The mechanism of coking pressure generation I: Effect of high volatile matter coking coal, semi-anthracite and coke breeze on coking pressure and plastic coal layer permeability

One of the most important aspects of the cokemaking process is to control and restrain the coking pressure since excessive coking pressure tends to lead to operational problems and oven wall damage. Therefore, in order to understand the mechanism of coking pressure generation, the permeability of the plastic coal layer and the coking pressure for the same single coal and the same blended coal were measured and the relationship between them was investigated. Then the ‘inert’ (pressure modifier) effect of organic additives such as high volatile matter coking coal, semi-anthracite and coke breeze was studied. The coking pressure peak for box charging with more uniform bulk density distribution was higher than that for top charging. It was found that the coking pressure peaks measured at different institutions (NSC and BHPBilliton) by box charging are nearly the same. The addition of high volatile matter coking coal, semi-anthracite and coke breeze to a low volatile matter, high coking pressure coal greatly increased the plastic layer permeability in laboratory experiments and correspondingly decreased the coking pressure. It was found that, high volatile matter coking coal decreases the coking pressure more than semi-anthracite at the same plastic coal layer permeability, which indicates that the coking pressure depends not only on plastic coal layer permeability but also on other factors. Coking pressure is also affected by the contraction behavior of the coke layer near the oven walls and a large contraction decreases the coal bulk density in the oven center and hence the internal gas pressure in the plastic layer. The effect of contraction on coking pressure needs to be investigated further.     

炼焦煤焦化特性评价指标探讨

介绍了奥亚膨胀度、基氏流动度、罗加指数、胶质层指数、黏结指数和镜质组平均最大反射率等冶金煤焦化特性评价指标的建立和测试方法,以及各评价指标在我国的应用情况。同时讨论了各个指标对胶质体的原生黏结性、膨胀程度引起的黏着性和焦炭残留物强度这3个焦化特性的表征。     

The effect of plastic size on coke quality and coking pressure in the co-carbonization of coal/plastic in coke oven

In the recycling process of waste plastics using coke ovens, coals and added plastics are carbonized and changed into coke, tar, oil and coke oven gas in a coke oven chamber. In this study, the effect of added plastic size on coke quality and the effect of plastic addition on coking pressure was investigated. In the case of a plastic addition rate of 2%, the coke strength reached a minimum at the particle size of 10 mm for polyethylene (PE) and 3 mm for polystyrene (PS). The mechanism was attributed to the weak coke structure formed on the interface between plastic and coal. The result indicates that large or small plastic particles are favorable in order to add waste plastics to blended coals for coke making without affecting coke strength . Furthermore, it was also shown that a 1% addition of large size agglomerated waste plastics to blended coals did not increase coking pressure. Based on this fundamental study, and considering the ease of handling plastics, we have determined that the size of waste plastic used in a commercial-scale recycling process of waste plastics using coke ovens is about 25 mm. Nippon Steel Corporation started to operate a waste plastic recycling process using coke ovens at Nagoya and Kimitsu works in 2000 and at Yawata and Muroran works in 2002. Now the total capacity is 120,000 tons per year as of 2003 and this process is operating smoothly.     

Influence of coal thermoplastic properties on coking pressure generation: Part 2 - A study of binary coal blends and specific additives

A number of coal blends and pitch/coal blends were evaluated using rheometry, thermogravimetric analysis and microscopy to confirm and further elucidate the coking pressure mechanism previously proposed by Duffy et al. (2007) [1]. We confirm that blending a low rank, high fluidity, low coking pressure coal, with a high rank, low fluidity, high coking pressure coal can significantly reduce the coking pressure associated with the latter. Interestingly, blending does not necessarily result in a fluidity that is midway between that of the two coals; sometimes the fluidity of the blend is less than that of the low fluidity coal, especially when the coals are significantly different in rank. This occurs because the increase in complex viscosity (η*) through resolidification of the low rank, high fluidity coal counteracts the reduction in η* resulting from softening of the high rank, low fluidity coal. It has also been confirmed that the η* of the resultant blend can be estimated from the η* of each component coal using a logarithmic additivity rule commonly employed for polymer blends.Polarised light microscopy has indicated that the degree of mixing between coals of different rank is minimal, with fusion restricted to the particle surface. It is therefore inappropriate to think of such a coal blend in the same way as a single coal, since each component coal behaves relatively independently. This limited fusion is important for understanding the coking pressure mechanism for blends. It is proposed here that the lower rank coal, which softens at lower temperature, is able to expand into the interparticle voids between the high rank coal that is yet to soften, and these voids can create channels for volatiles to traverse. Then, and importantly, when the high rank coal begins to expand, the pore structure developed in the resolidified structures of the low rank coal can facilitate removal of volatiles, while the resolidified material may also act as a suitable sorbent for volatile matter. This is considered to be the primary mechanism by which coal blending is able to alleviate coking pressure, and applies to addition of inert material also.Addition of a coal tar pitch was found to increase fluidity but also to extend the thermoplastic range to lower temperatures. This caused an increase in the swelling range, which was accompanied by a long plateau in η*, a feature which has previously been observed for certain high fluidity, high pressure coals. Elasticity and η* at the onset of expansion were also higher for both the pitch impregnated coals and the high pressure blends, which supports previous findings for singly charged high pressure coals, and confirms the potential use of such criteria for identifying potentially dangerous coals/blends.     

Influence of porosity and fissuring on coking pressure generation

Nine bituminous coals of different rank and geographical origin were carbonized at pilot scale coke oven (300 kg) in order to study the pressure generated during coking. At the same time their contraction/expansion was assessed by means of the Koppers-INCAR test. Semicokes were carefully recovered from the test so that their structure could be studied. The semicokes were separated into two parts, i.e. one that had been heated to 575 °C and the other that had been heated to 700 °C. The true and apparent density of the semicokes was measured together with their pore size distribution by means of mercury porosimetry and the results were related to the dangerousness of coals. The structure of the semicokes from safe and dangerous coals is different especially in those obtained at lower temperature. In addition, the fissures of the semicokes were evaluated. The area of the fissures was found to be greater in the case of non-dangerous coals.     

The mechanism of coking pressure generation II: Effect of high volatile matter coking coal, semi-anthracite and coke breeze on coking pressure and contraction

One of the most important aspects of the cokemaking process is to control and limit the coking pressure since excessive coking pressure can lead to operational problems and oven wall damage. Following on from a previous paper on plastic layer permeability we have studied the effect of contraction of semi-coke on coking pressure and the effect of organic additives on contraction. A link between contraction (or simulated contraction) outside the plastic layer and coking pressure was demonstrated. The interaction between this contraction, local bulk density around the plastic layer and the dependence of the permeability of the plastic layer on bulk density was discussed as possible mechanisms for the generation of coking pressure. The effect of blending either a high volatile matter coal or one of two semi-anthracites with low volatile matter, high coking pressure coals on the coking pressure of the binary blends has been explained using this mechanism.     

澳大利亚煤代替西曲焦煤的配煤炼焦研究

对澳大利亚煤和国内6种单种煤进行煤质分析和配煤炼焦实验,分析澳大利亚煤代替西曲煤进行炼焦的可行性。结果表明澳大利亚煤具有低灰低硫、高挥发分等特点,在炼焦中用澳大利亚煤替代西曲焦煤可降低焦炭的灰分和硫分,增大焦炭的各向异性指数,改善焦炭强度。     

Influence of thermoplastic properties on coking pressure generation: Part 1 - A study of single coals of various rank

In this study a number of high coking pressure coals with different fluidities were evaluated alongside a number of low pressure coals also with differing fluidities. This was to confirm findings from an earlier study using a limited selection of coals, and to establish rheological parameters within which a coal may be considered potentially dangerous with regards to coking pressure. The results have confirmed and elaborated on previous findings which show that parallel plate displacement (ΔL) and axial force profiles can be used to distinguish between high and low pressure coals, with peak values indicating cell rupture and subsequent pore network formation. This is thought to correspond with plastic layer compaction in the coke oven.For low pressure coals pore coalescence occurs quite early in the softening process when viscosity/elasticity are decreasing and consequently a large degree of contraction/collapse is observed. For higher pressure coals the process is delayed since pore development and consequently wall thinning progress at a slower rate. If or when a pore network is established, a lower degree of contraction/collapse is observed because the event occurs closer to resolidification, where viscosity and elasticity are increasing. For the higher fluidity, high coking pressure coals, a greater degree of swelling is observed prior to cell rupture, and this is considered to be the primary reason for the high coking pressure observed with these coals. An additional consequence of these events is that high pressure coals are likely to contain a higher proportion of closed cells both at and during resolidification, reducing permeability in both the semi-coke and high temperature plastic layers, respectively.Using a rheological mapping approach to follow viscoelastic changes during carbonisation it has been possible to identify specific regions associated with dangerous coals. These tend to be fusible coals where at the onset of expansion, δ (elasticity) < 54° and η* (complex viscosity) > 5 × 10⁵ Pa s, and where in most cases δ_MAX does not exceed 65°.     

Detection of natural oxidation of coking coal by TG-FTIR—mechanistic implications

The natural oxidation/weathering of coal continues to be a subject of interest both scientifically and industrially, in part due to the complexity of the molecular processes at hand as well as to the commercial implications involved. It is widely recognized that coking can be adversely affected by weathering whereas, combustion processes appear to be enhanced as result of oxidation.Combustion techniques are commonly used in the analysis of coal, and organic compounds in general, for the determination of elemental hydrogen, carbon and nitrogen. For oxygen, the method in common practice involves the determination by difference from directly determined values for moisture, ash, sulphur, hydrogen, carbon and nitrogen. This has led us to consider the use of thermogravimetry coupled to gas analysis by infrared spectroscopy (TG-FTIR) to measure organic oxygen in coal directly. Although this technique, developed by Solomon and coworkers, has been extensively used by our group and others, it appears not to have been considered for this particular purpose.Recently, we have shown that TG-FTIR is capable of measuring all the organic oxygen in both fresh and oxidized coal by simultaneous measurement of the three main oxygen-containing gases H₂O, CO and CO₂ evolved during rapid pyrolysis. This gives us a way of measuring quantitatively the oxygen introduced into the coal matrix during oxidation and at least a partial capability of establishing oxygen speciation.We have found, using TG-FTIR, that the early stages of coal oxidation results in the appearance of O-containing functional groups not present in the original coal. The nature of these functional groups is directly related to the oxidation reaction mechanism. These results will be presented and discussed in detail.     

贫瘦煤配煤炼焦试验及应用

对贫瘦煤进行工业分析、镜质组反射率分析和成焦分析,并对其进行20kg小焦炉的配煤炼焦试验和生产应用,研究分析表明,采用贫瘦煤配煤炼焦在合理调整配比的情况下能够生产出质量合格的焦炭。     

炼焦煤混煤特性对配煤炼焦的影响

针对炼焦煤混煤严重造成单种煤质量差异较大的问题,应用煤岩学观点从混煤的角度,探讨了混煤特性对配煤炼焦质量的影响。试验结果显示国内炼焦煤有2/3以上属于混煤,混煤是造成焦炭质量波动的主要原因。按国标以反射率方差S₀来表征混煤特性,其对炼焦煤的粘结性能有一定影响,对焦炭的冷热强度无明显的影响趋势。以微强粘比WQN作为混煤特性指标,对炼焦煤的粘结性影响较小,而对焦炭冷强度M40和焦炭热强度CSR有明显的影响趋势。     

Understanding the mechanisms behind coking pressure: Relationship to pore structure

During carbonisation coal undergoes both physical and chemical changes that result in the generation of gas and tar and the formation of an intermediate plastic state. This transformation is known to generate high internal gas pressures for some coals during carbonisation that translate to high pressures at the oven wall. In this study, three low volatile coals A, B and C with oven wall pressures of 100 kPa, 60 kPa and 20 kPa respectively were investigated using high-temperature rheometry, ¹H NMR, thermogravimetric analysis and SEM, with the primary aim to better understand the mechanisms behind the coking pressure phenomenon. Rheometer plate displacement measurements (ΔL) have shown differences in the expansion and contraction behaviour of the three coals, which seem to correlate with changes in rheological properties; while SEM images have shown that the expansion process coincides with development of pore structure. It is considered that the point of maximum plate height (ΔL_max) prior to contraction may be indicative of a cell opening or pore network forming process, based on analogies with other foam systems. Such a process may be considered important for coking pressure since it provides a potential mechanism for volatile escape, relieving internal gas pressure and inducing charge contraction. For coal C, which has the highest fluidity ΔL_max occurs quite early in the softening process and consequently a large degree of contraction is observed; while for the lower fluidity coal B, the process is delayed since pore development and consequently wall thinning progress at a slower rate. When ΔL_max is attained, a lower degree of contraction is observed because the event occurs closer to resolidification where the increasing viscosity/elasticity can stabilise the expanded pore structure. For coal A which is relatively high fluidity, but also high coking pressure, a greater degree of swelling is observed prior to cell rupture, which may be due to greater fluid elasticity during the expansion process. This excessive expansion is considered to be a potential reason for its high coking pressure.     

Modification of sub-bituminous coal by steam treatment: Caking and coking properties

A Chinese sub-bituminous Shenfu (SF) coal was steam treated under atmospheric pressure and the caking and coking properties of the treated coals were evaluated by caking indexes (G_RI) and crucible coking characterizations. The results show that steam treatment can obviously increase the G_RI of SF coal. When the steam treated coals were used in the coal blends instead of SF raw coal, the micro-strength index (MSI) and particle coke strength after reaction (PSR) of the coke increased, and particle coke reactivity index (PRI) decreased, which are beneficial for metallurgical coke to increase the gas permeability in blast furnace. The quality of the coke obtained from 8% of 200 °C steam treated SF coal in coal blends gets to that of the coke obtained from the standard coal blends, in which there was no SF coal addition in the coal blends. The removal of oxygen groups, especially hydroxyl group thus favoring the breakage of the coal macromolecules and allowing the treated coal formation of much more amount of hydrocarbons, may be responsible for the modified results. The mechanism of the steam treatment was proposed based on the elemental analysis, thermo gravimetric (TG) and FTIR spectrometer characterizations of the steam treated coal.     

Optimization of experimental conditions for recovery of coking coal fines by oil agglomeration technique

The significance of coking coal in the metallurgical sector as well as the meager coking coal reserves across the globe increase the necessity to recover coking coal fines from the fine coking coal slurries generated from coal preparation and utilization activities. Oil agglomeration studies were carried out by varying the experimental conditions for maximum recovery of coking coal fines i.e., yield of the agglomerates. The various operational parameters studied were oil dosage, agitation speed, agglomeration time and pulp density. By using Taguchi experimental design, oil dosage (20%), agitation speed (1100 rpm), agglomeration time (3 min) and pulp density (4.5%) were identified as the optimized conditions. A confirmation experiment has also been carried out at the optimized conditions. The percentage contribution of each parameter on agglomerate yield was analyzed by adopting analysis of variance (ANOVA) statistical method as well as multiple linear regression analysis. The order of influence of the parameters on the agglomerate yield is of the following order: pulp density > oil dosage > agitation speed > agglomeration time. A mathematical model was developed to fit the set of experimental conditions with the yield obtained at each test run and also at the optimized conditions. The experimentally obtained yield was compared with the predicted yield of the model and the results indicate a maximum error of 5% between the two. A maximum yield of 90.42% predicted at the optimized conditions appeared to be in close agreement with the experimental yield thus indicating the accuracy of the model in predicting the results.     

Comparative study of the performance of conventional and column flotation when treating coking coal fines

Investigations were carried out on coking coal fines by conventional cell and column flotation techniques. The effects of different operating parameters were evaluated for both conventional and column flotation. The coal fines were collected from Bhojudih washery, India. These coal fines averaged 24.4% ash, 19.8% volatile matter and 53.8% fixed carbon on a dry basis. A commercial grade sodium silicate, light diesel oil and pine oil were used as depressant, collector and frother respectively. The flotation performance was compared with release analysis. The conventional flotation results indicated that a clean coal with 14.4% ash could be obtained at 78.0% yield with 88.4% combustible recovery. The ash of the clean coal could be further reduced to 10.1% at 72.0% yield with 85.6% combustible recovery by using column flotation. The column flotation results were close to those obtained by release analysis.     

无焦煤炼焦时惰性物对焦炭微观强度的影响

在无焦煤的炼焦条件下,将徐州气煤和枣庄肥煤两种煤混合,以此混煤为基础煤料,添加不同粒度的焦粉、瘦煤、无烟煤进行坩埚焦炼焦实验,对所炼制的坩埚焦进行焦炭光学组织、焦炭显微强度和结构强度分析测试。结果表明,添加惰性物使焦炭的各向异性指数(OTI)增大,I+Mf+FF减小;3种惰性物中,添加焦粉对提高焦炭显微强度和结构强度效果最好;惰性物的粒度越小,提高焦炭质量效果越明显。     

捣固炼焦配入气肥煤的实践

10%～15%的澳大利亚气肥煤配合云南昭通地区10%的无烟煤,并以云南和贵州地区的焦煤作基础煤进行捣固炼焦,在提高煤气发生量的同时对焦炭的机械强度没有产生较大的影响,提高了化产品的回收率和甲醇的产量,取得了较好效果。     

煤岩显微类型对成焦性能的影响

分析测定了7种炼焦煤的煤岩显微组分、显微煤岩类型及其焦炭的显微结构,发现有机组分基质中,矿物质均匀分布有利于成焦,而团块或大颗粒矿物质分布不利于成焦,显微矿化类型比例较高、显微矿质类型比例较低的高灰焦煤和肥煤仍具有良好的成焦性能,可作为配煤使用;气煤和瘦煤的显微矿化类型比例对其成焦性影响较小。     

低阶烟煤对常规炼焦煤热塑性影响规律研究

为提高低灰低硫低阶煤的配煤炼焦用量,在低阶烟煤煤岩分离的基础上,研究了低阶烟煤镜质组对气煤、肥煤、焦煤等常规炼焦煤塑性胶质体的流动性、膨胀性和黏结能力等影响。结果表明,随着低阶烟煤镜质组比例的增加,气煤、肥煤、焦煤塑性胶质体的流动性、膨胀性和黏结力指数下降。低阶煤镜质组的比例为12%时,所得气煤配低阶煤镜质组的流动性下降70.0%,黏结指数下降31.8%;所得肥煤配低阶煤镜质组的流动性下降66.1%,黏结指数下降9.4%;所得焦煤配低阶煤镜质组的流动性下降78.4%,黏结指数下降45.7%。配入低阶煤镜质组比例为8%时,气煤的膨胀度由1.3%降至-19.5%,肥煤的膨胀度由158.7%降至88.2%,焦煤的膨胀度由28.5%降至-1.2%,呈现劣化作用。所得配煤指标并不存在线性关系,对焦煤影响最大,交互作用偏负;对肥煤影响较小,交互作用偏正。     

Effect of coke shrinkage and plastic layer deformation on gas pressure in a coke oven

A plausible mechanism is provided for the generation of internal gas pressure (IGP) in the plastic layer of a coke charge, by relating the formation of low permeability at the outer edges of the plastic layer to the deformability of the plastic material. The amount of lateral shrinkage post-resolidification is shown to be important in constraining the deformation of the plastic material, and hence the generation of low permeability to gas flow. The mechanism provides a natural explanation for permeability variation in the plastic layer, as well as for the linkage between IGP/coking pressure generation and pushing difficulties.