Carbonization and liquid-crystal (mesophase) development. 21. Replacement of low-volatile caking coal by petroleum pitch in coal blends for metallurgical coke

Cokes were prepared in a 7 kg oven from blends of high-volatile and low-volatile caking coals, using ratios of 1:1 and 3:7. To the 1:1 blend was added 7.5% of either Ashland A240 or A170 petroleum pitch or SFBP petroleum pitch 1. Micum m₃₀ and m₁₀ indices were determined on cokes from the 7 kg oven, using the $\text{1}\text{5}$ Micum drum. Optical textures were assessed using polarized light microscopy of polished surfaces of cokes. The effect of additive is to increase the strength of cokes. The pitch can be an effective replacement of low-volatile caking coal. The analysis by optical microscopy shows that with the stronger cokes from the 7 kg oven there has occurred an interaction between the coal and pitch at the interface of coal particles to produce a solution or fluid phase which carbonizes to a coke with an optical texture of fine-grained mozaics. This material could be responsible for the enhancement of coke strength, being associated with pore wall material rather than with a change in porosity. The results agree with previous work using cokes prepared in the laboratory on a small scale.     

The possible role of fissure formation in the prevention of coking pressure generation

Fissure patterns have been studied for both high-volatile, low-coking pressure coals and low-volatile, high-coking pressure coals. The high-volatile coals form an extensive pattern of interconnected fissures, which seem to form early on and extend further toward the plastic region than the low-volatile coals, which form only a few fissures that do not extend very far into the charge. It is proposed that the combination of high-fluidity and extensive fissure network present for high-volatile coals may assist in allowing continual release of volatiles throughout the coking process and play a part in preventing the generation of high gas pressures. It was also found that a higher proportion of the volatiles for the low-volatile, high-coking pressure coals is released after the coal has been converted to semi-coke, which may play a role in preventing the effective release of gas.     

混煤燃烧过程中的交互作用:过量空气系数对混煤燃尽特性的影响

在沉降炉上对无烟煤、贫煤、烟煤、褐煤以及不同配比(25%、50%、75%)混煤开展了过量空气系数对混煤燃尽特性影响的实验研究。结果表明,混煤燃烧中挥发分高的煤对挥发分低的煤存在促进和抑制2种交互作用。"炉外"掺烧方式下,低挥发分煤与高挥发分煤掺混时,抢风抑制作用表现明显;尤其是掺烧75%高挥发分煤时,抢风抑制作用最为显著;提高过量空气系数可以改善贫氧气氛,减弱混煤燃烧中的抢风抑制作用,提高混煤燃尽率。"炉内"掺烧方式下,过量空气系数的增加,交互作用减弱,各单煤的燃烧独立性增强,混煤燃尽率逐渐接近计算线性燃尽率。     

Coal rank and coke reactivity

For a series of cokes prepared under identical carbonizing conditions it was shown that rates of reaction with carbon dioxide/nitrogen mixtures at about 1000°C were generally higher for the cokes prepared from coals of above 30% volatile matter. The reaction rates of cokes prepared from certain Durham coals were generally higher than anticipated. Gasification of cokes prepared from certain Durham coals were generally higher than anticipated. Gasification of the cokes resulted in loss of strength, decrease in apparent density, enlargement of pores and reduction of pore-wall thickness. It appeared that burn-off was primarily associated mineral constituents of the coke ash upon gasification rates was detected. A tentative relation was derived between gasification rates and the ratio of pore-wall thickness to pore diameter of the cokes. In turn, this latter ratio appeared to bear some relation to the oxygen content of the coal. The majority of the parameters studied were measured by established techniques. Porosity was determined by mercury pressure porosimetry and the Quantimet 720 automatic image analysis microscope was employed for structural studies. The accuracy of the measurement of pore and pore-wall dimensions was approximately ± 2%.     

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.     

Reactivity of heat-treated coals in air at 500 °C

Twenty one US coals, of widely ranging rank, have been carbonized under controlled conditions to 1000 °C, and the reactivity in air at 500 °C of the resulting chars or cokes has been measured by a gravimetric method. The reactivities lie within a well-defined band when plotted against rank of the parent coal. The lower-rank coal chars are more reactive than those prepared from high-rank coals. In extreme cases, the reactivity found for a Montana lignite char is some 100 times as great as that obtained for a char produced from a Pennsylvania low-volatile coal. Variation of reactivity with heat-treatment temperature (600 to 1000 °C) has been studied for three coals. As heat-treatment temperature increases, there is a decrease in reactivity. Some results are reported on the effects which mineral matter and pore structure have on the reactivity parameter. Chars containing high concentrations of magnesium and calcium impurities are most reactive. The amount of macro and transitional porosity in a char has a marked influence on reactivity.     

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.     

Correlation of reactivity and electrical resistivity of coke

A correlation between the reactivity and electrical resistivity of a series of cokes was determined using cokes prepared from blends of medium- and high-volatile coals. After correcting for the ash yield of the coke and the density of the resistivity specimen, a statistically significant relation between reactivity and resistivity was established. As the resistivity test requires much less time to perform than the reactivity test, it can be considered as an alternative approach to reactivity testing currently performed for quality control purposes.     

Effect of air-oxidation on the pore-structure of coals and cokes or chars obtained from oxidized coals

Seven kinds of coals (C, 77.8–89.8 wt% daf) were oxidized with air at 150 °C for 1, 5 and 10 h. The oxidized coals were heat-treated at various temperatures between 300 and 1500 °C with intervals of 50 or 100°C. The pore-structure of the oxidized coals and the cokes or chars obtained from the oxidized coals was compared with those of parent coals and their cokes or chars. True densities were measured in methanol and straight-chain hexane and pore volumes were determined by the Dubinin-Polanyi procedure. For the coals, the methanol-density increased with extent of oxidation; the hexane-density increased at first, but then decreased and again increased in the course of the oxidation. The air-oxidation of coals has a marked and controlling effect on the development of the pore-structure of cokes and chars in the course of the carbonization.     

Effect of moisture on the methane capacity of American coals

An investigation of methane sorption in bituminous coals ranging from low-volatile to high-volatile B has been carried out. Equilibrium sorption isotherms for dry and moist coal were measured at 30 °C and at pressures up to 60 atm. The natural oxygen content of a coal plays a major role in determining its methane capacity. Capacities of high-oxygen coals undergo a much greater reduction when saturated with moisture than do low-oxygen coals. Measurements of adsorbed-water saturation capacities clearly suggest that only adsorbed water affects the equilibrium capacity of a coal for methane; water present in excess of the adsorbed water has no effect on methane sorption. Excellent agreement has been found between the methane sorption data reported here and field measurements of methane emission from coal samples obtained during borehole drillings.     

Statistical analyses and prediction of coking properties of coal

Using regression analyses between the properties of coals and the strengths of their cokes several significant correlations are derived, which are useful to evaluate coals in the making of metallurgical coke. Slight but significant modification was necessary for their application to coal blends. For example, plasticities of the coal blends required a different equation from that derived for the single coals. The region of high coke-strength in the diagram of volatile matter vs. total dilatation was expanded considerably towards coals of lower caking properties by blending of coals, suggesting that the blending may serve to increase the coking properties of component coals. The coke strength, especially after the gasification was found to increase with the increasing inert maceral content in the parent coals up to 30 wt %. The high level of strength was maintained even above 35 wt % of inert content.     

Dependence of coal liquefaction behaviour on coal characteristics. 1. Vitrinite-rich samples

The liquefaction behaviour of a number of vitrinite-rich coals has been determined in batch autoclaves at temperatures of 385–425 °C and pressures of about 8.6 MPa (85 atm) of hydrogen. In one set of experiments, impregnated ammonium molybdate was used as catalyst, with no added liquid as vehicle. In a second set, a proprietary catalyst was used and anthracene oil served as vehicle. Lignites, sub-bituminous, medium-volatile and low-volatile bituminous coals gave relatively poor conversions. However, a lignite sample that had been subjected to ion-exchange treatments gave high conversion, and the viscosity and structural parameters of the products varied with the nature of the treatment. In general the highest conversions were observed for coals in the high-volatile bituminous range, but within this broad range and for the comparatively small set of samples studied neither these data nor the structural characteristics of the products show any very evident correlation with rank parameters or with the geological history of the sample. Two geologically young bituminous coals from the Pacific Coal Province gave excellent conversions; both had very high mineral-matter contents, a fact that may be very relevant.     

Catalytic graphitization of cokes by indigeneous mineral matter

Indigeneous mineral matter in coals acts catalytically towards graphitization during heat treatment of coals to 2273 K. Nineteen coals of a wide range of rank were demineralized by acid extraction. Original and demineralized coals were carbonized in the range 1073–2273 K, and the resulting cokes examined by optical microscopy, X-ray diffraction and phase-contrast high resolution electron microscopy. Optical microscopy indicated the extent of formation of anisotropic carbon in the resultant cokes. The (002) X-ray diffraction profiles indicated three types of catalytic effect, for which electron microscopy demonstrated different crystallite structures and interrelations. The importance of catalytic graphitization in metallurgical cokes in relation to their strength and reactivity is discussed.     

Fate of coal nitrogen during combustion

A total of 21 coals covering all ranks have been burned under a wide variety of conditions to ascertain the impact of coal properties on the fate of fuel nitrogen. Fuel NO was identified with a nitrogen-free oxidant consisting of Ar-O₂CO₂. In general, under fuel-lean conditions fuel NO formation increases with increasing fuel nitrogen content; however, other fuel properties also significantly affect the fate of fuel-bound nitrogen during combustion. In particular, fuel nitrogen conversion appears to be greater with coals containing a high fraction of volatile reactive nitrogen. Under fuel-rich conditions measurements of first-stage and exhaust-species concentrations suggest that the optimum stoichiometry for minimum emissions is a function of fuel composition. As first-stage stoichiometry is decreased, the NO formed in the first stage decreases, but other oxidizable gas nitrogen species increase as does nitrogen retention in the char. Total fixed nitrogen generally increases with increasing fuel nitrogen and correlates well with excess air exhaust emissions. The distribution of the total fixed nitrogen species leaving the first stage is strongly dependent upon the coal composition. Of the 12 coals tested in detail, only 1 (the high-volatile B bituminous from Utah) produced high HCN concentrations. The low-volatile Pennsylvania anthracite formed almost no HCN or NH₃ even under extremely fuel-rich conditions. In general, the first stage NO percentage decreased significantly with decreasing coal rank from anthracite to lignite. Conversely, the relative importance of NH₃ grew with decreasing rank. HCN was greater than NH₃ in all bituminous tests, but less than NH₃ with all subbituminous and lignite coals.     

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.     

水煤浆气化原料的成浆性研究

在实验室条件下研究了从低煤化度烟煤到高煤化度无烟煤,以及石油焦等不同气化原料煤的成浆性.为提高低煤化度烟煤的成浆浓度,在保证其混合原料灰熔融特征温度满足液态排渣前提下,将低煤化度烟煤与一种或两种煤化度较高的煤或者石油焦配比,考察了它们的成浆性.结果表明,煤化度适中的QD煤单独制浆浓度达到70%,黏度536mPa.s,流动性为A;通过不同煤种的级配,三种原料配合的料浆浓度为62%时,黏度在340mPa.s～550mPa.s之间,可以获得符合液态排渣气化要求的混合料水煤浆,扩大了气化原料来源.     

Hot-compaction behaviour of coals

Hot-compaction behaviour of semi-anthracite, low-, medium- and high-volatile caking, oxidized medium-volatile, sub-bituminous and lignite coals was studied under a constant load of 3.45 MPa. Generally, total compaction decreased and initial compaction temperatures increased with increasing coal rank. Initial compaction of the caking coals was noted at temperatures below their Ruhr dilatometer softening temperatures. For the high-volatile sample, this softening temperature was low enough, at about 250 °C, to imply possible thermal breakdown of such coals in preheating processes used before coke-making. The visco-elastic behaviour of coals of different rank over a range of temperature was examined. Densification mechanisms are suggested and the rate-controlling steps are discussed. Devolatilization is an important factor in particle-flow and shrinkage/dilatation mechanisms, as inferred from the similarities of the activation energies associated with these mechanisms and the reported values for devolatilization processes. Non-caking coals exhibit visco-elasticity at temperatures where they are expected to be semi-chars, in contrast with the caking coals.     

焦炭不同层次强度的影响因素研究

研究了7种单煤焦和1种配煤焦不同层次强度的影响因素。在焦炭显微特征中,影响因素较大的是焦炭平均壁厚B和光学各向异性指数H1。显微强度直接受焦炭孔壁的光学组织组成影响;结构强度受气孔率P和B影响较大;M40、M10主要取决于B和H1;CSR除受孔壁的厚度和孔壁的光学组成影响外,还与灰成分关系密切。对煤岩煤质指标参数影响较大的是炼焦煤的Rmax、S、∑I和G。     

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.     

Gasification studies of cokes from coals. The effects of carbonization pressure on optical texture and porosity

Ten coals were carbonized under various pressures (4 kPa, normal pressure and 10 MPa). Optical textures and physical structures of resultant cokes were monitored. The extent of optical anisotropy increased greatly with increasing carbonization pressure, such a trend being more pronounced with the lower-rank coals. Physical structure was also influenced by carbonization pressure. Gasification reactivities of the cokes with carbon dioxide and steam (1200 °C) were studied with respect to their optical anisotropy and physical structure. Gasification reactivities of optical textures were estimated using both the point-counting technique and regression analysis. The reactivities of cokes with the same optical texture produced from the same parent coal were similar. However, there were considerable differences when compared with cokes from different parent coals. Although the values estimated by regression analyses are consistent with those obtained by point-counting, except for the leaflet and inert textures, the physical locations of respective textures can be important in quantitative discussions of their reactivities.