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.     

Influence of the permeability of the coal plastic layer on coking pressure

Ten coals of different rank and coking pressure characteristics were chosen in order to study the time of occurrence of the phenomena that take place during the coking of a coal and the way they affect the generation of dangerous coking pressures. Parameters derived from thermoplastic, thermogravimetric and permeability tests were studied together with semicoke contraction and the coking pressure generated by the coals in a movable wall oven. It was found that for safe coals, the maximum evolution of volatile matter occurs near the temperature of maximum fluidity. The position of the maximum rate of volatile matter evolution with respect to the zone of low permeability varies depending on the coking pressure characteristics of the coals. In addition, the relationship between the period of low permeability to the resolidification temperature may serve to indicate the degree of dangerousness of a coal. The fissure pattern of the semicoke was found to be related to the coking pressure and semicoke contraction.     

The cause of the uneven carbonization process in wet coal charging in coke oven chamber

In the case of the wet coal charging process in coke oven chamber, it is known that the coking process is uneven and a local carbonization delay occurs. The reason was investigated through a laboratory-scale experiment and a quantitative estimation. A partial carbonization test in a test coke oven replicated the uneven plastic layer and local carbonization delay. It was revealed that most of the gas generated in the uncarbonized coal layer results from the evaporation of condensed water and that steam can break through the plastic layer in a test coke oven. Moreover, the order estimation implied that steam that generates in the uncarbonized coal layer and breaks through the plastic layer has sufficient heat capacity to cool the heating wall and delay the carbonization. It was also shown that the steam pressure peak measured in a commercial coke oven is much lower than the estimated steam pressure in this study assuming steam not breaking through the plastic layer. The above-mentioned results and quantitative investigation strongly support the ‘steam breaking through the plastic layer’ theory proposed by Dr. Rohde that an uneven carbonization process is caused by vaporized coal moisture breaking through the plastic layer at definite, unforeseeable points, which results in cooling of the wall by the steam flow.     

Basic study on separate charge of coal and waste plastics in coke oven chamber

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 and the recycling process is operating smoothly. In this process, coals and added plastics are carbonized and changed into coke, tar, oil and coke oven gas in a coke oven chamber. At present, upper limit of the addition rate of waste plastics to blended coals is 1% so that the plastic addition does not affect coke strength. However, the amount of waste plastics in Japan is as much as about 10 million tons per year and there is a real need for increasing the amount of waste plastics treated by the waste plastic recycling process using coke ovens. We investigated a method of increasing the addition rate of waste plastics without affecting coke strength by charging coal and plastic separately in a coke oven chamber. In the case of the same plastic addition rate, charging the plastic in the bottom or the top part of the coke oven chamber can decrease the deterioration of coke strength compared with charging a homogeneous mixture of coal and plastic. Charging the plastic in the bottom decreases the coke strength to a greater extent than charging the plastic in the top. This is because the decomposition of the plastic charged in the bottom decreases the bulk density of the upper coal layer. The results suggest that charging the coal and waste plastics separately increases the amount of waste plastics treated in the waste plastic recycling process using coke ovens. In order to commercialize this method, further studies are necessary concerning the charging method, device and the effect of this method on the coke oven operation.     

中高硫瘦煤配煤炼焦试验及应用研究

为扩大炼焦煤资源,降低配煤成本,采用鄂尔多斯盆地南部渭北煤田西部矿区10号煤层的中高硫瘦煤为试验煤样,分析了煤样基本性质,说明其具有高硫、低灰的特点,黏结指数和胶质层厚度较一般瘦煤高,活惰比接近2,黏结性和结焦性较好。通过中高硫瘦煤单独成焦试验、煤岩学模拟配煤、工业焦炉炼焦试验,验证中高硫瘦煤配煤炼焦的可行性,确定中高硫瘦煤配煤炼焦优化方案。结果表明:中高硫瘦煤配煤炼焦可行,应尽量控制中高硫瘦煤配入量在10%以下,多配入强黏结性煤,以提高焦炭的热态强度。中高硫瘦煤配煤炼焦工业应用表明:配入中高硫瘦煤3%~7%可生产出质量合格的焦炭,扩大了炼焦用煤范围,降低了配煤成本。     

Relation between the coking-chamber height, the coking pressure, and the packing density of regular or partially briquetted coal batch

Since coking coal is characterized by both elasticity and ductility in the plastic state, the coal charge of coke furnaces that contains a plastic layer exerts pressure (coking pressure) on the chamber walls. The pressure exerted depends on the height and mean density of the charge and on the elastoplastic (rheological) conditions. Formulas are obtained for the coking pressure as a function of the height and mean density of the charge and its properties. Those formulas are different for semiindustrial and industrial furnaces. They may be used in the analysis of semiindustrial and industrial coking data.     

不同炼焦方式对焦炭质量的影响

采用国内有代表性的焦炉进行配煤炼焦试验,探讨不同炼焦方式对焦炭质量的影响,结果表明,捣固、配型煤、配沥青炼焦均能改善焦炭质量。当入炉煤的挥发分较高、黏结性偏低时,焦炭质量改善的效果较为显著;采用捣固炼焦比配型煤、配沥青炼焦更能有效地改善焦炭的反应性和反应后强度。     

配煤炼焦试验研究

通过3个单种煤和9个配合煤,放入40kg试验焦炉中,进行常规炼焦、捣固炼焦试验后,对焦炭的质量进行了试验和分析,得到了生产优质焦炭的最佳配煤方案。     

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.     

耦合燃烧室的焦炉炭化室内热过程的数值分析

基于焦炉炼焦的流动、燃烧和传热过程,以质量守恒、能量守恒和动量守恒定律为基础,建立了焦炉燃烧室-炭化室耦合的三维非稳态数理模型,开展了数值模拟,将焦饼中心温度计算值与测量值进行了对比,进一步比较了3种不同水分蒸发模型的计算结果,分析了装炉煤水分含量、初始温度以及堆密度等工艺参数和操作参数对结焦时间的影响。研究结果表明,所建立的燃烧室-炭化室耦合模型能够较好地模拟焦饼的加热过程,并反映焦饼加热的高向均匀性,同时采用煤预热技术、煤调湿技术、煤料密实工艺可以很好地提高生产效率,为焦炉生产实际提供理论指导依据。     

炼焦煤粒度调整技术

分析了粒度对炼焦煤结焦性影响的研究进展,结合煤料显微组分特征、成焦机理等,介绍了粒度对煤料结焦性影响的主要研究方法。分析了原料煤粉碎粒度和焦炭质量的相关性,指出应充分利用煤料破碎后显微组分含量与粒度的关系。对煤料按粒度大小进行选择性利用,可以提高煤炭的利用率。超细粉碎灰分较高的煤料,可以使煤中无机物和有机物有效分离,提高选煤过程中的精煤产率,扩大炼焦煤源。但直接利用超细粒度煤料炼焦,可能会增加装炉操作压力、降低煤料黏结性。     

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.     

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。     

Feedstock recycling of plastic wastes/oil mixtures in cokemaking

Two lubricating oils, a plastic waste composed mainly of polyolefins (95%) and their mixtures (1:1 w/w) were assessed for possible use as minor components of coal blends for metallurgical coke production. The addition of 2 wt% plastic waste causes a decrease in the maximum fluidity of the coal developed during thermal heating between 400 and 500 °C. At the same addition rate, the two oils are good additives for mixing with coal/plastic blends in order to partially restore the caking ability of the co-carbonizing systems. Co-carbonizations of the coking blend with the different wastes were carried out in a movable wall oven of 15 kg capacity. Although the bulk density remained unchanged, the addition of the plastic waste produced an increase in coking pressure to values that were too high for it to be considered as a safe blend. At the same time the mechanical strength of the partially gasified coke was improved as reflected by the CSR index. The oils, however, had the effect of reducing bulk density and the coking pressure generated during the process. When blended with the coal and the plastics, the oils appeared to act as good solvents of the polyolefins and also proved to be effective in decreasing coking pressure without negatively affecting coke quality.     

Effects of weathered coal on coking properties and coke quality

Determinations of weathered coal by petrographic methods, and coking tests in an 18-inch ($\text{≈}\text{1}\text{2}\text{m}$) test oven were used to quantify the effects of weathered coal on coking properties and coke quality. The results show that the presence of weathered coal causes a decrease in coke stability and coking rate and an increase in coke reactivity and coke-breeze generation. Because these effects contribute to increased costs in both the coke plant and the blast furnace, every effort should be made to reduce the amount of weathered coal in coking coal mixes.     

Plastic wastes, lube oils and carbochemical products as secondary feedstocks for blast-furnace coke production

Two plastic wastes (polyolefin-enriched and multicomponent), two lube oils (paraffinic and synthetic) and one coal-tar were assessed as individual and combined additives to coal blends for the production of blast furnace coke. The effects of adding 2 wt.% of these additives or their mixtures (50:50 w/w) on the coking capacity of coal, coking pressure and coke quality parameters were investigated. It was found that the two plastic wastes reduce fluidity, whereas the addition of oils and tar helps to partially restore the fluidity of the coal-plastic blend. From the co-carbonization of the coking blend with the different wastes in a movable wall oven of over 15 kg capacity, it was deduced that polyolefins have a detrimental effect on coking pressure. The addition of oils and tar to the coal-plastic blend has different modifying effects. Whereas paraffinic oil eliminated the high coking pressure caused by the polyolefins, polyol-ester oil had a weak reducing effect unlike coal-tar which had a strong enhancing effect. The compatibility of the oils/tar with plastics and coal and the beneficial influence of these combinations on coking pressure is discussed in relation to the miscibility of the plastic and the oily and bituminous additives, and the amount and composition of the volatile matter evolved from each additive during pyrolysis as evaluated by thermal analysis. Furthermore, it was found that coke reactivity towards CO₂ (CRI) and coke strength after reaction with CO₂ (CSR) are heavily dependent on the composition of the plastic waste, with polystyrene (PS) and polyethylene terephthalate (PET) having a clear negative effect. The porosity of the cokes obtained from blends containing plastic wastes is always higher, but the pores are smaller in size.     

捣固炼焦值得关注的几个问题

叙述了捣同炼焦配煤与焦炭质量的关系及捣固强度与配煤的关联性.指出:焦炭质量的基础是配煤质量,不会因煤准备和炼焦工艺等有根本性的改变;捣固炼焦可多用低变质程度,高挥发分气煤类的炼焦煤生产出一定质量的焦炭;捣同炼焦配煤适当增加黏结性配煤比例可提高焦炭质量,但用接近顶装焦炉的配煤进行捣固炼焦将丧失其优势并引发问题.捣固焦炉的...     

Coal blending theory for dry coal charging process

Nippon Steel has successfully developed dry coal charging processes such as CMC and DAPS for cokemaking. In this report, the fundamental aspects of the coal blending theory for dry coal charging processes are investigated. The investigation has made it clear that even in cases of high coal bulk density due to dry coal charging processes, it is possible to control coking pressure by adjusting the blending ratio of a slightly caking and low rank coal; and it is also possible to produce high quality coke by adjusting the total dilatation of the blended coal at a suitable level.     

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°.     

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.