Impact of low-cost filler material on coke quality

An investigation was made to study the influence of low-cost filler material such as non-coking coals and refuse-derived fuel (RDF) on coke quality. Interfaces between textural components within the cokes were successfully characterised and the derived interface quality index showed some cokes contained more ‘good’ quality interfaces than others. The addition of filler coals and RDF to the coking coal increased the proportion of ‘poor’ interfaces’. A good correlation between coke strength, derived from a small drum test, and interface quality index was observed. During heat treatment of cokes at 1600 °C both metallic and non-metallic micro-constituents were found to undergo some transformation as revealed by the SEM surface morphology examination. Although heat treatment caused some fractures to enlarge and others to emerge, its effect on the quality of the coke was not significant. Based on the results from the samples studied, there were some indications of the potential use of RDF material in the production of coke as there were minimal adverse effects on the quality of coke produced.     

A semi-industrial scale study of petroleum coke as an additive in cokemaking

The addition of petroleum coke to a typical industrial coal blend used in the production of metallurgical coke was studied. Cokes were produced at semi-industrial scale at the INCAR coking plant, using petroleum coke of different particle size distribution as an additive. Special attention was paid to changes caused in the textural properties (porosity, pore size distribution, fissures at the interface between metallurgical coke and petroleum coke) which have been found to be responsible for variations in the metallurgical coke quality parameters (e.g., mechanical strength and reactivity towards CO₂). Variation in porosity was found to depend on particle size and the proportion of the additive. The decrease in the microporosity (i.e., pore radius<3.7 nm) of the metallurgical cokes observed when petroleum coke is added to the coal blend, is postulated to be one of the main factors responsible for the decrease in the reactivity of these cokes. The variation of the mechanical strength indices can be explained by the changes in porosity and the quality of the interfaces between petroleum coke and metallurgical coke.     

A scanning electron microscope study of fractured and etched metallurgical coke surfaces

The object of this work was to attempt to link more closely coke strength and structure by establishing whether features visible on fracture surfaces could be identified with coke carbon textural constituents revealed either by polarized light microscopy of polished surfaces or by scanning electron microscopy of atomic oxygen-etched surfaces. The cokes used were produced in a laboratory furnace from coals covering the whole range or rank normally encountered in metallurgical coke production in the UK. Fracture surfaces were created by tensile fracture during diametral compression. In all three surfaces examined, the appearance of components derived from reactive coal constituents varied with the rank of the coal carbonized. A clear similarity was evident between features visible in the etched and fracture surfaces. The marked variation of fracture features imply that the textural composition of the coke carbon may make some contribution, as yet unquantified, to the variation in strength among cokes.     

Preliminary studies of the relation between the carbon texture and the strength of metallurgical coke

Preliminary attempts to relate the carbon texture to the tensile strength of metallurgical cokes are described. Two series of cokes made by carbonizing blended coal charges in pilot scale ovens were examined. The diametral compression test was used to determine the tensile strength of the cokes and the composition of the coke carbon was measured by applying a point-counting technique to the examination of atomic-oxygen etched surfaces. The strengths and textural compositions could be related by a single equation derived by multi-linear regression analysis.     

The strength of industrial cokes. 5. Influence of coke breeze in a coal charge on tensile strength of coke

The purpose of this study was to determine the influence of different proportions and different particle sizes of coke breeze in a coke-oven charge on the tensile strength of the coke. The diametrical-compression test was used to determine the tensile strength of the coke produced in a 10-t test oven and the results obtained were considered in relation to the composition of the oven charge, the coke micum indices and to parameters describing the coke texture. It was established that breeze additions caused measurable but nonsystematic changes in the coke tensile strength and that decreasing the breeze particle size generally increased the coke tensile strength. These changes could not however be directly related to changes observed in the density, porosity, pore-wall thickness or mean pore size of the cokes. The previously established relations between micum indices and the tensile strength of foundry cokes were also found to be inapplicable. The conclusion was drawn that the behaviour described is associated with some, at present unestablished, factor of the blend composition, one possibility being the relative proportions and compatibility of the ‘binder’ and inert material acting through their influence on those aspects of the coke microstructure which control the coke breakage.     

Effects of additions of petroleum coke in coals upon strengths of resultant cokes

Coals of NCB rank 301, 401 and 502 were co-carbonized with pitch-coke breeze pre-carbonized to temperatures between 900–1200 K, in the ratio 9:1. The objective was to provide fundamental information concerning the effect of inert components upon strength of metallurgical coke; these inert components occur naturally in coals and may also be added to coking blends as coke breeze. Polished surfaces of resultant cokes were examined by optical microscopy and fracture surfaces were examined by SEM to investigate the coal-coke/pitch-coke interface for bonding between components and fissure propagation across the interface. Strengths of cokes were measured using a micro-strength apparatus. For three coals, pitch-coke breeze (900 K and highest volatile content) bonded best to the surrounding coal-coke. The interface became increasingly fissured with increasing pre-carbonization temperature of pitch-coke.     

The strength of industrial cokes: Part 4. The influence of carbonizing conditions on the tensile strength

The objective of this study was to ascertain if the observed differences in strength behaviour of blast-furnace and foundry cokes could be attributed to the different carbonizing conditions used in their production. Two coal blends, one being representative for blast-furnace coke production and the other for foundry coke production, were carbonized in a small-scale test oven using a wide range of heating conditions which included those used in the industrial production of the two types of coke. Coke tensile strengths were determined by the diametrical-compression test and a small-scale drum test was used to derive strength indices comparable to standard micum indices. The tensile strengths and material constants obtained by Weibull statistical analysis, when related to those drum-test indices which assess the resistance of coke to attrition and to corresponding data for equivalent commercial cokes, demonstrated that the cokes fell into two distinct sets according to the coal blend used. It was concluded that changes in coke strength caused by different carbonizing conditions could not account for the different strength behaviour of blast-furnace and foundry cokes. The alternative hypothesis that the nature of the coal blend is the predominant factor is supported by the correlations established for each of the coal blends.     

单种煤结焦性能评价与焦炭质量预测

根据11种单种煤的性质,在实验室20kg小焦炉上进行了11种单种煤和8种配煤方案的炼焦实验,并对焦炭进行了筛分组成、冷态强度、焦炭热性质等分析,初步建立了焦炭质量预测模型。实验结果表明:单一的煤质指标与焦炭强度的关系不是很明显,选用煤质多因素指标进行焦炭质量预测,其预测效果较好;单一的煤质指标(Vd、R0max、Ad)与焦炭反应性之间有较好的关系,且焦炭的反应性随反应温度的升高而增大。     

Influence of pitch additions on coal carbonization: Characterization of pitches

The technique of etching polished coke surfaces and examination by SEM was used to compare the abilities of a series of pitches to modify the carbon texture of cokes prepared from two low-rank coals. Cokes prepared from the pitches were similarly examined and a numerical texture index, the magnitude of which increased with increasing content of the larger textural components, was found to provide a useful measure of the ability of the pitches to modify the coke carbon texture.     

The strength of industrial cokes. 6. Further studies of the influence of coke breeze in a coal charge on tensile strength of coke

The objective of this investigation was to ascertain if there was any pattern in the dependence of the tensile strength of coke on the proportion and particle size of coke-breeze in an oven charge and to establish if it was possible to interpret the changes in tensile strength in terms of coke structural features. Using a small-scale oven in order to obtain the optimum in close control of the charge preparation and carbonization conditions, cokes were prepared from each of two coking coals blended with coke breeze. The tensile strength of these cokes was determined by the diametrical-compression test and some details of their porous nature were determined from density measurements, mercury porosimetry and optical microscopy. The results clearly demonstrate that the tensile strength of coke is, in general, systematically reduced with increasing breeze content of the oven charge, the more coarsely ground breeze leading to a greater reduction of the tensile strength at any level of breeze addition. But very finely ground breeze at relatively low levels of addition can lead to an improvement in the tensile strength. These changes correlate with variations in the apparent density and the total porosity and possibly also with the average pore size.     

The strength of industrial cokes. 7. Further studies of the influence of additives in a coke-oven charge on the tensile strength of coke

The objective of this investigation was to determine if the previously established dependence of the tensile strength of coke on the breeze content and particle size of coke breeze in the coke-oven charge was applicable to different types of breeze additives when used in a size range similar to that of commercial practice. Using a small-scale oven to obtain the desired close control of the charge preparation and carbonization conditions, cokes were prepared from a Yorkshire strongly-caking coal blended with either coke-oven breeze, petroleum-coke breeze, or silica sand. The tensile strength of the cokes was determined by the diametral-compression test and some details of their porous structure were obtained from density measurements and mercury pressure porosimetry. The results confirm that the tensile strength of coke varies systematically with the coke-oven breeze content of the oven charge, and in the present case, for a breeze of the particle size range used in commercial practice the tensile strength is increased at low additions and then progressively reduced at higher levels of addition. Different sources of coke-oven breeze behave in a similar manner and appear to act as an inert filler material. On the other hand petroleum-coke breeze additions progressively increase the coke tensile strength, the additive being bonded into the walls of the coke matrix. The changes in tensile strength are accompanied by systematic variations in apparent density and in porosity.     

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.     

SEM study of binder coke in electrode carbon

As part of a study on the use of coal-tar pitches as binders in electrode carbons for the aluminium industry, the object of the present work was to attempt to identify textural components in electrode binder cokes. Accordingly, four experimental carbons, made using the same petroleum coke as filler and four coal-tar pitches of differing characteristics as binders, were examined in a scanning electron microscope (SEM) after embedding in resin, polishing and etching in atomic oxygen. Binder cokes could be distinguished from the filler particles and could be characterized in terms of the content of three components, termed lamellar, intermediate and granular. The textural composition of the binder cokes was dependent upon the character of the pitch and the type of quinoline-insoluble components present. Pitch cokes, made by carbonizing the pitches alone, differed markedly from the corresponding binder cokes. The findings are considered in terms of current ideas on pitch carbonization.     

Thermal degradation of metallurgical coke

To understand the drastic deterioration of coke at the 3–5 m level above the tuyeres in the blast furnace, studies were made of the tensile strength in relation to the JIS drum strength, and measurements were made of the tensile strength of coke at high temperatures (maximum 2300 °C). Plastic deformation was observed above 2000 °C and this could be estimated from the microstrength and the porosity of coke. It was concluded that the factors affecting the resistivity to plastic deformation were the coke bonding between inerts and reactives and the existence of amorphous texture, and that the thermal degradation of the matrix strength must contribute, to some extent, to the drastic reduction in drum strength in the lower part of blast furnace.     

Influence of gas composition on the reactivity of cokes

The NSC reactivity test is often criticised for not being able to accurately predict the performance of cokes in the blast furnace. One explanation proposed for this inaccuracy is that the gas used in the NSC test, pure carbon dioxide, is different to the gas that coke is exposed to in blast furnaces, which is a complex mixture including carbon monoxide, carbon dioxide and nitrogen. The aim of this work was to see to what extent different cokes behaved differently during the NSC test under different gases.Nine Australian coals, used in coking blends, were selected to cover a wide range of rank, maceral composition and elemental composition of the mineral matter. These coals were coked and their relative reactivities in a series of gas mixtures were compared. The time for the reaction of the coke in a 30% CO₂/70% CO mixture was set to eight hours to give about the same weight loss as two hours exposure to 100% CO₂.The main conclusion of this study was that gas composition (using mixtures of CO, CO₂ and N₂) had little effect on the relative rate of gasification of cokes over a wide compositional range of gas and of coke (although of course the absolute reaction rate decreased with decreasing CO₂ levels). The previous studies that suggested changes in gas composition affect the relative reaction rate of different cokes were misleading because they performed the studies at constant burnoff time (2 h) rather than ensuring the cokes were reacted to about the same weight loss.Thus any differences between the behaviour of cokes in the NSC test and in the blast furnace are not due to differences in gas composition between the two.The CSR value was found to be a combination of strength and reactivity: for the data in this study, CSR was determined by a two-component fit involving CRI and the I600/10 index (the strength of the unreacted coke as measured by the CSR tumble test).     

Effect of pore diffusion on reactivity of lump coke

The gasification of fine-grained and lump coke in CO₂ atmosphere was measured in a laboratory reactor. Owing to the effect of pore diffusion, the reactivity of lump coke at 1050 °C was found to be considerably lower than that of fine-grained coke at otherwise the same conditions. The diffusion effect is more significant for cokes produced by a stamping method than for cokes from top charges. To express the gasification of lump coke, a simplified mathematical model considering the effect of pore diffusion on gasification rate was adopted. Kinetic parameters were determined from experiments with fine-grained coke, while effective diffusivity was evaluated from experiments with a single coke particle. The model fits experimental data of lump coke well. The relation between reactivity and textural characteristics was also examined: the correlation between effective diffusivity and the fraction of largest pore volume was found.     

溶损反应动力学对焦炭溶损后强度的影响

为了分析溶损反应动力学行为对焦炭反应后强度的影响,在1 050℃～1 300℃的范围内,对三种典型焦炭进行了等温溶损反应实验.当焦炭的溶损失重率为25%时停止反应,通过I型转鼓检测焦炭的溶损后强度.结果表明,不同反应性的焦炭发生最严重的劣化梯度反应时的温度不同,高反应性焦炭为1 100℃左右,低于其他两种焦炭.虽然高反应性焦炭发生最严重的劣化梯度反应后的强度很低,但改变反应温度使反应速率加快或减慢都能使高反应性焦炭的反应后强度显著地提高.     

Effect of low-volatile additives on the structure and strength of cokes

Blends of medium-volatile or high-volatile coals have been carbonized in a 7 kg oven with low-volatile coals (6–16% VM, dmmf). A comparison is made of the strength and structural properties of these cokes with those of the cokes made under corresponding conditions from the medium or high-volatile coals alone. With increasing levels of addition of the low-volatile coals the tensile strength of the blend cokes generally attains a maximum and then decreases. These strength changes are related to changes in porosity, pore-wall thickness and pore dimensions. Coals which display some degree of plasticity and which are weakly caking improve coke quality by altering the pore-structure due to the combined effects of decreasing the pore diameter and slightly increasing the wall thickness. Those additives which are non-caking act primarily as wall thickeners.     

Bonding of solid additives in cokes from coals: a microscopy study

Cortonwood Silkstone (NCB class 401) and Betteshanger (NCB class 301 a/204) coals were co-carbonized with solid additives such as anthracite, coke breeze, green and calcined petroleum cokes. The resultant carbonization products (cokes) were examined by optical microscopy and SEM was used to investigate polished surfaces etched by chromic acid and fracture surfaces. For both coals only the anthracite and green petroleum coke become bonded to the coal cokes. This probably results from softening and interaction of interfaces of the anthracite and green coke with the fluid coal via a mechanism of hydrogenating solvolysis during the carbonization process. The coke breeze and calcined petroleum cokes were interlocked into the matrix of coal coke.     

Examination of some effects of dry cooling on coke quality

A series of coals were carbonized, on the 250 kg scale under standardized conditions, to provide both dry-cooled and wet-quenched cokes which were subsequently subjected to reactivity and strength testing. The data from the tests of reactivity to CO₂ at ≈1000 °C support the view that dry coke cooling leads to lower reactivity, but examination of the porous structure in the pore size range > 5.45 μm and of the optical anisotropy of the coke carbon revealed no changes to account for this effect. Although the micum test indices were sometimes improved by dry coke cooling, the differences were not statistically significant. On the other hand, there was clear evidence of increased coke tensile strength. The effect of dry coke cooling on coke properties appears to be sufficient to exert some influence on the blast furnace coke rate and thereby on the economy of the dry-cooling process.