As the world’s largest steel producer, China has seen a continuous optimization of its graphite electrode industry’s product structure in recent years, leading to an increasing share of ultra-high-power graphite electrodes and a growing demand for needle coke. Although China's needle coke has undergone nearly... Over the past 30 years, research and development have led to industrial-scale production, and capacity has been rapidly expanding worldwide. However, product quality still lags somewhat behind that of imported products. This article primarily explores how these factors influence the quality of needle coke from the following five aspects, with the aim of providing guidance for industrial production.

I. In the raw asphalt The Influence of QI on the Formation of Needle-Like Coke

In the production of needle coke, the raw asphalt generally needs to meet the following requirements: The QI content is below 0.1%. However, in actual production, due to the considerable difficulty in pre-treating asphalt, it is very challenging to maintain the QI content in the raw asphalt below 0.1% for an extended period. Typically, it is sufficient to keep the QI content within 0.3%. When the raw asphalt has a high QI content, the reaction in the coking tower can be regarded as heterogeneous nucleation. The key characteristics of this reaction are as follows: In the early stages of the reaction, the QI present in the raw asphalt acts as a “nucleus,” accelerating the formation rate of small mesophase spheres within the system. Moreover, the small mesophase spheres formed via heterogeneous reactions tend not to coalesce easily at first and hardly generate new spheres. Later, once these spheres have grown and merged, they exhibit poor thermal deformability and, in the absence of external forces, are unlikely to develop into needle-like structures. Asphalts with high QI content—i.e., cokes formed through heterogeneous reactions—are predominantly characterized by an interlocking structure, with very few fibrous structures. Such cokes appear black, and after calcination, they lack the silvery-gray metallic luster typically associated with high-quality cokes. On the other hand, when the raw asphalt has a low QI content, the reaction in the coking tower can be considered homogeneous nucleation. The defining feature of this reaction is that, at any stage during the initial reaction period, the formation, growth, and merging of small mesophase spheres occur simultaneously. The resulting fused structures possess strong thermal deformability and readily develop needle-like structures under the influence of external forces, such as the overflow of high-temperature hydrocarbons. Cokes formed from such low-QI raw asphalts generally exhibit a broad, stream-lined structure, and after calcination, they display a silvery-gray metallic luster.

II. The Impact of Feed Distribution Methods on Needle Coke Production

In a coking tower without a feed distributor, the feed inlet is located at the center of the bottom cover of the tower. After feeding begins, the material spreads outward in a wave-like pattern, with the feed channel situated at the center of the tower. Due to the relatively narrow feed pipe and the relatively wide diameter of the coking tower, as the material spreads outward following the feed, it takes longer for the material to reach the tower walls. During this process, on the one hand, the larger spreading surface and the extended spreading time result in a faster overflow rate of light components within the system. As the material spreads further toward the periphery, the system viscosity increases, making it more difficult for intermediate-phase microspheres to form and coalesce. Moreover, as the material moves farther toward the periphery, the system viscosity rises even higher, while the light components become increasingly smaller, leading to insufficient generation of coke oven gas and consequently poor coke structure—especially at the tower walls. On the other hand, the prolonged spreading time causes an uneven radial temperature distribution within the coking tower, which is detrimental to the reaction process. By modifying the feed method at the bottom of the coking tower so that the feed spreads outward and forms a feed channel positioned somewhere between the center and the wall of the tower—in a cylindrical distribution—the following advantages are achieved: Once the material enters the tower, it quickly covers the entire radial cross-section, resulting in a uniform temperature distribution and consistent material viscosity. This creates favorable conditions for the formation and coalescence of intermediate-phase microspheres. Furthermore, the broader distribution of the feed channel not only provides optimal conditions during the blowing of oil and gas but also helps address the issue of poor coke quality along the tower walls.

3. The Influence of Coking Tower Operating Pressure on the Formation of Needle-Like Coke

During the production of needle coke, the process must be carried out under relatively high reaction pressure. During operation, the coking tower needs to undergo pressure-raising procedures. In the later stages of feedstock introduction, pressure reduction can be considered as appropriate, but such reductions should be small in magnitude. Since the properties of raw materials and equipment parameters vary among different units, the optimal reaction pressure required will also differ; therefore, the specific pressure conditions should be determined based on the unique circumstances of each unit.

The effect of operating pressure on needle coke production is as follows: The formation, growth, and coalescence of mesophase spherules require a system with relatively low viscosity. If the coking tower pressure is too low, once the feedstock enters the coking tower, the light components will quickly vaporize as oil and gas, increasing the viscosity within the reaction system and hindering the mutual contact between mesophase spherules. In the later stages of the reaction, since there is insufficient oil and gas in the system to facilitate coke drawing, this poses a challenge to the formation of fibrous structures. structure It will also have an impact. Maintaining low viscosity in the reaction system over a longer period facilitates the growth and coalescence of intermediate-phase microspheres. Increasing the pressure in the coking tower can slow down the overflow rate of light components within the system, thereby extending their residence time. This extended residence time allows the oil and gas to undergo directional coke formation, promoting the transformation of coalesced microspheres into wide-area structures. —Streamlined structure. In the later stage of feeding, to enable rapid reaction and the formation of a needle-like structure, appropriately reducing the pressure can increase the rate of oil-gas overflow, thereby enhancing the external pulling force and accelerating the time required for fiber structure formation.

IV. The Influence of Temperature on the Formation of Needle-Like Coke

During coking production, the temperature requirement generally refers to the outlet temperature of the heating furnace. After the feedstock enters the coking tower, the inlet temperature varies depending on differences in pipeline insulation among various units, insulation of the coking tower itself, and heat loss. However, the general principle to follow is this: during the initial stage of feed introduction, the temperature should be kept relatively low; during the later stages of feed introduction, the temperature should be kept relatively high. The effect of temperature on needle coke production can be summarized as follows: the process by which pitch ultimately transforms into needle coke can be roughly described as involving polycyclic aromatic hydrocarbons and large-molecular aromatics. —Intermediate-phase microspheres—The microspheres grow and coalesce—airflow draws the coalesced particles into a fibrous structure—then they solidify into coke. Since the reaction cycle for delayed coking in a single tower is about 36 hours, the feed material at the initial stage has the longest reaction time; therefore, the temperature control at the early stage should be relatively low to provide sufficient reaction time for the growth and coalescence of intermediate-phase microspheres. As feeding continues, the reaction time of the material gradually shortens, necessitating a corresponding increase in reaction temperature to reduce the time required for the intermediate-phase microspheres to develop into a fibrous structure. Thus, during the later stages of feeding, a higher reaction temperature is needed. If the reaction temperature is too low, it will adversely affect indicators such as the strength and yield of the coke; if the reaction temperature is too high, the reaction will proceed too rapidly, disrupting the orderly arrangement of the needle-like coke fiber structure.

V. The Impact of Cycle Ratio on the Formation of Needle-Like Coke

Since the properties of raw materials vary from region to region, the recycling ratios used also differ accordingly. The recycling ratio primarily affects the viscosity and the tensile strength within the reaction system. If the recycling ratio is too high, on the one hand, it may lead to non-homogeneous reactions occurring within the raw asphalt; on the other hand, due to an excessively rapid rate of oil and gas overflow, the drawing process can become overly vigorous, compromising the regularity of the needle coke fiber structure. Conversely, if the recycling ratio is too low, on the one hand, it may cause the viscosity within the system to become excessively high, hindering the growth and coalescence of mesophase spheres; on the other hand, it may also result in insufficient oil and gas supply for drawing, thereby impeding the formation of a well-defined fiber structure.

Conclusion

Through extensive production practice, this paper primarily explores the content of quinoline-insoluble substances in raw asphalt from the perspective of industrialized production. The following conclusions were drawn regarding the effects of factors such as QI, the feed distribution method in the coking tower, the outlet temperature of the coking heater, the pressure in the coking tower, and the circulation ratio on the formation and structural texture of needle coke:

( 1) The lower the QI content in the raw asphalt, the better—generally, it should not exceed 0.3%.

( 2) The feed to the coking tower should be distributed evenly around its perimeter, which will improve the reaction environment inside the tower. Typically, a feed distributor is required.

( 3) The outlet temperature of the heater should be controlled as much as possible between 470 and 500℃. If the temperature is too low, the reaction will be incomplete; if it’s too high, material coking will accelerate and the risk of coking in the heater will increase.

( 4) The operating pressure should be determined based on the properties of the materials used in your facility and should be moderate. Both excessively low and excessively high pressures are detrimental to the formation of needle coke.

( 5) The circulation ratio is generally maintained around 1.

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