes. The influence of such spherules or inclusions in the parent pitch upon properties of resultant cokes is briefly commented upon.     

The structure and behaviour of QI material in pitch

The QI material in coal-tar pitches is characterized by optical and scanning electron microscopy. Five main groups of QI are distinguished, primary, secondary, isotropic, foreign inclusions and material of unusual shape. Isotropic QI is considered to consist of asphaltene-like molecules dispersed in the pitch. It is found in air and acid-treated pitches and increases the coke yield compared with untreated feedstock pitches. Mesophase extracted from heat treated A240 pitch is heat treated in isolation and after mixing with fresh A240 pitch. At 375°C, mesophase partially dissolves in the fresh pitch if decacyclene is present. At 400°C, mesophase spheres heated in isolation coalesce only to a limited extent. The presence of fresh pitch is necessary for unhindered coalescence of extracted mesophase spheres and the development of optical texture of maximum size.